DOPAMINE AND THE NEUROBIOLOGY
OF BIPOLAR DISORDER
In 1965 Schildkraut[17] proposed the first neurobiological hypothesis of depression, suggesting that depression could be due to a dysregulation of serotonin and noradrenaline function, but not dopamine.
The first observation suggestive of an involvement of dopamine in the mechanism of action of antidepressants and in the pathogenesis of mood disorders has been reported by Serra et al[18]. A great deal of pharmacological evidence and clinical observations, confirming the important role of dopamine in the therapeutic effect of antidepressants and in the pathogenesis of mood disorders, has been reported in the last decades[19-22].
Moreover, a large body of clinical evidences has been accumulated indicating that antidepressant treatments can induce episodes of mania/hypomania, not only in bipolar but also in unipolar patients[23,24]. In a recent meta-analytic review Tondo et al[25] reported a rate of antidepressant- induced switching of 12.5%.
Early reports of a possible link between this effect of antidepressants and the induction of a rapid cycling course of bipolar disorders were made by Kukopulos et al[26] and Wehr et al[27]. The term rapid-cycling bipolar disorder was coined by Dunner et al[28] in 1974 to identify lithium non-responders (further research has confirmed that rapid cycling is a factor of poor prognosis). Although some controversies exist[29], it is now accepted that antidepressants can induce mania/hypomania[25] and rapid-cycling bipolar disorder[30]. In keeping with these observations Ghaemi[30] suggests viewing antidepressants as “mood destabilizers”. Since 1990 we and other groups re-evaluated the effect of chronic antidepressants on dopamine receptor sensitivity and observed that chronic antidepressant treatments sensitize dopamine D2 receptors selectively in the dopaminergic reward system, supporting the hypothesis that an increase activity of this system could underlying both the therapeutic effect and the ability to cause mania/hypomania of antidepressants[31-41].
Thus, the dopamine receptor sensitization induced by antidepressant should be considered a useful animal model of mania. In fact, it fulfils the McKinney et al[42] criteria to validate a human mental disorder animal model: it resembles the condition it models in its aetiology, biochemistry, symptomatology and treatment. The model is induced by the same treatment that can induce mania in humans, is associated with an increase dopaminergic transmission and, like other models of mania, with an increase protein kinase C (PKC) activity[43], which appear to be associated with mania. The animal behaviour showed an increase sexual activity[44,45] and aggressivity (unpublished results), manic symptoms that can be easy observed also in rats. Finally it is sensitive to treatments that seem to have an antimanic effect in humans.