The effect of short and skeleton long photoperiods on the plasma concentrations of prolactin and cortisol in sheep.

Suffolk-cross lambs, aged eight weeks, were exposed to either short photoperiods (8L : 16D, SP; 7 castrated males and 6 females) or skeleton long photoperiods (7L : 10D : 1L : 6D, SLP; 6 castrated males and 7 females). They were fed individually on a complete pelleted diet at 70 g/kg live weight 0.75/day. Blood samples were taken monthly by jugular puncture for three months and, after 39 days on experiment, jugular catheters were inserted for frequent sampling over a 24 h period. The samples were assayed for prolactin using radioimmunoassay and for cortisol by competitive protein binding. SLP caused a large and significant increase in plasma prolactin throughout the experiment, with no effect on sex. During the 24 h period there were consistent peaks of prolactin at the start of both dark phases under SLP, with lower levels around the start of both light phases; under SP, prolactin tended to be higher in the middle of the light phase and the middle of the dark phase than at other times. Considerable hourly fluctuations were observed in cortisol levels but an obvious 24 h rhythm was not. Mean levels were, however, significantly higher under SP (38.3 nmol/l) than under SLP (26.8 nmol/l).