Immunohistochemical localization of aspartate in corticofugal pathways

In the present study we used immunohistochemistry, with an antibody directed against aspartate (Asp), in conjunction with wheat germ agglutinin-horseradish peroxidase (WGA-HRP) histochemistry to examine the Asp-containing neurons in the motor and somatosensory cortices of the rat that project to the caudate/putamen or the pons. Injections of WGA-HRP in the caudate/putamen labelled cells in layer V of both cortical areas. In the motor cortex, retrogradely labelled neurons were distributed throughout layer V; 59% of these cells were also Asp-immunoreactive. In the somatosensory cortex labelled neurons were found mainly in layer Vb; 57% of these cells were double labelled. A similar percentage of cells were double labelled in this cortical area following WGA-HRP injections into the pontine nuclei. These observations taken together with previous findings (Exp. Neurol., 105 (1989) 36-44) suggest that nearly equal proportions of neurons that give rise to the corticostriatal and corticopontine pathways may use either Asp or glutamate (Glu) as putative neurotransmitters.