Prolactin-Brain Interactions and Reproductive Function

SYNOPSIS. There is mounting evidence that prolactin influencesreproductive function in several vertebrate species via directaction on the central nervous system. In most instances, however,the basic properties of these prolactin-induced alterationsin brain function, the neurochemical mechanisms underlying theseeffects, and the identity of the prolactinlike molecules thatmight normally promote these changes have yet to be adequatelycharacterized. Several properties of the prolactin-brain relationshipthat have emerged from our work in the ring dove (Streptopeliarisoria) are described in this paper and discussed in relationto similar work in other species. Receptor binding studies usingradiolabelled mammalian prolactin preparations consistentlyindicate the existence of saturable, high affinity binding sitesfor prolactin in the brains of several vertebrate species. Theapparent concentration of these sites in the diencephalon iscorroborated by recent mapping studies in rabbits, ring doves,and three song birds in which in vitro autoradiography and densitometrywas employed. Nevertheless, direct comparisons among the threesongbird species suggest some differences in the precise distributionand/or concentration of prolactin binding sites within the preoptic-hypothalamiccontinuumthat may relate to species differences in the display of prolactin-related incubation behavior and parental activities. Althoughdefinitive evidence is currently lacking, it would appear thatblood-borne prolactin gains access to binding sites in the centralnervous system and that a receptor-mediated, blood-to-cerebrospinalfluid transport process in the choroid plexus may be a majorconduit for such uptake. Recent findings from several vertebratespecies also suggest that the brain may synthesize one or moreprolactin-like molecules that could conceivably interact withthese binding sites. Direct actions of prolactin and relatedhormones on brain mechanisms underlying reproductive processesare suggested by the profound dose-dependent suppression ofgonadotropin secretion and gonadal activity in ring doves byintracerebroventricular administration of prolactin or growthhormone at doses that are below those required for significantperipheral actions. A major challenge for future investigationwill be to identify the brain site(s) at which these and othereffects of prolactin are exerted and to determine how blood-borneprolactin of pituitary origin, prolactin-like molecules of brainorigin, and growth hormone-like molecules interact with eachother and with other neurochemical systems to promote thesetypes of changes under normal physiological conditions.