Graduate Thesis Or Dissertation

The vagal inhibitory reflex in the rat stomach

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  • Electrical stimulation of the vagus nerves produces an excitatory contraction response in the isolated rat stomach. This contraction response was abolished by atropine to unmask the vagal inhibitory response (VIR) elicited during low frequency vagal stimulation. Babring and Gershon (1967), after working on guinea-pigs and mice, hypothesized that both serotonin (5-HT) and acetylcholine (ACh) are neurotransmitters acting on the same ganglion cell in the vagal inhibitory pathway. This hypothesis for the role of 5-HT was tested in the rat after the VIR of the isolated stomach was characterized. The hypothesis was tested with 5-HT depletory, parachlorophenylalanine (PCPA) and reserpine, and a ganglionic blocking agent, hexamethonium (C₆). If the hypothesized role for 5-HT is true, the depletion of 5-HT from stomach tissues should influence the VIR. Furthermore, if both 5-HT and ACh are neurotransmitters acting on the same ganglion cell in the vagal inhibitory pathway, the addition of C₆ should only block the action of ACh but not the actions of 5-HT in the ganglia. Thus, vagal stimulation after the addition of C₆, in the presence of atropine, should still produce a VIR due to the action of 5-HT. The VIR was present in all groups of rats tested and remained unchanged even though large amounts of 5-HT were depleted from the stomach tissues. The possibility still exists that the small amounts of 5-HT remaining after treatment with PCPA and reserpine could be sufficient to maintain the vagal inhibitory pathway. However, the VIR of the rat was completely abolished by C₆. These results indicate that serotonin is not involved either pre- or postganglionically in the vagal inhibitory pathway of the rat stomach. Stimulation of the vagus nerves of the in situ rat stomach elicited a gastric relaxation response in contrast to the contraction response observed with the isolated stomach. This in situ relaxation response was elicited by stimulation of the intact vagi as well as by peripheral vagal stimulation. The in situ relaxation response was characterized and an effort was made to determine the relationship between the in situ relaxation response and the VIR. Some experimental conditions which may influence the in situ relaxation response were investigated. The influence of age and sex, lowered body temperature, starvation and partial evisceration by removal of the intestines, on the in situ relaxation response were examined. None of the above conditions appeared to affect this response except the partial evisceration of the animal. The in situ relaxation response was abolished following the removal of the intestines and succeeding stimulation of the vagi elicited only gastric contractions. Subsequent treatment with guanethidine, reserpine and 6-hydroxydopamine (6- OHDA), all resulted in the reversal of the in situ relaxation response into a contraction response to vagal stimulation. Atropine abolished the contraction response to reveal the VIR. The evidence indicates that the in situ relaxation response is a composite response by three different gastric responses manifested in the following order of dominance: (1) an adrenergic inhibitory response, (2) a cholinergic excitatory response, (3) a non-adrenergic vagal inhibitory response.
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