PHOTIC ENTRAINMENT IN HAMSTERS - EFFECTS OF SIMULATED TWILIGHTS AND NEST BOX AVAILABILITY

Entrainment of wheel-running activity rhythms was compared in hamsters exposed to daily light-dark (LD) cycles with abrupt transitions betwe en 0 and 10 lux or with artificial twilights simulating summer solstic e conditions at 41 degrees N latitude but truncated at 10 lux. The pho toperiod in LD-rectangular was set at 16.24 h, equating the total ligh t (in lux . min) emitted under the two schedules. The LD cycles were m aintained for 35 days and were followed by 14 days of constant darknes s (DD). Half the animals in each condition had access to a dark nest b ox connected to the outer compartment by a tunnel, the remaining anima ls being confined to a single compartment. Body temperature and locomo tor activity inside the nest boxes were recorded by telemetry. Movemen ts between the nest box and the outer compartment were monitored and t he data were used to calculate Light exposure at different times of th e day. In all groups, the phase angle difference between wheel-running onset and dusk was more positive than that between activity offset an d dawn. Hamsters with access to nest boxes, however, had later onsets, earlier offsets, and shorter activity durations (alpha s) than those without. These effects could be accounted for by the difference in lig ht exposure between the nest and no-nest animals, particularly light e xposure in the morning. The inclusion of twilights also resulted in la ter onsets and shorter alpha s, but the differences were relatively sm all and were only observed in the nest animals. The day-to-day variabi lity in activity onset was negatively correlated with onset time and w as smaller in the twilight/nest animals than in the other three groups . Most animals showed an expansion of alpha, during the first few days of DD, resulting from a rapid advance of activity onsets relative to offsets. The period of the rhythms, determined from the first five act ivity onsets in DD, was negatively correlated with the balance of even ing and morning Light exposure. These results are discussed in the con text of nonparametric entrainment of compound pacemakers.