Long-lasting changes in GABA responsiveness in cultured neurons

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

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.

Rapid regulation of tonic GABA currents in cultured rat hippocampal neurons

Subacute and chronic changes in tonic GABAergic inhibition occur in human and experimental epilepsy. Less is known about how tonic inhibition is modulated over shorter time frames (seconds). We measured endogenous tonic GABA currents from cultured rat hippocampal neurons to evaluate how they are affected by 1) transient increases in extracellular GABA concentration ([GABA]), 2) transient postsynaptic depolarization, and 3) depolarization of presynaptic cells. Transient increases in [GABA] (1 μM) reduced tonic currents; this reduction resulted from GABA-induced shifts in the reversal potential for GABA currents (E(GABA)). Transient depolarization of postsynaptic neurons reversed the effects of exogenous GABA and potentiated tonic currents. The voltage-dependent potentiation of tonic GABA currents was independent of E(GABA) shifts and represented postdepolarization potentiation (PDP), an intrinsic GABA(A) receptor property (Ransom CB, Wu Y, Richerson GB. J Neurosci 30: 7672-7684, 2010). Inhibition of vesicular GABA release with concanamycin A (ConA) did not affect tonic currents. In ConA-treated cells, transient application of 12 mM K(+) to depolarize presynaptic neurons and glia produced a persistent increase in tonic current amplitude. The K(+)-induced increase in tonic current was reversibly inhibited by SKF89976a (40 μM), indicating that this was caused by nonvesicular GABA release from GABA transporter type 1 (GAT1). Nonvesicular GABA release due to GAT1 reversal also occurred in acute hippocampal brain slices. Our results indicate that tonic GABA currents are rapidly regulated by GABA-induced changes in intracellular Cl(-) concentration, PDP of extrasynaptic GABA(A) receptors, and nonvesicular GABA release. These mechanisms may influence tonic inhibition during seizures when neurons are robustly depolarized and extracellular GABA and K(+) concentrations are elevated.

Efficacy of synaptic inhibition depends on multiple, dynamically interacting mechanisms implicated in chloride homeostasis

Chloride homeostasis is a critical determinant of the strength and robustness of inhibition mediated by GABA(A) receptors (GABA(A)Rs). 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 GABA(A)R-mediated inhibition, but increasing GABA(A)R 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 GABA(A)R 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.

Postdepolarization potentiation of GABAA receptors: a novel mechanism regulating tonic conductance in hippocampal neurons

Ambient GABA in the brain activates GABA(A) receptors to produce tonic inhibition. Membrane potential influences both GABA transport and GABA(A) receptors and could thereby regulate tonic inhibition. We investigated the voltage dependence of tonic currents in cultured rat hippocampal neurons using patch-clamp techniques. Tonic GABA(A) conductance increased with depolarization from 15 +/- 3 pS/pF at -80 mV to 29 +/- 5 pS/pF at -40 mV. Inhibition of vesicular or nonvesicular GABA release did not prevent voltage-dependent increases of tonic conductance. Currents evoked with exogenous GABA (1 mum) were outwardly rectifying, similar to tonic currents caused by endogenous GABA. These results indicate that the voltage-dependent increase of tonic conductance was attributable to intrinsic GABA(A) receptor properties rather than an elevation of ambient GABA. After transient depolarization to +40 mV, endogenous tonic currents measured at -60 mV were increased by 75 +/- 17%. This novel form of tonic current modulation, termed postdepolarization potentiation (PDP), recovered with a time constant of 63 s, was increased by exogenous GABA and inhibited by GABA(A) receptor antagonists. Measurements of E(GABA) showed PDP was caused by increased conductance and not a change in the anion gradient. To assess the functional significance of PDP, we used voltage-clamp waveforms that replicated epileptiform activity. PDP was produced by this pathophysiological depolarization. These data show that depolarization produces prolonged potentiation of tonic conductance attributable to voltage-dependent properties of GABA(A) receptors. These properties are well suited to limit excitability during pathophysiological depolarization accompanied by rises in ambient GABA, such as occur during seizures and ischemia.

Differential contribution of the Na(+)-K(+)-2Cl(-) cotransporter NKCC1 to chloride handling in rat embryonic dorsal root ganglion neurons and motor neurons

Plasma membrane chloride (Cl(-)) pathways play an important role in neuronal physiology. Here, we investigated the role of NKCC1 cotransporters (a secondary active Cl(-) uptake mechanism) in Cl(-) handling in cultured rat dorsal root ganglion neurons (DRGNs) and motor neurons (MNs) derived from fetal stage embryonic day 14. Gramicidin-perforated patch-clamp recordings revealed that DRGNs accumulate intracellular Cl(-) through a bumetanide- and Na(+)-sensitive mechanism, indicative of the functional expression of NKCC1. Western blotting confirmed the expression of NKCC1 in both DRGNs and MNs, but immunocytochemistry experiments showed a restricted expression in dendrites of MNs, which contrasts with a homogeneous expression in DRGNs. Both MNs and DRGNs could be readily loaded with or depleted of Cl(-) during GABA(A) receptor activation at depolarizing or hyperpolarizing membrane potentials. After loading, the rate of recovery to the resting Cl(-) concentration (i.e., [Cl(-)](i) decrease) was similar in both cell types and was unaffected by lowering the extracellular Na(+) concentration. In contrast, the recovery on depletion (i.e., [Cl(-)](i) increase) was significantly faster in DRGNs in control conditions but not in low extracellular Na(+). The experimental observations could be reproduced by a mathematical model for intracellular Cl(-) kinetics, in which DRGNs show higher NKCC1 activity and smaller Cl(-)-handling volume than MNs. On the basis of these results, we conclude that embryonic DRGNs show a higher somatic functional expression of NKCC1 than embryonic MNs. The high NKCC1 activity in DRGNs is important for maintaining high [Cl(-)](i), whereas lower NKCC1 activity in MNs allows large [Cl(-)](i) variations during neuronal activity.

Comparable GABAergic Mechanisms of Hippocampal Seizurelike Activity in Posttetanic and Low-Mg2+ Conditions

It is known that GABA is a major inhibitory neurotransmitter in mature mammalian brains, but the effect of this substance is sometimes converted into depolarizing or even excitatory when the postsynaptic Cl– concentration becomes high. Recently we have shown that seizurelike afterdischarge induced by tetanic stimulation in normal extracellular fluid (posttetanic afterdischarge) is mediated through GABAergic excitation in mature hippocampal CA1 pyramidal cells. In this study, we examined the possible contribution of similar depolarizing/excitatory GABAergic input to the CA1 pyramidal cells to the seizurelike afterdischarge induced in a low extracellular Mg2+ condition, another experimental model of epileptic seizure activity (low-Mg2+ afterdischarge). Perfusion of the GABAA antagonist bicuculline abolished the low-Mg2+ afterdischarge, but not the interictal-like activity, in most cases. Each oscillatory response during the low-Mg2+ afterdischarge was dependent on Cl– conductance and contained an F–-insensitive depolarizing component in the pyramidal cells, thus indicating that the afterdischarge response may be mediated through both GABAergic and nonGABAergic transmissions. In addition, local GABA application to the recorded cells revealed that GABA responses were indeed depolarizing during the low-Mg2+ afterdischarge. Furthermore, the GABAergic interneurons located in the strata pyramidale and oriens fired in oscillatory cycles more actively than those in other layers of the CA1 region. These results suggest that the depolarizing GABAergic input may facilitate oscillatory synchronization among the hippocampal CA1 pyramidal cells during the low-Mg2+ afterdischarge in a manner similar to the expression of the posttetanic afterdischarge.