D₃ dopamine receptor mRNA is widely expressed in the human brain

Considerable attention has been given to the association of the D₃ dopamine receptor subtype and limbic function based on the abundant localization of D₃ receptor sites and mRNA expression in the islands of Calleja and nucleus accumbens in experimental animals. Though most human anatomical studies have focused on the role of D₃ receptors in limited brain structures, detailed information about the overall anatomical organization of the D₃ receptor in the human brain is still, however, not available. In the current study, we examined the anatomical distribution of D₃ receptor mRNA expression at different levels of the human brain in whole hemisphere horizontal cryosections using in situ hybridization. This approach made it possible to establish for the first time the wide and heterogenous expression of the D₃ receptor gene throughout the human brain. As expected, the most abundant D₃ mRNA expression levels were found in the islands of Calleja and discrete cell cluster populations within the ventral striatum/nucleus accumbens region. High levels were also evident within the dentate gyms and striate cortex. Low to moderate D₃ mRNA expression levels were apparent in most brain areas including all other cortical regions (highest in the anterior cingulate/subcallosal gyms), caudate nucleus, putamen, anterior and medial thalamic nucleus, mammillary body, amygdala, hippocampal CA region, lateral geniculate body, substantia nigra pars compacta, locus coeroleus, and raphe nuclei. While the current anatomical map of D₃ receptor mRNA expression in the human brain does confirm previous reports that D₃ receptors may play important roles in limbic-related functions such as emotion and cognition, the findings also suggest other non-limbic functions for D₃ mRNA-expressing cell populations such as processing of motor and sensory information.