Relationships between 5-hydroxytryptamine and vagal relaxation in the dog stomach.

It has been reported that after treatment with atropine, stimulation of the vagus nerve causes a relaxation of the stomach which is possibly mediated by a non-adrenergic inhibitory mechanism (1-4). Biilbring and Gershon (5) proposed that 5-HT, with acetylcholine, seemed to be a neuro-transmitter substance in the vagal inhibitory pathways of the guinea-pig stomach. According to them, in the presence of hyoscine, 5-HT causes a similar relaxation of the stomach to that produced by vagal stimulation, and the 5-HT and vagal responses are similarly affected by the same antagonists. In the present experiment, the possible con tribution of tryptaminergic nerve mechanisms to the vagally induced relaxation of the dog stomach has been examined. The experiments were performed on adult dogs. The animals were anesthetized with pentobarbital sodium (25-30 mg/kg i.v.). The dorsal vagal trunk and the perivascular nerve attached to the coeliac artery were stimulated for 10-30 seconds with a supramaximal pulse of 1 millisecond duration at a frequency of 20 Hz. The mechanical activity of the stomach was displayed on a recording paper through a mechano-electronic transducer system as a volume change in a water balloon introduced into the fundus or body. Further details of the experimental methods have been described in the previous paper (4). 5-HT injected into the gastric artery always caused a contraction of the stomach. The threshold dose was about 0.1 fag. With an increase of the dose, amplitude of the contraction