Participación de los receptores 5-HT3 en el desarrollo y mantenimiento de la hiperalgesia y alodinia secundarias inducidas con formalina

Abstract

The 5-HT3 receptor belongs to the super family of ligand gated ion channels, consists of five subunits that are assembled so as to form a central ion channel. In human 5 subunits have been cloned (5-HT3A/B/C/D/E/Ea) however, only the homopentámers (five 5-HT3A subunits) and heteromers (two 5-HT3A subunits and three 5-HT3B subunits forming the receptor 5-HT3AB) have been characterized and considered functional. In the nervous system, the mRNA encoding 5-HT3 receptor and the protein for this receptor have been detected in the dorsal root ganglia and the dorsal horn of the spinal cord. On the other hand, through electrophysiological studies is known that 5-HT3 receptor leads to influx of non-selective cations (Na+, K+, Ca2+) and it excludes anions, allowing depolarization of the membrane, which has been linked with nociceptive transmission. In this regard, it´s known that tissue injury releases chemical mediators including 5-hidroxytryptamine (serotonin), which participate in the nociceptive transmission and induction of allodynia and hyperalgesia through its different receptors. The aim of this study was assess the role of peripheral and spinal 5-HT3 receptors on formalin-induced secondary allodynia and hyperalgesia. Female Wistar rats with body weight between 180-220 g were injected with 50 μl of formalin (0.5 or 1%) into the dorsal surface of right hind paw. For local peripheral and spinal evaluation rats received a subcutaneous or intrathecal injection, respectively, of 5-HT or m-chlorophenylbiguanide (mCPBG, selective agonist of 5-HT3 receptor) or ondansetron (selective antagonist of 5-HT3 receptor) 10 min before administration of formalin. We also performed co-administration of each agonist to antagonist. Also, the ondansetron and mCPBG were administered 6 days after the formalin injection. We recorded the average of both paws withdrawals when stimulated on the plantar surface with von Frey filaments of 10 to 250 mN. Formalin (0.5 or 1%) produced acute nociceptive behaviors (flinching and licking) followed by long-term secondary mechanical allodynia and hyperalgesia. Ipsilateral peripheral pre-treatment (- 10 min) with exogenous serotonin (30-100 nmol/paw) or the selective 5-HT3 receptor agonist (30-300 nmol/paw) significantly increased secondary allodynia and hyperalgesia in both paws on day 6 after injection of formalin. In contrast, ipsilateral peripheral pre-treatment with the selective 5-HT3 antagonist ondansetron (300 nmol/paw) significantly prevented formalin-induced secondary allodynia and hyperalgesia in both paws. The pronociceptive effect of serotonin (100 nmol/paw) or mCPBG (300 nmol/paw) was completely prevented by ondansetron (10 nmol/paw). However, ipsilateral peripheral post-treatment with ondansetron (300 nmol/paw) did not affect formalin-induced secondary mechanical allodynia and hyperalgesia. On the other hand, we assessed the role of spinal 5-HT3 receptors in the development and maintenance of these long-term behaviors. Spinal pre-treatment (- 10 min) with selective 5-HT3 receptor agonist mCPBG (300 nmol/rat) dose-dependently increased secondary allodynia and hyperalgesia in both paws on day 6 after peripheral injection of formalin. In constrast, spinal pre-treatment with the selective 5-HT3 antagonist ondansetron (30-100 nmol/rat) significantly prevented formalin-induced secondary allodynia and hyperalgesia in both paws. The pronociceptive effect of mCPBG (300 nmol/rat) was completely prevented by ondansetron (1 nmol/rat). Spinal post-treatment with ondansetron (30-300 nmol/rat) dose-dependently reversed formalin-induced secondary allodynia and hyperalgesia in both paws. Overall results suggest that a barrage of afferent input induced by 5-HT at peripheral 5-HT3 receptors participate in the development and but not in maintenance of these long-term behaviors, however the spinal 5-HT3 receptors have an important role in the development and maintenance of formalin-induced long-term secondary allodynia and hyperalgesia in the rat.