Lehrstuhl für Neurobiologie, Fakultät für Biologie, Universität Bielefeld, D-33501 Bielefeld, Germany
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ABSTRACT |
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Kurtz, Rafael, Volker Dürr, and Martin Egelhaaf. Dendritic Calcium Accumulation Associated With Direction-Selective Adaptation in Visual Motion-Sensitive Neurons In Vivo. J. Neurophysiol. 84: 1914-1923, 2000. Motion adaptation in directionally selective tangential cells (TC) of the fly visual system has previously been explained as a presynaptic mechanism. Based on the observation that adaptation is in part direction selective, which is not accounted for by the former models of motion adaptation, we investigated whether physiological changes located in the TC dendrite can contribute to motion adaptation. Visual motion in the neuron's preferred direction (PD) induced stronger adaptation than motion in the opposite direction and was followed by an afterhyperpolarization (AHP). The AHP subsides in the same time as adaptation recovers. By combining in vivo calcium fluorescence imaging with intracellular recording, we show that dendritic calcium accumulation following motion in the PD is correlated with the AHP. These results are consistent with a calcium-dependent physiological change in TCs underlying adaptation during continuous stimulation with PD motion, expressing itself as an AHP after the stimulus stops. However, direction selectivity of adaptation is probably not solely related to a calcium-dependent mechanism because direction-selective effects can also be observed for fast moving stimuli, which do not induce sizeable calcium accumulation. In addition, a comparison of two classes of TCs revealed differences in the relationship of calcium accumulation and AHP when the stimulus velocity was varied. Thus the potential role of calcium in motion adaptation depends on stimulation parameters and cell class.
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INTRODUCTION |
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Many sensory receptors and neurons adapt to a constant input or to repeated stimulation. Adaptation leads to a gradual decrease of response amplitude with time. This can reflect a shift in the operating range of the cell. Sensory systems thus retain their ability to detect small deviations of stimulus intensity from the adapting background level over a large range of stimulus intensities.
Decreased responsiveness on prolonged exposure to moving stimuli has
been demonstrated in visual motion-sensitive neurons of insects and
mammals (e.g., rabbit: Barlow and Hill 1963; cat: Vautin and Berkley 1977
; fly: Maddess and
Laughlin 1985
; monkey: Petersen et al. 1985
;
butterfly: Maddess et al. 1991
; wallaby: Ibbotson
et al. 1998
). Increased sensitivity to changes in velocity around an adapting level has been observed psychophysically in humans
(Clifford and Langley 1996b
; Clifford and
Wenderoth 1999
) and electrophysiologically in the fly's H1
cell (Jian and Horridge 1991
; Maddess and
Laughlin 1985
). This neuron is 1 of more than 30 motion-sensitive tangential cells (TCs), which have been individually identified in the lobula plate, a part of the third visual neuropil (review articles: Egelhaaf and Borst 1993a
;
Hausen 1984
; Hausen and Egelhaaf 1989
).
Movement detection is thought to be based on correlation of input from
one retinal location with temporally delayed input from a neighboring
location. A fully direction-selective element, being excited by motion
in the preferred direction (PD) and inhibited by motion in the opposite
direction ("null direction," ND), is obtained by subtracting the
output signals of such a correlator from that of its mirror-symmetrical
copy. This is called an "elementary motion detector," EMD (review
articles: Borst and Egelhaaf 1989
; Egelhaaf and
Borst 1993b
; Srinivasan et al. 1999
). The vast
dendrites of TCs are assumed to receive retinotopically arranged input
from numerous local EMDs, thus being able to respond to certain types of optic flow in a large part of the visual field (Hausen
1981
; Krapp and Hengstenberg 1996
).
In the H1 neuron, motion adaptation remains restricted to that part of
the receptive field in which an adapting stimulus was presented
(Maddess and Laughlin 1985). Since no cell intrinsic local mechanism was known then, the associated physiological changes were thought to be located presynaptic to H1. Shortening of the EMD
filter time constant, and thus tuning the motion detector to higher
velocities, was proposed to underlie motion adaptation (Clifford
and Langley 1996a
; de Ruyter van Steveninck et al.
1986
). A recent investigation on another class of TC, the
horizontal system (HS) cells (Harris et al.
1999a
), casts doubt on this model of motion adaptation:
prolonged movement did not cause changes in the optimal velocity tuning
as would be expected from adaptation of the EMD filter time constant.
Therefore the physiological mechanism leading to motion adaptation and
its location is still unclear.
In this study, we direct our attention to intrinsic physiological
changes in the motion-sensitive TCs induced by prolonged stimulation.
More specifically, we focus on a possible functional role of
intracellular Ca2+. Because in many receptor
cells and neurons the intracellular Ca2+
concentration affects cellular excitability, e.g., by regulating Ca2+ dependent conductances (review articles:
neurons, Sah 1996; photoreceptors, Koutalos and
Yau 1996
; olfactory receptors, Menini 1999
; hair cells, Fettiplace and Fuchs 1999
), we investigate the
relationship between adaptation and Ca2+
concentration changes and motion adaptation in TCs. Motion in restricted parts of the receptive field has been shown to lead to local
Ca2+ accumulation, which remains restricted to
the part of the dendrite getting input from the stimulated region of
the receptive field (Borst and Egelhaaf 1992
;
Dürr and Egelhaaf 1999
; Egelhaaf and Borst
1995
). Hence, a Ca2+-based mechanism of
motion adaptation in the TC could, similar to the formerly proposed
presynaptic mechanism, account for the localized adaptive characteristics.
A Ca2+-based mechanism would further imply that
motion adaptation is direction selective. This expectation results from
the observation, that major increases in dendritic
Ca2+ concentration of TCs are only elicited by
motion in the PD but not in the ND (Dürr 1998; Egelhaaf
and Borst 1995
). However, motion adaptation has not been
analyzed before with test stimuli moving in the ND because former
studies were mainly based on extracellular recordings of H1, which
ceases spiking during motion in the ND. In our study, motion adaptation
is tested both in the PD and in the ND by recording intracellularly
from HS cells, in which graded membrane-potential changes can be
recorded even in the axon (Hausen 1976
, 1982a
,b
). We
investigate the relation of calcium concentration changes to the
electrical response characteristics underlying adaptation in this cell
class and, additionally, in another class of TC, the centrifugal
horizontal (CH) cells (Eckert and Dvorak 1983
;
Egelhaaf et al. 1993
; Hausen 1976
). The
latter cell class is supposed to possess an input organization similar
to HS cells but to differ with respect to its output organization: its
dendrites do not only form postsynaptic sites to EMDs but also
dendritic presynaptic sites (Gauck et al. 1997
)
mediating inhibition to other TCs within the lobula plate
(Warzecha et al. 1993
). Therefore in the CH cell,
Ca2+ might not only be relevant for modulatory
functions but also for the control of transmitter release.
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METHODS |
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Electrical and optical recordings were performed in vivo from
the three HS cells (Hausen 1982a,b
) and the two CH cells
(Eckert and Dvorak 1983
; Egelhaaf et al.
1993
; Hausen 1976
) in female blowflies
(Calliphora erythrocephala), aged 1-3 days. Data were pooled for each cell class because the major difference within a class
is the receptive field location with known physiological differences
being only of minor significance (Eckert and Dvorak 1983
; Hausen 1982a
,b
). For information about the
animal preparation and closer details concerning the further
experimental procedures, see Dürr and Egelhaaf
(1999)
. Data are presented as means ± SE.
Electrophysiology
Ringer solution (containing, in mM, 128.3 NaCl, 5.4 KCl, 1.9 CaCl2, 4.8 NaHCO3, 3.3 Na2HPO4, 3.4 KH2PO4, 13.9 glucose, pH 7.0; all chemicals from Merck) was used to prevent desiccation of the
recording region and for filling a wide-tip glass pipette used as
indifferent electrode. Electrodes for intracellular recording were made
from borosilicate glass (GC100F-10, Clark Electromedical) on a
Brown-Flaming puller (P97, Sutter Instruments) and had resistances of
20-40 M when filled with 1 M KCl. Voltage records (Axoclamp 2A,
Axon Instruments) were low-pass filtered at 2 kHz, and AID converted at
a sampling rate of 1 kHz with an amplitude resolution of 0.0244 mV
(DT2801A, Data Translation). Part of the recordings were performed
during iontophoretic injection of the fluorescent dye by a weak tonic
hyperpolarizing current (see following text). No significant
differences in the electrical properties were observed between the
cells hyperpolarized in this way and the cells recorded without current injection.
Imaging changes in intracellular Ca2+ concentration
The calcium-sensitive dye Fura-2 was injected iontophoretically
into single neurons by a hyperpolarizing current of 0.8 nA, with
electrodes containing (in mM) 33.3 KCl (Sigma), 1.7 KOH (Merck), 20.0 fura-2 pentapotassium salt (Molecular Probes), and 33.3 HEPES (Sigma),
pH 7.3. Relative changes of intracellular ionic free Ca2+ concentration
([Ca2+]i) were
determined during or immediately following electrophysiological recording by a single wavelength measurement according to
Vranesic and Knöpfel (1991)
. Epifluorescence
measurements were performed under an upright microscope (Universal
III-RS, Zeiss). Fluorescence was elicited at 380 nm (light emitted from
a Hg arc lamp, HBO 100W, Osram, and band-pass filtered with a bandwidth
of 10 nm) and measured after passage of a 410-nm dichroic mirror and a
500- to 530-nm band-pass filter. Images were magnified 20-fold by the microscope objective (UD20, Zeiss) and acquired with a CCD camera (PXL,
Photometrics) controlled by PMIS software (GKR Computer Consulting).
Acquisition rate was approximately 3 Hz and spatial resolution was 9 µm.
F/F was determined relative to an image obtained prior to visual stimulation. The values were averaged in a
region covering the entire dendritic tree. Increases in
[Ca2+]i lead to a
decrement in fluorescence. For convenience when comparing the calcium
signals with the electrical signals of a cell, the former were inverted
in the figures. Increments in
[Ca2+]i are thus
indicated by positive
F/F values. Background
fluorescence as determined for each image in a region surrounding the
dendrite was subtracted from the fluorescence images.
Visual stimuli
TCs were stimulated by two stripe patterns projected through a
circular aperture on a translucent hemisphere and moving horizontally in the cells' receptive field. The stimulus was presented
ipsilaterally and was characterized by the following parameters:
diameter, 66°; spatial wavelength, ca. 33°; contrast, 0.68; mean
light intensity, 12 cd m2
with a spectral composition of a Hg arc lamp (HBO 100W, Osram) filtered
by a blue filter (BG1, Schott). The stimulus velocity depended on the
protocol and is given in the text and figure legends as temporal
frequency (cycles per second; 2-Hz temporal frequency thus corresponds
to an angular velocity of 66°
s
1).
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RESULTS |
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Adaptation of TC responses
The HS and CH cells of the fly respond to visual motion with
graded membrane potential changes
(EM). In HS cells, depolarizing responses are usually superimposed by spike-like rapid depolarizations (Hausen 1982a
). When a stripe pattern in the receptive
field starts to move at a constant velocity in the PD of the cell, the
membrane potential of both HS and CH cells rapidly shifts to a
depolarized state followed by a slow decline to a steady-state level
(Fig. 1). This transient behavior can, in
part, be attributed to the mechanism of motion detection itself
(Egelhaaf and Borst 1989
) but additionally reflects
changes in the movement processing system dependent on stimulus
history, i.e., motion adaptation (Maddess and Laughlin
1985
). The influence of stimulus history becomes obvious if a
test stimulus is preceded by an adapting stimulus: if the interval
between these two stimuli is sufficiently short (e.g., 0.1 s, as
in Fig. 1), the early component of the response to the test stimulus is
attenuated, rendering the test response less transient than the
adapting response. The latter can serve as reference in our
experimental procedure (Fig. 1). Motion adaptation, measured as the
attenuation of the early component of the test response, can also be
observed in both HS and CH cells for stimuli moving in the ND.
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Direction selectivity of adaptation
Adaptation is weaker and recovery from adaptation is faster for
motion in the ND compared with the PD. This was shown quantitatively for HS cells by presenting adapting and test stimuli, separated by
variable intervals. The experiments were done both for motion in the PD
and in the ND (Fig. 2, left).
To allow easier comparison of EM in
response to PD motion (top traces in each row), and
EM in response to ND motion
(bottom traces in each row), ND traces are shown inverted
and scaled by a factor of 1.5. Direction dependency is most prominent
at the medium interval length (0.5 s), where the attenuation of the
early component of the test response is pronounced for PD motion but
hardly detectable for ND motion.
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In Fig. 3A, the amplitudes of
the early component (200-ms period, starting 100 ms after stimulus
onset, see Fig. 1) of adapting and test responses are plotted for the
PD and the ND. Adaptation is marked by a weaker early component of the
response to the test than to the adapting stimulus. For motion in the
PD, this amplitude reduction is significant for the 0.1- and 0.5-s
inter-stimulus intervals. No significant amplitude difference between
adapting and test response can be observed if both are separated by a
4-s interval. In each cell tested, R, i.e., the
difference between the early component of the response to the adapting
and to the test stimulus, respectively (see Fig. 1), is positive for
both the 0.1- and the 0.5-s inter-stimulus interval (averaged values:
RPD = 4.08 ± 0.50 mV for the
0.1-s interval and
RPD = 2.63 ± 0.32 mV for the 0.5-s interval, n = 12). Statistical
significance of the difference between adapting response and test
response was confirmed by a Wilcoxon rank test (P < 0.005 for both 0.1- and 0.5-s inter-stimulus interval,
n = 12). In contrast, an adapting stimulus in the ND
leads to an attenuation of the early component of the test response
(leading to negative
R values) in only 8 of 12 cells for
both the 0.1- and the 0.5-s interval. Averaged for all cells, the
absolute values of
R for the ND
(
RND =
0.76 ± 0.40 mV for
the 0.1-s interval and
RND =
0.45 ± 0.32 mV for the 0.5-s interval, n = 12)
are almost an order of magnitude lower than the corresponding values
for the PD (see preceding results). Hence for ND motion, a statistical
difference between adapting response and test response is given only
for the 0.1-s interval at a low significance level (Wilcoxon rank test,
P < 0.1, n = 12).
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To assess the directional selectivity of adaptation, we determined the attenuation of the early response component by a relative measure (r = test response/adapting response). Ratios calculated for PD (rPD) and ND motion (rND), respectively, were statistically different both for the 0.1- and the 0.5-s inter-stimulus interval. Thus adaptation contains a directionally selective component (0.1-s interval: rPD = 0.645 ± 0.036, rND = 0.887 ± 0.074; 0.5-s interval: rPD = 0.762 ± 0.028, rND = 0.970 ± 0.085, H0: rPD = rND, Wilcoxon rank test, P < 0.05 in both cases, n = 12).
Regardless of the direction of motion (PD or ND), the late component of
the test response (last second of the 2-s stimulation period, see Fig.
1) is not greatly diminished by adaptation (Fig. 3A).
Nevertheless, probably due to the better signal-to-noise ratio
resulting from the larger time window, for an inter-stimulus interval
of either 0.1 or 0.5 s, there is significant attenuation of the
late response for PD and for ND motion (0.1-s interval: RPD = 0.57 ± 0.23 mV,
RND =
0.56 ± 0.14 mV; 0.5-s
interval:
RPD = 0.65 ± 0.26 mV,
RND =
0.25 ± 0.10 mV,
Wilcoxon rank test, P < 0.05 in all cases,
n = 12). No direction selectivity was found for
adaptation of the late response components (0.1-s interval: rPD = 0.945 ± 0.034, rND = 0.900 ± 0.034; 0.5-s
interval: rPD = 0.923 ± 0.034, rND = 0.953 ± 0.020, 4-s
interval: rPD = 1.106 ± 0.107, rND = 1.091 ± 0.044;
H0: rPD = rND, Wilcoxon rank test, P > 0.05 in all cases, n = 12).
Since PD motion elicits, on average, stronger responses than ND motion, direction-selective effects could be mimicked by processes that are in fact nonlinearly dependent on response size. Therefore we also compared adaptation in HS cells with approximately equally sized PD and ND responses. As has been found for the whole cell sample, the attenuation of the early response component by adapting stimuli was more pronounced and longer lasting for PD motion than for ND motion in this subset of neurons. Thus adaptation in TCs has a genuine direction-selective component.
In conclusion, our results indicate that motion adaptation in HS cells is characterized by both direction-selective and nondirectional mechanisms. Direction selectivity is reflected in a more pronounced attenuation of the early component of the test response to PD motion compared with ND motion. Additionally, recovery from adaptation is faster for PD than for ND motion. Compared to direction-selective adaptation, the nondirectional component of motion adaptation is weak but does result in a statistically significant reduction of the late component of the test response.
Adaptation in CH cells, tested experimentally in the same way for a smaller cell sample, followed a similar pattern. This, however, does not preclude quantitative differences in the characteristics of adaptation between the two cell classes.
After-reactions reflect motion adaptation in its strength and recovery time course
After PD motion stops, the membrane potentials of HS and CH cells hyperpolarize, a phenomenon referred to here as afterhyperpolarization (AHP) (Figs. 1 and 3B). In contrast, there is a comparatively weak and more transient depolarizing after-reaction following ND motion. An attenuation of the early component of the test response is only detectable if the test response occurs while the after-reaction is still present (compare Fig. 3, A and B). Likewise, the small adaptation amplitude and its fast recovery during motion in the ND is paralleled by a low amplitude and a fast recovery time course of the after-reaction following ND motion (see Fig. 3).
These results suggest, that direction-selective adaptation is based on a postsynaptic cellular mechanism in the TCs, for example an increased K+ conductance or a decreased nonselective cation conductance, which on the one hand leads to a reduction in response strength during prolonged motion stimulation and on the other hand results in an AHP on stimulus cessation.
Correlation of AHP with dendritic Ca2+ accumulation
Motion stimulation in the PD has previously been shown to increase
the Ca2+ concentration in TCs (Borst and
Egelhaaf 1992; Dürr and Egelhaaf 1999
;
Egelhaaf and Borst 1995
; Egelhaaf et al.
1993
; Single and Borst 1998
). By optical
recording of fluorescence changes of the Ca2+
indicator fura-2 in the dendrites of HS and CH cells, we measured the
relative changes in the intracellular Ca2+
concentration
(
[Ca2+]i) in vivo
during visual stimulation. HS cells showed significant rises in
[Ca2+]i in response to
adapting and test stimuli moving in the PD but almost no changes for ND
motion (Fig. 2). As was shown previously,
[Ca2+]i differs from
EM in its time course: The slope of
[Ca2+]i is less steep
than that of
EM and continues to
rise when the latter already reaches a steady-state plateau
(Dürr and Egelhaaf 1999
). Furthermore, whereas the
decay of
EM after the stimulus stops takes place within tens or hundreds of milliseconds, the decay of
[Ca2+]i is characterized
by time constants of several seconds.
To test a potential role of calcium in motion adaptation, we
analyzed whether the strength of the AHP correlates with
[Ca2+]i in the
dendrites of HS and CH cells. In response to PD motion stimuli of
increasing duration, both cell classes show dendritic
[Ca2+]i with
increasing peak amplitudes (Fig.
4A). The time course of both
the increase in [Ca2+]i
during motion stimulation and the decrease of
[Ca2+]i after termination
of stimulus motion is faster in CH than in HS cells as has been
described earlier (Dürr and Egelhaaf 1999
). During
4 s of ND motion (Fig. 4A, bottom), little
deviation from baseline is seen in the mean
[Ca2+]i. A systematic
investigation revealed that CH but not HS cells show, on average,
decrements in dendritic
[Ca2+]i in response to ND
motion (unpublished results).
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After-reactions of the membrane potential were quantified in a time
window of 1.8 s, starting 0.2 s after the stimulus stopped. An AHP occurs only when PD motion lasts for at least 0.5 s. If a
shorter stimulus is presented, the depolarization decays rather slowly
and the after-reaction is on a depolarized level (Fig. 4A).
Plotting the amplitude of the after-reaction
(EAR) to PD motion versus the
logarithm of stimulus duration reveals that for both cell classes
EAR steadily decreases with
increasing stimulus duration (Fig. 4B). In contrast, for ND
motion, HS cells show small after-depolarizations, the amplitudes of
which do not depend on stimulus duration. Although CH cells show
hyperpolarizing after-reactions to ND motion, again, there is no
monotonic dependency of
EAR on
stimulus duration.
Finally, there is a monotonic relationship between AHP and
Ca2+ accumulation for PD motion both for HS and
CH cells. This is demonstrated by plotting
EAR as a function of the relative
fluorescence change
F/F at the end of the
motion stimulus (Fig. 4C).
Dependence on stimulus velocity of Ca2+ accumulation and motion adaptation
The response of visual motion-sensitive neurons such as HS and CH
cells is not only determined by the duration of the stimulus but also
by other stimulus parameters, such as velocity. Therefore we
investigated whether a correlation between AHP and
[Ca2+]i also exists if
the stimulus strength is altered by varying the velocity of PD motion.
Due to the nature of motion detection, HS and CH cells show
membrane-potential responses of maximal size at a certain temporal
frequency, below and beyond which the amplitudes decline
(Dürr 1998; Egelhaaf and Borst
1989
; Hausen 1982b
). In HS cells, the
corresponding velocity tuning of
[Ca2+]i is known to
follow a similar characteristic (Dürr 1998
). In contrast, CH cells exhibit a discrepancy between the velocity tuning of
the [Ca2+]i response and
that of the electrical response (Dürr 1998
). In a
plot of
EAR values versus
F/F, both determined in the same way as for
Fig. 4C, but for motion stimuli of constant duration varying
in temporal frequency, a monotonic relationship between the two
parameters is obtained for HS but not for CH cells (Fig. 5).
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At very high velocities (24-Hz temporal frequency), HS cells
respond with a distinct, though transient, membrane depolarization, which is accompanied by little or no
[Ca2+]i increases
(Dürr 1998). As expected if the AHP depends on
cytosolic Ca2+, membrane potential changes
induced by 24-Hz stimuli are not followed by an AHP (Fig.
6A). In contrast, an
after-depolarization follows, comparable in strength and time course to
the one seen after ND motion.
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Although adaptation renders membrane potential responses more
transient, the time course by itself cannot serve as an unequivocal indicator of adaptation because, inherent to the mechanism of motion
detection, responses are more transient for fast stimuli than for slow
stimuli (cf. Figs. 1 and 6A) (Egelhaaf and Borst 1989). However, differences in the time course of responses to PD and ND motion may indicate direction-selective adaptation. The
responses to fast moving stimuli shown in Fig. 6A exhibit two direction-selective characteristics: first, the response to ND
motion remains on a slightly larger steady-state membrane potential level than the PD response. Second, the after-reactions of responses to
PD and ND motion are not mirror symmetrical, particularly because responses to both PD and ND motion are followed by
after-depolarizations. Since stimuli moving at a temporal frequency of
24 Hz were shown to cause at most very slight
[Ca2+]i,
Ca2+-dependent adaptation cannot be assumed to be
the sole determinant of direction-selective effects.
Direction-selective adaptation potentially independent of Ca2+ accumulation
As outlined in the preceding paragraph, fast moving stimuli
cause very transient membrane potential responses. During PD motion, these are accompanied by at most very small
[Ca2+]i increases
(Dürr 1998). To assess the strength and direction selectivity of
motion adaptation caused by a fast moving stimulus, hence without
pronounced concurrent Ca2+ increase, HS cells
were stimulated by the following protocol: First, 2-Hz motion was
presented for 2 s as a reference stimulus. After 1.9 s, 24-Hz
motion was presented for 2 s as the adapting stimulus, followed
after 100 ms by a 2-Hz test stimulus (Fig. 6B). During PD
motion, the transient response to the test stimulus is attenuated
relative to that elicited by the reference stimulus. The size of this
effect renders it unlikely that it is completely due to the initial
2-Hz stimulus because adaptation elicited by such a stimulus has
recovered after an interval of 4 s (see Fig. 3A). Thus
the attenuation is at least partially due to the 24-Hz stimulus. The
adapting effect of 24-Hz motion is probably direction selective because
during ND motion, the transient response to a test stimulus is even
slightly enhanced in 9 of 13 recordings if a 24-Hz stimulus is
presented first (Fig. 6B). The time course of
[Ca2+]i during 24-Hz
motion (Fig. 6B, · · · ) is consistent
with the finding that fast moving stimuli have only little influence on [Ca2+]i (Dürr
1998
). Nevertheless
[Ca2+]i increases that
are hardly detectable by measuring
[Ca2+]i across the
entire dendrite could be fairly large in close proximity to the cell
membrane. Because such
[Ca2+]i increases could
not be detected with our available imaging techniques, it appears at
present most plausible to conclude that fast moving stimuli cause
direction-selective adaptation by a mechanism that is probably
independent of
[Ca2+]i increases.
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DISCUSSION |
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Prolonged visual motion stimulation reduces the amplitude of a response to subsequent stimuli in directionally selective neurons of the fly. The effect, although present in both directions of motion, is much more pronounced and longer lasting for motion in the PD of the neuron. This direction selectivity is also observed in the after-reactions: an AHP, increasing in size with stimulus duration, is built up by PD motion, whereas comparable after-depolarizations are not elicited by ND motion. Adaptation and AHP could be based on the same physiological mechanism, for instance, a reduced excitatory or an increased inhibitory conductance. We propose dendritic Ca2+ as a physiological mediator of such a process. In our approach, direction selectivity of motion adaptation helps to discriminate mechanisms that are potentially based on Ca2+ from Ca2+-independent ones.
Role of Ca2+ for the modulation of responsiveness
When stimulus duration is varied, AHP and dendritic
[Ca2+]i are correlated
in HS and CH cells (Fig. 4C). Although there are several
possible mechanisms to cause an AHP, Ca2+ seems a
likely candidate, given the evidence from other preparations. AHPs were
shown to be mediated by Ca2+-dependent
K+ currents in a multitude of cell types (review:
Sah 1996
). This physiological mechanism is responsible
for spike frequency adaptation in neurons (e.g., Chitwood and
Jaffe 1998
; Lo et al. 1998
). Similarly, sensitivity changes in sensory cells are often mediated by ion channels
that are directly or indirectly regulated by Ca2+
(reviews: Fettiplace and Fuchs 1999
; Koutalos and
Yau 1996
; Menini 1999
). For example, in
vertebrate photoreceptors Ca2+ regulates
sensitivity by both decreasing the affinity of the cGMP-gated cation
channel for cGMP and inhibiting guanylate cyclase.
However, the correlation of the AHP and
[Ca2+]i observed in
TCs does not prove that Ca2+ controls the AHP. To
provide unequivocal evidence, it would be necessary to interfere
effectively with [Ca2+]i
in TCs. Single (1998)
reported an increase in the
frequency of spike-like discharges superimposed on graded membrane
potential responses by iontophoretic injection of the
Ca2+ chelator
1,2-bis(o-aminophenoxy)ethane-N,N,N',N'-tetra-acetic acid (BAPTA) into the cytoplasm of TCs. This hints at a role of Ca2+ in the modulation of cellular excitability.
In our own experiments, injection of BAPTA into single HS cells caused
no significant change in AHP and adaptation. It has to be noted,
however, that in vivo recording of TCs cannot yet be done with patch
electrodes. Thus intracellular buffer concentrations cannot be
controlled; something that would be necessary to rule out the existence
of a BAPTA-blockable, thus Ca2+-dependent,
mechanism in motion adaptation.
Possible Ca2+-independent mechanisms leading to
an AHP are for example slowly inactivating K+
conductances, the Na+/K+
pump (e.g., Gerster et al. 1997; Jansen and
Nicholls 1973
; Jansonius 1990
) or
Na+-dependent K+ channels
(e.g., Kim and McCormick 1998
; Schwindt et al.
1989
), the latter of which have been concluded to exist in
blowfly HS neurons (Haag et al. 1997
). Measuring the
conductance change following motion stimulation and comparing its time
course with that the AHP and of
[Ca2+]i could help to
determine the physiological mechanism underlying adaptation. However,
due to ion channel kinetics and to putative differences in bulk
cytosolic and near-membrane Ca2+ dynamics, the
time course of a
[Ca2+]i is not
necessarily identical to that of a Ca2+-dependent
conductance (Jahromi et al. 1999
; Sah and
Clements 1999
). Furthermore because different physiological
mechanisms could cause similar conductance changes, an unequivocal
identification of the conductance leading to AHP would additionally
require pharmacological studies, which are beyond the scope of this study.
Comparison of HS and CH cells
In HS cells, the AHP and dendritic
[Ca2+]i are
correlated, regardless of whether stimulus strength is altered by
varying the duration or the temporal frequency of motion. CH cells, on
the other hand, exhibit a monotonic relationship between AHP and
[Ca2+]i only for
increases in stimulus duration but not for changes in temporal
frequency (Figs. 4C and 5). This is reminiscent of the
finding that the temporal frequency tuning of electrical and Ca2+ responses is similar in HS cells but
different in CH cells (Dürr 1998
). Clearly,
[Ca2+]i is not the only
determinant of the AHP in CH cells because AHPs were also present
following ND stimuli (Fig. 4B). Further differences in
motion-induced
[Ca2+]i
between HS and CH cells, as found with respect to the time course and
the spatial spread of Ca2+ accumulation, and the
efficacy of ND stimuli in eliciting Ca2+
decrements have been found (Dürr 1998
;
Dürr and Egelhaaf 1999
). At present it is unclear
to what extent these differences correspond to cell-class-specific
properties of motion adaptation, e.g., with respect to its temporal and
spatial characteristics. So far, the differences between cell classes
have been discussed to reflect a purely modulatory role of
Ca2+ in HS cells and a putative involvement in
transmitter release at the dendrodendritic synapses of CH cells.
Implications for concepts of motion adaptation in TCs
In the past, only presynaptic changes have been discussed to cause
motion adaptation in TCs of the fly. One possible mechanism, although
controversially discussed, is a decrease of EMD delay time constants,
which would shift the temporal frequency tuning of TCs to higher
velocities (de Ruyter van Steveninck et al. 1986; but
see also Harris et al. 1999a
). Borst and Egelhaaf
(1987)
showed that adaptation cannot only be elicited by motion
stimulation but also by temporal modulation of luminance (flicker).
However, the fact that motion stimuli were more effective than flicker stimuli in causing motion adaptation can be interpreted as a first hint
that more than a single mechanism underlies motion adaptation in TCs.
Recently Harris et al. (1999b)
provided evidence for a reduction of contrast gain in HS cells induced by presenting adapting motion in any direction, including orthogonal motion, but not by
presenting flicker. The most plausible explanation for this observation
is an adaptation mechanism located presynaptic to TCs.
Here we suggest a postsynaptic mechanism for motion adaptation. We show
that motion adaptation is to a certain extent associated specifically
with PD motion. The TC itself is a likely location for the
physiological mechanisms associated with direction-selective adaptation
because it is regarded as the stage of motion processing where full
direction selectivity is computed from only weakly direction-selective
inputs (Borst and Egelhaaf 1990; Borst et al.
1995
; Egelhaaf et al. 1989
; Single et al.
1997
). However, direction-selective motion adaptation is not
simply determined by the excitation of the TC because membrane
potential responses exhibit much faster time courses compared with the
buildup of adaptation and recovery from it. Rather, temporally
integrating the membrane potential response would be necessary to
predict the strength of adaptation. Due to the time course of its
accumulation, Ca2+ could mediate such a
computational function. Nevertheless direction-selective motion
adaptation seems not to be caused exclusively by a
Ca2+-dependent mechanism. Fast-moving stimuli are
shown to produce adaptation in a direction-selective manner without
leading to considerable Ca2+ accumulation, thus
providing further evidence for a heterogeneous origin of motion adaptation.
Comparison with motion-sensitive neurons in other species
Motion adaptation and its underlying mechanisms have also been
investigated in motion-sensitive neurons of other species. Additionally, psychophysical studies (e.g., Clifford and Langley 1996b; Clifford and Wenderoth 1999
) and
functional magnetic resonance imaging (Tootell et al.
1995
) have been performed on man to elucidate the nature of
motion adaptation and its connection with the well-studied motion
aftereffect. In this psychophysical phenomenon, prolonged viewing of a
pattern moving in one direction results in illusory perception of
movement in the opposite direction after the stimulus stops (review:
Sekuler et al. 1978
). In a direction-selective neuron
such as a HS or CH cell, ND motion is signaled by membrane hyperpolarization. An AHP following prolonged PD motion might be
"misinterpreted" by a subsequent processing stage as ND motion and
could thus serve as a physiological explanation of the motion aftereffect.
The first physiological study on motion adaptation was by Barlow
and Hill (1963) in the retinae of rabbits. They found that direction-selective ganglion cells exhibit a drop of discharge frequency below the spontaneous rate when movement stops after a
long-lasting stimulation period.
Maddess et al. (1991) report that adaptation induces a
shift of the velocity response curve toward higher velocities in
motion-sensitive neurons of the butterfly Papilio aegeus.
They propose the idea of piece-wise linearity, i.e., neurons responding
linearly to changes in stimulus strength within each adaptation range.
Similar changes in the temporal frequency tuning were reported for
directional neurons in the nucleus of the optic tract (NOT) of the
wallaby Macropus eugenii (Ibbotson et al.
1998
). Adaptation, as assessed by determining decay time
constants of responses to step-wise grating displacements (impulsive
stimuli), was pronounced for PD motion but only weak with adapting
stimuli moving in the ND or modulating temporally in luminance.
In the visual cortex of cats (area 17), adaptation also altered the
temporal frequency tuning of motion-sensitive neurons, but instead of
shifting the response curves, the strongest aftereffect occurred at the
adapting frequency, with smaller effects both above and below it
(Saul and Cynader 1989). This result is difficult to
reconcile with a model incorporating adaptation-induced changes of time
constants in EMDs unless there exist several kinds of EMDs with
different delay filters. Also in cat visual cortex, Giaschi et
al. (1993)
report that in simple cells two processes are
evident in the time courses of both the decrement of responsiveness during PD adaptation and its subsequent recovery: a rapid
direction-independent and an additional slow, direction-selective
process. This is very similar to the results on motion adaptation in
fly TCs as described in the present study. Several further studies also
show direction selectivity of motion adaptation in cat visual cortex
(e.g., Hammond et al. 1988
; Marlin et al.
1988
) and area MT of the owl monkey (Petersen et al.
1985
). However, the meaning of direction selectivity of
adaptation in their and in our study is not completely congruent because most of the cortical neurons investigated are not fully direction selective in a strict sense. When stimulated with ND motion,
the cortical neurons show weak response increments or no responses
rather than decrements below resting activity as is found in fly TCs.
Along with the different response characteristics of motion-sensitive
cells, there probably exists a variety of mechanisms mediating motion
adaptation. The present study presents evidence for a postsynaptic
mechanism, namely dendritic Ca2+ accumulation,
which can account for several features of motion adaptation in TCs of
the fly, such as its direction selectivity, time course, and local restriction.
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ACKNOWLEDGMENTS |
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We thank B. Kimmerle for useful discussions of this work and R. Feiler for support in the programming of the A/D converter. The constructive comments of two anonymous referees are gratefully acknowledged.
This work was supported by the Deutsche Forschungsgemeinschaft.
Present address of V. Dürr: Abteilung für biologische Kybernetik und theoretische Biologie, Fakultät für Biologie, Universität Bielefeld, Postfach 10 01 31, D-33501 Bielefeld, Germany.
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FOOTNOTES |
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Address for reprint requests: R. Kurtz, Lehrstuhl für Neurobiologie, Fakultät für Biologie, Universität Bielefeld, Postfach 10 01 31, D-33501 Bielefeld, Germany (E-mail: rafael.kurtz{at}biologie.uni-bielefeld.de).
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 solely to indicate this fact.
Received 12 January 2000; accepted in final form 6 July 2000.
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REFERENCES |
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