Systemic paracetamol-induced analgesic and antihyperalgesic effects through activation of descending serotonergic pathways involving spinal 5-HT7 receptors

Although some studies have shown the essential role of descending serotonergic pathways and spinal 5-HT1A, 5-HT2A, or 5-HT3 receptors in the antinociceptive effects of paracetamol, other studies have presented conflicting results, and the particular subtype of spinal 5-HT receptors involved in paracetamol-induced analgesia remains to be clarified. Recent studies have demonstrated the importance of spinal 5-HT7 receptors in descending serotonergic pain inhibitory pathways. In this study, we investigated the role of descending serotonergic pathways and spinal 5-HT7 receptors compared with 5-HT3 and 5-HT2A receptors in the antinociceptive and antihyperalgesic effects of paracetamol. Tail-flick, hot plate and plantar incision tests were used to determine nociception in male BALB/c mice. Lesion of serotonergic bulbospinal pathways was performed by intrathecal (i.th.) injection of 5,7-dihydroxytryptamine (5,7-DHT), and spinal 5-HT levels were measured by HPLC. To evaluate the particular subtypes of the spinal 5-HT receptors, the selective 5-HT7, 5-HT3 and 5HT(2A) receptor antagonists SB 269970, ondansetron and ketanserin, respectively, were given i.th. after oral administration of paracetamol. Oral paracetamol (200, 400 and 600 mg/kg) elicits dose-dependent antinociceptive and antihyperalgesic effects. I.th. pretreatment with 5,7-DHT (50 mu g) sharply reduced 5-HT levels in the spinal cord. Depletion of spinal 5-HT totally abolished the antinociceptive and antihyperalgesic effects of paracetamol. I.th. injection of SB 2669970 (10 mu g) blocked the antinociceptive and antihyperalgesic effects of paracetamol, but ondansetron and ketanserin (10 mu g) did not. Our findings suggest that systemic administration of paracetamol may activate descending serotonergic pathways and spinal 5-HT7 receptors to produce a central antinociceptive and antihyperalgesic effects. (C) 2011 Elsevier B.V. All rights reserved.