CIRCADIAN PHASE-RESPONSE CURVES FOR SIMULATED DAWN AND DUSK TWILIGHTSIN HAMSTERS

Phase shifts of the circadian rhythm of wheel-running activity were co mpared in Syrian hamsters maintained in constant darkness and exposed to l-h naturalistic dawn or dusk twilight ramps (0.003-10 Ix), or to l -h rectangular light pulses (I Ix) providing equal photon exposure. Th e phase-response curves (PRCs) for dusk and rectangular pulses were vi rtually identical and resembled the PRC for dawn pulses, except that t he mean phase advance caused by dawn pulses at circadian time 19 (CT 1 9) was approximately I h smaller. This difference could not be account ed for by differences in the amount of wheel-running observed during l ight pulse exposure, because the animals ran more during dusk pulses t han during either of the other two pulse types. In a second experiment , 15-min rectangular light pulses (I Ix) immediately preceded by a 47- min dawn ramp caused smaller phase delays at CT 13 than rectangular pu lses alone, despite a 40% increase in total photon exposure, but phase advances at CT 19 did not differ between the two light treatments. Th ese results indicate that phase shifts of the circadian pacemaker in h amsters are determined primarily, though not entirely, by total photon exposure. They also indicate that dawn pulses may be less effective t han dusk or rectangular purses at certain circadian phases, possibly d ue to light adaptation during the early portion of the dawn twilight. Copyright (C) 1996 Elsevier Science Inc.