In order to elucidate the possible role of estrogens in the sex-dependent modulation of met-enkephalin-induced prolactin secretion, and the cellular response at adenohypophyseal level, analyses of serum prolactin levels together with a morphometric study of prolactin-immunoreactive cells were carried out following intraventricular administration of the opioid to estrogenprimed male rats. Administration of the opioid led to an increase in the serum levels of the hormone and, morphometrically, a significant increase in the cellular, cytoplasmic and nuclear areas was observed. When naloxone was administered previously, the above, mentioned changes did not appear, and the values found were similar to those observed in the animals treated only with estrogens. The results suggest that the met-enkephalin-induced prolactin secretion is modulated by the gonadal steroid environment.
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