Analgesic effects of intrathecally-applied alpha 2-adrenoceptor agonists in conscious, unrestrained sheep.

Intrathecal injections of small volumes of the alpha 2-adrenoceptor agonists, xylazine and clonidine, into the cervical region of the spinal cord of conscious unrestrained sheep produced a dose-dependent analgesia of the forelimbs as measured using a mechanical pressure device. Intravenous injection of the alpha 2-adrenoceptor antagonist, idazoxan completely abolished the analgesic effects of the intrathecally applied alpha 2-adrenoceptor agonists. Subsequent studies using [3H] clonidine injected at a similar dose and volume via the intrathecal catheters, indicated that the volume of drug used, 100 microliter, gave a localisation of the drug limited to about five vertebral segments around the catheter tip.