Seasonal and circadian variations of behavioural response to antidepressants in the forced swimming test in rats.

In order to verify whether the variability reported to occur in the effectiveness of antidepressants in the forced swimming test (FST) was due to seasonal and/or circadian variations, the anti-immobility effect induced by several antidepressants was assessed in rats every month throughout the year or every 4 hours for 2 consecutive days. Amineptine (20 mg/kg), amitriptyline (15 mg/kg), desipramine (20 mg/kg), imipramine (15 mg/kg) and mianserin (15 mg/kg) exerted the maximal effect in March. The effect of nomifensine (5 mg/kg) was constant throughout the year. Unlike imipramine and mianserin, nomifensine induced an anti-immobility effect that was maximal during the light phase of the light/dark cycle and was constant in continuous darkness.