Elsevier

Molecular Brain Research

Volume 142, Issue 1, 7 December 2005, Pages 58-64
Molecular Brain Research

Research Report
Regulation of two rat mas-related genes in a model of neuropathic pain

https://doi.org/10.1016/j.molbrainres.2005.09.014Get rights and content

Abstract

The mas-related gene (Mrg) family is a large family of G-protein-coupled receptors which are variable in number depending on species. The so-called sensory-neuron-specific receptors (SNSRs) make up a subset of the Mrg family, and several of these have been implicated in nociceptive processes. To verify their specific localization in sensory ganglia, we have determined the expression patterns of two of them, rMrgA and rMrgC, in a panel of rat tissues. The quantitative PCR results in the rat tissue panel indicate that, while several non-neuronal tissues contain significant levels of mRNA for both receptors, these two receptors are most highly expressed in dorsal root ganglia and trigeminal ganglia. Given this, we have examined the effects of spinal nerve ligation (SNL) on the expression of these genes. Peripheral neuropathy induced by ligation of spinal nerves at L5 and L6 resulted in a pronounced mechanical allodynia. These behavioral changes in tactile sensitivity were accompanied by significant decreases (10- to 100-fold) in the mRNA expression of both rMrgA and rMrgC exclusively in the L5 and L6 dorsal root ganglia ipsilateral to the SNL. In situ hybridization studies demonstrated that this decrease did not result from neuronal loss but rather from a reduction in the hybridization signals for rMrgC over small-to-medium diameter L5 and L6 dorsal root ganglia neurons. While the functional implications of the altered regulation of rMrgA and rMrgC in neuropathic pain models remain unclear, the results suggest that therapeutics targeting these receptors may have limited utility.

Introduction

The Mrg (mas-related gene) family is a complex group of genes which exhibit considerable interspecies differences, both in the number and homology of Mrgs found in a given species [1], [5], [12], [16], [17]. Interspecies differences may have resulted from positive selection for nociceptive functional diversification [4] as has been seen for taste receptors [14]. To further complicate the nomenclature, several Mrgs have been described as sensory-neuron-specific receptors (SNSR), based on their preferential localization in neurons of the dorsal root ganglia [5], [12]. While many are still classified as orphan receptors, a number of these receptors have been “de-orphaned” in the past several years by the identification of ligands which activate their cognate receptor(s). These receptor/ligand pairs include the activation of human MrgX2 by cortistatin and proadrenomedullin-derived peptides [10], [13], activation of human MrgX1 (aka SNSR4), SNSR3, and rat rMrgC by BAM22 (bovine adrenal medulla peptide) [6], [12], activation of MrgA1, MrgA4, and MrgC11 by a variety of RF-amide neuropeptides [5], [8], and activation of the rat version of MrgA10 by adenine [1].

In spite of the considerable complexity presented by the interspecies differences in this gene family, the expression of a subset of these genes in sensory neurons of the dorsal root ganglia makes them a compelling target for pain research. Chronic pain syndromes in particular represent a large unmet medical need, and the search for novel mechanisms may provide innovative therapeutics devoid of the side effects known for currently available remedies. In the present study, we have examined the effects of spinal nerve ligation on the expression of two members of the rat Mrg family, termed here rMrgA and rMrgC, in dorsal root ganglia. Our results indicate that these two genes are more widely expressed than previously demonstrated and, more importantly, that both are downregulated in a neuropathic pain model.

Section snippets

Experimental animals

Experiments were carried out on male Sprague–Dawley rats (Charles River, Calco, Italy) weighing 150–175 g at time of surgery. Rats were housed in groups of 6 per cage and supplied with food and water ad libitum, under a 12 h day/night cycle at a constant room temperature of 21 °C. The animals were kept 2 days under these conditions before surgery. All experimental procedures involving rat manipulation were performed according to Schering-Plough ACUC guidelines, in compliance with both the

Note on nomenclature

In our review of the literature, it became clear that following the “SNSR” nomenclature would be counterproductive. First, as shown below, the two liganded members of the rat Mrg family are not sensory-neuron-specific. Second, the existing Mrg nomenclature covers all of the known family members and provides an interspecies point of reference for all. A ClustalW multiple sequence alignment [3] of the two rat receptors of interest and their closest mouse and human relatives resulted in the

Discussion

In the present study, the tissue distribution of two related rat Mrg receptors has been elucidated, and we have demonstrated that the expression of these genes in the DRG is regulated in response to a neuropathic stimulus. The results of the quantitative PCR studies suggest that rMrgC and rMrgA are more widely distributed than previously documented, although both are present in the DRG and trigeminal ganglion. In addition, the qPCR results indicate that the expression of both genes is

Acknowledgments

The authors would like to acknowledge the support of Dr. Marvin Bayne, Dr. John Hunter, and Dr. Catherine Strader in performing these studies.

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