MELATONIN AND LIGHT INDUCE PHASE-SHIFTS OF CIRCADIAN ACTIVITY RHYTHMSIN THE C3H HEN MOUSE/

This study examines the effect of light pulses and administration of t he pineal hormone melatonin on the circadian activity rhythm of C3H/He N mice. Mice were housed in constant dark in cages equipped with runni ng wheels. Phase shifts in the circadian rhythm of wheel-running activ ity were measured following treatment with a 15-min pulse of light (30 0 lux) or administration of vehicle (ethanol/saline) or melatonin (90 mu g, sc). Light treatment induced phase changes in circadian activity rhythms; specifically, delays during early subjective night (circadia n time [CT] 12.5 to CT 18.5) and advances during late subjective night (CT 0.5). A single dose of melatonin administered at various CTs had no consistent effect on free-running circadian activity rhythms. By co ntrast, melatonin administration for 3 consecutive days at the same cl ock time induced advances in circadian activity rhythms by more than 1 h when the first dose was administered at CT 10 and induced delays in circadian activity rhythms by up to 1 h when the first dose was admin istered between CT 24 and CT 2. With the caveat that multi- ple melato nin treatments are required to induce phase shifts, the results sugges t that the circadian timing system controlling the rhythm of wheel-run ning activity in the C3H/HeN mouse is responsive to both light and mel atonin.