Ca2+/calmodulin-kinase II enhances channel conductance of alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionate type glutamate receptors

Subunit-specific rules governing AMPA receptor trafficking to synapses in hippocampal pyramidal neurons

AMPA-type glutamate receptors (AMPA-Rs) mediate a majority of excitatory synaptic transmission in the brain. In hippocampus, most AMPA-Rs are hetero-oligomers composed of GluR1/GluR2 or GluR2/GluR3 subunits. Here we show that these AMPA-R forms display different synaptic delivery mechanisms. GluR1/GluR2 receptors are added to synapses during plasticity; this requires interactions between GluR1 and group I PDZ domain proteins. In contrast, GluR2/GluR3 receptors replace existing synaptic receptors continuously; this occurs only at synapses that already have AMPA-Rs and requires interactions by GluR2 with NSF and group II PDZ domain proteins. The combination of regulated addition and continuous replacement of synaptic receptors can stabilize long-term changes in synaptic efficacy and may serve as a general model for how surface receptor number is established and maintained.

Calcium/calmodulin-dependent protein kinase II inhibitor protein: localization of isoforms in rat brain

A second isoform of Ca2+/calmodulin-dependent-kinase II inhibitor protein (CaM-KIIN) has been identified using the yeast two-hybrid screen. The 1.8kb message encodes a 78 residue CaM-KIINalpha that is 65% identical in its putative open-reading frame and 95% identical in its inhibitory domain to the previously characterized CaM-KIINbeta. CaM-KIINalpha exhibits inhibitory properties towards recombinant mouse CaM-kinase IIalpha indistinguishable from CaM-KIINbeta. The 27 amino acid inhibitory peptide (CaM-KIINtide) derived from CaM-KIIN has the ability to inhibit brain CaM-kinase II activity from multiple organisms including rat, Drosophila and goldfish. Northern analysis of various rat tissues indicates that CaM-KIINalpha is specific to brain whereas CaM-KIINbeta message is also present in testis. In situ hybridization shows a general distribution of both isoforms in rat brain with stronger localization of CaM-KIINbeta in cerebellum and hindbrain and CaM-KIINalpha in frontal cortex, hippocampus and inferior colliculus. An antibody that recognizes both isoforms shows a distribution of CaM-KIIN in rat brain that correlates with immunoreactivity of CaM-kinase II. In cultured mature hippocampal neurons, CaM-KIIN is present in cell bodies and dendrites but, unlike CaM-kinase II, does not display punctate staining at synapses. These results suggest a localized function for CaM-KIIN in inhibiting specialized pools of CaM-kinase II.

Postsynaptic Modulation of AMPA Receptor-Mediated Synaptic Responses and LTP by the Type 3 Ryanodine Receptor

The precise function of ryanodine receptors (RyRs) in synaptic transmission is unknown, but three of their subtypes are expressed in the brain. We examined the roleof RyRs in excitatory synaptic transmission in hippocampal slices, using type 3 RyR (RyR3)-deficient mice. The alpha-amino-3-hydroxy-5-methyl-4-isoxozolepropionic acid (AMPA) receptor-mediated basal synaptic responses in the CA1 region of mutant mice were smaller than those of wild-type mice, while there was no difference in N-methyl-d-aspartate receptor-mediated responses, suggesting selective postsynaptic modification of AMPA receptors by RyR3. The expression of synaptic AMPA receptor subunits examined by Western blotting or immunohistochemistry was indistinguishable, suggesting that the smaller AMPA synaptic responses in mutant mice were not due to the reduced number of synaptic AMPA receptors. Although the initial potentiation following tetanic stimulation of afferent fibers was similar, long-term potentiation (LTP) was smaller in mutant mice. There were no differences in presynaptic electrophysiological properties. We thus conclude that RyR3 postsynaptically regulates the properties of AMPA receptors and LTP.

Is persistent activity of calcium/calmodulin-dependent kinase required for the maintenance of LTP?

Calcium/calmodulin-dependent protein kinase II (CaMKII) is concentrated in the postsynaptic density (PSD) and plays an important role in the induction of long-term potentiation (LTP). Because this kinase is persistently activated after the induction, its activity could also be important for LTP maintenance. Experimental tests of this hypothesis, however, have given conflicting results. In this paper we further explore the role of postsynaptic CaMKII in induction and maintenance of LTP. Postsynaptic application of a CaMKII inhibitor [autocamtide-3 derived peptide inhibitor (AC3-I), 2 mM] blocked LTP induction but had no detectable affect on N-methyl-D-aspartate (NMDA)-mediated synaptic transmission, indicating that the primary function of CaMKII in LTP is downstream from NMDA channel function. We next explored various methodological factors that could account for conflicting results on the effect of CaMKII inhibitors on LTP maintenance. In contrast to our previous work, we now carried out experiments at higher temperature (33 degrees C), used slices from adult animals, and induced LTP using a tetanic stimulation. However, we still found that LTP maintenance was not affected by postsynaptic application of AC3-I. Furthermore the inhibitor did not block LTP maintenance under conditions designed to enhance the Ca(2+)-dependent activity of protein phosphatases 1 and 2B (elevated Ca(2+), calmodulin, and an inhibitor of protein kinase A). We also tested the possibility that CaMKII inhibitor might not be able to affect CaMKII once it was inserted into the PSD. In whole-brain extracts, AC3-I blocked autophosphorylation of both soluble and particulate/PSD CaMKII with similar potencies although the potency of the inhibitor toward other CaMKII substrates varied. Thus we were unable to demonstrate a functional role of persistent Ca(2+)-independent CaMKII activity in LTP maintenance. Possible explanations of the data are discussed.

Chronic exposure to nicotine upregulates the human (alpha)4((beta)2 nicotinic acetylcholine receptor function

Widely expressed in the brain, the alpha4beta2 nicotinic acetylcholine receptor (nAChR) is proposed to play a major role in the mechanisms that lead to and maintain nicotine addiction. Using the patch-clamp technique and pharmacological protocols, we examined the consequences of long-term exposure to 0.1-10 micrometer nicotine in K-177 cells expressing the major human brain alpha4beta2 receptor. The acetylcholine dose-response curves are biphasic and revealed both a high- and a low-affinity component with apparent EC(50) values of 1.6 and 62 micrometer. Ratios of receptors in the high- and low-affinity components are 25 and 75%, respectively. Chronic exposure to nicotine or nicotinic antagonists [dihydro-beta-erytroidine (DHbetaE) or methyllycaconitine (MLA)] increases the fraction of high-affinity receptors up to 70%. Upregulated acetylcholine-evoked currents increase by twofold or more and are less sensitive to desensitization. Functional upregulation is independent of protein synthesis as shown by the lack of effect of 20 micrometer cycloheximide. Single-channel currents recorded with 100 nm acetylcholine show predominantly high conductances (38.8 and 43.4 pS), whereas additional smaller conductances (16.7 and 23.5 pS) were observed with 30 micrometer acetylcholine. In addition, long-term exposure to dihydro-beta-erytroidine increases up to three times the frequency of channel openings. These data indicate, in contrast to previous studies, that human alpha4beta2 nAChRs are functionally upregulated by chronic nicotine exposure.

Spine changes associated with long-term potentiation

High-frequency stimulation of excitatory synapses in many regions of the brain triggers a lasting increase in the efficacy of synaptic transmission referred to as long-term potentiation (LTP) and believed to contribute to learning and memory. One hypothesis proposed to account for the stability and properties of this functional plasticity is a structural remodeling of spine synapses. This possibility has recently received support from several studies. It has been found that spines are highly dynamic structures, that they can be formed very rapidly, and that synaptic activity and calcium modulate changes in spine shape and formation of new spines. Ultrastructural analyses bring additional support to these observations and suggest that LTP is associated with a remodeling of the postsynaptic density (PSD) and a process of spine duplication. This new information is reviewed and interpreted in light of other recent advances concerning the mechanisms of LTP and especially the role of postsynaptic glutamate receptor turnover in this form of plasticity. Taken together, a view is emerging that suggests that morphologic changes of spine synapses are associated with LTP and that they not only correlate with, but probably also contribute to the increase in synaptic transmission.

Modification of AMPA receptor clustering regulates cerebellar synaptic plasticity

Cerebellar long-term depression (LTD) induced at parallel fiber-Purkinje neuron synapses is proposed to underlie certain types of motor learning. alpha-Amino-3-hydroxy-5-methylisoxazole-4-propionate (AMPA) receptors, which mediate chemical transmission in these synapses, are clustered on the postsynaptic membrane. By increasing local density of the receptors, clustering is believed to increase synaptic efficacy. This article focuses on molecular mechanisms regulating the synaptic AMPA receptor clustering in Purkinje cells, which could underlie the expression of cerebellar LTD. Synaptic AMPA receptor clusters in dendritic spines of Purkinje cells are disrupted upon protein kinase C (PKC)-mediated phosphorylation of serine 880 in the C-terminal domain of GluR2. Phosphorylation of this residue causes significant reduction in the affinity of GluR2 C-terminal tail for glutamate receptor interacting protein (GRIP), a molecule known to be crucial for AMPA receptor clustering. Consequently, AMPA receptors on the synaptic membrane are destabilized and internalized by endocytosis. Based on these findings, a model for the expression of cerebellar LTD is proposed, in which a decrease in the number of postsynaptic AMPA receptors, initiated by phosphorylation of GluR2 serine 880, is the major mechanism underlying cerebellar LTD.

AMPA receptor current density, not desensitization, predicts selective motoneuron vulnerability

Spinal motoneurons are more susceptible to AMPA receptor-mediated injury than are other spinal neurons, a property that has been implicated in their selective degeneration in amyotrophic lateral sclerosis (ALS). The aim of this study was to determine whether this difference in vulnerability between motoneurons and other spinal neurons can be attributed to a difference in AMPA receptor desensitization and/or to a difference in density of functional AMPA receptors. Spinal motoneurons and dorsal horn neurons were isolated from embryonic rats and cultured on spinal astrocytes. Single-cell RT-PCR quantification of the relative abundance of the flip and flop isoforms of the AMPA receptor subunits, which are known to affect receptor desensitization, did not reveal any difference between the two cell populations. Examination of AMPA receptor desensitization by patch-clamp electrophysiological measurements on nucleated and outside-out patches and in the whole-cell mode also yielded similar results for the two cell groups. However, AMPA receptor current density was two- to threefold higher in motoneurons than in dorsal horn neurons, suggesting a higher density of functional AMPA receptors in motoneuron membranes. Pharmacological reduction of AMPA receptor current density in motoneurons to the level found in dorsal horn neurons eliminated selective motoneuron vulnerability to AMPA receptor activation. These results suggest that the greater AMPA receptor current density of spinal motoneurons may be sufficient to account for their selective vulnerability to AMPA receptor agonists in vitro.

Activation of synaptic NMDA receptors induces membrane insertion of new AMPA receptors and LTP in cultured hippocampal neurons

Long-term potentiation (LTP) of excitatory transmission in the hippocampus likely contributes to learning and memory. The mechanisms underlying LTP at these synapses are not well understood, although phosphorylation and redistribution of AMPA receptors may be responsible for this form of synaptic plasticity. We show here that miniature excitatory postsynaptic currents (mEPSCs) in cultured hippocampal neurons reliably demonstrate LTP when postsynaptic NMDA receptors are briefly stimulated with glycine. LTP of these synapses is accompanied by a rapid insertion of native AMPA receptors and by increased clustering of AMPA receptors at the surface of dendritic membranes. Both LTP and glycine-facilitated AMPA receptor insertion are blocked by intracellular tetanus toxin (TeTx), providing evidence that AMPA receptors are inserted into excitatory synapses via a SNARE-dependent exocytosis during LTP.

Postsynaptic organization and regulation of excitatory synapses

Dynamic regulation of synaptic efficacy is one of the mechanisms thought to underlie learning and memory. Many of the observed changes in efficacy, such as long-term potentiation and long-term depression, result from the functional alteration of excitatory neurotransmission mediated by postsynaptic glutamate receptors. These changes may result from the modulation of the receptors themselves and from regulation of protein networks associated with glutamate receptors. Understanding the interactions in this synaptic complex will yield invaluable insight into the molecular basis of synaptic function. This review focuses on the molecular organization of excitatory synapses and the processes involved in the dynamic regulation of glutamate receptors.

Involvement of neurogranin in the modulation of calcium/calmodulin-dependent protein kinase II, synaptic plasticity, and spatial learning: a study with knockout mice

Neurogranin/RC3 is a neural-specific Ca(2+)-sensitive calmodulin (CaM)-binding protein whose CaM-binding affinity is modulated by phosphorylation and oxidation. Here we show that deletion of the Ng gene in mice did not result in obvious developmental or neuroanatomical abnormalities but caused an impairment of spatial learning and changes in hippocampal short- and long-term plasticity (paired-pulse depression, synaptic fatigue, long-term potentiation induction). These deficits were accompanied by a decreased basal level of the activated Ca(2+)/CaM-dependent kinase II (CaMKII) ( approximately 60% of wild type). Furthermore, hippocampal slices of the mutant mice displayed a reduced ability to generate activated CaMKII after stimulation of protein phosphorylation and oxidation by treatments with okadaic acid and sodium nitroprusside, respectively. These results indicate a central role of Ng in the regulation of CaMKII activity with decisive influences on synaptic plasticity and spatial learning.

Concentration-dependent substate behavior of native AMPA receptors

AMPA-type glutamate receptors mediate most excitatory postsynaptic currents (EPSCs) at central synapses, and their conductance determines in part the size of EPSCs. The conductance of a recombinant AMPA receptor depends on the number of agonist molecules bound to the channel. Here we tested whether native AMPA and kainate receptors show this behavior in outside-out patches from neurons in situ by measuring conductance levels of single channels over a wide range of agonist concentrations. We found that the conductance of AMPA, but not kainate, receptors depended strongly on agonist concentration. Our results suggest that alterations in the glutamate concentration in the synaptic cleft may change the apparent unitary conductance of postsynaptic AMPA receptors.

AMPA receptor current density, not desensitization, predicts selective motoneuron vulnerability

Spinal motoneurons are more susceptible to AMPA receptor-mediated injury than are other spinal neurons, a property that has been implicated in their selective degeneration in amyotrophic lateral sclerosis (ALS). The aim of this study was to determine whether this difference in vulnerability between motoneurons and other spinal neurons can be attributed to a difference in AMPA receptor desensitization and/or to a difference in density of functional AMPA receptors. Spinal motoneurons and dorsal horn neurons were isolated from embryonic rats and cultured on spinal astrocytes. Single-cell RT-PCR quantification of the relative abundance of the flip and flop isoforms of the AMPA receptor subunits, which are known to affect receptor desensitization, did not reveal any difference between the two cell populations. Examination of AMPA receptor desensitization by patch-clamp electrophysiological measurements on nucleated and outside-out patches and in the whole-cell mode also yielded similar results for the two cell groups. However, AMPA receptor current density was two- to threefold higher in motoneurons than in dorsal horn neurons, suggesting a higher density of functional AMPA receptors in motoneuron membranes. Pharmacological reduction of AMPA receptor current density in motoneurons to the level found in dorsal horn neurons eliminated selective motoneuron vulnerability to AMPA receptor activation. These results suggest that the greater AMPA receptor current density of spinal motoneurons may be sufficient to account for their selective vulnerability to AMPA receptor agonists in vitro.

Regulation of AMPA receptors by phosphorylation

The AMPA receptors for glutamate are oligomeric structures that mediate fast excitatory responses in the central nervous system. Phosphorylation of AMPA receptors is an important mechanism for short-term modulation of their function, and is thought to play an important role in synaptic plasticity in different brain regions. Recent studies have shown that phosphorylation of AMPA receptors by cAMP-dependent protein kinase (PKA) and Ca2+- and calmodulin-dependent protein kinase II (CaMKII) potentiates their activity, but phosphorylation of the receptor subunits may also affect their interaction with intracellular proteins, and their expression at the plasma membrane. Phosphorylation of AMPA receptor subunits has also been investigated in relation to processes of synaptic plasticity. This review focuses on recent advances in understanding the molecular mechanisms of regulation of AMPA receptors, and their implications in synaptic plasticity.

Impaired synaptic plasticity and cAMP response element-binding protein activation in Ca2+/calmodulin-dependent protein kinase type IV/Gr-deficient mice

The Ca(2+)/calmodulin-dependent protein kinase type IV/Gr (CaMKIV/Gr) is a key effector of neuronal Ca(2+) signaling; its function was analyzed by targeted gene disruption in mice. CaMKIV/Gr-deficient mice exhibited impaired neuronal cAMP-responsive element binding protein (CREB) phosphorylation and Ca(2+)/CREB-dependent gene expression. They were also deficient in two forms of synaptic plasticity: long-term potentiation (LTP) in hippocampal CA1 neurons and a late phase of long-term depression in cerebellar Purkinje neurons. However, despite impaired LTP and CREB activation, CaMKIV/Gr-deficient mice exhibited no obvious deficits in spatial learning and memory. These results support an important role for CaMKIV/Gr in Ca(2+)-regulated neuronal gene transcription and synaptic plasticity and suggest that the contribution of other signaling pathways may spare spatial memory of CaMKIV/Gr-deficient mice.

The role of alpha-CaMKII autophosphorylation in neocortical experience-dependent plasticity

Calcium/calmodulin kinase type II (CaMKII) is a major postsynaptic density protein. CaMKII is postulated to act as a 'molecular switch', which, when triggered by a transient rise in calcium influx, becomes active for prolonged periods because of its ability to autophosphorylate. We studied experience-dependent plasticity in the barrel cortex of mice carrying a point mutation of the alpha-CaMKII gene (T286A), which abolishes this enzyme's ability to autophosphorylate. Plasticity was prevented in adult and adolescent mice homozygous for the mutation, but was normal in heterozygotes and wild-type littermates. These results provide evidence that the molecular switch hypothesis is valid for neocortical experience-dependent plasticity.

Impaired synaptic plasticity and cAMP response element-binding protein activation in Ca2+/calmodulin-dependent protein kinase type IV/Gr-deficient mice

The Ca(2+)/calmodulin-dependent protein kinase type IV/Gr (CaMKIV/Gr) is a key effector of neuronal Ca(2+) signaling; its function was analyzed by targeted gene disruption in mice. CaMKIV/Gr-deficient mice exhibited impaired neuronal cAMP-responsive element binding protein (CREB) phosphorylation and Ca(2+)/CREB-dependent gene expression. They were also deficient in two forms of synaptic plasticity: long-term potentiation (LTP) in hippocampal CA1 neurons and a late phase of long-term depression in cerebellar Purkinje neurons. However, despite impaired LTP and CREB activation, CaMKIV/Gr-deficient mice exhibited no obvious deficits in spatial learning and memory. These results support an important role for CaMKIV/Gr in Ca(2+)-regulated neuronal gene transcription and synaptic plasticity and suggest that the contribution of other signaling pathways may spare spatial memory of CaMKIV/Gr-deficient mice.

Mechanism and regulation of calcium/calmodulin-dependent protein kinase II targeting to the NR2B subunit of the N-methyl-D-aspartate receptor

Calcium influx through the N-methyl-d-aspartate (NMDA)-type glutamate receptor and activation of calcium/calmodulin-dependent kinase II (CaMKII) are critical events in certain forms of synaptic plasticity. We have previously shown that autophosphorylation of CaMKII induces high-affinity binding to the NR2B subunit of the NMDA receptor (Strack, S., and Colbran, R. J. (1998) J. Biol. Chem. 273, 20689-20692). Here, we show that residues 1290-1309 in the cytosolic tail of NR2B are critical for CaMKII binding and identify by site-directed mutagenesis several key residues (Lys(1292), Leu(1298), Arg(1299), Arg(1300), Gln(1301), and Ser(1303)). Phosphorylation of NR2B at Ser(1303) by CaMKII inhibits binding and promotes slow dissociation of preformed CaMKII.NR2B complexes. Peptide competition studies imply a role for the CaMKII catalytic domain, but not the substrate-binding pocket, in the association with NR2B. However, analysis of monomeric CaMKII mutants indicates that the holoenzyme structure may also be important for stable association with NR2B. Residues 1260-1316 of NR2B are sufficient to direct the subcellular localization of CaMKII in intact cells and to confer dynamic regulation by calcium influx. Furthermore, mutation of residues in the CaMKII-binding domain in full-length NR2B bidirectionally modulates colocalization with CaMKII after NMDA receptor activation, suggesting a dynamic model for the translocation of CaMKII to postsynaptic targets.

Regulation of the phosphorylation state of the AMPA receptor GluR1 subunit in the postsynaptic density

1. Changes in the phosphorylation state of AMPA-type glutamate receptors are thought to underlie activity-dependent synaptic modification. It has been established that the GluR1 subunit is phosphorylated on two distinct sites, Ser-831 and Ser-845, by CaMKII and by PKA, respectively, and that phosphorylation by either kinase correlates with an increase in the AMPA receptor-mediated current. GluR1 is concentrated in postsynaptic densities and it is expected that this particular receptor pool is involved in synaptic modification. The present study describes the regulation of the phosphorylation state of GluR1 in isolated postsynaptic densities. 2. Addition of Ca2+/calmodulin to the postsynaptic density fraction promotes phosphorylation of GluR1, and under these conditions, dephosphorylation is prevented by the inclusion of phosphatase type 1 inhibitors, microcystin-LR and Inhibitor-1. CaMKII and phosphatase type 1 are also found to be enriched in the PSD fraction compared to the parent fractions. 3. On the other hand, the addition of cAMP, either by itself or with exogenous PKA, does not change the phosphorylation state of GluR1. Prior incubation of PSDs under dephosphorylating conditions results in only a small PKA-mediated phosphorylation of GluR1. 4. These results support the hypothesis that PSDs contain the molecular machinery to promote the phosphorylation as well as the dephosphorylation of GluR1 on Ser-831, while Ser-845, the site phosphorylated by PKA, appears to be mostly occluded. Thus, it is possible that a large pool of PSD-associated GluR1 is regulated through modification of the phosphorylation state of the Ser-831 site only.

Targeting of PKA to glutamate receptors through a MAGUK-AKAP complex

Compartmentalization of glutamate receptors with the signaling enzymes that regulate their activity supports synaptic transmission. Two classes of binding proteins organize these complexes: the MAGUK proteins that cluster glutamate receptors and AKAPs that anchor kinases and phosphatases. In this report, we demonstrate that glutamate receptors and PKA are recruited into a macromolecular signaling complex through direct interaction between the MAGUK proteins, PSD-95 and SAP97, and AKAP79/150. The SH3 and GK regions of the MAGUKs mediate binding to the AKAP. Cell-based studies indicate that phosphorylation of AMPA receptors is enhanced by a SAP97-AKAP79 complex that directs PKA to GluR1 via a PDZ domain interaction. As AMPA receptor phosphorylation is implicated in regulating synaptic plasticity, these data suggest that a MAGUK-AKAP complex may be centrally involved.

Regulation of phosphorylation of the GluR1 AMPA receptor in the neostriatum by dopamine and psychostimulants in vivo

The activation of cAMP-dependent protein kinase regulates the physiological activity of AMPA-type glutamate receptors. In this study, phosphorylation of the AMPA receptor subunit GluR1 at Ser(845) was increased in neostriatal slices by activation of D1-type dopamine receptors and by inhibitors of protein phosphatase 1/protein phosphatase 2A. In contrast, Ser(831), a residue which, when phosphorylated by protein kinase C or calcium/calmodulin-dependent kinase II, increases AMPA receptor channel conductance, was unaffected by either D1 or D2 receptor agonists in neostriatal slices. The phosphorylation of Ser(845), but not Ser(831), was strongly increased in neostriatum in vivo in response to the psychostimulants cocaine and methamphetamine. The effects of dopamine and psychostimulants on the phosphorylation of GluR1 were attenuated in dopamine and cAMP-regulated phosphoprotein M(r) 32 kDa (DARPP-32) knock-out mice. These results identify DARPP-32 and AMPA-type glutamate receptors as likely essential cellular effectors for psychostimulant actions.

Regulation of distinct AMPA receptor phosphorylation sites during bidirectional synaptic plasticity

Bidirectional changes in the efficacy of neuronal synaptic transmission, such as hippocampal long-term potentiation (LTP) and long-term depression (LTD), are thought to be mechanisms for information storage in the brain. LTP and LTD may be mediated by the modulation of AMPA (alpha-amino-3-hydroxy-5-methyl-4-isoxazloe proprionic acid) receptor phosphorylation. Here we show that LTP and LTD reversibly modify the phosphorylation of the AMPA receptor GluR1 subunit. However, contrary to the hypothesis that LTP and LTD are the functional inverse of each other, we find that they are associated with phosphorylation and dephosphorylation, respectively, of distinct GluR1 phosphorylation sites. Moreover, the site modulated depends on the stimulation history of the synapse. LTD induction in naive synapses dephosphorylates the major cyclic-AMP-dependent protein kinase (PKA) site, whereas in potentiated synapses the major calcium/calmodulin-dependent protein kinase II (CaMKII) site is dephosphorylated. Conversely, LTP induction in naive synapses and depressed synapses increases phosphorylation of the CaMKII site and the PKA site, respectively. LTP is differentially sensitive to CaMKII and PKA inhibitors depending on the history of the synapse. These results indicate that AMPA receptor phosphorylation is critical for synaptic plasticity, and that identical stimulation conditions recruit different signal-transduction pathways depending on synaptic history.

Regulation of phosphorylation of the GluR1 AMPA receptor in the neostriatum by dopamine and psychostimulants in vivo

The activation of cAMP-dependent protein kinase regulates the physiological activity of AMPA-type glutamate receptors. In this study, phosphorylation of the AMPA receptor subunit GluR1 at Ser(845) was increased in neostriatal slices by activation of D1-type dopamine receptors and by inhibitors of protein phosphatase 1/protein phosphatase 2A. In contrast, Ser(831), a residue which, when phosphorylated by protein kinase C or calcium/calmodulin-dependent kinase II, increases AMPA receptor channel conductance, was unaffected by either D1 or D2 receptor agonists in neostriatal slices. The phosphorylation of Ser(845), but not Ser(831), was strongly increased in neostriatum in vivo in response to the psychostimulants cocaine and methamphetamine. The effects of dopamine and psychostimulants on the phosphorylation of GluR1 were attenuated in dopamine and cAMP-regulated phosphoprotein M(r) 32 kDa (DARPP-32) knock-out mice. These results identify DARPP-32 and AMPA-type glutamate receptors as likely essential cellular effectors for psychostimulant actions.

Brain-derived neurotrophic factor differentially regulates excitatory and inhibitory synaptic transmission in hippocampal cultures

Brain-derived neurotrophic factor (BDNF) has been postulated to be a key signaling molecule in regulating synaptic strength and overall circuit activity. In this context, we have found that BDNF dramatically increases the frequency of spontaneously initiated action potentials in hippocampal neurons in dissociated culture. Using analysis of unitary synaptic transmission and immunocytochemical methods, we determined that chronic treatment with BDNF potentiates both excitatory and inhibitory transmission, but that it does so via different mechanisms. BDNF strengthens excitation primarily by augmenting the amplitude of AMPA receptor-mediated miniature EPSCs (mEPSCs) but enhances inhibition by increasing the frequency of mIPSC and increasing the size of GABAergic synaptic terminals. In contrast to observations in other systems, BDNF-mediated increases in AMPA-receptor mediated mEPSC amplitudes did not require activity, because blocking action potentials with tetrodotoxin for the entire duration of BDNF treatment had no effect on the magnitude of this enhancement. These forms of synaptic regulations appear to be a selective action of BDNF because intrinsic excitability, synapse number, and neuronal survival are not affected in these cultures. Thus, although BDNF induces a net increase in overall circuit activity, this results from potentiation of both excitatory and inhibitory synaptic drive through distinct and selective physiological mechanisms.

Synaptic plasticity and dynamic modulation of the postsynaptic membrane

The biochemical composition of the postsynaptic membrane and the structure of dendritic spines may be rapidly modulated by synaptic activity. Here we review these findings, discuss their implications for long-term potentiation (LTP) and long-term depression (LTD) and propose a model of sequentially occurring expression mechanisms.

In and out of the postsynaptic region: signalling proteins on the move

Reversible translocation of signalling proteins to and from their sites of action has emerged as an important theme in signal transduction. The recent findings of the stimulus-induced translocation of Ca2+-calmodulin-dependent protein kinase II (CaMKII) and the alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptor to and from the postsynaptic region are model cases for understanding how the dynamic localization of signalling proteins is used to regulate signal transduction.

CaM-kinases: modulators of synaptic plasticity

Calcium signaling is crucial for several aspects of plasticity at glutamatergic synapses, and studies over the past two to three years have identified key functions for Ca(2+)/calmodulin-dependent protein kinases II and IV (CaM-KII and CaM-KIV). Sustained activation of CaM-KII localized at the postsynaptic density results in phosphorylation of numerous synaptic substrates including ion channels, other signaling molecules and scaffolding proteins, to modulate synaptic transmission within minutes. More prolonged responses may be effected through enhanced dendritic protein synthesis of CaM-KII and regulation of nuclear gene transcription by CaM-KIV.

LTP mechanisms: from silence to four-lane traffic

Brief periods of strong neuronal activity induce long-lasting changes in synaptic function. This synaptic plasticity is thought to play important roles in learning and memory. One example--long-term potentation in the CA1 region of the hippocampus--has been studied extensively, and conflicting views regarding the underlying mechanisms have emerged. Recent findings, regarding basic properties of synaptic transmission, appear to reconcile these diverging views.

Brain-derived neurotrophic factor differentially regulates excitatory and inhibitory synaptic transmission in hippocampal cultures

Brain-derived neurotrophic factor (BDNF) has been postulated to be a key signaling molecule in regulating synaptic strength and overall circuit activity. In this context, we have found that BDNF dramatically increases the frequency of spontaneously initiated action potentials in hippocampal neurons in dissociated culture. Using analysis of unitary synaptic transmission and immunocytochemical methods, we determined that chronic treatment with BDNF potentiates both excitatory and inhibitory transmission, but that it does so via different mechanisms. BDNF strengthens excitation primarily by augmenting the amplitude of AMPA receptor-mediated miniature EPSCs (mEPSCs) but enhances inhibition by increasing the frequency of mIPSC and increasing the size of GABAergic synaptic terminals. In contrast to observations in other systems, BDNF-mediated increases in AMPA-receptor mediated mEPSC amplitudes did not require activity, because blocking action potentials with tetrodotoxin for the entire duration of BDNF treatment had no effect on the magnitude of this enhancement. These forms of synaptic regulations appear to be a selective action of BDNF because intrinsic excitability, synapse number, and neuronal survival are not affected in these cultures. Thus, although BDNF induces a net increase in overall circuit activity, this results from potentiation of both excitatory and inhibitory synaptic drive through distinct and selective physiological mechanisms.

Heterogeneous conductance levels of native AMPA receptors

The single-channel properties of AMPA receptors can affect information processing in neurons by influencing the amplitude and kinetics of synaptic currents, yet little is known about the unitary properties of native AMPA receptors in situ. Using whole-cell and outside-out patch-clamp recordings from granule cells in acute cerebellar slices, we found that migrating granule cells begin to express AMPA receptors before they arrive in the internal granule cell layer and receive synaptic input. At saturating agonist concentrations, the open probability of channels in outside-out patches from migrating cells was very high, allowing us to identify patches that contained only one or two active channels. Analysis of the single-channel activity in these patches showed that individual AMPA receptors exhibit as many as four distinguishable conductance levels. The conductance levels observed varied substantially for different channels, although on average the values fell within the range of unitary conductances estimated previously for synaptic AMPA receptors. In contrast to patches from migrating granule cells, we rarely observed directly resolvable single-channel currents in patches excised from the somata of granule cells in the internal granular layer, even though these cells gave large AMPA receptor whole-cell currents. We did, however, detect AMPA receptors with apparent unitary conductances of <1 pS in patches from both migrating and mature granule cells. Our results suggest that granule cells express a heterogeneous population of AMPA receptors, a subset of which are segregated to postsynaptic sites after synaptogenesis.

Driving AMPA receptors into synapses by LTP and CaMKII: requirement for GluR1 and PDZ domain interaction

To elucidate mechanisms that control and execute activity-dependent synaptic plasticity, alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionate receptors (AMPA-Rs) with an electrophysiological tag were expressed in rat hippocampal neurons. Long-term potentiation (LTP) or increased activity of the calcium/calmodulin-dependent protein kinase II (CaMKII) induced delivery of tagged AMPA-Rs into synapses. This effect was not diminished by mutating the CaMKII phosphorylation site on the GluR1 AMPA-R subunit, but was blocked by mutating a predicted PDZ domain interaction site. These results show that LTP and CaMKII activity drive AMPA-Rs to synapses by a mechanism that requires the association between GluR1 and a PDZ domain protein.

Heterogeneous conductance levels of native AMPA receptors

The single-channel properties of AMPA receptors can affect information processing in neurons by influencing the amplitude and kinetics of synaptic currents, yet little is known about the unitary properties of native AMPA receptors in situ. Using whole-cell and outside-out patch-clamp recordings from granule cells in acute cerebellar slices, we found that migrating granule cells begin to express AMPA receptors before they arrive in the internal granule cell layer and receive synaptic input. At saturating agonist concentrations, the open probability of channels in outside-out patches from migrating cells was very high, allowing us to identify patches that contained only one or two active channels. Analysis of the single-channel activity in these patches showed that individual AMPA receptors exhibit as many as four distinguishable conductance levels. The conductance levels observed varied substantially for different channels, although on average the values fell within the range of unitary conductances estimated previously for synaptic AMPA receptors. In contrast to patches from migrating granule cells, we rarely observed directly resolvable single-channel currents in patches excised from the somata of granule cells in the internal granular layer, even though these cells gave large AMPA receptor whole-cell currents. We did, however, detect AMPA receptors with apparent unitary conductances of <1 pS in patches from both migrating and mature granule cells. Our results suggest that granule cells express a heterogeneous population of AMPA receptors, a subset of which are segregated to postsynaptic sites after synaptogenesis.

Activation of CA(2+)/calmodulin-dependent protein kinase IV in cultured rat hippocampal neurons

Ca(2+)/calmodulin-dependent protein kinase IV (CaM kinase IV) is a multifunctional enzyme that is abundantly present in the nuclei of neurons. We report the properties of phosphorylation and activation of CaM kinase IV in comparison to CaM kinase II in cultured rat hippocampal neurons. Phosphorylation and activity of CaM kinase IV as well as CaM kinase II were increased by treatment of neurons either with glutamate or high K(+). Glutamate-induced phosphorylation and activity of CaM kinase IV were blocked by N-methyl-D-asparate (NMDA) antagonists, and NMDA application instead of glutamate did increase CaM kinase IV phosphorylation. CaM kinase IV phosphorylation was also increased by alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionate (AMPA), and was blocked by an inhibitor of NMDA receptor. The AMPA-induced phosphorylation was blocked by tetrodotoxin, a Na(+) channel blocker, that was expected to block endogenous glutamate transmission indirectly. On the other hand, high K(+)-induced phosphorylation and activation were not blocked by inhibitors of glutamate receptors, and effectively blocked by nifedipine, an L-type Ca(2+) channel blocker. These properties were similar between CaM kinase IV and CaM kinase II.

Postsynaptic protein phosphorylation and LTP

Prolonged changes in synaptic strength, such as those that occur in LTP and LTD, are thought to contribute to learning and memory processes. These complex phenomena occur in diverse brain structures and use multiple, temporally staged and spatially resolved mechanisms, such as changes in neurotransmitter release, modulation of transmitter receptors, alterations in synaptic structure, and regulation of gene expression and protein synthesis. In the CA1 region of the hippocampus, the combined activation of SRC family tyrosine kinases, protein kinase A, protein kinase C and, in particular, Ca2+/calmodulin-dependent protein kinase II results in phosphorylation of glutamate-receptor-gated ion channels and the enhancement of subsequent postsynaptic current. Crosstalk between these complex biochemical pathways can account for most characteristics of early-phase LTP in this region.

Control of GluR1 AMPA receptor function by cAMP-dependent protein kinase

Modulation of postsynaptic AMPA receptors in the brain by phosphorylation may play a role in the expression of synaptic plasticity at central excitatory synapses. It is known from biochemical studies that GluR1 AMPA receptor subunits can be phosphorylated within their C terminal by cAMP-dependent protein kinase A (PKA), which is colocalized with the phosphatase calcineurin (i.e., phosphatase 2B). We have examined the effect of PKA and calcineurin on the time course, peak open probability (P(O, PEAK)), and single-channel properties of glutamateevoked responses for neuronal AMPA receptors and homomeric GluR1(flip) receptors recorded in outside-out patches. Inclusion of purified catalytic subunit Calpha-PKA in the pipette solution increased neuronal AMPA receptor P(O,PEAK) (0.92) compared with recordings made with calcineurin included in the pipette (P(O,PEAK) 0.39). Similarly, Calpha-PKA increased P(O,PEAK) for recombinant GluR1 receptors (0. 78) compared with patches excised from cells cotransfected with a cDNA encoding the PKA peptide inhibitor PKI (P(O,PEAK) 0.50) or patches with calcineurin included in the pipette (P(O,PEAK) 0.42). Neither PKA nor calcineurin altered the amplitude of single-channel subconductance levels, weighted mean unitary current, mean channel open period, burst length, or macroscopic response waveform for recombinant GluR1 receptors. Substitution of an amino acid at the PKA phosphorylation site (S845A) on GluR1 eliminated the PKA-induced increase in P(O,PEAK), whereas the mutation of a Ca(2+), calmodulin-dependent kinase II and PKC phosphorylation site (S831A) was without effect. These results suggest that AMPA receptor peak response open probability can be increased by PKA through phosphorylation of GluR1 Ser845.

Control of GluR1 AMPA receptor function by cAMP-dependent protein kinase

Modulation of postsynaptic AMPA receptors in the brain by phosphorylation may play a role in the expression of synaptic plasticity at central excitatory synapses. It is known from biochemical studies that GluR1 AMPA receptor subunits can be phosphorylated within their C terminal by cAMP-dependent protein kinase A (PKA), which is colocalized with the phosphatase calcineurin (i.e., phosphatase 2B). We have examined the effect of PKA and calcineurin on the time course, peak open probability (P(O, PEAK)), and single-channel properties of glutamateevoked responses for neuronal AMPA receptors and homomeric GluR1(flip) receptors recorded in outside-out patches. Inclusion of purified catalytic subunit Calpha-PKA in the pipette solution increased neuronal AMPA receptor P(O,PEAK) (0.92) compared with recordings made with calcineurin included in the pipette (P(O,PEAK) 0.39). Similarly, Calpha-PKA increased P(O,PEAK) for recombinant GluR1 receptors (0. 78) compared with patches excised from cells cotransfected with a cDNA encoding the PKA peptide inhibitor PKI (P(O,PEAK) 0.50) or patches with calcineurin included in the pipette (P(O,PEAK) 0.42). Neither PKA nor calcineurin altered the amplitude of single-channel subconductance levels, weighted mean unitary current, mean channel open period, burst length, or macroscopic response waveform for recombinant GluR1 receptors. Substitution of an amino acid at the PKA phosphorylation site (S845A) on GluR1 eliminated the PKA-induced increase in P(O,PEAK), whereas the mutation of a Ca(2+), calmodulin-dependent kinase II and PKC phosphorylation site (S831A) was without effect. These results suggest that AMPA receptor peak response open probability can be increased by PKA through phosphorylation of GluR1 Ser845.

Differential regulation of synaptic GABAA receptors by cAMP-dependent protein kinase in mouse cerebellar and olfactory bulb neurones

1. It has been demonstrated that the regulation of recombinant GABAA receptors by phosphorylation depends on the subunit composition. Here we studied the regulation of synaptic GABAA receptor function by cAMP-dependent protein kinase (PKA) in neurones expressing distinct receptor subtypes. 2. Light microscopic immunocytochemistry revealed that granule cells of the olfactory bulb express only the beta3 as the beta subunit variant, whereas cerebellar stellate and basket cells express only the beta2 as the beta subunit. 3. In cerebellar interneurones, intracellular application of 20 microM microcystin, a protein phosphatase 1/2A inhibitor, prolonged (63 +/- 14 %; mean +/- s.e.m.) the decay time course of miniature IPSCs (mIPSCs) without significantly affecting their amplitude, rise time and frequency. The effect of microcystin could be blocked by co-applying PKA inhibitory peptide (PKA-I, 1 microM). 4. No significant changes in any of the mIPSC parameters could be detected after intracellular application of PKA-I alone or following the inhibition of calcineurin with FK506 (50 nM). 5. In granule cells of the olfactory bulb expressing the beta3 subunit fast and slowly rising mIPSCs were detected, resulting in a bimodal distribution of the 10-90 % rise times, suggesting two distinct populations of events. Fast rising mIPSCs (mIPSCFR) had a 10-90 % rise time of 410 +/- 50 micros, an amplitude of 68 +/- 6 pA, and a weighted decay time constant (tauw) of 15.8 +/- 2.9 ms. In contrast, slowly rising mIPSCs (mIPSCSR) displayed an approximately threefold slower rise time (1.15 +/- 0.12 ms), 57 % smaller amplitude (29 +/- 1.7 pA), but had a tauw (16.8 +/- 3.0 ms) similar to that of the fast events. 6. mIPSCs in olfactory granule cells were not affected by the intracellular perfusion of microcystin. In spite of this, intracellular administration of constitutively active PKA caused a small, gradual, but significant increase (18 +/- 5 %) in the amplitude of the events without changing their time course. 7. These findings demonstrate a cell-type-dependent regulation of synaptic inhibition by protein phosphorylation. Furthermore, our results show that the effect of PKA-mediated phosphorylation on synaptic inhibition depends upon the subunit composition of postsynaptic GABAA receptors.

Role of AMPA receptor cycling in synaptic transmission and plasticity

Compounds known to disrupt exocytosis or endocytosis were introduced into CA1 pyramidal cells while monitoring excitatory postsynaptic currents (EPSCs). Disrupting exocytosis or the interaction of GluR2 with NSF caused a gradual reduction in the AMPAR EPSC, while inhibition of endocytosis caused a gradual increase in the AMPAR EPSC. These manipulations had no effect on the NMDAR EPSC but prevented the subsequent induction of LTD. These results suggest that AMPARs, but not NMDARs, cycle into and out of the synaptic membrane at a rapid rate and that certain forms of synaptic plasticity may utilize this dynamic process.

Analysis of NMDA-independent long-term potentiation induced at CA3-CA1 synapses in rat hippocampus in vitro

1. Excitatory postsynaptic currents (EPSCs) were evoked at synapses formed by Schaffer collaterals/commissural (CA3) axons with CA1 pyramidal cells using the rat hippocampal slice preparation. Long-term potentiation (LTP) was induced at these synapses using a pairing protocol, with 50 microM d,l-APV present in the artificial cerebrospinal fluid (ACSF). 2. Quantal analysis of the amplitudes of the control and conditioned EPSCs showed that the enhancement of synaptic strength was due entirely to an increase in quantal content of the EPSC. No change occurred in the quantal current. 3. These results were compared with those obtained from a previous quantal analysis of LTP induced in normal ACSF, where both quantal current and quantal content increased. The results suggest that calcium entering via NMDA receptors initiates the signalling cascade that results in enhanced AMPA currents because it is adding to cytoplasmic calcium from other sources to reach a threshold for this signalling pathway, or because calcium entering via NMDA receptors specifically activates this signalling pathway.

Long-term potentiation--a decade of progress?

Long-term potentiation of synaptic transmission in the hippocampus is the leading experimental model for the synaptic changes that may underlie learning and memory. This review presents a current understanding of the molecular mechanisms of this long-lasting increase in synaptic strength and describes a simple model that unifies much of the data that previously were viewed as contradictory.

Rapid spine delivery and redistribution of AMPA receptors after synaptic NMDA receptor activation

To monitor changes in alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionate (AMPA) receptor distribution in living neurons, the AMPA receptor subunit GluR1 was tagged with green fluorescent protein (GFP). This protein (GluR1-GFP) was functional and was transiently expressed in hippocampal CA1 neurons. In dendrites visualized with two-photon laser scanning microscopy or electron microscopy, most of the GluR1-GFP was intracellular, mimicking endogenous GluR1 distribution. Tetanic synaptic stimulation induced a rapid delivery of tagged receptors into dendritic spines as well as clusters in dendrites. These postsynaptic trafficking events required synaptic N-methyl-D-aspartate (NMDA) receptor activation and may contribute to the enhanced AMPA receptor-mediatedtransmission observed during long-term potentiation and activity-dependent synaptic maturation.