Prolactin-Brain Interactions and Reproductive Function

Abstract

SYNOPSIS. There is mounting evidence that prolactin influences reproductive function in several vertebrate species via direct action on the central nervous system. In most instances, however, the basic properties of these prolactin-induced alterations in brain function, the neurochemical mechanisms underlying these effects, and the identity of the prolactinlike molecules that might normally promote these changes have yet to be adequately characterized. Several properties of the prolactin-brain relationship that have emerged from our work in the ring dove ( Streptopelia risoria ) are described in this paper and discussed in relation to similar work in other species. Receptor binding studies using radiolabelled mammalian prolactin preparations consistently indicate the existence of saturable, high affinity binding sites for prolactin in the brains of several vertebrate species. The apparent concentration of these sites in the diencephalon is corroborated by recent mapping studies in rabbits, ring doves, and three song birds in which in vitro autoradiography and densitometry was employed. Nevertheless, direct comparisons among the three songbird species suggest some differences in the precise distribution and/or concentration of prolactin binding sites within the preoptic-hypothalamiccontinuum that may relate to species differences in the display of prolactin- related incubation behavior and parental activities. Although definitive evidence is currently lacking, it would appear that blood-borne prolactin gains access to binding sites in the central nervous system and that a receptor-mediated, blood-to-cerebrospinal fluid transport process in the choroid plexus may be a major conduit for such uptake. Recent findings from several vertebrate species also suggest that the brain may synthesize one or more prolactin-like molecules that could conceivably interact with these binding sites. Direct actions of prolactin and related hormones on brain mechanisms underlying reproductive processes are suggested by the profound dose-dependent suppression of gonadotropin secretion and gonadal activity in ring doves by intracerebroventricular administration of prolactin or growth hormone at doses that are below those required for significant peripheral actions. A major challenge for future investigation will be to identify the brain site(s) at which these and other effects of prolactin are exerted and to determine how blood-borne prolactin of pituitary origin, prolactin-like molecules of brain origin, and growth hormone-like molecules interact with each other and with other neurochemical systems to promote these types of changes under normal physiological conditions.