The role of VIP/PACAP receptor subtypes in spinal somatosensory processing in rats with an experimental peripheral mononeuropathy

Abstract

Peripheral nerve damage often results in the development of chronic pain states, resistant to classical analgesics. Since vasoactive intestinal polypeptide (VIP) and pituitary adenylate cyclase-activating polypeptide (PACAP) are up-regulated in dorsal root ganglion cells following peripheral nerve injury, we investigated the expression and influence of VPAC₁, VPAC₂ and PAC₁ receptors in rat spinal dorsal horn following a chronic constriction injury (CCI). Electrophysiological studies revealed that selective antagonists of VPAC₁, VPAC₂ and PAC₁ receptors inhibit mustard oil-, but not brush-induced activity of dorsal horn neurones in CCI animals, while cold-induced neuronal activity was attenuated by VPAC₁ and PAC₁, but not VPAC₂ receptor antagonists. Ionophoresis of selective agonists for the receptor subtypes revealed that the VPAC₂ receptor agonist excited twice as many cells in CCI compared to normal animals, while the number of cells excited by the VPAC₁ receptor agonist decreased and responses to PACAP-38 remained unchanged. In situ hybridisation histochemistry (ISHH) confirmed an increase in the expression of VPAC₂ receptor mRNA within the ipsilateral dorsal horn following neuropathy, while VPAC₁ receptor mRNA was seen to decrease and that for PAC₁ receptors remained unchanged. These data indicate that VIP/PACAP receptors may be important regulatory factors in neuropathic pain states.

Introduction

Neuropathic pain arising from peripheral tissue damage or direct trauma to nerve, is characterised by lasting spontaneous pain, allodynia, and mechanical and thermal hyperalgesia (Bonica, 1990). The mechanisms underlying neuropathic pain are poorly understood, but inflammatory mediators released at the periphery (Tracey and Walker, 1995), central changes within the spinal dorsal horn (Woolf et al., 1995), and biochemical and structural changes within the damaged nerve itself (Coggeshall et al., 1993, Hokfelt et al., 1994) are all thought to contribute.

The chronic constriction injury (CCI) model of neuropathic pain (produced by loose ligation of the rat common sciatic nerve with chromic cat gut ligatures) brings about lowered response thresholds and exaggerated nociceptive responses to mechanical, thermal and chemical stimuli in the affected hindlimb (Bennett and Xie, 1988, Attal et al., 1990). Cold allodynia appears to be a striking feature of neuropathic pain in humans (Frost et al., 1988), yet very little is known about the underlying mechanisms. Unlike normal rats, the majority of animals with a CCI show aversive behavioural responses to cutaneous thermal stimuli in the range 0–5°C (Bennett and Xie, 1988, Attal et al., 1990), signifying a change in sensitivity.

Following peripheral nerve injury, a number of characteristic changes in neuropeptide expression occur in the dorsal root ganglia (DRG) and spinal cord (Hokfelt et al., 1994). The expression of vasoactive intestinal polypeptide (VIP), pituitary adenylate cyclase-activating polypeptide (PACAP), neuropeptide Y, galanin and cholecystokinin increases (Villar et al., 1989, Nahin et al., 1994, Zhang et al., 1995a, Zhang et al., 1995b) while the levels of other peptides such as calcitonin gene-related polypeptide (CGRP), substance P (SP) and somatostatin are seen to decrease (Villar et al., 1989, Nahin et al., 1994, Kajander and Xu, 1995).

VIP and the two alternatively processed forms of the PACAP precursor (PACAP-27 and PACAP-38) are recognised by a family of three G protein-coupled receptors: namely the PACAP or PAC₁ receptor, which displays a much greater affinity for the two forms of PACAP than for VIP (Shivers et al., 1991, Hashimoto et al., 1993), and the VIP₁ or VPAC₁ (Ishihara et al., 1992) and VIP₂ or VPAC₂ (Lutz et al., 1993) receptors which display no marked selectivity for any one of the peptide ligands. These receptors appear to have distinct distributions throughout the peripheral and central nervous systems (Cauvin et al., 1991, Usdin et al., 1994), suggesting that they may have distinct functional roles.

In a previous study, we have shown both VIP and PACAP to play a role in modulating sensory processing within the spinal dorsal horn of normal animals (Dickinson et al., 1997), despite the fact that these ligands are normally present in low abundance (Nahin et al., 1994, Zhang et al., 1995a). Here we investigated whether the role of these peptides changes in neuropathic animals, when their levels are markedly increased. Of particular interest is the possible role these peptides may play during the development of hyperalgesia and allodynia to normally innocuous mechanical and cold stimuli. The development of new agonists and antagonists with significant selectivity for the three receptor subtypes has enabled us to address this in electrophysiological experiments, while parallel in situ hybridisation histochemistry (ISHH) studies compared the distribution of mRNA for the three receptor subtypes in the dorsal horn of neuropathic and control animals.