mGlu5 receptors and nociceptive function II. mGlu5 receptors functionally expressed on peripheral sensory neurones mediate inflammatory hyperalgesia
Introduction
The actions of glutamate are mediated either through interaction with ionotropic glutamate (iGlu) receptor channels or by G protein-coupled metabotropic glutamate (mGlu) receptors that are linked to the modulation of second messenger systems (Salt, 1986, Knopfel et al., 1995, Conn and Pin, 1997). A specific role for group I mGlu receptors (mGlu1 and mGlu5) in nociceptive processing has been demonstrated by pharmacological, immunohistochemical and in situ hybridization studies (Fisher and Coderre, 1998, Salt and Turner, 1998, Boxall et al., 1998a, Jia et al., 1999). In addition to a role in pain transmission within the central nervous system, glutamate also excites peripheral nociceptive neurones (Carlton et al., 1995, Jackson et al., 1995, Lawand et al., 1997, Fiorentino et al., 1999). This effect has been attributed to the activation of peripherally expressed ionotropic glutamate receptors (Carlton et al., 1995, Jackson et al., 1995, Zhou et al., 1996, Davidson et al., 1997, Lawand et al., 1997).
In our first study (see Walker et al., 2001), we examined the effects of the selective mGlu5 receptor antagonist, 2-methyl-6-(phenylethynyl)-pyridine (MPEP: Gasparini et al., 1999) in models of pain. These experiments demonstrated that systemic administration of MPEP produced effective reversal of the hyperalgesia associated with inflammation, without affecting the normal behavioural responses to noxious stimulation in naive rats (see Walker et al., 2001).
The expression of the mGlu5 receptor in various regions of the brain and spinal cord has been studied in detail (Shigemoto et al., 1993, Romano et al., 1995, Valerio et al., 1997, Boxall et al., 1998b). Interestingly, Valerio and colleagues have shown that mGlu5 receptor protein is also expressed in small diameter rat DRG cells, and the superficial layers of the rat dorsal horn, suggesting that mGlu5 receptors are expressed at the central terminals of nociceptive afferents (Valerio et al., 1997). However, we hypothesized that mGlu5 receptors might also be functionally expressed at the peripheral terminals of primary nociceptive afferent neurones.
In this study we carried out pharmacological, electrophysiological and immunohistochemical experiments to further explore the possible site and mechanism of mGlu5 receptor-mediated antihyperalgesia. The purpose of these experiments was to: (1) determine the local CNS or peripheral site of action of MPEP that produces the most effective reversal of inflammatory hyperalgesia; (2) provide pharmacological evidence for the function of mGlu5 receptors, and their role in nociceptive processes, outside the CNS; and (3) provide immunohistological evidence for the expression of mGlu5 receptors by neurones in rat skin.
Section snippets
Drugs
MPEP was synthesized as described previously (Gasparini et al., 1999), and was used in all experiments as its hydrochloride salt (MW=229.7). (RS)-2-chloro-5-hydroxyphenylglycine (CHPG, MW=201.61), 3,5-dihydroxyphenylglycine (DHPG, MW=183.16), α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA, MW=186.17), N-methyl-d-aspartic acid (NMDA, MW=147.13), (S)-4-carboxy-phenylglycine (4-CPG, MW=195.17) and l-(+)-2-amino-4-phosphonobutyric acid (L-AP4, MW=183.1) are all described by Schoepp and
Effects of central vs. peripherally administered MPEP in rat models of inflammatory pain
The central vs. peripheral effects of MPEP were compared by examining the reversal of FCA-induced mechanical hyperalgesia produced by the local microinjection of MPEP via i.c.v., i.t. or i.pl. routes. I.c.v. or i.t. administration of morphine (10 nmole) reversed FCA-induced hyperalgesia 1 h following administration (Fig. 1(a)). In contrast, neither i.t. nor i.c.v. microinjection of MPEP (10–300 nmole) produced a significant effect on paw withdrawal thresholds in the inflamed rat hind paw 1 or 3
Discussion
The expression of the mGlu5 receptor in various regions of the brain and spinal cord has been studied in detail (Shigemoto et al., 1993, Romano et al., 1995, Valerio et al., 1997, Boxall et al., 1998a). In the rat spinal cord mGlu5 receptors are predominantly expressed in the superficial layers of the dorsal horn where nociceptive primary afferent neurons terminate (Valerio et al., 1997, Berthele et al., 1999, Jia et al., 1999). We compared the central vs. peripheral effects of MPEP by
Acknowledgements
We would like to thank Prof. Roderick Scott, University of Aberdeen, for his help during the preparation of this manuscript.
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