Nociceptin stimulates thyrotropin secretion in rats

Effects of nociceptin on thyrotropin (TSH) and thyrotropin-releasing hormon e (TRH) secretion in rats were studied. Nociceptin (150 mu g/kg) was inject ed intravenously and rats were serially decapitated after the injection. Th e effects of nociceptin on TRH release from the hypothalamus and TSH releas e from the anterior pituitary in vitro were also investigated. TRH and thyr oid hormones were measured by individual radioimmunoassays. TSH was determi ned by enzyme immunoassay. TRH contents in the hypothalamus decreased signi ficantly after nociceptin injection, whereas plasma TRH concentrations show ed no changes. Plasma TSH concentrations increased significantly in a dose- related manner. The TRH release from the hypothalamus was enhanced signific antly in a dose-related manner with the addition of nociceptin. The TSH rel ease from the anterior pituitary in vitro was not affected by the addition of nociceptin. The plasma thyroxine and 3,3',5-triiodothyronine levels did not change significantly after nociceptin administration. The inactivation of TRH by plasma or hypothalamus in vitro after nociceptin injection did no t differ from that of controls. The findings suggest that nociceptin acts o n the hypothalamus to stimulate TRH and TSH secretion.