Babu et al. Synaptic activity induces neurogenesis (Figure 4G). Incubating neutralizing antibody against NT3 also resulted in a decrease in neuronal differentiation from the precursor cells in co-culture even though its secretion was not regulated by the glycine-induced LTP-like synaptic activity (Figure 4F). These results proved that BDNF released by neurons into the co-culture medium was responsible for the increase in neuronal differentiation from adult precursor cells in response to an LTP-like stimulus. DISCUSSION The present study indicates that structural cellular plasticity brought about by neurogenesis in the adult hippocampus could be the product of a complex and dynamically changing micro- environment, in which the neurons play a critical role. We have shown here ex vivo that adult hippocampal precursor cells respond to coherent neuronal network activity by initiating neuronal dif- ferentiation. This induction of neuronal differentiation from pre- cursor cells was dependent on the presence of mature neurons in that it was the neuronal activity-dependent secretion of BDNF from the mature neurons that mediated the effect on the precursor cells. For this paracrine signaling mechanism, synaptic activation of NMDA receptors and CaMK activation was required, followed by Ca2+ inﬂux into the neurons. We thus propose that cellular plasticity originating from the precursor cells is linked to the network activity based on synaptic transmission between mature neurons. This does not principally rule out that additional effects based on direct neurotransmission to the precursor cells may play a role as previously suggested by others (Deisseroth et al., 2004). In fact some effects of excitation on radial glia-like type-1 cells in vivo might depend on the pres- ence of glutamate receptors on these cells. Type-1 cells respond to seizures induced by the application of kainic acid (Huttmann et al., 2003; Steiner et al., 2008). The largest population of precursor cells in vitro, however, apparently corresponds to the type-2a precursor cells in vivo, but no exact match of phenotypes in vivo and in vitro has yet been reached. Mature neurons also undergo Ca2+ oscillations that may regu- late synaptic strength (Berridge, 1998; Wang et al., 2000b). This is especially interesting in the adult DG where mature functional neurons exhibiting network activity coexist with precursor cells. Neuronally determined precursor cells represent a prospective pool of potentially recruitable neurons. Subsequently, more spe- ciﬁc stimuli, presumably ﬁrst GABAergic excitation then excitatory input from the entorhinal cortex recruit new neurons from this available pool for further integration and maturation (Tozuka et al., 2005; Ge et al., 2007). Thus different types of activity-dependent controls follow each other in the course of adult neurogenesis. Our present study, albeit in vitro, addressed the very ﬁrst stage at which undetermined precursor cells become neuronally determined in response to neuronal network activity. In vivo, GABA would then drive the next step of maturation until the new neurons have made appropriate contacts, received correct glutamatergic input and have made axonal connection to CA3. Activity dependent synaptic plas- ticity in the new neurons themselves would control the terminal maturation and network integration. We identiﬁed BDNF as key mediator between active neurons undergoing LTP-like synaptic plasticity and the trkB-expressing precursor cells. Several studies have shown that the release of BDNF is regulated by stimulus patterns with high frequency bursts being most effective (Balkowiec and Katz, 2002) besides backpropagating action potentials (Kuczewski et al., 2008). We used a stimulus that generated changes similar to the above- referred study, thus supporting the view that BDNF release from mature neurons was associated with patterned neuronal activity. A key question is whether BDNF is not only necessary but also sufﬁcient to mediate neuronal differentiation evoked by synaptic network activity, in particular in the in vivo situation. Paracrine neurotrophin release might be a relatively non-speciﬁc means, only signaling the incidence of neuronal network activity. As one caveat, neurons might respond LTP-like stimuli with a paracrine release of BDNF even in the absence of synaptic plasticity. In such case, the release of BDNF would trigger neuronal differentiation in anticipation of neural network reorganization not the reor- ganization itself. BDNF might also be released in situations other than LTP in the network so that this mechanism is unlikely to be speciﬁc. In addition, the exact location within the DG where the release of BDNF can activate the precursor cells has not been identiﬁed. Immunohistochemical studies have shown that precur- sor cells are morphologically variable with several cells project- ing processes well into the molecular layer (Filippov et al., 2003). Precursor cells with processes could detect BDNF released from the neuronal dendrites far away from their cell bodies. Once the new neurons are more mature, speciﬁc synaptic input becomes dominant in determining the ultimate survival and terminal integration of the new cells. BDNF could also be involved in modulating electrophysiological properties of ion-channels dur- ing neuronal stem cells differentiation and development in our model (Leng et al., 2009). The reductionist setting of our model system might also obscure the relevance of additional mediators. Most notably, the serotoner- gic system has been suggested to be involved in the control of adult neurogenesis at this level. An accepted limitation of tissue culture preparation of neurons as used in this study is the alteration in the expression pattern of receptors and signaling molecules. Besides, hippocampal neuronal cultures besides dentate granule cells also include cells from CA1, CA3 and possibly other neighboring regions, which have different properties related to synaptic plasticity and may thus might inﬂuence the results. Hippocampal neuronal cul- ture also contain non-neuronal cell, albeit in a minor proportion, which have the potential to interfere with in vivo extrapolation of tissue culture results. Irrespective of this, our present study clariﬁes the contribution of synchronous network activity to the initiation of neuronal development from adult hippocampal precursor cells. Therefore, stimulation of neuronal synaptic activity might offer new opportunities to modulate stem cell-based plasticity in the adult hippocampus. ACKNOWLEDGMENTS We would like to thank Wolfgang Uckert for providing with the retrovirus used in this study. We thank Ruth Zarmstorff and Siegne Knespel for technical support. The study was ﬁnanced with gen- erous grants from Deutsche Forschungsgemeinschaft (DFG), Volkswagenstiftung, and Thyssenstiftung. www.frontiersin.org September 2009 | Volume 3 | Article 49 | 9