Characterization of the 5-HT(7) receptor as a new therapeutic target for the treatment of pain

• Autores: Alex Brenchat Barberà
• Directores de la Tesis: José Miguel Vela Hernández (dir. tes.), Antonio Zorzano Olarte (dir. tes.), Luz Romero Alonso (dir. tes.)
• Lectura: En la Universitat de Barcelona ( España ) en 2012
• Idioma: español
• Tribunal Calificador de la Tesis: Xavier Navarro Acebes (presid.), Carme Lluís Biset (secret.), Miquel Martín Sánchez (voc.)
• Materias:
  • Química
    • Bioquímica
  • Ciencias de la vida
    • Biología molecular
  • Ciencias médicas
    • Farmacodinámica
      • Lugar de acción activa, receptores
    • Farmacología
• Enlaces
  • Tesis en acceso abierto en:  TDX  Dipòsit Digital de la UB 
• Resumen

  • The work showed in this Thesis has been part of the “5-HT7 and neuropathic pain” project in the pharmaceutical company Esteve. Thus, the aim of this Thesis was in line with the goal of the 5-HT7 project at Esteve, focused on drug discovery of 5-HT7 receptor ligands for the treatment of neuropathic pain. Taking the advantage of a genetic approach (5-HT7 receptor knockout mice) and pharmacological tools (5-HT7 receptor ligands) we investigated at the preclinical level the role of 5-HT7 receptors in nociception and the therapeutic interest of 5-HT7 receptor ligands on pain treatment. The 5-HT7 receptor ligands used were SB-258719 and SB-269970 as 5- HT7 receptor antagonists, and AS-19, MSD-5a, E-55888 and E-57431 as 5-HT7 receptor agonists. E-55888 and E-57431 developed by Esteve were described for the first time and their binding profile and functionality (cAMP formation) were examined. In vivo behavioural studies were performed in mice and rats subjected to nociceptive, inflammatory, neurogenic or neuropathic pain conditions. Our results showed that 5-HT7 receptors per se were not involved in the nociceptive response to a normally noxius stimulus, although when co-activated together with opioid receptors potentiated the opioidergic analgesic response in nociceptive pain conditions. Indeed, 5-HT7 receptor knockout and wild-type mice showed similar sensitivity to a noxious heat stimulus, and systemic administration of the 5-HT7 receptor agonist E-55888 or the 5-HT7 receptor antagonist SB-258719 showed no effects on acute nociceptive pain using the tail flick test in mice. However, the 5-HT7 receptor agonist E-55888 enhanced the morphine-induced analgesia in this test and this potentiation was significantly reversed by the 5-HT7 receptor antagonist SB-258719. On the other hand, we studied the role of 5-HT7 receptors in pain conditions involving central sensitization. We showed that the 5-HT7 receptor agonists AS-19, E-55888 and E-57431 inhibited capsaicin-induced mechanical hypersensitivity, nerve injury-induced mechanical and thermal hypersensitivity and reduced the phase II formalin-induced nociception. In contrast, a promotion of mechanical hypersensitivity after administration of the 5-HT7 receptor antagonists SB-258719 and SB-269970 was observed. This reduction of hypersensitivity by agonists and promotion of hypersensitivity by antagonists was reversed by antagonists and agonists, respectively. It is important to note that effectiveness of the treatment with 5-HT7 receptor agonists was not masked by non-specific motor effects, as no motor incoordination was found in the rota-rod test at the doses used and no tolerance to the effect was evidenced following repeated systemic administrations. The antinociceptive effects exerted by systemic 5-HT7 receptor agonists seemed to be mediated by 5-HT7 receptors localized in the spinal cord. We found that intrathecal administration of the 5-HT7 receptor agonist E-57431 inhibited mechanical hypersensitivity secondary to capsaicin injection and nerve injury-induced mechanical hypersensitivity. In contrast, a pronociceptive effect was observed after local intraplantar injection of the selective 5-HT7 receptor agonist E-57431 in the capsaicin model. Thus, the antinociceptive role mediated by central 5-HT7 receptors seems to predominate over their pronociceptive role at the periphery, resulting in an overall analgesic effect when 5-HT7 receptor agonists are administered by a systemic route. In line with these spinal antinociceptive effects, we found an increased immunoreactivity of 5-HT7 receptors in the ipsilateral dorsal horn of the spinal cord in sciatic nerve-injured mice. This increased 5-HT7 receptor expression in the dorsal horn induced by nerve injury could represent a physiological, compensatory, protective spinal mechanism relevant to the control of nociception in neuropathic pain conditions. We observed that 5-HT7 receptors co-localized with GABAergic neurons in the ipsilateral dorsal horn of the spinal cord. Therefore, we suggested that an indirect action through activation of 5-HT7 receptors localized on inhibitory interneurons may be responsible of the antinociceptive effects observed after administration of 5-HT7 receptor agonists. Finally, using 5-HT7 receptor knockout mice, we demonstrated that the 5-HT7 receptor agonists AS-19, E-55888 and E-57431 exerted in vivo target specific effects on pain control. We observed that systemic administration of these 5-HT7 receptor agonists reduced phase II formalin-induced nociception in wild-type but not in 5-HT7 receptor knockout mice. Taken together, these data add a piece of knowledge to the role played by 5-HT7 receptors in the control of pain and point to a new potential use of 5-HT7 receptor agonists as promising drugs for the treatment of neuropathic pain.