Light control of prolactin secretion in sheep. Evidence for a photoinducible phase during a diurnal rhythm

The secretion of prolactin comes under important seasonal variations in sheep.Thus, in the ram the highest levels are in summer and the lowest in winter.These variations of secretion are under light control because illumination of long duration (long photoperiods) produces high prolactin levels (220 -L 2 ng) ; short photoperiods produce low levels (12 ! 2 ng).Using the hypothesis of Bunning, we study a photoinducible phase during diurnal rhythm.For this purpose, rams preconditioned either in short photoperiods or in long pho- toperiods, receive 8 hours of illumination in two fractions of 7 and 1-hour durations.The beginning of the 7-hour period is considered to be a subjective dawn, and the 1-hour light pulse is given at various times after dawn : at hours 11, 14, 17 and 20.The group receiving the light pulse during hour 17 after dawn has significantly higher prolactin levels than all the other groups.For example, in rams preconditioned in short photoperiods and after 5 weeks of light pulse treatment, the mean prolactin levels are 71 ! 4 ng (hour 11)) ; 68 ! 5 ng (hour 14) ; 225 !22 ng (hour 17) and 110 !16 ng (hour 20).The prolactin level is higher during the dark phase than during the light phase (272 ! 15 ng vs 126 ! 8 ng in the group with light pulse at hour 17).This increase is essentially due to one important release resulting from the change in the light-dark sequence.Another release only occurs after the light pulse of one hour, if given at hour 17.However, these two prolactin releases are not sufficient to entirely explain this increase.In domestic mammals, prolactin secretion can be influenced by different factors.In the male or the female, stress (Raud et al., 1971), estrogens (Schams and Reinhardt, 1973) and TRH (Debeliuk et al., 1973) cause an increase in the level of plasma prolac- tin.Secretion of this hormone also depends on external factors such as ambient tempe- rature and relative humidity (Tucker and Wetteman, 1976).However, one external factor-light-has been shown to be important in the ram (Pelletier, 1973) and in the bull (Bourne and Tucker, 1975), but has not received much attention in other species.This paper describes the effects of light on the secretion of prolactin in the sheep.