Noradrenergic rather than GABAergic processes as the common mediation of the antidepressant profile of GABA agonists and imipramine-like drugs in animals.

The present study was aimed at investigating in rats whether a common mechanism might underlie the reversal of depressive-like behaviors by classical antidepressants and by GABA agonists such as muscimol. Blockade of GABA transmission with picrotoxin (1 mg/kg IP) abolished the muscimol (0.5-1 mg/kg)-induced reduction of immobility in the swimming test and the reversal of escape failures in the learned helplessness paradigm. Conversely, picrotoxin was found not to reduce the efficacy of imipramine-like drugs in these same animal models. The combination of muscimol and tricyclics given at subeffective doses resulted in behavioral changes that can be accounted for by an additive interaction between these two classes of drugs. These data confirm the antidepressant-like profile of GABA agonists but suggest that it is unlikely that the primary antidepressant mechanism of conventional antidepressants involves GABA-A receptors. In the swimming test, prazosin (2 mg/kg), an alpha adrenoceptor blocker, antagonized the reduction of immobility produced by both muscimol and imipramine-like drugs. In the learned helplessness paradigm, penbutolol (0.25-0.5 mg/kg) and, though to a lesser extent prazosin, counter-acted the reversal of escape failures caused by muscimol and imipramine. On the basis of these data, it is tempting to speculate that increased transmitter outflow at noradrenergic receptors may be an essential component in the mechanism of action of imipramine-like drugs but also of GABA agonists.