DISTRIBUTION OF PARVALBUMIN-IMMUNOREACTIVE CELLS AND FIBERS IN THE MONKEY TEMPORAL-LOBE - THE HIPPOCAMPAL-FORMATION

The distribution of parvalbumin-immunoreactive cells and fibers in the various fields of the hippocampal formation was studied in the macaque monkey. Parvalbumin-immunoreactive neurons had aspiny or sparsely spiny dendrites that often had a beaded appearance; most resembled classically identified interneurons. Parvalbumin-immunoreactive fibers and terminals were confined to certain laminae in each field and generally had a pericellular distribution. In the dentate gyrus, there was a dense pericellular plexus of immunoreactive terminals in the granule cell layer. Except for a narrow supragranular zone, there was a marked paucity of terminals in the molecular and polymorphic cell layers. Immunoreactive neurons were mainly located immediately subjacent to the granule cell layer and comprised a variety of morphological cell types. The three fields of the hippocampus proper (CA3, CA2, and CA1) demonstrated differences in their parvalbumin staining characteristics. In CA3, there was a prominent pericellular terminal plexus in the pyramidal cell layer that was densest distally (closer to CA2). Immunoreactive cells were located either in the pyramidal cell layer, where many had a pyramidal shape and prominent apical and basal dendrites, or in stratum oriens. CA2 had a staining pattern similar to that in CA3, though both the number of labeled cells and the density of the pericellular terminal plexus were greater in CA2. In CA1, there was a markedly lower number of parvalbumin-labeled cells than in CA3 and CA2 and the cells tended to be located in the deep part of the pyramidal cell layer or in stratum oriens. The pyramidal cell layer of CA1 contained a pericellular terminal plexus that was substantially less dense than in CA3 and CA2. At the border between CA1 and the subiculum there was a marked increase in the number of parvalbumin-immunoreactive neurons. The positive cells were scattered throughout the pyramidal cell layer of the subiculum and comprised a variety of sizes and shapes. Terminal labeling was higher in the pyramidal cell layer of the subiculum than in CA1. Layer II of the presubiculum had one of the highest densities of fiber and terminal labeling in the hippocampal formation. The density of staining was lower in the superficial portion of the layer where linear cartridges of presumed axo-axonic synapses were common. A large number of parvalbumin-immunoreactive cells were scattered throughout layer II of the presubiculum; small, spherical, multipolar cells were commonly observed in layer I. The parasubiculum had a somewhat lower density of positive cells and fibers than the presubiculum. The entorhinal cortex demonstrated a rostrocaudal and mediolateral gradient to its distribution of parvalbumin-immunoreactive cells, fibers, and terminals. The density of positive structures became increasingly greater along a rostrocaudal gradient and, at all levels, staining was denser in the lateral portion of the entorhinal cortex. This study provides a comprehensive analysis of the distribution of parvalbumin-immunoreactive profiles in the nonhuman primate hippocampal formation. Since parvalbumin has been found to colocalize with certain classes of GABAergic neurons, this study also provides information on the organization of one component of the inhibitory circuitry of the monkey hippocampal formation.