Biology subjects

Burman, R. J.

Publications and source records attributed to Burman, R. J..

10 recordsLinked to original sources

bioRxiv2026

Opposing intrinsic and synaptic plasticity mechanisms stabilize altered cortical networks during sleep deprivation

Sleep deprivation alters brain activity and impairs performance, yet many aspects of behaviour are preserved in this state. In the cortex for example, sleep deprivation increases neuronal firing rates and low-frequency oscillatory activity, but cortical circuits continue to process information. Recent work has implicated synaptic inhibition in these changes, with sleep deprivation causing cortical GABAA receptor (GABAAR) signalling to become depolarizing due to changes in chloride gradients that determine the GABAAR reversal potential (EGABAAR). The impact in the intact brain remains unclear, however, as both the degree and effects of EGABAAR changes depend on network activity and intrinsic properties of neurons. To address this, we perform in vivo gramicidin recordings from cortical pyramidal neurons in sleep-deprived mice. We reveal that synaptic EGABAAR is depolarised to values sufficient to support bona fide excitatory GABAAR signalling, through combined network-dependent and cell-autonomous effects. In the face of this depolarizing drive, cortical neurons engage spike threshold adaptation mechanisms that limit their excitability. These opposing effects stabilise activity in simulations of sleep-deprived cortical networks and reproduce the increased firing rates and low-frequency oscillatory activity. This interplay between intrinsic and synaptic mechanisms may help maintain network stability during sleep deprivation, while reducing flexibility for subsequent adaptation.

neuroscience↗

bioRxiv2024

Local cortical inhibitory subnetworks are shaped by pyramidal neuron progenitor type

The degree to which cortical neurons share inhibitory synaptic input determines their co-activity within a network. However, the principles by which inhibition is shared between neurons are not known. Here we combine in utero labeling with in vivo two-photon targeted patch-clamp recordings in mature cortex to reveal that a layer 2/3 (L2/3) pyramidal neurons local inhibitory input reflects the embryonic progenitor type from which the neuron is born. In contrast to neighboring neurons, pyramidal neurons derived from intermediate progenitors receive synaptic inhibition that is weakly coupled to local network activity. The underlying mechanisms do not depend upon the amount of inhibitory input received from different interneuron subclasses. Rather, progenitor type defines how much inhibitory input a neuron shares with its neighbors, which is reflected in how individual interneurons target pyramidal neurons according to progenitor type. These findings reveal new significance for progenitor diversity and identify ontogenetic origins of fine-scale inhibitory cortical subnetworks.

neuroscience↗

bioRxiv2024

Network models incorporating chloride dynamics predict optimal strategies for terminating status epilepticus

Seizures that continue for beyond five minutes are classified as status epilepticus (SE) and constitute a medical emergency. Benzodiazepines, the current first-line treatment, attempt to terminate SE by increasing the conductance of chloride-permeable type-A GABA receptors (GABAARs). Despite their widespread use, benzodiazepines are ineffective in over a third of cases. Previous research in animal models has demonstrated that changes in intraneuronal chloride homeostasis and GABAAR physiology may underlie the development of benzodiazepine resistance in SE. However, there remains a need to understand the effect of these changes at a network level to improve translation into the clinical domain. Therefore, informed by data from human EEG recordings of SE and experimental brain slice recordings, we used a large spiking neural network model that incorporates chloride dynamics to investigate and address the phenomenon of benzodiazepine resistance in SE. We found that the GABAAR reversal potential (EGABA) sets SE-like bursting and determines the response to GABAAR conductance modulation, with benzodiazepines being anti-seizure at low EGABA and ineffective or pro-seizure at high EGABA. The SE-like activity and EGABA depended on a non-linear relationship between the strength of Cl- extrusion and GABAAR conductance, but not on the initial EGABA of neurons. Independently controlling Cl- extrusion in the pyramidal and interneuronal cell populations revealed the critical role of pyramidal cell Cl- extrusion in determining the severity of SE activity and the response to simulated benzodiazepine application. Finally, we demonstrate the models utility for considering improved therapeutic approaches for terminating SE in the clinic.

neuroscience↗

bioRxiv2023

All-optical reporting of chloride ion driving force in the nervous system.

Ionic driving forces provide the net electromotive force for ion movement across receptors, channels, and transporters, and are a fundamental property of all cells. In the brain for example, fast synaptic inhibition is mediated by chloride permeable GABAA receptors, and single-cell intracellular recordings have been the only method for estimating driving forces across these receptors (DFGABAA). Here we present a new tool for quantifying inhibitory receptor driving force named ORCHID: all-Optical Reporting of CHloride Ion Driving force. We demonstrate ORCHIDs ability to provide accurate, high-throughput measurements of resting and dynamic DFGABAA from genetically targeted cell types over multiple timescales. ORCHID confirms theoretical predictions about the biophysical mechanisms that establish DFGABAA, reveals novel differences in DFGABAA between neurons and astrocytes, and affords the first in vivo measurements of intact DFGABAA. This work extends our understanding of inhibitory synaptic transmission and establishes a precedent for all-optical methods to assess ionic driving forces.

neuroscience↗

bioRxiv2023

Optogenetic determination of dynamic and cell-type-specific chloride equilibrium potentials

Optogenetics has revolutionized neurobiological research by providing tools for modulating neuronal activity. As these tools utilise light-activated ion fluxes, they afford new opportunities to examine the nature of transmembrane ion gradients. Traditional investigation into the equilibrium potential for chloride (ECl) has been limited to studying endogenous chloride-permeable receptors. Here we demonstrate the utility of using a light-activated chloride channel, stGtACR2, to probe somatic ECl in rodent. This agonist-independent optogenetic strategy is validated in vitro and in vivo, captures differences in ECl dynamics following manipulations of endogenous chloride fluxes, and reveals distinct resting ECl across genetically-defined neuronal subpopulations. Using this approach to challenge chloride homeostasis, we uncover cell-specific ECl dynamics that are supported by the differential expression of endogenous handling mechanisms. Our findings establish an optical method for investigating transmembrane chloride gradients and thereby expand the repertoire of optogenetics.

neuroscience↗

bioRxiv2023

Active cortical networks promote shunting fast synaptic inhibition in vivo

Fast synaptic inhibition determines neuronal response properties in the mammalian brain and is mediated by chloride-permeable ionotropic GABA-A receptors (GABAARs). Despite their fundamental role, it is still not known how GABAARs signal in the intact brain. Here we use in vivo gramicidin recordings to investigate synaptic GABAAR signaling in mouse cortical pyramidal neurons under conditions that preserve native transmembrane chloride gradients. In anaesthetized cortex, synaptic GABAARs exert classic hyperpolarizing effects. In contrast, GABAAR-mediated synaptic signaling in awake cortex is found to be predominantly shunting. This is due to more depolarized GABAAR equilibrium potentials (EGABAAR), which are shown to result from the high levels of synaptic activity that characterize awake cortical networks. The EGABAAR observed in awake cortex can facilitate the decoupling of local networks, which improves the ability of the network to discriminate stimuli. Our findings therefore suggest that GABAAR signaling adapts to optimize cortical functions.

neuroscience↗

bioRxiv2019

Divergent paths to seizure-like events.

AimMuch debate exists about how the brain transitions into an epileptic seizure. One source of confusion is that there are likely to be critical differences between experimental seizure models. To address this, we compared the evolving activity patterns in two, widely used, in vitro models of epileptic discharges.\n\nMethodsWe compared brain slices, prepared in the same way from young adult mice, that were bathed either in 0 Mg2+, or 100{micro}M 4AP, artificial cerebrospinal fluid.\n\nResultsWe find that while local field potential recordings of epileptiform discharges in the two models appear broadly similar, patch-clamp analysis reveals an important difference in the relative degree of glutamatergic involvement. 4AP affects parvalbumin-expressing interneurons more than other cortical populations, destabilizing their resting state and inducing spontaneous bursting behavior. Consequently, the most prominent pattern of transient discharge (\"interictal event\") in this model is almost purely GABAergic, although the transition to seizure-like events (SLEs) involves pyramidal recruitment. In contrast, interictal discharges in 0 Mg2+ are only maintained by a very large glutamatergic component that also involves transient discharges of the interneurons. Seizure-like events in 0 Mg2+ have significantly higher power in the high gamma frequency band (60-120Hz) than these events do in 4AP, and are greatly delayed in onset by diazepam, unlike 4AP events.\n\nConclusionsThe 0 Mg2+ and 4AP models display fundamentally different levels of glutamatergic drive, demonstrating how ostensibly similar pathological discharges can arise from different sources. We contend that similar interpretative issues will also be relevant to clinical practice.

neuroscience↗

bioRxiv2018

Excitatory GABAergic signalling is associated with acquired benzodiazepine resistance in status epilepticus

Status epilepticus (SE) is defined as a state of unrelenting seizure activity. It is associated with a rapidly rising mortality rate, and thus constitutes a medical emergency. Benzodiazepines, which act as positive modulators of chloride (Cl-) permeable GABAA receptors, are indicated as the first line of treatment, but this is ineffective in many cases. We found that 48% of children presenting with SE were unresponsive to benzodiazepine treatment, and critically, that the duration of SE at the time of treatment is an important predictor of non-responsiveness. We therefore investigated the cellular mechanisms that underlie acquired benzodiazepine resistance, using rodent organotypic brain slices. Removing Mg2+ ions leads to an evolving pattern of epileptiform activity, and eventually to a persistent state of repetitive discharges that strongly resembles clinical EEG recordings of SE. We show that the persistent SE-like activity is associated with a reduction in GABAA receptor conductance and Cl- extrusion capability. We explored the effect on intraneuronal Cl- using both gramicidin, perforated-patch clamp recordings and Cl- imaging. This showed that during SE-like activity, reduced Cl- extrusion capacity was further exacerbated by activity-dependent Cl- loading, resulting in a persistently high intraneuronal Cl-. Consistent with these results, we found that optogenetic stimulation of GABAergic interneurons in the SE-like state, actually enhanced epileptiform activity in a GABAAR dependent manner. Together our findings describe a novel potential mechanism underlying benzodiazepine-resistant SE, with relevance to how this life-threatening condition should be managed in the clinic.

neuroscience↗

bioRxiv2018

Transcriptional and electrophysiological aberrations in an induced pluripotent stem cell-derived model of spinocerebellar ataxia type 7

Spinocerebellar ataxia type 7 (SCA7) is an inherited neurodegenerative disease caused by a polyglutamine repeat expansion in the ATXN7 gene. Patients with this disease suffer from a degeneration of their cerebellar Purkinje neurons and retinal photoreceptors that result in a progressive ataxia and loss of vision. As with many neurodegenerative diseases, studies of pathogenesis have been hindered by a lack of disease-relevant models. To this end, we have generated induced pluripotent stem cells (iPSCs) from a cohort of SCA7 patients in South Africa. First, we differentiated the SCA7 affected iPSCs into neurons which showed evidence of a transcriptional phenotype affecting components of STAGA (ATXN7 and KAT2A) and the heat shock protein pathway (DNAJA1 and HSP70). We then performed electrophysiology on the SCA7 iPSC-derived neurons and found that these cells show features of functional aberrations. Lastly, we were able to differentiate the SCA7 iPSCs into retinal photoreceptors that also showed similar transcriptional aberrations to the SCA7 neurons. Our findings demonstrate that iPSC-derived neurons and photoreceptors from SCA7 patients express molecular and electrophysiological differences that are indicative of impaired neuronal health. We hope that these findings will contribute towards the ongoing efforts to establish the cell-derived models of neurodegenerative diseases that are needed to develop patient-specific treatments.

neuroscience↗

bioRxiv2017

Biophysical models reveal the relative importance of transporter proteins and impermeant anions in chloride homeostasis

Fast synaptic inhibition in the nervous system depends on the transmembrane flux of Cl- ions based on the neuronal Cl- driving force. Established theories regarding the determinants of Cl- driving force have recently been questioned. Here we present biophysical models of Cl- homeostasis using the pump-leak model. Using numerical and novel analytic solutions, we demonstrate that the Na+/K+-ATPase, ion conductances, impermeant anions, electrodiffusion, water fluxes and cation-chloride cotransporters (CCCs) play roles in setting the Cl- driving force. Our models, together with experimental validation, show that while impermeant anions can contribute to setting [Cl-]i in neurons, they have a negligible effect on the driving force for Cl- locally and cell-wide. In contrast, we demonstrate that CCCs are well-suited for modulating Cl- driving force and hence inhibitory signalling in neurons. Our findings reconcile recent experimental findings and provide a framework for understanding the interplay of different chloride regulatory processes in neurons.

neuroscience↗