Figure 4.

TH-positive cell numbers in the SN and the VTA that constitute the midbrain ascending projections to the forebrain. A, VTA neurons lie medially and project mainly to nonstriatal targets, whereas the laterally located SN cells preferentially innervate the striatum, as shown in a nonlesioned side of the brain. B, Striatal injection of the 6-OHDA toxin leads to lesion of the striatal dopamine terminals and a retrograde degeneration of the SN cells, leaving most of the VTA neurons intact. C, However, injection of the same toxin at the level of the medial forebrain bundle damages axons from both pathways and, thus, leads to a combined lesion affecting both projection systems. D, E, The total numbers of TH-positive cells present in each nuclei was quantified using computerized stereological estimation tools. We found that the striatal lesion in groups 1 and 3 lead to a comparable lesion of ∼75% in the SN. In these groups the VTA cell numbers were not affected. In the complete lesion animals (groups 2 and 4), however, not only the SN was completely depleted, but also the TH-positive cells in the VTA were reduced by ∼50%. SNpc, Substantia nigra pars compacta; SNpr, substantia nigra pars reticulata. Scale bar: (in C) A–C, 1 mm. *Different from intact side; ⁺different from its control group. Error bars indicate SEM.