EVIDENCE THAT GABAARECEPTORS MEDIATE RELAXATION OF RAT DUODENUM BY ACTIVATING INTRAMURAL NONADRENERGIC-NON-CHOLINERGIC NEURONES

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Abstract
GABA produced rapid and transient relaxation of rat duodenum. Homotaurine (3-aminopropansulfonic acid) but not (.+-.)-baclofen had a GABA-like effect. GABA-induced relaxation was almost completely inhibited by tetrodotoxin but was unaffected by atropine. Cross desensitization developed between GABA and homotaurine but not between GABA and (.+-.)-baclofen. The concentration response curve to the relaxant effects of GABA was shifted to the right by both bicuculline and picrotoxin. Maximal relaxation was still produced by GABA in the presence of bicuculline but not in the presence of picrotoxin. GABA-induced relaxation was not affected by prazosin, yohimbine, propranolol or reserpine pretreatment. Field stimulation (0.1 Hz) of rat isolated duodenum in the presence of atropine and guanethidine produced relaxation similar to that produced by GABA. The ganglionic stimulant DMPP [1,1-dimethyl-4-phenylpiperazinium halide] produced a similar effect. Neither Met-enkephalin, noradrenaline [norepinephrine], 5-HT [5-hydroxytryptamine], histamine, VIP [vasoactive intestinal peptide] or arachidonic acid could be held responsible for GABA-induced neurogenic relaxation of rat duodenum. ATP produced relaxations which closely mimicked those produced by either GABA or field stimulation. Exposure to ATP desensitized responses to both GABA and field stimulation to about the same extent. ATP, GABA and field stimulation-induced relaxation was unaffected by either theophylline or indomethacin, but was significantly and selectively antagonized by apamin. Evidently, GABA-induced relaxation of rat isolated duodenum is largely dependent upon activation of intra-mural nonadrenergic-noncholinergic neurons. The GABA receptor involved appears to be of the GABAA subtype. ATP might be the endogenous substance released by GABA.