Effects of enkephalins infusion on hormonal levels in man: Inhibition of the episodic secretion of LH by met-enkephalin

Administration of a long-acting met-enkephalin derivative has been shown to induce alterations of pituitary hormone level in man. As native enkephalin may circulate undegraded in blood, we investigated whether met- or leu-enkephalin infusion could also modify hormonal blood levels. Four normal male volunteers were submitted to a 10 min infusion of either met-enkephalin (1000 μg in 50 ml saline) alone or together wth 2 mg naloxone. The following hormonal levels were determined in plasma before, during and after the infusions: PP, IRSRIF, T3, T4, TSH, LH, FSH, PRL, ACTH and GH; they were compared, by covariance analysis, to these obtained in a control experiment with saline infusion. All the levels remained in the physiological range, and, except LH, did not change with the treatments. The LH levels were significantly lowered during met-enkephalin infusion; this effect was overcome by naloxone. In contrast, leu-enkephalin administration did not affect the LH levels. These results suggest that met-enkephalin is a sensitive modulator of LH secretion. The lack of effect on the other hormone secretions might suggest that opioid receptors with different specificity and sensitivity could be involved in their secretion.SCOPUS: NotDefined.j