Thyrotropin-releasing hormone and histidyl-proline diketopiperazine: stimulation of secretion from hypothalamic fragments in vitro by depolarizing agents.

The influence of depolarizing agents on the release of TRH and histidyl-proline diketopiperazine [C(HP)] from rat hypothalami was examined in vitro. Thin hypothalamic fragments from two separate animals were incubated in Krebs-Ringer bicarbonate media. Immunoreactive TRH and C(HP) released into medium were determined by specific RIAs. Potassium and ouabain, a Na+/K+-ATPase inhibitor, stimulated TRH and C(HP) secretion in a graded fashion, but the magnitudes of C(HP) secretory responses were less than those of TRH. The stimulation by KCl and ouabain was inhibited by the addition of verapamil, a Ca2+ channel blocker, or omission of Ca2+ from medium in the presence of EGTA. Veratridine, a Na+ channel activator, also stimulated TRH and C(HP) release from rat hypothalami, and this stimulation was completely blocked by tetrodotoxin, a Na+ channel blocker. The results of this study indicate that TRH and C(HP) release from rat hypothalami are stimulated by membrane depolarization in vitro and depend on Na+ and Ca2+ influx.