Expression of D₁ receptor mRNA in projections from the forebrain to the ventral tegmental area

In situ hybridization was combined with Fluoro-Gold retrograde labeling to determine if cells projecting from the forebrain to the ventral tegmental area (VTA) express D₁ receptor mRNA. Cell counts were made in the prefrontal cortex, shell of the nucleus accumbens, and ventral pallidum to estimate the percentage of neurons projecting to the VTA that express D₁ receptor mRNA. Retrogradely labeled cells were observed in the infralimbic and prelimbic regions of the prefrontal cortex, and up to 37% of the retrogradely labeled cells expressed D₁ receptor mRNA. Double-labeled cells constituted up to 89% of retrogradely labeled neurons in the rostral shell and up to 68% in the caudal shell of the nucleus accumbens. The number of retrogradely labeled cells in the ventral pallidum that were double-labeled ranged from 13% in the rostral to less than 10% in the caudal portions. These data provide anatomical support for a role of D₁ receptors in the reciprocal innervation between the forebrain and VTA.