Receptor interactions with n-type calcium channels in the modulation of neuropathic pain

Abstract

This work was undertaken to investigate the effectiveness and mechanism of action of agents that interact with the N-Type Voltage Gated Calcium Channel (VGCC) in acute and neuropathic pain in a rat model. A range of agents was employed that enabled characterization of receptor targets, sites of action and levels of interaction between drugs. A well-known technique was used for establishment of neuropathy (spinal nerve ligation) , and a modified intrathecal catheterisation technique was developed to minimize spinal cord trauma. Investigations into strategies for improving the likelihood of surgical neuropathy development were undertaken. Initial studies used spinally administered selective N-type VGCC blockers, the co¬conotoxins, and compared them with morphine. These demonstrated the efficacy of co-conotoxins in neuropathic pain (tactile allodynia). An advantage of co-conotoxin CVID over other members of this class of drug (co-conotoxins GVIA and MVIIA) was also shown by greater separation of therapeutic and toxic doses and also by synergism with morphine. Furthermore, unlike morphine, the co-conotoxins resulted in differential antinociception because they did not affect acute nociception (thermal tail flick latency). These findings suggest that neuropathic pain is conducted via different pathways in the spinal cord compared with acute pain, and that selective antinociception is possible. Cannabinoids were also investigated due to their antinociceptive effect which is known to be at least patdy mediated via coupling with N-type VGCCs. The non-selective synthetic cannabinoid CP55,940 was investigated amongst others (WIN55,212 and AM577). Antinociceptive efficacy was demonstrated for CP55,940 in both acute and neuropathic pain, with the spinal route being more effective than peripheral administration. Adverse behavioural effects occurred at doses similar to those for antinociception, and thus low-dose interactions were explored with morphine and 0)¬conotox1n CVID showing synergism with tactile allodynia and also tail flick antinociception. The receptor targets were assessed using highly selective antagonists to CB-l and CB-2 receptors. Antinociception to acute pain was found to be mediated by CB-l receptors located at spinal and supraspinal sites and peripheral CB-2 sites. In contrast, antinociception to neuropathic pain was found to be contributed to by spinal CB-l and peripheral CB-2 sites. As CB-2 agonists have few behavioural side effects, this represents an important finding. The interaction of O)-conotox1n CVID and an alpha-2 adtenoceptor agonist, dexmedetomidine was also assessed (in collaboration with Professor Duncan Blake). Dexmedetomidine was less potent than CVID in tactile allodynia antinociception, but there was a synergistic interaction between the two. Alpha-2 adtenoceptor pathways ate associated with central cannabinoid mediated antinociception and thus a further lead for multimodal therapy was demonstrated.