Molecular mechanism of neuronal plasticity: induction and maintenance of long-term potentiation in the hippocampus

Recent studies have demonstrated that activation of enzymes can be observed in living cells in response to stimulation with neurotransmitters, hormones, growth factors, and so forth. Thus, the activation of enzymes was shown to be closely related to the dynamic states of various cell functions. The development of new experimental methodologies has enabled researchers to study the molecular basis of neuronal plasticity in living cells. In 1973, Bliss and his associates identified the phenomena of long-term potentiation (LTP). Since it was thought to be a model for neuronal plasticity such as learning and memory, its molecular mechanism has been extensively investigated. The mechanism was found to involve a signal transduction cascade that includes release of glutamate, activation of the NMDA glutamate receptors, Ca(2+) entry, and activations of Ca(2+)/calmodulin-dependent protein kinases (CaM kinases) II and IV and mitogen-activated protein kinase (MAPK). Consequently, AMPA glutamate receptors were activated by phosphorylation by CaM kinase II, resulting in an increase of Ca(2+) entry into postsynaptic neurons. Furthermore, activation of CaM kinase IV and MAPK increased phosphorylation of CREB (cyclic AMP response element binding protein) and expression of c-Fos by stimulation of gene expression. These results suggest that LTP induction and maintenance would be models of short- and long-term memory, respectively.     

L-glutamate and its ionotropic receptors in the nervous system of cephalopods

In several species of cephalopod molluscs there is good evidence for the presence of L-glutamate in the central and peripheral nervous system and evidence for both classes of ionotropic receptor, AMPA/kainate and NMDA.The best evidence for glutamate being a transmitter in cephalopods comes from pharmacological, immunohistochemical and molecular investigations on the giant fibre system in the squid stellate ganglion. These studies confirm there are AMPA/kainate-like receptors on the third-order giant axon. In the (glial) Schwann cells associated with the giant axons both classes of glutamate receptor occur.Glutamate is an excitatory transmitter in the chromatophores and in certain somatic muscles and its action is mediated primarily via AMPA/kainate-like receptors, but at some chromatophores there are NMDA-like receptors.In the statocysts the afferent crista fibres are also glutamatergic, acting at non-NMDA receptors.In the brain (of Sepia) a neuronal NOS is activated by glutamate with subsequent production of nitric oxide and elevation of cGMP levels. This signal transduction pathway is blocked by D-AP-5, a specific antagonist of the NMDA receptor.Recently immunohistochemical analysis has demonstrated (in Sepia and Octopus) the presence of NMDAR2A /B - like receptors in motor centres, in the visual and olfactory systems and in the learning system. Physiological experiments have shown that glutamatergic transmission is involved in long term potentation (LTP) in the vertical lobe of Octopus, a brain area involved in learning. This effect seems to be mediated by non-NMDA receptors. Finally in the CNS of Sepia NMDA-mediated nitration of tyrosine residues of cytoskeletal protein such as alpha-tubulin, has been demonstrated.     

Nitric oxide release and long term potentiation at synapses in autonomic ganglia

• 1.1. Long-term potentiation (LTP) of synaptic transmission in autonomic ganglia is reviewed, together with the possible role of nitric oxide (NO) in this process.
• 2.2. Calcium levels in preganglionic nerve terminals are elevated during at least the induction phase of LTP following a tetanus as well as during LTP induced by transmitter substances acting on the nerve terminals. Of the large number of calcium-dependent processes in the nerve terminal that might affect transmitter release, only calcium-calmodulin has been shown to be important in both the induction and maintenance of LTP.
• 3.3. The possibility that there is a decrease in the open time of nerve-terminal potassium channels following a tetanus, leading to an increase in duration of the terminal action potential and hence an increase in calcium influx and transmitter release is considered. There is little evidence for such an effect as yet for preganglionic nerve terminals.
• 4.4. Phosphorylation of potassium channels by cAMP-dependent protein kinase can lead to their inactivation with consequent action potential broadening in some systems. Exogenous cAMP enhances synaptic efficacy at preganglionic nerve terminals. Whether this occurs through an inactivation of potassium channels is not known.
• 5.5. Nitric oxide (NO) synthase is present in both sympathetic ganglia and the ciliary ganglia. NO increases synaptic efficacy in both ganglia. In at least the case of ciliary ganglion this is due to elevation of quantal secretion.
• 6.6. NO can in some conditions increase the terminal action potential duration in ciliary ganglia, probably through decrease in the Ic potassium current. There is evidence that this happens through cGMP modulating cAMP phosphodiesterases, thereby affecting cAMP phosphorylation of the Ic channel.
• 7.7. Blocking NO synthase markedly decreases LTP following a tetanus in the ciliary ganglion. The possibility is considered that NO is released from the terminal during a tetanus and through altering cAMP phosphorylation of Ic enhances transmitter release.

     

Astrocyte origin of activity-dependent release of ATP and glutamate in hippocampal slices: real-time measurement utilizing microelectrode biosensors

It is well known that astrocytic and neuronal transmitter release processes are important for signalling, and that activity-dependent release of adenosine nucleotides and transmitters occurs after stimulation. Neurons and astrocytes can account for the source of ATP efflux. In this issue of the BJP, Heinrich et al. characterized K⁺ depolarization-evoked release of ATP, adenosine and glutamate in hippocampal slices, utilizing microelectrode biosensors for simultaneous real-time recordings of multiple transmitter effluxes. They demonstrated efflux of ATP, adenosine and glutamate from hippocampus slices, in response to K⁺-depolarization, with distinct kinetics and mechanisms, suggesting a coordinated pattern of transmitter release. Surprisingly, it turned out that a considerable amount of the transmitter efflux measured under these conditions had a glial origin. For a long time, it was believed that the glial cell did not play a major role in neurotransmission, but the latter results somewhat change this view. The release of ATP and glutamate from glial cells under these conditions involved P2X7 receptors, and a source of adenosine accumulation independent of the metabolism of extracellular ATP was identified. This study also highlighted a novel use of multi-enzymatic microelectrode biosensors, which enabled a better characterization of transmitter release processes with higher temporal and spatial resolution than obtained previously. This technique was originally developed and used for the detection of purine release. In the present study, it was modified to identify the interplay between different transmitters, measured simultaneously in hippocampal slices.

LINKED ARTICLE

This article is a commentary on Heinrich et al., pp. 1003–1020 of this issue. To view this paper visit http://dx.doi.org/10.1111/j.1476-5381.2012.01932.x     

The effects of sigma ligands on the release of glutamate from rat striatal slices

This study investigated the effects of sigma receptor ligands on the release of endogenous amino acid neurotransmitters from rat striatal slices. The effect of haloperidol on release in slices prepared from 6-hydroxydopamine lesioned animals was also tested. Haloperidol, the (±) reduced metabolite of haloperidol, rimcazole and ifenprodil specifically reduced potassium-stimulated release of glutamate with IC₅₀ values between 20–60 M. The release of aspartate, -aminobutyric acid (GABA) and glycine was not affected. Haloperidol also reduced glutamate release from slices prepared from lesioned animals. The neuroleptic drug a-flupenthixol and the putative sigma receptor ligand R(+)3-(3-hydroxyphenyl)-N(n-propyl) piperidine (3-PPP) had no effect on release.These effects of the sigma ligands show that the inhibition of glutamate release is specific to this amino acid and also that it is not due to dopamine receptor blockade as those ligands which have low affinity for dopamine receptors were also effective in reducing release. A presynaptic location for sigma receptor sites, possibly associated with ion channels, could account for the effects of these ligands on transmitter release. Correspondence to: J. A. Davies at the above address     

Candidate mechanisms for inhibition of neurotransmitter release by narcotic analgesics and endorphins

Some neurones are endowed with receptors for endorphins and narcotic analgesics. Activation of these receptors results in a depression of the release of transmitter per impulse. It is currently believed that narcotic analgesics and endorphins depress the stimulus-induced influx of calcium (Ca2+) into the terminal and thereby modify the amount of the ion which triggers the release of the transmitter from intracellular stores. The influx of Ca2+ is largely governed by the Ca2+ "channel", which opens during depolarization of the neuronal membrane either after an action potential (electrical stimulus) or in the presence of high extracellular potassium (K+) or nicotinic stimulants (chemical stimulus). The evoked influx of Ca2+ can be affected by a direct action on the Ca2+ "channel" or by primary actions on other membrane properties that subsequently regulate the Ca2+ "channel". In many tissues narcotic analgesics and endorphins fail to inhibit transmitter release. This may be accounted for by the possibility that either such neurones lack presynaptic opiate receptors or that the function of existing receptors remains latent under the experimental conditions employed. Currently, there is insufficient evidence for endorphins physiologically modulating transmitter release.     

Dopamine D1 receptors and group I metabotropic glutamate receptors contribute to the induction of long-term potentiation in the nucleus accumbens

Long-term changes in the efficacy of glutamatergic synaptic transmission in the striatal complex are proposed to underlie motor learning and neuroadaptations leading to addiction. Dopamine and glutamate play key roles in the induction of long-term potentiation (LTP) and long-term depression (LTD) in the dorsal striatum, but their contribution to synaptic plasticity in the ventral striatum (nucleus accumbens, NAc) has been less extensively studied. We have examined the role of dopamine, glutamate and GABA in the induction of LTP in mouse brain slices containing the NAc. High-frequency stimulation of glutamatergic inputs elicited LTP of field excitatory postsynaptic potentials/population spikes (fEPSP/PSs) in the core region of the NAc. GABA did not seem to participate in LTP induction because LTP was not altered in the presence of either a GABA(A)- (bicuculline) or a GABA(B)- (CGP 55845) receptor antagonist. However, the dopamine D1 receptor antagonist SCH 23390, but not the dopamine D2 receptor antagonist sulpiride, impaired LTP. The dopamine reuptake blocker nomifensine also inhibited LTP induction. We found that group I metabotropic glutamate receptors (mGluRs) contribute to LTP induction because the mGluR1 antagonist LY 367385, or the mGluR5 antagonist MPEP, blocked LTP induction. Furthermore, the glutamate reuptake blocker DL-TBOA also impaired LTP. The present results demonstrate that dopamine and glutamate play critical roles in the mechanisms of induction of LTP in the NAc through the activation of dopamine D1 receptors and group I mGluRs. However, LTP is negatively regulated when endogenous levels of dopamine or glutamate are elevated.     

GM1 ganglioside administration partially counteracts the morphological changes associated with haloperidol treatment within the dorsal striatum of the rat

Haloperidol, a typical antipsychotic drug, causes an increase in the mean percentage of synapses within the striatum containing a discontinuous, or perforated, postsynaptic density (PSD) following 1 month of treatment (Meshul et al. 1994). This effect is not observed with the atypical antipsychotic drug, clozapine, following subchronic administration (Meshul et al. 1992a). This morphological change is also associated with an increase in the density of dopamine D₂ receptors. The synapses containing the perforated PSD are asymmetrical and the nerve terminals contain the neurotransmitter, glutamate, as demonstrated by immunocytochemistry. We have also shown that subchronic treatment with haloperidol (0.5 mg/kg per day, 30 days) results in a decrease in the density of glutamate immunoreactivity within asymmetric nerve terminals associated with perforated and non-perforated PSDs (Meshul and Tan 1994). This could be due to an increase in glutamate release, perhaps due to activation of corticostriatal synapses. Agnati et al. (1983a) reported that administration of GM1 ganglioside blocks the increase in dopamine D₂ receptors following haloperidol treatment. GM1 has also been shown to attenuate the release of glutamate (Nicoletti et al. 1989). In order to determine if similar treatment with ganglioside could block the haloperidol-induced ultrastructural changes noted above, rats were coadministered GM1 (10 mg/kg per day) and haloperidol (0.5 mg/kg per day) for 30 days. We report that GM1 blocked the haloperidol-induced increase in striatal asymmetric synapses containing a perforated PSD, but had no effect on the increase in dopamine D₂ receptors or the decrease in nerve terminal glutamate immunoreactivity. GM1, either alone or co-administered with haloperidol, also caused a small, but significant, increase in the density of all asymmetric synapses within the striatum. It is possible that the effect of GM1 in attenuating the haloperidol-induced change in glutamate synapses with perforated PSDs is primarily postsynaptic, since GM1 did not block the change in density of glutamate immunoreactivity within asymmetric nerve terminals.     

Rat hippocampal NMDA receptor binding as a function of chronic lead exposure level

Chronic developmental lead (Pb) exposure is known to impair cognitive ability in children and young animals. These findings have led to research examining exposure effects on long-term potentiation (LTP), a model of synaptic plasticity, and on NMDA receptor function. This study determined the changes occurring in hippocampal 3H-MK-801 binding as a function of exposure level for comparison to changes in LTP previously reported from this laboratory. Dams were exposed to 0.1%, 0.2%, 0.5% and 1.0% Pb in the drinking water beginning at parturition, and male offspring were weaned to the same solutions as their dams and maintained on these regimens until assessment as adults. A crude membrane fraction was prepared from hippocampal tissue, and Scatchard analysis conducted in the presence of saturating concentrations of glutamate and glycine. NMDA receptor density was elevated as a result of Pb exposure with significant increases in the 0.2% (38%) and 0.5% (30%) groups compared to control group values. No changes were observed in the 0.1% and 1.0% animals, thus constituting a biphasic dose-effect relationship. These findings are an approximate reflection of analogous relationships reported for hippocampal LTP and glutamate release, suggesting that the diminished glutamate release is one cause of the receptor up-regulation. However, since increases in receptor number were uncovered, it is unlikely that changes in NMDA receptor density constitute a primary mechanism whereby Pb impairs hippocampal LTP.     

Presynaptic lonotropic receptors

The release of transmitters through vesicle exocytosis from nerve terminals is not constant but is subject to modulation by various mechanisms, including prior activity at the synapse and the presence of neurotransmitters or neuromodulators in the synapse. Instantaneous responses of postsynaptic cells to released transmitters are mediated by ionotropic receptors. In contrast to metabotropic receptors, ionotropic receptors mediate the actions of agonists in a transient manner within milliseconds to seconds. Nevertheless, transmitters can control vesicle exocytosis not only via slowly acting metabotropic, but also via fast acting ionotropic receptors located at the presynaptic nerve terminals. In fact, members of the following subfamilies of ionotropic receptors have been found to control transmitter release: ATP P2X, nicotinic acetylcholine, GABA(A), ionotropic glutamate, glycine, 5-HT(3), andvanilloid receptors. As these receptors display greatly diverging structural and functional features, a variety of different mechanisms are involved in the regulation of transmitter release via presynaptic ionotropic receptors. This text gives an overview of presynaptic ionotropic receptors and briefly summarizes the events involved in transmitter release to finally delineate the most important signaling mechanisms that mediate the effects of presynaptic ionotropic receptor activation. Finally, a few examples are presented to exemplify the physiological and pharmacological relevance of presynaptic ionotropic receptors.     

An analysis of low level doses of cholinesterase inhibitors in cultured neurons and hippocampal slices of rats

Recent data indicate that the neurotoxic effects of organophosphate compounds, including those of the nerve agents VX and sarin, are not solely due to irreversible cholinesterase inhibition. In this study we applied the patch clamp technique to hippocampal neurons in culture and slices to investigate the effects of VX, sarin and huperzine A on transmitter release and the mechanisms related with such effects. The nerve agents VX and sarin at very low concentrations significantly reduced the evoked release of GABA and glutamate. This effect was dependent of the activation of muscarinic receptors. In the presence or absence of the Na(+)-channel blocker tetrodotoxin (TTX), VX increased the frequency of spontaneous glutamate and GABA-induced postsynaptic currents. The effect of VX on TTX-insensitive spontaneous currents appears to be unrelated to cholinesterase inhibition, because it could be detected even after cholinesterase was blocked by high concentrations of the nerve agent soman. The ability of the nerve gases to decrease evoked release of GABA and increase spontaneous transmitter release may underlie some of the neurotoxic effects of the compounds. Huperzine A did not affect spontaneous or evoked release of GABA and glutamate, suggesting that this compound may be a pure cholinesterase inhibitor and had no effect on postsynaptic GABAA or AMPA receptors.     

Differential roles of NR2A and NR2B-containing NMDA receptors in LTP and LTD in the CA1 region of two-week old rat hippocampus

The role of NMDA receptors in the induction of long-term potentiation (LTP) and long-term depression (LTD) is well established but which particular NR2 subunits are involved in these plasticity processes is still a matter of controversy. We have studied the effects of subtype selective NMDA receptor antagonists on LTP induced by high frequency stimulation (100 Hz for 1s) and LTD induced by low frequency stimulation (1 Hz for 15 min) in the CA1 region of hippocampal slices from 14 day old Wistar rats. Against recombinant receptors in HEK293 cells NVP-AAM077 (NVP) was approximately 14-fold selective for NR2A vs NR2B receptors, whilst Ro 25-6981 (Ro) was highly selective for NR2B receptors. On NMDA receptor-mediated EPSCs from Schaffer collaterals in CA1 neurones, NVP and Ro both reduced the amplitude but differentially affected the time constant of decay. The data are compatible with the selective effect of NVP (0.1 microM) and Ro (4 microM) on native NR2A and NBR2B receptors, respectively. NVP reduced both LTP and LTD whereas Ro reduced only LTP. Thus, LTP was reduced by 63% at 0.1 microM NVP and almost completely at 0.4 microM whereas 5 microM Ro reduced LTP by 45%. These data are consistent with a role for both NR2A and NR2B in the induction of LTP, under our experimental conditions. In comparison, LTD was unaffected by Ro (5 microM) even in the presence of a glutamate uptake inhibitor threo-beta-benzylaspartic acid (TBOA) to increase the concentration of glutamate at NR2B containing receptors. NVP (0.2-0.4 microM), however, produced a concentration dependent inhibition of LTD which was complete at 0.4 microM. The lack of effect of 0.1 microM NVP on LTD contrasts with its marked effect on LTP and raises the possibility that different NVP-sensitive NR2 subunit-containing NMDA receptors are required for LTP and LTD in this preparation.     

Amphetamine depresses excitatory synaptic transmission at prefrontal cortical layer V synapses

Dopamine modulates the function of glutamatergic synapses in prefrontal cortex, modifying synaptic strength and influencing synaptic plasticity. Here we have explored the ability of endogenous dopamine, present in slices containing the prefrontal cortex, to influence excitatory synaptic transmission. We found that 10 microM amphetamine, which releases and blocks the reuptake of dopamine from dopaminergic nerve terminals, significantly depressed excitatory field potentials recorded in layer V during stimulation of layer II/III. The depression was reversible, dose dependent and correlated with increased paired pulse facilitation, suggesting that amphetamine inhibits the presynaptic release of glutamate. Pharmacological dissection of this response showed that dopamine D1 receptors are likely to mediate the effects of endogenous dopamine on excitatory synaptic transmission, with little effect of alpha2 adrenergic receptors, serotonin receptors, or D2 dopamine receptors. The time to peak amphetamine effect was longer than expected based on diffusion, suggesting that to raise dopamine levels in brain slices amphetamine may need to be transported into the presynaptic terminals. These results provide evidence that D1/D5 receptors depress glutamate release at this cortical synapse, and suggest that amphetamine will have profound and persistent effects on PFC functioning in vivo. Dysregulation of this mechanism could contribute to the impairment in cognitive performance associated with abnormal PFC dopamine receptor activation.     

Glutamate receptor agonists cause efflux of endogenous neuroactive amino acids from cerebellar neurons in culture

Cultured neurons from rat cerebellum were used to examine the effects of glutamate receptor agonists on the release of endogenous amino acids and adenosine. Kainic acid exposure resulted in the release of glutamate, taurine, GABA and alanine in a dose- and calcium-dependent manner. Stimulation with quisqualic acid resulted in the dose- and calcium-dependent release of GABA. N-Methyl aspartic acid did not elicit the release of any neuroactive amino acids. These findings suggest that N-methyl aspartate receptors are not coupled to transmitter release in these cultures, and that kainate and quisqualate receptors may have different neuronal distributions.     

Facilitatory and inhibitory muscarine receptors on the rat phrenic nerve: effects of pirenzepine and dicyclomine

Summary Neuronal transmitter stores of the rat phrenic nerve were labelled by an incubation with [³H]choline. Release of [³H]acetylcholine was elicited either by a short (100 pulses, 5 Hz) or by a long (1500 pulses, 5 or 25 Hz) period of electrical nerve stimulation. Pirenzepine and dicyclomine enhanced transmitter release evoked by the short stimulation period. Both antagonists reduced transmitter release evoked by the long stimulation period. Pirenzepine reduced transmitter release at low concentrations (1 nmol/l) whereas a higher concentration was necessary for the enhancing effect; the opposite pattern was found for dicyclomine. A low concentration of oxotremorine (10 nmol/l) enhanced and a high concentration (1 mol/l) reduced transmitter release evoked by the short stimulation period. Both effects could be prevented by a low concentration of pirenzepine (10 nmol/l). It is concluded that facilitatory and inhibitory muscarine receptors are present on the motor nerve. A short stimulation period activates predominantly the negative muscarinic feedback, whereas during a long period of continuous nerve stimulation the positive muscarinic feedback mechanism is additionally activated. Both the facilitatory and inhibitory receptors might be regarded as M1-receptors but differences in the pharmacological properties between both receptor populations appear possible.This work was supported by the Deutsche Forschungsgemeinschaft. The paper contains part of the Dr. med. thesis of A. D. and M. O. Send offprint requests to I. Wessler at the above address     

Increased expression, but not postsynaptic localisation, of ionotropic glutamate receptors during the late-phase of long-term potentiation in the dentate gyrus in vivo

Long-term potentiation (LTP) is extensively studied as a cellular mechanism of information storage in the brain. The induction and early expression mechanisms of LTP depend on activation and rapid modulation of ionotropic glutamate receptors. However, the mechanisms that underlie maintenance of LTP over the course of days or longer are poorly understood. Here, we have investigated the overall expression of AMPA- and NMDA-type glutamate receptors (AMPARs and NMDARs, respectively), as well as their levels at the synaptic surface membrane and in the postsynaptic density (PSD), in the dentate gyrus at 48 h following the induction of LTP at perforant path synapses in awake rats. We found a high-frequency stimulation-dependent increase in the overall levels of AMPAR subunits GluA1 and GluA2, but not GluA3 in the dentate gyrus. The increases in GluA1 and GluA2 levels were partially NMDAR-dependent, but were not found in biochemically isolated synaptic surface membrane or PSD fractions. In contrast, we found that the core NMDAR subunit, GluN1, increased in the synaptic surface-membrane fraction but it also was not targeted to the PSD. The GluA1 and GluA2 expression and the surface localisation of GluN1 returned to baseline levels by 2 weeks post-LTP induction. These data suggest that the late-phase LTP is not mediated by an overt increase in the AMPAR content of perforant path synapses. The increase in surface expression NMDARs may influence thresholds for future plasticity events.     

Neurotensin inhibits glutamate-mediated synaptic inputs onto ventral tegmental area dopamine neurons through the release of the endocannabinoid 2-AG

Neurotensin (NT), a neuropeptide abundant in the ventral midbrain, is known to act as a key regulator of the mesolimbic dopamine (DA) system, originating in the ventral tegmental area (VTA). NT activates metabotropic receptors coupled to Gq heterotrimeric G proteins, a signaling pathway often triggering endocannabinoid (EC) production in the brain. Because ECs act as negative regulators of many glutamate synapses and have also been shown recently to gate LTP induction in the VTA, we examined the hypothesis that NT regulates glutamate-mediated synaptic inputs to VTA DA neurons. We performed whole cell patch-clamp recordings in VTA DA neurons in TH-EGFP transgenic mouse brain slices and found that NT induces a long-lasting decrease of the EPSC amplitude that was mediated by the type 1 NT receptor. An antagonist of the CB1 EC receptor blocked this decrease. This effect of NT was not dependent on intracellular calcium, but required G-protein activation and phospholipase C. Blockade of the CB1 receptor after the induction of EPSC depression reversed synaptic depression, an effect not mimicked by blocking NT receptors, thus suggesting the occurrence of prolonged EC production and release. The EC responsible for synaptic depression was identified as 2-arachidonoylglycerol, the same EC known to gate LTP induction in VTA DA neurons. However, blocking NT receptors during LTP induction did not facilitate LTP induction, suggesting that endogenously released NT is not a major source of EC production during LTP inducing stimulations.     

NMDA GluN2A and GluN2B receptors play separate roles in the induction of LTP and LTD in the amygdala and in the acquisition and extinction of conditioned fear

Synaptic plasticity mediated by NMDA glutamate receptors is thought to be a primary mechanism underlying the formation of new memories. Activation of GluN2A NMDA receptor subunits may induce long-term potentiation (LTP), whereas low-frequency stimulation of GluN2B receptors induces long-term depression (LTD). In the present study, we show that blockade of GluN2A, but not GluN2B receptors with NVP-AAM077 and Ro25-6981 respectively, prevented LTP of auditory thalamic inputs to the lateral amygdala. Conversely, LTD induction in this pathway was prevented by blockade of GluN2B, but not GluN2A receptors. As this pathway plays a critical role in the acquisition, retrieval and extinction of a learned auditory-cue fear association, we next examined the effects of blockade of GluN2A and GluN2B receptors on the development and retention of a conditioned fear response. Administration of NVP-AAM077, but not Ro25-6981, prior to conditioning disrupted the expression of conditioned fear 24h later. Conversely, Ro25-6981 but not NVP-AAM077 impaired extinction of the conditioned fear response. These data expand on previous work showing that LTP/D in the thalamic-lateral amygdala pathway is dependent on NMDA receptors, by demonstrating selective roles for GluN2A and GluN2B NMDA receptor subunits in LTP and LTD respectively. Furthermore, GluN2A receptor activation and associated LTP may be involved specifically in the initial formation and/or stabilization of a learned fear response, whereas GluN2B receptor activation and associated LTD may facilitate the suppression of Pavlovian fear responses during extinction. This article is part of a Special Issue entitled 'Post-Traumatic Stress Disorder'.     

Postsynaptic silent synapses: evidence and mechanisms

In this review I discuss the evidence that some glutamatergic synapses exist that lack surface-expressed postsynaptic AMPA receptors (AMPARs) but contain NMDA receptors opposed to a functional release site. I have summarised the electrophysiological, anatomical and cell biological evidence for such postsynaptically silent synapses, and data that support the idea of rapid AMPAR insertion at silent synapses during long-term potentiation (LTP). I also discuss recent findings suggesting that developmental and activity-dependent alteration in the postsynaptic glutamate receptor composition is a general principle that occurs for other receptor subtypes. This review is not intended to provide a full discussion of possible presynaptic mechanisms for silent synapses; these are covered in the accompanying recent article [Voronin and Cherubini (this issue)].     

Rat hippocampal glutamate and GABA release exhibit biphasic effects as a function of chronic lead exposure level.

Previous work has suggested that the lead (Pb) exposure-induced decrease in K(+)-evoked hippocampal glutamate (GLU) release is an important factor in the elevated threshold and diminished magnitude reported for hippocampal long-term potentiation (LTP) in exposed animals. In addition, complex dose-effect relationships between Pb exposure level and LTP have been reported. This investigation was conducted to determine the effects of Pb on hippocampal GLU and GABA release as a function of exposure level. Rats were continuously exposed to 0.1, 0.2, 0.5, or 1.0% Pb in the drinking water beginning at gestational day 15-16. Hippocampal transmitter release was induced in adult males by perfusion of 150 mM K(+) in the presence of Ca(+2) (total release) through a microdialysis probe in one test session, followed by perfusion through a contralateral probe in the absence of Ca(+2) (Ca(+2)-independent release) in the second session. Chronic exposure produced decreases in total K(+)-stimulated hippocampal GLU and GABA release at exposure levels of 0.1-0.5% Pb. Maximal effects were seen in the 0.2% group (blood Pb = 40 microg/100 ml), and changes in total release could be directly traced to alterations in the Ca(+2)-dependent component. However, these effects were less evident in the 0.5% group and were no longer present in the 1.0% Pb group, thus defining U-shaped dose-effect relationships. Moreover, in the absence of Ca(+2) in the dialysis perfusate, K(+)-induced release was elevated in the 2 highest exposure groups, suggesting a Pb(+2)-induced enhancement in evoked release. This pattern of results indicates the presence of 2 actions of Pb on in vivo transmitter release: a more potent suppression of stimulated release seen at lower exposure levels (27-62 microg/100 ml) combined with Ca+2-mimetic actions to independently induce exocytosis that is exhibited at higher exposure levels (> or =62 microg/100 ml). Furthermore, significant similarities in the dose-effect relationships uncovered in measures of evoked GLU release and hippocampal LTP (M. E. Gilbert et al., 1999b, Neurotoxicology 20, 71-82) reinforce the conclusion that exposure-related changes in GLU release play a significant role in the Pb-induced effects seen in this model of synaptic plasticity.