Regulation of seasonal reproductive activity in the stallion, ram and hamster

This review considers seasonal reproduction in male animals with emphasis o n the stallion, ram and hamster, The pineal hormone melatonin is the common link between photoperiod and reproduction. An increase in the daily diurna l period of melatonin secretion is associated with a decrease in GnRH relea se in long-day breeders, but an increase in GnRH release in short-day breed ers. Melatonin influences GnRH release within or close to the mediobasal hy pothalamus in rams; whereas melatonin receptors have not been found in the hypothalamus of horses. Prolactin release is positively correlated with day length, Prolactin concentrations are consequently low during the breeding s eason of sheep and high during the breeding season of horses and hamsters, Prolactin stimulates testicular function in rams, Seasonal changes in GnRH release in the horse are regulated by changes in a GnRH-inhibitory opioider gic tone. Opioids are at least, in part, responsible for the decrease in te sticular function during winter, An opioidergic inhibition of LH release is present during the breeding season in rams; but dopaminergic pathways inhi bit LH release during long daylight hours, A dopaminergic inhibition of LH release does not exist in stallions. (C) 2000 Elsevier Science B.V. All rig hts reserved.