Evidence for crossed catecholaminergic nigrostriatal projections by combining wheat germ agglutinin-horseradish peroxidase retrograde transport and tyrosine hydroxylase immunocytochemistry.

After unilateral injections of wheat germ agglutinin-horseradish peroxidase into the rat caudate-putamen, a few retrogradely labeled neurons were found in the contralateral pars compacta of the substantia nigra. These contralaterally projecting nigral cell bodies also immunoreacted positively to a specific tyrosine hydroxylase antiserum. We conclude that crossed catecholaminergic nigrostriatal projections may contribute to the reciprocal regulation exerted by the two nigrostriatal dopaminergic systems.