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.     

Presynaptic induction of heterosynaptic associative plasticity in the mammalian brain

The induction of associative synaptic plasticity in the mammalian central nervous system classically depends on coincident presynaptic and postsynaptic activity. According to this principle, associative homosynaptic long-term potentiation (LTP) of excitatory synaptic transmission can be induced only if synaptic release occurs during postsynaptic depolarization. In contrast, heterosynaptic plasticity in mammals is considered to rely on activity-independent, non-associative processes. Here we describe a novel mechanism underlying the induction of associative LTP in the lateral amygdala (LA). Simultaneous activation of converging cortical and thalamic afferents specifically induced associative, N-methyl-D-aspartate (NMDA)-receptor-dependent LTP at cortical, but not at thalamic, inputs. Surprisingly, the induction of associative LTP at cortical inputs was completely independent of postsynaptic activity, including depolarization, postsynaptic NMDA receptor activation or an increase in postsynaptic Ca2+ concentration, and did not require network activity. LTP expression was mediated by a persistent increase in the presynaptic probability of release at cortical afferents. Our study shows the presynaptic induction and expression of heterosynaptic and associative synaptic plasticity on simultaneous activity of converging afferents. Our data indicate that input specificity of associative LTP can be determined exclusively by presynaptic properties.     

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.     

Associative long-term depression in the hippocampus induced by hebbian covariance

A brief, high-frequency activation of excitatory synapses in the hippocampus produces a long-lasting increase in synaptic strengths called long-term potentiation (LTP). A test input, which by itself does not have a long-lasting effect on synaptic strengths, can be potentiated through association when it is activated at the same time as a separate conditioning input. Neural network modelling studies have also predicted that synaptic strengths should be weakened when test and conditioning inputs are anti-correlated. Evidence for such heterosynaptic depression in the hippocampus has been found for inputs that are inactive or weakly active during the stimulation of a conditioning input, but this depression does not depend on any pattern of test input activity and does not seem to last as long as LTP. We report here an associative long-term depression (LTD) in field CA1 that is produced when a low-frequency test input is negatively correlated in time with a high-frequency conditioning input. LTD of synaptic strength is also produced by activating presynaptic terminals while a postsynaptic neuron is hyperpolarized. This confirms theoretical predictions that the mechanism for associative LTD is homosynaptic and follows a hebbian covariance rule.     

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.     

An essential role for postsynaptic calmodulin and protein kinase activity in long-term potentiation

The phenomenon of long-term potentiation (LTP), a long lasting increase in the strength of synaptic transmission which is due to brief, repetitive activation of excitatory afferent fibres, is one of the most striking examples of synaptic plasticity in the mammalian brain. In the CA1 region of the hippocampus, the induction of LTP requires activation of NMDA (N-methyl-D-aspartate) receptors by synaptically released glutamate with concomitant postsynaptic membrane depolarization. This relieves the voltage-dependent magnesium block of the NMDA-receptor ion channel, allowing calcium to flow into the dendritic spine. Although calcium has been shown to be a necessary trigger for LTP (refs 11, 12), little is known about the immediate biochemical processes that are activated by calcium and are responsible for LTP. The most attractive candidates have been calcium/calmodulin-dependent protein kinase II (CaM-KII) (refs 13-16), protein kinase C (refs 17-19), and the calcium-dependent protease, calpain. Extracellular application of protein kinase inhibitors to the hippocampal slice preparation blocks the induction of LTP (refs 21-23) but it is unclear whether this is due to a pre- and/or postsynaptic action. We have found that intracellular injection into CA1 pyramidal cells of the protein kinase inhibitor H-7, or of the calmodulin antagonist calmidazolium, blocks LTP. Furthermore, LTP is blocked by the injection of synthetic peptides that are potent calmodulin antagonists and inhibit CaM-KII auto- and substrate phosphorylation. These findings demonstrate that in the postsynaptic cell both activation of calmodulin and kinase activity are required for the generation of LTP, and focus further attention on the potential role of CaM-KII in LTP.     

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.     

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.     

Spike-timing-dependent synaptic plasticity depends on dendritic location

In the neocortex, each neuron receives thousands of synaptic inputs distributed across an extensive dendritic tree. Although postsynaptic processing of each input is known to depend on its dendritic location, it is unclear whether activity-dependent synaptic modification is also location-dependent. Here we report that both the magnitude and the temporal specificity of spike-timing-dependent synaptic modification vary along the apical dendrite of rat cortical layer 2/3 pyramidal neurons. At the distal dendrite, the magnitude of long-term potentiation is smaller, and the window of pre-/postsynaptic spike interval for long-term depression (LTD) is broader. The spike-timing window for LTD correlates with the window of action potential-induced suppression of NMDA (N-methyl-D-aspartate) receptors; this correlation applies to both their dendritic location-dependence and pharmacological properties. Presynaptic stimulation with partial blockade of NMDA receptors induced LTD and occluded further induction of spike-timing-dependent LTD, suggesting that NMDA receptor suppression underlies LTD induction. Computer simulation studies showed that the dendritic inhomogeneity of spike-timing-dependent synaptic modification leads to differential input selection at distal and proximal dendrites according to the temporal characteristics of presynaptic spike trains. Such location-dependent tuning of inputs, together with the dendritic heterogeneity of postsynaptic processing, could enhance the computational capacity of cortical pyramidal neurons.     

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.     

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.     

Long-term potentiation and NMDA receptors in rat visual cortex

In the hippocampus, which is phylogenetically older than the cerebral neocortex, high frequency stimulation of afferent pathways leads to long-term potentiation (LTP) of synaptic transmission. This use-dependent malleability is of considerable interest because it may serve as a substrate for memory processes. However, in the neocortex, whose involvement in learning is undisputed, attempts to demonstrate LTP have remained inconclusive. Here we use intracellular recording techniques to show that LTP can be induced by high frequency stimulation of the optic radiation in slices of the visual cortex of adult rats. We identify as a necessary prerequisite for the induction of LTP the activation of the membrane channel that is associated with the NMDA (N-methyl-D-aspartate) receptor. Selective blockade of this receptor system with DL-2-amino-5-phosphonovalerate consistently prevents LTP as in most hippocampal pathways. In most cortical neurons the activation of the NMDA mechanism and hence the induction of LTP in these experiments requires a concomitant reduction of GABAergic inhibition by low doses of the GABAA antagonist bicuculline. This indicates that in the neocortex the activation threshold of the NMDA-mechanism and consequently the susceptibility to LTP, are strongly influenced by inhibitory processes.     

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.     

Persistent protein kinase activity underlying long-term potentiation

Long-term potentiation (LTP) of synaptic transmission in the hippocampus is a much-studied example of synaptic plasticity. Although the role of N-methyl-D-aspartate (NMDA) receptors in the induction of LTP is well established, the nature of the persistent signal underlying this synaptic enhancement is unclear. Involvement of protein phosphorylation in LTP has been widely proposed, with protein kinase C (PKC) and calcium-calmodulin kinase type II (CaMKII) as leading candidates. Here we test whether the persistent signal in LTP is an enduring phosphoester bond, a long-lived kinase activator, or a constitutively active protein kinase by using H-7, which inhibits activated protein kinases and sphingosine, which competes with activators of PKC (ref. 17) and CaMKII (ref. 18). H-7 suppressed established LTP, indicating that the synaptic potentiation is sustained by persistent protein kinase activity rather than a stably phosphorylated substrate. In contrast, sphingosine did not inhibit established LTP, although it was effective when applied before tetanic stimulation. This suggests that persistent kinase activity is not maintained by a long-lived activator, but is effectively constitutive. Surprisingly, the H-7 block of LTP was reversible; evidently, the kinase directly underlying LTP remains activated even though its catalytic activity is interrupted indicating that such kinase activity does not sustain itself simply through continual autophosphorylation (see refs 9, 13, 15).     

Novel form of long-term potentiation produced by a K+ channel blocker in the hippocampus.

Long-term potentiation (LTP) of synaptic transmission in the hippocampus is a widely studied model of memory processes. In the CA1 region, LTP is triggered by the entry of Ca2+ through N-methyl-D-aspartate (NMDA) receptor channels and maintained by the activation of Ca2(+)-sensitive intracellular messengers. We now report that in CA1, a transient block by tetraethylammonium of IC, IM and the delayed rectifier (IK) produces a Ca2(+)-dependent NMDA-independent form of LTP. Our results suggest that this new form of LTP (referred as to LTPK) is induced by a transient enhanced release of glutamate which generates a depolarization by way of the non-NMDA receptors and the consequent activation of voltage-dependent Ca2+ channels.     

Postsynaptic NMDA receptor-mediated calcium accumulation in hippocampal CA1 pyramidal cell dendrites

In the CA1 hippocampal region, intracellular calcium is a putative second messenger for the induction of long-term potentiation (LTP), a persistent increase of synaptic transmission produced by high frequency afferent fibre stimulation. Because LTP in this region is blocked by the NMDA (N-methyl-D-aspartate) receptor antagonist AP5 (DL-2-amino-5-phosphonovaleric acid) and the calcium permeability of NMDA receptors is controlled by a voltage-dependent magnesium block, a model has emerged that suggests that the calcium permeability of NMDA receptor-coupled ion channels is the biophysical basis for LTP induction. We have performed microfluorometric measurements in individual CA1 pyramidal cells during stimulus trains that induce LTP. In addition to a widespread component of postsynaptic calcium accumulation previously described, we now report that brief high frequency stimulus trains produce a transient component spatially localized to dendritic areas near activated afferents. This localized component is blocked by the NMDA receptor antagonist AP5. The results directly confirm the calcium rise predicted by NMDA receptor models of LTP induction.     

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.     

Long-term potentiation in the hippocampus is blocked by tyrosine kinase inhibitors.

Long-term potentiation (LTP) in the hippocampus is thought to contribute to memory formation. In the Ca1 region, LTP requires the NMDA (N-methyl-D-aspartate) receptor-dependent influx of Ca2+ and activation of serine and threonine protein kinases. Because of the high amount of protein tyrosine kinases in hippocampus and cerebellum, two regions implicated in learning and memory, we examined the possible additional requirement of tyrosine kinase activity in LTP. We first examined the specificity in brain of five inhibitors of tyrosine kinase and found that two of them, lavendustin A and genistein, showed substantially greater specificity for tyrosine kinase from hippocampus than for three serine-threonine kinases: protein kinase A, protein kinase C, and Ca2+/calmodulin kinase II. Lavendustin A and genistein selectively blocked the induction of LTP when applied in the bath or injected into the postsynaptic cell. By contrast, the inhibitors had no effect on the established LTP, on normal synaptic transmission, or on the neurotransmitter actions attributable to the actions of protein kinase A or protein kinase C. These data suggest that tyrosine kinase activity could be required postsynaptically for long-term synaptic plasticity in the hippocampus. As Ca2+ calmodulin kinase II or protein kinase C seem also to be required, the tyrosine kinases could participate postsynaptically in a kinase network together with serine and threonine kinases.     

Presynaptic enhancement shown by whole-cell recordings of long-term potentiation in hippocampal slices

Long-term potentiation (LTP) of synaptic transmission in the hippocampus is a widely studied model system for understanding the cellular mechanisms of memory. In region CA1, LTP is triggered postsynaptically by Ca2(+)-dependent activation of protein kinases, but the locus of persistent modification remains controversial. Statistical analysis of synaptic variability has been proposed as a means of settling this debate, although a major obstacle has been the poor signal-to-noise ratio of conventional intracellular recordings. We have applied the whole-cell voltage clamp technique to study synaptic transmission in conventional hippocampal slices (compare refs 28-30). Here we report that robust LTP can be recorded with much improved signal resolution and biochemical access to the postsynaptic cell. Prolonged dialysis of the postsynaptic cell blocks the triggering of LTP, with no effect on expression of LTP. The improved signal resolution unmasks a large trial-to-trial variability, reflecting the probabilistic nature of transmitter release. Changes in the synaptic variability, and a decrease in the proportion of synaptic failures during LTP, suggest that transmitter release is significantly enhanced.     

Long-term potentiation is associated with increases in quantal content and quantal amplitude.

Long-term potentiation (LTP) of synaptic transmission in CA1 neurons of the hippocampus, elicited by the conjunction of presynaptic firing and postsynaptic depolarization, is an important model of plasticity, which may underlie memory storage. Although induction of LTP takes place in the postsynaptic cell, it is not clear whether it is expressed through an enhancement of transmitter release or through an increased postsynaptic response to the same amount of transmitter. Analysis of the trial-to-trial amplitude fluctuations of synaptic signals, that is quantal analysis, gives an important insight into the probabilistic mechanisms of transmission, although attempts to apply it to the mode of expression of LTP have so far yielded inconsistent results, at least in part because they have relied on models of transmitter release that have not been confirmed experimentally. Here we report clear evidence for quantal fluctuation in a subset of cells. Induction of LTP in these cells causes abrupt increases in either quantal content or quantal amplitude, or both. This shows that two different mechanisms can underlie the maintenance of LTP.