NMDA receptors activate the arachidonic acid cascade system in striatal neurons

Receptors for excitatory amino-acid transmitters on nerve cells fall into two main categories associated with non-selective cationic channels, the NMDA (N-methyl-D-aspartate) and non-NMDA (kainate and quisqualate) receptors. Special properties of NMDA receptors such as their voltage-dependent blockade by Mg2+ (refs 3, 4) and their permeability to Na+, K+ as well as to Ca2+ (refs 5, 6), have led to the suggestion that these receptors are important in plasticity during development and learning. They have been implicated in long-term potentiation (LTP), a model for the study of the cellular mechanisms of learning. We report here that glutamate and NMDA, acting at typical NMDA receptors, stimulate the release of arachidonic acid (as well as 11- and 12-hydroxyeicosatetraenoic acids from striatal neurons probably by stimulation of a Ca2+-dependent phospholipase A2. Kainate and quisqualate, as well as K+-induced depolarization were ineffective. Our results provide direct evidence in favour of the hypothesis, that arachidonic acid derivatives, produced by activation of the postsynaptic cell, could be messengers that cross the synaptic cleft to modify the presynaptic functions known to be altered during LTP. In addition, we suggest that NMDA receptors are the postsynaptic receptors which trigger the synthesis of these putative transynaptic messengers.     

Long-term potentiation of NMDA receptor-mediated synaptic transmission in the hippocampus.

Neurotransmission at most excitatory synapses in the brain operates through two types of glutamate receptor termed alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionate (AMPA) and N-methyl-D-aspartate (NMDA) receptors; these mediate the fast and slow components of excitatory postsynaptic potentials respectively. Activation of NMDA receptors can also lead to a long-lasting modification in synaptic efficiency at glutamatergic synapses; this is exemplified in the CA1 region of the hippocampus, where NMDA receptors mediate the induction of long-term potentiation (LTP). It is believed that in this region LTP is maintained by a specific increase in the AMPA receptor-mediated component of synaptic transmission. We now report, however, that a pharmacologically isolated NMDA receptor-mediated synaptic response can undergo robust, synapse-specific LTP. This finding has implications for neuropathologies such as epilepsy and neurodegeneration, in which excessive NMDA receptor activation has been implicated. It adds fundamentally to theories of synaptic plasticity because NMDA receptor activation may, in addition to causing increased synaptic efficiency, directly alter the plasticity of synapses.     

Arachidonic acid induces a prolonged inhibition of glutamate uptake into glial cells

Activation of NMDA (N-methyl-D-aspartate) receptors by neurotransmitter glutamate stimulates phospholipase A2 to release arachidonic acid. This second messenger facilitates long-term potentiation of glutamatergic synapses in the hippocampus, possibly by blocking glutamate uptake. We have studied the effect of arachidonic acid on glutamate uptake into glial cells using the whole-cell patch-clamp technique to monitor the uptake electrically. Micromolar levels of arachidonic acid inhibit glutamate uptake, mainly by reducing the maximum uptake rate with only small effects on the affinity for external glutamate and sodium. On removal of arachidonic acid a rapid (5 minutes) phase of partial recovery is followed by a maintained suppression of uptake lasting at least 20 minutes. Surprisingly, the action of arachidonic acid is unaffected by cyclo-oxygenase or lipoxygenase inhibitors suggesting that it inhibits uptake directly, possibly by increasing membrane fluidity. As blockade of phospholipase A2 prevents the induction of long-term potentiation (LTP), inhibition of glutamate uptake by arachidonic acid may contribute to the increase of synaptic gain that occurs in LTP. During anoxia, release of arachidonic acid could severely compromise glutamate uptake and thus contribute to neuronal death.     

Positive feedback of glutamate exocytosis by metabotropic presynaptic receptor stimulation.

Glutamate is important in several forms of synaptic plasticity such as long-term potentiation, and in neuronal cell degeneration. Glutamate activates several types of receptors, including a metabotropic receptor that is sensitive to trans-1-amino-cyclopenthyl-1,3-dicarboxylate, coupled to G protein(s) and linked to inositol phospholipid metabolism. The activation of the metabotropic receptor in neurons generates inositol 1,4,5-trisphosphate, which causes the release of Ca2+ from intracellular stores and diacylglycerol, which activates protein kinase C. In nerve terminals, the activation of presynaptic protein kinase C with phorbol esters enhances glutamate release. But the presynaptic receptor involved in this protein kinase C-mediated increase in the release of glutamate has not yet been identified. Here we demonstrate the presence of a presynaptic glutamate receptor of the metabotropic type that mediates an enhancement of glutamate exocytosis in cerebrocortical nerve terminals. Interestingly, this potentiation of glutamate release is observed only in the presence of arachidonic acid, which may reflect that this positive feedback control of glutamate exocytosis operates in concert with other pre- or post-synaptic events of the glutamatergic neurotransmission that generate arachidonic acid. This presynaptic glutamate receptor may have a physiological role in the maintenance of long-term potentiation where there is an increase in glutamate release mediated by postsynaptically generated arachidonic acid.     

Kainate receptors are involved in synaptic plasticity

The ability of synapses to modify their synaptic strength in response to activity is a fundamental property of the nervous system and may be an essential component of learning and memory. There are three classes of ionotropic glutamate receptor, namely NMDA (N-methyl-D-aspartate), AMPA (alpha-amino-3-hydroxy-5-methyl-4-isoxazole-4-propionic acid) and kainate receptors; critical roles in synaptic plasticity have been identified for two of these. Thus, at many synapses in the brain, transient activation of NMDA receptors leads to a persistent modification in the strength of synaptic transmission mediated by AMPA receptors. Here, to determine whether kainate receptors are involved in synaptic plasticity, we have used a new antagonist, LY382884 ((3S, 4aR, 6S, 8aR)-6-((4-carboxyphenyl)methyl-1,2,3,4,4a,5,6,7,8,8a-decahydro isoquinoline-3-carboxylic acid), which antagonizes kainate receptors at concentrations that do not affect AMPA or NMDA receptors. We find that LY382884 is a selective antagonist at neuronal kainate receptors containing the GluR5 subunit. It has no effect on long-term potentiation (LTP) that is dependent on NMDA receptors but prevents the induction of mossy fibre LTP, which is independent of NMDA receptors. Thus, kainate receptors can act as the induction trigger for long-term changes in synaptic transmission.     

Temporally distinct pre- and post-synaptic mechanisms maintain long-term potentiation

Long-term potentiation (LTP) in the hippocampus is widely studied as the mechanisms involved in its induction and maintenance are believed to underlie fundamental properties of learning and memory in vertebrates. Most synapses that exhibit LTP use an excitatory amino-acid neurotransmitter that acts on two types of receptor, the N-methyl-D-aspartate (NMDA) and quisqualate receptors. The quisqualate receptor mediates the fast synaptic response evoked by low-frequency stimulation, whereas the NMDA receptor system is activated transiently by tetanic stimulation, leading to the induction of LTP. The events responsible for maintaining LTP once it is established are not known. We now demonstrate that the sensitivity of CA1 neurons in hippocampal slices to ionophoretically-applied quisqualate receptor ligands slowly increases following the induction of LTP. This provides direct evidence for a functional post-synaptic change and suggests that pre-synaptic mechanisms also contribute, but in a temporally distinct manner, to the maintenance of LTP.     

Potentiation of NMDA receptor currents by arachidonic acid.

Arachidonic acid is released by phospholipase A2 when activation of N-methyl-D-aspartate (NMDA) receptors by neurotransmitter glutamate raises the calcium concentration in neurons, for example during the initiation of long-term potentiation and during brain anoxia. Here we investigate the effect of arachidonic acid on glutamate-gated ion channels by whole-cell clamping isolated cerebellar granule cells. Arachidonic acid potentiates, and makes more transient, the current through NMDA receptor channels, and slightly reduces the current through non-NMDA receptor channels. Potentiation of the NMDA receptor current results from an increase in channel open probability, with no change in open channel current. We observe potentiation even with saturating levels of agonist at the glutamate- and glycine-binding sites on these channels; it does not result from conversion of arachidonic acid to lipoxygenase or cyclooxygenase derivatives, or from activation of protein kinase C. Arachidonic acid may act by binding to a site on the NMDA receptor, or by modifying the receptor's lipid environment. Our results suggest that arachidonic acid released by activation of NMDA (or other) receptors will potentiate NMDA receptor currents, and thus amplify increases in intracellular calcium concentration caused by glutamate. This may explain why inhibition of phospholipase A2 blocks the induction of long-term potentiation.     

NMDA and non-NMDA receptors are co-localized at individual excitatory synapses in cultured rat hippocampus

A CENTRAL assumption about long-term potentiation in the hippocampus is that the two classes of glutamate-receptor ion channel, the N-methyl-D-aspartate (NMDA) and the kainate/quisqualate (non-NMDA) subtypes, are co-localized at individual excitatory synapses. This assumption is important because of the perceived interplay between NMDA and non-NMDA receptors in the induction and expression of long-term potentiation: the NMDA class, by virtue of its voltage-dependent channel block by magnesium and calcium permeability, provides the trigger for the induction of long-term potentiation, whereas the actual enhancement of synaptic efficacy is thought to be provided by the non-NMDA class. If both receptor subtypes are present at the one synapse, such cross-modulation could occur rapidly and locally through diffusible factors. By measuring miniature synaptic currents in cultured hippocampal neurons we show that the majority (approximately 70%) of the excitatory synapses on a postsynaptic cell possess both kinds of receptor, although to different extents. Of the remaining excitatory synapses, approximately 20% contain only the non-NMDA subtype and the rest possess only NMDA receptors. This finding provides direct evidence for co-localization of glutamate-receptor subtypes at individual synapses, and also points to the possibility that long-term potentiation might be differentially expressed at each synapse according to the mix of receptor subtypes at that synapse.     

Single cocaine exposure in vivo induces long-term potentiation in dopamine neurons

How do drugs of abuse modify neural circuitry and thereby lead to addictive behaviour? As for many forms of experience-dependent plasticity, modifications in glutamatergic synaptic transmission have been suggested to be particularly important. Evidence of such changes in response to in vivo administration of drugs of abuse is lacking, however. Here we show that a single in vivo exposure to cocaine induces long-term potentiation of AMPA (alpha-amino-3-hydroxy-5-methyl-isoxazole propionic acid)-receptor-mediated currents at excitatory synapses onto dopamine cells in the ventral tegmental area. Potentiation is still observed 5 but not 10 days after cocaine exposure and is blocked when an NMDA (N-methyl-d-aspartate) receptor antagonist is administered with cocaine. Furthermore, long-term potentiation at these synapses is occluded and long-term depression is enhanced by in vivo cocaine exposure. These results show that a prominent form of synaptic plasticity can be elicited by a single in vivo exposure to cocaine and therefore may be involved in the early stages of the development of drug addiction.     

NMDA application potentiates synaptic transmission in the hippocampus

The NMDA (N-methyl-D-aspartate) class of glutamate receptor plays a critical role in a variety of forms of synaptic plasticity in the vertebrate central nervous system. One extensively studied example of plasticity is long-term potentiation (LTP), a remarkably long-lasting enhancement of synaptic efficiency induced in the hippocampus by brief, high-frequency stimulation of excitatory synapses. LTP is a strong candidate for a cellular mechanism of learning and memory. The site of LTP induction appears to be the postsynaptic cell and induction requires both activation of NMDA receptors by synaptically released glutamate and depolarization of the postsynaptic membrane. It is proposed that this depolarization relieves a voltage-dependent Mg2+ block of the NMDA receptor channel, resulting in increased calcium influx which is the trigger for the induction of LTP. This model predicts that application of a large depolarizing dose of NMDA should be sufficient to evoke LTP. In agreement with a previous study, we have found that NMDA or glutamate application does potentiate synaptic transmission in the hippocampus. This agonist-induced potentiation is, however, decremental and short-lived, unlike LTP. It is occluded shortly after the induction of LTP and a similar short-term potentiation can be evoked by synaptically released glutamate. We thus propose that LTP has two components, a short-term, decremental component which can be mimicked by NMDA receptor activation, and a long-lasting, non-decremental component which, in addition to requiring activation of NMDA receptors, requires stimulation of presynaptic afferents.     

Opioids block long-term potentiation of inhibitory synapses

Excitatory brain synapses are strengthened or weakened in response to specific patterns of synaptic activation, and these changes in synaptic strength are thought to underlie persistent pathologies such as drug addiction, as well as learning. In contrast, there are few examples of synaptic plasticity of inhibitory GABA (gamma-aminobutyric acid)-releasing synapses. Here we report long-term potentiation of GABA(A)-mediated synaptic transmission (LTP(GABA)) onto dopamine neurons of the rat brain ventral tegmental area, a region required for the development of drug addiction. This novel form of LTP is heterosynaptic, requiring postsynaptic NMDA (N-methyl-d-aspartate) receptor activation at glutamate synapses, but resulting from increased GABA release at neighbouring inhibitory nerve terminals. NMDA receptor activation produces nitric oxide, a retrograde signal released from the postsynaptic dopamine neuron. Nitric oxide initiates LTP(GABA) by activating guanylate cyclase in GABA-releasing nerve terminals. Exposure to morphine both in vitro and in vivo prevents LTP(GABA). Whereas brief treatment with morphine in vitro blocks LTP(GABA) by inhibiting presynaptic glutamate release, in vivo exposure to morphine persistently interrupts signalling from nitric oxide to guanylate cyclase. These neuroadaptations to opioid drugs might contribute to early stages of addiction, and may potentially be exploited therapeutically using drugs targeting GABA(A) receptors.     

Long-term potentiation of electrotonic coupling at mixed synapses

Long-term potentiation of chemical synapses is closely related to memory and learning. Studies of this process have concentrated on chemically mediated excitatory synapses. By contrast, activity-dependent modification of gap junctions, which also widely exist in higher structures such as hippocampus and neocortex, has not been described. Here we report that at mixed synapses between sensory afferents and an identified reticulospinal neuron, the electrotonic coupling potential can be potentiated, as well as the chemically mediated excitatory postsynaptic potential, for a prolonged time period using a stimulation paradigm like that which produces long-term potentiation in hippocampus. The effect on coupling is due to an increase in gap-junctional conductance. Our data indicate that the potentiation of both synaptic components requires an increase in intracellular calcium, involves activation of NMDA (N-methyl-D-aspartate) receptors, and is specific to the tetanized pathway.     

Conservation of total synaptic weight through balanced synaptic depression and potentiation

Memory is believed to depend on activity-dependent changes in the strength of synapses. In part, this view is based on evidence that the efficacy of synapses can be enhanced or depressed depending on the timing of pre- and postsynaptic activity. However, when such plastic synapses are incorporated into neural network models, stability problems may develop because the potentiation or depression of synapses increases the likelihood that they will be further strengthened or weakened. Here we report biological evidence for a homeostatic mechanism that reconciles the apparently opposite requirements of plasticity and stability. We show that, in intercalated neurons of the amygdala, activity-dependent potentiation or depression of particular glutamatergic inputs leads to opposite changes in the strength of inputs ending at other dendritic sites. As a result, little change in total synaptic weight occurs, even though the relative strength of inputs is modified. Furthermore, hetero- but not homosynaptic alterations are blocked by intracellular dialysis of drugs that prevent Ca2+ release from intracellular stores. Thus, in intercalated neurons at least, inverse heterosynaptic plasticity tends to compensate for homosynaptic long-term potentiation and depression, thus stabilizing total synaptic weight.     

Clustering of L-type Ca2+ channels at the base of major dendrites in hippocampal pyramidal neurons

Integration and processing of electrical signals in individual neurons depend critically on the spatial distribution of ion channels on the cell surface. In hippocampal pyramidal neurons, voltage-sensitive calcium channels have important roles in the control of Ca2(+)-dependent cellular processes such as action potential generation, neurotransmitter release, and epileptogenesis. Long-term potentiation of synaptic transmission in the hippocampal pyramidal cell, a form of neuronal plasticity that is thought to represent a cellular correlate of learning and memory, is dependent on Ca2+ entry mediated by synaptic activation of glutamate receptors that have a high affinity for NMDA (N-methyl(-D-aspartate) and are located in distal dendrites. Stimuli causing long-term potentiation at these distal synapses also cause a large local increase in cytosolic Ca2+ in the proximal regions of dendrites. This increase has been proposed to result from activation of voltage-gated Ca2+ channels. At least four types of voltage-gated Ca2+ channels, designated N, L. T and P, may be involved in these processes. Here we show that L-type Ca2+ channels, visualized using a monoclonal antibody, are located in the cell bodies and proximal dendrites of hippocampal pyramidal cells and are clustered in high density at the base of major dendrites. We suggest that these high densities of L-type Ca2+ channels may serve to mediate Ca2+ entry into the pyramidal cell body and proximal dendrites in response to summed excitatory inputs to the distal dendrites and to initiate intracellular regulatory events in the cell body in response to the same synaptic inputs that cause long-term potentiation at distal dendritic synapses.     

Developmental and activity-dependent regulation of kainate receptors at thalamocortical synapses.

Most of the fast excitatory synaptic transmission in the mammalian brain is mediated by ionotrophic glutamate receptors, of which there are three subtypes: AMPA (alpha-amino-3-hydroxyl-5-methyl-4-isoxazolepropionate), NMDA (N-methyl-D-aspartate) and kainate. Although kainate-receptor subunits (GluR5-7, KA1 and 2) are widely expressed in the mammalian central nervous system, little is known about their function. The development of pharmacological agents that distinguish between AMPA and kainate receptors has now allowed the functions of kainate receptors to be investigated. The modulation of synaptic transmission by kainate receptors and their synaptic activation in a variety of brain regions have been reported. The expression of kainate receptor subunits is developmentally regulated but their role in plasticity and development is unknown. Here we show that developing thalamocortical synapses express postsynaptic kainate receptors as well as AMPA receptors; however, the two receptor subtypes do not colocalize. During the critical period for experience-dependent plasticity, the kainate-receptor contribution to transmission decreases; a similar decrease occurs when long-term potentiation is induced in vitro. This indicates that during development there is activity-dependent regulation of the expression of kainate receptors at thalamocortical synapses.     

RIM1alpha is required for presynaptic long-term potentiation.

Two main forms of long-term potentiation (LTP)-a prominent model for the cellular mechanism of learning and memory-have been distinguished in the mammalian brain. One requires activation of postsynaptic NMDA (N-methyl d-aspartate) receptors, whereas the other, called mossy fibre LTP, has a principal presynaptic component. Mossy fibre LTP is expressed in hippocampal mossy fibre synapses, cerebellar parallel fibre synapses and corticothalamic synapses, where it apparently operates by a mechanism that requires activation of protein kinase A. Thus, presynaptic substrates of protein kinase A are probably essential in mediating this form of long-term synaptic plasticity. Studies of knockout mice have shown that the synaptic vesicle protein Rab3A is required for mossy fibre LTP, but the protein kinase A substrates rabphilin, synapsin I and synapsin II are dispensable. Here we report that mossy fibre LTP in the hippocampus and the cerebellum is abolished in mice lacking RIM1alpha, an active zone protein that binds to Rab3A and that is also a protein kinase A substrate. Our results indicate that the long-term increase in neurotransmitter release during mossy fibre LTP may be mediated by a unitary mechanism that involves the GTP-dependent interaction of Rab3A with RIM1alpha at the interface of synaptic vesicles and the active zone.     

Arachidonic acid induces a long-term activity-dependent enhancement of synaptic transmission in the hippocampus

Long-term potentiation (LTP) is a widely studied model of the synaptic basis of information storage in the mammalian brain. The induction of LTP is triggered by the postsynaptic entry of calcium through the channel associated with the N-methyl-D-aspartate (NMDA) receptor, whereas its maintenance is mediated, at least in part, by presynaptic mechanisms. To explain how postsynaptic events can lead to an increase in transmitter release, we have postulated the existence of a retrograde messenger to carry information from the postsynaptic side of the synapse to recently active presynaptic terminals. Candidates for a retrograde messenger include arachidonic acid or one of its lipoxygenase metabolites. Here we report that weak activation of the perforant path, when given in the presence of arachidonic acid, leads to a slow-onset persistent increase in synaptic efficacy both in vivo and in vitro. The activity-dependent potentiation thus produced is accompanied by an increase in the release of glutamate, and is non-additive with tetanus-induced LTP. These observations indicate a role for arachidonic acid as a retrograde messenger in the later, but not the initial, stages of LTP.     

GABA autoreceptors regulate the induction of LTP.

Understanding the mechanisms involved in long-term potentiation (LTP) should provide insights into the cellular and molecular basis of learning and memory in vertebrates. It has been established that in the CA1 region of the hippocampus the induction of LTP requires the transient activation of the N-methyl-D-aspartate (NMDA) receptor system. During low-frequency transmission, significant activation of this system is prevented by gamma-aminobutyric acid (GABA) mediated synaptic inhibition which hyperpolarizes neurons into a region where NMDA receptor-operated channels are substantially blocked by Mg2+ (refs. 5, 6). But during high-frequency transmission, mechanisms are evoked that provide sufficient depolarization of the postsynaptic membrane to reduce this block and thereby permit the induction of LTP. We now report that this critical depolarization is enabled because during high-frequency transmission GABA depresses its own release by an action on GABAB autoreceptors, which permits sufficient NMDA receptor activation for the induction of LTP. These findings demonstrate a role for GABAB receptors in synaptic plasticity.     

Frequency-dependent involvement of NMDA receptors in the hippocampus: a novel synaptic mechanism

Acidic amino acids, such as l-glutamate, are believed to be excitatory neurotransmitters in the mammalian brain and exert effects on several different receptors named after the selective agonists kainate, quisqualate and N-methyl-D-aspartate (NMDA). The first two receptors collectively termed non-NMDA receptors, have been implicated in the mediation of synaptic transmission in many excitatory pathways in the central nervous system (CNS), whereas NMDA receptors, with few exceptions do not appear to be involved; this is typified in the hippocampus where there is a high density of NMDA receptors yet selective NMDA receptor antagonists, such as D-2-amino-5-phosphonovalerate (APV), do not affect synaptic potentials. NMDA receptors have, however, been shown to be involved in long-term potentiation (LTP) in the hippocampus, a form of synaptic plasticity which may be involved in learning and memory. NMDA receptors have also been found to contribute to epileptiform activity in this region. We now describe how NMDA receptors can participate during high-frequency synaptic transmission in the hippocampus, their involvement during low-frequency transmission being greatly suppressed by Mg2+. A frequency dependent alleviation of this blockade provides a novel synaptic mechanism whereby a single neurotransmitter can transmit very different information depending on the temporal nature of the input. This mechanism could account for the involvement of NMDA receptors in the initiation of LPT and their contribution, in part, to epileptic activity.     

Calcium stores regulate the polarity and input specificity of synaptic modification

Activity-induced synaptic modification is essential for the development and plasticity of the nervous system. Repetitive correlated activation of pre- and postsynaptic neurons can induce persistent enhancement or decrement of synaptic efficacy, commonly referred to as long-term potentiation or depression (LTP or LTD). An important unresolved issue is whether and to what extent LTP and LTD are restricted to the activated synapses. Here we show that, in the CA1 region of the hippocampus, reduction of postsynaptic calcium influx by partial blockade of NMDA (N-methyl-D-aspartate) receptors results in a conversion of LTP to LTD and a loss of input specificity normally associated with LTP, with LTD appearing at heterosynaptic inputs. The induction of LTD at homo- and heterosynaptic sites requires functional ryanodine receptors and inositol triphosphate (InsP3) receptors, respectively. Functional blockade or genetic deletion of type 1 InsP3 receptors led to a conversion of LTD to LTP and elimination of heterosynaptic LTD, whereas blocking ryanodine receptors eliminated only homosynaptic LTD. Thus, postsynaptic Ca2+, deriving from Ca2+ influx and differential release of Ca2+ from internal stores through ryanodine and InsP3 receptors, regulates both the polarity and input specificity of activity-induced synaptic modification.