Efficacy of Synaptic Inhibition Depends on Multiple, Dynamically Interacting Mechanisms Implicated in Chloride Homeostasis

Abstract

Chloride homeostasis is a critical determinant of the strength and robustness of inhibition mediated by GABAA receptors (GABAARs). The impact of changes in steady state Cl− gradient is relatively straightforward to understand, but how dynamic interplay between Cl− influx, diffusion, extrusion and interaction with other ion species affects synaptic signaling remains uncertain. Here we used electrodiffusion modeling to investigate the nonlinear interactions between these processes. Results demonstrate that diffusion is crucial for redistributing intracellular Cl− load on a fast time scale, whereas Cl−extrusion controls steady state levels. Interaction between diffusion and extrusion can result in a somato-dendritic Cl− gradient even when KCC2 is distributed uniformly across the cell. Reducing KCC2 activity led to decreased efficacy of GABAAR-mediated inhibition, but increasing GABAAR input failed to fully compensate for this form of disinhibition because of activity-dependent accumulation of Cl−. Furthermore, if spiking persisted despite the presence of GABAAR input, Cl− accumulation became accelerated because of the large Cl− driving force that occurs during spikes. The resulting positive feedback loop caused catastrophic failure of inhibition. Simulations also revealed other feedback loops, such as competition between Cl− and pH regulation. Several model predictions were tested and confirmed by [Cl−]i imaging experiments. Our study has thus uncovered how Cl− regulation depends on a multiplicity of dynamically interacting mechanisms. Furthermore, the model revealed that enhancing KCC2 activity beyond normal levels did not negatively impact firing frequency or cause overt extracellular K− accumulation, demonstrating that enhancing KCC2 activity is a valid strategy for therapeutic intervention.

Allosteric modulation of metabotropic glutamate receptors by chloride ions

“…For example, the thin diameter of a distal dendrite will slow Cl 2 diffusion toward the soma, exacerbating local accumulation (72). Such dynamic fluctuations should also occur in the extracellular space because its convoluted geometry limits diffusion and compartmentalizes ionic concentrations (29,30), especially near synapses that are often wrapped in a dense network of extracellular matrix (73).…”

Synaptic Inhibition and Disinhibition in the Spinal Dorsal Horn

“…Disinhibition caused by chloride dysregulation does not imply that benzodiazepines or other means of augmenting transmission will necessarily exacerbate pain, but it does predict that such treatments will be less effective compared with their efficacy against pain arising from impaired synaptic transmission. 34,145 On the other hand, enhancing KCC2 function is not predicted to 34 and nor does it 48 enhance inhibition unless inhibition has been compromised by chloride dysregulation. This discussion highlights that when trying to reduce pain caused by disinhibition, one must identify the precise disinhibitory mechanism and choose therapy accordingly.…”

Activation of 5-HT2A receptors upregulates the function of the neuronal K-Cl cotransporter KCC2

“…The expression or the function of KCC2 is reduced in several neurological disorders (2,4), and the resulting slight increase in [Cl -] i (depolarizing shift of the chloride equilibrium potential, E Cl ) dramatically compromises the inhibitory control of firing rate and excitatory inputs (5)(6)(7). Given the role of KCC2 in regulating the strength of inhibitory synaptic transmission, identifying tools that may increase KCC2 function and, hence, restore endogenous inhibition in pathological conditions is of particular importance.…”