Role of GABAA receptors in rat hindbrain nuclei controlling gastric motor function

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Abstract
It has been shown in cats that gastric motor control by the dorsal vagal complex and nucleus ambiguus is under a tonic GABAergic influence. Since much more work has been performed in rats to define vago-vagal reflexes controlling gastrointestinal function, an understanding of the potential inhibition by candidate neurotransmitters such as GABA (gamma aminobutyric acid) in the rat dorsal vagal complex (DVC) is essential to assess. Multiple-barrelled micropipettes were used to apply to the dorsal vagal complex the GABAA antagonist, bicuculline methiodide (0.1-1 nmol), and a GABAA agonist, muscimol (10 nmol) prior to micro-injection of the GABAA antagonist. Micro-injections of bicuculline (353 pmol and 1 nmol), which were localized primarily in the dorsal motor nucleus of the vagus, produced significant increases in intragastric pressure and pyloric motility. These responses were abolished by vagotomy and by a prior micro-injection of muscimol. To determine whether GABAergic blockade in the dorsal vagal complex results in gastric motor excitation through excitatory amino acid receptors, kynurenic acid (5 nmol), a kainate/NMDA (N-methyl D-aspartic acid) receptor antagonist, was micro-injected prior to bicuculline. This abolished the increase in gastric motor function normally evoked by bicuculline. In the other two important hindbrain nuclei controlling gastric function, the nucleus raphe obscurus and nucleus ambiguus, bicuculline (353 pmol) significantly increased intragastric pressure via vagally mediated pathways. These data demonstrate that all three rat hindbrain nuclei known to influence gastric function via the vagus nerve are under tonic GABAergic control. In addition, in the dorsal vagal complex, relief from GABAergic inhibition results in increases in gastric motor function through kainate/NMDA receptor-mediated excitation.