The action of four proposed chloride movement modifiers on acetylcholine responses of central neurones of Helix aspersa

Abstract

1. Intracellular recordings were made from identified neurones in the visceral, (abdominal) ganglion of the snail, Helix aspersa. The hyperpolarising response of neurone E4 is a pure chloride event and has a reversal potential, (E_Cl), of −69.7 ± 1.7mV, (n = 4). This can be compared to depolarising responses of neurones E1 and E2 which are sodium mediated.

2. Four membrane transport system antagonists, all thought to affect trans-membrane chloride movement, were investigated with respect to the two different acetylcholine responses described.

3. Piretanide irreversibly blocks the hyperpolarising response in cell E4, (1–10 μM), increasing its inhibition with time but without changing E_Cl.

4. Furosemide irreversibly blocks both types of acetylcholine responses at concentrations in the nanomolar range; no change in E_Cl or E_Na was associated with the inhibition but the “passive” membrane resistance appeared to decrease. Inhibition increased with time, normally causing a significant effect after 10–30 min.

5. S.I.T.S. and ethacrynic acid appear very similar in effect; both reversibly block the two responses to acetylcholine and recovery after washing is virtually complete. The onset of antagonism is rapid and both compounds are slightly more effective against the hyperpolarising (“H”), response than the depolarising (“D”) response (threshold ∼10⁻⁵ M compared to ∼ 10⁻⁴ M). No change in E_Cl or E_Na was noted.

6. Piretanide and furosemide probably exert their effect by interactions with the neuronal cell membrane, disrupting the integrity of this structure, whereas S.I.T.S. and ethacrynic acid may interact more specifically with the acetylcholine receptor protein.