Antinociceptive effects of the hydrophilic alpha 2-adrenoceptor agonist ST-91 in different test circumstances after intrathecal administration to Wistar rats.

The antinociceptive and motor effects of the hydrophilic alpha 2-adrenoceptor agonist ST-91 were studied after intrathecal administration to male Wistar rats in different heat-pain tests and different test settings. Intrathecal administration of ST-91 caused a dose-dependent increase in hind paw licking latency in the hot-plate test, while in contrast with morphine it had a much lower efficacy in the tail-flick test in freely moving conditions. Sprague-Dawley rats gave similar results to those for Wistar rats in this setting. However, when the tail-flick test was performed under chronic restraint conditions (after a 1-h restraining period), the compound caused a significant antinociception. No signs of motor impairment and no changes in electromyographic activity were detected after ST-91 administration. The results indicate a characteristic analgesic profile for ST-91. In the interpretation of ST-91 data, consideration should be paid both to test model differences and to test conditions.