Antinociceptive actions of intrathecal xylazine: interactions with spinal cord opioid pathways.

We have studied rats with chronically implanted subarachnoid catheters. Xylazine, an alpha 2 adrenoceptor agonist, was injected intrathecally and nociceptive thresholds measured at two skin sites: the tail and the neck. Intrathecal xylazine (dose range 24.3-389 nmol) produced increases in electrical thresholds for nociception in the tail without any change in the neck; this observation suggested that the antinociceptive action of this drug was confined to the caudal part of the spinal cord responsible for tail innervation. The magnitude of this effect was dose-dependent. Tail flick latency also increased in these rats and the antinociceptive effects were antagonized in a dose-dependent manner by the selective alpha 2 adrenoceptor antagonist idazoxan (dose range 6.7-540 nmol). Intrathecal idazoxan also suppressed the increase in tail flick latency caused by the mu opioid agonist fentanyl (0.74 nmol) given intrathecally. This effect was also dose-dependent. The idazoxan dose-response curve for this suppression of fentanyl antinociception assessed with tail flick latency was the same as that for suppression of xylazine. In contrast, the antinociceptive effects of intrathecal xylazine were not affected by concurrent administration of opioid or GABAA antagonists. We conclude that intrathecal xylazine produced spinally mediated antinociceptive effects by combination with spinal cord alpha 2 adrenoceptors and that neither opioid nor GABA-containing propriospinal neurones were involved in the mediation of this effect. However, alpha 2 adrenoceptors in the spinal cord appear to be involved with antinociception produced by intrathecal fentanyl.