Morphology of CA3 non-pyramidal cells in the developing rat hippocampus

Although several investigations have shown that the local GABAergic circuit in the rat hippocampus is functional very early in development, this resul t has not been yet completed by the investigation of the full dendritic and axonal arborization of the neonatal interneurones. In the present study, i ntracellular injection of biocytin was used to assess the branching pattern of interneurones in the hippocampal CA3 region of rat between 2 and 6 days of age. Based on their dendritic morphology, the biocytin-filled interneur ones were divided into four classes: bipolar, stellate, pyramidal-like and fusiform interneurones. About half of the biocytin-filled neonatal interneu rones exhibited dendritic or somatic filopodial processes. The axonal arbor s of the filled-interneurones were widely spread into the CA3 region, and i n four out of nine cases extended beyond the CA3 region to branch into the CA1 region. These results show that. despite immature features. the filopod ial processes, the hippocampal interneurones are well developed early in de velopment at a time when their target cells, the pyramidal neurones, are st ill developing. These observations are consistent with a trophic role that GABA may play early in development. (C) 2001 Elsevier Science B.V. All righ ts reserved.