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The Journal of Neuroscience, November 1, 1998, 18(21):9078-9087
Light Responsiveness of the Suprachiasmatic Nucleus: Long-Term
Multiunit and Single-Unit Recordings in Freely Moving Rats
Johanna H.
Meijer1,
Kazuto
Watanabe2,
Jeroen
Schaap1,
Henk
Albus1, 3, and
László
Détári4
1 Department of Physiology, Leiden University Medical
Centre, 2300 RC Leiden, The Netherlands, 2 Department of
Physiology, Dokkyo University School of Medicine, 321-02 Mibu, Japan,
3 Instituut voor Epilepsiebestrijding, 2100AA Heemstede,
The Netherlands, and 4 Department of Comparative
Physiology, Eötvös Loránd University, H-1088
Budapest, Hungary
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ABSTRACT |
The suprachiasmatic nuclei (SCN) of the hypothalamus contain a
pacemaker that generates circadian rhythms in many functions. Light is
the most important stimulus that synchronizes the circadian pacemaker
to the environmental cycle. In this paper we have characterized the
baseline neuronal firing patterns of the SCN as well as their response
to light in freely moving rats. Multiunit and single-unit recordings
showed that SCN neurons increase discharge during daytime and decrease
discharge at night. Discharge levels of individual neurons that were
followed throughout the circadian cycle appeared in phase with the
population and were characterized by low discharge rates (often below 1 Hz), with a twofold increase during the day. The effect of light on the
multiunit response was dependent on the duration of light exposure and
on light intensity, with light thresholds of ~0.1 lux. The light
response level showed a strong dependency on time of day, with large
responsiveness at night and low responsiveness during day. At both
phases of the circadian cycle, the response level could be raised by an
increase in light intensity. Single-unit measurements revealed that the
time-dependent light response of SCN neurons was present also at the
level of single units. The results show that the basic light response
characteristics that were observed at the multiunit level result from
an integrated response of similarly behaving single units. Research at
the single-unit level is therefore a useful approach for investigating
the basic principles of photic entrainment.
Key words:
suprachiasmatic nucleus; circadian; entrainment; photic; rat; multiunit activity; single cells; electrophysiology
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INTRODUCTION |
Substantial evidence indicates that
the suprachiasmatic nuclei (SCN) of the hypothalamus function as a
circadian pacemaker for many behavioral and physiological functions in
mammals (Moore and Eichler, 1972 ; Rusak and Zucker, 1979; Meijer and
Rietveld, 1989; Ralph et al., 1990). In constant conditions, these
functions have rhythms slightly different from 24 hr and are "free
running." To adopt the environmental periodicity, the SCN require
light input at specific times of the day. At the beginning of the
night, light produces phase delays of the circadian pacemaker, whereas at the end it produces phase advances (Pittendrigh and Daan, 1976). The
magnitude of light-induced phase delays and advances is dependent on
light intensity and on duration of light exposure (Nelson and Takahashi, 1991; Meijer et al., 1992). During the day, light does not
induce shifts in the pacemaker's phase.
Several neuronal pathways transmit light information from the eyes to
the SCN. Of these, the direct retinohypothalamic tract is the most
important for photic entrainment (Moore, 1973; Cassone et al., 1988).
Other visual pathways arise from the intergeniculate leaflet
(Harrington et al., 1987), the pretectum (Mikkelsen and Vrang, 1994),
and possibly the raphe nuclei (Foote et al., 1978). All pathways send
their information to the ventral and sometimes also to the lateral
borders of the SCN.
Single-unit recordings in anesthetized hamsters and rats have indicated
that ~30% of SCN neurons are light responsive, with a majority that
is light-activated and a minority that is light-suppressed (Groos and
Mason, 1980; Meijer et al., 1986). These neurons are mainly located
within the retinorecipient areas of the SCN. They give sustained
responses and code for light intensity according to a sigmoid-shaped
curve. However, thresholds for light responses in these neurons were
found to be higher by two magnitudes than the threshold for behavioral
phase shifting (Meijer et al., 1986). This discrepancy has been
attributed to the use of anesthetics, which presumably reduce the
sensitivity of SCN neurons to light (Dyer and Rigdon, 1987; Meijer et
al., 1992). A further, major problem with anesthetics is their putative
effect on the pacemaker's functioning (Colwell et al., 1993).
Consequently, light response of SCN neurons cannot be studied as a
function of circadian time (CT) in the presence of anesthetics.
In a previous paper, we reported our first data on the light responses
of SCN neurons in freely moving (i.e., unanesthetized) rats and
found maximum responsiveness during the night (Meijer et al., 1996). In
the present paper we characterize SCN light response as a function of
circadian phase, light intensity, and light duration. The results
demonstrate that at any phase, the combination of light intensity and
circadian phase determine the final response level.
Long-term single-unit recordings have so far been performed only in
cultured systems (Mirmiran et al., 1995; Welsh et al., 1995; Herzog et
al., 1997). In these studies it was found that individual SCN neurons
have the capacity to oscillate. The importance of in vivo
recordings was recently indicated in a study that showed differences in
the characteristics of electrical multiunit activity (MUA)
discharge rhythms in vivo versus in vitro (Meijer
et al., 1997). Because of stabilization difficulties, it has never been possible to perform long-term recordings of single neurons in freely
moving animals. We have now performed nine long-term single-unit recordings in freely moving animals for >48 hr, and we demonstrate for
these neurons a low-amplitude rhythm in spontaneous discharge rate with
increased discharge during the day. Moreover we show that individual
SCN neurons exhibit oscillations in their light response with maximum
responsiveness during the night.
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MATERIALS AND METHODS |
Male Wistar rats were implanted with tripolar electrodes
(Formvar insulated stainless steel, diameter 0.125 mm) (Plastics One,
Roanoke, VA). Two electrodes were aimed at the SCN, with an
interelectrode distance of 0.4 mm. The third electrode was cut shorter
and its insulation was partly removed before it was placed in the white
matter for reference. After the surgery, the animals were put back in
the colony room with a light regimen of 12 hr light (100-150 lux)
alternating with 12 hr dark for at least 10 d.
At the onset of the experiment, the animals were transferred to the
experimental room and connected to the recording system. A flexible
cable attached to a counterbalanced swivel system was used to
minimalize any effect on the animal's freedom of movement. Recordings
were performed through one electrode at a time. The details of
recording have been described previously (Meijer at al., 1996).
Briefly, after appropriate amplification and filtering (500 Hz-5 kHz),
multiunit activity was converted to pulses by window discriminators and
was counted by a computer every 10 sec. Signal-to-noise ratio varied
over different recordings (Fig. 1). The
first window discriminator was set at such a level that signal-to-noise ratio varied from 2:1 to 8:1 or more. The second window discriminator was used to isolate a single unit from the multiunit signal to record
the unit for at least two circadian cycles. This was only possible when
one of the cells that made up the multiunit signal was very close to
the electrode tip and therefore produced spikes of large and
distinctive amplitude (Fig. 1). A second criterion was uniformity of
the shape of the spike during the recording session.
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Figure 1.
Representative example of an oscilloscope trace
with multiunit activity. The spike indicated by a dot
could be separated from the other spikes and could be recorded for
several days. The inset shows the characteristic
waveform of this spike. Horizontal calibration: 5 msec
(oscilloscope trace) and 0.3 msec
(inset). Vertical calibration: 10 µV throughout.
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A third window discriminator was set at a higher level to record
artifacts that occurred as a result of vigorous head movements and
feeding behavior. The amplitudes of these artifacts were much larger
than the amplitude of the action potentials. Those 10 sec bins in which
artifacts occurred were excluded from the analysis.
Recordings were performed in constant darkness. To characterize the
response of the SCN, light pulses of various intensity and duration
were applied. The timing of the light pulse applications was controlled
by a computer, whereas light intensities were regulated by a series of
neutral density filters. To determine SCN light response as a function
of CT, 6 min light pulses were applied every hour. Because this
protocol is expected to affect circadian period, the animal's drinking
rhythm was recorded throughout the experiment. The CT of light
responses was calculated per day, on the basis of the onset of the
drinking activity (CT 12).
At the end of the experiment, animals were deeply anesthetized with
Nembutal and a positive current (20 µA for 50 sec) was passed through
one of the electrodes to deposit a small amount of iron at the
recording site. After this procedure, rats were perfused through the
heart with physiological saline followed by a 4% Formalin solution.
The Formalin solution contained 1.5% potassium ferrocyanide to obtain
a blue spot by Prussian blue reaction with the iron deposit. The brain
was removed and blocked to prepare freeze-cut sections of 40 µm.
Sections were stained with cresyl violet for final inspection.
The presence of rhythmicity in the MUA activity was investigated by
Lomb periodogram analysis for nonequidistant data (i.e., data sets with
missing values). The level of statistical significance was assessed by
a Monte Carlo simulation (Lomb, 1976; Horne and Baliunas, 1986). To
quantify light responses, the steady-state response level was taken.
This implies that transient increases in activity at the onset of light
pulses and transient decreases in firing rate that were correlated with
the animal's behavior were excluded from the analysis.
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RESULTS |
In the present study, 149 successful hypothalamic recordings were
performed for at least one circadian cycle in 104 rats. In 52 cases, no
circadian variation was seen in the multiunit activity. In the
remaining 97 cases, a significant circadian rhythm was present
(p < 0.01). Of these, 27 recordings showed a
rhythm with high discharge rate during the day and low discharge rate during the night (Fig. 2). This discharge
pattern is opposite to the animal's behavioral activity cycle.
Histology confirmed in these rats that the recording site was within
the SCN, with the exception of a few cases in which the blue spot could
not be found in the sections. Reversed rhythms with high activity during the night and low activity during the day were recorded in 70 cases. Recording sites were located lateral, anterior, or posterior
from the SCN, with a maximum distance of 0.8 mm to the SCN border. In
17 recordings we succeeded in recording a single unit for more than 48 hr, which met the criterion for a single unit. Nine units were located
in the SCN, and eight were located outside the SCN.
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Figure 2.
Long-term multiunit activity as a function of
circadian time. Circadian time was determined on the basis of drinking
actograms. The recordings were performed in constant darkness. The
onset of drinking activity is the onset of the subjective night and
corresponds with CT 12. The subjective night is shaded.
Multiunit discharge inside (A) and outside
(B) the SCN was counted every 10 sec.
Dots represent mean values per minute. At the bottom of
each figure, movement artifacts have been plotted. The gaps in the
multiunit traces are the consequence of the exclusion of data bins in
which movement artifacts occurred.
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SCN light responsiveness
After a light stimulus, two multiunit response types were recorded
inside the SCN. In most cases (n = 21) light induced an increase in discharge; in four cases no light response occurred, and in
two cases a decrease was observed (Fig.
3). The light-activated response usually
started with a fast transient increase in MUA activity. The discharge
rate then gradually decreased, and after a few minutes a steady-state
response was seen. Similarly, the termination of the light pulse very
often induced a sudden drop in the activity, which decreased well below
the baseline level. The original activity level returned within a few
minutes (see Figs. 5A-C, 6). These three components of the
light-activated response usually occurred together, but they also could
appear separately. The pattern of the light-depressed response was very different from the light-activated ones. No transients were seen, but
instead there was a gradual decrease in MUA activity when the light was
turned on. The discharge rate reached the steady-state level after 1-2
min. A similar, slow buildup of activity was observed at the end of the
light pulse.
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Figure 3.
Multiunit light response of SCN neurons. The
timing of the light pulse is indicated in the step
diagram above the records. The light-activated
(A) and light-suppressed
(B) responses represent the mean of five and
seven responses, respectively, obtained during two circadian
cycles.
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The effect of stimulus duration on the response pattern was
investigated by presenting light pulses of different length ranging from 2 to 30 min (Fig. 4). Responses
appeared sustained in all cases and lasted for the full duration of the
light stimulus. The amplitude of the response was usually constant, but
in a few instances a small, gradual increase or decrease was seen.
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Figure 4.
Responses to light pulses of different duration.
Light responses of SCN neurons are sustained for 2, 6, and 30 min light
pulses. The drop in discharge at the end of the 30 min light pulse is
accidental. Discontinuity in the data, immediately after the onset of a
light pulse, resulted from movement artifacts.
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To investigate the effect of light intensity on discharge level, 6 min
light pulses were used at different light intensities ranging from
10 4 to 140 lux. The light intensities were spread
evenly on a logarithmic scale. At any circadian time a monotonic
relation was found between light intensity and neuronal discharge. The
monotonic relation holds for both the light-activated and the
light-suppressed responses (Fig. 5). To
quantify light response of SCN neurons as a function of light
intensity, circadian time had to be taken into account.
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Figure 5.
Light responses to increasing light intensities.
Activated (A-C) and suppressed
(D-F) response types with three to five traces
averaged per light intensity. Intensities are indicated above the step
diagram.
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Rhythmic light response
When light response was studied as a function of circadian time, a
significant difference in light response emerged between day and night
(Fig. 6). During the day, the baseline
discharge was relatively high, with a maximum at mid subjective
day. Light presentations during this phase elicited only small
responses. During the night, when baseline discharge was in its lower
phase, large light responses were obtained. The variation in light
response emerged not as a sudden transition in responsiveness but as a gradual process (Fig. 7A), and
it displayed significant circadian rhythmicity
(p < 0.001). From mid subjective day onward, a
steady increase in responsiveness was observed lasting until late
subjective night. From there a fast decline started, leading to small
responsiveness during mid subjective day. The steady-state multiunit
response to a light pulse applied at night exceeded the light response during the day (in all but one cases), despite the fact that baseline activity during night was lower. The direction of the response never
changed in the course of the day.
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Figure 6.
Light responses as a function of circadian time.
Light intensity was set at 0.15 lux. Above the step diagram, circadian
time (CT) of each response has been given. The
change in magnitude of the light response throughout the circadian
cycle is clearly visible, as is the circadian rhythm in basal
MUA.
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Figure 7.
Light responses during long-term multiunit
activity recordings inside (A) and outside
(B) the SCN. The subjective night of the animal
is shaded. Light pulses were given every hour for 6 min,
with an intensity of 0.15 lux. In between the light presentations, the
animal was in total darkness. The mean discharge rates per minute
during the periods of darkness are indicated by a dot.
The mean discharge rates per minute during the light intervals are
indicated by triangles. Light responses marked by an
arrow are shown on a shorter time scale in Figure
10.
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The threshold of the light responses was ~0.1 lux at any phase of the
circadian cycle. When light intensity was increased, the change in
response depended on the circadian time. At mid subjective night, when
sensitivity was high, responses grew rapidly as a function of light
intensity, and saturation occurred between 1 and 10 lux. At mid
subjective day, responses increased only slowly as a function of
intensity, and saturation occurred above 100 lux (Fig.
8).
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Figure 8.
Averaged light sensitivity of activated SCN
neurons during day ( ) and night ( ). The maximum light response
obtained in each animal was normalized to 1. The standardized light
response is plotted against the light intensity and was obtained by
taking the difference between baseline activity in the dark immediately
before the light pulse, and discharge during light presentation. The
difference was averaged for the mid subjective day ±3 hr () and for
mid subjective night ±3 hr (). Data points were fitted with a
Michaelis equation: y = xa/xa + ba. The fitted parameters were
a = 0.6, b = 9 lux and
a = 1.2, b = 0.5 lux for the
day and the night, respectively.
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A complete description of the steady-state light response should be
based on its dependence on light intensity and CT at any phase of the
circadian cycle. We have exposed the animals to different light
intensities on consecutive circadian cycles. Often, one light intensity
was presented for two circadian cycles. The results indicate that at
any circadian phase the response of the SCN is determined by the
combination of light intensity and circadian time (Fig.
9).
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Figure 9.
Light response of SCN neurons as a function of
both light intensity and circadian time in a particular animal. The
averaged response level was plotted against circadian time (assessed by
the drinking rhythm) and against the logarithm of light intensity (in
lux). The response level was determined by taking the difference
between the dark discharge level before the light pulse and the
discharge level during light and was averaged per 2 hr. For every light
intensity used, the circadian variation in light response has been
fitted by a free three-order polynomial, whereas the average is
indicated by a dashed line, to enable
visualization.
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Outside SCN-type rhythms were obtained in 70 recordings, with 23 cases
showing light responsiveness. These light responses were relatively
weak and always of the light-activated type. When light response was
measured as a function of circadian time, high responses appeared
during mid subjective night and low responses appeared during mid
subjective day (Fig. 7B). Thus light sensitivity rhythms
were in the same phase inside and outside the SCN, whereas baseline
activity rhythms were in opposite phase (Fig.
10).
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Figure 10.
Light response inside (A)
and outside (B) the SCN. Circadian time of the
light pulses is specified above the step diagram. The long-term
recording traces from which these examples are taken are given in
Figure 7A,B. The timing of the light pulses is indicated
by the arrows in Figure 7. Circadian rhythms in baseline
discharge inside and outside the SCN are in opposite phase.
Nevertheless, the rhythm in light response is similar inside and
outside the SCN. Furthermore, it can be observed that light responses
in the SCN were larger than outside the SCN, which was a general
observation.
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Single-unit recordings
Long-term single-unit recordings in which an individual unit could
be recorded for at least 48 hr were performed in 17 cases, nine inside
the SCN and eight outside the SCN (Fig.
11). In all SCN neurons, baseline
discharge patterns of single units showed increased activity during the
day and decreased activity during the night, with no exception (Table
1). The mean discharge rate of SCN
neurons at mid subjective night was 0.7 Hz and ranged from 0.01 to 4 Hz, depending on the neuron. In seven of nine neurons, discharge did
not exceed 0.2 Hz during the night. The mean discharge during mid
subjective day was 1.39 Hz and ranged from 0.02 to 8 Hz. In seven of
nine cases, discharge did not raise above 0.7 Hz during the day.
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Figure 11.
Simultaneous multiunit and single unit recording.
A, Multiunit baseline discharge pattern () and
discharge rate during hourly 6 min light presentation ( ). The
trace at the bottom indicates the
occurrence of movement artifacts. B, Single-unit
baseline discharge () varies between a mean of 0.05 Hz during night
and 0.2 Hz during day. The light response level () varies between
mean levels of ~1 Hz during night and 0.5 Hz during day. After a
recording time of ~36 hr the single unit was lost. Circadian time
12 corresponds to the onset of drinking activity.
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Table 1.
Discharge rates of individual SCN neurons during the
subjective day and night when the animal was in darkness
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In four long-term recordings of single SCN units, light-activated
responses were observed. All light-activated SCN neurons showed a
significant circadian rhythm in light response, with high
responsiveness during night and low responsiveness during day
(p < 0.001). In fact, the rhythm in light
response was more pronounced than the rhythm in baseline discharge
(Fig. 12). SCN light response during
the day reached a mean value of 7.15 Hz and during the night a mean
value of 13.05 Hz.
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Figure 12.
Long-term single-unit recording inside the SCN.
A, Two circadian cycles of multiunit baseline activity
indicated per minute in dots, with mean values of ~1.5
Hz during night and 2.5 Hz during day. Responses to hourly light pulses
of 6 min, 0.15 lux are indicated per minute by triangles
and vary between 16 Hz during night and 8 Hz during day. Circadian time
12 corresponds to the onset of drinking activity of the
animal. B, Example of a light response during daytime at
a different scale. The example is taken from the recording depicted in
A and represents the mean response during mid subjective
day (CT 6 ± 2 hr). C, Light response during mid
subjective night (CT 18 ± 2 hr).
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Outside the SCN none of the eight neurons exhibited significant
circadian discharge patterns in baseline firing frequencies, and none
of the neurons were light responsive. The mean discharge rate outside
the SCN was 0.44 Hz.
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DISCUSSION |
Baseline rhythms
This is the first study in which the behavior of single SCN
neurons was followed throughout the circadian cycle and could be linked
to the animal's circadian activity rhythm. Single-unit analysis was
performed only in those cases in which a neuron that was apparently
close to the recording electrode generated a spike that was much larger
than all others in the multiunit signal. If the shape of this
spike varied, however, we supposed that it was generated by more than
one neuron, and consequently single-unit analysis was not possible. On
the basis of these criteria, we demonstrated the presence of circadian
rhythmicity in the discharge pattern of nine individual SCN neurons.
For all of these neurons, discharge was high during day and low at
night, which is similar to the multiunit discharge pattern. Previous
long-term recordings in cultured systems (Mirmiran et al., 1995; Herzog
et al., 1997) and in dissociated cultured systems (Welsh et al., 1995)
have indicated rhythms in spontaneous discharge, but no relation with subjective day or night could be made. In two of these studies, neurons
were recorded simultaneously and found to differ occasionally in phase
or period, especially in the dissociated cultured system. In contrast,
our single units were always synchronized with the multiunit discharge
pattern, which suggests that they were also mutually synchronized. It
is possible that dissociated cultured systems lack essential
possibilities to synchronize (Welsh et al., 1995). Nevertheless we feel
that the number of single-unit recordings in vivo is too
small to generalize at this point and additional measurements are
required.
As an important difference with isolated in vitro
preparations, we found that discharge rates were very low, in most
cases below 1 Hz. In vitro studies generally describe
discharge rates between 1 and 10 Hz (Green and Gillette, 1982; Groos
and Hendriks, 1982; Shibata, 1982; Thomson et al., 1984; Mason, 1991).
Possibly the absence of inhibitory afferents in vitro (such
as those from the raphe and geniculate nucleus) lead to increases in
firing rate. Other factors such as increased concentrations of
potassium and calcium in the artificial medium may further increase
neuronal discharge.
A second difference with dissociated cultured systems is the shape of
the circadian discharge pattern. In all of our neurons, discharge
during daytime increased approximately twofold. This amplitude is less
than those obtained with multiunit recordings in vitro
(Bouskila and Dudek, 1993; Meijer et al., 1997). In vitro, the system is deafferented from the brain, which results in a loss of
synaptic input. Local interactions may be somewhat reduced as well. In
dissociated culture systems, cellular interactions are expected to be
even lower. In such cultures, the rhythm in discharge appeared even
more pronounced and increased 5- to 10-fold in the course of the cycle
(Welsh et al, 1995). In the cell culture system of Herzog et al.
(1997), amplitudes varied among the various examples. In those cases in
which synchronized cell firing rhythms were shown in the figures,
rhythm amplitude was smaller as compared with those instances in which
cells did not fire in synchrony. Our results suggest that a decrease in
cellular communication leads to a decrease in synchronization and to an
increase in amplitude of an individual neuron's firing rhythm.
SCN light response
Almost all multiunit recordings from inside the SCN (23 of 27)
showed clear responses to light. These responses were positive, with
the exception of two recordings in which a decrease in discharge was
observed. Short-term recordings have indicated that most
light-responsive neurons are activated by light and only a minority are
light-suppressed. Therefore, it is expected that light-activated
responses dominate the response pattern in multiunit recordings in
which hundreds of neurons are recorded simultaneously. The presence of
a light-suppressed response in two of our multiunit recordings may
indicate a clustering of cell types within the SCN.
Retinal input to the SCN has been shown to use glutamate as a
transmitter (DeVries at al, 1993, 1994; Ding et al., 1994). Excitatory amino acids are known to have fast actions on the
firing behavior of neurons, which is in good agreement with the
abruptness of the light-activated responses observed in the present
experiments. On the other hand, the slow buildup and decay of the
light-suppressed responses suggest that different pathways are
responsible for this response type.
The direction of response never changed in the course of the circadian
cycle. In other words, both the light-suppressed and the
light-activated responses retained their sign. These results extend an important study of Inouye (1984) in which light-activated responses were similar at four different times of the circadian cycle.
This phenomenon, which can be examined only in long-term recordings, is
of critical importance in understanding photic entrainment.
Apparently no relation exists between the positive or negative
sign of the photic response and the advance/delay zone of the pacemaker. Even during subjective day, when light stimuli do not phase
shift the pacemaker, light responses could be obtained in SCN neurons.
For yet unknown reasons, neuronal discharge does not impinge on the
pacemaker during daytime.
In this respect, the organization of the circadian pacemaker appears
similar to the organization of the pacemaker in the marine snail
Bulla gouldiana (Block et al., 1993). Electrophysiological recordings in this snail have shown that throughout the circadian cycle, pacemaker cells respond to light with an increase in discharge, despite the fact that phase delays and advances are obtained only during the first and second part of the subjective night, respectively, as is the case in mammals. However, depolarization results in an
opening of calcium channels only during the night when these channels
are normally closed. It remains to be determined whether calcium plays
a similar role in the phase-resetting mechanism of the mammalian
pacemaker. Current lines of investigation will reveal whether calcium
plays a similar role in the more complex circadian system of mammals
(van den Pol et al., 1992; Chen and van den Pol, 1998).
We have demonstrated that light-activated responses in the multiunit
signal could also be measured outside the SCN, although these were much
weaker than inside the SCN. This is consistent with the observation
that hypothalamic areas adjacent to the SCN also receive retinal
innervation, but this is relatively sparse (Levine et al., 1991).
Light-suppressed responses were never observed outside the SCN.
Rhythmic light response
Although light responses are present throughout the circadian
cycle, they are not constant. During daytime, when baseline activity is
high, the light response of the SCN is low. In the night, when baseline
activity is low, light response is high. Thus, the increase in light
response occurs when baseline decreases. Noticeably, the light response
pattern outside the SCN was also circadian, with large responsiveness
at night. Thus outside the SCN, the rhythm in light response follows
the rhythm in baseline discharge, and high responsiveness coincides
with the phase when baseline activity is high.
In a previous paper we speculated that the increased light
responsiveness at night might be the result of the hyperpolarized membrane potentials of SCN neurons at this phase of the cycle, resulting in increased driving force and therefore increased light response (Meijer et al., 1996). The present recordings from areas surrounding the SCN suggest instead that the rhythm in light response is independent from the baseline discharge rhythm. Therefore, the
origin of the light-response rhythm should be upstream from the SCN,
along the light-input pathway.
Light-responsive SCN cells receive information from retinal ganglion
cells. In the ocular pacemaker of the rat, circadian rhythms have been
observed in the expression of transducin, which mediates the initial
stages in the phototransduction cascade (Brann and Cohen, 1987).
Furthermore, disk shedding and electroretinogram sensitivity exhibit
circadian rhythms (LaVail, 1980; Besharse, 1982; White and Hock, 1990;
for review, see Remé et al., 1991). Also in the isolated retina,
circadian rhythms have been observed in melatonin production (Tosini
and Menaker, 1996). So far, it is not known whether these ocular
rhythms have consequences for the processing of light information. The
present findings are a first indication that retinal pacemakers
modulate light transmission to the brain and have functional
importance.
Conclusive evidence was obtained for rhythmicity in light
response of single SCN neurons. Light responses of single units showed
increased responsiveness at night and low responsiveness during day.
Thus, the light-response pattern of individual SCN neurons was similar
to the pattern observed in multiple unit recordings. The results
indicate that the electrophysiological characteristics of the SCN are
present also at the level of the individual neurons. Therefore,
long-term single-unit recording is a promising new approach for
learning more about the organization and synchronization of the
circadian pacemaker.
| |
FOOTNOTES |
Received March 20, 1998; revised Aug. 11, 1998; accepted Aug. 14, 1998.
This work was supported by the Nederlandse Organisatie voor
Wetenschappelijk Onderzoek (Visiting Grant B91-248 to K.W.) and the
European Community (Grant ERB-CIPA-CT92-2277). We thank Mary Harrington
and Benjamin Rusak for their useful comments on this manuscript, and we
thank Jan Janse and Hans Duindam for engineering and technical
assistance.
Correspondence should be addressed to Dr. J. H. Meijer, Department
of Physiology, P.O. Box 9604, 2300 RC Leiden, The
Netherlands.
| |
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Top
Abstract
Introduction
