Pharmacological and Electrophysiological Characterization of a Postsynaptic Muscarinic Receptor in the Central Nervous System of the Cockroach Available to Purchase

The properties of the postsynaptic muscarinic receptors of a ventral giant interneurone in the sixth abdominal ganglion of the cockroach were studied using the single-fibre oil-gap technique.

1. Pressure-ejections of 10⁻⁴ mol l⁻¹ arecoline (ARE) and muscarine evoked a small (approximately 1mV), prolonged slow depolarization whereas the muscarinic agonist McN-A-343 (10⁻³ mol l⁻¹) elicited only a fast transient depolarization.

2. At a higher concentration, ARE (10⁻³ mol l⁻¹) produced a biphasic depolarization composed of a fast depolarization followed by the slow depolarization.

3. The fast depolarization was specifically inhibited by the nicotinic antagonist d-tubocurarine (dTC; 5×10⁻⁵ mol l⁻¹) and the slow depolarization was blocked by muscarinic antagonists such as atropine (ATR; 10⁻⁵ mol l⁻¹), scopolamine (10⁻⁶ mol l⁻¹) and quinuclidinyl benzilate (10⁻⁶ mol l⁻¹).

4. The ARE-induced slow depolarization was reduced by 10⁻⁵ mol l⁻¹ pirenzepine, but neither methoctramine (10⁻⁵ mol l⁻¹) nor 4-DAMP (10⁻⁵ mol l⁻¹) modified the slow depolarization. The McN-A-343-induced depolarization was fully blocked by dTC.

5. The slow depolarization was tetrodotoxin-insensitive and was unchanged when the external Na⁺ concentration was reduced by half. Tetraethylammonium (5×10⁻³ moll⁻¹) and Ba²⁺ (5.4×10⁻³ moll⁻¹) inhibited the slow depolarization. The inward K⁺ current induced by pressure-ejections of high-K⁺ saline was reduced by ARE but no increase of the membrane resistance was observed. The calcium channel blockers Co²⁺ (2×10⁻³ mol l⁻¹), Cd²⁺ (10⁻³ moll⁻¹) and La³⁺ (10⁻³ moll⁻¹) did not modify the muscarinic response.

6. The threshold of action potentials triggered by presynaptic stimulation was reduced by ARE and increased by ATR.

These results suggest that muscarinic receptors are present on cockroach ventral giant interneurones and that they can reduce a K⁺ conductance and increase an unknown conductance. The physiological role of these receptors might be to reduce the spike threshold and consequently to modify the integrative properties of giant interneurones.