NMDA receptors in the visual cortex of young kittens are more effective than those of adult cats

Acidic amino acids, such as glutamate and aspartate, are thought to be excitatory transmitters in the cerebral neocortex and hippocampus. Receptors for these amino acids can be classified into at least three types on the basis of their agonists. Quisqualate-preferring receptors and kainate-preferring receptors are implicated in the mediation of synaptic transmission in many regions including the hippocampus and visual cortex, whereas N-methyl-D-aspartate (NMDA)-preferring receptors are thought to be involved in modulating synaptic efficacy, for example in longterm potentiation, a form of synaptic plasticity in the hippocampus. In the visual cortex of the cat and monkey, it is well established that synaptic plasticity, estimated by susceptibility of binocular responsiveness of cortical neurons to monocular visual deprivation, disappears after the 'critical' period of postnatal development. Here we report that during the critical period in young kittens, a selective NMDA-receptor antagonist blocks visual responses of cortical neurons much more effectively than it does in the adult cat. This suggests that NMDA receptors may be involved in establishing synaptic plasticity in the kitten visual cortex.     

NMDA receptors of dentate gyrus granule cells participate in synaptic transmission following kindling

In the mammalian central nervous system, receptors for the excitatory amino-acid neurotransmitters are divided into three subtypes depending on their sensitivity to three specific agonists: kainate, quisqualate and N-methyl-D-aspartate (NMDA). The ionophores operated by NMDA are gated by Mg2+ in a voltage-dependent manner and allow passage of several cations, including Ca2+ which may be important in plastic alterations of neuronal excitability. Indeed, specific antagonists of NMDA receptors effectively block spatial learning, long-term potentiation and some animal models of chronic epilepsy. Despite their abundance on central neurons, NMDA receptors, with a few noteworthy exceptions, do not generally seem to be involved in low-frequency synaptic transmission. Here we report for the first time that NMDA receptors of the dentate gyrus, where they do not normally contribute to the generation of synaptic potentials, become actively involved in synaptic transmission following long-lasting neuronal changes induced by daily electrical stimulation (kindling) of the amygdala or hippocampal commissures. In contrast to controls, the excitatory postsynaptic potentials (e.p.s.ps) of granule cells in hippocampal slices obtained from kindled animals displayed characteristics typical of an NMDA-receptor-mediated component. The involvement of NMDA receptors in synaptic transmission may underlie the long-lasting changes in neuronal function induced by kindling.     

Mediation of thalamic sensory input by both NMDA receptors and non-NMDA receptors

Excitatory amino acids such as L-glutamate and L-aspartate are well established as neurotransmitter candidates in the mammalian central nervous system, and three types of receptor for these substances have been proposed, characterized by the agonists N-methyl-D-aspartate (NMDA), kainate and quisqualate. All these receptors have been suggested to have synaptic roles in excitatory transmission in the brain. Here I demonstrate that NMDA receptors play a crucial role in the observed response of ventrobasal thalamus (VB) neurones to natural stimulation of somatosensory afferents, but do not appear to be responsible for the short-latency excitation seen on electrical stimulation of the afferents which is apparently mediated by excitatory amino-acid receptors of the non-NMDA type. This result indicates an involvement of NMDA and non-NMDA receptors in the responses of VB neurones to stimulation of somatosensory somatosensory afferents, depending on the mode of stimulation of the pathway.     

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.     

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.     

Micromolar concentrations of Zn2+ antagonize NMDA and GABA responses of hippocampal neurons

NMDA (N-methyl-D-aspartate) receptors serve as modulators of synaptic transmission in the mammalian central nervous system (CNS) with both short-term and long-lasting effects. Divalent cations are pivotal in determining this behaviour in that Mg2+ blocks the ion channel in a voltage-dependent manner, and Ca2+ permeates NMDA channels. Zn2+ could also modulate neuronal excitability because it is present at high concentrations in brain, especially the synaptic vesicles of mossy fibers in the hippocampus and is released with neuronal activity. Both proconvulsant and depressant actions of Zn2+ have been reported. We have found that zinc is a potent non-competitive antagonist of NMDA responses on cultured hippocampal neurons. Unlike Mg2+, the effect of Zn2+ is not voltage-sensitive between -40 and +60 mV, suggesting that Zn2+ and Mg2+ act at distinct sites. In addition, we have found that Zn2+ antagonizes responses to the inhibitory transmitter GABA (gamma-aminobutyric acid). Our results provide evidence for an additional metal-binding site on the NMDA receptor channel, and suggest that Zn2+ may regulate both excitatory and inhibitory synaptic transmission in the hippocampus.     

NMDA receptor agonists selectively block N-type calcium channels in hippocampal neurons.

The modulation of voltage-dependent calcium channels by various neurotransmitters has been demonstrated in many neurons. Because of the critical role of Ca2+ in transmitter release and, more generally, in transmembrane signalling, this modulation has important functional implications. Hippocampal neurons possess low-threshold (T-type) Ca2+ channels and both L- and N-type high voltage-activated Ca2+ channels. N-type Ca2+ channels are blocked selectively by omega-conotoxin and adenosine. These substances both block excitatory synaptic transmission in the hippocampus, whereas dihydropyridines, which selectively block L-type channels, are ineffective. Excitatory synaptic transmission in the hippocampus displays a number of plasticity phenomena that are initiated by Ca2+ entry through ionic channels operated by N-methyl-D-aspartate (NMDA) receptors. Here we report that NMDA receptor agonists selectively and effectively depress N-type Ca2+ channels which are involved in neurotransmitter release from presynaptic sites. The inhibitory effect is eliminated by the competitive NMDA antagonist D-2-amino-5-phosphonovalerate, does not require Ca2+ entry into the cell, and is probably receptor-mediated. This phenomenon may provide a negative feedback between the liberation of excitatory transmitter and entry of Ca2+ into the cell, and could be important in presynaptic inhibition and in the regulation of synaptic plasticity.     

Developmental regulation of NMDA receptor-mediated synaptic currents at a central synapse.

The central nervous system has extraordinary plasticity in early life. This is thought to involve N-methyl-D-aspartate (NMDA) receptors which, along with the non-NMDA receptors, mediate fast excitatory synaptic transmission. Although NMDA receptors may be transiently enhanced early in life, it has not been possible to demonstrate directly a functional change in the NMDA receptor-mediated synaptic response because of the voltage-dependence of the NMDA conductance and the overlapping inhibitory synaptic conductances. Here I report that the duration of evoked NMDA-receptor-mediated excitatory postsynaptic currents (e.p.s.cs) in the superior colliculus is several times longer at early developmental stages compared to that measured in older animals. In contrast, the amplitude of NMDA-receptor-mediated miniature e.p.s.cs does not change during development. The kinetic response of excised membrane patches to a brief activation of NMDA receptors is similar to that of the NMDA e.p.s.c, which suggests that the time course of the NMDA e.p.s.c. in the superior colliculus reflects slow NMDA channel properties as in the hippocampus. Therefore, these data indicate that the molecular properties of NMDA receptors are developmentally regulated and thus may be controlling the ability of synapses to change in early life.     

Selective impairment of learning and blockade of long-term potentiation by an N-methyl-D-aspartate receptor antagonist, AP5

Recent work has shown that the hippocampus contains a class of receptors for the excitatory amino acid glutamate that are activated by N-methyl-D-aspartate (NMDA) and that exhibit a peculiar dependency on membrane voltage in becoming active only on depolarization. Blockade of these sites with the drug aminophosphonovaleric acid (AP5) does not detectably affect synaptic transmission in the hippocampus, but prevents the induction of hippocampal long-term potentiation (LTP) following brief high-frequency stimulation. We now report that chronic intraventricular infusion of D,L-AP5 causes a selective impairment of place learning, which is highly sensitive to hippocampal damage, without affecting visual discrimination learning, which is not. The L-isomer of AP5 did not produce behavioural effects. AP5 treatment also suppressed LTP in vivo. These results suggest that NMDA receptors are involved in spatial learning, and add support to the hypothesis that LTP is involved in some, but not all, forms of learning.     

Glutamate activates multiple single channel conductances in hippocampal neurons

There is considerable evidence that glutamate is the principal neurotransmitter that mediates fast excitatory synaptic transmission in the vertebrate central nervous system. This single transmitter seems to activate two or three distinct types of receptors, defined by their affinities for three selective structural analogues of glutamate, NMDA (N-methyl-D-aspartate), quisqualate and kainate. All these agonists increase membrane permeability to monovalent cations, but NMDA also activates a conductance that permits significant calcium influx and is blocked in a voltage-dependent manner by extracellular magnesium. Fast synaptic excitation seems to be mediated mainly by kainate/quisqualate receptors, although NMDA receptors are sometimes activated. We have investigated the properties of these conductances using single-channel recording in primary cultures of hippocampal neurons, because the hippocampus contains all subtypes of glutamate receptors and because long-term potentiation of synaptic transmission occurs in this structure. We find that four or more distinct single-channel currents are evoked by applying glutamate to each outside-out membrane patch. These conductances vary in their ionic permeability and in the agonist most effective in causing them to open. Clear transitions between all the conductance levels are observed. Our observations are compatible with the model that all the single channel conductances activated by glutamate reflect the operation of one or two complex molecular entities.     

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.     

Two components of long-term potentiation induced by different patterns of afferent activation

Long-term potentiation (LTP) of excitatory synaptic transmission could be a mechanism underlying memory. Induction of LTP requires Ca2+ influx into postsynaptic neurons through ion channels gated by NMDA (N-methyl-D-aspartate) receptors in hippocampus (area CA1 and dentate gyrus) and neocortex. Here we report that a component of LTP not requiring the activation of NMDA receptors can be induced in area CA1. The component is dependent on tetanus frequency, requires increases in postsynaptic intracellular Ca2+ concentrations, and is suppressed by an antagonist of voltage-dependent Ca2+ channels.     

Anatomical distributions of four pharmacologically distinct 3H-L-glutamate binding sites

Glutamate is thought to serve as a major excitatory neurotransmitter throughout the central nervous system (CNS); electrophysiological studies indicate that its action is mediated by multiple receptors. Four receptors have been characterized by their selective sensitivity to N-methyl-D-aspartate (NMDA), kainic acid (KA), quisqualic acid (QA) and 2-amino-4-phosphonobutyric acid (APB). Electrophysiological evidence indicates that these receptors are all present in the rat hippocampus and that the anatomically discrete synaptic fields within the hippocampus exhibit differential sensitivity to the selective excitatory amino acid agents. Thus, we have used the hippocampus as a model system to investigate possible subpopulations of 3H-L-glutamate binding sites. By using quantitative autoradiography, the pharmacological specificity of 3H-L-glutamate binding in discrete terminal fields was determined. We report here that there are at least four distinct classes of 3H-L-glutamate binding sites which differ in their anatomical distribution, pharmacological profile and regulation by ions. Two of these sites seem to correspond to the KA and NMDA receptor classes, and a third site may represent the QA receptor. The fourth binding site does not conform to present receptor classifications. None of these binding sites corresponds to the major glutamate binding site observed in biochemical studies.     

Glycine enhances NMDA-receptor mediated synaptic potentials in neocortical slices

One class of excitatory amino-acid receptors, the N-methyl-D-aspartate (NMDA) receptors, mediates transmission at a small, but important, group of synapses in the neocortex. These receptors are implicated in neuronal plasticity during development in young mammals and in memory acquisition in adults. Recently, responses of isolated membrane patches to NMDA were shown to be greatly enhanced by glycine. This, together with the demonstration that the strychnine-insensitive glycine-binding site is distinct from, but linked to, the NMDA receptor has excited intense interest in glycine as a synaptic modulator. Before proposing a physiological function, however, it is important to determine whether glycine could enhance synaptic responses to NMDA receptor activation in intact, adult tissue. An earlier study failed to demonstrate enhancement of NMDA responses when glycine was applied and it was proposed that in intact tissue the high-affinity glycine site was already saturated by endogenous glycine. It remained possible that glycine concentrations can be maintained at low levels close to synaptic receptors. We have examined responses of neurons in slices of adult neocortex to focal applications of excitatory amino acids and glycine and report enhancement by glycine of NMDA receptor-mediated excitatory postsynaptic potentials.     

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.     

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.     

Synaptic activity at calcium-permeable AMPA receptors induces a switch in receptor subtype

Activity-dependent change in the efficacy of transmission is a basic feature of many excitatory synapses in the central nervous system. The best understood postsynaptic modification involves a change in responsiveness of AMPAR (alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid receptor)-mediated currents following activation of NMDA (N-methyl-D-aspartate) receptors or Ca2+-permeable AMPARs. This process is thought to involve alteration in the number and phosphorylation state of postsynaptic AMPARs. Here we describe a new form of synaptic plasticity--a rapid and lasting change in the subunit composition and Ca2+ permeability of AMPARs at cerebellar stellate cell synapses following synaptic activity. AMPARs lacking the edited GluR2 subunit not only exhibit high Ca2+ permeability but also are blocked by intracellular polyamines. These properties have allowed us to follow directly the involvement of GluR2 subunits in synaptic transmission. Repetitive synaptic activation of Ca2+-permeable AMPARs causes a rapid reduction in Ca2+ permeability and a change in the amplitude of excitatory postsynaptic currents, owing to the incorporation of GluR2-containing AMPARs. Our experiments show that activity-induced Ca2+ influx through GluR2-lacking AMPARs controls the targeting of GluR2-containing AMPARs, implying the presence of a self-regulating mechanism.     

Proton inhibition of N-methyl-D-aspartate receptors in cerebellar neurons.

Mammalian neurons contain at least three types of excitatory amino-acid receptors, selectively activated by N-methyl-D-aspartate (NMDA) or aspartate, (S)-alpha-amino-3-hydroxy-5-methyl-4-isoxazole proprionate ((S)-AMPA) and kainate. An important aspect of NMDA receptors is their regulation by a variety of factors such as glycine, Mg2+ and Zn2+ that are present in vivo. We show here that NMDA receptor responses are selectively inhibited by protons, with a 50% inhibitory concentration (IC50) that is close to physiological pH, implying that NMDA receptors are not fully active under normal conditions. (S)-AMPA and kainate responses remain unchanged at similar pH levels. Proton inhibition is voltage-insensitive and does not result either from fast channel block, a change in channel conductance, or an increase in the 50% excitatory concentration (EC50) of aspartate/NMDA or glycine. Instead, protons seem to decrease markedly the opening frequency of 30-50 pS NMDA channels, and reduce the relative proportion of longer bursts. This feature of NMDA receptors could be relevant to neurotoxic activation of NMDA receptors during ischaemia, as well as to seizure generation, as extracellular proton changes occur during both of these pathological situations. Furthermore, these results may have implications for normal NMDA receptor function as transient changes in extracellular protons occur during synaptic transmission.     

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.     

Different voltage-dependent thresholds for inducing long-term depression and long-term potentiation in slices of rat visual cortex

In the hippocampus and neocortex, high-frequency (tetanic) stimulation of an afferent pathway leads to long-term potentiation (LTP) of synaptic transmission. In the hippocampus it has recently been shown that long-term depression (LTD) of excitatory transmission can also be induced by certain combinations of synaptic activation. In most hippocampal and all neocortical pathways studied so far, the induction of LTP requires the activation of N-methyl-D-aspartate (NMDA) receptor-gated conductances. Here we report that LTD can occur in neurons of slices of the rat visual cortex and that the same tetanic stimulation can induce either LTP or LTD depending on the level of depolarization of the postsynaptic neuron. By applying intracellular current injections or pharmacological disinhibition to modify the depolarizing response of the postsynaptic neuron to tetanic stimulation, we show that the mechanisms of induction of LTD and LTP are both postsynaptic. LTD is obtained if postsynaptic depolarization exceeds a critical level but remains below a threshold related to NMDA receptor-gated conductances, whereas LTP is induced if this second threshold is reached.