Pharmacological studies on acetylcholine and other transmitter receptors from invertebrate muscle and central neurones

Abstract

Muscle tension recordings were made from earthworm body wall muscle and the action of acetylcholine, cholinomimtics and cholinolytics examined. The effect of cholinergic agents on muscle twitches induced following field stimulation was also investigated. Carbachol was 8 times more active than acetylcholine in the presence of physostigmine.

α-Bungarotoxin, d-tubocurarine, gallamine, atropine, mecamylamine and hexamethonium reduced both electrical stimulation of the muscle and the acetylcholine response while β-bungarotoxin reduced the electrically induced twitch but enhanced the acetylcholine contraction. 0.4µM Hemicholinium abolished the electrically induced twitch while having no effect on the acetylcholine response, but at 4.0µM, did reduce the acetylcholine response. These results provide further evidence for cholinergic excitatory innervation of earthworm body wall muscle.

Intracellular recordings were made from identifiable central neurones of Helix aspersa and the action of anthelmintic compounds investigated. The anthelmintics pyrantel, morantel and deacylated amidantel mimicked acetylcholine induced excitation"D" and inhibition "H" had the same ionic mechanism and were blocked by d-tubocurarine. This suggests these compounds interact with acetylcholine receptors on Helix neurones. Levamisole only inhibited the activity.

A series of glutamate analogues was tested on Helix neurones which were either excited or inhibited by Lglutamate. The only analogue with clear glutamate-like activity was thio-glutamic acid. In normal saline Lglutamate hyperpolarises the membrane potential of cell F-1. This event is chloride mediated and is reversed to a depolarisation followed by hyperpolarisation in low external chloride. This afterhyperpolarisation is reduced in sodium or potassium free saline or following application of strophanthidin, 1.0-100µM.

The local anaesthetics procaine and tetracaine mimicked the "H" and "D" effects of acetylcholine on certain neurones. Tetracaine, 0.01µM, gradually and reversibly reduced both "H" and "D" responses of acetylcholine and the "H" response to dopamine. This provides evidence that local anaesthetics can interact with responses linked to chloride, sodium and potassium ion channels.

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