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

Spinal protein kinase A and phosphorylated extracellular signal-regulated kinase signaling are involved in the antinociceptive effect of phytohormone abscisic acid in rats

OBJECTIVE

The phytohormone abscisic acid (ABA) as a signaling molecule exists in various types of organisms from early multicellular to animal cells and tissues. It has been demonstrated that ABA has an antinociceptive effect in rodents. The present study was designed to assess the possible role of PKA and phosphorylated ERK (p-ERK) on the antinociceptive effects of intrathecal (i.t.) ABA in male Wistar rats.

METHODS

The animals were cannulated intrathecally and divided into different experimental groups (n=6‒7): Control (no surgery), vehicle (received ABA vehicle), ABA-treated groups (received ABA in doses of 10 or 20 µg/rat), ABA plus H.89 (PKA inhibitor)-treated group which received the inhibitor 15 min prior to the ABA injection. Tail-flick and hot-plate tests were used as acute nociceptive stimulators to assess ABA analgesic effects. p-ERK was evaluated in the dorsal portion of the spinal cord using immunoblotting.

RESULTS

Data showed that a microinjection of ABA (10 and 20 µg/rat, i.t.) significantly increased the nociceptive threshold in tail flick and hot plate tests. The application of PKA inhibitor (H.89, 100 nM/rat) significantly inhibited ABA-induced analgesic effects. Expression of p-ERK was significantly decreased in ABA-injected animals, which were not observed in the ABA+H.89-treated group.

CONCLUSIONS

Overall, i.t. administration of ABA (10 µg/rat) induced analgesia and p-ERK down-expression likely by involving the PKA-dependent mechanism.

Potentiation of the glycine response by serotonin on the substantia gelatinosa neurons of the trigeminal subnucleus caudalis in mice

The lamina II, also called the substantia gelatinosa (SG), of the trigeminal subnucleus caudalis (Vc), is thought to play an essential role in the control of orofacial nociception. Glycine and serotonin (5-hydroxytryptamine, 5-HT) are the important neurotransmitters that have the individual parts on the modulation of nociceptive transmission. However, the electrophysiological effects of 5-HT on the glycine receptors on SG neurons of the Vc have not been well studied yet. For this reason, we applied the whole-cell patch clamp technique to explore the interaction of intracellular signal transduction between 5-HT and the glycine receptors on SG neurons of the Vc in mice. In nine of 13 neurons tested (69.2%), pretreatment with 5-HT potentiated glycine-induced current (I_Gly). Firstly, we examined with a 5-HT₁ receptor agonist (8-OH-DPAT, 5-HT_1/7 agonist, co-applied with SB-269970, 5-HT₇ antagonist) and antagonist (WAY-100635), but 5-HT₁ receptor agonist did not increase I_Gly and in the presence of 5-HT₁ antagonist, the potentiation of 5-HT on I_Gly still happened. However, an agonist (α-methyl-5-HT) and antagonist (ketanserin) of the 5-HT₂ receptor mimicked and inhibited the enhancing effect of 5-HT on I_Gly in the SG neurons, respectively. We also verified the role of the 5-HT₇ receptor by using a 5-HT₇ antagonist (SB-269970) but it also did not block the enhancement of 5-HT on I_Gly. Our study demonstrated that 5-HT facilitated I_Gly in the SG neurons of the Vc through the 5-HT₂ receptor. The interaction between 5-HT and glycine appears to have a significant role in modulating the transmission of the nociceptive pathway.

PKA and PKC Modulators Affect Ion Channel Function and Internalization of Recombinant Alpha1 and Alpha1-Beta Glycine Receptors

Glycine receptors (GlyRs) are important mediators of fast inhibitory neurotransmission in the mammalian central nervous system. Their function is controlled by multiple cellular mechanisms, including intracellular regulatory processes. Modulation of GlyR function by protein kinases has been reported for many cell types, involving different techniques, and often yielding contradictory results. Here, we studied the effects of protein kinase C (PKC) and cAMP-dependent protein kinase A (PKA) on glycine induced currents in HEK293 cells expressing human homomeric α1 and heteromeric α1-β GlyRs using whole-cell patch clamp techniques as well as internalization assays. In whole-cell patch-clamp measurements, modulators were applied in the intracellular buffer at concentrations between 0.1 μM and 0.5 μM. EC₅₀ of glycine increased upon application of the protein kinase activators Forskolin and phorbol-12-myristate-13-acetate (PMA) but decreased in the presence of the PKC inhibitor Staurosporine aglycon and the PKA inhibitor H-89. Desensitization of recombinant α1 receptors was significantly increased in the presence of Forskolin. Staurosporine aglycon, on the other hand decreased desensitization of heteromeric α1-β GlyRs. The time course of receptor activation was determined for homomeric α1 receptors and revealed two simultaneous effects: cells showed a decrease of EC₅₀ after 3–6 min of establishing whole-cell configuration. This effect was independent of protein kinase modulators. All modulators of PKA and PKC, however, produced an additional shift of EC₅₀, which overlay and eventually exceeded the cells intrinsic variation of EC₅₀. The effect of kinase activators was abolished if the corresponding inhibitors were co-applied, consistent with PKA and PKC directly mediating the modulation of GlyR function. Direct effects of PKA- and PKC-modulators on receptor expression on transfected HEK cells were monitored within 15 min of drug application, showing a significant increase of receptor internalization with PKA and PKC activators, while the corresponding inhibitors had no significant effect on receptor surface expression or internalization. Our results confirm the observation that phosphorylation via PKA and PKC has a direct effect on the GlyR ion channel complex and plays an important role in the fine-tuning of glycinergic signaling.

Effects of ethanol on glycinergic synaptic currents in mouse spinal cord neurons

Ethanol increased the frequency of miniature glycinergic currents [miniature inhibitory postsynaptic currents (mIPSCs)] in cultured spinal neurons. This effect was dependent on intracellular calcium augmentation, since preincubation with BAPTA (an intracellular calcium chelator) or thapsigargin [a sarco(endo)plasmic reticulum Ca(2+)-ATPase (SERCA) pump inhibitor] significantly attenuated this effect. Similarly, U73122 (a phospholipase C inhibitor) or 2-aminoethoxydiphenyl borate [2-APB, an inositol 1,4,5-trisphosphate (IP₃) receptor (IP3R) inhibitor] reduced this effect. Block of ethanol action was also achieved after preincubation with Rp-cAMPS, inhibitor of the adenylate cyclase (AC)/PKA signaling pathway. These data suggest that there is a convergence at the level of IP₃R that accounts for presynaptic ethanol effects. At the postsynaptic level, ethanol increased the decay time constant of mIPSCs in a group of neurons (30 ± 10% above control, n = 13/26 cells). On the other hand, the currents activated by exogenously applied glycine were consistently potentiated (55 ± 10% above control, n = 11/12 cells), which suggests that ethanol modulates synaptic and nonsynaptic glycine receptors (GlyRs) in a different fashion. Supporting the role of G protein modulation on ethanol responses, we found that a nonhydrolyzable GTP analog [guanosine 5'-O-(3-thiotriphosphate) (GTPγS)] increased the decay time constant in ∼50% of the neurons (28 ± 12%, n = 11/19 cells) but potentiated the glycine-activated Cl(-) current in most of the neurons examined (83 ± 29%, n = 7/9 cells). In addition, confocal microscopy showed that α1-containing GlyRs colocalized with Gβ and Piccolo (a presynaptic cytomatrix protein) in ∼40% of synaptic receptor clusters, suggesting that colocalization of Gβγ and GlyRs might account for the difference in ethanol sensitivity at the postsynaptic level.

Inhibition of the ethanol-induced potentiation of α1 glycine receptor by a small peptide that interferes with Gβγ binding

BACKGROUND

Gβγ interaction with GlyR is an important determinant in ethanol potentiation of this channel.

RESULTS

A small peptide, RQH(C7), can inhibit ethanol potentiation of GlyR currents.

CONCLUSION

Results with RQH(C7) indicate that ethanol mediated potentiation of GlyR is in part by Gβγ activation.

SIGNIFICANCE

Molecular interaction between Gβγ and GlyR could be used as a target for pharmacological modification of ethanol effects. Previous studies indicate that ethanol can modulate glycine receptors (GlyR), in part, through Gβγ interaction with basic residues in the intracellular loop. In this study, we show that a seven-amino acid peptide (RQH(C7)), which has the primary structure of a motif in the large intracellular loop of GlyR (GlyR-IL), was able to inhibit the ethanol-elicited potentiation of this channel from 47 ± 2 to 16 ± 4%, without interfering with the effect of Gβγ on GIRK (G protein activated inwardly rectifying potassium channel) activation. RQH(C7) displayed a concentration-dependent effect on ethanol action in evoked and synaptic currents. A fragment of GlyR-IL without the basic amino acids did not interact with Gβγ or inhibit ethanol potentiation of GlyR. In silico analysis using docking and molecular dynamics allowed to identify a region of ~350Å(2) involving aspartic acids 186, 228, and 246 in Gβγ where we propose that RQH(C7) binds and exerts its blocking action on the effect of ethanol in GlyR.

Melatonin potentiates glycine currents through a PLC/PKC signalling pathway in rat retinal ganglion cells

In vertebrate retina, melatonin regulates various physiological functions. In this work we investigated the mechanisms underlying melatonin-induced potentiation of glycine currents in rat retinal ganglion cells (RGCs). Immunofluorescence double labelling showed that rat RGCs were solely immunoreactive to melatonin MT(2) receptors. Melatonin potentiated glycine currents of RGCs, which was reversed by the MT(2) receptor antagonist 4-P-PDOT. The melatonin effect was blocked by intracellular dialysis of GDP-beta-S. Either preincubation with pertussis toxin or application of the phosphatidylcholine (PC)-specific phospholipase C (PLC) inhibitor D609, but not the phosphatidylinositol (PI)-PLC inhibitor U73122, blocked the melatonin effect. The protein kinase C (PKC) activator PMA potentiated the glycine currents and in the presence of PMA melatonin failed to cause further potentiation of the currents, whereas application of the PKC inhibitor bisindolylmaleimide IV abolished the melatonin-induced potentiation. The melatonin effect persisted when [Ca(2+)](i) was chelated by BAPTA, and melatonin induced no increase in [Ca(2+)](i). Neither cAMP-PKA nor cGMP-PKG signalling pathways seemed to be involved because 8-Br-cAMP or 8-Br-cGMP failed to cause potentiation of the glycine currents and both the PKA inhibitor H-89 and the PKG inhibitor KT5823 did not block the melatonin-induced potentiation. In consequence, a distinct PC-PLC/PKC signalling pathway, following the activation of G(i/o)-coupled MT(2) receptors, is most likely responsible for the melatonin-induced potentiation of glycine currents of rat RGCs. Furthermore, in rat retinal slices melatonin potentiated light-evoked glycine receptor-mediated inhibitory postsynaptic currents in RGCs. These results suggest that melatonin, being at higher levels at night, may help animals to detect positive or negative contrast in night vision by modulating inhibitory signals largely mediated by glycinergic amacrine cells in the inner retina.

Bradykinin enhances AMPA and NMDA receptor activity in spinal cord dorsal horn neurons by activating multiple kinases to produce pain hypersensitivity

Bradykinin potentiates synaptic glutamate release and action in the spinal cord via presynaptic and postsynaptic B(2) receptors, contributing thereby to activity-dependent central sensitization and pain hypersensitivity (Wang et al., 2005). We have now examined the signaling pathways that are responsible for the postsynaptic modulatory actions of bradykinin on glutamatergic action and transmission in superficial dorsal horn neurons. B(2) receptors are coexpressed in dorsal horn neurons with protein kinase A (PKA) and the delta isoform of protein kinase C (PKC), and we find that the augmentation by bradykinin of AMPA and NMDA receptor-mediated currents in lamina II neurons requires coactivation of both PKC and PKA. The activation of PKA is downstream of COX1 (cyclooxygenase-1). Extracellular signal-regulated kinase (ERK) activation is involved after the PKC and PKA coactivation, and intrathecal administration of bradykinin induces a thermal hyperalgesia in vivo, which is reduced by inhibition of ERK, PKA, and PKC. We conclude that bradykinin, by activating multiple kinases in dorsal horn neurons, potentiates glutamatergic synaptic transmission to produce pain hypersensitivity.

Mechanisms of homomeric alpha1 glycine receptor endocytosis

Little is known regarding the mechanism(s) by which glycine receptors are endocytosed. Here we examined the endocytosis of homomeric alpha1 glycine receptors expressed in HEK 293 cells using immunofluorescence/confocal microscopy and whole-cell patch-clamp recordings. Our studies demonstrate that constitutive endocytosis of glycine receptors is blocked by the dominant negative dynamin construct K44A and that intracellular dialysis with peptide P4, a dynamin/amphiphysin-disrupting peptide, increased whole-cell glycine-gated chloride currents. To examine whether receptor endocytosis could be regulated by PKC, experiments with the PKC activator PMA (phorbol 12-myristate 13-acetate) were performed. PMA, but not its inactive analogue PMM (phorbol 12-monomyristate), stimulated receptor endocytosis and inhibited glycine-gated chloride currents. Similar to constitutive endocytosis, PKC-stimulated endocytosis was blocked by dynamin K44A. Mutation of a putative AP2 adaptin dileucine motif (L314A, L315A) present in the receptor cytoplasmic loop blocked PMA-stimulated receptor endocytosis and also prevented PMA inhibition of glycine receptor currents. In patch-clamp experiments, intracellular dialysis of a 12-amino acid peptide corresponding to the region of the receptor containing the dileucine motif prevented PKC modulation of wild-type glycine receptors. Unlike PKC modulation of the receptor, constitutive endocytosis was not affected by mutation of this dileucine motif. These results demonstrate that PKC activation stimulates glycine receptor endocytosis, that both constitutive endocytosis and PKC-stimulated endocytosis are dynamin-dependent, and that PKC-stimulated endocytosis, but not constitutive endocytosis, occurs via the dileucine motif (L314A, L315A) within the cytoplasmic loop of the receptor.

A transient receptor potential vanilloid 4-dependent mechanism of hyperalgesia is engaged by concerted action of inflammatory mediators

The transient receptor potential vanilloid 4 (TRPV4) is a primary afferent transducer that plays a crucial role in neuropathic hyperalgesia for osmotic and mechanical stimuli, as well as in inflammatory mediator-induced hyperalgesia for osmotic stimuli. In view of the clinical importance of mechanical hyperalgesia in inflammatory states, the present study investigated the role of TRPV4 in mechanical hyperalgesia induced by inflammatory mediators and the second-messenger pathways involved. Intradermal injection of either the inflammogen carrageenan or a soup of inflammatory mediators enhanced the nocifensive paw-withdrawal reflex elicited by hypotonic or mechanical stimuli in rat. Spinal administration of TRPV4 antisense oligodeoxynucleotide blocked the enhancement without altering baseline nociceptive threshold. Similarly, in TRPV4(-/-) knock-out mice, inflammatory soup failed to induce any significant mechanical or osmotic hyperalgesia. In vitro investigation showed that inflammatory mediators engage the TRPV4-mediated mechanism of sensitization by direct action on dissociated primary afferent neurons. Additional behavioral observations suggested that multiple mediators are necessary to achieve sufficient activation of the cAMP pathway to engage the TRPV4-dependent mechanism of hyperalgesia. In addition, direct activation of protein kinase A or protein kinase C epsilon, two pathways that mediate inflammation-induced mechanical hyperalgesia, also induced hyperalgesia for both hypotonic and mechanical stimuli that was decreased by TRPV4 antisense and absent in TRPV4(-/-) mice. We conclude that TRPV4 plays a crucial role in the mechanical hyperalgesia that is generated by the concerted action of inflammatory mediators present in inflamed tissues.

Temperature: an important experimental variable in studying PKC modulation of ligand-gated ion channels

Amphibian oocyte and mammalian heterologous expression systems are often used to investigate the function of recombinant ion channels using electrophysiological techniques. Although both systems have yielded important information, the results obtained in these systems are sometimes conflicting. Oocytes and mammalian cells differ in their physiological temperature requirements. While room temperature is within the physiological temperature range for oocytes, this temperature is far below that required by mammalian cells. Since electrophysiological studies are often performed in both oocytes and mammalian cells at room temperature, we sought to determine if recording temperature could be a factor in some disparate results obtained in these cell types. For these studies, we examined phorbol ester modulation of GABA(A) and glycine receptors. Consistent with the literature, at room temperature, PMA (phorbol 12-myristate 13-acetate) produced a large reproducible decrease in the peak amplitude of GABA and glycine-gated currents in Xenopus oocytes. In contrast, PMA was ineffective in modulating these heterologously expressed receptors at room temperature in human embryonic kidney (HEK) 293 cells. However, when electrophysiological experiments were performed at 35 degrees C in HEK 293 cells, PMA decreased the function of these receptors. Our results indicate that the temperature at which electrophysiological studies are conducted is an important experimental variable. To determine the extent to which electrophysiological recordings are performed at physiological temperatures in HEK 293 cells, a PubMed search was conducted using the search terms "patch clamp" and "HEK" for the years 2003-2004. This search revealed that only 15% of the patch clamp studies were reported to have been conducted in the temperature range of 32-37 degrees C. The results of our study indicate that temperature is an important experimental variable that requires rational consideration in the design of electrophysiological experiments.

Calcium and endocannabinoids in the modulation of inhibitory synaptic transmission

Synapses in the central nervous system can be highly plastic devices, being able to modify their efficacy in relaying information in response to several factors. Calcium ions are often fundamental in triggering synaptic plasticity. Here, we will shortly review the effects induced by postsynaptic increases of calcium concentration at GABAergic and glycinergic synapses. Both postsynaptic and presynaptic mechanisms mediating changes in synaptic strength will be examined. Particular attention will be devoted to phenomena of retrograde signaling and, specifically, to the recently discovered role, played by the endocannabinoid system in retrograde synaptic modulation.

Dual Ca2+ modulation of glycinergic synaptic currents in rodent hypoglossal motoneurones

Glycinergic synapses are implicated in the coordination of reflex responses, sensory signal processing and pain sensation. Their activity is pre- and postsynaptically regulated, although mechanisms are poorly understood. Using patch-clamp recording and Ca2+ imaging in hypoglossal motoneurones from rat and mouse brainstem slices, we address here the role of cytoplasmic Ca2+ (Ca(i)) in glycinergic synapse modulation. Ca2+ influx through voltage-gated or NMDA receptor channels caused powerful transient inhibition of glycinergic IPSCs. This effect was accompanied by an increase in both the failure rate and paired-pulse ratio, as well as a decrease in the frequency of mIPSCs, suggesting a presynaptic mechanism of depression. Inhibition was reduced by the cannabinoid receptor antagonist SR141716A and occluded by the agonist WIN55,212-2, indicating involvement of endocannabinoid retrograde signalling. Conversely, in the presence of SR141716A, glycinergic IPSCs were potentiated postsynaptically by glutamate or NMDA, displaying a Ca2(+)-dependent increase in amplitude and decay prolongation. Both presynaptic inhibition and postsynaptic potentiation were completely prevented by strong Ca(i) buffering (20 mm BAPTA). Our findings demonstrate two independent mechanisms by which Ca2+ modulates glycinergic synaptic transmission: (i) presynaptic inhibition of glycine release and (ii) postsynaptic potentiation of GlyR-mediated responses. This dual Ca2(+)-induced regulation might be important for feedback control of neurotransmission in a variety of glycinergic networks in mammalian nervous systems.

Characterization of strychnine-sensitive glycine receptor in the intact frog retina: modulation by protein kinases

We studied 3H-glycine and 3H-strychnine specific binding to glycine receptor (GlyR) in intact isolated frog retinas. To avoid glycine binding to glycine uptake sites, experiments were performed at low ligand concentrations in a sodium-free medium. The binding of both radiolabeled ligands was saturated. Scatchard analysis of bound glycine and strychnine revealed a KD of 2.5 and 2.0 microM, respectively. Specific binding of glycine was displaced by beta-alanine, sarcosine, and strychnine. Strychnine binding was displaced 50% by glycine, and sarcosine. Properties of the strychnine-binding site in the GlyR were modified by sarcosine. Binding of both radioligands was considerably reduced by compounds that inhibit or activate adenylate cyclase and increased cAMP levels. A phorbol ester activator of PKC remarkably decreased glycine and strychnine binding. These results suggest modulation of GlyR in response to endogenous activation of protein kinases A and C, as well as protein phosphorylation modulating GlyR function in retina.

Binding site stoichiometry and the effects of phosphorylation on human alpha1 homomeric glycine receptors

The kinetic properties of the human alpha1 homomeric glycine receptor were investigated. Receptors were expressed in HEK 293 cells, and glycine was applied to outside-out membrane patches with sub-millisecond solution exchange. The activation time course of the glycine response was used to investigate receptor stoichiometry. The unbinding of three strychnine molecules and the cooperative binding of two glycine molecules were required to activate the channel. The effects of phosphorylation on glycine receptor kinetics were investigated by pretreating cells with phosphorylators or with phosphatases. Phosphorylation accelerated desensitisation, but slowed deactivation and recovery from desensitisation. A chemical-kinetic model was developed that reproduced the experimental observations. The model suggests that only three binding sites on the glycine channel are functional, while the remaining two binding sites are 'silent', possibly due to strong negative cooperativity.

Tyrosine kinases enhance the function of glycine receptors in rat hippocampal neurons and human alpha(1)beta glycine receptors

Glycine receptors (GlyRs) are transmitter-gated channels that mediate fast inhibitory neurotransmission in the spinal cord and brain. The GlyR beta subunit contains a putative tyrosine phosphorylation site whose functional role has not been determined. To examine if protein tyrosine kinases (PTKs) regulate the function of GlyRs, we analysed whole-cell currents activated by applications of glycine to CA1 hippocampal neurons and spinal neurons. The role of a putative site for tyrosine phosphorylation at position 413 of the beta subunit was examined using site-directed mutagenesis and expression of recombinant (alpha(1)beta(Y413F)) receptors in human embryonic kidney (HEK 293) cells. Lavendustin A, an inhibitor of PTKs, depressed glycine-evoked currents (I(Gly)) in CA1 neurons and spinal neurons by 31 % and 40 %, respectively. In contrast, the intracellular application of the exogenous tyrosine kinase, cSrc, enhanced I(Gly) in CA1 neurons by 56 %. cSrc also accelerated GlyR desensitization and increased the potency of glycine 2-fold (control EC(50) = 143 microM; cSrc EC(50) = 74 microM). Exogenous cSrc, applied intracellularly, upregulated heteromeric alpha(1)beta receptors but not homomeric alpha(1) receptors. Substitution mutation of the tyrosine to phenylalanine at position beta-413 prevented this enhancement. Furthermore, a selective inhibitor of the Src family kinases, PP2, down-regulated wild-type alpha(1)beta but not alpha(1)beta(Y413F) receptors. Together, these findings indicate that GlyR function is upregulated by PTKs and this modulation is dependent on the tyrosine-413 residue of the beta subunit.

Role of protein kinase C in opioid modulation of glycine-gated Cl(-) current in rat periaqueductal gray neuron

The Role of protein kinase C in the modulatory effect of a mu-opioid receptor agonist, [D-Ala(2),N-Me-Phe(4),Gly(5)-ol]enkephalin (DAMGO), on the glycine-gated Cl(-) current was examined in acutely dissociated rat periaqueductal gray neurons. Using the nystatin-perforated patch-clamp technique, the neurons were voltage-clamped at -60 mV. The glycine-gated Cl(-) current (I(Gly)) was sensitive to strychnine. On pretreatment with 1 microM DAMGO, the 30-microM glycine response increased with time and showed a maximum amplitude of 209+/-37% of control. After a protein kinase C activator, phorbol-12-myristate-13-acetate (PMA, 0.1 microM) as pretreatment, I(Gly) increased to 138+/-6% of control. The DAMGO potentiation of I(Gly) was not altered by coapplication with PMA. Although protein kinase C inhibitors, chelerythrine (3 microM) and 2-[1-(3-dimethylaminopropyl)indol-3-yl]-3-(indol-3-yl) maleimide (GF109203X, 1 microM), did not alter I(Gly), the DAMGO-induced potentiation of I(Gly) was reduced to 161+/-21% or 164+/-31% of the control after coapplication with chelerythrine or GF109203X, respectively. These results indicate that the potentiation of I(Gly) by a mu-opioid receptor agonist is partly mediated by activation of protein kinase C.

Alteration in the voltage dependence of NMDA receptor channels in rat dorsal horn neurones following peripheral inflammation

1. It has been proposed that the activation of NMDA receptors and upregulation of protein kinase C (PKC) underlie the exaggerated and persistent pain experienced in the inflammatory state. However, there is no direct evidence to show that inflammation alters the function of NMDA receptors. 2. We examined the voltage-dependent properties of NMDA receptor channels in rat dorsal horn neurones that receive sensory inputs from an inflamed hindpaw. 3. Peripheral inflammation was induced by injections of complete Freund's adjuvant (CFA). Membrane currents were measured using the perforated patch-clamp technique. 4. After CFA treatment, the current-voltage relationship of NMDA receptor channels was shifted in the hyperpolarized direction. This resulted in enhanced NMDA responses at negative potentials. 5. The change was mediated by PKC because the voltage shift was blocked by the selective PKC inhibitors chelerythrine and bisindolylmaleimide I. 6. Furthermore, the Mg(2+) blockade of NMDA receptors was reduced. This reduction could account for the shift in the voltage dependence of NMDA receptor channels. 7. These results indicate that NMDA receptor channel characteristics in the dorsal horn are altered by inflammation, and that the changes observed could contribute to the hyperalgesia and allodynia associated with tissue injury.

Gabapentin actions on N-methyl-D-aspartate receptor channels are protein kinase C-dependent

Gabapentin (Neurontin) (GBP) is a widely prescribed analgesic used in treating pain patients with peripheral nerve injuries, diabetic neuropathy and cancer. To understand the mechanism of its action, we used the whole-cell patch recording technique to study the effects of GBP on N-methyl-D-aspartate (NMDA)-evoked currents in single dorsal horn neurons isolated from normal rats and from rats with inflammation induced by the injection of complete Freund adjuvant (CFA) to the hindpaw. We found that GBP enhanced NMDA currents in normal neurons only when protein kinase C (PKC) was added to these cells. The enhancement resulted from an increase in the affinity of glycine for NMDA receptors by GBP. In contrast, in neurons isolated from CFA-treated rats, GBP enhanced NMDA responses without any PKC treatment. Since endogenous PKC in inflamed tissue is elevated, these results suggest that GBP exerts its effects only on those cells affected by inflammatory injuries. Thus, the effects of GBP on NMDA receptors are plastic; they depend on the phosphorylation states of cells or receptors. These observations point to a new strategy for drug design. A chemical whose action depends on the state of cells would maximize its effectiveness while keeping its side-effects to a minimum.

Isolation and characterization of an alpha 2-type zebrafish glycine receptor subunit

The complementary DNA for a novel alpha subunit of the glycine receptor, alphaZ2, was isolated from a zebrafish adult brain library. The molecular characteristics, phylogenetic relationships and messenger RNA length of this alphaZ2 subunit show it to be an alpha2-type glycine receptor subunit isoform. The leader peptide however, diverges from those of known glycine receptor alpha isoforms. Recombinantly expressed in Xenopus oocytes, alphaZ2 formed functional glycine receptor channels. These homomeric channels were activated by glycine and taurine, with apparent affinities similar to those reported for zebrafish alphaZ1 glycine receptor, and were also effectively antagonized by nanomolar concentrations of strychnine. However, during prolonged applications of agonists, ionic currents of alphaZ2 receptor channels declined to a much lower steady-state level than those of alphaZ1, indicating different desensitization properties. Analysis of messenger RNA revealed that alphaZ2 is specifically expressed in adult brain tissue and present in both adult and embryonic zebrafish. This report contributes to the characterization of the diversity of glycine receptor isoforms in vertebrates.

Effect of protein kinase C activation on the glycine evoked Cl− current in spinal cord neurons

We investigated whether the effect of phorbol-12-myristate-13-acetate (PMA) was altered by a kinase inhibitor and by down-regulation of protein kinase C (PKC) in order to determine if glycine receptors in mouse spinal neurons, unlike those in hippocampal and trigeminal neurons, can be inhibited by PKC. To examine the above, electrophysiological and immunofluorescence studies were carried out in mouse spinal neurons kept in culture for up to 3 weeks. The inhibition of the glycine activated current by PMA (1 microM) increased from 12+/-3% during week 1 to 27+/-6% during week 3. The effect of PMA was completely blocked by the PKC selective inhibitor RO 31-8220 (1 microM). After culturing the cells with 1 microM PMA for 24 h, the inhibitory effect of acute application of PMA disappeared altogether, suggesting that the effect of PMA was via PKC. Immunofluorescence studies showed that a short stimulation with PMA translocated the enzyme to the periphery whereas longer term stimulation (24 h) down regulated the PKC signal. These results indicate that activation of PKC by PMA inhibits the glycine receptor in cultured spinal neurons and that its sensitivity changes during neuronal development.

Fast potentiation of glycine receptor channels of intracellular calcium in neurons and transfected cells

Inhibitory glycine receptors (GlyRs) are mainly expressed in the spinal cord and in the midbrain, where they control motor and sensory pathways. We describe here a fast potentiation of GlyR by intracellular Ca2+. This phenomenon was observed in rat spinal cord neurons and in transfected human cell lines. Potentiation develops in <100 ms, is proportional to Ca2+ influx, and is characterized by an increase in GlyR apparent affinity for glycine. Phosphorylation and G protein pathways appear not to be involved in the potentiation mechanism. Single-channel recordings in cell-attached and excised patches, as well as whole-cell data suggest the presence of a diffusible cytoplasmic factor that modulates the GlyR channel gating properties. Ca2+-induced potentiation may be important for rapid modulation of glycinergic synapses.

Common components of patch-clamp internal recording solutions can significantly affect protein kinase A activity

Common components of whole-cell internal recording solutions were tested both in vitro and in patch-clamp experiments for their effects on the activity of cAMP-dependent protein kinase. Potassium fluoride (KF), 440 mM trimethylamine chloride and exclusion of bovine serum albumin (BSA) decreased the activity of the enzyme, while ethylene glycol-bis (beta-aminoethyl ether) N,N,N',N'-tetraacetic acid (EGTA) and the potassium salts of aspartate, gluconate, methylsulfate and monobasic phosphate increased its activity. Addition of KF to the internal solution produced a hyperpolarizing shift in the V1/2 of Ih channel activation, consistent with the KF-induced reduction of protein kinase A activity. Therefore, consideration of the composition of internal solutions is warranted when studying channel physiology by patch-clamp techniques.

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.