NBQX, an improved non-NMDA antagonist studied in retinal ganglion cells

NMDAR-dependent presynaptic homeostasis in adult hippocampus: Synapse growth and cross-modal inhibitory plasticity

Homeostatic plasticity (HP) encompasses a suite of compensatory physiological processes that counteract neuronal perturbations, enabling brain resilience. Currently, we lack a complete description of the homeostatic processes that operate within the mammalian brain. Here, we demonstrate that acute, partial AMPAR-specific antagonism induces potentiation of presynaptic neurotransmitter release in adult hippocampus, a form of compensatory plasticity that is consistent with the expression of presynaptic homeostatic plasticity (PHP) documented at peripheral synapses. We show that this compensatory plasticity can be induced within minutes, requires postsynaptic NMDARs, and is expressed via correlated increases in dendritic spine volume, active zone area, and docked vesicle number. Further, simultaneous postsynaptic genetic reduction of GluA1, GluA2, and GluA3 in triple heterozygous knockouts induces potentiation of presynaptic release. Finally, induction of compensatory plasticity at excitatory synapses induces a parallel, NMDAR-dependent potentiation of inhibitory transmission, a cross-modal effect consistent with the anti-epileptic activity of AMPAR-specific antagonists used in humans.

Alternative splicing of GluN1 gates glycine site-dependent nonionotropic signaling by NMDAR receptors

N-methyl-D-aspartate receptors (NMDARs), which are critical in the brain, are increasingly being shown to signal without ion flux (i.e., “metabotropically”). What controls the metabotropic function of NMDARs is unknown. We discovered that a form of metabotropic signaling—glycine priming—is controlled by alternative splicing of the mRNA encoding one NMDAR subunit, GluN1. Our discovery was surprising because the spliced exon encodes a peptide cassette in the extracellular region of GluN1 far from the plasma membrane, and yet, metabotropic function requires signaling across the neuronal membrane. Moreover, we found that this metabotropic function of NMDARs is neuron cell–type specific: excitatory neurons show glycine priming, whereas inhibitory neurons do not. These findings have widespread implications for NMDARs in health and disease.

N-methyl-D-aspartate (NMDA) receptors (NMDARs), a principal subtype of excitatory neurotransmitter receptor, are composed as tetrameric assemblies of two glycine-binding GluN1 subunits and two glutamate-binding GluN2 subunits. NMDARs can signal nonionotropically through binding of glycine alone to its cognate site on GluN1. A consequence of this signaling by glycine is that NMDARs are primed such that subsequent gating, produced by glycine and glutamate, drives receptor internalization. The GluN1 subunit contains eight alternatively spliced isoforms produced by including or excluding the N1 and the C1, C2, or C2’ polypeptide cassettes. Whether GluN1 alternative splicing affects nonionotropic signaling by NMDARs is a major outstanding question. Here, we discovered that glycine priming of recombinant NMDARs critically depends on GluN1 isoforms lacking the N1 cassette; glycine priming is blocked in splice variants containing N1. On the other hand, the C-terminal cassettes—C1, C2, or C2’—each permit glycine signaling. In wild-type mice, we found glycine-induced nonionotropic signaling at synaptic NMDARs in CA1 hippocampal pyramidal neurons. This nonionotropic signaling by glycine to synaptic NMDARs was prevented in mice we engineered, such that GluN1 obligatorily contained N1. We discovered in wild-type mice that, in contrast to pyramidal neurons, synaptic NMDARs in CA1 inhibitory interneurons were resistant to glycine priming. But we recapitulated glycine priming in inhibitory interneurons in mice engineered such that GluN1 obligatorily lacked the N1 cassette. Our findings reveal a previously unsuspected molecular function for alternative splicing of GluN1 in controlling nonionotropic signaling of NMDARs by activating the glycine site.

Transcriptional modulation of calcium-permeable AMPA receptor subunits in glioblastoma by MEK-ERK1/2 inhibitors and their role in invasion

Glioblastoma is the most common primary brain tumor. Glioblastoma cells secrete a significant amount of glutamate, which serve as a potential growth factor in glioma pathobiology through their specific receptor subtypes including α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptor (AMPAR). Glioblastoma express AMPAR subunits; however, its regulation and activation with downstream intracellular signaling are not well-understood. Phosphorylated-extracellular signaling-regulated kinase (ERK)1/2 is known to regulate the ionotropic glutamate receptors in cortical neurons. The mitogen-activated protein kinase cascade is frequently activated in several tumors, including glioma. Nonetheless, the association of ERK signaling with AMPAR subunits in glioblastoma is undetermined. Here, we demonstrated potential role of AMPAR in invasion, and the modulation of AMPAR subunits at transcript level by ERK signaling in glioblastoma cells. Inhibition of ERK signaling specifically downregulated the expression of calcium-permeable AMPAR subunits, GluA1 and GluA4, and upregulated calcium-impermeable AMPAR subunit GluA2 implying differential regulation of the expression of calcium-permeable AMPAR subunits of glioblastoma. Concomitantly, it significantly decreased the invasion of U87MG cells. Taken together, these findings suggest that the AMPAR enhances invasion of glioblastoma, and ERK signaling modulates the differential expression of calcium-permeable AMPAR phenotype that might play a crucial role in the invasive propensity of glioblastoma cells.

Restoring Light Sensitivity in Blind Retinae Using a Photochromic AMPA Receptor Agonist

Retinal degenerative diseases can have many possible causes and are currently difficult to treat. As an alternative to therapies that require genetic manipulation or the implantation of electronic devices, photopharmacology has emerged as a viable approach to restore visual responses. Here, we present a new photopharmacological strategy that relies on a photoswitchable excitatory amino acid, ATA. This freely diffusible molecule selectively activates AMPA receptors in a light-dependent fashion. It primarily acts on amacrine and retinal ganglion cells, although a minor effect on bipolar cells has been observed. As such, it complements previous pharmacological approaches based on photochromic channel blockers and increases the potential of photopharmacology in vision restoration.

Sigma receptors [σRs]: biology in normal and diseased states

This review compares the biological and physiological function of Sigma receptors [σRs] and their potential therapeutic roles. Sigma receptors are widespread in the central nervous system and across multiple peripheral tissues. σRs consist of sigma receptor one (σ₁R) and sigma receptor two (σ₂R) and are expressed in numerous regions of the brain. The sigma receptor was originally proposed as a subtype of opioid receptors and was suggested to contribute to the delusions and psychoses induced by benzomorphans such as SKF-10047 and pentazocine. Later studies confirmed that σRs are non-opioid receptors (not an µ opioid receptor) and play a more diverse role in intracellular signaling, apoptosis and metabolic regulation. σ₁Rs are intracellular receptors acting as chaperone proteins that modulate Ca²⁺ signaling through the IP₃ receptor. They dynamically translocate inside cells, hence are transmembrane proteins. The σ₁R receptor, at the mitochondrial-associated endoplasmic reticulum membrane, is responsible for mitochondrial metabolic regulation and promotes mitochondrial energy depletion and apoptosis. Studies have demonstrated that they play a role as a modulator of ion channels (K⁺ channels; N-methyl-d-aspartate receptors [NMDAR]; inositol 1,3,5 triphosphate receptors) and regulate lipid transport and metabolism, neuritogenesis, cellular differentiation and myelination in the brain. σ₁R modulation of Ca²⁺ release, modulation of cardiac myocyte contractility and may have links to G-proteins. It has been proposed that σ₁Rs are intracellular signal transduction amplifiers. This review of the literature examines the mechanism of action of the σRs, their interaction with neurotransmitters, pharmacology, location and adverse effects mediated through them.

Paraventricular nucleus of the hypothalamus glutamate neurotransmission modulates autonomic, neuroendocrine and behavioral responses to acute restraint stress in rats

In the present study, the involvement of paraventricular nucleus of the hypothalamus (PVN) glutamate receptors in the modulation of autonomic (arterial blood pressure, heart rate and tail skin temperature) and neuroendocrine (plasma corticosterone) responses and behavioral consequences evoked by the acute restraint stress in rats was investigated. The bilateral microinjection of the selective non-NMDA glutamate receptor antagonist NBQX (2 nmol/ 100 nL) into the PVN reduced the arterial pressure increase as well as the fall in the tail cutaneous temperature induced by the restraint stress, without affecting the stress-induced tachycardiac response. On the other hand, the pretreatment of the PVN with the selective NMDA glutamate receptor antagonist LY235959 (2 nmol/100 nL) was able to increase the stress-evoked pressor and tachycardiac response, without affecting the fall in the cutaneous tail temperature. The treatment of the PVN with LY235959 also reduced the increase in plasma corticosterone levels during stress and inhibited the anxiogenic-like effect observed in the elevated plus-maze 24h after the restraint session. The present results show that NMDA and non-NMDA receptors in the PVN differently modulate responses associated to stress. The PVN glutamate neurotransmission, via non-NMDA receptors, has a facilitatory influence on stress-evoked autonomic responses. On the other hand, the present data point to an inhibitory role of PVN NMDA receptors on the cardiovascular responses to stress. Moreover, our findings also indicate an involvement of PVN NMDA glutamate receptors in the mediation of the plasma corticosterone response as well as in the delayed emotional consequences induced by the restraint stress.

Kappa opioid receptors regulate stress-induced cocaine seeking and synaptic plasticity

Stress facilitates reinstatement of addictive drug seeking in animals and promotes relapse in humans. Acute stress has marked and long-lasting effects on plasticity at both inhibitory and excitatory synapses on dopamine neurons in the ventral tegmental area (VTA), a key region necessary for drug reinforcement. Stress blocks long-term potentiation at GABAergic synapses on dopamine neurons in the VTA (LTPGABA), potentially removing a normal brake on activity. Here we show that blocking kappa opioid receptors (KORs) prior to forced-swim stress rescues LTPGABA. In contrast, blocking KORs does not prevent stress-induced potentiation of excitatory synapses nor morphine-induced block of LTPGABA. Using a kappa receptor antagonist as a selective tool to test the role of LTPGABA in vivo, we find that blocking KORs within the VTA prior to forced-swim stress prevents reinstatement of cocaine seeking. These results suggest that KORs may represent a useful therapeutic target for treatment of stress-triggered relapse in substance abuse.

Interphotoreceptor retinoid-binding protein as the physiologically relevant carrier of 11-cis-retinol in the cone visual cycle

Cones function in constant light and are responsible for mediating daytime human vision. Like rods, cones use the photosensitive molecule 11-cis-retinal to detect light, and in constant illumination, a continuous supply of 11-cis-retinal is needed. A retina visual cycle is thought to provide a privileged supply of 11-cis-retinal to cones by using 11-cis-retinol generated in Müller cells. In the cycle, 11-cis-retinol is transported from Müller cells to cone inner segments, where it is oxidized to 11-cis-retinal. This oxidation step is only performed in cones, thus rendering the cycle cone-specific. Interphotoreceptor retinoid-binding protein (IRBP) is a retinoid-binding protein in the subretinal space that binds 11-cis-retinol endogenously. Cones in Irbp(-/-) mice are retinoid-deficient under photopic conditions, and it is possible that 11-cis-retinol supplies are disrupted in the absence of IRBP. We tested the hypothesis that IRBP facilitates the delivery of 11-cis-retinol to cones by preserving the isomeric state of 11-cis-retinol in light. With electrophysiology, we show that the cone-like photoreceptors of Nrl(-/-) mice use the cone visual cycle similarly to wild-type cones. Then, using oxidation assays in isolated Nrl(-/-)Rpe65(-/-) retinas, we show that IRBP delivers 11-cis-retinol for oxidation in cones and improves the efficiency of the oxidation reaction. Finally, we show that IRBP protects the isomeric state of 11-cis-retinol in the presence of light. Together, these findings suggest that IRBP plays an important role in the delivery of 11-cis-retinol to cones and can facilitate cone function in the presence of light.

Mesopic background lights enhance dark-adapted cone ERG flash responses in the intact mouse retina: a possible role for gap junctional decoupling

The cone-driven flash responses of mouse electroretinogram (ERG) increase as much as twofold over the course of several minutes during adaptation to a rod-compressing background light. The origins of this phenomenon were investigated in the present work by recording preflash-isolated (M-)cone flash responses ex vivo in darkness and during application of various steady background lights. In this protocol, the cone stimulating flash was preceded by a preflash that maintains rods under saturation (hyperpolarized) to allow selective stimulation of the cones at varying background light levels. The light-induced growth was found to represent true enhancement of cone flash responses with respect to their dark-adapted state. It developed within minutes, and its overall magnitude was a graded function of the background light intensity. The threshold intensity of cone response growth was observed with lights in the low mesopic luminance region, at which rod responses are partly compressed. Maximal effect was reached at intensities sufficient to suppress ∼ 90% of the rod responses. Light-induced enhancement of the cone photoresponses was not sensitive to antagonists and agonists of glutamatergic transmission. However, applying gap junction blockers to the dark-adapted retina produced qualitatively similar changes in the cone flash responses as did background light and prevented further growth during subsequent light-adaptation. These results are consistent with the idea that cone ERG photoresponses are suppressed in the dark-adapted mouse retina by gap junctional coupling between rods and cones. This coupling would then be gradually and reversibly removed by mesopic background lights, allowing larger functional range for the cone light responses.

Stimulation of lateral hypothalamic AMPA receptors may induce feeding in rats

Glutamate or its ionotropic receptor (iGluR) agonists, N-methyl-D-aspartate (NMDA), alpha-amino-3-hydroxy-5-methyl-4-isoxale propionate (AMPA), and kainate (KA) elicit feeding when microinjected into the lateral hypothalamus (LH) of satiated rats. In the present study we investigated the contributions of AMPA and KA receptors (AMPARs and KARs) to feeding initiation. Intense feeding was elicited by LH injection of RS-AMPA (1 and 10 nmol) but not by the isolated, inactive R-AMPA enantiomer (1 and 10 nmol). Further, LH pretreatment with either the non-selective AMPAR/KAR antagonist, 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX, 4 nmol) or the selective AMPAR antagonist, GYKI 52466 (10 nmol), suppressed AMPA-elicited food intake and, when combined, blocked AMPA-elicited food intake. These findings suggest that LH AMPARs mediate AMPA injection-elicited feeding with a possible contribution by KARs. In contrast, CNQX or GYKI 52466 injected into the LH at the onset of the nocturnal period or into fasted rats did not suppress the feeding produced by either condition. RS-AMPA injected into the LH of fasted or nocturnal feeding subjects elicited eating in both conditions; however, the magnitude of the increase was greater in fasted rats. These data suggest that selective stimulation of AMPAR in the LH is sufficient to elicit feeding. In contrast, the results did not provide evidence that AMPAR stimulation is necessary for deprivation-induced or nocturnal eating; however, they did suggest that modulatory interactions may exist between these receptors and these forms of naturally occurring eating behavior.

Intra-retinal visual cycle required for rapid and complete cone dark adaptation

Daytime vision is mediated by retinal cones which, unlike rods, remain functional even in bright light and dark-adapt rapidly. These cone properties are enabled by rapid regeneration of their pigment. This in turn requires rapid chromophore recycling which may not be achieved by the canonical retinal pigment epithelium visual cycle. Recent biochemical studies have suggested the presence of a second, cone-specific visual cycle, although its physiological function remains to be established. Here we report that the Müller cells within the salamander neural retina promote cone-specific pigment regeneration and dark adaptation that are independent of the pigment epithelium. Without this pathway, dark adaptation of cones is slow and incomplete. Interestingly, the rates of cone pigment regeneration by the retina and pigment epithelium visual cycles are essentially identical suggesting a possible common rate-limiting step. Finally, we also observed cone dark adaptation in the isolated mouse retina.

D1 and D4 dopaminergic receptor interplay mediates coincident G protein-independent and dependent regulation of glutamate NMDA receptors in the lateral amygdala

Dopamine (DA) receptor and NMDA receptor (NMDAR) activation in the lateral (LA) nucleus of the amygdala plays a critical role in emotional processing. Several distinct mechanisms regulate the molecular cross-talk between DA receptors and NMDARs in different brain regions; however, the cellular mechanism through which DA modulates NMDARs in LA projection neurons has not been studied. Here, we investigated the effect of DA receptor activation on NMDAR currents in LA projection neurons recorded in amygdala slices obtained from young rats. We found that DA reduces NMDAR current amplitudes in an additive manner through the activation of both D1-like and D2-like receptors. The reduction of NMDAR current amplitudes by D1-like receptor activation is mediated by a protein-protein interaction between the D1R and the NMDAR, while the regulation of NMDAR activity by D2-like receptors is elicited through a G protein-dependent pathway controlled by D4R. The results of our investigation show for the first time a functional interplay between D1R and D4R that mediates coincident G protein-independent and dependent regulation of NMDARs.

The expression of contextual fear conditioning involves activation of an NMDA receptor-nitric oxide pathway in the medial prefrontal cortex

The ventral portion of medial prefrontal cortex (vMPFC) is involved in contextual fear-conditioning expression in rats. In the present study, we investigated the role of local N-methyl-D-aspartic acid (NMDA) glutamate receptors and nitric oxide (NO) in vMPFC on the behavioral (freezing) and cardiovascular (increase of arterial pressure and heart rate) responses of rats exposed to a context fear conditioning. The results showed that both freezing and cardiovascular responses to contextual fear conditioning were reduced by bilateral administration of NMDA receptor antagonist LY235959 (4 nmol/200 nL) into the vMPFC before reexposition to conditioned chamber. Bilateral inhibition of neuronal NO synthase (nNOS) by local vMPFC administration of the Nω-propyl-L-arginine (N-propyl, 0.04 nmol/200 nL) or the NO scavenger carboxy-PTIO (1 nmol/200 nL) caused similar results, inhibiting the fear responses. We also investigated the effects of inhibiting glutamate- and NO-mediated neurotransmission in the vMPFC at the time of aversive context exposure on reexposure to the same context. It was observed that the 1st exposure results in a significant attenuation of the fear responses on reexposure in vehicle-treated animals, which was not modified by the drugs. The present results suggest that a vMPFC NMDA–NO pathway may play an important role on expression of contextual fear conditioning.

Interaction between glutamatergic and nitrergic mechanisms mediating cardiovascular responses to l-glutamate injection in the diagonal band of Broca in anesthetized rats

In a previous study, we reported depressor and bradycardiac responses after L-glutamate (L-glu) microinjection into the diagonal band of Broca (dbB) in anesthetized rats. Here, we report the glutamatergic-receptor subtype mediating the cardiovascular effects evoked by L-glu injection into the dbB and the involvement of local nitric oxide (NO) mechanisms as well as peripheral effectors. Microinjections of 100 nL of L-glu (1, 27, 81, 130 or 200 nmol) into the dbB of urethane-anesthetized rats caused short-lasting depressor and bradycardiac responses. Responses were dose-related, with an ED(50) of approximately 81 nmol. This dose was used in later experiments. The cardiovascular responses to L-glu in the dbB were abolished by local pretreatment (100 nL) with the selective N-methyl-D-aspartic acid (NMDA) receptor antagonist LY235959 (4 nmol) but were not affected by pretreatment with the selective non-NMDA receptor antagonist NBQX (4 nmol). Responses to L-glu in the dbB were blocked by local pretreatment with the selective neuronal NO-synthase (nNOS) inhibitor N(omega)-propyl-L-arginine (NPLA, 0.04 nmol); the NO scavenger carboxy-PTIO (C-PTIO, 1 nmol) or the guanylate cyclase inhibitor ODQ (1 nmol). These results suggest that the microinjection of L-glu into the dbB of urethane-anesthetized rats causes dose-related depressor and bradycardiac responses through the NMDA receptor-NO-guanylate cyclase pathway.

Acute effects of AMPA-type glutamate receptor antagonists on intermale social behavior in two mouse lines bidirectionally selected for offensive aggression

Involvement of AMPA-type glutamate receptors in the regulation of social behavior has been suggested by experiments with mice deficient for the GluR-A subunit-containing AMPA receptors showing reduced intermale aggression. In the present study, effects of AMPA receptor antagonists on mouse social behavior towards unfamiliar Swiss-Webster males on a neutral territory were tested using male subjects from the Turku Aggressive (TA) and Turku Non-Aggressive (TNA) mouse lines bidirectionally selected for high and low levels of offensive aggression. The drugs were the competitive antagonists 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX) and 2,3-dioxo-6-nitro-1,2,3,4-tetrahydrobenzo[f]quinoxaline-7-sulfonamide (NBQX), and the non-competitive antagonist 4-(8-methyl-9H-1,3-dioxolo[4,5-h][2,3]benzodiazepin-5-yl)-benzenamine (GYKI 52466). In TA mice, CNQX and NBQX decreased the biting component of aggressive structure, while GYKI 52466 suppressed all aggressive manifestations. All drugs increased anxiety-like behavior towards the partner. In TNA mice, NBQX activated mouse social behavior and ambivalent aggression, while CNQX and GYKI 52466 only increased anxiety. Thus, AMPA receptor antagonists affect aggressive behaviors in TA mice supporting the idea that AMPA receptors are involved in the modulation of agonistic impulsive behavioral pattern. GYKI 52466 appeared to be the most selective and efficacious in suppressing the aggression.

Endogenous N-acetylaspartylglutamate reduced NMDA receptor-dependent current neurotransmission in the CA1 area of the hippocampus

N-Acetylaspartylglutamate (NAAG) is a neuropeptide found in high concentrations in the brain. Using whole-cell recordings of CA1 pyramidal neurons in acute hippocampal slices, we found that either (i) the application of exogenous NAAG or (ii) an increase of endogenous extracellular NAAG, caused by the inhibition of its catabolic enzyme glutamate carboxypeptidase II (GCP II), resulted in a significant reduction in the amplitude of the isolated NMDA receptor (NMDAR) component of the evoked excitatory postsynaptic current (EPSC). Conversely, reduction of endogenous extracellular NAAG caused by either (i) perfusion with a soluble form of pure human GCP II or (ii) affinity purified antibodies against NAAG, enhanced the amplitude of the isolated NMDAR current. Bath application of GCP II inhibitor induced a progressive loss of spontaneous NMDAR miniatures. Furthermore, NAAG blocked the induction of long-term potentiation at Schaffer collateral axons-CA1 pyramidal neuron synapses. All together, these results suggest that NAAG acts as an endogenous modulator of NMDARs in the CA1 area of the hippocampus.

Effect of Conantokin G on NMDA receptor-mediated spontaneous EPSCs in cultured cortical neurons

Conantokin G (Con G), derived from the venom of Conus geographus, is the most characterized natural peptide antagonist targeted to N-methyl-D-aspartate (NMDA) receptors. Although Con G is known to bind to the glutamate binding site on the NR2 subunit of the receptor, it is unclear whether it can allosterically modulate the function of the receptor through the glycine binding site on the NR1 subunit. This study was designed to evaluate the action of Con G on NMDA receptor-mediated spontaneous excitatory postsynaptic currents (sEPSCs) and its modulation by glycine in cultured cortical neurons (13-19 days in vitro) using the whole cell patch-clamp technique. Con G inhibited NMDA receptor-mediated sEPSCs in a concentration-dependent manner. Also, the potency of Con G decreased as a function of time in culture. The inhibition of EPSCs observed after application of Con G in the presence of high (10 microM) and nominal (no added) concentrations of glycine was not different at 13 days in vitro (DIV). Furthermore, similar results were obtained with experiments on Con G-induced inhibition of NMDA-evoked whole cell currents. These results indicate that glycine concentrations do not have a direct effect on Con G-induced inhibition of NMDA currents. In addition, age dependency in the action of Con G on cortical neurons in vitro suggests that this model system would be useful in examining the effects of different agonists/antagonists on native synaptic NMDA receptors.

AMPA receptors regulate transcription of the plasticity-related immediate-early gene Arc

Learning and memory depend critically on long-term synaptic plasticity, which requires neuronal gene expression. In the prevailing view, AMPA receptors mediate fast excitatory synaptic transmission and effect short-term plasticity, but they do not directly regulate neuronal gene expression. By studying regulation of Arc, a gene required for long-term plasticity, we uncovered a new role for AMPA receptors in neuronal gene expression. Spontaneous synaptic activity or activity induced by brain-derived neurotrophic factor (BDNF) elicited Arc expression in cultures of rat cortical neurons and in organotypic brain slices. Notably, inhibiting AMPA receptors strongly potentiated activity-dependent Arc expression. We found that AMPA receptors negatively regulate Arc transcription, but not translation or stability, through a mechanism involving a pertussis toxin-sensitive G protein. These results provide insights into the activity-dependent mechanisms of Arc expression and suggest that, in addition to effecting short-term plasticity, AMPA receptors regulate genes involved in long-term plasticity.

Ionotropic glutamate receptors mediate excitatory drive to caudal medullary expiratory neurons in the rabbit

Most of the neurons of the caudal ventral respiratory group (cVRG) are bulbospinal expiratory neurons that receive their main excitatory drive from more rostral, but not yet defined regions. This study was devoted to investigate the functional role of ionotropic excitatory amino acid (EAA) receptors in the excitatory drive transmission to cVRG expiratory neurons during eupnoeic breathing and some respiratory reflexes including cough induced by mechanical stimulation of the tracheobronchial tree. The experiments were performed on spontaneously breathing rabbits under pentobarbitone anesthesia making use of microinjections (30--50 nl) of EAA receptor antagonists into the cVRG. Phrenic nerve and abdominal muscle activities were recorded. Bilateral microinjections of 50 mM kynurenic acid (KYN), a broad-spectrum EAA antagonist, and 10 mM 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX), a non-NMDA antagonist, or 5 mM 6-nitro-7-sulphamoylbenzo(f)quinoxaline-2,3-dione (NBQX), a more specific non-NMDA antagonist, completely suppressed spontaneous rhythmic abdominal activity and reflex expiratory responses either to tracheal occlusion at end-inspiration (Breuer-Hering inflation reflex) or to expiratory threshold loading (5 cm H(2)O); they also suppressed both the inspiratory and expiratory components of the cough reflex. Spontaneous rhythmic abdominal activity and the reflex respiratory responses were strongly reduced, but not completely abolished by microinjections of 10 mM d(-)-2-amino-5-phosphonopentanoic acid (D-AP5), an NMDA antagonist. The results provide evidence that the excitatory drive to cVRG bulbospinal expiratory neurons during eupnoeic breathing and the investigated respiratory reflexes is mediated by EAA receptors. They also support the view that neurons located in the cVRG are not merely elements of the expiratory output system.

Involvement of NMDA receptors in the hypotensive response to the injection of l-glutamate into the lateral hypothalamus of unanesthetized rats

We report that microinjections of L-glutamate (L-glu) or N-methyl-D-aspartic acid (NMDA) in the lateral hypothalamus (LH) of unanesthetized rats caused a hypotensive response. Guide cannulas were stereotaxically placed in the LH 3 days before the experiments, under tribromoethanol anesthesia. One day before the experiments, the femoral artery was cannulated for pulsatile arterial pressure (PAP), mean arterial pressure (MAP) and heart rate (HR) measurements. In the first experiment, unanesthetized rats received microinjections of 2.5, 5.0 or 10.0 nmol/100 nL of L-glu in the LH. Dose-dependent hypotensive responses were observed, without significant concomitant changes in heart rate. In a second group of experiments, 5.0 nmol of L-glu was microinjected into the LH before and 10 min after pretreatment with glutamatergic antagonists. Pretreatments with the non-selective ionotropic glutamatergic-receptor antagonist kynurenic acid or the selective NMDA receptor antagonists AP-7 and LY235959 significantly reduced the hypotensive response to microinjection of L-glu in the LH. Pretreatment with the selective AMPA-receptor antagonist NBQX or with vehicle did not affect the hypotensive response. The present results suggest that the hypotensive response to the injection of L-glu into the LH is mediated by NMDA receptors.

Reduced glycine transporter type 1 expression leads to major changes in glutamatergic neurotransmission of CA1 hippocampal neurones in mice

To investigate the effects of persistent elevation of synaptic glycine at Schaffer collateral-CA1 synapses of the hippocampus, we studied the glutamatergic synaptic transmission in acute brain slices from mice with reduced expression of glycine transporter type 1 (GlyT1+/-) as compared to wild type (WT) littermates using whole-cell patch-clamp recordings of CA1 pyramidal cells. We observed faster decay kinetics, reduced ifenprodil sensitivity and increased zinc-induced antagonism in N-methyl-d-aspartate receptor (NMDAR) currents of GlyT1+/- mice. Moreover, the ratio alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid receptor (AMPAR)/NMDAR was decreased in mutants compared to WT. Surprisingly, this change was associated with a reduction in the number of AMPARs expressed at the CA1 synapses in the mutants compared to WT. Overall, these findings highlight the importance of GlyT1 in regulating glutamatergic neurotransmission.

Glycine transporter type 1 blockade changes NMDA receptor-mediated responses and LTP in hippocampal CA1 pyramidal cells by altering extracellular glycine levels

Long-term potentiation (LTP) in the hippocampal CA1 region requires the activation of NMDA receptors (NMDARs). NMDAR activation in turn requires membrane depolarization as well as the binding of glutamate and its coagonist glycine. Previous pharmacological studies suggest that the glycine transporter type 1 (GlyT1) maintains subsaturating concentrations of glycine at synaptic NMDARs. Antagonists of GlyT1 increase levels of glycine in the synaptic cleft and, like direct glycine site agonists, can augment NMDAR currents and NMDAR-mediated functions such as LTP. In addition, stimulation of the glycine site initiates signalling through the NMDAR complex, priming the receptors for clathrin-dependent endocytosis. We have used a new potent GlyT1 antagonist, CP-802,079, with whole-cell patch-clamp recordings in acute rat hippocampal slices to determine the effect of GlyT1 blockade on LTP. Reverse microdialysis experiments in the hippocampus of awake, freely moving rats, showed that this drug elevated only the extracellular concentration of glycine. We found that CP-802,079, sarcosine and glycine significantly increased the amplitude of the NMDAR currents and LTP. In contrast, application of higher concentrations of CP-802,079 and glycine slightly reduced NMDAR currents and did not increase LTP. Overall, these data suggest that the level of glycine present in the synaptic cleft tightly regulates the NMDAR activity. This level is kept below the 'set point' of the NMDAR internalization priming mechanism by the presence of GlyT1-dependent uptake.

Ganglion cell contributions to the rat full-field electroretinogram

The purpose of this study was to determine what contributions are made to the rat full-field electroretinogram (ERG) by ganglion cells (GCs). To that end, the ERG was assessed longitudinally following optic nerve transection (ONTx). Additional studies were conducted using intravitreal injections of pharmacologically active substances. The ERG was recorded simultaneously from both eyes of anaesthetized adult Brown-Norway rats (ketamine: xylazine: acepromazine, 55: 5: 1 mg kg(-1)) using custom silver chloride electrodes. Stimuli were brief, white xenon discharges delivered via a Ganzfeld under dark-adapted and light-adapted conditions (150 cd m(-2)). ERGs were obtained 1, 2, 3, 4 and 9 weeks after ONTx (n = 8) or sham (n = 8) operations. ONTx reduced both positive and negative components of the scotopic threshold response (pSTR and nSTR). Scotopic ERG responses to brighter flashes, including a-waves, b-waves and oscillatory potentials (OPs) were unaffected by ONTx. ONTx reduced the photopic b-wave and OPs. TTX (6 microM) reduced the pSTR and nSTR, but not the scotopic a-wave, b-wave or OPs. TTX had dramatic effects on the photopic ERG, surpassing the effects of ONTx. TTX application 9 weeks post-ONTx had little additional effect on the STR. Inhibition of inner retinal responses using GABA (10 mM) or NMDA (0.8 mM) reduced the nSTR substantially. Similar results were obtained with antagonists of AMPA/KA ionotropic glutamate receptors 6-cyano-7-nitroquinoxaline-2,3(1H,4H)-dione (CNQX, 0.2 mM) or cis-2,3-piperidinedicarboxylic acid (PDA, 5 mm); however, both also reduced the scotopic b-wave by approximately 40 %. By contrast, the NMDA receptor antagonist D(-)-2-amino-7-phosphonoheptanoic acid (D-AP7, 0.2 mM) had no effect alone, but the combination of D-AP7 and CNQX completely abolished the STR. The results of this study indicate that: (1) both pSTR and nSTR components in the rat depend directly upon intact GC responses, and that amacrine cell contributions to these components are relatively small; (2) scotopic ERG response components to brighter flashes receive little influence from GCs; (3) the rat photopic ERG also reflects GC signals and may serve as an additional useful test of GC function; (4) TTX had dramatic effects on the rat photopic ERG that were not attributable to GC currents, but rather to voltage-gated sodium currents in amacrine or interplexiform cells; (5) a small residual negative STR persisted after ONTx that was likely to be generated by graded responses of third-order retinal cells, most likely amacrine cells.

D-serine and serine racemase are present in the vertebrate retina and contribute to the physiological activation of NMDA receptors

d-serine has been proposed as an endogenous modulator of N-methyl-d-aspartate (NMDA) receptors in many brain regions, but its presence and function in the vertebrate retina have not been characterized. We have detected d-serine and its synthesizing enzyme, serine racemase, in the retinas of several vertebrate species, including salamanders, rats, and mice and have localized both constituents to Müller cells and astrocytes, the two major glial cell types in the retina. Physiological studies in rats and salamanders demonstrated that, in retinal ganglion cells, d-serine can enhance excitatory currents elicited by the application of NMDA, as well as the NMDA receptor component of light-evoked synaptic responses. Application of d-amino acid oxidase, which degrades d-serine, reduced the magnitude of NMDA receptor-mediated currents, raising the possibility that endogenous d-serine serves as a ligand for setting the sensitivity of NMDA receptors under physiological conditions. These observations raise exciting new questions about the role of glial cells in regulating the excitability of neurons through release of d-serine.

D-Serine differently modulates NMDA receptor function in rat CA1 hippocampal pyramidal cells and interneurons

The organization of the neuronal hippocampal network depends on the tightly regulated interaction between pyramidal cells (PCs) and interneurons (Ints). NMDA receptor (NMDAR) activation requires the binding of glutamate and co-activation of the 'glycine site'. It has been reported that D-serine is a more potent endogenous agonist than glycine for that site. While many studies have focused on NMDAR function in PCs, little is known regarding the modulation of NMDARs in Ints. We studied the modulatory effect of D-serine on NMDAR EPSCs in PCs and in stratum radiatum Ints using whole-cell patch-clamp recording in rat acute hippocampal slices. We found that D-serine enhances NMDAR function and differently modulates NMDAR currents in both cell types. The augmentation of NMDAR currents by D-serine was significantly larger in PCs compared with Ints. Moreover, we found differences in the kinetics of NMDAR currents in PCs and Ints. Our findings indicate that regulation of NMDAR through the 'glycine site' depends on the cell types. We speculate that the observed differences arise from assemblies of diverse NMDAR subunits. Overall, our data suggest that D-serine may be involved in regulation of the excitation-inhibition balance in the CA1 hippocampal region.

A kainate receptor increases the efficacy of GABAergic synapses

Brain functions are based on the dynamic interaction of excitatory and inhibitory inputs. Spillover of glutamate from excitatory synapses may diffuse to and modulate nearby inhibitory synapses. By recording unitary inhibitory postsynaptic currents (uIPSCs) from cell pairs in CA1 of the hippocampus, we demonstrated that low concentrations of Kainate receptor (KAR) agonists increased the success rate (P(s)) of uIPSCs, whereas high concentrations of KAR agonists depressed GABAergic synapses. Ambient glutamate released by basal activities or stimulation of the stratum radiatum increases the efficacy of GABAergic synapses by activating presynaptic KARs, which facilitate Ca(2+)-dependent GABA release. The results suggest that glutamate released from excitatory synapses may also function as an intermediary between excitatory and inhibitory synapses to protect overexcitation of local circuits.

Blocking the trigeminal EPSP in rat abducens motoneurons in vivo with the AMPA antagonists NBQX and GYKI 53655

In pentobarbitone-anaesthetized rats, the effects of two AMPA receptor antagonists, the competitive antagonist 2, 3-dihydroxy-6-nitro-7-sulfamoyl-benzo-(F)-quinoxaline (NBQX) and the non-competitive 2,3-benzodiazepine GYKI 53655, were compared on excitatory synaptic transmission of trigeminal origin in intracellularly-recorded abducens motoneurons. The effects of both antagonists were also investigated on the alpha-amino-3-hydroxy-5-methyl isoxazole-4-propionic acid (AMPA)-, kainate-, and N-methyl-D-aspartate (NMDA)-induced depression of extracellular antidromic field potentials in the abducens motor nucleus. Microiontophoretic application (< or =100 nA) or intravenous injection of NBQX (< or =5 mg/kg) affected both AMPA- and kainate-induced depressions whereas GYKI 53655 (< or =100 nA; < or =4 mg/kg) blocked only the AMPA-induced depression. Neither NBQX or GYKI 53655 affected NMDA-induced depressions of antidromic field potentials. Using low intravenous (i.v.) doses of the antagonists NBQX or GYKI 53655 (2-2.5 mg/kg), a complete blockade of the composite disynaptic trigeminal excitatory post-synaptic potential (EPSP) was obtained without any changes in membrane potential, input resistance and antidromic action potentials in abducens motoneurons. GYKI 53655 was more potent at low i.v. doses (0.5-1.8 mg/kg) but NBQX had longer-lasting effects. The results show the existence of differences between the blocking action of NBQX and GYKI 53655 on AMPA-mediated receptor EPSP in abducens motoneurons.

Modulation of cardiovascular effects produced by nitric oxide and ionotropic glutamate receptor interaction in the nucleus tractus solitarii of rats

Both nitric oxide (NO) and glutamate in the brain stem nuclei are involved in central cardiovascular regulation. In the present study, we investigated possible functional interactions between NO and glutamate in the modulation of cardiovascular function in the nucleus tractus solitarii (NTS) of anesthetized rats. In Sprague-Dawley rats, intra-NTS unilateral microinjections of L-glutamate (0.1 nmol/60 nl) and its ionotropic agonists NMDA (5 pmol) and AMPA (2 pmol) resulted in significant decreases in mean blood pressure (MBP) and heart rate (HR). The cardiovascular effects of L-glutamate, NMDA and AMPA were significantly blocked by prior administration of the neuronal NO synthase (nNOS) inhibitor, 7-nitroindazole (7-NI, 0.5 nmol), or by the soluble guanylyl cyclase (sGC) inhibitor, 1H-[1.2.4]oxadiazolo[4,3,-a]quinoxalin-1-one (ODQ, 0.03-1 pmol). Conversely, a depressor and bradycardic effect was elicited by microinjection of either the NO precursor L-arginine (10 nmol) or the NO donor sodium nitroprusside (SNP, 0.2 nmol) into the NTS. Prior administration of the NMDA receptor antagonists MK-801 (0.1-1 nmol) and APV (0.1-4 nmol) significantly attenuated these effects of L-arginine. Similarly, cardiovascular responses to L-arginine in the NTS were inhibited by pre-injections with the non-NMDA receptor antagonists CNQX (10-330 pmol) and NBQX (2-10 pmol). Furthermore, APV (4 nmol) and CNQX (330 pmol) attenuated the depressor and bradycardic effects of SNP, respectively. This study demonstrates that baroreflex-like responses to microinjections of L-glutamate and its ionotropic agonists into the NTS involve synthesis of NO and activation of sGC. Reciprocally, central cardiovascular effects of NO also depend on responsive ionotropic glutamate receptors.

The network-selective actions of quinoxalines on the neurocircuitry operations of the rabbit retina

We examined the contribution of N-methyl-D-aspartate (NMDA) and alpha-amino-3-hydroxy-5-methyl-4-isoxalole-4-propionic acid (AMPA)/kainate (KA) receptors to the light-responses of rabbit retinal neurons. In the outer retina, bath application of the AMPA/KA receptor antagonists 6,7-dinitro-quinoxaline-2,3-dione (DNQX) and 2,3,dihydroxy-6-nitro-7-sulfamoyl-benzo-f-quinoxaline (NBQX) blocked the light-responses of horizontal cells. Application of quinoxalines enhanced ON-bipolar cell light-responses, and was associated with a hyperpolarization of their resting potentials. In the inner retina, application of both AMPA/KA and NMDA antagonists to AII amacrine-like cells only partially blocked their light-responses. Their residual responses may reflect electrical coupling to neighboring ON-center cone bipolar cells, and can inhibit OFF-center ganglion cells. ON-sustained ganglion cells were highly sensitive to the quinoxalines, which reduced their light-evoked firing, while the firing of ON-transient cells remained as NMDA-mediated light-responses. Quinoxalines had differential effects on the firing rates of ON- and OFF-center ganglion cells: ON-cells were reduced, while OFF-cells were increased. In contrast, firing rates of ON-OFF ganglion cells were not excited by NBQX, and showed a recovered light-response mediated by NMDA receptors. The receptive field surround was lost in ganglion cells. For comparison, NMDA antagonists had only moderate effects on all ganglion cell light-responses. Our results indicate that NMDA and AMPA/KA receptors both contribute to ganglion cell light-responses. However, AMPA/KA receptors also significantly effect the light-response of neurons presynaptic to retinal ganglion cells, altering the observed pharmacology at the ganglion cell level.

Regulation of calcium/calmodulin-dependent protein kinase II in the adult rat retina is mediated by ionotropic glutamate receptors

This study is concerned with the transmitter-mediated regulation of the alpha(50 kDa) and beta(60 kDa) subunits of calcium calmodulin dependent protein kinase II (CamKII) in the adult rat retina. The level of antibody binding to the CamKII and the activity of CamKII were found to be increased after intravitreal injection of glutamate. Changes in the levels of the antibody-binding to the subunits of CamKII were observed in different subcellular fractions of the retina with a maximum response observed in crude synaptic membrane fractions. The glutamate mediated increases in CamKII were specific and blocked by 3,5-Dimethyl-1 adamantanamine; 3,5-Dimethylamantadine (Memantine), (+/-) 2-Amino-5-Phosphopentonic (AP-5) and 6-Cyano-7-Nitroquinoxaline-2,3-Dione (CNQX) but not with dl -2-Amino-3-Phosphono-Propionic (AP-3). The results indicate that the retinal neurotransmitter, glutamate, can regulate retinal CamKII activity through ionotropic but not metabotropic glutamate receptors. NMDA-receptors were found to be necessary but insufficient to stimulate CamKII. A model in which cooperative interaction between NMDA and non-NMDA glutamate receptors/ion channels is presented to explain the glutamate stimulated increases in CamKII activity in the retina.

N-methyl-D-aspartate receptors are indispensable for the formation of long-term potentiation in the rat suprachiasmatic nucleus in vitro

Optic nerve (ON) stimulation caused a postsynaptic field potential in the suprachiasmatic nucleus (SCN) of rat hypothalamic slices. The postsynaptic field potential was suppressed by 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX), a non-NMDA receptor antagonist, in a concentration-dependent manner, but not affected by D-amino-5-phosphonovaleric acid (APV), a competitive NMDA receptor antagonist. Tetanic stimulation to the ON induced long-term potentiation (LTP) in the SCN. Application of APV at 50 microM inhibited the induction of LTP by tetanic stimulation but CNQX at lower dose (5 microM) didn't inhibit it. These results suggest that NMDA receptors are indispensable for the induction of LTP after tetanic stimulation.

Estimating the contributions of NMDA and non-NMDA currents to EPSPs in retinal ganglion cells

Whole-cell recordings were obtained from retinal ganglion cells of the tiger salamander (Ambystoma tigrinum) in a superfused slice preparation to evaluate contributions of NMDA (N-methyl-D-aspartate) and KA/AMPA (kainate/alpha-amino-3-hydroxy-5-methyl-4-isoxalone propionic acid) receptors to excitatory postsynaptic potentials (EPSPs) of retinal ganglion cells. Synaptic activation of retinal ganglion cells was achieved through the use of a brief pressure pulse of hyperosmotic Ringer (Ringer + sucrose) delivered through a microelectrode visually placed in the inner plexiform layer while whole-cell recordings were obtained from adjacent cells in the ganglion cell layer. Separation of NMDA and KA/AMPA excitatory postsynaptic currents (EPSCs) was achieved through the application of the antagonists NBQX and D-AP7, while inhibitory currents were blocked by strychnine and picrotoxin. Simple addition of the two independent EPSCs showed, most often, that the sum of the KA/AMPA and NMDA currents was less than the control response, but in some cases the sum of the two currents exceeded the magnitude of the control response. Neither result was consistent with expectations based on voltage-clamp principles and the assumption that the two currents were independent; for this reason, we considered the possibility of nonlinear interactions between KA/AMPA and NMDA receptors. Computer simulations were carried out to evaluate the summation experiments. We used both an equivalent cylinder model and a more realistic, compartmental model of a ganglion cell constrained by a passive leakage conductance, a linear KA/AMPA synaptic current, and a nonlinear NMDA current based on the well-known, voltage-sensitive Mg2+ block. Computer simulation studies suggest that the hypo- and hyper-summation of NMDA and KA/AMPA currents, observed physiologically, can be accounted for by a failure to adequately space clamp the neuron. Clamp failure leads to enhanced NMDA currents as the ion channels are relieved of the Mg2+ block; their contribution is thus exaggerated depending on the magnitude of the conductance change and the spatial location of the synaptic input.

Sensorimotor gating in rats is regulated by different dopamine-glutamate interactions in the nucleus accumbens core and shell subregions

The amplitude of the acoustic startle reflex is normally reduced when the startling stimulus is preceded by a weak click or "prepulse'. Prepulse inhibition (PPI) of acoustic startle has been used as an operational measure of sensorimotor gating or inhibition, and is reduced in schizophrenia patients and in rats with central dopamine (DA) activation. The DA agonist-induced disruption of PPI in rats may thus offer a useful animal model to study impaired sensorimotor gating in schizophrenia. We have previously reported that DA-glutamate interactions in the nucleus accumbens (NAC) regulate PPI. The NAC has at least two major subregions-the core and shell-that have distinct anatomical and neurochemical properties. In this study, we compared changes in PPI after manipulations of DA-glutamate activity in these two NAC subregions. Consistent with previous findings, infusion of the non-NMDA agonist AMPA into the NAC core subregion significantly reduced PPI, and this effect was opposed by systemic administration of the D2 antagonist haloperidol. Also consistent with previous reports, infusion of the non-NMDA antagonist CNQX into the NAC core subregion did not alter PPI, but its co-infusion with D-amphetamine (AMPH) attenuated the AMPH-disruption of PPI. In contrast, while PPI was reduced after AMPA infusion into the NAC shell subregion, this effect of AMPA could not be blocked by pretreatment with haloperidol. Infusion of either AMPH or CNQX into the NAC shell subregion reduced PPI independently. The PPI-disruptive effects of intra-shell CNQX infusion were not blocked by haloperidol. The present results suggest striking differences between the NAC core and shell subregions in their neurochemical modulation of sensorimotor gating of acoustic startle in the rat.

The mechanism by which NBQX enhances NMDA currents in retinal ganglion cells

When the quinoxaline NBQX (2,3-dihydroxy-6-nitro-7-sulfamoylbenzo (F) quinoxaline), a KA/AMPA antagonist, is bath applied to the tiger salamander retina, a paradoxical action is evident in the light-evoked synaptic responses of ganglion cells: NBQX enhances excitatory synaptic currents at light onset observed under whole-cell voltage-clamp conditions in a perfused retinal slice preparation. This observation was surprising because synaptic inputs into ganglion cells that are mediated by KA/AMPA receptors are entirely blocked by NBQX. Thus, the NBQX-enhanced current is entirely mediated by NMDA receptors. The purpose of this study was to determine the mechanism(s) by which blocking KA/AMPA receptors appears to enhance NMDA currents. Using hyperosmotic sucrose stimulation to activate neurotransmitter release from the inner retina, we observed that NBQX augmented the sucrose-evoked response, suggesting that at least a component of this enhancement may reside in the inner retina. NBQX does not enhance NMDA currents activated by bath applied NMDA, demonstrating that the NBQX-induced enhancement does not result from modulation of NMDA receptors. Voltage-clamp studies, carried out at the appropriate holding potential, indicate that NBQX enhances glutamatergic transmission and reduces inhibitory inputs onto ganglion cells. In the presence of strychnine and picrotoxin, the NBQX-induced enhancement of NMDA currents is eliminated, suggesting that NBQX facilitates the expression of NMDA currents by a selective and partial reduction of inhibitory mechanisms. Additional studies suggest that part of the NMDA enhancement by NBQX is evident at the postsynaptic level, but a presynaptic component probably also participates, perhaps at the level of bipolar cell terminals. One way to account for this observation is to assume that a subpopulation of inhibitory amacrine cells requires KA/AMPA receptors exclusively for their synaptic activation: previous studies of sustained amacrine cells support this interpretation. Thus the NBQX-induced enhancement phenomenon may reflect a network-selective distribution of NMDA and KA/AMPA receptors among third-order neurons.