2-3-dioxo-6-nitro-7-sulfamoylbenzo(f)quinoxaline and 1-amino-1-3-dicarboxycyclopentane

By monitoring changes in the cytosolic [Ca2+](i) and rates of juvenile hormone (JH) synthesis in response to L-glutamate agonists and antagonists, we identified and characterized glutamate receptor subtypes in corpus allatum (CA) cells of the cockroach, Diploptera punctata. During the first ovarian cycle, corpora allata exhibited a cycle of changes in sensitivity to L-glutamate correlated to cyclic changes in rates of JH synthesis. When exposed to 60 microM L-glutamate in vitro, the active corpora allata of day-4 mated females produced 60% more JH, while inactive corpora allata at other ages showed 10-20% stimulatory response. Pharmacological characterization using various L-glutamate receptor agonists and antagonists indicated that several ionotropic subtypes of L-glutamate receptors were present in the CA. The CA showed an increase in rates of JH synthesis in response to NMDA, kainate, and quisqualate, but not to AMPA in both L-15 medium and minimum incubation medium. In contrast, applications of the metabotropic receptor-specific agonist trans-ACPD failed to elicit a change in the cytosolic [Ca2+](i) and JH production. An elevation of cytosolic calcium concentration, followed by 20-30% rise in JH production, was observed when active CA cells were exposed to 10-40 microM kainate. Kainate had no stimulatory effect on JH synthesis in calcium-free medium. The kainate-induced JH synthesis was blocked by 20 microM CNQX but was not affected by 20 microM NBQX. Kainate-stimulated JH production was not suppressed by MK-801 (a specific blocker of NMDA-receptor channel), nor was NMDA-stimulated JH production affected by CNQX (a specific antagonist of kainate receptor). These data suggest that active CA cells are stimulated to synthesize more JH by a glutamate-induced calcium rise via NMDA-, kainate- and/or quisqualate-sensitive subtypes of ionotropic L-glutamate receptors. The metabotropic-subtype and ionotropic AMPA-subtype L-glutamate receptors are unlikely to be present on active CA cells.

Topics: 6-Cyano-7-nitroquinoxaline-2,3-dione; alpha-Amino-3-hydroxy-5-methyl-4-isoxazolepropionic Acid; Animals; Calcium; Cockroaches; Corpora Allata; Cycloleucine; Cytosol; Excitatory Amino Acid Agonists; Excitatory Amino Acid Antagonists; Glutamates; Juvenile Hormones; Kainic Acid; Quinoxalines; Quisqualic Acid; Receptors, Glutamate; Receptors, Metabotropic Glutamate

The potential toxic effects of the metabotropic glutamate receptor agonist (1S,3R)-1-aminocyclopentane-1,3-dicarboxylic acid (ACPD) and its interactions with the N-methyl-D-aspartate (NMDA) receptor were studied in hippocampal brain slice cultures, using densitometric measurements of the cellular uptake of propidium iodide (PI) to quantify neuronal degeneration. Cultures exposed to ACPD, showed a concentration (2-5 mM) and time (1-4 days) dependent increase in PI uptake in CA1, CA3 and dentate subfields after 24 h and 48 h of exposure, with CA1 pyramidal cells being most sensitive. The neurodegeneration induced by 2 mM ACPD was completely abolished by addition of 10 microM of the NMDA receptor antagonist (5R,10S)-(+)-5-methyl-10,11-dihydro-5H-dibenzo[a,d]cyclohepten-5,10-imine (MK-801), while 20 microM of the 2-amino-3-hydroxy-5-methyl-4-isoxazole propionate (AMPA)/kainic acid receptor antagonist 2,3-dioxo-6-nitro-1,2,3,4-tetrahydrobenzo[f]quinoxaline-7-sulfonamide (NBQX) had no effect. Co-exposing cultures to a subtoxic dose of 300 microM ACPD together with 10 microM NMDA, which at this dose is known to induce a fairly selective degeneration of CA1 pyramidal cells, significantly increased the PI uptake in both CA1 and CA3, compared to cultures exposed to 10 microM NMDA only. Adding the 300 microM ACPD as pretreatment for 30 min followed by a 30 min wash in normal medium before the ACPD/NMDA co-exposure, eliminated the potentiation of NMDA toxicity. The potentiation was also blocked by addition of 10 or 100 microM 2-methyl-6-(phenylethynyl)pyridine (MPEP) (mGluR5 antagonist) during the co-exposure, while a corresponding addition of 10 or 100 microM 7-(hydroxyimino)cyclopropa[b]chromen-1a-carboxylate ethyl ester (CPCCOEt) (mGluR1 antagonist) had no effect. We conclude that, stimulation of metabotropic glutamate receptors with ACPD at concentrations of 2 mM or higher induces a distinct subfield-related and time and concentration dependent pattern of hippocampal degeneration, and that ACPD at subtoxic concentrations modulates NMDA-induced excitotoxicity through the mGluR5 receptor in a time dependent way.

Topics: Animals; Coloring Agents; Cycloleucine; Dizocilpine Maleate; Excitatory Amino Acid Agonists; Excitatory Amino Acid Antagonists; Hippocampus; Immunohistochemistry; In Vitro Techniques; Microtubule-Associated Proteins; N-Methylaspartate; Nerve Tissue Proteins; Neurotoxins; Propidium; Quinoxalines; Rats; Rats, Wistar; Receptors, Metabotropic Glutamate; Receptors, N-Methyl-D-Aspartate; Staining and Labeling

gamma (30-100 Hz) and beta (10-30 Hz) oscillations follow tetanic stimulation in the CA1 region of the rat hippocampal slice. Pyramidal neurons undergo a slow depolarization after the tetanus and generate synchronous action potentials. The slow depolarization was previously attributed to metabotropic glutamate receptor (mGluR) activation. However, we found that this event was mediated by GABA(A) receptors, being blocked by bicuculline (50 microM) and accompanied by a dramatic drop in input resistance. Experiments with NMDA and non-NMDA glutamate receptor antagonists revealed that fast synaptic excitation was not necessary for oscillations. IPSPs were strongly depressed during the oscillations. Instead, synchronization was caused by field effects, as shown by: (1) Action potentials of pyramidal neurons proximal (<200 micrometer) to the stimulation site were often preceded by negative deflections of the intracellular potential that masked a net transmembrane depolarization caused by the population spike. (2) Pyramidal neurons located on the surface of the slice, where field effects are weak, fired repetitively but were not synchronized to the network activity. (3) A moderate decrease (50 mOsm) in artificial CSF (ACSF) osmolality did not affect the slow depolarization but increased oscillation amplitude and duration and recruited previously silent neurons into oscillations. (4) 50 mOsm increase in ACSF osmolality dramatically reduced, or abolished, post-tetanic oscillations. Phasic IPSPs, not detectable in proximal neurons, were present, late in the oscillation, in cells located 200-400 micrometer from the stimulation site and possibly contributed to slowing the rhythm during the gamma to beta transition.

Topics: Action Potentials; Animals; Bicuculline; Cycloleucine; Electric Stimulation; Excitatory Amino Acid Antagonists; gamma-Aminobutyric Acid; Hippocampus; In Vitro Techniques; Male; Models, Neurological; Oscillometry; Pyramidal Cells; Quinoxalines; Rats; Rats, Sprague-Dawley; Reaction Time; Receptors, GABA-A; Synapses; Synaptic Transmission

To better characterize the roles of metabotropic glutamate receptors (mGluRs) in physiological and pathophysiological processes, there is an important need to learn more about the structural features relevant to the design of novel, high-affinity ligands that are family and subtype specific. To date, many of the biological studies that have been conducted in the area of mGluR research have made use of the agonist (1S,3R)-ACPD. This compound has been shown to act as an agonist at both the group I and group II receptors while showing little selectivity among the four subtypes belonging to these two groups. Moreover, (1S,3S)-ACPD, the cis isomer, shows negligible activity at group I receptors and is a good agonist of mGluR2. Since ACPD is itself somewhat flexible, with four distinctive conformations being identified from molecular modeling studies for the trans isomer and five conformations for the cis isomer, we believed that it would be of interest to examine the activity of an ACPD analogue that has been constrained through the introduction of a single carbon atom bridge. Accordingly, we have prepared an aminobicyclo[2.1.1]hexanedicarboxylic acid (ABHxD-I) analogue of ACPD. The synthesis of this compound was accomplished by use of an intramolecular [2 + 2] photocycloaddition reaction, in which four distinct isomers were isolated. Of these four compounds, only a single isomer, ABHxD-I (6a), was found to be a potent agonist of the mGluRs. This compound, which expresses the fully extended glutamate conformation, was found to be more potent than ACPD at all six of the eight mGluR subtypes that were investigated and to be comparable to or more potent than the endogenous ligand, glutamate, for these receptors. Interestingly, despite its fixed conformation, ABHxD-I, like glutamate, shows little subtype selectivity. Through modeling studies of ABHxD-I (6a), ABHD-VI, LY354740, (1S,3R)-ACPD, (1S, 3S)-ACPD, and l-glutamate, we conclude that the aa conformation of l-glutamate is the active conformation for both group I and group II mGluRs. Moreover, the modeling-based comparisons of these ligands suggest that the selectivity exhibited by LY354740 between the group I and group II mGluRs is not a consequence of different conformations of L-glutamate being required for recognition at these mGluRs but rather is related to certain structural elements within certain regions having a very different impact on the group I and group II mGluR activity. The enhanced potency of A

Topics: Animals; Bridged Bicyclo Compounds; Calcium; Cells, Cultured; Cerebellum; CHO Cells; Cricetinae; Cyclic AMP; Cycloleucine; Dicarboxylic Acids; Excitatory Amino Acid Agonists; Glutamic Acid; Inositol Phosphates; Kidney; Ligands; Models, Molecular; Molecular Conformation; Norbornanes; Rats; Receptors, Metabotropic Glutamate; Stereoisomerism

In an in vitro spinal cord slice preparation whole cell electrophysiological recordings of rat superficial dorsal horn neurons responding differentially to glutamate (Glu) and N-methyl-D-aspartate (NMDA) were investigated systematically for the role of kainate (KA) receptors in modulating their activity. In these neurons, coapplication of Glu and NMDA, as well as application of Glu immediately before NMDA, induced long- and short-lasting depressions of NMDA-induced currents as well as depression of NMDA-receptor-mediated excitatory postsynaptic currents. KA applied before NMDA mimicked Glu-induced attenuating effects. Furthermore, the low-affinity KA receptor antagonist 5-nitro-6,7,8,9- tetrahydrobenzo[G]indole-2,3-dione-3-oxime potentiated Glu-induced NMDA-receptor-mediated currents in neurons responding differentially to Glu and NMDA. These results provide evidence for a novel mechanism, which may relate to classical long-term depression, involving low-affinity KA receptors in long-lasting modulation of NMDA-receptor-mediated currents. This implies a physiological role of KA receptors in long-term modulation of sensory transmission in the superficial dorsal horn of rat spinal cord.

Topics: Action Potentials; Animals; Calcium; Cycloleucine; Electrophysiology; Excitatory Amino Acid Agonists; Excitatory Amino Acid Antagonists; Ganglia, Spinal; Glutamic Acid; Indoles; Ion Channel Gating; Kainic Acid; Male; N-Methylaspartate; Neurons, Afferent; Neuroprotective Agents; Oximes; Quinoxalines; Rats; Rats, Sprague-Dawley; Receptors, Kainic Acid; Receptors, N-Methyl-D-Aspartate; Spinal Cord; Spinal Nerve Roots; Synaptic Transmission

The functional relevance of presynaptic glutamate receptors in controlling presynaptic Ca2+ influx and thereby transmitter release is unknown. To test if presynaptic Ca2+ entry in the hippocampus is controlled by glutamate autoreceptors, we created a hippocampal slice preparation for investigation of presynaptic Ca2+ signals with Ca(2+)-sensitive microelectrodes after lesioning of neurons by glucose deprivation or kainate. Stratum radiatum and alveus stimulation-induced postsynaptic field potential components were irreversibly abolished in areas CA1 and CA3 of lesioned slices, whereas stratum radiatum stimulation still evoked afferent volleys. Repetitive stimulation of the stratum radiatum still induced decreases in extracellular Ca2+ concentration. Repetitive stimulation of the alveus no longer induced decreases in extracellular Ca2+ concentration, suggesting complete damage of pyramidal cells. The stratum radiatum stimulation-induced decreases in extracellular Ca2+ concentration in lesioned slices were comparable to those elicited during application of the glutamate antagonists 6-cyano-7-nitroquinoxaline-2,3-dione and L-2-amino-5-phosphonovalerate. In lesioned slices the stimulus-induced presynaptic Ca2+ influx was reversibly reduced by kainate. RS-alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionate (AMPA), N-methyl-D-aspartate and glutamate without effects on afferent volleys. The kainate and N-methyl-D-aspartate effects on presynaptic Ca2+ signals were partly sensitive to 2,3-dihydroxy-6-nitro-7-sulphamoyl-benzo(f)quinoxaline and L-2-amino-5-phosphonovalerate, respectively, while the AMPA effects were not significantly affected by 2,3-dihydroxy-6-nitro-7-sulphamoyl-benzo(f)quinoxaline, suggesting involvement of a novel glutamate receptor subtype. The involvement of a novel glutamate receptor subtype was supported by our findings that ionotropic glutamate receptor agonists also reduce presynaptic Ca2+ influx under conditions of blocked synaptic transmission by 6-cyano-7-nitroquinoxaline-2,3-dione and L-2-amino-5-phosphonovalerate. 1-Aminocyclopentane-trans-1,3-dicarboxylic acid had no significant effect on presynaptic Ca2+ entry. Also, the presynaptic Ca2+ influx was not influenced by the glutamate receptor antagonists 6-cyano-7-nitroquinoxaline-2,3-dione, 2,3-dihydroxy-6-nitro-7-sulphamoyl-benzo(f)quinoxaline and L-2-amino-5-phosphonovalerate when applied alone. Low kainate concentrations (5 microM) reduced presynaptic Ca2+ signals in area CA3 but no

Topics: 6-Cyano-7-nitroquinoxaline-2,3-dione; alpha-Amino-3-hydroxy-5-methyl-4-isoxazolepropionic Acid; Animals; Calcium; Cycloleucine; Electrophysiology; Excitatory Amino Acid Agonists; Excitatory Amino Acid Antagonists; Female; Glucose; Hippocampus; Kainic Acid; Male; N-Methylaspartate; Neuroprotective Agents; Organ Culture Techniques; Quinoxalines; Rats; Rats, Wistar; Receptors, Glutamate; Synaptic Transmission

Although glia are known to be sensitive to exogenously applied neurotransmitter substances, little is known about how glia respond to neuronal activity on the millisecond time scale of individual synaptic events. We have explored the electrophysiological effects of excitatory neuronal signaling on glial cells in rat hippocampal microcultures. Exogenous applications of glutamate analogs to islands of glia revealed a large ionotropic AMPA receptor-mediated current and a smaller current mediated by electrogenic glutamate uptake. Glia demonstrated no evidence of NMDA or metabotropic receptor-mediated currents or membrane conductance changes. Despite the dominant contribution of AMPA receptors to exogenous glutamate applications in glia, AMPA receptor currents contributed only approximately 20% to the response of glia to endogenous glutamate release from solitary excitatory neurons. Electrogenic glutamate uptake contributed strongly to glial responses to neuronal stimulation. In addition, some glia exhibited a residual current is response to neuronal stimulation that was not attributable to calcium-dependent transmitter release or to neuronal potassium efflux. These results help provide a context for understanding the role of glial transporters and receptors in nervous system signaling.

Topics: Animals; Anti-Anxiety Agents; Benzodiazepines; Cells, Cultured; Cycloleucine; Electrophysiology; Excitatory Amino Acid Antagonists; Glutamic Acid; Hippocampus; Microglia; Neurons; Neurotoxins; Quinoxalines; Rats; Rats, Sprague-Dawley; Receptors, AMPA; Receptors, Metabotropic Glutamate; Receptors, N-Methyl-D-Aspartate; Synapses; Tacrine

This study examined the binding of (S)-[3H]AMPA, the radiolabelled active isomer of AMPA, to rat brain synaptic membranes. Under non-chaotropic conditions specific binding of 10 nM (S)-[3H]AMPA represented 33 +/- 2% of the total; this increased to 74 +/- 1% in the presence of 100 mM KSCN. (S)-[3H]AMPA binding was inhibited by non-NMDA receptor agonists and the antagonists NBQX and CNQX, with the following rank order of potency: NBQX > (S)-AMPA > or = quisqualate > CNQX > L-glutamate > domoate > or = kainate > (R)-AMPA. NMDA, and the metabotropic glutamate receptor agonist (1S,3R)-ACPD, up to 100 microM, did not inhibit (S)-[3H]AMPA binding. A number of willardiine analogues all effectively inhibited (S)-[3H]AMPA binding with the rank order of potency: (S)-5-fluorowillardiine > (S)-5-nitrowillardiine > (S)-5-trifluoromethylwillardiine > (S)-5-bromowillardiine approximately (S)-5-chlorowillardiine > (S)-5-cyanowillardiine > (S)-willardiine > (S)-5-iodowillardiine > (S)-6-methylwillardiine > (S)-5-methylwillardiine. This rank order closely reflects data from equilibrium measurements made, under voltage clamp, on cultured hippocampal neurons. In contrast the respective (R)-enantiomers and the racemate mixtures of (R,S)-3, 5 and 6-isowillardiine were relatively inactive. Similar IC50 values and thus rank orders of potency for the willardiines were observed in the presence of 100 mM KSCN.

Topics: 6-Cyano-7-nitroquinoxaline-2,3-dione; Alanine; alpha-Amino-3-hydroxy-5-methyl-4-isoxazolepropionic Acid; Animals; Brain; Cycloleucine; Male; Pyrimidinones; Quinoxalines; Radioligand Assay; Rats; Rats, Wistar; Receptors, AMPA; Stereoisomerism; Synaptic Membranes; Tritium; Uracil

(S)-Bromohomoibotenic acid [(S)-BrHIbo] stereoselectively antagonized glutamate-stimulated phosphoinositide (PI) hydrolysis in baby hamster kidney (BHK) cells expressing mGluR1a in a competitive manner with an IC50 of 250 microM. However, (S)-BrHIbo did not inhibit (1S,3R)-1-aminocyclopentane-1,3-dicarboxylic acid [(1S,3R)-ACPD]-induced PI hydrolysis in rat hippocampal slices (S)- or (R)-BrHIbo did not show any effects on forskolin-stimulated cAMP-formation in BHK cells expressing mGluR2 or mGluR4 but did displace [3H]2-amino-4-phosphonobutyrate ([3H]AP4) binding from rat corticalmembranes with high affinities (IC50 = 1.0 microM and 1.1 microM, respectively). These data suggest that (S)-BrHIbo may interest with multiple PI-coupled glutamate receptors, however, at concentrations that are several fold higher than for displacement of [3H]AP4 binding from rat cortical membranes.

Topics: Aminobutyrates; Animals; Binding, Competitive; Cell Line; Cerebral Cortex; Cricetinae; Cyclic AMP; Cycloleucine; Dose-Response Relationship, Drug; Glutamic Acid; Hippocampus; Hydrolysis; Ibotenic Acid; Kidney; Membranes; Phosphatidylinositols; Quinoxalines; Receptors, AMPA; Receptors, Metabotropic Glutamate; Stereoisomerism

Intracerebroventricular (i.c.v.) infusion in mice of the selective metabotropic excitatory amino acid receptor agonist 1S,3R-1- aminocyclopentane-1,3-dicarboxylate (1S,3R-ACPD) (0.6-575 nmol/min) dose dependently induced face washing and scratching. In contrast, the subtype-specific ionotropic excitatory amino acid receptor agonists N-methyl-D-aspartate (NMDA), kainate and alpha-amino-3-hydroxy-5-methylisoxazole-4-propionate (AMPA) (0.3-3.0 nmol/min) dose dependently induced clonic convulsions. I.c.v. infusion of the non-selective metabotropic receptor agonists ibotenate (6 nmol/min) or quisqualate (30 nmol/min) induced clonic convulsions. However, when ionotropic receptors were blocked with (+)-5-methyl-10,11-dihydro-5H-dibenzo-(a,d)cyclohepten-5,10-imine maleate (MK-801, dizoclipine) (3 nmol/min) or 2,3-dihydroxy-6-nitro-7- sulfamoyl-benzo(f)-quinoxaline (NBQX) (9 nmol/min), respectively, face washing and scratching behavior emerged. Neither MK-801 or NBQX (ED50 value > 100 nmol/min), nor the putative metabotropic receptor antagonist L-amino-3-phosphoro-propionic acid (L-AP3) (> 176 nmol/min); nor the dopamine receptor antagonists SCH 23390 (> 74 nmol/min), metoclopramide (> 89 nmol/min) and haloperidol (> 27 nmol/min) antagonized 1S,3R-ACPD-induced scratching (144 nmol/min). These results suggest that the behavioral consequences of i.c.v. infusion of 1S,3R-ACPD in mice reflect a selective activation of metabotropic receptors that differs from the behavioral changes observed with i.c.v. infusion of ionotropic receptor agonists.

Topics: alpha-Amino-3-hydroxy-5-methyl-4-isoxazolepropionic Acid; Animals; Behavior, Animal; Cycloleucine; Dizocilpine Maleate; Dose-Response Relationship, Drug; Ibotenic Acid; Kainic Acid; Male; Mice; N-Methylaspartate; Quinoxalines; Quisqualic Acid; Receptors, Glutamate; Seizures

1. Characterization of excitatory amino acid-induced accumulation of [3H]-phosphoinositides was carried out in primary cerebrocortical cultures isolated from foetal rats. 2. All of the excitatory amino acid receptor agonists examined caused concentration-dependent enhancement of phosphoinositide (PI) formation. The most potent excitatory amino acid receptor agonists were quisqualate, (1S,3R)-1-aminocyclopentane-1,3-dicarboxylic acid ((1S,3R)-ACPD), ibotenate and glutamate with mean EC50 values of 0.9 +/- 0.4 microM, 15 +/- 5 microM, 15 +/- 3 microM and 41 +/- 8 microM respectively. 3. The selective ionotropic receptor antagonists kynurenic acid (1 mM), 2,3-dihydroxy-6-nitro-7-sulphamoyl-benzo(F)quinoxaline (NBQX, 10 microM) and (+/-)-4-(3-phosphonopropyl)-2 piperazinecarboxylic acid (CPP, 100 microM), failed to block responses to quisqualate, (1S,3R)-ACPD or glutamate. D,L-2-Amino-3-phosphonopropionate (D,L-AP3) did not block 1S,3R-ACPD or quisqualate-induced PI turnover, but had an additive effect with quisqualate or (1S,3R)-ACPD. 4. Exposure of cultures to agonists in the absence of added extracellular calcium reduced the maximal quisqualate response by approximately 45%, revealing a two-component concentration-response curve. Concentration-response curves to ibotenate and glutamate became flattened by omission of extracellular calcium, whereas (1S,3R)-ACPD-stimulated PI turnover was unaffected. 5. Pretreatment of cultures with pertussis toxin markedly inhibited PI responses evoked by (1S,3R)-ACPD. 6. These results suggest that excitatory amino acid-stimulated PI turnover in cerebrocortical cultures is independent of ionotropic receptor activation and is mediated via specific G-protein-linked metabotropic receptors. The partial dependence of the responses to quisqualate, ibotenate and glutamate on the presence of extracellular calcium suggests that the effects of these agonists may be mediated by more than one receptor subtype.

Topics: Animals; Calcium; Cells, Cultured; Cerebral Cortex; Cycloleucine; Female; GTP-Binding Proteins; Kynurenic Acid; Pertussis Toxin; Phosphatidylinositols; Piperazines; Pregnancy; Quinoxalines; Quisqualic Acid; Rats; Receptors, Amino Acid; Receptors, Glutamate; Receptors, N-Methyl-D-Aspartate; Virulence Factors, Bordetella

The electrophysiological actions of the metabotropic glutamate receptor agonist 1S,3R-1-aminocyclopentane-1,3-dicarboxylic acid (ACPD) on Purkinje and granule cells were studied in immature and adult cerebellar slices. ACPD elicited a depolarising response when applied to Purkinje cells (EC50 approximately 20 microM). Granule cells hyperpolarised when exposed to low (3-10 microM) concentrations of ACPD; higher concentrations produced a depolarisation (EC50 approximately 40 microM) that was rapidly curtailed by a hyperpolarisation. The hyperpolarisation was abolished when Ca2+ was removed. In Purkinje cells, the amplitude of the depolarisation was greater in adult slices compared to those in immature slices. The responses were not blocked by ionotropic glutamate receptor antagonists or (L)-2-amino-3-phosphonopropionate (AP3).

Topics: Age Factors; alpha-Amino-3-hydroxy-5-methyl-4-isoxazolepropionic Acid; Animals; Calcium; Cerebellum; Cycloleucine; Dose-Response Relationship, Drug; Electrophysiology; Excitatory Amino Acid Antagonists; Ibotenic Acid; In Vitro Techniques; Purkinje Cells; Quinoxalines; Rats; Receptors, Glutamate