Retrograde transport of D-[3H]-aspartate injected into the monkey amygdaloid complex

The possibility that certain of the afferents of the primate amygdaloid complex use an excitatory amino acid transmitter was evaluated by injecting D-[3H]-aspartate into the amygdala of two Macaca fascicularis monkeys. The distribution of D-[3H]-aspartate labeled neurons was compared with those labeled with the nonselective retrograde tracer WGA-HRP injected at the same location as the isotope. Retrogradely labeled cells of both types were observed in a variety of cortical and subcortical structures observed in a variety of cortical and subcortical structures and in discrete regions within the amygdala. D-[3H]-aspartate labeled neurons were observed in layers III and V of the frontal, cingulate, insular and temporal cortices. In the hippocampal formation, heavily labeled cells were observed in the CA1 region and in the deep layers of the entorhinal cortex. Of the subcortical afferents, the claustrum and the midbrain peripeduncular nucleus contained the greatest number of D-[3H]-aspartate labeled cells. Subcortical afferents that are not thought to use excitatory amino acids, such as the cholinergic neurons of the basal nucleus of Meynert, did not retrogradely transport the isotope. Within the amygdala, the most conspicuous labeling was in the paralaminar nucleus which forms the rostral and ventral limits of the amygdala. When the D-[3H]-aspartate injection involved the basal nucleus, many labeled cells were also observed in the lateral nucleus. Retrograde transport of D-[3H]-aspartate injected into the amygdala, therefore, appears to demonstrate a subpopulation of inputs that may use an excitatory amino acid transmitter.