DIFFERENTIAL ANTINOCICEPTIVE EFFECTS OF SPINAL OPIOIDS ON FOOT WITHDRAWAL RESPONSES EVOKED BY C-FIBER OR A-DELTA NOCICEPTOR ACTIVATION

1 Intrathecal application of mu, delta, and kappa opioids attenuate re sponses on several tests of animal nociception. However, the potency o f 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 dif ferentially sensitive to opiate inhibition To examine this hypothesis, we used a recently developed behavioural test which allows for differ ential assessment of nociception evoked by the activation of myelinate d (A delta) and unmyelinated C thermonociceptors. 2 Administration of a kappa-selective agonist was ineffective on either type of response. delta(1) drugs were slightly more potent on C fibre-mediated responses than on A delta-mediated responses. 3 Intrathecal mu and delta(2) dru gs were antinociceptive on both A delta and C nociceptor-mediated resp onses. However. unlike the delta(1) effects, the dose-response curves for mu and delta(2) drugs were significantly more steep for A delta th an for C fibre-mediated responses, potentially indicating differences in the mechanisms by which the drugs act on these 2 response types.