Comparative patch clamp studies on the kinetics and selectivity of glutamate receptor antagonism by 2,3-dihydroxy-6-nitro-7-sulfamoyl-benzo(F)quinoxaline (NBQX) and 1-(4-amino-phenyl)-4-methyl-7,8-methyl-endioxyl-5H-2,3-benzodiaze pine (GYKI 52466)

Unique Features of Network Bursts Emerge From the Complex Interplay of Excitatory and Inhibitory Receptors in Rat Neocortical Networks

Spontaneous network activity plays a fundamental role in the formation of functional networks during early development. The landmark of this activity is the recurrent emergence of intensive time-limited network bursts (NBs) rapidly spreading across the entire dissociated culture in vitro. The main excitatory mediators of NBs are glutamatergic alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptors (AMPARs) and N-Methyl-D-aspartic-acid receptors (NMDARs) that express fast and slow ion channel kinetics, respectively. The fast inhibition of the activity is mediated through gamma-aminobutyric acid type A receptors (GABA_ARs). Although the AMPAR, NMDAR and GABA_AR kinetics have been biophysically characterized in detail at the monosynaptic level in a variety of brain areas, the unique features of NBs emerging from the kinetics and the complex interplay of these receptors are not well understood. The goal of this study is to analyze the contribution of fast GABA_ARs on AMPAR- and NMDAR- mediated spontaneous NB activity in dissociated neonatal rat cortical cultures at 3 weeks in vitro. The networks were probed by both acute and gradual application of each excitatory receptor antagonist and combinations of acute excitatory and inhibitory receptor antagonists. At the same time, the extracellular network-wide activity was recorded with microelectrode arrays (MEAs). We analyzed the characteristic NB measures extracted from NB rate profiles and the distributions of interspike intervals, interburst intervals, and electrode recruitment time as well as the similarity of spatio-temporal patterns of network activity under different receptor antagonists. We show that NBs were rapidly initiated and recruited as well as diversely propagated by AMPARs and temporally and spatially maintained by NMDARs. GABA_ARs reduced the spiking frequency in AMPAR-mediated networks and dampened the termination of NBs in NMDAR-mediated networks as well as slowed down the recruitment of activity in all networks. Finally, we show characteristic super bursts composed of slow NBs with highly repetitive spatio-temporal patterns in gradually AMPAR blocked networks. To the best of our knowledge, this study is the first to unravel in detail how the three main mediators of synaptic transmission uniquely shape the NB characteristics, such as the initiation, maintenance, recruitment and termination of NBs in cortical cell cultures in vitro.

Endogenous Glutamate Excites Myenteric Calbindin Neurons by Activating Group I Metabotropic Glutamate Receptors in the Mouse Colon

Glutamate is a classic excitatory neurotransmitter in the central nervous system (CNS), but despite several studies reporting the expression of glutamate together with its various receptors and transporters within the enteric nervous system (ENS), its role in the gut remains elusive. In this study, we characterized the expression of the vesicular glutamate transporter, vGluT2, and examined the function of glutamate in the myenteric plexus of the distal colon by employing calcium (Ca²⁺)-imaging on Wnt1-Cre; R26R-GCaMP3 mice which express a genetically encoded fluorescent Ca²⁺ indicator in all enteric neurons and glia. Most vGluT2 labeled varicosities contained the synaptic vesicle release protein, synaptophysin, but not vesicular acetylcholine transporter, vAChT, which labels vesicles containing acetylcholine, the primary excitatory neurotransmitter in the ENS. The somata of all calbindin (calb) immunoreactive neurons examined received close contacts from vGluT2 varicosities, which were more numerous than those contacting nitrergic neurons. Exogenous application of L-glutamic acid (L-Glu) and N-methyl-D-aspartate (NMDA) transiently increased the intracellular Ca²⁺ concentration [Ca²⁺]_i in about 25% of myenteric neurons. Most L-Glu responsive neurons were calb immunoreactive. Blockade of NMDA receptors with APV significantly reduced the number of neurons responsive to L-Glu and NMDA, thus showing functional expression of NMDA receptors on enteric neurons. However, APV resistant responses to L-Glu and NMDA suggest that other glutamate receptors were present. APV did not affect [Ca²⁺]_i transients evoked by electrical stimulation of interganglionic nerve fiber tracts, which suggests that NMDA receptors are not involved in synaptic transmission. The group I metabotropic glutamate receptor (mGluR) antagonist, PHCCC, significantly reduced the amplitude of [Ca²⁺]_i transients evoked by a 20 pulse (20 Hz) train of electrical stimuli in L-Glu responsive neurons. This stimulus is known to induce slow synaptic depolarizations. Further, some neurons that had PHCCC sensitive [Ca²⁺]_i transients were calb immunoreactive and received vGluT2 varicosities. Overall, we conclude that electrically evoked release of endogenous glutamate mediates slow synaptic transmission via activation of group I mGluRs expressed by myenteric neurons, particularly those immunoreactive for calb.

Perampanel inhibition of AMPA receptor currents in cultured hippocampal neurons

Perampanel is an aryl substituted 2-pyridone AMPA receptor antagonist that was recently approved as a treatment for epilepsy. The drug potently inhibits AMPA receptor responses but the mode of block has not been characterized. Here the action of perampanel on AMPA receptors was investigated by whole-cell voltage-clamp recording in cultured rat hippocampal neurons. Perampanel caused a slow (τ∼1 s at 3 µM), concentration-dependent inhibition of AMPA receptor currents evoked by AMPA and kainate. The rates of block and unblock of AMPA receptor currents were 1.5×105 M-1 s-1 and 0.58 s-1, respectively. Perampanel did not affect NMDA receptor currents. The extent of block of non-desensitizing kainate-evoked currents (IC50, 0.56 µM) was similar at all kainate concentrations (3-100 µM), demonstrating a noncompetitive blocking action. Parampanel did not alter the trajectory of AMPA evoked currents indicating that it does not influence AMPA receptor desensitization. Perampanel is a selective negative allosteric AMPA receptor antagonist of high-affinity and slow blocking kinetics.

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.

Rhythm generation through period concatenation in rat somatosensory cortex

Rhythmic voltage oscillations resulting from the summed activity of neuronal populations occur in many nervous systems. Contemporary observations suggest that coexistent oscillations interact and, in time, may switch in dominance. We recently reported an example of these interactions recorded from in vitro preparations of rat somatosensory cortex. We found that following an initial interval of coexistent gamma ( approximately 25 ms period) and beta2 ( approximately 40 ms period) rhythms in the superficial and deep cortical layers, respectively, a transition to a synchronous beta1 ( approximately 65 ms period) rhythm in all cortical layers occurred. We proposed that the switch to beta1 activity resulted from the novel mechanism of period concatenation of the faster rhythms: gamma period (25 ms)+beta2 period (40 ms) = beta1 period (65 ms). In this article, we investigate in greater detail the fundamental mechanisms of the beta1 rhythm. To do so we describe additional in vitro experiments that constrain a biologically realistic, yet simplified, computational model of the activity. We use the model to suggest that the dynamic building blocks (or motifs) of the gamma and beta2 rhythms combine to produce a beta1 oscillation that exhibits cross-frequency interactions. Through the combined approach of in vitro experiments and mathematical modeling we isolate the specific components that promote or destroy each rhythm. We propose that mechanisms vital to establishing the beta1 oscillation include strengthened connections between a population of deep layer intrinsically bursting cells and a transition from antidromic to orthodromic spike generation in these cells. We conclude that neural activity in the superficial and deep cortical layers may temporally combine to generate a slower oscillation.

Kynurenic acid has a dual action on AMPA receptor responses

Glutamate is the predominant excitatory neurotransmitter in the central nervous system. The receptors that bind glutamate, including N-methyl-D-aspartate (NMDA) and alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid (AMPA) receptor subtypes, are strongly implicated in higher cognitive processes, especially learning and memory. Loss of glutamate receptor function impairs the ability to acquire and retain information in some patients subsequent to stroke or brain injury, and positive allosteric modulators of glutamate receptors can restore learning and memory in some of these patients. Here we demonstrate that kynurenic acid (KYNA), an endogenous tryptophan metabolite, acts upon heterologous AMPA receptors via two distinct mechanisms. Low (nanomolar to micromolar) concentrations of KYNA facilitate AMPA receptor responses, whereas high (millimolar) concentrations of KYNA competitively antagonize glutamate receptors. Low concentrations of KYNA appear to increase current responses through allosteric modulation of desensitization of AMPA receptors. These findings suggest the possibility that low concentrations of endogenous KYNA acting at AMPA receptors may be a positive modulator of excitatory synaptic transmission.

Coantagonism of glutamate receptors and nicotinic acetylcholinergic receptors disrupts fear conditioning and latent inhibition of fear conditioning

The present study investigated the hypothesis that both nicotinic acetylcholinergic receptors (nAChRs) and glutamate receptors (alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionate receptors (AMPARs) and N-methyl-d-aspartate glutamate receptors (NMDARs)) are involved in fear conditioning, and may modulate similar processes. The effects of the nAChR antagonist mecamylamine administered alone, the AMPAR antagonist NBQX administered alone, and the NMDAR antagonist MK-801 administered alone on cued fear conditioning, contextual fear conditioning, and latent inhibition of cued fear conditioning were examined. In addition, the effects of coadministration of either mecamylamine and NBQX or mecamylamine and MK-801 on these behaviors were examined. Consistent with previous studies, neither mecamylamine nor NBQX administered alone disrupted any of the tasks. However, coadministration of mecamylamine and NBQX disrupted both contextual fear conditioning and latent inhibition of cued fear conditioning. In addition, coadministration of mecamylamine with a dose of MK-801 subthreshold for disrupting either task disrupted both contextual fear conditioning and latent inhibition of cued fear conditioning. Coadministration of mecamylamine and NBQX, and coadministration of mecamylamine with a dose of MK-801 subthreshold for disrupting fear conditioning had little effect on cued fear conditioning. These results suggest that nAChRs and glutamate receptors may support similar processes mediating acquisition of contextual fear conditioning and latent inhibition of fear conditioning.

Xenon reduces glutamate-, AMPA-, and kainate-induced membrane currents in cortical neurones

BACKGROUND

The anaesthetic, analgesic, and neuroprotective effects of xenon (Xe) are believed to be mediated by a block of the NMDA (N-methyl-D-aspartate) receptor channel. Interestingly, the clinical profile of the noble gas differs markedly from that of specific NMDA receptor antagonists. The aim of this study was, therefore, to investigate whether Xe might be less specific, also inhibiting the two other subtypes of glutamate receptor channels, such as the alpha-amino-3-hydroxy-5-methyl-4-isoxazolole propionate (AMPA) and kainate receptors.

METHODS

The study was performed on voltage-clamped cortical neurones from embryonic mice and SH-SY5Y cells expressing GluR6 kainate receptors. Drugs were applied by a multi-barreled fast perfusion system.

RESULTS

Xe, dissolved at approximately 3.45 mM in aqueous solution, diminished the peak and even more the plateau of AMPA and glutamate induced currents. At the control EC(50) value for AMPA (29 microM) these reductions were by about 40 and 56% and at 3 mM glutamate the reductions were by 45 and 66%, respectively. Currents activated at the control EC(50) value for kainate (57 microM) were inhibited by 42%. Likewise, Xe showed an inhibitory effect on kainate-induced membrane currents of SH-SY5Y cells transfected with the GluR6 subunit of the kainate receptor. Xe reduced kainate-induced currents by between 35 and 60%, depending on the kainate concentration.

CONCLUSIONS

Xe blocks not only NMDA receptors, but also AMPA and kainate receptors in cortical neurones as well as GluR6-type receptors expressed in SH-SY5Y cells. Thus, Xe seems to be rather non-specific as a channel blocker and this may contribute to the analgesic and anaesthetic potency of Xe.

Stratum-by-stratum projection of light response attributes by retinal bipolar cells of Ambystoma

The visual system processes light images by projecting various representations of the visual world to segregated regions in the brain through parallel channels. Retinal bipolar cells constitute the first parallel channels that carry different light response attributes to different parts of the inner plexiform layer (IPL). Here we present a systematic study on detailed axonal morphology and light response characteristics of over 200 bipolar cells in dark-adapted salamander retinal slices by the whole-cell voltage clamp and Lucifer yellow fluorescence (with a confocal microscope) techniques. Four major groups of bipolar cells were identified according to the patterns of axon terminal ramification in the IPL: 36% were narrowly monostratified (whose axon terminals ramified in one of the 10 strata of the IPL), 27% were broadly monostratified, 19% were multistratified, and 18% bore pyramidally branching axons. By analysing the bipolar cells with narrowly monostratified axon terminals in each of the 10 strata of the IPL, we found that several key light response attributes are highly correlated with the strata in which the cells' axon terminals ramify. The 10 IPL strata appear to be the basic building blocks for attributes of light-evoked signal outputs in all bipolar cells, and several general stratum-by-stratum rules were identified by analysing the broadly monostratified, multistratified and pyramidally branching cells. These rules not only uncover mechanisms by which third-order retinal cells integrate and compute bipolar cell signals, but also shed considerable light on how bipolar cells in other vertebrates process visual information and how physiological signals may shape the morphology and projection of output synapses of visual neurones during development.

Characterization of spontaneous synaptic transmission in rat medial vestibular nucleus

Spontaneous synaptic currents of medial vestibular nucleus (MVN) neurons were studied using whole cell recording in slices prepared from 13- to 17-day-old rats. The spontaneous inhibitory postsynaptic currents (sIPSCs) were significantly reduced by the GABAA antagonist bicuculline (20 microM), but were not affected by the glycine antagonist strychnine (1 microM). The frequency, amplitude, and decay time constant of sIPSCs were 4.3 +/- 0.9 Hz, 18.1 +/- 2.0 pA and 8.9 +/- 0.4 ms, respectively. The specific AMPA receptor antagonist GYKI-52466 (50 microM) completely blocked the non-NMDA-mediated spontaneous excitatory postsynaptic currents (sEPSCs), indicating that they are mediated by an AMPA-preferring receptor. The AMPA-mediated sEPSCs were characterized by low frequency (1.5 +/- 0.4 Hz), small amplitude (13.9 +/- 1.9 pA) and rapid decay kinetics (2.8 +/- 0.2 ms). The majority (15/21) displayed linear I-V relationships, suggesting the presence of GluR2-containing AMPA receptors.

The non-NMDA receptor antagonist NBQX does not affect rats performance in the object recognition task

Though the AMPA receptor has been implicated in several neurodegenerative processes (epilepsy, ischemia, spasticity), its role in cognition is yet to be clarified. The aim of this study was to assess in the rat the effects of the AMPA receptor antagonist NBQX (3.5, 7, 10, 20 and 30 mgkg(-1), i.p.) on learning and memory. For this purpose, the object recognition task was chosen. NBQX, at the higher doses used (20 and 30 mgkg(-1)) caused respectively, depression of motility and ataxia, while given at lower doses (3.5, 7 and 10 mgkg(-1)) it did not influence animals performance in the object recognition paradigm. All rats acquired similarly well the task. In conclusion, these results would support and broaden previous observations on the lack of major involvement of AMPA receptors in the rat working memory mechanisms.

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.

Characteristics of AMPA receptor-mediated responses of cultured cortical and spinal cord neurones and their correlation to the expression of glutamate receptor subunits, GluR1-4

Electrophysiological recordings have been used to characterize responses mediated by AMPA receptors expressed by cultured rat cortical and spinal cord neurones. The EC(50) values for AMPA were 17 and 11 microM, respectively. Responses of cortical neurones to AMPA were inhibited competitively by NBQX (pK(i)=6.6). Lower concentrations of NBQX (< or =1 microM) also potentiated the plateau responses of spinal cord neurones to AMPA, which could be attributed to a depression of desensitization to AMPA. GYKI 52466 inhibited responses of spinal cord neurones to AMPA to about twice the extent of responses of cortical neurones. Blockade of AMPA receptor desensitization by cyclothiazide (CTZ) potentiated responses of spinal cord neurones (6.8 fold) significantly more than responses of cortical neurones (4.8 fold). Responses of cortical neurones to KA were potentiated 3.5 fold by CTZ, while responses of spinal cord neurones were unaffected. Ultra-fast applications of AMPA to outside-out patches showed responses of spinal cord neurones desensitized by 97.5% and exhibit marked inward rectification, whereas cortical neurones desensitized by 91% and exhibited slight outward rectification. The time constants of deactivation and desensitization were about twice as fast in spinal cord than cortical neurones. In cortical neurones, single-cell RT - PCR showed GluR2 and GluR1 accounted for 91% of all subunits and were expressed together in 67% of neurones, predominantly as the flip variants (78%). GluR2 was detected alone in 24% of neurones. GluR3 and GluR4 were present in only 14 and 29% of neurones, respectively. For spinal cord neurones, GluR4(o) was detected in 81% of neurones, whereas predominantly flop versions of GluR1, 2 and 3 were detected in 38, 13 and 13% of neurones, respectively. These expression patterns are related to the respective pharmacological and mechanistic properties.

GYKI 52466 has positive modulatory effects on AMPA receptors

The 2,3-benzodiazepine derivative GYKI 52466 has been well characterized as a negative modulator of AMPA-type glutamate receptors. The present study re-examined the effects of GYKI 52466 on AMPA receptor-mediated currents in patches excised from pyramidal neurons in the hippocampal CA1 field and found that this drug has positive modulatory effects in addition to its receptor blocking action. A low concentration of GYKI 52466 (10 microM) reliably increased the steady-state current by about three-fold, while the peak current was reduced by 30% only. Higher drug concentrations produced parallel reductions in both the steady-state and peak currents. The increase in the steady-state current was not accompanied by a change in the deactivation time constant and thus, is more likely to result from a change in desensitization than a slowing of channel closing. The results indicate that GYKI 52466 modulates AMPA receptor-mediated currents in a complex manner, perhaps by acting through more than one binding site.

Characterization of audiogenic-like seizures in naive rats evoked by activation of AMPA and NMDA receptors in the inferior colliculus

The role of glutamate receptors in the inferior colliculus (IC) in audiogenic and audiogenic-like seizures was investigated in adult rats with transient neonatal hypothyroidism by 0.02% propylthiouracil (PTU) treatment through mother's milk (PTU rats) and in naive rats treated intracisternally with N-methyl-d-aspartate (NMDA), alpha-amino-3-hydroxy-5-methyl-4-isoxazole-proprionic acid (AMPA), or cyclothiazide, an inhibitor of rapid AMPA receptor desensitization. All rats showed audiogenic or audiogenic-like seizures characterized by running fit (RF) and generalized tonic-clonic seizures (GTCS). While systemically administered MK-801 inhibited GTCS, intracisternally administered NBQX inhibited RF and GTCS in both audiogenic and audiogenic-like seizures. Auditory stimulation shortened the latency to GTCS induced by AMPA, but not NMDA, at a subclinical dose and further elongated the shortened duration of RF, but not GTCS, induced by MK-801 pretreatment. Furthermore, Northern blot analysis was used to evaluate the expression of the immediate-early gene c-fos in the IC following induction of audiogenic or audiogenic-like seizures. The significant induction of c-fos mRNA by audiogenic seizures in PTU rats or by AMPA- or cyclothiazide-induced seizures in naive rats was prominent in the IC. MK-801 suppressed c-fos mRNA expression in the IC induced by audiogenic seizures in PTU rats or by AMPA-induced seizures in naive rats. NBQX suppressed the expression of c-fos mRNA in the IC induced by AMPA-induced seizures but did not suppress c-fos mRNA in PTU rats or rats with cyclothiazide-induced seizures. Auditory stimuli failed to affect c-fos mRNA induction by AMPA. The present study suggests that audiogenic-like seizures can be reproduced by glutamate receptor agonists in which AMPA receptors are primarily linked to the initiation of audiogenic seizures (RF) while NMDA receptors presumably located within the IC are involved in the propagation of GTCS in audiogenic seizures.

Negative allosteric modulators of AMPA-preferring receptors inhibit [(3)H]GABA release in rat striatum

The effect of alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA), a selective glutamate receptor agonist, on the release of previously incorporated [(3)H]GABA was examined in superfused striatal slices of the rat. The slices were loaded with [(3)H]GABA in the presence of beta-alanine (1 mM) and superfused with Krebs-bicarbonate buffer containing nipecotic acid (0.1 mM) and aminooxyacetic acid (0.1 mM) to inhibit GABA uptake and metabolism. AMPA (0.01 to 3 mM) increased basal [(3)H]GABA outflow and nipecotic acid potentiated this effect. The [(3)H]GABA releasing effect of AMPA was an external Ca(2+)-dependent process in the absence but not in the presence of nipecotic acid. Cyclothiazide (0.03 mM), a positive modulator of AMPA receptors, failed to evoke [(3)H]GABA release by itself, but it dose-dependently potentiated the [(3)H]GABA releasing effect of AMPA. The AMPA (0.3 mM)-induced [(3)H]GABA release was antagonized by NBQX (0.01 mM) in a competitive fashion (pA(2) 5.08). The negative modulator of AMPA receptors, GYKI-53784 (0.01 mM) reversed the AMPA-induced [(3)H]GABA release by a non-competitive manner (pD'(2) 5.44). GYKI-53784 (0. 01-0.1 mM) also decreased striatal [(3)H]GABA outflow on its own right, this effect was stereoselective and was not influenced by concomitant administration of 0.03 mM cyclothiazide. GYKI-52466 (0. 03-0.3 mM), another negative modulator at AMPA receptors, also inhibited basal [(3)H]GABA efflux whereas NBQX (0.1 mM) by itself was ineffective in alteration of [(3)H]GABA outflow. The present data indicate that AMPA evokes GABA release from the vesicular pool in neostriatal GABAergic neurons. They also confirm that multiple interactions may exist between the agonist binding sites and the positive and negative modulatory sites but no such interaction was detected between the positive and negative allosteric modulators. Since GYKI-53784, but not NBQX, inhibited [(3)H]GABA release by itself, AMPA receptors located on striatal GABAergic neurons may be in sensitized state and phasically controlled by endogenous glutamate. It is also postulated that these AMPA receptors are located extrasynaptically on GABAergic striatal neurons.

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.

Developmental regulation of AMPA-receptor properties in CA1 pyramidal neurons of rat hippocampus

AMPA-receptor (AMPA-R) currents were recorded from CA1 pyramidal neurons in situ and after acute isolation from the hippocampus of 3- to 45-day-old rats. Membrane currents were analyzed by combining the patch clamp method with fast application techniques. The complete block of receptor currents by GYKI 53655 and the absence of modulation by Concanavalin A indicated that the cells exclusively expressed non-NMDA glutamate receptors of the AMPA subtype while functional kainate receptors could not be detected. The lowest sensitivity to kainate and NBQX was observed at postnatal day (p) 18. These changes might reflect a lower abundance of GluR1 at that developmental stage. A decrease of potentiation of receptor currents by cyclothiazide (CTZ), an acceleration of the recovery from CTZ potentiation and a faster and more complete desensitization of glutamate-evoked currents suggest an up-regulation of flop splice variants with increasing age. These functional data indicate that AMPA-R expression in CA1 pyramidal neurons varies during postnatal development which can be expected to influence the kinetics of synaptic transmission and the excitotoxic vulnerability as well.

Late embryonic expression of AMPA receptor function in the CA1 region of the intact hippocampus in vitro

Studies in slices suggest that alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptor-mediated synaptic currents are not present in CA1 (Cornu ammonis) pyramidal neurons at birth (P0). We have re-examined this issue in the rat intact hippocampal formation (IHF) in vitro. Injections of biocytin or carbocyanine show that the temporo-ammonic, commissural and Schaffer collateral pathways are present at birth in the marginal zone of CA1. Electrical stimulation of these pathways evoked field excitatory postsynaptic potentials (fEPSPs) in the marginal zone of CA1 from embryonic day 19 (E19) to postnatal day 9 (P9). These fEPSPs are mediated by synaptic AMPA receptors as they are reduced or completely blocked by: (i) tetrodotoxin; (ii) high divalent cation concentrations; (iii) the adenosine A1 receptor agonist CPA; (iv) anoxic episodes; (v) the selective AMPA receptor antagonist 1-(4-aminophenyl)-3-methylcarbamyl-4-methyl-7, 8-methylenedioxy-3,4-dihydro-5H-2,3-benzodiazepine (GYKI-53655) or the mixed AMPA-kainate receptor antagonists 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX) and 6-nitro-7-sulphamoylbenzo[f]quinoxaline-2,3-dione (NBQX). The amplitude of the fEPSPs is also reduced by D(-)-2-amino-5-phosphonopentanoic acid (D-APV) and its duration is increased by bicuculline suggesting the participation of N-methyl-D-aspartate (NMDA) and GABAA (gamma-aminobutyric acid) receptors. Finally, AMPA receptor-mediated fEPSPs are also recorded in P0 slices, but they are smaller and more labile than in the IHF. Our results suggest that in embryonic CA1 neurons, glutamate acting on AMPA receptors already provides a substantial part of the excitatory drive and may play an important role in the activity-dependent development of the hippocampus. Furthermore, the IHF may be a convenient preparation to investigate the properties of the developing hippocampus.

Role of glutamate receptors and voltage-dependent calcium channels in glutamate toxicity in energy-compromised cortical neurons

We have examined the effect of glutamate receptor antagonists and voltage-dependent calcium channel blockers on the neuronal injury induced by the combination of a low concentration of N-methyl-D-aspartate (NMDA) or kainate and energy compromise resulting from the use of glucose-free incubation buffer. Toxicity induced by NMDA or kainate was enhanced in the glucose-free buffer. NMDA-or non-NMDA-receptor antagonists added to the glucose-free buffer at the same time inhibited the neuronal cell death induced by each agonist. An NMDA-receptor antagonist, MK-801, but not non-NMDA-receptor antagonists, inhibited the toxicity when added to the culture medium after exposure of the cells to the agonists. P/Q-type calcium channel blockers, omega-agatoxin IVA and omega-agatoxin TK, and an N-type calcium channel blocker, omega-conotoxin GVIA, significantly attenuated the neuronal injury, although an L-type calcium channel blocker, nifedipine, showed little neuroprotective effect. A combination of calcium channel blockers of the three subtypes showed the most prominent neuroprotective effect. These observations suggest that the overactivation of NMDA and non-NMDA receptors and consequent activation of the voltage-dependent calcium channels lead to neuronal cell death in energy-compromised cortical neurons.

Novel potent AMPA analogues differentially affect desensitisation of AMPA receptors in cultured hippocampal neurons

The agonist actions of two AMPA receptor analogues, (RS)-2-amino-3-(3-carboxy-5-methyl-4-isoxazolyl)propionic acid (ACPA) and (RS)-2-amino-3-(3-hydroxy-5-trfluoromethyl-4-isoxazolyl)prop ionic acid (Tri-F-AMPA) have been studied on cultured rat hippocampal neurons. Whole-cell recordings with semi-rapid application of the agonists were used to study steady-state (plateau) responses. ACPA was the most potent agonist (EC50, 1.2 microM), followed by AMPA (4.3 microM) and Tri-F-AMPA (4.6 microM), corresponding to a potency ratio of 4:1:1. Hill coefficients were close to 1 for AMPA and ACPA and close to 2 for Tri-F-AMPA, respectively. Plateau responses to maximal concentrations of the three agonists varied more than 2-fold. ACPA responses were 2.1 times greater and responses to Tri-F-AMPA were 1.6 times greater than responses to AMPA, respectively. Peak responses and desensitization were studied by using a fast piezoelectric device to apply agonists rapidly to outside-out patches. The time constants of desensitization were 8 ms for AMPA, 12 ms for Tri-F-AMPA and 17 ms for ACPA. There were no significant differences in the time-to-peak and 10-90% rise-time of the responses. The results indicate that of the three agonists tested, ACPA is the most potent at AMPA receptors expressed in cultured hippocampal neurons and that the maximum response to the agonists is inversely related to the rate of desensitization.

Studies of the antagonist actions of (RS)-2-amino-3-[5-tert-butyl-3-(phosphonomethoxy)-4-isoxazolyl] propionic acid (ATPO) on non-NMDA receptors in cultured rat neurones

Whole-cell patch-clamp recordings from single cultured cortical neurones have been used to study the action of (RS)-2-amino-3-[5-tert-butyl-3-(phosphonomethoxy)-4-isoxazolyl+ ++]propionic acid (ATPO), which has previously been proposed to be a potent selective antagonist of 2-amino-3-(3-hydroxy-5-methyl-4-isoxazolyl)propionic acid (AMPA) receptors. ATPO competitively reduced peak responses evoked by semi-rapid applications of AMPA (Ki = 16 microM) but had variable effects on plateau responses, which were on average unchanged. Following blockade of AMPA receptor desensitization by cyclothiazide (CTZ, 100 microM), the plateau responses were reduced by ATPO to a similar extent as the peak responses, indicating that ATPO reduces desensitization of AMPA receptors. Semi-rapid application of kainic acid (KA) and the KA receptor-selective agonist, (2S,4R)-4-methylglutamic acid (MeGlu) evoked non-desensitizing responses which were competitively antagonized by ATPO (Ki values: 27 and 23 microM, respectively). Responses to MeGlu were unaffected by CTZ (100 microM), but potentiated 3 fold following blockade of KA receptor desensitization by concanavalin A (Con A, 300 microg ml(-1)). Responses of spinal cord neurones to MeGlu were blocked by ATPO to a similar extent before and after blockade of KA receptor desensitization by Con A. Although selectively potentiated by Con A, plateau responses to MeGlu were reduced by 69.6% by the AMPA selective antagonist, GYKI 53655 (10 microM). The remaining component was further reduced by ATPO with a Ki of 36 microM, which was not significantly different from that in the absence of GYKI 53655, but was greater than that on responses to AMPA. It is concluded that ATPO is a moderate-potency competitive inhibitor of naturally expressed non-NMDA receptors.

Interactions of GYKI 52466 and NBQX with cyclothiazide at AMPA receptors: experiments with outside-out patches and EPSCs in hippocampal neurones

In outside-out patches from cultured hippocampal neurones, glutamate (1 mM) applied for 1 ms evoked currents which rose rapidly (tau(on) 451 +/- 31 micros) to a peak and then deactivated with slower kinetics (1.95 +/- 0.13 ms). Offset time constants were significantly slower with longer application durations (tau(off) 3.10 +/- 0.19, 3.82 +/- 0.25, 4.80 +/- 0.65 and 7.56 +/- 0.65 ms with 10, 20, 100 and 500 ms applications respectively). Desensitization was complete within 100 ms with a similar rate for all application durations (4.74 +/- 0.34 ms with 100 ms applications). GYKI 52466 reduced inward peak currents with an IC50 of 11.7 +/- 0.6 microM and had similar potency on steady-state currents to longer glutamate applications. GYKI 52466 had no significant effect on desensitization or deactivation time constants but caused a modest and significant prolongation of onset kinetics at higher concentrations. Cyclothiazide (100 microM) potentiated steady-state currents 25-fold at 100 ms and caused a modest but significant slowing in onset kinetics (601 +/- 49 micros with 1 ms applications) but a more pronounced prolongation of deactivation time constants (5.55 +/- 0.66 ms with 1 ms applications). In 50% of neuronal patches cyclothiazide completely eliminated desensitization. In those patches with residual desensitization, the rate was not significantly different to control (5.36 +/- 0.43 ms with 100 ms applications). Following 100 ms applications of glutamate, GYKI 52466 had IC50s of 11.7 +/- 1.1 microM and 75.1 +/- 7.0 microM in the absence and presence of cyclothiazide (100 microM) respectively. Onset kinetics were slowed from 400 +/- 20 micros to 490 +/- 30 micros by cyclothiazide (100 microM) and then further prolonged by GYKI 52466 (100 microM) to a double exponential function (tau(on1) 1.12 +/- 0.13 ms and tau(on2) 171.5 +/- 36.5 ms). GYKI 52466 did not re-introduce desensitization but concentration-dependently weakened cyclothiazide's prolongation of deactivation time constants (1 ms applications: 5.01 +/- 0.71, 4.47 +/- 0.80 and 2.28 +/- 0.64 ms with GYKI 52466 30, 100 and 300 microM respectively). NBQX reduced peak current responses with an IC50 of 28.2 +/- 1.3 nM. Paradoxically, steady-state currents with 500 ms applications of glutamate were potentiated from 3.3 +/- 1.2 pA to 29.4 +/- 6.4 pA by NBQX (1 nM). Higher concentrations of NBQX then antagonized this potentiated response. The potency of NBQX in antagonizing steady-state currents to 500 ms applications of glutamate (IC50 120.9 +/- 30.2 nM) was 2-fold less than following 100 ms applications (IC50 67.7 +/- 2.6 nM). NBQX had no effect on rapid onset, desensitization or deactivation time constants. However, a slow relaxation of inhibition was seen with longer applications. NBQX was 2-5-fold less potent against inward currents in the presence of cyclothiazide (100 microM) depending on the application duration but had no effect on the rapid onset, desensitization or deactivation time constants. The same relaxation of inhibition was seen as with NBQX alone. NBQX (1 microM) reduced AMPA receptor-mediated EPSC amplitude to 7 +/- 1% of control with no effect on kinetics. Cyclothiazide (330 microM) caused a 2.8-fold prolongation of the decay time constant (control 26.6 +/- 2.2 ms, cyclothiazide 74.2 +/- 7.6 ms, n = 9). Additional application of NBQX (1 microM) partly reversed this prolongation to 1.9 fold (47.7 +/- 2.5 ms, n = 5). These results support previous findings that cyclothiazide also allosterically influences AMPA receptor agonist/antagonist recognition sites. There were no interactions between NBQX and cyclothiazide on desensitization or deactivation time constants of glutamate-induced currents but clear interactions on EPSC deactivation kinetics. This raises the possibility that the interactions of NBQX, GYKI 52466 and cyclothiazide on AMPA-receptor-mediated EPSC kinetics observed are due to modulation of glutamate-release at presynaptic AMPA receptors.

In vivo and in vitro evaluation of AMPA receptor antagonists in rat hippocampal neurones and cultured mouse cortical neurones

The effects of four glutamate receptor antagonists on alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid (AMPA)- and N-methyl-D-aspartate (NMDA)-responses were evaluated using both in vitro and in vivo electrophysiological techniques: whole cell patch-clamp recordings from cultured mouse cortical neurones and microiontophoresis in the rat hippocampus. The compounds tested were NBQX (2,3-dihydroxy-6-nitro-7-sulfamoyl-benzo(F)quinoxaline), GYKI 52466 (1-(4-amino-phenyl)-4-methyl-7,8-methyl-endioxyl-5H-2,3-benzodiaze pine), PNQX (pyrido[3, 4-f]quinoxaline-2,3-dione, 1,4,7,8,9,10-hexahydro-9-methyl-6-nitro-, methanesulfonate), NS377 (7-ethyl-5-phenyl-1,6,7,8-tetrahydro-1,7-diaza-as-indacene-2 ,3-dione), and MK-801 ((+)-5-methyl-10,11-dihydro-5H-dibenz(a,d)cycloheptene-5,10-imine hydrogen maleate). In vitro, the IC50 values (in microM) for inhibition of AMPA-evoked inward currents were approximately 0.4 for NBQX, approximately 7.5 for GYKI 52466, approximately 1 for PNQX and approximately 15 for NS377. PNQX and NS377 also inhibited NMDA-induced currents with IC50 values at approximately 5 and approximately 18 microM, respectively, while NBQX at 60 microM and GYKI 52466 at 100 microM had only weak effects. The ED50 values in micromol/kg i.v. for inhibition of AMPA-evoked hippocampal neuronal spike activity in vivo were approximately 32 for NBQX, approximately 19 for GYKI 52466, approximately 17 for PNQX and approximately 11 for NS377 with efficacy values (maximal inhibition) between 71% and 81%. The ED50 values (in [Lmol/kg i.v.) and efficacy values for inhibition of NMDA-evoked hippocampal neuronal spike activity were approximately 28 with an efficacy of 61% for NBQX, approximately 16 with 35% for PNQX and approximately 6 with 61% for NS377. GYKI 52466 did not significantly affect NMDA responses, whereas MK-801 showed NMDA specificity in vivo.

Postsynaptic responses of horizontal cells in the tiger salamander retina are mediated by AMPA-preferring receptors

The postsynaptic responses of sign-preserving second-order retinal neurons (horizontal cells (HCs) and off-bipolar cells) are mediated by CNQX-sensitive AMPA/KA glutamate receptors. In this study we used receptor-specific allosteric regulators of desensitization and selected antagonists to determine the glutamate receptor subtypes in tiger salamander horizontal cells. Two approaches were employed in this study. The first was to measure postsynaptic currents induced by exogenously applied glutamate under voltage clamp conditions in living retinal slices; and the second was to record voltage responses controlled by endogenous glutamate released from photoreceptors in whole retinas. Application of 100 microM cyclothiazide (a specific AMPA receptor desensitization blocker) enhanced the glutamate-induced current by about 5 fold. In contrast, 300 microgram ml-1 Co nA (a specific kainate receptor desensitization blocker), had no effect. GYKI 52466 (a specific AMPA receptor antagonist) at 30 microM almost completely suppressed the glutamate-induced inward current in HCs. Cyclothiazide at 100 microM depolarized the HC dark membrane potential by about 5 mV and reduced the amplitudes of the voltage responses to dim lights, but enhanced the voltage responses to bright lights. Cyclothiazide had no effect on either the dark potential or the light responses of rods and cones. Con A at 300 microgram ml-1 had no effect on either the dark potential or the light responses of the HC. GYKI 52466 (30 microM) hyperpolarized the HC dark membrane potential by about 55 mV and almost completely suppressed the light responses. We conclude from these results that the postsynaptic glutamate- and light-induced responses in the tiger salamander retinal horizontal cells are mediated by AMPA-preferring, and not kainate-preferring glutamate receptors. The functional roles of AMPA receptors and their desensitization kinetics in visual information processing are discussed.

Behavioral effects of NBQX, a competitive antagonist of the AMPA receptors

NBQX, a specific and potent AMPA receptor antagonist has been found to be neuroprotective in various models of ischemia and to have anticonvulsant properties in different models of epilepsy. In this experiment, the neurobehavioral effects of NBQX were studied. In an open field, an important ataxia was emphasized at a dose of 60 mg/kg. In a swimming task, an increase of the escape latencies was noted on the third day at a dose of 40 mg/kg. In a Morris water maze task, doses devoid of effects on locomotion were used (10, 20, and 30 mg/kg). There was no effect on the acquisition of the task at 10 mg/kg and a slight impairment at 20 mg/kg, but the rats did not learn the task at 30 mg/kg. This impairment was reversible, as shown by the increasing performance of this group without treatment. No impairment was noted in the retention phase of the Morris water maze task. The results are discussed relative to the role of the AMPA receptor in memory processes.

Distribution and properties of kainate receptors distinct in the CA3 region of the hippocampus of the guinea pig

To characterize the nature of kainate (KA) receptors distinct in the CA3 region of the hippocampus, properties of depolarizations induced by pulses of KA or AMPA (alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionate) applied to dendrites of CA3 neurons with micropipettes were studied in thin transverse slices of the guinea pig hippocampus. KA induced depolarizations at negligible latencies only when administered to the most proximal dendritic areas. The depolarization was unaffected by tetrodotoxin or by a decrease in Ca2+ and an increase in Mg2+ concentrations. The declining slope of the KA-induced depolarization was significantly slower than that of the AMPA-induced depolarization. In comparison with the AMPA-induced depolarization, the KA-induced depolarization was much less susceptible to antagonists such as 6-cyano-7-nitroquinoxaline-2, 3-dione (CNQX) and 1-(4-aminophenyl)-4-methyl-7, 8-methylenedioxy-5H-2,3-benzodiazepine hydrochloride (GYKI52466). 6, 7,8,9-Tetrahydro-5-nitro-1H-benz[g]indole-2,3-dione-3-oxime (NS-102) and (2S,4R)-4-methylglutamate (SYM 2081) were without effects. The threshold concentration of pressure-ejected KA to induce depolarizations was about 200 nM. Excitatory postsynaptic potentials elicited by mossy fiber stimulation were more potently suppressed by CNQX than by GYKI52466. These results indicate that receptors responsible for the slow KA depolarization in the CA3 region of the hippocampus are not AMPA receptors but KA receptors. They are localized in the most proximal part of the apical dendrite and distinct from those observed in primary cultures of hippocampal neurons.

Cyclothiazide and AMPA receptor desensitization: analyses from studies of AMPA-induced release of [3H]-noradrenaline from hippocampal slices

1. Responses in brain produced by the activation of the alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionate (AMPA) subtype of ionotropic receptor for L-glutamate are often rapidly desensitizing. AMPA-induced desensitization and its characteristics, and the potentiating effect of cyclothiazide were investigated in vitro by analysing AMPA-induced release of [3H]-noradrenaline from prisms of rat hippocampus. 2. AMPA (1-1000 microM) stimulated the release of [3H]-noradrenaline in a concentration-dependent manner that was both calcium-dependent and tetrodotoxin-sensitive, and attenuated by the AMPA-selective antagonists, NBQX (1 and 10 microM), LY 293558 (1 and 10 microM) and GYKI 52466 (10 and 30 microM). 3. By use of an experimental procedure with consecutive applications of AMPA (100 microM, 28 min apart), the second response was reduced, indicative of receptor desensitization, and was reversed by cyclothiazide in a concentration-dependent manner (1-300 microM). The concentration-response curve for AMPA-induced release of [3H]-noradrenaline was shifted leftwards, but the reversal by cyclothiazide of the desensitized response was partial and failed to reach the maximal response of the first stimulus. 4. Observations made with various schedules of cyclothiazide application indicated that the initial AMPA-evoked response was already partially desensitized (150% potentiation by 100 microM cyclothiazide) and that the desensitization was not likely to be due to a time-dependent diminution and was longlasting (second application of cyclothiazide was ineffective). 5. Co-application of a number of drugs with actions on second messenger systems, in association with the second AMPA stimulus, revealed significant potentiation of the AMPA-induced release of [3H]-noradrenaline: forskolin (10 microM, +78%), Rp-cAMPS (100 microM, +65%), Ro 31-8220 (10 microM, +163%) and thapsigargin (100 pM, + 161%). 6. The AMPA receptor-mediated response regulating the release of [3H]-noradrenaline from rat hippocampal slices was desensitized and cyclothiazide acted to reverse partially the desensitization in a concentration-dependent manner. Since the time-course of desensitization was longer lasting than that noted in previous electrophysiological studies, multiple events may be involved in the down-regulation of AMPA receptor activity including receptor phosphorylation and depletion of intracellular Ca2+ stores.

Fear conditioning induces a lasting potentiation of synaptic currents in vitro

The amygdala plays a critical role in the mediation of emotional responses, particularly fear, in both humans and animals. Fear conditioning, a conditioned learning paradigm, has served as a model for emotional learning in animals, and the neuroanatomical circuitry underlying the auditory fear-conditioning paradigm is well characterized. Synaptic transmission in the medial geniculate nucleus (MGN) to lateral nucleus of the amygdala (LA) pathway, a key segment of the auditory fear conditioning circuit, is mediated largely through N-methyl-D-aspartate (NMDA) and non-NMDA (such as alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA)) glutamate receptors; the potential for neural plasticity in this pathway is suggested by its capacity to support long-term potentiation (LTP). Here we report a long-lasting increase in the synaptic efficacy of the MGN-LA pathway attributable to fear-conditioning itself, rather than an electrically induced model of learning. Fear-conditioned animals show a presynaptic facilitation of AMPA-receptor-mediated transmission, directly measured in vitro with whole-cell recordings in lateral amygdala neurons. These findings represent one of the first in vitro measures of synaptic plasticity resulting from emotional learning by whole animals.

Evidence for a role of presynaptic AMPA receptors in the control of neuronal glutamate release in the rat forebrain

The role of presynaptic alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptors in controlling the neuronal release of excitatory amino acids has been investigated. Stimulation of presynaptic AMPA receptors by the endogenous agonist L-glutamate, or by (R,S)-AMPA, dose-dependently enhanced the Ca(2+)-dependent, tetrodotoxin-insensitive, electrically-stimulated release of [3H]D-aspartate from rat forebrain slices. This AMPA receptor-mediated response showed marked stereoselectivity with the activity residing solely in the (S)-isomer. (R)-AMPA was inactive in this respect. AMPA-evoked responses were significantly enhanced in the presence of the AMPA receptor desensitization inhibitor, cyclothiazide (10 microM). Moreover, responses to both AMPA and glutamate were inhibited by competitive (NBQX) and non-competitive (GYKI 52466) AMPA receptor-selective antagonists in a dose-dependent manner. These results provide strong support for the existence of presynaptic AMPA receptors acting to enhance the synaptic release of excitatory amino acids in the mammalian forebrain. Such a positive feedback system may play an important functional role in physiological (e.g., long-term potentiation) and/or pathological (e.g., epileptogenesis) processes in the mammalian central nervous system. AMPA-type autoreceptors may provide new targets for drug action.

GYKI 53665, a 2,3-benzodiazepine, non-competitively protects cultured neurones against AMPA toxicity

The nature of the neuroprotection by the competitive alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptor antagonist, 6-nitro-7-sulfamoyl-benzo[f]quinoxaline-2,3-dione (NBQX), and the non-competitive AMPA receptor antagonist, 1-(4-aminophenyl)-3-methylcarbamoyl-4-methyl-3,4-dihydro-7,8-methy lenedioxy-5H-2,3-benzodiazepine (GYKI 53655), was investigated in mature telencephalic neurone cultures of the rat. NBQX protected cultured neurones against AMPA-induced delayed toxicity in a competitive manner: the AMPA concentration-response curve was shifted to the right in parallel and concentration dependently. In contrast, GYKI 53655 decreased the maximal neurotoxic effect of AMPA considerably but without affecting the EC50, for AMPA toxicity, which indicated the non-competitive mode of its action. Thus we found a clear relationship between the nature of in vitro neuroprotection and the mode of AMPA channel block.

Comparison of neuroprotective efficacy of competitive and non-competitive AMPA antagonists in vitro

The neuroprotective efficacy of the most potent competitive α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) antagonist 2,3-dihydroxy-6-nitro-7-sulfamoyl-benzo(F)quinoxaline (NBQX) and three recently developed 2,3-benzodiazepine non-competitive AMPA antagonists (GYKI 52466, 53405 and 53655) was investigated in primary cultures of rat telencephalic neurons. NBQX protected cultured neurons against AMPA (20 μM for 21-23 h) induced toxicity with EC(50) of 0.5 μM. In the same test GYKI 52466, 53405 and 53655 had EC(50) values of 10.6, 9.3 and 5.1 μM, respectively. Thus we found the competitive antagonist NBQX to be ten times more effective neuroprotectant in vitro than the most potent non-competitive GYKI compound (GYKI 53655).

The effects of copper ions on glutamate receptors in cultured rat cortical neurons

Copper plays an important role in the function of many physiological processes and can affect different neurotransmitter systems. In this study, we used the patch-clamp technique to investigate the effect of copper ions on glutamate receptors in cultured rat cortical neurons. Cu2+ inhibited (S)-alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA)/kainate receptors with an IC50 of 4.3 +/- 0.6 microM (with 100 microM kainate, holding potential -60 mV). The concentration-response could be best described by a two-site binding model. Moreover, copper reduced the efficacy of kainate at the AMPA receptor: in the presence of 30 microM Cu2+, the EC50 of kainate was shifted from 100.3 +/- 2.0 microM to 329.9 +/- 31.4 microM. The block by copper ions was not use-dependent. Complete recovery only occurred after the application of a high agonist concentration, or in the presence of the antioxidant dithiotreitol (DTT). A high concentration of histidine, a physiological ligand for Cu2+, did not augment the recovery. The kinetics of block were compared to those induced by 2,3-dihydro-6-nitro-7-sulfamoyl-benz(F)quinoxaline (NBQX), a well-described competitive antagonist of AMPA receptors. The onset, as well as the offset of block by NBQX could be well approximated by single exponential functions with time constants of 0.28 +/- 0.02 and 0.87 +/- 0.09 s, respectively. Within seconds of wash-out of the antagonist, the response to kainate completely recovered. The kinetics of copper block were more complex: the block developed more slowly, and the onset, as well as the offset could be described by two exponential functions with quite different time constants (tau(on1), 0.8 +/- 0.13 s; tau(on2), 8.32 +/- 1.13 s; tau(off1), 0.17 +/- 0.01 s; tau(off2), 69 +/- 36.3 s). In addition to the described effects, Cu2+ also blocked currents induced by the application of N-methyl-D-aspartate (IC50: 15.0 +/- 2.6 microM with 50 microM NMDA). Based on these findings, a modulatory role of copper ions on the neurotransmission by excitatory amino acids is discussed.

Characterization of YM90K, a selective and potent antagonist of AMPA receptors, in rat cortical mRNA-injected Xenopus oocytes

The inhibitory potencies of 6-(1H-imidazol-1-yl)-7-nitro-2,3(1H,4H)-quinoxalinedione hydrochloride (YM90K), 2-3,dihydroxy-6-nitro-7-sulfamoyl-benzo(F)quinoxaline (NBQX) and 1-(4-amino-phenyl)-4-methyl-7,8-methyl-endioxyl-5H-2,3-benzodiazep ine (GYKI 52466) at excitatory amino acid receptors were examined in rat cortical mRNA-injected Xenopus oocytes using a two-electrode voltage clamp. Schild analysis of YM90K and NBQX inhibition of kainate currents yielded pA2 values of 6.83 +/- 0.01 and 7.24 +/- 0.01, respectively. GYKI 52466 reduced the maximum kainate response and increased the kainate EC50 in a dose-dependent manner, suggesting that the antagonism of AMPA receptors by GYKI 52466 is mixed competitive and non-competitive for kainate. Schild analysis of YM90K and NBQX inhibition of kainate currents in the presence of 30 microM cyclothiazide yielded pA2 values of 6.62 +/- 0.03 (slope: 1.02 +/- 0.01) and 7.10 +/- 0.02 (slope: 1.00 +/- 0.02), respectively, consistent with competitive antagonism. Cyclothiazide potentiated the AMPA response as well as the kainate response and increased the apparent Hill coefficients in a concentration-dependent manner. The potency of YM90K to inhibit AMPA-induced current could be reduced by increasing the concentration of cyclothiazide. We showed that YM90K is a potent and competitive antagonist for AMPA receptors and the apparent affinity of competitive antagonists was reduced by cyclothiazide. Cyclothiazide can affect the interaction between receptors and both agonists and antagonists, suggesting that it might allosterically alter the affinity of agonists and competitive antagonists for their binding site on the AMPA receptor complex.

A venus flytrap mechanism for activation and desensitization of alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid receptors

Desensitization of the alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid (AMPA) subtype of glutamate receptor channels is an important process shaping the time course of synaptic excitation. Upon desensitization, the receptor channel closes and the agonist affinity increases. So far, the nature of the structural rearrangements leading to these events was unknown. On the basis of the structural homology of the ligand binding domains of AMPA receptors and of the bilobated bacterial periplasmic proteins, we now show that agonist interaction with one lobe of the GluR1 subunit of homomeric AMPA receptors controls channel activation while additional interactions with the other lobe cause channel desensitization. Accordingly, we suggest that the transition of the AMPA receptor channel to the desensitized state involves the agonist-mediated stabilization of the closed lobe conformation of its binding domain and is a process akin to that used by the venus flytrap.

NMDA as well as non-NMDA receptors mediate the neurotransmission of inspiratory drive to phrenic motoneurons in the adult rat

The neurotransmission of bulbospinal respiratory drive is believed to involve primarily non-NMDA receptors located in the phrenic motonucleus (PMN). This conclusion is based on studies carried out mainly on in vitro brainstem-spinal cord preparations of the neonatal rat. The present study was undertaken to investigate the transmitter/receptor mechanisms in the PMN which are involved in the neurotransmission of inspiratory drive, using an in vivo adult rat model. Microinjections of glutamate, NMDA and AMPA into the PMN elicited an increase in the phrenic nerve (PN) background discharge. These injections did not alter significantly the frequency of spontaneously occurring PN bursts confirming that mechanisms responsible for respiratory rhythm reside in the supraspinal structures. Microinjections of an NMDA receptor blocker (AP-7), in concentrations that did not alter the responses to a non-NMDA receptor agonist (AMPA), reduced the PN amplitude significantly. Similarly, microinjections of a potent non-NMDA receptor blocker (NBQX), in concentrations that did not alter responses to NMDA, reduced the PN amplitude significantly. Sequential microinjections, within an interval of 5 min, of AP-7 and NBQX into the PMN, resulted in a dramatic reduction in the spontaneous PN bursts. The reduction of PN amplitude started immediately after the microinjection of AP-7 and NBQX, either alone or in combination, and reached a maximum within 5-10 min. These results indicate that, unlike in the neonatal rat, both NMDA and non-NMDA receptors located in the PMN play a significant role in the neurotransmission of the inspiratory drive in the adult rat.

Zinc changes AMPA receptor properties: results of binding studies and patch clamp recordings

The influence of zinc ions on alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptors was investigated using binding studies with [3H]AMPA to rat cortical membranes and patch clamp recordings from cultured superior colliculus neurones. In Tris-HCl buffer, zinc (1-10 mM) significantly increased the specific binding of [3H]AMPA whereas this increase was negligible in the presence of CaCl2 (2.5 mM) and KSCN (100 mM). This effect was associated with a dramatic increase in Bmax but a decrease in both agonist and antagonist affinity. Association and dissociation experiments showed that equilibrium [3H]AMPA binding is reached with faster kinetics in the presence of zinc. At low concentrations (0.3 mM) zinc also concentration-dependently potentiated both peak and plateau components of whole cell current responses to AMPA (100 microM). This effect was accompanied by a reduction of the degree, and slowing of the rate, of AMPA receptor desensitisation. In contrast, higher concentrations of zinc (1-3.0 mM) inhibited AMPA responses to some degree, but slowed desensitisation further. This ability of zinc to change AMPA receptor properties may be relevant to neurotoxicity associated with AMPA receptor activation.

Interactions of 2,3-benzodiazepines and cyclothiazide at AMPA receptors: patch clamp recordings in cultured neurones and area CA1 in hippocampal slices

1. The 2,3-benzodiazepines GYKI 52466, GYKI 53405 and GYKI 53655 antagonized AMPA-induced currents in cultured superior colliculus neurones in a non use-dependent manner (steady state IC50s: GYKI 52466 9.8 +/- 0.6 microM; GYKI 53405 3.1 +/- 0.6 microM; GYKI 53655 0.8 +/- 0.1 microM). 2. Higher concentrations of all three antagonists slowed the onset kinetics and quickened the offset kinetics of AMPA-induced currents indicative of an allosteric interaction with the AMPA recognition site. 3. Cyclothiazide (3-300 microM) dramatically slowed desensitization of AMPA-induced currents and potentiated steady state currents (EC50 10.0 +/- 2.5 microM) to a much greater degree than peak currents. Both tau on and tau off were also increased by cyclothiazide in a concentration-dependent manner (EC50: tau on 42.1 +/- 4.5 microM; tau off 31.6 +/- 6.6 microM). 4. Cyclothiazide (10-100 microM) shifted the concentration-response curves of the 2,3-benzodiazepines to the right. For example, with 10 microM cyclothiazide the IC50s of GYKI 52466 and GYKI 53405 on steady-state AMPA-induced currents were 57.9 +/- 9.5 and 41.6 +/- 1.5 microM, respectively. 5. GYKI 53405 and GYKI 52466 concentration-dependently reversed the effects of cyclothiazide (100 microM) on offset kinetics (GYKI 53405 IC50 16.6 +/- 4.2 microM). However, the 2,3-benzodiazepines were unable to reintroduce desensitization in the presence of cyclothiazide and even concentration-dependently slowed the onset kinetics of AMPA responses further (GYKI 53405 EC50 8.0 +/- 2.8 microM). 6. GYKI 52466 decreased the peak amplitude of hippocampal area CA1 AMPA receptor-mediated excitatory postsynaptic currents (e.p.s.cs) (IC50 10.8 +/- 0.8 microM) with no apparent effect on response kinetics. Cyclothiazide prolonged the decay time constant of AMPA receptor-mediated e.p.s.cs (EC50 35.7 +/- 6.5 microM) with less pronounced effects in slowing e.p.s.c. onset kinetics and increasing e.p.s.c. amplitude. 7. Cyclothiazide (330 microM) shifted the concentration-response curve for the effects of GYKI 52466 on AMPA receptor-mediated e.p.s.c. peak amplitude to the right (GYKI 52466 IC50 26.9 +/- 9.4 microM). Likewise, GYKI 52466 (30-100 microM)) shifted the concentration-response curve for the effects of cyclothiazide on AMPA receptor-mediated e.p.s.c. decay time constants to the right. 8. In conclusion, cyclothiazide and the 2,3-benzodiazepines seem to bind to different sites on AMPA receptors but exert strong allosteric interactions with one another and with other domains such as the agonist recognition site. The interactions of GYKI 52466 and cyclothiazide on AMPA receptor-mediated e.p.s.cs in area CA1 of hippocampal slices provide evidence that the decay time constant of these synaptic events are not governed by desensitization.

New developments in the molecular pharmacology of alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionate and kainate receptors

Separation of non-N-methyl-D-aspartate subtypes of glutamate receptors, known as alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionate (AMPA) and kainate receptors, is traced through conventional pharmacology to molecular biology. The physiology and pharmacology of recombinant receptor subtypes (GluR1-7 and KA1-2) are described. Competitive antagonists, e.g., the quinoxalinedione, 2,3-dihyroxy-6-nitro-7-sulphamoyl-benz(F)quinoxaline, and the decahydroisoquinoline, 3S,4aR,6R, 8aR-6-[2-(1(2)H-tetrazol-5-yl)ethyl]-decahydroisoquinolin e-3-carboxylate, have a broad antagonist spectrum, except that the latter is inactive on GluR6. The 2,3-benzodiazepines noncompetitively antagonise the AMPA receptor GluR1-4. Desensitisation of AMPA (GluR1-4) and kainate (GluR5-7 and KA1-2) receptors is blocked by cyclothiazide and concanavalin A, respectively. Polyamine toxins block AMPA receptors not containing GluR2 and unedited kainate receptors (GluR5-6). These data correlate well with results on native neurons characterised by techniques such as in situ hybridisation.

Effects of decahydroisoquinoline-3-carboxylic acid monohydrate, a novel AMPA receptor antagonist, on glutamate-induced CA2+ responses and neurotoxicity in rat cortical and cerebellar granule neurons

In this study, we examined the effects of a novel water-soluble, putative AMPA receptor antagonist, (-)(3S,4aR,6R,8aR)-6-[2-(1(2)H-tetrazole-5-yl)ethyl]-1,2,3, 4,4a,5,6,7,8,8a-decahydroisoquinoline-3-carboxylic acid monohydrate (LY326325), on glutamate-, N-methyl-D-aspartic acid (NMDA), alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA)-, and kainic acid (KA)-induced elevations of intracellular Ca2+ concentrations ([Ca2+]i) and 45Ca2+ uptake, as well as glutamate agonist-induced neurotoxicity in primary cultures of intact rat cortical and cerebellar granule neurons. In some experiments, the actions of LY326325 were tested in the presence of cyclothiazide, a compound that is known to block glutamate-induced desensitization of AMPA-preferring subtypes of glutamate receptors, thereby largely potentiating the functional effects of AMPA. LY326325 fully blocked the elevations of [Ca2+]i induced by NMDA and non-NMDA glutamate receptor agonists in both cortical and cerebellar granule neurons. The application of increasing concentrations of cyclothiazide was not able to reverse the LY326325-induced blockade of glutamate receptors in cortical neurons. In contrast, the same cyclothiazide treatment fully reversed the blockade produced by the noncompetitive AMPA/KA receptor antagonist 1-(4-aminophenyl)-4-methyl-7,8-methylenedioxy-5H-2, 3-benzodiazepine HCl (GYKI 52466). In 45Ca2+ uptake studies. LY325325 inhibited the NMDA-, AMPA-, and KA-induced enhancement of 45Ca2+ uptake in a concentration-dependent fashion in both cortical and cerebellar granule cells. In analogy to the results obtained with [Ca2+]i recordings, cyclothiazide failed to counteract the LY326325-induced blockade of KA-stimulated 45Ca2+ uptake in cerebellar granule neurons, whereas the blockade induced by the noncompetitive AMPA/KA receptor blocking agent GYKI 52466 was fully reversed by cyclothiazide. Because a similar, although not identical pattern of actions was seen following the application of the competitive AMPA/KA receptor antagonist 6-nitro-7-sulphamoyl-benzo(f)quinoxaline-2-3-dione (NBQX), it is suggested that the inhibitory actions of LY326325 are similar to those produced by NBQX but clearly differ from those caused by the noncompetitive AMPA/KA receptor antagonist GYKI 52466. Finally, when the neuroprotective actions of LY326325 on glutamate agonist-induced neurotoxicity were examined in cerebellar granule neurons, we found that LY326325 almost completely blocked the neurotoxic actions of NMDA, AMPA, and KA, respectively, whereas it produced only a partial blockade of glutamate-induced neurotoxicity. Taken together, our current results suggest that although LY326325 blocked both nonNMDA and NMDA-induced Ca2+ responses, it still displayed a preferential affinity of nonNMDA receptors as compared to NMDA receptors. However, LY326325 appears to be a less selective AMPA/KA receptor antagonist than NBQX and GYKI52466, respectively.

Preferential block of desensitizing AMPA receptor in hippocampal neurons by gamma-D-glutamylaminomethylsulfonic acid

The (RS)-alpha-Amino-3-hydroxy-5-methyl-4-isoxazolopropionic acid (AMPA)/kainate receptor-channel complex mediates fast components of excitatory synaptic currents in the central nervous system. Distinguishing between these components is a difficult pharmacological task. As was recently reported, gamma-D-glutamylaminomethylsulfonic acid (GAMS) may be a selective kainate receptor antagonist. We have tested this possibility in experiments which were carried out on acutely isolated rat hippocampal neurons. It appeared that 1 mM GAMS first blocked 83 +/- 1% of the fast desensitizing 128 microM AMPA-gated current, but only 38 +/- 6% of the non-desensitizing current component and reached, at higher GAMS concentrations, a plateau at about 50% of the control steady state current level. In contrast to the blocking action of GAMS on AMPA-gated currents, 4-fold higher concentrations of GAMS were needed to block currents elicited by 256 microM kainate application. It is suggested that several subunit compositions of the AMPA-gated receptor could coexist on a single hippocampal cell. Furthermore, GAMS has a certain preference for subunit assemblies which could mediate fast desensitizing and, a portion of, the non-desensitizing current component.

Modulation of fast excitatory synaptic transmission by cyclothiazide and GYKI 52466 in the rat hippocampus

The effects of cyclothiazide, a drug which potentiates AMPA receptor-mediated responses and GYKI 52466, a non-competitive AMPA receptor antagonist, were studied on fast glutamatergic transmission in rat hippocampal slices. Cyclothiazide prolonged the decay of AMPA receptor-mediated EPSCs (AMPA-EPSCs) in a concentration-dependent manner. GYKI 52466 reduced the peak amplitude of AMPA-EPSCs and blocked the induction of LTP. When GYKI 52466 was applied in the presence of cyclothiazide it still reduced the peak amplitude of AMPA-EPSCs but was not able to reverse the cyclothiazide induced prolongation of AMPA-EPSC duration. These data suggest that GYKI 52466 and cyclothiazide probably mediate their effects on the AMPA receptor via different binding sites.