Fast synaptic inhibition is a critical determinant of neuronal output, with subcellular targeting of synaptic inhibition able to exert different transformations of the neuronal input-output function. At the receptor level, synaptic inhibition is primarily mediated by chloride-permeable Type A GABA receptors. Consequently, dynamics in the neuronal chloride concentration can alter the functional properties of inhibitory synapses. How differences in the spatial targeting of inhibitory synapses interact with intracellular chloride dynamics to modulate the input-output function of neurons is not well understood. To address this, we developed computational models of multi-compartment neurons that incorporate experimentally parametrised mechanisms to account for neuronal chloride influx, diffusion, and extrusion. We found that synaptic input (either excitatory, inhibitory, or both) can lead to subcellular variations in chloride concentration, despite a uniform distribution of chloride extrusion mechanisms. Accounting for chloride changes resulted in substantial alterations in the neuronal input-output function. This was particularly the case for peripherally targeted dendritic inhibition where dynamic chloride compromised the ability of inhibition to offset neuronal inputoutput curves. Our simulations revealed that progressive changes in chloride concentration mean that the neuronal input-output function is not static but varies significantly as a function of the duration of synaptic drive. Finally, we found that the observed effects of dynamic chloride on neuronal output were mediated by changes in the dendritic reversal potential for GABA. Our findings provide a framework for understanding the computational effects of chloride dynamics on dendritically targeted synaptic inhibition.

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 signaling in neurons. Our findings reconcile recent experimental findings and provide a framework for understanding the interplay of different chloride regulatory processes in neurons.

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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.

This study investigates the effects of early exposure to ethanol on cognitive function and neural plasticity-related proteins in the rat brain. Sprague-Dawley rats were administered 12% ethanol solution (4 g/kg/day i.p.) or saline from P4 to P9. Vinpocetine, a phosphodiesterase type 1 inhibitor, was tested to determine whether it could reverse any changes induced by early ethanol exposure. Hence, from P25 to P31, ethanol-exposed male rats were injected with vinpocetine (20 mg/kg/day i.p.) or vehicle (DMSO) prior to undergoing behavioral testing in the open field and Morris water maze (MWM) tests. Ethanol exposure did not adversely affect spatial memory in the MWM. A key finding in this study was a significant ethanol-induced change in the function of the phosphorylated extracellular signal-related kinase (P-ERK) signaling pathway in the prefrontal cortex (PFC) and dorsal hippocampus (DH) of rats that did not display overt behavioral deficits. The P-ERK/ERK ratio was decreased in the PFC and increased in the DH of ethanol-exposed rats compared with controls. Rats that received vinpocetine in addition to ethanol did not display any behavioral changes but did show alterations in neural plasticity-related proteins. Mitogen-activated protein kinase phosphatase was increased, whereas brain-derived neurotrophic factor was decreased, in the PFC of vinpocetine-treated ethanol-exposed rats, and phosphorylated-glycogen synthase kinase β and synaptophysin were increased in the DH of these rats. This study provides insight into the long-term effects of early ethanol exposure and its interaction with vinpocetine in the rat brain. © 2016 Wiley Periodicals, Inc.

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Fast synaptic inhibition is a critical determinant of neuronal output, with subcellular targeting of synaptic inhibition able to exert different transformations of the neuronal input-output function. At the receptor level, synaptic inhibition is primarily mediated by chloride-permeable Type A GABA receptors. Consequently, dynamics in the neuronal chloride concentration can alter the functional properties of inhibitory synapses. How differences in the spatial targeting of inhibitory synapses interact with intracellular chloride dynamics to modulate the input-output function of neurons is not well understood. To address this, we developed computational models of multi-compartment neurons that incorporate experimentally parametrised mechanisms to account for neuronal chloride influx, diffusion, and extrusion. We found that synaptic input (either excitatory, inhibitory, or both) can lead to subcellular variations in chloride concentration, despite a uniform distribution of chloride extrusion mechanisms. Accounting for chloride changes resulted in substantial alterations in the neuronal inputoutput function. This was particularly the case for peripherally targeted dendritic inhibition where dynamic chloride compromised the ability of inhibition to offset neuronal input-output curves. Our simulations revealed that progressive changes in chloride concentration mean that the neuronal input-output function is not static but varies significantly as a function of the duration of synaptic drive. Finally, we found that the observed effects of dynamic chloride on neuronal output were entirely mediated by changes in the dendritic reversal potential for GABA. Our findings provide a framework for understanding the computational effects of chloride dynamics on dendritically targeted synaptic inhibition. Author SummaryThe fundamental unit of computation in the brain is the neuron, whose output reflects information within the brain. A determining factor in the transfer and processing of information in the brain is the modulation of activity by inhibitory synaptic inputs. Fast synaptic inhibition is mediated by the neurotransmitter GABA binding to GABAA receptors, which are permeable to chloride ions. How changes in chloride ion concentration affect neuronal output is an important consideration for information flow in the brain that is currently not being thoroughly investigated. In this research, we used multi-compartmental models of neurons to link the deleterious effects that accumulation of chloride ions can have on inhibitory signalling with changes in neuronal ouput. Together, our results highlight the importance of accounting for changes in chloride concentration in theoretical and computer-based models that seek to explore the computational properties of inhibition.

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