Intracellular recording and labeling of mossy cells and proximal CA3 pyramidal cells in macaque monkeys

Little is known about the morphological characteristics and intracellular e lectrophysiological properties of neurons in the primate hippocampus and de ntate gyrus. We have therefore begun a program of studies using intracellul ar recording and biocytin labeling in hippocampal slices from macaque monke ys. In the current study, we investigated mossy cells and proximal CA3 pyra midal cells. As in rats, macaque mossy cells display fundamentally differen t traits than proximal CA3 pyramidal cells. Interestingly, macaque mossy ce lls and CA3 pyramidal neurons display some morphological differences from t hose in rats. Macaque monkey mossy cells extend more dendrites into the mol ecular layer of the dentate gyrus, have more elaborate thorny excrescences on their proximal dendrites, and project more axon collaterals into the CA3 region. In macaques, three types of proximal CA3 pyramidal cells are found : classical pyramidal cells, neurons with their dendrites confined to the C A3 pyramidal cell layer, and a previously undescribed cell type, the "denta te" CA3 pyramidal cell, whose apical dendrites extend into and ramify withi n the hilus, granule cell layer, and molecular layer of the dentate gyrus. The basic electrophysiological properties of mossy cells and proximal CA3 c ells are similar to those reported for the rodent. Mossy cells have a highe r frequency of large amplitude spontaneous depolarizing postsynaptic potent ials, and proximal CA3 pyramidal cells are more likely to discharge bursts of action potentials. Although mossy cells and CA3 pyramidal cells in macaq ue monkeys display many morphological and electrophysiological features des cribed in rodents, these findings highlight significant species differences , with more heterogeneity and the potential for richer interconnections in the primate hippocampus. J. Comp. Neurol. 430:264-281, 2001. (C) 2001 Wiley -Liss. Inc.