1. ¹Department of Psychiatry, Clinical Sciences Institute, National University of Ireland, Galway, Ireland
2. ²Mood and Anxiety Disorders Program, Molecular Imaging Branch, National Institutes of Mental Health, NIH, Bethesda, MD, USA

Correspondence: Dr DM Cannon, Room 201 Comerford Building, Clinical Sciences Institute, National University of Ireland, Galway, Co-Galway, Ireland. Tel: +353 9149 5692; Fax: +353 9149 4563; E-mail: dara.cannon@nuigalway.ie

Received 8 May 2008; Revised 8 September 2008; Accepted 16 September 2008; Published online 22 October 2008.

Abstract

The dopamine type-1 receptor has been implicated in major depressive disorder (MDD) by clinical and preclinical evidence from neuroimaging, post mortem, and behavioral studies. To date, however, selective in vivo assessment of D₁ receptors has been limited to the striatum in MDD samples manifesting anger attacks. We employed the PET radioligand, [¹¹C]NNC-112, to selectively assess D₁ receptor binding in extrastriatal and striatal regions in a more generalized sample of MDD subjects. The [¹¹C]NNC-112 nondisplaceable binding potential (BP_ND) was assessed using PET in 18 unmedicated, currently depressed subjects with MDD and 19 healthy controls, and compared between groups using MRI-based region-of-interest analysis. The mean D₁ receptor BP_ND was reduced (14%) in the left middle caudate of the MDD group relative to control group (p<0.05). Among the MDD subjects D₁ receptor BP_ND in this region correlated negatively with illness duration (r=-0.53; p=0.02), and the left-to-right BP_ND ratio correlated inversely with anhedonia ratings (r=-0.65, p=0.0040). The D₁ receptor BP_ND was strongly lateralized in striatal regions (p<0.002 for main effects of hemisphere in accumbens area, putamen, and caudate). In post hoc analyses, a group-by-hemisphere-by-gender interaction was detected in the dorsal putamen, which was accounted for by a loss of the normal asymmetry in depressed women (F=7.33, p=0.01). These data extended a previous finding of decreased striatal D₁ receptor binding in an MDD sample manifesting anger attacks to a sample selected more generally according to MDD criteria. Our data also more specifically localized this abnormality in MDD to the left middle caudate, which is the target of afferent neural projections from the orbitofrontal and anterior cingulate cortices where neuropathological changes have been reported in MDD. Finally, D₁ receptor binding was asymmetrical across hemispheres in healthy humans, compatible with evidence that dopaminergic function in the striatum is lateralized during reward processing, voluntary movement, and self-stimulation behavior.