THE SITE OF THE 5‐HYDROXYTRYPTAMINE RECEPTOR ON THE INTRAMURAL NERVOUS PLEXUS OF THE GUINEA‐PIG ISOLATED ILEUM

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Abstract
Dose/response measurements were made on the guinea‐pig isolated ileum with six agonists, acetylcholine, 5‐hydroxytryptamine, nicotine, dimethylphenylpiperazinium, choline phenyl ether and histamine. The dose effects were repeated in the presence of each of twelve antagonists and one anticholinesterase. Acetylcholine and histamine were chosen because of their direct mode of action on smooth muscle, nicotine, dimethylphenylpiperazinium and choline phenyl ether were used as examples of drugs that act at the ganglionic acetylcholine receptor. 5‐Hydroxytryptamine was the drug investigated. Hyoscine blocked the contractions caused by acetylcholine, 5‐hydroxytryptamine and the ganglion‐stimulants but left the responses to histamine unchanged. The anticholinesterase N,N'‐diisopropylphosphorodiamidic fluoride (mipafox) potentiated all the agonists except histamine. The strength of potentiation decreased in the order 5‐hydroxytryptamine, nicotine, dimethylphenylpiperazinium and choline phenyl ether, and acetylcholine. The local anaesthetic procaine inhibited to the same extent contractions elicited by 5‐hydroxytryptamine, nicotine, dimethyl‐phenylpiperazinium and choline phenyl ether. These results showed that 5‐hydroxytryptamine, like nicotine, choline phenyl ether and dimethylphenylpiperazinium, mediated its response through the nervous plexus. Of those tested 5‐hydroxytryptamine was the only specific antagonist to 5‐hydroxytryptamine; lysergic acid derivatives produced spasm and prolonged changes in tone; phenoxybenzamine caused non‐specific block. The diverse modes of action of a number of ganglion‐blocking agents were selectively used. Thus hexamethonium, pentolinium, and nicotine in its competitive phase, blocked contractions due to nicotine, dimethylphenylpiperazinium and choline phenyl ether and left those due to 5‐hydroxytryptamine, acetylcholine and histamine unchanged. The depolarizing ganglion‐blocking agents, dimethylphenylpiperazinium and nicotine, inhibited the responses to all the indirectly acting drugs. Furthermore, mecamylamine, a drug with a less well‐defined mode of action, partially inhibited contractions due to 5‐hydroxytryptamine in a concentration that blocked those due to nicotine, dimethylphenylpiperazinium and choline phenyl ether. Pempidine, known to act like mecamylamine, did not antagonize 5‐hydroxytryptamine. It is concluded that 5‐hydroxytryptamine activates specific receptors sited at the intramural parasympathetic ganglion cells.