Ca(2+)-independent reduction of N-methyl-D-aspartate channel activity by protein tyrosine phosphatase

Src in synaptic transmission and plasticity

In the central nervous system (CNS), Src and other Src family kinases are widely expressed and are abundant in neurons. Src has been implicated in proliferation and differentiation during the development of the CNS. But Src is highly expressed in fully differentiated neurons in the developed CNS, implying additional functions of this kinase. Over the past decade, a large body of evidence has accumulated showing that a main function of Src is to upregulate the activity of N-methyl-D-aspartate (NMDA) receptors and other ion channels. NMDA receptors (NMDARs) are a principal subtype of glutamate receptors, which mediate fast excitatory transmission at most central synapses. In this review, we focus on Src as a regulator of NMDARs and on the role of Src in NMDAR-dependent synaptic plasticity. We also describe recent studies that give insights into the regulation of Src itself at glutamatergic synapses. By upregulating the function of NMDARs, Src gates the production of NMDAR-dependent synaptic potentiation and plasticity. Thus, Src may be critical for processes underlying physiological plasticity, including learning and memory, and pathological plasticity, such as pain and epilepsy.

Subunit-specific regulation of NMDA receptor endocytosis

At excitatory synapses, both NMDA and AMPA receptors are localized to the postsynaptic density (PSD). However, unlike AMPA receptors, synaptic NMDA receptors are stable components of the PSD. Even so, surface-expressed NMDA receptors undergo endocytosis, which is more robust early in development and declines during synaptic development. We investigated the subunit-specific contributions to NMDA receptor endocytosis, specifically defining the endocytic motifs and endocytic pathways preferred by the NR2A and NR2B subunits. We find that NR2A and NR2B have distinct endocytic motifs encoded in their distal C termini and that these interact with clathrin adaptor complexes with differing affinities. We also find that NR2A and NR2B sort into different intracellular pathways after endocytosis, with NR2B preferentially trafficking through recycling endosomes. In mature cultures, we find that NR2B undergoes more robust endocytosis than NR2A, consistent with previous studies showing that NR2A is more highly expressed at stable synaptic sites. Our findings demonstrate fundamental differences between NR2A and NR2B that help clarify developmental changes in NMDA receptor trafficking and surface expression.

Cardiac Na+-Ca2+ exchanger current induced by tyrphostin tyrosine kinase inhibitors

Tyrosine kinase (TK) inhibitors genistein and tyrphostin A23 (A23) inhibited Ca(2+) currents in guinea-pig ventricular myocytes investigated under standard whole-cell conditions (K(+)-free Tyrode's superfusate; EGTA-buffered (pCa-10.5) Cs(+) dialysate). However, the inhibitors (100 microM) also induced membrane currents that reversed between -40 and 0 mV, and the objective of the present study was to characterize these currents. Genistein-induced current behaved like Cl(-) current, and was unaffected by either the addition of divalent cations (0.5 mM Cd(2+); 3 mM Ni(2+)) that block the Na(+)-Ca(2+) exchanger (NCX), or the removal of external Na(+) and Ca(2+). A23-induced current was independent of Cl(-) driving force, and strongly suppressed by addition of Cd(2+) and Ni(2+), and by removal of either external Na(+) or Ca(2+). These and other results suggested that A23 activated an NCX current driven by submembrane Na(+) and Ca(2+) concentrations higher than those in the bulk cytoplasm. Improved control of intracellular Na(+) and Ca(2+) concentrations was obtained by suppressing cation influx (10 microM verapamil) and raising dialysate Na(+) to 7 mM and dialysate pCa to 7. Under these conditions, stimulation by A23 was described by the Hill equation with EC(50) 68 +/- 4 microM and coefficient 1.1, tyrphostin A25 was as effective as A23, and TK-inactive tyrphostin A1 was ineffective. Phosphotyrosyl phosphatase inhibitor orthovanadate (1 mM) antagonized the action of 100 microM A23. The results suggest that activation of cardiac NCX by A23 is due to inhibition of genistein-insensitive TK.

Dual neuroprotective signaling mediated by downregulating two distinct phosphatase activities of PTEN

The tumor suppressor PTEN (phosphatase and tensin homolog deleted on chromosome 10) is a lipid and protein phosphatase. We report here that PTEN physically associates with the NR1 and NR2B subunits of NMDA receptors (NMDARs) in rat hippocampus. Downregulating the protein expression of PTEN inhibits the function of extrasynaptic NMDARs and decreases NMDAR surface expression, suggesting a crucial role for endogenous PTEN in the modulation of NMDAR-mediated neuronal function. Reducing PTEN expression also enhances Akt/Bad phosphorylation in hippocampal neurons. Importantly, suppressing lipid and protein phosphatase activity of PTEN, respectively, activates Akt and inhibits extrasynaptic NMDAR activity and thereby protects against ischemic neuronal death in vitro and in vivo. Thus, our study reveals a dual neuroprotective mechanism by which Akt/Bad and extrasynaptic NMDARs are regulated via downregulation of two distinct PTEN phosphatase activities and present the possibility of PTEN as a potential therapeutic target for stroke treatment.

Intracellular spermine decreases open probability of N-methyl-D-aspartate receptor channels

Spermine and related polyamines have been shown to be endogenous regulators of several ion channel types including ionotropic glutamate receptors. The effect of spermine on N-methyl-d-aspartate (NMDA) receptors in cultured rat hippocampal neurons was studied using single-channel and whole-cell patch clamp recordings. Intracellular spermine resulted in the dose-dependent inhibition of NMDA-induced responses. Spermine reversibly inhibited the single NMDA receptor channel activity in inside-out patches suggesting a membrane-delimited mechanism of action. Open probability of NMDA receptor channels was decreased in a dose-dependent manner. Mechanism of spermine-induced inhibition of NMDA receptor was different from that of intracellular Ca(2+)-induced NMDA receptor inactivation. Both pharmacological studies and single channel analysis indicate that in contrast to the effect of extracellular spermine the intracellular spermine effect is not dependent on the NMDA receptor subunit composition. We propose that intracellular spermine has a direct inhibitory effect on NMDA receptors that is different from calcium-induced NMDA receptor inactivation and spermine-induced voltage-dependent inhibition of AMPA/kainate receptors. Spermine-induced tonic change in the open probability of NMDA receptor channels may play a role in mechanisms underlying short-term changes in the synaptic efficacy.

Cellular mechanisms of serotonin 5-HT2A receptor-mediated cGMP formation: the essential role of glutamate

The current study explores the mechanisms by which activation of serotonin(2A) (5-HT(2A)) receptors increase production of cyclic guanosine monophosphate (cGMP) in slices of rat frontal cortex. Contrary to results in cortical slices, stimulation of 5-HT(2A) receptors in cells stably expressing this serotonin receptor did not alter cGMP levels. In cortical slices, stimulation of cGMP formation by 2,5-dimethoxy-4-methylamphetamine (DOM), a 5-HT(2A/2C) receptor agonist, was blocked by tetanus toxin, a substance that prevents vesicular neurotransmitter release. However, this stimulation was not altered by tetrodotoxin, an agent that inhibits depolarization-induced neurotransmitter release. Addition of an N-methyl-d-aspartate (NMDA) receptor antagonist, d-AP-7, but not of an AMPA/kainate receptor antagonist CNQX, completely inhibited DOM-mediated cGMP production in the slices. Combined application of maximally effective concentrations of NMDA and DOM elicited a greater increase in cGMP content than either drug alone. The present study shows that 5-HT(2A) receptors do not directly stimulate cGMP formation, but rather that 5-HT(2A) receptor-mediated cGMP production is dependent on extracellular glutamate activating NMDA receptors. The results indicate that 5-HT(2A) receptor-mediated cGMP production could be at least partially attributed to potentiation of NMDA receptor-mediated cGMP formation.

Ethanol Sensitivity of Recombinant Homomeric and Heteromeric AMPA Receptor Subunits Expressed in Xenopus Oocytes

BACKGROUND

Ethanol is known to acutely inhibit AMPA receptor function, and sensitivity of AMPA receptors to ethanol is dependent on subunit composition in vivo and in vitro. A commonly used in vitro expression system for studying recombinant receptor subunits is the Xenopus laevis oocyte and two-electrode voltage-clamp electrophysiological recording. To date, ethanol sensitivity of injected receptor subunit complementary RNA (cRNA) has not been shown to be correlated with the actual expression of receptor subunits in oocytes. In this study, we compared ethanol sensitivity of homomeric and heteromeric AMPA receptor subunits microinjected into Xenopus oocytes and confirmed subunit expression in oocytes by immunoblot.

METHODS

cRNAs coding for the "flop" type AMPA GluR1 or GluR3 (homomeric), GluR2/GluR3 (heteromeric combination), and GluR1/2/3 (heteromeric combination) were microinjected in equimolar amounts of 16 to 20 ng into oocytes, which were studied for their sensitivity to ethanol. Oocytes injected with cRNA for homomeric or heteromeric subunit combinations were homogenized and the expressed subunits quantified with anti-GluR1, anti-GluR2, and anti-GluR2/3 antibodies.

RESULTS

Ethanol concentrations of 10 to 500 mM consistently inhibited currents activated in oocytes by 200 microM kainic acid. The expressed homomeric GluR1 receptor and heteromeric GluR1/2/3 receptor combination currents showed similar sensitivity to ethanol inhibition with half-maximal inhibition values of 170 +/- 12 mM and 176 +/- 8 mM, respectively. The expressed homomeric GluR3 receptor and heteromeric GluR2/3 receptor combination currents were differentially sensitive to ethanol inhibition with respective IC50 values of 238 +/- 9 mM and 338 +/- 16 mM.

CONCLUSION

The expressed homomeric and heteromeric "flop" type AMPA receptors were differentially sensitive to ethanol, which may in part explain differential ethanol sensitivity in native neurons.

Identification of PSD-93 as a substrate for the Src family tyrosine kinase Fyn

In order to study the role of tyrosine kinase signaling in the post-synaptic density (PSD), tyrosine-phosphorylated proteins associated with the PSD-95/NMDA receptor complex were analyzed. The NMDA receptor complex from the mouse brain was successfully solubilized with deoxycholate and immunopurified with anti-PSD-95 or anti-phosphotyrosine antibody. Immunoblot analyses revealed that the predominantly tyrosine-phosphorylated proteins in the NMDA receptor complex are the NR2A/B subunits and a novel 120 kDa protein. Purification and microsequencing analysis showed that the 120 kDa protein is mouse PSD-93/Chapsyn-110. Recombinant PSD-93 was phosphorylated by Fyn in vitro, and Tyr-384 was identified as a major phosphorylation site. Tyrosine phosphorylation of PSD-93 was greatly reduced in brain tissue from Fyn-deficient mice compared with wild-type mice. Furthermore, an N-terminal palmitoylation signal of PSD-93 was found to be essential for its anchoring to the membrane, where Fyn is also localized. In COS7 cells, exogenously expressed PSD-93 was phosphorylated, dependent on its membrane localization. In addition, tyrosine-phosphorylated PSD-93 was able to bind to Csk, a negative regulator of Src family kinases, in vitro as well as in a brain lysate. These results suggest that PSD-93 serves as a membrane-anchored substrate of Fyn and plays a role in the regulation of Fyn-mediated modification of NMDA receptor function.

Phosphorylation of vanilloid receptor 1 by Ca2+/calmodulin-dependent kinase II regulates its vanilloid binding

Vanilloid receptor 1 (VR1), a capsaicin receptor, is known to play a major role in mediating inflammatory thermal nociception. Although the physiological role and biophysical properties of VR1 are known, the mechanism of its activation by ligands is poorly understood. Here we show that VR1 must be phosphorylated by Ca2+-calmodulin dependent kinase II (CaMKII) before its activation by capsaicin. In contrast, the dephosphorylation of VR1 by calcineurin leads to a desensitization of the receptor. Moreover, point mutations in VR1 at two putative consensus sites for CaMKII failed to elicit capsaicin-sensitive currents and caused a concomitant reduction in VR1 phosphorylation in vivo. Such mutants also lost their high affinity binding with [3H]resiniferatoxin, a potent capsaicin receptor agonist. We conclude that the dynamic balance between the phosphorylation and dephosphorylation of the VR1 channel by CaMKII and calcineurin, respectively, controls the activation/desensitization states by regulating VR1 binding. Furthermore, because sensitization by protein kinase A and C converge at these sites, phosphorylation stress in the cell appears to control a wide range of excitabilities in response to various adverse stimuli.

Possible mechanisms underlying the protective effects of SY-21, an extract of a traditional Chinese herb, on transient brain ischemia/reperfusion-induced neuronal death in rat hippocampus

We examined the neuroprotective actions of SY-21, a potent ingredient of a traditional Chinese herb, histologically. Transient brain ischemia (15 min) was induced by four-vessel occlusion in Sprague-Dawley rats. Administration of SY-21 20 min before or 6 h after brain ischemia significantly increased the number of surviving hippocampal CA1 pyramidal cells in rats subjected to transient brain ischemia followed by 5 days of reperfusion. Neuronal cell death resulting from ischemic events is associated with abnormal activation of the NMDA receptors. Tyrosine phosphorylation of the NMDA receptor subunit NR2A by Src or Fyn has been implicated in the up-regulation of NMDA receptor activity. In order to investigate the possible mechanism of the neuroprotective action of SY-21, we examined the effects of SY-21 on the ischemia/reperfusion-induced increases in tyrosine phosphorylation of the NMDA receptor subunit 2A (NR2A) and on the interactions involving NR2A, PSD-95 and Src/Fyn. We found that the increase in the tyrosine phosphorylation of NR2A induced by brain ischemia/reperfusion is suppressed by SY-21 administered 15 min before, or instantly after, brain ischemia. Also, SY-21 attenuated the increased interactions involving NR2A, PSD-95, Fyn and Src. These results demonstrate that SY-21 has a prominent neuroprotective action against brain ischemic insult, and the mechanism may involve the regulation of the tyrosine phosphorylation of NR2A by changing the above interactions.

Acute ethanol affects phosphorylation state of the NMDA receptor complex: implication of tyrosine phosphatases and protein kinase A

Phosphorylation has been shown to regulate N-methyl-D-aspartic acid receptor (NMDAR) function. The inhibitory effect of ethanol on NMDAR function could be due, at least in part, to a change in NMDAR phosphorylation states. In order to investigate the effect of ethanol on phosphorylation of NR1 and NR2 subunits, NMDAR complexes were immunoprecipitated from cortical slices pre-exposed to ethanol. Acute ethanol, 100 and 200 mM, significantly decreased the tyrosine phosphorylation of NR2 subunits (Tyr-NR2). Treatment with a tyrosine phosphatase inhibitor reduced the inhibition of Tyr-NR2 phosphorylation caused by 100 mM ethanol. This suggests an involvement of tyrosine phosphatases in ethanol-induced inhibition of Tyr-NR2 phosphorylation. Slices pre-exposed to 100 and 200 mM ethanol exhibited a significant increase in the phosphorylation of NR1 by PKA at serine 897 (Ser897-NR1), which was blocked by a PKA inhibitor. Moreover, at 200 mM, ethanol produced a significant increase in PKA activity. Together, these results indicate that ethanol may increase Ser897-NR1 phosphorylation by activating PKA. However, ethanol did not affect phosphorylation of NR1 subunits by PKC at serine 896. We conclude that ethanol has the ability to modulate phosphorylation of both NR2 and NR1 subunits and these effects appear to implicate tyrosine phosphatases and PKA, respectively.

Control of synaptic strength, a novel function of Akt

Akt (also known as PKB), a serine/threonine kinase involved in diverse signal-transduction pathways, is highly expressed in the brain. Akt is known to have a strong antiapoptotic action and thereby to be critically involved in neuronal survival, but its potential role in the dynamic modulation of synaptic transmission is unknown. Here we report that Akt phosphorylates, both in vitro and in vivo, the type A gamma-aminobutyric acid receptor (GABA(A)R), the principal receptor mediating fast inhibitory synaptic transmission in the mammalian brain. Akt-mediated phosphorylation increases the number of GABA(A)Rs on the plasma membrane surface, thereby increasing the receptor-mediated synaptic transmission in neurons. These results identify the GABA(A)R as a novel substrate of Akt, thereby linking Akt to the regulation of synaptic strength. This work also provides evidence for the rapid regulation of neurotransmitter receptor numbers in the postsynaptic domain by direct receptor phosphorylation as an important means of producing synaptic plasticity.

Role of protein kinase A in phosphorylation of NMDA receptor 1 subunits in dorsal horn and spinothalamic tract neurons after intradermal injection of capsaicin in rats

Protein phosphorylation is a major mechanism for regulation of N-methyl-D-aspartate (NMDA) receptor function. The NMDA receptor 1 subunit (NR1) is phosphorylated by protein kinase A (PKA) on serine 890 and 897. We have recently reported that there is enhanced phosphorylation of NR1 on serine 897 in dorsal horn and spinothalamic tract (STT) neurons after intradermal injection of capsaicin (CAP) in rats [Zou et al. (2000) J. Neurosci. 20, 6989-6997]. Whether or not this phosphorylation, which develops during central sensitization following CAP injection, is mediated by PKA remains to be determined. In this study, western blots and immunofluorescence staining were employed to observe if pretreatment with a PKA inhibitor, N-[2-((p-bromocinnamyl)amino)ethyl]-5-isoquinolinesulfonamide, HCl (H89), blocks the enhanced phosphorylation of NR1 on serine 897 following injection of CAP into the glabrous skin of one hind paw of anesthetized rats. Western blots showed that pretreatment with H89 caused a decrease in CAP-induced phosphorylation of NR1 protein in spinal cord segments L(4)-S(1). In experiments using immunofluorescence staining, the numbers of phospho-NR1-like immunoreactive (p-NR1-LI) neurons seen after CAP injection were significantly decreased in the dorsal horn of the L(4)-L(5) segments on the side ipsilateral to the injection after PKA was inhibited. When STT cells were labeled by microinjection of the retrograde tracer, fluorogold, we found that the proportion of p-NR1-LI STT cells on the side ipsilateral to the injection in the superficial laminae of spinal cord segments L(4)-L(5) was markedly reduced when H89 was administered intrathecally before CAP injection. However, the proportion of p-NR1-LI STT cells in deep laminae was unchanged unless the PKC inhibitor, chelerythrine chloride, was co-administered with H89. Combined with our previous findings, the present results indicate that NR1 in spinal dorsal horn neurons, including the superficial dorsal horn STT cells, is phosphorylated following CAP injection and that this phosphorylation is due to the action of PKA. However, the phosphorylation of deep STT cells involves both PKA and PKC.

Switching between the two action modes of the dual-affinity nitrate transporter CHL1 by phosphorylation

To counteract fluctuating nutrient environments, plants have evolved high- and low-affinity uptake systems. These two systems were traditionally thought to be genetically distinct, but, recently, two Arabidopsis transporters, AtKUP1 and CHL1, were shown to have dual affinities. However, little is known about how a dual-affinity transporter works and the advantages of having a dual-affinity transporter. This study demonstrates that, in the case of CHL1, switching between the two modes of action is regulated by phosphorylation at threonine residue 101; when phosphorylated, CHL1 functions as a high-affinity nitrate transporter, whereas, when dephosphorylated, it functions as a low-affinity nitrate transporter. This regulatory mechanism allows plants to change rapidly between high- and low-affinity nitrate uptake, which may be critical when competing for limited nitrogen. These results demonstrate yet another regulatory role of phosphorylation in plant physiology.

Chronic NMDA receptor blockade from birth delays the maturation of NMDA currents, but does not affect AMPA/kainate currents

The activity of the N-methyl-D-aspartate receptor (NR) regulates the composition of excitatory synapses and mediates multiple forms of synaptic and structural plasticity. In the superficial superior colliculus (sSC) of the rat, NR activity is essential for the full refinement of retinotopy during development. We have examined the NR's role in synaptic development by chronically treating the sSC from birth with the competitive antagonist (+/-)-2-amino-5-phosphonopentanoic acid (AP5) released by the slow-release polymer Elvax. Whole-cell voltage-clamp recordings were used to characterize excitatory postsynaptic potentials (EPSCs) in slices from postnatal day (P)12-20 sSC. Chronic NR blockade reduced the ratio of AMPA/kainate receptor (AMPAR) to NR peak current amplitudes of both spontaneous (s)EPSCs and evoked EPSCs. Spontaneous NR current amplitude was increased following treatment, while spontaneous AMPAR currents were identical to those of controls, indicating that the ratio change was due to an increased NR current. Comparison of sEPSC frequency, AMPAR current rectification, and quantitative Western blots indicated that the characteristics of AMPARs at the synapse are normal following AP5 treatment. In the sSC, NR currents show a rapid decrease in decay time on P11 and previous studies in slices indicate this change results from a NR-mediated activation of the phosphatase calcineurin. Consistent with this in vitro finding, the down-regulation failed to occur in sSC chronically treated with AP5 in vivo. Together the present data show that NR function is necessary for subsequent NR current regulation in vivo, but it is not essential for the developmental expression of normal AMPAR currents.

Transient ischemia enhances tyrosine phosphorylation and binding of the NMDA receptor to the Src homology 2 domain of phosphatidylinositol 3-kinase in the rat hippocampus

Tyrosine phosphorylation of the NMDA receptor has been implicated in the regulation of the receptor channel. We investigated the effects of transient (15 min) global ischemia on tyrosine phosphorylation of NMDA receptor subunits NR2A and NR2B, and the interaction of NR2 subunits with the SH2 domain of phosphatidylinositol 3-kinase (PI3-kinase) in vulnerable CA1 and resistant CA3/dentate gyrus of the hippocampus. Transient ischemia induced a marked increase in the tyrosine phosphorylation of NR2A in both regions. The tyrosine phosphorylation of NR2B in CA3/dentate gyrus after transient ischemia was sustained and greater than that in CA1. PI3-kinase p85 was co-precipitated with NR2B after transient global ischemia. The SH2 domain of the p85 subunit of PI3-kinase bound to NR2B, but not to NR2A. Binding to NR2B was increased following ischemia and the increase in binding in CA3/dentate gyrus (4.5-fold relative to sham) was greater than in CA1 (1.7-fold relative to sham) at 10 min of reperfusion. Prior incubation of proteins with an exogenous protein tyrosine phosphatase or with a phosphorylated peptide (pYAHM) prevented binding. The results suggest that sustained increases in tyrosine phosphorylation and increased interaction of NR2B with the SH2 domain of PI3-kinase may contribute to altered signal transduction in the CA3/dentate gyrus after transient ischemia.

Differential regulation of synaptic and extra-synaptic NMDA receptors

A variety of processes limit NMDA (N-methyl-D-aspartate) receptor (NMDAR) activity in response to agonist exposure, including rundown--the decline of peak current with repeated, sustained agonist application. Here we report that calcium and tyrosine phosphorylation differentially regulate rundown of synaptic versus extrasynaptic NMDAR-mediated current in rat hippocampal pyramidal neurons.

Modulation of the expression of GABA(A) receptors in rat cerebellar granule cells by protein tyrosine kinases and protein kinase C

The expression of GABA(A) receptors in rat cerebellar granules in culture has been studied by beta(2/3) subunit immunocytochemistry and fluorescence confocal microscopy. These cells show labeling all over the cell bodies' plasma membrane and dendrites. Treatment with the protein tyrosine kinase (PTK) inhibitor genistein results in a decrease of the labeling associated with the beta(2/3) subunit in both cell bodies and dendrites. No effect was found with an inactive genistein analogue, daidzein. A similar effect was found with a protein kinase C (PKC) activator, phorbol myristate acetate (PMA). The effects of genistein and PMA are additive.The interpretation of the results is that PTK inhibition blocks exocytotic deposit of newly synthesized GABA(A) receptors onto the neuronal plasma membrane. On the other hand, PKC activation speeds up endocytotic removal of GABA(A) receptors.

Tyrosine dephosphorylation underlies DHPG-induced LTD

A form of long-term depression (LTD) of synaptic transmission can be induced by bath application of the group I metabotropic glutamate (mGlu) receptor agonist (RS)-3,5-dihydroxyphenylglycine (DHPG). The mechanisms responsible for the induction and expression of DHPG-induced LTD in the CA1 region of the hippocampus are currently the subject of intense investigation. Here we show that two protein tyrosine kinase (PTK) inhibitors (10 microM lavendustin A or 30 microM genistein) have little effect on DHPG-induced LTD. In contrast two protein tyrosine phosphatase (PTP) inhibitors (1 mM orthovanadate or 15 microM phenyl-arsine oxide) significantly inhibited DHPG-induced LTD. These data suggest that DHPG-induced LTD involves activation of a protein tyrosine phosphatase.

Gain control of N-methyl-D-aspartate receptor activity by receptor-like protein tyrosine phosphatase alpha

Src kinase regulation of N-methyl-D-aspartate (NMDA) subtype glutamate receptors in the central nervous system (CNS) has been found to play an important role in processes related to learning and memory, ethanol sensitivity and epilepsy. However, little is known regarding the mechanisms underlying the regulation of Src family kinase activity in the control of NMDA receptors. Here we report that the distal phosphatase domain (D2) of protein tyrosine phosphatase alpha (PTPalpha) binds to the PDZ2 domain of post-synaptic density 95 (PSD95). Thus, Src kinase, its activator (PTPalpha) and substrate (NMDA receptors) are linked by the same scaffold protein, PSD95. Removal of PTPalpha does not affect the association of Src with NMDA receptors, but turns off the constitutive regulation of NMDA receptors by the kinase. Further more, we found that application of the PTPalpha catalytic domains (D1 + D2) into neurones enhances NMDA receptor-mediated synaptic responses. Conversely, the blockade of endogenous PTPalpha inhibits NMDA receptor activity and the induction of long-term potentiation in hippocampal neurones. Thus, PTPalpha is a novel up-regulator of synaptic strength in the CNS.

Tyrosine phosphatase STEP is a tonic brake on induction of long-term potentiation

The functional roles of protein tyrosine phosphatases (PTPs) in the developed CNS have been enigmatic. Here we show that striatal enriched tyrosine phosphatase (STEP) is a component of the N-methyl-D-aspartate receptor (NMDAR) complex. Functionally, exogenous STEP depressed NMDAR single-channel activity in excised membrane patches. STEP also depressed NMDAR-mediated synaptic currents whereas inhibiting endogenous STEP enhanced these currents. In hippocampal slices, administering STEP into CA1 neurons did not affect basal glutamatergic transmission evoked by Schaffer collateral stimulation but prevented tetanus-induced long-term potentiation (LTP). Conversely, inhibiting STEP in CA1 neurons enhanced transmission and occluded LTP induction through an NMDAR-, Src-, and Ca(2+)-dependent mechanism. Thus, STEP acts as a tonic brake on synaptic transmission by opposing Src-dependent upregulation of NMDARs.

Effects of nerve and fibroblast growth factors on the production of nitric oxide in experimental model of Huntington's disease

The role of nitric oxide (NO) in neurological diseases represents one of the most studied, yet controversial subjects in physiology. The aim was to examine the effects of intrastriatal injection neurotrophins (nerve growth factors-NGF, fibroblast growth factors-FGF) in order to investigate the possible involvement of NO in quinolinic acid (QA) induced striatal toxicity in the rat model of Huntington's disease (HD). QA was administered unilaterally into the striatum of adult Wistar rats in a single dose of 150 nM. The other two groups of animals were pretreated immediately before QA application with NGF and FGF, respectively. Control group was treated with 0.9% saline solution in the same manner. Animals were decapitated 7 days after the treatment. Nitrite levels were significantly decreased both in the ipsi- and contralateral striatum and forebrain cortex of NGF- and FGF-treated animals compared with QA treatment. These results indicated a temporal and spatial propagation of oxidative stress and spread protective effects of NGF and FGF on the forebrain cortex, the distant structure, but tightly connected with striatum, the place of direct neurotoxic damage. Neurotrophins could be the potential neuroprotective agents in HD.

Regulation of ion channels by protein tyrosine phosphorylation

Ion channels are regulated by protein phosphorylation and dephosphorylation of serine, threonine, and tyrosine residues. Evidence for the latter process, tyrosine phosphorylation, has increased substantially since this topic was last reviewed. In this review, we present a comprehensive summary and synthesis of the literature regarding the mechanism and function of ion channel regulation by protein tyrosine kinases and phosphatases. Coverage includes the majority of voltage-gated, ligand-gated, and second messenger-gated channels as well as several types of channels that have not yet been cloned, including store-operated Ca2+ channels, nonselective cation channels, and epithelial Na+ and Cl- channels. Additionally, we discuss the critical roles that channel-associated scaffolding proteins may play in localizing protein tyrosine kinases and phosphatases to the vicinity of ion channels.

Tyrosine phosphorylation of ionotropic glutamate receptors by Fyn or Src differentially modulates their susceptibility to calpain and enhances their binding to spectrin and PSD-95

Both tyrosine phosphorylation and calpain-mediated truncation of ionotropic glutamate receptors are important mechanisms for synaptic plasticity. Previous work from our laboratory has shown that calpain activation results in truncation of the C-terminal domains of several glutamate receptor subunits. To test whether and how tyrosine phosphorylation of glutamate ionotropic receptor subunits modulates calpain susceptibility, synaptic membranes were phosphorylated by Fyn or Src, two members of the Src family tyrosine kinases. Tyrosine phosphorylation of synaptic membranes by Src significantly reduced calpain-mediated truncation of both NR2A and NR2B subunits of NMDA receptors, but not of GluR1 subunits of AMPA receptors. In contrast, phosphorylation with Fyn significantly protected calpain-mediated truncation of GluR1 subunits of AMPA receptors, but enhanced calpain-mediated truncation of NR2A subunits of NMDA receptors. Similar results were observed with NR2A and NR2B C-terminal domain fusion proteins phosphorylated by Fyn or Src before incubation with calpain and calcium. In addition, phosphorylation of NR2A and NR2B C-terminal fusion proteins by Fyn or Src enhanced their binding to spectrin and PSD-95. Thus, tyrosine phosphorylation impairs or facilitates calpain-mediated truncation of glutamate receptor subunits, depending on which tyrosine kinase is activated. Such mechanisms could serve to regulate receptor integrity and location, in addition to modulating channel properties.

Modulation of ionotropic glutamate receptor channels

Glutamate is the major excitatory neurotransmitter in the brain. It acts at ligand-gated cationic channels (NMDA, AMPA and kainate receptors) and at G protein-coupled metabotropic glutamate receptors as well. The glutamatergic transmission is suggested to be involved in development, learning and memory. Its dysfunction can be detected in epilepsy, stroke, neurodegenerative disorders and drug abuse. This paper summarizes the present knowledge on the modulation of glutamate-gated ion channels in the central nervous system by phosphorylation. An inhibitory interaction between adenosine A2A receptors and NMDA receptors in the neostriatum is described as an example. mediated by the phospholipase C/inositol trisphosphate/calmodulin and calmodulin kinase II pathway.

A use-dependent tyrosine dephosphorylation of NMDA receptors is independent of ion flux

Tyrosine phosphorylation can upregulate NMDA receptor activity during pathological and physiological alterations of synaptic strength. Here we describe downregulation of recombinant NR1/2A receptors by tyrosine dephosphorylation that requires agonist binding, but is independent of ion flux. The tyrosine residues involved in this new form of NMDA receptor modulation likely form a 'ring' adjacent to the last transmembrane domain. The downregulation was due to a reduction in the number of functional channels, and was blocked by co-expressing a dominant-negative mu2-subunit of the clathrin-adaptor protein AP-2. Our results provide a mechanism by which synaptic NMDA receptors can be modulated in a use-dependent manner even when the postsynaptic membrane is not sufficiently depolarized to relieve channel block by magnesium ions.

Confocal microscopic study of GABA(A) receptors in Xenopus oocytes after rat brain mRNA injection: modulation by tyrosine kinase activity

The expression of GABA(A) receptors in Xenopus oocytes injected with rat brain mRNA was studied by immunocytochemistry and evaluation of the distribution of fluorescent probes at the confocal microscope. The beta(2/3) subunit distributed exclusively on the membrane at the animal pole of the oocytes. Treatment of oocytes for 20 min with the protein tyrosine kinase inhibitor genistein, 200 microM, resulted in a lower presence of GABA(A) receptors on the membrane. The inactive genistein analogue daidzein, 200 microM, had no effect even with a 30 min treatment. Alkaline phosphatase but not a protein tyrosine phosphatase, when injected into oocytes, reduced GABA(A) receptor membrane expression. The data indicate that protein tyrosine phosphorylation modulates the expression on the plasma membrane of presynthesized GABA(A) receptors.

Enhanced phosphorylation of NMDA receptor 1 subunits in spinal cord dorsal horn and spinothalamic tract neurons after intradermal injection of capsaicin in rats

The functional enhancement of NMDA receptors after peripheral tissue injury is proposed to contribute to the sensitization of spinothalamic tract (STT) cells and hyperalgesia. Protein phosphorylation is a major mechanism for the regulation of NMDA receptor function. In this study, Western blots, immunofluorescence double labeling, and the retrograde tracing method were used to examine whether phosphorylation of NMDA receptor 1 (NR1) subunits increases in spinal cord tissue and spinal dorsal horn neurons, especially in STT cells, after injection of capsaicin (CAP) into the glabrous skin of one hindpaw of anesthetized rats. Western blots showed that phosphorylated NR1 protein in spinal cord tissue was increased 30 min after CAP injection. Immunofluorescence double-labeling staining showed no significant difference in the number of the NR1-like immunoreactive neurons in laminae I-VII in the lumbosacral segments (L(4)-S(1)) on the ipsilateral and the contralateral sides 30 min after CAP or vehicle injection. However, the numbers of phospho-NR1-like immunoreactive neurons were significantly increased on the ipsilateral side compared with the vehicle injection group. STT cells were labeled by bilateral microinjections of the retrograde tracer fluorogold into the lateral thalamus, including the ventral-posterior lateral nucleus. Immunofluorescence staining was performed at 30, 60, and 120 min after CAP injection or at 30 min after vehicle injection. There was a significant increase in the proportion of STT cells with phosphorylated NR1 subunits compared either with the contralateral side 30 and 60 min after CAP injection or either side of animals after intradermal injection of vehicle. These results provide direct evidence that NMDA receptors in STT cells are phosphorylated after CAP injection.

Enhanced phosphorylation of NMDA receptor 1 subunits in spinal cord dorsal horn and spinothalamic tract neurons after intradermal injection of capsaicin in rats

The functional enhancement of NMDA receptors after peripheral tissue injury is proposed to contribute to the sensitization of spinothalamic tract (STT) cells and hyperalgesia. Protein phosphorylation is a major mechanism for the regulation of NMDA receptor function. In this study, Western blots, immunofluorescence double labeling, and the retrograde tracing method were used to examine whether phosphorylation of NMDA receptor 1 (NR1) subunits increases in spinal cord tissue and spinal dorsal horn neurons, especially in STT cells, after injection of capsaicin (CAP) into the glabrous skin of one hindpaw of anesthetized rats. Western blots showed that phosphorylated NR1 protein in spinal cord tissue was increased 30 min after CAP injection. Immunofluorescence double-labeling staining showed no significant difference in the number of the NR1-like immunoreactive neurons in laminae I-VII in the lumbosacral segments (L(4)-S(1)) on the ipsilateral and the contralateral sides 30 min after CAP or vehicle injection. However, the numbers of phospho-NR1-like immunoreactive neurons were significantly increased on the ipsilateral side compared with the vehicle injection group. STT cells were labeled by bilateral microinjections of the retrograde tracer fluorogold into the lateral thalamus, including the ventral-posterior lateral nucleus. Immunofluorescence staining was performed at 30, 60, and 120 min after CAP injection or at 30 min after vehicle injection. There was a significant increase in the proportion of STT cells with phosphorylated NR1 subunits compared either with the contralateral side 30 and 60 min after CAP injection or either side of animals after intradermal injection of vehicle. These results provide direct evidence that NMDA receptors in STT cells are phosphorylated after CAP injection.

Src-mediated tyrosine phosphorylation of NR2 subunits of N-methyl-D-aspartate receptors protects from calpain-mediated truncation of their C-terminal domains

Src-mediated tyrosine phosphorylation of N-methyl-d-aspartate receptor subunits has been shown to modify the functional properties of N-methyl-d-aspartate receptors. Moreover, calpain-mediated truncation of N-methyl-d-aspartate receptor subunits has been found to alter the structure of the receptors. In the present study, we first used immunoprecipitation with a variety of antibodies against N-methyl-d-aspartate receptor subunits and anti-phosphotyrosine antibodies to show that tyrosine-phosphorylated subunits of N-methyl-d-aspartate receptor are protected against calpain-mediated truncation of their C-terminal domains. A GST fusion protein containing the C-terminal domain of NR2A was used to identify the calpain cutting sites in the C-terminal domain. One site was identified at residues 1278-1279, corresponding to one of the preferred calpain truncation sites. This site is adjacent to a consensus sequence for Src-mediated tyrosine phosphorylation, and Src-mediated tyrosine phosphorylation of the GST-NR2A C-terminal fusion protein also inhibited calpain-mediated truncation of the fusion protein. We propose that phosphorylation of NR2 subunits and the resulting inhibition of calpain-mediated truncation of their C-terminal domains provide for the stabilization of the N-methyl-d-aspartate receptors in postsynaptic structures.

The Dopamine/D1 receptor mediates the phosphorylation and inactivation of the protein tyrosine phosphatase STEP via a PKA-dependent pathway

The striatal-enriched protein tyrosine phosphatase (STEP) family is expressed within dopaminoceptive neurons of the CNS and is particularly enriched within the basal ganglia and related structures. Alternative splicing produces several isoforms that are found in a number of subcellular compartments, including postsynaptic densities of medium spiny neurons. The variants include STEP(61), a membrane-associated protein, and STEP(46), a cytosolic protein. The C terminals of these two isoforms are identical, whereas the N-terminal domain of STEP(61) contains a novel 172 amino acid sequence that includes several structural motifs not present in STEP(46). Amino acid sequencing revealed a number of potential phosphorylation sites in both STEP isoforms. Therefore, we investigated the role of phosphorylation in regulating STEP activity. Both STEP(61) and STEP(46) are phosphorylated on seryl residues by a cAMP-dependent protein kinase (PKA)-mediated pathway in striatal homogenates. The specific residues phosphorylated in STEP(61) were identified by site-directed mutagenesis and tryptic phosphopeptide mapping as Ser160 and Ser221, whereas the major site of phosphorylation in STEP(46) was shown to be Ser49. Ser160 is located within the unique N terminal of STEP(61). Ser221 and Ser49 are equivalent residues present in STEP(61) and STEP(46), respectively, and are located at the center of the kinase-interacting motif that has been implicated in protein-protein interactions. Phosphorylation at this site decreases the activity of STEP in vitro by reducing its affinity for its substrate. In vivo studies using striatal slices demonstrated that the neurotransmitter dopamine leads to the phosphorylation of STEP via activation of D1 receptors and PKA.

The Dopamine/D1 receptor mediates the phosphorylation and inactivation of the protein tyrosine phosphatase STEP via a PKA-dependent pathway

The striatal-enriched protein tyrosine phosphatase (STEP) family is expressed within dopaminoceptive neurons of the CNS and is particularly enriched within the basal ganglia and related structures. Alternative splicing produces several isoforms that are found in a number of subcellular compartments, including postsynaptic densities of medium spiny neurons. The variants include STEP(61), a membrane-associated protein, and STEP(46), a cytosolic protein. The C terminals of these two isoforms are identical, whereas the N-terminal domain of STEP(61) contains a novel 172 amino acid sequence that includes several structural motifs not present in STEP(46). Amino acid sequencing revealed a number of potential phosphorylation sites in both STEP isoforms. Therefore, we investigated the role of phosphorylation in regulating STEP activity. Both STEP(61) and STEP(46) are phosphorylated on seryl residues by a cAMP-dependent protein kinase (PKA)-mediated pathway in striatal homogenates. The specific residues phosphorylated in STEP(61) were identified by site-directed mutagenesis and tryptic phosphopeptide mapping as Ser160 and Ser221, whereas the major site of phosphorylation in STEP(46) was shown to be Ser49. Ser160 is located within the unique N terminal of STEP(61). Ser221 and Ser49 are equivalent residues present in STEP(61) and STEP(46), respectively, and are located at the center of the kinase-interacting motif that has been implicated in protein-protein interactions. Phosphorylation at this site decreases the activity of STEP in vitro by reducing its affinity for its substrate. In vivo studies using striatal slices demonstrated that the neurotransmitter dopamine leads to the phosphorylation of STEP via activation of D1 receptors and PKA.

Signaling pathways of the acute hypoxic ventilatory response in the nucleus tractus solitarius

The nucleus tractus solitarii (nTS) provides the initial central synaptic relay to peripheral chemoreceptor afferent inputs elicited by changes in oxygen tension. Insofar, the overall cumulative evidence pointing towards the N-methyl-D-aspartate (NMDA) glutamate receptor as the critical receptor underlying the early component of the hypoxic ventilatory response (HVR) is reviewed in detail. In addition, we will present recent findings supporting a role for platelet-derived growth factor (PDGF) beta receptor activation in modulation of the late phase of HVR. This evidence underscores the proposal of a working model for intracellular signaling pathways, downstream to the NMDA glutamate and PDGF-beta receptors in nTS neurons, which may contribute to both the ventilatory characteristics of the acute hypoxic response and to subsequently occurring functional adaptations and synaptic plasticity phenomena.

FGF-2 potentiates Ca(2+)-dependent inactivation of NMDA receptor currents in hippocampal neurons

Peptide growth factors such as the neurotrophins and fibroblast growth factors have potent effects on synaptic transmission, development, and cell survival. We report that chronic (hours) treatment with basic fibroblast growth factor (FGF-2) potentiates Ca(2+)-dependent N-methyl-D-aspartate (NMDA) receptor inactivation in cultured hippocampal neurons. This effect is specific for the NMDA-subtype of ionotropic glutamate receptor and FGF-2. The potentiated inactivation requires ongoing protein synthesis during growth factor treatment and the activity of protein phosphatase 2B (PP2B or calcineurin) during agonist application. These results suggest a mechanism by which FGF-2 receptor signaling may regulate neuronal survival and synaptic plasticity.

Subunit- and site-specific pharmacology of the NMDA receptor channel

N-Methyl-D-aspartate (NMDA) receptor channels play important roles in various physiological functions such as synaptic plasticity and synapse formation underlying memory, learning and formation of neural networks during development. They are also important for a variety of pathological states including acute and chronic neurological disorders, psychiatric disorders, and neuropathic pain syndromes. cDNA cloning has revealed the molecular diversity of NMDA receptor channels. The identification of multiple subunits with distinct distributions, properties and regulation, implies that NMDA receptor channels are heterogeneous in their pharmacological properties, depending on the brain region and the developmental stage. Furthermore, mutation studies have revealed a critical role for specific amino acid residues in certain subunits in determining the pharmacological properties of NMDA receptor channels. The molecular heterogeneity of NMDA receptor channels as well as their dual role in physiological and pathological functions makes it necessary to develop subunit- and site-specific drugs for precise and selective therapeutic intervention. This review summarizes from a molecular perspective the recent advances in our understanding of the pharmacological properties of NMDA receptor channels with specific references to agonists binding sites, channel pore regions, allosteric modulation sites for protons, polyamines, redox agents, Zn2+ and protein kinases, phosphatases.

Subtype-dependence of NMDA receptor channel open probability

NMDA receptor-mediated calcium transients play a critical role in synaptogenesis, synaptic plasticity, and excitotoxicity. NMDA receptors are heteromeric complexes of NR1A combined with NR2A, NR2B, NR2C, and/or NR2D subunits. The NR2 subunits determine a variety of electrophysiological and pharmacological properties of the NMDA receptor complex. In this report, we provide evidence for the first time that there is also a significant difference in peak channel open probability (P(o)) between NMDA receptors composed of NR1A/NR2A and those of NR1A/NR2B subunits. First, whole-cell patch-clamp recordings from human embryonic kidney (HEK) 293 cells expressing NMDA receptors revealed that NR1A/NR2A-mediated peak current densities are approximately four times larger than those of NR1A/NR2B. We show that this fourfold difference is unlikely caused by differences in receptor surface expression, since these levels were similar for the two subtypes by Western blot analysis. To determine whether P(o) contributed to the difference in peak current densities, we used two different open channel antagonists, MK-801 and 9-aminoacridine, in a variety of experimental paradigms. Our results indicate that peak P(o) is significantly higher (twofold to fivefold) for NR1A/NR2A than NR1A/NR2B, with estimated values of approximately 0.35 and 0.07, respectively. These results suggest that a change in the relative expression levels of NR2A and NR2B can regulate peak amplitude of NMDA receptor-mediated excitatory postsynaptic potentials and therefore may play a role in mechanisms underlying synaptic plasticity.

Subtype-dependence of NMDA receptor channel open probability

NMDA receptor-mediated calcium transients play a critical role in synaptogenesis, synaptic plasticity, and excitotoxicity. NMDA receptors are heteromeric complexes of NR1A combined with NR2A, NR2B, NR2C, and/or NR2D subunits. The NR2 subunits determine a variety of electrophysiological and pharmacological properties of the NMDA receptor complex. In this report, we provide evidence for the first time that there is also a significant difference in peak channel open probability (P(o)) between NMDA receptors composed of NR1A/NR2A and those of NR1A/NR2B subunits. First, whole-cell patch-clamp recordings from human embryonic kidney (HEK) 293 cells expressing NMDA receptors revealed that NR1A/NR2A-mediated peak current densities are approximately four times larger than those of NR1A/NR2B. We show that this fourfold difference is unlikely caused by differences in receptor surface expression, since these levels were similar for the two subtypes by Western blot analysis. To determine whether P(o) contributed to the difference in peak current densities, we used two different open channel antagonists, MK-801 and 9-aminoacridine, in a variety of experimental paradigms. Our results indicate that peak P(o) is significantly higher (twofold to fivefold) for NR1A/NR2A than NR1A/NR2B, with estimated values of approximately 0.35 and 0.07, respectively. These results suggest that a change in the relative expression levels of NR2A and NR2B can regulate peak amplitude of NMDA receptor-mediated excitatory postsynaptic potentials and therefore may play a role in mechanisms underlying synaptic plasticity.

The effect of transient global ischemia on the interaction of Src and Fyn with the N-methyl-D-aspartate receptor and postsynaptic densities: possible involvement of Src homology 2 domains

Transient ischemia increases tyrosine phosphorylation of N-methyl-D-aspartate (NMDA) receptor subunits NR2A and NR2B in the rat hippocampus. The authors investigated the effects of this increase on the ability of the receptor subunits to bind to the Src homology 2 (SH2) domains of Src and Fyn expressed as glutathione-S-transferase-SH2 fusion proteins. The NR2A and NR2B bound to each of the SH2 domains and binding was increased approximately twofold after ischemia and reperfusion. Binding was prevented by prior incubation of hippocampal homogenates with a protein tyrosine phosphatase or by a competing peptide for the Src SH2 domain. Ischemia induced a marked increase in the tyrosine phosphorylation of several proteins in the postsynaptic density (PSD), including NR2A and NR2B, but had no effect on the amounts of individual NMDA receptor subunits in the PSD. The level of Src and Fyn in PSDs, but not in other subcellular fractions, was increased after ischemia. The ischemia-induced increase in the interaction of NR2A and NR2B with the SH2 domains of Src and Fyn suggests a possible mechanism for the recruitment of signaling proteins to the PSD and may contribute to altered signal transduction in the postischemic hippocampus.

Src, a molecular switch governing gain control of synaptic transmission mediated by N-methyl-D-aspartate receptors

The N-methyl-D-aspartate (NMDA) receptor is a principal subtype of glutamate receptor mediating fast excitatory transmission at synapses in the dorsal horn of the spinal cord and other regions of the central nervous system. NMDA receptors are crucial for the lasting enhancement of synaptic transmission that occurs both physiologically and in pathological conditions such as chronic pain. Over the past several years, evidence has accumulated indicating that the activity of NMDA receptors is regulated by the protein tyrosine kinase, Src. Recently it has been discovered that, by means of up-regulating NMDA receptor function, activation of Src mediates the induction of the lasting enhancement of excitatory transmission known as long-term potentiation in the CA1 region of the hippocampus. Also, Src has been found to amplify the up-regulation of NMDA receptor function that is produced by raising the intracellular concentration of sodium. Sodium concentration increases in neuronal dendrites during high levels of firing activity, which is precisely when Src becomes activated. Therefore, we propose that the boost in NMDA receptor function produced by the coincidence of activating Src and raising intracellular sodium may be important in physiological and pathophysiological enhancement of excitatory transmission in the dorsal horn of the spinal cord and elsewhere in the central nervous system.

Tyrosine kinase inhibitors modulate the ventilatory response to hypoxia in the conscious rat

Tyrosine kinases (TKs) exert multiple regulatory roles in neuronal activity and synaptic plasticity and could be involved in modulation of cardiovascular and respiratory control mechanisms within the dorsocaudal brain stem. To study this issue, the cardioventilatory responses to 1-microl microinjection within the dorsocaudal brain stem of either vehicle (Veh), the inactive TK inhibitor analog tyrphostin A1 (A1; 1 mM), or the active TK inhibitors genistein (Gen; 10 mM) and tyrphostin A25 (A25; 1 mM) were assessed by whole body plethysmography in unrestrained Sprague-Dawley adult rats. No changes in minute ventilation, heart rate, or mean arterial pressure occurred with Veh, A1, Gen, or A25 during room air breathing (P not significant). However, Gen and A25 attenuated the peak hypoxic ventilatory responses (HVR) to 10% O(2) (P < 0.006 vs. Veh), whereas A1 did not modify HVR (P not significant). HVR reductions by Gen and A25 were primarily due to diminished respiratory frequency enhancements (P < 0.002). No changes in heart rate or mean arterial pressure responses occurred during hypoxia with TK inhibition. In addition, increases in tyrosine phosphorylation of the NR2A/B subunits, but not of the NR2C subunit, of the N-methyl-D-aspartate receptor occurred at 5, 30, and 60 min of hypoxia in the dorsocaudal brain stem and returned to baseline values at 120 min. We conclude that hypoxia induces tyrosine phosphorylation of the N-methyl-D-aspartate glutamate receptor, and TK inhibition within the dorsocaudal brain stem attenuates components of HVR in conscious rats.

Brain-derived neurotrophic factor enhances association of protein tyrosine phosphatase PTP1D with the NMDA receptor subunit NR2B in the cortical postsynaptic density

Our recent studies revealed that brain-derived neurotrophic factor (BDNF) rapidly enhances tyrosine phosphorylation and dephosphorylation of the NMDA receptor subunit, NR2B, in the postsynaptic density (PSD), potentially regulating synaptic plasticity. To explore the molecular mechanisms underlying synaptic NR2B signaling, we examined the protein tyrosine phosphatase, PTP1D; BDNF reportedly increases association of PTP1D with tyrosine phosphorylated proteins in cortical neurons and PC 12 cells. We now report that PTP1D is an intrinsic component of the rat cerebrocortical PSD, based on Western blot analysis using specific anti-PTP1D antibodies. In addition, NR2B was co-immunoprecipitated with PTP1D using anti-NR2B antibodies or anti-PTP1D antibodies, indicating physical association of the subunit with PTP1D. Moreover, treatment of the purified PSD with BDNF for 5 min elicited a two-fold increase in the association of NR2B with PTP1D. The BDNF action appeared to be specific, since nerve growth factor, another member of the neurotrophin gene family, did not alter the association. Finally, an overlay assay revealed that BDNF caused a two-fold increase in binding of blotted PSD NR2B proteins to PTP1D-SH2 domains, revealing molecular mechanisms mediating the PTP1D-NR2B binding. Taken together, our results raise the possibility that PTP1D participates in BDNF-mediated NR2B signaling cascades at the postsynaptic site, thereby regulating synaptic plasticity.

L-type Ca2+ current in guinea pig ventricular myocytes treated with modulators of tyrosine phosphorylation

Guinea pig ventricular myocytes in whole cell configuration were treated with tyrosine kinase (TK) inhibitors [genistein (Gst), tyrphostin A23 (T23), and tyrphostin A25 (T25)] and with inactive analogs [daidzein, genistin, and tyrphostin A1 (T1)] to measure effects on L-type Ca2+ current (ICa,L). Gst inhibited ICa,L (IC50 = 47 microM) without affecting its time course or shifting the ICa, L-voltage relationship. At the highest concentration of isoflavone tested (200 microM), ICa,L was inhibited by 66 +/- 7% (Gst), 22 +/- 2% (daidzein), and 1 +/- 3% (genistin). Inhibition of ICa,L by the active tyrphostins was significantly larger than inhibition by T1; at 200 microM the inhibitions were 72 +/- 6% (T23), 71 +/- 6% (T25), and 27 +/- 6% (T1). The phosphotyrosine phosphatase inhibitor orthovanadate (1 mM) had a small stimulatory effect (6 +/- 2%) on basal ICa,L and blocked the inhibition of ICa,L by TK inhibitors. The data suggest a role for the TK-phosphotyrosine phosphatase system in the regulation of cardiac Ca2+ channels.

Insulin causes a transient tyrosine phosphorylation of NR2A and NR2B NMDA receptor subunits in rat hippocampus

NMDA receptors play a critical role in various aspects of CNS function. Hence, it is important to identify mechanisms that regulate NMDA receptor activity. We have shown previously that insulin rapidly potentiates NMDA receptor activity in both native and recombinant expression systems. Here we report that insulin causes a transient phosphorylation of NR2A and NR2B NMDA receptor subunits on tyrosine residues. Rat hippocampal slices were exposed to 1 microM insulin for 20 and 60 min and then solubilized. NR2A and NR2B subunits were immunoprecipitated and probed for tyrosine phosphorylation. Insulin incubation of hippocampal slices for 20 min elicited an increase in tyrosine phosphorylation to 176 +/- 16% (NR2A) and 203 +/- 15% (NR2B) of control levels. In contrast, 60 min of insulin incubation did not alter NR2 tyrosine phosphorylation levels (NR2A: 85 +/- 13% of control; NR2B: 93 +/- 10% of control). Although the consequence of insulin-stimulated tyrosine phosphorylation is unknown, it is possible that this site(s) is responsible for insulin potentiation of NMDA receptor activity. This possibility is consistent with our earlier finding that insulin potentiates hippocampal NMDA receptor activity after 20 min, but not after 60 min, of insulin exposure.

The junction-associated protein AF-6 interacts and clusters with specific Eph receptor tyrosine kinases at specialized sites of cell-cell contact in the brain

The AF-6/afadin protein, which contains a single PDZ domain, forms a peripheral component of cell membranes at specialized sites of cell-cell junctions. To identify potential receptor-binding targets of AF-6 we screened the PDZ domain of AF-6 against a range of COOH-terminal peptides selected from receptors having potential PDZ domain-binding termini. The PDZ domain of AF-6 interacts with a subset of members of the Eph subfamily of RTKs via its COOH terminus both in vitro and in vivo. Cotransfection of a green fluorescent protein-tagged AF-6 fusion protein with full-length Eph receptors into heterologous cells induces a clustering of the Eph receptors and AF-6 at sites of cell-cell contact. Immunohistochemical analysis in the adult rat brain reveals coclustering of AF-6 with Eph receptors at postsynaptic membrane sites of excitatory synapses in the hippocampus. Furthermore, AF-6 is a substrate for a subgroup of Eph receptors and phosphorylation of AF-6 is dependent on a functional kinase domain of the receptor. The physical interaction of endogenous AF-6 with Eph receptors is demonstrated by coimmunoprecipitation from whole rat brain lysates. AF-6 is a candidate for mediating the clustering of Eph receptors at postsynaptic specializations in the adult rat brain.

Gain control of NMDA-receptor currents by intracellular sodium

The influx of Na+ is fundamental to electrical signalling in the nervous system and is essential for such basic signals as action potentials and excitatory postsynaptic potentials. During periods of bursting or high levels of discharge activity, large increases in intracellular Na+ concentration ([Na+]i) are produced in neuronal soma and dendrites. However, the intracellular signalling function of raised postsynaptic [Na+]i is unknown. Here we show that [Na+]i regulates the function of NMDA (N-methyl-D-aspartate) receptors, a principal subtype of glutamate receptor. NMDA-receptor-mediated whole-cell currents and NMDA-receptor single-channel activity were increased by raising [Na+]i and channel activity decreased upon lowering [Na+]i; therefore, the activity of NMDA channels tracks changes in [Na+]i. We found that the sensitivity of the channel to Na+ was set by a Src kinase that is associated with the channel. Raising [Na+]i selectively increased synaptic responses mediated by NMDA receptors, but not by non-NMDA receptors. Thus, the change in postsynaptic [Na+]i that occurs during neuronal activity is a signal for controlling the gain of excitatory synaptic transmission. This mechanism may be important for NMDA-receptor-dependent plasticity and toxicity in the central nervous system.

Ethanol tolerance and synaptic plasticity

Current concepts of the mechanisms underlying many of the pharmacological effects of ethanol on the CNS involve disruption of ion channel function via the interaction of ethanol with specific hydrophobic sites on channel subunit proteins. Of particular clinical importance is the development of tolerance and dependence to ethanol, and it is likely that adaptive changes in synaptic function in response to ethanol's actions on ion channels play a role in this process. In this article, Judson Chandler, Adron Harris and Fulton Crews discuss potential mechanisms of ethanol-induced changes in synaptic function that might provide a cellular basis for ethanol tolerance and dependence. It is proposed that multiple mechanisms are involved that include both transcriptional and post-translational modifications in NMDA and GABAA receptors.

NMDA receptor subunits in the postsynaptic density of rat brain: expression and phosphorylation by endogenous protein kinases

N-methyl-D-aspartate (NMDA) receptors (NRs) play critical roles in diverse synaptic processes in the brain. However, subcellular distribution, spatiotemporal expression and regulation of NR subunits in brain synapses are unknown. We report that NR1 and NR2A-2C subunits are all enriched in the postsynaptic density (PSD), which plays critical roles in trophin-mediated synaptic plasticity. Significant expression of NRs was observed the first two weeks after birth, during synaptogenesis, and in adulthood. Functional diversity of NRs, resulting from heterogeneous composition, was supported by the finding that different NR2 subunits were associated in a region-specific manner with NR1. Phosphorylation of NR1, a key subunit of the NMDA receptor-channel complex, was significantly enhanced by activators of calmodulin (CaM) kinases (CKs) or protein kinase C (PKC), but not by those of PKA. Co-immunoprecipitation studies revealed that NR1 was physically associated with functionally active PKCgamma and the major PSD protein (mPSDp) through noncovalent interactions. Our results suggest that NMDA receptors play roles in postsynaptic mechanisms in a subunit-, composition-, brain region- and developmental-specific manner. Our findings also indicate that the PSD is a coherent functional unit containing protein kinases that potentially regulate NMDA receptor function via phosphorylation.