The role of the Circadian system in mediating photoperiodic responses in Siberian hamsters

Date of Completion

January 1996


Biology, Animal Physiology




The goal of the present study was to determine the role of the circadian system in mediating short photoperiod responsivness in Siberian hamsters. The first study indicated that variation in photoperiodic responsiveness exists in Siberian hamsters, and that the proportion of photoperiod nonresponders (PNR) increases with age. Furthermore, this was the first study in which it was shown that PNR Siberian hamsters fail to exhibit inhibition of serum follicle stimulating hormone and prolactin concentrations, in addition to failing to exhibit testicular regression and the molt to winter type pelage. The second set of experiments indicated that PNR hamsters exhibit differences in their circadian organization as compared to short day responsive hamsters. PNR hamsters exhibited shorter durations of activity when exposed to short day lengths, and they began locomotor activity later in the dark period, as compared to short day responsive hamsters. PNR hamsters also exhibited a longer free running period lengths of activity onset in constant darkness as compared to responsive hamsters. This difference in the circadian system is thought to be partly responsible for mediating photoperiod nonresponsiveness. Running wheel access in short days elicited short day responses in PNR hamsters. The neural pathway involved in relaying this nonphotic stimulus to the circadian system was examined in a third set of experiments. It was shown that chemical lesions of the intergeniculate leaflet of the thalamus block the effect of running wheel activity in PNR hamsters. It was also shown that the intergeniculate leaflet may be involved in day length measurement in responsive hamsters. Finally, it was shown that altering the phase angle of entrainment to short light pulses in short day-responsive hamsters can result in photostimulation of the testes. ^