Browsing by Author "Raimondo, Joseph V"

Now showing 1 - 2 of 2
  • Thumbnail Image
    Item
    Open Access
    Exploring the determinants of chloride homeostasis in neurons using biophysical models
    (2018) Düsterwald, Kira M; Raimondo, Joseph V
    Fast synaptic inhibition in the nervous system depends on the transmembrane flux of Cl ions via activated GABAA and glycine receptors. As a result, changes to the neuronal driving force for Cl- are thought to play pivotal roles in many physiological and pathological brain processes. Established theories regarding the determinants of Cl- driving force have recently been questioned based on new experimental data. However, it is experimentally difficult to distinguish the respective contributions of the multiple, dynamically interacting mechanisms which may be important in Cl- homeostasis. Here I present biophysical models of Cl- homeostasis using the pump-leak formulation. By means of numerical and novel analytic solutions, I 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. Importantly, I 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, I demonstrate that CCCs are well-suited for modulating Cl- driving force and hence inhibitory signalling in neurons. This prediction is supported by a meta-analysis of multiple experimental studies, which demonstrates a strong correlation between the expression of the cationchloride cotransporter KCC2 and intracellular Cl concentration. My findings reconcile recent experimental findings and provide a framework for understanding the interplay of different chloride regulatory processes in neurons.
  • Thumbnail Image
    Item
    Open Access
    Investigating excitatory GABAergic signalling & benzodiazepine resistance in an in vitro model of status epilepticus
    (2018) Burman, Richard J; Raimondo, Joseph V; Katz, Arieh A
    Status epilepticus (SE) describes a state of persistent seizures which are unrelenting. First- line treatment for status epilepticus uses a group of drugs, the benzodiazepines, that promote the action of the major inhibitory neurotransmitter within the brain, gamma (γ)-aminobutyric acid (GABA). In a subset of patients however, benzodiazepines prove to be ineffective in terminating SE. Previous data from in vitro models has demonstrated that during single seizures, instead of being inhibitory, activation of the GABAA receptor can have an excitatory effect on neurons. To date, it is unknown whether this shift in GABAergic function contributes to SE, nor how it may modulate the anticonvulsant properties of benzodiazepines. In this thesis I explore the role of excitatory GABAergic signaling in an in vitro model of SE and how this may affect the anticonvulsant efficacy of the benzodiazepine, diazepam. Firstly, I confirm that benzodiazepine-resistant SE is prevalent in a South African paediatric population. Secondly, consistent with its established mechanism of action, I show that diazepam enhances GABAAR synaptic currents. Thirdly, using the in vitro 0 Mg²⁺ model of status epilepticus I show that whilst early application of diazepam has anticonvulsant properties, this is lost when the drug is applied during prolonged epileptiform activity. Fourthly, to investigate this phenomenon I use optogenetic activation of GABAergic interneurons to show that interneurons can drive epileptiform discharges during SE-like activity in vitro. Finally, I confirm that during seizure-like events there is a transient shift in GABAergic signaling that is caused by activity driven changes in the transmembrane Cl⁻ gradient. This thesis provides insight into how excitatory GABAergic signaling during prolonged seizures may contribute towards benzodiazepine resistance in SE. I believe that these results are relevant for understanding of the pathophysiology of SE and may help inform optimal treatment protocols for this condition.