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 g iant interneurone in the sixth abdominal ganglion of the cockroach wer e studied using the single-fibre oil-gap technique. 1. Pressure-ejecti ons of 10(-4) Mol l-1 arecoline (ARE) and muscarine evoked a small (ap proximately 1 mV), prolonged slow depolarization whereas the muscarini c agonist McN-A-343 (10(-3) Mol l-1) elicited only a fast transient de polarization. 2. At a higher concentration, ARE (10(-3) Mol l-1) produ ced a biphasic depolarization composed of a fast depolarization follow ed by the slow depolarization.3. The fast depolarization was specifica lly inhibited by the nicotinic antagonist d-tubocurarine (dTC; 5 X 10( -5) Mol l-1) and the slow depolarization was blocked by muscarinic ant agonists such as atropine (ATR; 10(-5) Mol l-1), scopolamine (10(-6) m ol l-1) and quinuclidinyl benzilate (10(-6) mol l-1). 4. The ARE-induc ed 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) mod ified the slow depolarization. The McN-A-343-induced depolarization wa s fully blocked by dTC. 5. The slow depolarization was tetrodotoxin-in sensitive and was unchanged when the external Na+ concentration was re duced by half Tetraethylammonium (5 x 10(-3) Mol l-1) and Ba2+ (5.4 x 10(-3) Mol l-1) inhibited the slow depolarization. The inward K+ curre nt induced by pressure-ejections of high-K+ saline was reduced by ARE but no increase of the membrane resistance was observed. The calcium c hannel blockers Co2+ (2 x 10(-3) Mol l-1), Cd2+ (10(-3) Mol l-1) and L a3+ (10(-3) Mol l-1) did not modify the muscarinic response. 6. The th reshold of action potentials triggered by presynaptic stimulation was reduced by ARE and increased by ATR. These results suggest that muscar inic receptors are present on cockroach ventral giant interneurones an d that they can reduce a K+ conductance and increase an unknown conduc tance. The physiological role of these receptors might be to reduce th e spike threshold and consequently to modify the integrative propertie s of giant interneurones.