The habenula couples the dopaminergic and the serotonergic systems: application to depression in Parkinson's disease

A high percentage of patients with Parkinson's disease suffer from depression in addition to their motor disabilities. However, the etiology of this depression and its relation to Parkinson's disease are unknown. Within the framework of the monoamine deficiency hypothesis of depression, we propose that the dopaminergic and serotonergic systems are coupled by the lateral habenula, and argue that altered basal ganglia activity leads to lateral habenula hyperactivity, which in turn down-regulates the serotonergic system, resulting in depressive symptoms in patients with Parkinson's disease. We tested this hypothesis using the unilateral 6-hydroxydopamine hemiparkinsonian rat model of Parkinson's disease. Behavior was assessed using the novelty suppressed feeding and forced swim tests, and the effective connectivity of the serotonergic system was estimated by manganese-enhanced magnetic resonance imaging of the raphe nuclei. The results show depression-like behaviors and reduced raphe connectivity with the lateral habenula, dentate gyrus of the hippocampus, thalamus and hypothalamus in the 6-hydroxydopamine rat groups. More importantly, partial restoration of the raphe connectivity and partial normalization of behavior were achieved by dopamine replacement therapy (apomorphine, 10 mg/kg, s.c. daily). Furthermore, nearly complete behavioral normalization was reached after a bilateral electric lesion of the lateral habenula. These findings provide a plausible link between Parkinson's disease and depression and open up avenues for new therapeutic interventions in depression and possibly in Parkinson's disease.