The effects of histamine on the concentrations of immunoreactive thyrotropin-releasing hormone in the stomach and hypothalamus in rats.

The effects of histamine and its related compounds on the concentrations of immunoreactive thyrotropin-releasing hormone (ir-TRH) in the stomach, gastric juice and hypothalamus in rats were studied. Histamine, ranitidine or ethanolamine was injected intraperitoneally, and the rats were decapitated at various times after the injection. Ir-TRH concentrations in the stomach, gastric juice and hypothalamus were measured by a radioimmunoassay. Ir-TRH concentrations in the stomach decreased significantly after histamine injection and increased significantly after ranitidine injection in a dose-dependent manner, but did not change with ethanolamine. Ir-TRH concentrations in the gastric juice increased in a dose-dependent manner, peaking at 30 min after histamine injection, and its effect was blocked with ranitidine. Ir-TRH concentrations in the hypothalamus elevated significantly after histamine injection and reduced significantly after ranitidine injection, but did not change with ethanolamine. The effects of histamine on ir-TRH concentrations in the stomach and hypothalamus were significantly blocked with ranitidine, but not with ethanolamine. These findings suggest that histamine stimulates ir-TRH release from the stomach and inhibits ir-TRH release from the hypothalamus, and that these effects of histamine on ir-TRH release are mediated via an H2-receptor.