The effect of monoamine oxidase inhibitors compared with classical tricyclic antidepressants on learned helplessness paradigm.

The present study investigated the effect of monoamine oxidase inhibitors, nialamide which is a non specific monoamine oxidase inhibitor (MAOI), toloxatone which is a A type MAOI and L-deprenyl which is a B type MAOI compared with classical tricyclic antidepressants (clomipramine, desipramine and imipramine), on the escape deficit induced by inescapable shocks (learned helplessness paradigm). Rats were first exposed to inescapable shock pretreatment (60 shocks, 15 sec duration, 0.8 mA, every minute + 15 sec) and 48 h later, shuttle box training (30 trials/day, 15 min.) was initiated in order to evaluate interference effect. Rats with inescapable shocks exhibited escape and avoidance deficits when tested for subsequent responding in a shuttle-box. Daily i.p. injections of nialamide (8 and 16 mg/kg), toloxatone (16 and 32 mg/kg), L-deprenyl (32 and 64 mg/kg) and tricyclic antidepressants (clomipramine: 16 and 32 mg/kg, desipramine: 16 and 24 mg/kg, imipramine: 16 and 32 mg/kg) eliminated escape deficits. In rat exposed to inescapable shocks and treated with L-deprenyl (16 mg/kg/day), nialamide (32 mg/kg/day) or toloxatone (64 mg/kg/day), avoidance responses are significantly increased as compared with non drugged rats preexposed to inescapable shocks. These data extend previous results bearing on the similarity of action of MAOI and tricyclic antidepressants in learned helplessness paradigm (Sherman et al., 1982) and are in agreement with data obtained in other animal models of depression.