PRESERVED NUMBER OF ENTORHINAL CORTEX LAYER-II NEURONS IN AGED MACAQUE MONKEYS

The perforant path, which consists of the projection from the layer II neurons of the entorhinal cortex to the outer molecular layer of the dentate gyrus, is a critical circuit involved in learning and memory f ormation. Accordingly, disturbances in this circuit may contribute to age-related cognitive deficits. In a previous study, we demonstrated a decrease in N-methyl-D-aspartate receptor subunit 1 immunofluorescenc e intensity in the outer molecular layer of aged macaque monkeys. In t his study, we used the optical fractionator, a stereological method, t o determine if a loss of layer II neurons occurred in the same animals in which the N-methyl-D-aspartate receptor subunit 1 alteration was o bserved. Our results revealed no significant differences in the number of layer II neurons between juvenile, young adult, and aged macaque m onkeys. These results suggest that the circuit-specific decrease in N- methyl-D-aspartate receptor subunit I reported previously occurs in th e absence of structural compromise of the perforant path, and thus may be linked to an age-related change in the physiological properties of this circuit. (C) 1997 Elsevier Science Inc.