The adult brain is highly plastic and tends to undergo substantial reorganization after injury to compensate for the lesion effects. It has been shown that such reorganization mainly relies on anatomical and biochemical modifications of the remaining cells which give rise to a network rewiring without reinstating the original morphology of the damaged region. However, few studies have analyzed the neurorepair potential of a neurogenic structure. Thus, the aim of this work was to analyze if the DG could restore its original morphology after a lesion and to establish if the structural reorganization is accompanied by behavioral and electrophysiological recovery. Using a subepileptogenic injection of kainic acid (KA), we induced a focal lesion in the DG and assessed in time (1) the loss and recovery of dependent and non dependent DG cognitive functions, (2) the anatomical reorganization of the DG using a stereological probe and immunohistochemical markers for different neuronal maturation stages and, (3) synaptic plasticity as assessed through the induction of in vivo long-term potentiation (LTP) in the mossy fiber pathway (CA3-DG). Our results show that a DG focal lesion with KA leads to a well delimited region of neuronal loss, disorganization of the structure, the loss of associated mnemonic functions and the impairment to elicit LTP. However, behavioral and synaptic plasticity expression occurs in a time dependent fashion and occurs along the morphological restoration of the DG. These results provide novel information on neural plasticity events associated to functional reorganization after damage.

AbstractHippocampal neurogenesis occurs in the adult brain in various species, including humans. A compelling question that arose when neurogenesis was accepted to occur in the adult dentate gyrus (DG) is whether new neurons become functionally relevant over time, which is key for interpreting their potential contributions to synaptic circuitry. The functional state of adult-born neurons has been evaluated using various methodological approaches, which have, in turn, yielded seemingly conflicting results regarding the timing of maturation and functional integration. Here, we review the contributions of different methodological approaches to addressing the maturation process of adult-born neurons and their functional state, discussing the contributions and limitations of each method. We aim to provide a framework for interpreting results based on the approaches currently used in neuroscience for evaluating functional integration. As shown by the experimental evidence, adult-born neurons are prone to respond from early stages, even when they are not yet fully integrated into circuits. The ongoing integration process for the newborn neurons is characterised by different features. However, they may contribute differently to the network depending on their maturation stage. When combined, the strategies used to date convey a comprehensive view of the functional development of newly born neurons while providing a framework for approaching the critical time at which new neurons become functionally integrated and influence brain function.

The dentate gyrus (DG) is a neurogenic structure that exhibits functional and structural reorganization after injury. Neurogenesis and functional recovery occur after brain damage, and the possible relation between both processes is a matter of study. We explored whether neurogenesis and the activation of new neurons correlated with DG recovery over time. We induced a DG lesion in young adult rats through the intrahippocampal injection of kainic acid and analyzed functional recovery and the activation of new neurons after animals performed a contextual fear memory task (CFM) or a control spatial exploratory task. We analyzed the number of BrdU+ cells that co-localized with doublecortin (DCX) or with NeuN within the damaged DG and evaluated the number of cells in each population that were labelled with the activity marker c-fos after either task. At 10 days post-lesion (dpl), a region of the granular cell layer was devoid of cells, evidencing the damaged area, whereas at 30 dpl this region was significantly smaller. At 10 dpl, the number of BrdU+/DCX+/c-fos positive cells was increased compared to the sham-lesion group, but CFM was impaired. At 30 dpl, a significantly greater number of BrdU+/NeuN+/c-fos positive cells was observed than at 10 dpl, and activation correlated with CFM recovery. Performance in the spatial exploratory task induced marginal c-fos immunoreactivity in the BrdU+/NeuN+ population. We demonstrate that neurons born after the DG was damaged survive and are activated in a time- and task-dependent manner and that activation of new neurons occurs along functional recovery.

GABAergic inhibition is critical for the precision of neuronal spiking and the homeostatic regulation of network activity in the brain. Adult neurogenesis challenges network homeostasis because new granule cells (GCs) integrate continuously in the functional dentate gyrus. While developing, adult‐born GCs undergo a transient state of enhanced excitability due to the delayed maturation of perisomatic GABAergic inhibition by parvalbumin interneurons (PV‐INs). The mechanisms underlying this delayed synaptic maturation remain unknown. We examined the morphology and function of synapses formed by PV‐INs onto new GCs over a 2‐month interval in young adult mice, and investigated the influence of the synaptic adhesion molecule neuroligin‐2 (NL2). Perisomatic appositions of PV‐IN terminals onto new GCs were conspicuous at 2 weeks and continued to grow in size to reach a plateau over the fourth week. Postsynaptic knockdown of NL2 by expression of a short‐hairpin RNA (shNL2) in new GCs resulted in smaller size of synaptic contacts, reduced area of perisomatic appositions of the vesicular GABA transporter VGAT, and the number of presynaptic active sites. GCs expressing shNL2 displayed spontaneous GABAergic responses with decreased frequency and amplitude, as well as slower kinetics compared to control GCs. In addition, postsynaptic responses evoked by optogenetic stimulation of PV‐INs exhibited slow kinetics, increased paired‐pulse ratio and coefficient of variation in GCs with NL2 knockdown, suggesting a reduction in the number of active synapses as well as in the probability of neurotransmitter release (Pr). Our results demonstrate that synapses formed by PV‐INs on adult‐born GCs continue to develop beyond the point of anatomical growth, and require NL2 for the structural and functional maturation that accompanies the conversion into fast GABAergic transmission.

The adult brain is plastic and able to reorganize structurally and functionally after damage. Growth factors are key molecules underlying the recovery process and among trophic molecules, Insulin-Like Growth Factor-I (IGF-I) is of particular interest given that it modulates neuronal and glial responses in the hippocampus including neurogenesis, which has been proposed as a mechanism of neurorepair. In this study we analyzed the effect of intracerebroventricular chronic infusion of IGF-I on functional recovery and morphological restoration after the induction of an excitotoxic lesion in the dentate gyrus (DG) of young-adult rats. Our results show that the lesion impairs contextual fear memory which is a DG dependent task, but not cued fear memory or performance in the open field motor task, which are independent of the DG integrity. Chronic administration of IGF-I, but not vehicle, promotes functional recovery to control levels in injured subjects. Analysis in NeuN immunoprocessed tissue revealed that the lesion volume was not different between groups and that the DG was not evidently restructured in the IGF-I treated group. Glial fibrillary acidic protein (GFAP) analysis revealed an increased astrocytic response in the injured region in both groups and Doublecortin (DCX) analysis showed a similar increase in number of newly born neurons in both groups. However, a remarkable increase in young neurons dendritic arborization was observed in the IGF-I treated group. These results provide evidence for IGF-I as a molecule mediating functional and cellular plasticity during a reorganization process after damage to a neurogenic niche.