THE NORADRENALINE PRECURSOR L-THREO-3,4-DIHYDROXYPHENYLSERINE EXHIBITS ANTINOCICEPTIVE ACTIVITY VIA CENTRAL ALPHA-ADRENOCEPTORS IN THE MOUSE

1 Systemic (s.c. or p.o.) administration of L-threo-3,4-dihydroxyphenylserine (droxidopa, L-threoDOPS; L-DOPS), a noradrenaline precursor, at a dose-range of 100-800 mg kg(-1), produced naloxone-resistant 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 (400mgkg(-1), 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 alpha-blocker, phentolamine, significantly reduced L-DOPS-induced antinociception. 4 I.c.v. administration of the alpha(1)-blocker, prazosin, but not the alpha(2)-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 alpha(1)-adrenoceptors and by spinal alpha 1- and alpha(2)-adrenoceptors and may predict additional therapeutic applications of L-DOPS as an analgesic.