Cholinergic transmission in cat parasympathetic ganglia.

Abstract

Intracellualr electrical recording techniques were used to study the ionic mechanisms of cholinergic synaptic transmission in cat vesical pelvic ganglia (VPG). Orthodromic nerve stimulation as well as ionophoretic application of acetylcholine (ACh) resulted in, first a fast excitatory post-synaptic potential (FEPSP) and secondly, a slow inhibitory post-synaptic potential (SIPSP). These distinct post-synaptic responses were direct actions of ACh and not mediated through an interneuron. In addition, a slow excitatory post-synaptic potential (SEPSP) was observed in 44% of the cells. The FEPSP, mediated via nicotinic receptors, had a reversal potential of -10 mV and resembled the conventional rapid depolarization in other ganglia. The SIPSP, mediated by muscarinic receptors, had a reversal potential of .apprx. -100 mV and resulted from an increase in K conductance. The slow muscarinic hyperpolarization could be observed in the absence of antagonists and it was elicited at stimulus frequencies in the physiological range (2-10 Hz). The SIPSP induced orthodromically or ionophoretically inhibited firing in spontaneously active neurons. The muscarinic hyperpolarization may occur under physiological conditions and have sufficient magnitude to be inhibitory to neuronal activity.