Elsevier

Neuroscience

Volume 293, 7 May 2015, Pages 151-156
Neuroscience

Group II metabotropic glutamate receptor antagonism prevents the antiallodynic effects of R-isovaline

https://doi.org/10.1016/j.neuroscience.2015.02.022Get rights and content

Highlights

  • Isovaline, the mGluR II agonist, LY354740, and baclofen prevented PGE2-induced allodynia.

  • Isovaline antiallodynia was blocked by mGluRII antagonism.

  • Isovaline antiallodynia was potentiated by an mGluRII modulator.

  • Isovaline represents a new class of peripheral analgesic.

Abstract

We previously showed that isovaline is a peripheral analgesic which acts in vivo and in brain slices as an atypical metabotropic GABAB agonist. Peripheral inhibitory group II and III metabotropic glutamate receptors (mGluRs) belong to the same family C as GABAB receptors; therefore, we hypothesized that isovaline’s analgesic effects could include their activation. We examined the effects of R-isovaline on mechanical allodynia produced by prostaglandin E2 in the mouse paw. Subcutaneous R-isovaline produced dose-dependent antiallodynia restricted to the injected hindlimb. This antiallodynia was blocked by co-injection with a selective group II mGluR antagonist, LY341495, but not a group III mGluR antagonist (MAP-4). The antiallodynic effect of R-isovaline was potentiated by co-administration of a group II mGluR-positive allosteric modulator, LY487379. Injection of a group II mGluR agonist (LY354740) produced an antiallodynic effect which was completely reversed by group II antagonism, but was not affected by group III or GABAB (CGP35348) antagonism. Similarly, group II mGluR antagonism did not alter the antiallodynia produced by the prototypical GABAB agonist, baclofen. Hence, there was no apparent crosstalk between group II mGluRs and GABAB receptors. Previous studies have demonstrated that peripheral GABAB receptor activation by isovaline produces antiallodynia. In addition, the present results indicate that activation of peripheral group II mGluRs by R-isovaline produces antiallodynia.

Introduction

Isovaline is an unusual amino acid that has antinociceptive properties in acute and chronic pain models. Consistent with its structural similarity to γ-aminobutyric acid (GABA), isovaline produces analgesia at least in part by activating GABAB receptors in the periphery (Whitehead et al., 2012). Although intrathecal application has antinociceptive effects, peripheral application results in analgesia without central nervous system (CNS) side effects due to limited passage of isovaline across the blood–brain barrier (Shiba et al., 1989, MacLeod et al., 2010).

Our initial studies on brain slices showed that isovaline bath application increases membrane K+ conductance of neurons by activating GABAB receptors (Cooke et al., 2009). The resulting inhibition of excitability by isovaline is attenuated by intracellular non-hydrolyzable GTP analogs, suggesting G-protein mediation. We subsequently found the in vivo pharmacology of isovaline to be similar to GABAB agonists, baclofen or GABA when applied intraplantarly (Whitehead et al., 2012) and some in vitro actions were atypical (Cooke et al., 2012). Co-application of isovaline and baclofen does not result in additive or synergistic effects. Many baclofen-sensitive neurons do not respond to isovaline (Cooke et al., 2012). In in vivo studies, isovaline’s analgesic actions are incompletely antagonized by a GABAB antagonist, CGP52432 ((3-[[[(3,4-dichlorophenyl)methyl]amino]propyl]diethoxymethyl)phosphinic acid), in contrast to baclofen- or GABA-mediated analgesia, suggesting that isovaline may interact with another receptor mechanism (cf. Neale and Salt, 2006, Whitehead et al., 2012). Therefore, we considered the possibility that isovaline may interact with other G-protein-coupled receptors (GPCRs) that mediate inhibitory effects.

Metabotropic glutamate receptors (mGluRs) belong to the same family C of GPCRs as GABAB receptors. While group I mGluRs are excitatory, group II and III inhibit nociceptive neurons by increasing K+ conductance (Dutar et al., 2000, Yang and Gereau, 2003). Group II and III mGluRs are negatively coupled to adenylyl cyclase (Schwartz and Alford, 2000, Goudet et al., 2009) and may provide a novel therapeutic approach to relieve pain (Urban and Thompson, 1994, Simmons et al., 1998). Our objective in the present studies was to test the hypothesis that isovaline’s analgesic actions result from mGluR activation in a murine model of allodynia induced by intraplantar injection of prostaglandin E2 (PGE2).

Section snippets

Animals

The Animal Care Committee of The University of British Columbia approved these experiments. Adult female CD-1 mice (weight, 25–30 g) were maintained under conditions of a 12-h dark–light cycle and room temperature of 21 °C. Food and water were available ad libitum and each animal was used only once in the experiments which were conducted between 9:00 am and 5:00 pm.

Drugs

The group II mGluR agonist LY354740 ((1S,2S,5R,6S)-2-aminobicyclo[3.1.0]hexane-2,6-dicarboxylic acid), antagonist LY341495

Antiallodynic effects of R-isovaline

We first determined a dose–response relationship for R-isovaline-induced antiallodynia, obtaining an estimation of the ED50. Application of R-isovaline prevented PGE2-induced allodynia in a dose-dependent manner (Fig. 1). Applying R-isovaline in single s.c. doses of 1, 3.5, 7, 14, and 28 μmol yielded an ED50 of 6 μmol (95% CI 3–12). Fig. 2 shows that R-isovaline (7 μmol) co-injected with PGE2 prevented the allodynic response. As in a previous report using RS-isovaline (Whitehead et al., 2012), the

Discussion

Our objective was to determine if isovaline’s antiallodynic actions involve mGluR activation, complementing the known mediation by peripheral GABAB receptors. In this study, we have demonstrated that isovaline prevented allodynia through mGluR activation in a PGE2-allodynia model. The effects were confined to the site of injection and did not occur in the contralateral paw indicating a peripheral action. The specific activation of the group II mGluRs was independent of GABAB receptors. However,

Conclusion

The present studies demonstrate that, in addition to GABAB receptors, peripheral group II mGluRs mediate the antiallodynic actions of isovaline. Acting at both peripheral metabotropic glutamate and GABAB receptor targets and apparently devoid of CNS side effects, isovaline is a prototype of a new class of analgesics (cf. Singh et al., 2013, Fowler et al., 2014).

Acknowledgements

This research was supported in part by the Dr. Jean Templeton Hugill Endowment for Anesthesia Memorial Fund, the Canadian Institutes for Health Research, and a scholarship from King Khalid University (Abha, Saudi Arabia) to Khalid A. Asseri. Stephan K. W. Schwarz gratefully acknowledges the research support by the Canadian Anesthesia Research Foundation. We appreciate the gift of R-isovaline from Nagase & Co. Ltd.

References (29)

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  • Differential effects of R-isovaline and the GABA<inf>B</inf>agonist, baclofen, in the guinea pig ileum

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    Subsequent investigations have demonstrated an additional involvement of group II metabotropic glutamate receptors in R-isovaline analgesia in vivo. LY341495, a group II metabotropic glutamate receptor antagonist nearly abolished the antiallodynic effect of R-isovaline which was potentiated by LY487379, a positive allosteric group II metabotropic glutamate receptor modulator (Asseri et al., 2015). On the basis of these findings, we undertook the present experiments to shed further light on the receptor mechanisms of R-isovaline's effects in the guinea pig ileum which expresses functional GABAB and group II metabotropic glutamate receptors (Chen and Kirchgessner, 2002; Larzabal et al., 1999; Kirchgessner, 2001; Hyland and Cryan, 2010).

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