Imipramine administered before the first of two forced swim sessions results in reduced immobility in the second session 24 h later.

A previous study investigating the effects of dopamine receptor antagonists administered before the first of two 24-h apart forced swim test (FST) sessions, provided evidence suggesting that evaluation of response efficacy - dependent on dopamine D-like receptors - might play a role in setting the balance between active behaviours and immobility in this test. Regardless of the underlying mechanisms, the observation that the effects of drugs in the first session have consequences in the second session might be relevant for a better understanding of the FST in behavioural/functional terms. Thus, the first objective of this study was to investigate the consequences in the second session of the administration of the prototypic antidepressant drug imipramine before the first of two sessions. A second objective was to investigate the effect of dopamine D-like and D-like receptor blockade on the effects of imipramine. Imipramine (20 mg/kg) was administered 24-h, 6-h and 30-min before the first of two FST sessions performed 24-h apart. SCH 23390 (0.01, 0.04 mg/kg) or raclopride (0.0125, 0.25 mg/kg) were administered 30-min before the first session. Imipramine increased active behaviours both in the first and in the second session. Raclopride attenuated and SCH 23390 potentiated imipramine effects only in the first session and to a limited extent. These results show that imipramine administration before the first of two FST sessions induces an increase in active behaviours in the second session, and suggest that this effect is the consequence of the behavioural effects of imipramine in the first session.