Zinc-containing afferent projections to the rat corticomedial amygdaloid complex: a retrograde tracing study

The mammalian amygdaloid complex is densely innervated by zinc-containing neurons. The distribution of the terminals throughout the region has been described, but the origins of these zinc-containing fibers have not. The present work describes the origins of one major component of the zinc-containing innervation of the amygdaloid complex, namely, the component that innervates the corticomedial complex. Selective labeling of zinc-containing axons was accomplished by intracerebral microinfusion of selenium anions (SeO3(2-)), a procedure that produces a ZnSe precipitate in zinc-containing axonal boutons with subsequent retrograde transport to the neurons of origin. After infusions of SeO3(2-) into combinations of cortical, medial, or amygdalohippocampal regions, retrogradely labeled zinc-containing somata were found in all amygdaloid nuclei except for the medial and central nuclei, the bed nucleus of the accessory olfactory tract, the nucleus of the lateral olfactory tract, and the anterior amygdaloid area. Extrinsic zinc-containing projections to the same amygdaloid terminal fields were found to originate from the infralimbic, cingulate, piriform, perirhinal and entorhinal cortices, and from the prosubiculum and CA1. Commissural zinc-containing projections were found to originate from the posterolateral and posteromedial cortical nuclei and from the posterior part of the basomedial nucleus. Zinc-containing neurons have been implicated in the pathophysiology of epilepsy, in cell death after seizure or stroke, and in Alzheimer's disease, all clinical conditions that involve the amygdaloid complex. Identification of the zinc-containing pathways is a prerequisite to the elucidation of zinc's role in these disorders.