Effects of photoperiod and continuous administration of melatonin on plasma concentrations of prolactin in cashmere goats

Abstract

The objective of this study was to determine whether continuous melatonin administration by means of implants (Regulin) would provide a short-day photoperiodic signal and modulate prolactin secretion in goats maintained under controlled environmental conditions. Mean plasma concentrations of prolactin in goats maintained under long-day (LD) photoperiods was greater (79 ng/ml; P=0.01) than for goats maintained under short-day (SD) photoperiods (41.9 ng/ml; SEM=3.8 ng/ml). There was a day-length-by-time interaction ( P<0.01). A nocturnal peak in circulating concentrations of prolactin was observed in goats exposed to LD but not to SD photoperiods. There also was a day-length-by-sex interaction ( P=0.06). Male goats had higher levels of prolactin than females during LD (98.6 and 59.3 ng/ml, respectively). During SD, plasma prolactin levels were higher in female compared to male goats (48.3 and 35.5 ng/ml, respectively; SEM=5.4 ng/ml). Prolactin secretion was reduced ( P=0.14) in goats receiving melatonin (50.9 ng/ml) compared with those not receiving melatonin implants (70.3 ng/ml; SEM=3.8 ng/ml). The nocturnal rise in prolactin secretion in goats subjected to LD photoperiods was not prevented by melatonin. Therefore, a continuous melatonin release, administered by means of implants near the end of the normal breeding season, did not significantly impact prolactin secretion in goats.