THE ROLE OF THE PINEAL-GLAND AND MELATONIN IN REPRODUCTION IN MALE DOMESTIC RUMINANTS

The role of the pineal gland and melatonin in photoperiodic influences on reproduction is reviewed for male sheep, deer, goats and cattle. A role for the pineal was established by studies in which pinealectomy and superior cervical ganglionectomy (SCGx) caused a disruption of normal reproductive responses to changes in natural and artificial day lengths. Melatonin is a major secretory product of the pineal and is considered to be involved in photoperiodic time measurement. Circulating concentrations of melatonin follow light-dark cycles with significantly elevated secretion during darkness. A disruption of the melatonin rhythm is associated with photo-refractoriness. Also, melatonin treatments which mimic patterns of secretion under long or short days, induce corresponding reproductive responses in normal, pinealectomized and SCGx animals. The pineal gland and melatonin do not generate reproductive cycles, but rather entrain endogenous reproductive rhythms to light-dark cycles.