fg-9041 and 2-amino-7-phosphonoheptanoic-acid

In the mammalian retina, excitatory and inhibitory circuitries enable retinal ganglion cells (RGCs) to signal the occurrence of visual features to higher brain areas. This functionality disappears in certain diseases of retinal degeneration because of the progressive loss of photoreceptors. Recent work in a mouse model of retinal degeneration (rd1) found that, although some intraretinal circuitry is preserved and RGCs maintain characteristic physiological properties, they exhibit increased and aberrant rhythmic activity. Here, extracellular recordings were made to assess the degree of aberrant activity in adult rd1 retinas and to investigate the mechanism underlying such behavior. A multi-transistor array with thousands of densely packed sensors allowed for simultaneous recordings of spiking activity in populations of RGCs and of local field potentials (LFPs). The majority of identified RGCs displayed rhythmic (7-10 Hz) but asynchronous activity. The spiking activity correlated with the LFPs, which reflect an average synchronized excitatory input to the RGCs. LFPs initiated from random positions and propagated across the retina. They disappeared when ionotrophic glutamate receptors or electrical synapses were blocked. They persisted in the presence of other pharmacological blockers, including TTX and inhibitory receptor antagonists. Our results suggest that excitation-transmitted laterally through a network of electrically coupled interneurons-leads to large-scale retinal network oscillations, reflected in the rhythmic spiking of most rd1 RGCs. This result may explain forms of photopsias reported by blind patients, while the mechanism involved should be considered in future treatment strategies targeting the disease of retinitis pigmentosa.

Topics: 2-Amino-5-phosphonovalerate; Action Potentials; Age Factors; Animals; Carbenoxolone; Cyclooxygenase Inhibitors; Disease Models, Animal; Evoked Potentials, Visual; Excitatory Amino Acid Antagonists; GABA Antagonists; gamma-Aminobutyric Acid; Gap Junctions; Glutamic Acid; Glycine; In Vitro Techniques; Light; Male; Meclofenamic Acid; Mice; Mice, Inbred C3H; Mice, Inbred C57BL; Mice, Neurologic Mutants; Nerve Net; Neural Inhibition; Periodicity; Pyridazines; Quinoxalines; Retinal Degeneration; Retinal Rod Photoreceptor Cells; Sodium Channel Blockers; Statistics as Topic; Tetrodotoxin

The nucleus cuneiformis (CnF), located just ventrolateral to the periaqueductal gray, is part of the descending pain modulatory system. Neurons in the CnF project to medullary nucleus raphe magnus (NRM), which plays an important role on pain modulation. In this study, we investigated the effect of microinjection of the non-competitive NMDA receptor antagonist MK-801, the competitive NMDA receptor antagonist AP-7, and the kainate/AMPA receptor antagonist DNQX, alone or in combination with morphine into the nucleus cuneiformis on morphine-induced analgesia to understand the role of glutamatergic receptors in the modulating activity of morphine. Antinociception was assessed with the tail-flick test. Morphine (10, 20, 40 microg in 0.5 microl saline) had an antinociceptive effect, increasing tail-flick latency in a dose-dependent manner. Microinjection of MK-801 (10 microg/0.5 microl saline) and AP7 (3 microg/0.5 microl saline) prior to morphine microinjection (10 microg/0.5 microl saline) attenuated the antinociceptive effects of morphine, whereas DNQX (0.5 microg/0.5 microl saline) showed a partial antinociceptive effect and potentiated the analgesic effect of morphine. These results indicated that the NMDA receptor partially potentiates the antinociceptive effect of morphine. Our results suggest that NMDA but not non-NMDA receptors are involved in the antinociception produced by morphine in the CnF. The non-NMDA receptors in this area may have a facilitatory effect on nociceptive transmission. The fact that morphine's effect was potentiated by NMDA receptor suggests that projection neurons within the CnF are under tonic, glutamatergic input and when the influence of this input is blocked, the descending inhibitory system is inactivated.

Topics: 2-Amino-5-phosphonovalerate; Analgesics, Opioid; Animals; Dizocilpine Maleate; Excitatory Amino Acid Antagonists; Male; Microinjections; Morphine; Nociceptors; Pain; Quinoxalines; Rats; Rats, Inbred Strains; Reaction Time; Receptors, AMPA; Receptors, Glutamate; Receptors, Kainic Acid; Receptors, N-Methyl-D-Aspartate; Tegmentum Mesencephali

AMPA (EC50 = 1.0 x 10(-6) M) and NMDA (EC50 = 1.3 x 10(-4) M) trigger 45Ca2+ influx in 13-day chick embryonic retinal explants. This agonist-driven cationic flux is specifically inhibited by typical competitive antagonists, such as 6,7-dinitroquinoxaline-2,3-dione (DNQX) and 2-amino-7-phosphonoheptanoate (AP7), respectively. Guanine nucleotides, with different degrees of phosphorylation, namely 5'-GMP, guanosine 5'-O-(2-thiodiphosphate) (GDPbetaS), guanosine 5'-O-(3-thiotriphosphate) (GTPgammaS) and 5'-guanylyl-imidodiphosphate (GppNHp), are also efficient blockers of 45Ca2+ influx. These results confirm the antagonistic behavior of guanine nucleotides towards ionotropic glutamate receptors and suggest a convenient experimental approach for screening of novel agonists and antagonists.

Topics: 2-Amino-5-phosphonovalerate; alpha-Amino-3-hydroxy-5-methyl-4-isoxazolepropionic Acid; Animals; Biological Transport; Calcium; Calcium Radioisotopes; Chick Embryo; Excitatory Amino Acid Antagonists; Guanine Nucleotides; Guanosine 5'-O-(3-Thiotriphosphate); Guanosine Diphosphate; Guanosine Monophosphate; N-Methylaspartate; Organ Culture Techniques; Quinoxalines; Receptors, AMPA; Receptors, N-Methyl-D-Aspartate; Retina; Thionucleotides

In order to study the functional role of the trans-synaptic neuronal interaction between glutamatergic afferents and mesolimbic dopaminergic neurons in internal reward processes, BALB/c male mice were unilaterally implanted with a guide-cannula, the tip of which was positioned 1.5 mm above the ventral tegmental area (VTA). On each day of the following experimental period, a stainless steel injection cannula was inserted into the VTA in order to study the eventual self-administration behaviour of either the competitive N-methyl-D-aspartate antagonist, D(-)-2-amino-7-phosphonoheptanoic acid (AP-7) or the alpha-amino-3-hydroxy-5-methyl-isoxazole-4-propionic acid antagonist, 6,7-dinitroquinoxaline-2,3-dione (DNQX) (3 ng/50 nL) using a spatial discrimination task in a Y maze. Mice rapidly discriminated between the arm enabling a microinjection of either of these glutamatergic antagonists and the neutral arm of the maze, and a robust self-administration of either of these compounds was observed from the first session of acquisition. These data provide strong evidence that the intra-VTA microinjection of either of these subclasses of glutamatergic antagonist produces an effect which is interpreted centrally by the experimental subjects as being highly rewarding. Once the self-administration response had been fully acquired by the experimental subjects, preinjection of the dopaminergic D2 antagonist, sulpiride (50 mg/kg i.p.), 30 min before the test, produced a rapid extinction of the self-administration response. This latter result demonstrates the dopaminergic D2 receptor dependence of this intra-VTA self-administration of both of these subclasses of glutamatergic antagonist. We conclude that the different glutamatergic afferent neuronal inputs to the VTA globally exert, in vivo, via the mediation of interposed endogenous GABAergic interneurons, a tonic trans-synaptic inhibitory regulation of neuronal activity in the mesolimbic dopaminergic pathway and that this complex neuronal interaction in the VTA plays a significant functional part in the modulation of internal reward processes.

Topics: 2-Amino-5-phosphonovalerate; Animals; Dopamine Antagonists; Excitatory Amino Acid Antagonists; Male; Mice; Mice, Inbred BALB C; Microinjections; Quinoxalines; Receptors, AMPA; Receptors, N-Methyl-D-Aspartate; Reward; Self Administration; Sulpiride

The cholinergic amacrine cells of the rabbit retina can be labeled with [3H]choline and the activity of the cholinergic population monitored by following the release of [3H]acetylcholine. It has been proposed that L-homocysteate may be the main endogenous transmitter released onto cholinergic amacrine cells by bipolar cells. Therefore, we have examined the effects of the isomers of homocysteate on the release of [3H]acetylcholine. In magnesium-free medium, D-homocysteate was slightly more potent than the L-isomer. The addition of magnesium, which blocks responses mediated by NMDA receptors, preferentially reduced but did not eliminate, the response to L-homocysteate. 2-Amino-7-phosphonoheptanoate, a potent NMDA antagonist, preferentially blocked L-homocysteate evoked responses. 6,7-Dinitroquinoxaline-2,3-dione, a potent kainate antagonist, preferentially blocked D-homocysteate-evoked responses. Therefore, in the rabbit retina, L-homocysteate is an NMDA-preferring agonist, whereas D-homocysteate is a kainate-preferring agonist. In addition, we found that L-homocysteate can activate the physiologically activated kainate receptor but only when used in millimolar concentrations and under conditions that minimize NMDA-receptor activation. However, the low potency of L-homocysteate combined with low affinity for the glutamate transporter, lack of immunocytochemical localization in bipolar cells, and low retinal content place serious limitations on the role of L-homocysteate at the bipolar-to-cholinergic amacrine cell synapse.

Topics: 2-Amino-5-phosphonovalerate; Acetylcholine; Amino Acids; Animals; Homocysteine; Magnesium; Neurotransmitter Agents; Quinoxalines; Rabbits; Receptors, Kainic Acid; Receptors, N-Methyl-D-Aspartate; Retina; Stereoisomerism; Stimulation, Chemical

In pentobarbital-anaesthetized rats early somatosensory evoked potentials (SEPs) were recorded from the sensory cortex in response to electrical stimulation of the contralateral forepaw and visual evoked potentials (VEPs) from the primary visual cortex in response to single light flashes. Microapplication of the specific non-NMDA antagonist 6,7-dinitroquinoxaline-2,3-dione (DNQX) into the ventro-basal thalamus (VB) resulted in a pronounced decrease in amplitude and an increase in latency of SEPs, whereas injection of DNQX into the dorsal lateral geniculate nucleus (DGL) induced a pronounced decrease in amplitude and an increase in latency of VEPs. These changes were: (1) dose-dependent (DNQX 0.01-1.0 nmol), (2) receptor-specific, and (3) site-specific. In contrast, the specific NMDA antagonist 2-amino-7-phosphonoheptanoate (AP7; 0.5-5 nmol) did not affect SEPs after microapplication into the BV and less potently reduced the amplitude and increased the latency of VEPs after microapplication into the DGL. The present findings are consistent with the assumption that an excitatory amino acid serves as transmitter at synapses in the rat thalamus mediating the nervous impulses responsible for the generation of SEPs and of VEPs. In addition the results suggest that this transmitter preferentially interacts with non-NMDA receptors.

Topics: 2-Amino-5-phosphonovalerate; Amino Acids; Animals; Electric Stimulation; Evoked Potentials, Somatosensory; Evoked Potentials, Visual; Male; Photic Stimulation; Quinoxalines; Rats; Rats, Wistar; Receptors, Glutamate; Receptors, N-Methyl-D-Aspartate; Thalamus

The present study examined the mechanisms by which GYKI 52466 (1-(amino-phenyl)-4-methyl-7,8-methyldioxy-5H-2,3-benzodiazepine) exerts its muscle relaxant effects. Intrathecal injection of the specific N-methyl-D-aspartate (NMDA) receptor antagonist (-)-2-amino-7-phosphonoheptanoate (AP7, 50-500 nmol) and systemic application of the benzodiazepine diazepam (0.2-5 mg/kg) dose dependently reduced the integrated area of the polysynaptic flexor reflex without affecting the monoxynaptic H-reflex. In contrast, intrathecal administration of the non-NMDA receptor antagonist 6,7-dinitroquinoxaline-2,3-dione (DNQX, 0.1-10 nmol) depressed the H-reflex in a dose-dependent manner without affecting the flexor reflex. The depressant effect of GYKI 52466 on the flexor reflex was reduced by coadministration with flumazenil (5 mg/kg i.p.), an antagonist at the benzodiazepine receptor, whereas coadministration of the non-NMDA receptor agonist alpha-amino-3-hydroxy-5-tertbutyl-4-isoxazole-propionic acid (ATPA, 0.1 pmol) with GYKI 52466 attenuated the reduction of the H-reflex induced by GYKI 52466. The chosen doses of flumazenil and ATPA did not affect spinal reflex transmission when given alone. These data suggest that GYKI 52466 depresses spinal reflex transmission via an action on non-NMDA receptors and on benzodiazepine receptors.

Topics: 2-Amino-5-phosphonovalerate; Amino Acids; Animals; Anti-Anxiety Agents; Anticonvulsants; Benzodiazepines; Depression, Chemical; Diazepam; Dose-Response Relationship, Drug; Electric Stimulation; H-Reflex; Injections, Spinal; Male; Quinoxalines; Rats; Rats, Wistar; Receptors, AMPA; Receptors, GABA-A; Receptors, N-Methyl-D-Aspartate; Reflex; Synaptic Transmission

The role of excitatory amino acid receptors in the rostral ventrolateral medulla (RVLM) in mediating a somatosympathetic pressor response (SPR) was studied. Rats were anesthetized with urethane, bilaterally vagotomized, paralyzed and respirated. Increases in mean arterial pressure were evoked by 10-s trains of electrical stimulation of sciatic nerve afferents before and after bilateral microinjections into the RVLM of the N-methyl-D-aspartic acid (NMDA) receptor antagonist D-2-amino-7-phosphono-heptanoic acid (D-AP7) or the non-NMDA receptor antagonist 6,7-dinitroquinoxaline-2,3-dione (DNQX). DNQX reversed or markedly attenuated the SPR. In contrast, the SPR was not significantly altered by blockade of NMDA receptors in the RVLM with D-AP7. However, prior administration of D-AP7 prevented reversal of the SPR by DNQX, while administration of D-AP7 after DNQX partially restored the SPR. These results indicate that pressor responses evoked by electrical stimulation of sciatic nerve afferents require synaptic activation of non-NMDA receptors in the RVLM. A somatic depressor response, revealed after blockade of non-NMDA receptors within the RVLM, may be mediated by activation of NMDA receptors in this region of the brainstem.

Topics: 2-Amino-5-phosphonovalerate; alpha-Amino-3-hydroxy-5-methyl-4-isoxazolepropionic Acid; Amino Acids; Animals; Blood Pressure; Electric Stimulation; Female; Histocytochemistry; Ibotenic Acid; Kainic Acid; Medulla Oblongata; Neurons, Afferent; Quinoxalines; Rats; Rats, Sprague-Dawley; Receptors, Amino Acid; Receptors, N-Methyl-D-Aspartate; Sciatic Nerve; Sympathetic Nervous System; Vagotomy

The present study examined whether the antimicrobial agent pentamidine exerts an antagonistic action at the N-methyl-D-aspartate (NMDA) receptor as tested on spinal reflexes in rats in vivo. After intrathecal injection both the specific NMDA antagonist (-)-2-amino-7-phosphonoheptanoate and pentamidine dose-dependently reduced the magnitude of the polysynaptic flexor reflex without affecting the monosynaptic H-reflex. In contrast, the non-NMDA antagonist 6,7-dinitroquinoxaline-2,3-dione depressed the H-reflex in a dose-dependent manner without affecting the flexor reflex. The depressant effect of pentamidine on the flexor reflex was prevented by coadministration with NMDA but not with the non-NMDA agonist alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid. These data suggest that pentamidine exerts an antagonistic action at the NMDA receptor in vivo.

Topics: 2-Amino-5-phosphonovalerate; alpha-Amino-3-hydroxy-5-methyl-4-isoxazolepropionic Acid; Amino Acids; Animals; Drug Interactions; Electromyography; H-Reflex; Ibotenic Acid; Injections, Spinal; Male; N-Methylaspartate; Pentamidine; Quinoxalines; Rats; Rats, Wistar; Receptors, N-Methyl-D-Aspartate; Reflex, Stretch; Tibial Nerve

The present study examined whether the antitussive agent dextromethorphan, which is an antagonist at the N-methyl-D-aspartate (NMDA) receptor, depresses spinal reflexes in rats. Injection of both the specific NMDA antagonist (-)-2-amino-7-phosphonoheptanoate and of dextromethorphan dose-dependently reduced the magnitude of the polysynaptic flexor reflex without affecting the monosynaptic H-reflex. In contrast, the non-NMDA antagonist 6,7-dinitroquinoxaline-2,3-dione depressed the H-reflex in a dose-dependent manner without affecting the flexor reflex. The depressant effect of dextromethorphan on the flexor reflex was prevented by co-administration with NMDA but not with the non-NMDA agonist alpha-amino-3-hydroxy-5-terthyl-4-isoxazole-propionic acid. These data suggest that dextromethorphan exerts a muscle relaxant action via the NMDA receptor.

Topics: 2-Amino-5-phosphonovalerate; Amino Acids; Animals; Anticonvulsants; Dextromethorphan; Electric Stimulation; Electromyography; H-Reflex; Injections, Intraperitoneal; Injections, Spinal; Male; Muscle Relaxation; Muscles; N-Methylaspartate; Quinoxalines; Rats; Rats, Wistar; Reflex, Stretch

We investigated the hypothesis that stimulation of metabotropic excitatory amino acid receptors in the ventrolateral medulla evokes cardiovascular responses. Thus, (1S,3R)-1-aminocyclopentane-1,3-dicarboxylic acid [(1S,3R)-ACPD], a selective agonist of metabotropic excitatory amino acid receptors, was microinjected into the rostral or caudal ventrolateral medulla of halothane-anesthetized Sprague-Dawley rats. Microinjections of (1S,3R)-ACPD (100 pmol-1 nmol) into the rostral ventrolateral medulla produced dose-dependent increases in mean arterial pressure (+20 +/- 4 mm Hg by 100 pmol and +35 +/- 2 mm Hg by 1 nmol, p < 0.01 versus artificial cerebrospinal fluid) and integrated splanchnic sympathetic nerve activity (+17 +/- 3% and +46 +/- 4%, respectively, p < 0.01), whereas (1S,3+)-ACPD microinjected into the caudal ventrolateral medulla decreased mean arterial pressure (-28 +/- 2 mm Hg by 100 pmol and -48 +/- 6 mm Hg by 1 nmol, p < 0.01 versus artificial cerebrospinal fluid) and splanchnic sympathetic nerve activity (-24 +/- 4% and -49 +/- 5%, p < 0.01). The blockade of ionotropic excitatory amino acid receptors by the combined injection of 2-amino-7-phosphonoheptanoic acid (200 pmol) and 6,7-dinitroquinoxaline-2,3-dione (200 pmol), which effectively blocked the responses elicited by either N-methyl-D-aspartate (20 pmol) or alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid (5 pmol), failed to affect the responses evoked by either (1S,3R)-ACPD (100 pmol) or L-glutamate (2 nmol) microinjected in the rostral and caudal ventrolateral medulla. These results suggest that metabotropic receptors are present and mediate cardiovascular responses evoked by L-glutamate injections into the rostral and caudal ventrolateral medulla.

Topics: 2-Amino-5-phosphonovalerate; Alanine; Amino Acids; Animals; Blood Pressure; Cycloleucine; Dose-Response Relationship, Drug; Drug Combinations; Male; Medulla Oblongata; Quinoxalines; Rats; Rats, Sprague-Dawley; Receptors, Glutamate; Stimulation, Chemical

1. Excitatory inputs to amacrine cells in the salamander retinal slice preparation were examined using whole-cell patch pipette voltage-clamp techniques. In strychnine (500 nM) and bicuculline (100 microM), two types of amacrine cell were easily distinguished by their light-evoked excitatory responses: transient and sustained. 2. In transient amacrine cells the current-voltage (I-V) relation for the peak light-evoked current was non-linear with a negative slope region between -50 and -70 mV. Responses reversed near +10 mV and were prolonged at more positive holding potentials. 3. In DL-2-amino-phosphonoheptanoate (AP7, 30 microM), a selective N-methyl-D-aspartate (NMDA) receptor antagonist, both the negatively sloped region of the light I-V relation and the prolongation of the response at positive potentials were eliminated. In 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX, 2 microM), a selective non-NMDA receptor antagonist, light-evoked currents at the most hyperpolarized holding potentials were eliminated. At potentials positive to -85 mV the light-evoked currents lacked a fast onset. The light I-V relation in CNQX had a negative slope region between -35 and -80 mV. 4. With synaptic transmission blocked, kainate evoked responses in transient cells with a resultant I-V relation that was nearly linear, whereas glutamate and NMDA elicited responses with non-linear I-V relations. 5. Light-evoked currents in sustained amacrine cells had a nearly linear I-V relation and reversed near +10 mV. AP7 at a concentration of 30 microM did not affect the light-evoked currents in sustained cells, but 2 microM-CNQX eliminated all light-evoked currents in these cells. 6. With synaptic transmission blocked, sustained amacrine cells responded only to glutamate and kainate, not NMDA. The resultant I-V relations were linear. 7. We conclude that the light-evoked responses of transient amacrine cells are mediated by concomitant activation of both non-NMDA and NMDA receptors whereas the responses of sustained amacrine cells are mediated only by non-NMDA receptors. Furthermore, these data provide supportive evidence that the primary light-evoked excitatory neurotransmitter activating amacrine cells is glutamate.

Topics: 2-Amino-5-phosphonovalerate; 6-Cyano-7-nitroquinoxaline-2,3-dione; Amino Acids; Animals; Anticonvulsants; Evoked Potentials, Visual; Photic Stimulation; Quinoxalines; Receptors, N-Methyl-D-Aspartate; Retina; Urodela

Quisqualic acid-mediated excitotoxicity has been attributed essentially to the activation of non-N-methyl-D-aspartate (non-NMDA) receptors. In the present study we demonstrate the possible involvement of both NMDA and non-NMDA receptors in quisqualic acid-induced toxicity in mouse brain slices, in vitro. Incubation of mouse brain sagittal slices with various concentrations of quisqualic acid resulted in significant increase in the leakage of lactate dehydrogenase and potassium from the slices into the medium. Prior incubation of mouse brain slices with NMDA (MK-801 or AP7) or non-NMDA receptor antagonists (GDEE or quinoxalinediones) protected against quisqualic acid-mediated toxicity. Slices prepared from animals pretreated in vivo with MK-801 (5 mg/kg b.wt.) were also resistant to the toxic effects of quisqualic acid, indicating the possible involvement of NMDA receptors in quisqualic acid toxicity.

Topics: 2-Amino-5-phosphonovalerate; 6-Cyano-7-nitroquinoxaline-2,3-dione; Amino Acids; Animals; Brain; Dizocilpine Maleate; Excitatory Amino Acid Antagonists; Glutamates; Glutamic Acid; Ion Channel Gating; L-Lactate Dehydrogenase; Mice; Potassium; Quinoxalines; Quisqualic Acid; Receptors, AMPA; Receptors, Glutamate; Receptors, Kainic Acid; Receptors, N-Methyl-D-Aspartate

The effects of glutamate receptor antagonists on sexual receptivity induced by progesterone and LHRH were examined in ovariectomized, estradiol-primed rats (OVX-EB). Enhancement of lordosis/mounts quotient (L/M) by progesterone (0.5 mg) or LH-RH (150 ng; third ventricle, IVT) in OVX-EB rats was significantly decreased by IVT injection of (+) 2-amino-7-phosphonoheptanoic acid a competitive N-methyl-D-aspartic acid (NMDA) receptor antagonist. On the contrary, there were no changes in L/M quotient after IVT injection of 6,7-dinitroquinoxaline-2,3,dione at two dose levels, a Non-NMDA receptor antagonist. The NMDA antagonist did not modify lordosis behavior in OVX-EB rats. The results indicate that the NMDA type of glutamate receptors appears to mediate progesterone and LHRH facilitatory actions and suggest that glutamatergic synapses may be involved in lordosis-facilitating neural mechanisms.

Topics: 2-Amino-5-phosphonovalerate; Amino Acids; Animals; Anticonvulsants; Estradiol; Gonadotropin-Releasing Hormone; Injections, Intraventricular; Male; Neural Inhibition; Progesterone; Quinoxalines; Rats; Rats, Sprague-Dawley; Receptors, N-Methyl-D-Aspartate; Sexual Behavior, Animal

The aim of this study was to evaluate the involvement of dicarboxylic amino acid neurotransmission in the periodic discharges of respiratory neurones. Respiratory neurones of the ventral and dorsal respiratory groups in the medulla of the cat were subjected to iontophoretic applications of (1) N-methyl-D-aspartate (NMDA) and a blocker of the NMDA subtype of glutamate receptor, D-2-amino-7-phosphonoheptanoic acid (AP7) and (2) an agonist and an antagonist of the non-NMDA subtypes of receptor: quisqualate and 6,7-dinitroquinoxaline-2,3-dione (DNQX), respectively. All five main types of respiratory neurones (all-, early- and late-inspiratory, transitional "off-switch", late expiratory) were excited by NMDA and quisqualate. Both agonists increased the peak firing rate but exerted different effects on the discharge pattern of respiratory neurones, within the respiratory cycle. Quisqualate induced discharges in the "silent" period of the neurone more readily than did NMDA which, in turn had a more pronounced effect during the burst period of the neurone. The effects of quisqualate and NMDA were suppressed by prior application of their selective antagonists, AP7 and DNQX. These antagonists decreased the spontaneous neuronal discharge of all cell types, throughout the entire firing phase, by a maximum of 24-63% with AP7 and by 30-50% with DNQX. The non-selective antagonist, gamma-D-glutamyl-glycine and the selective NMDA antagonists, CPP and MK-801, were also effective. It is concluded that respiratory neurones, of all types, within the medullary respiratory network are subjected to endogenous glutamate-like excitations, which may possibly shape the respiratory train of action potentials through the sequential activation of non-NMDA and NMDA subtypes of receptor.

Topics: 2-Amino-5-phosphonovalerate; Amino Acids; Animals; Cats; Decerebrate State; Iontophoresis; Medulla Oblongata; Membrane Potentials; N-Methylaspartate; Neurons; Quinoxalines; Quisqualic Acid; Receptors, Glutamate; Receptors, N-Methyl-D-Aspartate; Receptors, Neurotransmitter; Respiratory Center

The effects of elevating the potassium concentration of the growth medium of neocortical explants was studied. Under control conditions, 10 mM potassium resulted in ca 20% decrease in the number of surviving neurons. The same potassium concentration, however, was clearly neurotrophic in tetrodotoxin-grown cultures: tetrodotoxin-induced neuronal death was significantly reduced. Both effects could be mimicked by the addition of 10 microM N-methyl-D-aspartate (NMDA); lower concentrations were without effect; higher concentrations were neurotoxic under both control and tetrodotoxin conditions. The neurotoxic, as well as the neurotrophic effects of 10 mM potassium appear to be mediated through depolarization-induced glutamate release since they could be influenced by the application of glutamate receptor antagonists. The addition of the NMDA receptor antagonist D-2-amino-7-phosphonoheptanoate (APH) blocked the trophic effect of 10 mM potassium in tetrodotoxin-grown cultures, resulting in low survival. On the other hand, the addition of the non-NMDA antagonist 6,7-dinitroquinoxaline-2,3-dione (DNQX) resulted in neuronal survival similar to control cultures, indicating that it blocked the toxic effects of glutamate, leaving the trophic effects on the NMDA receptor untouched. Under control (non-TTX) conditions, neither DNQX nor APH showed significant effects on 10 mM potassium-induced cell death, indicating that stimulation of the non-NMDA, as well as the NMDA receptors is neurotoxic. This differential effect of NMDA receptor stimulation on neuronal survival is discussed with respect to the maturational and/or functional state of the neurons in the culture.

Topics: 2-Amino-5-phosphonovalerate; Amino Acids; Animals; Cell Survival; Cerebral Cortex; Culture Techniques; Electrophysiology; Glutamates; N-Methylaspartate; Neurons; Potassium; Quinoxalines; Rats; Rats, Inbred Strains; Receptors, Glutamate; Receptors, Neurotransmitter; Tetrodotoxin