Maturation and Functional Integration of New Granule Cells...

Maturation and Functional Integration of NewGranule Cells into the Adult Hippocampus

Nicolas Toni1 and Alejandro F. Schinder2

1Department of Fundamental Neurosciences, University of Lausanne, CH-1005 Lausanne, Switzerland2Fundacion Instituto Leloir (IIBBA–CONICET), 1405 Buenos Aires, Argentina

Correspondence: [email protected]; [email protected]

The adult hippocampus generates functional dentate granule cells (GCs) that release gluta-mate onto target cells in the hilus and cornus ammonis (CA)3 region, and receive glutama-tergic and g-aminobutyric acid (GABA)ergic inputs that tightly control their spiking activity.The slow and sequential development of their excitatory and inhibitory inputs makes themparticularly relevant for information processing. Although they are still immature, newneurons are recruited by afferent activity and display increased excitability, enhanced activ-ity-dependent plasticity of their input and output connections, and a high rate of synapto-genesis. Once fully mature, new GCs show all the hallmarks of neurons generated duringdevelopment. In this review, we focus on how developing neurons remodel the adult dentategyrus and discuss key aspects that illustrate the potential of neurogenesis as a mechanism forcircuit plasticity and function.

Why is there neurogenesis in the dentategyrus of the adult hippocampus and not

in many other cortical regions of the mamma-lian brain with a high demand for plasticity?There are two sides to this question. One isthe mechanistic aspect, which relates to under-standing which cellular and molecular substratesallow the subgranular zone of the dentate gyrusto act as a discrete neurogenic niche in whichadult neural stem cells can give rise to neurons,whereas other areas cannot support neuro-genesis under physiological (nonpathological)conditions. These topics are discussed in depthin the literature (Beckervordersandforth et al.2015; Choe et al. 2015; Gotz et al. 2015; Kuhn2015). The other side to this question focuseson how neurogenesis might influence hippo-

campal function, taking into account the func-tion of the dentate gyrus as a whole, the prop-erties of the network in which newly generatedneurons are incorporated, and the functionthese new neurons might undertake.

Evidence accumulated over the last twodecades has shown that dentate granule cells(GCs) are the only neuronal type generated inthe adult hippocampus. A new GC developingin the adult hippocampus faces the challenge ofproperly integrating in a complex network andprocessing information with functional rele-vance. Neurogenesis begins when radial glia-like (RGL) neural stem cells of the subgranularzone exit the quiescent stage and become am-plifying neural progenitor cells that finally di-vide to adopt a neuronal fate. There are several

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hallmarks that can be distinguished as almostdiscrete events of neuronal maturation: estab-lishment of neuronal polarity and migration, g-aminobutyric acid (GABA)ergic synapse for-mation onto apical dendrites, glutamatergicsynaptogenesis, circuit integration, followed bythe final steps of maturation and refinement.Completing this process in the adult mamma-lian brain requires several weeks. In this article,we will go in depth into this process to under-stand how neuronal development provides aunique mechanism of plasticity that will ulti-mately determine the influence of adult-bornneurons on information processing in the hip-pocampus.

STRUCTURE AND CIRCUITS OF THEDENTATE GYRUS

The dentate gyrus forms a V-shaped structureembedded into the curved cornus ammonis(CA), which itself is composed of CA3, CA2,and CA1 regions of the hippocampus (Fig. 1).It can be subdivided into three layers. The GClayer (GCL) is the middle stratum, formed by sixto eight layers of densely packed cell bodies ofGCs, the principal neurons of the dentate gyrus.The GCL is divided into the suprapyramidal andinfrapyramidal blades, reflecting the portions ofthe layer located above or below the CA3 region,and the tip of the V-shaped structure is known as

HilusGCL

Perforantpathway

Dentate gyrus

CA1

Mossy fiberpathway

Schaffer collateralpathway

Hilus

CA3 Lateral EC

Medial EC

Basket cell

Mossy cell

Granule cell

Pyramidalcell

Pyramidal cell

Figure 1. Circuits of the hippocampus. Schematic view of a transversal slice through the hippocampus depictingthe dentate gyrus, cornus ammonis (CA)3, and CA1. The principal cells located in densely packed layers aresynaptically connected via the so-called trisynaptic circuit. The main entry site for this circuit is the perforantpath, composed of axons originating from lateral and medial entorhinal cortex (EC) and contacting the outerand medial molecular layer, respectively. Granule neurons then project mossy fibers to CA3 pyramidal neurons,which then project Schaffer collaterals to CA1 pyramidal neurons. Other input to granule neurons includeinterneurons located in the molecular layer and in the hilus, principally composed of basket and mossy cells.

N. Toni and A.F. Schinder

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the apex or the crest (Amaral et al. 2007). GCsdisplay an inverted cone-shaped tree of spinyapical dendrites that project through the “stra-tum moleculare” (molecular layer), the mostsuperficial layer of the dentate gyrus. This layermainly contains axons of the perforant pathwaythat originate from the entorhinal cortex andform synaptic contacts with dendrites of GCs.It also contains GABAergic interneurons andafferent fibers of inputs extrinsic to the dentategyrus. The innermost layer is the hilus, alsocalled polymorphic layer, which contains the ax-ons of GCs as well as GABAergic and glutama-tergic interneurons, the most abundant of whichare the mossy cells. The border between the GCLand the hilus is the subgranular zone, a region inwhich adult neurogenesis occurs, which con-tains neural stem cells and progenitor cells, aswell as the bodies of GABAergic basket cells.

Input Pathways

The dentate gyrus is the main entry point ofthe classical trisynaptic hippocampal network,which comprises a lamellar set of unidirectionalconnections connecting the entorhinal cortex tothe dentate gyrus, the dentate gyrus to the CA3,the CA3 to the CA1, and the CA1 to the ento-rhinal cortex. It should be kept in mind, howev-er, that the so-called trisynaptic network is notentirely linear, because the perforant pathwayalso impinges directly onto CA3 pyramidal cells,thus bypassing the dentate gyrus. Inputs to thedentate gyrus bring sensory information fromthe cortex that will ultimately lead to the pro-duction of episodic memories. The major excit-atory/glutamatergic input to the dentate gyrusenters through the perforant pathway from theentorhinal cortex. Distinct areas of the entorhi-nal cortex project toward and innervate differentdendritic portions of GCs. The medial entorhi-nal cortex (layer II, with some contribution oflayers III, V, and VI) targets the middle third ofthe dendritic tree of GCs, whereas projectionsfrom the lateral entorhinal cortex contact theouter third of those dendrites (van Strien et al.2009). GC dendrites of the inner third of themolecular layer are mostly innervated by associ-ational and commissural fibers that arise from

glutamatergic mossy cells of the ipsilateral andcontralateral hilus (Frotscher et al. 1991; Buck-master et al. 1992). Because presynaptic gluta-matergic terminals contact postsynaptic den-dritic spines, the number of spines may serveas an estimate of the total number of gluta-matergic inputs of a given neuron. GCs have atotal dendritic tree length of between 2800 to3500 mm, depending on whether they are locat-ed in the infra- or suprapyramidal blades, and adendritic spine density of �2 spines/mm (asevaluated by confocal microscopy). Thus, thereare �6000 to 7000 glutamatergic terminals im-pinging onto each GC (Desmond and Levy1985; Claiborne et al. 1990).

GCs also receive inputs from several types ofGABAergic interneurons (Amaral 1978; Hanet al. 1993; Freund and Buzsaki 1996; Houser2007; Hosp et al. 2013). These include the basketcells that are located in the subgranular zone andcontact primarily the cell bodies and axon initialsegments of GCs, as well as their proximal den-drites in the molecular layer. In addition to re-leasing GABA, they also express the calcium-binding protein parvalbumin and are namedafter the basket-shaped axonal plexus they formaround the cell bodies of GCs. Two main typesof GABAergic interneurons reside in the hilus.The hilar perforant path-associated (HIPP)cells, some of which express the peptide soma-tostatin) that project to the outer two-thirdsof the molecular layer, and the hilar commissur-al-associational pathway-related (HICAP) cellsthat project to the inner third of the molecularlayer. In the molecular layer, molecular layer per-forant path-associated (MOPP) cells project tothe outer two thirds of the molecular layer,whereas the axo-axonic cells innervate the initialsegments of granule neurons axons.

The dentate gyrus also receives extrinsic in-put from structures lying outside the hippo-campus. The presubiculum and the parasubic-ulum project glutamatergic fibers to the secondthird of the molecular layer. The septal nuclei(mainly the medial septal nucleus and the diag-onal band of Broca) project GABAergic fibers tothe hilus and cholinergic fibers to the innerthird of the molecular layer. The supramammil-lary area projects glutamatergic fibers (some of

Functional Maturation of Adult-Born Granule Cells

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which also colocalize with substance P) to themost proximal dendritic portions of GCs. Thelocus coeruleus also projects noradrenergic fi-bers to the dentate gyrus, mainly into the hilus.Dopaminergic projections from the ventral teg-mental area also innervate the hilus, as do theserotonergic projections from the raphe nucle-us, in addition to their innervation of the sub-granular zone.

Output Pathways

GCs project their axons (mossy fibers) throughthe stratum lucidum, across the hilus, reachingthe CA3 region and contact pyramidal cells withtheir characteristically large and irregularlyshaped mossy fiber terminals. Mossy fiber syn-apses are sparse; each CA3 pyramidal cell is con-tacted by fewer than 50 GCs (Amaral et al.1990). However, mossy fiber terminals are veryeffective at activating their pyramidal target cells(Henze et al. 2002). Mossy fibers also have col-laterals (�6–7 per fiber) that innervate mossycells and GABAergic interneurons through thinterminals (Acsady et al. 1998). Each GC pos-sesses �160 to 200 axonal varicosities alongits collaterals and �20 mossy fiber terminals.Of these, the mossy fiber–GABA interneu-ron–CA3 pyramidal cell disynaptic circuit un-derlies a potent feedforward inhibition thatregulates the excitability of the CA3 area (Tor-borg et al. 2010). Individual mossy fiber termi-nals are morphologically distinct dependingon the nature of the target cell, but also displaydistinct functional features in terms of trans-mission and plasticity (McBain 2008).

DEVELOPMENT AND MATURATIONOF ADULT-BORN DENTATE GCs

Methods Used to Investigate the Maturationof Adult-Born Dentate GCs

Over the past decade, the functional propertiesof newly generated GCs have been mostly inves-tigated using retroviral-labeling techniquescombined with electrophysiological recordingsperformed in acute slices (van Praag et al. 2002).The most commonly used oncoretroviruses, de-rived from the Moloney murine leukemia virus,

can be delivered directly into the dentate gyrusof anesthetized adult rodents through stereo-taxic injection, and transduce dividing progen-itor cells (Tashiro et al. 2006). Retroviruses canonly enter the nucleus and integrate into thehost genome during the prophase–prometa-phase transition, when the nuclear envelopebreaks down and, therefore, they effectivelytransduce fast-dividing neural progenitor cells.Fluorescent reporters and other transgenes ofinterest can be thus readily expressed in theprogeny of neural progenitor cells. Expressionof fluorescent reporters in the progeny of neuralstem cells has enabled the characterization ofthe development, maturation, and integrationof adult-generated GCs at the morphologicaland functional levels (Ge et al. 2006; Laplagneet al. 2006; Zhao et al. 2006; Toni et al. 2007).

Other methods, such as fluorescent report-ers expressed under specific promoters in trans-genic mice (such as the pro-opiomelanocortin[POMC]-green fluorescent protein [GFP] orthe doublecortin [DCX]–GFP mice), havealso contributed substantially to understandingthe structure and function of newborn GCs(Couillard-Despres et al. 2006; Overstreet-Wa-diche et al. 2006a). These methods allow GCsto be readily identified under fluorescence mi-croscopy in live or chemically fixed tissue. Con-focal and electron microscopy in chemicallyfixed sections has been used extensively tocharacterize the expression of early and lateneuronal markers, and also to create completemorphological reconstructions of GCs at differ-ent developmental stages. Whole-cell patchclamp recordings have been used to describethe physiology of new neurons, from basicmembrane properties to synaptic inputs andoutputs throughout development. Used aloneor in combination, these methods enabled greatadvances in our understanding of the matura-tion of adult-born GCs and their integrationinto the hippocampal circuit.

The Subgranular Zone and the Transition fromNeural Stem Cell to Postmitotic Neuron

Adult-born GCs originate from neural stemcells that are located in the subgranular zone.





This occurs through several intermediate steps,which can be characterized using marker ex-pression (Kronenberg et al. 2003) and clonalanalysis (Fig. 2) (Bonaguidi et al. 2011). Neuralstem cells have a radial morphology, with anapical process extending through the GCL andbranching into the molecular layer. They areoften referred to as RGL cells or type 1 cellsand express markers, such as nestin, sox2, and

glial fibrillary acidic protein (GFAP). RGL cellsrarely divide, but when they do, they self-renewand give rise to intermediate progenitor cells(IPCs) (Bonaguidi et al. 2012). IPCs are alsolocated in the subgranular zone, have short pro-cesses, and proliferate rapidly. IPCs include type2a cells that still express nestin and sox2, andtype 2b cells that express tbr2 but not nestin.Type 2 cells then give rise to type 3 cells, which

Subgranularzone

Granularlayer

Molecularlayer

~1–3 d

HilusCA3

GABA input tonic

GABA input dendritic

Glutamate input spine

Glutamate output hilus

Glutamate output CA3

GABA input soma

~1 wk~2 wk

~3 wk

~4 wk

Figure 2. Developmental stages of adult-born granule cells (GCs). Schematic representation and confocalmicrographs of the different stages of maturation, from stem cell to mature neuron. The bottom panel indicatesthe approximate timeline of the major input and output (based on data in Toni and Sultan 2011). CA, Cornusammonis; GABA, g-aminobutyric acid.





no longer express tbr2, but instead express theimmature neuronal markers DCX and Prox1.They have a short vertical process and are al-ready committed to the neuronal lineage. Final-ly, type 3 cells differentiate into GCs, which, onmaturation, express the neuronal marker cal-bindin, extend an axon toward the CA3 regionof the hippocampus and dendrites into themolecular layer.

The functional integration of newborn neu-rons is not expected to have functional conse-quences for the hippocampal network beforethe formation of the first output synapses,which occurs by the end of the second week afterdivision. However, at earlier time points in thedevelopment of newborn neurons, synaptic-likeinputs play a significant role; indeed, even RGLcells express GABA and glutamate receptors(Tozuka et al. 2005; Wang et al. 2005). Althoughsynaptic currents are absent from RGL cells,GABAergic axon terminals from parvalbumin-expressing neurons are found in close proximi-ty, and their activity induces a tonic response inRGL cells, leading to their quiescence. Single-cell deletion of the g2 subunit of the GABAreceptor induces RGL cells to rapidly exit quies-cence and begin symmetrical division. Con-versely, optogenetic stimulation of parvalbuminaxons reduces their rate of division (Song et al.2012). Later in the developmental process,IPCs, although still dividing, receive immatureGABAergic input from parvalbumin interneu-rons that can quickly mature on intense neuro-nal activity and promote the cell survival (Songet al. 2012). Thus, at these early stages, synapticinput couples network activity to the regulationof progenitor/stem-cell proliferation.

Early Postmitotic Neurons

GCs born in the adult hippocampus follow apattern of morphological and functional matu-ration that closely resembles what occurs in newneurons during embryonic development, al-though it occurs at a much slower pace (Joneset al. 2003; Esposito et al. 2005; Overstreet-Wadiche et al. 2006a). During the first fewdays, postmitotic neurons remain in the prolif-erative (subgranular) zone and begin to display

a polarized shape, with neurites that begin toextend in a bipolar fashion parallel to the GCL.These young neurons display very high inputresistance (several gigaohms), because of thelow density of Kþ channels in the plasma mem-brane, and express early neuronal markers, suchas the microtubule-associated protein DCX andpolysialic acid neural cell-adhesion molecule(PSA-NCAM) (Schmidt-Hieber et al. 2004; Es-posito et al. 2005; Ge et al. 2006). Although theylack synaptic inputs, they do express GABAA

and glutamate (both a-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid [AMPA] andN-methyl-D-aspartate [NMDA]) receptors aswell as voltage-dependent Naþ and Kþ channelsat a low density. Thus, depolarizing steps elicitimmature action potentials (single spikes withsmall amplitude and long duration) in currentclamp recordings.

Similar to what occurs in the developingbrain, GABAA-mediated currents in immatureGCs of the adult dentate gyrus are depolarizingbecause of the reverse Cl2 gradient (Ben Ari2002; Overstreet-Wadiche et al. 2005; Tozukaet al. 2005). Intracellular Cl2 regulation in neu-rons depends primarily on the balance betweenthe activity of the Naþ/Kþ/Cl2 cotransporterNKCC1 that drives Cl2 influx, and the Cl2/Kþ

cotransporter KCC2 that drives Cl2 extrusion(Blaesse et al. 2009). Although the expression ofNKCC1 in immature neurons is high, KCC2levels are low, leading to a high intracellularCl2 concentration and, in turn, a depolarizedequilibrium potential for Cl2. Thus, GABAA

receptor activation in young neurons (includingimmature adult-born GCs) induces Cl2 effluxwith the consequent membrane depolariza-tion, which also triggers Ca2þ influx throughvoltage-gated Ca2þ channels. As neurons ma-ture, KCC2 levels increase and GABA becomeshyperpolarizing. In adult-born GCs, this tran-sition seems to occur throughout the initial4 wk of development, such that by the fourthweek GABA becomes hyperpolarizing (Ge et al.2006; Karten et al. 2006), acting as a bona fideinhibitory transmitter. The depolarizing effectsof GABA are required for proper maturationand synaptic integration of new GCs (Ge et al.2006).





As development proceeds through the sec-ond week, GCs continue to express immatureneuronal markers (DCX/PSA-NCAM), displaygigaohm input resistance, and increase the den-sity of voltage-gated Naþ and Kþ channels thatsupport the maturation of membrane excitabil-ity. This is the period of maximal migration inwhich GCs establish their final position withinthe inner half of the GCL (Kempermann et al.2003; Esposito et al. 2005; Mathews et al. 2010).They begin to acquire a polarized neuronalmorphology, extending their axons throughthe hilus toward the CA3 region, and their den-drites through the molecular layer (Sun et al.2013). GABAergic terminals begin to make thefirst functional synapses onto the dendrites ofnew GCs. Presynaptic GABA release elicits post-synaptic currents (PSCs) with slow kinetics thatexert a depolarizing effect (Esposito et al. 2005;Overstreet-Wadiche et al. 2005, 2006a; Tozukaet al. 2005; Ge et al. 2006). One important pop-ulation of GABAergic neurons responsible forthe initial contacts with slow kinetics are theneurogliaform interneurons, whose connectiv-ity with adult-born GCs is initially sparse andincreases several-fold as GCs mature (Mark-wardt et al. 2011).

The Onset of Neuronal Activity

When GCs enter the third week, their cell bodiesbecome positioned within the GCL and extendspineless dendrites through the molecular layer.This stage corresponds to the onset of glutama-tergic synaptogenesis, when perforant path ter-minals begin to establish contacts onto the na-ked dendrites of newborn cells. Synaptogenesishas been characterized in detail using both mor-phological and electrophysiological approachesperformed in retrovirally labeled GCs and inPOMC transgenic mice (Esposito et al. 2005;Ge et al. 2006; Zhao et al. 2006; Toni et al.2007). The mechanisms that trigger glutamater-gic synaptogenesis onto new GCs have recentlybeen tackled. Chancey and collaborators haveshown that immature GCs bear NMDA recep-tors only—containing (silent) synapses that canbe converted into NMDA/AMPA (active) syn-apses after coincident GABA-mediated depol-

arization and NMDA receptor activation bypresynaptic glutamate release (Chancey et al.2013). This work also showed that synaptic “un-silencing” can be induced in mice by a 2-h ex-posure to an enriched environment in a mannerthat is dependent on depolarizing GABAergicactivity, revealing that a brief behavioral experi-ence can produce the fast functional integrationof new GCs into the network; an outstandinglevel of circuit plasticity. It remains to be under-stood whether or not all glutamatergic synapto-genesis requires activity-dependent unsilencingor perhaps some active synapses may formspontaneously.

From the initial moment of excitatory syn-apse formation at 2 wk of age, glutamatergicsynaptogenesis onto developing GCs takes sev-eral weeks. As GCs mature, dendrites grow andbranch, and spine density also increases, gener-ating a continuous “demand” of active termi-nals (Esposito et al. 2005; Ge et al. 2006; Zhaoet al. 2006; Toni et al. 2007; Piatti et al. 2011; Sunet al. 2013). This morphological development isaccompanied by a substantial increase in theamplitude of postsynaptic responses that canbe visualized on activation of the perforantpathway in acute slices. The maximal size ofexcitatory postsynaptic responses is reached af-ter �6 wk (Laplagne et al. 2006; Mongiat et al.2009). The integration of new neurons results inthe formation of new networks and the modi-fication of preexisting ones. Three-dimensionalanalyses of dendritic protrusions of new neu-rons at the electron microscopic level indicatethat, instead of synapsing with new axonal ter-minals, neurons primarily connect to preexist-ing terminals, as evidenced by the presence of amature dendritic spine on the contacted termi-nals (Toni et al. 2007). These axon terminals arethus defined as multiple synapse boutons, asthey are contacted by two postsynaptic partners.As new GCs grow older, the proportion of mul-tiple synapse boutons decreases, suggesting thatthey are transitory structures formed while newspines replace older spines, thus restructuringthe network they formed before the generationof the new GC.

The velocity of maturation is also a highlydynamic process. It has been long known that





electroconvulsive seizures increase the rate ofneuronal proliferation and accelerate the pro-cess of neuronal development and maturationin the adult hippocampus (Parent et al. 1997;Overstreet-Wadiche et al. 2006b). These initialfindings, obtained in models of brain disorders,have in fact revealed a set of complex mecha-nisms that control many aspects of adult neuro-genesis under physiological conditions. Bothproliferation and maturation are acceleratedby running, which increases electrical activityin the hippocampus (van Praag et al. 1999;Piatti et al. 2011). It is likely that other con-ditions also modulate maturation in an activi-ty-dependent manner. As an example, spatiallearning has been recently shown to exert aremarkable effect in dendritic remodeling (Tro-nel et al. 2010).

Thus, by residing in the middle of the den-tate gyrus, RGL cells and their progeny are stra-tegically positioned at the crossroads of severalfeedforward and feedback network loops. As aconsequence, every step of maturation, fromstem-cell division to synaptic integration, isprone to modulation by neuronal activity andis therefore highly responsive to behavioral orpathological interference (reviewed in Kemper-mann 2015; Kuhn 2015; Lucassen et al. 2015).

Activity-Dependent Synaptic Plasticity

During this period of intense functional synap-togenesis, glutamatergic synapses onto newGCs are highly prone to activity-dependentplasticity, a mechanism that has long been im-plicated in learning and memory (Neves et al.2008). The synapse between perforant path ter-minals and new GCs displays a low threshold forthe induction of long-term potentiation (LTP).Work from several laboratories has shown thatconditions that would normally elicit little or noplasticity in synapses onto mature GCs can po-tentiate inputs onto young GCs: (1) weak stim-uli that elicit coincident pre- and postsynapticactivity are sufficient to induce LTP (Schmidt-Hieber et al. 2004); (2) LTP induction can beachieved leaving GABAergic inhibition intact(Wang et al. 2000; Snyder et al. 2001; Ge et al.2008; Massa et al. 2011; Li et al. 2012); and (3)

the levels of synaptic potentiation are largelyincreased (Ge et al. 2007). In this latter study,it is nicely shown that synaptic plasticity is en-hanced only in excitatory inputs to neurons be-tween 4 and 6 wk of age. After 7 wk, the levelof plasticity in newborn GCs becomes similarto that of mature GCs. Activity-dependent plas-ticity is also sensitive to neuromodulators. Forinstance, dopamine can decrease not only thesize of postsynaptic glutamatergic responses butalso the extent of LTP in immature neurons (Muet al. 2011).

Different synaptic and network mechanismsseem to contribute to the enhancement in ac-tivity-dependent synaptic plasticity observedin immature GCs. The key mechanism that hasbeen described so far is the switch in the NMDAreceptor composition that occurs during neuro-nal maturation. Immature GCs express NR2B-containing NMDA receptors, which display ahigh affinity for CaMKII and are responsiblefor the increased LTP levels (Snyder et al. 2001;Barria and Malinow 2005; Ge et al. 2007). Asneurons mature, they switch toward NR2A-con-taining NMDA receptors that present lower af-finity for CaMKII and, consequently, decreasedlevels of LTP expression. Interestingly, the en-hanced plasticity seen for excitatory GC inputshas recently been found for the outputs of new-born GCs, at the level of mossy fibers (see below)(Gu et al. 2012). It is still unclear whether suchenhanced plasticity is required for tailoring theconnectivity of immature GCs that are beingintegrated into the network, or whether plastic-ity of input synapses onto new GCs is, in itself, amechanism involved in processing and/or stor-age of incoming information.

FUNCTIONAL PROPERTIES ANDCONNECTIVITY NEW GCs AT THE MATURESTAGE

New GCs develop and mature for several weeksto reach a fully mature neuronal phenotype(Fig. 2). Most evidence indicates that new GCsreach a plateau for morphological and function-al development between the fourth and sixthweeks of age. At that stage, these neurons displaya dendritic spine density of �2 spines/mm





(Zhao et al. 2006), a total length of dendritesof �1500 mm, and a primary axonal lengthof 2 mm, with �10 axonal boutons/mm (Sunet al. 2013). However, discrete morphologicalparameters continue to mature up to 3 months,such as the number of presynaptic vesicles andcontacted spines per mossy fiber terminal (Toniet al. 2008).

For a long time, it has been hypothesizedthat adult-born GCs might acquire functionalproperties that are unique and different com-pared with those of neurons born in develop-ment for several reasons. Adult neurogenesisoccurs in a radically different environment,neuronal maturation in the adult hippocampusoccurs at a much slower pace, and neural pro-genitor cells might express a different program.To understand whether all mature GCs ulti-mately belong to the same functional popula-tion or whether adult-born neurons maintain adistinctive functional phenotype, double retro-viral labeling was used to distinguish betweenneurons generated in the developing and thosegenerated in the adult dentate gyrus (Laplagneet al. 2006, 2007). A retrovirus expressing GFPwas delivered to label neural progenitor cellsdividing during early postnatal development.A second retroviral injection was then per-formed during adulthood to express a red fluo-rescent protein (RFP) in adult-born neurons ofthe same hippocampus. Electrophysiological re-cordings in acute slices were used to carry out aquantitative comparison of GABAergic inputsfrom different origins (dendritic and somatic),as well as glutamatergic inputs from the medialand lateral perforant paths. Remarkably, all ma-ture GCs (regardless of their birth date) dis-played GABAergic and glutamatergic inputs ofsimilar kinetics and strength, and showed sim-ilar firing responses to evoked glutamatergicexcitation. The only exception to the similar-ity is that GCs generated during embryonicdevelopment seemed to display lower valuesfor input resistance, which might in turn de-crease their overall activity levels (Laplagneet al. 2007). Therefore, developmental and adultneurogenesis produces neurons that acquiresimilar functional properties. However, as dis-cussed below, adult-born neurons may become

engaged in circuit activity while still immature,at which time they display unique functionalproperties.

Data on input connectivity obtained byelectrophysiological recordings has been furthersubstantiated by anatomical studies using retro-grade monosynaptic tracing with pseudotypedrabies virus. This method allows tracing connec-tions between specific types of neurons andtheir first-order presynaptic partners (Wicker-sham et al. 2007). To search for the presynapticconnectome of adult-born GCs, Vivar and col-leagues (2012) and Deshpande and colleagues(2013) combined retroviral transduction ofneural progenitor cells with the rabies virus ap-proach. Retroviral vectors were used to trans-duce progenitor cells of the adult dentate gyruswith an avian receptor for the envelope proteinEnvA, the rabies virus glycoprotein G (whichallows retrograde transsynaptic viral transfer topresynaptic neurons), and a fluorescent report-er. Subsequent delivery of the EnvA-pseudo-typed rabies virus with another fluorescentreporter was used to assemble the rabies virusin new GCs carrying the EnvA receptor and gly-coprotein G (the starter neuron), allowing thevirus to jump and label first-order presynapticpartners. Consistent with previous functionalstudies, hilar GABAergic interneurons werefound among the most abundant presynapticpartners at early developmental stages (withinthe first 2 wk), followed by molecular layer in-terneurons that seemed to appear somewhatlater. GABA neurons included basket, HIPPand MOPP cells, which remained connectedlong after the maturation of new GCs (Li et al.2013). Intriguingly, hilar mossy cells were alsofound to target new GCs only at early develop-mental time points (1–2 wk). After �3 wk, glu-tamatergic neurons from the medial and lateralentorhinal cortex, the most well-characterizedinput to GCs, established their connectivity, to-gether with cholinergic neurons, from the me-dial septum and diagonal band of Broca.

Recent work by Berninger and colleagueshas revealed that the quantity and quality ofinput connections to adult-born GCs can bedefined by experience, but only within a criticalperiod of GC development encompassed within





the second and the sixth weeks (Bergami et al.2015). Exposure to enriched environment dur-ing the 4-wk critical period produced a transientincrease in GABAergic interneurons innervat-ing new GCs and a permanent enhancementin innervation from the entorhinal cortex,which might be a result of the increased levelsof synaptic potentiation described for 4-wk-oldGCs (Ge et al. 2007).

OUTPUT OF ADULT-BORN GCs:AXONS, SYNAPSES, ANDNEUROTRANSMITTERS

Functional integration in the dentate gyrus net-work depends not only on establishing appro-priate afferent connections that will allow newGCs to integrate an excitatory signal to elicit aspike, but also on the capability to transmit theencoded information onto postsynaptic targetcells. Axons can be observed in the hilar regionand proximal CA3 area (CA3c) during the sec-ond week of development (Fig. 3). They arrive atthe more distal CA3 area (CA3a) by the earlydays of the third week and reach a plateau intheir growth during the fourth week, with theprimary axon reaching a maximum length of�1.8 mm (Zhao et al. 2006; Sun et al. 2013). Itis interesting that, although axonal growth pro-ceeds at a relatively fast pace (an average of�70 mm/day over 3 wk), morphological matu-ration of presynaptic terminals is a slow process.For instance, cross-sectional areas of presynap-tic terminals onto hilar interneurons reach ma-ture values by the fourth week, and terminalsimpinging onto CA3 pyramidal cells seem torequire even longer periods (Faulkner et al.2008; Toni et al. 2008). In fact, more than 8 wkare needed for new terminals to reach sizes anddensities that are similar to those of mature GCs.This relatively slow maturation may be causedby the synaptic rearrangements induced by theformation of new mossy fiber terminals, whichmay interfere with preexisting ones (Fig. 3).During synapse formation in the CA3 area, sev-eral mossy fiber terminals contact thorny excres-cences that are already contacted by other mossyfibers (Toni et al. 2008). Such a mechanism re-sembles the synaptic competition occurring be-

tween axons at the developing neuromuscularsynapse and may represent a mechanism of ac-tivity-dependent refinement of the connectivi-ty of newborn neurons (Buffelli et al. 2003).Thus, maturation of the connectivity of new-born neurons induces modifications in theconnectivity of preexisting neurons and maycontribute to the pruning of older, perhapsless-effective synapses.

The question of whether mossy fiber termi-nals of newborn cells establish functional syn-apses onto target neurons was approached usingoptogenetics, namely, retroviral transduction ofnewborn GCs with the light-activated cationchannel channelrhodopsin-2 (ChR2) (Boydenet al. 2005; Li et al. 2005). Specific expression ofChR2 in the progeny of dividing progenitor cellsallowed the selective activation of labeled new-born GCs, by delivery of blue light onto hippo-campal slices, while performing whole-cellrecordings in putative postsynaptic cells (Toniet al. 2008). This approach revealed that fullymature newborn GCs make functional gluta-matergic synapses onto hilar interneurons,mossy cells, and CA3 pyramidal cells, and excit-atory postsynaptic responses were observed inpyramidal cells as early as 2 to 3 wk after retro-viral delivery (Gu et al. 2012). The demonstra-tion that glutamatergic inputs elicit spiking ofadult-born GCs (Laplagne et al. 2006), and thatoptogenetic stimulation elicits glutamate re-lease from mossy fibers, indicate that matureadult-born GCs neurons are fully integrated inthe hippocampal network and can, presumably,process information in a functionally appropri-ate fashion.

ChR2 expression was also used to investigateactivity-dependent plasticity (specifically, LTP)of mossy fiber synapses onto CA3 pyramidalcells. Gu and colleagues (2012) implanted anoptical fiber in the dentate gyrus region to de-liver optical stimulation and recorded fieldexcitatory postsynaptic potentials in the CA3region in vivo. They elicited u burst stimulationof newborn GCs at 3, 4, and 8 wk of age andrecorded excitatory postsynaptic potential fieldactivity before and after the induction of LTP.They observed that 4-wk-old neurons displayedthe highest sensitivity to LTP induction, in





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Figure 3. Synaptogenesis in new GCs. (A–C) Afferent synapses from perforant path axons. (A) Electronmicrographs of dark immunolabeled dendritic spines from new GCs contacting a multiple synapse bouton(left) or a single synapse bouton (right). (B) Three-dimensional reconstructions from serial-section electronmicrographs of dendritic protrusions from new GCs (in green). Left panel: filopodia approaching and making aphysical contact with a synapse. Middle panel: dendritic spine synapsing with a multiple synapse bouton. Rightpanel: dendritic spine synapsing with a single synapse bouton. (C) Schematic representation of the hypothesison the development of dendritic spine connectivity: Left panel: filopodia from new GCs (green) grow prefer-entially toward preexisting synapses. On stabilization, some filopodia transform into dendritic spines, therebyforming multiple synapse boutons (middle panel). Over maturation time, multiple synapse boutons transforminto single-synapse boutons, probably through the retraction of the preexisting spine. Green, dendritic protru-sion from new GCs; red, dendritic spine from other GCs; blue, axonal boutons (based on Toni and Sultan 2011).(D–F) Efferent synapses on mossy-fiber terminals in the CA3. (D) Electron micrographs of immunolabeledmossy fiber terminals from new GCs at 17 dpi (left panel) and 30 dpi (right panel). Note the thorny excrescenceprotruding into the terminal on the right panel. (E) Three-dimensional reconstructions of terminals from newGCs synapsing directly on the dendrite (left panel), with a shared thorny excrescence with another, nonimmu-nolabeled GCs (middle panel) or with an individual thorny excrescence (right panel). Green, terminal from anew GC; red, dendrite from a CA3 pyramidal neuron; blue, terminal from a nonimmunolabeled GC. (F)Schematic depicting the hypothetical development of output connectivity from new GCs. Left panel: theterminal of a new GC synapses first with the dendrite of a CA3 pyramidal cell. Middle panel: on maturation(30 dpi), some thorny excrescences are shared between new and preexisting terminals. Right panel: after 75 dpi,all terminals from new GCs synapse with individual thorny excrescences (based on Toni and Sultan 2011). Scalebars, 1 mm.



terms of both the percent of neurons displayingLTP and the extent of LTP expression. Theseresults showed that the output of new GCs ishighly sensitive to LTP induction within thesame time window when excitatory inputs tonew GCs are most sensitive to the same typeof plasticity. Thus, newly generated GCs seemto be able to tune the strength of specific excit-atory inputs and outputs to their level of ac-tivity. This might determine the number andidentity of those connections that will bestrengthened by the activity itself, and will ulti-mately determine which of those synapses willremain or disappear.

Optogenetic and chemogenetic approacheswere recently used to compare the feedforwardand feedback networks activated by 4- versus 8-wk-old (mature) GCs (Temprana et al. 2015). Itwas found that new GCs recruit powerful feed-back inhibition onto the GCL and feedforwardinhibition onto CA3 pyramidal cells. It is, how-ever, remarkable that new neurons control distalfeedforward networks at the younger stages, butthey can only activate substantial feedback inhi-bition after they become mature. The functionalimplications of such a delay in the establish-ment of proximal networks are discussed inthe next section.

WHAT IS UNUSUAL ABOUTADULT-BORN GCs?

The previous section highlights that, on reach-ing maturation, adult-born GCs become fullyintegrated in the preexisting network (inputsand outputs) in a manner that is similar to allother GCs generated during development. Con-sequently, new GCs appear ready to process in-coming information. For some time, the ideathat the adult hippocampus would continueto generate neurons throughout life and pro-duce cells with no special or differential charac-teristics has remained puzzling. We discuss be-low recent findings that have contributed tochange this view.

In recent years, the role of adult-born GCsin the dentate gyrus network has been interro-gated in different manners. Focal X-ray irradia-tion was used to eliminate new GCs from the

dentate gyrus (up to 10-wk-old neurons). Sub-sequent in vivo electrophysiological recordingsin anesthetized mice were then used to assess theimpact of their loss on dentate gyrus activity(Lacefield et al. 2010). Responses to perforantpath activation were reduced in mice lackingnew GCs, indicating that this neuronal popu-lation contributes to overall dentate activity.Interestingly, the absence of new GCs increasedthe amplitude of spontaneous g-frequencybursts, suggesting that new GCs might some-how disorganize recurrent activity in the den-tate gyrus.

Work of several laboratories has shownthat developing GCs of the adult dentate gyrusdisplay electrophysiological properties that aredifferent from mature GCs in terms of theirincreased excitability, reduced GABAergic in-hibition, and weak excitatory connectivity(Schmidt-Hieber et al. 2004; Esposito et al.2005; Overstreet-Wadiche et al. 2005, 2006a,b;Couillard-Despres et al. 2006; Ge et al. 2006;Karten et al. 2006; Stocca et al. 2008). It wasthen investigated whether new GCs acquire in-formation-processing capabilities before reach-ing a fully mature developmental stage. Studiesin acute slices showed that immature GCs withhigh input resistance can trigger action poten-tials in response to small current steps thatwould not activate mature neurons because oftheir low input resistance (Mongiat et al. 2009).In regard to the activation of perforant path ax-ons, immature GCs display weak excitatory in-puts owing to the low number of afferent syn-apses. Yet, those few glutamatergic contacts aresufficient to trigger action potentials in a sub-stantial proportion of immature GCs by thethird developmental week. These experimentswere done in the presence of GABAA receptorantagonists to isolate excitatory inputs andmonitor the intrinsic neuronal properties ofnew neurons. Notably, the increased membraneresistance was sufficient to compensate for theweak afferent connectivity. The mechanisms un-derlying such compensation include the rapiddevelopment of voltage-gated Naþ and Kþ

channels and the delayed expression of the in-ward rectifier Kþ conductance that decreasesmembrane resistance (Mongiat et al. 2009).





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Figure 4. Functional significance of immature granule cells (GCs). (A–D) Experiments in acute hippocampalslices. (A) A stimulating electrode was placed in the perforant pathway. Stimulation of the medial perforant path(mPP) evoked monosynaptic excitation and disynaptic inhibition via g-aminobutyric acid (GABA)ergic inter-neurons (INs) onto GCs. Spiking and the underlying synaptic currents were recorded in individual cells by loosepatch followed by whole cell. (B) Example curves of spiking probability versus input strength with exampletraces. (C) Cumulative distributions of input strength required to elicit 50% spiking probability (activated cells).(D) Activation of GCs evoked by trains and raster plots depicting spikes recorded in response 10 pulses at 10 Hz(five trials/neuron). (From Marin-Burgin et al. 2012; modified, with permission, from The American Associ-ation for the Advancement of Science # 2012.) (E,F) In vivo single unit recording in the rat dentate gyrus.Examples of cells with single (E) and multiple (F) place fields. Rate maps and plots showing spikes (Kunze et al.2009) from the cell superimposed on the rat’s trajectory (gray) are shown in the left and right columns. For therate maps, blue represents no firing and red represents peak firing (data kindly provided by J.P. Neunuebel andJ.J. Knierim).





Thus, the extent of afferent excitation and in-trinsic excitability would allow adult-born GCsat 3 to 4 wk of age to contribute to informationprocessing.

To investigate whether immature GCs arecapable of information processing, the activa-tion of 4-wk-old neurons and mature GCs werecompared (Fig. 4) (Li et al. 2012; Marin-Burginet al. 2012). Activation (spiking) in response tostimulation of the perforant path was assessedusing Ca2þ imaging to monitor entire neuronalpopulations and loose patch recordings of indi-vidual cells in acute hippocampal slices. For anygiven stimulus, the proportion of active neuronswas substantially higher for immature than formature GCs. Weak stimuli primarily activatedimmature neurons. The mechanism underlyingthe difference in activation threshold involvedfeedforward GABAergic inhibition. Whole-cellrecordings showed that the excitation/inhibi-tion balance at the time of spiking is higher inimmature cells owing to the delayed develop-ment of perisomatic GABAergic inhibition (Es-posito et al. 2005). Thus, neuronal activity inthe dentate gyrus is biased toward the immaturepopulation of principal neurons that bypass in-hibitory control, while the activation of matureneurons is limited by inhibition. The observedfunctional properties of immature GCs suggestthat activity reaching the dentate gyrus couldundergo differential decoding through imma-ture neuronal cohorts that are highly responsiveand integrative and, in parallel, through a largepopulation of mature GCs with sparse activityand high input specificity.

It is interesting to note that a recent studyby Neunuebel and Knierim (2012) found thatthere are at least two classes of neurons in thedentate gyrus in freely moving rats. One class,which they suggest corresponds to mature GCs,fires very sparsely, with a small number of activecells firing at single locations within an environ-ment like CA1 and CA3 place cells. The secondclass, which they suggest might correspond tonewborn GCs and/or hilar mossy cells, firesmore promiscuously, with a high fraction ofcells active in an environment and with multiplespatial subfields. Although this latter profilewould nicely reflect the low-activation thresh-

old displayed by immature GCs, this possibilityis yet to be investigated.

CONCLUDING REMARKS

Adult-born GCs develop very slowly, over sev-eral weeks. This is in striking contrast to the fewdays that are required to reach maturation dur-ing perinatal development. Thus, developingGCs dwell at different stages for long intervals,allowing them to sense information while theyremain immature, and become substrates of in-tense activity-dependent synaptic remodeling.Their delayed coupling to inhibitory networksmake them particularly sensitive to incomingsignals and also suggests that they might con-tribute to information encoding once they be-come mature, when they can increase thethreshold for activation of neighbor cells.From this point of view, adult neurogenesismay work as a mechanism to continuously gen-erate pools of new neurons with an unusualcapacity to tailor their connectivity based oncurrent experience. The thousands of new cellsadded every day will contribute to new cohortsof neurons that are ready to sense hippocampalinputs that will make their connectivity unique.Understanding how different stages of GC de-velopment contribute to discrimination of sim-ilar inputs, a basic function of the dentate gyrus,is the next challenge in this field.

ACKNOWLEDGMENTS

We thank Jonathan Moss for critical commentson the manuscript and Jamie Simon and Geor-gina Davies-Sala for artwork. This work wasfunded by grants from the National Institutesof Health (NIH) (Fogarty International Re-search Collaboration Award [FIRCA]), theHoward Hughes Medical Institute (Senior In-ternational Research Grant), and the AgenciaNacional de Promocion Cientıfica y Tecnolo-gica (PICT 2010, Argentina) to A.F.S., andfrom the Swiss National Science Foundation(PP00P3-150753) to N.T. A.F.S. is a principalinvestigator from the Argentine Research Coun-cil (CONICET).





REFERENCES�Reference is also in this collection.

Acsady L, Kamondi A, Sik A, Freund T, Buzsaki G. 1998.GABAergic cells are the major postsynaptic targets ofmossy fibers in the rat hippocampus. J Neurosci 18:3386–3403.

Amaral DG. 1978. A Golgi study of cell types in the hilarregion of the hippocampus in the rat. J Comp Neurol 182:851–914.

Amaral DG, Ishizuka N, Claiborne B. 1990. Neurons, num-bers and the hippocampal network. Prog Brain Res 83:1–11.

Amaral DG, Scharfman HE, Lavenex P. 2007. The dentategyrus: Fundamental neuroanatomical organization (den-tate gyrus for dummies). Prog Brain Res 163: 3–22.

Barria A, Malinow R. 2005. NMDA receptor subunit com-position controls synaptic plasticity by regulating bind-ing to CaMKII. Neuron 48: 289–301.

� Beckervordersandforth R, Zhang C-L, Lie DC. 2015.Transcription factor-dependent control of adult hippo-campal neurogenesis. Cold Spring Harb Perspect Biol doi:10.1101/cshperspect.a018879.

Ben Ari Y. 2002. Excitatory actions of GABA during devel-opment: The nature of the nurture. Nat Rev Neurosci 3:728–739.

Bergami M, Masserdotti G, Temprana SG, Motori E, Eriks-son TM, Gobel J, Yang SM, Conzelmann KK, SchinderAF, Gotz M, et al. 2015. A critical period for experience-dependent remodeling of adult-born neuron connectiv-ity. Neuron 85: 710–717.

Blaesse P, Airaksinen MS, Rivera C, Kaila K. 2009. Cation-chloride cotransporters and neuronal function. Neuron61: 820–838.

Bonaguidi MA, Wheeler MA, Shapiro JS, Stadel RP, Sun GJ,Ming GL, Song H. 2011. In vivo clonal analysis revealsself-renewing and multipotent adult neural stem cellcharacteristics. Cell 145: 1142–1155.

Bonaguidi MA, Song J, Ming GL, Song H. 2012. A unifyinghypothesis on mammalian neural stem cell properties inthe adult hippocampus. Curr Opin Neurobiol 22: 765–761.

Boyden ES, Zhang F, Bamberg E, Nagel G, Deisseroth K.2005. Millisecond-timescale, genetically targeted opticalcontrol of neural activity. Nat Neurosci 8: 1263–1268.

Buckmaster PS, Strowbridge BW, Kunkel DD, Schmiege DL,Schwartzkroin PA. 1992. Mossy cell axonal projections tothe dentate gyrus molecular layer in the rat hippocampalslice. Hippocampus 2: 349–362.

Buffelli M, Burgess RW, Feng G, Lobe CG, Lichtman JW,Sanes JR. 2003. Genetic evidence that relative synapticefficacy biases the outcome of synaptic competition. Na-ture 424: 430–434.

Chancey JH, Adlaf EW, Sapp MC, Pugh PC, Wadiche JI,Overstreet-Wadiche LS. 2013. GABA depolarization isrequired for experience-dependent synapse unsilencingin adult-born neurons. J Neurosci 33: 6614–6622.

� Choe Y, Pleasure SJ, Mira H. 2015. Control of adult neuro-genesis by short-range morphogenic signaling molecules.

Cold Spring Harb Perspect Biol doi: 10.1101/cshperspect.a018887.

Claiborne BJ, Amaral DG, Cowan WM. 1990. Quantitative,three-dimensional analysis of granule cell dendrites inthe rat dentate gyrus. J Comp Neurol 302: 206–219.

Couillard-Despres S, Winner B, Karl C, Lindemann G,Schmid P, Aigner R, Laemke J, Bogdahn U, Winkler J,Bischofberger J, et al. 2006. Targeted transgene expressionin neuronal precursors: Watching young neurons in theold brain. Eur J Neurosci 24: 1535–1545.

Deshpande A, Bergami M, Ghanem A, Conzelmann KK,Lepier A, Gotz M, Berninger B. 2013. Retrograde mono-synaptic tracing reveals the temporal evolution of inputsonto new neurons in the adult dentate gyrus and olfac-tory bulb. Proc Natl Acad Sci 110: E1152–E1161.

Desmond NL, Levy WB. 1985. Granule cell dendritic spinedensity in the rat hippocampus varies with spine shapeand location. Neurosci Lett 54: 219–224.

Esposito MS, Piatti VC, Laplagne DA, Morgenstern NA,Ferrari CC, Pitossi FJ, Schinder AF. 2005. Neuronal dif-ferentiation in the adult hippocampus recapitulates em-bryonic development. J Neurosci 25: 10074–10086.

Faulkner RL, Jang MH, Liu XB, Duan X, Sailor KA, Kim JY,Ge S, Jones EG, Ming GL, Song H, et al. 2008. Develop-ment of hippocampal mossy fiber synaptic outputs bynew neurons in the adult brain. Proc Natl Acad Sci 105:14157–14162.

Freund TF, Buzsaki G. 1996. Interneurons of the hippocam-pus. Hippocampus 6: 347–470.

Frotscher M, Seress L, Schwerdtfeger WK, Buhl E. 1991. Themossy cells of the fascia dentata: A comparative study oftheir fine structure and synaptic connections in rodentsand primates. J Comp Neurol 312: 145–163.

Ge S, Goh EL, Sailor KA, Kitabatake Y, Ming GL, Song H.2006. GABA regulates synaptic integration of newly gen-erated neurons in the adult brain. Nature 439: 589–593.

Ge S, Yang CH, Hsu KS, Ming GL, Song H. 2007. A criticalperiod for enhanced synaptic plasticity in newly generat-ed neurons of the adult brain. Neuron 54: 559–566.

Ge S, Sailor KA, Ming GL, Song H. 2008. Synaptic integra-tion and plasticity of new neurons in the adult hippo-campus. J Physiol 586: 3759–3765.

� Gotz M, Nakafuku M, Petrik D. 2015. Neurogenesis in thedeveloping adult brain—Similarities and key differences.Cold Spring Harb Perspect Biol doi: 10.1101/cshperspect.a018853.

Gu Y, Arruda-Carvalho M, Wang J, Janoschka SR, JosselynSA, Frankland PW, Ge S. 2012. Optical controlling revealstime-dependent roles for adult-born dentate granulecells. Nat Neurosci 15: 1700–1706.

Han ZS, Buhl EH, Lorinczi Z, Somogyi P. 1993. A highdegree of spatial selectivity in the axonal and dendriticdomains of physiologically identified local-circuit neu-rons in the dentate gyrus of the rat hippocampus. Eur JNeurosci 5: 395–410.

Henze DA, Wittner L, Buzsaki G. 2002. Single granule cellsreliably discharge targets in the hippocampal CA3 net-work in vivo. Nat Neurosci 5: 790–795.

Hosp JA, Struber M, Yanagawa Y, Obata K, Vida I, Jonas P,Bartos M. 2013. Morpho-physiological criteria divide





dentate gyrus interneurons into classes. Hippocampus 24:189–203.

Houser CR. 2007. Interneurons of the dentate gyrus: Anoverview of cell types, terminal fields and neurochemicalidentity. Prog Brain Res 163: 217–232.

Jones SP, Rahimi O, O’Boyle MP, Diaz DL, Claiborne BJ.2003. Maturation of granule cell dendrites after mossyfiber arrival in hippocampal field CA3. Hippocampus13: 413–427.

Karten YJ, Jones MA, Jeurling SI, Cameron HA. 2006.GABAergic signaling in young granule cells in the adultrat and mouse dentate gyrus. Hippocampus 16: 312–320.

� Kempermann G. 2015. Activity dependency and aging in theregulation of adult neurogenesis. Cold Spring Harb Per-spect Biol doi: 10.1101/cshperspect.a018929.

Kempermann G, Gast D, Kronenberg G, Yamaguchi M,Gage FH. 2003. Early determination and long-term per-sistence of adult-generated new neurons in the hippo-campus of mice. Development 130: 391–399.

Kronenberg G, Reuter K, Steiner B, Brandt MD, Jessberger S,Yamaguchi M, Kempermann G. 2003. Subpopulations ofproliferating cells of the adult hippocampus respond dif-ferently to physiologic neurogenic stimuli. J Comp Neurol467: 455–463.

� Kuhn HG. 2015. Control of cell survival in adult neurogen-esis. Cold Spring Harb Perspect Biol doi: 10.1101/cshperspect.a018895.

Kunze A, Congreso MR, Hartmann C, Wallraff-Beck A,Huttmann K, Bedner P, Requardt R, Seifert G, RedeckerC, Willecke K, et al. 2009. Connexin expression by radialglia-like cells is required for neurogenesis in the adultdentate gyrus. Proc Natl Acad Sci 106: 11336–11341.

Lacefield CO, Itskov V, Reardon T, Hen R, Gordon JA. 2010.Effects of adult-generated granule cells on coordinatednetwork activity in the dentate gyrus. Hippocampus 22:106–116.

Laplagne DA, Esposito MS, Piatti VC, Morgenstern NA,Zhao C, van Praag H, Gage FH, Schinder AF. 2006. Func-tional convergence of neurons generated in the develop-ing and adult hippocampus. PLoS Biol 4: e409.

Laplagne DA, Kamienkowski JE, Esposito MS, Piatti VC,Zhao C, Gage FH, Schinder AF. 2007. Similar GABAergicinputs in dentate granule cells born during embryonicand adult neurogenesis. Eur J Neurosci 25: 2973–2981.

Li X, Gutierrez DV, Hanson MG, Han J, Mark MD, Chiel H,Hegemann P, Landmesser LT, Herlitze S. 2005. Fast non-invasive activation and inhibition of neural and networkactivity by vertebrate rhodopsin and green algae chan-nelrhodopsin. Proc Natl Acad Sci 102: 17816–17821.

Li Y, Aimone JB, Xu X, Callaway EM, Gage FH. 2012.Development of GABAergic inputs controls the contri-bution of maturing neurons to the adult hippocampalnetwork. Proc Natl Acad Sci 109: 4290–4295.

Li Y, Stam FJ, Aimone JB, Goulding M, Callaway EM, GageFH. 2013. Molecular layer perforant path-associated cellscontribute to feed-forward inhibition in the adult den-tate gyrus. Proc Natl Acad Sci 110: 9106–9111.

� Lucassen PJ, Oomen CA, Naninck EFG, Fitzsimons CP, vanDam AM, Czeh B, Korosi A. 2015. Regulation of adultneurogenesis and plasticity by (early) stress, glucocorti-

coids, and inflammation. Cold Spring Harb Perspect Bioldoi: 10.1101/cshperspect.a021303.

Marin-Burgin A, Mongiat LA, Pardi MB, Schinder AF. 2012.Unique processing during a period of high excitation/inhibition balance in adult-born neurons. Science 335:1238–1242.

Markwardt SJ, Dieni CV, Wadiche JI, Overstreet-Wadiche L.2011. Ivy/neurogliaform interneurons coordinate activ-ity in the neurogenic niche. Nat Neurosci 14: 1407–1409.

Massa F, Koehl M, Wiesner T, Grosjean N, Revest JM, PiazzaPV, Abrous DN, Oliet SH. 2011. Conditional reduction ofadult neurogenesis impairs bidirectional hippocampalsynaptic plasticity. Proc Natl Acad Sci 108: 6644–6649.

Mathews EA, Morgenstern NA, Piatti VC, Zhao C, Jess-berger S, Schinder AF, Gage FH. 2010. A distinctive lay-ering pattern of mouse dentate granule cells is generatedby developmental and adult neurogenesis. J Comp Neurol518: 4479–4490.

McBain CJ. 2008. Differential mechanisms of transmissionand plasticity at mossy fiber synapses. Prog Brain Res 169:225–240.

Mongiat LA, Esposito MS, Lombardi G, Schinder AF. 2009.Reliable activation of immature neurons in the adult hip-pocampus. PLoS ONE 4: e5320.

Mu Y, Zhao C, Gage FH. 2011. Dopaminergic modulation ofcortical inputs during maturation of adult-born dentategranule cells. J Neurosci 31: 4113–4123.

Neunuebel JP, Knierim JJ. 2012. Spatial firing correlates ofphysiologically distinct cell types of the rat dentate gyrus.J Neurosci 32: 3848–3858.

Neves G, Cooke SF, Bliss TV. 2008. Synaptic plasticity, mem-ory and the hippocampus: A neural network approach tocausality. Nat Rev Neurosci 9: 65–75.

Overstreet-Wadiche LS, Bromberg DA, Bensen AL, West-brook GL. 2005. GABAergic signaling to newborn neu-rons in dentate gyrus. J Neurophysiol 94: 4528–4532.

Overstreet-Wadiche LS, Bensen AL, Westbrook GL. 2006a.Delayed development of adult-generated granule cells indentate gyrus. J Neurosci 26: 2326–2334.

Overstreet-Wadiche LS, Bromberg DA, Bensen AL, West-brook GL. 2006b. Seizures accelerate functional integra-tion of adult-generated granule cells. J Neurosci 26:4095–4103.

Parent JM, Yu TW, Leibowitz RT, Geschwind DH, SloviterRS, Lowenstein DH. 1997. Dentate granule cell neuro-genesis is increased by seizures and contributes to aber-rant network reorganization in the adult rat hippocam-pus. J Neurosci 17: 3727–3738.

Piatti VC, Davies-Sala MG, Esposito MS, Mongiat LA, Trin-chero MF, Schinder AF. 2011. The timing for neuronalmaturation in the adult hippocampus is modulated bylocal network activity. J Neurosci 31: 7715–7728.

Schmidt-Hieber C, Jonas P, Bischofberger J. 2004. Enhancedsynaptic plasticity in newly generated granule cells of theadult hippocampus. Nature 429: 184–187.

Snyder JS, Kee N, Wojtowicz JM. 2001. Effects of adult neu-rogenesis on synaptic plasticity in the rat dentate gyrus. JNeurophysiol 85: 2423–2431.

Song J, Zhong C, Bonaguidi MA, Sun GJ, Hsu D, Gu Y,Meletis K, Huang ZJ, Ge S, Enikolopov G, et al. 2012.





Neuronal circuitry mechanism regulating adult quiescentneural stem-cell fate decision. Nature 489: 150–154.

� Song J, Olsen RHJ, Sun J, Ming G-L, Song H. 2015. Neuronalcircuitry mechanisms regulating adult mammalian neu-rogenesis. Cold Spring Harb Perspect Biol doi: 10.1101/cshperspect.a018937.

Stocca G, Schmidt-Hieber C, Bischofberger J. 2008. Differ-ential dendritic Ca2þ signalling in young and maturehippocampal granule cells. J Physiol 586: 3795–3811.

Sun GJ, Sailor KA, Mahmood QA, Chavali N, Christian KM,Song H, Ming GL. 2013. Seamless reconstruction of in-tact adult-born neurons by serial end-block imaging re-veals complex axonal guidance and development in theadult hippocampus. J Neurosci 33: 11400–11411.

Tashiro A, Zhao C, Gage FH. 2006. Retrovirus-mediatedsingle-cell gene knockout technique in adult newbornneurons in vivo. Nat Protoc 1: 3049–3055.

Temprana SG, Mongiat LA, Yang SM, Trinchero MF, AlvarezDD, Kropff E, Giacomini D, Beltramone N, Lanuza GM,Schinder AF. 2015. Delayed coupling to feedback inhibi-tion during a critical period for the integration of adult-born granule cells. Neuron 85: 116–130.

Toni N, Sultan S. 2011. Synapse formation on adult-bornhippocampal neurons. Eur J Neurosci 33: 1062–1068.

Toni N, Teng EM, Bushong EA, Aimone JB, Zhao C, Con-siglio A, van Praag H, Martone ME, Ellisman MH, GageFH. 2007. Synapse formation on neurons born in theadult hippocampus. Nat Neurosci 10: 727–734.

Toni N, Laplagne DA, Zhao C, Lombardi G, Ribak CE, GageFH, Schinder AF. 2008. Neurons born in the adult dentategyrus form functional synapses with target cells. NatNeurosci 11: 901–907.

Torborg CL, Nakashiba T, Tonegawa S, McBain CJ. 2010.Control of CA3 output by feedforward inhibition despitedevelopmental changes in the excitation–inhibition bal-ance. J Neurosci 30: 15628–15637.

Tozuka Y, Fukuda S, Namba T, Seki T, Hisatsune T. 2005.GABAergic excitation promotes neuronal differentiationin adult hippocampal progenitor cells. Neuron 47: 803–815.

Tronel S, Fabre A, Charrier V, Oliet SH, Gage FH, AbrousDN. 2010. Spatial learning sculpts the dendritic arbor ofadult-born hippocampal neurons. Proc Natl Acad Sci107: 7963–7968.

van Praag H, Kempermann G, Gage FH. 1999. Runningincreases cell proliferation and neurogenesis in the adultmouse dentate gyrus. Nat Neurosci 2: 266–270.

van Praag H, Schinder AF, Christie BR, Toni N, Palmer TD,Gage FH. 2002. Functional neurogenesis in the adulthippocampus. Nature 415: 1030–1034.

van Strien NM, Cappaert NL, Witter MP. 2009. The anato-my of memory: An interactive overview of the parahip-pocampal-hippocampal network. Nat Rev Neurosci 10:272–282.

Vivar C, Potter MC, Choi J, Lee JY, Stringer TP, CallawayEM, Gage FH, Suh H, van Praag H. 2012. Monosynapticinputs to new neurons in the dentate gyrus. Nat Commun3: 1107.

Wang S, Scott BW, Wojtowicz JM. 2000. Heterogenous prop-erties of dentate granule neurons in the adult rat. J Neuro-biol 42: 248–257.

Wang LP, Kempermann G, Kettenmann H. 2005. A sub-population of precursor cells in the mouse dentate gyrusreceives synaptic GABAergic input. Mol Cell Neurosci 29:181–189.

Wickersham IR, Lyon DC, Barnard RJ, Mori T, Finke S,Conzelmann KK, Young JA, Callaway EM. 2007. Mono-synaptic restriction of transsynaptic tracing from single,genetically targeted neurons. Neuron 53: 639–647.

Zhao C, Teng EM, Summers RG Jr, Ming GL, Gage FH.2006. Distinct morphological stages of dentate granuleneuron maturation in the adult mouse hippocampus. JNeurosci 26: 3–11.





December 4, 20152016; doi: 10.1101/cshperspect.a018903 originally published onlineCold Spring Harb Perspect Biol

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