Heterogenous properties of dentate granule neurons in the adult rat

Postnatal neurogenesis contributes substantially to the neuronal population of the adult dentate gyrus. We report here that the neurons located in the deep aspects of the granule cell layer, near the proliferative zone, have different properties from those located in the superficial layers. The form er group of neurons, tentatively designated as young, can be readily identi fied in a standard hippocampal slice preparation by morphological, immunohi stochemical, and electrophysiological criteria. Electrophysiological record ings and imaging with Lucifer yellow from these neurons in the standard hip pocampal slice preparation showed one or two main dendrites and conically s haped branches possessing varicose protrusions. These features are in agree ment with the appearance of the same population of young neurons immunoposi tive for TOAD-64, a marker for immature neurons. In disinhibited slices, th ese putative young neurons are distinguished from the mature neurons, locat ed in the superficial granule cell layer, by shelving paired pulse facilita tion and having a lower threshold for induction of long-term potentiation. The putative young neurons are completely unaffected by GABA(A) inhibition and always display robust long-term potentiation. In contrast, the mature n eurons never produce long-term potentiation when the GABA(A) inhibition is intact. We propose that the heterogeneity of the functional properties of t he granule neurons is related to the ongoing neurogenesis in the adult anim als. (C) 2000 John Wiley & Sons, Inc.