Hypothalamic Prolactin: Characterization by Radioimmunoassay and Bioassay and Response to Hypophysectomy and Restraint Stress

Abstract

Prompted by immunohistochemical reports of prolactin-like immunoreactivity in cell bodies within the rat hypothalamus, a study was undertaken to quantitate the immunologic and biologic activity of this material. Hypothalamic concentrations of prolactin-like immunoreactivity averaged 402 ± 23 pg/mg of protein (n = 30). 97% recovery of rat prolactin standards added to homogenates of hypothalamus insured that neuronal tissue, as prepared for these studies, did not interfere with the radioimmunoassay of rat prolactin. Examination of the elution profile from Sephadex G-75 columns of the prolactin-like immunoreactivity in hypothalamic extracts showed that the majority of hypothalamic prolactin-like substance was of a larger molecular size than pituitary prolactin. While increasing amounts of brain extract progressively displaced more I¹²⁵ prolactin from antibody-binding sites, the displacement curve produced by adding hypothalamic extract was not parallel to that produced by the addition of increasing amounts of anterior pituitary prolactin standards of rat origin. Hypothalamic extracts from hypophysectomized animals, analyzed for biologic activity in the Nb₂ lymphoma cell assay, revealed prolactin-like bioactivity, but the bioactivity/immunoactivity (B/I) ratios for hypothalamic extracts were significantly lower than the B/I ratios for pituitary prolactin (0.71 ± 0.04 for pituitary, vs. 0.19 ±0.06 in the hypothalamus; p < 0.001). Hypophysectomy, which led to the expected fall in serum prolactin to undetectable levels, and restraint stress, which resulted in a statistically significant 4-fold rise in serum prolactin, caused no change in prolactin concentrations in the hypothalamus, indicating that brain prolactin-like substance is regulated independently of pituitary prolactin and circulating serum prolactin levels.

Prolactin, Central Nervous System and Behavior: A Critical Review

“…Numerous studies using radioimmunoassay demon strated the presence of a substance resembling immunoreactive prolactin in the brain [13]. In fact, consistent with electron-microscopic immunocytochemistry, prolactin like immunoreactivity could be localized to synaptosomes [14].…”

The prolactin gene is expressed in the hypothalamic-neurohypophyseal system and the protein is processed into a 14-kDa fragment with activity like 16-kDa prolactin.

“…Production of brain PRL has been supported by immunocytochemical, RIA, and bioassay data from normal and hypophysectomized animals (18)(19)(20). Using RT-PCR, Emanuele and colleagues (21) demonstrated the presence of PRL mRNA in whole hypothalamus and in several brain extrahypothalamic regions, indicating the expression of PRL in the brain.…”

Immunoreactive Prolactin in the Hypothalamus and Cerebrospinal Fluid of Male and Female Rats

“…In addition, the failure of hypothalamic ir-PRL to parallel changes in either plasma or CSF ir-PRL indicates that hypothalamic ir-PRL is not derived from the plasma or CSF. In contrast, this study suggests that hypothalamic ir-PRL may be one source of CSF PRL.Studies using a specific radioimmunoassay for rat prolactin have detected immunoreactive prolactin (ir-PRL) in the brain [2,5] and cerebrospinal fluid (CSF) [I, 12]. ir-PRL concentrated from selected brain regions, such as the hypothalamus, hippo campus, and ponsmedulla displays physicochemical properties similar to those of pituitary ir-PRL [2,3].…”