Millisecond light pulses make mice stop running, then display prolonged sleep-like behavior in the absence of light

Abstract

Masking, measured as a decrease in nocturnal rodent wheel running, is a visual system response to rod/cone and retinal ganglion cell photoreception. Here, the authors show that a few milliseconds of light are sufficient to initiate masking, which continues for many minutes without additional photic stimulation. C57J/B6 mice were tested using flash stimuli previously shown to elicit large circadian rhythm phase shifts. Ten flashes, 2 msec each and equally distributed over 5 min, activate locomotor suppression that endures for an additional 25 to 35 min in the dark and does not differ in magnitude or duration from that elicited by 5-min saturating light pulse. Locomotor activity by mice without access to running wheels is also suppressed by light flashes. The effects of various light flash patterns on mouse locomotor suppression are similar to those previously described for hamster phase shifts. Video analysis of active mice indicates that light flashes initiated at ZT13 rapidly induce an interval of behavioral quiescence that lasts about 10 min at which time the animals assume a typical sleep posture that is maintained for an additional 25 min. Thus, the period coincident with light-induced wheel running suppression appears to consist of two distinct behavioral states, one interval during which locomotor quiescence is initiated and maintained, followed by a second interval characterized by behavioral sleep. Given this sequence effected by light stimulation, we suggest that it be referred to as "photosomnolence," the term reflecting upon both the nature of the stimulus and the associated behavioral change.

Figure 1

Interval of wheelrunning suppression and phase shift are simultaneously induced by millisecond light flash stimulation in a mouse. (A) Running record of a mouse in constant dark. Solid lines indicate activity onsets before and after the light stimulus which consisted of 10 flashes, each 2 msec, delivered equally spaced across 5 min. (B) Enlarged portion of the running record showing the time of light stimulation, consequent interval of locomotor suppression and the associated phase shift (ΔΦ). (C) Plot of the wheel revolutions per 5 min bin during the interval corresponding to the running record in (B) on the day of light stimulation. Flashes are associated with a rapid drop in wheelrunning. The asterisk (*) in (A-C) indicates the interval of wheelrunning suppression. The black arrows in (B) and (C) indicate the time of the stimulus.

Figure 2

Wheelrunning suppression varies with the number of flashes and also depends on the IFI. (A) Expt. 1: As the number of flashes delivered over a 5 min interval increases from 1 to 5, there is little change in the extent of wheelrunning suppression. Response jumps to maximal following 10 flashes per 5 min, suggesting a step function. All individuals responded to 10 flashes. (B) Expt. 2: Wheelrunning suppresion was more effectively achieved with 4-5 flashes presented with the IFI fixed at 16 sec. The difference between the Expts. 1 and 2 was that the IFI varied inversely with the number of flashes in the first, but remained fixed at 16 sec for the second. The solid lines indicate eye-estimated curves fitted to the data. The two experiments were conducted and analyzed independently. Groups sharing letter (Expt. 1) or number (Expt. 2) identifiers did not differ (p>.05). The groups receiving zero flashes experienced all sounds associated with delivery of 10 flashes, but no light.

Figure 3

Wheelrunning suppression is impaired when IFIs are short. Magnitude, indicated by the maximal percent change from baseline, is equivalent for all groups, but duration varies with the IFI of the stimulus pattern. The effects of flashes are shown in comparison with that elicited by a 5 min light pulse (□) administered beginning at time zero, as indicated by the open bar. All other groups received 10 flashes delivered with the IFIs indicated, the first flash delivered at time zero.

Figure 4

Wheelrunning suppression is impaired when the IFIs are long. (A) Ten flashes with a 30, 60, 120 or 240 sec IFI rapidly reduce wheelrunning to zero, with duration of locomotor suppression greater with a 240 sec IFI. The extent of suppression when IFI was 480 sec was prolonged but highly variable. Each group received its initial flash at time 0 and the timing all flashes administered to each group is indicated by the symbols below the abscissa. (B) The data from (A) are re-plotted relative to the time of the last flash to show the time to recovery from that flash. Recovery latency is inversely related to the IFI. The legend in (B) applies to both graphs. The last flash was 4.5, 9, 18, 36 and 72 min after the first for 30, 60, 120, 240 and 480 sec IFI groups, respectively.

Figure 5

The duration of wheelrunning suppression increases in relation to the number of supplementary light flashes. A 5 min light pulse administered at time zero induced typical robust suppression of wheelrunning. One or more flashes (IFI fixed at 16 sec) administered 25 min after the onset of the 5 min pulse and during the interval of locomotor suppression induced by the pulse, significantly increased the zero counts during the next 40 min. The group receiving 0 flashes was exposed only to the initial 5 min light pulse. Groups with shared letter identifiers do not statistically differ; otherwise, p<.05.

Figure 6

Still images extracted from a video stream captured under infrared illumination at 640×480 pixel resolution show characteristic events during the three behavioral states. (A) While wheelrunning; (B) Grooming, about 6.5 min after the first flash, during which the left ear was noticeable as the animal’s head was bobbing up and down; (C) Behavioral sleep, which began (for this animal) 8.3 min after the first flash, during which the left ear is now downward, the mouse having assumed a reclining position on its left side with snout facing away from the camera. Behavioral sleep lasted 18.7 min. A video clip summarizing the behavior of this animal is shown in Supplementary Online Material SOM2. Left arrow points to the top head just rostral to the ears; right arrow points to the spine.