The physiological properties, shapes, projections and neurochemistries of Dogiel type II neurons have been thoroughly investigated in the guinea-pig intestine in which these neurons have been identified as intrinsic primary afferent neurons. Dogiel type II neurons in the myenteric ganglia of mice have similar physiological properties to those in guinea-pigs but whether other features of the neurons are similar is unknown. We have used intracellular dye-filling, retrograde tracing, immunohistochemistry and nerve lesions to determine salient features of Dogiel type II neurons of the mouse colon. Dye-filling showed that the neurons provide profuse terminal networks in the myenteric ganglia and also have axons that project towards the mucosa. Retrograde tracing and lesion studies showed that these axons provide direct innervation to the mucosa. High proportions of the neurons had immunoreactivity for calretinin, calbindin, choline acetyltransferase, the purine P2X2 receptor and calcitonin gene-related peptide (CGRP). CGRP was the most selective marker of the neurons. Following surgery to remove an area of myenteric plexus, the CGRP-immunoreactive nerve fibres in the mucosa degenerated. Thus, Dogiel type II neurons in mice have similar shapes and projections but some differences in chemistry from those in guinea-pigs. The close similarities between the two species in the shapes, projections and electrophysiology of these neurons suggest that they serve the same functions in both species.

The P2X(2) subtype of purine receptor was localised by immunohistochemistry to nerve cells of the myenteric ganglia of the stomach, small and large intestines of the guinea-pig, and nerve cells of submucosal ganglia in the intestine. Nerve cells with strong and with weak immunoreactivity could be distinguished. Immunoreactivity in both strongly and weakly immunoreactive neurons was absorbed with P2X(2) receptor peptide. In the myenteric plexus, strong immunoreactivity was in nitric oxide synthase (NOS)- and in calbindin-immunoreactive neurons. In all regions, over 90% of NOS-immunoreactive neurons were strongly P2X(2) receptor immunoreactive. The intensity of reaction varied in calbindin neurons; in the ileum, 90% were immunoreactive for the receptor, about one-third having a strong reaction. In the submucosal ganglia, all vasoactive intestinal peptide-immunoreactive neurons were P2X(2) receptor immunoreactive, but there was no receptor immunoreactivity of calretinin or neuropeptide Y neurons. Varicose nerve fibres with P2X(2) receptor immunoreactivity were found in the gastric myenteric ganglia. These fibres disappeared after vagus nerve section. It is concluded that the P2X(2) receptor is expressed by specific subtypes of enteric neurons, including inhibitory motor neurons, non-cholinergic secretomotor neurons and intrinsic primary afferent neurons, and that the receptor also occurs on the endings of vagal afferent fibres in the stomach.

Cholinergic neurons have been revealed in the enteric nervous system by functional and biochemical studies but not by antibodies that provide excellent localisation of the synthesising enzyme, choline acetyltransferase (ChAT), in the central nervous system. In order to determine whether a newly described peripheral form of ChAT (pChAT) is a ChAT enzyme of enteric neurons, we have compared pChAT distribution with that of the common form of ChAT, cChAT, by quantitative analysis of the co-localisation of pChAT and cChAT with other neurochemical markers in enteric neurons of the guinea-pig ileum. We found classes of neuron with strong pChAT immunoreactivity (IR) and others with strong cChAT-IR. In myenteric ganglia, strong pChAT-IR was in calbindin-positive intrinsic primary afferent neurons (IPANs), whereas cChAT-IR of these neurons was weak. Calretinin neurons were immunoreactive for cChAT, but not pChAT. Only 4% of nitric oxide synthase (NOS) neurons (possibly interneurons) were pChAT-immunoreactive, similar to observations with cChAT. NOS-immunoreactive inhibitory motor neurons stained with neither cChAT nor pChAT antisera. In the submucosal ganglia, pChAT-IR was strongly expressed in IPANs (identified by cytoplasmic staining for the neuronal nuclear marker, NeuN) and in neuropeptide Y (NPY)-immunoreactive secretomotor neurons, but not in calretinin-immunoreactive neurons. cChAT-IR occurred weakly in submucosal IPANs and also labelled NPY- and calretinin-immunoreactive neurons. Submucosal vasoactive-intestinal-peptide-immunoreactive neurons (non-cholinergic secretomotor neurons) were not reactive for either form of ChAT.

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We investigated the responses of morphologically identified myenteric neurons of the guinea-pig ileum to inflammation that was induced by the intraluminal injection of trinitrobenzene sulphonate, 6 or 7 days previously. Electrophysiological properties were examined with intracellular microelectrodes using in vitro preparations from the inflamed or control ileum. The neurons were injected with marker dyes during recording and later they were recovered for morphological examination. A proportion of neurons with Dogiel type I morphology, 45% (32/71), from the inflamed ileum had a changed phenotype. These neurons exhibited an action potential with a tetrodotoxin-resistant component, and a prolonged after-hyperpolarizing potential followed the action potential. Of the other 39 Dogiel type I neurons, no changes were observed in 36 and 3 had increased excitability. The afterhyperpolarizing potential (AHP) in Dogiel type I neurons was blocked by the intermediate conductance, Ca 2+ -dependent K + channel blocker TRAM-34. Neurons which showed these phenotypic changes had anally directed axonal projections. Neither a tetrodotoxin-resistant action potential nor an AHP was seen in Dogiel type I neurons from control preparations. Dogiel type II neurons retained their distinguishing AH phenotype, including an inflection on the falling phase of the action potential, an AHP and, in over 90% of neurons, an absence of fast excitatory transmission. However, they became hyperexcitable and exhibited anodal break action potentials, which, unlike control Dogiel type II neurons, were not all blocked by the h current (I h ) antagonist Cs + . It is concluded that inflammation selectively affects different classes of myenteric neurons and causes specific changes in their electrophysiological properties.