Muscarinic inhibition of high-voltage-activated calcium channels in excised membranes of rat hippocampal neurons.

The effect of muscarine on voltage-gated calcium channels was investigated in outside-out patches from rat hippocampal neurons in culture. By clamping the excised patches at -60 mV holding potential, single and multiple Ca channel currents were recorded, and these displayed features similar to the high-voltage-activated Ca current, with unitary conductance of 6.4 pS in 50 mM external Ca2+. These channels turned out to be insensitive both to Bay K 8644 and to omega-conotoxin. In excised patches muscarine caused a marked reduction in the probability of opening of this class of Ca channels without significant changes in the unitary current amplitude. Interestingly, the degree of current inhibition was reduced by depolarization, thus suggesting a voltage-dependent inhibitory action of the agonist. We conclude that in hippocampal neurons one of the possible ways of HVA Ca channel modulation by muscarine occurs through activation of a substratum localized within the plasma membrane of the cell, independent of the involvement of diffusible intracellular messengers.