administration of L-threo-3,4-dihydroxyphenylserine (droxidopa, L-threo-DOPS; L-DOPS), a noradrenaline precursor, at a dose-range of 100-800 mg kg-', produced naloxoneresistant antinociception in a dose-dependent manner in the mouse, as assessed by the tail flick test, kaolin-induced writhing test and formalin-induced nociception test. 2 Antinociception elicited by L-DOPS (400 mg kg-', s.c.) was not affected by s.c. injection of benserazide, a peripherally preferential L-aromatic amino acid decarboxylase inhibitor, but was suppressed by its intracerebroventricular (i.c.v.) injection. 3 I.c.v. or intrathecal (i.t.) administration of the non-selective a-blocker, phentolamine, significantly reduced L-DOPS-induced antinociception. 4 I.c.v. administration of the a1-blocker, prazosin, but not the a2-blocker, yohimbine, abolished the antinociceptive effects of L-DOPS. In contrast, both blockers, when administered i.t., exhibited significant inhibitory effects. 5 These results suggest that systemic L-DOPS produces opioid-independent antinociception, mediated by supraspinal a1-adrenoceptors and by spinal a1-and a2-adrenoceptors and may predict additional therapeutic applications of L-DOPS as an analgesic.
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