Department of Biology, Washington University, St. Louis, Missouri 63130
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ABSTRACT |
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Ji, Weiqing, Enquan Gao, and Nobuo Suga. Effects of Acetylcholine and Atropine on Plasticity of Central Auditory Neurons Caused by Conditioning in Bats. J. Neurophysiol. 86: 211-225, 2001. In the big brown bat (Eptesicus fuscus), conditioning with acoustic stimuli followed by electric leg-stimulation causes shifts in frequency-tuning curves and best frequencies (hereafter BF shifts) of collicular and cortical neurons, i.e., reorganization of the cochleotopic (frequency) maps in the inferior colliculus (IC) and auditory cortex (AC). The collicular BF shift recovers 180 min after the conditioning, but the cortical BF shift lasts longer than 26 h. The collicular BF shift is not caused by conditioning, as the AC is inactivated during conditioning. Therefore it has been concluded that the collicular BF shift is caused by the corticofugal auditory system. The collicular and cortical BF shifts both are not caused by conditioning as the somatosensory cortex is inactivated during conditioning. Therefore it has been hypothesized that the cortical BF shift is mostly caused by both the subcortical (e.g., collicular) BF shift and the activity of nonauditory systems such as the somatosensory cortex excited by an unconditioned leg-stimulation and the cholinergic basal forebrain. The main aims of our present studies are to examine whether acetylcholine (ACh) applied to the AC augments the collicular and cortical BF shifts caused by the conditioning and whether atropine applied to the AC abolishes the cortical BF shift but not the collicular BF shift, as expected from the preceding hypothesis. In the awake bat, we made the following findings. ACh applied to the AC augments not only the cortical BF shift but also the collicular BF shift through the corticofugal system. Atropine applied to the AC reduces the collicular BF shift and abolishes the cortical BF shift which otherwise would be caused. ACh applied to the IC significantly augments the collicular BF shift but affects the cortical BF shift only slightly. ACh makes the cortical BF shift long-lasting beyond 4 h, but it cannot make the collicular BF shift long-lasting beyond 3 h. Atropine applied to the IC abolishes the collicular BF shift. It reduces the cortical BF shift but does not abolish it. Our findings favor the hypothesis that the BF shifts evoked by the corticofugal system, and an increased ACh level in the AC evoked by the basal forebrain are both necessary to evoke a long-lasting cortical BF shift.
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INTRODUCTION |
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In the big brown bat
(Eptesicus fuscus), repetitive focal electrical stimulation
of the auditory cortex (AC) with 0.2 ms-long, 100-nA electric pulses
for 30 min evokes changes in the best frequencies (BFs),
frequency-tuning curves, and auditory responses of collicular and
cortical neurons. These changes recover 180 min after the stimulation
(Chowdhury and Suga 2000; Ma and Suga
2001
; Yan and Suga 1998
). Repetitive acoustic
stimuli delivered to the bat without any electrical stimulation also
evoke small short-lasting collicular (Gao and Suga 1998
;
Yan and Suga 1998
) and cortical changes
(Chowdhury and Suga 2000
). Auditory conditioning with
repetitive acoustic stimuli followed by electric leg-stimulation evokes
the conditioned response of the bat and causes large collicular changes
that recover 180 min after the conditioning (Gao and Suga
1998
) and large cortical changes that last more than 26 h
after the conditioning (Gao and Suga 2000
). For these
changes caused by the conditioning, the AC and the somatosensory cortex
are both necessary (Gao and Suga 1998
, 2000
).
Gao and Suga (1998, 2000
) incorporated their findings
with the hypothesis proposed by Weinberger and his coworkers
(1990)
and proposed the following working hypothesis of the
corticofugal function for cortical plasticity. When behaviorally
irrelevant acoustic stimuli are delivered to an animal, the AC and the
corticofugal system perform egocentric selection, which is a small and
short-term modulation of subcortical signal processing. Accordingly,
the small and short-term cortical change is evoked (Fig.
1, left column). When the
acoustic stimuli are paired with electric leg stimulation, the auditory
and somatosensory signals ascend from the periphery to the auditory and
somatosensory cortices, respectively (Fig. 1, middle
column), and then to the amygdala through association cortices.
These signals are perhaps associated in the association cortices and
the amygdala, which is essential for evoking a conditioned behavioral
response. Therefore the acoustic stimuli become behaviorally relevant
to the animal. The amygdala sends the "associated" signal to the
cholinergic basal forebrain, which increases the cortical acetylcholine
level. Then the change in the AC is augmented (Fig. 1, right
column) (Kilgard and Merzenich 1998
;
Weinberger 1998
). Accordingly, egocentric selection is
augmented and the subcortical change becomes larger, so that the
cortical change becomes larger and long term. This positive feedback
loop is controlled by inhibition mediated by the thalamic reticular
nucleus.
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The importance of the amygdala for conditioned response (reviews by
LeDoux 2000; Weinberger 1998
), of the
cholinergic basal forebrain for plasticity of the AC (Bakin and
Weinberger 1996
; Bjordahl et al. 1998
;
Kilgard and Merzenich 1998
; review by Rasmusson 2000
), and of the corticofugal auditory system for collicular plasticity (Gao and Suga 1998
, 2000
) has been well
demonstrated. However, it has not yet been studied whether ACh applied
to the AC augments collicular and cortical plasticity caused by
auditory conditioning as hypothesized by Gao and Suga
(2000)
.
There has been a large number of papers on the auditory, somatosensory,
and visual cortices indicating that the cholinergic basal forebrain and
acetylcholine (ACh) in the cortex play an important role in cortical
plasticity, as reviewed by Buonomano and Merzenich
(1998), Sarter and Bruno (2000)
, and
Rasmusson (2000)
. The papers on the somatosensory and
visual systems are reviewed in DISCUSSION, but those on the
auditory system are briefly reviewed in the following text.
In the AC of the guinea pig, plasticity in frequency tuning (i.e., BF
shift) is caused by a tone burst paired with electrical stimulation of
the cholinergic basal forebrain but not by the tone burst or the
electrical stimulation alone, and it is similar to that caused by
behavioral learning (Bakin and Weinberger 1996; Bjordahl et al. 1998
). In the cat's AC, massive
progressive reorganization of the cochleotopic map (i.e., BF shift) is
caused by electrical stimulation of the basal forebrain paired with a
tone burst (Kilgard and Merzenich 1998
). In the big
brown bat, electrical stimulation of the basal forebrain augments
collicular and cortical BF shifts evoked by a train of acoustic stimuli
or electrical stimulation of the AC (Ma and Suga 2000
).
In the ACs of the squirrel monkey (Foote et al. 1975)
and the guinea pig (Metherate et al. 1990
), ACh
iontophoretically applied to cortical neurons augments their responses
to acoustic stimuli. In the ACs of cats (McKenna et al.
1989
; Metherate and Weinberger 1989
, 1990
), an
iontophoretic application of ACh to cortical neurons paired with tone
bursts reduces their responses at the frequency of the tone bursts but
increases their responses at other frequencies. Such effects of ACh are
antagonized by atropine iontophoretically applied to these cortical
neurons. In the inferior colliculus (IC) of the horseshoe bat
(Rhinolophus ferrumequinum), ACh iontophoretically applied
to collicular neurons does not change their frequency-tuning curves but
increases the auditory responses of 37% of them and reduces the
auditory responses of 16% of them. On the other hand, atropine
decreases the auditory responses of 62% of the neurons studied
(Habbicht and Vater 1996
).
ACh apparently plays an important role in cortical plasticity and has
excitatory and/or inhibitory effects on collicular and cortical
neurons. However, there have been no direct studies about the role of
ACh in BF shifts (i.e., reorganization of the cochleotopic map) caused
by auditory conditioning. The aim of our present paper is to report the
effects of ACh and/or atropine applied to the AC or IC on collicular
and cortical BF shifts (plasticity) caused by the conditioning. We
specifically examined the following four questions: whether ACh applied
to the AC augments cortical plasticity as expected from the hypothesis
that the cholinergic basal forebrain causes cortical plasticity
(Weinberger 1998; Weinberger et al. 1990
)
and also collicular plasticity as expected from the hypothesis that the
corticofugal system plays a role in collicular plasticity (Gao
and Suga 1998
, 2000
); whether atropine applied to the AC abolishes or dramatically reduces cortical plasticity, as expected from
the hypothesis proposed by Weinberger and his coworkers
(1990)
, but not collicular plasticity as expected from both
Gao and Suga's hypotheses (1998
, 2000
) and the fact
that the corticofugal system as well as cortical signal processing does
not depend on ACh (Tsumoto 1990
); whether ACh applied to
the IC does not augment cortical plasticity; and whether atropine
applied to the IC does not abolish cortical BF shift.
Auditory conditioning causes short-term collicular plasticity and
long-term cortical plasticity in the big brown bat (Gao and Suga
2000). It also causes short-term thalamic plasticity (Edeline and Weinberger 1991
) and long-term cortical
plasticity in the guinea pig (Weinberger et al. 1993
).
In the big brown bat, focal electric stimulation of the AC evokes
brief-term collicular plasticity (Jen et al. 1998
) or
short-term collicular (Ma and Suga 2001
; J Yan
and Suga 1996
; W Yan and Suga 1998
; Zhang
and Suga 2000
) and cortical plasticity (Chowdhury and
Suga 2000
; Ma and Suga 2001
). We have a
hypothesis that ACh is necessary to change cortical plasticity from the
short term to the long term. In the studies of plasticity, the time
course as well as the magnitude and direction of plastic changes is the
important measure, as demonstrated by Gao and Suga
(2000)
. Therefore our present studies were particularly focused
on the effects of ACh or atropine on the time course of collicular and
cortical plasticity. We found that ACh and atropine have profound
effects on collicular and cortical plasticity caused by auditory
conditioning and that ACh changes short-term plasticity into long-term
plasticity in the AC but not in the IC. (Here, the brief-, short-, and
long-term plasticities are, respectively, defined as the changes that
last not more than 10 s, not more than 4 h, and longer than 4 h after the cessation of cortical electric stimulation, auditory
conditioning, or repetitive acoustic simulation.)
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METHODS |
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Preparation
Experiments were performed with 42 adult big brown bats,
E. fuscus (body weight: 18-24g). Procedures for animal
preparation, acoustic stimulation, electric leg-stimulation, recording
of action potentials, and drug applications were the same as those
previously described (Gao and Suga 1998, 2000
). The
protocol for this research was approved by the animal studies committee
of Washington University.
Under neuroleptanalgesia (Innovar, 4.08 mg/kg body wt), a 15-mm-long metal post was glued to the dorsal surface of the bat's skull. Experiments for recording auditory responses from single neurons, conditioning animals, and drug application began 3-4 days after the surgery. The awake bat was placed in a polyethylene-foam body mold suspended by an elastic band at the center of a soundproof room maintained at a temperature of about 31°C. The temperature monitored with a thermister placed between the bat and body mold was 37°C. The head was immobilized by fixing the metal post glued on the skull onto a metal rod with set screws, and it was adjusted to face directly at a loudspeaker located 74 cm away. The bat was neither anesthetized nor tranquilized during experiments.
Electrodes for recording action potentials
For electrode penetrations into the AC or IC, holes of about 50 µm in diameter were made in the skull with the tip of a sharpened needle under a dissection microscope (40 magnification). [For a drug application, a hole of ~1.0 mm in diameter was made in the skull (see following text).] The awake animals showed no signs of distress with this procedure yet responded vigorously to accidental touching of the exposed surgical wounds or the face. This indicated their awareness and ability to express pain if so needed.
For recording of action potentials, a sharpened, vinyl-coated tungsten-wire microelectrode with a tip diameter of ~7 µm was inserted orthoganally into the AC or dorsoventrally into the IC through the holes. An indifferent tungsten-wire electrode was placed on the dura mater near the recording electrode.
Acoustic and/or electric stimuli
Acoustic stimuli (20-ms-long tone bursts with a 0.5-ms rise-decay time) were delivered to the bat at a rate of 5/s. Their frequency and amplitude were manually varied to measure the BF and minimum threshold of a given neuron. The tone bursts were also computer-controlled. The sharpness of the manually measured tuning curve was used to determine the step size (0.2-0.5 kHz) of a computer-controlled frequency scan: the sharper the tuning curve, the smaller the step. The frequency scan consisted of 34 150-ms-long time blocks. In each scan, a single tone burst was delivered at the beginning of each block, and the frequency of the tone burst was shifted in 33 steps across the BF of the neuron. In the last time block, no simulation was presented to count background discharges. The amplitude of the tone bursts in the scan was always set at 10 dB above the minimum threshold of the neuron so as to easily detect a BF shift. An identical frequency scan was delivered 50 times to obtain an array of peristimulus-time (PST) histograms as a function of frequency. BFs and frequency-response curves were obtained by counting the total numbers of impulses discharged to 50 identical tone bursts.
To evoke the BF shifts of cortical and collicular neurons, the bat was
conditioned with a 1.0-s-long train of acoustic stimuli (ASt), followed by a 1.0-s gap and then by an
electric leg-stimulus (ESl; hereafter,
ASt + ESl).
ASt and ESl were the
conditioned and unconditioned stimuli, respectively. In
ASt, the tone bursts were 50 dB SPL and 10-ms
long and were delivered at a rate of 33/s. Their frequencies were set
5.0 kHz lower than the BF of a given cortical or collicular neuron to
be studied because in the big brown bat, ASt
evokes the largest BF shift for cortical (Chowdhury and Suga
2000) and collicular neurons (Gao and Suga 1998
;
Yan and Suga 1998
) with a BF ~5.0 kHz higher than
ASt. ESl was a 50-ms-long
monophasic electric pulse. The intensity of ESl was 0.15-0.57 mA, just above the threshold for eliciting a leg flexion. A single (ASt or
ESl) or a paired stimulus
(ASt + ESl) was delivered
every 30 s for 15 min (30 times in total) or 30 min (60 times in
total). To minimize a cumulative effect of conditioning, only one
neuron was studied in a 1-day experiment, and the same animal was used
with a 1- to 3-day interval. In each 1-day experiment, tone bursts
alone were delivered at a rate of 5/s over 2-3 h to record
single-unit activity and to obtain data in the control condition. This
period presumably caused extinction of BF shifts, if any, remaining
after a previous conditioning experiment.
Drug applications
To investigate the role of ACh in BF shift caused by the
conditioning, ACh and/or atropine were applied to the IC or AC before or after the conditioning. The AC was first electrophysiologically mapped to locate its approximate center (Dear et al.
1993). In the big brown bat, the dorsal surface of the IC was
directly visible through the skull. A 1.0-mm-diam hole was made in the
skull at the center of this visible portion for a drug application. The solutions of 1 M acetylcholine chloride (pH 4.5; dissolved in distilled
water) and/or 0.4 M atropine sulfate (pH 5; dissolved in 0.9% saline)
were applied to the exposed surface of the AC or IC with a 1.0 µl
Hamilton syringe or syringes. The supplier of these drugs was Sigma
Chemical in St. Louis, MO. A saline solution (0.9% sodium chloride)
was also applied to the AC or IC for sham experiments.
Data acquisition and processing
The responses of single cortical and collicular neurons to tone bursts were respectively recorded at 200-700 µm depths (layers III-V) in the AC and 300-2,000 µm depths in the central nucleus of the IC with tungsten-wire microelectrodes (~7 µm tip diameter). A BAK time-amplitude window discriminator was used to select action potentials from a single neuron.
The waveform of an action potential of a single neuron was stored on a digital storage oscilloscope at the beginning of the data acquisition and was used as a template. Action potentials discharged by the neuron were continuously monitored together with the template on the screen of the digital storage oscilloscope during data acquisition: before, during, and after the conditioning and/or drug application. Data were acquired every 15 min over 225 min after the conditioning as far as action potentials visually matched the template. Data were stored on a hard drive of a personal computer and were used for off-line analysis.
Off-line data processing included plotting PST or peristimulus time-cumulative (PSTC) histograms displaying responses of a single collicular or cortical neuron to 50 identical acoustic stimuli and frequency-response curves based on 50 frequency scans obtained before, during, and after the conditioning and/or drug application. The magnitude of auditory responses was expressed by a number of impulses per 50 identical stimuli after subtracting background discharges counted in the last block of the frequency scan. The BF of the neuron was defined as the frequency at which the frequency-response curve was peaked (e.g., Fig. 2). To study the time course of BF shift, BFs determined from the frequency-response curves obtained every 15 min were plotted as a function of time. The time courses of BF shifts obtained from several neurons were averaged (e.g., Fig. 3). Therefore each data point in an averaged time course represents the mean and standard error based on 50 responses multiplied by a number of neurons (N) used for averaging. A t-test was used to test the difference between the auditory responses obtained before and after the conditioning and/or drug application and to test the difference between the responses of collicular and cortical neurons.
The following criteria were used for a shift in the frequency-response
curve or BF of a collicular or cortical neuron evoked by the
conditioning and/or drug application. If a shifted frequency-response curve or BF of a collicular neuron did not recover by more than 50%,
the data were excluded from the analysis. In stable, long recording
conditions, all curves shifted by the conditioning and/or drug
application recovered by more than 50%. This recovery itself helped
prove that the shift was significant and that the shift was not due to
recording action potentials from different neurons. However, this
criterion was not necessarily applied to cortical BF shift because it
lasted several hours after a 30-min-long conditioning. Cortical BF
shifts as well as collicular ones were highly specific to the frequency
of a conditioned tone (Gao and Suga 1998, 2000
; present
study). This indicates that BF shifts were not due to random change in
recording action potentials from different neurons.
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RESULTS |
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In 42 bats, 136 collicular and 149 cortical neurons were studied before and after auditory conditioning and/or drug application. Of the 285, different numbers of neurons were examined for the effect of ACh and/or atropine applied to the AC or IC as indicated in individual figures representing the time courses of BF shifts. BF shifts caused by the conditioning with or without an application of saline solution were obtained from 54 collicular and 51 cortical neurons. Effects of ACh applied to the AC or IC were studied on the BF shifts of 39 collicular and 44 cortical neurons; the effects of atropine applied to the AC or IC were studied on the BF shifts of 36 collicular and 49 cortical neurons; and the effects of ACh-atropine mixture applied to the AC or IC were studied on the BF shifts of 7 collicular and 5 cortical neurons. BF shifts at subthreshold (see DISCUSSION) were caused by a 15-min-long conditioning session to examine the effects of ACh on them, and BF shifts above threshold were caused by a 30-min-long conditioning session to examine the effects of atropine on them.
Collicular and cortical changes caused by conditioning with or without an application of saline solution (control data to drug-application experiments)
When a train of acoustic stimuli (ASt)
followed by an electric leg-stimulation (ESl) was
delivered to the animal for 30 min, collicular and cortical neurons
with a BF 5.0 kHz higher than the frequency of
ASt showed the largest BF shift toward the
frequency of ASt, as previously reported by
Gao and Suga (1998). Therefore all the data reported in
the present paper were obtained from neurons tuned to a frequency 5.0 kHz higher than the frequency of ASt.
Figure 2A shows the responses
to tone bursts and the frequency-response curves of a single collicular
neuron which was tuned to 37.0 kHz (Fig. 2A, curve 1 in
c). When the animal was conditioned with a 32.0-kHz
ASt paired with ESl, the
response (number of impulses per 50 stimuli) of the neuron decreased by
28% to a 37.0-kHz tone burst (Fig. 2A; compare a,
2 with 1), but increased by 103% to a 35.5-kHz tone
burst (Fig. 2A; compare b, 2 with 1).
As a result of these frequency-dependent changes, the
frequency-response curve and BF of the neuron shifted from 37.0 to 35.5 kHz (Fig. 2A, curve 2 in c). Both the response
and the frequency-response curve returned (hereafter recovered) to
those in the control condition 170 min after the conditioning (Fig.
2A, a4, b4, and curve 4 in c). The response at the BF in the shifted condition was 10% less than that at
the BF in the control condition as previously reported by Gao
and Suga (1998). All 13 collicular neurons studied for the
30-min-long conditioning showed changes that were basically the same as
those described in the preceding text. The mean BF shift and the mean
decrease in response at the shifted BF were 1.35 ± 0.23 kHz and
11.6 ± 5.78% (n = 13), respectively.
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Figure 2B shows the responses to tone bursts and the frequency-response curves of a single cortical neuron tuned to 35.0 kHz (Fig. 2B, curve 1 in c). When conditioned with a 30.0-kHz ASt followed by ESl, the response of the neuron immediately after the conditioning decreased by 14% to a 35.0-kHz tone burst (Fig. 2B; compare a, 2 with 1) and increased by 9.2% to a 34.5-kHz tone burst (Fig. 2B; compare b, 2 with 1). As a result, the BF of the neuron shifted down to 34.5 kHz from 35.0 kHz (Fig. 2B, curve 2 in c). About 180 min after the onset of the conditioning, the response decreased further by 27% at 35.0 kHz (Fig. 2B; compare a, 3 with 1) and increased by 22% at 33.5 kHz (Fig. 2B; compare b, 3 with 1). Because of these changes, the frequency-response curve shifted further down to 33.5 kHz (Fig. 2B, curve 3 in c). The response at the shifted BF was 10% less than that at the control BF. The changes in response and frequency-response curve of the neuron showed no sign of recovery even 360 min after the conditioning (Fig. 2B, a4, b4, and curve 4 in c). All 12 cortical neurons studied for the 30-min-long conditioning showed changes that were basically the same as those described in the preceding text. The mean BF shift and the mean decrease in response at the maximally shifted BF were 1.43 ± 0.19 kHz and 10.3 ± 2.1% (n = 12), respectively. These cortical changes were very similar to the collicular changes (P > 0.05). However, the time course of BF shift was dramatically different between collicular and cortical neurons.
The mean time courses of the BF shifts of 13 collicular (Fig.
3A, ) and 12 cortical
neurons studied (Fig. 3B,
) showed the following
differences between them: the collicular BF shift was largest at the
end of the conditioning, and monotonically recovered ~180 min after
the conditioning; the cortical BF shift developed slowly, reached a
plateau ~60 min after the conditioning, and plateaued for more than
210 min; and immediately after the 30-min-long conditioning, the
cortical BF shift was smaller than the collicular BF shift (0.83 ± 0.11 vs. 1.15 ± 0.09 kHz; P < 0.01).
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As described later, ACh or atropine was applied to the AC or IC before or after the conditioning. Therefore sham experiments were performed with a saline solution. An application of saline solution to the AC 5 min prior to or 75 min after the onset of the 30-min-long conditioning session did not affect any of the auditory responses, frequency-response curves, and time courses of BF shifts of 15 collicular and 17 cortical neurons caused by the conditioning session. Figure 3, A and B, shows that the mean time courses of the collicular and cortical BF shifts caused by the 30-min-long conditioning were not affected by a saline application.
A 15-min-long conditioning session caused no changes in the responses,
frequency-tuning curves, and BFs of 10 collicular and 8 cortical
neurons studied (Fig. 3, C and D, and
). A
saline solution applied to the AC before or after the 15-min-long
conditioning also evoked no changes in 16 collicular and 14 cortical
neurons studied (Fig. 3, C and D,
,
, and
×).
Effects of ACh or atropine applied to the AC on collicular and cortical responses
ACh applied to the AC augmented responses to tone bursts and
background discharges of 26 collicular neurons studied. The mean maximal increases in response at the BF and background discharges of
the collicular neurons were 49.8 ± 11.6 and 9.7 ± 6.5%
(n = 26), respectively, which occurred 60 min after the
ACh application. These augmentations disappeared ~150 min after the
application (Fig. 4A, ).
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ACh applied to the AC also augmented auditory responses and background
discharges of 36 cortical neurons studied. The mean maximal increases
in response and background discharges were 61.7 ± 8.1 and
18.5 ± 7.8% (n = 36), respectively, which also
occurred ~60 min after the ACh application. The augmentation
disappeared ~150 min after the application (Fig. 4A, ).
The time course of the cortical augmentation was thus similar to that
of the collicular augmentation.
ACh applied to the AC augmented the auditory responses of collicular and cortical neurons at all frequencies of tone bursts. Figure 5A shows the frequency-response curve of a collicular neuron tuned to 30.5 kHz (curve 1). The height of the curve gradually increased up to 51% over 60 min after the ACh application (curves 2 and 3) and then gradually decreased back to the original height over 120 min thereafter (curves 4 and 5). The overall shape of the frequency-response curve and the BF did not change regardless of a prominent change in response magnitude. Figure 5B shows the frequency-response curve of a cortical neuron tuned to 36.5 kHz (curve 1). Just like the collicular neuron described in the preceding text, the height of the curve of the cortical neuron increased up to 49% over 60 min after an ACh application (curves 2 and 3) and then decreased back to the original height over 120 min thereafter (curves 4 and 5). The overall shape of the frequency-response curve and BF did not change. Since a nonfocal application of ACh to the AC carried no frequency information, the preceding results were completely expected. As described later, however, when the ACh application was paired with a conditioned acoustic stimulus, ACh evoked BF shifts that were specifically related to the frequency of the conditioned acoustic stimulus.
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In the big brown bat, the IC protrudes between the cerebrum and the cerebellum so that its dorsal surface is located immediately posterior to the visual cortex. ACh applied to the visual cortex, which is located dorsoposteriorly to the AC, 5 min before the 15-min-long conditioning evoked neither BF shift nor increase in response and background discharges of any of six collicular neurons studied. Therefore the augmentation of auditory responses of collicular neurons by ACh applied to the AC was not due to ACh that might be diffused to the IC.
Atropine applied to the AC reduced collicular and cortical responses
for 45 min (Fig. 4A, and
). The amount of reduction was, however, statistically insignificant for 17 collicular neurons studied (4.7 ± 3.2%; P > 0.05), but was
statistically significant for 32 cortical neurons studied (10.0 ± 6.8%; P < 0.05). The difference in reduction between
the collicular and cortical neurons was insignificant (P > 0.05).
Effects of ACh or atropine applied to the IC on collicular and cortical responses
ACh applied to the IC augmented both collicular and cortical
responses to tone bursts, as that applied to the AC did (Fig. 4B, and
). Atropine applied to the IC reduced both
collicular and cortical responses to tone bursts also as did that
applied to the AC (Fig. 4B,
and
). The amount and
time course of the augmentation or reduction were nearly the same for
collicular and cortical responses (P > 0.05). Since
ACh applied to the visual cortex had no effect on collicular responses,
the augmentation and reduction of cortical auditory responses evoked by
ACh or atropine applied to the IC were not due to the drugs, which
might be diffused to the AC, but due to the change in the IC, i.e., the
change in the ascending auditory system.
Effects of ACh applied to the AC on collicular and cortical plasticity
The 15-min-long conditioning session caused no BF shifts of collicular and cortical neurons (Fig. 3, C and D). However, it caused significant BF shifts of these neurons when ACh was applied to the AC 5 min prior to the conditioning. Figure 6A shows the responses (a and b) and frequency-tuning curves (c) of a single collicular neuron that was tuned to 28.0 kHz (curve 1 in c). When ACh was applied to the AC prior to the 15-min-long conditioning, frequency-dependent changes in responses were first evoked. That is, 30 min after the onset of the conditioning, the response was augmented at 27.5 kHz and frequencies lower than that, but was reduced at 28.0 kHz and frequencies higher than that (Fig. 6A, a2, b2, and curve 2 in c). Accordingly, the BF shifted toward the frequency of the conditioned tone burst, which was 23.0 kHz. This BF shift was, however, small and temporary. The response to the 27.5-kHz tone burst further increased with time because of ACh applied to the AC: 154% increase 60 min after and 95% increase 90 min after the onset of the conditioning (Fig. 6Ab, 3 and 4). The response to the 28.0-kHz tone burst also increased: 74% 60 min after and 43% 90 min after the onset of the conditioning (Fig. 6Aa, 3 and 4). Sixty minutes after the onset of the conditioning session, the frequency-response curve was raised and the small BF shift recovered (Fig. 6A, curve 3 in c). All 26 collicular neurons studied with ACh paired with the conditioning showed changes that were basically the same as those described in the preceding text. The mean BF shift and the mean maximal increase in the response at the BF were 0.82 ± 0.22 kHz and 52.7 ± 9.3% (n = 26), respectively.
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The cortical neuron in Fig. 6B was tuned to 26.0 kHz (curve
1 in c). When ACh was applied to the AC 5 min prior to the
15-min-long conditioning, its responses to tone bursts were augmented
and its BF gradually shifted over 60 min. Sixty minutes after ACh application, augmentation was largest for the response to 24.5 kHz, a
143% increase (Fig. 6Bb3), and the frequency-response curve and BF shifted to 24.5 kHz from 26.0 kHz, that is, toward the frequency
of the 21.0-kHz conditioned tone (Fig. 6B, curve 3 in c). Ninety minutes after the onset of the conditioning, the
augmented responses to tone bursts slightly decreased, but the shifted
BF stayed the same (Fig. 6B, a4, b4, and curve 4 in c). The augmented response returned to normal 150 min
after the conditioning (Fig. 4A, ), but the shifted BF
stayed the same even 210 min after the conditioning (Fig.
8A,
). All 36 cortical neurons studied with ACh paired
with the conditioning showed basically the same changes as described in
the preceding text. The mean maximal cortical BF shift evoked by the
conditioning paired with ACh was much larger than the mean maximal
collicular BF shift: 1.52 ± 0.23 (n = 18) versus
0.82 ± 0.22 kHz (n = 8), P < 0.01.
The augmentation of auditory responses by ACh was mostly due to an increase not in initial discharges but in late discharges. In Fig. 6, an increase is the largest at the BF shifted by the conditioning paired with ACh: 2.5 times increase in A (b3 and curve 3 in c) and 2.4 times increase in B (b3 and curve 3 in c). In spite of such a large increase, the height of the PST histograms displaying the collicular and cortical responses stay nearly the same. That is, the initial discharges stayed almost unchanged. To further substantiate this observation, the PST histograms in Fig. 6, Ab and Bb, are replotted as PSTC histograms (Fig. 7). In Fig. 7, the PSTC histograms for the control condition and for 60 min after the conditioning with ACh are the same for the initial 2.0 ms (A) or 1.3 ms (B). Thereafter, the histogram for 60 min after the conditioning becomes 2.5 (A) or 2.4 times (B) higher than that for the control. When the response started to recover, the PSTC histogram becomes lower without a decrease in its initial portion (Fig. 7, curves 3 in A and B).
|
The mean time courses of BF shifts of 8 collicular and 18 cortical
neurons caused by the 15-min-long conditioning preceded by an ACh
application are shown in Fig.
8A. The collicular BF shift
peaked 30 min after the onset of the conditioning and then recovered 60 min thereafter (). On the other hand, the cortical BF shift reached
a plateau 60 min after the onset of the conditioning, that is, the time
when the collicular BF shift had almost recovered. The cortical BF
shift was much larger and lasted much longer than the collicular BF
shift. The cortical BF shift showed little sign of recovery even 4 h after the conditioning. Therefore the time course and the amount of
the cortical BF shift assisted by ACh are very similar to those caused
by the 30-min-long conditioning alone (see Fig. 3B). It is
also noticed that the time course of the cortical BF shift is quite
different from that of the augmentation of auditory responses evoked by
ACh applied to the AC (see Fig. 4A).
|
ACh applied to the AC 5 min after the 15-min-long conditioning evoked neither collicular nor cortical BF shift (Fig. 8B), although it augmented collicular and cortical responses to tone bursts, and augmentation lasted ~150 min as shown in Fig. 4A.
The 30-min-long conditioning caused short-term collicular and long-term
cortical BF shifts as reported by Gao and Suga (2000) and as shown in Fig. 3, A and B. ACh applied to
the AC 40 min after the 30-min-long conditioning evoked no additional
BF shifts of collicular and cortical neurons (Fig. 8C),
although it augmented the responses of collicular and cortical neurons
to tone bursts over ~150 min as shown in Fig. 4A.
An application of both atropine and ACh to the AC 5 min before the 15-min-long conditioning evoked neither BF shift nor increase in the auditory response of four cortical neurons studied but evoked a 0.5-kHz BF shift of one cortical neuron that lasted ~20 min. An application of both atropine and ACh to the IC 5 min before the 15-min-long conditioning also evoked neither BF shift nor increase in the auditory response of seven collicular neurons studied. Therefore the effects of ACh on collicular and cortical BF shifts and responses were antagonized by atropine.
Effects of atropine applied to the AC on collicular and cortical plasticity
Atropine applied to the AC 5 min prior to the 30-min-long conditioning session reduced the collicular BF shift caused by the conditioning slightly, whereas it completely abolished the cortical BF shift that otherwise would be caused by the conditioning. The collicular neuron in Fig. 9A was tuned to 23.0 kHz (curve 1 in c). When atropine was applied to the AC 5 min prior to the 30-min-long conditioning, the response of the neuron to a tone burst decreased by 47% at 23.0 kHz and increased by 77% at 21.5 kHz 30 min after the onset of the conditioning (Fig. 9A, a2 and b2). Such frequency-dependent changes shifted the BF of the neuron toward the frequency of the conditioned tone, 18.0 kHz. Figure 9Ac, curves 1-4, respectively, shows the frequency-response curves of the collicular neuron obtained before and 30, 60, and 90 min after the onset of the conditioning with an atropine application to the AC 5 min prior to the conditioning. These curves show that the BF of the neuron shifted from 23.0 to 21.5 kHz 30 min after (curve 2), recovered to 22.0 kHz 60 min after (curve 3), and recovered to 22.5 kHz 90 min after (curve 4) the onset of the conditioning. These changes were similar to, but not the same as, those caused by the conditioning without atropine (Fig. 2A) in amounts of changes in response and BF shift. The maximum decrease in response at the shifted BF from that at the control BF was 23.4 ± 6.85% (n = 8) for the conditioning with atropine, but it was 11.3 ± 4.72% (n = 7) for the conditioning with saline. The BF shift was 1.23 ± 0.23 kHz (n = 8) for the conditioning with atropine, but it was 1.62 ± 0.24 kHz (n = 7) for the conditioning with saline (Fig. 10A). These differences were statistically significant (P < 0.05).
|
|
The cortical neuron in Fig. 9B was tuned to 26.0 kHz (curve 1 in b). Atropine applied to the AC 5 min prior to the 30-min-long conditioning slightly reduced the response of the neuron to 26.0 kHz (a) and abolished a BF shift of the neuron that otherwise would be caused by the conditioning (b). The data shown in Fig. 9 indicate that the collicular BF shift can be evoked without the cortical BF shift.
Compared with the collicular BF shift caused by the conditioning
without atropine, the collicular BF shift caused by the conditioning with atropine developed slowly and peaked 45 min after the onset of the
conditioning instead of 30 min after. The amount of the BF shift was
25% smaller (P < 0.05), and its recovery time was 60 min shorter (P < 0.05) than those caused by the
conditioning without atropine (Fig. 10A, ).
Atropine applied to the AC 70 min after the onset of the 30-min-long
conditioning (i.e., during the initial portion of the recovery phase of
the collicular BF shift caused by the conditioning without atropine)
had no effect on the collicular BF shift caused by the conditioning, so
that the collicular BF shift monotonically recovered 180 min after the
conditioning (Fig. 10B, ). On the other hand, the
cortical BF shift that had nearly developed to the plateau after the
conditioning was drastically affected by atropine. On the average from
the data obtained from 10 cortical neurons, the BF shift recovered
73 ± 18.5% (n = 10) almost immediately after an
atropine application and then developed back to the plateau 60 min
thereafter. The plateau was maintained as was that which had been
caused by the conditioning without atropine (Fig. 10B,
).
When atropine was applied to the AC 210 min after the onset of the
conditioning (i.e., after the recovery of the collicular BF shift), the
cortical BF shift was unaffected by atropine (Fig. 10C,
). These data indicate that the cortical BF shift was not stabilized
at ~70 min after the onset of the conditioning but was stabilized at
~210 min after.
Effects of ACh applied to the IC on collicular and cortical plasticity
ACh applied to the IC 5 min prior to the 15-min-long conditioning
session augmented the responses to tone bursts and evoked the BF shifts
of all 13 collicular neurons studied. The collicular neuron in Fig.
11A was tuned to 28.0 kHz
(curve 1 in c). When ACh was applied to the IC 5 min prior
to the 15-min-long conditioning, the neuron's response decreased at
28.0 kHz (a2) and increased at 27.0 kHz (b2), and
its BF shifted from 28.0 to 27.0 kHz 30 min after the onset of the
conditioning (curve 2 in c). Sixty minutes after the onset
of the conditioning, the response of the neuron increased to all sounds
from 23.5 to 31.5 kHz (a3, b3 and curve 3 in c).
The shifted BF stayed at 27.0 kHz. These changes recovered ~150 min
after the conditioning (Fig.
12A, ). On the average,
the auditory response augmented by ACh was 56.0 ± 8.9% (n = 13) larger than that in the control condition, and
the BF shift was 1.06 ± 0.32 kHz (n = 13),
instead of no BF shift for the 15-min-long conditioning alone. The BF
shift plateaued over ~45 min and then recovered 180 min after the end
of the conditioning (Fig. 12A,
). The collicular and
cortical BF shifts could, respectively, be evoked by the 15-min-long
conditioning when ACh was applied to the IC or AC prior to the
conditioning. However, there was a clear difference in duration of BF
shift. That is, the collicular BF shift recovered in 3 h (Fig.
12A,
), but the cortical BF shift showed no sign of
recovery even 3.5 h after the conditioning (Fig. 8A,
).
|
|
ACh applied to the IC 5 min prior to the 15-min-long conditioning
augmented the auditory responses of all of eight cortical neurons
studied, but it evoked the BF shifts of only three of the eight
neurons. The cortical neuron in Fig. 11B was tuned to 33.0 kHz (curve 1 in c). When ACh was applied to the IC 5 min prior to the 15-min-long conditioning, the neuron shifted its BF from
33.0 to 32.5 kHz 30 min after the conditioning (curve 2 in
c). The shifted BF quickly recovered to 33.0 kHz, but the response of the neuron increased to all sounds from 28.0 to 38.0 kHz 60 min after the conditioning (a, b, and curve 3 in
c). On the average, the response augmented by ACh was
49.8 ± 11.7% (n = 8) larger than the response in
the control condition. The BF shift observed in the three cortical
neurons was largest 30 min after the onset of the conditioning, and the
mean BF shift was 0.52 ± 0.29 kHz (n = 3). This
BF shift was small but significant (P < 0.05). The
data shown in Figs. 11B and 12A () indicate
that the cortical BF did not shift according to the collicular BF shift.
Effects of atropine applied to the IC on collicular and cortical plasticity
Atropine applied to the IC 5 min prior to the 30-min-long conditioning session abolished the changes in response, frequency-response curve and BF that otherwise would be caused by the conditioning for all nine collicular neurons studied. In other words, they were not affected by the conditioning at all, and their function in encoding tone bursts was also not affected by atropine. The collicular neuron in Fig. 13A was tuned to 25.5 kHz (curve 1 in b). Its responses to a 25.5-kHz tone burst (a1) and frequency-response curve (curve 1 in b) were not changed by the 30-min-long conditioning following an atropine application to the IC (a2-a4 and curves 2-4 in b). The only change noticed was a small decrease in auditory response evoked by atropine.
|
On the other hand, atropine applied to the IC decreased, but did not
abolish, a cortical BF shift caused by the conditioning. In other
words, the cortical BF shift could be evoked without the collicular BF
shift. The cortical neuron in Fig. 13B was tuned to 25.0 kHz
(curve 1 in c). For the conditioning following an atropine
application, the neuron's response decreased at 25.0 kHz (a,
2 and 3) but increased at 23.0 kHz (b, 2 and
3). Its frequency-response curve and BF shifted from 25.0 to
23.0 kHz, i.e., toward the frequency of the conditioned tone, 20.0 kHz
(Fig. 13B, curve 3 in c). Compared with the BF
shifts of the 10 cortical neurons caused by the conditioning with a
saline application (Fig. 3B), the BF shifts of 10 cortical neurons caused by the conditioning with atropine (Fig. 12B,
) were small and short-lasting. The mean maximal BF shift was
1.10 ± 0.21 kHz (n = 10) instead of 1.75 ± 0.15 kHz (n = 10). This difference is significant
(P < 0.05). The mean recovery time of the BF shift was
~70 min (Fig. 12B,
) instead of longer than 3 h,
i.e., showing no sign of recovery (Fig. 3B). The amount and time course of the cortical BF shift caused by the 30-min-long conditioning with atropine (Fig. 12B,
) were also small
and short compared with those caused by the 15-min-long conditioning
following a cortical ACh application (Fig. 8A,
).
Therefore the collicular BF shift, as well as an increased ACh level in
the AC, is necessary to evoke the large and long-lasting cortical BF shift.
When atropine was applied to the IC 40 min after the end of the
30-min-long conditioning session, i.e., at the middle of the recovery
period of the collicular BF shift, the collicular BF shift rapidly
recovered (Fig. 12C, ). On the other hand, the cortical BF shift was unaffected and showed no sign of recovery even 210 min
after the conditioning (Fig. 12C,
). In other words, the
cortical BF shift that has developed to the plateau does not require
the collicular BF shift for its maintenance.
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DISCUSSION |
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In the following, we discuss the significance of our individual
data in relation to the model proposed by Gao and Suga (1998, 2000
) and Weinberger and his coworkers (1990)
.
We then briefly review the data obtained from the auditory system of
non-bat species and from nonauditory systems in relation to our present studies.
Effects of ACh and/or atropine applied to the AC or IC on collicular and cortical responses
ACh applied to the AC augmented not only cortical, but also
collicular, responses to tone bursts (Fig. 4A). ACh applied
to the visual cortex, which is located immediately anterior to the colliculus, did not affect collicular responses to tone bursts at all.
Therefore collicular augmentation by ACh applied to the AC was not due
to diffusion of ACh from the AC to the IC but was due to the
augmentation of the corticofugal activity because there was no other
neural pathway from the AC to the IC. This collicular augmentation
agrees with the data that cortical neurons augment subcortical auditory
responses through positive feedback mediated by the corticofugal system
(Gao and Suga 1998; Yan and Suga 1996
, 1999
; Zhang and Suga 1997
; Zhang et al.
1997
).
ACh applied to the IC augmented not only collicular but also cortical auditory responses (Fig. 4B). The augmentation developed at the same time and to the same amount for collicular and cortical neurons. The augmented cortical response was apparently evoked by the augmented collicular response transmitted from the IC to the AC through the medial geniculate body. The augmentation of collicular and cortical responses by ACh was antagonized by atropine applied together with ACh to the IC or AC so that ACh affects collicular or cortical neurons via muscarinic receptors.
Atropine applied to the AC (Fig. 4A) or IC (Fig. 4B) reduced the auditory responses of collicular and cortical neurons only slightly and for a short time. Therefore the auditory responses of collicular and cortical neurons only slightly depend on ACh. However, as discussed later, BF shifts (plasticity) of the neurons caused by the conditioning greatly depend on ACh.
Nonfocal ACh application to the IC or AC, without auditory
conditioning, always augmented both collicular and cortical auditory responses but did not evoke BF shifts (Fig. 5). This is quite reasonable because without auditory conditioning, there is no frequency
information necessary for BF shifts. Augmentation of auditory responses
by iontophoretically applied ACh was also found in the ACs of the
squirrel monkey (Foote et al. 1975) and the guinea pig
(Metherate et al. 1990
). In addition to augmentation, however, suppression of auditory responses by iontophoretically applied
ACh was found in the IC of the horseshoe bat (Habbicht and Vater
1996
).
It was also found that whether ACh evokes augmentation or suppression
of auditory responses of cortical neurons in the cat and guinea pig
depends on the frequency of acoustic stimuli: suppression at the
frequency of a tone burst paired with an iontophoretic ACh application
but augmentation at other frequencies (McKenna et al.
1989; Metherate and Weinberger 1989
, 1990
). In
our present experiments, ACh applied to the IC or AC immediately prior
to the conditioning augmented the changes in the frequency-response curves of collicular and cortical neurons (Fig. 6). Such changes resulted from frequency dependent suppression and facilitation of their
auditory responses caused by the conditioning. Therefore our findings
are not contradictory to those by others.
Effects of ACh or atropine applied to the AC on collicular and cortical plasticity
A 30-min-long conditioning session without an ACh application to
the AC evokes noticeable collicular and cortical BF shifts (Gao
and Suga 1998, 2000
) (Figs. 2 and 3, A and
B). A nonfocal ACh application to the AC or IC alone did not
evoke collicular and cortical BF shifts at all (Fig. 5). A 15-min-long
conditioning alone also did not evoke the BF shifts (Fig. 3,
C and D). However, the 15-min-long conditioning
following the ACh application evoked BF shifts (Figs. 6 and
8A). The frequency information necessary for the BF shifts
was not in the ACh applied, but in the conditioning, i.e., in the
conditioned tone bursts that excited auditory neurons from the
periphery through the AC. ACh would not evoke the BF shifts if nothing
was evoked in the IC and AC by the 15-min-long conditioning. It was
most likely that small BF shifts evoked by the 15-min-long conditioning
would be detected if the step-size of the frequency scan was smaller
than 0.2 kHz. Therefore it may be defined that the 15-min-long
conditioning evoked "subthreshold" BF shifts in the IC and AC.
ACh applied to the AC immediately prior to the 15-min-long conditioning
augmented the subthreshold collicular and cortical BF shifts caused by
the conditioning: the collicular BF shift developed up to half of that
caused by the 30-min-long conditioning alone, and the cortical BF shift
became almost the same in amount and duration as that caused by the
30-min-long conditioning alone (Figs. 6 and 8A). Atropine
applied to the AC prior to the 30-min-long conditioning reduced the
collicular BF shift by 43% and completely abolished the cortical BF
shift that otherwise would be caused by the conditioning (Figs. 9 and
10A). These data indicate that ACh is necessary to evoke the
long-lasting cortical BF shift, that ACh augments the collicular BF
shift through augmented activity of the corticofugal system, and that
the collicular BF shift only partially depends on the cortical BF
shift. The importance of the corticofugal system in the collicular
plasticity can be justified because it has been known that the
corticofugal system forms highly frequency-specific feedback loops and
evokes collicular BF shifts (Yan and Suga 1998;
Zhang and Suga 2000
; Zhang et al. 1997
),
that no pathways other than the corticofugal system have been known to
evoke collicular changes due to cortical changes, and that the
collicular plasticity caused by auditory conditioning is abolished as
the AC is inactivated (Gao and Suga 2000
).
An ACh application to the AC after the 15-min-long conditioning
augmented neither the subthreshold collicular BF shift nor the
subthreshold cortical BF shift (Fig. 8B). ACh applied to the AC 45 min after the 30-min-long conditioning did not affect the collicular and cortical BF shifts (Fig. 8C). Therefore an
increase in ACh level during the conditioning is essential. It has been hypothesized that an increase in ACh level in the AC caused by the
excitation of the cholinergic basal forebrain is essential for auditory
cortical plasticity (Weinberger 1998; Weinberger et al. 1990
). Electrical stimulation of the basal forebrain
indeed evokes large plastic changes in the AC (Bakin and
Weinberger 1996
; Kilgard and Merzenich 1998
).
Our present data favor the hypotheses that an increase in ACh level in
the AC is necessary for the development of cortical plasticity caused
by conditioning (Weinberger 1998
; Weinberger et
al. 1990
), that collicular plasticity is caused by the
corticofugal system (Gao and Suga 1998
, 2000
), and that collicular and cortical plasticity evoked by a behaviorally irrelevant sound is augmented by an increase in the cortical ACh level as the
sound becomes behaviorally relevant.
Atropine applied to the AC 45 min after the 30-min-long conditioning did not affect the collicular BF shift at all but temporarily reduced the cortical BF shift (Fig. 10B). Atropine applied to the AC 180 min after a 30-min-long conditioning affects neither the collicular BF shift nor the cortical BF shift (Fig. 10C). This suggests that the cortical BF shift was not yet consolidated at ~1.5 h after the conditioning, i.e., at the time when the BF shift just reached a plateau, and that the recovery phase of the collicular BF shift is not influenced by the cortical BF shift but totally depends on a collicular change itself.
In the cat's AC, activation of muscarinic receptors by
acetyl-beta-methylcholine (MCh) enhances
N-methyl-D-aspartate (NMDA)-mediated neurotransmission and reduces GABA-mediated inhibition (Aramakis et al. 1997a,b
) so that ACh facilitates the NMDA dependent
plasticity. We have not yet studied the effects of NMDA and
2-amino-5-phosphonovaleric acid on the collicular and cortical BF
shifts caused by conditioning.
Effects of ACh or atropine applied to the IC on cortical and collicular plasticity
ACh applied to the IC 5 min prior to the 15-min-long conditioning
augmented the subthreshold collicular BF shift to be somewhat similar
to that caused by the 30-min-long conditioning, but the augmentation of
the cortical BF shift was very small and short-lasting (Figs. 11 and
12A). This means that the collicular BF shift alone is not
enough to evoke a large and long-lasting cortical BF shift and that an
increase of ACh level in the AC is essential for the long-lasting
cortical BF shift. Therefore our data support Gao and Suga's
(1998) hypothesis that the collicular BF shift augments the
cortical BF shift, which is further augmented by an increased ACh level
(Fig. 1). It also supports Weinberger's hypothesis (Weinberger 1998
; Weinberger et al. 1990
) that an increase
in an ACh level in the AC is essential for cortical plasticity caused
by fear conditioning (Fig. 1).
Atropine applied to the IC prior to the 30-min-long conditioning did
not change the auditory responses and frequency-response curves of
collicular neurons but abolished the collicular BF shift that otherwise
would be caused by the conditioning (Figs. 12B and 13A). It reduced the cortical BF shift caused by the
conditioning (Figs. 12B and 13B). These data
indicate that endogenous ACh is important for the development of the
collicular BF shift, that the collicular BF shift is not a result of BF
shifts that might be caused in the subcollicular nuclei and the cochlea
by the conditioning, and that a small and short-lasting cortical BF
shift can be caused by conditioning without collicular BF shift. The
corticofugal system forms feedback loops not only through the IC but
also through the medial geniculate body. It is likely that the cortical
BF shift is augmented by both the cortico-thalamic and -collicular feedback loops. One of the critical experiments for further testing of
Gao and Suga's hypothesis (1998), in which the BF
shifts in the IC and medial geniculate body both are abolished during
the conditioning with atropine and the cortical BF shift is examined, remains to be performed.
Several studies have demonstrated that the excitatory
neurotransmitters in the cerebral cortex (Tsumoto 1990)
and the neurotransmitters of the corticofugal neurons are glutamate
and/or aspartate but not acetylcholine (Dori et al.
1992
; Fonnum et al. 1981
; Karlsen and
Fonnum 1978
; Nieoullon and Dusticier 1983
;
Tsumoto 1990
). Therefore it is explainable that atropine
applied to the AC (Fig. 9B) and the IC (Fig.
13A), respectively, did not affect cortical and collicular
auditory responses and frequency-response curves, and it is expected
that atropine applied to the AC or IC did not abolish the activity of
the corticofugal fibers descending to and beyond the IC. Atropine
applied to the AC prior to the 30-min-long conditioning completely
abolishes the cortical BF shift but not the collicular BF shift (Figs.
9 and 10A). The collicular BF shift that would be caused by
the 30-min-long conditioning is completely abolished by inactivation of
the AC with muscimol (Gao and Suga 1998
). These data
clearly indicate that the collicular BF shift is not the result of the
cortical BF shift and that it is evoked by the corticofugal system. As
already discussed, the collicular BF shift is not a result of a BF
shift that might be evoked in the subcollicular auditory nuclei and the
cochlea by the conditioning. The data presented in our paper support
the hypothesis proposed by Gao and Suga (1998
, 2000
).
Subcortical plasticity based on corticofugal modulation in the visual, somatosensory, and auditory systems
It has been well known that the auditory, visual, and
somatosensory systems, respectively, have cochleotopic (frequency), retinotopic, and somatotopic maps in their central neural pathways and
that these topographic maps are modified by deprivation, injury, and
experience even in adult animals (reviews by Buonomano and Merzenich 1998; Irvine and Rajan 1996
;
Kaas et al. 1990
; Weinberger 1998
). Such
plasticity has been explained by changes in divergent and convergent
projections of the ascending sensory system. The contribution of the
massive corticofugal system to the plasticity of sensory systems had
been given little consideration until very recently although there had
been a great deal of data demonstrating the corticofugal modulation of
the responses of subcortical neurons to sensory stimuli.
In the visual system, the contribution of the corticofugal system to
visual signal processing has been extensively studied (e.g.,
Murphy et al. 1999; Sillito et al. 1993
,
1994
; Tsumoto et al. 1978
). However,
reorganization of the retinotopic map (i.e., shift in receptive field)
in the thalamus by the corticofugal system has not yet been shown.
In the somatosensory system, reorganization of the somatotopic
map due to somatosensory experience is evident in the thalamus as well
as in the cortex (Garraghty and Kaas 1991a,b
;
Pollin and Albe-Fessard 1979
). A treatment of the
somatosensory cortex for several months with an NMDA receptor agonist
induces a large change in the somatotopic map in the thalamus
(Ergenzinger et al. 1998
). Inactivation of the
somatosensory cortex by muscimol (GABA agonist) infused through a
cannula immediately increases or decreases the size of the receptive
fields of thalamic neurons (Krupa et al. 1999
). These
findings indicate that the corticofugal system modulates the thalamic
somatotopic map.
In the auditory system, it has been known for a long time that electric
stimulation of the AC evokes excitation or inhibition of collicular and
thalamic auditory neurons (e.g., Jen et al. 1998;
Sun et al. 1989
, 1996
; Watanabe et al.
1966
). However, the finding that the corticofugal system shifts
the tuning curves of subcortical neurons (i.e., reorganizes the
physiological maps in the subcortical nuclei) was rather recent.
Yan and Suga (1996)
found that focal inactivation of the
FM-FM area of the AC of the mustached bat with lidocaine immediately
shifts the delay-response curves of collicular FM-FM neurons. Their
finding indicates that the normal functional organization of the IC is
maintained by the corticofugal system. They also found that focal
activation of the FM-FM area with electric pulses shifts the
delay-response curves of collicular neurons and that the shifts last a
few hours after the cessation of the electric stimulation. Therefore it is clear that the corticofugal system is involved in the plasticity of
the IC. The plasticity of the central auditory system has been further
studied by Suga and his coworkers in relation to corticofugal modulation. It has been demonstrated that reorganization of the cochleotopic map in the IC is evoked by focal electric stimulation of
the AC (Ma and Suga 2001
; Yan and Suga
1998
; Zhang and Suga 2000
), repetitive acoustic
stimulation (Gao and Suga 1998
; Yan and Suga
1998
), or auditory conditioning (Gao and Suga 1998
,
2000
). Our present data also indicate that the corticofugal
system plays an important role in collicular plasticity.
Comparative studies with the mustached bat (Sakai and Suga
2001; Zhang and Suga 2000
), big brown bat
(Chowdhury and Suga 2000
; Gao and Suga 1998
,
2000
; Ma and Suga 2001
; Yan and Suga
1998
), and Mongolian gerbil (Sakai and Suga
2001
) indicate that reorganization of the cochleotopic maps in
the IC and AC is different between species and between the specialized
and nonspecialized areas of the same species (Sakai and Suga
2001
; review by Suga et al. 2000
).
Effects of ACh on neurons in the sensory cortices of non-bat species
ACh facilitates excitatory responses in the
somatosensory cortex (Donoghue and Carroll 1987;
Krujevic and Phillis 1963
; Lamour et al.
1988
; Metharate et al. 1987
) and those in the
visual cortex (Murphy and Sillito 1991
; Sato et
al. 1987
; Sillito and Kemp 1983
). Therefore the
effect of ACh on these sensory cortices are the same as that on the AC
(Figs. 4A and 5B). In the AC of cats, ACh evokes
facilitation or suppression of auditory responses depending on
frequencies of stimulus tones (McKenna et al. 1989
;
Metherate and Weinberger 1989
, 1990
). The data obtained
from the AC of cats appear to be different from those obtained from the
somatosensory and visual cortices. However, this is probably not the
case. In our present experiments, augmentation of cortical auditory
responses to tone bursts disappears 150 min after an ACh application
(Fig. 4A). However, the cortical BF shift lasts much longer
than 150 min (Fig. 8A). The BF shift (i.e., the shift in
frequency-tuning curve) is evoked by a decrease in the response at the
BF and an increase in the responses at other frequencies (Fig.
2B). Therefore our data are not contradictory at all to the
data obtained from the cat's AC. In other words, augmentation of a
response evoked by ACh and a shift in receptive field evoked by a
sensory stimulus have different time courses from each other. Therefore
no difference in ACh effect will be found between sensory cortices if
the changes in response magnitude and receptive field are further
studied as a function of time after an ACh application and a
conditioning stimulus.
Cholinergic basal forebrain and cortical plasticity
It has been well established that the cholinergic basal
forebrain releases ACh in the cerebral cortex, facilitates cortical responses to sensory stimuli, and causes cortical plasticity (review by
Rasmusson 2000). This conclusion is based on a number of
findings, enumerated in the following text. Cholinergic neurons in the
basal forebrain project to the cerebral cortex (Semba and
Fibiger 1989
). Electrical stimulation of the basal forebrain
elicits ACh release in the cortex (Casamenti et al.
1986
; Kurosawa et al. 1989
). Electrical stimulation of the basal forebrain evokes long-lasting facilitation of
cutaneous responses in the somatosensory cortex (Howard and Simmons 1994
; Rasmusson and Dykes 1988
;
Tremblay et al. 1990a
,b
; Webster et al.
1991
) and auditory responses in the rat's AC (Edeline et al. 1994
; Hars et al. 1993
). It was also
demonstrated that electric stimulation of the basal forebrain together
with tone burst stimulation evokes over-representation of the frequency of the tone burst in the rat's AC (Kilgard and Merzenich
1998
).
In the big brown bat, Ma and Suga (2000) found
that cortical and collicular plasticities evoked by focal electric
stimulation of the AC are augmented by electric stimulation of the
basal forebrain. In our present paper, we demonstrated that
subthreshold cortical and collicular plasticities caused by a
15-min-long auditory conditioning are augmented by ACh applied to the
AC (Figs. 6B and 8A). Therefore our data further
support the hypothesis that the cholinergic basal forebrain plays an
important role in cortical plasticity by increasing cortical ACh level.
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ACKNOWLEDGMENTS |
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We thank N. Laleman for editing the manuscript.
This work was supported by a research grant from the National Institute on Deafness and Other Communication Disorders (DC-00175).
Present address of E. Gao: Dept. of Neurology, General Hospital of the Navy, Beijing 100037, P. R. China.
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FOOTNOTES |
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Address for reprint requests: N. Suga, Dept. of Biology, Washington University, One Brookings Dr., St. Louis, MO 63130 (E-mail: suga{at}biology.wustl.edu).
Received 6 November 2000; accepted in final form 27 March 2001.
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REFERENCES |
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