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 (ICa) 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 omega-conotoxin GVIA (omega-CgTx)-sensitive channels and omega-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 inactive G-proteins or other unknown proteins involved in inhibitory muscarinic actions on ICa. 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.
Department of General Physiology and Biochemistry, University of Milan, Italy.
E
E
Wanke
L
L
Bianchi
M
M
Mantegazza
E
E
Guatteo
E
E
Mancinelli
A
A
Ferroni
D000200:Action Potentials / Q000187:drug effects; D000818:Animals; D002121:Calcium Channel Blockers / Q000494:pharmacology; D015220:Calcium Channels / Q000187:drug effects*; D004553:Electric Conductivity; D066298:In Vitro Techniques; D007700:Kinetics; D008564:Membrane Potentials / Q000187:drug effects; D008978:Mollusk Venoms / Q000494:pharmacology; D009116:Muscarine / Q000494:pharmacology; D009475:Neurons, Afferent / Q000187:drug effects*; D010455:Peptides / Q000494:pharmacology; D047428:Protein Kinase Inhibitors; D051381:Rats; D017207:Rats, Sprague-Dawley; D011976:Receptors, Muscarinic / Q000187:drug effects*; D015398:Signal Transduction / Q000187:drug effects*; D020960:omega-Conotoxins
D002121:Calcium Channel Blockers; D015220:Calcium Channels; D008978:Mollusk Venoms; D010455:Peptides; D047428:Protein Kinase Inhibitors; D011976:Receptors, Muscarinic; D020960:omega-Conotoxins; C053423:Conus magus toxin; D009116:Muscarine