We have previously shown that

We have previously shown that the antinociceptive effect of tramadol, an analgesic that, like paracetamol is able to increase serotonin levels within CNS, is potentiated or antagonized respectively by a 5-HT1A/B nonspecific gw web synthesis blockade or activation (Rojas-Corrales et al., 2000). Moreover, it has been shown that the antinociceptive effect of clomipramine, 5-HT and NA re-uptake inhibitor, is also enhanced by the specific blockade of 5-HT1A receptors (Ardid et al., 2001). In other study, we have shown that the selective blockade of the 5-HT1A or 5-HT1B potentiate the antinociceptive effect of paracetamol in the hot plate test, while this antinociceptive effect of paracetamol can antagonized by specific agonist of these autoreceptors, 5-HT1A and 5-HT1B (Roca-Vinardell et al., 2003). The hot plate test is one of the most commonly used tests of analgesic measure of analgesic drugs that act at the level of spine and higher centres (Vogel, 2002). As both central as well as peripheral mechanisms of paracetamol has been proposed, in the current study we employed the formalin test to assess the effect of blockade or activation of 5-HT1A or 5-HT1B receptors, by specific antagonist or agonist, on antinociceptive action of paracetamol in rats.
Material and methods
Results
Discussion In this study, we evaluated whether the 5-HT1A and 5-HT1B receptors are involved in the antinociceptive effect of paracetamol in the rat model of formalin-induced pain. Flinches were used to quantify formalin-induced behaviours since they provide a reliable correlation of pain in the awake, freely moving rat. The behavioural response to the injection of formalin is biphasic, with an acute phase followed by tonic phase. It has been suggested that the early phase is caused by a direct effect of formalin on nociceptors, whereas the second phase is due to an inflammatory process (Le Bars et al., 2001). Therefore, the antinociceptive activity of paracetamol can be evaluated immediately after formalin injection. Our results showed a weak antinociceptive effect at doses of 125 and 250 mg/kg of paracetamol, and a strong analgesic effect at doses of 500 and 1000 mg/kg of paracetamol in both phases. In line with previously published data, our results confirmed that paracetamol is able to induce a dose-dependent antinociceptive activity in the formalin test in rat (Dogrul et al., 2012, Gong et al., 2011). To study whether the 5-HT1A and 5-HT1B receptors are involved in the antinociceptive effect of paracetamol, we evaluated if the blockade of the 5-HT1A or 5-HT1B autoreceptors by different antagonists (WAY 100,635 and SB 216,641, respectively) can potentiate the antinociceptive effect of paracetamol and, in contrast, the activation of the 5-HT1A or 5-HT1B by different agonists (8-OH-DPAT and CP 93,129, respectively) reduced the antinociceptive effect induced by paracetamol. Our data clearly show that WAY 100,635, selective 5-HT1A antagonist, potentiates the antinociceptive effect of paracetamol in the formalin test. While 8-OH-DPAT, selective 5-HT1A agonist, reduced its analgesic effect in the same test. However, our results not show a clear effect on the antinociceptive effect of paracetamol when is administered SB 216,641, selective 5-HT1B antagonist, or CP 93,129, selective 5-HT1B agonist. The 5-HT1A receptors have a somatodendritic location on 5-HT neurons of the midbrain raphe nuclei (autoreceptors) and on neurons postsynaptic to 5-HT nerve terminals, mainly in cortico-limbic areas that exerts a pronounced inhibitory influence upon the release of 5-HT throughout the CNS. Also, 5-HT1A can be localised at the spinal cord, a diversity of analgesiometric paradigms has been employed and numerous behavioural studies have reported hyperalgesia upon spinal administration (Alhaider and Wilcox, 1993, Bardin et al., 2000). Stimulation of 5-HT1A receptors also attenuates induction of antinociception by the antidepressant, clomipramine (Ardid et al., 2001). While some authors (Bardin et al., 2003, Colpaert et al., 2002) have demonstrated that a 5-HT1A agonist, F13640, induced central analgesia in different analgesimetric test. Our results showed that the 5-HT1A antagonist, WAY 100,635, induced an increase of antinociceptive effect of paracetamol in the formalin test at a low dose. These results are supported by different studies showing that alprenolol and WAY 100,635 induced antinociception in the writhing test mice (Millan, 1994, Millan et al., 1996). Also, other study show paracetamol or venlafaxine with WAY 100,635 led to a significant antinociceptive effect (Bonnefont et al., 2005). A recent study, show the role of 5-HT1A in the antinociceptive effect of paracetamol, but suggest that spinal 5-HT7 receptors are involved in a central antinociceptive and antihyperalgesic effect of paracetamol (Dogrul et al., 2012). However, these results are conflicting, because some studies have shown that 5-HT1A agonists induced antinociception. For instance, the antinociceptive effect of several 5-HT1A agonist, as 8-OH-DPAT, has been demonstrated in the formalin test (Granados-Soto et al., 2010) as well as the antinociceptive effect of buspirone increased the licking latency in the hot-plate test in mice (Liang et al., 2003). These data seem to indicate that the results obtained in the experiments depends on the nature of the noxious stimuli and, consequently, to the nature of the afferent fibre involved and the administration route of the drug, thus, the serotonergic system pharmacologic is very complex in controlling nociceptive pathways.