Elsevier

Autonomic Neuroscience

Volume 83, Issues 1–2, 1 September 2000, Pages 37-48
Autonomic Neuroscience

Roles of central glutamate, acetylcholine and CGRP receptors in gastrointestinal afferent inputs to vagal preganglionic neurones

https://doi.org/10.1016/S0165-1838(00)00109-0Get rights and content

Abstract

It is unknown which neurotransmitter receptors are involved in the transfer of sensory information from the digestive tract to the brainstem. We examined the potential involvement of central glutamate, acetylcholine, and CGRP receptors in vagal pathways arising from gut chemo- and mechanosensitive afferents. Unitary recordings were made from 21 vagal preganglionic efferent neurones. Neuronal discharge showed either excitation or inhibition in response to oesophageal balloon distension (1–2 ml), gastric distension (40–60 ml in whole stomach or 20 ml in proximal stomach), cholecystokinin-8s (100 pmol close intra-arterially) and bradykinin (18 nmol close intra-arterially). Effects of glutamatergic non-NMDA, muscarinic M1, and CGRP1 receptor antagonism on efferent responses were investigated using CNQX (75–155 nmol i.c.v.), pirenzepine (2.5–5.0 μmol/kg i.v.), and hCGRP8-37 (3.2–6.4 nmol i.c.v.), respectively. CNQX, pirenzepine, and hCGRP8-37, respectively, altered efferent responses in 65%, 23% and 41% of neurones. When both CNQX and hCGRP8-37 were administered, a further 58% of responses were reduced. CNQX and hCGRP8-37 reduced a proportion of efferent responses to all stimuli whereas pirenzepine selectively reduced only efferent responses to gastric distension. We conclude that central CGRP1 and non-NMDA receptors are involved in mediating a range of upper gastrointestinal mechano- and chemo-sensitive afferent inputs onto vagal efferents. M1 receptors, on the other hand, are selectively involved in neurotransmission from gastric mechanoreceptors.

Introduction

The peripheral endings of vagal and spinal gastrointestinal primary afferents are found in all three (mucosal, muscular and serosal) layers of the gut wall, from the oesophagus to the colon (Clerc and Mei, 1983, Grundy and Scratcherd, 1989, Blackshaw and Grundy, 1990, Blackshaw and Grundy, 1991, Sengupta et al., 1992, Page and Blackshaw, 1998, Lynn and Blackshaw, 1999). Vagal and spinal afferents may have direct or indirect central connections with vagal preganglionic neurones found throughout the dorsal motor vagal nucleus (DMVN) (Blackshaw and Grundy, 1988, Rinaman et al., 1989), to form a reflex loop which projects to the abdominal and thoracic viscera. Although few studies have concentrated on the neurotransmitter content of gastrointestinal afferents, it is known that spinal and vagal primary afferents in general contain a large number of neuroactive substances (Dietrich et al., 1982, Kalia et al., 1984, Gibbins et al., 1985, Green and Dockray, 1988, Falempin et al., 1989, Sykes et al., 1994). These include glutamate, acetylcholine, calcitonin gene-related peptide (CGRP), adrenaline, noradrenaline, 5-hydroxytryptamine, γ-amino-butyric acid (GABA), substance P, and cholecystokinin.

It is known in cardiovascular and respiratory afferents that transmitter content provides functional specificity (Wilson et al., 1996, Paton, 1998). For example, the neurokinin-1 (NK-1) receptor is selectively involved in mediating vagal inputs from cardiac fibres to neurones in the nucleus tractus solitarius (NTS) — the main site of vagal afferent termination in the brainstem, but not those inputs from peripheral chemoreceptors or pulmonary C-fibres (Paton, 1998). On the other hand, non-NMDA (N-methyl-d-aspartate) receptors in the NTS are important in the transmission of information from pulmonary C-fibres (Wilson et al., 1996). From the gastrointestinal tract, so far we have only determined that NK-1 receptors are unlikely to be involved in transfer of sensory information (Partosoedarso and Blackshaw, 1997). Therefore it is not known which transmitters are released by mucosal and muscular afferents from different regions of the gut, and if each type releases different transmitters. This may be clinically relevant in the treatment of functional disorders such as irritable bowel syndrome and functional dyspepsia. We used an established in vivo electrophysiological technique where inputs from different afferent populations to vagal preganglionic neurones could be selectively activated (Iggo and Leek, 1967, Partosoedarso and Blackshaw, 1997), and combined this with pharmacological blockade of central neurotransmitter receptors. We focused on the role of the putative transmitters glutamate (via non-NMDA receptors), acetylcholine (via M1 muscarinic receptors), and CGRP (via CGRP1 receptors).

Section snippets

General

Experiments were performed on 21 male and female ferrets (Mustela putorius furo L.) weighing 0.5–1.4 kg, each initially anaesthetized with 1.25 g/kg urethane intraperitoneally. They were fed a standard carnivore diet with free access to water but were deprived of food for ∼18 h before experimentation. Studies were conducted in accordance with guidelines of the Animal Ethics Committee of the Royal Adelaide Hospital and Institute of Medical and Veterinary Sciences. Experiments were terminated by

Resting neuronal activity

In the absence of any intentional stimulus, vagal efferent neurones were either silent or possessed a low level of discharge (2.6±0.7 Hz, n=21). This basal discharge was irregular and not correlated with the cardiovascular, respiratory or gastrointestinal motility rhythms of the animal and remained steady prior to the administration of drug antagonists when not associated with a particular stimulus.

Efferent responses to peripheral stimuli

Responses to oesophageal balloon distension (1–2 ml air for 30 s) were evoked in 15/21 neurones.

Discussion

This study provides evidence for the involvement of at least three major transmitter mechanisms in central pathways from gastrointestinal afferents to vagal preganglionic neurones. These are acetylcholine acting via M1 muscarinic receptors, glutamate acting via non-NMDA receptors and CGRP acting via CGRP1 receptors. M1 muscarinic receptors were specifically involved in gastric mechanoreceptor inputs. Non-NMDA and CGRP1 receptors were not involved in mediating any particular type of afferent

Conclusions

There are at least three excitatory central neurotransmitter mechanisms involved in mediating afferent inputs to vagal preganglionic neurones. We found that non-NMDA and CGRP1 receptors together mediate inputs from the majority of oesophageal and gastric mechanoreceptors, gastrointestinal mucosal afferents and bradykinin-sensitive afferents. M1 muscarinic receptors are selectively involved in the transmission of inputs from gastric mechanoreceptors. Identification of selectivity in mechanisms

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