Light responsiveness of the suprachiasmatic nucleus: long-term multiunit and single-unit recordings in freely moving rats

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 approximately 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.

Fig. 1.

Representative example of an oscilloscope trace with multiunit activity. The spike indicated by a dotcould 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.

Fig. 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.

Fig. 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.

Fig. 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.

Fig. 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.

Fig. 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.

Fig. 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 anarrow are shown on a shorter time scale in Figure10.

Fig. 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 =x^a/x^a+ b^a. The fitted parameters werea = 0.6, b = 9 lux anda = 1.2, b = 0.5 lux for the day and the night, respectively.

Fig. 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.

Fig. 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.

Fig. 11.

Simultaneous multiunit and single unit recording.A, Multiunit baseline discharge pattern (•) and discharge rate during hourly 6 min light presentation (▴). Thetrace 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 time12 corresponds to the onset of drinking activity.

Fig. 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 trianglesand vary between 16 Hz during night and 8 Hz during day. Circadian time12 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 inA and represents the mean response during mid subjective day (CT 6 ± 2 hr). C, Light response during mid subjective night (CT 18 ± 2 hr).