Cannabinoid suppression of noxious heat-evoked activity in wide dynamic range neurons in the lumbar dorsal horn of the rat.

The effects of cannabinoid agonists on noxious heat-evoked firing of 62 spinal wide dynamic range (WDR) neurons were examined in urethan-anesthetized rats (1 cell/animal). Noxious thermal stimulation was applied with a Peltier device to the receptive fields in the ipsilateral hindpaw of isolated WDR neurons. To assess the site of action, cannabinoids were administered systemically in intact and spinally transected rats and intraventricularly. Both the aminoalkylindole cannabinoid WIN55,212-2 (125 microg/kg iv) and the bicyclic cannabinoid CP55,940 (125 microg/kg iv) suppressed noxious heat-evoked activity. Responses evoked by mild pressure in nonnociceptive neurons were not altered by CP55,940 (125 microg/kg iv), consistent with previous observations with another cannabinoid agonist, WIN55,212-2. The cannabinoid induced-suppression of noxious heat-evoked activity was blocked by pretreatment with SR141716A (1 mg/kg iv), a competitive antagonist for central cannabinoid CB1 receptors. By contrast, intravenous administration of either vehicle or the receptor-inactive enantiomer WIN55,212-3 (125 microg/kg) failed to alter noxious heat-evoked activity. The suppression of noxious heat-evoked activity induced by WIN55,212-2 in the lumbar dorsal horn of intact animals was markedly attenuated in spinal rats. Moreover, intraventricular administration of WIN55,212-2 suppressed noxious heat-evoked activity in spinal WDR neurons. By contrast, both vehicle and enantiomer were inactive. These findings suggest that cannabinoids selectively modulate the activity of nociceptive neurons in the spinal dorsal horn by actions at CB1 receptors. This modulation represents a suppression of pain neurotransmission because the inhibitory effects are selective for pain-sensitive neurons and are observed with different modalities of noxious stimulation. The data also provide converging lines of evidence for a role for descending antinociceptive mechanisms in cannabinoid modulation of spinal nociceptive processing.