CONTROL OF THE CIRCANNUAL RHYTHM OF REPRODUCTION BY MELATONIN IN THE EWE

Annual variations in day length are responsible for seasonal changes i n reproductive activity in sheep, However, in constant photoperiodic c onditions, ewes express an endogenous rhythm characterized by alternat ions of reproductive activity and quiescence that are not synchronized among animals. Thus, the main role of photoperiod in the natural envi ronment appears to be the synchronization of this endogenous rhythm, P hotoperiodic information is processed through a complex nervous and en docrine pathway to modulate reproductive activity, Light information p erceived at the level of the retina is transformed through neural proc essing into an endocrine signal by the pineal gland: the nocturnal inc rease in melatonin release, Recent studies strongly suggest that melat onin has a hypothalamic target to modulate the reproductive neuroendoc rine axis, Most LHRH perikarya are located in the preoptic area, but t his region is devoid of melatonin receptors, and microimplants of mela tonin placed in the preoptic area do not effect LHRH release, Thus, me latonin influences LHRH neurones indirectly and must involve interneur ons. Good evidence now exists to demonstrate that a population of dopa minergic neurons with axons projecting to the median eminence is one o f these interneurons. (C) 1997 Elsevier Science Inc.