Showing papers on "Long-term potentiation published in 1991"

A requirement for the intercellular messenger nitric oxide in long-term potentiation

Abstract: Long-term potentiation (LTP) of synaptic transmission is a widely studied model of neuronal plasticity. The induction of LTP is known to require processes in the postsynaptic neuron, while experimental evidence suggests that the expression of LTP may occur in the presynaptic terminal. This has led to speculation that a retrograde messenger travels from the post- to the presynaptic cell during induction of LTP. Extracellular application or postsynaptic injection of two inhibitors of nitric oxide synthase, N-nitro-L-arginine or NG-methyl-L-arginine, blocks LTP. Extracellular application of hemoglobin, which binds nitric oxide, also attenuates LTP. These findings suggest that nitric oxide liberated from postsynaptic neurons may travel back to presynaptic terminals to cause LTP expression.

Tests of the roles of two diffusible substances in long-term potentiation: evidence for nitric oxide as a possible early retrograde messenger.

Abstract: Although long-term potentiation (LTP) in the CA1 region of the hippocampus is initiated postsynaptically by the influx of Ca2+ through N-methyl-D-aspartate receptor channels, the maintenance of LTP seems to be at least in part presynaptic. This suggests that the postsynaptic cell releases a retrograde messenger to activate the presynaptic terminals. It is likely that this messenger is membrane-permeant and reaches the presynaptic neuron by diffusion. We therefore have investigated two major membrane-permeant candidate retrograde messengers, arachidonic acid and nitric oxide (NO). Consistent with arachidonic acid or a lipoxygenase metabolite being a retrograde messenger, the phospholipase A2 and lipoxygenase inhibitor nordihydroguaiaretic acid blocked LTP in the guinea pig CA1 region in vitro. However, arachidonic acid (up to 100 microM) did not reliably produce activity-independent LTP, and activity-dependent potentiation by arachidonic acid was blocked by DL-aminophosphonovaleric acid. Since nordihydroguaiaretic acid also interferes with signal transduction involving NO, we next examined whether inhibitors of NO synthase block LTP. NG-Nitro-L-arginine blocked LTP when given in the bath, and this inhibition was partially overcome by high concentrations of L-arginine, suggesting that the inhibitor is specific to NO synthase. NG-Nitro-L-arginine and NG-methyl-L-arginine (but not NG-methyl-D-arginine) also blocked LTP when injected intracellularly, indicating that NO synthase is located in the postsynaptic cell. The NO, in turn, seems to be released into the extracellular space, since bathing the slice with hemoglobin, a protein that binds NO and is not taken up by cells, also blocked LTP. Moreover, NO enhances spontaneous presynaptic release of transmitter from hippocampal neurons in dissociated cell culture. These data favor the idea that NO might be a retrograde messenger in LTP.

Mechanisms underlying long-term potentiation of synaptic transmission.

Abstract: A curious property of excitatory synapses in the hippocampus and some other neural tissues is that when they are heavily used, they undergo a long­ lasting increase in their efficacy. Brief repetitive activation of hippocampal excitatory synapses results in a substantial increase in synaptic strength that can last for several hours and has been detected even weeks after induction. This use-dependent strengthening of a synapse is known as long-term potentiation, or more commonly, LTP (Bliss & Lorna 1973, Lomo 1966, Bliss & Lynch 1988). LTP occurs most prominently in the hippocampus, where consolidation of experience into long-term memory is thought to occur. LTP is the most compelling and widely studied model

GABA B autoreceptors regulate the induction of LTP

Abstract: UNDERSTANDING the mechanisms involved in long-term potenti-ation (LTP) should provide insights into the cellular and molecular basis of learning and memory in vertebrates1. 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 system2. During low-frequency transmission, significant activation of this system is prevented by γ-aminobutyric acid (GABA) mediated synaptic inhibition3,4 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 block7 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.

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

Abstract: 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.

Dendritic spines as individual neuronal compartments for synaptic Ca2+ responses.

Abstract: The possibility that postsynaptic spines on neuronal dendrites are discrete biochemical compartments for Ca(2+)-activated processes involved in synaptic plasticity is a widely proposed concept that has eluded experimental demonstration. Using microfluorometry on CA3 neurons in hippocampal slices, we show here that with weak presynaptic stimulation of associative/commissural fibres, Ca2+ accumulates in single postsynaptic spines but not in the parent dendrite. Stronger stimulation also promotes changes in dendrites. The NMDA-receptor antagonist AP-5 blocks changes in Ca2+ in spines. Sustained steep Ca2+ gradients between single spines and the parent dendrite, often lasting several minutes, develop with repeated stimulation. The observed compartmentalization allows for the specificity, cooperativity and associativity displayed by memory models such as long-term potentiation.

Agonists at metabotropic glutamate receptors presynaptically inhibit EPSCs in neonatal rat hippocampus.

Abstract: 1. The effects of metabotropic glutamate receptor agonists on excitatory synaptic transmission in the CA1 region of rat hippocampal slices (11-30 days) were studied using extracellular and whole-cell patch-clamp recording techniques. 2. Trans-1-amino-1,3-cyclopentanedicarboxylic acid (trans-ACPD; 25-100 microM) reversibly depressed excitatory postsynaptic currents (EPSCs) without affecting presynaptic fibre excitability or EPSC reversal potential. 3. Ibotenate (25 microM) or L-glutamate (250 microM), in the presence of the N-methyl-D-aspartate (NMDA) receptor antagonist, D-2-amino-5-phosphonovaleric acid (APV, 50-75 microM), depressed the EPSC amplitude while inducing no detectable inward current. L-2-Amino-4-phosphonobutyrate (L-AP4, 25-100 microM), the phosphonic derivative of glutamate, also depressed EPSC amplitude and caused no detectable inward current. 4. The NMDA receptor-mediated component of the EPSC recorded in the presence of the non-NMDA receptor antagonist, 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX, 20-30 microM) was depressed by trans-ACPD, L-AP4, or quisqualate (1-2 microM). 5. The response to ionophoretic application of alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid (AMPA) was unaffected by trans-ACPD or L-AP4 although the simultaneously recorded EPSC was strongly depressed. In addition, paired-pulse facilitation (50-75 ms interstimulus interval) was reversibly enhanced by trans-ACPD or L-AP4. These results indicate that the depression of synaptic transmission likely was mediated by a presynaptic 'autoreceptor'. 6. The effects of trans-ACPD or L-AP4 on synaptic transmission decreased significantly over ages 12-30 days and were minimal in adult (greater than 80 days) slices. 7. The depression of synaptic transmission caused by trans-ACPD or L-AP4 was not altered following the induction of long-term potentiation (LTP). 8. The results indicate that metabotropic glutamate receptor agonists suppress excitatory synaptic transmission in CA1 pyramidal cells by an action at a presynaptic site. This effect is developmentally regulated and is maximally expressed during the first postnatal month.

Inhibition of proteolysis protects hippocampal neurons from ischemia.

Abstract: Intense proteolysis of cytoskeletal proteins occurs in brain within minutes of transient ischemia, possibly because of the activation of calcium-sensitive proteases (calpains). This proteolytic event precedes overt signs of neuronal degeneration, is most pronounced in regions of selective neuronal vulnerability, and could have significant consequences for the integrity of cellular function. The present studies demonstrate that (i) the early phase of enhanced proteolysis is a direct response to hypoxia rather than other actions of ischemia, (ii) it is possible to pharmacologically inhibit the in vivo proteolytic response to ischemia, (iii) inhibition of proteolysis is associated with a marked reduction in the extent of neuronal death, and (iv) protected neurons exhibit normal-appearing electrophysiological responses and retain their capacity for expressing long-term potentiation, a form of physiological plasticity thought to be involved in memory function. These observations indicate that calcium-activated proteolysis is an important component of the post-ischemic neurodegenerative response and that targeting this response may be a viable therapeutic strategy for preserving both the structure and function of vulnerable neurons.

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

Abstract: 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.

Altered synaptic plasticity in Drosophila memory mutants with a defective cyclic AMP cascade

Abstract: Synaptic transmission was examined in Drosophila mutants deficient in memory function These mutants, dunce and rutabaga, are defective in different steps of the cyclic adenosine 3',5'-monophosphate (cAMP) cascade In both dunce and rutabaga larvae, voltage-clamp analysis of neuromuscular transmission revealed impaired synaptic facilitation and post-tetanic potentiation as well as abnormal responses to direct application of dibutyryl cAMP In addition, the calcium dependence of transmitter release was shifted in dunce The results suggest that the cAMP cascade plays a role in synaptic facilitation and potentiation and indicate that synaptic plasticity is altered in Drosophila memory mutants

Facilitation of the induction of long-term potentiation by GABAB receptors

Abstract: Long-term potentiation (LTP), an in vitro model of learning, was induced in hippocampal slices by 5-hertz stimulation. During induction, gamma-aminobutyric acid A (GABAA) inhibition decreased, causing the N-methyl-D-aspartate receptor-mediated excitation to increase. 2-OH Saclofen, a GABAB receptor antagonist, prevented the reduction of inhibition, the increase of excitation, and the induction of LTP. Therefore, disinhibition caused by GABAB receptors is required for induction of LTP by 5-hertz stimulation. GABAB receptor modulation of synaptic plasticity occurs at frequencies in the range of the endogenous hippocampal theta rhythm, which has been shown to modulate LTP in vivo.

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

Abstract: LONG-term potentiation (LTP) of synaptic transmission in the hippocampus is a widely studied model of memory processes1. In the CA1 region, LTP is triggered by the entry of Ca2+ through N-methyl-D-aspartate (NMD A) receptor channels and maintained by the activation of Ca2+-sensitive intracelluar messengers2,3. 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-NMD A receptors and the consequent activation of voltage -dependent Ca2+ channels.

N-methyl-D-aspartate receptor antagonist APV blocks acquisition but not expression of fear conditioning.

Abstract: The role of N-methyl-o-aspartate (NMDA) receptors in Pavlovian fear conditioning was examined using the NMDA antagonist DL-2-amino-5-phosphonovaleric acid (APV). Either APV (5 #g/rat) or saline was administered before the training phase, the testing phase, or both. APV completely blocked acquisition but not expression of fear conditioning. The L enantiomer of APV did not affect the acquisition of conditional fear. To separate encoding from consolidation processes, APV was administered either before or immediately after the footshock unconditional stimulus (US) during the training phase. The results indicate that APV must be present during the US to produce its effects on fear conditioning. The behavioral effect of the drug is not due to analgesic action because APV did not alter pain sensitivity. The data suggest that NMDA receptors are critical for the acquisition but not expression of fear conditioning. These effects on fear conditioning are parallel to the in vitro effects of APV on the acquisition but not expression of long-term potentiation (LTP) and suggest that endogenously generated NMDAdependent LTP participates in the neural plasticity underlying fear conditioning. Fear conditioning is a rapidly acquired and persistent form of simple associative learning. When a rat is placed in a novel chamber and presented with a mild footshock, shortly afterward the animal freezes. For rats, this freezing response is the dominant species-specific defense reaction to conditional

Enhancement of spinothalamic neuron responses to chemical and mechanical stimuli following combined micro-iontophoretic application of N-methyl-D-aspartic acid and substance P.

Abstract: A role for sensitization of nociceptors in the generation of primary hyperalgesia is well documented. More recent work has begun to define a role of an increased excitability of neurons within the spinal cord in the generation of secondary hyperalgesia. The present study demonstrates increased responses of primate spinothalamic neurons following co-administration of N-methyl-D-aspartic acid (NMDA) and substance P (SP) by micro-iontophoresis. Wide dynamic range and high threshold STT neurons in laminae I-VI showed an increased frequency of discharges following application of NMDA which was characterized by a slow onset to peak discharge rate and a slow return to background levels of discharge. Combined application of NMDA with SP resulted in an enhancement of responses to NMDA that often long outlasted the administration of SP. This increase in response of the cells to NMDA was not produced by repeated application of NMDA alone or following combined application of NMDA with an SP analog. NMDA responses were reduced or prevented in all cases by co-application of an NMDA-receptor antagonist. Finally, long-lasting potentiation of NMDA responses by SP was paralleled by enhanced responses to mechanical stimulation of skin. It is proposed that a mechanism involving the combined synaptic release of excitatory amino acids and peptides leads to secondary hyperalgesia.

N-methyl-D-aspartate receptor activation increases cAMP levels and voltage-gated Ca2+ channel activity in area CA1 of hippocampus.

Abstract: Tetanic stimulation of the Schaffer collateral inputs into area CA1 of the hippocampus causes N-methyl-D-aspartate (NMDA) receptor activation, an effect that contributes to the induction of long-term potentiation (LTP) in this region. The present studies demonstrate that LTP-inducing tetanic stimulation in rat hippocampal area CA1 elicited increased levels of cAMP. The elevation of cAMP was blocked by the NMDA receptor antagonist DL-2-amino-5-phosphonovaleric acid (APV). Bath application of NMDA also resulted in an increase in cAMP in CA1, an effect that was blocked by both APV and removal of extracellular Ca2+. These findings suggest that activation of NMDA receptors elicits a Ca(2+)-dependent increase in cAMP, and taken together with the data from tetanic stimulation, suggest that NMDA-receptor-mediated increases in cAMP could play a role in the induction of LTP in area CA1. One role for cAMP may be to increase Ca2+ influx through voltage-gated Ca2+ channels, as it was observed that application of either 8-bromo-cAMP or NMDA increased the fractional open time of high-threshold Ca2+ channels in CA1 pyramidal cells. Our results raise the possibility that a positive-feedback loop for Ca2+ influx in area CA1 exists. In this model, NMDA receptor-mediated Ca2+ influx leads to an enhancement of further Ca2+ influx via intermediate steps of increased cAMP and subsequent increased voltage-gated Ca2+ channel activity.

The role of immediate early genes in the stabilization of long-term potentiation.

Abstract: Immediate early genes (IEGs) are a class of genes that show rapid and transient but protein synthesisindependent increases in expression to extracellular signals such as growth factors and neurotransmitters. Many IEGs code for transcription factors that have been suggested to govern the growth and differentiation of many cell types by regulating the expression of other genes. IEGs are expressed in adult neurons both constitutively and in response to afferent activity, and it has been suggested that during learning, IEGs may play a role in the signal cascade, resulting in the expression of genes critical for the consolidation of long-term memory. Long-term potentiation (LTP) is a persistent activity-dependent form of synaptic plasticity that stands as a good candidate for the mechanism of associative memory. A number of IEGs coding for transcription factors have been shown to transiently increase transcription in the dentate gyrus of rats following LTP-inducing afferent stimulation. These includezif/268 (also termedNGFI-A, Krox-24, TIS-8, andegr-l),c-fos-related genes,c-jun, junB, and junD. Of these,zif/268 appears to be the most specifically related to LTP since it is evoked under virtually all LTP-inducing situations and shows a remarkably high correlation with the duration of LTP. There are a number of outstanding questions regarding the role ofzif/268 and other IEGs in LTP, including which second messenger systems are important for activating them, which “late effector” genes are regulated by them, and the exact role these genes play, if any, in the stabilization and maintenance of LTP.

Transmission between pairs of hippocampal slice neurons: quantal levels, oscillations, and LTP.

Abstract: Long-term potentiation (LTP) of synaptic transmission after coincident pre- and postsynaptic activity is considered a cellular model of changes underlying learning and memory. In intact tissue, LTP has been observed only between populations of neurons, making analysis of mechanisms difficult. Transmission between individual pre- and postsynaptic hippocampal cells was studied, suggesting quantal amplitude distributions with little variability in quantal size. LTP between such pairs is manifested by large, persistent, and synapse-specific potentiation with a shift in amplitude distribution that suggests presynaptic changes. Oscillations in amplitude of transmission, apparently of presynaptic origin, are common and can be triggered by LTP.

Ethanol potentiation of 5-hydroxytryptamine3 receptor-mediated ion current in neuroblastoma cells and isolated adult mammalian neurons.

Abstract: Recent studies indicate that ethanol (EtOH) potentiates ion current through the channel associated with the 5-hydroxytryptamine3 (5-HT3)-type serotonin receptor. The present study was designed to determine 1) whether such potentiation occurs in adult mammalian neurons expressing 5-HT3 receptors; 2) whether potentiation is selective for the 5-HT3 receptor, relative to other ligand-gated ion channels; and 3) possible mechanisms by which EtOH potentiates this response. EtOH potentiated 5-HT3 receptor-mediated ion current in freshly isolated nodose ganglion neurons at concentrations similar to those previously reported to be effective in neuroblastoma cells (25-100 mM). Current was blocked by the selective 5-HT3 antagonist ICS 205-930 even in the presence of EtOH, and current activated by a 5-HT3 agonist (2-methyl-5-HT) was potentiated by EtOH. Thus, EtOH appears to produce potentiation via an alteration in the function of 5-HT3 receptors and not through an independent effect. gamma-Aminobutyric acidA receptor-mediated Cl- current was not potentiated by EtOH in neurons in which potentiation of responses to 5-HT was observed. Methanol potentiated 5-HT3 receptor-mediated current with a potency lower than that of EtOH. Potentiation by EtOH decreased with increasing 5-HT concentration. In addition, EtOH increased the decay rate of current. EtOH did not alter the reversal potential of the 5-HT3 receptor-mediated current. These observations indicate that intoxicating concentrations of EtOH selectively potentiate 5-HT3 receptor-mediated responses by increasing the apparent potency of 5-HT for activating ion current.

Long-term potentiation : a debate of current issues

Abstract: Part 1 Molecular and cellular processes in synaptic plasticity: mGluR1 and mGluR5 glutamate receptors - molecular biology, pharmacology and roles on hippocampal synaptic transmission, Joel Bockaert et al AMPA-glutamate receptor regulation and synaptic plasticity, Steve Standley et al the expression of LTP of glutamate receptor-mediated synaptic transmission as determined by the redox state of NMDA receptors and the extent of NMDA receptor activation during a tetanus, June C. Hirsch et al analysis of synaptic plasticity and memory in the mammalian brain with the gene-knockout technology, Chong Chen and Susumu Tonegawa. Part 2 LTP and LTD mechanisms: involvement of AMPA receptors in LTP mechanisms and memory, John Larson and Peter W. Vanderklish mechanisms of homosynaptic LTD in the hippocampus, Robert C. Malenka homosynaptic LTD and depotentiation in the hippocampus in vivo, Serge Laroche et al LTP and LTD in the visual cortex, Alfredo Kirkwood and Mark F. Bear electroresponsive properties of cerebellar purkinje cells in mGlur1 gene-lacking mice, Francis Crepel et al LTD and LTP at the corticostriatal synapse, Antonio Pisani et al. Part 3 Synaptic plasticity in network processes: generation of temporal correlations in firing of pyramidal cells by networks of inhibitory neurons, Roger D. Traub et al LTP and LTD and the encoding of memory in small ensembles of hippocampal neurons, Robert E. Hampson and Sam A. Deadwyler the constraint of synaptic potentiation and memory formation by entorhinal-hippocampal network dynamics, James J. Chrobak and Gyorgy Buzsaki network determinants of hippocampal synaptic plasticity, Theodore W. Berger et al. Part 4 Synaptic plasticity in network models: what LTP, LTD and cortical receptive fields tell us about synaptic modification, Harel Shouval and Leon M. Cooper linking LTP to network function - a simulation of episodic memory in the hippocampal formation, Michael E. Hasselmo and Chantal E. Stern adaptive stimulus representations in a computational model of cortical-hippocampal function, Mark A. Gluck and Catherine E. Myers adapting recurrent cortical excitation, Christof Koch et al.

Potentiation of the effects of reward-related stimuli by dopaminergic-dependent mechanisms in the nucleus accumbens.

Abstract: Three experiments examined the behavioural, pharmacological and neural specificity of the previously reported potentiation of responding with conditioned reinforcement following intra-accumbensd-amphetamine, by studying the effects of intraaccumbens dopamine (DA) and noradrenaline, using an acquisition of a new response procedure. In experiment 1, the effects of intra-cerebral DA infusions (5, 20, 50 µg/2 µl) were compared in four conditions: (i) intra-accumbens DA following positive pairing of the conditioned stimulus (CS) and water during training; (ii) as (i) but also following a systemic dose of the DA receptor antagonist alpha-flupenthixol; (iii) intra-accumbens DA following random pairing of the CS and water during training; and (iv) as (i) but with intra-caudate rather than intra-accumbens DA. The results showed that only with intra-accumbens DA in the positive pairing condition was there a significant dose-dependent increase in responding. In experiment 2, the effects of a higher range of doses (20, 100, 200 µg) and smaller infusion volume (5, 25, 50 µg/l µl) of intra-accumbens DA were studied, in comparison with a similar range of doses (5, 25, 50 µg/l µl) of intra-accumbens noradrenaline (NA). Only DA produced a selective, dose-dependent increase in responding with conditioned reinforcement. In experiment 3 neurotoxic lesions of the dorsal noradrenergic bundle (DNAB) using 6-hydroxydopamine producing profound (about 90%) depletion of cortical and nucleus accumbens NA levels had no effect on the increased responding with conditioned reinforcement produced by intra-accumbensd-amphetamine (3, 10, 30 µg/l µl). The results are discussed in terms of the neurochemical mediation of the potentiation of the effects of conditioned reinforcers byd-amphetamine and the role of DA-dependent mechanisms of the nucleus accumbens in reward-related processes.