Effect of CP55,940 on mechanosensory spinal neurons following chronic inflammation

Cannabinoid receptor agonists have previously been shown to produce antinociceptive effects in rodent models of inflammatory pain. In the present study, we characterized responses of spinal dorsal horn neurons receiving sensory input from the hind paw in rats that had received intraplantar injection of complete Freund's adjuvant (CFA), and examined effects of the nonselective CB1/2 receptor agonist CP55,940 on spinal neuron responses. Systemic (i.v.) administration of CP55,940 failed to attenuate responses of dorsal horn neurons to noxious mechanical stimulation in naïve rats, but significantly reduced responses in CFA-inflamed rats to 25.78 ± 13.7% of vehicle control at a cumulative dose of 0.8 mg/kg (ID50 = 0.28 ± 0.02 mg/kg). Additionally, local administration of CP55,940 (10 μM) to the spinal cord reduced responses of mechanosensory dorsal horn neurons in CFA-inflamed rats to 67.15 ± 7.1% of vehicle control. The inhibitory action of CP55,940 on spinal dorsal horn neurons in CFA-inflamed rats was mediated by CB1 receptors since local pretreatment with the CB1 receptor antagonist AM251 (10 μM) blocked this effect, while the CB2 receptor antagonist AM630 (10 μM) was ineffective. Our results suggest that following inflammation, the inhibition of spinal nociceptive transmission by CP55,940 is mediated in part by spinal CB1 receptors, and not spinal CB2 receptors.