Effects of histamine implants in several brain regions on the release of prolactin in conscious adult male rats.

The levels of prolactin in the plasma of conscious male rats were determined at various times after an acute administration of histamine or a histamine releasing agent, compound 48/80, in three brain regions. The brain structures that were examined were, the caudal part of the preoptic area and anterior part of the anterior hypothalamic area (POA-AHA), the arcuate nucleus-ventromedial nucleus region (ARC-VMN) and the medial-basal amygdaloid nucleus of the limbic system (AME). A marked increase in plasma levels of prolactin was observed when implants of histamine were in the POA-AHA region. A more consistent increase was found when 1 microgram histamine was injected in the same region; values of prolactin were about 3.6 times greater than in their controls injected with 0.9% saline. Such increased hormone levels lasted up to 2 h. A similar rise in prolactin level was found when the implants of histamine were located in the ARC-VMN region. When compound 48/80 or empty cannulae were placed in those brain regions that were examined, no changes in plasma levels of prolactin were induced. Both histamine and compound 48/80 elicited a delayed and long-lasting decrease of the high plasma level of prolactin present in rats bearing cannulae in the AME region. The results suggest that in the male rat, histaminergic sites, located in rostral and mediobasal hypothalamus and in the central area of the amygdala, are involved in the mechanisms controlling prolactin secretion.