Expression of D₁ receptor, D₂ receptor, substance P and enkephalin messenger RNAs in the neurons projecting from the nucleus accumbens

In situ hybridization was combined with FluoroGold retrograde labelling to determine the distribution of messenger RNAs for the D₁ dopamine receptor, D₂ dopamine receptor, β-preprotachykinin or preproenkephalin in the neurons projecting from the nucleus accumbens to the ventral pallidum and the ventral tegmental area. Neurons were quantified in both the core and the shell of the nucleus accumbens to estimate the proportion of neurons projecting to the ventral pallidum or ventral tegmental area that contain transcripts for D₁ receptors, D₂ receptors, β-preprotachykinin or preproenkephalin. Following the deposition of FluoroGold into the central ventral pallidum, both the core and the shell of the nucleus accumbens were retrogradely labelled, while deposits into the ventral tegmental area selectively labelled cells in the shell. A high percentage of nucleus accumbens neurons innervating the ventral tegmental area expressed messenger RNAs for D₁ receptors (72%) and β-preprotachykinin (62%), while less than 3% of the neurons contained messenger RNAs for preproenkephalin or D₂ receptors. The neurons projecting to the ventral pallidum did not show the discrete distribution of transcripts as was observed in the accumbens- ventral tegmental area projection. Preproenkephalin messenger RNA was identified in 46% of the neurons innervating the ventral pallidum, and D₂ receptor messenger RNA was found in approximately 40% of the cells. A large minority of neurons projecting from the nucleus accumbens to the ventral pallidum also expressed messenger RNAs for D₁ receptors (37%) and β- preprotachykinin (35%). While a higher percentage of D₁ receptor, and β- preprotachykinin messenger RNA expressing cells were located in the shell than in the core of the nucleus accumbens, the percentage tended to be higher in the core for cells expressing D₂ receptors or preproenkephalin messenger RNA. These data indicate that messenger RNAs for D₂ receptors and enkephalin are selectively expressed in the accumbens-pallidal projection while transcripts encoding D₁ receptors and substance P are contained in the efferent projections to both the ventral pallidum and ventral tegmental area. The presence of D₁ receptor and β-preprotachykinin messenger RNAs in both mesencephalic and pallidal projections contrasts output from the striatum where the expression of D₁ receptor and β-preprotachykinin messenger RNAs is primarily restricted to the mesencephalic projection.