Muscarinic Regulation of Ca2+ Currents in Rat Sensory Neurons: Channel and Receptor Types, Dose - response Relationships and Cross-talk Pathways

We studied, in rat sensory neurons, the modulation of high voltage-activated Ca²⁺ currents (I_Ca mediated by the pertussis toxin-sensitive activation of muscarinic receptors, which were found to be of subtypes M₂, or M₄. Muscarine reversibly blocked somatic Ca²⁺ spikes but strong predepolarizations only partially relieved the inhibited Ca²⁺ current. On the other hand, the putative coupling messenger could not rapidly diffuse towards channels whose activity was recorded from a macro-patch. The perforated patch technique virtually prevented the response rundown present during whole-cell experiments. Both ω-conotoxin GVIA (ω-CgTx)-sensitive channels and ω-CgTx- and dihydropyridine-resistant channels are coupled to the muscarinic receptor, but not the L-channel. When measured in the same neuron, dose - response relationships for the first and subsequent agonist applications differed; maximal inhibition, the reciprocal of half-maximal concentration and the Hill coefficient were always highest in the first trial. Muscarine and oxotremorine exhibited monotone dose - response curves, but oxotremorine-M showed non-linear relationships which became monotonic when cells were intracellularly perfused with inhibitors of protein kinase A (PKA) and C (PKC), suggesting that either PKA or receptor-induced PKC could phosphorylate and thus inactivate G-proteins or other unknown proteins involved in inhibitory muscarinic actions on I_Ca. In summary, these data provide a preliminary pharmacological characterization of the muscarinic inhibition of the Ca²⁺ channels in sensory neurons, with implications about agonist specificity and the interplay between signalling pathways.