Electrophysiological effects

Abstract

Responses of cultured rat superior cervical sympathetic neurons to local applications of cholinergic drugs were studied using whole cell, cell attached and outside—out patch versions of the patch clamp technique. All neurons tested were activated by three nicotinic agonists with the following rank order: DMPP > Nicotine> ACh. With 100 μM each of these agonists, the peak responses ranged from 100 pA to 4 nA at a membrane potential of —60 mV. The nicotinic responses were blocked by hexamethonium, mccamylamine, curare, but not by α—bungarotoxin. The blockade of mecamylamine was use—dependent. Whole cell currents evoked by the three nicotinic agonists showed strong rectification at positive membrane potentials. The nicotinic responses declined progressively in the presence of agonist, a phenomena was known as desensitization. The rate of desensitization was both dose and voltage—dependent. The single nicotinic channels are studied using DMPP as an agonist. In outside— out patch, simultaneous openings of multi— channels was immediately detectable with the onset of DMPP perfusion. In cell— attached patch, DMPP activated single channel events of many different conductance: 15 ps, 27 ps and 38 ps, and with the 27 ps being the most frequently seen conductance. DMPP—activated single channels generally have two gating modes, the brief opening mode and a burst of long opening mode. The open and closed durations are distributed according to double exponentials. After 2 weeks in culture, some neurons showed spontaneous synaptic currents, which could be blocked by hexamethonium. It provides functional evidence for a cholinergic nature of synaptic transmission between cultured sympathetic neurons.