Two muscarinic depolarizing mechanisms in mammalian sympathetic neurons.

A voltage-sensitive outward membrane current ('M') and a consequent change in conductance (delta G) appear with a slow time-constant, in principal neurons of rabbit superior cervical ganglion (SCG), only when membrane potentials (Vm) are depolarized to less than -60 mV. Effects of muscarine on the voltage-current curves indicate that, in this depolarized range of less than -60 mV, suppression of M-current could contribute a muscarinic depolarization accompanied by a decrease in G; but that, at all Vms tested (about -90 to -40 mV), there is an additional larger muscarinic depolarization with no delta G. Thus, the muscarinic depolarizing response and the equivalent slow excitatory postsynaptic potential in the rabbit SCG may consist of two different components: one is due to the suppression of M-current and is substantial only in the depolarized range; the other is probably mediated via an intracellular increase in cyclic GMP and can account for most or all of the response at Vms more negative than -55 mV.