Regulatory role of galanin in control of hypothalamic-anterior pituitary function

The role of the neuropeptide galanin in the regulation of anterior pituitary function was studied in vivo in conscious male rats and in vitro with cultured anterior pituitary cells. Galanin (50-200 ng; 15-60 pmol) injected into the third cerebral ventricle of rats produced highly significant, dose-related increases of plasma growth hormone (GH) concentrations, whereas galanin increased prolactin (PRL) and decreased thyroid-stimulating hormone (TSH) levels only at the highest dose (60 pmol) tested. Intravenous galanin failed to alter PRL and TSH levels in these rats. In contrast with the results with intraventricular injection of the peptide, intravenous injection of 30 or 300 pmol of galanin produced small, brief, dose-related increases in plasma GH. The response to the 300-pmol dose was less than that induced by a factor-of-20-lower intraventricular dose, which establishes a central action of galanin. Galanin in concentrations ranging from 1 nM to 1 microM failed to alter significantly GH, PRL, or TSH release from dispersed anterior pituitary cells. It also failed to alter GH secretion in response to 100 nM GH-releasing hormone; however, at this dose galanin did potentiate the effect of 100 nM TSH-releasing hormone on TSH and PRL release. Thus, the effects of third-ventricular injection of the peptide are mediated by the hypothalamus. To determine the physiological significance of galanin in control of pituitary hormone release, highly specific antiserum against galanin was injected intraventricularly. Third-ventricular injection of 3 microliter of galanin antiserum resulted in a dramatic decrease in plasma GH values as compared with those of normal rabbit serum-injected controls within 15 min, which persisted until the end of the experiment (5 hr postinjection). Galanin antiserum did not decrease plasma PRL or TSH levels at any time period after its third-ventricular injection; however, a transient increase of plasma TSH levels occurred after 30 and 60 min in comparison with TSH levels in normal rabbit serum-injected controls. Since there was no effect of the antiserum on plasma PRL and only a transient elevation in TSH, galanin may not be physiologically significant enough during resting conditions to alter PRL and TSH release in the male rat. The results of the experiments with galanin antiserum indicate that endogenous galanin has a tonic action within the hypothalamus to stimulate GH release. The rapidity of onset of the effects of galanin and the antiserum directed against it suggest that it acts to stimulate release of GH-releasing hormone from periventricular structures, which then stimulates the release of GH.