Hippocampal gabaergic inhibitory interneurons

This is the author accepted manuscript. The final version is available from American Physiological Society via the DOI in this record In the hippocampus GABAergic local circuit inhibitory interneurons represent only ~10–15% of the total neuronal population; however, their remarkable anatomical and physiological diversity allows them to regulate virtually all aspects of cellular and circuit function. Here we provide an overview of the current state of the field of interneuron research, focusing largely on the hippocampus. We discuss recent advances related to the various cell types, including their development and maturation, expression of subtype-specific voltage-and ligand-gated channels, and their roles in network oscillations. We also discuss recent technological advances and approaches that have permitted high-resolution, subtype-specific examination of their roles in numerous neural circuit disorders and the emerging therapeutic strategies to ameliorate such pathophysiological conditions. The ultimate goal of this review is not only to provide a touchstone for the current state of the field, but to help pave the way for future research by highlighting where gaps in our knowledge exist and how a complete appreciation of their roles will aid in future therapeutic strategies.National Institute of Child Health and Human Developmen

Membrane Properties and the Balance between Excitation and Inhibition Control Gamma-Frequency Oscillations Arising from Feedback Inhibition

Computational studies as well as in vivo and in vitro results have shown that many cortical neurons fire in a highly irregular manner and at low average firing rates. These patterns seem to persist even when highly rhythmic signals are recorded by local field potential electrodes or other methods that quantify the summed behavior of a local population. Models of the 30–80 Hz gamma rhythm in which network oscillations arise through ‘stochastic synchrony’ capture the variability observed in the spike output of single cells while preserving network-level organization. We extend upon these results by constructing model networks constrained by experimental measurements and using them to probe the effect of biophysical parameters on network-level activity. We find in simulations that gamma-frequency oscillations are enabled by a high level of incoherent synaptic conductance input, similar to the barrage of noisy synaptic input that cortical neurons have been shown to receive in vivo. This incoherent synaptic input increases the emergent network frequency by shortening the time scale of the membrane in excitatory neurons and by reducing the temporal separation between excitation and inhibition due to decreased spike latency in inhibitory neurons. These mechanisms are demonstrated in simulations and in vitro current-clamp and dynamic-clamp experiments. Simulation results further indicate that the membrane potential noise amplitude has a large impact on network frequency and that the balance between excitatory and inhibitory currents controls network stability and sensitivity to external inputs

Hippocampal GABAergic inhibitory interneurons

In the hippocampus GABAergic local circuit inhibitory interneurons represent only ~10–15% of the total neuronal population; however, their remarkable anatomical and physiological diversity allows them to regulate virtually all aspects of cellular and circuit function. Here we provide an overview of the current state of the field of interneuron research, focusing largely on the hippocampus. We discuss recent advances related to the various cell types, including their development and maturation, expression of subtype-specific voltage- and ligand-gated channels, and their roles in network oscillations. We also discuss recent technological advances and approaches that have permitted high-resolution, subtype-specific examination of their roles in numerous neural circuit disorders and the emerging therapeutic strategies to ameliorate such pathophysiological conditions. The ultimate goal of this review is not only to provide a touchstone for the current state of the field, but to help pave the way for future research by highlighting where gaps in our knowledge exist and how a complete appreciation of their roles will aid in future therapeutic strategies

Distinctive biophysical features of human cell-types: insights from studies of neurosurgically resected brain tissue

Electrophysiological characterization of live human tissue from epilepsy patients has been performed for many decades. Although initially these studies sought to understand the biophysical and synaptic changes associated with human epilepsy, recently, it has become the mainstay for exploring the distinctive biophysical and synaptic features of human cell-types. Both epochs of these human cellular electrophysiological explorations have faced criticism. Early studies revealed that cortical pyramidal neurons obtained from individuals with epilepsy appeared to function “normally” in comparison to neurons from non-epilepsy controls or neurons from other species and thus there was little to gain from the study of human neurons from epilepsy patients. On the other hand, contemporary studies are often questioned for the “normalcy” of the recorded neurons since they are derived from epilepsy patients. In this review, we discuss our current understanding of the distinct biophysical features of human cortical neurons and glia obtained from tissue removed from patients with epilepsy and tumors. We then explore the concept of within cell-type diversity and its loss (i.e., “neural homogenization”). We introduce neural homogenization to help reconcile the epileptogenicity of seemingly “normal” human cortical cells and circuits. We propose that there should be continued efforts to study cortical tissue from epilepsy patients in the quest to understand what makes human cell-types “human”

Optogenetic investigation of cortical network dynamics in epilepsy

Ph. D. ThesisUnderstanding the cortical network properties which determine the susceptibility of cortex to the onset of seizures remains a major goal of epilepsy research. The determinants of seizure risk in cortical networks are dynamic, showing dependency on intrinsic cortical activity and environmental influences. The failure to identify reliable electrographic indicators of imminent seizure onset suggests that the contributory factors may not be electrographically obvious. A strong candidate for such a property is the activity dependent disinhibition of the excitatory network which results from increases in intracellular chloride concentration. Chloride loading has been shown previously to occur during periods of intense neuronal activity, resulting from concomitant excitatory and inhibitory synaptic transmission. To explore how network dynamics evolve from a stable healthy state to one permissive for the onset and propagation of seizures, I used an optogenetic approach to selectively interrogate dynamic changes to excitatory transmission between the principal cells of the cortical circuit following an acute ictogenic challenge, both in vitro and in vivo. Using ultra-low frequency optogenetic stimulation genetically targeted to the pyramidal cells of neocortex, I demonstrate that epileptiform activity, which develops spontaneously following an acute chemoconvulsant challenge, can both be reduced and monitored, using an active probing strategy. Delivering continuous and focal optogenetic stimulations to superficial neocortex and regions of the hippocampal formation evokes glutamatergic responses in the LFP which can be used to assay dendritic excitability in the network. At ultralow frequencies, between 0.1-0.033 Hz, optogenetic stimulation markedly reduced the rate of evolution of epileptiform activity, when delivered to neocortex or hippocampal structures, in acutely prepared adult mouse brain slices bathed in 0Mg2+ perfusate. The response evoked by these test pulses undergoes an all-or-nothing transformation observable in the LFP which reliably telegraphed the onset of ictal activity in two models of epilepsy. Using electrophysiological tools and 2-photon calcium imaging of individual dendrites, I demonstrate that this phenomenon likely reflects a reduction in the threshold for dendritic spikes. Using an anatomically realistic computational model pyramidal cell I show that this effect is reproduced by modest positive shifts in the GABAergic reversal potential in distal pyramidal cell dendrites. Finally, I report preliminary data demonstrating a potential mechanism for the diurnal modulation of seizure risk. Diurnal periodicity in seizure susceptibility have been observed longitudinal recordings from both patients and chronically epileptic experimental animals. Using the optical chloride sensor ClopHensor I examine steady-state pyramidal cell chloride concentration over the diurnal period and show that periodicity in chloride homeostasis is consistent with the phase of diurnally modulated seizure risk. In this thesis I use a range of optical and electrophysiological tools to explore the contribution of dynamic chloride concentration in pyramidal cells in determining cortical susceptibility to seizures onset. Using two acute epilepsy models I demonstrate that an assayable increase in dendritic excitability precedes ictogenesis, and demonstrate a potential mechanism by which variation in [Cl-]i can give rise to this effect. I go on to show diurnal variation in [Cl-]i in cortical pyramidal cells, and link this to circadian modulation of susceptibility to chemoconvulsants, suggesting a functional mechanism for the dynamic seizure risk observed in epileptic patients

Computational aspects of parvalbumin-positive interneuron function

The activity of neurons is dependent on the manner in which they process synaptic inputs from other cells. In the event of clustered synaptic input, neurons can respond in a nonlinear manner through synaptic and dendritic mechanisms. Such mechanisms are well established in principal excitatory neurons throughout the brain, where they increase neuronal computational ability and information storage capacity. In contrast for parvalbumin-positive (PV+) interneurons, the most common cortical class of in- hibitory interneuron, synaptic integration is thought to be either linear or sub-linear in nature, facilitating their role as mediators of precise and fast inhibition. This thesis addresses situations in which PV+ interneurons integrate synaptic inputs in a nonlinear manner, and explores the functions of this synaptic processing. First, I describe a form of cooperative supralinear synaptic integration by local excitatory inputs onto PV+ interneurons, and I extend these results to show how this augments the computational capability of PV+ cells within spiking neuron networks. I also explore the importance of polyamine-modulation of synaptic receptors in mediating sublinear synaptic integration, and discuss how this expands the array of mechanisms known to perform similar functions in PV+ cells. Finally, I present work manipulating PV+ cells experimentally during epilepsy. I consider these findings together with recent scientific advances and suggest how they account for a number of open questions and previously contradictory theories of PV+ interneuron function

Interplay of chemical neurotransmitters regulates developmental increase in electrical synapses

Coupling of neurons by electrical synapses (gap junctions) transiently increases during embryonic and/or early postnatal development of the mammalian central nervous system and plays an important role in a number of developmental events. A previous study revealed the mechanisms that control the developmental uncoupling of neuronal gap junctions, however, developmental regulation of neuronal gap junction coupling is largely unknown and is addressed in this dissertation. The current study revealed that the developmental increase in neuronal gap junction coupling is regulated by the interplay between the activity of group II metabotropic glutamate receptors (mGluR) and GABAA receptors (GABAAR). Specifically, the experiments including dye coupling, electrotonic coupling, western blots and siRNA technology in the rat and mouse hypothalamus and cortex in vivo and in vitro demonstrated that activation of group II mGluRs augments, and inactivation prevents, the developmental increase in neuronal gap junction coupling and connexin36 (Cx36, neuronal gap junction protein) expression. In contrast, changes in GABAA receptor activity have the opposite effects. The regulation by group II mGluRs is through cyclic AMP/protein kinase A-dependent signaling, while the GABAAR-dependent regulation is via influx of Ca2+ through L-type voltage-gated Ca2+ channels and activation of protein kinase C-dependent signaling. Further, the receptor mediated up-regulation of Cx36 requires a neuron-restrictive silencer element in the Cx36 gene promoter and the down-regulation involves the 3' untranslated region of the Cx36 mRNA, as shown using real-time quantitative polymerase chain reaction and luciferase reporter activity analysis. In addition, the methyl thiazolyl tetrazolium analysis indicates that mechanism for the developmental increase in neuronal gap junction coupling directly control the death/survival mechanisms in developing neurons. Altogether, the results suggest a multi-tiered strategy for chemical synapses in developmental regulation of electrical synapses

Chemical, electrical, hormonal and nutrient signaling in the mammalian central nervous system.

[Please refer to thesis

Underlying Mechanisms of Epilepsy

This book is a very provocative and interesting addition to the literature on Epilepsy. It offers a lot of appealing and stimulating work to offer food of thought to the readers from different disciplines. Around 5% of the total world population have seizures but only 0.9% is diagnosed with epilepsy, so it is very important to understand the differences between seizures and epilepsy, and also to identify the factors responsible for its etiology so as to have more effective therapeutic regime. In this book we have twenty chapters ranging from causes and underlying mechanisms to the treatment and side effects of epilepsy. This book contains a variety of chapters which will stimulate the readers to think about the complex interplay of epigenetics and epilepsy

Presynaptic action potential modulation in a neurological channelopathy

Channelopathies are disorders caused by inherited mutations of specific ion channels. Neurological channelopathies in particular offer a window into fundamental physiological functions such as action potential modulation, synaptic function and neurotransmitter release. One such channelopathy Episodic Ataxia type 1 (EA1), is caused by a mutation to the gene that encodes for the potassium channel subunit Kv1.1. This channel is predominantly found in presynaptic terminals and EA1 mutations have previously been shown to result in increased neuronal excitability and neurotransmitter release. A possible reason is that presynaptic action potential waveforms are affected in EA1. Thus far, direct electrophysiological recording of presynaptic terminals has been limited to large specialised synapses e.g. mossy fibre boutons, or axonal blebs, unnatural endings of transected axons. This is not representative of the vast majority of small synapses found in the forebrain. Using a novel technique termed Hopping Probe Ion Conductance Microscopy (HPICM) I have been able to directly record action potentials from micrometer sized boutons in hippocampal neuronal culture. I have shown that in a knockout model of Kv1.1 and in a knockin model of the V408A EA1 mutation, presynaptic action potentials are broader than in wild type; however action potentials are unaffected in the cell body. Finally in some central synapses neurotransmitter release has been shown to depend on not only action potentials received in the presynaptic terminal, but also on slow subthreshold membrane potential fluctuations from the soma, termed analogue digital signalling. Kv1 channels have been implicated in partly mediating this form of signalling. I have shown via dual recordings from the soma and small presynaptic boutons, that analogue-digital signalling occurs in wild type and knockout of Kv1.1, but is abolished in the V408A EA1 mutation. This implies that analogue-digital signalling may not depend on Kv1.1 in particular, rather a change in the stoichiometry of the Kv1 channel