Phosphorylation and regulation of glutamate receptors by calcium/calmodulin-dependent protein kinase II

Regulation of ionotropic receptors by protein phosphorylation

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

Characterization of multiple phosphorylation sites on the AMPA receptor GluR1 subunit

We have characterized the phosphorylation of the glutamate receptor subunit GluR1, using biochemical and electrophysiological techniques. GluR1 is phosphorylated on multiple sites that are all located on the C-terminus of the protein. Cyclic AMP-dependent protein kinase specifically phosphorylates SER-845 of GluR1 in transfected HEK cells and in neurons in culture. Phosphorylation of this residue results in a 40% potentiation of the peak current through GluR1 homomeric channels. In addition, protein kinase C specifically phosphorylates Ser-831 of GluR1 in HEK-293 cells and in cultured neurons. These results are consistent with the recently proposed transmembrane topology models of glutamate receptors, in which the C-terminus is intracellular. In addition, the modulation of GluR1 by PKA phosphorylation of Ser-845 suggests that phosphorylation of this residue may underlie the PKA-induced potentiation of AMPA receptors in neurons.

Prevention of trauma-induced neurodegeneration in infant and adult rat brain: glutamate antagonists

The mechanisms of neuronal degeneration following traumatic head injury are not well understood and no adequate treatment is currently available for the prevention of traumatic brain damage in humans. Seven day old rat pups were subjected to mechanical percussion of the head. Cortical damage in infant rats was reduced by pre-treatment with the N-methyl-D-aspartate (NMDA) antagonists dizocilpine (MK-801) or 3-((+/-)-2-carboxypiperazin-4-yl)-propyl-I-phosphonate (CPP). The AMPA antagonist 2,3-dihydroxy-6-nitro-7-sulfamoyl-benzo (f) quinoxaline (NBQX) did not significantly suppress cortical damage in infant rats. In adult rats, traumatic head injury leads to primary (at impact-cortex) and secondary (distant-hippocampus) damage to the brain. Morphometric analysis demonstrated that both cortical and hippocampal damage was mitigated by pre-treatment with either the NMDA antagonist CPP or the non-NMDA antagonist NBQX. Neither treatment prevented primary damage in the cortex when therapy was started after trauma. Delayed treatment of rats with NBQX, but not with CPP, beginning between 1 and 7 h after trauma prevented the hippocampal damage. No protection was seen when therapy with NBQX was started 10 h after trauma. These data indicate that NMDA antagonists may possess better neuroprotective properties against excitotoxic processes triggered by traumatic brain injury in young individuals whereas AMPA antagonists may be more beneficial in adults.

Traumatic brain damage prevented by the non-N-methyl-D-aspartate antagonist 2,3-dihydroxy-6-nitro-7-sulfamoylbenzo[f] quinoxaline

The mechanisms of neuronal degeneration following traumatic head injury are not well understood and no adequate treatment is currently available for the prevention of traumatic brain damage in humans. Traumatic head injury leads to primary (at impact) and secondary (distant) damage to the brain. Mechanical percussion of the rat cortex mimics primary damage seen after traumatic head injury in humans; no animal model mimicking the secondary damage following traumatic head injury has yet been established. Rats subjected to percussion trauma of the cortex showed primary damage in the cortex and secondary damage in the hippocampus. Morphometric analysis demonstrated that both cortical and hippocampal damage was mitigated by pretreatment with either the N-methyl-D-aspartate (NMDA) antagonist 3-((+/-)- 2-carboxypiperazin-4-yl)-propyl-1-phosphonate (CPP) or the non-NMDA antagonist 2,3-dihydroxy-6-nitro-7-sulfamoyl-benzo[f]quinoxaline (NBQX). Neither treatment prevented primary damage in the cortex when therapy was started after trauma. Surprisingly, delayed treatment of rats with NBQX, but not with CPP, beginning between 1 and 7 hr after trauma prevented hippocampal damage. No protection was seen when therapy with NBQX was started 10 hr after trauma. These data indicate that both NMDA- and non-NMDA-dependent mechanisms contribute to the development of primary damage in the cortex, whereas non-NMDA mechanisms are involved in the evolution of secondary damage in the hippocampus in rats subjected to traumatic head injury. The wide therapeutic time-window documented for NBQX suggests that antagonism at non-NMDA receptors may offer a novel therapeutic approach for preventing deterioration of the brain after head injury.

Messengers from the synapses to the nucleus (MSNs) that establish late phase of long-term potentiation (LTP) and long-term memory

The late stage of long-term potentiation (LTP) and long-term memory is believed to be largely governed by altered gene expression for its generation and maintenance, while the early stages of LTP and memory are controlled mainly by the phosphorylation-dephosphorylation of the synaptic proteins. For the altered gene expression, synaptic information must be transmitted from the synaptic sites to the nucleus. This article describes the presence of specific messenger molecules that transmit synaptic information to the nucleus; these molecules are referred to as MSNs (Messengers from Synapse to the Nucleus). In addition, recent studies have indicated that certain transcription factors localize at postsynaptic sites as well as the nucleus, and may function as MSNs.

CaM kinase II in long-term potentiation

The observation that autophosphorylation converts CaM kinase II from the Ca(2+)-dependent form to the Ca(2+)-independent form has led to speculation that the formation of the Ca(2+)-independent form of the enzyme could encode frequency of synaptic usage and serve as a molecular explanation of "memory". In cultured rat hippocampal neurons, glutamate elevated the Ca(2+)-independent activity of CaM kinase II through autophosphorylation, and this response was blocked by an NMDA receptor antagonist, D-2-amino-5-phosphonopentanoate (AP5). In addition, we confirmed that high, but not low frequency stimulation, applied to two groups of CA1 afferents in the rat hippocampus, resulted in LTP induction with concomitant long-lasting increases in Ca(2+)-independent and total activities of CaM kinase II. In experiments with 32P-labeled hippocampal slices, the LTP induction in the CA1 region was associated with increases in autophosphorylation of both alpha and beta subunits of CaM kinase II 1 h after LTP induction. Significant increases in phosphorylation of endogenous CaM kinase II substrates, synapsin I and microtubule-associated protein 2 (MAP2), which are originally located in presynaptic and postsynaptic regions, respectively, were also observed in the same slice. All these changes were prevented when high frequency stimulation was applied in the presence of AP5 or a calmodulin antagonist, calmidazolium. Furthermore, in vitro phosphorylation of the AMPA receptor by CaM kinase II was reported in the postsynaptic density and infusion of the constitutively active CaM kinase II into the hippocampal neurons enhanced kainate-induced response. These results support the idea that CaM kinase II contributes to the induction of hippocampal LTP in both postsynaptic and presynaptic regions through phosphorylation of target proteins such as the AMPA receptor, MAP2 and synapsin I.

Spatial learning alters hippocampal calcium/calmodulin-dependent protein kinase II activity in rats

This study investigated the role of hippocampal CaM-kinase II (calcium/calmodulin-dependent protein kinase II) in spatial learning. In Experiment I, three groups of rats received 1, 2 or 5 days of training on a spatial task in the Morris water maze with a hidden platform, while a control group was trained on a nonspatial task with a visible platform. The acquisition rate in the spatial task was slower than that in the nonspatial task. However, rats receiving 5 days of spatial training had the highest Ca(2+)-independent activity of CaM-kinase II compared with the controls receiving nonspatial training and rats having 1 or 2 days of spatial training. Furthermore, the level of hippocampal Ca2+-independent CaM-kinase II activity was correlated with the final performance on the spatial task. In Experiment II, rats received intra-hippocampal injections of a specific CaM-kinase II inhibitor-KN-62-before each training session. In comparison with the vehicle-injected controls, pretraining injection of KN-62 retarded acquisition in the spatial task but had no effect on the nonspatial task. These results, taken together, indicated that the activation of CaM-kinase II in the hippocampus is not only correlated to the degree of spatial training on the Morris water maze, but may also underlie the neural mechanism subserving spatial memory.

Expression of constitutively active CaMKII in target tissue modifies presynaptic axon arbor growth

Calcium/calmodulin-dependent protein kinase II (CaMKII) can be regulated by synaptic activity and could therefore be involved in activity-dependent control of neuronal growth. We tested whether increased CaMKII activity in postsynaptic optic tectal neurons can modify the development of retinotectal axons in Xenopus. The elaboration of individual presynaptic retinal axons was observed in vivo before and up to 3 days after infecting the tectal cells with vaccinia virus carrying the gene for constitutively active truncated CaMKII (tCaMKII). Elevated postsynaptic CaMKII activity prevented the axons from developing the complexity of normal arbors by increasing the normal rate of branch retractions. Some effects of tCaMKII on arbor morphology were seen 1 day after infection, but they became more dramatic by the third day. The results suggest that postsynaptic CaMKII plays a role in the development of presynaptic arbor structure.

Suppression by calmodulin of glutamate-induced potassium current in identified snail neurons

Using the voltage-clamp technique combined with pressure injection, we examined the inhibitory action of glutamate (Glu) in identified snail neurons. Calimidazolium and N-(6-aminohexyl)-5-chloro-1-naphthalenesulfonamide (W-7), calmodulin inhibitors, enhanced a slow outward current (Glu) current induced by Glu in a dose-dependent manner. This Glu current was suppressed by intracellular injection of calmodulin. However, no significant change in the Glu current was observed when either CaCl2 or EGTA was intracellularly applied. These results suggest a novel calmodulin-mediated process, through which Glu induces an outward current.

Ionotropic glutamate receptors. Their possible role in the expression of hippocampal synaptic plasticity

In the brain, most fast excitatory synaptic transmission is mediated through L-glutamate acting on postsynaptic ionotropic glutamate receptors. These receptors are of two kinds--the alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionate (AMPA)/kainate (non-NMDA) and the N-methyl-D-aspartate (NMDA) receptors, which are thought to be colocalized onto the same postsynaptic elements. This excitatory transmission can be modulated both upward and downward, long-term potentiation (LTP) and long-term depression (LTD), respectively. Whether the expression of LTP/LTD is pre-or postsynaptically located (or both) remains an enigma. This article will focus on what postsynaptic modifications of the ionotropic glutamate receptors may possibly underly long-term potentiation/depression. It will discuss the character of LTP/ LTD with respect to the temporal characteristics and to the type of changes that appears in the non-NMDA and NMDA receptor-mediated synaptic currents, and what constraints these findings put on the possible expression mechanism(s) for LTP/LTD. It will be submitted that if a modification of the glutamate receptors does underly LTP/LTD, an increase/ decrease in the number of functional receptors is the most plausible alternative. This change in receptor number will have to include a coordinated change of both the non-NMDA and the NMDA receptors.

Modulation of amino acid-gated ion channels by protein phosphorylation

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

Characterization of protein kinase and phosphatase systems in chick ciliary ganglion

The aim of the present study was to characterize the second messenger activated protein kinase and phosphatase systems in chick ciliary ganglion using biochemical and immunochemical techniques. Using synthetic peptide substrates cyclic-AMP-, cyclic-GMP-, Ca2+/calmodulin- and Ca2+/phospholipid-dependent protein kinase activities were detected in homogenates of ciliary ganglion dissected from 15-16-day-old embryos. Autophosphorylation of the alpha and beta subunits of Ca2+/calmodulin-dependent protein kinase II in the presence of Ca2+/calmodulin or 5 mM ZnSO4 was detected by sodium dodecyl sulphate-polyacrylamide gel electrophoresis and autoradiography. Protein kinase C was shown to be present using a monoclonal antibody. Two cyclic-AMP binding proteins whose molecular weights corresponded to the regulatory subunits of cyclic AMP-dependent protein kinase (RI and RII) were detected in ciliary ganglia using 8-azido-cyclic-AMP. The most heavily labelled band following incubation with [gamma-32P]ATP under most conditions had an apparent molecular weight of 65,000 which corresponds to the chicken form of myristoylated alanine-rich C kinase substrate, a known substrate of protein kinase C. Another substrate for protein kinase C was a 45,000 molecular weight protein which was tentatively identified as neuromodulin (B-50/GAP-43). Although no endogenous substrate proteins for cyclic-GMP-dependent protein kinase were detected, protein kinase A strongly labelled a 40,000 molecular weight protein. Using 32P(i)-labelled glycogen phosphorylase, protein phosphatases 1 and 2A were identified in ciliary ganglia homogenates at levels which were indistinguishable from forebrain at the same age. The major endogenous protein substrates in ciliary ganglion homogenates from 15-16-day-old embryos were also labelled to a similar extent in homogenates of ciliary ganglia from newly hatched chickens. Intact ciliary ganglia remained viable for several hours after dissection and, after incubation with 32P(i), responded to phorbol ester stimulation by an increased endogenous phosphorylation of several proteins, but especially myristoylated alanine-rich C kinase substrate. These results represent the first systematic characterization of the protein phosphorylation systems in chicken ciliary ganglion and provide a basis for future studies on the biochemical mechanisms responsible for regulating synaptic transmission in this tissue.

A role of Ca2+/calmodulin-dependent protein kinase II in the induction of long-term potentiation in hippocampal CA1 area

Long-term potentiation (LTP) in the CA1 area of the hippocampus is considered to be a synaptic model for learning and memory. The induction of LTP is initiated by activation of the NMDA glutamate receptor in the postsynaptic membrane and a subsequent increase in Ca2+ -influx into the neurons following glutamate release. The action of Ca2+ has been proposed to be mediated by Ca2+ -dependent protein kinases. Recent studies indicate that, among the protein kinases, Ca2+/calmodulin-dependent protein kinase II is implicated in the induction of LTP in the hippocampus.

Localization of AMPA-selective glutamate receptor subunits in the adult cat visual cortex

We have studied the presence and distribution of alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionate (AMPA)-selective glutamate receptor subunits (GluR1, 2, 3, and 4) in the adult cat visual cortical areas 17, 18, 19, and the lateral suprasylvian areas (LSA). Reverse transcription-polymerase chain reaction (RT-PCR) amplification indicated that the genes encoding GluR1, 2, 3, and 4 are expressed in these areas and Western blot analysis revealed that the size of the corresponding peptides is similar to those described in the rat brain. In situ hybridization (ISH) using digoxigenin-labeled riboprobes showed that mRNAs coding for GluR1 and GluR3 were located in cells in all layers of the areas examined and also in the underlying white matter. GluR1 mRNA was relatively abundant throughout layers II-VI while GluR3 mRNA revealed a more laminated pattern of expression, preferentially labeling cells in layers II, III, V, and VI. The distribution of AMPA-selective receptor subunit peptides was studied by immunohistochemistry using subunit specific antibodies and found to be consistent with ISH results. In addition, we observed that most of the cells strongly labeled by the anti-GluR1 antibody were non-pyramidal neurons and that intense GluR2/3 immunoreactivity was seen preferentially in pyramidal neurons. Interestingly, double-labeling experiments indicated that neurons expressing gamma-aminobutyric acid (GABA) as well as the GluR1 subunit were particularly abundant in deeper layers. The GluR4 peptide was predominantly found in a relatively low number of layer III and layer V neurons with either pyramidal or non-pyramidal morphology. Finally, the distribution of neurons expressing the various receptor subunits was similar in all the visual cortical areas studied. These findings indicate a high expression of GluR1-3 subunits in the cat visual cortex and that GluR1 and GluR2/3 subunits are particularly abundant in non-pyramidal and pyramidal neurons, respectively. In addition, the results described here provide a reference for future studies dealing with the effect of visual deprivation on the expression of this receptor type.

Antibody specific for phosphorylated AMPA-type glutamate receptors at GluR2 Ser-696

Possible phosphorylation sites on the Purkinje cell alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionate (AMPA)-type glutamate receptor subunits were identified using in vitro kinase assays of 17 synthetic peptides derived from the transmembrane-3 (TM3) domain to the end of C-terminal of a rat glutamate receptor 2 (GluR2). Only two peptides containing Ser-662 and Ser-696 were found to be efficiently phosphorylated by protein kinase C (PKC). The peptide including Ser-696 was also phosphorylated by protein kinase G (PKG). Another peptide containing Thr-692 of a rat GluRA, clone almost identical to GluR1, was phosphorylated by PKC but not by PKG. Antisera recognizing phosphorylated AMPA receptor subunits at GluR2 Ser-696 or the homologous sites of GluR1/3/4 were produced, and the specificity of one of them, named 12P3, was established by enzyme-linked immunosorbent assay (ELISA), immunoblot and immunoprecipitation analyses. 12P3-immunocytochemistry on cerebellar slices demonstrated an AMPA-induced transient AMPA receptor phosphorylation, which appeared in Purkinje cell dendrites as well as somata immediately after AMPA treatment and disappeared after 20 min. This antibody may be a useful tool to study the role of AMPA receptor phosphorylation in producing synaptic plasticity.

High-resolution immunogold localization of AMPA type glutamate receptor subunits at synaptic and non-synaptic sites in rat hippocampus

The cellular and subcellular localization of the GluRA, GluRB/C and GluRD subunits of the alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionate (AMPA) type glutamate receptor was determined in the rat hippocampus using polyclonal antipeptide antibodies in immunoperoxidase and immunogold procedures. For the localization of the GluRD subunit a new polyclonal antiserum was developed using the C-terminal sequence of the protein (residues 869-881), conjugated to carrier protein and absorbed to colloidal gold for immunization. The purified antibodies immunoprecipitated about 25% of 3[H]AMPA binding activity from the hippocampus, cerebellum or whole brain, but very little from neocortex. These antibodies did not precipitate a significant amount of 3[H]kainate binding activity. The antibodies also recognize the GluRD subunit, but not the other AMPA receptor subunits, when expressed in transfected COS-7 cells and only when permeabilized with detergent, indicating an intracellular epitope. All subunits were enriched in the neuropil of the dendritic layers of the hippocampus and in the molecular layer of the dentate gyrus. The cellular distribution of the GluRD subunit was studied more extensively. The strata radiatum, oriens and the dentate molecular layer were more strongly immunoreactive than the stratum lacunosum moleculare, the stratum lucidum and the hilus. However, in the stratum lucidum of the CA3 area and in the hilus the weakly reacting dendrites were surrounded by immunopositive rosettes, shown in subsequent electron microscopic studies to correspond to complex dendritic spines. In the stratum radiatum, the weakly reacting apical dendrites contrasted with the surrounding intensely stained neuropil. The cell bodies of pyramidal and granule cells were moderately reactive. Some non-principal cells and their dendrites in the pyramidal cell layer and in the alveus also reacted very strongly for the GluRD subunit. At the subcellular level, silver intensified immunogold particles for the GluRA, GluRB/C and GluRD subunits were present at type 1 synaptic membrane specializations on dendritic spines of pyramidal cells throughout all layers of the CA1 and CA3 areas. The most densely labelled synapses tended to be on the largest spines and many smaller spines remained unlabelled. Immunoparticle density at type 1 synapses on dendritic shafts of some non-principal cells was consistently higher than at labelled synapses of dendritic spines of pyramidal cells. Synapses established between dendritic spines and mossy fibre terminals, were immunoreactive for all studied subunits in stratum lucidum of the CA3 area. The postembedding immunogold method revealed that the AMPA type receptors are concentrated within the main body of the anatomically defined type 1 (asymmetrical) synaptic junction. Often only a part of the membrane specialization showed clustered immunoparticles. There was a sharp decrease in immunoreactive receptor density at the edge of the synaptic specialization. Immunolabelling was consistently demonstrated at extrasynaptic sites on dendrites, dendritic spines and somata. The results demonstrate that the GluRA, B/C and D subunits of the AMPA type glutamate receptor are present in many of the glutamatergic synapses formed by the entorhinal, CA3 pyramidal and mossy fibre terminals. Some interneurons have a higher density of AMPA type receptors in their asymmetrical afferent synapses than pyramidal cells. This may contribute to a lower activation threshold of interneurons as compared to principal cells by the same afferents in the hippocampal formation.

Postsynaptic cAMP pathway gates early LTP in hippocampal CA1 region

The role of the cAMP pathway in LTP was studied in the CA1 region of hippocampus. Widely spaced trains of high frequency stimulation generated cAMP postsynaptically via NMDA receptors and calmodulin, consistent with the Ca2+/calmodulin-mediated stimulation of postsynaptic adenylyl cyclase. The early phase of LTP produced by the same pattern of high frequency stimulation was dependent on postsynaptic cAMP. However, synaptic transmission was not increased by postsynaptic application of cAMP. Early LTP became cAMP-independent when protein phosphatase inhibitors were injected postsynaptically. These observations indicate that in early LTP the cAMP signaling pathway, instead of transmitting signals for the generation of LTP, gates LTP through postsynaptic protein phosphatases.

Calcium/calmodulin-dependent kinase II and long-term potentiation enhance synaptic transmission by the same mechanism

Ca(2+)-sensitive kinases are thought to play a role in long-term potentiation (LTP). To test the involvement of Ca2+/calmodulin-dependent kinase II (CaM-K II), truncated, constitutively active form of this kinase was directly injected into CA1 hippocampal pyramidal cells. Inclusion of CaM-K II in the recording pipette resulted in a gradual increase in the size of excitatory postsynaptic currents (EPSCs). No change in evoked responses occurred when the pipette contained heat-inactivated kinase. The effects of CaM-K II mimicked several features of LTP in that it caused a decreased incidence of synaptic failures, an increase in the size of spontaneous EPSCs, and an increase in the amplitude of responses to iontophoretically applied alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionate. To determine whether the CaM-K II-induced enhancement and LTP share a common mechanism, occlusion experiments were carried out. The enhancing action of CaM-K II was greatly diminished by prior induction of LTP. In addition, following the increase in synaptic strength by CaM-K II, tetanic stimulation failed to evoke LTP. These findings indicate that CaM-K II alone is sufficient to augment synaptic strength and that this enhancement shares the same underlying mechanism as the enhancement observed with LTP.

Asynchronous release of synaptic vesicles determines the time course of the AMPA receptor-mediated EPSC

The contribution of intersynaptic transmitter diffusion to the AMPA receptor EPSC time course was studied in cultured CA1 hippocampal neurons. Reducing release probability 20-fold with cadmium did not affect the time course of the averaged AMPA receptor EPSC, even when receptor desensitization was blocked by cyclothiazide, suggesting that individual synapses contribute independently to the AMPA receptor-mediated EPSC. Deconvolution of the averaged miniature EPSC from the evoked EPSC showed that release probability decays only slightly faster than the EPSC, suggesting that the AMPA receptor EPSC time course is determined primarily by the asynchrony of vesicle release. Further experiments demonstrated that cyclothiazide, previously thought to affect only AMPA receptor kinetics, also enhances synaptic release.

Excitatory amino acid induced currents of isolated murine hypothalamic neurons and their suppression by 2,3-butanedione monoxime

Ionic currents induced by excitatory amino acids were investigated for freshly isolated murine hypothalamic neurons with whole cell recording techniques. L-glutamate or N-methyl-D-aspartate (NMDA), in combination with glycine, resulted in a rapidly rising current which decayed in the continued presence of agonist. In contrast, kainate currents did not decay. While quisqualate-induced current maintained a steady amplitude in the continued presence of agonist, a rapid decay phase appeared at holding potentials negative to -50 mV. Co-application of 2,3-butanedione monoxime (BDM) reversibly inhibited the currents due to each agonist. Detailed study of BDM suppression of kainate-induced current revealed two components. A component with a rapid onset did not involve phosphatase action since 500 microM ATP-gamma-S or a protein kinase inhibitor (H-7, 200 microM) did not alter current suppression or recovery after BDM. Thus, the probable mechanism for this component of BDM's effect is direct block of the kainate-activated ion channel. However, preincubating neurons with 30 mM BDM reduced their subsequent response to kainate alone. This persistent effect of BDM was not seen for neurons dialyzed with a solution containing ATP-gamma-S during conventional whole cell recording. Furthermore, exposure to H-7 prevented recovery of the kainate response suppressed by preincubation in BDM. These findings suggest that BDM causes sustained suppression of the kainate response of hypothalamic neurons via a "chemical phosphatase" action.

Alterations of Ca2+/calmodulin-dependent protein kinase II and its messenger RNA in the rat hippocampus following normo- and hypothermic ischemia

The change in the subcellular distribution of Ca2+/calmodulin-dependent protein kinase II was studied in the rat hippocampus following normothermic and hypothermic transient cerebral ischemia of 15 min duration. A decrease in immunostaining of Ca2+/calmodulin-dependent protein kinase II was observed at 1 h of reperfusion which persisted until cell death in the CA1 region. In the CA3 and dentate gyrus areas immunostaining recovered at one to three days of reperfusion. The CA2+/calmodulin-dependent protein kinase II was translocated to synaptic junctions during ischemia and reperfusion which could be due to a persistent change in the intracellular calcium ion homeostasis. The expression of the messenger RNA of the alpha-subunit of Ca2+/calmodulin-dependent protein kinase II decreased in the entire hippocampus during reperfusion, and was most marked in the dentate gyrus at 12 h of reperfusion. This decrease could be a feedback downregulation of the mRNA due to increased Ca2+/calmodulin-dependent protein kinase II activation. Intraischemic hypothermia protected against ischemic neuronal damage and attenuated the ischemia-induced decrease of Ca2+/calmodulin-dependent protein kinase II immunostaining in all hippocampal regions. Hypothermia also reduced the translocation of Ca2+/calmodulin-dependent protein kinase II and restored Ca2+/calmodulin-dependent protein kinase II alpha messenger RNA after ischemia. The data suggest that ischemia leads to an aberrant Ca2+/calmodulin-dependent protein kinase II mediated signal transduction in the CA1 region, which is important for the development of delayed neuronal damage. Hypothermia enhances the restoration of the Ca2+/calmodulin-dependent protein kinase II mediated cell signalling.

Contrasting properties of two forms of long-term potentiation in the hippocampus

Activity-dependent enhancement of synaptic transmission, referred to as long-term potentiation (LTP), is observed at many synapses in the central nervous system. In the hippocampus two distinct forms of LTP have been identified. One involves the activation of the NMDA (N-methyl-D-aspartate) subtype of glutamate receptor and a rise in postsynaptic Ca2+, whereas the other, which is found at mossy fibre synapses, is independent of NMDA receptors but does require a rise in presynaptic Ca2+. Although it is now generally accepted that mossy fibre LTP is expressed presynaptically, the locus of expression for NMDA-receptor-dependent LTP is controversial. Here the two forms of LTP are compared and it is argued that the balance of evidence favours a postsynaptic locus for NMDA-receptor-dependent LTP.

Inability to restore resting intracellular calcium levels as an early indicator of delayed neuronal cell death

The hippocampus is especially vulnerable to excitotoxicity and delayed neuronal cell death. Chronic elevations in free intracellular calcium concentration ([Ca2+]i) following glutamate-induced excitotoxicity have been implicated in contributing to delayed neuronal cell death. However, no direct correlation between delayed cell death and prolonged increases in [Ca2+]i has been determined in mature hippocampal neurons in culture. This investigation was initiated to determine the statistical relationship between delayed neuronal cell death and prolonged increases in [Ca2+]i in mature hippocampal neurons in culture. Using indo-1 confocal fluorescence microscopy, we observed that glutamate induced a rapid increase in [Ca2+]i that persisted after the removal of glutamate. Following excitotoxic glutamate exposure, neurons exhibited prolonged increases in [Ca2+]i, and significant delayed neuronal cell death was observed. The N-methyl-D-aspartate (NMDA) channel antagonist MK-801 blocked the prolonged increases in [Ca2+]i and cell death. Depolarization of neurons with potassium chloride (KCl) resulted in increases in [Ca2+]i, but these increases were buffered immediately upon removal of the KCl, and no cell death occurred. Linear regression analysis revealed a strong correlation (R = 0.973) between glutamate-induced prolonged increases in [Ca2+]i and delayed cell death. These data suggest that excitotoxic glutamate exposure results in an NMDA-induced inability to restore resting [Ca2+]i (IRRC) that is a statistically significant indicator of delayed neuronal cell death.

Transient and persistent phosphorylation of AMPA-type glutamate receptor subunits in cerebellar Purkinje cells

We generated a polyclonal antibody, 12P3, specifically recognizing rat AMPA-type glutamate receptor (GluR) subunits phosphorylated at Ser-696 of GluR2 or at the homologous sites in GluR1, GluR3, and GluR4. Using 12P3, we demonstrate that a brief exposure of a rat cerebellar slice to AMPA leads to transient phosphorylation of the GluR subunits in Purkinje cell dendrites. Persistent phosphorylation over 30 min was obtained when exposure to AMPA was preceded by a 15 min perfusion of the slice with 8-bromo-cGMP, dibutyryl-cGMP, or calyculin A but not phorbol 12,13-diacetate. These results indicate that Ser-696 of GluR2, or the corresponding sites in other AMPA receptor subunits, is a specific site at which phosphorylation takes place when AMPA-type GluRs are activated by agonists, especially under the influence of certain second messenger activities.

Decreased expression of the alpha subunit of Ca2+/ calmodulin-dependent protein kinase type II mRNA in the adult rat CNS following recurrent limbic seizures

Calcium/calmodulin-dependent protein kinase type II (CamKII) is a ubiquitous brain enzyme implicated in a wide variety of neuronal processes. Understanding CamKII has become increasingly complicated with the recent identification of multiple gene transcripts coding for separate subunits. Previous studies have shown that mRNA for the alpha subunit of CamKII can be increased by reduction of afferent input. In this study we have examined the regulation of alpha CamKII mRNA following increased activity due to seizures. Using in situ hybridization with a cRNA probe against the rat alpha CamKII sequence we found reduced levels of hybridization following limbic seizures induced by lesions of the hilus of the dentate gyrus. Hybridization was most dramatically reduced in the granule cells of the dentate gyrus and the pyramidal cells of hippocampal region CA1. There were also significant reductions in hybridization in the superficial layers of neocortex and piriform cortex. In each of these region hybridization was decreased in the molecular layers which is consistent with the reported dendritic localization of alpha CamKII mRNA. All changes in mRNA content were transient, with maximal reductions at 24 h following lesion placement and a return to control levels by 96 h. These findings demonstrate the negative regulation of alpha CamKII mRNA by seizure activity and raise the possibility that synthesis of this kinase may be regulated by normal physiological activity.

Long-term glutamate desensitization in locus coeruleus neurons and its role in opiate withdrawal

During opiate withdrawal, there is an elevated and prolonged efflux of glutamate and aspartate in the locus coeruleus (LC). The enhanced excitatory amino acid (EAA) release is thought to contribute to the withdrawal-induced activation of LC neurons and to the expression of the physical withdrawal syndrome. In this study, prolonged bath applications of glutamate to LC neurons in brain slices resulted in a slowly developing long-term glutamate desensitization (LTGD). LTGD was observed during extracellular recordings or in neurons voltage-clamped to -60mV, in both cases reaching a maximum of about a 50% reduction in the glutamate response. Responses in the desensitized cells gradually recovered within 3 h. Cyclothiazide, an inhibitor of rapid glutamate receptor desensitization did not prevent LTGD. LTGD could not be induced by prolonged applications of EAA agonists other than glutamate, either alone or in various combinations. However, after induction by glutamate, there was cross-desensitization to quisqualate but not to AMPA or NMDA. LTGD was blocked by either lowering extracellular Ca2+ concentrations or by treatment with the protein kinase C inhibitor chelerythrine but not by inhibitors of calcium/calmodulin-dependent kinase or nitric oxide synthase. Applications of the protein kinase C activator phorbol diacetate did not cause a decrease in glutamate responses indicating that an activation of protein kinase C may not be sufficient for desensitization to occur. A decrement of the glutamate response resembling LTGD occurred after treatment by the protein phosphatase inhibitors okadaic acid or calyculin A. LC neurons in brain slices prepared from opiate-withdrawn rats exhibited glutamate responses that were initially desensitized and recovered within 3 h after withdrawal. These results suggest that LTGD in LC neurons may occur during opiate withdrawal and could contribute to the time course of LC hyperactivity and the associated behavioral withdrawal syndrome.

Postsynaptic injection of CA2+/CaM induces synaptic potentiation requiring CaMKII and PKC activity

CA2+-regulated protein kinases play critical roles in long-term potentiation (LTP). To understand the role of Ca2+/calmodulin (CaM) signaling pathways in synaptic transmission better, Ca2+/CaM was injected into hippocampal CA1 neurons. Ca2+/CaM induced significant potentiation of excitatory synaptic responses, which was blocked by coinjection of a CaM-binding peptide and was not induced by injections of Ca2+ or CaM alone. Reciprocal experiments demonstrated that Ca2+/CaM-induced synaptic potentiation and tetanus-induced LTP occluded one another. Pseudosubstrate inhibitors or high-affinity substrates of CaMKII or PKC blocked Ca2/CaM-induced potentiation, indicating the requirement of CaMKII and PKC activities in synaptic potentiation. We suggest that postsynaptic levels of free Ca2+/CaM is a rate limiting factor and that functional cross-talk between Ca2+/CaM and PKC pathways occurs during the induction of LTP.

The biochemistry of learning and memory

An overview of some of the biochemical and molecular events involved in the process of learning and memory are presented in a short review. Two invertebrate models of learning are considered: the gill-withdrawal reflex of Aplysia and avoidance learning in Drosophila melanogaster. Particular attention is paid to the biochemical mechanisms underlying both the development of long-term potentiation (LTP) and passive avoidance learning (PAL) in the young chick. The role of several biological molecules in learning and memory are considered, for example, protein kinase C (PKC), Ca(++)-Calmodulin kinase II (CaMKII), GAP-43, and glutamate receptors.

CaMKII regulates the frequency-response function of hippocampal synapses for the production of both LTD and LTP

To investigate the function of the autophosphorylated form of CaMKII in synaptic plasticity, we generated transgenic mice that express a kinase that is Ca2+ independent as a result of a point mutation of Thr-286 to aspartate, which mimics autophosphorylation. Mice expressing the mutant form of the kinase show an increased level of Ca(2+)-independent CaMKII activity similar to that seen following LTP. The mice nevertheless exhibit normal LTP in response to stimulation at 100 Hz. However, at lower frequencies, in the range of 1-10 Hz, there is a systematic shift in the size and direction of the resulting synaptic change in the transgenic animals that favors LTD. The regulation of this frequency-response function by Ca(2+)-independent CaMKII activity seems to account for two previously unexplained synaptic phenomena, the relative loss of LTD in adult animals compared with juveniles and the enhanced capability for depression of facilitated synapses.

Effect of cerebral ischemia on calcium/calmodulin-dependent protein kinase II activity and phosphorylation

The effects of cerebral ischemia on calcium/calmodulin-dependent kinase II (CaM kinase II) were investigated using the rat four-vessel occlusion model. In agreement with previous results using rat or gerbil models of cerebral ischemia or a rabbit model of spinal cord ischemia, this report demonstrates that transient forebrain ischemia leads to a reduction in CaM kinase II activity within 5 min of occlusion onset. Loss of activity from the cytosol fractions of homogenates from the neocortex, striatum, and hippocampus correlated with a decrease in the amount of CaM kinase alpha and beta isoforms detected by immunoblotting. In contrast, there was an apparent increase in the amount of CaM kinase alpha and beta in the particulate fractions. The decrease in the amount of CaM kinase isoforms from the cytosol but not the particulate fractions was confirmed by autophosphorylation of CaM kinase II after denaturation and renaturation in situ of the blotted proteins. These results indicate that ischemia causes a rapid inhibition of CaM kinase II activity and a change in the partitioning of the enzyme between the cytosol and particulate fractions. CaM kinase II is a multifunctional protein kinase, and the loss of activity may play a critical role in initiating the changes leading to ischemia-induced cell death. To identify a structural basis for the decrease in enzyme activity, tryptic peptide maps of CaM kinase II phosphorylated in vitro were compared. Phosphopeptide maps of CaM kinase alpha from particulate fractions of control and ischemic samples revealed not only reduced incorporation of phosphate into the protein but also the absence of a limited number of peptides in the ischemic samples. This suggested that certain sites are inaccessible, possibly due to a conformational change, a covalent modification of CaM kinase II, or steric hindrance by an associated molecule. Verifying one of these possibilities should help to elucidate the mechanism of ischemia-induced modulation of CaM kinase II.

Interaction of autophosphorylated Ca2+/calmodulin-dependent protein kinase II with neuronal cytoskeletal proteins. Characterization of binding to a 190-kDa postsynaptic density protein

Subcellular localization of Ca2+/calmodulin-dependent protein kinase II (CaMKII) by interaction with specific anchoring proteins may be an important mechanism contributing to the regulation of CaMKII. Proteins capable of binding CaMKII were identified by the use of a gel overlay assay with recombinant mouse CaMKII alpha (mCaMKII alpha) or Xenopus CaMKII beta (xCaMKII beta) 32P-autophosphorylated at Thr286/287 as a probe. Numerous [32P]CaMKII-binding proteins were identified in various whole rat tissue extracts, but binding was most prominent to forebrain proteins of 190 kDa (p190) and 140 kDa (p140). Fractionation of forebrain extracts localized p190 and p140 to a crude particulate/cytoskeletal fraction and isolated postsynaptic densities. [32P]m-CaMKII alpha-bound to p190 with an apparent Kd of 609 nM (subunit concentration) and a Bmax of 7.0 pmol of mCaMKII alpha subunit bound per mg of P2 protein, as measured using the overlay assay. Binding of 100 nM [32P]m-CaMKII alpha to p190 was competed by nonradioactive mCaMKII alpha autophosphorylated on Thr286 (EC50% = 200 nM), but to a much lesser extent by nonradioactive mCaMKII alpha autophosphorylated on Thr306 (EC50% > 2000 nM). In addition, nonphosphorylated mCaMKII alpha was a poor competitor for [32P]mCaMKII alpha binding to p190. The competition data indicate that Ca2+/CaM-dependent autophosphorylation at Thr286 promotes binding to p190, whereas, Ca2+/CaM-independent autophosphorylation at Thr306 does not enhance binding. Therefore, CaMKII may become localized to postsynaptic densities by p190 following its activation by an increase of dendritic Ca2+ concentration.

Calcium signaling in neurons: molecular mechanisms and cellular consequences

Neuronal activity can lead to marked increases in the concentration of cytosolic calcium, which then functions as a second messenger that mediates a wide range of cellular responses. Calcium binds to calmodulin and stimulates the activity of a variety of enzymes, including calcium-calmodulin kinases and calcium-sensitive adenylate cyclases. These enzymes transduce the calcium signal and effect short-term biological responses, such as the modification of synaptic proteins and long-lasting neuronal responses that require changes in gene expression. Recent studies of calcium signal-transduction mechanisms have revealed that, depending on the route of entry into a neuron, calcium differentially affects processes that are central to the development and plasticity of the nervous system, including activity-dependent cell survival, modulation of synaptic strength, and calcium-mediated cell death.

Structural determinants of allosteric regulation in alternatively spliced AMPA receptors

The flip and flop splice variants of AMPA receptors show strikingly different sensitivity to allosteric regulation by cyclothiazide; heteromers assembled from GluR-A and GluR-B also exhibit splice variant-dependent differences in efficacy for activation by glutamate and kainate. The sensitivity for attenuation of desensitization by cyclothiazide for homomeric GluR-A was solely dependent upon exchange of Ser-750 (flip) and Asn-750 (flop), and was unaffected by mutagenesis of other divergent residues. In contrast, substantial alteration of the relative efficacy of glutamate versus kainate required mutation of multiple residues in the flip/flop region. Modulation by cyclothiazide was abolished by mutation of Ser-750 to Gin, the residue found at the homologous site in kainate-preferring subunits, whereas introduction of Ser at this site in GluR6 imparted sensitivity to cyclothiazide.

Unraveling the modular design of glutamate-gated ion channels

Glutamate receptors that function as ligand-gated ion channels are essential components of cell-cell communication in the nervous system. Despite a wealth of information concerning these receptors, details of their structure are just beginning to emerge. We propose that glutamate receptors comprise four modules: two modules that are related to bacterial periplasmic-binding proteins, one module that is related to the pore-forming region of K+ channels, and one regulatory module of unknown origin. A K(+)-channel-like domain inserted into a crucial region of a periplasmic-binding protein-like domain suggests a mechanism for transduction of binding energy to channel opening. This modular design also suggests an evolutionary link between a ligand-gated ion-channel family and voltage-gated ion channels.

Increased phosphorylation of Ca2+/calmodulin-dependent protein kinase II and its endogenous substrates in the induction of long-term potentiation

Induction of long-term potentiation in the CA1 region of hippocampal slices is associated with increased activity of Ca2+/calmodulin-dependent protein kinase II (CaM kinase II) (Fukunaga, K., Stoppini, L., Miyamoto, E., and Muller, D. (1993) J. Biol. Chem. 268, 7863-7867). Here we report that application of high but not low frequency stimulation to two groups of afferents in the CA1 region of 32P-labeled slices resulted in the phosphorylation of two major substrates of this enzyme, synapsin I and microtubule-associated protein 2, as well as in the autophosphorylation of CaM kinase II. Furthermore, immunoblotting analysis revealed that long term potentiation induction was associated with an increase in the amount of CaM kinase II in the same region. All these changes were prevented when high frequency stimulation was applied in the presence of the N-methyl-D-aspartate receptor antagonist, D-2-amino-5-phosphonopentanoate. These results indicate that activation of CaM kinase II is involved in the induction of synaptic potentiation in both the postsynaptic and presynaptic regions.

Identification of a Ca2+/calmodulin-dependent protein kinase II regulatory phosphorylation site in non-N-methyl-D-aspartate glutamate receptors

Glutamate receptor ion channels are colocalized in postsynaptic densities with Ca2+/calmodulin-dependent protein kinase II (CaM-kinase II), which can phosphorylate and strongly enhance non-N-methyl-D-aspartate (NMDA) glutamate receptor current. In this study, CaM-kinase II enhanced kainate currents of expressed glutamate receptor 6 in 293 cells and of wild-type glutamate receptor 1, but not the Ser-627 to Ala mutant, in Xenopus oocytes. A synthetic peptide corresponding to residues 620-638 in GluR1 was phosphorylated in vitro by CaM-kinase II but not by cAMP-dependent protein kinase or protein kinase C. The 32P-labeled peptide map of this synthetic peptide appears to be the same as the two-dimensional peptide map of alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionate (AMPA) glutamate receptors phosphorylated in cultured hippocampal neurons by CaM-kinase II described elsewhere. This CaM-kinase II regulatory phosphorylation site is conserved in all AMPA/kainate-type glutamate receptors, and its phosphorylation may be important in enhancing postsynaptic responsiveness as occurs during synaptic plasticity.

Topology profile for a glutamate receptor: three transmembrane domains and a channel-lining reentrant membrane loop

We investigated the transmembrane topology of the GluR3 subunit that was translated in rabbit reticulocytes supplemented with microsomal membranes. A prolactin reporter epitope was fused to GluR3 at six locations, bracketing each of the proposed transmembrane domains. The sidedness of the epitope in the microsomal membrane was then assessed by proteinase K sensitivity. The N terminus and the entire region between M3 and M4 was extracellular, and the C terminus was intracellular by this method. Four native N-linked glycosylation sites in the amino terminus and one introduced site between M3 and M4 were utilized, confirming the extracellular location of these regions. Epitopes inserted upstream and downstream of M2 were protease sensitive and thus intracellular. Our results support a topological model for glutamate receptor subunits that consists of three transmembrane domains, M1, M3, and M4, and another domain, the proposed channel-lining M2, which forms a reentrant membrane segment with both ends facing the cytoplasm.

Are NMDA or AMPA/kainate receptor antagonists more efficacious in the delayed treatment of excitotoxic neuronal injury?

At which time-point and to what extent do N-methyl-D-aspartate (NMDA) receptors, alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid (AMPA)/kainate receptors and L-type voltage-sensitive Ca2+ channels (VSCC) contribute to glutamate-induced neuronal injury? To address this question, we induced glutamate neurotoxicity in two neuronal culture systems, chick telencephalic neurons and rat hippocampal neurons, and tested selective antagonists for their neuroprotective activity when administered either during the excitotoxic insult (acute treatment) or during the recovery period (posttreatment). In cultured chick telencephalic neurons exposed to 1 mM L-glutamate for 60 min, both the NMDA receptor antagonist dizocilpine (MK-801; 0.1 microM) and the AMPA/kainate receptor antagonist 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX; 1 microM) completely blocked glutamate-induced neuronal injury when applied concomitantly with glutamate. If the antagonists were applied during the recovery period, dizocilpine at concentrations up to 10 microM only moderately increased cell viability, whereas CNQX showed a neuroprotective activity comparable to that observed in the case of the acute treatment. In cultured rat hippocampal neurons, excitotoxic injury was induced by a 30-min exposure to 1 microM glutamate. Treatment with dizocilpine during the glutamate exposure could rescue the hippocampal neurons from the excitotoxic insult, whereas acute treatment with the AMPA/kainate receptor antagonist 2,3-dihydroxy-6-nitro-7-sulfamoylbenzo(F)-quinoxaline (NBQX) or the L-type VSCC blocker nimodipine showed no protection. In contrast, all three drugs showed neuroprotective activity when applied 30, 60 or 120 min after the glutamate exposure. Surprisingly, when the onset of the treatment was delayed for even 240 min, only NBQX and nimodipine led to a reduction in excitotoxic neuronal injury. We conclude that activation of AMPA/kainate receptors and L-type VSCC is critically involved in a late stage of glutamate neurotoxicity, thereby allowing pharmacological intervention at a time when blockade of NMDA receptors becomes less efficacious.

Decoding Ca2+ signals to the nucleus by multifunctional CaM kinase

Multifunctional Ca2+/calmodulin-dependent protein kinase (CaM kinase) is one of the major protein kinases coordinating cellular responses to neurotransmitters and hormones. CaM kinase transduces changes in intracellular free Ca2+ into changes in the phosphorylation state and activity of target proteins involved in neurotransmitter synthesis and release, neuronal plasticity and gene expression. Structure/function analyses of the kinase reveal the kinase is kept inactive in its basal state by a regulatory domain that is displaced by the binding of Ca2+/calmodulin. Once activated by Ca2+/calmodulin, autophosphorylation occurs if a pair of proximate subunits of the decameric kinase have calmodulin bound. The frequency of Ca2+ oscillations or spikes may be decoded by CaM kinase via this autophosphorylation. Calmodulin is essentially trapped by autophosphorylation which converts CaM kinase into a high affinity calmodulin-binding protein. Repetitive stimulation of the kinase may promote recruitment of calmodulin to the kinase so that it becomes increasingly active with each stimulus in a frequency-dependent manner. The association domain at the C-terminal end of CaM kinase contains a variable region that targets isoforms of the kinase to the nucleus or cytoskeleton and assembles the kinase into a decameric structure. Alternative splicing introduces a short nuclear localization signal that targets transfected kinase to the nucleus where it may regulate nuclear functions. The regulatory properties of CaM kinase provide for molecular potentiation of Ca2+ signals and frequency detection whereas its association domain should enable it to decode such Ca2+ fluctuations in the nucleus.

Potentiated transmission and prevention of further LTP by increased CaMKII activity in postsynaptic hippocampal slice neurons

Calcium-calmodulin-dependent protein kinase II (CaMKII) is a necessary component of the cellular machinery underlying learning and memory. Here, a constitutively active form of this enzyme, CaMKII(1-290), was introduced into neurons of hippocampal slices with a recombinant vaccinia virus to test the hypothesis that increased postsynaptic activity of this enzyme is sufficient to produce long-term synaptic potentiation (LTP), a prominent cellular model of learning and memory. Postsynaptic expression of CaMKII(1-290) increased CaMKII activity, enhanced synaptic transmission, and prevented more potentiation by an LTP-inducing protocol. These results, together with previous studies, suggest that postsynaptic CaMKII activity is necessary and sufficient to generate LTP.