Pineal-independent regulation of photo-nonresponsiveness in the Siberian hamster (Phodopus sungorus)

The pineal hormone melatonin influences circadian rhythms and also mediates reproductive responses to photoperiod. The authors tested whether pinealec tomy influences circadian oscillators responsible for induction of nonrespo nsiveness to short day lengths by preventing normal short-day patterns of c ircadian entrainment. Adult male Siberian hamsters were pinealectomized or sham operated, maintained in either 18 h light per day (18L) or 15L for 10 weeks, and then tested for responsiveness to 10L. Because pinealectomized h amsters do not show gonadal regression in short day lengths, responsiveness was assessed by measuring phase angle of entrainment and the length of the nightly activity period following transfer to 10L. The incidence of nonres ponsiveness was significantly higher in 18L hamsters than in 15L hamsters b ut was unaffected by pineal status. Fully 88% of 18L hamsters failed to ent rain to 10L in the normal short-day manner; the duration of nightly activit y remained compressed, and the phase angle of entrainment was large and neg ative relative to lights off. The 15L hamsters entrained normally to 10L. E xposure to constant Light after 10L treatment was equally effective in indu cing arrhythmicity in pinealectomized and intact hamsters. Changes in the p eriod of morning and evening circadian oscillators subsequent to 18L treatm ent did not predict circadian responsiveness to short photoperiod. Long-day induction of photo-nonresponsiveness, which prevents winter responses to s hort day lengths, occurs independently of pineal melatonin feedback on the circadian system.