Evidence that 5-hydroxytryptamine may exert both facilitatory and inhibitory control of electrical field stimulation-evoked contractions in longitudinal muscle taken from the body of guinea-pig stomach

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Abstract
Electrical field stimulation (FS) of guinea-pig stomach body longitudinal muscle strips caused frequency-related contractions mediated via cholinergic mechanisms. Metoclopramide (10−8–10−5 M), MDL 72222 (10−9–10−7 M) and 5-hydroxytryptophan (5-HTP) (3 × 10−7–2.4 × 10−4 M) enhanced these contractions in all tissues, whereas 5-hydroxytryptamine (5-HT) (3 × 10−8–3 × 10−5 M) enhanced the contractions, but only in approximately 15% of the tissues tested. FS-induced contractions were also enhanced in tissues treated in-vitro with p-chlorophenylalanine (2.5 × 10−7–2.5 × 10−5 M) or monofluoromethyldopa (6 × 10−7–10−4 M) or in tissues taken from animals having received p-chlorophenylalanine or monofluoromethyldopa. It is concluded that cholinergic-mediated contractions of stomach strips are subject to 5-HT modulation in two ways. The predominant action of endogenous 5-HT is to exert an inhibitory tone mediated via a metoclopramide and MDL 72222-sensitive 5-HT neuronal receptor. Exogenously applied 5-HT has little overt action to increase the essentially maximal inhibitory action of endogenous 5-HT, but acts on a 5-HT facilitatory receptor system to enhance contractions. Therefore, the actions of 5-HT agonists and antagonists to modify contractions in stomach strips will reflect the balance between 5-HT inhibitory and facilitatory influences, and the specificity of action of the compounds for the two 5-HT receptor systems.