Department of Neurobiology, University of Alabama at Birmingham, Birmingham, Alabama 35294
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ABSTRACT |
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Zhou, Fu-Ming and John J. Hablitz. Dopamine modulation of membrane and synaptic properties of interneurons in rat cerebral cortex. Dopamine (DA) is an endogenous neuromodulator in the mammalian brain. However, it is still controversial how DA modulates excitability and input-output relations in cortical neurons. It was suggested that DA innervation of dendritic spines regulates glutamatergic inputs to pyramidal neurons, but no experiments were done to test this idea. By recording individual neurons under direct visualization we found that DA enhances inhibitory neuron excitability but decreases pyramidal cell excitability, through depolarization and hyperpolarization, respectively. Accordingly, DA also increased the frequency and amplitude of spontaneous inhibitory postsynaptic currents (sIPSCs). In the presence of TTX, DA did not affect the frequency, amplitude, or kinetics of miniature IPSCs and excitatory postsynaptic currents in inhibitory interneurons or pyramidal cells. Our results suggest that DA can directly excite cortical interneurons, but there is no detectable DA gate to regulate spontaneous GABA and glutamate release or the properties of postsynaptic GABA and glutamate receptors in neocortical neurons.
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INTRODUCTION |
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Dopamine (DA) is believed to be an endogenous
neuromodulator in the cerebral cortex and to be important for normal
brain function (Bjorklund and Lindvall 1986;
Williams and Goldman-Rakic 1995
). Clinical and
experimental studies also implicated DA in the pathogenesis of a number
of psychiatric disorders, schizophrenia in particular (Creese et
al. 1976
; Dolan et al. 1995
; Okubo et al.
1997
; for review see Andreason 1996
; Egan
and Weinberger 1997
; Grace et al. 1997
;
Jaskiw and Weinberger 1992
). Multiple DA receptor types are expressed in the CNS. According to their pharmacological and physiological properties, DA receptors were originally classified broadly into D1 and D2 types, which are positively and negatively coupled to adenylate cyclase, respectively (Kebabian and Calne 1979
).
Since the initial discovery of DA in rat cerebral cortex by Thierry et
al. (1973), a wealth of information regarding cortical DA innervation
has accumulated (for review see Berger 1992
;
Bjorklund and Lindvall 1986
). It is now clear that rat
cerebral cortex, the prefrontal cortex in particular, receives a
substantial DA innervation originating from midbrain DA neurons. In
monkey and human brain, DA innervation was greatly expanded, and the
distribution of DA axons is more extensive (Berger 1992
;
Smiley et al. 1992
; Williams and Goldman-Rakic
1993
). Immunohistochemical studies show that both cortical
inhibitory interneurons and excitatory pyramidal neurons in rat and
primate cortex are targets for DA innervation (Benes et al.
1993
; Sesack et al. 1995
; Verney et al.
1990
; Williams and Goldman-Rakic 1993
). It was
also suggested that pyramidal neurons and interneurons may express
different DA receptor types (Mrzljak et al. 1996
;
Vincent et al. 1993
1995
), providing an
anatomic basis for differential DA modulation of cortical neurons.
Therefore dopaminergic activation may shift the balance between
excitation and inhibition in neuronal circuits, which may be related to
clinical and experimental observations, suggesting a potential link
between DA and GABA systems (Starr 1996
; Trimble
1996
).
The cellular neurophysiology of the cerebral DA system is far from
being established. In vivo extracellular recordings showed that DA
depressed spontaneous firing of rat prefrontal pyramidal neurons
(Sesack and Bunney 1989; Thierry et al.
1992
). Indirect evidence also suggests that a GABAergic
mechanism may be partly responsible for the DA-induced inhibition of
frontal pyramidal neurons (Penit-Soria et al. 1987
;
Pirot et al. 1992
). However, a detailed direct
characterization of DA effects on frontal GABAergic neurons is lacking.
On the basis of immunohistochemical electron microscopic studies in
primate cerebral cortex (Bergson et al. 1995;
Goldman-Rakic et al. 1989
; Smiley and
Goldman-Rakic 1993
; Smiley et al. 1994
), it was
proposed that DA terminals, together with glutamatergic axon terminals,
form so called synaptic triads on dendritic spines of pyramidal neurons
such that DA activation can gate or inhibit excitatory synaptic inputs
to pyramidal neurons (Goldman-Rakic 1992
;
Williams and Goldman-Rakic 1995
). It was further
suggested that these synaptic triads may be involved in the
pathophysiology of schizophrenia because D1 receptors on dendritic
spines may be reduced in schizophrenics (Nestler 1997
;
Okubo et al. 1997
). However, the exact physiological
function of these synaptic triads was not examined.
We therefore set out to examine the cellular effects of DA on cortical
neurons and to test the triad model by making high-resolution, patch-clamp recording from visually identified cortical interneurons and pyramidal neurons. Specifically, we sought to 1) examine
if DA, the endogenous neurotransmitter, can alter the excitability of
cortical neurons, inhibitory neurons in particular, and 2) test if DA affects spontaneously occurring, miniature inhibitory and
excitatory postsynaptic currents (mIPSCs and mEPSCs) as predicted by
Goldman-Rakic model. We chose the so-called prefrontal cortex because
this area receives a relatively dense DA innervation and is important
for cognition, which requires proper DA functions (Dolan et al.
1995; Weinberger and Berman 1996
;
Williams and Goldman-Rakic 1995
). We paid particular
attention to layer I neurons because they are mostly GABAergic
(Gabbott and Somogyi 1986
), and layer I receives a
particularly dense DA innervation (Berger 1992
;
Sesack et al. 1995
; Smiley et al. 1992
;
Williams and Goldman-Rakic 1993
). Our results indicated
that DA increased spontaneous action potential firing and enhanced
spontaneous inhibitory synaptic transmission but generally decreased
pyramidal neuron excitability through a small hyperpolarization. We
also found that DA had no effect on the frequency or amplitude of
mIPSCs and mEPSCs, indicating that DA did not modulate the spontaneous
release of glutamate and GABA or the properties of postsynaptic
glutamate and GABA receptors. Therefore there was no detectable DA gate
of spontaneous synaptic activity in cortical neurons under the
conditions employed in this study. Instead our results suggest that a
major action of DA in rat prefrontal cerebral cortex is to enhance
interneuron excitability, possibly through nonsynaptic DA receptors
(Descarries et al. 1991
).
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METHODS |
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Experiments were conducted in brain slice preparations. The
methods to prepare brain slices were described by Zhou and Hablitz (1996). Briefly, 15- to 22-day-old Sprague-Dawley rats were decapitated under ketamine anesthesia, and brains were dissected out quickly. Coronal brain slices (250- to 300-µm thick) were then cut from the
anterior portion of the brain on a Vibratome. Slices were kept in a
storage chamber at room temperature (~22°C) for 1 h before
recording. The normal extracellular bathing solution contained (in mM)
125 NaCl, 3.5 KCl, 2.5 CaCl2, 1.3 MgCl2, 26 NaHCO3, and 10 D-glucose and was bubbled with
95% O2-5% CO2 to maintain pH at ~7.4.
During actual recording KCl was raised to 4 mM to induce a small
depolarization such that potential DA effects on action potential-dependent spontaneous synaptic events were more detectable.
The anterior cingulate cortex and the shoulder region or Fr2 region of
the frontal cortex (Paxinos and Watson 1986) were the target of this study. These two areas make up a large portion of the
prefrontal cortex (Kolb 1990
). An Olympus BX50WI upright microscope equipped with Nomarski optics, a ×40 water immersion lens,
and infrared illumination was used to view cells in slices. Layer I
neurons were reliably identified by their depth below the pial surface
(Zhou and Hablitz 1996
). Fast spiking interneurons in
layer II/III were identified by their nonpyramidal appearance and fast
spiking characteristics (McCormick et al. 1985
;
Zhou and Hablitz 1997b
). Pyramidal neurons in layers
II-V were identified by their pyramidal shape, prominent apical
dendrites, and regular spiking properties.
DA was used as the agonist and was bath applied. Sodium meta-bisulfite
(Na2S2O5, 10-100 µM) was used as
an antioxidant to protect DA (Sutor and ten Bruggencate
1990). Control recordings from four layer I neurons and two
pyramidal neurons showed that 10-100 µM
Na2S2O5 had no effect on either
intrinsic membrane properties or spontaneous synaptic transmission.
D(
)2-Amino-5-phosphonovaleric acid (D-APV, 20 µM) and 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX, 10 µM) were
used to block ionotropic glutamate receptors. Bicuculline methiodide
(Bic) was used to block GABAA receptor-mediated IPSCs. To
study membrane properties and their modulation by DA in isolation, D-APV, CNQX, and Bic were included in bathing solution to
block synaptic activity.
Whole cell patch recording techniques described by Edwards et al.
(1989) were used. Tight seals (>2 G
before breaking into whole cell
mode) were obtained without first cleaning the cell. Patch electrodes
had an open tip resistance of ~3 M
. Series resistance during
recording varied from 6 to 15 M
among different neurons and was not
compensated. Recordings were terminated whenever significant increases
(>20%) in series resistance occurred. Recordings obtained with series
resistance >10 M
were not optimal for kinetic analysis (Zhou
and Hablitz 1997a
) but were still included in analysis for pharmacological purposes as long as series resistance was stable. The
intracellular solution for recording synaptic currents contained (in
mM) 135 KCl or CsCl, 10 HEPES, 2 Mg-ATP, 0.2 Na-GTP, and 0.5 EGTA; pH
and osmolarity were adjusted to 7.3 and 285 mosm, respectively. All
voltage-clamp recordings were made at a holding potential
70 mV. KCl-
and K-isothionate-based intracellular solutions were used to record
action potentials in current-clamp mode. Possible liquid junction
potentials developed after going whole cell mode were not subtracted
from the data presented. All recordings were made at room temperature
(~22°C).
Electrical signals were recorded with an Axopatch-200A amplifier controlled by Clampex software (Axon Instruments). Continuous recordings were made to videotape via a tape recorder (Neuro-corder, Neuro Data Instruments). Individual synaptic events (400-15,000) were captured with a threshold detector in SCAN software (provided by Dr. J. Dempster, University of Strathclyde). Neurons used for statistical analysis of DA effects were required to have synaptic events with stable frequency and amplitude during both control and DA application. Statistical comparisons of the frequency and amplitude of synaptic currents before, during, and after DA were made with the Kolmogorov-Smirnov (K-S) test with StatMost software (DataMost, Salt Lake City, UT); P < 0.01 was considered significant. Numerical values were expressed as means ± SD.
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RESULTS |
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DA depolarizes cortical interneurons
Under direct visualization, 28 layer I neurons and 4 deeper layer
fast spiking interneurons in anterior cingulate cortex and frontal
cortex were recorded in current clamp. No difference between the
neurons from the two brain area was found, and data were pooled. These
neurons had a resting membrane potential of 56 ± 2.5 mV. At
this resting potential, the apparent input resistance was 360 ± 120 M
. Spontaneous action potentials (spikes) were rare at rest.
Injection of suprathreshold depolarizing current pulses evoked a
single spike or a train of nonadaptive spikes of short duration (base
duration ~2 ms) followed by a strong fast afterhyperpolarization (fAHP). In contrast, pyramidal neurons (n = 24) from
the same brain areas fired long-duration (base duration ~4.5 ms)
spikes that showed adaptation during long pulses followed by a weak
fAHP. These spiking properties are characteristic of cortical fast
spiking interneurons and pyramidal neurons (McCormick et al.
1985
; Zhou and Hablitz 1996
).
Bath application of DA (30-100 µM) caused a depolarization (4 ± 2.1 mV, Fig. 1,
A1-A3) in 17 of 28 layer I neurons and 2 of 4 deeper layer fast spiking interneurons. Because of this depolarization, the same amount of injected depolarizing current evoked more spikes in
the presence of DA. The apparent input resistance of these depolarized
neurons, monitored by a constant hyperpolarizing current pulse, was
also slightly increased (by ~20%) by DA application. This apparent
increase in input resistance was not directly caused by DA. When the
membrane potential was maintained at resting levels with current
injection (n = 3), DA caused a ~15% decrease in
input resistance (see also Sutor and Hablitz 1989).
Spontaneous spikes were observed in 12 of 19 depolarized neurons,
although the frequency was still low (<0.5 Hz) and variable (Fig. 1,
A4 and A5). However, in 5 of 28 layer I neurons,
DA (30-100 µM) caused a small hyperpolarization (2 ± 0.8 mV)
accompanied by a slight decrease (24 ± 6%) in input resistance.
In the remaining cells application of 30-100 µM DA induced no
discernible change in membrane potential. These results indicate that
DA can directly increase excitability of the majority of cortical fast
spiking interneurons.
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DA was found to have weaker and less consistent effects on cortical pyramidal neurons. In 11 of 24 pyramidal neurons tested DA (30-100 µM) induced a small hyperpolarization of 3.5 ± 1 mV and a decrease (by ~25%) in input resistance (Fig. 1, B1-B3). In these cells the number of spikes evoked by a constant depolarizing current was also decreased (Fig. 1, B1-B3). In 4 of 24 pyramidal neurons, DA (30-100 µM) caused a small depolarization (2-3 mV), which was not strong enough to cause spontaneous spike firing in these cells. The rest of the pyramidal neurons showed no detectable response to DA application. These results indicate that DA may cause an overall reduction in pyramidal cell excitability.
DA enhances sIPSCs in pyramidal and layer I neurons
Previous immunohistochemical studies have shown that cortical
GABAergic interneurons receive DA innervation and express DA receptors,
suggesting that activation of DA system may modulate the activity of
GABAergic interneurons (Sesack et al. 1995;
Verney et al. 1990
). Our current-clamp data presented in
the previous section indicate that DA can enhance the excitability of
GABAergic interneurons. Therefore we hypothesized that DA can enhance
action potential-dependent GABAergic inhibitory synaptic transmission. To test this idea, we recorded spontaneous IPSCs (sIPSCs) under voltage-clamp conditions in the absence of TTX and in the presence of
20 µM D-APV and 10 µM CNQX. Voltage-clamp recordings
from 17 layer I neurons and 4 pyramidal neurons showed that bath
application of DA (10-100 µM) increased the frequency, calculated as
the reciprocal of interevent interval, of sIPSCs by 250 ± 85%
(Figs. 2, A-D, and
3, A-C). In the
presence of DA there were also more large-amplitude events, presumably
because of the increased spontaneous action potentials induced by DA,
such that the amplitude of averaged sIPSCs was also increased to
155 ± 25% of the control (Figs. 2, A-C
and inset of D; 3, A, B,
and inset of C). The enhancement of sIPSCs was
more pronounced when the percentage of action potential-dependent events in control was small. Dose-response relations were not studied
because of the fact that cells needed
10 min wash to recover, and
responses to a same dose of DA varied to a great extent among different
cells. When recovery was obtained and recordings were reasonably
stable, neurons (n = 4) responded to repeated DA
applications (Fig. 2D). The DA effects were also persistent and showed no desensitization during 10- to 30-min applications. Averaged sIPSCs had 10-90% rise time of 0.85 ± 0.14 ms and
0.88 ± 0.15 ms, double-exponential decay with time constants of
3.8 ± 0.3 ms and 19.4 ± 3.5 ms and 3.9 ± 0.3 ms and
20 ± 3.4 ms in control and during DA application, respectively
(n = 21, P > 0.6; to make the
comparison meaningful, overlapping and large events were excluded) (see
Zhou and Hablitz 1997a
,b
), indicating that DA did not
affect the gating of postsynaptic GABAA receptors. The time
needed for DA to exert its effects after switching to DA-containing
solution was
2 min, a time course similar to that needed for DA to
induce a depolarization and for 0.3 µM TTX to block action
potentials. These results strongly support the idea that DA increases
the firing of cortical interneurons innervating both interneurons and
pyramidal cells and therefore enhances action potential-dependent
GABAergic inhibitory synaptic transmission.
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DA had no effect on sEPSCs in pyramidal and layer I neurons
In vivo studies reported that DA inhibits pyramidal cell firing
(Bunney and Chiodo 1984; Thierry et al.
1992
), whereas in vitro intracellular studies suggest that DA
may increase frontal pyramidal cell excitability (Shi et al.
1997
; Yang and Seemans 1996
). Our current-clamp
data presented previously indicated that DA may induce a small overall
decrease in pyramidal neuron excitability. The reason for these
discrepancies is unknown. We reasoned that, if DA can increase the
overall excitability of cortical pyramidal neurons, we should see a
DA-induced increase in sEPSCs. If DA has only weak and mixed effects on
pyramidal neuron excitability as seen in our current-clamp recordings,
we should expect that DA has only minimal effects on sEPSCs,
particularly when most sEPSCs in neocortical neurons in vitro are
action potential independent (Zhou and Hablitz 1997a
).
To test these ideas, we recorded sEPSCs from seven pyramidal neurons
and eight layer I neurons. The bathing solution contained 10 µM Bic
and no D-APV, CNQX, or TTX. As shown in Figs.
4 and 5, in
contrast to the enhancing effects of DA on sIPSCs, bath application of
DA (10-100 µM for 10-25 min) failed to induce any significant
change in the frequency or amplitude of sEPSCs in layer I neurons
(P > 0.01, K-S tests; Fig. 4) and pyramidal neurons
(P > 0.01, K-S tests, Fig. 5). The kinetics of these
sEPSCs were also not altered. These results support the idea that DA
does not increase the overall pyramidal neuron excitability.
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DA had no effect on mIPSCs in pyramidal neurons and layer I neurons
GABAergic interneurons receive DA innervation (Sesack et
al. 1995; Verney et al. 1990
; Williams
and Goldman-Rakic 1993
). DA receptors also were detected on the
presynaptic terminals of GABAergic axons (Bergson et al.
1995
). Therefore DA could potentially modulate the release of
GABA as well as the properties of postsynaptic GABA receptors. To test
these two potential DA effects, we recorded mIPSCs in six pyramidal
neurons and seven layer I neurons in the presence of 0.3 µM TTX to
block sodium action potentials and 20 µM D-APV and 10 µM CNQX to block ionotropic glutamate receptors. In all pyramidal
cells and layer I neurons tested, bath application of 20 or 50 µM DA
for 5-15 min did not alter the frequency or amplitude of mIPSCs, as
indicated by K-S tests performed in individual cells (P > 0.01, Fig. 6,
A-D). The waveform of mIPSCs was also not
altered by DA application.
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DA did not affect mEPSCs in pyramidal neurons and layer I neurons
DA receptors were found at glutamatergic axon terminals
(Bergson et al. 1995), suggesting that DA may modulate
the release of glutamate vesicles. It was proposed that DA may regulate
excitatory synaptic inputs to cortical neurons, based on findings that
DA receptors are found on dendritic spines (Goldman-Rakic
1992
). We reasoned that such a DA gate could either modulate
the spontaneous glutamate vesicle release and/or the postsynaptic
glutamate receptors. To test this idea experimentally, we recorded
mEPSCs in 10 pyramidal neurons and 9 layer I neurons. As shown in Fig.
7, bath application of 20 or 50 µM DA
for 8-20 min was unable to induce any visible change in the frequency,
amplitude, or shape of mEPSCs. The frequency and amplitude of mEPSCs in
the presence of DA was 97 ± 2% and 98 ± 2% of control,
respectively. K-S tests also showed that distributions of mEPSC
frequency and amplitude in individual cells were not different before
and during DA application (P > 0.01, Fig. 7, A1-A4 and B1-B4). These
results indicate that DA activation does not modulate the spontaneous
glutamate vesicle release or the sensitivity of postsynaptic glutamate
receptors in cortical pyramidal neurons and fast spiking interneurons.
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DISCUSSION |
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The main finding of this study is that DA can directly excite cortical fast spiking interneurons. A DA gate to modulate the spontaneous release of GABA and glutamate was not observed in these experiments. The amplitude and kinetic properties of GABA- and glutamate-mediated spontaneous synaptic currents in rat cortical pyramidal neurons and interneurons were also not altered.
DA enhances interneuron excitability
Our ability to make recordings routinely from cortical fast
spiking interneurons enabled us to directly examine the effects of DA
on these interneurons. Therefore we were able to demonstrate that DA
can directly depolarize the majority of cortical interneurons in
frontal and cingulate areas, increase their excitability and spontaneous spike firing, and enhance action potential-dependent spontaneous GABAergic transmission. These results provide direct evidence for the suggestion that a GABAergic component is involved in
mediating the inhibitory effects of ventral tegmental DA neuron activation on prefrontal pyramidal neurons (Pirot et al.
1992). Our results are consistent with early brief reports
showing that DA increased sIPSPs in rat prefrontal neurons
(Penit-Soria et al. 1987
) and in rat piriform cortex
(Gellman and Aghajanian 1993
). It is also generally
consistent with immunohistochemical studies showing that cortical
interneurons receive DA innervation (Krimer et al. 1997
;
Sesack et al. 1995
; Smiley and Goldman-Rakic
1993
; Verney et al. 1990
). Our result that DA
can induce a direct depolarization in interneurons suggests that the
most likely site of DA effects is the soma and proximal dendrites. This
agrees particularly well with findings that DA varicosities
preferentially make contacts with the somata of rat prefrontal
interneurons and pyramidal neurons (Benes et al. 1993
)
and that DA receptors are more concentrated in the somata in these two
cell types (Vincent et al. 1993
).
The endogenous neurotransmitter DA was used to maximize the probability
of seeing the possible effects mediated by different DA receptors.
Further studies are needed to identify the receptor subtypes
responsible for the observed DA effects on cortical interneurons. Also,
these studies were conducted at room temperature. Studies by Hardingham
and Larkman (1998) reported temperature-dependent changes in the
reliability of excitatory synaptic transmission in rat visual cortex.
It remains to be determined if possible neuromodulatory actions are
also temperature dependent.
DA decreases pyramidal neuron excitability
Our results show that the direct effects of DA on the
cortical pyramidal neurons tested are weak and more variable. A
majority of the pyramidal neurons responded to DA with a small
hyperpolarization accompanied with a small decrease in input
resistance, and some cells showed a small depolarization, whereas
others showed no response at all. Therefore the overall effect of DA on
cortical pyramidal neurons was a small decrease in excitability. This
conclusion is further supported by the failure of DA to induce any
change in sEPSCs. If DA can enhance the overall excitability of
pyramidal neurons as reported in a number of studies
(Penit-Soria et al. 1987; Shi et al.
1997
; Yang and Seamans 1996
), one should expect that DA application, in the presence of Bic, would increase action potential-dependent sEPSCs in a fashion similar to the DA-induced enhancement of sIPSCs discussed in the previous section. However, our
results clearly show that DA did not enhance sEPSCs in pyramidal neurons and interneurons. These data are particularly informative because sEPSCs are a reliable index of activity in the pyramidal neurons not disturbed by the recording pipette. Therefore a lack of
effect of DA on sEPSCs strongly suggests that DA does not increase pyramidal neuron excitability. Instead, DA may slightly decrease the
overall excitability of pyramidal neurons. This is consistent with in
vivo extracellular studies showing that DA caused an inhibition of
pyramidal cells in prefrontal cortex (Bunney and Chiodo
1984
; Sesack and Bunney 1989
; Thierry et
al. 1992
) and with in vitro intracellular studies showing that
DA induced a hyperpolarization in hippocampal pyramidal cells
(Benardo and Prince 1982
; Berretta et al.
1990
).
The relative heterogeneity of DA effects on membrane potentials, also
observed by Yang and Seamans (1996), may result from the fact that DA
was used instead of subtype-specific agonists and that pyramidal
neurons may express multiple DA receptor types (Bergson et al.
1995
; Vincent et al. 1993
, 1995
), which may
induce either inhibition or excitation, as demonstrated in monkey
frontal cortex (Sawaguchi et al. 1986
), rat hippocampus
(Berretta et al. 1990
; Smialowski and Buak
1987
), and rat nucleus accumbens (Uchimura et al.
1986
).
DA does not affect spontaneous GABA and glutamate release or the sensitivity of these postsynaptic receptors
Our study showed no effect of DA application on mIPSCs and mEPSCs.
Because the frequency of mIPSCs and mEPSCs is an index of the frequency
of GABA and glutamate vesicles being released from the presynaptic
terminals, our results suggest that there may be no functionally
detectable presynaptic DA receptors in GABA and glutamate axon
terminals in cerebral cortex. This is generally in agreement with
immunohistochemical electron microscopic studies showing that DA
receptors were not frequently detected in presynaptic axonal terminals
in primate cerebral cortex (Bergson et al. 1995). Verney
et al. (1990)
also reported that there are no axoaxonic synapses
between GABAergic terminals and DA terminals in rat frontal cortex. Our
data showing that DA did not alter the amplitude or kinetics of mIPSCs
and mEPSCs indicate that DA activation may not modulate the sensitivity
of postsynaptic GABA and glutamate receptors in these cortical neurons,
in contrast to the triad model that proposes that DA activation gates
excitatory synaptic inputs (Goldman-Rakic 1992
;
Williams and Goldman-Rakic 1995
). However, DA may affect
evoked transmitter release because of effects on membrance excitability
(our current study) and calcium signaling (Missale et al.
1998
).
One possible reason for our failure to detect any acute effect of DA on
mEPSCs and mIPSCs could be inadequate recording conditions. However,
the enhancement of sIPSCs by 10 µM DA indicates that there should be
sufficient amount of DA, when bath applied at 20-100 µM, reaching
neurons in the depths of slice. Second, the fast kinetics of mIPSCs and
mEPSCs recorded in this study, similar to those reported in our
previous studies (Zhou and Hablitz 1997a,b
), indicate
adequate voltage clamp. Finally, bath application of the metabotropic
glutamate receptor agonist 1S,3R-1-aminocyclopentane-1,3-dicarboxylic acid induced a reversible decrease (~50%) in the frequency of mEPSCs
in these same cortical neurons under the same recording conditions used
for DA experiments (Zhou and Hablitz 1997c
). This indicates that changes in synaptic activity caused by activation of
G-protein-coupled receptors are detectable under the current recording
conditions. The relative scarcity of DA terminals and synapses in the
cortex may be the major factor contributing to our failure to detect
any DA effect on mIPSCs and mEPSCs. In the macaque prefrontal cortex, a
recent quantitative three-dimensional analysis (Krimer et al.
1997
) found that a pyramidal neuron and a GABAergic neuron
receive only ~90 and 45 catecholaminergic appositions, respectively.
These numbers are very small compared with the total synapses a
pyramidal neuron typically receives, which are ~10,000-30,000 (Peters 1987
). Other quantitative studies also indicate
that non-GABA nonglutamate axon terminals, i.e., DA, serotonin,
noradrenaline, and acetycholine terminals, only constitute <1% of the
total nerve terminals in the cerebral cortex (Descarries and
Umbriaco 1995
; Micheva and Beaulieu 1996
).
Interestingly, in nucleus accumbens, which has a much higher DA
innervation (Bjorklund and Lindvall 1986
) and DA
receptor density (Boyson et al. 1986
) than those in
cerebral cortex, it was reported that DA activation depressed the
frequency of mEPSCs but not mIPSCs (Nicola and Malenka
1997
). It is also possible that DA may have long-term effects
on synaptic inputs at synaptic triads, which are beyond the detection
of the techniques employed here.
Clinical and experimental observations suggested that there may
be an intrinsic link between epilepsy, a disorder primarily associated
with dysfunction of GABAergic inhibition, and schizophrenia, a disorder
closely involving DA system (Starr 1996; Trimble
1996
). More recent studies indicate that there may be modest
abnormalities in GABAergic system in prefrontal cortex of schizophrenic
patients (Akbarian et al. 1995
; Benes et al.
1996
). Our results show that DA activation can directly enhance
cortical GABAergic outputs. Therefore we speculate that DA modulation
of cortical GABAergic inhibitory systems could play a role in both
epilepsy and schizophrenia.
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ACKNOWLEDGMENTS |
---|
We thank A. Margolies for excellent technical assistance.
This study was supported by National Institute of Neurological Disorders and Stroke Grants NS-18145 and NS-22373, and by a grant from the National Alliance for Research on Schizophrenia and Depression.
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FOOTNOTES |
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Present address and address for reprint requests: F.-M. Zhou, Dept. of Anatomy and Neurobiology, Health Sciences Facility, Room 280-L, 685 W. Baltimore St., University of Maryland at Baltimore, Baltimore, MD 21201.
The costs of publication of this article were defrayed in part by the payment of page charges. The article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. Section 1734 soley to indicate this fact.
Received 23 July 1998; accepted in final form 4 November 1998.
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REFERENCES |
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