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

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 oilgap technique. 1. Pressure-ejections of 1024 mol l21 arecoline (ARE) and muscarine evoked a small (approximately 1mV), prolonged slow depolarization whereas the muscarinic agonist McN-A-343 (1023 mol l21) elicited only a fast transient depolarization. 2. At a higher concentration, ARE (1023 mol l21) 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; 531025 mol l21) and the slow depolarization was blocked by muscarinic antagonists such as atropine (ATR; 1025 mol l21), scopolamine (1026 mol l21) and quinuclidinyl benzilate (1026 mol l21). 4. The ARE-induced slow depolarization was reduced by 1025 mol l21 pirenzepine, but neither methoctramine (1025 mol l21) nor 4-DAMP (1025 mol l21) 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 (531 023 m o l l21) and Ba2 + ( 5 . 431 023 m o l l21) inhibited the slow depolarization. The inward K+ c u r r e n t 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 + ( 231 023 m o l l21) , C d2 + ( 1 023 m o l l21) and La3 + ( 1 023 m o l l21) 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.