ENVIRONMENTAL INDUCTION OF PHOTONONRESPONSIVENESS IN THE SIBERIAN HAMSTER, PHODOPUS-SUNGORUS

In seasonally breeding rodent species, a fraction of the population is unresponsive to short day lengths (DL) and remains reproductively com petent during winter. We previously observed that incidence of nonresp onsiveness to short days was affected by photoperiodic history. Here w e tested whether exposure to long DL (18h light/day; 18L) renders anim als unresponsive to short DL (10L). Hamsters, maintained from birth in 10L, were transferred at week 6 to 18L or 14L. Ten weeks later (week 16), groups were transferred to 10L for 10 wk. All hamsters maintained in short DL from birth had undeveloped testes at week 6. At week 26, however, 92% of hamsters previously kept in 18L failed to undergo comp lete gonadal regression in 10L, compared with only 10% of hamsters pre viously in 14L. Entrainment of locomotor activity in 10L in nonrespons ive hamsters resembled that typically observed under long DL. Exposure to 18L may induce nonresponsiveness by altering interactions of compo nent circadian oscillators that mediate gonadal regression in short DL .