Absolute Number of Thalamic Parafascicular and Striatal Cholinergic Neurons, and the Three-Dimensional Spatial Array of Striatal Cholinergic Neurons, in the Sprague-Dawley Rat.

The absolute number of neurons and their spatial distribution yields important information about brain function and species comparisons. We studied thalamic parafascicular neurons and striatal cholinergic interneurons (CINs) because the parafascicular neurons are the main excitatory input to the striatal CINs. This circuit is of increasing interest due to research showing its involvement in specific types of learning and behavioral flexibility. In the Sprague-Dawley rat, the absolute number of thalamic parafascicular neurons and striatal CINs is unknown. They were estimated in this study using modern stereological counting methods. From each of six young adult rats, complete sets of serial 40 µm glycol methacrylate sections were used to quantify neuronal numbers in the right parafascicular nucleus (PFN). From each of five young adult rats, complete sets of serial 20 µm frozen sections were immunostained and used to quantify cholinergic neuronal numbers in the right striatum. The spatial distribution, in three dimensions, of striatal CINs was also determined from exhaustive measurement of the x, y, z coordinates of each large interneuron in 40 µm glycol methacrylate sections in sampled sets of five consecutive serial sections from each of two rats. Statistical analysis of spatial distribution was conducted by comparing observed three-dimensional data with computer models of 10,000 pseudorandom distributions, using measures of nearest neighbor distance and Ripley's K-function for inhomogeneous samples. We found that the right PFN consisted, on average, of 30,073 neurons (with a coefficient of variation of 0.11). The right striatum consisted, on average, of 10,778 CINs (0.14). The statistical analysis of spatial distribution showed no evidence of clustering of striatal CINs in three dimensions in the rat striatum, consistent with previous findings in the mouse striatum. The results provide important data for the transfer of information through the PFN and striatum, species comparisons, and computer modeling.