rats, amounting to 0.1% of the population of the entire DG, with a survival rate of about 50% (Cameron and McKay, 2001). Neuroblasts born in these niches migrate out and are able to populate the olfactory bulb (OB) and the granule layer of the DG, respectively, where they differentiate into glial cells and neurons. Newly formed neurons in the GCL send axonal projections to the CA3 subfield of the hippocampus and dendrites to the molecular layer (Zhao et al., 2006). New neurons in the adult hippocampus receive a variety of inputs from existing mature neurons, and preferentially contact and form synapses with preexisting boutons (Toni et al., 2007). It is thought that during maturation, newly formed granule neurons transiently express the Na+-K+-2Cl- transporter NKCC1 for the enhancement of synaptic integration as they form glutamatergic synapses around 2-3 weeks after birth (Ge et al., 2006). Newly generated dentate granule cells have decreased thresholds for activation, increased resting potentials and they undergo long term potentiation more rapidly (van Praag et al., 2002). While the precise physiological functions of neurogenesis, as well as the full spectrum of functional implications of this extent of plasticity in the adult brain are still under intense investigation,
