RESUMO
Time of delivery was studied in normal (N) or enucleated (E) rats. Isolated (1/cage) at mating (day 1 of gestation), they were all anaesthetized on day 8 and some of them were then blinded. After anaesthesia, they were kept either alone (N and E) or in homogeneous (NN and EE) and heterogeneous (NE) pairs either under a 12L-12D or a 2L-22D light-dark (LD) cycle. Activity, monitored throughout the rest of pregnancy in 2 homogeneous (NN and EE) and heterogeneous (NE) pairs of rats, kept a marked circadian rhythm under each light regimen. Birth distributions differed according to LD cycle in normal (N or NN) but not in enucleated (E and EE) rats. Under the 12L-12D regimen, parturitions of N and E rats were divided into 2 parts, the majority occurred on the afternoon of day 22 before 21:00 h, the remainder were observed after 6:00 h on day 23. Under the 2L-22D regimen, N rats gave birth over one period, mainly on day 23, whereas E rats had the same birth distribution as those subjected to the 12L-12D LD cycle. In heterogeneous pairs of rats (NE), birth times were affected by photoperiod; under the 2L-22D regimen it was intermediate between those of homogeneous pairs (NN and EE). These results indicate that the eyes were the first link of the nervous chain by which photoperiod influenced birth time. Social conditions may also modulate the photodependent mechanism in ways which remain to be determined.