GABAA and GABAB-receptor agonists evoked vagal nerve efferent transmission in the rat

The effect of GABA agonists on vagal efferent activity was studied in anesthetized rats. PCPGABA, a GABAB agonist (4 and 8 mg/kg, s.c.), markedly activated the neural efferent discharge of the vagus. Muscimol, a GABAA agonist (0.1 and 0.3 mg/kg, i.v.), also facilitated vagal activity. Both agonists caused significant gastric acid hyperacidity. Bicuculline (0.25 mg/kg, i.v.) or picrotoxin (0.5 mg/kg, i.v.) given 10 min prior to each agonist had no effect on the frequency of vagal nerve firing elicited by PCPGABA (4 mg/kg, s.c.) or muscimol (0.3 mg/kg, i.v.). Pretreatment with scopolamine (0.25 mg/kg, i.v.) abolished PCPGABA stimulated vagal activity and gastric acid secretion. Methscopolamine (0.25 mg/kg, i.v.) inhibited only the hyperacidity evoked by PCPGABA, but not vagal activation. These results suggest that PCPGABA and muscimol may cause gastric acid secretion through central cholinergic descending mechanisms that are resistant to GABAA and GABAB antagonists.