Direct projections from the central amygdaloid nucleus to the globus pallidus and substantia nigra in the cat.

Employing both anterograde and retrograde axonal tracing, we investigated direct projections from the central amygdaloid nucleus to the basal ganglia in the cat. The anterograde axonal tracing of Phaseolus vulgaris-leucoagglutinin revealed that projection fibers from the central amygdaloid nucleus to the basal ganglia ended in the globus pallidus (the feline homolog to the external segment of the globus pallidus of primates) and substantia nigra. The amygdalopallidal fibers terminated chiefly in the medial most part of the globus pallidus at its caudal level. The amygdalonigral fibers terminated densely in the substantia nigra pars lateralis, and moderately in the dorsolateral part of the substantia nigra pars reticulata; none of them were found to end in the substantia nigra pars compacta. Both of the amygdalopallidal and amygdalonigral projections were ipsilateral. These neuronal connections were confirmed by retrograde axonal tracing of cholera toxin B subunit in the second set of the experiments: The cells of origin of the amygdalopallidal and amygdalonigral projections were located predominantly in the lateral part of the central amygdaloid nucleus, and additionally in the intercalated cell islands of the amygdala. Most of them were of small bipolar or multipolar type. The cells projecting to the globus pallidus were preferentially distributed at the rostral levels of the central nucleus and intercalated cell islands of the amygdaloid complex, while those projecting to the substantia nigra were mainly located at the caudal levels of these amygdaloid subdivisions. In the third set of the experiments, sequential double-antigen immunofluorescence histochemistry for transported cholera toxin B subunit and horseradish peroxidase showed that some single neurons in the lateral part of the central amygdaloid nucleus, particularly at its middle level, issued axon collaterals to both the globus pallidus and substantia nigra pars lateralis. The results of the present study indicate that the central amygdaloid nucleus sends projection fibers to the globus pallidus and substantia nigra possibly to exert a limbic influence upon forebrain motor mechanisms.