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Precocious development of parvalbumin‐like immunoreactive interneurons in the hippocampal formation and entorhinal cortex of the fetal cynomolgus monkey

The calcium‐binding protein parvalbumin (PV), a reliable marker of the hippocampal basket and chandelier cells, is first expressed on embryonic day 83 (E83), corresponding to midgestation of the macaque monkey, in restricted hippocampal groups of immature neurons (Berger and Alvarez [1996] J. Comp. Neurol. 366:674–699). In the present study, PV‐like immunoreactivity (LIR) was used to follow the further development of this subclass of interneurons. Asynchronous area‐specific developmental sequences were observed, predominating initially in the caudal half of the hippocampal formation and the laterocaudal division of the entorhinal cortex and occurring relatively simultaneously in the interconnected hippocampal and entorhinal subfields. Dendritic elongation of PV‐like immunoreactive interneurons and perisomatic distribution of PV‐like immunoreactive terminal boutons on their cellular targets were first observed in the subiculum around E127; then from E127 to E142 in CA3/CA2 and layers III–V of the entorhinal cortex and, to a lesser extent in CA1, the dentate hilus and deep granule cell layer; and finally from E156 to postnatal day 12 in the rest of the dentate gyrus, the presubiculum and parasubiculum, and layers III‐II‐I of the entorhinal cortex. These data provide the first indication that a population of basket cells, a major γ‐aminobutyric acid (GABA)ergic component of the hippocampal intrinsic inhibitory circuitry, reaches its cellular targets several weeks before birth in primates in contrast to rodents. The role of the prenatal PV expression in the hippocampal formation of nonhuman primates and whether it coincides with the onset of postsynaptic inhibitory potentials or is accompanied or preceded by a period of γ‐aminobutyric acid‐–mediated excitatory effects as in rat pups, are crucial questions. They underline the need to pursue direct investigations on primates to be able to legitimately extrapolate the data obtained in rodents. J. Comp. Neurol. 403:309–331, 1999. © 1999 Wiley‐Liss, Inc.