Induction of prolactin release by H1- or H2-histamine agonists in maturing male and female rats

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Abstract
Previous evidence has shown that histamine (HA) increases prolactin (PRL) release in adult male and female rats. This response is depending on the age of the animals and some differences were appreciated between male and female rats. In addition, at 20 days of age the PRL response was greater than earlier ages. The purpose of the present communication was to study further this response and to define the type of HA receptor that may be involved in the mediation of this PRL response. Animals of both sexes, at 5 or 20 days of age, were injected into the lateral brain ventricles with three log-doses of histamine (HA), 2,2, pyridil-ethylamine (2 PEA, and H1-histamine agonist), 4 methyl-histamine (4 MHA, an H2-histamine agonist) or saline as control. Fifteen minutes later a sample of blood was taken from the jugular vein and plasma PRL levels were determined by RIA. In complete agreement with previous work, results obtained indicate that HA is able to induce an increased PRL release in 20-day-old animals. This response was not identical in rats of both sexes. Female rats appeared to be more sensitive to HA than male rats. 2 PEA, was not able to reproduce the response in male rats but it was effective in female animals. 4 MHA induced the response in both sexes in a similar way than HA. At 5 days of age in male rats HA had no effect on PRL release. In female rats, instead, only the greater dose was able to induce a small but statistically significant response. In general, nor 2 PEA or 4 MHA were able to stimulate the PRL release at this age. The present results suggest that H2-histamine receptors are present in male rats and in female animals both types of receptors may be involved in the control of PRL release.