We investigated the role of histamine (HA) in the neuroendocrine regulation of PRL secretion in conscious male rats. Blood samples were obtained by decapitation of the animals at 0 min. Intracerebroventricular infusion of HA (34-540 nmol at -15 min) stimulated PRL secretion dose dependently with an ED50 of approximately 135 nmol. Inhibition of neuronal HA synthesis by the specific histidine decarboxylase-inhibitor (S)alpha-fluoro-methylhistidine (alpha FMH; 100 mumol/kg ip at -6 h or 400 mumol/kg ip at -20 h and -6 h) caused a 50% reduction (P less than 0.01) in the PRL response to 5 min of restraint stress. Inhibition of neuronal HA metabolism by the specific histamine-methyltransferase-inhibitor SKF-91488 (400 and 800 nmol icv at -20 min) augmented the restraint stress-induced PRL release 26% and 37%, respectively (P less than 0.05). The two enzyme inhibitors had no or only modest effect on the basal PRL secretion. Pretreatment with a specific antiserum (0.5 ml) to arginine vasopressin (AVP) or an AVP antagonist (25 nmol) administered iv at -20 min inhibited the PRL response to HA (270 nmol icv) 80% and 45%, respectively (P less than 0.01) and inhibited the PRL response to restraint stress 70% (P less than 0.01). In contrast, pretreatment with a specific oxytocin (OT) antagonist (50 nmol) had no effect on the HA- or stress-induced PRL release. The AVP antiserum showed less than 0.0003% cross-reactivity with OT, and radiolabeled OT did not bind to serial dilutions of plasma from rats treated with the AVP antiserum. The AVP antiserum or the AVP antagonist almost prevented the PRL release induced by iv infusion of 800 pmol AVP or a posterior pituitary extract. Infusion of AVP (24-800 pmol iv at -15 min) stimulated PRL secretion dose dependently. However, the dose of AVP (800 pmol) required to induce an increase in plasma PRL similar to that obtained by HA-stimulation, led to a rise in plasma AVP which was approximately 1000-fold higher than that induced by HA, which increased plasma AVP 2-fold. Restraint stress had no effect on the plasma AVP level. We conclude that neuronal HA participates in the mediation of the PRL response to stress and that the stress- and HA-induced release of PRL may involve AVP, whereas an involvement of OT seems unlikely.(ABSTRACT TRUNCATED AT 400 WORDS)