Optical studies of nicotinic acetylcholine receptor subtypes in the guinea-pig enteric nervous system

Abstract

Nicotinic transmission in the enteric nervous system (ENS) is extensive, but the role of individual nicotinic acetylcholine receptor (nAChR) subtypes in the functional connectivity of its plexuses has been elusive. Using monoclonal antibodies (mAbs) against neuronal alpha3-, alpha4-, alpha3/alpha5-, beta2-, beta4- and alpha7-subunits, combined with radioimmunoassays and immunocytochemistry, we demonstrate that guinea-pig enteric ganglia contain all of these nAChR-subunits with the exception of alpha4, and so, differ from mammalian brain. This information alone, however, is insufficient to establish the functional role of the identified nAChR-subtypes within the enteric networks and, ultimately, their specific contributions to gastrointestinal physiology. We have used voltage-sensitive dyes and a high-speed CCD camera, in conjunction with specific antagonists to various nAChRs, to elucidate some of the distinct contributions of the individual subtypes to the behaviour of enteric networks. In the guinea-pig, the submucous plexus has the extraordinary advantage that it is virtually two-dimensional, permitting optical recording, with single cell resolution, of the electrical activity of all of its neurones. In this plexus, the block of alpha3beta2-, alpha3beta4- and/or alpha7-nAChRs always results in a decrease in the magnitude of the synaptic response. However, the magnitude of the fast excitatory post-synaptic potentials (epsps) evoked by electrical stimulation of a neighbouring ganglion varies from cell to cell, reflecting the differential expression of subunits already observed using mAbs, as well as the strengths of the activated synaptic inputs. At the same time, we observe that submucous neurones have a substantial mecamylamine (Mec)-insensitive (non-nicotinic) component to their fast epsps, which may point to the presence of purinergic or serotonergic fast epsps in this system. In the myenteric plexus, on the other hand, the antagonist-induced changes in the evoked synaptic response vary depending upon the location of the stimulating electrode with respect to the ganglion under study. The range of activity patterns that follows sequential pharmacological elimination of individual subtypes suggests that nAChRs may be capable of regulating the activity of both excitatory and inhibitory pathways, in a manner similar to that described in the central nervous system.