Mesostriatal projections in BALB/c and CBA mice: a quantitative retrograde neuroanatomical tracing study.

A major portion of the midbrain dopamine (DA)-containing neurons project to the striatum and make up the mesostriatal DA system. The purpose of the present experiment was to map the location and quantify the density of mesostriatal neurons within two inbred mouse strains (BALB/c and CBA) known to possess different numbers of midbrain DA neurons. Computer-assisted reconstructions were made of both the wheat germ agglutinin-conjugated horseradish peroxidase (WGA-HRP) striatal injection site and the retrogradely labeled midbrain cells. There was no strain differences in the major source or topographical pattern of innervation of the striatum from the midbrain cellular regions. Even following small striatal injections, the labeled midbrain cells were found throughout most of the rostrocaudal extent of the midbrain DA nuclei; some labeled cells were found within the substantia nigra, the ventral tegmental area and the adjacent retrorubral field. Although the BALB/c strain has 20-25% more midbrain DA neurons than the CBA, given comparable striatal injection volumes, there was no significant difference in the number of HRP-filled mesostriatal neurons between the two mouse strains. These data suggest that the mesostriatal neurons give rise to comparable axonal branching within the striatum in the two mouse strains.