The Posterior Pituitary Contains a Potent Prolactin-Releasing Factor:In VivoStudies*

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Abstract
The posterior pituitary contains a PRL-releasing factor (PRF), a small (<500 mol wt) peptide which is distinct from known PRL secretagogues. The objective of this study were to determine if posterior pituitary extracts specifically stimulate PRL release in vivo and to assess the relative contributions of oxytocin (OT), arginine vasopressin (AVP), and .beta.-endorphin (.beta.END) to the PRF activity of the extract. Rat posterior pituitaries or cerebellar tissue were extracted with 1.0 N acetic acid, boiled, and ultrafiltered through 5000 mol wt cut-off membranes. The eluates were treated with performic acid (which oxidizes bonds and methionine residues), lyophilized, and reconstituted in saline. Jugular blood was collected from conscious ovariectomized rats before and after intracarotid injection of test substances and was analyzed for PRL, LH, and GH by RIA. Injection of 0.3, 1.0, and 3.0, posterior pituitary equivalents increased plasma PRL levels by 2- 8-, and 22-fold respectively. PRL levels peaked within 5 min after the injection and returned to basal levels by 30 min. Plasma LH levels decreased slightly, and GH was unchanged. Cerebellar extracts did not affect plasma hormone levels. Injection of OT induced a 4-fold rise in plasma PRL levels. Oxidation of OT as well as AVP with performic acid abolished any PRL-releasing activity. Injection of .beta.END increased plasma PRL levels by 7-fold. Treatment of .beta.END with perfomic acid caused a 60% loss in its ability to release PRL. Pretreatment of rats with naloxone abolished the PRL-releasing effect of .beta.END, but did not alter the PRF activity of posterior pituitary extracts. We conclude that posterior pituitary extracts stimulate PRL release in vico in the presence of an intact dopaminergic inhibition. This stimulation is rapid, dose dependent, and hormone specific. OT, AVP, and .beta.END do not contribute significantly to the PRF activity in the posterior pituitary extract.