EFFECTS OF PHOTOPERIOD AND CONTINUOUS ADMINISTRATION OF MELATONIN ON PLASMA-CONCENTRATIONS OF PROLACTIN IN CASHMERE GOATS

The objective of this study was to determine whether continuous melato nin administration by means of implants (Regulin) would provide a shor t-day photoperiodic signal and modulate prolactin secretion in goats m aintained under controlled environmental conditions. Mean plasma conce ntrations of prolactin in goats maintained under long-day (LD) photope riods was greater (79 ng/ml; P=0.01) than for goats maintained under s hort-day (SD) photoperiods (41.9 ng/ml; SEM=3.8 ng/ml). There was a da y-length-by-time interaction (P<0.01). A nocturnal peak in circulating concentrations of prolactin was observed in goats exposed to LD but n ot to SD photoperiods. There also was a day-length-by-sex interaction (P=0.06). Male goats had higher levels of prolactin than females durin g 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 nocturn al rise in prolactin secretion in goats subjected to LD photoperiods w as not prevented by melatonin. Therefore, a continuous melatonin relea se, administered by means of implants near the end of the normal breed ing season, did not significantly impact prolactin secretion in goats.