European journal of pharmacology
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The intrathecal (i.t.) injection of 100 nmol of oxymetazoline to male, Sprague-Dawley rats significantly increased tail flick latency and paw pressure threshold for 10 h as compared to i.t. saline-treated rats. Oxymetazoline-induced antinociception was accompanied by a 2 degree C decrease in rectal temperature and a delayed but mild sedative effect. Intrathecal phentolamine (50 micrograms), injected 8 h after i.t. oxymetazoline, completely reversed the analgesic and hypothermic effects but did not affect sedation. ⋯ For i.t. morphine alone, the ED50 and 95% confidence interval (95% CI) was 3.8 nmol (2.8-5.6) in the tail flick test and 7.7 nmol (5.4-12.8) in the paw pressure test. In the combination, the ED50 (95% CI) of i.t. morphine was 0.7 nmol (0.6-0.8) in the tail flick test and 1.2 nmol (1.1-1.4) in the paw pressure test, corresponding to an approximate 6-fold increase in potency. The data indicate that: (1) the antinociceptive and hypothermic effects of i.t. oxymetazoline at 8 h are mediated by spinal alpha-adrenoceptors; (2) peripheral sites, and probably supraspinal sites, do not contribute to i.t. oxymetazoline-induced antinociception [corrected]; and (3) oxymetazoline potentiates the analgesic effects of morphine in the spinal cord of the naive rat.