Influence of photoperiod on the time of parturition in the rat .

Parturition time has been chosen to investigate whether a photoinducible phase exists during the light-dark cycle in the rat. A 1-hr light stimulation was given during the dark phase at various times after the beginning of the principal photoperiod or conventional dawn. The rats kept under 8 hrs of light and 16 hrs of darkness from mating (day 1 of pregnancy) gave birth to most of their pups on the afternoon of day 22 of pregnancy when the light pulse was applied 9 to 18 hrs or 22 to 23 hrs after dawn. Parturition was delayed till day 23 when the light pulse was given 18 to 22 hrs after dawn. In the same manner, under 16 hrs of light and 8 of darkness, most of the parturitions occurred on day 23 of pregnancy when a light pulse was given 22 hrs after dawn ; they extended over two preferential periods one on day 22 (about 50 p. 100) and the other on day 23 when the light pulse was given 16 to 17 hrs or 18 to 19 hrs after dawn. Thus, the stimulation of the photoinducible phase, found at the end of the dark period, resulted in retarded parturition. That delay was also observed when the light pulse was only given during the final days of pregnancy. Most parturitions occurred on day 22 of pregnancy under a short photoperiod to which 5 hrs of light were added during the dark phase at a time not coinciding with the photoinducible phase. This suggested that the effect of the principal light regime and the stimulation of the photoinducible phase could be dissociated.