The actions of nicotinic and muscarinic cholinomimetics and a series of choline esters on two identified neurones in the brain of Helix aspersa

Abstract

1. Experiments have been performed using intracellular recording techniques on two identifiable neurones, E4 and F1, in the brain of the snail Helix aspersa. Cell E4 is inhibited by acetylcholine while cell F1 is excited by acetylcholine. 2. Using a range of cholinomimetics the potency ratios of these compounds compared with acetylcholine as standard have been obtained. 3. Certain nicotinic agents are far more potent on cell E4 than on cell F1, for example, decamethonium, 2(3-pyridyl) 1, 4, 5, 6-tetrahydropyrimidine, carnitine nitrile and nicotine, with potency ratios between the two cells of 3, 700, 320, 150 and 28 respectively. 4. Muscarine was found to be a potent cholinomimetric on cell F1 but was inactive on cell E4. On cell F1 muscarine has an equipotent molar ratio (EPMR) of 3.6. 5. From a range of choline esters tested only one was clearly more potent on cell F1 compared with cell E4, that is, acetyl-β-methylcholine. This has an EPMR of 23 on F1 and an EPMR of 10,000 on E4. A number of the choline esters were considerably more potent on E4 than on F1, for example, butyrylcholine, nicotinoylcholine 2-pentenoylcholine, succinylcholine, iso-valerylcholine and benzoylcholine, with potency ratios of 77.5, 200, 1017, 1200, 1450 and 350, 000 respectively. 6. From these studies it is suggested that the acetylcholine receptors on cell E4 are more “nicotinic” while those on cell F1 are more “muscarinic”.