Pharmacological characterization of non-NMDA subtypes of glutamate receptor in the neonatal rat hemisected spinal cord in vitro

Combined Glia Inhibition and Opioid Receptor Agonism Afford Highly Potent Analgesics without Tolerance

Commonly prescribed opioid analgesics produce tolerance upon chronic use due in part to induction of hyperalgesia. Given that two reported bivalent ligands (MMG22 and MCC22) produce potent antinociception without tolerance only in inflamed mice, we have investigated the possible cellular and receptor targets of these ligands. The selective microglia inhibitors, minocycline and SB290157, antagonized intrathecal (i.t.) MCC22 antinociception orders of magnitude more potently than MMG22, suggesting that MCC22 selectively targets activated microglia. The astrocyte toxin, l-α-aminoadipic acid antagonized MMG22 antinociception 126-fold without reducing the potency of MCC22, indicating that activated astrocytes are targets of MMG22. MK-801 and Ro25-6981 antagonism of MMG22 antinociception, but not MCC22, is consistent with selective inhibition of activated NMDAR in astrocytes. The antinociception produced by i.t. MMG22 or MCC22 were both antagonized by the selective mu opioid receptor antagonist, β-FNA, implicating interaction of these ligands with MOR in spinal afferent neurons. MCC22 antinociception was potently blocked by kainate or AMPA ion channel antagonists (LY382884; NBQX), in contrast to MMG22. It is concluded that i.t. MMG22 and MCC22 produce exceptional antinociception via potent inhibition of activated spinal glia, thereby leading to desensitization of spinal neurons and enhanced activation of neuronal MOR. Thus, the present study suggests a new approach to treatment of chronic inflammatory pain without tolerance through a single molecular entity that simultaneously inhibits activated glia and stimulates MOR in spinal neurons.

A high-throughput model for investigating neuronal function and synaptic transmission in cultured neuronal networks

Loss of synapses or alteration of synaptic activity is associated with cognitive impairment observed in a number of psychiatric and neurological disorders, such as schizophrenia and Alzheimer’s disease. Therefore successful development of in vitro methods that can investigate synaptic function in a high-throughput format could be highly impactful for neuroscience drug discovery. We present here the development, characterisation and validation of a novel high-throughput in vitro model for assessing neuronal function and synaptic transmission in primary rodent neurons. The novelty of our approach resides in the combination of the electrical field stimulation (EFS) with data acquisition in spatially separated areas of an interconnected neuronal network. We integrated our methodology with state of the art drug discovery instrumentation (FLIPR Tetra) and used selective tool compounds to perform a systematic pharmacological validation of the model. We investigated pharmacological modulators targeting pre- and post-synaptic receptors (AMPA, NMDA, GABA-A, mGluR2/3 receptors and Nav, Cav voltage-gated ion channels) and demonstrated the ability of our model to discriminate and measure synaptic transmission in cultured neuronal networks. Application of the model described here as an unbiased phenotypic screening approach will help with our long term goals of discovering novel therapeutic strategies for treating neurological disorders.

Cell diversity and network dynamics in photosensitive human brain organoids

In vitro models of the developing brain such as three-dimensional brain organoids offer an unprecedented opportunity to study aspects of human brain development and disease. However, the cells generated within organoids and the extent to which they recapitulate the regional complexity, cellular diversity and circuit functionality of the brain remain undefined. Here we analyse gene expression in over 80,000 individual cells isolated from 31 human brain organoids. We find that organoids can generate a broad diversity of cells, which are related to endogenous classes, including cells from the cerebral cortex and the retina. Organoids could be developed over extended periods (more than 9 months), allowing for the establishment of relatively mature features, including the formation of dendritic spines and spontaneously active neuronal networks. Finally, neuronal activity within organoids could be controlled using light stimulation of photosensitive cells, which may offer a way to probe the functionality of human neuronal circuits using physiological sensory stimuli.

Distinct receptors underlie glutamatergic signalling in inspiratory rhythm-generating networks and motor output pathways in neonatal rat

Despite the enormous diversity of glutamate (Glu) receptors and advances in understanding recombinant receptors, native Glu receptors underlying functionally identified inputs in active systems are poorly defined in comparison. In the present study we use UBP-302, which antagonizes GluR5 subunit-containing kainate (KA) receptors at < or = 10 microm, but other KA and AMPA receptors at > or = 100 microm, and rhythmically active in vitro preparations of neonatal rat to explore the contribution of non-NMDA receptor signalling in rhythm-generating and motor output compartments of the inspiratory network. At 10 microm, UBP-302 had no effect on inspiratory burst frequency or amplitude. At 100 microm, burst amplitude recorded from XII, C1 and C4 nerve roots was significantly reduced, but frequency was unaffected. The lack of a frequency effect was confirmed when local application of UBP-302 (100 microm) into the pre-Bötzinger complex (preBötC) did not affect frequency but substance P evoked a 2-fold increase. A UBP-302-sensitive (10 microm), ATPA-evoked frequency increase, however, established that preBötC networks are sensitive to GluR5 activation. Whole-cell recordings demonstrated that XII motoneurons also express functional GluR5-containing KA receptors that do not contribute to inspiratory drive, and confirmed the dose dependence of UBP-302 actions on KA and AMPA receptors. Our data provide the first evidence that the non-NMDA (most probably AMPA) receptors mediating glutamatergic transmission within preBötC inspiratory rhythm-generating networks are pharmacologically distinct from those transmitting drive to inspiratory motoneurons. This differential expression may ultimately be exploited pharmacologically to separately counteract depression of central respiratory rhythmogenesis or manipulate the drive to motoneurons controlling airway and pump musculature.

The effects of AMPA receptor antagonists in models of stroke and neurodegeneration

Alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptor antagonists have been shown to have neuroprotective effects in stroke models and although clinical trials with some agents are still ongoing, published results have not been favourable. We therefore wished to compare the effects of GYKI 52466, GYKI 53405, EGIS-8332 and EGIS-10608, non-competitive AMPA receptor antagonists with homophthalazine chemical structures, in standard animal stroke models with effects in a neurodegenerative model--excitoxicity in newborn mice. All compounds inhibited the S-AMPA-induced spreading depression in the chicken retina, in vitro, and were potent anticonvulsants against maximal electroshock in mice, in vivo. The AMPA receptor antagonists prevented domoate-induced cell death of motoneurons, in vitro, and reduced infarct size in a dose-dependent manner in the permanent middle cerebral artery occlusion model in mice, in vivo. In newborn mice (P5, histopathology at P10), local injection of the AMPA receptor agonist S-bromo-willardiine at day 5 after birth induced cortical damage and white matter damage, which was reduced in a dose-dependent manner by the AMPA receptor antagonists. EGIS 10608 was a very powerful receptor antagonist of white matter damage. In contrast, GYKI 52466 did not antagonize cortical and white matter damage induced by ibotenic acid. These models allow quantification of the effects of AMPA receptor antagonists in vitro and in vivo.

The role of AMPA and metabotropic glutamate receptors on morphine withdrawal in infant rats

Glutamate receptors, especially N-methyl-d-aspartate (NMDA) receptors, are hypothesized to play key roles in opiate tolerance and withdrawal. There is also accumulating evidence that alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptor antagonists and group II metabotropic glutamate receptor (mGluR) agonists attenuate opiate withdrawal. However, most existing data are derived from adult animal models. Glutamate receptor types undergo dramatic developmental changes during early life. Thus, the pharmacological effects on opiate withdrawal of NMDA receptor, AMPA receptor, and mGluR antagonists in the developing organism may not be comparable to those in the adult. Indeed, NMDA receptor antagonists do not block morphine tolerance or withdrawal in the 7-day-old rat, but are partially effective in the 14-day-old, and fully effective in the 21-day-old. Thus, there is a transition period around the second post-natal week for NMDA receptor antagonists to suppress opiate tolerance and withdrawal. A combination of in vivo and in vitro assays was used in the present studies to test the effect of drugs acting on AMPA and group II mGlu receptors on morphine withdrawal in rats at 7, 14, and 21 days of age. These ages represent the critical periods when various glutamate receptor subunits undergo differential change. In contrast to NMDA receptor antagonists' early ineffectiveness in suppressing morphine withdrawal, the AMPA receptor antagonist and the group II mGluR agonists were effective at all ages tested. Thus, for the human infant patient, pharmacotherapies to reduce opiate tolerance and withdrawal should focus on non-NMDA ionotropic and metabotropic receptors.

Specific involvement in neuropathic pain of AMPA receptors and adapter proteins for the GluR2 subunit

Chronic pain states arise from peripheral nerve injury and are inadequately treated with current analgesics. Using intrathecal drug administration in a rat model of neuropathic pain, we demonstrate that AMPA receptors play a role in the central sensitisation that is thought to underpin chronic pain. The GluR2 subunit of the AMPA receptor binds to a number of intracellular adapter proteins including GRIP, PICK1 and NSF, which may link the receptor to proteins with signalling, scaffolding and other roles. We implicate for the first time a possible role for GRIP, PICK1 and NSF in neuropathic sensitisation from experiments with cell-permeable blocking peptides mimicking their GluR2 interaction motifs and also demonstrate differential changes in expression of these proteins following peripheral nerve injury. These studies suggest a critical involvement of protein:protein complexes associated with the AMPA receptor in neuropathic pain, and the possibility that they may have potential as novel therapeutic targets.

The novel antagonist 3-CBW discriminates between kainate receptors expressed on neonatal rat motoneurones and those on dorsal root C-fibres

1. The natural product willardiine is a selective AMPA receptor agonist. We report that an N(3)-substituted analogue of willardiine, (S)-3-(4-carboxybenzyl)willardiine 3-CBW, antagonizes AMPA and kainate receptors expressed on motoneurones and dorsal root C-fibres, respectively. 2. Reduction of the fast component of the dorsal root-evoked ventral root potential (fDR-VRP) has been used as a novel method to compare AMPA receptor antagonists. 3-CBW, NBQX and GYKI53655 depressed the fDR-VRP with IC(50) values of 10.3+/-2.4, 0.214+/-0.043 and 4.03+/-0.31 micro M, respectively. That 3-CBW depressed the fDR-VRP by acting at AMPA and not metabotropic glutamate receptors was demonstrated by the lack of effect of LY341495 (100 micro M). 3. The Schild plot for antagonism of responses to (S)-5-fluorowillardiine on motoneurones by 3-CBW had a slope of 1.11+/-0.13 giving a pA(2) value of 4.48. The Schild plot for antagonism of kainate responses on the dorsal root by 3-CBW had a slope of 1.05+/-0.05 giving a pA(2) value of 4.96. 4. On neonatal rat motoneurones 3-CBW (200 micro M) almost completely abolished responses to AMPA while responses to NMDA, kainate and DHPG were 101.6+/-11.6%, 39.4+/-5.8% and 110.5+/-9.0% of control, respectively. 3-CBW can therefore be used to isolate kainate receptor responses from those mediated by AMPA receptors. 5 3-CBW antagonized kainate-induced responses on dorsal root C-fibres with a pA(2) value of 4.96 whereas kainate receptor mediated responses (isolated by including GYKI53655 in the medium) on motoneurones were not completely blocked by 200 micro M 3-CBW, substantiating evidence that kainate receptors on neonatal rat motoneurones differ from those on dorsal root C-fibres.

Role of spin trapping and P2Y receptor antagonism in the neuroprotective effects of 2,2'-pyridylisatogen tosylate and related compounds

2,2'-Pyridylisatogen tosylate (PIT) is both an allosteric modulator of P2Y receptors, and an immine oxide, acting as a spin trap for free radicals. PIT (10 mg kg(-1), i.p.) was found to be a powerful neuroprotective agent in protecting against the lesions induced by 15 micro g S-bromo-willardiine injected into the cortex or white matter of 5-day-old mice pups. As the multiple effects of PIT may induce both beneficial and deleterious effects, a reanalysis of the structure-activity relationship was undertaken. PIT (50 micro M) and 2,3'-pyridylisatogen were potent antagonists of responses to ATP in the taenia preparation of the guinea-pig caecum, but 2,3'-nitrophenylisatogen was not. The reactive immine oxide group could be substituted by a keto moiety (N-(2'-pyridyl)phthalide) while maintaining antagonism of responses to ATP, equivalent to PIT. Thus, antagonism of P2Y receptors was not restricted to the isatogen nucleus. Other spin traps did not antagonise P2Y receptors, although dimethyl-pyrroline-N-oxide (DMPO) increased the sensitivity of responses to ATP. Both N-(2'-pyridyl)phthalide and 2,3'-nitrophenylisatogen was less neuroprotective than PIT (10 mg kg(-1), i.p.) in protecting against the S-bromo-willardiine-induced lesions in mice, implying that both antagonism of P2Y receptors and the immine oxide moiety may be important for the neuroprotective effects of PIT. However, the usefulness of the neuroprotection was limited because, in motoneurones obtained from rat embryos, PIT (10-100 micro M) exacerbated cell death.

Inhibition of neuronal Ca(2+) influx by gabapentin and pregabalin in the human neocortex

Gabapentin and pregabalin (S-(+)-3-isobutylgaba) produced concentration-dependent inhibitions of the K(+)-induced [Ca(2+)](i) increase in fura-2-loaded human neocortical synaptosomes (IC(50)=17 microM for both compounds; respective maximal inhibitions of 37 and 35%). The weaker enantiomer of pregabalin, R-(-)-3-isobutylgaba, was inactive. These findings were consistent with the potency of these drugs to inhibit [(3)H]-gabapentin binding to human neocortical membranes. The inhibitory effect of gabapentin on the K(+)-induced [Ca(2+)](i) increase was prevented by the P/Q-type voltage-gated Ca(2+) channel blocker omega-agatoxin IVA. The alpha 2 delta-1, alpha 2 delta-2, and alpha 2 delta-3 subunits of voltage-gated Ca(2+) channels, presumed sites of gabapentin and pregabalin action, were detected with immunoblots of human neocortical synaptosomes. The K(+)-evoked release of [(3)H]-noradrenaline from human neocortical slices was inhibited by gabapentin (maximal inhibition of 31%); this effect was prevented by the AMPA receptor antagonist NBQX (2,3-dioxo-6-nitro-1,2,3,4-tetrahydro[f]quinoxaline-7-sulphonamide). Gabapentin and pregabalin may bind to the Ca(2+) channel alpha 2 delta subunit to selectively attenuate depolarization-induced Ca(2+) influx of presynaptic P/Q-type Ca(2+) channels; this results in decreased glutamate/aspartate release from excitatory amino acid nerve terminals leading to a reduced activation of AMPA heteroreceptors on noradrenergic nerve terminals.

Thiopentone and methohexital, but not pentobarbitone, reduce early focal cerebral ischemic injury in rats

PURPOSE

Although barbiturates are considered to be cerebral protectants, little is known regarding the relative efficacy of different barbiturates to reduce ischemic brain injury. In a model of middle cerebral artery occlusion (MCAo), we compared the relative effects of 1.0 and 0.4 burst-suppression doses of thiopentone, methohexital, and pentobarbitone on cerebral infarct.

METHODS

During isoflurane anesthesia, MCAo was achieved via a temporal craniotomy. Thirty minutes before MCAo the rats were randomized to receive one of the following which was maintained throughout the study. Halothane (n=20)-1.2 MAC halothane, thiopentone (n=20), methohexital (n=20), or pentobarbitone (n=20). The first ten animals in each barbiturate group received the respective barbiturate in a dose sufficient to maintain burst-suppression of the electroencephalogram (3-5 bursts x min(-1)). The subsequent ten animals in each barbiturate group received 40% of the burst-suppression dose. After 180 min of MCAo and 120 min of reperfusion, cerebral injury was assessed.

RESULTS

For the burst-suppression animals, injury volume (mm3, mean +/- SD) was less in the thiopentone group (88 +/- 14) than the halothane (133 +/- 17), methohexital (126 +/- 19), or pentobarbitone (130 +/- 17) groups (P <0.05). For 0.4 burst-suppression animals, injury volume was less for the methohexital group (70 +/- 22) than the halothane (124 +/- 24), thiopentone (118 +/- 15), or pentobarbitone (121 +/- 20) groups (P <0.05).

CONCLUSIONS

These data are inconsistent with the longstanding assumption that electrophysiologically comparable doses of the various classes of barbiturates have equivalent protective efficacy. They in turn suggest that mechanisms other than, or at least in addition to, metabolic suppression may contribute to the protective effect of barbiturates.

Possible linkage between glutamate transporter and mitogen-activated protein kinase cascade in cultured rat cortical astrocytes

The mitogen-activated protein kinases (MAPKs) play a pivotal role in the mediation of cellular responses to a variety of signalling molecules. In the present study, we investigated possible linkage between glutamate signalling and the MAPK cascade in cultured rat cortical astrocytes. Exposure of the cells to L-glutamate (100-1000 microM) resulted in an increase in phosphorylated p44/42 MAPK (ERK1/2) in a concentration- and time-dependent manner. The glutamate-induced ERK1/2 phosphorylation was blocked by U0126 and PD98059, specific inhibitors of the MAPK-activating enzyme MEK. Furthermore, L-glutamate-induced ERK1/2 phosphorylation was not mimicked by glutamate receptor agonists and was not blocked by glutamate receptor antagonists. In contrast, the effect of L-glutamate was mimicked by D- and L-aspartate and transportable glutamate uptake inhibitors. These results suggest that the MEK/ERK cascade is activated by a mechanism related to glutamate transporters. We propose that the glutamate transporter functions as a receptor transmitting extracellular glutamate signal to intracellular messengers.

Patterns of spontaneous activity and morphology of interneuron types in organotypic cortex and thalamus-cortex cultures

The physiological and morphological properties of interneurons in infragranular layers of rat visual cortex have been studied in organotypic cortex monocultures and thalamus-cortex co-cultures using intracellular recordings and biocytin injections. Cultures were prepared at the day of birth and maintained for up to 20 weeks. Twenty-nine interneurons of different types were characterized, in addition to 170 pyramidal neurons. The cultures developed a considerable degree of synaptically driven "spontaneous" bioelectric activity without epileptiform activity. Interneurons in cortex monocultures and thalamus-cortex co-cultures had the same physiological and morphological properties, and also pyramidal cell properties were not different in the two culture conditions. All interneurons and the majority of pyramidal cells displayed synaptically driven action potentials. The physiological group of fast-spiking interneurons included large basket cells, columnar basket cells (two cells with an arcade axon) and horizontally bitufted cells. The physiological group of slow-spiking interneurons included Martinotti cells and a "long-axon" cell. Analyses of the temporal patterns of activity revealed that fast-spiking interneurons have higher rates of spontaneous activity than slow-spiking interneurons and pyramidal cells. Furthermore, fast-spiking interneurons fired spontaneous bursts of action potentials in the gamma frequency range. We conclude from these findings that physiological and morphological properties of interneurons in organotypic mono- and co-cultures match those of interneurons characterized in vivo or in acute slice preparations, and they maintain in long-term cultures a well-balanced state of excitation and inhibition. This suggests that cortex-intrinsic or cell-autonomous mechanisms are sufficient for the expression of cell type-specific electrophysiological properties in the absence of afferents or sensory input.

Modulation of kainate-induced responses by pentobarbitone and GYKI-53784 in rat abducens motoneurons in vivo

The modulation of kainate-induced responses by pentobarbitone and the 2,3-benzodiazepine GYKI-53784 (LY303070), a potent non-competitive AMPA antagonist, was studied in vivo using both extracellular recordings of antidromic field potentials and intracellular recordings from abducens motoneurons in ketamine/diazepam-anesthetized rats. In previous studies on pentobarbitone-anesthetized rats [M. Ouardouz, J. Durand, GYKI-52466 antagonizes glutamate responses but not NMDA and kainate responses in rat abducens motoneurons, Neurosci. Lett. 125 (1991) 5-8; M. Ouardouz, J. Durand, Involvement of AMPA receptors in trigeminal postsynaptic potentials recorded in rat abducens motoneurons in vivo, Eur. J. Neurosci. 6 (1994) 1662-1668; A. Ruiz, J. Durand, Blocking the trigeminal EPSPs in rat abducens motoneurons in vivo with the AMPA antagonists, NBQX and GYKI-53655, J. Neurophysiol. (1998) submitted], we showed that 2,3-benzodiazepines do not affect kainate-induced depolarizations in abducens motoneurons. Here, we tested whether pentobarbitone is involved in the pharmacological discrimination by 2,3-benzodiazepines between AMPA- and kainate-induced responses. Kainate-induced depolarizations were reversibly depressed after application of either GYKI-53784 and pentobarbitone. However, kainate-induced depolarizations were not inhibited by GYKI-53784 with pentobarbitone; they were even potentiated sometimes. Using extracellular recordings, we confirmed that in the presence of pentobarbitone, GYKI-53784 counteracts the effects of AMPA but not of kainate on antidromic field potentials in the abducens nucleus. Blockade of kainate-induced responses by GYKI-53784 was reversed with pentobarbitone, which appears relevant to the discrimination between AMPA- and kainate receptor-mediated responses in vivo. In the presence of pentobarbitone, kainate would depolarize motoneurons mainly via kainate receptors since kainate-induced responses were not depressed by 2,3-benzodiazepines. This finding strongly favors the existence of kainate receptors in adult motoneurons but their role is still unknown.

Pharmacological differentiation of kainate receptors on neonatal rat spinal motoneurones and dorsal roots

The objectives of this study, conducted on neonatal rat spinal cord and dorsal roots in vitro, were to characterise the actions of a range of willardiine analogues on GluR5-containing kainate receptors present in dorsal roots, to determine whether GluR5-containing receptors are also present on motoneurones, and to differentiate responses mediated by kainate receptors from those mediated by AMPA receptors on motoneurones. (S)-5-Trifluoromethyl-willardiine, (S)-5-iodowillardiine, (S)-5-iodo-6-azawillardiine and ATPA were found to be potent agonists of kainate receptors on dorsal roots (EC50 values 0.108 +/- 0.002, 0.127 +/- 0.010, 0.685 +/- 0.141 and 1.3 +/- 0.3 microM, respectively) being more potent but of lower efficacy than kainate (EC50 value 14.8 +/- 1.8 microM). (S)-5-Iodo-6-azawillardiine blocked kainate-induced depolarisations of the dorsal root, probably via its desensitising action. Kainate-induced responses of dorsal roots were weakly antagonised by (RS)-3,5-dicarboxyphenylglycine (DCPG) (apparent KD 1.5 +/- 0.4 mM). Kainate receptors containing GluR5 subunits do not appear to be present on motoneurones since (RS)-3,5-DCPG (1 mM) potentiated rather than antagonised kainate-induced depolarisations of motoneurones. Although (S)-5-iodowillardiine (a potent and selective agonist at GluR5-containing kainate receptors) depolarised motoneurones (EC50 value 5.8 +/- 0.6 microM), such depolarisations were antagonised by both (RS)-3,4- and (RS)-3,5-DCPG, which are selective AMPA receptor antagonists at motoneurones, showing a KD value of 73 microM (Schild slope, 0.96 +/- 0.09) and an apparent KD value of 123 +/- 38 microM, respectively. This accords with the previously reported activity of willardiine analogues at AMPA receptors. Since neither (RS)-3,4- nor (RS)-3,5-DCPG antagonised kainate-induced motoneuronal depolarisations but cyclothiazide enhanced and GYK153655 blocked these responses it is possible that a component of the kainate response may be mediated by a population of DCPG-insensitive AMPA receptors on motoneurones. However, it is also possible that a population of kainate receptors other than those containing GluR5 subunits, are responsible for these effects. The new compounds introduced in this study are likely to be useful tools for studying the physiological role of kainate receptors in CNS function.

Synaptic activity, induced rhythmic discharge patterns, and receptor subtypes in enriched primary cultures of embryonic rat motoneurones

Long-term cultures of ventral horn neurones from embryonic rat spinal cord were established, after enrichment using density gradient centrifugation, to give a high proportion of cells (>82%) with motoneurone characteristics. Neurones were grown on spinal cord glial monolayers for 4-83 days and investigated using whole-cell patch clamp. Synaptic activity interrupted by periods of quiescence increased in frequency with culture age and was suppressed by 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX) and strychnine. However, strychnine (10 µM) or bicuculline (10-30 µM) or removal of Mg2+ alone induced patterned rhythmic bursting. Glutamate (3-300 µM), alpha -amino-3-hydroxy-5-methyl-4-isoxazole propionic acid (AMPA, 0.3-30 µM), and kainate (1-300 µM) evoked inward currents, as did N-methyl-D-aspartic acid (NMDA, 100 µM) in the absence of Mg2+ and presence of glycine (3-10 µM). Inward currents carried by Cl- were elicited by glycine (10-300 µM) and GABA (1-300 µM), while adenosine (1-10 µM) and cyclopentyladenosine (10 nM - 1 µM) evoked a K+-dependent hyperpolarization. 5-HT, GABAB, purine A, and metabotropic glutamate receptors modulated synaptic excitation of presumed motoneurones. The results suggest that long-term cultures, containing more than 82% developing motoneurones, are able to generate rhythmic bursting; they respond to many of the neurotransmitters that are likely to be released onto motoneurones developing in vivo.Key words: embryonic rat motoneurones, culture, amino acid receptors, adenosine, spinal cord.

Excitatory amino acid antagonists alleviate convulsive and toxic properties of lindane in mice

Pesticides acting at GABAA receptors may induce convulsions in man and animals, but the mechanisms responsible for their convulsant activity are not fully explained. The following excitatory amino acid antagonists were studied for their protective action in mice intoxicated with chlorinated hydrocarbon insecticide lindane (gamma-hexachlorocyclohexane): the competitive NMDA antagonist: 3-(2-carboxypiperazine-4-yl)propenyl-1- phosphonic acid (D-CPPene, 20 mg/kg), the non-competitive NMDA antagonist: dizocilpine (MK-801, 0.4 mg/kg), the glycine site antagonist of NMDA receptor: 2-phenyl-1,3-propane-diol dicarbamate (felbamate, 400 mg/kg) and the competitive AMPA antagonist: 2,3-dihydroxy-6-nitro-7-sulfamoyl-benzo(F)quinoxaline (NBQX, 100 mg/kg). Systemic administration of an antagonist prior to lindane resulted in a strong anticonvulsant effect. D-CPPene, MK-801 and NBQX produced a marked increase of CD50 values of lindane for clonic convulsions. All the antagonists protected animals against tonic convulsions. Toxicity of lindane was potently reduced, as assessed 2, 24 and 120 hr after administration of the pesticide. Our results demonstrate that excitatory amino acid antagonists reduce convulsant properties and toxicity of lindane, suggesting that excitatory amino acid neurotransmission may be involved in its central action.

Norketamine, the main metabolite of ketamine, is a non-competitive NMDA receptor antagonist in the rat cortex and spinal cord

The enantiomers of the potent non-competitive NMDA receptor antagonist ketamine and its major metabolite, norketamine were evaluated as NMDA receptor antagonists using the rat cortical wedge preparation and the neonatal rat spinal cord preparation, respectively, for electrophysiological studies and [3H](RS)-5-methyl-10,11-dihydro-5H-dibenzo[a,d]cyclohepten-5,10-im ine ([3H]MK801) in homogenate binding experiments. In agreement with earlier studies (S)-ketamine (Ki 0.3 microM) was found to possess a 5 times higher affinity for the NMDA receptor complex than (R)-ketamine (Ki 1.4 microM). (S)-Norketamine (Ki 1.7 microM) had approximately an 8 times higher affinity than (R)-norketamine (Ki 13 microM) in the inhibition of [3H]MK-801 binding. All compounds inhibited responses to NMDA in the rat cortical wedge preparation and the hemisected neonatal rat spinal cord, being approximately four times more potent in the cortex than in the spinal cord except for (R)-norketamine being only twice as potent. In light of the clinically obtained concentrations of norketamine after oral administration of ketamine, these data strongly suggest that (S)-norketamine may contribute significantly to the clinical activity of (S)-ketamine, especially when given orally.

Regulation of glutamate release by presynaptic kainate receptors in the hippocampus

Most reported actions of kainate are mediated by AMPA (alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionate) receptors. Here we report that, unlike AMPA which stimulates, kainate elicits a dose-dependent decrease in L-glutamate release from rat hippocampal synaptosomes and also depresses glutamatergic synaptic transmission. Brief exposure to kainate inhibited Ca(2+)-dependent [3H]L-glutamate release by up to 80%. Inhibition was reversed by kainate antagonists but not by the AMPA-selective non-competitive antagonist 1-(4-aminophenyl)-4-methyl-7,8-methylenedioxy-5H-2,3-benzodiazepine (GYKI 52466). A corresponding reversible kainate-evoked depression of NMDA (N-methyl-D-aspartate) receptor-mediated excitatory postsynaptic currents (e.p.s.cs) was observed when AMPA receptors were blocked by GYKI 52466. The synaptic depression was preceded by a brief period of enhanced release and a small inward current was also observed. The effects of kainate were unaffected by metabotropic glutamate (mGlu), GABAA, GABAB, glycine and adenosine receptor antagonists. These results indicate that glutamate release can be modulated directly by kainate autoreceptors.

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.

Ketobemidone, methadone and pethidine are non-competitive N-methyl-D-aspartate (NMDA) antagonists in the rat cortex and spinal cord

The opiate agonists, ketobemidone, methadone and pethidine, were evaluated as N-methyl-D-aspartate (NMDA) receptor antagonists using the rat cortical wedge preparation and the neonatal rat spinal cord preparation for electrophysiological studies and [3H](RS)-5-methyl-10,11-dihydro-5H-dibenzo[a,d]cyclohepten-5,10-im ine ([3H]MK-801) binding experiments using rat forebrain homogenates. Ketobemidone, methadone and pethidine were inhibitors of [3H]MK-801 binding with Ki values of 26 microM, 0.85 microM and 47 microM, respectively. In the cortex, 1 mM ketobemidone and 1 mM methadone reduced NMDA responses, but not (RS)-2-amino-3-(3-hydroxy-5-methylisoxazol-4-yl) propionic acid (AMPA) or kainate responses in an use-dependent manner, whereas 1 mM pethidine was devoid of antagonist activity. In the spinal cord preparation, the activities of ketobemidone and methadone were weaker than in cortex. In contrast, pethidine was equipotent with ketobemidone in the spinal cord. These results suggest that ketobemidone and methadone may be useful therapeutic agents in conditions where a combined opiate agonist and NMDA antagonist treatment is desired.

Participation of NMDA and non-NMDA excitatory amino acid receptors in the mediation of spinal reflex potentials in rats: an in vivo study

1. The effect of various intravenously administered excitatory amino acid (EAA) antagonists on the dorsal root stimulation-evoked, short latency (up to 10 ms) spinal root reflex potentials of chloralose-urethane anaesthetized C1 spinal rats was studied, in order to gain information on the involvement of non-NMDA (AMPA/kainate; AMPA = alpha-amino-3-hydroxy-5-methyl-isoxazole-4-propionate) and NMDA (N-methyl-D-aspartate) receptors in their mediation. The competitive non-NMDA antagonist, 2,3-dihydroxy-6-nitro-7-sulphamoyl-benzo(F)quinoxaline (NBQX; 1-32 mg kg-1), the non-competitive non-NMDA antagonist, 1-(amino)phenyl-4-methyl-7,8-methylendioxy-5H-2,3-benzodiazepine (GYKI 52466; 0.5-8 mg kg-1), the competitive NMDA antagonist 3-((+/-)-2-carboxypiperazin-4-yl)-propyl-l-phosphonic acid (CPP, 2-8 mg kg-1) and two non-competitive NMDA antagonists: MK-801 (0.5-2 mg kg-1) and ketamine (2-32 mg kg-1) were used as pharmacological tools. 2. Validating the applied pharmacological tools regarding selectivity at the applied doses, their effects were tested on direct (electrical) as well as on synaptic excitability of motoneurones evoked by intraspinal stimulation. Furthermore, their effect was investigated on the responses elicited by microiontophoretic application of EAA agonists (AMPA, kainate and NMDA) into the motoneurone pool, where the extracellular field potential evoked by antidromic stimulation of the ventral root was recorded to detect the effects of EAA agonists. 3. NBQX and GYKI 52466 were able to abolish completely the mono-, di- and polysynaptic ventral root reflexes (MSR, DSR, PSR) and the synaptic excitability of motoneurones, while hardly influencing direct excitability of motoneurones. They markedly attenuated AMPA and kainate responses whilst having little or no effect on NMDA responses. 4. Apparently 'supramaximal' doses of CPP and MK-801 slightly inhibited MSR (by about 10%) moderately reduced DSR and PSR (by about 20-30%) and did not influence excitability of motoneurones. They selectively blocked responses to NMDA. 5. Ketamine dose-dependently inhibited MSR, DSR and PSR. Nevertheless, diminution of none of the responses exceeded 50%. It reduced both direct and synaptic excitability of motoneurones, thus displaying a local anaesthetic-like effect, which may contribute to its reflex inhibitory action. It depressed responses to NMDA whilst having negligible effects on responses to AMPA and kainate. 6. We conclude that non-NMDA receptors play a substantial role in the mediation of MSR, DSR and PSR, while NMDA receptors contribute little to this. Neither MSR nor PSR is mediated exclusively by non-NMDA or NMDA receptors, respectively. 7. The drugs investigated in this study, with the exception of ketamine, proved to be useful tools for elucidation of the involvement of EAA receptors in various processes in vivo Keywords: Glutamate receptors; AMPA; kainate; NMDA; NBQX; GYKI 52466; CPP; MK-801; spinal reflex; spinal cord

Characteristics and localization of high-affinity kainate sites in slide-mounted sections of rat cerebellum

The characteristics and localization of high-affinity, kainic acid (KA)-sensitive glutamate sites have been investigated using a radioreceptor procedure to provide insights into specific high-affinity KA receptors identified in molecular biological studies. Binding sites identified by employing [3H]KA. in the presence of the AMPA-selective antagonist NBQX (1 microM), and slide-mounted, coronal sections of rat cerebellum were of high-affinity (Kd 6 nM) and possessed an unique pharmacological profile. Specific binding was to a single population of sites and fully inhibited by kainoids and glutamate, but essentially insensitive to AMPA and willardiines. Autoradiography revealed that the high-affinity KA sites were localized to the granule cell layer of cerebellum. The KA site resembled both the KA receptor found on spinal motoneurones and the KA-2 type of receptor.

A comparison of intravenous NBQX and GYKI 53655 as AMPA antagonists in the rat spinal cord

1. The effects of intravenous administration of two alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid (AMPA) antagonists were studied on responses of single neurones to iontophoretically applied excitatory amino acids. The tests were performed on spinal neurones in alpha-chloralose anaesthetized, spinalized rats. 2. Both the quinoxaline, NBQX (2-16 mg kg-1) and the 2,3-benzodiazepine, GYKI 53655 (2-8 mg kg-1) dose-dependently decreased responses to AMPA. 3. Both compounds were short acting, with half-recovery times of 15 min for NBQX and 7 min for GYKI 53655. 4. The selectivity for responses to AMPA over those to N-methyl-D-aspartate (NMDA) was significantly poorer for systemic NBQX than for either systemic GYKI 53655 or iontophoretic NBQX, suggesting that systemic NBQX may be converted to a less selective metabolite. 5. GYKI 53655 is therefore likely to be a more valuable tool than NBQX for the study of AMPA receptor-mediated processes in vivo.

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)

The glutamate antagonistic effects of NBQX [2,3-dihydroxy-6-nitro-7-sulfamoyl-benzo(F)quinoxaline] and GYKI 52466 [1-(4-amino-phenyl)-4-methyl-7,8-methyl-endioxyl-5H-2,3-benzodiaze pine] were compared on inward current responses of cultured superior collicular and hippocampal neurones with the whole cell patch clamp technique. Both NBQX (8 microM) and GYKI 52466 (33 microM) selectively reduced responses to AMPA [(S)-alpha-amino-3-hydroxy-5-methyl-4- isoxazolepropionic acid, 50 microM] and kainate (50 microM) whilst having little effect on responses to NMDA (N-methyl-D-aspartate, 100 microM). The effects of the two antagonists on the kinetics of AMPA (50 microM) responses were, however, very different--NBQX dramatically slowed the rise time of responses so that peak currents (IC50 60.4 +/- 4.2 nM) were markedly more effected than desensitized plateau currents (IC50 706 +/- 99 nM) whereas GYKI 52466 antagonized plateau responses (IC50 4.44 +/- 0.21 microM) somewhat more than peak responses (IC50 6.87 +/- 0.46 microM) and had only marginal effects on kinetics. In fact, low concentrations of NBQX (50-250 nM) actually potentiated plateau AMPA responses--an effect likely to be due to a reduction in the degree of AMPA-induced desensitization. Similar effects on response kinetics, were seen with kainate such that the IC50s for NBQX in antagonizing initial and plateau components of current responses to kainate 400 microM were 18.1 +/- 2.9 nM and 298 +/- 27 nM respectively whereas the IC50s for GYKI 52466 against kainate 50 microM were 17.3 +/- 1.8 microM and 15.5 +/- 3.3 microM respectively. These differences are likely to be due to the different modes of action of the two antagonists--NBQX shifted kainate concentration responses curves to the right in a parallel fashion indicative of competitive antagonism whereas the effects of GYKI 52466 were largely noncompetitive. There was, however, some indication for a small allosteric influence of GYKI 52466 on the affinity of the glutamate recognition site of the AMPA/kainate receptor. Estimation of Kbs using the Cheng-Prussoff relationship revealed little difference in the affinity of NBQX in antagonizing plateau responses to AMPA (Kb 23.2 nM) and kainate (Kb 57.1 nM) and indicate that the effects of these two agonists are mediated at a common receptor under the experimental conditions used. Moreover, the differential effects of NBQX on peak and plateau components of AMPA (50 microM) responses was associated with a desensitization-induced, paradoxical increase in the agonist affinity and was probably not due to any change in the affinity of NBQX.(ABSTRACT TRUNCATED AT 400 WORDS)

Allosteric potentiation by diazoxide of AMPA receptor currents and synaptic potentials

Diazoxide (100-560 microM) reversibly increased the amplitude and duration of excitatory post-synaptic field potentials recorded in the dentate gyrus of hippocampal slices following stimulation of the perforant pathway. In rat cortex mRNA-injected Xenopus oocytes diazoxide (1-1000 microM) alone had little effect on membrane current, but rapidly and reversibly increased (up to 5-fold) current responses to (R,S)-alpha-amino-3-hydroxy-5-methyl-4-isoxazole-propionic acid (AMPA, 30 microM), L-glutamate (100 microM), quisqualate (3 microM), kainate (100 microM) and domoate (3 microM), an effect that was neither mimicked by other activators of ATP-sensitive potassium channels nor blocked by glibenclamide. Diazoxide increased current amplitudes for all concentrations of the 'inactivating' ligands, AMPA, L-glutamate and quisqualate but had little effect on their EC50 values. In contrast, diazoxide increased the apparent potency of the 'non-inactivating' ligands, kainate and domoate, but increased the efficacy of saturating concentrations by only 10-20%. Diazoxide did not modify the competitive inhibition of AMPA and kainate currents by 6-nitro-7-sulfamoylbenzo[f]quinoxaline-2,3-dione (NBQX) and thus does not compete for the agonist site as do AMPA and kainate. Similarly, diazoxide neither inhibited the binding of [3H]AMPA or [3H]kainate to rat cortical membranes in competition experiments nor consistently modified the apparent [3H]AMPA affinity (Kd) or receptor density (Bmax) in saturation experiments. These data suggest that diazoxide acts at an allosteric site on the AMPA receptor/channel to potentiate activation in a manner dependent upon the properties of the excitatory agonist.

Use-dependent pentobarbital block of kainate and quisqualate currents

The effects of pentobarbital on whole-cell excitatory amino acid-induced currents were studies in cultured rat cortical neurons. Currents evoked by 40 microM kainate were reversibly inhibited by pentobarbital with an IC50 value of 50 microM. The block of the kainate response by pentobarbital was use dependent, requiring kainate stimulation. In the absence of kainate activation, 10 min perfusions of 100 microM pentobarbital inhibited kainate-induced currents less than 10%. Recovery from pentobarbital block also exhibited use dependence, reversing in 5-10 s with kainate stimulation, while persisting 10 min or more in the absence of agonist. Pentobarbital inhibition of the kainate response was not voltage dependent. Responses evoked by 10 microM quisqualate consisted of a peak current desensitizing to a smaller steady-state current. The co-application of 100 microM pentobarbital reduced the steady-state current by 49 +/- 5%. The peak current before desensitization, however, was inhibited less than 10%. Currents evoked by 25 microM N-methyl-D-aspartate were not significantly inhibited by co-application of 100 microM pentobarbital. The results suggest that the pentobarbital-induced inhibition of kainate responses involves open channel block and that the block of quisqualate currents primarily involve non-desensitizing receptor channels that generate steady-state currents.

Methohexitone antagonises kainate and epileptiform activity in rat neocortical slices

Using a grease-seal technique on cortical slices, methohexitone (10-316 microM) dose dependently and reversibly reduced depolarising responses to kainate more than those to alpha-amino-3-hydroxy-4-isoxazolepropionate (AMPA) and N-methyl-D-aspartate (NMDA). The respective pA2 values were 4.9 +/- 0.07, 3.6 +/- 0.03 and 4.0 +/- 0.05 whereas, for 6-nitro,7-sulphamoylbenz[F]quinoxalinedione (NBQX), they were 5.8 +/- 0.06, 6.7 +/- 0.05 and < 4.0. Methohexitone was also more effective than NBQX in reducing the spontaneous epileptiform activity occurring in these cortical slices. Thus 10 and 20 microM of this short-acting barbiturate reduced afterpotentials and burst frequencies respectively by about 50% whereas NBQX 10 microM only reduced burst frequency by some 15%. The results are discussed with respect to a putative methohexitone- and kainate-sensitive autoreceptor which facilitates presynaptic glutamate release.