Babu et al. Synaptic activity induces neurogenesis FIGURE 3 | Glycine-induced synaptic plasticity changes are associated with neuronal differentiation from neural precursor cells. (A) Experimental time line for glycine induced synaptic plasticity changes and associated neuronal differentiation. (B) Glycine induced an increase in the frequency of mEPSCs in hippocampal neurons [158 ± 24% (n = 4) compared to the controls].The recordings were performed before and after the addition of glycine.There was no change in the amplitude of the mEPSCs.The experiments were performed on neurons that were 12 days old. (C)This increase in the frequency of mEPSCs was blocked by the inhibition of NMDA receptors by APV and also by the chelating intracellular Ca2+ with EGTA. (D,E) Glycine induced increased neuronal differentiation from GFP- labeled neural precursor cells in co-culture with primary hippocampal neurons (Ctr; 1.0, LTP; 1.79 ± 0.22, P < 0.01).The co-cultures were stimulated with Mg2+-free glycine solution the day after the co-cultures were started as shown in (A).The number of cells per ﬁeld of view remained unchanged in between the experimental conditions.There was no change in neuronal differentiation when precursor cells were stimulated with glycine in Mg2+-free media in the absence of hippocampal neurons. Experiments were performed at least three times in duplicates. (F)The increase in neuronal differentiation is not due to activation of GABAergic receptors on precursor cells by released GABA from neurons. Inhibiting GABA receptors with SR95531 paradoxically increases neuronal differentiation compared to control possibly by blocking synaptic GABA receptors (Ctr, 1.0; SR95531, 1.63 ± 0.20; P < 0.03). Experiments were done at least three times in duplicates. (G)The increase in neuronal differentiation is blocked by NMDA inhibitor APV and CaMK inhibitor KN-93. Each experiment was done at least three times. underlying mechanism of this effect was not subject of our study but it is likely that seizure-like activity due to the complete disinhi- bition of the neuronal network was the cause. Independent of this, the results conﬁrm that neuronal differentiation can occur in the absence of GABAergic stimulation of the precursor cells. BLOCKING SYNAPTIC PLASTICITY WITH NMDA-ANTAGONISTS LEADS TO SUPPRESSION OF NEUROGENESIS Blocking NMDA receptors abolishes LTP-like synaptic plasticity in vitro (Figurov et al., 1996; Lu et al., 2001; Patterson et al., 2001). If our culture conditions were indeed inducing neurogenesis because the network of neurons in the cultures underwent LTP-like synaptic activity, then blocking the induction of synaptic plasticity should also block the positive regulation of neurogenesis. To address this question we added APV (2-Amino-5-Phosphonopentanoic acid) within the same stimulation paradigm described above to block activated NMDA receptors. APV along with glycine reduced the rise in mEPSC frequency observed to prestimulation levels (Figure 3B). As expected, addition of APV along with glycine to the co-cul- tures abolished the increase in neuronal differentiation (Figure 3G; www.frontiersin.org September 2009 | Volume 3 | Article 49 | 7