Receptor-mediated Excitatory Synaptic Transmission Research Articles

A potential of methoxpropamine to be a widespread recreational drug: it blocks NMDA receptors and inhibits NMDA receptor-mediated synaptic transmission in a brain preparation of mice

The activity of N-methyl-d-aspartate (NMDA) receptors in the central nervous system is affected by many psychoactive drugs, such as 2-(2-chlorophenyl)-2-(methylamino)cyclohexan-1-one (ketamine) and its analog 2-(3-methoxyphenyl)-2-(amino)cyclohexan-1-one (methoxetamine, MXE). Recreational use of MXE can cause acute toxicity, such as dissociative state and tachycardia. After MXE use, confusion, agitation, ataxia, and nystagmus are induced. Moreover, MXE-related deaths have been reported, and this compound is banned in many countries. Recently, MXE’s derivative, 2-(3-methoxyphenyl)-2-(propylamino)cyclohexan-1-one (methoxpropamine, MXPr), was reported as a new psychoactive substance and sold online as a designer drug. The aims were to determine how MXPr affects NMDA receptors in neurons and to compare the potency with MXE. Cartwheel cells in the dorsal cochlear nucleus of mice were used as a model of NMDA receptor-expressing neurons, and patch-clamp method was performed for the recordings. NMDA-induced inward current was initially evoked by microiontophoresis application of NMDA onto the cartwheel cells, and the effects of MXPr, MXE, and (5R,10S)-( +)-5-methyl-10,11-dihydro-5H-dibenzo[a,d]cyclohepten-5,10-imine (MK-801, as a positive control substance) on NMDA-induced response were examined. Moreover, the effects of MXPr on NMDA receptor-mediated excitatory postsynaptic currents were also investigated. The IC50 of MXPr, MXE, and MK-801 on NMDA-induced response decreased in the following order: MXPr (1.647 μM) > MXE (0.841 μM) > MK-801 (0.060 μM), indicating that MXPr and MXE act as potent antagonists of NMDA receptors. MXPr suppressed NMDA receptor-mediated excitatory synaptic transmission in a dose-dependent manner. MXPr, which may cause health and social damages to humans by blocking NMDA receptors, is a serious concern like MXE.

Enhancement of postsynaptic GABAA and extrasynaptic NMDA receptor-mediated responses in the barrel cortex of Mecp2-null mice.

Rett syndrome (RTT) is a neurodevelopmental disorder that results from mutations in the X-linked gene for methyl-CpG-binding protein 2 (MECP2). The underlying cellular mechanism for the sensory deficits in patients with RTT is largely unknown. This study used the Bird mouse model of RTT to investigate sensory thalamocortical synaptic transmission in the barrel cortex of Mecp2-null mice. Electrophysiological results showed an excitation/inhibition imbalance, biased toward inhibition, due to an increase in efficacy of postsynaptic GABAA receptors rather than alterations in inhibitory network and presynaptic release properties. Enhanced inhibition impaired the transmission of tonic sensory signals from the thalamus to the somatosensory cortex. Previous morphological studies showed an upregulation of NMDA receptors in the neocortex of both RTT patients and Mecp2-null mice at early ages [Blue ME, Naidu S, Johnston MV. Ann Neurol 45: 541-545, 1999; Blue ME, Kaufmann WE, Bressler J, Eyring C, O'Driscoll C, Naidu S, Johnston MV. Anat Rec (Hoboken) 294: 1624-1634, 2011]. Although AMPA and NMDA receptor-mediated excitatory synaptic transmission was not altered in the barrel cortex of Mecp2-null mice, extrasynaptic NMDA receptor-mediated responses increased markedly. These responses were blocked by memantine, suggesting that extrasynaptic NMDA receptors play an important role in the pathogenesis of RTT. The results suggest that enhancement of postsynaptic GABAA and extrasynaptic NMDA receptor-mediated responses may underlie impaired somatosensation and that pharmacological blockade of extrasynaptic NMDA receptors may have therapeutic value for RTT.

Neonatal ketamine exposure causes impairment of long-term synaptic plasticity in the anterior cingulate cortex of rats

Ketamine, a dissociative anesthetic most commonly used in many pediatric procedures, has been reported in many animal studies to cause widespread neuroapoptosis in the neonatal brain after exposure in high doses and/or for a prolonged period. This neurodegenerative change occurs most severely in the forebrain including the anterior cingulate cortex (ACC) that is an important brain structure for mediating a variety of cognitive functions. However, it is still unknown whether such apoptotic neurodegeneration early in life would subsequently impair the synaptic plasticity of the ACC later in life. In this study, we performed whole-cell patch-clamp recordings from the ACC brain slices of young adult rats to examine any alterations in long-term synaptic plasticity caused by neonatal ketamine exposure. Ketamine was administered at postnatal day 4–7 (subcutaneous injections, 20mg/kg given six times, once every 2h). At 3–4weeks of age, long-term potentiation (LTP) was induced and recorded by monitoring excitatory postsynaptic currents from ACC slices. We found that the induction of LTP in the ACC was significantly reduced when compared to the control group. The LTP impairment was accompanied by an increase in the α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptor-mediated excitatory synaptic transmission and a decrease in GABA inhibitory synaptic transmission in neurons of the ACC. Thus, our present findings show that neonatal ketamine exposure causes a significant LTP impairment in the ACC. We suggest that the imbalanced synaptic transmission is likely to contribute to ketamine-induced LTP impairment in the ACC.

Long‐term depression of synaptic transmission in the adult mouse insular cortex in vitro

The insular cortex (IC) is known to play important roles in higher brain functions such as memory and pain. Activity-dependent long-term depression (LTD) is a major form of synaptic plasticity related to memory and chronic pain. Previous studies of LTD have mainly focused on the hippocampus, and no study in the IC has been reported. In this study, using a 64-channel recording system, we show for the first time that repetitive low-frequency stimulation (LFS) can elicit frequency-dependent LTD of glutamate receptor-mediated excitatory synaptic transmission in both superficial and deep layers of the IC of adult mice. The induction of LTD in the IC required activation of the N-methyl-d-aspartate (NMDA) receptor, metabotropic glutamate receptor (mGluR)5, and L-type voltage-gated calcium channel. Protein phosphatase 1/2A and endocannabinoid signaling are also critical for the induction of LTD. In contrast, inhibiting protein kinase C, protein kinase A, protein kinase Mζ or calcium/calmodulin-dependent protein kinase II did not affect LFS-evoked LTD in the IC. Bath application of the group I mGluR agonist (RS)-3,5-dihydroxyphenylglycine produced another form of LTD in the IC, which was NMDA receptor-independent and could not be occluded by LFS-induced LTD. Our studies have characterised the basic mechanisms of LTD in the IC at the network level, and suggest that two different forms of LTD may co-exist in the same population of IC synapses.

Autism-linked neuroligin-3 R451C mutation differentially alters hippocampal and cortical synaptic function

Multiple independent mutations in neuroligin genes were identified in patients with familial autism, including the R451C substitution in neuroligin-3 (NL3). Previous studies showed that NL3(R451C) knock-in mice exhibited modestly impaired social behaviors, enhanced water maze learning abilities, and increased synaptic inhibition in the somatosensory cortex, and they suggested that the behavioral changes in these mice may be caused by a general shift of synaptic transmission to inhibition. Here, we confirm that NL3(R451C) mutant mice behaviorally exhibit social interaction deficits and electrophysiologically display increased synaptic inhibition in the somatosensory cortex. Unexpectedly, however, we find that the NL3(R451C) mutation produced a strikingly different phenotype in the hippocampus. Specifically, in the hippocampal CA1 region, the NL3(R451C) mutation caused an ∼1.5-fold increase in AMPA receptor-mediated excitatory synaptic transmission, dramatically altered the kinetics of NMDA receptor-mediated synaptic responses, induced an approximately twofold up-regulation of NMDA receptors containing NR2B subunits, and enhanced long-term potentiation almost twofold. NL3 KO mice did not exhibit any of these changes. Quantitative light microscopy and EM revealed that the NL3(R451C) mutation increased dendritic branching and altered the structure of synapses in the stratum radiatum of the hippocampus. Thus, in NL3(R451C) mutant mice, a single point mutation in a synaptic cell adhesion molecule causes context-dependent changes in synaptic transmission; these changes are consistent with the broad impact of this mutation on murine and human behaviors, suggesting that NL3 controls excitatory and inhibitory synapse properties in a region- and circuit-specific manner.

Postsynaptic and Presynaptic Group II Metabotropic Glutamate Receptor Activation Reduces Neuronal Excitability in Rat Midline Paraventricular Thalamic Nucleus

Drugs that interact with group II metabotropic glutamate receptors (mGluRs) are presently being evaluated for a role in the treatment of anxiety disorders and symptoms of schizophrenia. Their mechanism of action is believed to involve a reduction in excitatory neurotransmission in limbic and forebrain regions commonly associated with these mental disorders. In rodents, the glutamatergic neurons in the midline paraventricular thalamic nucleus (PVT) provide excitatory inputs to the limbic system and forebrain. PVT also displays a high density of group II mGluRs, predominantly the metabotropic glutamate 2 receptor (mGluR2). Because the role of group II mGluRs in regulating cellular and synaptic excitability in this location has yet to be determined, we used whole-cell patch-clamp recording and acute rat brain slice preparations to evaluate PVT neuron responses to a selective group II mGluR agonist, (1R,4R,5S,6R)-4-amino-2-oxabicyclo[3.1.0]hexane-4,6-dicarboxylic acid (LY 379268). LY 379268 consistently induced membrane hyperpolarization and suppressed firing by postsynaptic receptor-mediated activation of a barium-sensitive background K(+) conductance. This effect could be blocked by (2S)-2-amino-2-[(1S,2S)-2-carboxycycloprop-1-yl]-3-(xanth-9-yl)propanoic acid (LY 341495), a selective group II mGluR antagonist. In addition, LY 379268 acted at presynaptic receptors to reduce ionotropic glutamate receptor-mediated excitatory synaptic transmission. An mGluR2-positive allosteric modulator, 2,2,2-trifluoro-N-[4-(2-methoxyphenoxy)phenyl]-N-(3-pyridinylmethyl)ethanesulfonamide hydrochloride (LY 487379), resulted in leftward shifts of the LY 379268 dose-response curve for both postsynaptic and presynaptic actions. The data demonstrate that activation of postsynaptic and presynaptic group II (presumably mGluR2) mGluRs reduces neuronal excitability in midline thalamus, an action that may contribute to the effectiveness of mGluR2-activating drugs in rodent models of anxiety and psychosis.

Homeostatic regulation of glutamatergic transmission by dopamine D 4 receptors

Alterations of synaptic transmission have been considered a core feature of mental disorders; thus, we examined the role of dopamine D(4) receptors, which is highly implicated in attention-deficit hyperactivity disorder and schizophrenia, in regulating synaptic functions of prefrontal cortex, a brain region critical for cognitive and emotional processes. We found that D(4) stimulation caused a profound depression or potentiation of AMPA receptor-mediated excitatory synaptic transmission in prefrontal cortex pyramidal neurons when their activity was elevated or dampened, respectively, which was accompanied by a D(4)-induced decrease or increase of AMPARs at synapses. The dual effects of D(4) on AMPAR trafficking and function was dependent on the D(4)-mediated bidirectional regulation of CaMKII activity via coupling to distinct signaling pathways, which provides a unique mechanism for D(4) receptors to serve as a homeostatic synaptic factor to stabilize cortical excitability.

Levetiracetam inhibits glutamate transmission through presynaptic P/Q‐type calcium channels on the granule cells of the dentate gyrus

Levetiracetam is an effective anti-epileptic drug in the treatment of partial and generalized seizure. The purpose of the present study was to investigate whether levetiracetam regulates AMPA and NMDA receptor-mediated excitatory synaptic transmission and to determine its site of action in the dentate gyrus (DG), the area of the hippocampus that regulates seizure activities. Whole-cell patch-clamp method was used to record the AMPA and NMDA receptor-mediated excitatory postsynaptic currents (EPSC(AMPA) and EPSC(NMDA)) in the presence of specific antagonists, from the granule cells in the DG in brain slice preparations from young Wistar rats (60-120 g). Levetiracetam (100 microM) inhibited both evoked EPSC(AMPA) and EPSC(NMDA) to an equal extent (80%), altered the paired-pulse ratio (from 1.39 to 1.25), decreased the frequency of asynchronous EPSC and prolonged the inter-event interval of miniature EPSC(AMPA) (from 2.7 to 4.6 s) without changing the amplitude, suggesting a presynaptic action of levetiracetam. The inhibitory effect of levetiracetam on evoked EPSC(AMPA) was blocked by omega-agatoxin TK (100 nM), a selective P/Q-type voltage-dependent calcium channel blocker, but not by nimodipine (10 microM) or omega-conotoxin (400 nM). These results suggest that levetiracetam modulated the presynaptic P/Q-type voltage-dependent calcium channel to reduce glutamate release and inhibited the amplitude of EPSC in DG. This effect is most likely to contribute to the anti-epileptic action of levetiracetam in patients.

Enhanced NMDA Receptor-Mediated Synaptic Transmission, Enhanced Long-Term Potentiation, and Impaired Learning and Memory in Mice Lacking IRSp53

IRSp53 is an adaptor protein that acts downstream of Rac and Cdc42 small GTPases and is implicated in the regulation of membrane deformation and actin filament assembly. In neurons, IRSp53 is an abundant postsynaptic protein and regulates actin-rich dendritic spines; however, its in vivo functions have not been explored. We characterized transgenic mice deficient of IRSp53 expression. Unexpectedly, IRSp53(-/-) neurons do not show significant changes in the density and ultrastructural morphologies of dendritic spines. Instead, IRSp53(-/-) neurons exhibit reduced AMPA/NMDA ratio of excitatory synaptic transmission and a selective increase in NMDA but not AMPA receptor-mediated transmission. IRSp53(-/-) hippocampal slices show a markedly enhanced long-term potentiation (LTP) with no changes in long-term depression. LTP-inducing theta burst stimulation enhances NMDA receptor-mediated transmission. Spatial learning and novel object recognition are impaired in IRSp53(-/-) mice. These results suggest that IRSp53 is involved in the regulation of NMDA receptor-mediated excitatory synaptic transmission, LTP, and learning and memory behaviors.

Lamotrigine inhibits postsynaptic AMPA receptor and glutamate release in the dentate gyrus

The dentate gyrus (DG) is a gateway that regulates seizure activity in the hippocampus. We investigated the site of action of lamotrigine (LTG), an effective anticonvulsant, in the regulation of alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid (AMPA) and N-methyl-D-aspartic acid (NMDA) receptor-mediated excitatory synaptic transmission on DG. Evoked AMPA and NMDA receptor-mediated excitatory postsynaptic currents (eEPSCampa and eEPSCnmda) were recorded by whole-cell patch-clamp recording from the granule cells of DG in brain slice preparation of young Wistar rats (60-120 g). Exogenously applied AMPA and NMDA-induced currents and AMPA receptor-mediated miniature EPSC (mEPSCampa) were recorded in the presence of specific antagonists. LTG inhibited both eEPSCampa and eEPSCnmda, and had no effect on exogenously applied NMDA-induced current indicating LTG inhibited glutamate release. Previous studies demonstrated that alteration in glutamate concentration in synaptic cleft causes parallel changes of eEPSCampa and eEPSCnmda. Our results showed that LTG inhibited eEPSCampa significantly more than eEPSCnmda (p < 0.05), suggesting that LTG may also have blocked the postsynaptic AMPA receptor. The hypothesis is further supported by the facts that; (1) LTG (30-100 microM) inhibited direct exogenously applied AMPA-induced currents (to 90%), (2) LTG significantly reduced the amplitude, but not the frequency of mEPSCampa and asynchronous (EPSC), and (3) LTG-induced reduction of eEPSCampa was not associated with a modification of the paired-pulse ratio. To sum up, LTG exerts a postsynaptic inhibitory mechanism on the AMPA receptor. Our results demonstrate that LTG suppresses postsynaptic AMPA receptors and reduces glutamate release in granule cells of DG. The postsynaptic effect can be one of the underlying mechanisms of LTG's anticonvulsant action.

Inactivation of adenosine A 2A receptor impairs long term potentiation in the accumbens nucleus without altering basal synaptic transmission

The nucleus accumbens is considered to be critically involved in the control of complex motivated behaviors. By modulating its glutamatergic excitatory input, mesolimbic dopaminergic afferents have been implicated in the reinforcing properties of drugs of abuse. However, they might not represent the only path for influencing the accumbens output. The aim of this study was to investigate possible modulation of synaptic transmission at this glutamatergic synapse by adenosine receptors. The standard field potential recording technique was used on brain slices from wild-type and A 2A receptor-deficient mice. Neither the stimulus–response relationship nor paired-pulse facilitation was altered in the mutant mice. In both genotypes, the activation of A 1 receptors by 2-chloro- N 6-cyclopentyladenosine reduced the field excitatory postsynaptic potential (fEPSP) slope to a similar extent. In wild-type slices, activation or blockade of A 2A receptors by 2-[4-(carboxyethyl)phenylethylamino]-5′- N-ethylcarboxamidoadenosine and 4-(2-[7-amino-2-(2-furyl)[1,2,4]-triazolo-[2,3-a][1,3,5]triazin-5-ylamino]ethyl)phenol, respectively, did not modify the synaptic transmission. Moreover, a long lasting pre-activation of these A 2A receptors did not influence the A 1 receptor-mediated reduction in fEPSP slope. Long term potentiation (LTP) of the α-amino-3-hydroxy-5-methyl-4-isoxazole proprionate (AMPA) receptor-mediated synaptic transmission could be elicited in both wild-type and A 2A receptor-deficient mice. However, LTP appeared to be quantitatively modulated by the A 2A receptor pathway since the level of potentiation was reduced in A 2A receptor-deficient mice as well as in slices of wild-type mice in which the A 2A receptor pathway was blocked. The involvement of the cAMP-dependent protein kinase was supported by the reduction in potentiation level in slices of wild-type mice treated with adenosine 3′,5′-cyclic monophosphorothiotate, 8-(4-chlorophenylthio)-Rp isomer, an inhibitor of this enzyme. These data provide evidence that the adenosine acting at the A 2A receptor is implicated in events directly or indirectly related to LTP induction in the accumbens whereas it is not involved in the regulation of the basal AMPA receptor-mediated excitatory synaptic transmission.

Modulation of synaptic transmission by nociceptin/orphanin FQ and nocistatin in the spinal cord dorsal horn of mutant mice lacking the nociceptin/orphanin FQ receptor.

Nociceptin/orphanin FQ (N/OFQ) and nocistatin (NST) are two neuropeptides derived from the same precursor protein that exhibit opposing effects on spinal neurotransmission and nociception. Here, we have used whole-cell, patch-clamp recordings from visually identified neurons in spinal cord dorsal horn slices of genetically modified mice to investigate the role of the N/OFQ receptor (N/OFQ-R) in the modulatory action of both peptides on excitatory glutamatergic and inhibitory glycinergic and gamma-aminobutyric acid (GABA)-ergic synaptic transmission. In wild-type mice, N/OFQ selectively suppressed excitatory transmission in a concentration-dependent manner but left inhibitory synaptic transmission unaffected. In contrast, NST reduced only inhibitory but not alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid (AMPA) receptor-mediated excitatory synaptic transmission. N/OFQ-mediated inhibition of excitatory transmission was completely absent in N/OFQ-R receptor-deficient (N/OFQ-R(-/-)) mice and significantly reduced in heterozygous (N/OFQ-R(+/-)) mice, whereas the action of NST on inhibitory neurotransmission was completely retained. To test for the relevance of these results for spinal nociception, we investigated the effects of intrathecally injected N/OFQ in the mouse formalin test, an animal model of tonic pain. N/OFQ (3 nmol/mouse) induced significant antinociception in wild-type mice, but had no antinociceptive effects in N/OFQ-R(-/-) mice. These results indicate that the inhibitory action of N/OFQ on excitatory glutamatergic synaptic transmission and its spinal antinociceptive action are mediated via the N/OFQ receptor, whereas the action of NST is independent of this receptor.

Exaggerated responses to chronic nociceptive stimuli and enhancement of N-methyl- d-aspartate receptor-mediated synaptic transmission in mutant mice lacking d-amino-acid oxidase

Formalin-induced nociceptive behaviors and N-methyl- d-aspartate (NMDA) subtype glutamate receptor-mediated excitatory synaptic transmission were analyzed in mutant mice lacking d-amino-acid oxidase, which catalyzes the oxidative deamination of d-amino acids. The second phase of the formalin-induced licking response, a part of which is known to be mediated by NMDA receptors in the spinal cord, was significantly augmented in mutant mice. NMDA receptor-mediated excitatory postsynaptic currents recorded from spinal cord dorsal horn neurons by tight-seal whole-cell methods were significantly potentiated in mutant mice. The present observations provide another line of evidence that d-serine functions as an endogenous coagonist at the glycine site of NMDA receptors, and raise the possibility that d-amino-acid oxidase exerts a neuromodulatory function by controlling the concentration of d-serine in the central nervous system.

Development of NMDA and non-NMDA receptor-mediated excitatory synaptic transmission in geniculocortical and corticocortical connections in organotypic coculture preparations.

Development of N-methyl-D-aspartate (NMDA) and non-NMDA receptor-mediated excitatory synaptic transmission was studied in the visual cortex using organotypic slice cocultures. A slice of visual cortex (VC) dissected from newborn rats was cocultured with either a chunk of embryonic lateral geniculate nucleus (LGN) or another VC. During 7-38 days in vitro (DIV), geniculocortical monosynaptic excitatory postsynaptic potentials (EPSPs) were recorded from layer IV neurons in response to stimulation of the LGN in cocultures of the VC with the LGN. Similarly, corticocortical monosynaptic EPSPs were recorded from layers II/III and V/VI neurons in cocultures of two VCs when stimulating the partner VC. The initial slopes of the non-NMDA and NMDA receptor-mediated components of the EPSPs, which were dissociated pharmacologically, were assessed and compared among three different culture stages, early (7-11 DIV), middle (12-15 DIV) and late (17-38 DIV). With progression of the culture stage, the non-NMDA component tended to increase in both the geniculocortical and corticocortical connections. In contrast, the NMDA component exhibited distinct developmental changes. The NMDA component in layer IV neurons, which receive geniculate inputs, showed a transient increase in the middle stage. In the corticocortical connection, the magnitude of the NMDA component was large in the early stage and maintained through all culture stages in layer V/VI cells, whereas in layer II/III cells it decreased sharply by the late stage. Our results suggest that glutamatergic transmission in the visual cortex develops differently in the geniculocortical and corticocortical connections.

Excitatory Synaptic Transmission Mediated by NMDA Receptors Is More Sensitive to Isoflurane than Are Non-NMDA Receptor-mediated Responses

Effects of volatile anesthetic agents on N-methyl-D-aspartate (NMDA) receptor-mediated excitatory synaptic transmission have not been well characterized. The authors compared effects produced by halothane and isoflurane on electrophysiologic properties of NMDA and non-NMDA receptor-mediated synaptic responses in slices from the rat hippocampus. Field excitatory postsynaptic potentials (fEPSPs) in the CA1 area were recorded with extracellular electrodes after electrical stimulation of Schaffer-collateral-commissural fiber inputs. NMDA or non-NMDA receptor-mediated fEPSPs were pharmacologically isolated using selective antagonists. Clinically relevant concentrations of halothane or isoflurane were applied to slices in an artificial cerebrospinal fluid perfusate. Paired pulse facilitation was used as a measure of presynaptic effects of the anesthetic agents. Clinically relevant concentrations of halothane (1.2 vol% approximately 0.35 mM) depressed fEPSP amplitudes mediated by NMDA receptors and non-NMDA receptors to a similar degree (mean +/- SD: 63.3 +/- 14.0% of control, n = 5; 60.2 +/- 7.3% of control, n = 7, respectively). In contrast, isoflurane (1.4 vol% approximately 0.50 mM) preferentially depressed fEPSP amplitudes mediated by NMDA receptors (44.0 +/- 7.4% of control, n = 6, P < 0.001) compared with those for non-NMDA receptors (68.7 +/- 5.4% of control n = 6), indicating a selective, additional postsynaptic effect. Paired pulse facilitation of fEPSPs was increased significantly by both anesthetic agents from 1.37 +/- 0.13 to 1.91 +/- 0.25 (n = 5, P < 0.05 for halothane) and from 1.44 +/- 0.04 to 1.64 +/- 0.08 (n = 5, P < 0.01 for isoflurane), suggesting that presynaptic mechanisms are also involved in fEPSP depression produced by the anesthetic agents. Neither rise times nor decay times of fEPSPs were changed in the presence of the anesthetic agents. These results indicate that fEPSPs mediated by postsynaptic NMDA receptors are more sensitive to clinically relevant concentrations of isoflurane than are non-NMDA receptor-mediated responses, but this selective effect was not observed for halothane. Both agents also appeared to depress release of glutamate from nerve terminals via presynaptic actions.

Reduction of voltage-dependent currents by ethanol contributes to inhibition of NMDA receptor-mediated excitatory synaptic transmission

Previous studies have shown inhibitory effects of EtOH on NMDA receptor-mediated synaptic transmission in several brain regions. We examined this effect of EtOH under both current clamp and voltage clamp conditions in the basolateral amygdala because of the putative role of the amygdala in mediating anxiolytic effects of EtOH. We found that EtOH reduced NMDA receptor-mediated synaptic responses. In addition, we found that NMDA receptor-mediated depolarizations could also activate a voltage-dependent regenerative potential which was also sensitive to EtOH. Pharmacological characterization of this current was consistent with a high-threshold Ca 2+ current. This current also exhibited a pronounced tendency towards transient enhancement upon withdrawal of EtOH.

A Comparison of the Effect of Halothane on N-Methyl-D-Aspartate and Non-N-Methyl-D-Aspartate Receptor-Mediated Excitatory Synaptic Transmission in the Hippocampus

A Comparison of the Effect of Halothane on N-Methyl-D-Aspartate and Non-N-Methyl-D-Aspartate Receptor-Mediated Excitatory Synaptic Transmission in the Hippocampus

A comparison of the effect of halothane on N-methyl-D-aspartate and non-N-methyl-D-aspartate receptor-mediated excitatory synaptic transmission in the hippocampus.

Halothane depresses synaptic transmission in the rat brain. First we determined the concentration of halothane which decreased the amplitude of the population spike recorded in the CA1 region of the hippocampus to 50% of the control value (105 +/- 4.9 micrograms/mL [0.53 mM] halothane). Hippocampal glutamate receptors are divided into N-methyl-D-aspartate (NMDA) and alpha-amino-3-hydroxy-5-methyl-4-isoxazole proprionate (AMPA) and kainate (non-NMDA) subtypes. The NMDA and non-NMDA receptors were blocked with (+/-)-2-amino-5-phosphonopentanoic acid (AP5) (30 microM), and 6,7-dinitroquinoxaline-2,3-dione (DNQX) (10 microM), respectively, to allow observation of the effects of halothane on the NMDA and non-NMDA receptors, respectively. gamma-Aminobutyric acid type A (GABAA) receptors were blocked in all studies with picrotoxin (PTX) (40 microM). When the non-NMDA receptors were blocked a halothane concentration of 38.1 +/- 5.6 mg/mL was required to produce a further 50% decrease in population spike amplitude. When NMDA receptors were blocked with AP5 or only GABAA receptors were blocked the halothane concentrations needed to produce 50% block were higher than needed for the control (160.8 +/- 17.8 and 190.2 +/- 12.1 microgram/mL, respectively). These studies indicate that the NMDA receptors are more sensitive to the effects of halothane than the non-NMDA receptors.