The somatodendritic domain of substantia nigra pars reticulata projection neurons in the rat

We have examined the morphology of the somatodendritic domain of projection neurons located in different sectors of rat substantia nigra pars reticulata (SNr) or having distinct axonal arborizations. Forty-three neurons – 23 located in the dorsal half and 20 in the ventral half of SNr – were injected with biotinylated dextran amine and their somatodendritic domain was reconstructed from serial sagittal sections with a camera lucida. The axonal arborization of 14 neurons was also reconstructed. Dorsally located SNr neurons had a larger perikaryon, a higher number of primary dendrites and a more extensive dendritic arbor than the ventrally located ones. However, irrespective of their location in the SNr, the somatodendritic domain was always longer along the rostrocaudal axis than along the dorsoventral and mediolateral axes. Specific correlations between somatodendritic morphology and axonal arborization could be established for some SNr neurons, but among SNr neurons with similar efferent projections, those lying dorsally always exhibited a larger perikaryon and a more widespread dendritic arbor than those located ventrally. These results indicate that the morphology of the somatodendritic domain of SNr projection neurons is related to the location of their perikaryon within the structure rather than to the pattern of their axonal projections.