The action of K-agonists on the nociceptive responses of neurones in the medullary dorsal horn of the anaesthetized rat.

Responses of medullary dorsal horn neurones to both mechanical and thermal noxious stimuli were recorded in urethane anaesthetized rats. Opiates with reported activity at K receptors (tifluadom, BL 5572M, and U50488) were found to reduce responses to both noxious stimuli, and in this respect, were indistinguishable from the mu agonist fentanyl. These observations are in contrast to the behavioural antinociceptive effects of K agonists as these substances are active in tests using mechanical noxious stimuli but in those using thermal stimuli have little effect. It is therefore possible that the modality decoding seen in behavioural experiments occurs at a supraspinal level.