Evidence for the involvement of serotonin in the antinociception induced by electrical or chemical stimulation of the mesencephalic tectum.

A great deal of evidence has shown that electrical stimulation or microinjections of GABAA blockers, such as bicuculline, into the midbrain tectum (MT) produce escape behavior, which has been associated to fear. This study was aimed to examine the characteristics of the analgesia that follows the escape behavior induced by electrical (freezing and escape thresholds) and chemical (bicuculline microinjections) stimulation of the midbrain tectum. Immediately after the expression of the aversive responses the rats were submitted to the tail-flick test. The obtained results show that analgesia always follows aversive responses integrated at the MT level regardless of the kind of stimulation applied. The antinociceptive effects induced by either electrical or chemical stimulation of the MT were not antagonized by central microinjections of naloxone. On the other hand, the non-specific serotonin antagonist methysergide microinjected into the MT was effective in antagonizing the analgesia induced by any of the aversive stimulations. Based on these results we suggest that serotonin, but not opioid mechanisms, may be involved in the integration of antinociceptive responses to stimulation of the midbrain tectum.