Contribution of Resting Conductance, GABA_A-Receptor Mediated Miniature Synaptic Currents and Neurosteroid to Chloride Homeostasis in Central Neurons

Abstract

Maintenance of a low intraneuronal Cl– concentration, [Cl–]i, is critical for inhibition in the CNS. Here, the contribution of passive, conductive Cl– flux to recovery of [Cl–]i after a high load was analyzed in mature central neurons from rat. A novel method for quantifying the resting Cl– conductance, important for [Cl–]i recovery, was developed and the possible contribution of GABAA and glycine receptors and of ClC-2 channels to this conductance was analyzed. The hypothesis that spontaneous, action potential-independent release of GABA is important for [Cl–]i recovery was tested. [Cl–]i was examined by gramicidin-perforated patch recordings in medial preoptic neurons. Cells were loaded with Cl– by combining GABA or glycine application with a depolarized voltage, and the time course of [Cl–]i was followed by measurements of the Cl– equilibrium potential, as obtained from the current recorded during voltage ramps combined with GABA or glycine application. The results show that passive Cl– flux contributes significantly, in the same order of magnitude as does K+-Cl– cotransporter 2 (KCC2), to [Cl–]i recovery and that Cl– conductance accounts for ∼ 6% of the total resting conductance. A major fraction of this resting Cl– conductance is picrotoxin (PTX)-sensitive and likely due to open GABAA receptors, but ClC-2 channels do not contribute. The results also show that when the decay of GABAA receptor-mediated miniature postsynaptic currents (minis) is slowed by the neurosteroid allopregnanolone, such minis may significantly quicken [Cl–]i recovery, suggesting a possible steroid-regulated role for minis in the control of Cl– homeostasis.

Extracellular Matrix Regulates Neuronal Chloride Concentration via K+-Cl−-Cotransporter 2

“…Thus, the two independent tests, with different procedures to affect the ECM, suggest a role of the ECM in controlling the rate of Clextrusion. [Cl -] i recovery measured previously with similar methods in acutely dissociated preoptic neuronal cell bodies, where we expect even less ECM, is still slower (499 ± 49 s, n = 19;Yelhekar et al, 2017;comparison with present control values: p = 6.0 10 -7 , Mann-Whitney test), further strengthening this idea (Fig. 4E).…”

Mathematical model of Na-K-Cl homeostasis in ictal and interictal discharges

“…The cotransporters do not balance chloride influx during an IID1. The duration of such events is sufficient for a millimolar increase in the chloride concentration, whereas transporter-mediated recovery takes minutes [ 21 , 38 ]. Thus, it leads to chloride accumulation and an elevation of the resting membrane potential of pyramidal cells of the hyperpolarized subpopulation.…”

“…The Na + /K + pump [ 13 , 14 ], KCC2 [ 1 , 13 , 38 , 39 ], and NKCC1 [ 13 , 39 ] transporter currents were calculated as follows: …”

Computational model of interictal discharges triggered by interneurons

“…The reversal potential of GABAergic currents in neurons of human and rat epileptic brains is depolarized from rest towards the action potential threshold [ 44 ], [ 13 ], [ 45 ], [ 18 ], [ 46 ]. The main factors that result in the depolarized GABA reversal potential are the chloride flux via GABAergic synapses, the downregulation and reversed operation of potassium chloride cotransporter 2 (KCC2) and voltage-gated chloride channels (ClC) [ 45 ], [ 61 ]. Sodium- and potassium-coupled chloride transport (NKCC1) may contribute to the up- and downregulations of the chloride gradient [ 44 ], [ 45 ].…”