Test-dependent changes in nociception after administration of the putative serotonin antagonist metitepin in mice.

Intraperitoneal administration of the putative serotonin receptor antagonist metitepin (0.06-1.0 mg/kg) in mice induced dose-dependent antinociception in the increasing temperature hot-plate test and the formalin test, but elicited hyperalgesia in the tail-flick test. Reduced motor activity was observed after injection of the largest dose of metitepin, but did not influence the behavioural responses in the tests. Selective lesions of ascending serotonergic pathways induced by administration of the neurotoxin p-chloroamphetamine 5 and 6 days before testing (40 mg/kg each day) did not directly affect the responsiveness in any of the tests but enhanced the metitepin-induced antinociception in the hot-plate and formalin tests. The hyperalgesia in the tail-flick test was not affected by the lesions. The results suggest that metitepin may alter nociception in mice by exhibiting both agonist and antagonist properties on central serotonergic receptors.