Subsets of midbrain dopaminergic neurons in monkeys are distinguished by different levels of mRNA for the dopamine transporter: comparison with the mRNA for the D2 receptor, tyrosine hydroxylase and calbindin immunoreactivity.

The midbrain dopamine system can be divided into two groups of cells based on chemical characteristics and connectivity. The dorsal tier neurons, which include the dorsal pars compacta and the ventral tegmental area, are calbindin-positive, and project to the shell of the nucleus accumbens. The ventral tier neurons are calbindin-negative and project to the sensorimotor striatum. This study examined the distribution of the mRNAs for the dopamine transporter molecule (DAT) and the D2 receptor in the midbrain of monkeys by using in situ hybridization. The distribution patterns were compared to that of tyrosine hydroxylase and calbindin immunohistochemistry. The results show that high levels of hybridization for DAT and the D2 receptor mRNA are found in the ventral tier, calbindin-negative neurons and relatively low levels are found in the dorsal, calbindin-positive tier. Within the dorsal tier, the dorsal substantia nigra pars compacta has the least amount of both messages. These results show that in monkeys, the ventral tegmental area and the dorsal pars compacta form a dorsal continuum of dopamine neurons which express lower levels of mRNA for DAT and D2 receptor than the ventral tier. DAT has been shown to be involved in the selective neurotoxicity of N-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP). Different levels of DAT mRNA and calbindin may explain the differential effects of MPTP neurotoxicity.