Background. Neurocircuits comprising limbic, striato–pallidal and thalamo–cortical brain areas are assumed to be involved in the pathophysiology of mood disorders. All these brain regions receive serotonergic afferents arising from the rostral raphe, mainly the dorsal raphe. Although serotonergic systems appear to be involved in the pathology of mood disorders, there is uncertainty as to whether structural alterations in raphe nuclei exist alongside a functional dysregulation of the serotonergic system.

Methods. In the brains of 12 patients with mood disorders (major depressive disorder N = 6, bipolar disorder N = 6) and 12 normal subjects we performed a morphometric post-mortem study on neuronal morphology in all subnuclei of the dorsal raphe nucleus using Nissl stained 20 μm axial serial sections of the brainstem.

Results. The number of neurones of the ventrolateral subnucleus of the dorsal raphe was reduced by 31% in patients with mood disorders compared with non-psychiatric control subjects. Ventrally located subnuclei of the rostral dorsal raphe (ventrolateral, ventral, interfascicular) taken together also showed a smaller number of neurones. Neurone numbers of the dorsal and the caudal subnucleus and volumes of all single subnuclei appeared to be unchanged. Analysis of morphological neuronal types revealed a smaller number of triangular neurones in the ventrolateral subnucleus. Numbers of ovoid and round neurones in the ventrolateral subnucleus also showed a trend to reduction. No correlation was found between neurone numbers in any subnucleus of the dorsal raphe and duration of illness. Neurone numbers did not differ in any subnucleus between patients with unipolar and those with bipolar affective disorder.

Conclusions. Results indicate that patients with primary mood disorders have a circumscribed numerical neuronal deficiency in the dorsal raphe. This structural deviation may contribute to impaired serotonergic innervation of brain regions which are involved in the pathology of mood disorders.