Postsynaptic NMDA Receptors Research Articles

The importance of N-methyl-d-aspartate (NMDA) receptors in subtraction of electrosensory reafference in the dorsal nucleus of skates

The dorsal nucleus of the little skate is a cerebellum-like sensory structure that adaptively filters out predictable electrosensory inputs. The filter's plasticity is mediated by anti-Hebbian associative depression at the synapses between parallel fibers and ascending efferent neurons (AENs). Changes in synaptic strength are indicated by the formation of a cancellation signal which is initiated by co-activation of parallel fibers and AENs, and can be reversed by parallel fiber activity in the absence of AEN activation. In other cerebellum-like sensory structures, the formation of the cancellation signal requires activation of postsynaptic NMDA receptors on the principal neurons. We demonstrate here by immunohistochemistry that the somas and the initial portion of both apical and basal dendrites of the AENs are labeled with antibodies raised against the NR1 subunit of NMDA receptors from a South American electric fish. In in vivo physiological experiments, we show that the formation of the cancellation signal induced by coupling an electrosensory stimulus to ventilatory movements or direct parallel fiber stimulation is blocked when either of the NMDA receptor antagonists 2-amino-5-phosphonovaleric acid (APV) or MK801 is injected into the molecular layer above the recorded AEN. Blocking NMDA receptors prevented formation of a cancellation signal in 79% (15/19; APV) and 60% (3/5; MK801) of the AENs. This blockage was reversible in 40% (6/15) of the AENs after APV removal. Thus, in the dorsal nucleus, the activity-dependent, long-lasting but reversible change in synaptic strength of the parallel fiber-AEN synapses appears to be an NMDA receptor-dependent process.

Input-specific synaptic plasticity in the amygdala is regulated by neuroligin-1 via postsynaptic NMDA receptors

Despite considerable evidence for a critical role of neuroligin-1 in the specification of excitatory synapses, the cellular mechanisms and physiological roles of neuroligin-1 in mature neural circuits are poorly understood. In mutant mice deficient in neuroligin-1, or adult rats in which neuroligin-1 was depleted, we have found that neuroligin-1 stabilizes the NMDA receptors residing in the postsynaptic membrane of amygdala principal neurons, which allows for a normal range of NMDA receptor-mediated synaptic transmission. We observed marked decreases in NMDA receptor-mediated synaptic currents at afferent inputs to the amygdala of neuroligin-1 knockout mice. However, the knockout mice exhibited a significant impairment in spike-timing-dependent long-term potentiation (STD-LTP) at the thalamic but not the cortical inputs to the amygdala. Subsequent electrophysiological analyses indicated that STD-LTP in the cortical pathway is largely independent of activation of postsynaptic NMDA receptors. These findings suggest that neuroligin-1 can modulate, in a pathway-specific manner, synaptic plasticity in the amygdala circuits of adult animals, likely by regulating the abundance of postsynaptic NMDA receptors.

Aktuelle Erkenntnisse zur Pathophysiologie des CRPS I

Knowledge about the pathophysiology underlying the complex regional pain syndrome (CRPS) has increased over the last years. Classically, CRPS has been considered to be mainly driven by sympathetic dysfunction with sympathetically maintained pain being its major pathogenetic mechanism. Currently, the disease is understood as result of a complex interplay between altered somatosensory, motor, autonomic and inflammatory systems. Peripheral and central sensitization is a common feature in CRPS as in other neuropathic pain syndromes. One important mechanism is the sensitization of spinal dorsal horn cells via activation of postsynaptic NMDA-receptors by chronic C-fiber input. Differential activity of endogenous pain modulating systems may play a pivotal role in the development of CRPS, too. Neuronal plasticity of the somatosensory cortex accounts for central sensory signs. Also the motor system is subject to central adaptive changes in patients with CRPS. Calcitonin-gene related peptide (CGRP) and substance P mediate neurogenic inflammation. Additionally other proinflammatory cytokines involved in the inflammatory response in CRPS have been identified. In terms of the sympathetic nervous system, recent evidence rather points to a sensitization of adrenergic receptors than to increased efferent sympathetic activity. Particularly the expression of alpha (1)-adrenoceptors on nociceptive C-fibers may play a major role. These pathophysiological ideas do not exclude each other. In fact they complement one another. The variety of the involved systems may explain the versatile clinical picture of CRPS.

Enhancement of Learning and Memory by Elevating Brain Magnesium

Learning and memory are fundamental brain functions affected by dietary and environmental factors. Here, we show that increasing brain magnesium using a newly developed magnesium compound (magnesium-L-threonate, MgT) leads to the enhancement of learning abilities, working memory, and short- and long-term memory in rats. The pattern completion ability was also improved in aged rats. MgT-treated rats had higher density of synaptophysin-/synaptobrevin-positive puncta in DG and CA1 subregions of hippocampus that were correlated with memory improvement. Functionally, magnesium increased the number of functional presynaptic release sites, while it reduced their release probability. The resultant synaptic reconfiguration enabled selective enhancement of synaptic transmission for burst inputs. Coupled with concurrent upregulation of NR2B-containing NMDA receptors and its downstream signaling, synaptic plasticity induced by correlated inputs was enhanced. Our findings suggest that an increase in brain magnesium enhances both short-term synaptic facilitation and long-term potentiation and improves learning and memory functions.

Effects of 17β-estradiol on glutamate synaptic transmission and neuronal excitability in the rat medial vestibular nuclei

We investigated the effects of the neurosteroid 17β-estradiol (E2) on the evoked and spontaneous activity of rat medial vestibular nucleus (MVN) neurons in brainstem slices. E2 enhances the synaptic response to vestibular nerve stimulation in type B neurons and depresses the spontaneous discharge in both type A and B neurons. The amplitude of the field potential, as well as the excitatory post-synaptic potential (EPSP) and current (EPSC), in type B neurons, are enhanced by E2. Both effects are long-term phenomena since they outlast the drug washout. The enhancement of synaptic response is mainly due to facilitation of glutamate release mediated by pre-synaptic n-methyl-d-aspartate receptors (NMDARs), since the reduction of paired pulse ratio (PPR) and the increase of miniature EPSC frequency after E2 are abolished under D-(−)-2-amino-5-phosphonopentanoic acid (AP-5). E2 also facilitates post-synaptic NMDARs, but it does not affect directly α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptors (AMPARs) and group I-metabotropic glutamate receptors (mGluRs-I). In contrast, the depression of the spontaneous discharge of type A and type B neurons appears to depend on E2 modulation of intrinsic ion conductances, as the effect remains after blockade of glutamate, GABA and glycine receptors (GlyRs). The net effect of E2 is to enhance the signal-to-noise ratio of the synaptic response in type B neurons, relative to resting activity of all MVN neurons. These findings provide evidence for a novel potential mechanism to modulate the responsiveness of vestibular neurons to afferent inputs, and so regulate vestibular function in vivo.

Properties of glutamatergic synapses in immature layer Vb pyramidal neurons: coupling of pre- and postsynaptic maturational states

Following initial contact formation, glutamatergic synapses in cortical neurons undergo pronounced functional maturation. These maturational events, occurring both pre- and postsynaptically, have been well described in the developing hippocampus. In this paper, we characterized glutamatergic synapses in immature layer Vb pyramidal neurons of the mouse somatosensory cortex during early postnatal development. At postnatal day 7, a significant subpopulation of glutamatergic synapses exhibited a low release probability that was accompanied by strong paired-pulse facilitation of AMPA EPSCs (paired-pulse ratio C > or = 2). Increasing extracellular Ca(2+) concentration increased release probability and led to paired-pulse depression. During further postnatal development, these functionally immature synapses disappeared. As shown pharmacologically,these synapses expressed postsynaptic NMDA receptors containing NR2B subunits, while NMDA receptors with NR2A subunits were lacking. Taken together, a low release probability presynaptically was coupled to postsynaptic NR2B signaling. This subpopulation of neocortical synapses thus differed from the majority of synapses in the developing hippocampus, where high release probability is coupled to NR2B signaling. The novel type of functionally immature glutamatergic synapse described here might play an important role in early developmental synapse elimination and in the activity-dependent refinement of the neocortical synaptic microcircuitry.

Fragile X mental retardation protein is required for chemically‐induced long‐term potentiation of the hippocampus in adult mice

Fragile X syndrome (FXS), a common form of inherited mental retardation, is caused by the lack of fragile X mental retardation protein (FMRP). The animal model of FXS, Fmr1 knockout mice, have deficits in the Morris water maze and trace fear memory tests, showing impairment in hippocampus-dependent learning and memory. However, results for synaptic long-term potentiation (LTP), a key cellular model for learning and memory, remain inconclusive in the hippocampus of Fmr1 knockout mice. Here, we demonstrate that FMRP is required for glycine induced LTP (Gly-LTP) in the CA1 of hippocampus. This form of LTP requires activation of post-synaptic NMDA receptors and metabotropic glutamateric receptors, as well as the subsequent activation of extracellular signal-regulated kinase (ERK) 1/2. However, paired-pulse facilitation was not affected by glycine treatment. Genetic deletion of FMRP interrupted the phosphorylation of ERK1/2, suggesting the possible role of FMRP in the regulation of the activity of ERK1/2. Our study provide strong evidences that FMRP participates in Gly-LTP in the hippocampus by regulating the phosphorylation of ERK1/2, and that improper regulation of these signaling pathways may contribute to the learning and memory deficits observed in FXS.

Endogenous nonneuronal modulators of synaptic transmission control cortical slow oscillations in vivo.

Gliotransmission, the release of molecules from astrocytes, regulates neuronal excitability and synaptic transmission in situ. Whether this process affects neuronal network activity in vivo is not known. Using a combination of astrocyte-specific molecular genetics, with in vivo electrophysiology and pharmacology, we determined that gliotransmission modulates cortical slow oscillations, a rhythm characterizing nonrapid eye movement sleep. Inhibition of gliotransmission by the expression of a dominant negative SNARE domain in astrocytes affected cortical slow oscillations, reducing the duration of neuronal depolarizations and causing prolonged hyperpolarizations. These network effects result from the astrocytic modulation of intracortical synaptic transmission at two sites: a hypofunction of postsynaptic NMDA receptors, and by reducing extracellular adenosine, a loss of tonic A1 receptor-mediated inhibition. These results demonstrate that rhythmic brain activity is generated by the coordinated action of the neuronal and glial networks.

Presynaptic NR2A-containing NMDA receptors implement a high-pass filter synaptic plasticity rule

The detailed characterization of synaptic plasticity has led to the replacement of simple Hebbian rules by more complex rules depending on the order of presynaptic and postsynaptic action potentials. Here, we describe a mechanism endowing a plasticity rule with additional computational complexity--a dependence on the pattern of presynaptic action potentials. The classical Hebbian rule is based on detection of conjunctive presynaptic and postsynaptic activity by postsynaptic NMDA receptors, but there is also accumulating evidence for the existence of presynaptic NMDA receptors in several brain structures. Here, we examine the role of presynaptic NMDA receptors in defining the temporal structure of the plasticity rule governing induction of long-term depression (LTD) at the cerebellar parallel fiber-Purkinje cell synapse. We show that multiple presynaptic action potentials at frequencies between 40 Hz and 1 kHz are necessary for LTD induction. We characterize the subtype, kinetics, and role of presynaptic NMDA receptors involved in the induction of LTD, showing how the kinetics of the NR2A subunits expressed by parallel fibers implement a high-pass filter plasticity rule that will selectively attenuate synapses undergoing high-frequency bursts of activity. Depending on the type of NMDA receptor subunit expressed, high-pass filters of different corner frequencies could be implemented at other synapses expressing NMDA autoreceptors.

Postsynaptic Mechanisms Govern the Differential Excitation of Cortical Neurons by Thalamic Inputs

Thalamocortical (TC) afferents relay sensory input to the cortex by making synapses onto both excitatory regular-spiking principal cells (RS cells) and inhibitory fast-spiking interneurons (FS cells). This divergence plays a crucial role in coordinating excitation with inhibition during the earliest steps of somatosensory processing in the cortex. Although the same TC afferents contact both FS and RS cells, FS cells receive larger and faster excitatory inputs from individual TC afferents. Here, we show that this larger thalamic excitation of FS cells occurs via GluR2-lacking AMPA receptors (AMPARs), and results from a fourfold larger quantal amplitude compared with the thalamic inputs onto RS cells. Thalamic afferents also activate NMDA receptors (NMDARs) at synapses onto both cells types, yet RS cell NMDAR currents are slower and pass more current at physiological membrane potentials. Because of these synaptic specializations, GluR2-lacking AMPARs selectively maintain feedforward inhibition of RS cells, whereas NMDARs contribute to the spiking of RS cells and hence to cortical recurrent excitation. Thus, thalamic afferent activity diverges into two routes that rely on unique complements of postsynaptic AMPARs and NMDARs to orchestrate the dynamic balance of excitation and inhibition as sensory input enters the cortex.

Saffron extract and trans-crocetin inhibit the glutamatergic synaptic transmission on rat cortical neurones

Crocus sativus L. is a small perennial plant from the Iridaceae family. The stigmata, commonly known as saffron, were used over centuries as a spice and dye but also as a medicinal plant. It has been shown that an ethanolic (80 vol.-%) saffron extract and trans-crocetin, a carotinoid from saffron, interact with the phencyclidine binding site of the NMDA receptor. The aim of the present study was to examine the influence of the ethanolic saffron extract CSE and trans-crocetin on the glutamatergic synaptic transmission in rat cortical brain slices. Postsynaptic potentials (PSPs) were elicited by electrical field stimulation in pyramidal cells of the cingulate cortex and recorded using intracellular placed microelectrodes. PSPs are induced by glutamate released from presynaptic terminals which activates postsynaptic NMDA and non-NMDA receptors. Additionally, glutamate induces a membrane depolarisation when applied directly to the brain slices. CSE (10–100µg/ml) decreased the glutamate-induced membrane depolarisation and inhibited the evoked PSPs. In further experiments, the non-NMDA component of the PSPs was separated by application of the NMDA receptor antagonist APV (10µM) and the NMDA component by application of the non-NMDA receptor antagonist CNQX (10µM). CSE (100µg/ml) decreased the isolated non-NMDA and NMDA component of the PSPs. CSE decreased the kainate (1µM) induced membrane depolarisation, whereas the AMPA (1µM) induced membrane depolarisation was not affected. Our results indicate that CSE has an antagonistic effect on NMDA and kainate receptors. AMPA receptors seem to be not involved. Trans-crocetin (1–50µM) investigated under the same conditions as CSE induced inhibitory effects on the membrane depolarisation and evoked PSPs comparable to CSE. The inhibition of the glutamatergic synaptic transmission in rat cingulate cortex by CSE and trans-crocetin represents a possible new pathway by which Crocus sativus L. acts within the CNS.

The ameliorating effects of NMDA receptor agonists on histamine H 1 antagonist-induced memory and hippocampal theta disruptions are prevented by the H 3 receptor agonist in rats

In the present study, we investigated the effect of imetit, a selective histamine H 3 agonist, on the improvement of NMDA receptor agonists in H 1 antagonist-induced working memory deficit and the decrease of hippocampal theta activity in rats. Pyrilamine (35 mg/kg, i.p.) impaired spatial memory and decreased hippocampal theta activity during the radial maze task. In addition, intrahippocampal injection of d-cycloserine (1 μg/side) and spermidine (10 μg/side) improved the pyrilamine-induced working memory deficit and the decrease of hippocampal theta activity. The improvement effects of d-cycloserine and spermidine were antagonized by intrahippocampal injection of imetit (10 μg/side). These results indicate that, the facilitation of neurotransmitter release from the presynaptic terminals besides direct stimulation of postsynaptic NMDA receptors take part in the improvements of NMDA receptor agonists on pyrilamine-induced spatial memory deficit and the decrease of hippocampal theta activity.

Double Dissociation of Spike Timing–Dependent Potentiation and Depression by Subunit-Preferring NMDA Receptor Antagonists in Mouse Barrel Cortex

Spike timing–dependent plasticity (STDP) is a strong candidate for an N-methyl-D-aspartate (NMDA) receptor-dependent form of synaptic plasticity that could underlie the development of receptive field properties in sensory neocortices. Whilst induction of timing-dependent long-term potentiation (t-LTP) requires postsynaptic NMDA receptors, timing-dependent long-term depression (t-LTD) requires the activation of presynaptic NMDA receptors at layer 4-to-layer 2/3 synapses in barrel cortex. Here we investigated the developmental profile of t-LTD at layer 4-to-layer 2/3 synapses of mouse barrel cortex and studied their NMDA receptor subunit dependence. Timing-dependent LTD emerged in the first postnatal week, was present during the second week and disappeared in the adult, whereas t-LTP persisted in adulthood. An antagonist at GluN2C/D subunit–containing NMDA receptors blocked t-LTD but not t-LTP. Conversely, a GluN2A subunit–preferring antagonist blocked t-LTP but not t-LTD. The GluN2C/D subunit requirement for t-LTD appears to be synapse specific, as GluN2C/D antagonists did not block t-LTD at horizontal cross-columnar layer 2/3-to-layer 2/3 synapses, which was blocked by a GluN2B antagonist instead. These data demonstrate an NMDA receptor subunit-dependent double dissociation of t-LTD and t-LTP mechanisms at layer 4-to-layer 2/3 synapses, and suggest that t-LTD is mediated by distinct molecular mechanisms at different synapses on the same postsynaptic neuron.

Effect of Corticotropin-Like Intermediate Lobe Peptide on Presynaptic and Postsynaptic Glutamate Receptors and Postsynaptic GABA Receptors in Rat Brain

We studied the effect of corticotropin-like intermediate lobe peptide (CLIP) on presynaptic NMDA receptors and postsynaptic GABA, NMDA, and AMPA receptors in rat brain. CLIP inhibited presynaptic and postsynaptic NMDA receptors, but potentiated postsynaptic GABA and AMPA receptors. Our results indicate that CLIP modulates function of ionotropic receptors for glutamate and GABA.

Coincident Pre- and Postsynaptic Activation Induces Dendritic Filopodia via Neurotrypsin-Dependent Agrin Cleavage

The synaptic serine protease neurotrypsin is essential for cognitive function, as its deficiency in humans results in severe mental retardation. Recently, we demonstrated the activity-dependent release of neurotrypsin from presynaptic terminals and proteolytical cleavage of agrin at the synapse. Here we show that the activity-dependent formation of dendritic filopodia is abolished in hippocampal neurons from neurotrypsin-deficient mice. Administration of the neurotrypsin-dependent 22 kDa fragment of agrin rescues the filopodial response. Detailed analyses indicated that presynaptic action potential firing is necessary for the release of neurotrypsin, whereas postsynaptic NMDA receptor activation is necessary for the neurotrypsin-dependent cleavage of agrin. This contingency characterizes the neurotrypsin-agrin system as a coincidence detector of pre- and postsynaptic activation. As the resulting dendritic filopodia are thought to represent precursors of synapses, the neurotrypsin-dependent cleavage of agrin at the synapse may be instrumental for a Hebbian organization and remodeling of synaptic circuits in the CNS.

Changes in hippocampal morphology and neuroplasticity induced by adolescent THC treatment are associated with cognitive impairment in adulthood

Marijuana and hashish are the illicit drugs most frequently used by human adolescents. Given the continued neurodevelopment throughout adolescence, adolescents may be more vulnerable than adults to certain neural consequences of heavy marijuana use. This study aimed to assess whether an experimental model of adolescent chronic exposure to Delta9-tetrahydrocannabinol (THC), may induce lasting effects on learning and memory. Adolescent rats have been treated with THC or its vehicle from 35 to 45 postnatal days (PND) and left undisturbed until their adulthood (75 PND) when aversive and spatial memory was assessed using the passive avoidance and radial maze tasks. No alteration was found in aversive memory, but in the radial maze THC pretreated animals exhibited a worse performance than vehicles, suggesting a deficit in spatial working memory. To correlate memory impairment to altered neuroplasticity, level of marker proteins was investigated in the hippocampus, the most relevant area mediating spatial memory. A significant decrease in the astroglial marker glial fibrillar acid protein was found as well as in pre- and postsynaptic protein expression (VAMP2, PSD95) and NMDA receptor levels in pretreated rats. To parallel these changes to alteration in dendritic morphology, Golgi-Cox staining was performed in the hippocampal dentate gyrus. Pretreated rats had a significantly lower total dendritic length and number than vehicles, as well as reduced spine density. Our data suggest that THC pretreated rats may establish less synaptic contacts and/or less efficient synaptic connections throughout the hippocampus and this could represent the molecular underpinning of the cognitive deficit induced by adolescent THC treatment.

Evaluating statistical methods used to estimate the number of postsynaptic receptors

Calcium levels in spines play a significant role in determining the sign and magnitude of synaptic plasticity. The magnitude of calcium influx into spines is highly dependent on influx through N-methyl D-aspartate (NMDA) receptors, and therefore depends on the number of postsynaptic NMDA receptors in each spine. We have calculated previously how the number of postsynaptic NMDA receptors determines the mean and variance of calcium transients in the postsynaptic density, and how this alters the shape of plasticity curves. However, the number of postsynaptic NMDA receptors in the postsynaptic density is not well known. Anatomical methods for estimating the number of NMDA receptors produce estimates that are very different than those produced by physiological techniques. The physiological techniques are based on the statistics of synaptic transmission and it is difficult to experimentally estimate their precision. In this paper we use stochastic simulations in order to test the validity of a physiological estimation technique based on failure analysis. We find that the method is likely to underestimate the number of postsynaptic NMDA receptors, explain the source of the error, and re-derive a more precise estimation technique. We also show that the original failure analysis as well as our improved formulas are not robust to small estimation errors in key parameters.

Presynaptic and postsynaptic modulation of glutamatergic synaptic transmission by activation of α1‐ and β‐adrenoceptors in layer V pyramidal neurons of rat cerebral cortex

Adrenergic agonists have different modulatory effects on excitatory synaptic transmission depending on the receptor subtypes involved. The present study examined the loci of alpha(1)- and beta-adrenoceptor agonists, which have opposite effects on excitatory neural transmission, involved in modulation of glutamatergic transmission in layer V pyramidal cells of rat cerebral cortex. Phenylephrine, an alpha(1)-adrenoceptor agonist, suppressed the amplitude of AMPA receptor-mediated excitatory postsynaptic currents evoked by repetitive electrical stimulation (eEPSCs, 10 pulses at 33 Hz). The coefficient of variation (CV) of the 1st eEPSC amplitude and paired-pulse ratio (PPR), which were sensitive to extracellular Ca(2+) concentration, were not affected by phenylephrine. Phenylephrine suppressed miniature EPSC (mEPSC) amplitude without changing its frequency. In contrast, isoproterenol, a beta-adrenoceptor agonist, strongly increased the amplitude of the 1st eEPSC compared with that of the 2nd to 10th eEPSCs, which resulted in a decrease in PPR. Isoproterenol-induced enhancement of eEPSC amplitude was accompanied by a decrease in CV. Isoproterenol increased the frequency of mEPSCs without significant effect on amplitude. Phenylephrine suppressed inward currents evoked by puff application of glutamate, AMPA, or NMDA, whereas isoproterenol application was not accompanied by significant changes in these inward currents. These findings suggest that phenylephrine decreases eEPSCs through postsynaptic AMPA or NMDA receptors, while the effects of isoproterenol are mediated by facilitation of glutamate release from presynaptic terminals without effect on postsynaptic glutamate receptors. These two different mechanisms of modulation of excitatory synaptic transmission may improve the "signal-to-noise ratio" in cerebral cortex.

Presynaptic and Postsynaptic NMDA Receptors Mediate Distinct Effects of Brain-Derived Neurotrophic Factor on Synaptic Transmission

In addition to its effects on neuronal survival and differentiation, brain-derived neurotrophic factor (BDNF) plays an important role in modulating synaptic transmission and plasticity in many brain areas, most notably the neocortex and hippocampus. These effects may underlie a role for BDNF in learning and memory as well as developmental plasticity. Consistent with localization of the tropomyosin-related kinase B receptor to both sides of the synapse, BDNF appears to have pre- and postsynaptic effects, but the underlying cellular mechanisms are unclear and it is not known whether pre- and postsynaptic modulations by BDNF occur simultaneously. To address these issues, we recorded dual-component (alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid [AMPA] and N-methyl-D-aspartate [NMDA]) miniature excitatory postsynaptic currents (mEPSCs) from cortical and hippocampal pyramidal neurons and dentate gyrus granule cells from acute brain slices. BDNF had no effect on the fast component of mEPSC decay or on the peak amplitude, suggesting that BDNF did not modulate postsynaptic AMPA receptors, although BDNF rapidly modulated NMDA receptors, as seen by an enhancement of the slow component of mEPSC decay that was prevented by blocking postsynaptic NMDA receptors. At the same time, BDNF acted presynaptically to enhance mEPSC frequency. Surprisingly, the effect on frequency was also NMDA receptor dependent, but required activation of presynaptic, not postsynaptic, NMDA receptors. BDNF also enhanced action potential-dependent glutamate release via presynaptic NMDA receptors, an effect that was unmasked when voltage-gated calcium channels were partially inhibited. Our results indicate that BDNF acutely modulates presynaptic release and postsynaptic responsiveness through simultaneous effects on pre- and postsynaptic NMDA receptors.

Anchors aweigh: NMDA receptors set sail from their presynaptic port

NMDA receptors are unevenly expressed in neuronal cell membranes, reaching their highest concentration in the postsynaptic density (PSD), in register with presynaptic active zones. This confined distribution partly arises from biochemical interactions that the large cytosolic tail of NMDA receptors (> 600 amino acids) establishes with subsynaptic scaffold proteins of the PSD (PSD-95, SAP102), which ultimately anchor them to the intracellular cytoskeleton. In part, it is also due to neighbouring proteins and lipids that, through weak molecular interactions and steric hindrance, prevent NMDA receptors from drifting away from the PSD. All these interactions are transient and reversible in nature, and most NMDA receptors do not spend their entire lifetime docked at the postsynaptic harbour (Choquet & Triller, 2003). Occasionally, they remove the mooring and start sailing across extrasynaptic seas until they reach a neighbouring harbour where to spend a few seconds of their life. In cultured hippocampal neurons, at room temperature, optical tracking of Cy3-conjugated antibodies against the NR1 subunit shows that lateral diffusion of synaptic NMDA receptors is around 0.021 μm2 s−1, slower than that of extrasynaptic ones (0.037 μm2 s−1) (Groc et al. 2004). These estimates have recently been re-evaluated in view of the subunit composition of NMDA receptors with different subcellular locations (Groc et al. 2006). The proportion of immobile NMDA receptors is greater for those containing the NR2A subunit (Groc et al. 2006), although the biochemical mechanisms underlying this behaviour remain a matter of current investigation. It also remains to be established to what extent this correlates with different geometries of spine heads and necks or with developmental changes in the properties of the molecular interactions between NMDA receptors and neighbouring proteins. Since sailing needs a breeze, something over the cell membrane has to aid the NMDA receptor boat. Indeed, proteins of the extracellular matrix, like reelin, appear to favour NMDA receptor navigation (Groc et al. 2007). If postsynaptic NMDA receptors move, there is little reason, at least in principle, why they should not do so even at presynaptic sites. In the cortex, presynaptic NMDA receptors have been proposed to exist in specific layers of the entorhinal, visual and somatosensory region (Woodhall et al. 2001; Brasier & Feldman, 2008). This has been generally inferred from electrophysiological recordings where pharmacological block of NMDA receptors is associated with increase in the paired-pulse ratio of evoked EPSCs or decrease in miniature EPSC frequency. Whether these effects are directly consequent to prevention of Ca2+ influx through NMDA receptors or indirectly due to inhibition of a depolarization-induced recruitment of voltage-gated Ca2+ channels, they do indicate that at certain synapses NMDA receptor activation enhances the probability of neurotransmitter release. By using a similar approach in acute slices from adult rats, in layer V of the entorhinal cortex, Yang et al. (2008) (this issue of The Journal of Physiology) have recently caught sight of NMDA receptors cruising amidst the waves of presynaptic terminals. The conclusion is based on the observation that if postsynaptic NMDA receptors are blocked, frequency-dependent facilitation of evoked EPSCs and frequency of spontaneous EPSCs recover from the irreversible, use-dependent block by the NMDA receptor antagonist MK-801. A definitive proof of this interpretation would inevitably require a multidisciplinary effort, in which functional electrophysiological approaches are combined with anatomical evidence for presynaptic NMDA receptors and live imaging of their trajectories. But in the absence of yet-unidentified presynaptic anchoring proteins, this remains a feasible hypothesis provided the interactions with neighbouring proteins inside and underneath the cell membrane are not strong enough to constantly drive NMDA receptors aground. One would then speculate that lateral diffusion takes place between active zones, where neurotransmitter vesicles fusing with the presynaptic membrane continuously push receptors away, and the sites of exo-endocytosis, where receptor trafficking occurs. What implications on synaptic transmission would such a dynamic behaviour of presynaptic NMDA receptors have? It could work as a homeostatic mechanism to maintain constant the baseline level of release probability. For this to happen, lateral diffusion should occur within the time frame of NMDA receptor deactivation (∼100 ms) and a more quantitative analysis in the future will help to elucidate this scenario. In general though, it could provide neurons with an additional tool to regulate synaptic strength. It would be interesting to know whether NMDA receptor activation per se alters the mobility of receptors nearby: Ca2+ influx through NMDA receptors could lead to protein kinase C activation, which has been shown to increase their own mobility (Groc et al. 2004). Finally, I wonder whether glutamate escape from active terminals would also alter the dynamic behaviour of receptors at adjacent synapses. The notion of receptor mobility has definitely changed our understanding of synaptic function. No doubt the extraordinary adventures of NMDA receptors will soon lead us to better shores, with clearer views of the physiological processes studied so far.