Galanin and vasopressin response to hyperosmotic stimulation: in vitro study

Galanin (Gal)--a neuropeptide present in the nervous system and peripheral tissues--may be involved in the regulation of hypothalamo-neurohypophysial system function. It was shown that centrally injected galanin inhibits osmotically stimulated vasopressin (VP) secretion into the blood and reduces VP mRNA level in the supraoptic (SON) and paraventricular (PVN) hypothalamic nuclei. The aim of the present study in vitro was to investigate the influence of Gal on vasopressin release from isolated rat hypothalamus (Hth), neurohypophysis (NH) or hypothalamo-neurohypophysial explants (Hth-NH). The effect of Gal on VP secretion was studied under conditions of direct osmotic (i.e., Na⁺-evoked) (series 1) as well as nonosmotic (i.e., K⁺-evoked) (series 2) stimulation. In series 3, vasopressin response to Gal was studied using the neural tissues obtained from animals drinking 2 percent NaCl solution for eight days (indirect osmotic stimulation). Gal in a concentration of 10⁻¹⁰ M and 10⁻⁸ M inhibited basal VP release from Hth, NH and Hth-NH explants isolated from euhydrated rats as well as from Hth-NH complex of osmotically challenged animals. When the neural tissues obtained from previously salt-loaded rats were incubated in K⁺-enriched medium the inhibitory effect of Gal was completely blocked. It may be concluded that the effect of Gal is depending on the current functional status of the hypothalamo-neurohypophysial system.