Neurogenesis in the dentate gyrus begins before birth but then continues into adulthood. Consequently, many newborn granule cells must integrate into a preexisting hippocampal network. Little is known about the timing of this process or the characteristics of the first established synapses. We used mice that transiently express enhanced green fluorescent protein in newborn granule cells to examine their synaptic input. Although newborn granule cells had functional glutamate receptors, evoked and spontaneous synaptic currents were exclusively GABAergic with immature characteristics including slow rise and decay phases and depolarized reversal potentials. Synaptic currents in newborn granule cells were relatively insensitive to the GABAA receptor modulator zolpidem compared with neighboring mature granule cells. Consistent with the kinetics and pharmacology, newborn granule cells isolated by fluorescent cell sorting lacked the α1 GABAA receptor subunit. Our results indicate that newborn granule cells initially receive only GABAergic synapses even in the adult.
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