Metabotropic glutamate receptors inhibit mechanosensitivity in vagal sensory neurons

doi:10.1053/j.gastro.2004.11.062

Gastroenterology

Volume 128, Issue 2, February 2005, Pages 402-410

https://doi.org/10.1053/j.gastro.2004.11.062 Get rights and content

Background & Aims: Inhibitory G-protein-coupled receptors have demonstrated potential in treatment of gastroesophageal reflux disease (GERD) through actions on vagal afferent signaling. Metabotropic glutamate receptors (mGluR) belong to this receptor family and have great pharmacologic and molecular diversity, with 8 subtypes. We investigated mGluR in the vagal system of humans and other species. Methods: Expression of mGluR1-8 in human, dog, ferret, and rodent nodose ganglia was investigated by reverse-transcription polymerase chain reaction. mGluR1-8 immunohistochemistry was performed in combination with retrograde tracing of vagal afferents from ferret proximal stomach to nodose ganglia. Transport of mGluR peripherally was investigated by vagal ligation, followed by immunohistochemistry. Glutamate receptor pharmacology of ferret and rodent gastroesophageal vagal afferents was investigated by testing single fiber responses to graded mechanical stimuli during drug application to their peripheral endings. Results: Messenger RNA for several mGluR was detected in the nodose ganglia of all species. Retrograde tracing indicated that ferret gastric vagal afferents express mGluR protein. Accumulation of immunoreactivity proximal to a ligature showed that mGluR were transported peripherally in the vagus nerves. Glutamate (1–30 μmol/L with kynurenate 0.1 mmol/L) concentration dependently inhibited vagal afferent mechanosensitivity. This was mimicked by selective group II and III mGluR agonists but not by a group I agonist. Conversely, a group III mGluR antagonist increased mechanosensitivity to intense stimuli. Conclusions: Both exogenous and endogenous glutamate inhibits mechanosensitivity of vagal afferents. Group II (mGluR2 and 3) and group III mGluR (mGluR4, 6, 7, 8) are novel targets for inhibition of vagal signaling with therapeutic potential in, for example, GERD.

Section snippets

RT-PCR analysis

Total RNA was isolated from nodose ganglia and brain from rat, dog, ferret, and human, using Trizol (Life Technologies, Inc.). One to 2.5 μg total RNA was reverse transcribed at 42°C for 50 minutes in the presence of 0.5 μg/μL oligo (dT)12, 13, 14, 15, 16, 17, 18 primer, 10 × RT buffer (200 mmol/L Tris-HCl, pH 8.4, 500 mmol/L KCl, 25 mmol/L MgCl₂), 10 mmol/L DTT, 10 mmol/L of each dNTP, 40 U Recombinant Ribonuclease Inhibitor and 50 U Reverse Transcriptase (SuperScript II, from Life

Results

We compared mGluR expression in vagal afferent cell bodies in the nodose ganglia of humans, dogs, ferrets, and rats. All 8 mGluR transcripts were detected in rat and dog nodose ganglion and brain using primers designed to detect human and rat sequences (Figure 1A and 1B), and identity of products was confirmed by sequencing. In human and in ferret, all except mGluR 3 and 6 could be detected in nodose, and mGluR6 was not evident in nodose or brain specimens.

Retrograde tracing from the ferret

Discussion

Interest is intensifying in pharmacologic approaches to reducing input from afferent neurons to the brain and spinal cord, in particular from groups of afferent neurons that may contribute to diseased states or to pain. Vagal afferents provide input to pathways in the brain that are involved in triggering of behavioral responses,22, 23 reflexes controlling digestive function,24 and, importantly, motor patterns that lead to gastroesophageal reflux.25, 26 Therefore, any means of reducing this

References (32)

K.A. Moore et al.
Synaptic transmission and plasticity in the superficial dorsal horn
Prog Brain Res
(2000)
A. Lehmann et al.
Activation of the GABA(B) receptor inhibits transient lower esophageal sphincter relaxations in dogs
Gastroenterology
(1999)
I. Lidums et al.
Control of transient lower esophageal sphincter relaxations and reflux by the GABA(B) agonist baclofen in normal subjects
Gastroenterology
(2000)
W.L. Hasler
Baclofen effects on esophageal functiona possible therapy for GERD?
Gastroenterology
(2002)
D.D. Schoepp et al.
Pharmacological agents acting at subtypes of metabotropic glutamate receptors
Neuropharmacology
(1999)
J.A. McRoberts et al.
Role of peripheral N-methyl-D-aspartate (NMDA) receptors in visceral nociception in rats
Gastroenterology
(2001)
N.K. Thomas et al.
(S)-3,4-DCPG, a potent and selective mGlu8a receptor agonist, activates metabotropic glutamate receptors on primary afferent terminals in the neonatal rat spinal cord
Neuropharmacology
(2001)
L.A. Blackshaw et al.
The pharmacology of gastrointestinal nociceptive pathways
Curr Opin Pharmacol
(2002)
D. Grundy
Vagal control of gastrointestinal function
Baillieres Clin Gastroenterol
(1988)
R.K. Mittal et al.
Transient lower esophageal sphincter relaxation
Gastroenterology
(1995)

M. Raab et al.

Vesicular glutamate transporter 2 immunoreactivity in putative vagal mechanosensor terminals of mouse and rat esophagusindication of a local effector function?

Cell Tissue Res

(2003)

J. Cartmell et al.

Regulation of neurotransmitter release by metabotropic glutamate receptors

J Neurochem

(2000)

C.D. Fiorillo et al.

Glutamate mediates an inhibitory postsynaptic potential in dopamine neurons

Nature

(1998)

A.J. Page et al.

GABA(B) receptors inhibit mechanosensitivity of primary afferent endings

J Neurosci

(1999)

S.D. Smid et al.

GABA(B)R expressed on vagal afferent neurones inhibit gastric mechanosensitivity in ferret proximal stomach

Am J Physiol Gastrointest Liver Physiol

(2001)

L.A. Blackshaw et al.

Inhibition of transient LES relaxations and reflux in ferrets by GABA receptor agonists

Am J Physiol

(1999)

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: Supported by National Health and Medical Research Council grant 104814, AstraZeneca, and the University of Adelaide (J.R.C. and C.M.M).

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Basic-alimentary tractMetabotropic glutamate receptors inhibit mechanosensitivity in vagal sensory neurons

Section snippets

RT-PCR analysis

Results

Discussion

Prog Brain Res

Gastroenterology

Gastroenterology

Gastroenterology

Neuropharmacology

Gastroenterology

Neuropharmacology

Curr Opin Pharmacol

Baillieres Clin Gastroenterol

Gastroenterology

Vesicular glutamate transporter 2 immunoreactivity in putative vagal mechanosensor terminals of mouse and rat esophagusindication of a local effector function?

Cell Tissue Res

Regulation of neurotransmitter release by metabotropic glutamate receptors

J Neurochem

Glutamate mediates an inhibitory postsynaptic potential in dopamine neurons

Nature

GABA(B) receptors inhibit mechanosensitivity of primary afferent endings

J Neurosci

GABA(B)R expressed on vagal afferent neurones inhibit gastric mechanosensitivity in ferret proximal stomach

Am J Physiol Gastrointest Liver Physiol

Inhibition of transient LES relaxations and reflux in ferrets by GABA receptor agonists

Am J Physiol

Basic-alimentary tract
Metabotropic glutamate receptors inhibit mechanosensitivity in vagal sensory neurons