Modulation of inhibitory glycine receptors by phosphorylation by protein kinase C and cAMP-dependent protein kinase

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.

Glycine receptors in adult guinea pig brain stem auditory nuclei: regulation after unilateral cochlear ablation

In young adult guinea pigs, the effects of unilateral cochlear ablation were determined on the specific binding of [3H]strychnine measured in subdivisions of the cochlear nucleus (CN), the superior olivary complex, and the auditory midbrain, after 2, 7, 31, 60, and 147 postlesion days. Changes in binding relative to that in age-matched controls were interpreted as altered activity and/or expression of synaptic glycine receptors. Postlesion binding declined ipsilaterally in most of the ventral CN and in the lateral superior olive (LSO). Binding was modestly deficient in the ipsilateral dorsal CN and in the anterior part of the contralateral anteroventral CN. Binding was elevated in the contralateral LSO. Transient changes also occurred. Binding was elevated transiently, between 2 and 31 days, contralaterally in parts of the anteroventral CN, bilaterally in the medial superior olive (MSO), and bilaterally in most of the midbrain nuclei. Binding was deficient transiently, at 60 days, in most of the contralateral CN and bilaterally in the midbrain nuclei. The present findings, together with previously reported postlesion changes in glycine release, were consistent with persistently weakened glycinergic inhibitory transmission ipsilaterally in the ventral CN and the LSO and bilaterally in the dorsal CN. Glycinergic inhibitory transmission was strengthened in the contralateral LSO and transiently strengthened in the MSO bilaterally. A hypothetical model of the findings suggested that glycine receptor regulation may depend on excitatory and glycinergic input to auditory neurons. The present changes in glycine receptor activity may contribute to altered auditory functions, which often accompany hearing loss.

cAMP-dependent protein kinase modulation of glycine-activated chloride current in neurons freshly isolated from rat ventral tegmental area

Adenosine 3',5'cyclic monophosphate-(cAMP)-dependent protein kinase (PKA) modulation of glycine-activated Cl- currents (IGly) in single neurons freshly isolated from the rat ventral tegmental area (VTA) was studied using whole-cell patch-clamp technique. In the majority of cells tested with Mg-ATP in the internal solution, IGly induced by 3-10 microM glycine increased spontaneously (ran up). In the absence of internal ATP, IGly remained stable in six of seven cells. External perfusion of 8-Br-cAMP, a PKA activator, potentiated IGly only in cells showing run-up. 8-Br-cAMP potentiated IGly induced by low concentrations of glycine, but had no effect on the maximal current. When added to the pipette solution, H-89, a PKA inhibitor, blocked ATP and 8-Br-cAMP induced run-up of IGly. In contrast, dialysis with chelerythrine, a PKC inhibitor, did not alter the run-up of IGly. These results suggest that the PKA pathway modulates the activity of the glycine receptor/channel complex via enhancing the affinity of the receptor for glycine.

Enhancement of glycine receptor function by ethanol: role of phosphorylation

1. The effects of several kinase inhibitors (staurosporine, GF 109203X, H89, KN62, genistein) and of the phosphatase inhibitor calyculin A were studied on the ethanol potentiation and on the function of homomeric alpha1 glycine receptor expressed in Xenopus oocytes using a two electrode voltage clamp recording technique. 2. The function of the homomeric alpha1 glycine receptor was not modified in Xenopus oocytes pretreated with kinase inhibitors or with the phosphatase inhibitor calyculin A. 3. The potentiation of the glycine receptor function induced by ethanol (10-200 mM) was significantly reduced in Xenopus oocytes pretreated with the PKC inhibitors staurosporine or GF 109203X. 4. No differences in propofol (2.5 microM) or halothane (250 microM) actions were found after exposure of Xenopus oocytes to staurosporine. 5. No differences in ethanol sensitivity were found after exposure of Xenopus oocytes expressing glycine alpha1 receptors to H89, KN62, genistein or to the phosphatase inhibitor calyculin A. 6. The mutant alpha1 (S391A), in which the PKC phosphorylation site at serine 391 was mutated to alanine, was less sensitive to the effects of ethanol than was the alpha1 wild type receptor. Moreover, the ethanol potentiation of the glycine receptor function was not affected by treatment with staurosporine in oocytes expressing alpha1 (S391A). 7. The splice variant of the alpha1 glycine receptor subunit, alpha1ins, containing eight additional amino acids and a potential phosphorylation site for PKA, did not differ from wild type for sensitivity to ethanol. 8. These results indicate that phosphorylation by PKC of the homomeric alpha1 glycine receptor subunit modulates ethanol potentiation, but not the function of the glycine receptor.

The human glycine receptor subunit alpha3. Glra3 gene structure, chromosomal localization, and functional characterization of alternative transcripts

The neuronal glycine receptor is a ligand-gated chloride channel composed of ligand binding alpha and structural beta polypeptides. Homology screening of a human fetal brain cDNA library resulted in the identification of two alternative splice variants of the glycine receptor alpha3 subunit. The amino acid sequence predicted for the alpha3L variant was largely identical to the corresponding rat subunit. In contrast, the novel splice variant alpha3K lacked the coding sequence for 15 amino acids located within the cytoplasmic loop connecting transmembrane spanning region 3 (TM3) and TM4. Using P1 artificial chromosome (PAC) clones, the structure of the GLRA3 gene was elucidated and its locus assigned to human chromosomal bands 4q33-q34 by fluorescence in situ hybridization. Two transcripts of 2.4 and 9 kilobases, corresponding to alpha3L and alpha3K, respectively, were identified and found to be widely distributed throughout the human central nervous system. Structural analysis of the GLRA3 gene revealed that the alpha3K transcript resulted from a complex splice event where excision of the novel exon 8A comprising the alternative sequence of 45 base pairs coincides with the persistence of a large intronic sequence in the 3'-untranslated region. Functional expression in HEK 293 cells of alpha3L and alpha3K subunits resulted in the formation of glycine-gated chloride channels that differed significantly in desensitization behavior, thus defining the cytoplasmic loop as an important determinant of channel inactivation kinetics.

Kainate receptor modulation of GABA release involves a metabotropic function

The mechanism through which kainate receptors downregulate the release of GABA in the hippocampus is not known. We have found that the action of kainate on the hippocampal inhibitory postsynaptic current (IPSC) is mediated by a metabotropic process that is sensitive to Pertussis toxin (PTx) and independent of ion channel current. The downregulation of GABA IPSCs by kainate was also prevented in a dose-dependent manner by calphostin C, a specific inhibitor of PKC, and the inhibition of phospholipase C (PLC) drastically reduced the action of kainate. The effect of kainate was completely occluded by phorbol esters and by increasing extracellular Ca2+ but remained unaltered after inhibition or activation of protein kinase A (PKA). These results demonstrate that the activation of kainate receptors triggers a second messenger cascade, which results in the stimulation of PKC, and therefore document a metabotropic action of kainate receptors, which results in the inhibition of GABA release.

Protein kinases modulate two glycine currents in salamander retinal ganglion cells

1. Protein kinase modulation of glycine-activated currents was examined in acutely dissociated ganglion cells from tiger salamander retina using whole-cell voltage-clamp techniques. 2. Glycine (100 microM) induced an outward chloride current in cells clamped at 0 mV. Co-application of 50 microM forskolin made the glycine-induced current more transient. The combination of forskolin and glycine reduced the later portion of current response without changing the initial peak amplitude. 3. 3-Isobutyl-1-methylxanthine (IBMX) or 8-bromoadenosine 3',5'-cyclic monophosphate (8-Br-cAMP) produced effects similar to those of forskolin. H-89, a protein kinase A (PKA) inhibitor, blocked the effect of forskolin. 4. A protein kinase C (PKC) activator, OAG (1-oleoyl-2-acetyl-sn-glycerol), also made the glycine response more transient. Unlike PKA analogues, OAG enhanced the glycine peak response without changing the glycine late response. OAG effects were blocked by 1 microM GF-109203X, a PKC inhibitor. 5. Nanomolar concentrations of strychnine selectively blocked the fast phase of the glycine current and reversed the effect of OAG, but not that of forskolin. Conversely, forskolin occluded the effect of 5,7-dichlorokynurenic acid, which selectively suppresses the late phase of the glycine current. The action of OAG was not blocked by 5,7-dichlorokynurenic acid. 6. Thus, through a differential modulation, both protein kinase A and C shorten the decay time of the glycine current. PKA suppresses the slow component, while PKC potentiates the fast component.

Kinetic analysis of glycine receptor currents in ventral cochlear nucleus

Glycine plays an important role as an inhibitory neurotransmitter in the ventral cochlear nucleus. However, little is known about the kinetic behavior of glycine receptors. The present study examines the kinetics of the native inhibitory glycine receptors in neurons of the ventral cochlear nucleus, using outside-out patches from acutely dissociated cells and a fast flow system. Steps into 1 mM glycine revealed fast phases of desensitization with time constants of 13 and 129 ms, that together produced a 40% reduction in current from the peak response. Slower desensitization phases also were observed. After removal of glycine, currents deactivated with two time constants of 15 and 68 ms, and these rates were independent of the glycine concentration between 0.2 and 1 mM. Recovery from desensitization was slow relative to desensitization itself. These results demonstrate that glycine receptors can exhibit faster rates of desensitization and deactivation than previously reported.

Cross-modulation of glycine-activated Cl- channels by protein kinase C and cAMP-dependent protein kinase in the rat

1. The cross-modulation of glycine responses by cyclic-AMP-dependent protein kinase (PKA) and protein kinase C (PKC) was determined in acutely dissociated trigeminal neurons. 2. Whole-cell glycine-evoked Cl- current (IGly) was recorded using the patch clamp technique. Protein kinases and their inhibitors were intracellularly perfused into the cells. 3. Both PKA and PKC when applied separately potentiated IGly. 4. When PKA and PKC were sequentially applied, PKC could not increase the IGly any further after the glycine responses were enhanced by PKA. 5. In 42% of our cells, IGly increased spontaneously. Endogenous PKA was found to mediate the increase. PKC had no effects on IGly in these cells. 6. The effect of PKA on IGly was studied in PKC-pretreated cells. PKA failed to potentiate IGly in these cells, suggesting that the PKA action also depends on the activity of PKC inside the cells. 7. These results suggest that the PKC action on IGly is conditional upon the modulation of the currents by PKA and vice versa. This cross-regulation of ligand-gated channel activity by protein kinases may play a role in neuronal integration and synaptic plasticity.

Regulation of neuronal plasticity in the central nervous system by phosphorylation and dephosphorylation

Neuronal plasticity can be defined as adaptive changes in structure and function of the nervous system, an obvious example of which is the capacity to remember and learn. Long-term potentiation and long-term depression are the experimental models of memory in the central nervous system (CNS), and have been frequently utilized for the analysis of the molecular mechanisms of memory formation. Extensive studies have demonstrated that various kinases and phosphatases regulate neuronal plasticity by phosphorylating and dephosphorylating proteins essential to the basic processes of adaptive changes in the CNS. These proteins include receptors, ion channels, synaptic vesicle proteins, and nuclear proteins. Multifunctional kinases (cAMP-dependent protein kinase, Ca2+/phospholipid-dependent protein kinase, and Ca2+/calmodulin-dependent protein kinases) and phosphatases (calcineurin, protein phosphatases 1, and 2A) that specifically modulate the phosphorylation status of neuronal-signaling proteins have been shown to be required for neuronal plasticity. In general, kinases are involved in upregulation of the activity of target substrates, and phosphatases downregulate them. Although this rule is applicable in most of the cases studied, there are also a number of exceptions. A variety of regulation mechanisms via phosphorylation and dephosphorylation mediated by multiple kinases and phosphatases are discussed.

Biology of the postsynaptic glycine receptor

Glycine is one of the major inhibitory neurotransmitters, and upon binding to its receptor it activates chloride conductances. Receptors are accumulated immediately opposite release sites, at the postsynaptic differentiations, where they form functional microdomains. This review describes recent advances in our understanding of the structure-function relationships of the glycine receptor, a member of the ligand-gated ion channel superfamily. Following purification of the receptor complex and identification of its integral and peripheral membrane protein components, molecular cloning has revealed the existence of several subtypes of the ligand-binding subunit. This heterogeneity is responsible for the distinct pharmacological and functional properties displayed by the various receptor configurations that are differentially expressed and assembled during development. This review also focuses on the molecular aspects of glycinergic synaptogenesis, highlighting gephyrin, the peripheral component of the receptor. The role of this cytoplasmic protein in anchoring and maintaining the channel complex in postsynaptic clusters is discussed. The glycine receptor recently moved into the spotlight as a paradigm in the approach to cell biology of the formation of the postsynaptic membrane.

Spinal modulation of the induction of central sensitization

Peripheral tissue injury results in a change in the excitability of spinal dorsal horn neurons, central sensitization, and the behavioral correlate, hyperalgesia. It is proposed here that a dynamic balance exists between excitatory and inhibitory synaptic input to the spinal dorsal horn that functions to prevent central sensitization following brief, mild, noxious stimulation. Following more severe stimulation and injury, there is a loss of these inhibitory mechanisms that allow central sensitization to proceed. Single-unit recordings were made from L4-L5 deep dorsal horn neurons (wide dynamic range and nociceptive specific) from barbiturate-anesthetized rats that were non-inflamed or had a carrageenan-inflamed hindpaw. Baseline test responses to mechanical stimuli were obtained and normalized to 100%. An electrical conditioning stimulus (1 Hz, 20 s, C-fiber strength) was applied to the tibial nerve or the neuronal receptive field. Five seconds later the test stimulus was repeated and the magnitude of response compared to baseline. During the conditioning stimulus, 46% of the neurons from non-inflamed and inflamed rats showed wind-up although the magnitude of wind-up was significantly greater for inflamed rats. The remaining neurons showed no change (36-46%) or wind-down (8-18%). Five seconds following the end of the conditioning stimulus 67% of the neurons from non-inflamed rats had attenuated responses to mechanical stimuli (36% of baseline). The remaining neurons were either unaffected (30%) or facilitated (3%). Following inflammation significantly fewer neurons (28%) had attenuated responses and the magnitude of attenuation was significantly less than in non-inflamed rats (54% of baseline). The responses of the remaining neurons were unaffected (54%) or facilitated (18%). During subsequent test stimuli, the responses of 30% of the neurons from non-inflamed rats were facilitated to 140% of baseline. The responses of 46% of neurons from inflamed rats were facilitated to 160% of baseline. In these neurons there was significantly less initial attenuation following inflammation compared to non-inflamed rats. The response of the neuron during the electrical conditioning had no effect upon the response following conditioning. The conditioning stimulus given transcutaneously within the receptive field produced qualitatively similar results to tibial nerve stimulation. In non-inflamed rats, when the conditioning/test-stimulus interval was increased from 5 s to 10-30 s, the responses of 20% of the neurons were attenuated (compared to 67%) and the mean magnitude of attenuation was 52% of baseline (compared to 36% of baseline). However, the responses of only 33% of the neurons were ultimately facilitated (compared to 30%). The present study documents a short period following a low-frequency C-fiber input in which the response to natural stimuli is suppressed. It is suggested that this attenuation, whether or not expressed, prevents a significant portion of deep dorsal horn neurons from becoming sensitized to C-fiber input. This functions to prevent central sensitization when the noxious stimulus does not produce inflammation and it is not beneficial to the animal to become hyperalgesic (i.e., to alter its behavior in order to protect an injured limb and reduce painful sensations). Following injury-producing tissue damage and inflammation the mechanisms that produce the attenuation are reduced, with a concomitant increase in excitation to electrical and natural stimuli, suggesting that the attenuation is inhibitory modulation of nociceptive input and injury results in a disinhibition producing an increase in excitability and central sensitization.

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.

Molecular biology of glycinergic neurotransmission

Glycine is a major inhibitory neurotransmitter in the spinal cord and brainstem of vertebrates. Glycine is accumulated into synaptic vesicles by a proton-coupled transport system and released to the synaptic cleft after depolarization of the presynaptic terminal. The inhibitory action of glycine is mediated by pentameric glycine receptors (GlyR) that belong to the ligand-gated ion channel superfamily. The synaptic action of glycine is terminated by two sodium- and chloride-coupled transporters, GLYT1 and GLYT2, located in the glial plasma membrane and in the presynaptic terminals, respectively. Dysfunction of inhibitory glycinergic neurotransmission is associated with several forms of inherited mammalian myoclonus. In addition, glycine could participate in excitatory neurotransmission by modulating the activity of the NMDA subtype of glutamate receptor. In this article, we discuss recent progress in our understanding of the molecular mechanisms that underlie the physiology and pathology of glycinergic neurotransmission.

The glycine receptor

The inhibitory glycine receptor (GlyR) is a member of the ligand-gated ion channel receptor superfamily. The GlyR comprises a pentameric complex that forms a chloride-selective transmembrane channel, which is predominantly expressed in the spinal cord and brain stem. We review the pharmacological and physiological properties of the GlyR and relate this information to more recent insights that have been obtained through the cloning and recombinant expression of the GlyR subunits. We also discuss insights into our understanding of GlyR structure and function that have been obtained by the genetic characterisation of various heritable disorders of glycinergic neurotransmission.

The role of protein kinase C in cellular tolerance to ethanol

We have shown that ethanol inhibits uptake of adenosine by a specific nucleoside transporter in NG108-15 neuroblastoma x glioma cells and that cAMP-dependent protein kinase (PKA) activity is required for this inhibition. After chronic exposure to ethanol, adenosine uptake is no longer inhibited on rechallenge with ethanol, i.e. transport has become tolerant to ethanol. Here we show that protein kinase C (PKC) contributes to ethanol-induced tolerance of adenosine transport. Activation of PKC by phorbol esters in control cells results in an ethanol-tolerant phenotype, similar to that produced by chronic ethanol exposure. In addition, chronic exposure to ethanol increases the amounts of alpha, delta, and epsilon PKC. However, reducing PKC activity by inhibition with chelerythrine during chronic exposure to ethanol or down-regulation by phorbol esters prevents the development of ethanol-induced tolerance of adenosine transport. By contrast, the inhibition of PKA activity produces tolerance to ethanol inhibition of adenosine uptake. When protein phosphatase inhibitors are present, inhibiting PKA activity has no effect on ethanol sensitivity of adenosine uptake, suggesting a role for protein phosphatases in the regulation of ethanol sensitivity of uptake. Taken together, our results suggest that PKA and PKC have opposing effects on the ethanol sensitivity of adenosine transport; PKA activity is required for ethanol sensitivity, and PKC activation produces tolerance. Based on these data, we propose that chronic ethanol exposure increases PKC activity, leading to the activation of a protein phosphatase (1 or 2A). This phosphatase then dephosphorylates a PKA-phosphorylated site, which is required for ethanol to inhibit adenosine uptake. Therefore, the sensitivity of adenosine transport to ethanol appears to be maintained by a balance of PKA and protein phosphatase activities, and PKC may regulate phosphatase activity.

2,3-Butanedione monoxime modifies the glycine-gated chloride current of acutely isolated murine hypothalamic neurons

In this study, we explored the effect of the chemical phosphatase 2,3-butanedione monoxime (BDM) on glycine current (IGly) of murine ventromedial hypothalamic neurons. Co-application of 0.01 to 67 mM BDM increased IGly decay rate with little change of the peak amplitude. This effect was both rapid in onset and offset and required the presence of the agonist. Pretreatment with BDM alone did not alter-IGly decay. In addition, dialysis of neurons with 500 microM ATP-gamma-S did not alter the acute effect of BDM. Thus, this effect may result from open channel block rather than BDM-induced dephosphorylation of the receptor/channel protein. In contrast to the acute effect described above, relatively prolonged (i.e., greater than 80 s) pretreatment with BDM reduced peak IGly. The phorbol ester (PDBu), a protein kinase C (PKC) activator, mimicked this effect of BDM. Furthermore, chelerythrine, a specific PKC inhibitor, prevented this effect of BDM on peak IGly. Thus, activation of PKC may mediate this attenuating effect of BDM on IGly. For a sub-population of these pretreated neurons, there was a subsequent potentiation of IGly which followed the initial suppressant effect. This potentiation may be due to a phosphatase effect of BDM, since it was observed more frequently when neurons were also pretreated with the protein kinase inhibitors H7 or chelerythrine. These findings suggest that BDM alters protein kinase activity and acts as a phosphatase to regulate the activity of the glycine receptor/channel complex.

Serotonin 5-HT2 receptor activation potentiates N-methyl-D-aspartate receptor-mediated ion currents by a protein kinase C-dependent mechanism

Modulation of N-methyl-D-aspartate (NMDA) receptor-mediated ion currents by serotonin was investigated with a two-electrode voltage clamp technique in Xenopus oocytes injected with rat brain RNA. After a 1-min application of 200 nM serotonin a transient potentiation of the NMDA receptor-mediated ion currents was observed. The serotonin-induced enhancement was mimicked by the protein kinase C activators 1-oleoyl-2-acetyl-sn-glycerol (100 microM) and phorbol 12-myristate 13-acetate (10 nM), whereas the inactive phorbol ester 4-alpha-phorbol 12-myristate 13-acetate (10 nM) had no effect. From these observations it was concluded that protein kinase C was involved in the enhancement of NMDA-induced currents. In agreement with this conclusion, it was found that the serotonin effect was inhibited by the protein kinase C inhibitors sphingosine (1 microM) or staurosporine (1 microM) added 20 min before NMDA application and by oocyte injection of protein kinase C (PKC)-inhibitor peptide (500 ng/oocyte) 1 hr prior to recordings. The serotonin receptor involved was identified as a 5-HT2 receptor subtype by the finding that 200 nM of the selective 5-HT2 receptor agonist alpha-methyl-5-hydroxytryptamine mimicked the potentiation of NMDA-induced ion currents by serotonin. Furthermore, the observed potentiation was significantly reduced by co-application of serotonin with 100 microM of the selective 5-HT2 receptor antagonist ketanserin. These results indicate that 5-HT2 receptors enhance NMDA receptor function via phosphoinositol hydrolysis and subsequent stimulation of PKC.

Regulation of ionotropic receptors by protein phosphorylation

The regulation of synaptic signal transduction is of central importance to our understanding of normal and abnormal nervous system function. One mechanism by which signal transduction can be affected is the modification of cellular sensitivity by alterations of transmembrane receptor properties. For G-protein coupled receptors, protein phosphorylation is intimately involved in many stages of receptor regulation. This appears to be true for ionotropic receptors as well. Evidence of a role for protein kinase and protein phosphatase activity in the multi-staged ionotropic receptor regulation cascade is presented and a comparison to G-protein coupled receptor regulation is considered.

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.

Activation of endogenous protein kinase C enhances currents through alpha 1 and alpha 2 glycine receptor channels

The effects of Ca2+ /phospholipid dependent (PKC) phosphorylation on the current amplitudes of alpha 1 and alpha 2 glycine receptors expressed in Xenopus oocytes were examined by whole cell voltage clamp recording. In studies using phorbol esters, PKC phosphorylation has been shown to reduce glycine-induced currents. Endogenous PKC activation by pretreatment with serum, however, enhanced the currents to around 140% in both alpha 1 and alpha 2 glycine receptors. This effect was completely blocked by a specific PKC inhibitor, GF109203X. Instead, treatment with a potent PKC activator, 12-O-tetradecanoylphorbol-13-acetate (TPA) revealed a decrease in glycine-gated channel currents. Thus, the present results demonstrate that glycine receptor phosphorylation mediated by endogenous pathway of PKC activation potentiates glycine-induced currents and phorbol esters may have a direct action on glycine receptor channels independent of PKC activation.

The inhibitory glycine receptor: architecture, synaptic localization and molecular pathology of a postsynaptic ion-channel complex

Significant progress has been made towards the identification of functional domains of the inhibitory glycine receptor. Several residues crucial for ligand binding, ion-channel properties and stoichiometric subunit assembly have been identified. A major recent advance has been the finding that the biogenesis of postsynaptic glycine receptor clusters requires the tubulin-binding protein, gephyrin. Another area of exciting research has focused on mutations of glycine receptor alpha and beta subunit genes, which have been found to be causal for different hereditary motor disorders.