The effects of neurotensin on anterior pituitary hormone secretion

The present experiments were conducted to determine the effects of neurotensin on secretion of a variety of anterior pituitary hormones. Conscious rats with indwelling cannulae in the third ventricle and external jugular vein were used and the effects on plasma hormone levels measured by radioimmunoassay. Neurotensin was found to decrease plasma prolactin levels in ovariectomized females, normal males, and males in which prolactin levels had been elevated by ether or by a combination of fluoxetine and 5-hydroxytryptaphane. The prolactin-lowering effect was blocked by alpha-methyl-tyrosine to inhibit catecholamine synthesis and by the specific dopamine receptor blocker, spiroperidol. In ovariectomized females, neurotensin was also capable of suppressing LH and elevating growth hormone following its intraventricular injection. Intravenous injection of the peptide elevated prolactin but had no effect on the release of the other pituitary hormones. When hemipituitaries of ovariectomized rats were incubated in vitro, neurotensin elevated prolactin and TSH release into the medium. The minimal effective dose to elevate prolactin and TSH release was 50 ng/ml. Release of gonadotropins and growth hormone was unaffected. It is concluded that neurotensin inhibits prolactin release by a CNS, presumably hypothalamic action, to stimulate the tuberoinfundibular dopaminergic neurons. The dopamine released then inhibits prolactin release either by a direct action on the pituitary or by release of another prolactin-inhibiting factor. In addition, the peptide has a direct prolactin-releasing action on the pituitary. Neurotensin can inhibit LH and stimulate growth hormone presumably by a hypothalamic action since there was no effect on the release of these pituitary hormones by glands incubated in vitro. Although the peptide had no effect on TSH release following its intraventricular injection, it stimulated prolactin release by pituitaries incubated in vitro. The physiological significance of these results is not yet established; however, the presence of the peptide in regions concerned with pituitary control suggests that it may play a physiological role.