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Costa M, Furness JB. 
“On the Possibility that an Indoleamine is a Neurotransmitter in the Gastrointestinal Tract”. 
Biochem.Pharmacol.. 1979;28(5):565-71.
Evidence for an indoleamine neurotransmitter in the gastrointestinal tract is reviewed, with reference to studies in the guinea pig, mouse, rat, and cat, Histochemical studies using methyl tyrosine, 6 -hydroxydopamine, reserpine, guanethidine, fenclonine and R o-4 -4602 treated animals revealed the ability synthesize 5-hydroxytryptamine (5-HT) and the presence of somatic-1-amino acid decarboxylase, tryptophan hydroxylase and MAO, but not tyrosine hydroxylase or dopamine-betahydroxylase in intrinsic intestinal nerves. No 5-HT stores have been found, but specific 5-HT uptake systems blocked by chlorimipramine and desmethylimipramine are present, 5-HT Has similar effects on inhibitory nerves to the guinea-pig stomach to ACh or vagal stimulation but is not antagonized by pentolinium or hexamethonium. Transmural stimulation of mouse stomach causes tetrodotoxin-sensitive release of a 5-HT-like substance. Contractions of the canine esophagus elicited by transmural stimulation were blocked by bromolysergic acid and 5-HT desensitization but were not modified by atropine or eserine. Vagal stimulation re faxes the lower esophageal sphincter in the presence of hexamethonium and atropine and this relaxation is blocked by 5-methoxydimethyltryptamine. 5-HT Acts on smooth muscle receptors which are blocked by LSD, bromolysergic acid or methysergide and on enteric nerve receptors which are blocked by biguanides-and quaternary derivatives of 5-HT but not by morphine. Nicotine causes relaxation of the guinea-pig colon blocked by metoclopramide or desensitization of receptors to 5-HT. Inhibitory nerves to the colon were stimulated by 5-HT but it had no direct effect on the muscle. In the presence of hyoscine and tetrodotoxin, 5-HT causes a slow contraction of proximal colon blocked by methysergide and phentolamine. Vasodialator responses to mucosal stimulation were mimicked by 5-HT -a tetrodotoxin-sensitive manner and blocked by bromolysergic acid and dihydroergotamine. Atropine and blockers of nicotinic or alpha- or beta-adrenergic receptors had no effect.
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