Autoradiographic characterization and localization of quisqualate binding sites in rat brain using the antagonist [3H]6-cyano-7-nitroquinoxaline-2,3-dione: comparison with (R,S)-[3H]alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid binding sites

Effects of risperidone on glutamate receptor subtypes in developing rat brain

Levels of ionotropic glutamate (Glu) N-methyl-d-aspartic acid (NMDA), 2-amino-3-(3-hydroxy-5-methyl-isoxazol-4-yl)propionic acid (AMPA), and kainic acid (KA) receptors in forebrain regions of juvenile rats (age 42 days) were quantified after 3 weeks of treatment with three different doses of risperidone (0.3, 1.0 and 3.0 mg/kg) and compared findings to those in adult rats treated with risperidone (3.0 mg/kg/day) previously. Risperidone (at 0.3 mg/kg/day) did not alter levels of three ionotropic Glu receptors in all brain regions examined. Risperidone (at 1.0 and 3.0 mg/kg/day) significantly decreased NMDA binding in caudate-putamen of juvenile and adult animals. In contrast, the same two doses of risperidone decreased NMDA receptors in nucleus accumbens of juveniles and not adults. Risperidone (at 1.0 and 3.0 mg/kg/day) increased AMPA receptors in medial prefrontal cortex and caudate-putamen of juvenile animals, whereas risperidone (at 3.0 mg/kg) increased AMPA receptors in caudate-putamen and hippocampus of adults. Kainate receptors were not altered by any dose of risperidone in any brain region examined in developing and mature animals. The findings indicate that risperidone exerts dose-dependent effects on Glu receptor subtypes in developing animals, and that Glu receptor responses to repeated administration of risperidone are different in juvenile animals than adults.

Differential effects of two chronic diazepam treatment regimes on withdrawal anxiety and AMPA receptor characteristics

Withdrawal from chronic benzodiazepines is associated with increased anxiety and seizure susceptibility. Neuroadaptive changes in neural activity occur in limbo-cortical structures although changes at the level of the GABA(A) receptor do not provide an adequate explanation for these functional changes. We have employed two diazepam treatment regimes known to produce differing effects on withdrawal aversion in the rat and examined whether withdrawal-induced anxiety was accompanied by changes in AMPA receptor characteristics. Rats were given 28 days treatment with diazepam by the intraperitoneal (i.p.) route (5 mg/kg) and the subcutaneous (s.c.) route (15 mg/kg). Withdrawal anxiety in the elevated plus maze was evident in the group withdrawn from chronic s.c. diazepam (relatively more stable plasma levels) but not from the chronic i.p. group (fluctuating daily plasma levels). In the brains of these rats, withdrawal anxiety was accompanied by increased [3H]Ro48 8587 binding in the hippocampus and thalamus, and decreased GluR1 and GluR2 subunit mRNA expression in the amygdala (GluR1 and GluR2) and cortex (GluR1). The pattern of changes was different in the chronic i.p. group where in contrast to the chronic s.c. group, there was reduced [3H]Ro48 8587 binding in the hippocampus and no alterations in GluR1 and GluR2 subunit expression in the amygdala. While both groups showed reduced GluR1 mRNA subunit expression in the cortex overall, only the agranular insular cortex exhibited marked reductions following chronic i.p. diazepam. Striatal GluR2 mRNA expression was increased in the i.p. group but not the s.c. group. Taken together, these data are consistent with differential neuroadaptive processes in AMPA receptor plasticity being important in withdrawal from chronic benzodiazepines. Moreover, these processes may differ both at a regional and receptor function level according to the behavioral manifestations of withdrawal.

Isoflurane preconditioning decreases glutamate receptor overactivation-induced Purkinje neuronal injury in rat cerebellar slices

A brain slice model was used to test the hypothesis that preconditioning with isoflurane, a commonly used volatile anesthetic in clinical practice, reduces neuronal injury caused by overstimulation of glutamate receptors. Glutamate receptors were stimulated by various concentrations of glutamate for 20 min, N-methyl-d-aspartate (NMDA) for 15 min or alpha-amino-3-hydroxy-5-methyl-4-isoxazol propionic acid (AMPA) for 15 min. Morphology of Purkinje neurons in the cerebellar slices of adult male Sprague-Dawley rats was evaluated 5 h after the agonist stimulation. Glutamate, NMDA and AMPA induced a dose-dependent decrease in the percentage of morphologically normal Purkinje neurons. The concentration to induce the maximal neurotoxic effect was 300 microM for glutamate, 300 microM for NMDA and 30 microM for AMPA. Isoflurane preconditioning (2% isoflurane for 30 min and then a 15-min rest period before the agonist stimulation) significantly reduced the neurotoxicity induced by 300 microM glutamate, 300 microM NMDA or 30 microM AMPA. Isoflurane preconditioning-induced protection against glutamate neurotoxicity was abolished by two protein kinase C (PKC) inhibitors, calphostin C (0.5 microM) and chelerythrine (5 microM), or a nitric oxide synthase (NOS) inhibitor, l-nitro(G)-arginine methyl ester (l-NAME, 1.5 mM), but was not affected by an adenosine A1 receptor inhibitor, 8-cyclopentyl-1,3-dipropylxanthine (DPCPX, 300 nM), or a Gi protein inhibitor, pertussis toxin (PTX, 200 ng/ml). Isoflurane preconditioning-induced protection against NMDA neurotoxicity was also abolished by calphostin C, chelerythrine or l-NAME. Thus, isoflurane preconditioning reduced glutamate receptor overstimulation-induced neuronal injury/death. This neuroprotection may be PKC- and NOS-dependent.

Cerebellar AMPA/KA receptor antagonism by CNQX inhibits vestibuloocular reflex adaptation

Vestibuloocular reflex (VOR) performance and adaptation have been investigated during antagonism of cerebellar AMPA/quisqualate and kainate receptors (AMPA/KA) by 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX). Injection of CNQX into the vestibulo-cerebellum of the goldfish before adaptation significantly inhibited and, at the highest dosage, completely prevented acquisition of adaptive reflex gain increases and decreases during a 3-h training period. Injection of CNQX before initiation of VOR adaptive training did not affect pre-adapted baseline performance of the reflex. Injection of CNQX, 1 to 2 h after the initiation of training did not alter the performance of adaptive gain increases that occurred before the injection. If injection of CNQX occurred at the end of adaptive training, there was an accelerated loss of the previously adapted gain changes during the retention period when the animal remained stationary in the dark. CNQX injection did not produce any permanent or long-term deficits, because goldfish could be retrained 48 h later to produce adaptive VOR gain changes similar to control animals. Thus, this work demonstrates that the AMPA/KA receptors located in the vestibulo-cerebellum of the goldfish are necessary for acquisition of short-term adaptive VOR gain increases and decreases. The deficit in adaptive capability was not the result of a deficit in performance, because CNQX did not inhibit an adaptive change that had already occurred as long as the adapting vestibular and visual stimulation continued. This adaptive performance could possibly be maintained by other glutamatergic (metabotropic) receptors located on the Purkinje cells. The retention of adapted gain increases and decreases after CNQX application was inhibited because AMPA/KA antagonism accelerated VOR gain loss after the completion of training when no vestibular or visual stimulation was present. Because the AMPA/KA receptors are located only in the molecular layer of the goldfish cerebellum, these results are, presumably, the result of AMPA/KA receptor antagonism at synapses located on the Purkinje cell dendrite tree.

alpha-Amino-3-hydroxy-5-methylisoxazole-4-propionate receptor autoradiography in mouse brain after single and repeated withdrawal from diazepam

Withdrawal from chronic treatment with benzodiazepines is associated with increased neuronal excitability leading to anxiety, aversive effects and increased seizure sensitivity. After repeated withdrawal experiences, seizure sensitivity increases while withdrawal-induced anxiety and aversion decrease. We used autoradiographical methods employing [(3)H]Ro48 8587, a selective ligand for glutamatergic alpha-amino-3-hydroxy-5-methylisoxazole-4-propionate (AMPA) receptors, to study withdrawal-induced changes in AMPA receptor binding in areas of the mouse brain postulated to be involved in these responses. Mice were given 21 days treatment with diazepam (15 mg/kg, s.c. in sesame oil) followed by withdrawal (single withdrawal) or three blocks of 7 days treatment interspersed with 3-day periods to allow washout of drug (repeated withdrawal). In keeping with heightened excitability in withdrawal from chronic diazepam treatment, the single withdrawal group showed, 72 h after their final dose of diazepam, increased [(3)H]Ro48 8587 binding in several brain areas associated with emotional responses or seizure activity, including hippocampal subfields, amygdalar and thalamic nuclei and motor cortex. In contrast, the repeated withdrawal group showed no changes in [(3)H]Ro48 8587 binding in any brain area studied. These observations are consistent with up-regulation of AMPA receptor-mediated transmission being important in withdrawal-induced anxiety and aversion but not in increased seizure sensitivity associated with repeated withdrawal. As changes in AMPA receptor subunit expression alter the functionality of the receptor, future studies will address this possibility.

Reduced aggression in AMPA-type glutamate receptor GluR-A subunit-deficient mice

The importance of AMPA-type glutamate receptors has been demonstrated in neuronal plasticity and in adaptation to drugs of abuse. We studied the involvement of AMPA receptors in social interaction and anxiety and found that in several paradigms of agonistic behavior naïve male mice deficient for the GluR-A subunit- containing AMPA receptors are less aggressive than wild-type littermates. GluR-A deficient mice and wild-type littermates exhibited similar basic behavior and reflexes as monitored by observational Irwin's test, but they tended to be less anxious in elevated plus-maze and light-dark tests. Maternal aggression or male-female encounters were not affected which suggests that male hormones are involved in the expression of suppressed aggressiveness. However, testosterone levels and brain monoamines can be excluded and found to be similar between GluR-A deficient and wild-type littermates. The reduced AMPA receptor levels caused by the lack of the GluR-A subunit, and measured by a 30% reduction in hippocampal [3H]-S-AMPA binding, seem to be the reason for suppressed male aggressiveness. When we analyzed mice with reduced number of functional AMPA receptors mediated by the genomic introduced GluR-A(Q582R) channel mutation, we observed again male-specific suppressed aggression, providing additional evidence for GluR-A subunit-containing AMPA receptor involvement in aggression.

Long-term effects of olanzapine, risperidone, and quetiapine on ionotropic glutamate receptor types: implications for antipsychotic drug treatment

Levels of ionotropic glutamate (Glu) N-methyl-d-aspartate (NMDA), alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA), and kainic acid (KA) receptors in rat forebrain regions were compared by quantitative in vitro receptor autoradiography after continuous treatment for 28 days with the atypical antipsychotics olanzapine, risperidone, and quetiapine, or vehicle controls. All three treatments significantly decreased NMDA binding in caudate-putamen (CPu; by 30, 34, and 26%, respectively) but increased AMPA receptor levels in same region (by 22, 30, and 28%). Olanzapine and risperidone, but not quetiapine, also reduced NMDA receptor labeling in hippocampal CA1 (21 and 19%) and CA3 (23 and 22%) regions. KA receptors were unaltered by any treatment in the brain regions examined. These findings suggest that the antipsychotic effects of olanzapine and risperidone may be mediated in part by NMDA receptors in hippocampus, and perhaps AMPA receptors in CPu. The findings also support the hypothesis that down-regulation of NMDA receptors by atypical antipsychotic agents in CPu contributes to their low risk of extra-pyramidal side effects. Inability of olanzapine, risperidone, and quetiapine to alter KA receptors suggests their minimal role in mediating the central nervous system actions of these drugs.

Neuroadaptive processes in GABAergic and glutamatergic systems in benzodiazepine dependence

Knowledge of the neural mechanisms underlying the development of benzodiazepine (BZ) dependence remains incomplete. The gamma-aminobutyric acid (GABA(A)) receptor, being the main locus of BZ action, has been the main focus to date in studies performed to elucidate the neuroadaptive processes underlying BZ tolerance and withdrawal in preclinical studies. Despite this intensive effort, however, no clear consensus has been reached on the exact contribution of neuroadaptive processes at the level of the GABA(A) receptor to the development of BZ tolerance and withdrawal. It is likely that changes at the level of this receptor are inadequate in themselves as an explanation of these neuroadaptive processes and that neuroadaptations in other receptor systems are important in the development of BZ dependence. In particular, it has been hypothesised that as part of compensatory mechanisms to diazepam-induced chronic enhancement of GABAergic inhibition, excitatory mechanisms (including the glutamatergic system) become more sensitive [Behav. Pharmacol. 6 (1995) 425], conceivably contributing to BZ tolerance development and/or expression of withdrawal symptoms on cessation of treatment, including increased anxiety and seizure activity. Glutamate is a key candidate for changes in excitatory transmission mechanisms and BZ dependence, (1) since there are defined neuroanatomical relationships between glutamatergic and GABAergic neurons in the CNS and (2) because of the pivotal role of glutamatergic neurotransmission in mediating many forms of synaptic plasticity in the CNS, such as long-term potentiation and kindling events. Thus, it is highly possible that glutamatergic processes are also involved in the neuroadaptive processes in drug dependence, which can conceivably be considered as a form of synaptic plasticity. This review provides an overview of studies investigating changes in the GABAergic and glutamatergic systems in the brain associated with BZ dependence, with particular attention to the possible differential involvement of N-methyl-D-aspartate and alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid receptors in these processes.

Increased density of hippocampal kainate receptors but normal density of NMDA and AMPA receptors in a rat model of prenatal protein malnutrition

The postnatal development of excitatory amino acid receptor types including kainate, N-methyl-D-aspartate (NMDA), and alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionate (AMPA) was assessed in the hippocampus, entorhinal cortex, and adjacent neocortex in normal and prenatally protein malnourished rats ages 15, 30, 90, and 220 postnatal days by quantitative autoradiography. Tritiated ligands used to measure binding site density were (3)[H]kainate, (3)[H]MK-801, and (3)[H]AMPA, respectively. Kainate receptors showed statistically significant increases in binding density in stratum lucidum of CA3 (hippocampal mossy fiber zone) in 90- and 220-day-old malnourished rats compared with age- and sex-matched controls but not in 15- or 30-day-old malnourished rats. Compared with previous anatomic studies, these results are mostly in agreement with a significantly decreased hippocampal mossy fiber plexus in 15-, 90-, and 220-day-old rats but not in 30-day-old rats. These results suggested that the increased density of postsynaptic kainate receptors located mainly on proximal apical dendrites of CA3 pyramidal cells may be compensatory to decreased glutamate release due to the reduction in mossy fiber plexus. In contrast, the density of putative NMDA and AMPA receptors quantified in prenatally malnourished rats was comparable to the density quantified in age- and sex-matched control rats, as were all three receptor types in entorhinal cortex and adjacent neocortex. Thus, the selectivity of the compensation of (3)[H]kainate-labeled mossy fiber plexus in adult but not in early postnatal developing malnourished rats may help ensure continued breeding and survival of the species under otherwise adverse environmental conditions.

Isoflurane decreases AMPA-induced dark cell degeneration and edematous damage of Purkinje neurons in the rat cerebellar slices

This study was designed to investigate whether isoflurane, a commonly used volatile anesthetic with neuroprotective property, reduces alpha-amino-3-hydroxy-5-methyl-4-isoxazol propionic acid (AMPA)-induced neurotoxicity in a concentration- and time (when isoflurane was applied in relation to the AMPA exposure)-dependent manner. Cerebellar slices from postnatal 10-14-day-old rats were exposed to 30 microM AMPA for 30 min followed by a 120-min AMPA-free recovery period at 37 degrees C. This protocol resulted in dark cell degeneration (DCD) in the majority of Purkinje neurons (60.8+/-6.9%). Fewer Purkinje neurons (31.6+/-5.2%) had edematous damage (ED) characters. Application of isoflurane (1, 2, or 3%) during both the AMPA exposure and recovery periods significantly increased the percentage of morphologically normal Purkinje neurons (neurons without DCD or ED changes) but the effects were apparently not dose-dependent. Isoflurane (3%) applied before, during or after the AMPA exposure period also significantly increased the percentage of morphologically normal Purkinje neurons. These isoflurane-induced increases in the percentage of morphologically normal Purkinje neurons were mainly due to fewer cells with DCD changes. Isoflurane decreased AMPA-induced ED significantly only when isoflurane (1 or 2%) was present during both the AMPA exposure and recovery periods. Isoflurane applied before, during or after the AMPA exposure period did not significantly affect the percentage of cells with ED changes. These results suggest that isoflurane time-dependently but not concentration-dependently reduce AMPA-induced neurotoxicity. These effects may be one mechanism for the isoflurane-induced neuroprotection demonstrated in previous studies.

AMPA Neurotoxicity in Rat Cerebellar and Hippocampal Slices: Histological Evidence for Three Mechanisms

Excitatory amino acid-induced death of central neurons may be mediated by at least two receptor types, the so-called NMDA (N-methyl-d-aspartate) and AMPA (alpha-amino-3-hydroxy-5-methyl-4-isoxazoleproprionate) receptors. We have studied the neurodegenerative mechanisms set in motion by AMPA receptor activation using incubated slices of 8-day-old rat cerebellum and hippocampus. In both preparations, AMPA induced a pattern of degeneration that differed markedly from the one previously shown to be elicited by NMDA. In cerebellar slices, AMPA induced the degeneration of most Purkinje cells together with a population of Golgi cells; in hippocampal slices the neurons were affected in the order CA3 > CA1 > dentate granule cells. Three mechanisms could be discerned: an acute one in which neurons (e.g. cerebellar Golgi cells) underwent a rapid degeneration; a delayed one in which the neurons (Purkinje cells and hippocampal neurons) appeared to be only mildly affected immediately after a 30 min exposure but then underwent a protracted degeneration during the postincubation period (1.5 - 3 h); and finally a slow toxicity, which took place during long (2 h) exposures to AMPA (3 - 30 microM). Although Purkinje cells were vulnerable in both cases, the efficacy of AMPA was higher for the delayed mechanism than for the slow one. The pathology displayed by the acutely destroyed Golgi neurons was a classical oedematous necrosis, whereas most neurons vulnerable to the delayed and slow mechanisms displayed a 'dark cell degeneration', whose cytological features bore a close resemblance to those of neurons irreversibly damaged by ischaemia, hypoglycaemia or status epilepticus in vivo.

Alteration of glutamate receptors in the striatum of dyskinetic 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine-treated monkeys following dopamine agonist treatment

The effects of 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP)-induced nigrostriatal lesion and dopaminomimetic treatment on parameters of glutamatergic activity within the basal ganglia of monkeys were studied in relation with the development of dyskinesias. Drug-naive controls, saline-treated MPTP monkeys, as well as MPTP monkeys treated with either a long-acting D2 agonist (cabergoline) or a D1 agonist (SKF-82958) given by intermittent injections or continuous infusion, were included in this study. 3H-L-glutamate, 3H-alpha-amino-3-hydroxy-5-methylisoxasole-4-propionate (AMPA), 3H-glycine, 3H-CGP39653 (an N-methyl-D-aspartate, NMDA, antagonist selective for NR1/NR2A assembly) and 3H-Ro 25-6981 (an NMDA antagonist selective for NR1/NR2B assembly), specific binding to glutamate receptors, the expression of the NR1 subunit of NMDA receptors and glutamate, glutamine and glycine concentrations were studied by autoradiography, in situ hybridization and high-performance liquid chromatography (HPLC), respectively. Pulsatile SKF-82958 and cabergoline treatment relieved parkinsonian symptoms, whereas animals continuously treated with SKF-82958 remained akinetic. Pulsatile SKF-82958 induced dyskinesias in two of the three animals tested, whereas cabergoline did not. MPTP induced no significant changes of striatal specific binding of the radioligands used, NR1 mRNA expression and amino acid concentrations. In the putamen, pulsatile SKF-82958 treatment was associated with decreased content of glycine and glutamate, whereas only glycine was decreased in cabergoline-treated monkeys. Cabergoline and continuous administration of SKF-82958 led to lower levels of NR1 mRNA in the caudate in comparison to pulsatile SKF-82958 administration. The development of dyskinesias following a D1 agonist treatment was associated with an upregulation of 3H-glutamate [+49%], 3H-AMPA [+38%], 3H-CGP39653 [+ 111%], 3H-glycine [+ 26%, nonsignificant] and 3H-Ro 25-6981 [+ 33%] specific binding in the striatum in comparison to nondyskinetic MPTP monkeys. Our data suggest that supersensitivity to glutamatergic input in the striatum might play a role in the pathogenesis of dopaminomimetic-induced dyskinesias and further support the therapeutic potential of glutamate antagonists in Parkinson's disease.

Comparison of excitotoxic profiles of ATPA, AMPA, KA and NMDA in organotypic hippocampal slice cultures

The excitotoxic profiles of (RS)-2-amino-3-(3-hydroxy-5-tert-butylisoxazol-4-yl)propionic acid (ATPA), (RS)-2-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid (AMPA), kainic acid (KA) and N-methyl-D-aspartate (NMDA) were evaluated using cellular uptake of propidium iodide (PI) as a measure for induced, concentration-dependent neuronal damage in hippocampal slice cultures. ATPA is in low concentrations a new selective agonist of the glutamate receptor subunit GluR5 confined to KA receptors and also in high concentrations an AMPA receptor agonist. The following rank order of estimated EC(50) values was found after 2 days of exposure: AMPA (3.7 mM)>NMDA (11 mM)=KA (13 mM)>ATPA (33 mM). Exposed to 30 microM ATPA, 3 microM AMPA and 10 microM NMDA, CA1 was the most susceptible subfield followed by fascia dentata and CA3. Using 8 microM KA, CA3 was the most susceptible subfield, followed by fascia dentata and CA1. In 100 microM concentrations, all four agonists induced the same, maximal PI uptake in all hippocampal subfields, corresponding to total neuronal degeneration. Using glutamate receptor antagonists, like GYKI 52466, NBQX and MK-801, inhibition data revealed that AMPA excitotoxicity was mediated primarily via AMPA receptors. Similar results were found for a high concentration of ATPA (30 microM). In low GluR5 selective concentrations (0.3-3 microM), ATPA did not induce an increase in PI uptake or a reduction in glutamic acid decarboxylase (GAD) activity of hippocampal interneurons. For KA, the excitotoxicity appeared to be mediated via both KA and AMPA receptors. NMDA receptors were not involved in AMPA-, ATPA- and KA-induced excitotoxicity, nor did NMDA-induced excitotoxicity require activation of AMPA and KA receptors. We conclude that hippocampal slice cultures constitute a feasible test system for evaluation of excitotoxic effects and mechanisms of new (ATPA) and classic (AMPA, KA and NMDA) glutamate receptor agonists. Comparison of concentration-response curves with calculation of EC(50) values for glutamate receptor agonists are possible, as well as comparison of inhibition data for glutamate receptor antagonists. The observation that the slice cultures respond with more in vivo-like patterns of excitotoxicity than primary neuronal cultures, suggests that slice cultures are the best model of choice for a number of glutamate agonist and antagonist studies.

Hypoxia induces an excitotoxic-type of dark cell degeneration in cerebellar Purkinje neurons

In the rat cerebellar slice preparation, exposure to hypoxia elicited by a 30 min exposure to artificial cerebrospinal fluid continuously gassed with 95% N(2): 5% CO(2) induced a characteristic type of toxicity of Purkinje cells (PCs) resembling excitotoxic-mediated dark cell degeneration (DCD). Morphologically, PCs exhibited marked rounded appearance with cytoplasmic darkening, nuclear condensation and cytoplasmic vacuoles. Using gel electrophoresis, genomic DNA obtained from the cerebellar slice exhibited fragmentation. However, PCs failed to exhibit apoptotic bodies or evidence of phagocytosis, spherical- or crescent-shaped chromatin aggregations or TUNEL-positive staining. Ultrastructural analyses of granule cells revealed the presence of apoptotic bodies and discrete spherical collection of chromatin clumping as well as phagocytosis suggesting that the oligonucleosomal-sized DNA fragments primarily were derived from granule cells. PC-elicited toxicity was attenuated significantly in the presence of the competitive AMPA and NMDA antagonists CNQX and APV, respectively. The present study extends the involvement of excitotoxic processes in mediating hypoxic-induced toxicity of PCs in postnatal rats and suggests, in contrast to DCD elicited by direct application of excitotoxic agents, that DCD associated with acute hypoxic insults in PCs does not resemble classical apoptosis.

AMPA receptor binding in adult guinea pig brain stem auditory nuclei after unilateral cochlear ablation

This study determined if an asymmetric hearing loss, due to unilateral cochlear ablation, could induce the regulation of intracellular AMPA receptors in brain stem auditory nuclei. In young adult guinea pigs, the high-affinity specific binding of [(3)H]AMPA was measured in the cochlear nucleus (CN), the superior olivary complex (SOC), and the auditory midbrain at 2-147 postlesion days. After correction for tissue shrinkage, changes in specific binding relative to that in age-matched unlesioned controls were interpreted as altered numbers and/or activity of intracellular AMPA receptors. In the CN, transient elevations and/or deficits in binding were evident in most regions, which usually recovered by 147 days. However, persistently deficient binding was evident ipsilaterally in the anterior part of the anteroventral CN (AVCNa). In the SOC, transient elevations in binding were evident at 2 days in the medial limb of the lateral superior olive (LSOmed) and the medial superior olive. Between 7 and 147 days, most SOC nuclei exhibited transient, temporally synchronized postlesion deficits in binding. However, late in the survival period, deficits persisted ipsilaterally in the LSOmed and the lateral (LSOlat) limb of the lateral superior olive. In the midbrain, transient elevations and/or deficits in binding were evident in the dorsal nucleus of the lateral lemniscus as well as in the central and dorsal nucleus of the inferior colliculus. A persistent deficit was evident in the intermediate nucleus of the lateral lemniscus. The findings implied that auditory neurons contain regulatory mechanisms that control the numbers and/or activity of intracellular AMPA receptors. Regulation was induced by cochlear nerve destruction and probably by changes in the excitation of glutamatergic neurons. Many of the regulatory changes were transient, except in the ipsilateral AVCNa and LSO, where postlesion downregulations were persistent. The downregulation in the ipsilateral AVCNa was probably induced directly by the loss of cochlear nerve endings. However, other regulatory changes may have been induced by signals carried on pathways emerging from the ipsilateral CN and on centrifugal auditory pathways.

Regional and selective effects of oestradiol and progesterone on NMDA and AMPA receptors in the rat brain

We investigated the effect of 10 months ovariectomy and a correction therapy, 2 weeks before the rats were killed, of oestradiol, progesterone or their combination on NMDA and AMPA receptor binding in the hippocampus, dentate gyrus, striatum, nucleus accumbens and frontal cortex of the rat brain as well as on amino acid levels in frontal cortex. NMDA and AMPA binding densities were assayed by autoradiography using, respectively, L-[3H]glutamate and [3H]AMPA; amino acid concentrations were measured by high performance liquid chromatograhy (HPLC) coupled with UV detection. Ovariectomy was without effect on NMDA and AMPA binding density in all brain regions assayed except in the hippocampal CA1 region and dentate gyrus where it decreased NMDA binding density compared to intact rats values. Oestradiol restored and increased NMDA binding density in the CA1 subfield and the dentate gyrus of ovariectomized rats but, by contrast, it decreased binding density in the striatum and in the frontal cortex while having no effect in the CA2/3 subfield of the hippocampus and in the nucleus accumbens. Oestradiol was without effect on AMPA binding density in the hippocampus and the dentate gyrus but it reduced AMPA binding density in the striatum, the frontal cortex and the nucleus accumbens. Progesterone, and oestradiol combined with progesterone, decreased NMDA but not AMPA binding density in the frontal cortex of ovariectomized rats, and they were without effect on these receptors in the other brain regions assayed. Amino acid concentrations in the frontal cortex were unchanged after ovariectomy or steroid treatments. The effect of oestradiol in the hippocampus confirmed in the present study and our novel findings in the frontal cortex, striatum and nucleus accumbens may have functional significance for schizophrenia and neurodegenerative diseases.

[3H]CNQX and NMDA-sensitive [3H]glutamate binding sites and AMPA receptor subunit RNA transcripts in the striatum of normal and weaver mutant mice and effects of ventral mesencephalic grafts

Levels of excitatory amino acid receptors were studied in the weaver mouse model of DA deficiency after unilateral intrastriatal transplantation of E12+/+ mesencephalic cell suspensions. Graft integration was verified by turning behavior tests and from the topographical levels of the DA transporter, tagged autoradiographically with 3 nM [3H]GBR 12935 (average increase in grafted dorsal striatum compared to nongrafted side, 60%). Autoradiography of 80 nM [3H]CNQX and 100 nM NMDA-sensitive [3H]glutamate binding was carried out to visualize the topography of non-NMDA and NMDA receptors, respectively, in +/+ mice and in recipient weaver mutants 3 months after grafting. Increases of 30% or more were found for [3H]CNQX binding in the dorsal nongrafted weaver striatum compared to +/+, and a further 6-9% increase in grafted weaver compared to nongrafted side. The added increase of non-NMDA receptors in the transplanted striatum might be explained by a presence of such receptors on DA presynaptic endings of graft origin. A 20% increase in NMDA-sensitive [3H]glutamate binding was measured in the dorsal nongrafted weaver striatum compared to +/+. NMDA-sensitive [3H]glutamate binding in the transplanted side of weaver mutants tended to be slightly higher in all areas of the striatal complex compared to the nongrafted side, without reaching conventional levels of statistical significance. Using in situ hybridization histochemistry with synthetic 32p labeled oligonucleotide probes, we investigated RNA transcripts encoding the four AMPA receptor subunits. RNA transcripts in the striatum are seen with a decreasing signal intensity in the following order: GluRB > GluRA > GluRC > GluRD. The weaver caudate-putamen shows a 12% increase in GluRA subunit mRNA compared to +/+, whereas mesencephalic neuron transplantation leads to slight increases (3%) in the levels of GluRB mRNA in the nucleus accumbens. The results are placed in the context of the important interaction between the converging glutamatergic corticostriatal and the DAergic nigrostriatal pathways in controlling the functional output of the basal ganglia in Parkinson's disease and in experimental models of DA deficiency.

AMPA receptor-mediated alterations of intracellular calcium homeostasis in rat cerebellar Purkinje cells in vitro: correlates to dark cell degeneration

In the rat cerebellar slice preparation in vitro, excessive DL-amino-3-hydroxy-5-methyl-isoxazole-4-propionic acid (AMPA)-receptor activation elicits a characteristic type of excitotoxicity of Purkinje cells (PCs) known as dark cell degeneration (DCD). DCD models neurotoxicity of PCs and hippocampal pyramidal neurons in vivo following hyperexcitable states. The intent of this study was to: a) determine whether AMPA-induced neurotoxicity of PCs is correlated with temporally and spatially restricted rises in intracellular Ca2+ and b) whether GYKI 52466 and nominal external Ca2+, conditions that reduced expression of AMPA-elicited DCD, altered the induced Ca2+ patterns. Employing the Ca2+-sensitive dye Fluo-3 and a confocal laser scanning microscope, we evaluated changes in intracellular Ca2+ within PCs in a cerebellar slice preparation. AMPA application alone (30 microM for 30 min) caused a significant initial rise in perinuclear and cytoplasmic Ca2+ that returned to control levels during the latter part of the AMPA exposure period. Following removal of AMPA (expression period), perinuclear and cytoplasmic Ca2+ displayed a significant delayed rise peaking transiently 60 min after AMPA removal. The efficacy of GYKI 52466 and nominal external Ca2+ conditions to attenuate AMPA-induced DCD was correlated to reductions in AMPA-induced transient elevations in perinuclear and cytoplasmic Ca2+ levels during the expression phase and to a lesser extent during the exposure period. The present data suggest that during the expression phase, the delayed perinuclear and cytoplasmic Ca2+ transient may be the harbinger of impending loss of Ca2+ homeostasis and cell damage.

Pharmacological characterization, anatomical distribution and sex differences of the non-NMDA excitatory amino acid receptors in the quail brain as identified by CNQX binding

The distribution of non-N-methyl-D-aspartate binding sites was studied in coronal and sagittal sections through the brain of adult Japanese quail by quantitative autoradiography, using tritiated 6-cyano-7-nitroquinoxaline-2,3-dione as a radioligand. Saturation binding experiments were, in addition, carried out in areas showing high levels of binding (cerebellar molecular layer, nucleus anterior medialis and nucleus infundibularis) and demonstrated that the binding of tritiated ligand was specific and saturable. Competition studies with alpha-amino-3-hydroxy-methyl-4-isoxazole propionic acid and kainic acid indicated that kainic acid strongly inhibited ligand binding in all brain areas. alpha-Amino-3-hydroxy-methyl-4-isoxazole propionic acid was only a weak inhibitor in the hypothalamic nuclei whereas in the cerebellar molecular layer both high and low affinity inhibitions were detected. The highest binding levels of tritiated ligand were observed in the molecular layer of the cerebellum. Very high levels of binding were detected in various preoptic/hypothalamic sites including the nucleus suprachiasmaticus pars medialis, nucleus anterior medialis hypothalami, nucleus infundibularis, nucleus mammillaris medialis, nucleus posteromediale hypothalami and nucleus hypothalami ventromedialis. High levels of binding were also detected in the bulbus olfactorius, bed nucleus commissuralis anterior, bed nucleus commissuralis pallii, nucleus accumbens, bed nucleus striae terminalis and nucleus interpeduncularis. In the preoptic area/hypothalamus, high levels of binding were clearly present in all areas that contain gonadotropin releasing hormone cells or fibers. In the pons and mesencephalon, moderate levels of binding were associated with catecholaminergic areas such as the area ventralis tegmentalis (area ventralis of Tsai) and the locus coeruleus. Saturation analysis demonstrated the presence of a higher number of binding sites in females than in males in the cerebellar molecular layer, nucleus infundibularis and nucleus anterior medialis. This latter difference was confirmed in the one point assays that also identified higher levels of specific binding in the nucleus suprachiasmaticus pars medialis of males as compared with females. These anatomical data suggest a possible implication of non-N-methyl-D-aspartate receptors in the synthesis and/or release of both gonadotropin releasing hormone and catecholaminergic neurotransmitters that should now be tested by pharmacological experiments.

3,5-Dihydro-4H-2,3-benzodiazepine-4-thiones: a new class of AMPA receptor antagonists

Synthesis and evaluation of anticonvulsant activity of a series of 2,3-benzodiazepin-4-ones (2) chemically related to 1-(4'-aminophenyl)-4-methyl-7,8-(methylenedioxy)-5H-2,3-benzodiazepine (1, GYKI 52466) have been reported in our recent publications. Compounds 2 manifested marked anticonvulsant properties acting as 2-amino-3-(3-hydroxy-5-methylisoxazol-4-yl)propionic acid (AMPA) receptor antagonists. In an attempt to better define the structure-activity relationships (SAR) and to obtain more potent and selective anticonvulsant agents, 1-aryl-3,5-dihydro-4H-2, 3-benzodiazepine-4-thiones 3 were synthesized from the corresponding isosteres 2. The evaluation is reported of their anticonvulsant effects, both in the audiogenic seizures test with DBA/2 mice and against the maximal electroshock- and pentylenetetrazole-induced seizures in Swiss mice. New derivatives 3 showed higher potency, less toxicity and longer-lasting anticonvulsant action than those of the parent compounds 2 in all tests employed. Analogous to derivatives 2, new compounds 3 do not affect the benzodiazepine receptor (BZR) while they do antagonize AMPA-induced seizures; their anticonvulsant activity is reversed by pretreatment with aniracetam but not with flumazenil, thus suggesting a clear involvement of AMPA receptors. Electrophysiological data indicate a noncompetitive blocking mechanism at the AMPA receptor sites for 3i, the most active of the series and over 5-fold more potent than 1.

Gabapentin potentiates the antiseizure activity of certain anticonvulsants in DBA/2 mice

Gabapentin (1-50 mg/kg, intraperitoneally (i.p.)) was able to antagonize audiogenic seizures in Dilute Brown Agouti DBA2J (DBA/2) mice in a dose-dependent manner. Gabapentin at dose of 2.5 mg/kg i.p., which per se did not significantly affect the occurrence of audiogenic seizures in DBA/2 mice, potentiated the anticonvulsant activity of carbamazepine, diazepam, felbamate, lamotrigine, phenytoin, phenobarbital and valproate against sound-induced seizures in DBA/2 mice. The potentiation induced by gabapentin was greatest for diazepam, phenobarbital and valproate, less for felbamate and phenytoin and least for carbamazepine and lamotrigine. The increase in anticonvulsant activity was associated with a comparable increase in motor impairment. However, the therapeutic index of combined treatment of the above drugs + gabapentin was more favourable than that of the same drugs + saline. Since gabapentin did not significantly influence the total and free plasma levels of the anticonvulsant drugs studied, we suggest that pharmacokinetic interactions, in terms of total or free plasma levels, are not probable. However, the possibility that gabapentin can modify the clearance from the brain of the anticonvulsant drugs studied can not be excluded. In addition, gabapentin did not significantly affect the hypothermic effects of the anticonvulsants tested. In conclusion, gabapentin showed an additive effect when administered in combination with certain classical anticonvulsants, most notably diazepam, phenobarbital, felbamate, phenytoin and valproate.

Molecular biology of glutamate receptors in the central nervous system and their role in excitotoxicity, oxidative stress and aging

Forty years of research into the function of L-glutamic acid as a neurotransmitter in the vertebrate central nervous system (CNS) have uncovered a tremendous complexity in the actions of this excitatory neurotransmitter and an equally great complexity in the molecular structures of the receptors activated by L-glutamate. L-Glutamate is the most widespread excitatory transmitter system in the vertebrate CNS and in addition to its actions as a synaptic transmitter it produces long-lasting changes in neuronal excitability, synaptic structure and function, neuronal migration during development, and neuronal viability. These effects are produced through the activation of two general classes of receptors, those that form ion channels or "ionotropic" and those that are linked to G-proteins or "metabotropic". The pharmacological and physiological characterization of these various forms over the past two decades has led to the definition of three forms of ionotropic receptors, the kainate (KA), AMPA, and NMDA receptors, and three groups of metabotropic receptors. Twenty-seven genes are now identified for specific subunits of these receptors and another five proteins are likely to function as receptor subunits or receptor associated proteins. The regulation of expression of these protein subunits, their localization in neuronal and glial membranes, and their role in determining the physiological properties of glutamate receptors is a fertile field of current investigations into the cell and molecular biology of these receptors. Both ionotropic and metabotropic receptors are linked to multiple intracellular messengers, such as Ca2+, cyclic AMP, reactive oxygen species, and initiate multiple signaling cascades that determine neuronal growth, differentiation and survival. These cascades of complex molecular events are presented in this review.

Antipsychotic drug regulation of AMPA receptor affinity states and GluR1, GluR2 splice variant expression

We recently reported that chronic administration of antipsychotic drugs dramatically elevated [3H]AMPA binding, with minimal elevation of [3H]CNQX binding in rat brain. The aim of the current study was to examine the mechanism of this effect. Chronic haloperidol minimally increased the total number of binding sites (total Bmax) compared to saline-injected animals. Specifically, haloperidol dramatically increased the proportion of high-affinity-site AMPA receptors (approximately 30% increase) without inducing a significant change in the low-affinity constant. In situ hybridization for flip and flop isoforms of GluR1 and GluR2 (AMPA receptors) was not altered in a pattern or degree that compared to the changes seen in AMPA receptor binding. These findings suggest that the long-term action of antipsychotic drugs may be to regulate AMPA receptor responsiveness to agonist stimulation via posttranscriptional means, and is unlikely to be related to GluR1 or GluR2 splice variant expression. This effect may have relevance to both the therapeutic effects and side effects of antipsychotic drugs in humans.

The regulation of AMPA receptor-binding sites

A wide variety of mechanisms have been identified that can regulate the alpha-amino-3-hydroxy-5-methylisoxazole-4-propionate (AMPA)-receptor complex. Modulation has been shown to occur at the nucleic acid level via RNA editing and alternative splicing. At the posttranslational level, processes such as phosphorylation, glycosylation, chemical modification of reactive groups on the receptor proteins, interaction with a putative receptor-associated modulatory protein, and changes in the lipid environment have been reported to regulate receptor binding and function. In this review, we discuss general aspects of the cell biology, pharmacology, and function of AMPA receptors. In particular, we focus on some factors shown to modulate agonist binding and discuss possible molecular mechanisms underlying the regulation observed.

Effects of monocular enucleation on receptor binding and innervation pattern of the noradrenergic system in the superior colliculus of the pigmented rat

To examine visual activity-dependent regulation of the central noradrenergic (NAergic) system, we carried out in vitro autoradiography for alpha2-, beta-adrenergic and NMDA, non-NMDA glutamatergic receptors together with immunohistochemical analysis for dopamine beta-hydroxylase in the major central visual structures, including the lateral geniculate nucleus, superior colliculus (SC), and visual cortex of pigmented rats, that had received monocular enucleation. Beta-adrenergic receptor (beta-AR) binding in the SC contralateral to the enucleated eye was significantly decreased (82%, P < 0.01) following monocular deprivation started at P12 and continued for 2 or 11 weeks. No significant change in beta-AR binding was found in other structures examined. The number of varicosities in NAergic fibers was significantly increased following longer enucleation, i.e. for 11 weeks, in the contralateral SC (197%, P <0.001), whereas that in the ipsilateral SC was reduced (75%, P <0.001). Changes in alpha2-adrenergic, NMDA, and non-NMDA glutamatergic receptor binding were small in these animals. Changes in neither beta-AR binding nor innervation pattern of NAergic fibers were found in one-year-old rats that had received a comparable period of monocular enucleation. Furthermore, neither unilateral ablation of the visual cortex to reduce a different set of major afferents nor neonatal enucleation, which induced anatomical reorganization of the afferents, was found to be effective. These findings suggest that beta-AR binding and innervation pattern of NAergic fibers in the SC are modified only when massive imbalance of retinal afferent activity is imposed during a limited period in early postnatal life (i.e. the sensitive period).

Development of MK-801, kainate, AMPA, and muscimol binding sites and the effect of dark rearing in rat visual cortex

We used quantitative autoradiography to determine whether the development of glutamate receptors correlates with the plastic period for monocular deprivation in rat visual cortex. To study glutamate receptors, we incubated sections of rat visual cortex with tritiated (+)-5-methyl-10,11-dihydro-5H-dibenzo[a,d]-cyclohepten-5,10imin e maleate (MK-801), tritiated kainate, and tritiated amino-3-hydroxy-5-methyl-isoxazole-4-propionic acid (AMPA). [3H]MK-801 is a noncompetitive ligand for the N-methyl-D-aspartate (NMDA) receptor. [3H]kainate and [3H]AMPA are competitive ligands for non-NMDA receptors. To compare glutamate binding sites with a nonglutamate binding site, we studied [3H]muscimol, which binds to gamma-aminobutyric acid (GABA)A receptors. [3H]MK-801 binding was maximal at postnatal day 26 (P26) and decreased in adulthood. [3H]AMPA binding was maximal at P18. [3H]kainate binding and [3H]muscimol binding were not age dependent. Dark rearing partially prevented the age-dependent decrease in [3H]MK-801 binding but had no effect on [3H]kainate or [3H]AMPA binding. Dark rearing decreased muscimol binding in adult animals. These results suggest that NMDA receptors, but not other glutamate receptors or GABAA receptors, are likely to be critical for developmental plasticity in rat visual cortex.

Changes in glutamate receptors, c-fos mRNA expression and activator protein-1 (AP-1) DNA binding activity in the brain of phenobarbital-dependent and -withdrawn rats

We studied changes in glutamate receptors, expression of immediate early genes, and AP-1 DNA binding activity in the brains of phenobarbital (PB)-dependent and -withdrawn rats to investigate the possible involvement of activation of glutamate receptors in PB withdrawal syndrome. PB-dependent rats were prepared by feeding drug-admixed food for 5 weeks. Autoradiographic analysis showed that binding of [3H(+)-5-methyl-10,11-dihydro-5H-dibenzo[a,d]cyclohepten-5,10-imin e (MK-801), an antagonist of N-methyl-D-aspartic acid (NMDA) receptors, increased significantly in the cerebral cortices of PB-dependent and 24-h-withdrawn rats. However, [3H]MK-801 binding in the hippocampus and [3H]6-cyano-7-nitroquinoxaline-2,3-dione (CNQX) and [3H]kainic acid binding in the hippocampus and cerebral cortex were essentially unchanged in both groups. PB withdrawal seizures were followed by increased expression of c-fos mRNA in the hippocampus and cerebral cortex and of c-jun mRNA in the cerebral cortex. The induction of c-fos and c-jun mRNA was suppressed by administration of MK-801. Furthermore, PB withdrawal enhanced AP-1 DNA binding activity in the brain. The present findings suggest functional enhancement of glutamatergic neurotransmission during the development of PB withdrawal syndrome.

Cerebellar Grafts Partially Reverse Amino Acid Receptor Changes Observed in the Cerebellum of Mice with Hereditary Ataxia: Quantitative Autoradiographic Studies

We used quantitative autoradiography of [3H]CNQX (200 nM), [3H]muscimol (13 nM), and [3H]flunitrazepam (10 nM) binding to study the distribution of non-NMDA and GABAA receptors in the cerebellum of pcd mutant mice with unilateral cerebellar grafts. Nonspecific binding was determined by incubation with 1 mM Glu, 200 μM GABA, or 1 μM clonazepam, respectively. Saturation parameters were defined in wild-type and mutant cerebella. In mutants, non-NMDA receptors were reduced by 38% in the molecular layer and by 47% in the granule cell layer. The reduction of non-NMDA receptors in the pcd cerebellar cortex supports their localization on Purkinje cells. [3H] CNQX binding sites were visualized at higher density in grafts that had migrated to the cerebellar cortex of the hosts (4.1 and 11.0 pmol/mg protein, respectively, at 23 and 37 days after grafting) than in grafts arrested intraparen-chymally (2.6 and 6.2 pmol/mg protein, respectively, at 23 and 37 days after grafting). The pattern of expression of non-NMDA receptors in cortical vs. parenchymal grafts suggests a possible regulation of their levels by transacting elements from host parallel fibers. GABAA binding levels in the grafts for both ligands used were similar to normal molecular layer. Binding was increased in the deep cerebellar nuclei of pcd mutants: the increase in [3H]muscimol binding over normal was 215% and the increase in [3H]flunitrazepam binding was 89%. Such increases in the pcd deep cerebellar nuclei may reflect a denervation-induced supersensitivity subsequent to the loss of Purkinje axon terminal innervation. In the deep nuclei of pcd mutants with unilateral cerebellar grafts, [3H]muscimol binding was 31% lower in the grafted side than in the contralateral nongrafted side at 37 days after transplantation; [3H]fluni-trazepam binding was also lower in the grafted side by 15% compared to the nongrafted side. Such changes in GABAA receptors suggest a significant, albeit partial, normalizing trend of cerebellar grafts on the state of postsynaptic supersensitive receptors in the host cerebellar nuclei.

1-Aryl-3,5-dihydro-4H-2,3-benzodiazepin-4-ones: novel AMPA receptor antagonists

Our previous publication (Eur. J. Pharmacol. 1995, 294, 411-422) reported preliminary chemical and biological studies of some 2,3-benzodiazepines, analogues of 1-(4-aminophenyl)-4-methyl-7,8-(methylenedioxy)-5H-2,3-benzodiazepine (1, GYKI 52466), which have been shown to possess significant anticonvulsant activity. This paper describes the synthesis of new 1-aryl-3,5-dihydro-4H-2,3-benzodiazepin-4-ones and the evaluation of their anticonvulsant effects. The observed findings extend the structure-activity relationships previously suggested for this class of anticonvulsants. The seizures were evoked both by means of auditory stimulation in DBA/2 mice and by pentylenetetrazole or maximal electroshock in Swiss mice. 1-(4'-Aminophenyl)- (38) and 1-(3'-aminophenyl)-3,5-dihydro-7,8-dimethoxy-4H-2,3-benzodiazepin- 4-one (39), the most active compounds of the series, proved to be more potent than 1 in all tests employed. In particular, the ED50 values against tonus evoked by auditory stimulation were 12.6 micromol/kg for derivative 38, 18.3 micromol/kg for 39, and 25.3 micromol/kg for 1. Higher doses were necessary to block tonic extension induced both by maximal electroshock and by pentylenetetrazole. In addition these compounds exhibited anticonvulsant properties that were longer lasting than those of compound 1 and were less toxic. The novel 2,3-benzodiazepines were also investigated for a possible correlation between their anticonvulsant activities against convulsions induced by 2-amino-3-(3-hydroxy-5-methylisoxazol-4-yl)propionic acid (AMPA) and their affinities for benzodiazepine receptors (BZR). The 2,3-benzodiazepines did not affect the binding of [3H]flumazenil to BZR, and conversely, their anticonvulsant effects were not reversed by flumazenil. On the other hand the 2,3-benzodiazepines antagonized seizures induced by AMPA and aniracetam in agreement with an involvement of the AMPA receptor. In addition, both the derivative 38 and the compound 1 markedly reduced the AMPA receptor-mediated membrane currents in guinea-pig olfactory cortical neurons in vitro in a noncompetitive manner. The derivatives 25 and 38-40 failed to displace specific ligands from N-methyl-D-aspartate (NMDA), AMPA/kainate, or metabotropic glutamate receptors.

Hippocampal N-methyl-D-aspartate and kainate binding in response to entorhinal cortex aspiration or 192 IgG-saporin lesions of the basal forebrain

Lesion models in the rat were used to examine the effects of removing innervation of the hippocampal formation on glutamate receptor binding in that system. Bilateral aspiration of the entorhinal cortex was used to remove the cortical innervation of the hippocampal formation and the dentate gyrus. The subcortical input to the hippocampus from cholinergic neurons of the basal forebrain was lesioned by microinjection of the immunotoxin 192 IgG-saporin into the medial septum and vertical limb of diagonal band. After a 30-day postlesion survival, the effects of these lesions on N-methyl-D-aspartate-displaceable [3H]glutamate and [3H]kainate binding in the hippocampus were quantified using in vitro autoradiography. The bilateral entorhinal lesion induced a sprouting response in the dentate gyrus, measured by an increase in the width of [3H]kainate binding. It also induced an increase in the density of [3H]kainate binding in CA3 stratum lucidum and an increase in N-methyl-D-aspartate binding throughout the hippocampus proper and the dentate gyrus. The selective lesion of cholinergic septal input did not have any effect on hippocampal [3H]kainate binding and induced only a moderate decrease in N-methyl-D-aspartate binding that was not statistically reliable. The entorhinal and cholinergic lesions were used as in vivo models of the degeneration of hippocampal input that occurs in normal aging and Alzheimer's disease. The results from the present lesion study suggest that some, but not all, of the effects on hippocampal [3H]kainate and N-methyl-D-aspartate binding induced by the lesions are consistent with the status of binding to these receptors in aging and Alzheimer's disease. Consistent with the effects of aging and Alzheimer's disease is an altered topography of [3H]kainate binding after entorhinal cortex lesion and a modest decline in N-methyl-D-aspartate binding after lesions of the cholinergic septal input to the hippocampus.

Quantitative autoradiography of 5-[3H]6-cyano-7-nitro-quinoxaline-2,3-dione and (+)-3-[3H]dizocilpine maleate binding in rat vestibular nuclear complex after unilateral deafferentation, with comparison to cochlear nucleus

The distributions of non-N-methyl-D-aspartate and N-methyl-D-aspartate receptors in the rat vestibular nuclear complex were estimated by quantitative autoradiography of 5-[3H]6-cyano-7-nitro-quinoxaline-2,3-dione and (+)-3-[3H]dizocilpine maleate binding, respectively. The binding of 5-[3H]6-cyano-7-nitro-quinoxaline-2,3-dione in the vestibular nuclear complex was also compared with that in the cerebellar cortex and cochlear nucleus. Measurements were made in control rats and in rats with unilateral destruction of the inner ear and removal of the vestibular ganglion. Compared to the unlesioned side, 5-[3H]6-cyano-7-nitro-quinoxaline-2,3-dione binding in the lesioned-side vestibular nuclear complex was decreased significantly in all regions at two to four postoperative days. However, the bilateral asymmetry disappeared in most regions by 30 days. 5-[3H]6-Cyano-7-nitro-quinoxaline-2,3-dione binding increased in the molecular layer of the cerebellar cortex at 30 days after lesion, although there were no clear changes at two to seven days. 5-[3H]6-Cyano-7-nitro-quinoxaline-2,3-dione binding in the cochlear nucleus decreased on the lesioned side, compared to the unlesioned side, in regions receiving significant auditory nerve innervation, but increased in the molecular layer of the dorsal cochlear nucleus. (+)-3-[3H]Dizocilpine maleate binding in regions of the vestibular nuclear complex was reduced on the lesioned side, compared to the unlesioned side, after deafferentation, with the largest reductions usually at 30 postoperative days. It is suggested that: (i) non-N-methyl-D-aspartate receptors are involved in synaptic transmission for both vestibular and auditory nerve fibers, while the involvement of N-methyl-D-aspartate receptors is less certain; (ii) unilateral deafferentation of the vestibular nuclear complex can result in bilateral asymmetries for non-N-methyl-D-aspartate and N-methyl-D-aspartate receptors, which are most prominent at earlier and later survival times, respectively; and (iii) vestibular compensation may involve regulation of both non-N-methyl-D-aspartate and N-methyl-D-aspartate receptors in the vestibular nuclear complex and activation of non-N-methyl-D-aspartate receptor-related processes in cerebellar cortex.

Chronic treatment with typical and atypical antipsychotics increases the AMPA-preferring form of AMPA receptor in rat brain

We assessed the effects of chronic (21 day) administration of antipsychotic drugs on the density of the alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptor in rat brain. We used two typical antipsychotic drugs, haloperidol and pimozide, and two atypical antipsychotic drugs, risperidone and clozapine. Antipsychotic drugs as a group significantly elevated the density of the AMPA receptor measured with an AMPA receptor agonist ([3H]AMPA), but not with an AMPA receptor antagonist, 6-cyano-7-nitro-quinoxaline-2,3-dione ([3H]CNQX). In all regions studied, the magnitude of the increase seen with chronic typical antipsychotic drugs was significantly greater than that seen with chronic atypical antipsychotic drugs. In frontal cortex and striatum, typical antipsychotics but not atypical antipsychotics elevated AMPA receptor binding over control. These findings suggest that antipsychotic drugs alter the agonist affinity of the AMPA receptor without altering the number of AMPA receptors. Typical antipsychotic drugs may be more potent in this effect than atypical antipsychotic drugs, especially in critical corticostriatal circuits.

Distinct distributions of alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionate (AMPA) receptor subunits and a related 53,000 M(R) antigen (GR53) in brain tissue

Polyclonal antibodies against specific carboxy-terminal sequences of known alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionate (AMPA) receptor subunits (GluR-4) were used to screen regional homogenates and subcellular fractions from rat brain. Affinity purified anti-GluR1 (against amino acids 877-899), anti-GluR2/3 (850-862), and anti-GluR4a and anti-GluR4b (868-881) labeled distinct subunits with the expected molecular weight of approximately 105,000. These antigens were shown to have distinct distributions in the brain. While GluR2/3 epitopes had a distribution profile similar to that of the presynaptic marker synaptophysin, GluR1 was notable for its abundance in the hippocampus and its relatively low density in neocortical areas, and GluR4 was highly enriched in cerebellar tissue. An additional antigen (glutamate receptor-related, GR53) of lower molecular weight (50,000-59,000) was recognized in rat, human, frog, chick and goldfish brain samples by anti-GluR4a as well as by anti-GluR1 at, an antibody that specifically recognizes the extracellular aminoterminal domain of GluR1 (amino acids 163-188). Both antibodies also labeled antigens of approximately 105,000 mol. wt in brain tissue from all species tested. The approximately 53,000 mol. wt antigen was concentrated 10-20-fold in synaptic membranes vs homogenates across rat brain regions. Both the 105,000 and the 53,000 mol. wt proteins were also concentrated in postsynaptic densities, and neither of the two antigens were evident in seven non-brain tissue samples. These data indicate that AMPA receptors have regionally different subunit combinations and that some AMPA receptor composites include proteins other than the conventional 105,000 mol. wt GluR subunits.

In vitro autoradiography of ionotropic glutamate receptors in hippocampus and striatum of aged Long-Evans rats: relationship to spatial learning

Using in vitro autoradiography, we investigated [3H] alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionate, [3H]kainate and [3H]N-methyl-D-aspartate binding in two forebrain regions, the hippocampus and striatum, of young (four months of age) and aged (24-25 months of age) Long-Evans rats that had previously been tested for spatial learning ability in the Morris water maze. Although there was substantial preservation of binding in the aged rats, reductions in binding were present in the aged rats that were specific to ligand and anatomical region. In the hippocampus of aged rats, [3H] alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionate binding in CA1 and [3H]kainate binding in CA3 were reduced. In contrast, N-methyl-D-aspartate binding was not significantly different between age groups. There was evidence of sprouting in the dentate gyrus molecular layer of aged rats, indicated by changes in the topography of [3H]kainate binding. Binding density was analysed with respect to patch/matrix compartmentalization in the striatum. The most striking result was a large decrease in N-methyl-D-aspartate binding in aged rats that was not limited to any dorsal/ventral or patch/matrix area of the striatum. Additionally, [3H]kainate binding in striatal matrix was modestly reduced in aged rats. Of these age effects, only N-methyl-D-aspartate binding in the striatum and [3H]kainate binding in the CA3 region of the hippocampus were correlated with spatial learning, with lower binding in the aged rats associated with better spatial learning ability. Age-related alterations in ionotropic glutamate receptors differ with respect to the receptor subtype and anatomical region examined. The age effects were not necessarily indicative of cognitive decline, as only two age-related binding changes were correlated with spatial learning. Interestingly, in these instances, lower binding in the aged rats was associated with preserved spatial learning, suggesting a compensatory reduction in receptor binding in a subpopulation of aged rats.

Changes in [3H]AMPA and [3H]kainate binding in rat caudate-putamen and nucleus accumbens after 6-hydroxydopamine lesions of the medial forebrain bundle: an autoradiographic study

The binding parameters of [3H] alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionate (AMPA) and [3H]kainate binding were examined in caudate-putamen and nucleus accumbens of rat striatum after unilateral lesions of the right medial forebrain bundle (MFB) using in vitro receptor autoradiography. Lesioning of the dopaminergic fibres in the MFB with 6-hydroxydopamine (6-OHDA) resulted, after one or four weeks, in a significant decrease in the levels of [3H]GBR 12935 (1-[2-diphenylmethoxy)-ethyl]-4-(3-phenylpropyl) piperazine) in ipsilateral caudate-putamen and nucleus accumbens (62 and 43%, respectively). A comparison of the dissociation constants (Kd) of [3H]AMPA and [3H]kainate binding in caudate-putamen and nucleus accumbens between control and MFB-lesioned side did not indicate any significant change. However, the maximum number of [3H]AMPA and [3H]kainate binding sites (Bmax) were significantly decreased in caudate-putamen and nucleus accumbens of the MFB-lesioned side of the brain. This decrease was between 17 and 26%. Our results suggested that at least one-fourth to one-fifth of AMPA and kainate receptors in rat caudate-putamen and nucleus accumbens are localized on the presynaptic endings of dopamine fibres that follow the MFB. A role of non-NMDA glutamate receptors in the presynaptic regulation of dopamine release in rat striatum is therefore supported.

Glutamate receptor binding in the human hippocampus and adjacent cortex during development and aging

Distinct patterns of age-related alterations in NMDA (MK801 binding) and non-NMDA, AMPA (CNQX), and kainate binding have been identified in human hippocampus and parahippocampal gyrus in normal individuals with no evidence of degenerative brain disease ranging in age from 24 gestational weeks to 94 years. Whereas MK801 binding did not alter substantially over this age range, CNQX binding rose from low levels in the fetus to maximum levels between neonate and middle age, and kainate binding declined extensively from the perinatal to adult stage. Following maturity, there were no significant changes in kainate binding, although MK801 binding increased in CA1 and CA3 and CNQX binding declined in several regions, particularly CA2 and subiculum. For each receptor binding the timing of these fluctuations ocurring during development and aging varied within different regions of the dentate gyrus, hippocampus proper, subicular complex, and entorhinal cortex examined. The transient peaks of receptor binding are likely to reflect processes of synaptogenesis and pruning and may provide clues regarding the role of the different glutamate receptor subtypes in various pathologies of the hippocampus and adjacent cortex associated with developmental disorders (of genetic origin or due to perinatal trauma or insult). The absence of substantial changes in any subtype examined from middle to old age suggests alterations in transmitter binding to these glutamate receptors are not involved in senescent neurodegeneration.

Comparison of the autoradiographic binding distribution of [3H]-gabapentin with excitatory amino acid receptor and amino acid uptake site distributions in rat brain

1. Gabapentin is a novel anticonvulsant with an unknown mechanism of action. Recent homogenate binding studies with [3H]-gabapentin have suggested a structure-activity relationship similar to that shown for the amino acid transport system responsible for the uptake of large neutral amino acids (LNAA). 2. The autoradiographic binding distribution of [3H]-gabapentin in rat brain was compared with the distributions for excitatory amino acid receptor subtypes and the uptake sites for excitatory and large neutral amino acids in consecutive rat brain sections. 3. Densitometric measurement of the autoradiographic images followed by normalisation with respect to the hippocampus CA1 stratum radiatum, was carried out before comparison of each binding distribution with that of [3H]-gabapentin by linear regression analysis. The correlation coefficients observed showed no absolute correlation was observed between the binding distributions of [3H]-gabapentin and those of the excitatory amino acid receptor subtypes. The acidic and large neutral amino acid uptake site distributions demonstrated a much closer correlation to the [3H]-gabapentin binding site distribution. The correlation coefficients for D-[3H]-aspartate, L-[3H]-leucine and L-[3H]-isoleucine binding site distributions were 0.76, 0.90 and 0.88 respectively. 4. Concentration-dependent inhibition by unlabelled gabapentin of autoradiographic binding of L-[3H]-leucine and L-[3H]-isoleucine was observed, with non-specific binding levels being reached at concentrations between 10 and 100 microM. 5. Excitotoxic quinolinic acid lesion studies in rat brain caudate putamen and autoradiography were carried out for the amino acid uptake sites mentioned above. The resulting glial infiltration of the lesioned areas was visualized by autoradiography using the peripheral benzodiazepine receptor specific ligand [3H]-PK11195. A significant decrease in binding density in the lesioned area compared with sham-operated animals was observed for D-[3H]-aspartate, L-[3H]-leucine, L-[3H]-isoleucine and [3H]-gabapentin, whilst [3H]-PK11195 showed a significant increase in binding density indicative of glial infiltration into the lesioned area. These results suggest that the gabapentin binding site and the acidic and LNAA uptake site may be present on cell bodies of a neuronal population of cells. 6. From these studies it appears that [3H]-gabapentin, L-[3H]-leucine and L-[3H]-isoleucine bind to the same site in rat brain. The inhibition of [3H]-gabapentin binding by the LNAA uptake system-specific ligand, BCH, suggests that [3H]-gabapentin may label this uptake site, termed system-L. Conversely these ligands could be labelling a novel site that coincidentally has a similar structure-activity relationship to this uptake site. These results suggest a novel mechanistically relevant site of action for gabapentin and may enable further anti-epileptic agents of this type to be developed.

Pharmacological characteristics of an outward current produced by beta-hydroxy-L-glutamic acid on a snail neurone

An outward current (Iout) was produced by stereoisomers of beta-hydroxy-L-glutamic acid (L-BHGA), an L-glutamic acid (L-Glu) derivative, applied by brief pneumatic pressure ejection on an identifiable neurone type, v-LCDN (ventral-left cerebral distinct neurone), of Achatina fulica Férussac. However, L- and D-Glu were almost ineffective on this neurone type. The pharmacological features of this Iout caused by L-BHGA were elucidated in the present study. According to the dose (pressure duration)-response studies on the L-BHGA stereoisomers that produced the Iout, the effective potency of threo-L-BHGA was approximately similar to that of erythro-L-BHGA. The dose (pressure duration)-response curve of quisqualic acid was shifted towards the left direction from those of threo-and erythro-L-BHGA, suggesting that the binding activity of quisqualic acid to the receptors would be stronger than those of the L-BHGA stereoisomers. GABA, glycine and L-homocysteic acid showed an inward current (Iin) on this neurone type, in contrast to the Iout caused by L-BHGA. beta-Alanine and taurine had absolutely no effect. Therefore, no amino acid inhibitory neurotransmitter candidate was found for this neurone type except for L-BHGA. It was assumed that L-BHGA, in either threo-or erythro-configuration, would be an inhibitory neurotransmitter for this neurone type. Mammalian L-Glu receptor antagonists. D(-)-AP-5, (+/-)-CPP, CNQX and L(+)-AP-3, applied by perfusion, showed no effect on the Iout of v-LCDN caused by threo-L-BHGA, indicating that the features of the inhibitory receptor activated by L-BHGA were much different from those of any type of the mammalian L-Glu receptors. Among the inhibitors of ATP-sensitive K+ channel, glipizide significantly inhibited the Iout caused by threo-L-BHGA, whereas tolbutamide did not. Inhibitors of intracellular signal transduction systems, H-7, H-8, H-9, staurosporine, calphostin C, KT5823 and W-7, had no effect on the Iout caused by threo-L-BHGA, suggesting that the receptors activated by threo-L-BHGA would be ionotropic.

Laminar distribution of MK-801, kainate, AMPA, and muscimol binding sites in cat visual cortex: a developmental study

We used quantitative autoradiography to determine whether the development of glutamate receptors correlates with the sensitive period for monocular deprivation in the visual cortex. To study glutamate receptors, we incubated sections of cat visual cortex with tritiated (+)-5-methyl-10,11-dihydro-5H-dibenzo[a,d]-cyclohepten-5,10imin e-maleate (MK-801), tritiated kainate, and tritiated amino-3-hydroxy-5-methyl-isoxazole-4-propionic acid (AMPA). [3H]MK-801 is a noncompetitive ligand for the N-methyl-D-aspartate (NMDA) receptor. [3H]kainate and [3H]AMPA are competitive ligands for non-NMDA receptors. We used [3H]muscimol, which binds to GABAA receptors, so that we would have one control ligand that binds to a nonglutamate receptor. When all layers were combined, the results confirmed our previous studies with homogenate binding. [3H]MK-801 and [3H]kainate binding were significantly greater at 42 days than at earlier or later times. [3H]AMPA and [3H]muscimol binding did not show such a peak. This suggests that MK-801 and kainate binding sites are more likely to be involved in plasticity than are AMPA and muscimol binding sites. In layers 2/3, MK-801 had the greatest age-dependent changes; in layers 5 and 6, kainate binding changed most with age. This suggests that the mechanisms of plasticity may vary with cortical layer.

Characterisation of the interaction between guanyl nucleotides and AMPA receptors in rat brain

Guanyl nucleotides inhibit the binding of the AMPA receptor agonists [3H]fluorowillardiine and [3H]AMPA and the competitive antagonist [3H]CNQX to rat brain cerebrocortical membranes. The rank order of inhibition for each of the radioligands tested was GTP > GDP > GMP. The nucleotides CTP and ATP showed no effect. GTP inhibition was unaffected by the presence or absence of NaCl and MgCl2. Pre-treatment of the membranes with GTP, and its removal before addition of radioligand, did not inhibit binding. Quantitative autoradiography demonstrated that GTP inhibition occurred throughout the brain. These results are consistent with guanyl nucleotides acting at an extracellular site present on all AMPA receptor subunits, occupation of which inhibits agonist and antagonist binding.

Identifiable Achatina giant neurones: their localizations in ganglia, axonal pathways and pharmacological features

1. An African giant snail (Achatina fulica Férussac), originally from East Africa, is now found abundantly in tropical and subtropical regions of Asia, including Okinawa in Japan. This is one of the largest land snail species in the world. The Achatina central nervous system is composed of the buccal, cerebral and suboesophageal ganglia. The 37 giant neurones were identified in these ganglia by the series of studies conducted over about 20 years. The identifications were made by the localization of these neurones in the ganglia, their axonal pathways and their pharmacological features. 2. In the left buccal ganglion, the four giant neurones, d-LBAN, d-LBMB, d-LBCN and d-LBPN, were identified. In the left and right cerebral ganglia, d-LCDN, d-RCDN, v-LCDN and v-RCDN were identified. The suboesophageal ganglia are further composed of the left and right parietal, the visceral, the left and right pleural, and the left and right pedal ganglia. In the right parietal ganglion, PON, TAN, TAN-2, TAN-3, RAPN, d-RPLN, BAPN, LPPN, LBPN, LAPN and v-RPLN were identified. In the visceral ganglion, VIN, FAN, INN, d-VLN, v-VLN, v-VAN, LVMN, RVMN and v-VNAN were identified. In the left parietal ganglion, v-LPSN was identified. In the left and right pedal ganglia, LPeNLN, RPeNLN, d-LPeLN, d-LPeCN, d-RPeAN, d-LPeDN, d-LPeMN and d-LPeEN were identified. 3. Of the small molecule compounds tested, dopamine, 5-hydroxytryptamine, GABA, L-glutamic acid, threo- or erythro-beta-hydroxy-L-glutamic acid were effective on the Achatina giant neurones. We suppose that these compounds act as the neurotransmitters for these neurones. 4. Of the neuroactive peptides, achatin-I(Gly-D-Phe-Ala-Asp). APGW-amide(Ala-Pro-Gly-Trp-NH2) and Achatina cardioexcitatory peptide (ACEP-1)(Ser-Gly-Gln-Ser-Trp-Arg-Pro-Gln-Gly-Arg-Phe-NH2) were proposed as neurotransmitters, because these were effective on the Achatina giant neurones and their presence was demonstrated in the Achatina ganglia. Further, myomodulin (Pro-Met-Ser-Met-Leu-Arg-Leu-NH2), buccalin (Gly-Met-Asp-Ser-Leu-Ala-Phe-Ser-Gly-Gly-Leu-NH2), FMRFamide (Phe-Met-Arg-Phe-NH2). [Ser2]-Mytilus inhibitory peptide ([Ser2]-MIP) (Gly-Ser-Pro-Met-Phe-Val-NH2), catch-relaxing peptide (CARP) (Ala-Met-Pro-Met-Leu-Arg-Leu-NH2), oxytocin (Cys-Tyr-Ile-Gln-Asn-Cys-Pro-Leu-Gly-NH2) and small cardioactive peptideB (SCPB) (Met-Asn-Tyr-Leu-Ala-Phe-Pro-Arg-Met-NH2) could also be neurotransmitters because these peptides were also effective on the Achatina giant neurones, though their presence in the ganglia of this animal has not yet been demonstrated. 5. Calcium current (ICa) was recorded from Achatina giant neurones in the Na(+)-free solution containing K(+)-channel blockers under voltage clamp. The Ca2+ antagonistic effects of brovincamine, verapamil, eperisone, diltiazem, monatepil, etc., were compared using the ICa of the Achatina neurones. 6. Almost all of the mammalian small molecule neurotransmitters were effective on the Achatina giant neurones, suggesting that these compounds are acting on the neurones of a wide variety of animal species. However, the pharmacological features of the Achatina neurone receptors to these compounds were not fully comparable to those of the mammalian receptors. For example, we proposed that beta-hydroxy-L-glutamic acid (either threo- or erythro-) could be an inhibitory neurotransmitter for an Achatina neurone. 7. In contrast, the Achatina giant neurones appear to have no receptor for the mammalian neuroactive peptides, except for oxytocin and Arg-vasotocin. On the other hand, many neuroactive peptides were isolated from invertebrate nervous tissues, including achatin-I, a neuroexcitatory tetrapeptide having a D-phenylalanine residue.

Characterization of the pharmacology and regional distribution of (S)-[3H]-5-fluorowillardiine binding in rat brain

1. This study examined the binding of the new radioligand (S)-[3H]-5-fluorowillardiine to rat brain synaptic membranes. Specific binding represented greater than 80% of the total binding and was increased by 10% in the presence of 100mM potassium thiocyanate (KSCN). 2. In the absence of KSCN, (S)-[3H]-5-fluorowillardiine identified two binding sites with KD1=22.5 nM, Bmax1=1.4 pmol mg(-1) protein and KD2=1.5 microM, Bmax2=10.8 pmol mg(-1) protein. In the presence of 100 mM KSCN the affinities of both the binding sites were increased, yielding values of KD1=6.9 nM and KD2=0.4 microM KSCN was without effect on the Bmax values. 3. (S)-[3H]-5-fluorowillardiine binding was displaced by non-NMDA receptor ligands with the rank order of potency: 2,3-dihydroxy-6-nitro-7-sulphamoyl-benzo(F)quinoxaline (NBQX) > domoate > (S)-alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionice acid (AMPA) = L-glutamate > 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX) > kainate >> (R)-5-fluorowellardiine. In contrast, both N-methyl-D-aspartate (NMDA) and the metabotropic glutamate receptor agonist, (1S,3R)-1-aminocyclopentane-1,3-dicarboxylic acid (ACPD) were inactive. 4. By use of quantitative autoradiography the regional distribution of (S)-[3H]-5-fluorowillardiine binding in rat brain was assessed. The highest levels of binding were in the dentate gyrus and the CA1 region of the hippocampus. Lower levels of binding were detected in the cerebral cortex, olfactory system, lateral septum, caudate putamen and nucleus accumbens. 5. We conclude that the pharmacological profile and regional distribution of (S)-[3H]-5-fluorowillardiine binding is consistent with its specific interaction with AMPA receptors.

Low nanomolar serotonin inhibits the glutamate receptor/nitric oxide/cyclic GMP pathway in slices from adult rat cerebellum

The function of serotonin afferents to the cerebellum has been investigated by monitoring the effects of serotoninergic drugs on the production of cyclic GMP elicited in cerebellar slices by activation of ionotropic glutamate receptors. Exposure of adult rat cerebellar slices to N-methyl-D-aspartate (1 nM to 1 microM) or to (RS)-alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionate (AMPA; 1 nM to 10 microM) elicited concentration-dependent and saturable rises in the levels of cyclic GMP. These responses were blocked by selective antagonists at the N-methyl-D-aspartate or AMPA receptors and by inhibiting nitric oxide synthase, but were insensitive to tetrodotoxin. When tested between 0.1 and 10 nM, serotonin, the serotonin1A receptor agonist (+/-)-8-hydroxy-2-(di-n-propylamino)tetralin and the serotonin2 receptor agonist (+/-)-1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane inhibited, concentration-dependently, the cyclic GMP responses evoked by near-maximal (0.1 microM) concentrations of N-methyl-D-aspartate or AMPA. The EC50 values (concentrations causing half-maximal effect) ranged between 0.7 and 2.1 nM. The actions of serotonin were totally abolished by methiothepin, a mixed-type serotonin receptor antagonist. Thus, the serotonergic cerebellar afferents may exert a potent inhibitory control on the excitatory transmission mediated by N-methyl-D-aspartate and AMPA receptors; the inhibition occurs through both serotonin1A and serotonin2 receptors. As the glutamate receptor-dependent cyclic GMP responses involve production of nitric oxide, a diffusible activator of guanylate cyclase, the above inhibitory serotonin receptors may have multiple localization.(ABSTRACT TRUNCATED AT 250 WORDS)

Developmental changes in alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionate receptor properties and expression in the rat hippocampal formation

The developmental changes in alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionate (AMPA) receptor properties in rat hippocampus were evaluated with quantitative autoradiography of ligand binding and in situ hybridization performed in adjacent sections with antisense oligonucleotides for AMPA receptor subunits (GluR1-3, flip and flop splice variants). Specific 3H-AMPA binding in different hippocampal subfields increased between postnatal day 7 and 15 and was higher in CA3 during the postnatal period when compared to adult levels. This effect was mostly due to high levels of high affinity binding sites in cell body layers during the developmental period. By contrast, autoradiograms of 3H-AMPA binding predominantly to the low affinity binding sites indicated an absence of these sites in cell body layers and the overall levels of binding exhibited little overshoot compared to adult levels during the developmental period. The changes in binding of the antagonist of the AMPA receptor, 6-nitro-7-cyanoquinoxaline-2,3-dione were markedly different from those for the high affinity AMPA binding sites but quite similar to those for the low affinity sites. The binding was extremely low at postnatal day 7 and increased rapidly between postnatal day 7 and 15 and slowly between postnatal day 15 and adult. Low levels of binding were observed in the cell body layer at every postnatal age. The changes in expression of messenger RNAs for the different subunits of the AMPA receptors were well correlated with the modifications in high affinity AMPA binding sites measured in the cell body layers also exhibiting an increased expression of the receptors at the transcriptional level during the developmental period as compared to adult levels. The relative expression of the GluR2 subunits decreased during the postnatal period and the time course for this reduction paralleled that for the increased vulnerability of hippocampal pyramidal neurons to a variety of insults. The results indicate that both the messenger RNAs for the subunits and the AMPA receptors exhibit increased levels of expression during the postnatal period compared to adult levels. They also suggest that nascent receptors might bind AMPA with high affinity before their insertion in membranes into functional receptors that have low affinity for agonists and high affinity for antagonists. The changes in subunit composition of the receptors during the postnatal period may have important implications for mechanisms of plasticity as well as of neuropathology.

Distribution of AMPA-selective glutamate receptor subunits in the human hippocampus and cerebellum

The distribution of AMPA-selective subunits, GluR1-4, was determined in the human hippocampus and cerebellum by in situ hybridization and immunocytochemistry. In the hippocampus, in situ hybridization revealed that GluR1 and GluR2 mRNAs were similarly distributed and highly expressed in the dentate gyrus, with lower levels in the CA regions. GluR3 and GluR4 mRNAs were expressed at very low levels. Immunocytochemical studies showed that GluR1- and GluR2/3-immunoreactivity were highest in the dentate molecular and granular layers. In the CA regions, GluR1 and GluR2/3 staining was observed in pyramidal cell bodies and surrounding neuropil and was more intense in CA4/3/2 compared with CA1. GluR4-immunoreactivity was low throughout the hippocampus. In the cerebellum, GluR1 and GluR4 transcripts were expressed in the granular and Purkinje cell/Bergmann glia layers. GluR2 mRNA was highly expressed in the granular layer and individual Purkinje cells, while GluR3 mRNA was not detectable in the cerebellum. GluR1- and GluR4-immunoreactivity were localized to Purkinje cells and putative Bergmann glia, as well as their processes extending into the molecular layer. GluR2/3 staining was intense in Purkinje cells, with moderate staining in the granular layer. Thus, GluR1-4 subunits are differentially distributed in the hippocampus and cerebellum. In addition, the distribution of subunit mRNA and protein correlate well with each other and with the glutamatergic neuroanatomy of the hippocampus and cerebellum.

Binding of the new radioligand (S)-[3H]AMPA to rat brain synaptic membranes: effects of a series of structural analogues of the non-NMDA receptor agonist willardiine

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.