INDUCTION OF PROLACTIN AND LUTEINIZING HORMONE RELEASE BY HISTAMINE IN MALE AND FEMALE RATS AND THE INFLUENCE OF BRAIN TRANSMITTER ANTAGONISTS

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Abstract
The levels of prolactin and LH in the plasma of rats were determined at various times after intraventricular injection of histamine. Doses of 5 and 60 μg histamine (free base) in male rats, anaesthetized with ether, induced an increase in the level of prolactin in the plasma, whilst producing a slight decrease in the concentration of LH. Injection of 5 μg histamine at 14.00 h into female rats at all stages of the oestrous cycle caused prolactin to be released; the effect was greatest at oestrus and at day 1 of dioestrus. Histamine also gave rise to a marked increase in the level of LH in the plasma when administered to pro-oestrous rats, but had no effect when injected at the other stages of the oestrous cycle. The effect of histamine on the release of prolactin in ovariectomized, oestradiol benzoate: progesterone-primed (OVX,OB:P) rats was found to be dose-related, and the level of LH in the plasma was increased by as little as 1·25 μg. Pretreatment with adrenergic (phenoxybenzamine and propranolol) and cholinergic (atropine) antagonists failed to block the stimulatory effects of histamine on prolactin secretion, but pretreatment with methysergide (serotonin antagonist) increased the histamine-induced release of prolactin in male rats. Antagonists did not modify the response of prolactin to histamine in OVX,OB:P-primed rats. The histamine-induced release of LH in OVX,OB:P-primed rats was slightly reduced by pretreatment with phenoxybenzamine, propranolol and atropine, but not by methysergide. These results indicate that histamine facilitates the release of prolactin. The stimulatory action of histamine on both pro-oestrous and OVX,OB:P-primed but not male rats suggests that histamine may be involved in LH release in the rat. Results obtained in animals pretreated with transmitter antagonists, which were unable to prevent histamine-induced hormone release, suggest that the actions of this amine are not mediated by cholinergic, noradrenergic or serotonergic mechanisms.

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