Changes In Glutamatergic Synapses Research Articles

Abstinence from Cocaine-Induced Conditioned Place Preference Produces Discrete Changes in Glutamatergic Synapses onto Deep Layer 5/6 Neurons from Prelimbic and Infralimbic Cortices.

Glutamatergic signaling in the medial prefrontal cortex (mPFC) plays a critical role in drug addiction and relapse. The mPFC is functionally subdivided into dorsal (prelimbic, PL) and ventral (infralimbic, IL) regions, and evidence suggests a differential role of these two divisions in the control of drug seeking and taking; however, there is a dearth of information on the cocaine-induced adaptations in PL- and IL-mPFC synaptic glutamate transmission and their regulation of behavioral responses to cocaine-associated stimuli. We tested male rats in a cocaine-induced conditioned place preference (CPP) paradigm. In vitro whole-cell recordings were performed at different abstinence intervals to investigate the neuroadaptations in synaptic glutamate transmission in PL- and IL-mPFC deep layer (5/6) pyramidal neurons. Our results show that in naïve animals, PL-mPFC neurons expressed higher frequency of spontaneous events (sEPSCs) than IL-mPFC neurons. Following cocaine-CPP and a short abstinence (SA) period (8 d), we observed decreases in the amplitude of sEPSCs in both mPFC regions. Longer abstinence periods (30 d), resulted in a sustained decrease in the frequency of sEPSCs and an increase in AMPA receptor rectification only in PL-mPFC neurons. In addition, PL-mPFC neurons expressed a decrease in the area under the curve of sEPSCs, suggesting altered receptor activation dynamics. Synaptic glutamate transmission was not significantly different between retested and naïve rats. These results suggest that retention of cocaine-CPP requires differential modulation of glutamate transmission between PL- and IL-mPFC neurons and that these adaptations are dependent on the abstinence interval and reexposure to the cocaine context.

Corticostriatal plasticity, neuronal ensembles, and regulation of drug-seeking behavior.

The idea that interconnected neuronal ensembles code for specific behaviors has been around for decades; however, recent technical improvements allow studying these networks and their causal role in initiating and maintaining behavior. In particular, the role of ensembles in drug-seeking behaviors in the context of addiction is being actively investigated. Concurrent with breakthroughs in quantifying ensembles, research has identified a role for synaptic glutamate spillover during relapse. In particular, the transient relapse-associated changes in glutamatergic synapses on accumbens neurons, as well as in adjacent astroglia and extracellular matrix, are key elements of the synaptic plasticity encoded by drug use and the metaplasticity induced by drug-associated cues that precipitate drug-seeking behaviors. Here, we briefly review the recent discoveries related to ensembles in the addiction field and then endeavor to link these discoveries with drug-induced striatal plasticity and cue-induced metaplasticity toward deeper neurobiological understandings of drug seeking.

Enhanced long term potentiation and decreased AMPA receptor desensitization in the acute period following a single kainate induced early life seizure

Neonatal seizures are associated with long term disabilities including epilepsy and cognitive deficits. Using a neonatal seizure rat model that does not develop epilepsy, but develops a phenotype consistent with other models of intellectual disability (ID) and autism spectrum disorders (ASD), we sought to isolate the acute effects of a single episode of early life seizure on hippocampal CA1 synaptic development and plasticity. We have previously shown chronic changes in glutamatergic synapses, loss of long term potentiation (LTP) and enhanced long term depression (LTD), in the adult male rat ~50days following kainic acid (KA) induced early life seizure (KA-ELS) in post-natal (P) 7day old male Sprague–Dawley rats. In the present work, we examined the electrophysiological properties and expression levels of glutamate receptors in the acute period, 2 and 7days, post KA-ELS. Our results show for the first time enhanced LTP 7days after KA-ELS, but no change 2days post KA-ELS. Additionally, we report that ionotropic α-amino-3-hydroxy-5-methyl-isoxazole-propionic acid type glutamate receptor (AMPAR) desensitization is decreased in the same time frame, with no changes in AMPAR expression, phosphorylation, or membrane insertion. Inappropriate enhancement of the synaptic connections in the acute period after the seizure could alter the normal patterning of synaptic development in the hippocampus during this critical period and contribute to learning deficits. Thus, this study demonstrates a novel mechanism by which KA-ELS alters early network properties that potentially lead to adverse outcomes.

Small Conductance Calcium-Activated Potassium Type 2 Channels Regulate Alcohol-Associated Plasticity of Glutamatergic Synapses

Small conductance calcium-activated potassium type 2 channels (SK2) control excitability and contribute to plasticity by reducing excitatory postsynaptic potentials. Recent evidence suggests that SK2 channels form a calcium-dependent negative-feedback loop with synaptic N-methyl-D-aspartate (NMDA) receptors. Addiction to alcohol and other drugs of abuse induces plastic changes in glutamatergic synapses that include the targeting of NMDA receptors to synaptic sites; however, the role of SK2 channels in alcohol-associated homeostatic plasticity is unknown. Electrophysiology, Western blot, and behavioral analyses were used to quantify changes in hippocampal small conductance calcium-activated potassium (SK) channel function and expression using well-characterized in vitro and in vivo models of chronic alcohol exposure. Chronic ethanol reduced apamin-sensitive SK currents in cornu ammonis 1 pyramidal neurons that were associated with a downregulation of surface SK2 channels. Blocking SK channels with apamin potentiated excitatory postsynaptic potentials in control but not ethanol-treated cornu ammonis 1 pyramidal neurons, suggesting that chronic ethanol disrupts the SK channel-NMDA receptor feedback loop. Alcohol reduced expression of SK2 channels and increased expression of NMDA receptors at synaptic sites in a mouse model. Positive modulation of SK function by 1-EBIO decreased alcohol withdrawal hyperexcitability and attenuated ethanol withdrawal neurotoxicity in hippocampus. The 1-EBIO also reduced seizure activity in mice undergoing withdrawal. These results provide evidence that SK2 channels contribute to alcohol-associated adaptive plasticity of glutamatergic synapses and that positive modulation of SK channels reduces the severity of withdrawal-related hyperexcitability. Therefore, SK2 channels appear to be critical regulators of alcohol-associated plasticity and may be novel therapeutic targets for the treatment of addiction.

Inhibition of glutamate transporters couples to Kv4.2 dephosphorylation through activation of extrasynaptic NMDA receptors

Activation of glutamate receptors is known to modulate K+ channel surface trafficking, phosphorylation, and function, and increasing evidence has implicated K+ channels in plastic changes in glutamatergic synapses. Kv4.2 channels control the amplitude of back-propagating action potentials and shape postsynaptic responses in hippocampus, and synaptic glutamate receptor activation leads to increased phosphorylation of Kv4.2 channels that is associated with enhanced synaptic plasticity. Thus, we investigated the possibility that activation of extrasynaptic NMDA-type glutamate receptors couples to Kv4.2 channel dephosphorylation. In hippocampal neurons, we found that selective activation of extrasynaptic NMDA receptors dephosphorylates Kv4.2 channels, and driving synaptic activity increases phosphorylation of Kv4.2. We also observed that Ca2+ entry through NMDA receptors is necessary for dephosphorylation of Kv4.2 channels. Consistent with a synaptic and extrasynaptic localization at hippocampal synapses, a fraction of Kv4.2 channel clusters was found to localize outside of pre- and postsynaptic markers. Excitatory amino acid transporters (EAATs) regulate ambient extracellular glutamate levels that active extrasynaptic NMDA receptors, and inhibition of glutamate uptake by blocking EAATs with the non-selective transporter inhibitor dl-threo-β-benzyloxyaspartic acid (TBOA) or the EAAT1/3 selective inhibitor l-serine O-sulfate (SOS) dephosphorylates Kv4.2 channels. These findings in conjunction with previous reports support the interesting possibility that synaptic and extrasynaptic NMDA receptors bi-directionally regulate phosphorylation levels of Kv4.2 channels in hippocampus. Moreover, we observed that EAAT activity controls extrasynaptic NMDA receptor modulation of Kv4.2 channel dephosphorylation.

Stability of surface NMDA receptors controls synaptic and behavioral adaptations to amphetamine

Plastic changes in glutamatergic synapses that lead to enduring drug craving and addiction are poorly understood. By focusing on the turnover and trafficking of NMDA receptors, we found that chronic exposure to the psychostimulant amphetamine induces selective downregulation of NMDA receptor NR2B subunits in the confined surface membrane pool of rat striatal neurons at synaptic sites. Remarkably, this downregulation is a long-lived event and results from the destabilization of surface-expressed NR2B due to accelerated ubiquitination and degradation of crucial NR2B-anchoring proteins by the ubiquitin-proteasome system. The biochemical loss of synaptic NR2B further translates to the significant modulation of synaptic plasticity in the form of long-term depression at cortico-accumbal glutamatergic synapses. Behaviorally, genetic disruption of NR2B induces, whereas restoration of NR2B loss prevents, behavioral sensitization to amphetamine. Our data identify NR2B as a key regulator in the remodeling of excitatory synapses and persistent psychomotor plasticity in response to amphetamine.

Molecular Changes in Glutamatergic Synapses Induced by Pb 2+: Association with Deficits of LTP and Spatial Learning

What are the molecular bases for the neurotoxicity that occurs after developmental exposure to low levels of Pb 2+, and are these effects persistent and detrimental in adults? Our inability to understand specific mechanisms behind Pb 2+ neurotoxicity has long been one of many problem areas of this preventable childhood disease. The sensitivity of the developing brain to Pb 2+-induced neurotoxicity is an outcome of the many unique characteristics that comprise the developing central nervous system. The developing brain can be exposed to significant concentrations of Pb 2+ during vulnerable periods of development such as synapse formation, gene and protein expression, and other diverse molecular changes associated with these processes. Recently, changes in NMDA receptor subunits were identified in animals that showed cognitive deficits induced by exposure to Pb 2+. This molecular association is important because it provides new evidence in the characterization of developmental Pb 2+ neurotoxicity that supports physiological findings of impairments in synaptic plasticity and behavior. This review updates information from molecular studies that can be directly associated with impairments of behavior and synaptic plasticity, and outlines the functional consequences of molecular differences in Pb 2+-exposed animals that illuminate potential mechanisms of Pb 2+-induced neurotoxicity.

Time-dependent changes in striatal glutamate synapses following a 6-hydroxydopamine lesion

The goal of this study was to investigate changes in glutamatergic synapses in the striatum of rats at two different time-points following a unilateral injection of 6-hydroxydopamine into the medial forebrain bundle. One month following this lesion of the nigrostriatal pathway, there was an increase (70%) in the mean percentage of asymmetrical synapses within the dorsolateral striatum containing a discontinuous, or perforated, postsynaptic density, possibly suggesting an increase in glutamatergic activity. This was correlated, in the same brain region, with a decrease (44%) in the density of glutamate immunoreactivity within nerve terminals associated with all asymmetrical synapses and also with those terminals associated with a perforated postsynaptic density. These morphological changes were consistent with an increase (>two-fold) in the basal extracellular level of striatal glutamate, as measured by in vivo microdialysis. The density of GABA immunolabeling within symmetrical nerve terminals was increased (25%) at this one month time-period. Dopamine levels within the lesioned striatum were >99% depleted. However, at three months, while an increase in the mean percentage of striatal perforated synapses was maintained, a significant increase (50%) in the density of striatal nerve terminal glutamate immunolabeling within all asymmetrical synapses and those associated with a perforated postsynaptic density was observed. This was correlated with a small, but significant, decrease (32%) in the basal extracellular level of striatal glutamate. The density of GABA immunolabeling within nerve terminals associated with a symmetrical contact remained elevated at this three month time-period, while striatal dopamine levels remained depleted. While the density of nerve terminal GABA immunolabeling remained elevated at both the one and three month time-periods, there appeared to be a differential effect on glutamatergic synapses. The in vivo microdialysis data suggest that glutamate synapses were more active at a basal level at one month and become less active compared to the control group at the three month time-period. These data suggest that there are compensatory changes in glutamatergic synapses within the striatum following a 6-hydroxydopamine lesion that appear to be independent of the level of striatal dopamine or GABA. We propose that changes in the activity of the thalamo-cortico-striatal pathway may help to explain the differential time-course change in striatal glutamatergic synaptic activity.