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 m otor behavior, motivation, and learning. Here we examine the genetic variat ion of mesencephalic DA neuron number in a quasi-congenic RQI mouse strain and its background partner and in a recombinant inbred strain with differen t levels of mesencephalic tyrosine hydroxylase activity (TH/MES). We used B 6.Cb4i5-alpha6/Vad, C57BL/6By, and CXBI, which are known to express high, i ntermediate, and low levels of TH/MES, respectively. Unbiased stereological sampling with optical di-sector counting methods were employed to estimate the number of TH-positive neurons in the A8-A9-A10 cell groups. Morphometr ic studies on the mesencephalic dopamine cell groups indicated that male mi ce of the B6.Cb4i5-alpha6/Vad strain were endowed with a significantly lowe r number of TH-positive cells than CXBI mice. In all strains studied, the r ight retrorubral field (A8 area) had a higher number of dopamine neurons co mpared to the left A8 area. The results suggest an inverse relationship bet ween TH/MES and number of dopamine neurons in the A9-A10 cell groups and si gnificant lateral asymmetry in the A8 cell group. A detailed anatomical atl as of the mesencephalic A8-A9-A10 dopaminergic cell groups in the mouse is also presented to facilitate the assignment of TH-positive neurons to speci fic cell groups.