Stimulus intensity and the comparative efficacy of mu- and kappa-opioid agonists on nociceptive spinal reflexes in the rat.

The influence of stimulus intensity was tested on the relative spinal efficacy of intravenously administered mu- (fentanyl) and kappa-opioid (U-50,488) agonists. Spinal reflexes were generated by different intensities of noxious electrical stimuli in alpha-chloralose anaesthetized, spinalized rats. Both drugs became less effective as the intensity of C-fibre generated responses was increased, but U-50,488 retained the ability to reduce responses to the same degree as fentanyl. The effects were naloxone reversible. The results indicate that kappa-opioid receptor activation has similar potential for spinal analgesia as does activation of mu-opioid receptors.