Differential effects of short light pulses in the dark phase of an L/D cycle on behavioral despair in male wistar rats

Abstract

The present study investigated the effect of a 10-min light pulse either early or late in the dark phase of the L/D cycle on behavioral despair in male Wistar rats. Independent groups of rats (n=8 each) maintained on a 12L/12D cycle (lights on at 06:00 h) received a 10-min light pulse (either 900 or 1350 lux provided by an incandescent lamp) 3 or 9 h after dark onset. A control group (n=8) was treated similarly except for light exposure. All animals then underwent a 15-min Forced Swim Test (FST) starting at 14:00 in the next light phase of the L/D cycle, followed by a 5-min FST 24 h later. Analysis of variance indicated that exposure to either intensity of light delivered in the late but not the early part of the dark phase of the L/D cycle has protective effect in behavioral despair as indicated by shortened durations of immobility in the second swim test compared to controls. The fact that light pulses in the early part of the dark phase had no ameliorative effect on durations of immobility suggests that the antidepressant property of the late pulses may be due to their differential effect in phase shifting the circadian rhythm. Analysis of c-Fos expression reported a significant difference between the control and light exposure groups. Light exposure at ZT21 and ZT15 induces dense c- Fos immunoreactivity in the core region of the SCN which indicates phase shifting. Slight spontaneous c-Fos immunoreactivity was detected in the shell region of the SCN of the control rats. These results suggest that light may exert its antidepressant effect via circadian phase shifting.|Keywords: depression, circadian rhythms, suprachiasmatic nucleus, light pulse, c-Fos