Stereologic analysis of cell number and size during postnatal development in the rat substantia nigra.

Parkinson's disease is characterized by age-related atrophy and loss of dopaminergic neurons within the compact portion of the substantia nigra (SNpc) projecting to neostriatum. Despite numerous studies using rodent models to examine mechanisms underlying this disorder, the fundamental question of whether development- or age-related changes occur in the rodent SNpc remains unanswered. The present study used a three-level, optical fractionator approach to estimate the number and size of SNpc neurons immunoreactive for tyrosine hydroxylase (TH) in eight young (2-month) and eight older (7-month) Sprague-Dawley rats. Following standard protocols for animal care and tissue harvesting, every eighth 60-microm section from a gapless coronal series was treated immunohistochemically for TH along with a thionin counterstain. Neither the ventral tegmental area nor the lateral part of the SN was included in the analysis. The total bilateral number of SNpc TH+ neurons (approximately 8000) was equivalent between groups, whereas mean TH+ neuronal volume decreased significantly in the older group (approximately 18%). In contrast, volume of the SNpc increased with age by 17%, as did volume of the entire brain (24%). TH+ cells in the SNpc were also significantly larger on the left versus right side of the brain. These data are consistent with the hypothesis that age-related volumetric expansion of the SNpc is accounted for by an increase in the ratio between neuropil and average neuron somal size during intermediate postnatal development.