Delayed onset and slow time course of the non-M-type muscarinic current in bullfrog sympathetic neurons.
The onset and time course of the muscarinic currents induced by brief applications of acetylcholine (ACh) were examined in voltage-clamped neurons of bullfrog sympathetic ganglia bathed in a solution containing d-tubocurarine. At a potential of -40 mV, the ACh-induced current (IACh) appeared within 1.2 s and rapidly increased to its peak with a half-activation time of 2.2 s. This initial current was termed the fast IACh and was blocked by 4 mM Ba2+. At a potential more negative than -60 mV, the fast IACh disappeared and the remaining IACh activated with a delay of 3.9 s and slowly increased to its peak with a half-activation time of 8.2 s. This delayed current was termed the slow IACh and is thought to be associated with inhibition of a K+ current, or IM, as well as activation of an inward current through non-M-type muscarinic cation channels. The slow IACh was not inhibited by Ba2+, but its amplitude was reduced with depolarization (the extrapolated reversal potential was +3 mV). In Na(+)-free solution, the amplitude of the slow IACh reduced, but its polarity did not reverse in the voltage region examined (-30 to -100 mV). The slow excitatory postsynaptic current was also recorded, and was shown to have a similar delay in onset and slow time course. The results demonstrate that ACh activates the non-M-type muscarinic current three times more slowly than it inhibits IM.
Minota
S
S
Department of Physiology, Saga Medical School, Japan.
D000109:Acetylcholine / Q000494:pharmacology*; D000320:Adrenergic Fibers / Q000502:physiology; D000818:Animals; D001285:Atropine / Q000494:pharmacology; D001464:Barium / Q000494:pharmacology; D005071:Evoked Potentials / Q000187:drug effects; D011892:Rana catesbeiana; D011976:Receptors, Muscarinic / Q000502:physiology*; D013997:Time Factors
D011976:Receptors, Muscarinic; D001464:Barium; D001285:Atropine; D000109:Acetylcholine