[The effect of serotonin on pain sensitivity and the nociceptive reactions of the blood pressure].

When injected intravenously (1 mg/kg) and intrathecally (100 micrograms) to rats, serotonin produced an analgesic effect in thermal pain and failed to affect nociceptive responses under electric irritation. When given in the analgesic dose, it unchanged the baseline level of blood pressure, but enhanced its nociceptive pressor responses. Naloxone (0.1 mg/kg) reduced the sedative action of serotonin and prevented the increase of pressor responses of blood pressure induced by its intrathecal administration.