Efficacy of atypical antipsychotics in depressive syndromes

Depression is a frequent symptom in psychiatry, either isolated (major depression) or entangled with other psychiatric symptoms (psychotic depression, depression of bipolar disorders). Many antidepressant drugs are available with different pharmacological profiles from different classes: tricyclic antidepressants, monoamine oxydase inhibitors, selective serotonin reuptake inhibitors (SSRI). However, there are some limitations with these drugs because there is a long delay before relief for symptoms, some patients with major depression are resistant to treatment, there is a risk to induce manic symptoms in patients with bipolar disorders and these drugs have no effect on the psychotic symptoms frequently associated to major depression. The leading hypothesis for the search of more efficient new antidepressants has been the amine deficit hypothesis: noradrenaline and/or serotonin deficit and more recently dopamine deficit. Moreover, a dopamine deficit has been also hypothesized as the central mechanism explaining the negative symptoms of schizophrenia. These symptoms are the consequence of a deficit of normal behaviours and include affective flattening, alogia, apathy, avolition and social withdrawal. There is thus a great overlap between symptoms of depression and negative symptoms of schizophrenia. Atypical antipsychotics, in contrast with conventional neuroleptics, have been shown to decrease negative symptoms, most probably through the release of dopamine in prefrontal cortex, thus improving psychomotor activity, motivation, pleasure, appetite, etc. The dopamine deficit in cortical prefrontal areas was thus an unifying hypothesis to explain both some symptoms of depression and negative symptoms of schizophrenia. Studies in animal confirm this view and show that the association of an atypical antipsychotic drug and an SSRI (olanzapine plus fluoxetine) increases synergistically the release of dopamine in prefrontal areas. Moreover, most of the atypical antipsychotics have a large action spectrum, beyond the only dopamine receptors: their effects on the serotonin receptors--particularly the 5-HT2A and 5-HT2C receptors--suggest that their association to SSRI could be a promising treatment for depression. Indeed, SSRI act mainly by increasing the serotonin level in the synapse, thus leading to a non specific activation of all pre- and post-synaptic serotonin receptors. Among them, 5-HT2A/2C receptors have been involved in some of the unwanted effects of SSRI: agitation, anxiety, insomnia, sexual disorders, etc. The inhibition of these receptors could be thus beneficial for patients treated with SSRI. Amisulpride is an unique atypical antipsychotic that selectively blocks dopamine receptors presynaptically in the frontal cortex, possibly enhancing dopaminergic transmission. The antidepressant effect of amisulpride was shown in dysthymia in many clinical studies versus placebo, tricyclic antidepressants, SSRI or others. However, a shorter delay for symptom relief was not demonstrated for amisulpride as compared to comparative antidepressants. Other atypical antipsychotics (clozapine, olanzapine), which act on a large variety of receptors, have shown antidepressant effects--mainly in association with SSRI--in different psychiatric diseases: treatment-resistant major depression, major depression with psychotic symptoms and depression of bipolar disorders, with no increase of manic symptoms in this latter case. Moreover, the delay for symptom relief was greatly shortened. More comparative double-blind studies are required to confirm and to precise the antidepressant effects of atypical antipsychotics. Nevertheless, these studies suggest that atypical anti-psychotics could be of great value in depressive conditions reputed for their resistance to treatment with usual antidepressants. Particularly, new strategies emerge that combine atypical antipsychotics and antidepressants for greater efficacy and more rapid relief of depression symptoms.