Differential antinociceptive effects of spinal opioids on foot withdrawal responses evoked by C fibre or A delta nociceptor activation.

1. Intrathecal application of mu, delta, and kappa opioids attenuate responses on several tests of animal nociception. However, the potency of these opioids differ depending on which tests were used. One factor contributing to these discrepancies is that different types of noxious stimuli activate different sets of nociceptor types, which may be differentially sensitive to opiate inhibition. To examine this hypothesis, we used a recently developed behavioural test which allows for differential assessment of nociception evoked by the activation of myelinated (A delta) and unmyelinated C thermonociceptors. 2. Administration of a kappa-selective agonist was ineffective on either type of response. Delta1 drugs were slightly more potent on C fibre-mediated responses than on A delta-mediated responses. 3. Intrathecal mu and delta2 drugs were antinociceptive on both A delta and C nociceptor-mediated responses. However, unlike the delta1 effects, the dose-response curves for mu and delta2 drugs were significantly more steep for A delta than for C fibre-mediated responses, potentially indicating differences in the mechanisms by which the drugs act on these 2 response types.