PHOTOPERIODISM IN HAMSTERS - ABRUPT VERSUS GRADUAL CHANGES IN DAY LENGTH DIFFERENTIALLY ENTRAIN MORNING AND EVENING CIRCADIAN OSCILLATORS

In studies of photoperiodism, animals typically are transferred abrupt ly from a long (e.g., 16 h light per day [16L]) to a short (8L) photop eriod, and circadian oscillators that regulate pineal melatonin secret ion are presumed to reentrain rapidly to the new photocycle. Among rat s and Siberian hamsters, however, reentrainment rates vary depending o n whether additional darkness is added to morning or evening, and a su bset of hamsters (nonresponders) fails ever to reentrain normally to s hort photoperiods. The authors assessed whether several short-day resp onses occurred at different rates when darkness was extended into morn ing versus evening hours and the effectiveness of abrupt versus gradua l shortening in day lengths (DLs). Entrainment patterns of photorespon sive hamsters also were compared to those of photononresponsive hamste rs. Responsive hamsters transferred on a single day from 16L to 8L und erwent more rapid gonadal regression, weight loss, decreases in follic le-stimulating hormone titers, and expansion of nocturnal locomotor ac tivity when darkness was added to morning versus evening. When the dar k phase was extended gradually by 8 h over 16 weeks, short-day respons es occurred at the same rate whether darkness was appended to morning or evening or was added symmetrically. Darkness added to evening promo ted more rapid short-day responses when it was added gradually rather than abruptly, despite the fact that average DLs were significantly sh orter for the latter group. Among nonresponders, morning extensions of darkness transiently increased activity duration, whereas evening ext ensions did not. Gradual and abrupt decreases in DL differentially aff ect entrainment of evening and morning circadian oscillators. The auth ors argue for the incorporation of simulated natural photoperiods in s tudies of photoperiodism.