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This thesis investigates how through experience the brain acquires and stores memories, and uses these to extract and modify knowledge. This question is being studied by both computational and experimental neuroscientists as it is of relevance for neuroscience, but also for artificial systems that need to develop knowledge about the world from limited, sequential data. It is widely assumed that new memories are initially stored in the hippocampus, and later are slowly reorganised into distributed cortical networks that represent knowledge. This memory reorganisation is called systems consolidation. In recent years, experimental studies have revealed complex hippocampal-neocortical interactions that have blurred the lines between the two memory systems, challenging the traditional understanding of memory processes. In particular, the prior existence of cortical knowledge frameworks (also known as schemas) was found to speed up learning and consolidation, which seemingly is at odds with previous models of systems consolidation. However, the underlying mechanisms of this effect are not known. In this work, we present a computational framework to explore potential interactions between the hippocampus, the prefrontal cortex, and associative cortical areas during learning as well as during sleep. To model the associative cortical areas, where the memories are gradually consolidated, we have implemented an artificial neural network (Restricted Boltzmann Machine) so as to get insight into potential neural mechanisms of memory acquisition, recall, and consolidation. We analyse the network's properties using two tasks inspired by neuroscience experiments. The network gradually built a semantic schema in the associative cortical areas through the consolidation of multiple related memories, a process promoted by hippocampal-driven replay during sleep. To explain the experimental data we suggest that, as the neocortical schema develops, the prefrontal cortex extracts characteristics shared across multiple memories. We call this information meta-schema. In our model, the semantic schema and meta-schema in the neocortex are used to compute consistency, conflict and novelty signals. We propose that the prefrontal cortex uses these signals to modulate memory formation in the hippocampus during learning, which in turn influences consolidation during sleep replay. Together, these results provide theoretical framework to explain experimental findings and produce predictions for hippocampal-neocortical interactions during learning and systems consolidation.

The study of cultured neuronal networks has opened up avenues for understanding the ion channels, receptor molecules, and synaptic plasticity that may form the basis of learning and memory. The hippocampal neurons prepared from Wistar rats and put in culture, show, after a few days, spontaneous activity with typical electrophysiological pattern ranging from stochastic spiking to synchronized bursting. Using a multi-electrode array (MEA) having 64 electrodes, the electrophysiological signals are acquired, and connectivity maps are constructed using correlation matrix to understand how the neurons in a network communicate during the burst. The response of the neuronal system to epilepsy caused by induced glutamate injury and subsequent exposure of the system to phenobarbital to form different connectivity networks is analyzed in this study. The correlation matrix of the neuronal network before and after administering glutamate as well as after administering phenobarbital is used to understand the neuronal and network level changes that take place in the system. In order to interface a neuronal network to a physical world, the major computations to be performed are noise removal, pattern recovery, pattern matching and clustering. These computations are to be performed in real time. The system should be able to identify a pattern and relate a physical task to the pattern in about 200-400 ms. Algorithms have been developed for the implementation of a real-time neuronal system on a multi-node digital processor system.

Epilepsy is characterized by the hyperexcitability of individual neurons and hyper synchronization of groups of neurons (networks). The acquired changes that take place at molecular, cellular and network levels are important for the induction and maintenance of epileptic activity in the brain. Epileptic activity is known to alter the intrinsic properties and signaling of neurons. Understanding acquired changes that cause epilepsy may lead to innovative strategies to prevent or cure this neurological disorder. Advances in in vitro electrophysiological techniques together with experimental models of epilepsy are indispensible tools to understand molecular, cellular and network mechanisms that underlie epileptiform activity. The aim of the study was to investigate the epileptiform activity induced alterations in Ca2+ dynamics in apical dendrites of hippocampal subicular pyramidal neurons in slices and changes in network properties of cultured hippocampal neurons. We have also made attempts to develop an in vitro model of epilepsy using organotypic hippocampal slice cultures. In the first part of the present study, investigations on the basic properties of dendritic Ca2+ signaling in subicular pyramidal neurons during epileptiform activity are described. Subiculum, a part of the hippocampal formation is present, adjacent to the CA1 subfield. It acts as a transition zone between the hippocampus and entorhinal cortex. It receives inputs directly from the CA1 region, the entorhinal cortex, subcortical and other cortical areas. Several forms of evidences support the role of subiculum in temporal lobe epilepsy. Pronounced neuronal loss has been reported in various regions of the hippocampal formation (CA1 and CA3) leaving the subiculum generally intact in human epileptic tissue. It has been observed that epileptic activity is generated in subiculum in cases where the CA3 and CA1 regions are damaged or even absent. However, it is not clear how subicular neurons protect themselves from epileptic activity induced neuronal death. It is widely accepted that epileptiform activity induced neuronal damage is a result of an abnormally large influx of Ca2+ into neuronal compartments. In the present study, combined hippocampus / entorhinal cortical brain slices were exposed to zero Mg2+ + 4-amino pyridine artificial cerebrospinal fluid (ACSF) to generate spontaneous epileptiform discharges. Whole cell current-clamp recordings combined with Ca2+ imaging experiments (by incorporating Oregon green BAPTA-1 in the recording pipette) were performed on subicular pyramidal neurons to understand the changes in [Ca2+]i transients elicited in apical dendrites, in response to spontaneous epileptic discharges. To understand the changes occurring with respect to control, experiments were performed (in both control and in vitro epileptic conditions) where [Ca2+]i transients in dendrites were elicited by back propagating action potentials following somatic current injections. The results show clear distance-dependent changes in decay kinetics of [Ca2+]i transients (τdecay), without change in the amplitude of the [Ca2+]i transients, in distal parts (95–110 µm) compared to proximal segments (30–45 µm) of apical dendrites of subicular pyramidal neurons under in vitro epileptic condition, but not in control conditions. Pharmacological agents that block Ca2+ transporters viz. Na+/Ca2+ exchangers (Benzamil), plasma membrane Ca2+-ATPase pumps (Calmidazolium) and smooth endoplasmic reticulum Ca2+-ATPase pumps (Thapsigargin) were applied locally to the proximal and distal part of the apical dendrites in both experimental conditions to understand the molecular aspects of the Ca2+ extrusion mechanisms. The relative contribution of Na+/Ca2+ exchangers in Ca2+ extrusion was higher in the distal apical dendrite in in vitro epileptic condition. Using computer simulations with NEURON, biophysically realistic models were built to understand how faster decay of [Ca2+]i transients in the distal part of apical dendrite associated with [Ca2+]i extrusion mechanisms affect excitability of the neurons. With a linear increase in the density of Na+/Ca2+ exchangers along the apical dendrite, the decrease in τ decay values of [Ca2+]i transients in distal regions seen in experimental epileptic condition was reproduced in simulation. This linear increase in Na+/Ca2+ exchangers lowered the threshold for firing in response to consecutive synaptic inputs to the distal apical dendrite. Our results thus, show the existence of a novel neuroprotective mechanism in distal parts of the apical dendrite of subicular pyramidal neurons under in vitro epileptic condition with the Na+/Ca2+ exchangers being the major contributors to this mechanism. Although the enhanced contribution of Na+/Ca2+ exchangers helps the neuron in removing excess [Ca2+]i loads, it paradoxically makes the neuron hyperexcitable to synaptic inputs in the distal parts of the apical dendrites. Thus, the Na+/Ca2+ exchangers may actually protect subicular pyramidal neurons and at the same time contribute to the maintenance of epileptiform activity. In the second part of the study, neuronal network topologies and connectivity patterns were explored in control and glutamate injury induced epileptogenic hippocampal neuronal networks, cultured on planar multielectrode array (8×8) probes. Hyper synchronization of neuronal networks is the hallmark of epilepsy. To understand hyper synchronization and connectivity patterns of neuronal networks, electrical activity from multiple neurons were monitored simultaneously. The electrical activity recorded from a single electrode mainly consisted of randomly fired single spikes and bursts of spikes. Simultaneous measurement of electrical activity from all the 64 electrodes revealed network bursts. A network burst represents the period (lasting for 0.1–0.2 s) of synchronized activity in the network and, during this transient period, maximum numbers of neurons interact with each other. The network bursts were observed in both control and in vitro epileptic networks, but the frequency of network bursts was more in the latter, compared to former condition. Time stamps of individual spikes (from all 64 electrodes) during such time-aligned network burst were collected and stored in a matrix and used to construct the network topology. Connectivity maps were obtained by analyzing the spike trains using cross-covariance analysis and graph theory methods. Analysis of degree distribution, which is a measure of direct connections between electrodes in a neuronal network, showed exponential and Gaussian distributions in control and in vitro epileptic networks, respectively. Quantification of number of direct connections per electrode revealed that the in vitro epileptic networks showed much higher number of direct connections per electrode compared to control networks. Our results suggest that functional two-dimensional neuronal networks in vitro are not scale-free (not a power law degree distribution). After brief exposure to glutamate, normal hippocampal neuronal networks became hyperexcitable and fired a larger number of network bursts with altered network topology. Quantification of clustering coefficient and path length in these two types of networks revealed that the small-world network property was lost once the networks become epileptic and this was accompanied by a change from an exponential to a Gaussian network. In the last part of the study, we have explored if an excitotoxic glutamate injury (20 µM for 10 min) that produces spontaneous, recurrent, epileptiform discharges in cultured hippocampal neurons can induce epileptogenesis in hippocampal neurons of organotypic brain slice cultures. In vitro models of epilepsy are necessary to understand the mechanisms underlying seizures, the changes in brain structure and function that underlie epilepsy and are the best methods for developing new antiseizure and antiepileptogenic strategies. Glutamate receptor over-activation has been strongly associated with epileptogenesis. Recent studies have shown that brief exposure of dissociated hippocampal neurons in culture to glutamate (20 µM for 10 min) induces epileptogenesis in surviving neurons. Our aim was to extend the in vitro model of glutamate injury induced epilepsy to the slice preparations with intact brain circuits. Patch clamp technique in current-clamp mode was employed to monitor the expression of spontaneous epileptiform discharges from CA1 and CA3 neurons using several combinations of glutamate injury protocols. The results presented here represent preliminary efforts to standardize the glutamate injury protocol for inducing epileptogenesis in organotypic slice preparations. Our results indicate that glutamate injury protocols that induced epileptogenesis in dissociated hippocampal neurons in culture failed to turn CA1 and CA3 neurons of organotypic brain slice cultures epileptic. We also found that the CA1 and CA3 neurons of organotypic brain slice cultures are resilient to induction of epileptogenesis by glutamate injury protocols with 10 times higher concentrations of glutamate (200µM) than that used for neuronal cultures and long exposure periods (upto 30 min). These results clearly show that the factors involved in induction of epileptiform activity after glutamate injury in neuronal cultures and those involved in making the neurons in organotypic slices resilient to such insults are different, and understanding them could give vital clues about epileptogenesis and its control. The resilience of CA1 and CA3 neurons seen could be due to differences in homeostatic plasticity that operate in both these experimental systems. However, further studies are required to corroborate this hypothesis.

The anterior thalamic nuclei (ATN) are a critical part of an extended hippocampal system that supports key elements of episodic memory. Damage or disconnection of the ATN is a component of clinical conditions associated with severe anterograde amnesisa such as the Korsakoff’s syndrome, thalamic stroke, and neurodegenerative disorders. Previous studies have demonstrated that the ATN and hippocampus are often interdependent, and that ATN damage can result in ‘covert pathology’ in ostensibly healthy distal regions of the extended hippocampal system. Adult male rats with neurotoxic bilateral ATN lesions or sham surgery were post-operatively housed in an enriched environment or standard housing after a lesion-induced spatial working memory deficit had been established. These rats were retested on cross-maze and then trained in radial-arm maze spatial memory tasks. Other enriched rats received pseudo-training only after the enrichment period. The detailed neuromorphology of neurons was subsequently examined in the hippocampal CA1. Soma characteristics were also examined in the retrosplenial granular b cortex and the prelimbic cortex. In Experiment 1, ATN lesions produced clear deficits in both the cross-maze and radial-arm maze tasks and reduced hippocampal CA1 dendritic complexity, length, and spine density, while increasing the average diameter of the dendrites. Post-operative enrichment reversed the ATN lesion-induced deficits in the cross-maze and radial-arm maze, and returned CA1 basal and apical spine density to a level comparable to that of sham standard housed trained rats. The sham enriched rats exhibited improved radial-arm maze performance and increased CA1 branching complexity and spine density in both basal and apical arbors compared to sham standard housed rats. The neuromorphological changes observed in the enriched ATN and sham rats may be in part responsible for the spatial working memory improvements observed. Experiment 2 provided support for this contention by demonstrating that the CA1 spine changes were explicitly relevant to spatial learning and memory, because trained enriched sham and ATN rats had increased spines, particularly in the basal tree when compared to closely comparable pseudo-trained enriched rats. Interestingly, spatial memory training increased the numbers of both thin and mushroom spines, whereas enrichment was only associated with an increase in thin spines. In Experiment 3, ATN lesions increased cell body size in layer II of the retrosplenial granular b cortex, whereas enrichment decreased cell body size in layer V of this region. Neither ATN lesions nor enrichment had any effect on cell body morphology in the prelimbic cortex. The current research provides some of the strongest evidence to date of ATN and hippocampal interdependence within the extended hippocampal system, and provides the first evidence of neuromorphological correlates of recovery after ATN lesions.

Intricate interactions among constituent components are defining hallmarks of biological systems and sculpt physiology across different scales spanning gene networks to behavioural repertoires. Whereas interactions among channels and receptors define neuronal physiology, interactions among different cells specify the characteristic features of network physiology. From a single-neuron perspective, it is now evident that the somato-dendritic plasma membrane of hippocampus pyramidal neurons is endowed with several voltage-gated ion channels (VGICs) with varying biophysical properties and sub cellular expression profiles. Structural and physiological interactions among these channels define generation and propagation of electrical signals, thereby transforming neuronal dendrites to actively excitable membrane endowed with complex computational capabilities. In parallel to this complex network of plasma membrane channels is an elegantly placed continuous intraneuronal membrane of the endoplasmic reticulum (ER) that runs throughout the neuronal morphology. Akin to the plasma membrane, the ER is also endowed with a variety of channels and receptors, prominent among them being the inositol trisphosphate (InsP3) receptors (InsP3Rs) and ryanodine receptors (RyR), both of which are calcium release channels. Physiological interactions among these receptors transform the ER into a calcium excitable membrane, capable of active propagation of calcium waves and of spatiotemporal integration of neuronal signals. Thus, a neuron is endowed with two continuously parallel excitable membranes that actively participate in the bidirectional flow of intraneuronal information, through interactions among different channels and receptors on either membrane. Although the interactions among sets of channels and receptors present individually on either membrane are very well characterized, our understanding of cross-membrane interactions among channels and receptors across these two membranes has been very limited. Recent literature has emphasized the critical nature of such cross-membrane interactions and the several physiological roles played by such interactions. Such cross-channel interactions include ER depletion-induced signaling involving store-operated calcium channels, generation and propagation of calcium waves through interactions between plasma membrane and ER membrane receptors, and the plasticity of plasma membrane VGICs and receptors induced by ER Ca2+. Such tight interactions between these two membranes have highlighted several roles of the ER in the integration of intraneuronal information, in regulating signalling microdomains and in regulating the downstream signaling pathways that are regulated by these Ca2+ signals. Yet, our understanding about the functional interactions between the ion channels and receptors present on either of these membranes is very limited from the perspective of the combinatorial possibilities that encompass the span of channels and receptors across these two membranes. In this context, the first part of this thesis deals with two specific instances of such cross-membrane functional interactions, presented as two subparts with each probing different direction of impact. Specifically, whereas the first of these subparts deals with the impact of plasma membrane VGICs on the physiology of ER receptors, the second subpart presents an instance of the effect of ER receptor activation on plasma membrane VGIC. In the first subpart of the thesis, we establish a novel role for the A-type potassium current in regulating the release of calcium through inositol triphosphate receptors (InsP3R) that reside on the endoplasmic reticulum (ER) of hippocampus pyramidal neurons. Specifically, the A-type potassium current has been implicated in the regulation of several physiological processes including the regulation of calcium influx through voltage-gated calcium channels (VGCCs). Given the dependence of InsP3R open probability on cytosolic calcium concentration ([Ca2+]c) we asked if this regulation of calcium influx by A-type potassium current could translate into the regulation of release of calcium through InsP3Rs by the A-type potassium current. To answer this, we constructed morphologically realistic, conductance-based neuronal models equipped with kinetic schemes that govern several calcium signalling modules and pathways, and constrained the distributions and properties of constitutive components by experimental measurements from these neurons. Employing these models, we establish a bell-shaped dependence of calcium release through InsP3Rs on the density of A-type potassium current, during the propagation of an intraneuronal calcium wave initiated through established protocols. Exploring the sensitivities of calcium wave initiation and propagation to several underlying parameters, we found that ER calcium release critically depends on dendrite diameter and wave initiation occurred at branch points as a consequence of high surface area to volume ratio of oblique dendrites. Further, analogous to the role of A-type potassium channels in regulating spike latency, we found that an increase in the density of A-type potassium channels led to increases in the latency and the temporal spread of a propagating calcium wave. Next, we incorporated kinetic models for the metabotropic glutamate receptor (miler) signalling components and a calcium-controlled plasticity rule into our model and demonstrate that the presence of mGluRs induced a leftward shift in a BCM-like synaptic plasticity profile. Finally, we show that the A-type potassium current could regulate the relative contribution of ER calcium to synaptic plasticity induced either through 900 pulses of various stimulus frequencies or through theta burst stimulation. These results establish a novel form of interaction between active dendrites and the ER membrane and suggest that A-type K+ channels are ideally placed for inhibiting the suppression of InsP3Rs in thin-caliber dendrites. Furthermore, they uncover a powerful mechanism that could regulate biophysical/biochemical signal integration and steer the spatiotemporal spread of signalling micro domains through changes in dendritic excitability. In the second subpart, we turned our focus to the role of calcium released through InsP3Rs in regulating the properties of VGICs present on the plasma membrane, thereby altering neuronal intrinsic properties that are dependent on these VGICs. Specifically, the synaptic plasticity literature has focused on establishing necessity and sufficiency as two essential and distinct features in causally relating a signalling molecule to plasticity induction, an approach that has been surprisingly lacking in the intrinsic plasticity literature. Here, we complemented the recently established necessity of inositol trisphosphate (InsP3) receptors (InsP3R) in a form of intrinsic plasticity by asking if ER InsP3R activation was sufficient to induce plasticity in intrinsic properties of hippocampus neurons. To do this, we employed whole-cell patch-clamp recordings to infuse D-myo-InsP3, the endogenous ligand for InsP3Rs, into hippocampus pyramidal neurons and assessed the impact of InsP3R activation on neuronal intrinsic properties. We found that such activation reduced input resistance, maximal impedance amplitude and temporal summation, but increased resonance frequency, resonance strength, sag ratio, and impedance phase lead of hippocampus pyramidal neurons. Strikingly, the magnitude of plasticity in all these measurements was dependent upon [InsP3], emphasizing the graded dependence of such plasticity on InsP3R activation. Mechanistically, we found that this InsP3-induced plasticity depended on hyperpolarization-activated cyclic-nucleotide gated (HCN) channels. Moreover, this calcium-dependent form of plasticity was critically reliant on the release of calcium through InsP3Rs, the influx of calcium through N-methyl-D -aspartate receptors and voltage-gated calcium channels, and on the protein kinase A pathway. These results delineate a causal role for InsP3Rs in graded adaptation of neuronal response dynamics through changes in plasma membrane ion channels, thereby revealing novel regulatory roles for the endoplasmic reticulum in neural coding and homeostasis. Whereas the first part of the thesis dealt with bidirectional interactions between ER membrane and plasma membrane channels/receptors within a neuron, second part focuses on cross-cellular interactions, specifically between ER membrane on astrocytes and dendritic plasma membrane of neurons. Specifically, the universality of ER-dependent calcium signalling ensures that its critical influence extends to regulating the physiology of astrocytes, an abundant form of glial cells in the hippocampus. Due to the presence of calcium release channels on ER membrane, astrocytes are calcium excitable, whereby they respond to neuronal activity by increase in their cytosolic calcium levels. Specifically, astrocytes respond to the release of neurotransmitters from neuronal presynaptic terminals through activation of metabotropic receptors expressed on their plasma membrane. Such activation results in the mobilization of cytosolic InsP3 and subsequent release of calcium through InsP3 on the astrocytes ER membrane. These ER-dependent [Ca2+]c elevations in astrocytes then result in the release of gliotransmitters from astrocytes, which bind to corresponding receptors located on neuronal plasma membrane resulting in voltage-deflections and/or activation of signaling pathways in the neuron. Although it is well established that gliotransmission constitutes an important communication channel between astrocytes and neurons, the impact of gliotransmission on neurons have largely been centered at the cell body of the neurons. Consequently, the impact of the activation of astrocytic InsP3R on neuronal dendrites, and the role of dendritic active conductances in regulating this impact have been lacking. This lacuna in mapping the spatial spread of gliotransmission in neurons is especially striking because most afferent synapses impinge on neuronal dendrites, and a significant proportion of information processing in neurons is performed in their dendritic arborization. Additionally, given that active dendritic conductances play a pivotal role in regulating the impact of fast synaptic neurotransmission on neurons, we hypothesized that such active-dendritic regulation should extend to the impact of slower extrasynaptic gliotransmission on neurons. The second part of the thesis is devoted to testing this hypothesis using dendritic and paired astrocyte-neuron electrophysiological recordings, where we also investigate the specific roles of active dendritic conductances in regulating the impact of gliotransmission initiated through activation of astrocytic InsP3Rs. In testing this hypothesis, in the second part of the thesis, we first demonstrate a significantly large increase in the amplitude of astrocytically originating spontaneous slow excitatory potentials (SEP) in distal dendrites compared to their perisomatic counterparts. Employing specific neuronal infusion of pharmacological agents, we show that blocking HCN channels increased the frequency, rise-time and width of dendritically-recorded spontaneous SEPs, whereas blockade of A-type potassium channels enhanced their amplitude. Next, through paired neuron-astrocytes recordings, we show that our conclusions on the differential roles of HCN and A-type potassium channels in modulating spontaneous SEPs also extended to SEPs induced through infusion of InsP3 in a nearby astrocyte. Additionally, employing subtype-specific receptor blockers during paired neuron-astrocyte recordings, we provide evidence that GluN2B-and GluN2D-containing NMDARs predominantly mediate perisomatic and dendritic SEPs, respectively. Finally, using morphologically realistic conductance-based computational models, we quantitatively demonstrate that dendritic conductances play an active role in mediating compartmentalization of the neuronal impact of gliotransmission. These results unveil an important role for active dendrites in regulating the impact of gliotransmission on neurons, and suggest astrocytes as a source of dendritic plateau potentials that have been implicated in localized plasticity and place cell formation. This thesis is organized into six chapters as follows: Chapter 1 lays the motivations for the questions addressed in the thesis apart from providing the highlights of the results presented here. Chapter 2 provides the background literature for the thesis, introducing facts and concepts that forms the foundation on which the rest of the chapters are built upon. In chapter 3, we present quantitative analyses of the physiological interactions between A-type potassium conductances and InsP3Rs in CA1 pyramidal neurons. In chapter 4, using electrophysiological recordings, we investigate the role of calcium released through InsP3Rs in induction of plasticity of intrinsic response dynamics, and demonstrate that this form of plasticity is consequent to changes in neuronal HCN channels. In chapter 5, we systematically map the spatial dynamics of the impact of gliotransmission on neurons across the somato-apical trunk, also unveiling the role of neuronal HCN and A-type potassium channels in compartmentalizing such impact. Finally, chapter 6 concludes the thesis highlighting its major contributions and discussing directions of future research.

Les expériences qui jalonnent la vie favorisent la survenue de modifications cérébrales durables et pouvant impacter les fonctions cognitives, ainsi que le développement de troubles cérébraux. L'hippocampe est une structure cérébrale qui joue un rôle essentiel dans l'apprentissage et la mémoire. Dans la première étude, nous avons montré comment l'activité neuronale sous-tendant les processus de la mémoire influence fortement l'intégration des nouveaux neurones hippocampiques dans le cerveau adulte, suggérant une modulation durable de la fonction hippocampique. Dans la deuxième étude, nous avons montré que le séjour en milieu enrichi qui prévient les déficits mnésiques liés à l'âge et induit également des modifications épigénétiques dans le cerveau sain et modèle de la maladie d'Alzheimer. Ceci suggére que des règulations épigénétiques durables pourraient soutenir les effets promnésiques de l'enrichissement environnemental. Ainsi, cette thèse a mis en évidence dans l'hippocampe, l'existence de plasticité dépendante de l'activité dans le cerveau sain et modèle de la maladie d'Alzheimer. Cette plasticité pourrait être une cible pertinente dans le traitement de certaines conditions pathologiques. / Throughout life, environmental challenges promote long-lasting changes within the brain that can affect cognitive function, as well as the development of brain disorders. Within the brain, the hippocampus plays a key role in learning and memory processes. In the first study, we demonstrate how neuronal activity triggered by the learning and memory enhances the synaptic integration of adult-born hippocampal neurons that could support hippocampal function. In the second study, we show that enriched environment prevents age-related memory deficits and induces epigenetic modifications in both healthy and Alzheimer's disease conditions. This suggests that long-lasting epigenetic regulations may participate in sustaining the promnesic effects of environmental enrichment. Altogether, this thesis provides evidence of activity-dependent plasticity in the hippocampus in healthy and diseased brain, and suggests that stimulating such plasticity may contribute to improve pathological conditions.

Systematic investigations through several experimental techniques have revealed that hippocampal pyramidal neurons express voltage gated ion channels (VGICs) with well-defined gradients along their dendritic arbor. These actively maintained gradients in various dendritic VGICs effectuate several stereotypic, topographically continuous functional gradients along the topograph of the dendritic arbor, and have been referred to as intraneuronal functional maps. The prime goal of my thesis was to understand the emergence and homeostasis of the several coexistent functional maps that express within hippocampal pyramidal neurons. In the first part of the thesis, we focus only on spatial interactions between ion channels and analyzed the role of such interactions in the emergence of functional maps. We developed a generalized quantitative framework, the influence field, to analyze the extent of influence of a spatially localized VGIC cluster. Employing this framework, we showed that a localized VGIC cluster could have spatially widespread influence, and was heavily reliant on the specific physiological property and background conductances. Using the influence field model, we reconstructed functional gradients from VGIC conductance gradients, and demonstrated that the cumulative contribution of VGIC conductances in adjacent compartments plays a critical role in determining physiological properties at a given location. These results suggested that spatial interactions among spatially segregated VGIC clusters are necessary for the emergence of the functional maps. In the second part of the thesis, we assessed the specific roles of only kinetic interactions between ion channels in determining physiological properties by employing a single-compartmental model. In doing this, we analyzed the roles of interactions among several VGICs in regulating intrinsic response dynamics. Using global sensitivity analysis, we showed that functionally similar models could be achieved even when underlying parameters displayed tremendous variability and exhibited weak pair-wise correlations. These results suggested that that response homeostasis could be achieved through several non-unique channel combinations, as an emergent consequence of kinetic interactions among these channel conductances. In the final part of the thesis, we analyzed the combined impact of both spatial and kinetic interactions among ion channel conductances on the emergence and homeostasis of functional maps in a neuronal model endowed with extensive dendritic arborization. To do this, we performed global sensitivity analysis on morphologically realistic conductance-based models of hippocampal pyramidal neurons that coexpressed six functional maps. We found topographically continuous functional maps to emerge from disparate model parameters with weak pair-wise correlations between parameters. These results implied that individual channel properties need not be set at constant values in achieving overall homeostasis of several coexistent functional maps. We suggest collective channelostasis, where several channels regulate their properties and expression profiles in an uncorrelated manner, as an alternative for accomplishing functional map homeostasis. Finally, we developed a methodology to assess the contribution of individual channel conductances to the various functional measurements employing virtual knockout simulations. We found that the deletion of individual channels resulted in variable, measurement-and location-specific impacts across the model population.

Cerebral ischemia is a highly debilitating condition where shortage of oxygen and glucose leads to profuse cell death. Insufficient blood supply to the brain leads to cerebral ischemia and increase in extracellular lactate concentrations. Rise in lactate concentration and the leak potassium channel TREK1 have been independently associated with cerebral ischemia. Lactate is a neuroprotective metabolite whose concentrations increase to 15-30 mM during ischemia and TREK1 is a neuroprotective potassium channel which is upregulated during ischemia. Recent literature suggests lactate to be neuroprotective and TREK1 knockout mice show an increased sensitivity to brain and spinal cord ischemia, however the connecting link between the two is missing. We hypothesized that lactate might interact with TREK1 channels and mediate neuroprotection. The aim of this study was to investigate the effect of lactate on activity and expression of TREK1 channels and evaluate the role of lactate-TREK1 interaction in conferring neuroprotection in the ischemia-prone hippocampus Ischemic concentrations (15-30 mM) of lactate at pH 7.4 increased whole cell TREK1 current in CA1 stratum radiator astrocytes and caused membrane hyperpolarization. We confirmed the intracellular action of lactate on TREK1 in hippocampal slices using mono carboxylate transporter blockers. The intracellular effect of lactate on TREK1 channels is specific since other mono carboxylates such as pyruvate at pH 7.4 failed to increase TREK1 current. We used immunostaining, western blot and electrophysiology to show that 15-30 mM of lactate increased functional TREK1 protein expression by 1.5-3 fold in hippocampal astrocytes. Next, we performed quantitative PCR to investigate if the increase in TREK1 protein expression was due to increased transcription and found that lactate stimulated TREK1 mRNA transcription to increase TREK1 protein. Lactate mediated increase in TREK1 expression was dependent on protein kinase A as inhibitors of protein kinase A abolished the increase in TREK1 mRNA and protein. The role of lactate-TREK1 interaction in neuroprotection was subsequently investigated using an in vitro oxygen glucose deprivation model of ischemia. Addition of 30 mM lactate to oxygen glucose deprived slices reduced neuronal death in the hippocampal CA1 pyramidal layer. However, 30 mM lactate failed to reduce cell death in rat hippocampal slices treated with TREK1 channel blockers signifying the requirement of active TREK1 channels for lactate mediated neuroprotection. However, lactate in the presence of protein kinase inhibitor failed to reduce cell death. This might be related to the role of protein kinase A in upregulation of TREK1 channels. We also estimated CA1 pyramidal neuronal TREK1 channel expression and found both lactate and oxygen glucose deprivation to decrease TREK1 channel expression that was surprisingly opposite to the effects on astrocytes. As TREK1 channel activation and upregulation decreases neuronal excitability, a decrease in neuronal TREK1 channel expression in response to lactate is expected to cause higher neuronal death and fails to explain lactate mediated neuroprotection. Since, lactate upregulated TREK1 channel expression and functional activity in CA1 stratum radiate astrocytes, we reasoned that the lactate mediated neuroprotection might be via astrocytic TREK1 channels requiring viable functional astrocytes. This was tested by disrupting astrocyte function using gliotoxin, and estimating cell death in oxygen glucose deprived hippocampal slices. Lactate failed to reduce cell death in presence of gliotoxin signifying the importance of viable astrocytes for lactate mediated neuroprotection. The above effects were specific to lactate as pyruvate failed to increase TREK1 expression and reduce cell death. TREK1 channels contribute to neuroprotection by enhancing potassium buffering and glutamate clearance capacity of astrocytes. We propose that lactate promotes neuronal survival in hippocampus by increasing TREK1 channel expression and activity in astrocytes during ischemia. This pathway serves as an alternate mechanism of neuroprotection.

A significant consequence of alcohol use disorders (AUDs) is hippocampal neurodegeneration. The hippocampus is responsible for learning and memory, and neurodegeneration in this brain region has been shown to result in cognitive deficits. Interestingly, some alcoholics demonstrate improvements in hippocampus-dependent functions, potentially due the phenomenon termed adult neurogenesis. Adult neurogenesis, the process by which neural stem cells (NSCs) proliferate, differentiate into neurons, migrate into the granule cell layer, and survive, occurs in two brain regions; however, this study examines only neurogenesis occurring in the subgranular zone of the hippocampal dentate gyrus. Four-day binge ethanol exposure in an animal model causes a decrease in neurogenesis during intoxication; however, there is a reactive increase in cell proliferation on day seven of abstinence. The purpose of this study was to determine the timing of increased cell proliferation. Furthermore, most alcoholics also smoke tobacco, and nicotine, the addictive component of tobacco, has also been shown to affect hippocampal neurogenesis. As many people initiate alcohol and tobacco use during adolescence, the second experiment herein examined the effect of nicotine coadministration on alcohol-induced reactive hippocampal cell proliferation.