Somatic and terminal CB1 receptors are differentially coupled to voltage-gated sodium channels in neocortical neurons.

Endogenous cannabinoid signaling is vital for important brain functions, and the same pathways can be modified pharmacologically to treat pain, epilepsy, and posttraumatic stress disorder. Endocannabinoid-mediated changes to excitability are predominantly attributed to 2-arachidonoylglycerol (2-AG) acting presynaptically via the canonical cannabinoid receptor, CB1. Here, we identify a mechanism in the neocortex by which anandamide (AEA), another major endocannabinoid, but not 2-AG, powerfully inhibits somatically recorded voltage-gated sodium channel (VGSC) currents in the majority of neurons. This pathway involves intracellular CB1 that, when activated by anandamide, decreases the likelihood of recurrent action potential generation. WIN 55,212-2 similarly activates CB1 and inhibits VGSC currents, indicating that this pathway is also positioned to mediate the actions of exogenous cannabinoids on neuronal excitability. The coupling between CB1 and VGSCs is absent at nerve terminals, and 2-AG does not block somatic VGSC currents, indicating functional compartmentalization of the actions of two endocannabinoids.

Direct whole-cell patch-clamp recordings from small boutons in rodent primary neocortical neuron cultures.

Direct electrical recordings from conventional boutons in the mammalian central nervous system have proven challenging due to their small size. Here, we provide a protocol for direct whole-cell patch-clamp recordings from small presynaptic boutons of primary dissociated cultured neurons of the rodent neocortex. We describe steps to prepare primary neocortical cultures and recording pipettes, followed by identifying boutons and establishing a whole-cell bouton recording. We then provide details on precise pipette capacitance compensation required for high-resolution current-clamp recordings from boutons. For further details on the use and execution of this protocol, please refer to Ritzau-Jost et al.1.

Enhanced excitability of cortical neurons in low-divalent solutions is primarily mediated by altered voltage-dependence of voltage-gated sodium channels.

Increasing extracellular [Ca2+] ([Ca2+]o) strongly decreases intrinsic excitability in neurons but the mechanism is unclear. By one hypothesis, [Ca2+]o screens surface charge, reducing voltage-gated sodium channel (VGSC) activation and by another [Ca2+]o activates Calcium-sensing receptor (CaSR) closing the sodium-leak channel (NALCN). Here we report that neocortical neurons from CaSR-deficient (Casr-/-) mice had more negative resting potentials and did not fire spontaneously in reduced divalent-containing solution (T0.2) in contrast with wild-type (WT). However, after setting membrane potential to -70 mV, T0.2 application similarly depolarized and increased action potential firing in Casr-/- and WT neurons. Enhanced activation of VGSCs was the dominant contributor to the depolarization and increase in excitability by T0.2 and occurred due to hyperpolarizing shifts in VGSC window currents. CaSR deletion depolarized VGSC window currents but did not affect NALCN activation. Regulation of VGSC gating by external divalents is the key mechanism mediating divalent-dependent changes in neocortical neuron excitability.

Large, Stable Spikes Exhibit Differential Broadening in Excitatory and Inhibitory Neocortical Boutons.

Presynaptic action potential spikes control neurotransmitter release and thus interneuronal communication. However, the properties and the dynamics of presynaptic spikes in the neocortex remain enigmatic because boutons in the neocortex are small and direct patch-clamp recordings have not been performed. Here, we report direct recordings from boutons of neocortical pyramidal neurons and interneurons. Our data reveal rapid and large presynaptic action potentials in layer 5 neurons and fast-spiking interneurons reliably propagating into axon collaterals. For in-depth analyses, we establish boutons of mature cultured neurons as models for excitatory neocortical boutons, demonstrating that the presynaptic spike amplitude is unaffected by potassium channels, homeostatic long-term plasticity, and high-frequency firing. In contrast to the stable amplitude, presynaptic spikes profoundly broaden during high-frequency firing in layer 5 pyramidal neurons, but not in fast-spiking interneurons. Thus, our data demonstrate large presynaptic spikes and fundamental differences between excitatory and inhibitory boutons in the neocortex.

Diadenosine-Polyphosphate Analogue AppCH2ppA Suppresses Seizures by Enhancing Adenosine Signaling in the Cortex.

Epilepsy is a multifactorial disorder associated with neuronal hyperexcitability that affects more than 1% of the human population. It has long been known that adenosine can reduce seizure generation in animal models of epilepsies. However, in addition to various side effects, the instability of adenosine has precluded its use as an anticonvulsant treatment. Here we report that a stable analogue of diadenosine-tetraphosphate: AppCH2ppA effectively suppresses spontaneous epileptiform activity in vitro and in vivo in a Tuberous Sclerosis Complex (TSC) mouse model (Tsc1+/-), and in postsurgery cortical samples from TSC human patients. These effects are mediated by enhanced adenosine signaling in the cortex post local neuronal adenosine release. The released adenosine induces A1 receptor-dependent activation of potassium channels thereby reducing neuronal excitability, temporal summation, and hypersynchronicity. AppCH2ppA does not cause any disturbances of the main vital autonomous functions of Tsc1+/- mice in vivo. Therefore, we propose this compound to be a potent new candidate for adenosine-related treatment strategies to suppress intractable epilepsies.

Strong G-Protein-Mediated Inhibition of Sodium Channels.

Voltage-gated sodium channels (VGSCs) are strategically positioned to mediate neuronal plasticity because of their influence on action potential waveform. VGSC function may be strongly inhibited by local anesthetic and antiepileptic drugs and modestly modulated via second messenger pathways. Here, we report that the allosteric modulators of the calcium-sensing receptor (CaSR) cinacalcet, calindol, calhex, and NPS 2143 completely inhibit VGSC current in the vast majority of cultured mouse neocortical neurons. This form of VGSC current block persisted in CaSR-deficient neurons, indicating a CaSR-independent mechanism. Cinacalcet-mediated blockade of VGSCs was prevented by the guanosine diphosphate (GDP) analog GDPßs, indicating that G-proteins mediated this effect. Cinacalcet inhibited VGSCs by increasing channel inactivation, and block was reversed by prolonged hyperpolarization. Strong cinacalcet inhibition of VGSC currents was also present in acutely isolated mouse cortical neurons. These data identify a dynamic signaling pathway by which G-proteins regulate VGSC current to indirectly modulate central neuronal excitability.

Distinct Actions of Voltage-Activated Ca2+ Channel Block on Spontaneous Release at Excitatory and Inhibitory Central Synapses.

At chemical synapses, voltage-activated calcium channels (VACCs) mediate Ca2+ influx to trigger action potential-evoked neurotransmitter release. However, the mechanisms by which Ca2+ regulates spontaneous transmission have not been fully determined. We have shown that VACCs are a major trigger of spontaneous release at neocortical inhibitory synapses but not at excitatory synapses, suggesting fundamental differences in spontaneous neurotransmission at GABAergic and glutamatergic synapses. Recently, VACC blockers were reported to reduce spontaneous release of glutamate and it was proposed that there was conservation of underlying mechanisms of neurotransmission at excitatory and inhibitory synapses. Furthermore, it was hypothesized that the different effects on excitatory and inhibitory synapses may have resulted from off-target actions of Cd2+, a nonselective VACC blocker, or other variations in experimental conditions. Here we report that in mouse neocortical neurons, selective and nonselective VACC blockers inhibit spontaneous release at inhibitory but not at excitatory terminals, and that this pattern is observed in culture and slice preparations as well as in synapses from acute slices of the auditory brainstem. The voltage dependence of Cd2+ block of VACCs accounts for the apparent lower potency of Cd2+ on spontaneous release of GABA than on VACC current amplitudes. Our findings indicate fundamental differences in the regulation of spontaneous release at inhibitory and excitatory synapses by stochastic VACC activity that extend beyond the cortex to the brainstem.SIGNIFICANCE STATEMENT Presynaptic Ca2+ entry via voltage-activated calcium channels (VACCs) is the major trigger of action potential-evoked synaptic release. However, the role of VACCs in the regulation of spontaneous neurotransmitter release (in the absence of a synchronizing action potential) remains controversial. We show that spontaneous release is affected differently by VACCs at excitatory and inhibitory synapses. At inhibitory synapses, stochastic openings of VACCs trigger the majority of spontaneous release, whereas they do not affect spontaneous release at excitatory synapses. We find this pattern to be wide ranging, holding for large and small synapses in the neocortex and brainstem. These findings indicate fundamental differences of the Ca2+ dependence of spontaneous release at excitatory and inhibitory synapses and heterogeneity of the mechanisms of release across the CNS.

Control of Inhibition by the Direct Action of Cannabinoids on GABAA Receptors.

Cannabinoids are known to regulate inhibitory synaptic transmission via activation of presynaptic G protein-coupled cannabinoid CB1 receptors (CB1Rs). Additionally, recent studies suggest that cannabinoids can also directly interact with recombinant GABAA receptors (GABAARs), potentiating currents activated by micromolar concentrations of γ-aminobutyric acid (GABA). However, the impact of this direct interaction on GABAergic inhibition in central nervous system is unknown. Here we report that currents mediated by recombinant GABAARs activated by high (synaptic) concentrations of GABA as well as GABAergic inhibitory postsynaptic currents (IPSCs) at neocortical fast spiking (FS) interneuron to pyramidal neuron synapses are suppressed by exogenous and endogenous cannabinoids in a CB1R-independent manner. This IPSC suppression may account for disruption of inhibitory control of pyramidal neurons by FS interneurons. At FS interneuron to pyramidal neuron synapses, endocannabinoids induce synaptic low-pass filtering of GABAAR-mediated currents evoked by high-frequency stimulation. The CB1R-independent suppression of inhibition is synapse specific. It does not occur in CB1R containing hippocampal cholecystokinin-positive interneuron to pyramidal neuron synapses. Furthermore, in contrast to synaptic receptors, the activity of extrasynaptic GABAARs in neocortical pyramidal neurons is enhanced by cannabinoids in a CB1R-independent manner. Thus, cannabinoids directly interact differentially with synaptic and extrasynaptic GABAARs, providing a potent novel context-dependent mechanism for regulation of inhibition.

Oxytocin-mediated GABA inhibition during delivery attenuates autism pathogenesis in rodent offspring.

We report that the oxytocin-mediated neuroprotective γ-aminobutyric acid (GABA) excitatory-inhibitory shift during delivery is abolished in the valproate and fragile X rodent models of autism. During delivery and subsequently, hippocampal neurons in these models have elevated intracellular chloride levels, increased excitatory GABA, enhanced glutamatergic activity, and elevated gamma oscillations. Maternal pretreatment with bumetanide restored in offspring control electrophysiological and behavioral phenotypes. Conversely, blocking oxytocin signaling in naïve mothers produced offspring having electrophysiological and behavioral autistic-like features. Our results suggest a chronic deficient chloride regulation in these rodent models of autism and stress the importance of oxytocin-mediated GABAergic inhibition during the delivery process. Our data validate the amelioration observed with bumetanide and oxytocin and point to common pathways in a drug-induced and a genetic rodent model of autism.

GRIN2A mutations in acquired epileptic aphasia and related childhood focal epilepsies and encephalopathies with speech and language dysfunction.

Epileptic encephalopathies are severe brain disorders with the epileptic component contributing to the worsening of cognitive and behavioral manifestations. Acquired epileptic aphasia (Landau-Kleffner syndrome, LKS) and continuous spike and waves during slow-wave sleep syndrome (CSWSS) represent rare and closely related childhood focal epileptic encephalopathies of unknown etiology. They show electroclinical overlap with rolandic epilepsy (the most frequent childhood focal epilepsy) and can be viewed as different clinical expressions of a single pathological entity situated at the crossroads of epileptic, speech, language, cognitive and behavioral disorders. Here we demonstrate that about 20% of cases of LKS, CSWSS and electroclinically atypical rolandic epilepsy often associated with speech impairment can have a genetic origin sustained by de novo or inherited mutations in the GRIN2A gene (encoding the N-methyl-D-aspartate (NMDA) glutamate receptor α2 subunit, GluN2A). The identification of GRIN2A as a major gene for these epileptic encephalopathies provides crucial insights into the underlying pathophysiology.

Tubacin prevents neuronal migration defects and epileptic activity caused by rat Srpx2 silencing in utero.

Altered development of the human cerebral cortex can cause severe malformations with often intractable focal epileptic seizures and may participate in common pathologies, notably epilepsy. This raises important conceptual and therapeutic issues. Two missense mutations in the sushi repeat-containing protein SRPX2 had been previously identified in epileptic disorders with or without structural developmental alteration of the speech cortex. In the present study, we aimed to decipher the precise developmental role of SRPX2, to have a better knowledge on the consequences of its mutations, and to start addressing therapeutic issues through the design of an appropriate animal model. Using an in utero Srpx2 silencing approach, we show that SRPX2 influences neuronal migration in the developing rat cerebral cortex. Wild-type, but not the mutant human SRPX2 proteins, rescued the neuronal migration phenotype caused by Srpx2 silencing in utero, and increased alpha-tubulin acetylation. Following in utero Srpx2 silencing, spontaneous epileptiform activity was recorded post-natally. The neuronal migration defects and the post-natal epileptic consequences were prevented early in embryos by maternal administration of tubulin deacetylase inhibitor tubacin. Hence epileptiform manifestations of developmental origin could be prevented in utero, using a transient and drug-based therapeutic protocol.

Subthalamic lesion or levodopa treatment rescues giant GABAergic currents of PINK1-deficient striatum.

Cellular electrophysiological signatures of Parkinson's disease described in the pharmacological 6-hydroxydopamine (6-OHDA) animal models of Parkinson's disease include spontaneous repetitive giant GABAergic currents in a subpopulation of striatal medium spiny neurons (MSNs), and spontaneous rhythmic bursts of spikes generated by subthalamic nucleus (STN) neurons. We investigated whether similar signatures are present in Pink1(-/-) mice, a genetic rodent model of the PARK6 variant of Parkinson's disease. Although 9- to 24-month-old Pink1(-/-) mice show reduced striatal dopamine content and release, and impaired spontaneous locomotion, the relevance of this model to Parkinson's disease has been questioned because mesencephalic dopaminergic neurons do not degenerate during the mouse lifespan. We show that 75% of the MSNs of 5- to 7-month-old Pink1(-/-) mice exhibit giant GABAergic currents, occurring either singly or in bursts (at 40 Hz), rather than the low-frequency (2 Hz), low-amplitude, tonic GABAergic drive common to wild-type MSNs of the same age. STN neurons from 5- to 7-month-old Pink1(-/-) mice spontaneously generated bursts of spikes instead of the control tonic drive. Chronic kainic acid lesion of the STN or chronic levodopa treatment reliably suppressed the giant GABAergic currents of MSNs after 1 month and replaced them with the control tonic activity. The similarity between the in vitro resting states of Pink1 MSNs and those of fully dopamine (DA)-depleted MSNs of 6-OHDA-treated mice, together with the beneficial effect of levodopa treatment, strongly suggest that dysfunction of mesencephalic dopaminergic neurons in Pink1(-/-) mice is more severe than expected. The beneficial effect of the STN lesion also suggests that pathological STN activity strongly influences striatal networks in Pink1(-/-) mice.

Early γ oscillations synchronize developing thalamus and cortex.

During development, formation of topographic maps in sensory cortex requires precise temporal binding in thalamocortical networks. However, the physiological substrate for such synchronization is unknown. We report that early gamma oscillations (EGOs) enable precise spatiotemporal thalamocortical synchronization in the neonatal rat whisker sensory system. Driven by a thalamic gamma oscillator and initially independent of cortical inhibition, EGOs synchronize neurons in a single thalamic barreloid and corresponding cortical barrel and support plasticity at developing thalamocortical synapses. We propose that the multiple replay of sensory input in thalamocortical circuits during EGOs allows thalamic and cortical neurons to be organized into vertical topographic functional units before the development of horizontal binding in adult brain.

Frequency-Dependent Cannabinoid Receptor-Independent Modulation of Glycine Receptors by Endocannabinoid 2-AG.

Endocannabinoids are known as retrograde messengers, being released from the postsynaptic neuron and acting on specific presynaptic G-protein-coupled cannabinoid (CB) receptors to decrease neurotransmitter release. Also, at physiologically relevant concentrations cannabinoids can directly modulate the function of voltage-gated and receptor-operated ion channels. Using patch-clamp recording we analyzed the consequences of the direct action of an endocannabinoid, 2-arachidonoylglycerol (2-AG), on the functional properties of glycine receptor channels (GlyRs) and ionic currents in glycinergic synapses. At physiologically relevant concentrations (0.1-1 µM), 2-AG directly affected the functions of recombinant homomeric α1H GlyR: it inhibited peak amplitude and dramatically enhanced desensitization. The action of 2-AG on GlyR-mediated currents developed rapidly, within ∼300 ms. Addition of 1 µM 2-AG strongly facilitated the depression of glycine-induced currents during repetitive (4-10 Hz) application of short (2 ms duration) pulses of glycine to outside-out patches. In brainstem slices from CB1 receptor knockout mice, 2-AG significantly decreased the extent of facilitation of synaptic currents in hypoglossal motoneurons during repetitive (10-20 Hz) stimulation. These observations suggest that endocannabinoids can modulate postsynaptic metaplasticity of glycinergic synaptic currents in a CB1 receptor-independent manner.

N-methyl-D-aspartate (NMDA) receptor NR2 subunit selectivity of a series of novel piperazine-2,3-dicarboxylate derivatives: preferential blockade of extrasynaptic NMDA receptors in the rat hippocampal CA3-CA1 synapse.

N-Methyl-d-aspartate (NMDA) receptor antagonists that are highly selective for specific NMDA receptor 2 (NR2) subunits have several potential therapeutic applications; however, to date, only NR2B-selective antagonists have been described. Whereas most glutamate binding site antagonists display a common pattern of NR2 selectivity, NR2A > NR2B > NR2C > NR2D (high to low affinity), (2S*,3R*)-1-(phenanthrene-2-carbonyl)piperazine-2,3-dicarboxylic acid (PPDA) has a low selectivity for NR2C- and NR2D-containing NMDA receptors. A series of PPDA derivatives were synthesized and then tested at recombinant NMDA receptors expressed in Xenopus laevis oocytes. In addition, the optical isomers of PPDA were resolved; the (-) isomer displayed a 50- to 80-fold greater potency than the (+) isomer. Replacement of the phenanthrene moiety of PPDA with naphthalene or anthracene did not improve selectivity. However, phenylazobenzoyl (UBP125) or phenylethynylbenzoyl (UBP128) substitution significantly improved selectivity for NR2B-, NR2C-, and NR2D-containing receptors over NR2A-containing NMDA receptors. Phenanthrene attachment at the 3 position [(2R*,3S*)-1-(phenanthrene-3-carbonyl)piperazine-2,3-dicarboxylic acid (UBP141); (2R*,3S*)-1-(9-bromophenanthrene-3-carbonyl)piperazine-2,3-dicarboxylic acid (UBP145); (2R*,3S*)-1-(9-chlorophenanthrene-3-carbonyl)piperazine-2,3-dicarboxylic acid (UBP160); and (2R*,3S*)-1-(9-iodophenanthrene-3-carbonyl)piperazine-2,3-dicarboxylic acid (UBP161)] displayed improved NR2D selectivity. UBP141 and its 9-brominated homolog (UBP145) both display a 7- to 10- fold selectivity for NR2D-containing receptors over NR2B- or NR2A-containing receptors. Schild analysis indicates that these two compounds are competitive glutamate binding site antagonists. Consistent with a physiological role for NR2D-containing receptors in the hippocampus, UBP141 (5 muM) displayed greater selectivity than PPDA for inhibiting the slow-decaying component of the NMDA receptor-mediated CA3-CA1 synaptic response in rat hippocampal slices. UBP125, UBP128, UBP141, and UBP145 may be useful tools for determining the function of NMDA receptor subtypes.

Diadenosine polyphosphate analog controls postsynaptic excitation in CA3-CA1 synapses via a nitric oxide-dependent mechanism.

Previously, we have described the modulatory effect of diadenosine polyphosphates Ap4A and Ap5A on synaptic transmission in the rat hippocampal slices mediated by presynaptic receptors (Klishin et al., 1994). In contrast, we now describe how nonhydrolyzable Ap4A analog diadenosine-5',5'''-P1,P4-[beta,beta'-methylene]tetraphosphate (AppCH2ppA) at low micromolar concentrations exerts strong nondesensitizing inhibition of orthodromically evoked field potentials (OFPs) without affecting the amplitude of excitatory postsynaptic currents and antidromically evoked field potentials, as recorded in hippocampal CA1 zone. The effects of AppCH2ppA on OFPs are eliminated by a P2 receptor antagonist pyridoxal-phosphate-6-azophenyl-2',4'-disulfonic acid (PPADS) but not mimicked by purinoceptor agonists alpha,beta-methylene-ATP and adenosine 5'-O-(3-thio)-triphosphate, indicating that a P2-like receptor is involved but not one belonging to the conventional P2X/P2Y receptor classes. Diadenosine polyphosphate receptor (P4) antagonist Ip4I (diinosine tetraphosphate) was unable to modulate AppCH2ppA effects. Thus, the PPADS-sensitive P2-like receptor for AppCH2ppA seems to control selectively dendritic excitation of the CA1 neurons. The specific nitric oxide (NO)-scavenger 2-phenyl-4,4,5,5-tetramethyl-imidazoline-1-oxyl-3-oxide is shown to significantly attenuate AppCH2ppA-mediated inhibitory effects, indicating that NO is involved in the cascade of events initiated by AppCH2ppA. Further downstream mediation by adenosine A1 receptors is also demonstrated. Hence, AppCH2ppA-mediated effects involve PPADS-sensitive P2-like receptor activation leading to the production of NO that stimulates intracellular synthesis of adenosine, causing in turn postsynaptic A1 receptor activation and subsequent postsynaptic CA1 dendritic inhibition. Such spatially selective postsynaptic dendritic inhibition may influence dendritic electrogenesis in pyramidal neurons and consequently mediate control of neuronal network activity.

Glycine receptors in CNS neurons as a target for nonretrograde action of cannabinoids.

At many central synapses, endocannabinoids released by postsynaptic cells act retrogradely on presynaptic G-protein-coupled cannabinoid receptors to inhibit neurotransmitter release. Here, we demonstrate that cannabinoids may directly affect the functioning of inhibitory glycine receptor (GlyR) channels. In isolated hippocampal pyramidal and Purkinje cerebellar neurons, endogenous cannabinoids anandamide and 2-arachidonylglycerol, applied at physiological concentrations, inhibited the amplitude and altered the kinetics of rise time, desensitization, and deactivation of the glycine-activated current (I(Gly)) in a concentration-dependent manner. These effects of cannabinoids were observed in the presence of cannabinoid CB1/CB3, vanilloid receptor 1 antagonists, and the G-protein inhibitor GDPbetaS, suggesting a direct action of cannabinoids on GlyRs. The effect of cannabinoids on I(Gly) desensitization was strongly voltage dependent. We also demonstrate that, in the presence of a GABA(A) receptor antagonist, GlyRs may contribute to the generation of seizure-like activity induced by short bursts (seven stimuli) of high-frequency stimulation of inputs to hippocampal CA1 region, because this activity was diminished by selective GlyR antagonists (strychnine and ginkgolides B and J). The GlyR-mediated rhythmic activity was also reduced by cannabinoids (anandamide) in the presence of a CB1 receptor antagonist. These results suggest that the direct inhibition of GlyRs by endocannabinoids can modulate the hippocampal network activity.

Acid sensing ionic channels: modulation by redox reagents.

Acid-sensing ion channels (ASICs) are widely expressed in mammalian sensory neurons and supposedly play a role in nociception and acid sensing. In the course of functioning the redox status of the tissue is subjected to changes. Using whole-cell patch-clamp/concentration clamp techniques we have investigated the effect of redox reagents on the ASIC-like currents in the sensory ganglia and hippocampal neurons of rat. The reducing agent dithiothreitol (DTT), when applied in the concentrations 1-2 mM, reversibly potentiates proton-activated currents, while the oxidizing reagent 5,5'-dithio-bis-(2-nitrobenzoic acid) (DTNB) causes their inhibition. The EC50 and Hill coefficient for the activation of ASIC-like currents by protons are not affected by DTT. Redox modulation of proton-activated currents is independent on the membrane potential and on the level of pH used for the current activation. The endogenous antioxidant tripeptide glutathione (its reduced form, g-l-glutamyl-l-cysteinyl-glycine, GSH) also potentiates proton-activated currents. Our results indicate that ASIC-like currents are susceptible to regulation by redox agents.

Post-synaptic N-methyl-d-aspartate signalling in hippocampal neurons of rat: spillover increases the impact of each spike in a short burst discharge.

High-frequency burst discharges in hippocampus typically consist of less than ten spikes fired at frequencies too high to be followed by a post-synaptic neuron. How significant are these numbers for synaptic signalling? We have measured the N-methyl-d-aspartate (NMDA) component of the excitatory post-synaptic current (EPSC(NMDA)) in hippocampal CA1 neurons of rat after burst discharge of variable duration. The synaptic facilitation is accompanied by a slow-down of the EPSC(NMDA) which develops on a spike-to-spike basis. Consequently the charge transferred by the after-burst EPSC(NMDA) is increased with each spike. The phenomenon is most probably due to the spillover-mediated recruitment of extrasynaptic NMDA receptors. In terms of post-synaptic signalling it dramatically increases the impact of each spike in a short burst discharge.

Extrasynaptic NR2B and NR2D subunits of NMDA receptors shape 'superslow' afterburst EPSC in rat hippocampus.

In conditions of facilitated synaptic release, CA3/CA1 synapses generate anomalously slow NMDA receptor-mediated EPSCs (EPSC(NMDA)). Such a time course has been attributed to the cooperation of synapses through glutamate spillover. Imitating a natural pattern of activity, we have applied short bursts (2-7 stimuli) of high-frequency stimulation and observed a spike-to-spike slow-down of the EPSC(NMDA) kinetics, which accompanied synaptic facilitation. It was found that the early component of the EPSC(NMDA) and the burst-induced late component of the EPSC(NMDA) have distinct pharmacological properties. The competitive NMDA antagonist R-(-)-3-(2-carboxypiperazine-4-yl)-propyl-1-phosphonic acid (D-CPP), which has higher affinity to NR2A than to NR2B subunits and lowest affinity at NR2D subunits, significantly slowed down the decay rate of the afterburst EPSC while leaving the kinetics of the control current unaffected. In contrast, ifenprodil, a highly selective NR2B antagonist, and [+/-]-cis-1-[phenanthren-2yl-carbonyl]piperazine-2,3-dicarboxylic acid (PPDA), a competitive antagonist that is moderately selective for NR2D subunits, more strongly inhibited the late component of the afterburst EPSC(NMDA). The receptors formed by NR2B and (especially) NR2D subunits are known to have higher agonist sensitivity and much slower deactivation kinetics than NR2A-containing receptors. Furthermore, NR2B is preferentially and NR2D is exclusively located on extrasynaptic membranes. As the density of active synapses increases, the confluence of released glutamate makes EPSC decay much longer by activating more extrasynaptic NR2B- and NR2D-subunit-containing receptors. Long-term potentiation (LTP) induced by successive rounds of burst stimulation is accompanied by a long-term increase in the contribution of extrasynaptic receptors in the afterburst EPSC(NMDA.)