Computational analysis of the impact of chronic stress on intrinsic and synaptic excitability in the hippocampus

Restoring social deficits in IRSp53-deleted mice: chemogenetic inhibition of ventral dentate gyrus Emx1-expressing cells

IRSp53 is a synaptic scaffold protein reported to be involved in schizophrenia, autism spectrum disorders, and social deficits in knockout mice. Identifying critical brain regions and cells related to IRSp53 deletion is expected to be of great help in the treatment of psychiatric problems. In this study, we performed chemogenetic inhibition within the ventral dentate gyrus (vDG) of mice with IRSp53 deletion in Emx1-expressing cells (Emx1-Cre;IRSp53 flox/flox). We observed the recovery of social deficits after chemogenetic inhibition within vDG of Emx1-Cre;IRSp53 flox/flox mice. Additionally, chemogenetic activation induced social deficits in Emx1-Cre mice. CRHR1 expression increased in the hippocampus of Emx1-Cre;IRSp53 flox/flox mice, and CRHR1 was reduced by chemogenetic inhibition. Htd2, Ccn1, and Atp61l were decreased in bulk RNA sequencing, and Eya1 and Ecrg4 were decreased in single-cell RNA sequencing of the hippocampus in Emx1-Cre;IRSp53 flox/flox mice compared to control mice. This study determined that the vDG is a critical brain region for social deficits caused by IRSp53 deletion. Social deficits in Emx1-Cre;IRSp53 flox/flox mice were recovered through chemogenetic inhibition, providing clues for new treatment methods for psychiatric disorders accompanied by social deficits.

Ion-channel degeneracy and heterogeneities in the emergence of signature physiological characteristics of dentate gyrus granule cells

Complex systems are neither fully determined nor completely random. Biological complex systems, including single neurons, manifest intermediate regimes of randomness that recruit integration of specific combinations of functionally specialized subsystems. Such emergence of biological function provides the substrate for the expression of degeneracy, the ability of disparate combinations of subsystems to yield similar function. Here, we present evidence for the expression of degeneracy in morphologically realistic models of dentate gyrus granule cells (GCs) through functional integration of disparate ion-channel combinations. We performed a 45-parameter randomized search spanning 16 active and passive ion channels, each biophysically constrained by their gating kinetics and localization profiles, to search for valid GC models. Valid models were those that satisfied 17 sub- and suprathreshold cellular-scale electrophysiological measurements from rat GCs. A vast majority (>99%) of the 15,000 random models were not electrophysiologically valid, demonstrating that arbitrarily random ion-channel combinations would not yield GC functions. The 141 valid models (0.94% of 15,000) manifested heterogeneities in and cross-dependencies across local and propagating electrophysiological measurements, which matched with their respective biological counterparts. Importantly, these valid models were widespread throughout the parametric space and manifested weak cross-dependencies across different parameters. These observations together showed that GC physiology could neither be obtained by entirely random ion-channel combinations nor is there an entirely determined single parametric combination that satisfied all constraints. The complexity, the heterogeneities in measurement and parametric spaces, and degeneracy associated with GC physiology should be rigorously accounted for while assessing GCs and their robustness under physiological and pathological conditions.NEW & NOTEWORTHY A recent study from our laboratory had demonstrated pronounced heterogeneities in a set of 17 electrophysiological measurements obtained from a large population of rat hippocampal granule cells. Here, we demonstrate the manifestation of ion-channel degeneracy in a heterogeneous population of morphologically realistic conductance-based granule cell models that were validated against these measurements and their cross-dependencies. Our analyses show that single neurons are complex entities whose functions emerge through intricate interactions among several functionally specialized subsystems.

Degeneracy in epilepsy: multiple routes to hyperexcitable brain circuits and their repair

Due to its complex and multifaceted nature, developing effective treatments for epilepsy is still a major challenge. To deal with this complexity we introduce the concept of degeneracy to the field of epilepsy research: the ability of disparate elements to cause an analogous function or malfunction. Here, we review examples of epilepsy-related degeneracy at multiple levels of brain organisation, ranging from the cellular to the network and systems level. Based on these insights, we outline new multiscale and population modelling approaches to disentangle the complex web of interactions underlying epilepsy and to design personalised multitarget therapies.

Active Dendrites and Local Field Potentials: Biophysical Mechanisms and Computational Explorations

Neurons and glial cells are endowed with membranes that express a rich repertoire of ion channels, transporters, and receptors. The constant flux of ions across the neuronal and glial membranes results in voltage fluctuations that can be recorded from the extracellular matrix. The high frequency components of this voltage signal contain information about the spiking activity, reflecting the output from the neurons surrounding the recording location. The low frequency components of the signal, referred to as the local field potential (LFP), have been traditionally thought to provide information about the synaptic inputs that impinge on the large dendritic trees of various neurons. In this review, we discuss recent computational and experimental studies pointing to a critical role of several active dendritic mechanisms that can influence the genesis and the location-dependent spectro-temporal dynamics of LFPs, spanning different brain regions. We strongly emphasize the need to account for the several fast and slow dendritic events and associated active mechanisms - including gradients in their expression profiles, inter- and intra-cellular spatio-temporal interactions spanning neurons and glia, heterogeneities and degeneracy across scales, neuromodulatory influences, and activitydependent plasticity - towards gaining important insights about the origins of LFP under different behavioral states in health and disease. We provide simple but essential guidelines on how to model LFPs taking into account these dendritic mechanisms, with detailed methodology on how to account for various heterogeneities and electrophysiological properties of neurons and synapses while studying LFPs.

Anti-Lingo-1 antibody ameliorates spatial memory and synapse loss induced by chronic stress

Chronic stress can induce cognitive impairment, and synapse number was significantly decreased in the hippocampus of rats suffering from chronic stress. Lingo-1 is a potent negative regulator of axonal outgrowth and synaptic plasticity. In the current study, the effects of anti-Lingo-1 antibody on the spatial learning and memory abilities and hippocampal synapses of stressed rats were investigated. After 4 weeks of stress exposure, the model group was randomly divided into a chronic stress group and an anti-Lingo-1 group. Then, the anti-Lingo-1 group rats were treated with anti-Lingo-1 antibody (8 mg/kg) for 3 weeks. The effects of anti-Lingo-1 antibody on the spatial learning and memory abilities were investigated with the Morris water maze test. Immunohistological staining and an unbiased stereological method were used to estimate the total number of dendritic spine synapses in the hippocampus. At the behavioral level, after 3 weeks of treatment, the anti-Lingo-1 group rats displayed significantly more platform location crossings in the Morris water maze test than the chronic stress group rats. Anti-Lingo-1 significantly prevented the declines in dendritic spine synapses and postsynaptic density protein-95 (PSD-95) expression in the dentate gyrus and the CA1 and CA3 regions of the hippocampus. The present results indicated that anti-Lingo-1 antibody may be a safe and effective drug for alleviating memory impairment in rats after chronic stress and protecting synapses in the hippocampus of stressed rats.

Race, Ethnicity, Family Socioeconomic Status, and Children's Hippocampus Volume

Introduction

The hippocampus has a significant role in memory, learning, and cognition. Although hippocampal size is highly susceptible to family socioeconomic status (SES) and associated stress, very little is known on racial and ethnic group differences in the effects of SES indicators on hippocampus volume among American children.

Purpose

This study explored the multiplicative effects of race, ethnicity, and family SES on hippocampus volume among American children.

Methods

Using data from the Adolescent Brain Cognitive Development (ABCD), we analyzed the functional Magnetic Resonance Imaging (fMRI) data of 9390 9-10 years old children. The main outcome was hippocampus volume. The predictor was parental education. Subjective family SES was the independent variable. Age, sex, and marital status were the covariates. Racial and ethnic group membership were the moderators. To analyze the data, we used regression models.

Results

High subjective family SES was associated with larger hippocampus volume. This effect was significantly larger for Whites than Black families.

Conclusions

The effect of subjective family SES on children's hippocampus volume is weaker in Black than White families.

Family Income Mediates the Effect of Parental Education on Adolescents' Hippocampus Activation During an N-Back Memory Task

Introduction: Hippocampus, a medial temporal lobe structure, has significant implications in memory formation and learning. Although hippocampus activity is believed to be affected by socioeconomic status (SES), limited knowledge exists on which SES indicators influence hippocampus function. Purpose: This study explored the separate and combined effects of three SES indicators, namely parental education, family income, and neighborhood income, on adolescents’ hippocampus activation during an N-Back memory task. As some of the effects of parental education may be through income, we also tested if the effect of parental education on hippocampus activation during our N-Back memory task is mediated by family or neighborhood income. Methods: The Adolescent Brain Cognitive Development (ABCD) study is a national multi-center investigation of American adolescents’ brain development. Functional magnetic resonance imaging (fMRI) data of a total sample of 3067 9–10-year-old adolescents were used. The primary outcome was left- hippocampus activation during the N-Back memory task (mean beta weight for N-Back run 1 2 back versus 0 back contrast in left hippocampus). The independent variable was parental education. Family income and neighborhood income were two possible mediators. Age, sex, and marital status were the covariates. To test mediation, we used hierarchical linear regression models first without and then with our mediators. Full mediation was defined according to Kenny. The Sobel test was used to confirm statistical mediation. Results: In the absence of family and neighborhood income in the model, higher parental educational attainment was associated with lower level of left hippocampus activation during the N-Back memory task. This effect was significant while age, sex, and marital status were controlled. The association between parental educational attainment and hippocampus activation during the N-Back memory task was no more significant when we controlled for family and neighborhood income. Instead, family income was associated with hippocampus activation during the N-Back memory task. These findings suggested that family income fully mediates the effect of parental educational attainment on left hippocampus activation during the N-Back memory task. Conclusions: The effect of parental educational attainment on adolescents’ hippocampus activation during an N-Back memory task is fully explained by family income. That means low family income is why adolescents with low-educated parents show highlighted hippocampus activation during an N-Back memory task. Given the central role of the hippocampus in learning and memory and as income is a modifiable factor by tax and economic policies, income-redistribution policies, fair taxation, and higher minimum wage may have implications for promotion of adolescent equality and social justice. There is a need to focus on family-level economic needs across all levels of neighborhood income.

Robust emergence of sharply tuned place-cell responses in hippocampal neurons with structural and biophysical heterogeneities

Hippocampal pyramidal neurons sustain propagation of fast electrical signals and are electrotonically non-compact structures exhibiting cell-to-cell variability in their complex dendritic arborization. In this study, we demonstrate that sharp place-field tuning and several somatodendritic functional maps concomitantly emerge despite the presence of geometrical heterogeneities in these neurons. We establish this employing an unbiased stochastic search strategy involving thousands of models that spanned several morphologies and distinct profiles of dispersed synaptic localization and channel expression. Mechanistically, employing virtual knockout models (VKMs), we explored the impact of bidirectional modulation in dendritic spike prevalence on place-field tuning sharpness. Consistent with the prior literature, we found that across all morphologies, virtual knockout of either dendritic fast sodium channels or N-methyl-D-aspartate receptors led to a reduction in dendritic spike prevalence, whereas A-type potassium channel knockouts resulted in a non-specific increase in dendritic spike prevalence. However, place-field tuning sharpness was critically impaired in all three sets of VKMs, demonstrating that sharpness in feature tuning is maintained by an intricate balance between mechanisms that promote and those that prevent dendritic spike initiation. From the functional standpoint of the emergence of sharp feature tuning and intrinsic functional maps, within this framework, geometric variability was compensated by a combination of synaptic democracy, the ability of randomly dispersed synapses to yield sharp tuning through dendritic spike initiation, and ion-channel degeneracy. Our results suggest electrotonically non-compact neurons to be endowed with several degrees of freedom, encompassing channel expression, synaptic localization and morphological microstructure, in achieving sharp feature encoding and excitability homeostasis.

Degeneracy in hippocampal physiology and plasticity

Degeneracy, defined as the ability of structurally disparate elements to perform analogous function, has largely been assessed from the perspective of maintaining robustness of physiology or plasticity. How does the framework of degeneracy assimilate into an encoding system where the ability to change is an essential ingredient for storing new incoming information? Could degeneracy maintain the balance between the apparently contradictory goals of the need to change for encoding and the need to resist change towards maintaining homeostasis? In this review, we explore these fundamental questions with the mammalian hippocampus as an example encoding system. We systematically catalog lines of evidence, spanning multiple scales of analysis that point to the expression of degeneracy in hippocampal physiology and plasticity. We assess the potential of degeneracy as a framework to achieve the conjoint goals of encoding and homeostasis without cross-interferences. We postulate that biological complexity, involving interactions among the numerous parameters spanning different scales of analysis, could establish disparate routes towards accomplishing these conjoint goals. These disparate routes then provide several degrees of freedom to the encoding-homeostasis system in accomplishing its tasks in an input- and state-dependent manner. Finally, the expression of degeneracy spanning multiple scales offers an ideal reconciliation to several outstanding controversies, through the recognition that the seemingly contradictory disparate observations are merely alternate routes that the system might recruit towards accomplishment of its goals.

Regulation of intrinsic excitability: Roles for learning and memory, aging and Alzheimer's disease, and genetic diversity

Plasticity of intrinsic neuronal excitability facilitates learning and memory across multiple species, with aberrant modulation of this process being linked to the development of neurological symptoms in models of cognitive aging and Alzheimer's disease. Learning-related increases in intrinsic excitability of neurons occurs in a variety of brain regions, and is generally thought to promote information processing and storage through enhancement of synaptic throughput and induction of synaptic plasticity. Experience-dependent changes in intrinsic neuronal excitability rely on activity-dependent gene expression patterns, which can be influenced by genetic and environmental factors, aging, and disease. Reductions in baseline intrinsic excitability, as well as aberrant plasticity of intrinsic neuronal excitability and in some cases pathological hyperexcitability, have been associated with cognitive deficits in animal models of both normal cognitive aging and Alzheimer's disease. Genetic factors that modulate plasticity of intrinsic excitability likely underlie individual differences in cognitive function and susceptibility to cognitive decline. Thus, targeting molecular mediators that either control baseline intrinsic neuronal excitability, subserve learning-related intrinsic neuronal plasticity, and/or promote resilience may be a promising therapeutic strategy for maintaining cognitive function in aging and disease. In this review, we discuss the complementary relationship between intrinsic excitability and learning, with a particular focus on how this relationship varies as a function of age, disease state, and genetic make-up, and how targeting these factors may help to further elucidate our understanding of the role of intrinsic excitability in cognitive function and cognitive decline.

Reciprocal interactions across and within multiple levels of monoamine and cortico-limbic systems in stress-induced depression: A systematic review

The monoamine hypothesis of depression, namely that the reduction in synaptic serotonin and dopamine levels causes depression, has prevailed in past decades. However, clinical and preclinical studies have identified various cortical and subcortical regions whose altered neural activities also regulate depressive-like behaviors, independently from the monoamine system. Our systematic review indicates that neural activities of specific brain regions and associated neural circuitries are adaptively altered after chronic stress in a specific direction, such that the neural activity in the infralimbic cortex, lateral habenula and amygdala is upregulated, whereas the neural activity in the prelimbic cortex, hippocampus and monoamine systems is downregulated. The altered neural activity dynamics between monoamine systems and cortico-limbic systems are reciprocally interwoven at multiple levels. Furthermore, depressive-like behaviors can be experimentally reversed by counteracting the altered neural activity of a specific neural circuitry at multiple brain regions, suggesting the importance of the reciprocally interwoven neural networks in regulating depressive-like behaviors. These results promise for reshaping altered neural activity dynamics as a therapeutic strategy for treating depression.

Degeneracy in the robust expression of spectral selectivity, subthreshold oscillations, and intrinsic excitability of entorhinal stellate cells

Biological heterogeneities are ubiquitous and play critical roles in the emergence of physiology at multiple scales. Although neurons in layer II (LII) of the medial entorhinal cortex (MEC) express heterogeneities in channel properties, the impact of such heterogeneities on the robustness of their cellular-scale physiology has not been assessed. Here, we performed a 55-parameter stochastic search spanning nine voltage- or calcium-activated channels to assess the impact of channel heterogeneities on the concomitant emergence of 10 in vitro electrophysiological characteristics of LII stellate cells (SCs). We generated 150,000 models and found a heterogeneous subpopulation of 449 valid models to robustly match all electrophysiological signatures. We employed this heterogeneous population to demonstrate the emergence of cellular-scale degeneracy in SCs, whereby disparate parametric combinations expressing weak pairwise correlations resulted in similar models. We then assessed the impact of virtually knocking out each channel from all valid models and demonstrate that the mapping between channels and measurements was many-to-many, a critical requirement for the expression of degeneracy. Finally, we quantitatively predict that the spike-triggered average of SCs should be endowed with theta-frequency spectral selectivity and coincidence detection capabilities in the fast gamma-band. We postulate this fast gamma-band coincidence detection as an instance of cellular-scale-efficient coding, whereby SC response characteristics match the dominant oscillatory signals in LII MEC. The heterogeneous population of valid SC models built here unveils the robust emergence of cellular-scale physiology despite significant channel heterogeneities, and forms an efficacious substrate for evaluating the impact of biological heterogeneities on entorhinal network function.

NEW & NOTEWORTHY We assessed the impact of heterogeneities in channel properties on the robustness of cellular-scale physiology of medial entorhinal cortical stellate neurons. We demonstrate that neuronal models with disparate channel combinations were endowed with similar physiological characteristics, as a consequence of the many-to-many mapping between channel properties and the physiological characteristics that they modulate. We predict that the spike-triggered average of stellate cells should be endowed with theta-frequency spectral selectivity and fast gamma-band coincidence detection capabilities.

T2N as a new tool for robust electrophysiological modeling demonstrated for mature and adult-born dentate granule cells

Compartmental models are the theoretical tool of choice for understanding single neuron computations. However, many models are incomplete, built ad hoc and require tuning for each novel condition rendering them of limited usability. Here, we present T2N, a powerful interface to control NEURON with Matlab and TREES toolbox, which supports generating models stable over a broad range of reconstructed and synthetic morphologies. We illustrate this for a novel, highly detailed active model of dentate granule cells (GCs) replicating a wide palette of experiments from various labs. By implementing known differences in ion channel composition and morphology, our model reproduces data from mouse or rat, mature or adult-born GCs as well as pharmacological interventions and epileptic conditions. This work sets a new benchmark for detailed compartmental modeling. T2N is suitable for creating robust models useful for large-scale networks that could lead to novel predictions. We discuss possible T2N application in degeneracy studies.

Synaptic Loss and the Pathophysiology of PTSD: Implications for Ketamine as a Prototype Novel Therapeutic

PURPOSE OF REVIEW

Studies of the neurobiology and treatment of PTSD have highlighted many aspects of the pathophysiology of this disorder that might be relevant to treatment. The purpose of this review is to highlight the potential clinical importance of an often-neglected consequence of stress models in animals that may be relevant to PTSD: the stress-related loss of synaptic connectivity.

RECENT FINDINGS

Here, we will briefly review evidence that PTSD might be a "synaptic disconnection syndrome" and highlight the importance of this perspective for the emerging therapeutic application of ketamine as a potential rapid-acting treatment for this disorder that may work, in part, by restoring synaptic connectivity. Synaptic disconnection may contribute to the profile of PTSD symptoms that may be targeted by novel pharmacotherapeutics.

Ndel1 and Reelin Maintain Postnatal CA1 Hippocampus Integrity

How the integrity of laminar structures in the postnatal brain is maintained impacts neuronal functions. Ndel1, the mammalian homolog of NuDE from the filamentous fungus Aspergillus nidulans, is an atypical microtubule (MT)-associated protein that was initially investigated in the contexts of neurogenesis and neuronal migration. Constitutive knock-out mice for Ndel1 are embryonic lethal, thereby necessitating the creation a conditional knock-out to probe the roles of Ndel1 in postnatal brains. Here we report that CA1 pyramidal neurons from mice postnatally lacking Ndel1 (Ndel1 conditional knock-out) exhibit fragmented MTs, dendritic/synaptic pathologies, are intrinsically hyperexcitable and undergo dispersion independently of neuronal migration defect. Secondary to the pyramidal cell changes is the decreased inhibitory drive onto pyramidal cells from interneurons. Levels of the glycoprotein Reelin that regulates MTs, neuronal plasticity, and cell compaction are significantly reduced in hippocampus of mutant mice. Strikingly, a single injection of Reelin into the hippocampus of Ndel1 conditional knock-out mice ameliorates ultrastructural, cellular, morphological, and anatomical CA1 defects. Thus, Ndel1 and Reelin contribute to maintain postnatal CA1 integrity.

SIGNIFICANCE STATEMENT

The significance of this study rests in the elucidation of a role for Nde1l and Reelin in postnatal CA1 integrity using a new conditional knock-out mouse model for the cytoskeletal protein Ndel1, one that circumvents the defects associated with neuronal migration and embryonic lethality. Our study serves as a basis for understanding the mechanisms underlying postnatal hippocampal maintenance and function, and the significance of decreased levels of Ndel1 and Reelin observed in patients with neurological disorders.

Chronic stress and hippocampal dendritic complexity: Methodological and functional considerations

The current understanding of how chronic stress impacts hippocampal dendritic arbor complexity and the subsequent relationship to hippocampal-dependent spatial memory is reviewed. A surge in reports investigating hippocampal dendritic morphology is occurring, but with wide variations in methodological detail being reported. Consequently, this review systematically outlines the basic neuroanatomy of relevant hippocampal features to help clarify how chronic stress or glucocorticoids impact hippocampal dendritic complexity and how these changes occur in parallel with spatial cognition. Chronic stress often leads to hippocampal CA3 apical dendritic retraction first with other hippocampal regions (CA3 basal dendrites, CA1, dentate gyrus, DG) showing dendritic retraction when chronic stress is sufficiently robust or long lasting. The stress-induced reduction in hippocampal CA3 apical dendritic arbor complexity often coincides with impaired hippocampal function, such as spatial learning and memory. Yet, when chronic stress ends and a post-stress recovery period ensues, the atrophied dendritic arbors and poor spatial abilities often improve. However, this process differs from a simple reversal of chronic stress-induced deficits. Recent reports suggest that this return to baseline-like functioning is uniquely different from non-stressed controls, emphasizing the need for further studies to enhance our understanding of how a history of stress subsequently alters an organism's spatial abilities. To provide a consistent framework for future studies, this review concludes with an outline for a quick and easy reference on points to consider when planning chronic stress studies with the goal of measuring hippocampal dendritic complexity and spatial ability.

The influence of stress and gonadal hormones on neuronal structure and function

This article is part of a Special Issue "SBN 2014". The brain is highly plastic, allowing us to adapt and respond to environmental and physiological challenges and experiences. In this review, we discuss the relationships among alterations in dendritic arborization, spine morphology, and behavior due to stress exposure, endogenous hormone fluctuation, or exogenous hormonal manipulation. Very few studies investigate structure-function associations directly in the same cohort of animals, and there are notable inconsistencies in evidence of structure-function relationships in the prefrontal cortex and hippocampus. Moreover, little work has been done to probe the causal relationship between dendritic morphology and neuronal excitability, leaving only speculation about the adaptive versus maladaptive nature of experience-dependent dendritic remodeling. We propose that future studies combine electrophysiology with a circuit-level approach to better understand how dendritic structure contributes to neuronal functional properties and behavioral outcomes.

Analogous synaptic plasticity profiles emerge from disparate channel combinations

An open question within the Bienenstock-Cooper-Munro theory for synaptic modification concerns the specific mechanism that is responsible for regulating the sliding modification threshold (SMT). In this conductance-based modeling study on hippocampal pyramidal neurons, we quantitatively assessed the impact of seven ion channels (R- and T-type calcium, fast sodium, delayed rectifier, A-type, and small-conductance calcium-activated (SK) potassium and HCN) and two receptors (AMPAR and NMDAR) on a calcium-dependent Bienenstock-Cooper-Munro-like plasticity rule. Our analysis with R- and T-type calcium channels revealed that differences in their activation-inactivation profiles resulted in differential impacts on how they altered the SMT. Further, we found that the impact of SK channels on the SMT critically depended on the voltage dependence and kinetics of the calcium sources with which they interacted. Next, we considered interactions among all the seven channels and the two receptors through global sensitivity analysis on 11 model parameters. We constructed 20,000 models through uniform randomization of these parameters and found 360 valid models based on experimental constraints on their plasticity profiles. Analyzing these 360 models, we found that similar plasticity profiles could emerge with several nonunique parametric combinations and that parameters exhibited weak pairwise correlations. Finally, we used seven sets of virtual knock-outs on these 360 models and found that the impact of different channels on the SMT was variable and differential. These results suggest that there are several nonunique routes to regulate the SMT, and call for a systematic analysis of the variability and state dependence of the mechanisms underlying metaplasticity during behavior and pathology.

Dendritic atrophy constricts functional maps in resonance and impedance properties of hippocampal model neurons

A gradient in the density of hyperpolarization-activated cyclic-nucleotide gated (HCN) channels is necessary for the emergence of several functional maps within hippocampal pyramidal neurons. Here, we systematically analyzed the impact of dendritic atrophy on nine such functional maps, related to input resistance and local/transfer impedance properties, using conductance-based models of hippocampal pyramidal neurons. We introduced progressive dendritic atrophy in a CA1 pyramidal neuron reconstruction through a pruning algorithm, measured all functional maps in each pruned reconstruction, and arrived at functional forms for the dependence of underlying measurements on dendritic length. We found that, across frequencies, atrophied neurons responded with higher efficiency to incoming inputs, and the transfer of signals across the dendritic tree was more effective in an atrophied reconstruction. Importantly, despite the presence of identical HCN-channel density gradients, spatial gradients in input resistance, local/transfer resonance frequencies and impedance profiles were significantly constricted in reconstructions with dendritic atrophy, where these physiological measurements across dendritic locations converged to similar values. These results revealed that, in atrophied dendritic structures, the presence of an ion channel density gradient alone was insufficient to sustain homologous functional maps along the same neuronal topograph. We assessed the biophysical basis for these conclusions and found that this atrophy-induced constriction of functional maps was mediated by an enhanced spatial spread of the influence of an HCN-channel cluster in atrophied trees. These results demonstrated that the influence fields of ion channel conductances need to be localized for channel gradients to express themselves as homologous functional maps, suggesting that ion channel gradients are necessary but not sufficient for the emergence of functional maps within single neurons.

High-conductance states and A-type K+ channels are potential regulators of the conductance-current balance triggered by HCN channels

An increase in the hyperpolarization-activated cyclic nucleotide-gated (HCN) channel conductance reduces input resistance, whereas the consequent increase in the inward h current depolarizes the membrane. This results in a delicate and unique conductance-current balance triggered by the expression of HCN channels. In this study, we employ experimentally constrained, morphologically realistic, conductance-based models of hippocampal neurons to explore certain aspects of this conductance-current balance. First, we found that the inclusion of an experimentally determined gradient in A-type K+ conductance, but not in M-type K+ conductance, tilts the HCN conductance-current balance heavily in favor of conductance, thereby exerting an overall restorative influence on neural excitability. Next, motivated by the well-established modulation of neuronal excitability by synaptically driven high-conductance states observed under in vivo conditions, we inserted thousands of excitatory and inhibitory synapses with different somatodendritic distributions. We measured the efficacy of HCN channels, independently and in conjunction with other channels, in altering resting membrane potential (RMP) and input resistance ( Rin) when the neuron received randomized or rhythmic synaptic bombardments through variable numbers of synaptic inputs. We found that the impact of HCN channels on average RMP, Rin, firing frequency, and peak-to-peak voltage response was severely weakened under high-conductance states, with the impinging synaptic drive playing a dominant role in regulating these measurements. Our results suggest that the debate on the role of HCN channels in altering excitability should encompass physiological and pathophysiological neuronal states under in vivo conditions and the spatiotemporal interactions of HCN channels with other channels.

Progressive alterations of hippocampal CA3-CA1 synapses in an animal model of depression

Major depressive disorder is the most prevalent psychiatric condition, but the cellular and molecular mechanisms underlying this disorder are largely unknown, although multiple hypotheses have been proposed. The aim of this study was to characterize the progressive alteration of neuronal plasticity in the male rat hippocampus during depression induced by chronic unpredictable mild stress (CUMS), an established animal model of depression. The data in the hippocampus were collected on days 7, 14 and 21 after the onset of three-week CUMS. When analyzed on day 21, three-week CUMS induced typically depressive-like behaviors, impaired LTP induction, and decreased basal synaptic transmission at hippocampal CA3-CA1 synapses recorded in vivo, which was accompanied by decreased density of dendritic spines in CA1 and CA3 pyramidal neurons. The levels of both Kalirin-7 and brain-derived neurotrophic factor (BDNF) in the hippocampus were decreased at the same time. On day 14 (middle phase), some depressive-like behaviors were observed, which was accompanied by depressed basal synaptic transmission and enhanced LTP induction at the CA3-CA1 synapses. However, BDNF expression was decreased without alteration of Kalirin7 expression in comparison with no-stress control. Depressed basal synaptic transmission occurred in the middle phase of CUMS may contribute to decreased expression of BDNF. On day 7, depressive-like behaviors were not observed, and LTP induction, spine density, Kalirin-7 and BDNF expression were not altered by CUMS in comparison with no-stress control. These results showed that the functional changes at CA3-CA1synapses occurred earlier than the structural alteration during three-week CUMS as a strategy of neural adaptation, and rats required three weeks to develop depressive-like behaviors during CUMS. Our results suggest an important role of Kalirin-7 in CUMS-mediated alterations in spine density, synaptic function and overall depressive-like behaviors on day 21.

The dynamic impact of repeated stress on the hippocampal spatial map

Stress alters the function of many physiological processes throughout the body, including in the brain. A neural circuit particularly vulnerable to the effects of stress is the hippocampus, a key component of the episodic and spatial memory system in both humans and rodents. Earlier studies have provided snapshots of morphological, molecular, physiological and behavioral changes in the hippocampus following either acute or repeated stress. However, the cumulative impact of repeated stress on in vivo hippocampal physiology remains unexplored. Here we report the stress-induced modulation of the spatially receptive fields of the hippocampal CA1 'place cells' as mice explore familiar and novel tracks after 5 and 10 days of immobilization stress. We find that similar to what has been observed following acute stress, five days of repeated stress results in decreased excitability of CA1 pyramidal cells. Following ten days of chronic stress, however, this decreased hippocampal excitability is no longer evident, suggesting adaptation may have occurred. In addition to these changes in neuronal excitability, we find deficient context discrimination, wherein both short-term and chronic stress impair the ability of the hippocampus to unambiguously distinguish novel and familiar environments. These results suggest that a loss of network flexibility may underlie some of the behavioral deficits accompanying chronic stress.