Multiple actions of 5‐hydroxytryptamine on myenteric neurones of the guinea‐pig ileum.

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Abstract
Intracellular recordings were made from neurons lying within ganglia isolated from the myenteric plexus of the guinea pig ileum. 5-Hydroxytryptamine (5-HT) (200 nM-10 .mu.M) was applied to 160 neurons by adding it to the perfusing solution. Depolarizations were observed in 27% of neurons and were more likely to be observed in S-cells [ganglion surface cells with a typical fast excitatory post-synaptic potential]. Hyperpolarizations were observed in 30% of neurons and were more likely to be seen in AH cells [cells whose soma action potential was followed by a prolonged hyperpolarizing after-potential]. Of the neurons, 52% were unaffected. 5-HT was applied to 104 neurons by microelectrophoresis from an electrode with its tip placed close to the soma membrane. Depolarizations were observed in 35% of cells and hyperpolarizations in 13%; 10% of neurons showed biphasic responses in membrane potential and the remaining cells were unaffected. The depolarizing responses to 5-HT were associated with an increase in neuronal input resistance and were probably due to inactivation of the membrane K conductance. The responses declined in amplitude during prolonged or repeated applications of 5-HT. The hyperpolarizing responses to 5-HT were associated with a fall in neuronal input resistance and were probably due to activation of the membrane K conductance. These responses persisted throughout the duration of application of 5-HT and did not show marked tachyphylaxis. The hyperpolarizing responses were reversibly abolished by Ca-free solutions. The depolarizing responses to 5-HT may underlie the release of acetylcholine due to 5-HT in the guinea pig ileum. The hyperpolarizing responses may contribute to the inhibition of the peristaltic reflex which was observed when 5-HT is applied to the serosal aspect of the intestine.