Nicotinic acetylcholine receptor-ion channels involved in synaptic currents in bullfrog sympathetic ganglion cells and effects of atropine

Abstract

The nicotinic acetylcholine receptor-ion channels (AChR channels) of the bullfrog sympathetic ganglion cells were studied with a two-electrode voltage clamp technique. The decay phase of the fast excitatory postsynaptic current (fast e.p.s.c.) in B-type neurones followed a double exponential function whose time constants were 3.2 and 8.0 ms at −60 mV and increased with membrane hyperpolarization. Likewise, the decay phase of the fast e.p.s.c. in C-type neurones was double-exponential with time constants of 4.4 and 12.3 ms (at −60 mV). The miniature e.p.s.c. in B-type neurones also decayed with a double exponential function (2.7 and 15.4 ms at −100 mV). Analysis of acetylcholine-induced current fluctuations revealed the power spectral density distribution of a double Lorentzian function which yielded the time constants of elementary events [τ_noise(f) and τ_noise(s): 1.7 and 29.7 ms, respectively, at −100 mV] and the averaged elementary conductance (γ: 7.8 pS).

The amplitude of fast e.p.s.c. and the time constant of the fast component of its decay phase decreased during the initial (“acute”) phase (within 15 min) of the action of atropine (3 μM), but recovered during the later (“chronic”) phase (more than 30 min after application) of the action. The slow component was affected by atropine in a manner similar to the fast component during the “acute” phase.

During the “chronic phase”, however, the slow time constant recovered and exceeded the control value. Furthermore, this prolongation remained for at least 1 h after the removal of atropine. τ_noise(s) during the “chronic” phase of the action was also prolonged to 50.3 ms, while [τ_noise(f) (2.1 ms) was similar to the control value. The amplitude and quantal size of the fast excitatory postsynaptic potential were also decreased during the “acute” phase of atropine action, and recovered during the “chronic” phase. Interestingly, they were reduced transiently during the course of removal of atropine from the bath.

These results revealed that the decay phases of the fast e.p.s.c. and miniature e.p.s.c. have two components which are explained either by the existence of two types of AChR channels having different open times or by a single type having three states with rate constants of certain relationships in the bullfrog sympathetic ganglion cells and suggested that their open forms are blocked by atropine only transiently and later desensitized to the blocking action.