Selective lesions of the bulbospinal serotonergic pathways reduce the analgesia induced by p-chloroamphetamine in the hot-plate test.

The antinociceptive effect of the serotonin releasing compound p-chloroamphetamine (PCA) was investigated in rats by means of the hot-plate method. Selective neurotoxic lesions of the descending serotonergic pathways induced by intrathecal injections of 5,6-dihydroxytryptamine (20 micrograms, 7 days before testing), produced hyperalgesia and significantly attenuated the analgesic effect of acutely administered PCA (2.5 mg/kg). The lesions reduced the spinal serotonin content by about 90% without significantly altering noradrenaline levels. It is concluded that the descending serotonergic pathways contribute to the expression of PCA-induced hot-plate analgesia.