MORPHINE AND NOREPINEPHRINE-INDUCED ANTINOCICEPTION AT THE SPINAL LEVEL IS MEDIATED BY ADENOSINE

THE purpose of this study was to examine whether adenosine or serotoni n is involved in mediation of the antinociception produced by norepine phrine at the spinal cord level. Aminophylline, an adenosine receptor antagonist and naloxone given intrathecally (i.t.) were used to test t he antinociception produced by i.t. norepinephrine, serotonin, morphin e or the adenosine receptor agonist 5'-N-ethylcarboxamidoadenosine (NE CA) by using the tail-flick assay in rats. It was observed that (1) am inophylline blocked the antinociception produced by norepinephrine, bu t exhibited no effect on the antinociception produced by serotonin, (2 ) aminophylline blocked the antinociception produced by morphine simil arly to naloxone, (3) aminophylline blocked the antinociception produc ed by NECA and (4) naloxone failed to block the antinociception produc ed by NECA and serotonin. The results suggest that adenosine is involv ed in mediation of the norepinephrine-produced antinociception at the spinal level and that norepinephrine and adenosine may act in a sequen tial manner in norepinephrine-induced antinociception.