LTP mechanisms: from silence to four-lane traffic

Hippocampal long term potentiation: silent synapses and beyond

Long-term, activity-driven synaptic plasticity allows neuronal networks to constantly and durably adjust synaptic gains between synaptic partners. These processes have been proposed to serve as a substrate for learning and memory. Long-term synaptic potentiation (LTP) has been observed at many central excitatory synapses and perhaps most extensively studied at Schaffer collaterals synapses onto hippocampal CA1 neurons. Multiple contradictory models were proposed to account for this form of LTP. However, recent evidence suggests that some synapses are initially devoid of functional AMPA receptors which can be incorporated during LTP. This new model appears to account for most, but not all, properties of this form of plasticity. Indeed, several mechanisms seem to act in parallel to specifically enhance AMPA-receptor mediated synaptic transmission.

Local control of AMPA receptor trafficking at the postsynaptic terminal by a small GTPase of the Rab family

The delivery of neurotransmitter receptors into the synaptic membrane is essential for synaptic function and plasticity. However, the molecular mechanisms of these specialized trafficking events and their integration with the intracellular membrane transport machinery are virtually unknown. Here, we have investigated the role of the Rab family of membrane sorting proteins in the late stages of receptor trafficking into the postsynaptic membrane. We have identified Rab8, a vesicular transport protein associated with trans-Golgi network membranes, as a critical component of the cellular machinery that delivers AMPA-type glutamatergic receptors (AMPARs) into synapses. Using electron microscopic techniques, we have found that Rab8 is localized in close proximity to the synaptic membrane, including the postsynaptic density. Electrophysiological studies indicated that Rab8 is necessary for the synaptic delivery of AMPARs during plasticity (long-term potentiation) and during constitutive receptor cycling. In addition, Rab8 is required for AMPAR delivery into the spine surface, but not for receptor transport from the dendritic shaft into the spine compartment or for delivery into the dendritic surface. Therefore, Rab8 specifically drives the local delivery of AMPARs into synapses. These results demonstrate a new role for the cellular secretory machinery in the control of synaptic function and plasticity directly at the postsynaptic membrane.

AMPA receptor regulation mechanisms: future target for safer neuroprotective drugs

The post-synaptic AMPA receptors play an important role in mediating fast excitatory transmission in the mammalian brain. Over-activated AMPA receptors induce excitotoxicity, implicated in a number of Chronic neurodegenerative disorders such as Parkinson's disease, Huntington's disease, and AIDS encephalitis. AMPA receptor antagonists offer protection against neurodegeneration in the experimental models even if they are given 24 h after the injury. Because AMPA receptors seem to be involved in the neurodegenerative diseases, modulating the activity of the AMPA receptors could be an attractive approach to reduce or prevent excitotoxicity. Studies conducted recently have exhibited a number of new mechanisms for AMPA receptor regulation. Modulations of these were found to have protective implications. AMPA receptor depolarization and desensitization are protective to the neurons. Receptor desensitization depends on the receptor subunit composition. The R/G editing site and the flip/flop cassettes in AMPA receptor subunits contribute to a great extent in receptor desensitization and recovery rates. Molecules that could quicken receptor desensitization or delay recovery could be of use. AMPA receptors limit neuronal entry of Ca2+ ions by regulating Ca2+-permeability. Ca2+-permeable receptor channels are made up of GluR1, GluR3, or GluR4 subunits, whereas presence of the GluR2 subunit restricts Ca2+ entry and renders the receptor Ca2+-impermeable. GluR2 levels, however, experience a fall after neuronal insult rendering the AMPA receptors Ca2+-permeable, thus factors that could interfere with this event might prove to be very beneficial against excitotoxicity. AMPA receptor clusters are stabilized by PSD-95, which requires palmitoylation at two sites. Targeting palmitoylation of the PSD-95 can also be a useful approach to disperse AMPA clusters at the synapse. In the perisynaptic region, mGluRs are present a little away from the synapse and are among the glutamate transporters, which require high-frequency firing for activation. On activation they might enhance the activity of NMDA receptors at the synapse to increase the levels of AMPA receptors. AMPA receptors surfaced at this juncture can contribute to heavy Ca2+ influx. Thus, blocking this pathway could be of considerable importance in preventing the excitotoxicity. A number of proteins such as the GRIP, PICK, and NSF also modulate the functions of AMPA receptors. Polyamines also block Ca2+ permeable AMPA receptors and thus are protective. NO and cGMP also play an important role in negatively regulating AMPA receptors and thus could offer protection. Modulation of AMPA receptor by different mechanisms has been discussed in the present review to implicate importance of these targets/pathways for safer and future neuroprotective drugs.

Removal of AMPA receptors (AMPARs) from synapses is preceded by transient endocytosis of extrasynaptic AMPARs

AMPA receptors (AMPARs) are dynamically regulated at synapses, but the time course and location of their exocytosis and endocytosis are not known. Therefore, we have used ecliptic pHluorin-tagged glutamate receptor 2 to visualize changes in AMPAR surface expression in real time. We show that synaptic and extrasynaptic AMPARs respond very differently to NMDA receptor activation; there is a rapid internalization of extrasynaptic AMPARs that precedes the delayed removal of synaptic AMPARs.

Stress generates emotional memories and retrograde amnesia by inducing an endogenous form of hippocampal LTP

Models of the neurobiology of memory have been based on the idea that information is stored as distributed patterns of altered synaptic weights in neuronal networks. Accordingly, studies have shown that post-training treatments that alter synaptic weights, such as the induction of long-term potentiation (LTP), can interfere with retrieval. In these studies, LTP induction has been relegated to the status of a methodological procedure that serves the sole purpose of disturbing synaptic activity in order to impair memory. This perspective has been expressed, for example, by Martin and Morris (2002: Hippocampus 12:609-636), who noted that post-training LTP impairs memory by adding "behaviorally meaningless" noise to hippocampal neural networks. However, if LTP truly is a memory storage mechanism, its induction should represent more than just a means with which to disrupt memory. Since LTP induction produces retrograde amnesia, the formation of a new memory should also produce retrograde amnesia. In the present report, we suggest that one type of learning experience, the storage of fear-related (i.e., stressful) memories, is consistent with this prediction. Studies have shown that stress produces potent effects on hippocampal physiology, generates long-lasting memories, and induces retrograde amnesia, all through mechanisms in common with LTP. Based on these findings, we have developed the hypothesis that a stressful experience generates an endogenous form of hippocampal LTP that substitutes a new memory representation for preexisting representations. In summary, our hypothesis implicates the induction of endogenous synaptic plasticity by stress in the formation of emotional memories and in retrograde amnesia.

Changes in AMPA receptor currents following LTP induction on rat CA1 pyramidal neurones

In the CA1 region of the hippocampus, LTP is thought to be initiated by a transient activation of NMDA receptors and is expressed as a persistent increase in synaptic transmission through AMPA receptors. To investigate the postsynaptic modifications of AMPA receptors involved in this enhanced synaptic transmission, the channel density and single-channel properties of extrasynaptic AMPA receptors located in synaptically active dendritic regions were examined following the induction of LTP. Following tetanic stimulation an outside-out patch was excised from the apical dendrite near the point of stimulation and saturating concentrations of glutamate were rapidly applied to the patch. AMPA current amplitude and duration were increased significantly in patches pulled from dendrites that expressed LTP. Non-stationary fluctuation analysis of AMPA currents indicated that AMPA channel number was nearly twofold larger than in controls, while single channel conductance and maximum open-probability were unchanged. Furthermore, while subtle changes in AMPA channel kinetics could also be observed, we did not find any evidence that receptor affinity or rectification properties were altered by LTP induction. Very similar results were found when CaMK-II activity was increased through the intracellular application of Ca/CaM. Together, we interpret our data to indicate that the stimuli used here produce an increased delivery of AMPA receptors to synaptically active regions of the apical dendrite without inducing any significant changes in their basic biophysical properties and that such delivery is a key element in this form of synaptic plasticity.

Biologic perspective on early erotic development

Neurobiologic researchers can understand children's sexuality in less moral and more biologic terms. Genetically programmed levels of neurotransmitters and hormones establish a baseline trajectory of erotic interest and activity across the lifespan. Environmental influences also contribute. Intense early stimulation can affect the brain and create a condition of hyper eroticism, whereas too little stimulation can limit the ability to bond and impair the sexual response. Children who are erotically challenged or challenging are viewed correctly as having a brain imbalance, rather than as victims or as being morally deficient. This should pave the way for more humane, objective, and effective interventions.

Synaptic and Molecular Mechanisms Regulating Plasticity during Early Learning

Many behaviors are learned most easily during a discrete developmental period, and it is generally agreed that these "sensitive periods" for learning reflect the developmental regulation of molecular or synaptic properties that underlie experience-dependent changes in neural organization and function. Avian song learning provides one example of such temporally restricted learning, and several features of this behavior and its underlying neural circuitry make it a powerful model for studying how early experience sculpts neural and behavioral organization. Here we describe evidence that within the basal ganglia-thalamocortical loop implicated in vocal learning, song acquisition engages N-methyl-d-aspartate receptors (NMDARs), as well as signal transduction cascades strongly implicated in other instances of learning. Furthermore, NMDAR phenotype changes in parallel with developmental and seasonal periods for vocal plasticity. We also review recent studies in the avian song system that challenge the popular notion that sensitive periods for learning reflect developmental changes in the NMDAR that alter thresholds for synaptic plasticity.

Independent functions of hsp90 in neurotransmitter release and in the continuous synaptic cycling of AMPA receptors

The delivery of neurotransmitter receptors into synapses is essential for synaptic function and plasticity. In particular, AMPA-type glutamate receptors (AMPA receptors) reach excitatory synapses according to two distinct routes: a regulated pathway, which operates transiently during synaptic plasticity, and a constitutive pathway, which maintains synaptic function under conditions of basal transmission. However, the specific mechanisms that distinguish these two trafficking pathways are essentially unknown. Here, we evaluate the role of the molecular chaperone hsp90 (heat shock protein 90) in excitatory synaptic transmission in the hippocampus. On one hand, we found that hsp90 is necessary for the efficient neurotransmitter release at the presynaptic terminal. In addition, we identified hsp90 as a critical component of the cellular machinery that delivers AMPA receptors into the postsynaptic membrane. Using the hsp90-specific inhibitors radicicol and geldanamycin, we show that hsp90 is required for the constitutive trafficking of AMPA receptors into synapses during their continuous cycling between synaptic and nonsynaptic sites. In contrast, hsp90 function is not required for either the surface delivery of AMPA receptors into the nonsynaptic plasma membrane or for the acute, regulated delivery of AMPA receptors into synapses during plasticity induction (long-term potentiation). The synaptic cycling of AMPA receptors was also blocked by an hsp90-binding tetratricopeptide repeat (TPR) domain, suggesting that the role of hsp90 in AMPA receptor trafficking is mediated by a TPR domain-containing protein. These results demonstrate new roles for hsp90 in synaptic function by controlling neurotransmitter release and, independently, by mediating the continuous cycling of synaptic AMPA receptors.

NMDA receptors and voltage-dependent calcium channels mediate different aspects of acquisition and retention of a spatial memory task

Activity dependent calcium entry into neurons can initiate a form of synaptic plasticity called long-term potentiation (LTP). This phenomenon is considered by many to be one possible cellular mechanism underlying learning and memory. The calcium entry that induces this phenomenon can occur when N-methyl-D-aspartate receptors (NMDARs) and/or voltage-dependent calcium channels (VDCCs) are activated. While much is known about synaptic plasticity and the mechanisms that are triggered by activation of these two Ca(2+) channels, it is unclear what roles they play in learning. To better understand the role activation of these channels may play in learning we systemically administered pharmacological antagonists to block NMDARs, VDCCs, or both during training trials and retention tests in a radial arm maze task. Wistar rats injected with the NMDAR antagonist MK-801 (0.1mg/kg) were impaired in the acquisition of this task. In contrast, rats injected with verapamil (10mg/kg), an antagonist to VDCCs, acquired the task at the same rate as control animals, but were impaired on a 10-day retention test. A group of animals injected with both antagonists were unable to learn the task. The results suggest that each of the calcium channels and the processes they trigger are involved in a different stage of memory formation or expression.

Single spine Ca2+ signals evoked by coincident EPSPs and backpropagating action potentials in spiny stellate cells of layer 4 in the juvenile rat somatosensory barrel cortex

The precise timing of presynaptic and postsynaptic activity results in synaptic modifications, which depend on calcium influx. [Ca2+] transients in the spines of spiny neurons in layer 4 (L4) of the somatosensory barrel cortex of young rats were investigated in thalamocortical brain slices by two-photon excitation microscopy to determine the spike timing dependence of the Ca2+ signal during near-coincident presynaptic and postsynaptic activity. [Ca2+] transients evoked by backpropagating action potentials (bAPs) were mediated by voltage-dependent Ca2+ channels and were of comparable size in a spine and adjacent dendritic shaft. They decreased with the distance of the spine from the soma. EPSP-evoked [Ca2+] transients were restricted to spine heads and were mediated almost entirely by Ca2+ influx through NMDA receptors (NMDARs). Their amplitude was independent of the position of the spine along the dendritic arbor. bAPs interacted with EPSPs to generate sublinear or supralinear Ca2+ signals in a spine when EPSP and bAP occurred within a time window of 50 msec. Synaptic stimulation, coincident with a bAP, evoked a large postsynaptic Ca2+ influx that was restricted to a single spine, even after EPSPs were blocked by the AMPA receptor antagonist NBQX that rendered synapses effectively "electrically silent." We conclude that the spines of L4 cells can act as sharply tuned detectors for patterns of APs occurring in the boutons of the afferents to L4 cells and the spines of L4 cell dendrites. The readout for near-coincident presynaptic and postsynaptic APs is a large transient Ca2+ influx into synaptically active spines mediated by the brief unblocking of NMDARs during the dendritic bAP.

Learning modifies subsequent induction of long-term potentiation-like and long-term depression-like plasticity in human motor cortex

Learning may alter rapidly the output organization of adult motor cortex. It is a long-held hypothesis that modification of synaptic strength along cortical horizontal connections through long-term potentiation (LTP) and long-term depression (LTD) forms one important mechanism for learning-induced cortical plasticity. Strong evidence in favor of this hypothesis was provided for rat primary motor cortex (M1) by showing that motor learning reduced subsequent LTP but increased LTD. Whether a similar relationship exists in humans is unknown. Here, we induced LTP-like and LTD-like plasticity in the intact human M1 by an established paired associative stimulation (PAS) protocol. PAS consisted of 200 pairs of electrical stimulation of the right median nerve, followed by focal transcranial magnetic stimulation of the hand area of the left M1 at an interval equaling the individual N20 latency of the median nerve somatosensory-evoked cortical potential (PAS(N20)) or N20-5 msec (PAS(N20-5)). PAS(N20) induced reproducibly a LTP-like long-lasting (>30 min) increase in motor-evoked potentials from the left M1 to a thumb abductor muscle of the right hand, whereas PAS(N20-5) induced a LTD-like decrease. Repeated fastest possible thumb abduction movements resulted in learning, defined by an increase in maximum peak acceleration of the practiced movements, and prevented subsequent PAS(N20)-induced LTP-like plasticity but enhanced subsequent PAS(N20-5)-induced LTD-like plasticity. The same number of repeated slow thumb abduction movements did not result in learning and had no effects on PAS-induced plasticity. Findings support the view that learning in human M1 occurs through LTP-like mechanisms.

Synaptic structural modification following changes in activity induced by tetanus neurotoxin in cat abducens neurons

A low or a high dose of tetanus neurotoxin (TeNT) injected in the lateral rectus muscle of the cat causes respectively, functional block of inhibitory synapses only or of both inhibitory and excitatory synapses simultaneously in abducens neurons (González-Forero et al. [2003] J. Neurophysiol. 89:1878-1890). As a consequence, neuronal firing activity increases (at low dose) or decreases (at high dose). We investigated possible structural modifications of inhibitory synapses in response to these activity alterations induced by TeNT. We used immunofluorescence against postsynaptic (gephyrin) and presynaptic (vesicular gamma-aminobutyric acid [GABA] transporter [VGAT]) markers of inhibitory synapses in combination with cell type markers for abducens motoneurons (calcitonin gene-related peptide or choline acetyltransferase) or internuclear neurons (calretinin). Seven days after high-dose treatment, the number of gephyrin-immunoreactive (IR) clusters per 100 microm of membrane perimeter was reduced on the soma of abducens motoneurons by 55.3% and by 60.1% on internuclear neurons. Proximal dendritic gephyrin-IR clusters were also significantly altered but to a lesser degree. Partial synaptic re-establishment was observed 15 days post injection, and complete recovery occurred after 42 days. Coverage by VGAT-IR terminals was reduced in parallel. In contrast, a low dose of TeNT caused no structural alterations. With electron microscopy we estimated that overall synaptic coverage was reduced by 40% in both types of neurons after a high dose of TeNT. However, F-type terminals with postsynaptic gephyrin were preferentially lost. Thus, the ratio between F and S terminals diminished from 1.28 to 0.39 on motoneurons and from 1.26 to 0.47 on internuclear neurons. These results suggest that the maintenance of proximal inhibitory synaptic organization on central neurons is best related to neuronal activity and not to the level of inhibitory synaptic function, which was equally blocked by the high or low dose of TeNT.

Orchestration of synaptic plasticity through AKAP signaling complexes

Significant progress has been made toward understanding the mechanisms by which organisms learn from experiences and how those experiences are translated into memories. Advances in molecular, electrophysiological and genetic technologies have permitted great strides in identifying biochemical and structural changes that occur at synapses during processes that are thought to underlie learning and memory. Cellular events that generate the second messenger cyclic AMP (cAMP) and activate protein kinase A (PKA) have been linked to synaptic plasticity and long-term memory. In this review we will focus on the role of PKA in synaptic plasticity and discuss how the compartmentalization of PKA through its association with A-Kinase Anchoring Proteins (AKAPs) affect PKA function in this process.

Postsynaptic density 95 controls AMPA receptor incorporation during long-term potentiation and experience-driven synaptic plasticity

The regulated delivery of AMPA-type glutamate receptors (AMPARs) to synapses is an important mechanism underlying synaptic plasticity. Here, we ask whether the synaptic scaffolding protein PSD-95 (postsynaptic density 95) participates in AMPAR incorporation during two forms of synaptic plasticity. In hippocampal slice cultures, the expression of PSD-95-green fluorescent protein (PSD-95-GFP) increases AMPAR currents by selectively delivering glutamate receptor 1 (GluR1)-containing receptors to synapses, thus mimicking long-term potentiation (LTP). Mutational analysis shows that the N terminal of PSD-95 including the first two PDZ [PSD-95/Discs large (Dlg)/zona occludens-1 (ZO-1)] domains is necessary and sufficient to mediate this effect. Further supporting a role in synaptic plasticity, wild-type PSD-95 occludes LTP and dominant negative forms block LTP. Moreover, we demonstrate that PSD-95 also participates in AMPAR delivery during experience-driven plasticity in vivo. In the barrel cortex from experience-deprived animals, the expression of PSD-95-GFP selectively increases AMPAR currents, mimicking experience-driven plasticity. In nondeprived animals, PSD-95-GFP produces no additional potentiation, indicating common mechanisms between PSD-95-mediated potentiation and experience-driven synaptic strengthening. A dominant negative form of PSD-95 blocks experience-driven potentiation of synapses. Pharmacological analysis in slice cultures reveals that PSD-95 acts downstream of other signaling pathways involved in LTP. We conclude that PSD-95 controls activity-dependent AMPAR incorporation at synapses via PDZ interactions not only during LTP in vitro but also during experience-driven synaptic strengthening by natural stimuli in vivo.

Silent synapses in the immature visual cortex: layer-specific developmental regulation

Central glutamatergic synapses are thought to initially form as immature, so-called silent synapses showing exclusively N-methyl-d-aspartate receptor-mediated synaptic transmission. Postsynaptic insertion of alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptors during further development leads to a conversion into functional, mature synapses. Here, we tested the hypothesis that, according to the "inside first-outside last" pattern of neocortical layer formation and synaptogenesis, pyramidal cells in the superficial layers might show a higher fraction of silent synapses compared with pyramidal cells in the deep layers. We performed an electrophysiological analysis of glutamatergic synapses in acute rat visual cortex slices during postnatal development. In layer VI pyramidal neurons the incidence of silent synapses was high during the first postnatal week and strongly declined during further development. Surprisingly, in superficial cortical plate pyramidal neurons (immature layers II/III), the fraction of silent synapses was initially very low and increased up to the second postnatal week. Thereafter, a similar decline as found in layer VI pyramidal neurons was observed. Thus the developmental regulation of silent synapses was clearly different in pyramidal neurons from different neocortical layers. The almost complete absence of silent synapses at early stages in layer II/III pyramidal neurons indicates that an initially formed subset of synapses is constitutively functional. This might be important to enable spontaneous activity and latter activity-dependent maturation of synapses.

A proportional but slower NMDA potentiation follows AMPA potentiation in LTP

Most excitatory glutamatergic synapses contain both AMPA and NMDA receptors, but whether these receptors are regulated together or independently during synaptic plasticity has been controversial. Although long-term potentiation (LTP) is thought to selectively enhance AMPA currents and alter the NMDA-to-AMPA ratio, this ratio is well conserved across synapses onto the same neuron. This suggests that the NMDA-to-AMPA ratio is only transiently perturbed by LTP. To test this, we induced LTP at rat neocortical synapses and recorded mixed AMPA-NMDA currents. We observed rapid LTP of AMPA currents, as well as delayed potentiation of NMDA currents that required previous AMPA potentiation. The delayed potentiation of NMDA currents restored the original NMDA-to-AMPA ratio within 2 h of LTP induction. These data suggest that recruitment of AMPA receptors to synapses eventually induces a proportional increase in NMDA current. This may ensure that LTP does not alter the relative contributions of these two receptors to synaptic transmission and information processing.

Dynamic interaction of stargazin-like TARPs with cycling AMPA receptors at synapses

Activity-dependent plasticity in the brain arises in part from changes in the number of synaptic AMPA receptors. Synaptic trafficking of AMPA receptors is controlled by stargazin and homologous transmembrane AMPA receptor regulatory proteins (TARPs). We found that TARPs were stable at the plasma membrane, whereas AMPA receptors were internalized in a glutamate-regulated manner. Interaction with AMPA receptors involved both extra- and intracellular determinants of TARPs. Upon binding to glutamate, AMPA receptors detached from TARPs. This did not require ion flux or intracellular second messengers. This allosteric mechanism for AMPA receptor dissociation from TARPs may participate in glutamate-mediated internalization of receptors in synaptic plasticity.

Synaptic activity-independent persistent plasticity in endogenously active mammalian motoneurons

Potentiation and depression of glutamate receptor function in hippocampal, cerebellar, and cortical neurons are examples of persistent changes in synaptic function that underlie important behavioral adaptations such as learning and memory. Persistent changes in synaptic function relevant for motor behaviors have not been demonstrated in mammalian motoneurons. We demonstrate that adaptive changes in (+/-)-alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid hydrobromide (AMPA) receptor function at endogenously active synapses occur in motoneurons in neonatal rodents. We found a form of serotonin (5-HT)-dependent synaptic plasticity in hypoglossal (XII) motoneurons, which control tongue muscles affecting upper airway function, that is metamodulated by metabotropic glutamate receptors. Episodic, but not continuous, activation of postsynaptic 5-HT type 2 (5-HT(2)) receptors on hypoglossal (XII) motoneurons leads to long-lasting increases in their AMPA receptor-mediated respiratory drive currents and associated XII nerve motor output. Antagonism of group-I metabotropic glutamate receptors blocks induction of the 5-HT-induced increase in excitability. We propose that this activity-independent postsynaptic 5-HT-mediated plasticity represents the cellular mechanism underlying long-term facilitation, i.e., persistent increases in respiratory motor output and ventilation seen in humans and rodents in response to episodic hypoxia. Loss of activity in XII motoneurons is common during sleep causing snoring and, in serious cases, airway obstruction that interrupts breathing, a condition known as obstructive sleep apnea. These results may provide the basis for rationale development of therapeutics for obstructive sleep apnea in humans.

Interferon-gamma induces characteristics of central sensitization in spinal dorsal horn neurons in vitro

Hyperexcitability of spinal dorsal horn neurons, also known as 'central sensitization', is a component of pain associated with pathological conditions in the nervous system. The aim of the present study was to analyze if the pro-inflammatory cytokine, interferon-gamma (IFN-gamma), which can be released for extended periods of time in the nervous system during inflammatory and infectious events, can alter synaptic activity in dorsal horn neurons and thereby contribute to such hyperexcitability. Treatment of cultured dorsal horn neurons with IFN-gamma for 2 weeks resulted in a significantly reduced clustering of alpha-amino-3-hydroxy-5-methylisoxazole (AMPA) receptor subunit 1 (GluR1) that was dependent on nitric oxide. The neurons displayed an increased frequency and amplitude of excitatory postsynaptic currents (EPSCs) upon IFN-gamma treatment. Treated dorsal horn neurons also exhibited increased responsiveness to stimulation of dorsal root ganglia (DRG) axons in a two-compartment model. Furthermore, disinhibition by the GABA(A) receptor antagonist picrotoxin (PTX) significantly increased EPSC frequency and induced bursting in untreated cultures but did not significantly increase the frequency in treated neurons, which displayed bursting even without PTX. GABA(A) agonists reduced activity more strongly in treated cultures and immunochemical staining for GABA(A) receptors showed no difference from controls. Since GluR1-containing AMPA receptors (AMPARs) occur predominantly on inhibitory neurons in the dorsal horn, we suggest that the IFN-gamma-mediated increase in spontaneous activity and responsiveness to DRG axon stimulation, decrease in sensitivity to PTX and tendency for EPSC bursting result from a reduced expression of GluR1 on these neurons and not from a reduction in active GABA(A) receptors in the network. IFN-gamma thereby likely causes disinhibition of synaptic activity and primary afferent input in the dorsal horn, which consequently results in central sensitization.

Glutamatergic plasticity by synaptic delivery of GluR-B(long)-containing AMPA receptors

Activity-driven delivery of AMPA receptors is proposed to mediate glutamatergic synaptic plasticity, both during development and learning. In hippocampal CA1 principal neurons, such trafficking is primarily mediated by the abundant GluR-A subunit. We now report a study of GluR-B(long), a C-terminal splice variant of the GluR-B subunit. GluR-B(long) synaptic delivery is regulated by two forms of activity. Spontaneous synaptic activity-driven GluR-B(long) transport maintains one-third of the steady-state AMPA receptor-mediated responses, while GluR-B(long) delivery following the induction of LTP is responsible for approximately 50% of the resulting potentiation at the hippocampal CA3 to CA1 synapses at the time of GluR-B(long) peak expression-the second postnatal week. Trafficking of GluR-B(long)-containing receptors thus mediates a GluR-A-independent form of glutamatergic synaptic plasticity in the juvenile hippocampus.

Coordinated PKA and PKC phosphorylation suppresses RXR-mediated ER retention and regulates the surface delivery of NMDA receptors

Endoplasmic reticulum (ER) retention mediated by the RXR (Arg-X-Arg) motif is an important quality control mechanism used by G-protein coupled receptors and ion channels, including N-methyl-D-aspartate (NMDA) receptors, to ensure the proper assembly and trafficking of multimeric complexes. During assembly, RXR motifs are masked by intersubunit interactions thereby allowing ER release. Here, we find that PKA and PKC phosphorylation sites flanking the RXR motif of the NMDA receptor NR1 subunit suppress ER retention and regulate receptor forward trafficking. These sites are differentially phosphorylated during the trafficking of NR1 subunits in vivo, and phosphorylation at these sites occurs in early secretory compartments. In addition, residues near the RXR motif not involved in phosphorylation are also required for ER retention. These results indicate that ER retention of NMDA receptors is tightly regulated, and suggest that coordinated phosphorylation by PKA and PKC mediates release of receptors from the ER for subsequent traffic to synapses. Phosphorylation-induced ER export of RXR-containing channels and receptors may serve as a novel quality control mechanism for creating a readily releasable pool of receptors sensitive to the activation of intracellular signaling pathways.

Targeting of GLUR4-containing AMPA receptors to synaptic sites during in vitro classical conditioning

The synaptic delivery of GluR4-containing AMPA receptors during in vitro classical conditioning of a neural correlate of an eyeblink response was examined by fluorescence imaging of punctate staining for glutamate receptor subunits and the presynaptic marker synaptophysin. There was a significant increase in GluR4-containing AMPA receptors to synaptic sites after conditioning as determined by colocalization of GluR4 subunit puncta with synaptophysin. Moreover, the trafficking of these receptor subunits requires NMDA receptor activation as it was blocked by D,L-2-amino-5-phosphonovaleric acid (AP-5). In contrast, colocalization of NR1 subunits with synaptophysin was stable regardless of whether the preparations had undergone conditioning or had been treated by AP-5. The enhanced colocalization of GluR4 and synaptophysin was accompanied by an increase in both the total number and size of puncta for both proteins, suggesting greater synthesis and aggregation during conditioning. Western blot analysis confirmed upregulation of synaptophysin and GluR4 following conditioning. These data support the hypothesis that GluR4-containing AMPA receptors are delivered to synaptic sites during conditioning. Further, they suggest coordinate presynaptic and postsynaptic modifications during in vitro classical conditioning.

In vivo evidence for an activity-independent maturation of AMPA/NMDA signaling in the developing hippocampus

Correlated pre- and postsynaptic activity is thought to promote maturation of excitatory synapses in the developing brain by directing AMPA receptors to pure NMDA synapses. However, this hypothesis has not been tested in vivo. Here, we have performed such test by inhibiting correlated neural activity in vivo using a single injection of tetanus toxin into the rat hippocampal CA1 area at postnatal day 1. When examined in the acute slice preparation (1-7 days post-injection), there was a strong reduction, down to 20% of control level, in the frequency of glutamatergic and GABAergic spontaneous postsynaptic currents (sPSCs). This activity deprivation led to a growth retardation of CA1 pyramidal neurons and to markedly faster decay kinetics of NMDA sPCSs. However, it did not alter the relationship between AMPA and NMDA sPSCs with respect to either their frequency or amplitude. Thus, although critical for certain aspects of neuronal development, correlated neural activity in the neonatal hippocampus does not seem to promote incorporation of AMPA receptors at pure NMDA synapses.

Aberrant formation of glutamate receptor complexes in hippocampal neurons of mice lacking the GluR2 AMPA receptor subunit

The number and type of receptors present at the postsynaptic membrane determine the response to the neurotransmitter released from the presynaptic terminal. Because most neurons receive multiple and distinct synaptic inputs and contain several different subtypes of receptors stimulated by the same neurotransmitter, the assembly and trafficking of receptors in neurons is a complex process involving many levels of regulation. To investigate the mechanism that neurons use to regulate the assembly of receptor subunits, we studied a GluR2 knock-out mouse. GluR2 is a critical subunit that controls calcium permeability of AMPA receptors and is present in most native AMPA receptors. Our data indicate that in the absence of GluR2, aberrant receptor complexes composed of GluR1 and GluR3 are formed in the hippocampus, and that there is an increased number of homomeric GluR1 and GluR3 receptors. We also show that these homomeric and heteromeric receptors are less efficiently expressed at the synapse. Our results show that GluR2 plays a critical role in controlling the assembly of AMPA receptors, and that the assembly of subunits may reflect the affinity of one subunit for another or the stability of intermediates in the assembly process. Therefore, GluR1 may have a greater preference for GluR2 than it does for GluR3.

Spinal glutamate receptor antagonists differentiate primary and secondary mechanical hyperalgesia caused by incision

Secondary mechanical hyperalgesia has been demonstrated in postoperative patients indicating that central sensitization occurs after surgery. However, the underlying mechanisms are unknown. Here, we studied the role of spinal N-methyl-D-aspartate and alpha-amino-3-hydroxy-5-methyl-4-isoxazole-propionic acid (AMPA)/kainate receptors for pain behaviors indicating secondary hyperalgesia caused by gastrocnemius incision in the rat. We further determined if Ca(2+) permeable AMPA/kainate receptors are important for secondary hyperalgesia after gastrocnemius incision and for pain behaviors indicating primary hyperalgesia and guarding behavior after plantar incision. Withdrawal thresholds (WTs) to punctate mechanical stimuli were assessed by applying calibrated monofilaments to the plantar hind paw before gastrocnemius incision. WTs were tested again 2 h after gastrocnemius incision and again after intrathecal (IT) injection of either dizocilpine maleate (MK-801), 2-amino-5-phosphonovaleric acid (AP5), 1,2,3,4-tetrahydro-6-nitro-2,3-dioxo[f]quinoxaline-7-sulfonamide (NBQX), or Joro spider toxin (JSTX). The doses used were: MK-801 (vehicle, 15, 30, 40 nmol), AP5 (vehicle, 10, 30 nmol), NBQX (vehicle, 5, 10 nmol), and JSTX (vehicle, 2, 5, 9 nmol). In the same rats, WTs were tested on postoperative day 2 before and after the same drugs were injected again. In other rats, WTs to monofilaments and response frequencies to a non-punctate mechanical stimulus or guarding behaviors were determined before, 1 h after plantar incision was made, and assessed again after JSTX (9 nmol or vehicle) was administered IT. Secondary mechanical hyperalgesia after gastrocnemius incision was dose-dependently blocked by NBQX but was only marginally affected by AP5 or MK-801. Only secondary mechanical hyperalgesia was reversed by JSTX; primary mechanical hyperalgesia and guarding behavior were unchanged. These results indicate that spinal sensitization contributing to behaviors for secondary hyperalgesia after incision requires Ca(2+) permeable AMPA/kainate receptors. The data further demonstrate that behaviors for secondary mechanical hyperalgesia after incision can be inhibited without affecting behaviors for primary mechanical hyperalgesia and guarding. Mechanisms for central sensitization causing secondary hyperalgesia in postoperative patients may therefore be separated from spontaneous pain and hyperalgesia that arises adjacent to the area of the incision.

Glutamate receptor subunit 2 Serine 880 phosphorylation modulates synaptic transmission and mediates plasticity in CA1 pyramidal cells

The cytoplasmic C termini of AMPA receptor subunits contain PDZ (postsynaptic density 95/Discs large/zona occludens 1) ligand domains that can control their synaptic trafficking during plasticity. The glutamate receptor subunit 2 (GluR2) PDZ ligand domain can be phosphorylated at serine 880 (S880), and this disrupts interactions with GRIP/ABP (glutamate receptor-interacting protein/AMPA-binding protein) but not with PICK1 (PKC-interacting protein 1). Here, the impact of GluR2 S880 phosphorylation on synaptic transmission and plasticity was explored by expressing, in hippocampal slice cultures, GluR2 subunits containing point mutations that mimic or prevent phosphorylation at this residue. Our results indicate that mimicking GluR2 S880 phosphorylation excludes these receptors from synapses, depresses transmission, and partially occludes long-term depression (LTD). Conversely, mutations that prevent phosphorylation reduce LTD. Disruption of the interaction between GluR2 and GRIP/ABP by S880 phosphorylation may thus facilitate removal of synaptic AMPA receptors and mediate some forms of activity-dependent synaptic depression.

Interaction of transmembrane AMPA receptor regulatory proteins with multiple membrane associated guanylate kinases

Surface expression of AMPA type glutamate receptors requires stargazin or a related transmembrane AMPA receptor regulatory protein (TARP). Furthermore, interaction of the cytosolic tail of TARPs with PDZ domains of PSD-95 targets AMPA receptors to postsynaptic densities. Here, we screened for additional proteins that might interact with the cytosolic domain of TARPs. Screening a rat brain cDNA library with the yeast two-hybrid system yielded six PDZ proteins that can bind tail of TARPs. These PDZ proteins include the four neuronal membrane associated guanylate kinases, PSD-95/SAP-90, PSD-93/Chapsyn-110, SAP-97/hDLG and SAP-102; the multi-PDZ protein, MUPP1; and the mitochondrial PDZ protein, OMP-25. Although all of these proteins can bind to TARPs in vitro, only two of these, PSD-95 and PSD-93 associate with TARPs in brain. We also found that all three PDZ domains from PSD-95 associate with the TARP C-termini with similar affinities. This work identifies biochemical promiscuity for interaction of the TARP C-termini with PDZ domains in vitro, but also shows that only specific synaptic PDZ proteins associate with TARPs in brain.

A model synapse that incorporates the properties of short- and long-term synaptic plasticity

We propose a general computer model of a synapse, which incorporates mechanisms responsible for the realization of both short- and long-term synaptic plasticity-the two forms of experimentally observed plasticity that seem to be very significant for the performance of neuronal networks. The model consists of a presynaptic part based on the earlier 'double barrier synapse' model, and a postsynaptic compartment which is connected to the presynaptic terminal via a feedback, the sign and magnitude of which depend on postsynaptic Ca(2+) concentration. The feedback increases or decreases the amount of neurotransmitter which is in a ready for release state. The model adequately reproduced the phenomena of short- and long-term plasticity observed experimentally in hippocampal slices for CA3-CA1 synapses. The proposed model may be used in the investigation of certain real synapses to estimate their physiological parameters, and in the construction of realistic neuronal networks.

Molecular mechanisms of calcium-dependent neurodegeneration in excitotoxicity

Excitotoxicity contributes to neuronal degeneration in many acute CNS diseases, including ischemia, trauma, and epilepsy, and may also play a role in chronic diseases, such as amyotrophic lateral sclerosis (ALS). Key mediators of excitotoxic damage are Ca ions (Ca(2+)), which under physiological conditions govern a multitude of cellular processes, including cell growth, differentiation, and synaptic activity. Consequently, homeostatic mechanisms exist to maintain a low intracellular Ca(2+) ion concentration so that Ca(2+) signals remain spatially and temporally localized. This permits multiple independent Ca-mediated signaling pathways to occur in the same cell. In excitotoxicity, excessive synaptic release of glutamate can lead to the disregulation of Ca(2+) homeostasis. Glutamate activates postsynaptic receptors, including the ionotropic N-methyl-D-aspartate (NMDA), 2-amino-3-(3-hydroxy-5-methylisoxazol-4-yl) proprionate (AMPA), and kainate receptors. Upon their activation, these open their associated ion channel to allow the influx of Ca(2+) and Na(+) ions. Although physiological elevations in intracellular Ca(2+) are salient to normal cell functioning, the excessive influx of Ca(2+) together with any Ca(2+) release from intracellular compartments can overwhelm Ca(2+)-regulatory mechanisms and lead to cell death. Although Ca(2+) disregulation is paramount to neurodegeneration, the exact mechanism by which Ca(2+) ions actually mediate excitotoxicity is less clear. One hypothesis outlined in this review suggests that Ca(2+)-dependent neurotoxicity occurs following the activation of distinct signaling cascades downstream from key points of Ca(2+) entry at synapses, and that triggers of these cascades are physically co-localized with specific glutamate receptors. Thus, we summarize the importance of Ca(2+) regulation in mammalian neurons and the excitotoxicity hypothesis, and focus on the molecular determinants of glutamate receptor-mediated excitotoxic mechanisms.

Trafficking of NMDA receptors

The NMDA receptor (NMDAR) plays a central role in the function of excitatory synapses. Recent studies have provided interesting insights into several aspects of the trafficking of this receptor in neurons. The NMDAR is not a static resident of the synapse. Rather, the number and composition of synaptic NMDARs can be modulated by several factors. The interaction of PDZ proteins, generally thought to occur at the synapse, appears to occur early in the secretory pathway; this interaction may play a role in the assembly of the receptor complex and its exit from the endoplasmic reticulum. This review addresses recent advances in our understanding of NMDAR trafficking and its synaptic delivery and maintenance.

Stargazin differentially controls the trafficking of alpha-amino-3-hydroxyl-5-methyl-4-isoxazolepropionate and kainate receptors

Synaptic plasticity at excitatory synapses in the brain is largely achieved by rapid changes in the number of synaptic alpha-amino-3-hydroxyl-5-methyl-4-isoxazolepropionate (AMPA) receptors. Stargazin, a membrane protein that interacts with AMPA receptors, is believed to play a pivotal role in trafficking AMPA receptors to the plasma membrane and targeting them to the synapse. However, it is unclear whether the trafficking of kainate receptors, which are structurally very similar to AMPA receptors, is also dependent on stargazin. Here we show that in both cerebellar granule cells and in Xenopus laevis oocytes expression system, surface delivery of kainate receptor is independent of stargazin. These results suggest that stargazin action is highly selective for AMPA receptors.

Long-term depression of presynaptic release from the readily releasable vesicle pool induced by NMDA receptor-dependent retrograde nitric oxide

Postsynaptic alterations are currently believed to be able to fully account for NMDA-receptor-dependent long-term depression (LTD) and long-term potentiation of synaptic strength, although there is also evidence supporting changes in presynaptic release. Using dualphoton laser scan microscopy of N-(3-triethylammoniumpropyl)-4-(4-(dibutylamino)styryl) pyridinium dibromide (FM1-43) to directly visualize presynaptic vesicular release at Schaffer collateral-CA1 excitatory synapses in hippocampal slices, we demonstrate reduced vesicular release associated with LTD. Selective loading, by hypertonic shock, of the readily releasable vesicle pool (RRP) showed that LTD of release is a selective modification of release from the RRP. Presynaptic LTD of RRP release required activation of NMDA receptors, production and extracellular diffusion of the intercellular messenger NO, and activation of cGMP-dependent protein kinase.

Real-time imaging of alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptor (AMPA receptor) movements in neurons

The mechanisms that regulate alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptor (AMPAR) synthesis, transport, targeting and surface expression are of fundamental importance for fast excitatory neurotransmission and synaptic plasticity in the mammalian central nervous system. It has become apparent that these control processes involve complex sets of protein-protein interactions and many of the proteins responsible have been identified. We have been working to visualize AMPAR movement in living neurons in order to investigate the effects of blocking protein interactions. Here we outline the approaches used and the results obtained thus far.

Role for calbindin-D28K in in vitro classical conditioning of abducens nerve responses in turtles

Intracellular calcium has a pivotal role in synaptic modifications that may underlie learning and memory. The present study examined whether there were changes in immunoreactivity levels of the AMPA receptor subunits GluR2/3 and calcium binding proteins during classical conditioning recorded in the abducens nerve of in vitro brain stem preparations from turtles. The results showed that abducens motor neurons in unconditioned turtle brain stems were immunopositive for GluR2/3, calbindin-D28K, and calmodulin, but were immunonegative for parvalbumin. After classical conditioning, immunoreactivity for calbindin-D28K in the abducens motor nuclei was significantly reduced, whereas there were no significant changes in GluR2/3, calmodulin, or parvalbumin. This reduction in calbindin-D28K immunoreactivity was not observed following conditioning in the NMDA receptor antagonist AP-5, which blocked conditioned responses, suggesting that these changes are NMDA receptor-dependent. Moreover, the degree of the decrease in calbindin-D28K immunoreactivity was negatively correlated with the level of conditioning. Consistent with the immunocytochemical findings, Western blot analysis showed that calbindin-D28K protein levels were reduced after classical conditioning. The results support the hypothesis that in vitro classical conditioning of abducens nerve responses utilizes intracellular calcium-dependent signaling pathways that require NMDA receptor function and suggest a specific role for the calcium binding protein calbindin-D28K.

Control of synaptic strength, a novel function of Akt

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

Partial reversal of the effect of maternal care on cognitive function through environmental enrichment

Maternal care influences hippocampal development in the rat. The offspring of mothers that exhibit increased levels of pup licking/grooming and arched-back nursing (High LG-ABN mothers) show increased hippocampal N-methyl-D-aspartate (NMDA) receptor binding and enhanced hippocampal-dependent spatial learning. In these studies we examined whether environmental enrichment from days 22-70 of life might reverse the effects of low maternal care. Environmental enrichment eliminated the differences between the offspring of High and Low LG-ABN mothers in both Morris water maze learning and object recognition. However, enrichment did not reverse the effect of maternal care on long-term potentiation in the dentate gyrus or on hippocampal NMDA receptor binding. In contrast, peripubertal enrichment did reverse the effects of maternal care on hippocampal alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptor binding. These findings provide evidence for the reversal of the effects of reduced maternal investment in early life on cognitive function in adulthood. Such effects might involve compensatory changes associated with peripubertal enrichment.

Functional studies and distribution define a family of transmembrane AMPA receptor regulatory proteins

Functional expression of alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptors in cerebellar granule cells requires stargazin, a member of a large family of four-pass transmembrane proteins. Here, we define a family of transmembrane AMPA receptor regulatory proteins (TARPs), which comprise stargazin, gamma-3, gamma-4, and gamma-8, but not related proteins, that mediate surface expression of AMPA receptors. TARPs exhibit discrete and complementary patterns of expression in both neurons and glia in the developing and mature central nervous system. In brain regions that express multiple isoforms, such as cerebral cortex, TARP-AMPA receptor complexes are strictly segregated, suggesting distinct roles for TARP isoforms. TARPs interact with AMPA receptors at the postsynaptic density, and surface expression of mature AMPA receptors requires a TARP. These studies indicate a general role for TARPs in controlling synaptic AMPA receptors throughout the central nervous system.

Activation of PI3-kinase is required for AMPA receptor insertion during LTP of mEPSCs in cultured hippocampal neurons

Hippocampal CA1 homosynaptic long-term potentiation (LTP) is expressed specifically at activated synapses. Increased insertion of postsynaptic alpha-amino-3-hydroxy-5-methyl-isoxazole-4-propionic acid receptors (AMPARs) appears to be crucial for CA1 LTP. However, the mechanism underlying AMPAR insertion during LTP remains largely unknown. We now report that phosphatidylinositol 3-kinase (PI3K) is complexed with AMPARs at synapses and activated by selective stimulation of synaptic N-methyl-D-aspartate (NMDA) receptors. Activation of the AMPAR-associated PI3K is required for the increased cell surface expression of AMPARs and LTP. Thus, our results strongly suggest that the AMPAR-PI3K complex may constitute a critical molecular signal responsible for AMPAR insertion at activated CA1 synapses during LTP, and consequently, this lipid kinase may serve to determine the polarity of NMDA receptor-dependent synaptic plasticity.

Hippocampal LTP is accompanied by enhanced F-actin content within the dendritic spine that is essential for late LTP maintenance in vivo

The dendritic spine is an important site of neuronal plasticity and contains extremely high levels of cytoskeletal actin. However, the dynamics of the actin cytoskeleton during synaptic plasticity and its in vivo function remain unclear. Here we used an in vivo dentate gyrus LTP model to show that LTP induction is associated with actin cytoskeletal reorganization characterized by a long-lasting increase in F-actin content within dendritic spines. This increase in F-actin content is dependent on NMDA receptor activation and involves the inactivation of actin depolymerizing factor/cofilin. Inhibition of actin polymerization with latrunculin A impaired late phase of LTP without affecting the initial amplitude and early maintenance of LTP. These observations suggest that mechanisms regulating the spine actin cytoskeleton contribute to the persistence of LTP.

Expression mechanisms underlying long-term potentiation: a postsynaptic view

This review summarizes the various experiments that have been carried out to determine if the expression of long-term potentiation (LTP), in particular N-methyl-D-aspartate (NMDA) receptor-dependent LTP, is presynaptic or postsynaptic. Evidence for a presynaptic expression mechanism comes primarily from experiments reporting that glutamate overflow is increased during LTP and from experiments showing that the failure rate decreases during LTP. However, other experimental approaches, such as monitoring synaptic glutamate release by recording astrocytic glutamate transporter currents, have failed to detect any change in glutamate release during LTP. In addition, the discovery of silent synapses, in which LTP rapidly switches on alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptor function at NMDA-receptor-only synapses, provides a postsynaptic mechanism for the decrease in failures during LTP. It is argued that the preponderance of evidence favours a postsynaptic expression mechanism, whereby NMDA receptor activation results in the rapid recruitment of AMPA receptors as well as a covalent modification of synaptic AMPA receptors.

Synaptic plasticity in the amygdala: comparisons with hippocampus

Long-term potentiation (LTP) is a widely studied form of synaptic plasticity, and a considerable amount of evidence indicates that it could be involved in learning and memory. Intensive investigation of this phenomenon in the hippocampus has yielded tremendous insight into the workings of synapses in the mammalian central nervous system, but important questions remain to be answered. The most important of these are: (1) whether LTP is the basis of learning and memory, and (2) how similar are the induction, maintenance, and expression mechanisms in the rest of the brain to those in the hippocampus. Because the most important strategy for linking LTP to learning involves disrupting the mechanisms of LTP and examining the consequences on behavior, it is likely that the first question cannot be answered until the second has been addressed. Recent evidence indicates that although the general processes have much in common, significant differences exist among forebrain structures, including the hippocampus, basolateral amygdala, and ventral striatum. It is clear that the roles of receptors and calcium channels, kinases, and transcription factors vary within these structures, reflecting the different functions of these brain regions.

Tyrosine dephosphorylation and ethanol inhibition of N-Methyl-D-aspartate receptor function

The inhibitory effect of ethanol on N-methyl-d-aspartate receptors (NMDARs) is well documented in several brain regions. However, the molecular mechanisms by which ethanol affects NMDARs are not well understood. In contrast to the inhibitory effect of ethanol, phosphorylation of the NMDAR potentiates channel currents (Lu, W. Y., Xiong, Z. G., Lei, S., Orser, B. A., Dudek, E., Browning, M. D., and MacDonald, J. F. (1999) Nat. Neurosci. 2, 331-338). We have previously shown that protein kinase C activators induce tyrosine phosphorylation and potentiation of the NMDAR (Grosshans, D. R., Clayton, D. R., Coultrap, S. J., and Browning, M. D. (2002) Nat. Neurosci. 5, 27-33). We therefore hypothesized that the ethanol inhibition of NMDARs might be due to changes in tyrosine phosphorylation of NMDAR subunits. In support of this hypothesis, we found that tyrosine phosphorylation of both NR2A and NR2B subunits was significantly reduced following in situ exposure of hippocampal slices to 100 mm ethanol. Specifically, phosphorylation of tyrosine 1472 on NR2B was reduced 23.5%. These data suggest a possible mechanism by which ethanol may inhibit the NMDAR via activation of a tyrosine phosphatase. Electrophysiological studies demonstrated that ethanol inhibited NMDAR field excitatory postsynaptic potential slope and amplitude to a similar degree as previously reported by our laboratory and others (Schummers, J., Bentz, S., and Browning, M. D. (1997) Alcohol Clin. Exp. Res. 21, 404-408). Inclusion of bpV(phen), a potent phosphotyrosine phosphatase inhibitor, in the recording chamber prior to and during ethanol exposure significantly reduced the inhibitory effect of ethanol on NMDAR field excitatory postsynaptic potentials. Taken together, these data suggest that phosphatase-mediated dephosphorylation of NMDAR subunits may play an important role in mediating the inhibitory effects of ethanol on the N-methyl-D-aspartate receptor.

APP processing and synaptic function

A large body of evidence has implicated Abeta peptides and other derivatives of the amyloid precursor protein (APP) as central to the pathogenesis of Alzheimer's disease (AD). However, the functional relationship of APP and its proteolytic derivatives to neuronal electrophysiology is not known. Here, we show that neuronal activity modulates the formation and secretion of Abeta peptides in hippocampal slice neurons that overexpress APP. In turn, Abeta selectively depresses excitatory synaptic transmission onto neurons that overexpress APP, as well as nearby neurons that do not. This depression depends on NMDA-R activity and can be reversed by blockade of neuronal activity. Synaptic depression from excessive Abeta could contribute to cognitive decline during early AD. In addition, we propose that activity-dependent modulation of endogenous Abeta production may normally participate in a negative feedback that could keep neuronal hyperactivity in check. Disruption of this feedback system could contribute to disease progression in AD.

Proteins interactions implicated in AMPA receptor trafficking: a clear destination and an improving route map

The mechanisms that regulate alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid receptor (AMPAR), synthesis, transport, targeting and surface expression are of fundamental importance to understand the molecular basis of fast excitatory neurotransmission and synaptic plasticity in the mammalian CNS. An area of intense current interest is how AMPARs are directed to the correct locations in the neuron as and when required. This is a multi-layered problem, which involves complex spatio-temporal coordination of multiple protein interactions. Considerable progress has been achieved in identifying a number of proteins that bind directly to AMPAR subunits and the functional consequences of blocking some of these interactions have been determined. This review highlights recent developments in the field.

Silence analysis of AMPA receptor mutated at the CaM-kinase II phosphorylation site

Direct phosphorylation of the GluR1 subunit of postsynaptic AMPA receptors by Ca(2+)/calmodulin-dependent protein kinase II (CaM-KII) is believed to be one of the major contributors to the enhanced strength of glutamatergic synapses in CA1 area of hippocampus during long-term potentiation. The molecular mechanism of AMPA receptor regulation by CaM-KII is examined here by a novel approach, silence analysis, which is independent of previously used variance analysis. I show that three fundamental channel properties-single-channel conductance, channel open probability, and the number of functional channels-can be measured in an alternative way, by analyzing the probability of channels to be simultaneously closed (silent). Validity of the approach was confirmed by modeling, and silence analysis was applied then to the GluR1 AMPA receptor mutated at S831, the site phosphorylated by CaM-KII during long-term potentiation. Silence analysis indicates that a negative charge at S831 is a critical determinant for the enhanced channel function as a charge carrier. Silence and variance analyses, when applied to the same sets of data, were in agreement on the receptor regulation upon mutations. These results provide independent evidences for the mechanism of AMPA receptor regulation by CaM-KII and further strengthens the idea how calcium-dependent phosphorylation of AMPA receptors can contribute to the plasticity at central glutamatergic synapses.