Antinociceptive effects of trazodone and m-chlorophenylpiperazine (mCPP) in mice: interaction with morphine.

1. The antidepressant trazodone and its main metabolite m-chlorophenylpiperazine (mCPP) were investigated for their analgesic properties and their sensitivity to a threshold dose of morphine in acetic acid abdominal constriction and hot plate tests. 2. The drugs elicited hypoalgesic effects at about the same doses in the two analgesic assays. 3. Naloxone (2 mg/kg i.p.) prevented the hypoalgesia of trazodone but not of mCPP in the hot plate test. The opiate antagonist did not affect the responses of both drugs to the writhing test. 4. Subanalgesic doses of the two drugs increased the sensitivity to morphine in both assays. The results further support the suggested role played by opioid and 5-HT systems on depression.