The midbrain dopaminergic system: anatomy and genetic variation in dopamine neuron number of inbred mouse strains.

The mesotelencephalic dopamine system is genetically variable and affects motor behavior, motivation, and learning. Here we examine the genetic variation of mesencephalic DA neuron number in a quasi-congenic RQI mouse strain and its background partner and in a recombinant inbred strain with different levels of mesencephalic tyrosine hydroxylase activity (TH/MES). We used B6.Cb4i5-alpha6/Vad, C57BL/6By, and CXBI, which are known to express high, intermediate, and low levels of TH/MES, respectively. Unbiased stereological sampling with optical disector counting methods were employed to estimate the number of TH-positive neurons in the A8-A9-A10 cell groups. Morphometric studies on the mesencephalic dopamine cell groups indicated that male mice of the B6.Cb4i5-alpha6/Vad strain were endowed with a significantly lower number of TH-positive cells than CXBI mice. In all strains studied, the right retrorubral field (A8 area) had a higher number of dopamine neurons compared to the left A8 area. The results suggest an inverse relationship between TH/MES and number of dopamine neurons in the A9-A10 cell groups and significant lateral asymmetry in the A8 cell group. A detailed anatomical atlas of the mesencephalic A8-A9-A10 dopaminergic cell groups in the mouse is also presented to facilitate the assignment of TH-positive neurons to specific cell groups.