Test-dependent relationship of the antidepressant and analgesic effects of amitriptyline

Antidepressants have been found to be of value in the treatment of pain of various etiologies. Nevertheless, the data are conflicting as it is often difficult to distinguish between the analgesic and antidepressant action. The analgesic effect of acutely administered amitriptyline at doses of 2.5, 5, 10, 20 and 40 mg/kg was investigated in four nociceptive tests involving physical (hot plate and tail flick tests), or noxious chemical stimuli (acetic acid and formalin tests). Relationships were established between the analgesic actions and the antidepressant effect of acutely administered amitriptyline at doses of 2.5, 5, 10 and 20 mg/kg in the forced swimming test. The results demonstrated a relationship between the antidepressant effect and the analgesic action in the tail flick test, but not in the hot plate, acetic acid and formalin tests. Thus, the type of noxious stimulus may be a determining factor in the relationship between these two pharmacological actions.