Calbindin-D28k immunoreactivity in the rat amygdala.

Calbindin-D28k (CB) is a calcium-binding protein whose exact function has yet to be elucidated. Because CB is contained in distinct cell types in the nervous system, it is a valuable marker for distinguishing specific nuclear subdivisions and neuronal populations. In the present study, immunohistochemical methods were used to localize CB in the rat amygdala. A subpopulation of nonpyramidal neurons in all nuclei of the basolateral amygdala (ABL) exhibited intense CB immunoreactivity (CB-ir). CB-positive puncta resembling axon terminals were observed surrounding pyramidal perikarya in the ABL. Pyramidal neurons in caudal and lateral portions of the ABL exhibited moderate CB-ir. Intensely stained nonpyramidal neurons resembling those of the ABL were also seen in the cortical nuclei, periamygdaloid cortex, and nucleus of the lateral olfactory tract; these nuclei also contained variable numbers of moderately stained pyramidal cells. Numerous CB-positive neurons were observed in all subdivisions of the medial nucleus. The posterodorsal subdivision of the medial nucleus exhibited a centrally located island of small CB-negative neurons and three cell-dense clusters of CB-positive neurons. The distribution of CB-ir in the central nuclear complex was very heterogeneous. The intermediate subdivision of the central nuclear complex exhibited the most robust staining, whereas the lateral subdivision contained relatively few CB-positive cells. Dorsal and ventral portions of the lateral capsular subdivision of the central nuclear complex could be readily distinguished on the basis of differing levels of CB-ir. These results indicate that CB is localized in discrete cell types and nuclear subdivisions in the rat amygdala and suggest that CB immunohistochemistry is a useful technique for identifying specific structural components in this brain region.