Reduction Of Excitatory Synaptic Transmission Research Articles

Remifentanil self-administration in mice promotes sex-specific prefrontal cortex dysfunction underlying deficits in cognitive flexibility

Opioid-based drugs are frequently used for pain management in both males and females despite the known risk of prefrontal cortex dysfunction and cognitive impairments. Although poorly understood, loss of cognitive control following chronic drug use has been linked to decreased activation of frontal cortex regions. Here, we show that self-administration of the potent opioid, remifentanil, causes a long-lasting hypoactive basal state evidenced by a decrease in ex vivo excitability that is paralleled by an increase in firing capacity of layer 5/6 pyramidal neurons in the prelimbic, but not infralimbic region of the medial prefrontal cortex. This phenomenon was observed in females after as few as 5 days and up to 25–30 days of self-administration. In contrast, pyramidal neurons in males showed increased excitability following 10–16 days of self-administration, with hypoactive states arising only following 25–30 days of self-administration. The emergence of a hypoactive, but not hyperactive basal state following remifentanil self-administration aligned with deficits in cognitive flexibility as assessed using an operant-based attentional set-shifting task. In females, the hypoactive basal state is driven by a reduction in excitatory synaptic transmission mediated by AMPA-type glutamate receptors. Alternatively, hyper- and hypoactive states in males align selectively with decreased and increased GABAB signaling, respectively. Chemogenetic compensation for this hypoactive state prior to testing restored cognitive flexibility, basal hypoactive state, and remifentanil-induced plasticity. These data define cellular and synaptic mechanisms by which opioids impair prefrontal function and cognitive control; indicating that interventions aimed at targeting opioid-induced adaptations should be tailored based on biological sex.

The role of p21-activated kinase in maintaining the fear learning-induced modulation of excitation/inhibition ratio in lateral amygdala

We study the relations between different learning paradigms and enduring changes in excitatory synaptic transmission. Here we show that auditory fear conditioning (AFC), but not olfactory fear conditioning (OFC) training, led to enduring enhancement in AMPA-mediated miniature EPSCs (mEPSCs). Moreover, olfactory unpaired training led to a stable significant reduction in excitatory synaptic transmission. However, olfactory discrimination learning (OD) did not modulate postsynaptic AMPA-mediated mEPSCs in LA. The p21-activated kinase (PAK) activity, previously shown to have a key role in maintaining persistent long-lasting enhancement in synaptic inhibition after OFC, has an opposing effect on excitatory synaptic transmission. PAK maintained the level of excitatory synaptic transmission in the amygdala in all experimental groups, except in neurons in the OFC trained rats. PAK also maintained excitatory synaptic transmission in all neurons of auditory fear conditioning and naïve training groups except in neurons of the auditory safety learning. Safety learning was previously shown in our study to enhance synaptic inhibition. We thus suggest that PAK maintains inhibitory synaptic transmission in a learning-dependent manner and on the other hand affects excitatory synaptic transmission only in groups where learning has not affected inhibitory transmission. Thus, PAK controls learning-induced changes in the excitation/inhibition balance.

Diminished excitatory synaptic transmission correlates with impaired spatial working memory in neurodevelopmental rodent models of schizophrenia

Neurodevelopmental abnormalities are associated with cognitive dysfunction in schizophrenia. In particular, deficits of working memory, are consistently observed in schizophrenia, reflecting prefrontal cortex (PFc) dysfunction. To elucidate the mechanism of such deficits in working memory, the pathophysiological properties of PFc neurons and synaptic transmission have been studied in several developmental models of schizophrenia. Given the pathogenetic heterogeneity of schizophrenia, comparison of PFc synaptic transmission between models of prenatal and postnatal defect would promote our understanding on the developmental components of the biological vulnerability to schizophrenia. In the present study, we investigated the excitatory synaptic transmission onto pyramidal cells localized in layer 5 of the medial PFc (mPFc) in two developmental models of schizophrenia: gestational methylazoxymethanol acetate (MAM) administration and post-weaning social isolation (SI). We found that both models exhibited defective spatial working memory, as indicated by lower spontaneous alternations in a Y-maze paradigm. The recordings from pyramidal neurons in both models exhibited decreased spontaneous excitatory postsynaptic current (sEPSC), representing the reduction of excitatory synaptic transmission in the mPFc. Interestingly, a positive correlation between the impaired spontaneous alternation behavior and the decreased excitatory synaptic transmission of pyramidal neurons was found in both models. These findings suggest that diminished excitatory neurotransmission in the mPFc could be a common pathophysiology regardless of the prenatal and postnatal pathogenesis in developmental models of schizophrenia, and that it might underlie the mechanism of defective working memory in those models.

Somatostatin enhances visual processing and perception by suppressing excitatory inputs to parvalbumin-positive interneurons in V1.

Somatostatin (SST) is a neuropeptide expressed in a major subtype of GABAergic interneurons in the cortex. Despite abundant expression of SST and its receptors, their modulatory function in cortical processing remains unclear. Here, we found that SST application in the primary visual cortex (V1) improves visual discrimination in freely moving mice and enhances orientation selectivity of V1 neurons. We also found that SST reduced excitatory synaptic transmission to parvalbumin-positive (PV+) fast-spiking interneurons but not to regular-spiking neurons. Last, using serial block-face scanning electron microscopy (SBEM), we found that axons of SST+ neurons in V1 often contact other axons that exhibit excitatory synapses onto the soma and proximal dendrites of the PV+ neuron. Collectively, our results demonstrate that the neuropeptide SST improves visual perception by enhancing visual gain of V1 neurons via a reduction in excitatory synaptic transmission to PV+ inhibitory neurons.

Presynaptic and Somatic Mechanisms of Opioid Depression in preBötC Inspiratory Circuits

Opioid‐induced respiratory depression (OIRD) is the primary cause of death from the overdose of opioid analgesics and drugs of abuse. We are interested in identifying molecular/cellular mechanisms underlying OIRD in central respiratory circuits, towards the aim of mitigating OIRD.Although multiple brainstem centers likely contribute to OIRD, we focused on the preBötzinger Complex (preBötC) since this locus is essential for generating rhythmic inspiratory drive. Using in vitro mouse brain slices containing the isolated preBötC, we confirmed that inspiratory burst frequency is depressed by bath‐applied DAMGO, a mu‐opioid receptor (MOR1) specific agonist. MOR1 is known to suppress excitability by signaling via the Gi/o pathway to suppress presynaptic calcium channels and activate GIRK K+ channels. The contribution of each effector pathway was tested with pharmacological and genetic tools. Unexpectedly, blockade of GIRK K+ channels (TertiapinQ) failed to rescue in vitro DAMGO‐depressed rhythms, and activation of GIRK channels (ML297) failed to mimic OIRD, suggesting a relatively modest role for MOR1‐mediated GIRK activation as a mechanism for OIRD. By contrast, slices derived from heterozygous CACNA1A (encoding P/Q‐type calcium channels) KO/+ animals were found to be 10‐fold sensitized to DAMGO depression, consistent with a major presynaptic component for OIRD. In addition, blockers of KCNQ K+ channels (XE991, Chromanol 293B) found in presynaptic terminals, rescued DAMGO‐depressed rhythms, and activators of KCNQ channels (ICA 69673, Retigabine) mimicked DAMGO‐mediated OIRD. Direct support for a presynaptic component of OIRD was provided by recording mEPSCs from genetically labelled Dbx1 neurons. Spontaneous excitatory mEPSCs frequency in TTX was suppressed by DAMGO (70% of cells) and restored by subsequent block of KCNQ channels (XE991) (~83% of cells), supporting the co‐localization of MOR1 and KCNQ K+ channels with P/Q‐type calcium channels to presynaptic terminals. In vivo plethysmography in unanesthetized animals combined with systemic injections with GIRK (ML297) and KCNQ (Retigabine) activators, along with morphine followed by KCNQ blocker (XE991), largely corroborated these in vitro findings. We hypothesize that preBötC OIRD is primarily due to MOR1‐mediated suppression of presynaptic calcium channels, modulated by co‐compartmentalized KCNQ K+ channels. By this model, decreased synaptic efficacy due to OIRD leads to collapse of the rhythmogenic capacity of the preBötC network. Although our results do not support a major role for GIRK K+ channels in OIRD, preliminary results suggest a secondary role for TWK‐type K+ leak channels in OIRD. Co‐expression of MOR1 and TREK2 in Xenopus oocytes produced currents that augmented ~2‐fold with DAMGO and blocked by fluoxetine. In vitro slice recordings revealed rescue of DAMGO‐mediated OIRD with fluoxetine and mimicking of OIRD with a TREK‐specific activator (BL 1249). Our results thus support a two‐tiered mechanism for OIRD: Primary suppression of network‐driven rhythmogenesis due to MOR1‐mediated reduction of excitatory synaptic transmission across the network, and a secondary role for intrinsic hyperpolarization driven by MOR1‐activated TREK K+ channels.Support or Funding InformationNIH R01HL126523, R01HL144801

UBE3A-mediated PTPA ubiquitination and degradation regulate PP2A activity and dendritic spine morphology

Deficiency in the E3 ubiquitin ligase UBE3A leads to the neurodevelopmental disorder Angelman syndrome (AS), while additional dosage of UBE3A is linked to autism spectrum disorder. The mechanisms underlying the downstream effects of UBE3A gain or loss of function in these neurodevelopmental disorders are still not well understood, and effective treatments are lacking. Here, using stable-isotope labeling of amino acids in mammals and ubiquitination assays, we identify PTPA, an activator of protein phosphatase 2A (PP2A), as a bona fide ubiquitin ligase substrate of UBE3A. Maternal loss of Ube3a (Ube3am-/p+) increased PTPA level, promoted PP2A holoenzyme assembly, and elevated PP2A activity, while maternal 15q11-13 duplication containing Ube3a down-regulated PTPA level and lowered PP2A activity. Reducing PTPA level in vivo restored the defects in dendritic spine maturation in Ube3am-/p+ mice. Moreover, pharmacological inhibition of PP2A activity with the small molecule LB-100 alleviated both reduction in excitatory synaptic transmission and motor impairment in Ube3am-/p+ mice. Together, our results implicate a critical role of UBE3A-PTPA-PP2A signaling in the pathogenesis of UBE3A-related disorders and suggest that PP2A-based drugs could be potential therapeutic candidates for treatment of UBE3A-related disorders.

Serotonin 5-HT1A Receptor-Mediated Reduction of Excitatory Synaptic Transmission in Layers II/III of the Parasubiculum

Serotonin (5-HT) has important effects on cognitive function within the hippocampal region where it modulates membrane potential and excitatory and inhibitory synaptic transmission. Here, we investigated how 5-HT modulates excitatory synaptic strength in layers II/III of the parasubiculum in rat brain slices. Bath-application of 1 or 10 μM 5-HT resulted in a strong, dose-dependent, and reversible reduction in the amplitude of field excitatory postsynaptic potentials (fEPSPs) recorded in the parasubiculum. The 5-HT reuptake blocker citalopram (10 μM) also reduced fEPSP amplitudes, indicating that 5-HT released within the slice inhibits synaptic transmission. The reduction of fEPSPs induced by 5-HT was blocked by the 5-HT1A receptor blocker NAN-190 (10 μM), but not by the 5-HT7 receptor blocker SB-269970 (10 μM). Moreover, the 5-HT1A agonist 8-OH-DPAT induced a reduction of fEPSP amplitude similar to that induced by 5-HT. The reduction was prevented by the 5-HT1A receptor blocker NAN-190. The reduction in fEPSPs induced by either 5-HT or by 8-OH-DPAT was accompanied by an increase in paired-pulse ratio, suggesting that it is due mainly to reduced glutamate release. Our data suggest that the effects of serotonin on cognitive function may depend in part upon a 5-HT1A-mediated reduction of excitatory synaptic transmission in the parasubiculum. This may also affect synaptic processing in the entorhinal cortex, which receives the major output projection of the parasubiculum.

BACE1 is necessary for experience-dependent homeostatic synaptic plasticity in visual cortex.

Alzheimer's disease (AD) is the most common form of age-related dementia, which is thought to result from overproduction and/or reduced clearance of amyloid-beta (Aβ) peptides. Studies over the past few decades suggest that Aβ is produced in an activity-dependent manner and has physiological relevance to normal brain functions. Similarly, physiological functions for β- and γ-secretases, the two key enzymes that produce Aβ by sequentially processing the amyloid precursor protein (APP), have been discovered over recent years. In particular, activity-dependent production of Aβ has been suggested to play a role in homeostatic regulation of excitatory synaptic function. There is accumulating evidence that activity-dependent immediate early gene Arc is an activity “sensor,” which acts upstream of Aβ production and triggers AMPA receptor endocytosis to homeostatically downregulate the strength of excitatory synaptic transmission. We previously reported that Arc is critical for sensory experience-dependent homeostatic reduction of excitatory synaptic transmission in the superficial layers of visual cortex. Here we demonstrate that mice lacking the major neuronal β-secretase, BACE1, exhibit a similar phenotype: stronger basal excitatory synaptic transmission and failure to adapt to changes in visual experience. Our results indicate that BACE1 plays an essential role in sensory experience-dependent homeostatic synaptic plasticity in the neocortex.

Highlights in basic autonomic neurosciences: Glia and neuromodulation

Adenosine is an important neuromodulator in the CNS. In response to insults, such as hypoxia, adenosine is released and exerts a neuroprotective effect by inhibiting excitatory synaptic transmission. Martin et al. investigated whether the hypoxia-induced excitatory synaptic depression might be mediated by adenosine release from astrocytes by using electrophysiological and calcium imaging techniques in hippocampal slices of transgenic mice with dominant negativesoluble N-ethylmaleimide-sensitive factor attachment protein receptor (dn-SNARE) and in pure astrocytic cultures,. The inhibition of glial cell metabolismwithfluorocitrate or theblockadeofA1 adenosine receptors with 8-cyclopentyltheophylline was effective in preventing the hypoxia-induced reduction in excitatory synaptic transmission. It was also shown that adenosine transporter inhibitors failed to modify hypoxiainduced effects, demonstrating that hypoxia-induced adenosine release was transporter independent. Moreover, adenosine release under hypoxic conditions was shown to be calcium independent. As under normoxic physiological conditions extracellular adenosine is mainly derived from extracellular catabolism of astrocytic ATP released by exocytosis, the presence of adenosine-mediated modulation of excitatory synaptic transmission during hypoxia in transgenic mice with impaired astrocytic ATP exocytosis (dn-SNARE) indicates that the adenosine mediating the hypoxic effects is not produced by extracellular ATP degradation. Thus, the authors conclude that under hypoxic conditions astrocytes regulate the excitatory synaptic transmission by direct release of adenosine, which acts on A1 receptors to reduce presynaptic transmitter release.

Synaptic removal of diacylglycerol by DGKζ and PSD-95 regulates dendritic spine maintenance

Diacylglycerol (DAG) is an important lipid signalling molecule that exerts an effect on various effector proteins including protein kinase C. A main mechanism for DAG removal is to convert it to phosphatidic acid (PA) by DAG kinases (DGKs). However, it is not well understood how DGKs are targeted to specific subcellular sites and tightly regulates DAG levels. The neuronal synapse is a prominent site of DAG production. Here, we show that DGKzeta is targeted to excitatory synapses through its direct interaction with the postsynaptic PDZ scaffold PSD-95. Overexpression of DGKzeta in cultured neurons increases the number of dendritic spines, which receive the majority of excitatory synaptic inputs, in a manner requiring its catalytic activity and PSD-95 binding. Conversely, DGKzeta knockdown reduces spine density. Mice deficient in DGKzeta expression show reduced spine density and excitatory synaptic transmission. Time-lapse imaging indicates that DGKzeta is required for spine maintenance but not formation. We propose that PSD-95 targets DGKzeta to synaptic DAG-producing receptors to tightly couple synaptic DAG production to its conversion to PA for the maintenance of spine density.

Neurons Skip a Beat during Fast Ripples

Fast ripples are EEG transients emanating from epileptic foci, but fast-ripple frequencies far exceed maximal neuronal firing rates. In this issue of Neuron, Menendez de la Prida and coworkers propose that out-of-phase firing of a subpopulation of neurons during physiological ripple activity effectively doubles the ripple frequency to produce fast ripples.

Emergent epileptiform activity in neural networks with weak excitatory synapses

Brain electrical activity recorded during an epileptic seizure is frequently associated with rhythmic discharges in cortical networks. Current opinion in clinical neurophysiology is that strongly coupled networks and cellular bursting are prerequisites for the generation of epileptiform activity. Contrary to expectations, we found that weakly coupled cortical networks can create synchronized cellular activity and seizure-like bursting. Evaluation of a range of synaptic parameters in a detailed computational model revealed that seizure-like activity occurs when the excitatory synapses are weakened. Guided by this observation, we confirmed experimentally that, in mouse neocortical slices, a pharmacological reduction of excitatory synaptic transmission elicited sudden onset of repetitive network bursting. Our finding provides powerful evidence that onset of seizures can be associated with a reduction in synaptic transmission. These results open a new avenue to explore network synchrony and may ultimately lead to a rational approach to treatment of network pathology in epilepsy.

Dopamine Depresses Excitatory Synaptic Transmission Onto Rat Subicular Neurons Via Presynaptic D1-Like Dopamine Receptors

Schizophrenia is considered to be associated with an abnormal functioning of the hippocampal output. The high clinical potency of antipsychotics that act as antagonists at dopamine (DA) receptors indicate a hyperfunction of the dopaminergic system. The subiculum obtains information from area CA1 and the entorhinal cortex and represents the major output region of the hippocampal complex. To clarify whether an enhanced dopaminergic activity alters the hippocampal output, the effect of DA on alveus- and perforant path-evoked excitatory postsynaptic currents (EPSCs) in subicular neurons was examined using conventional intracellular and whole cell voltage-clamp recordings. Dopamine (100 microM) depressed alveus-elicited (S)-alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptor-mediated EPSCs to 56 +/- 8% of control while perforant path-evoked EPSCs were attenuated to only 76 +/- 7% of control. Dopamine had no effect on the EPSC kinetics. Dopamine reduced the frequency of spontaneous miniature EPSCs without affecting their amplitudes. The sensitivity of subicular neurons to the glutamate receptor agonist (S)-alpha-amino-3-hydoxy-5-methyl-4-isoxazolepropionic acid was unchanged by DA pretreatment, excluding a postsynaptic mechanism for the observed reduction of excitatory synaptic transmission. The effect of DA on evoked EPSCs was mimicked by the D1 receptor agonist SFK 38393 and partially antagonized by the D1 receptor antagonist SCH 23390. While the D2 receptor agonist quinelorane failed to reduce the EPSCs, the D2 receptor antagonist sulpiride did not block the action of DA. The results indicate that DA strongly depresses the hippocampal and the entorhinal excitatory input onto subicular neurons by decreasing the glutamate release following activation of presynaptic D1-like DA receptors.

The effect of volatile anaesthetics on synaptic release and uptake of glutamate

1. Volatile anaesthetics seem to exert their effects on several parts of the neuronal conducting system. 2. The effect on synaptic excitation seems to be quantitatively the most important (Berg-Johnsen and Langmoen, Acta Physiol. Scand. 128, 1986, 613–618) as 1 minimum alveolar concentration (MAC) of isoflurane reduces the activity in thin unmyelinated afferent fibres by 18%, excitatory synapses by 27% and postsynaptic neurones by 24%. 3. The reduction in excitatory synaptic transmission is caused by a decreased amount of transmitter glutamate in the synaptic cleft caused by a reduced release and increased uptake of glutamate in the presynaptic terminals (Larsen et al., Brain Res. 663, 1994, 335–337; Larsen et al., Br. J. Anaesth. 78, 1997, 55–59).

Psychostimulants depress excitatory synaptic transmission in the nucleus accumbens via presynaptic D1-like dopamine receptors

The effects of dopamine (DA) and the psychostimulants cocaine and amphetamine on excitatory transmission in the nucleus accumbens (NAc) were examined in rat NAc slices using both extracellular-field and whole-cell patch-clamp recording. DA, cocaine, and amphetamine reversibly reduced the excitatory synaptic responses (EPSPs/EPSCs) elicited by stimulation of prelimbic cortical afferents. DA and amphetamine increased paired-pulse facilitation, reduced the frequency of spontaneous miniature EPSCs (mEPSCs), and had no effect on mEPSC amplitude, suggesting a presynaptic mechanism for the observed reduction in excitatory synaptic transmission. The effects of DA and amphetamine were attenuated by the D1 receptor antagonist SCH23390 but not by the D2 receptor antagonist sulpiride. The broad-spectrum DA receptor agonist 6,7-ADTN mimicked the effects of DA and the psychostimulants, but neither the D1 receptor agonists SKF38393 and SKF81297 nor the D2 receptor agonist quinpirole caused a significant reduction in EPSP magnitude. SKF38393 at a higher concentration (100 microM) was effective in reducing the EPSP, however, and this reduction was sensitive to SCH23390. There was no difference in the effects of DA in cells from mutant mice lacking D1a receptors and cells from wild-type control mice. Unilaterally lesioning the dopaminergic afferents to the NAc using 6-hydroxydopamine attenuated the amphetamine-induced reduction in EPSP magnitude in slices from the lesioned hemisphere but not the control (unlesioned) hemisphere. These results indicate that DA and psychostimulants (acting indirectly by increasing endogenous extracellular DA levels) reduce excitatory synaptic transmission in the NAc by activating presynaptic DA receptors with D1-like properties.

Analysis of adenosine actions on Ca2+ currents and synaptic transmission in cultured rat hippocampal pyramidal neurones.

1. The role of adenosine receptors in reducing calcium currents (ICa) and in triggering presynaptic inhibition was studied using whole-cell patch-clamp techniques to record ICa and synaptic currents from the cell bodies of cultured rat hippocampal pyramidal neurones. Recordings of intracellular Ca2+ using the indicator dye Fura-2 were used to obtain further insights into the actions of adenosine agonists. 2. The adenosine analogue 2-chloroadenosine (2-CA) reduced ICa in these neurones. This action was also evident when Ba2+ was used as the charge carrier through Ca2+ channels. Adenosine also reduced the influx of Ca2+ into the cell body during a depolarizing voltage-clamp pulse as measured with Fura-2. The potency of various adenosine receptor agonists was as follows: cyclopentyladenosine greater than cyclohexyl-adenosine greater than or equal to R-phenylisopropyladenosine greater than 2-CA greater than S-phenylisopropyladenosine, consistent with the pharmacological profile of an A1 adenosine receptor. 3. The specific A1 receptor antagonist cyclopentyltheophylline (CPT) blocked the actions of 2-CA on ICa in a competitive fashion. 4. The actions of 2-CA on ICa were abolished by pre-incubation of cultured cells with pertussis toxin (PTX; 250 ng/ml). Intracellular dialysis with the GTP analogue GTP-gamma-S (guanosine-5'-O-(3-thiotriphosphate] enhanced the actions of 2-CA and rendered the response irreversible. 5. Excitatory postsynaptic currents (EPSCs) were recorded from pyramidal neurones under whole-cell voltage clamp by stimulating nearby neurones with an extracellular electrode. 2-CA potently and reversibly reduced the amplitude of EPSCs. This action was shown to be due to presynaptic inhibition of neurotransmitter release. 6. The order of potency of different adenosine agonists in reducing EPSCs was as follows: cyclopentyladenosine greater than cyclohexyladenosine greater than or equal to R-phenylisopropyladenosine greater than 2-CA greater than S-phenylisopropyladenosine. CPT inhibited the action of 2-CA in a competitive fashion. 7. The effects of 2-CA on synaptic transmission were abolished by pre-treatment with 250 ng/ml PTX, indicating that a PTX-sensitive G-protein is involved in this action. 8. These results indicate that activation of adenosine receptors does induce a reduction in ICa in hippocampal pyramidal neurones. Furthermore, this effect and the reduction of excitatory synaptic transmission by adenosine analogues are both mediated by PTX-sensitive G-proteins and have identical pharmacological properties.