Modulation of inwardly rectifying currents in rat sympathetic neurones by muscarinic receptors

1. Intracellular recordings were made from rat sympathetic neurones in isolated superior cervical ganglia (SCG), coeliac ganglia (CG), and superior mesenteric ganglia (SMG). 2. Following classification of the firing properties of these neurones as either 'phasic' or 'tonic', single-electrode voltage-clamp recordings of the inwardly rectifying current were performed. The inward rectifier conductance was 6.4 times larger in tonic neurones than in phasic neurones. 3. The basic features of the inward rectifier in sympathetic neurones were similar to those of the classic inward rectifier described in several neuronal and non-neuronal preparations. The properties of the native channel were also similar to a subset of recently cloned inwardly rectifying channels. The reversal potential and the slope conductance were both dependent on external potassium ion concentration. The conductance was blocked by low concentrations of external Ba²⁺ and Cs⁺ ions. 4. A striking feature of the inward rectifier in sympathetic neurones was its modulation by muscarine. Application of 20 μM muscarine produced a mean 78 ± 1.4% inhibition of the current. From dose-response curves for muscarine a mean dissociation constant of K(D) = 1.95 ± 0.2 μM was determined. Schild plot analysis using the competitive antagonists pirenzepine and himbacine indicated that the effect of muscarine was mediated by the M₁ class of muscarinic receptors. 5. The inward rectifier was also inhibited by repetitive nerve stimulation which produced a block of the conductance similar to that seen in response to bath-applied muscarine. The onset of inhibition was relatively slow, 20-30 s, suggesting that it is mediated by a soluble second messenger pathway.