Granule cells born in the adult rat hippocampus can regulate the expression of GABAergic markers

The granule cells (GCs) of the dentate gyrus transiently express markers of the GABAergic phenotype early during development. However, GCs are generated throughout life, posing the question of whether the newborn neurons in the adult rodent recapitulate the development of the neurotransmitter phenotype of GCs generated during embryonic and early postnatal development. In this work we asked whether newborn GCs transiently express a GABAergic phenotype during their development in the adult rat. Using retroviral infection, we labeled dividing cells in the dorsal hippocampus with GFP, identified them as granule cells, and determined their expression of GABAergic markers at different developmental stages. We found that GFP-positive cells express Prox-1 and calbindin, identifying them as GCs. GABA or GAD67 was expressed in 13% of GFP-positive cells at 7 dpi, in 16% at 10 dpi and in 20% at 15 dpi. At 30 dpi, however, no GFP-positive cell somata containing GABAergic markers were detected, but their mossy fiber boutons did contain GAD67. Interestingly, developing GCs detected with doublecortin and PSA-NCAM in non-injected adult rats, did not express GABAergic markers, suggesting that retroviral injection/infection stimulates their transient expression. However, in non-injected rats, a number of mossy fiber boutons of newborn granule cells detected with PSA-NCAM did express GAD67. Our findings reveal that developing GCs born in the adult are able to transiently up-regulate the expression of GABAergic markers to be detected in their soma in response to insults, while they constitutively express GAD67 in their mossy fibers.

► We asked whether granule cells (GCs) born in the adult rat express GABAergic markers.
► Somata and axons of GCs infected with a retroviral vector expressed GAD/GABA.
► Somata of GCs identified by DCX or PSA-NCAM did not express GAD/GABA in intact rats.
► DCX- or PSA-NCAM positive somata expressed GAD/GABA in rats injected with PBS.
► Mossy fiber boutons of all experimental groups expressed GAD.