A note on the projections of pars compacta neurons within pars reticulata of the substantia nigra in the rat

The ascending projections of pars compacta (SNc) neurons displaced within the pars reticulata (SNr) of the substantia nigra in the rat were examined using a fluorescent retrograde tracing. Following unilateral injections of a tracer into the striatum, SNc cells within the SNr were retrogradely labeled predominantly in the caudal parts, and to a lesser extent in the rostral portions of the nucleus. These nigrostriatal projections arising from the SNc cells within the SNr were only ipsilateral. Injections of a tracer into the nucleus accumbens (Ace) did not produce any labeling of these displaced SNc cells in contrast with cells in the SNc proper. Given that the SNc neurons within the SNr project only to the striatum but not to the Ace, and that they contain dopamine but not cholecystokinin, they might be involved in the motor but not in the limbic function.