Endogenous phosphorylation of distinct gamma-aminobutyric acid type A receptor polypeptides by Ser/Thr and Tyr kinase activities associated with the purified receptor

Biochemical properties of the sensitivity to GABAAergic ligands, Cl-/HCO3--ATPase isolated from fish (Cyprinus carpio) olfactory mucosa and brain

This paper presents a comparative study of the roles of Cl^- and HCO₃^- in the functioning of the GABA_AR-associated Cl^-/HCO₃^--ATPase of the plasma membranes of the olfactory sensory neurons (OSNs) and mature brain neurons (MBNs) of fish. The ATPase activity of OSNs and its dephosphorylation were increased twofold by Cl^-(15-30 mmol l^-1), whereas the enzyme from MBNs was not significantly affected by Cl^-. By contrast, HCO₃^-(15-30 mmol l^-1) significantly activated the MBN enzyme and its dephosphorylation, but had no effect on the OSN ATPase. The maximum ATPase activity and protein dephosphorylation was observed in the presence of both Cl^-(15 mmol l^-1)/HCO₃^-(27 mmol l^-1) and these activities were inhibited in the presence of picrotoxin (100 μmol l^-1), bumetanide (150 μmol l^-1), and DIDS (1000 μmol l^-1). SDS-PAGE revealed that ATPases purified from the neuronal membrane have a subunit with molecular mass of ~ 56 kDa that binds [³H]muscimol and [³H]flunitrazepam. Direct phosphorylation of the enzymes in the presence of ATP-γ-³²P and Mg²⁺, as well as Cl^-/HCO₃^- sensitive dephosphorylation, is also associated with this 56 kDa peptide. Both preparations also showed one subunit with molecular mass 56 kDa that was immunoreactive with GABA_AR β3 subunit. The use of a fluorescent dye for Cl^- demonstrated that HCO₃^-(27 mmol l^-1) causes a twofold increase in Cl^- influx into proteoliposomes containing reconstituted ATPases from MBNs, but HCO₃^- had no effect on the reconstituted enzyme from OSNs. These data are the first to demonstrate a differential effect of Cl^- and HCO₃^- in the regulation of the Cl^-/HCO₃^--ATPases functioning in neurons with different specializations.

GAPDH in anesthesia

Thus far, two independent laboratories have shown that inhaled anesthetics directly affect GAPDH structure and function. Additionally, it has been demonstrated that GAPDH normally regulates the function of GABA (type A) receptor. In light of these literature observations and some less direct findings, there is a discussion on the putative role of GAPDH in anesthesia. The binding site of inhaled anesthetics is described from literature reports on model proteins, such as human serum albumin and apoferritin. In addition to the expected hydrophobic residues that occupy the binding cavity, there are hydrophilic residues at or in very close proximity to the site of anesthetic binding. A putative binding site in the bacterial analog of the human GABA (type A) receptor is also described. Additionally, GAPDH may also play a role in anesthetic preconditioning, a phenomenon that confers protection of cells and tissues to future challenges by noxious stimuli. The central thesis regarding this paradigm is that inhaled anesthetics evoke an intra-molecular protein dehydration that is recognized by the cell, eliciting a very specific burst of chaperone gene expression. The chaperones that are implicated are associated with conferring protection against dehydration-induced protein aggregation.

Mass spectrometric analysis of GABAA receptor subtypes and phosphorylations from mouse hippocampus

The brain GABA(A) receptor (GABA(A) R) is a key element of signaling and neural transmission in health and disease. Recently, complete sequence analysis of the recombinant GABA(A) R has been reported, separation and mass spectrometrical (MS) characterisation from tissue, however, has not been published so far. Hippocampi were homogenised, put on a sucrose gradient 10-69% and the layer from 10 to 20% was used for extraction of membrane proteins by a solution of Triton X-100, 1.5 M aminocaproic acid in the presence of 0.3 M Bis-Tris. This mixture was subsequently loaded onto blue native PAGE (BN-PAGE) with subsequent analysis on denaturing gel systems. Spots from the 3-DE electrophoretic run were stained with Colloidal Coomassie Brilliant Blue, and spots with an apparent molecular weight between 40 and 60 kDa were picked and in-gel digested with trypsin, chymotrypsin and subtilisin. The resulting peptides were analysed by nano-LC-ESI-MS/MS (ion trap) and protein identification was carried out using MASCOT searches. In addition, known GABA(A) R-specific MS information taken from own previous studies was used for searches of GABA(A) R subunits. β-1, β-2 and β-3, θ and ρ-1 subunits were detected and six novel phosphorylation sites were observed and verified by phosphatase treatment. The method used herein enables identification of several GABA(A) R subunits from mouse hippocampus along with phosphorylations of β-1 (T227, Y230), β-2 (Y215, T439) and β-3 (T282, S406) subunits. The procedure forms the basis for GABA(A) R studies at the protein chemical rather than at the immunochemical level in health and disease.

Dysfunction of GABAA receptor glycolysis-dependent modulation in human partial epilepsy

A reduction in GABAergic neurotransmission has been put forward as a pathophysiological mechanism for human epilepsy. However, in slices of human epileptogenic neocortex, GABAergic inhibition can be clearly demonstrated. In this article we present data showing an increase in the functional lability of GABAergic inhibition in epileptogenic tissue compared with nonepileptogenic human tissue. We have previously shown that the glycolytic enzyme GAPDH is the kinase involved in the glycolysis-dependent endogenous phosphorylation of the alpha1-subunit of GABA(A) receptor, a mechanism necessary for maintaining GABA(A) function. In human epileptogenic cortex obtained during curative surgery of patients with partial seizures, we demonstrate an intrinsic deficiency of GABA(A) receptor endogenous phosphorylation resulting in an increased lability of GABAergic currents in neurons isolated from this tissue when compared with neurons from nonepileptogenic human tissue. This feature was not related to a reduction in the number of GABA(A) receptor alpha1-subunits in the epileptogenic tissue as measured by [(3)H]flunitrazepam photoaffinity labeling. Maintaining the receptor in a phosphorylated state either by favoring the endogenous phosphorylation or by inhibiting a membrane-associated phosphatase is needed to sustain GABA(A) receptor responses in epileptogenic cortex. The increased functional lability induced by the deficiency in phosphorylation can account for transient GABAergic disinhibition favoring seizure initiation and propagation. These findings imply new therapeutic approaches and suggest a functional link to the regional cerebral glucose hypometabolism observed in patients with partial epilepsy, because the dysfunctional GABAergic mechanism depends on the locally produced glycolytic ATP.

Glyceraldehyde-3-phosphate dehydrogenase is a GABAA receptor kinase linking glycolysis to neuronal inhibition

Protein phosphorylation is crucial for regulating synaptic transmission. We describe a novel mechanism for the phosphorylation of the GABA(A) receptor, which mediates fast inhibition in the brain. A protein copurified and coimmunoprecipitated with the phosphorylated receptor alpha1 subunit; this receptor-associated protein was identified by purification and microsequencing as the key glycolytic enzyme glyceraldehyde-3-phosphate dehydrogenase (GAPDH). Molecular constructs demonstrated that GAPDH directly phosphorylates the long intracellular loop of GABA(A) receptor alpha1 subunit at identified serine and threonine residues. GAPDH and the alpha1 subunit were found to be colocalized at the neuronal plasma membrane. In keeping with the GAPDH/GABA(A) receptor molecular association, glycolytic ATP produced locally at plasma membranes was consumed for this alpha1 subunit phosphorylation, possibly within a single macrocomplex. The membrane-attached GAPDH is thus a dual-purpose enzyme, a glycolytic dehydrogenase, and a receptor-associated kinase. In acutely dissociated cortical neurons, the rundown of the GABA(A) responses was essentially attributable to a Mg(2+)-dependent phosphatase activity, which was sensitive to vanadate but insensitive to okadaic acid or fluoride. Rundown was significantly reduced by the addition of GAPDH or its reduced cofactor NADH and nearly abolished by the addition of its substrate glyceraldehyde-3-phosphate (G3P). The prevention of rundown by G3P was abolished by iodoacetamide, an inhibitor of the dehydrogenase activity of GAPDH, indicating that the GABA(A) responses are maintained by a glycolysis-dependent phosphorylation. Our results provide a molecular mechanism for the direct involvement of glycolysis in neurotransmission.

The difference between sleep and anaesthesia is in the intracellular signal: propofol and GABA use different subtypes of the GABA(A) receptor beta subunit and vary in their interaction with actin

BACKGROUND

Propofol is known to interact with the gamma-aminobutyric acidA (GABA(A)) receptor, however, activating the receptor alone is not sufficient for producing anaesthesia.

METHODS

To compare propofol and GABA, their interaction with the GABAA receptor beta subunit and actin were studied in three cellular fractions of cultured rat neurons using Western blot technique.

RESULTS

Propofol tyrosine phosphorylated the GABA(A) receptor beta2 (MW 54 and 56 kDa) and beta3 (MW 57 kDa) subtypes. The increase was shown in both the cytoskeleton (beta2(54) and beta2(56) subtypes) and the cell membrane (beta2(54) and beta3 subtypes). Concurrently the 56 kDa beta2 subtype was reduced in the cytosol. Propofol, but not GABA, also tyrosine phosphorylated actin in the cell membrane and cytoskeletal fraction. Without extracellular calcium available, the amount of actin decreased in the cytoskeleton, but tyrosine phosphorylation was unchanged. GABA caused increased tyrosine phosphorylation of beta2(56) and beta3 subtypes in the membrane and both beta2 subtypes in the cytoskeleton but no cytosolic tyrosine phosphorylation.

CONCLUSION

The difference between propofol and GABA at the GABA(A) receptor was shown to take place in the membrane, where the beta2(54) was increased by propofol and instead the beta2(56) subtype was increased by GABA. Only propofol also tyrosine phosphorylated actin in the cell membrane and cytoskeletal fraction. This interaction between the GABAA receptor and actin might explain the difference between anaesthesia and physiological neuronal inhibition.

GABA and GABA receptors in the central nervous system and other organs

Gamma-aminobutyrate (GABA) is a major inhibitory neurotransmitter in the adult mammalian brain. GABA is also considered to be a multifunctional molecule that has different situational functions in the central nervous system, the peripheral nervous system, and in some nonneuronal tissues. GABA is synthesized primarily from glutamate by glutamate decarboxylase (GAD), but alternative pathways may be important under certain situations. Two types of GAD appear to have significant physiological roles. GABA functions appear to be triggered by binding of GABA to its ionotropic receptors, GABA(A) and GABA(C), which are ligand-gated chloride channels, and its metabotropic receptor, GABA(B). The physiological, pharmacological, and molecular characteristics of GABA(A) receptors are well documented, and diversity in the pharmacologic properties of the receptor subtypes is important clinically. In addition to its role in neural development, GABA appears to be involved in a wide variety of physiological functions in tissues and organs outside the brain.

A tyrosine kinase regulates propofol-induced modulation of the beta-subunit of the GABA(A) receptor and release of intracellular calcium in cortical rat neurones

Propofol, an intravenous anaesthetic, has been shown to interact with the beta-subunit of the gamma-amino butyric acid(A) (GABA(A)) receptor and also to cause changes in [Ca2+]i. The GABA(A) receptor, a suggested target for anaesthetics, is known to be regulated by kinases. We have investigated if tyrosine kinase is involved in the intracellular signal system used by propofol to cause anaesthesia. We used primary cell cultured neurones from newborn rats, pre-incubated with or without a tyrosine kinase inhibitor before propofol stimulation. The effect of propofol on tyrosine phosphorylation and changes in [Ca2+]i were investigated. Propofol (3 microg mL(-1), 16.8 microM) increased intracellular calcium levels by 122 +/- 34% (mean +/- SEM) when applied to neurones in calcium free medium. This rise in [Ca2+]i was lowered by 68% when the cells were pre-incubated with the tyrosine kinase inhibitor herbimycin A before exposure to propofol (P < 0.05). Propofol caused an increase (33 +/- 10%) in tyrosine phosphorylation, with maximum at 120 s, of the beta-subunit of the GABA(A)-receptor. This tyrosine phosphorylation was decreased after pre-treatment with herbimycin A (44 +/- 7%, P < 0.05), and was not affected by the absence of exogenous calcium in the medium. Tyrosine kinase participates in the propofol signalling system by inducing the release of calcium from intracellular stores and by modulating the beta-subunit of the GABA(A)-receptor.

Multiple roles of protein kinases in the modulation of gamma-aminobutyric acid(A) receptor function and cell surface expression

gamma-Aminobutyric acid(A) (GABA(A)) receptors are ligand-gated ion channels that mediate the majority of fast synaptic inhibition in the brain and that are also important drug targets for benzodiazepines, barbiturates, and neurosteriods. These receptors are pentameric hetero-oligomers that can be assembled from 7 subunit classes with multiple members: alpha(1-6), beta(1-3), gamma(1-3), delta, epsilon, theta, and pi. Most receptor subtypes in the brain, however, are believed to be composed of alpha-, beta-, and gamma-subunits. Modifications of GABA(A) receptor function are continually implicated in a range of pathologies, including epilepsy, anxiety, insomnia, and substance abuse. Moreover, changes in the efficacy of synaptic inhibition mediated by GABA(A) receptors are believed to be play central roles in certain forms of synaptic plasticity, including rebound potentiation in the cerebellum, and hippocampal long-term potentiation. Given the critical role that GABA(A) receptors play as mediators of synaptic transmission, it is of fundamental importance to understand the endogenous mechanisms used by neurones to control the function of these receptors. This review will focus on the dynamic regulation of GABA(A) receptor phosphorylation state and channel function as mechanisms involved in determining the efficacy of synaptic inhibition. In addition, the possible role of GABA(A) receptor phosphorylation in controlling receptor internalization and recycling will also be explored.

Neurosteroid modulation of GABA IPSCs is phosphorylation dependent

The neurosteroid 3alpha-hydroxy-5alpha-pregnan-20-one (allopregnanolone) facilitates GABA(A) receptor-mediated ionic currents via allosteric modulation of the GABA(A) receptor. Accordingly, allopregnanolone caused an increase in the slow decay time constant of spontaneous GABA-mediated IPSCs in magnocellular neurons recorded in hypothalamic slices. The allopregnanolone effect on IPSCs was inhibited by a G-protein antagonist as well as by blocking protein kinase C and, to a lesser extent, cAMP-dependent protein kinase activities. G-protein and protein kinase C activation in the absence of the neurosteroid had no effect on spontaneous IPSCs but enhanced the effect of subsequent allopregnanolone application. These findings together suggest that the neurosteroid modulation of GABA-mediated IPSCs requires G-protein and protein kinase activation, although not via a separate G-protein-coupled steroid receptor.

Endogenous phosphorylation of the GABA(A) receptor protein is counteracted by a membrane-associated phosphatase

Incubation of bovine brain membranes with [gamma-33P]ATP phosphorylated mainly a 51-kDa band. Electrophoretic co-migration was observed for 33P- and [3H]flunitrazepam-labeled bands in both membrane fractions and in affinity-purified GABA(A) receptor (GABAA-R) preparations. An alpha-subunit monoclonal antibody adsorbed most of the radiolabeled-band, suggesting that the labeled-membrane polypeptide corresponds to the GABA(A)-R alpha1-subunit, which is the only GABA(A)-R subunit with a molecular weight of 51 kDa. The phosphorylation rate was much faster in membranes than in purified receptor. Dephosphorylation was detected in membranes only. The membrane-bound phosphatase was potently inhibited by vanadate and Zn2+>>Mn2+ , but was insensitive to okadaic acid (a phosphatase 1, 2 and 2B inhibitor), cyclosporin (specific calcineurin inhibitor) and phosphatase-1 inhibitor. Endogenous kinase was activated by divalent cations including calcium (Mg2- > Mn2+ > Ca2+), whilst dephosphorylation did not require the presence of Ca2+ ions. This suggests that at least one membrane-bound phosphatase counteracts the endogenous phosphorylation of the GABA(A)-R: the lack of dephosphorylation in the purified receptor preparation indicates that, in contrast to the endogenous kinase, no phosphatase is closely associated with the receptor protein complex.

Subunit-specific association of protein kinase C and the receptor for activated C kinase with GABA type A receptors

GABA receptors (GABA(A)) are the major sites of fast synaptic inhibition in the brain and can be assembled from five subunit classes: alpha, beta, gamma, delta, and epsilon. Receptor function can be regulated by direct phosphorylation of beta and gamma2 subunits, but how kinases are targeted to GABA(A) receptors is unknown. Here we show that protein kinase C-betaII (PKC-betaII) is capable of directly binding to the intracellular domain of the receptor beta1 and beta3 subunits, but not to those of the alpha1 or gamma2 subunits. Moreover, associating PKC-betaII is capable of specifically phosphorylating serine 409 in beta1 subunit and serines 408/409 within the beta3 subunit, key residues for modulating GABA(A) receptor function. The receptor for activated C kinase (RACK-1) was found also to bind to the beta1 subunit intracellular domain, but PKC binding appeared to be independent of this protein. Using immunoprecipitation, the association of PKC isoforms and RACK-1 with neuronal GABA(A) receptors was seen. Furthermore, PKC isoforms associating with neuronal receptors were capable of phosphorylating the receptor beta3 subunit. Together, these observations suggest GABA(A) receptors are intimately associated with PKC isoforms via a direct interaction with receptor beta subunits. This interaction may serve to localize PKC activity to GABA(A) receptors in neurons allowing the rapid regulation of receptor activity by cell-signaling pathways that modify PKC activity.

A novel serine kinase with specificity for beta3-subunits is tightly associated with GABA(A) receptors

Tuning of gamma-aminobutyric acid type A (GABA(A)) receptor function via phosphorylation of the receptor potentially allows neurons to modulate their inhibitory input. Several kinases, both of the serine-threonine kinase and the tyrosine kinase families, have been proposed as candidates for such a modulatory role in vivo. However, no GABA(A) receptor-phosphorylating kinase physically associated with the receptor has been identified so far on a molecular level. In this study, we demonstrate a GABA(A) receptor-associated protein serine kinase phosphorylating specifically beta3-subunits of native GABA(A) receptors. The characteristics of this novel kinase clearly distinguish it from enzymatic activities that have been shown so far to phosphorylate the GABA(A) receptor. We putatively identify this protein kinase as the previously described GTAP34 (GABA(A) receptor-tubulin complex-associated protein of molecular mass 34 kDa). Using expressed recombinant fusion proteins, we identify serine 408 as a major target of the phosphorylation reaction, whereas serine 407 is not phosphorylated. This demonstrates the high specificity of the kinase. Phosphorylation of serine 408 is known to result in a decreased receptor function. The direct association of this kinase with the receptor indicates an important physiological role.

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 nicotinic acetylcholine receptor activity in submucous neurons by intracellular messengers

The effects on acetylcholine-induced membrane currents (ACh currents), produced by agents known to modify the activity of intracellular messengers, were studied in the neurons of the guinea-pig ileum submucous plexus (SMP) using a whole-cell patch clamp recording method. The ACh currents were not affected by forskolin, the adenylate cyclase activator, regardless of whether or not ATP and GTP were present in the intracellular solution, and by phorbol 12-myristate 13-acetate, the protein kinase C activator. The ACh currents were strongly suppressed by thapsigargin, the microsomal calcium ATPase inhibitor, and genistein, the tyrosine protein kinase inhibitor. They were also suppressed by 3-isobutyl-1-methylxanthine, the cyclic-AMP phosphodiesterase inhibitor, regardless of the presence of forskolin in the extracellular solution and ATP and GTP in the intracellular solution. In addition, the currents were suppressed by activation of P2 purinoceptors with ATP, which could not be explained by a direct effect of ATP on nicotinic acetylcholine receptors (nAChRs). Reactive blue 2, the P2y purinoceptor antagonist, did not abolish inhibition of the ACh current by ATP. Alpha,beta-Imido-ATP and adenosine caused no membrane current responses and did not influence the ACh currents. These results suggest that the activity of the nAChRs in the SMP neurons is strongly suppressed by raised intracellular Ca2+ level, without involvement of protein kinases A and C, and may involve the participation of tyrosine kinase. The activity of nAChRs is also influenced by the activity of P2 purinoceptors; the mechanisms responsible for this influence are not yet clear. So, the activity of the SMP neuronal nAChRs is relatively independent on the intracellular signaling known to influence many other groups of transmitter-gated receptors of neuronal membrane.

Protein tyrosine phosphorylation: implications for synaptic function

The phosphorylation of proteins on tyrosine residues, initially believed to be primarily involved in cell growth and differentiation, is now recognized as having a critical role in regulating the function of mature cells. The brain exhibits one of the highest levels of tyrosine kinase activity in the adult animal and the synaptic region is particularly rich in tyrosine kinases and tyrosine phosphorylated proteins. Recent studies have described the effects of tyrosine phosphorylation on the activities of a number of proteins which are potentially involved in the regulation of synaptic function. Furthermore, it is becoming apparent that tyrosine phosphorylation is involved in the modification of synaptic activity, such as occurs during depolarization, the induction of long-term potentiation or long-term depression, and ischemia. Changes in the activities of tyrosine kinases and/or protein tyrosine phosphatases which are associated with synaptic structures may result in altered tyrosine phosphorylation of proteins located at the synapse leading to both short-term and long-lasting changes in synaptic and neuronal function.

Differential intracellular regulation of cortical GABA(A) and spinal glycine receptors in cultured neurons

Using patch-clamp techniques we studied several aspects of intracellular GABA(A) and glycine Cl- current regulation in cortical and spinal cord neurons, respectively. Activation of PKA with a permeable analog of cyclic AMP (cAMP) produced a potentiation of the Cl- current activated with glycine, but not of the current induced with GABA. The inactive analog was without effect. Activation of PKC with 1 microM PMA reduced the amplitude of the GABA(A) and glycine currents. Internal application of 1 mM cGMP, on the other hand, had no effect on the amplitude of either current. The amplitude of these inhibitory currents changed slightly during 20 min of patch-clamp recording. Internal perfusion of the neurons with 1 microM okadaic acid, a phosphatase inhibitor, induced potentiation in both currents. The amplitude of GABA(A) and glycine currents recorded with 1 mM internal CaCl2 and 10 mM EGTA (10 nM free Ca2+) decayed by less than 30% of control. Increasing the CaCl2 concentration to 10 mM (34 microM free Ca2+) induced a transient potentiation of the GABA(A) current. A strong depression of current amplitude was found with longer times of dialysis. The glycine current, on the contrary, was unchanged by increasing the intracellular Ca2+ concentration. Activation of G proteins with internal FAl4- induced an inhibition of the GABA(A) current, but potentiated the amplitude of the strychnine-sensitive Cl- current. These results indicate that GABA(A) and glycine receptors are differentially regulated by activation of protein kinases, G proteins and Ca2+. This conclusion supports the existence of selectivity in the intracellular regulation of these two receptor types.

Modulation of amino acid-gated ion channels by protein phosphorylation

The major excitatory and inhibitory amino acid receptors in the mammalian central nervous system are considered to be glutamate, gamma-aminobutyric acid type A (GABAA), and glycine receptors. These receptors are widely acknowledged to participated in fast synaptic neurotransmission, which ultimately is responsible for the control of neuronal excitability. In addition to these receptors being regulated by endogenous factors, including the natural neurotransmitters, they also form target substrates for phosphorylation by a number of protein kinases, including serine/threonine and tyrosine kinases. The process of phosphorylation involves the transfer of a phosphate group(s) from adenosine triphosphate to one or more serine, threonine, or tyrosine residues, which are invariably found in an intracellular location within the receptor Phosphorylation is an important means of receptor regulation since it represents a covalent modification of the receptor structure, which can have important implications for ion channel function. This chapter reviews the current molecular and biochemical evidence regarding the sites of phosphorylation for both native neuronal and recombinant glutamate, GABAA and glycine receptors, and also reviews the functional electrophysiological implications of phosphorylation for receptor function.