Intracellular regulation of GABAA-receptor function

Enhanced desensitization followed by unusual resensitization in GABAA receptors in phospholipase C-related catalytically inactive protein-1/2 double-knockout mice

Phospholipase C-related catalytically inactive proteins (PRIP-1/2) are previously reported to be involved in the membrane trafficking of GABAA receptor (GABAAR) and the regulation of intracellular Ca(2+) stores. GABAAR-mediated currents can be regulated by the intracellular Ca(2+). However, in PRIP-1/2 double-knockout (PRIP-DKO) mice, it remains unclear whether the kinetic properties of GABAARs are modulated by the altered regulation of intracellular Ca(2+) stores. Here, we investigated whether GABAAR currents (IGABA) evoked by GABA puff in layer 3 (L3) pyramidal cells (PCs) of the barrel cortex are altered in PRIP-DKO mice. The deletion of PRIP-1/2 enhanced the desensitization of IGABA but induced a hump-like tail current (tail-I) at the GABA puff offset. IGABA and the hump-like tail-I were suppressed by GABAAR antagonists. The enhanced desensitization of IGABA and the hump-like tail-I in PRIP-DKO PCs were mediated by increases in the intracellular Ca(2+) concentration and were largely abolished by a calcineurin inhibitor and ruthenium red. Calcium imaging revealed that Ca(2+)-induced Ca(2+) release (CICR) and subsequent store-operated Ca(2+) entry (SOCE) are more potent in PRIP-DKO PCs than in wild-type PCs. A mathematical model revealed that a slowdown of GABA-unbinding rate and an acceleration of fast desensitization rate by enhancing its GABA concentration dependency are involved in the generation of hump-like tail-Is. These results suggest that in L3 PCs of the barrel cortex in PRIP-DKO mice, the increased calcineurin activity due to the potentiated CICR and SOCE enhances the desensitization of GABAARs and slows the GABA-unbinding rate, resulting in their unusual resensitization following removal of GABA.

Hyperthermia decreases GABAergic synaptic transmission in hippocampal neurons of immature rats

The mechanisms underlying the generation of febrile seizures are poorly understood. We suggest that high temperature contributes to febrile seizures and specifically tested the hypothesis that hyperthermia suppressed GABAA-receptor-mediated inhibition in hippocampal neurons using whole-cell patch clamp recordings. We found that heating from a baseline temperature of 32 degrees C to 40 degrees C suppressed the peak amplitude of GABAA-receptor-mediated inhibitory postsynaptic currents (IPSCs) by 50+/-4.7% and decreased the decay time constant of IPSCs by 60.6+/-6.7% in immature CA1 neurons in the rat hippocampus. This inhibitory effect partly results from reduced IPSC conductance and increased GABA uptake, as demonstrated by the fact that GABA uptake blocker N-(4,4-diphenyl-3-butenyl)-3-piperidinecarboxylic acid (SKF89976A) significantly reduced the peak suppression and decay time decrease of the IPSC during hyperthermia. In addition, hyperthermia (40 degrees C) produced a significantly larger depression of the IPSC peak than the slope or peak of the excitatory postsynaptic current (EPSC), and IPSCs recovered slower than EPSCs after hyperthermia. The larger decrease in GABAA-receptor-mediated inhibition during and after hyperthermia, as compared with excitation, may shift the excitation/inhibition balance and contribute to the generation of febrile seizures.

Regulation of ligand-gated ion channels by protein phosphorylation

The studies discussed in this review demonstrate that phosphorylation is an important mechanism for the regulation of ligand-gated ion channels. Structurally, ligand-gated ion channels are heteromeric proteins comprised of homologous subunits. For both the AChR and the GABA(A) receptor, each subunit has a large extracellular N-terminal domain, four transmembrane domains, a large intracellular loop between transmembrane domains M3 and M4, and an extracellular C-terminal domain (Fig. 1B). All the phosphorylation sites on these receptors have been mapped to the major intracellular loop between M3 and M4 (Table 1). In contrast, glutamate receptors appear to have a very large extracellular N-terminal domain, one membrane hairpin loop, three transmembrane domains, a large extracellular loop between transmembrane domains M3 and M4, and an intracellular C-terminal domain (Fig. 1C). Most phosphorylation sites on glutamate receptors have been shown to be on the intracellular C-terminal domain, although some have been suggested to be on the putative extracellular loop between M3 and M4 (Table 1). A variety of extracellular factors and intracellular signal transduction cascades are involved in regulating phosphorylation of these ligand-gated ion channels (Fig. 2). Once again, the AChR at the neuromuscular junction is the most fully understood system. Phosphorylation of the AChR by PKA is stimulated synaptically by the neuropeptide CGRP and in an autocrine fashion by adenosine released from the muscle in response to acetylcholine. In addition, acetylcholine, via calcium influx through the AChR, appears to activate calcium-dependent kinases including PKC to stimulate serine phosphorylation of the receptor. Presently, agrin is the only extracellular factor known to stimulate phosphorylation of the AChR on tyrosine residues. For glutamate receptors, non-NMDA receptor phosphorylation by PKA is stimulated by dopamine, while NMDA receptor phosphorylation by PKA and PKC can be induced via the activation of beta-adrenergic receptors, and metabotropic glutamate or opioid receptors, respectively. In addition, Ca2+ influx through the NMDA receptor has been shown to activate PKC. CaMKII, and calcineurin, resulting in phosphorylation of AMPA receptors (by CaMKII) and inactivation of NMDA receptors (at least in part through calcineurin). In contrast to the AChR and glutamate receptors, no information is presently available regarding the identities of the extracellular factors and intracellular signal transduction cascades that regulate phosphorylation of the GABA(A) receptor. Surely, future studies will be aimed at further clarifying the molecular mechanisms by which the central receptors are regulated. The presently understood functional effects of ligand-gated ion channel phosphorylation are diverse. At the neuromuscular junction, a regulation of the AChR desensitization rate by both serine and tyrosine phosphorylation has been demonstrated. In addition, tyrosine phosphorylation of the AChR or other synaptic components appears to play a role in AChR clustering during synaptogenesis. For the GABA(A) receptor, the data are complex. Both activation and inhibition of GABA(A) receptor currents as a result of PKA and PKC phosphorylation have been reported, while phosphorylation by PTK enhances function. The predominant effect of glutamate receptor phosphorylation by a variety of kinases is a potentiation of the peak current response. However, PKC also modulates clustering of NMDA receptors. This complexity in the regulation of ligand-gated ion channels by phosphorylation provides diverse mechanisms for mediating synaptic plasticity. In fact, accumulating evidence supports the involvement of protein phosphorylation and dephosphorylation of AMPA receptors in LTP and LTD respectively. There has been a dramatic increase in our understanding of the nature by which phosphorylation regulates ligand-gated ion channels. However, many questions remain unanswered. (AB

Modulation of the glycine response by Ca2+-permeable AMPA receptors in rat spinal neurones

1. In acutely isolated rat sacral dorsal commisural nucleus (SDCN) neurones, application of kainate (KA) reversibly potentiated glycine-evoked Cl- currents (IGly) in a concentration-dependent manner. 2. The cellular events underlying the interaction between non-NMDA receptors and glycine receptors were studied by using nystatin-perforated patch and cell-attached single-channel recording modes. 3. The action of KA was not accompanied by a shift in the reversal potential for IGly. In dose-response curves, KA potentiated IGly without significantly changing glycine binding affinity. 4. GYKI 52466 blocked while NS-102 had no effect on the KA-induced potentiation of IGly. 5. The potentiation was reduced when KA was applied in a Ca2+-free extracellular solution or in the presence of BAPTA AM, and was independent of the activation of voltage-dependent Ca2+ channels. 6. Pretreatment with KN-62, a selective Ca2+-calmodulin-dependent protein kinase II (CaMKII) inhibitor, abolished the action of KA. Inhibition of calcineurin converted the KA-induced potentiation to a sustained one. 7. Single-channel recordings revealed that KA decreased the mean closing time of glycine-gated single-channel activity, resulting in an increase in the probability of channel opening. 8. It is proposed that Ca2+ entry through AMPA receptors modulates the glycine receptor function via coactivation of CaMKII and calcineurin in SDCN neurones. This interaction may provide a new postsynaptic mechanism for control of inhibitory synaptic signalling and represent one of the important regulatory mechanisms of spinal nociception.

Changes in intracellular calcium concentration affect desensitization of GABAA receptors in acutely dissociated P2-P6 rat hippocampal neurons

The whole cell configuration of the patch-clamp technique was used to study the effects of different cytosolic calcium concentrations [Ca2+]i on desensitization kinetics of gamma-aminobutyric acid (GABA)-activated receptors in acutely dissociated rat hippocampal neurons. Two different intrapipette concentrations of the calcium chelator 1, 2-bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid (BAPTA; 11 and 0.9 mM, respectively) were used to yield a low (1.2 x 10(-8) M) or a high (2.2 x 10(-6) M) [Ca2+]i. In low [Ca2+]i, peak values of GABA-evoked currents (20 microM) evoked at -30 mV, were significantly larger than those recorded in high calcium [2,970 +/- 280 (SE) pA vs. 1,870 +/- 150 pA]. The extent of desensitization, assessed from steady-state to peak ratio was significantly higher in high calcium conditions (0.14 +/- 0.007 vs. 0.11 +/- 0.008). Similar effects of -Ca2+-i on desensitization were observed with GABA (100 microM). Recovery from desensitization, measured at 30 s interval with double pulse protocol was significantly slower in high [Ca2+]i than in low [Ca2+]i (54 +/- 3% vs. 68 +/- 2%). The current-voltage relationship of GABA-evoked currents was linear in the potential range between -50 and 50 mV. The kinetics of desensitization process including the rate of onset, extent of desensitization, and recovery were voltage independent. The run down of GABA-evoked currents was faster with the higher intracellular calcium concentration. The run down process was accompanied by changes in desensitization kinetics: in both high and low [Ca2+]i desensitization rate was progressively increasing with time as the slow component of the desensitization onset was converted into the fast one. In excised patches, the desensitization kinetics was much faster and more profound than in the whole cell configuration, indicating the involvement of intracellular factors in regulation of this process. In conclusion, [Ca2+]i affects the desensitization of GABAA receptors possibly by activating calcium-dependent enzymes that regulate their phosphorylation state. This may lead to modifications in cell excitability because of changes in GABA-mediated synaptic currents.

Partial hippocampal kindling decreases efficacy of presynaptic GABAB autoreceptors in CA1

The effect of partial hippocampal kindling, a model of temporal lobe seizure, on monosynaptic inhibition mediated by GABA was studied. Kindled rats were given 15 nonconvulsive hippocampal afterdischarges, and control rats were given low frequency or no stimulations. At 1-2 d after kindling, paired-pulse depression (PPD) of the IPSCs recorded in CA1 neurons in vitro was significantly smaller in kindled as compared with control rats. The difference in PPD persisted for at least 21 d after kindling. The decrease in PPD of the IPSCs after partial hippocampal kindling was likely caused by a reduced GABA autoinhibition after downregulation of presynaptic GABAB receptors. The GABAB antagonist CGP35348 (1 mM) suppressed PPD of the IPSCs more strongly in control than in kindled rats. Direct activation of the presynaptic GABAB receptors by baclofen suppressed the monosynaptic IPSCs significantly more in control than in kindled rats. The decay rate of a single-pulse IPSC was faster in kindled than in control rats on day 1 or day 21 after partial kindling. The difference in IPSC decay between kindled and control rats was found with or without a GABAB receptor antagonist. The low efficacy of the presynaptic GABAB receptors in kindled rats may provide compensatory stabilization of the postsynaptic membrane against further seizures or plasticity.

The effects of raising intracellular calcium on synaptic GABAA receptor-channels

The effects of various calcium (Ca2+) loads imposed through whole-cell patch electrodes on dentate gyrus granule cells were investigated on synaptic GABAA receptor-channels. The kinetics of spontaneous inhibitory postsynaptic currents (sIPSCs) were similar when recorded without any exogenous Ca2+ buffers in the patch electrode or with up to 30 mM BAPTA in the pipette. Unbuffered Ca2+ concentrations of 20-100 microM in the patch pipettes induced a gradual prolongation of miniature IPSC (mIPSC) decays over the course of the recording (10-40 min) with no apparent change in their rise times, peak amplitudes, or frequency of occurrence. This effect was not mimicked by other divalent cations such as strontium. Infusion into the cells of free ionic Ca2+ concentrations buffered with various affinity chelators in the pipette had more pronounced effects on synaptic GABAA currents. Free ionic Ca2+ buffered in the range of 200-400 nM with BAPTA prolonged the decay time constant of mIPSCs. Introducing buffered Ca2+ into the neurons in excess of 1 microM, with a relatively low affinity buffer such as Br2BAPTA, resulted in a marked inhibition of mIPSCs. A similar effect was observed following release of Ca2+ from intracellular stores induced by caffeine (10 mM). We conclude that Ca2+ has a biphasic effect on synaptic GABAA receptor-channels. A high affinity potentiation, consistent with a prolongation of channel burst duration, and a low affinity depression of channel activity both contribute to a complex regulation of synaptic GABAA receptors by [Ca2+]i that has a profound bearing on cellular mechanisms of plasticity and pathological alterations in neuronal excitability.

From ion currents to genomic analysis: recent advances in GABAA receptor research

The gamma-aminobutyric acid type A (GABAA) receptor represents an elementary switching mechanism integral to the functioning of the central nervous system and a locus for the action of many mood- and emotion-altering agents such as benzodiazepines, barbiturates, steroids, and alcohol. Anxiety, sleep disorders, and convulsive disorders have been effectively treated with therapeutic agents that enhance the action of GABA at the GABAA receptor or increase the concentration of GABA in nervous tissue. The GABAA receptor is a multimeric membrane-spanning ligand-gated ion channel that admits chloride upon binding of the neurotransmitter GABA and is modulated by many endogenous and therapeutically important agents. Since GABA is the major inhibitory neurotransmitter in the CNS, modulation of its response has profound implications for brain functioning. The GABAA receptor is virtually the only site of action for the centrally acting benzodiazepines, the most widely prescribed of the anti-anxiety medications. Increasing evidence points to an important role for GABA in epilepsy and various neuropsychiatric disorders. Recent advances in molecular biology and complementary information derived from pharmacology, biochemistry, electrophysiology, anatomy and cell biology, and behavior have led to a phenomenal growth in our understanding of the structure, function, regulation, and evolution of the GABAA receptor. Benzodiazepines, barbiturates, steroids, polyvalent cations, and ethanol act as positive or negative modulators of receptor function. The description of a receptor gene superfamily comprising the subunits of the GABAA, nicotinic acetylcholine, and glycine receptors has led to a new way of thinking about gene expression and receptor assembly in the nervous system. Seventeen genetically distinct subunit subtypes (alpha 1-alpha 6, beta 1-beta 4, gamma 1-gamma 4, delta, p1-p2) and alternatively spliced variants contribute to the molecular architecture of the GABAA receptor. Mysteriously, certain preferred combinations of subunits, most notably the alpha 1 beta 2 gamma 2 arrangement, are widely codistributed, while the expression of other subunits, such as beta 1 or alpha 6, is severely restricted to specific neurons in the hippocampal formation or cerebellar cortex. Nervous tissue has the capacity to exert control over receptor number, allosteric uncoupling, subunit mRNA levels, and posttranslational modifications through cellular signal transduction mechanisms under active investigation. The genomic organization of the GABAA receptor genes suggests that the present abundance of subtypes arose during evolution through the duplication and translocations of a primordial alpha-beta-gamma gene cluster. This review describes these varied aspects of GABAA receptor research with special emphasis on contemporary cellular and molecular discoveries.

The effect of intracellular Ca2+ on GABA-activated currents in cerebellar granule cells in culture

The patch clamp technique was used to study the effects of intracellular free calcium ([Ca2+]i) on GABAA-evoked whole-cell and single channel currents of cultured cerebellar granule cells. Changes in [Ca2+]i were obtained by adding to the extracellular solution the calcium ionophore A23187 (2 microM). The relationship between [Ca2+]i and [Ca2+]o in the presence or absence of A23187 was assessed using fluorimetric measurements from Fura-2 loaded cells. In 2 mM [Ca2+]o and A23187, [Ca2+]i was about 1.5 microM, whereas in the absence of A23187 it was about 250 nM. In whole-cell experiments (symmetrical chloride concentrations) at -50 mV, GABA (0.5 microM) evoked inward currents that did not desensitize. Bath application of A23187 significantly reduced the steady-state amplitude of GABA currents by 37 +/- 6%. Single channel currents activated by GABA (0.5 microM) were also recorded in the outside-out configuration of the patch clamp technique. Kinetic analysis of single channel events revealed that A23187 significantly increased the long closed time constant (tau c3) without affecting the open time constants (tau o1 and tau o2) or the short and medium closed time constants (tau c1 and tau c2). Moreover, application of A23187 induced a significant reduction of burst duration (tau b). We conclude that a rise in [Ca2+]i by A23187 may decrease the binding affinity of GABA for the GABAA receptor.

The cisternal organelle as a Ca(2+)-storing compartment associated with GABAergic synapses in the axon initial segment of hippocampal pyramidal neurones

The axon initial segment of cortical principal neurones contains an organelle consisting of two to four stacks of flat, membrane-delineated cisternae alternating with electron-dense, fibrillar material. These cisternal organelles are situated predominantly close to the synaptic junctions of GABAergic axo-axonic cell terminals. To examine the possibility that the cisternal organelle is involved in Ca2+ sequestration, we tested for the presence of Ca(2+)-ATPase in the cisternal organelles of pyramidal cell axons in the CA1 and CA3 regions of the hippocampus. Electron microscopic immunocytochemistry using antibodies to muscle sarcoplasmic reticulum ATPase revealed immunoreactivity associated with cisternal organelle membranes. The localisation of Ca(2+)-ATPase in cisternal organelles was also confirmed by enzyme cytochemistry, which produced reaction product in the lumen of the cisternae. These experiments provide evidence for the presence of a Ca2+ pump in the cisternal organelle membrane, which may play a role in the sequestration and release of Ca2+. Cisternal organelles are very closely aligned to the axolemma and the outermost cisternal membrane is connected to the plasma membrane by periodic electron-dense bridges as detected in electron micrographs. It is suggested that the interface acts as a voltage sensor, releasing Ca2+ from cisternal organelles upon depolarisation of the axon initial segment, in a manner similar to the sarcoplasmic reticulum of skeletal muscle. The increase in intra-axonal Ca2+ may regulate the GABAA receptors associated with the axo-axonic cell synapses, and could affect the excitability of pyramidal cells.

GABAA receptor beta 1-3 subunit gene expression in the hippocampus of kindled rats

The effect of Schaffer collateral/commissural fiber kindling on the expression levels of GABAA receptor beta 1, beta 2 and beta 3 subunit mRNA in the pyramidal and granular neurons of the rat dorsal hippocampus was studied, using semi-quantitative in situ hybridization. In pyramidal neurons of CA1 and CA3, only small changes (10-15%) were found. In dentate granule neurons, the expression level of GABAA R-beta 3 mRNA was significantly, enhanced, bilaterally, in animals that were partial or fully kindled. At long-term, 4 weeks after the last convulsion no significant changes were found in pyramidal or granular neurons.