Immunoreactive Prolactin in the Cerebrospinal Fluid of Estrogen‐Treated and Lactating Rats as Determined by Push‐pull Perfusion of the Lateral Ventricles

Abstract

lmmunoreactive (ir) prolactin levels in the cerebrospinal fluid were measured in push-pull perfusion samples collected from the lateral cerebral ventricles in freely-moving ovariectomized rats prior to and after estrogen treatment, and in rats during pregnancy and lactation. Ir-prolactin levels in cerebrospinal fluid were elevated throughout the perfusion period in ovariectomized animals given estrogen and in Day 8 lactating females following the onset of nursing by their young. Cerebrospinal fluid perfusates collected from ovariectomized animals prior to estradiol treatment and from females on Days 18 and 22 of gestation contained levels of irprolactin that were below the limit of detectability of the prolactin radioimmunoassay. Data from previous studies in our laboratory suggest that pituitary prolactin is involved in the mediation of maternal responsiveness. Where and how prolactin exerts its actions in this regard remains to be determined. Results of the present study indicate that detectable levels of ir-prolactin are present in the cerebrospinal fluid of nulliparous females after exposure to an estradiol regimen known to shorten their latency to display maternal responsiveness and in lactating females actively exhibiting maternal behavior. Once in the cerebrospinal fluid, prolactin should have access to neural target sites behind the blood-brain barrier. The presence of ir-prolactin in the cerebrospinal fluid of females in the present study is consistent with a role for this protein hormone, acting at the level of the central nervous system, in the facilitation of maternal responsiveness as well as in the regulation of other neural and neuroendocrine processes.

Prolactin Receptors in Dove Brain: Autoradiographic Analysis of Binding Characteristics in Discrete Brain Regions and Accessibility to Blood-Borne Prolactin

“…A blood-to-CSF transport mechanism located in the cho roid plexus represents one possible route of access to such periventricular sites. This possibility is supported by evidence for PRL binding sites in the CP or ring doves [40] and a variety of other vertebrate species [41 ], by evi dence for concordance between changes in serum PRL concentrations and changes in C'SF level of PRL under some natural and experimental conditions [42][43][44], and by direct evidence for saturable uptake of radiolabelled PRL from blood to CSF in rats [45]. In an earlier study, intravenous injection of oPRL was found to be effective in inhibiting the uptake of radiolabelled oPRL in the dove CP at 5 min postinjeqtion [40].…”

Central prolactin infusions stimulate maternal behavior in steroid-treated, nulliparous female rats.

“…Evidence increases in blood and CSF PRL levels in the mother rats (13). CSF PRL concentrations are also higher at times of the reproductive cycle when females exhibit elevations in circulating PRL (11,12).…”

“…Interestingly, CSF concentrations of immunoreactive PRL are undetectable just prior to parturition (13). One possible explanation to account for the lack of PRL in the CSF at this time is that the high levels of rat placental lactogen II prepartum (9) (30,31) and estrogen receptors (32,33) in the MPOA prepartum may help stimulate the onset of maternal behavior.…”

“…The purpose of the present report was to investigate the possibility that PRL acts at the level of the central nervous system to stimulate maternal behavior. A central site of PRL action is feasible in light of a series of findings that indicate that circulating PRL can gain access to the brain through active transport across the blood-cerebrospinal fluid (CSF) barrier (10), that PRL, presumably of pituitary origin, is found in increased amounts within the CSF at times when circulating levels of PRL are elevated (11)(12)(13), and that receptors for PRL have been localized within the central nervous system (14)(15)(16)(17)(18).…”

Endocrine Communication between Conceptus and Mother: Placental Lactogen Stimulation of Maternal Behavior

“…the MPOA. It was somewhat surprising to be unable to detect mitogenic activity in CSF samples obtained from lactating rats, since the suckling stimuli received by the dams induce large increases in cir culating PRL secretion [24,25] and increased immunoreactive PRL concentrations in CSF [26]. One explanation for the apparent lack of mitogenic activity in the CSF of lactating rats in the present study is that transport of PRL into the CSF from plasma reduces the biological activity of the PRL molecule without affecting its immunologic activity in the PRL radioimmunoassay.…”