Action potentials produce a long-term enhancement of M-current in frog sympathetic ganglion

M-current is a voltage-gated K⁺ current that can be turned off by the muscarinic action of acetylcholine. We examined the effects of postsynaptic action potential firing on the level of M-current in B-cells of the bullfrog sympathetic ganglion. High frequency stimulation of action potentials induced an approximately two-fold increase in the level of the M-current that could last up to 35 min. The 'enhanced' M-current was similar to the 'resting' one in its time-dependence, voltage-dependence and sensitivity to neurotransmitters. Experiments were undertaken to examine the functional consequences of the enhanced M-current. Following high frequency stimulation the number of spikes evoked by depolarizing current was reduced. In addition, the excitatory postsynaptic potential (EPSP) evoked by maximal input became subthreshold, thereby blocking information flow through the ganglion cell. These results indicate that the enhancement of M-current by spikes provides a negative feedback mechanism for the control of excitability. It has been reported that postsynaptic stimulation of ganglion cells also produces a long-term increase in the nicotinic EPSP, but we were unable to confirm this observation.