Lateral hypothalamic injection of GABA(A) antagonist induces gastric vagus-mediated hypocalcemia in the rat.

Abstract

The involvement of lateral hypothalamic area (LHA) neurons in the regulation of blood calcium homeostasis was investigated in unanesthetized rats. The microinjection of the gamma-aminobutyric acid A receptor antagonist bicuculline methiodide (BM, 4-40 ng x 0.5 microl(-1) x 5 min(-1)) into the LHA decreased the blood concentration of ionized calcium. Total serum calcium also decreased after the BM injection. This hypocalcemic effect was eliminated by a bilateral vagotomy of the gastric branches. An intravenous injection of atropine methyl bromide (a muscarinic antagonist), nadolol (a beta-adrenergic blocker), or ranitidine (a histamine H2 blocker) suppressed the BM-induced hypocalcemia, whereas phenoxybenzamine (an alpha-adrenergic blocker) proved to be ineffective. Although the intra-LHA injection of BM increased the serum gastrin, which is known to have a hypocalcemic effect, neither secretin nor somatostatin (gastrin-release inhibitors) blocked the hypocalcemic response. These results suggest that the hypocalcemia observed after the excitation of LHA neurons was mediated by muscarinic, beta-adrenergic, and histamine H2 receptors through the gastric vagus.