AMPA receptors in cerebellar granule cells during development in culture

Slack K+ channels attenuate NMDA-induced excitotoxic brain damage and neuronal cell death

The neuronal Na⁺ -activated K⁺ channel Slack (aka Slo2.2, K_Na 1.1, or Kcnt1) has been implicated in setting and maintaining the resting membrane potential and defining excitability and firing patterns, as well as in the generation of the slow afterhyperpolarization following bursts of action potentials. Slack activity increases significantly under conditions of high intracellular Na⁺ levels, suggesting this channel may exert important pathophysiological functions. To address these putative roles, we studied whether Slack K⁺ channels contribute to pathological changes and excitotoxic cell death caused by glutamatergic overstimulation of Ca²⁺ - and Na⁺ -permeable N-methyl-D-aspartic acid receptors (NMDAR). Slack-deficient (Slack KO) and wild-type (WT) mice were subjected to intrastriatal microinjections of the NMDAR agonist NMDA. NMDA-induced brain lesions were significantly increased in Slack KO vs WT mice, suggesting that the lack of Slack renders neurons particularly susceptible to excitotoxicity. Accordingly, excessive neuronal cell death was seen in Slack-deficient primary cerebellar granule cell (CGC) cultures exposed to glutamate and NMDA. Differences in neuronal survival between WT and Slack KO CGCs were largely abolished by the NMDAR antagonist MK-801, but not by NBQX, a potent and highly selective competitive antagonist of α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA)-type ionotropic glutamate receptors. Interestingly, NMDAR-evoked Ca²⁺ signals did not differ with regard to Slack genotype in CGCs. However, real-time monitoring of K⁺ following NMDAR activation revealed a significant contribution of this channel to the intracellular drop in K⁺ . Finally, TrkB and TrkC neurotrophin receptor transcript levels were elevated in NMDA-exposed Slack-proficient CGCs, suggesting a mechanism by which this K⁺ channel contributes to the activation of the extracellular-signal-regulated kinase (Erk) pathway and thereby to neuroprotection. Combined, our findings suggest that Slack-dependent K⁺ signals oppose the NMDAR-mediated excitotoxic neuronal injury by promoting pro-survival signaling via the BDNF/TrkB and Erk axis.

Models of calcium dynamics in cerebellar granule cells

Intracellular calcium dynamics is critical for many functions of cerebellar granule cells (GrCs) including membrane excitability, synaptic plasticity, apoptosis, and regulation of gene transcription. Recent measurements of calcium responses in GrCs to depolarization and synaptic stimulation reveal spatial compartmentalization and heterogeneity within dendrites of these cells. However, the main determinants of local calcium dynamics in GrCs are still poorly understood. One reason is that there have been few published studies of calcium dynamics in intact GrCs in their native environment. In the absence of complete information, biophysically realistic models are useful for testing whether specific Ca(2+) handling mechanisms may account for existing experimental observations. Simulation results can be used to identify critical measurements that would discriminate between different models. In this review, we briefly describe experimental studies and phenomenological models of Ca(2+) signaling in GrC, and then discuss a particular biophysical model, with a special emphasis on an approach for obtaining information regarding the distribution of Ca(2+) handling systems under conditions of incomplete experimental data. Use of this approach suggests that Ca(2+) channels and fixed endogenous Ca(2+) buffers are highly heterogeneously distributed in GrCs. Research avenues for investigating calcium dynamics in GrCs by a combination of experimental and modeling studies are proposed.

Altered sensitivity to excitotoxic cell death and glutamate receptor expression between two commonly studied mouse strains

Alterations in glutamatergic synapse function have been implicated in the pathogenesis of many different neurological disorders, including ischemia, epilepsy, Parkinson's disease, Alzheimer's disease, and Huntington's disease. While studying glutamate receptor function in juvenile Batten disease on the C57BL/6J and 129S6/S(v)E(v) mouse backgrounds, we noticed differences unlikely to be due to mutation difference alone. We report here that primary cerebellar granule cell cultures from C57BL/6J mice are more sensitive to N-methyl-D-aspartate (NMDA)-mediated cell death. Moreover, sensitivity to AMPA-mediated excitotoxicity is more variable and is dependent on the treatment conditions and age of the cultures. Glutamate receptor surface expression levels examined in vitro by in situ ELISA and in vivo by Western blot in surface cross-linked cerebellar samples indicated that these differences in sensitivity likely are due to strain-dependent differences in cell surface receptor expression levels. We propose that differences in glutamate receptor expression and in excitotoxic vulnerability should be taken into consideration in the context of characterizing disease models on the C57BL/6J and 129S6/S(v)E(v) mouse backgrounds.

Membrane depolarization regulates AMPA receptor subunit expression in cerebellar granule cells in culture

The physiological responses of AMPA receptors can be modulated through the differential expression of their subunits and by modifying their number at the cell surface. Here we have studied the expression of AMPA receptor subunits (GluR1-4) mRNAs in cerebellar granule cells grown in depolarizing (25mMK(+)) medium, and we have evaluated the effect of decreasing the [K(+)] in the culture medium for 24 h on both GluR1-4 expression (both mRNA and protein) and their presence at the plasma membrane. The expression of the four AMPAR subunits increases as the [K(+)] decreases, although the increase in GluR2 and GluR3 was only observed in the cell soma but not in the dendrites. Calcium entry through L-type calcium channel and CaMKIV activation are responsible for the reduction in the expression of AMPA receptor subunits in cells cultured in depolarizing conditions. Indeed, prolonged reduction of extracellular [K(+)] or blockage of L-type calcium channels enhanced both the surface insertion of the four AMPAR subunits and the AMPA response measured through intracellular calcium increase. These findings reveal a balanced increase in functional AMPA receptors at the surface of cells that can trigger strong increases in calcium in response to the persistent reduction of calcium entry.

Reduced size of the dendritic tree does not protect Purkinje cells from excitotoxic death

Purkinje cell loss by excitotoxic damage is a typical finding in many cerebellar diseases. One important aspect of this high sensitivity of Purkinje cells to excitotoxic death might be the enormous size of their dendritic tree, with a high load of excitatory glutamate receptors. We have studied whether reduction in the size of the dendritic tree might confer resistance against excitotoxic death to Purkinje cells. We have grown Purkinje cells in organotypic cerebellar slice cultures under chronic activation of metabotropic glutamate receptors or of protein kinase C. Both treatments strongly reduced dendritic tree size. After this treatment, cells were exposed to the glutamate receptor agonist AMPA, which has a strong excitotoxic effect on Purkinje cells. We found that Purkinje cells with small dendritic trees were as sensitive to AMPA exposure as untreated control cells with large dendritic trees. Immunostaining against vesicular glutamate transporter 1 revealed that the small dendritic trees were densely covered by glutamatergic terminals. Our results indicate that the expansion of the dendritic tree and the total number of AMPA receptors per neuron do not play a major role in determining the susceptibility of Purkinje cells to excitotoxic death.

AMPA glutamate receptor subunits 1 and 2 regulate dendrite complexity and spine motility in neurons of the developing neocortex

Within neurons of several regions of the CNS, mature dendrite architecture is attained via extensive reorganization of arbor during the developmental period. Since dendrite morphology determines the firing patterns of the neuron, morphological refinement of dendritic arbor may have important implications for mature network activity. In the neocortex, a region of brain that is sensitive to activity-dependent structural rearrangement of dendritic arbor, the proportion of AMPA receptors increases over the developmental period. However, it is unclear whether changes in AMPA receptor expression contribute to maturation of dendritic architecture. To determine the effects of increasing AMPA receptor expression on dendrite morphology and connectivity within the neocortex, and to determine whether these effects are dependent on specific AMPA receptor subunits, we overexpressed the AMPA glutamate receptor subunit 1 (GluR1) and glutamate receptor subunit 2 (GluR2) in cultured rat neocortical neurons at the time that AMPA receptors would normally be incorporated into synapses. Following expression of GluR1 or GluR2 we observed increases in the length and complexity of dendritic arbor of cortical neurons, and a concurrent reduction in motility of spines. In addition, expression of either subunit was associated with an increased density of excitatory postsynaptic puncta. These results suggest that AMPA receptor expression is an important determinant of dendrite morphology and connectivity in neocortical neurons, and further, that contrary to other regions of the CNS, the effects of AMPA receptors on dendrite morphology are not subunit-specific.

Differential regulation of dendrite complexity by AMPA receptor subunits GluR1 and GluR2 in motor neurons

Activity-dependent developmental mechanisms in many regions of the central nervous system are thought to be responsible for shaping dendritic architecture and connectivity, although the molecular mechanisms underlying these events remain obscure. Since AMPA glutamate receptors are developmentally regulated in spinal motor neurons, we have investigated the role of activation of AMPA receptors in dendritic outgrowth of spinal motor neurons by overexpression of two subunits, GluR1 and GluR2, and find that dendrite outgrowth is differentially controlled by expression of these subunits. Overexpression of GluR1 was associated with greater numbers of filopodia, and an increase in the length and complexity of dendritic arbor. In contrast, GluR2 expression did not alter dendritic complexity, but was associated with a moderate increase in length of arbor, and decreased numbers of filopodia. Neither GluR1 nor GluR2 had any effect on the motility of filopodia. In addition, GluR1 but not GluR2 expression increased the density of dendritic puncta incorporating a GFP-labeled PSD95, suggesting that GluR1 may mediate its effect in part by augmenting the number of excitatory synapses within motor neuron dendrites. Together these results suggest that in spinal motor neurons, AMPA receptors composed of GluR1 subunits may facilitate neurotrophic mechanisms in these neurons, permitting sustained dendrite outgrowth and synaptogenesis, whereas expression of AMPA receptors containing GluR2 acts to preserve existing dendritic arbor. Thus, the observed downregulation of GluR1 in motor neurons during postnatal development may limit the formation of new dendrite segments and synapses, promoting stabilized synaptic connectivity.

Cyclothiazide selectively inhibits mGluR1 receptors interacting with a common allosteric site for non-competitive antagonists

Metabotropic glutamate receptors mGluR1 and mGluR5 stimulate phospholipase C, leading to an increased inositol trisphosphate level and to Ca(2+) release from intracellular stores. Cyclothiazide (CTZ), known as a blocker of AMPA receptor desensitization, produced a non-competitive inhibition of [Ca(2+)](i) increases induced by mGluR agonists in HEK 293 cells transfected with rat mGluR1a but had no effect on the [Ca(2+)](i) signals in cells expressing rat mGluR5a. In cells expressing mGluR1, CTZ also inhibited phosphoinositide hydrolysis, as well as cAMP accumulation and arachidonic acid release induced by mGluR1 agonists, indicating a direct inhibition of the receptor and not of a particular signal transduction system. However, CTZ failed to antagonize cAMP inhibition stimulated by rat mGluR2, -3, -4, -6, -7 and -8 receptors confirming its selectivity for mGluR1. The use of chimeric receptors with substituted N-terminal domains showed that CTZ did not interact with the N-terminal mGluR1a domain. Instead, mutation analysis revealed that CTZ interacts with the Thr-815 and Ala-818 residues, located at the 7th transmembrane domain, similarly as the mGluR1-selective antagonist CPCCOEt. In primary cultures of cerebellar granule neurons, expressing native metabotropic and ionotropic glutamate receptors, the final outcome of CTZ effects depended on its combined ability to potentiate AMPA receptors and inhibit mGluR1 receptors.

AMPA/kainate receptor-mediated up-regulation of GABAA receptor δ subunit mRNA expression in cultured rat cerebellar granule cells is dependent on NMDA receptor activation

We have studied the effects of AMPA/kainate receptor agonists on GABA(A) receptor subunit mRNA expression in vitro in cultured rat cerebellar granule cells (CGCs). Kainate (KA) (100 microM) and high K(+) (25 mM) dramatically up-regulated delta subunit mRNA expression to 500-700% of that in control cells grown in low K(+) (5 mM). KA or high K(+) had no effect on the expression of the other major GABA(A) receptor subunits alpha1, alpha6, beta2, beta3 or gamma2. Up-regulation of delta mRNA was also detected with the AMPA receptor-selective agonist CPW-399 and to a lesser extent with the KA receptor-selective agonist ATPA. AMPA/kainate receptor-selective antagonist DNQX completely inhibited KA-, CPW-399- and ATPA-induced delta mRNA up-regulation indicating that the effects were mediated via AMPA and KA receptor activation. NMDA receptor-selective antagonist MK-801 inhibited 76% of the KA- and 57% of the CPW-399-induced delta up-regulation suggesting that KA and CPW-399 treatments may induce glutamate release resulting in NMDA receptor activation, and subsequently to delta mRNA up-regulation. In CGCs, delta subunit is a component of extrasynaptic alpha6betadelta receptors that mediate tonic inhibition. Up-regulation of delta during prolonged glutamate receptor activation or cell membrane depolarization may be a mechanism to increase tonic inhibition to counteract excessive excitation.

Modulation of nicotinic acetylcholine and N-methyl-d-aspartate receptors by some Hymenopteran venoms

The effect of 19 venoms from solitary wasps, solitary bees, social wasps and ants were investigated for their effects on nicotinic acetylcholine receptors (nAChR) and ionotropic glutamate receptors (IGRs) of both the N-methyl-d-aspartate (NMDAR) and non-NMDAR type. Whole-cell patch clamp of human muscle TE671 cells was used to study nAChR, and of rat cortical and cerebellar granule cells for IGRs. Solitary wasp venoms caused significant voltage-dependent antagonism of nAChR responses to 10 microM ACh and NMDAR responses to 100 microM NMDA (+10 microM glycine) when co-applied at 1 microg/ml with the agonists. At positive holding potentials (V(H)) potentiation of these receptors was observed with some venoms. Solitary bee venoms only affected nAChR by causing either voltage-independent antagonism or potentiation of their responses to 10 microM ACh. Of four social wasp venoms, one acted on nAChR by potentiating responses to 10 ACh, while another generated an ACh-like response when applied alone. They had no effect on IGRs. Of the two ant venoms, one caused voltage-independent inhibition of nAChR. Neither affected IGRs. The data indicate the presence of nAChR agonists and antagonists and NMDAR antagonists in Hymenopteran venoms and warrant further investigation to separate and identify these venom components.

Kainate down-regulates a subset of GABAA receptor subunits expressed in cultured mouse cerebellar granule cells

The effect of kainate, an agonist selective for ionotropic AMPA/kainate type of glutamate receptors, on GABAA receptor subunit expression in cultured mouse cerebellar granule cells was studied using quantitative RT-PCR, ligand binding and electrophysiology. Chronic kainate treatment, without producing excitotoxicity, resulted in preferential, dose- and time-dependent down-regulation of alpha1, alpha6 and beta2 subunit mRNA expression, the expression of beta3, gamma2 and delta subunit mRNAs being less affected. The down-regulation was reversed by DNQX, an AMPA/kainate-selective glutamate receptor antagonist. A 14-day kainate treatment resulted in 46% decrease of total [3H]Ro 15-4513 binding to the benzodiazepine sites. Diazepam-insensitive [3H]Ro 15-4513 binding was decreased by 89% in accordance with very low amount of alpha6 subunit mRNA present. Diazepam-sensitive [3H]Ro 154513 binding was decreased only by 40%, contrasting >90% decrease in alpha1 subunit mRNA expression. However, this was consistent with lower potentiation of GABA-evoked currents in kainate-treated than control cells by the alpha1-selective benzodiazepine site ligand zolpidem, suggesting compensatory expression of alpha5 (and/or alpha2 or alpha3) subunits producing diazepam-sensitive but zolpidem-insensitive receptor subtypes. In conclusion, chronic kainate treatment of cerebellar granule cells selectively down-regulates oil, alpha6 and beta2 subunits resulting in altered GABAA receptor pharmacology.

RNA synthesis-dependent potentiation of alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionate receptor-mediated toxicity by antihistamine terfenadine in cultured rat cerebellar neurons

We have studied the effects of terfenadine on neurotoxicity and elevation of free cytoplasmic Ca2+ levels upon stimulation of alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionate (AMPA) receptors in cultured cerebellar neurons. Pre-exposure to terfenadine (5 microM, 5 h) significantly increased neuronal death following specific stimulation of receptors by 100 microM AMPA or by subtoxic concentrations of domoate (8 microM), stimuli that are non-toxic when applied to terfenadine-untreated sister cultures. Terfenadine potentiation was prevented by the transcription inhibitor actinomycin D and was significantly ameliorated by histamine (1 mM). In terfenadine-treated neurons, AMPA increased [Ca2+](i) by approximately five fold, while AMPA induced no significant increase in [Ca2+](i) in the absence of terfenadine. Terfenadine reduced neuronal steady-state concentrations of [Ca2+](i) by approximately 75%. Our results suggest a role for histamine H1 receptors and intracellular calcium in the modulation of the excitotoxic response via AMPA receptors.

Actions and interactions of extracellular potassium and kainate on expression of 13 gamma-aminobutyric acid type A receptor subunits in cultured mouse cerebellar granule neurons

Cerebellar granule neurons in culture are a popular model for studying neuronal signaling and development. Depolarizing concentrations of K(+) are routinely used to enhance cell survival, and kainate is sometimes added to eliminate GABAergic neurons. We have investigated the effect of these measures on expression of mRNA for gamma-aminobutyric acid type A (GABA(A)) receptor alpha1-6, beta1-3, gamma1-3, and delta subunits in cultures of mouse cerebellar granule neurons grown for 7 or 12 days in vitro (DIV) using semiquantitative reverse transcription-PCR. We detected mRNA for the alpha1, alpha2, alpha5, alpha6, beta2, beta3, gamma2, and delta subunits in all the cell cultures, but the expression levels of the alpha5-, alpha6-, and beta2-subunit mRNAs were significantly dependent on the composition of the culture medium. Both an increase of the extracellular K(+) concentration from 5 to 25 mm and the addition of 50 microm kainate immediately depolarized the neurons but prolonged exposure (7-8 DIV)-induced compensatory hyperpolarization. 25 mm K(+) caused a shift from alpha6 to alpha5 expression measured at 7 and 12 DIV, which was mimicked by kainate in 12 DIV cultures. The expression of beta2 was decreased by 25 mm K(+) in 7 DIV cultures and by kainate in 12 DIV cultures. The effects on beta2 expression could not be ascribed to depolarization. Alterations of alpha6 mRNA expression were reflected in altered sensitivity to GABA and furosemide of the resulting receptors. Our study has shown that a depolarizing K(+) concentration as well as kainate in the culture medium significantly disturbs maturation of GABA(A) receptor subunit expression.

Expression of AMPA/kainate receptors during development of chick embryo retina cells: in vitro versus in vivo studies

The activity and the subunit expression of alpha-amino-3-hydroxy-5-methylisoxazole-4-propionate (AMPA)/kainate ionotropic glutamate receptors were studied in retina cells developing in chick embryos and in retina cells cultured as retinospheroids, at the same stages of development. In the retinospheroids, the activity of the AMPA/kainate receptors was monitored by following the changes in the intracellular free calcium concentration ([Ca(2+)](i)), in response to AMPA, kainate or to L-glutamate, and the expression of the receptor subunits GluR1, GluR2/3, GluR4 and GluR6/7 was determined in the retinospheroids and in chick retinas by immunodetection using polyclonal antibodies. The changes in [Ca(2+)](i) in response to 400 microM kainate increased from 5h in vitro to 3 days, and remained constant until day 14, whereas the [Ca(2+)](i) in response to 500 microM L-glutamate or 400 microM AMPA increased from 5h in vitro to 3 days, and thereafter decreased slightly until day 14. The [Ca(2+)](i) responses to kainate are mainly due to AMPA receptor stimulation, since the signals were abolished by LY303070, the AMPA receptor antagonist, and were not affected by MK-801, the NMDA receptor antagonist. In retinospheroids, the levels of expression of GluR1 subunit increased from 5h in vitro until day 7, then decreased until day 14. The levels of expression of GluR2/3 and GluR4 subunits increased from 5h in vitro until day 10, and remained constant until day 14. The levels of kainate receptor subunits GluR6/7 increased from 5h in vitro until day 3, and thereafter decreased slightly until day 14. In the retinas, the expression of GluR1 and GluR6/7 subunits increased from day 8 until day 15, and then decreased until day 22 (post-natal 1). The subunits GluR2/3 and GluR4 increased from day 8 until day 18, and remained constant until day 22. The results suggest that AMPA/kainate receptors are expressed at early embryonic stages, although at low levels and before synapse formation (E12). However, the AMPA receptors are not completely functional at the first stage studied since they do not respond to the agonist AMPA. Also, the patterns of AMPA/kainate receptor subunit expression in retinospheroids of chick embryo retina cells cultured in vitro and in retina cells developing in the embryo (in vivo) were similar, indicating that the AMPA/kainate receptor subunits expression in these primary cultures mimics their expression in the developing chick retina.

Neurotoxic and neuroprotective effects of glutamate are enhanced by introduction of amyloid precursor protein cDNA

The physiological role of amyloid precursor protein (APP), whose anomalous metabolite is a putative pathogen for Alzheimer disease, remains unclear. From the enhanced responsiveness to glutamate in cultured hippocampal neurons after the introduction of cDNA of APP695 (an isoform of APP dominant in human brain) using an adenovirus vector, we have recently raised the hypothesis that APP modulates neuronal sensitivity to glutamate. To test this hypothesis, we utilized here the unique effects of glutamate on the survival of different types of neurons. It is known that hippocampal neurons undergo deterioration in 24 h after application of glutamate in a dose-dependent manner. This vulnerability was increased in the cells transfected with adenovirus carrying cDNA of APP695. By contrast, it is known that cerebellar granule neurons require for their survival the supplementation of NMDA to the medium. The dose of NMDA required for survival was reduced after the transfection of the APP-adenovirus to cerebellar granule neurons. These enhancing effects of APP on the glutamate-induced vulnerability in hippocampal neurons and the glutamate (NMDA)-dependent survival in cerebellar neurons were blocked by glutamate receptor inhibitors, and were not seen after application of a control adenovirus carrying cDNA of beta-galactosidase. Since the effects of glutamate were enhanced in both directions, the hypothesis became more likely that one of the physiological functions of cellular APP is the regulation of glutamate receptors.

Excitotoxic profiles of novel, low-affinity kainate receptor agonists in primary cultures of murine cerebellar granule cells

The involvement of low-affinity kainate (KA) receptors in neuronal injury was investigated by employing a variety of agonists active at GluR5-7. Their excitotoxic profiles were determined in primary cultures of cerebellar granule cells, which abundantly expressed low-affinity KA receptors, and in the absence of any AMPA receptor-mediated neurotoxicity. Neurotoxicity induced by these compounds was analysed by phase contrast microscopy, a cell viability assay, the TUNEL technique (apoptosis), and by employing propidium iodide (PI; necrosis). All agonists induced concentration-dependent neurotoxicity, with rank order (EC(50) values; microM): (S)-iodowillardiine (IW) 0.2>(2S,4R)-4-methylglutamate (4-MG) 36>(2S,4R,6E)-2-amino-4-carboxy-7-(2-naphthyl)hept-6-enoic acid (LY339434) 46>KA 74>(RS)-2-amino-3-(hydroxy-5-tert-butylisoxazol-4yl)propanoic acid (ATPA) 88. IW exposure resulted in apoptosis at lower concentrations (<30 microM) and necrosis at higher concentrations, both of which were attenuated by CNQX (50 microM), but not MK-801 (10 microM). ATPA-mediated neurotoxicity was purely apoptotic and was attenuated by the non-NMDA receptor antagonists. Both IW and ATPA induced injury with the morphological characteristics of apoptosis shown by the presence of TUNEL-positive neurones. LY339434-mediated neuronal injury was only attenuated by MK-801 and was necrotic in nature. Similarly, 4-MG (>30 microM) exposure caused necrosis that was partially attenuated by MK-801 (10 microM) and CNQX (50 microM). The patterns of neurotoxicity possessed a complex pharmacological profile, demonstrated an apoptotic-necrotic continuum and were inconsistent with past findings, further outlining the importance of characterizing novel compounds at native receptors. ATPA and to a lesser extent IW appear to be suitable drugs for low-affinity KA receptors. Since toxicity-mediated by low-affinity KA receptors seem likely to contribute to neurodegenerative conditions, our study importantly examines the excitotoxic profile of these novel agonists.

Glutamate receptor subunit expression in primary neuronal and secondary glial cultures

We report on the expression of ionotropic glutamate receptor subunits in primary neuronal cultures from rat cortex, hippocampus and cerebellum and of metabotropic glutamate (mGlu) receptor subtypes in these neuronal cultures as well as in cortical astroglial cultures. We found that the NMDA receptor (NR) subunits NR1, NR2A and NR2B were expressed in all three cultures. Each of the three cultures showed also expression of the four AMPA receptor subunits. Although RT-PCR detected mRNA of all kainate (KA) subunits in the three cultures, western blot showed only expression of Glu6 and KA2 receptor subunits. The expression analysis of mGlu receptors indicated the presence of all mGlu receptor subtype mRNAs in the three neuronal cultures, except for mGlu2 receptor mRNA, which was not detected in the cortical and cerebellar culture. mGlu1a/alpha, -2/3 and -5 receptor proteins were present in all three cultures, whereas mGlu4a and mGlu8a receptor proteins were not detected. Astroglial cultures were grown in either serum-containing or chemically defined medium. Only mGlu5 receptor protein was found in astroglial cultures grown in serum-containing medium. When astrocytes were cultured in chemically defined medium, mGlu3, -5 and -8 receptor mRNAs were detected, but at the protein level, still only mGlu5 receptor was found.

Cortical and striatal neuronal cultures of the same embryonic origin show intrinsic differences in glutamate receptor expression and vulnerability to excitotoxicity

Cortical and striatal cultures were prepared from the same embryonic rat brains and maintained in identical culture conditions. In this way, the intrinsic, genetically imprinted differences determine the responses of cortical and striatal neurons in comparative studies. Cortical and striatal neurons differed in their sensitivity to glutamate receptor-mediated neurotoxicity as measured by the MTT cell viability assay. On the 8th day in vitro, striatal cultures were less sensitive to N-methyl-d-aspartate (NMDA)-induced toxicity than cortical, although both cultures were equally vulnerable to alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionate (AMPA)- or kainate-induced toxicity. The AMPA receptor-mediated cell death in cortical cultures, however, was much more dependent on preventing AMPA receptor desensitization than in striatal cultures. Furthermore, glutamate-induced neurotoxicity was primarily mediated by NMDA receptors in cortical cultures, while blockade of either NMDA or AMPA receptors gave almost complete protection against glutamate in striatal cultures. To elucidate the molecular mechanisms responsible for the observed differences, we analyzed the expression of NMDA receptor subunits (NR1, NR2A-C) at the mRNA and the protein level in cortical and striatal cultures as well as in standard cerebellar granule cell cultures. The lowest expression level of NMDA receptor subunits was found in striatal cultures, thereby providing a possible explanation for their lower sensitivity to NMDA. Remarkable differences were found between the relative rates of mRNA and protein expression for NR1 and NR2B in the three cultures, indicative of intrinsic differences in the posttranscriptional regulation of NMDA receptor subunit expression in cultures from various brain regions.

Heterogeneous conductance levels of native AMPA receptors

The single-channel properties of AMPA receptors can affect information processing in neurons by influencing the amplitude and kinetics of synaptic currents, yet little is known about the unitary properties of native AMPA receptors in situ. Using whole-cell and outside-out patch-clamp recordings from granule cells in acute cerebellar slices, we found that migrating granule cells begin to express AMPA receptors before they arrive in the internal granule cell layer and receive synaptic input. At saturating agonist concentrations, the open probability of channels in outside-out patches from migrating cells was very high, allowing us to identify patches that contained only one or two active channels. Analysis of the single-channel activity in these patches showed that individual AMPA receptors exhibit as many as four distinguishable conductance levels. The conductance levels observed varied substantially for different channels, although on average the values fell within the range of unitary conductances estimated previously for synaptic AMPA receptors. In contrast to patches from migrating granule cells, we rarely observed directly resolvable single-channel currents in patches excised from the somata of granule cells in the internal granular layer, even though these cells gave large AMPA receptor whole-cell currents. We did, however, detect AMPA receptors with apparent unitary conductances of <1 pS in patches from both migrating and mature granule cells. Our results suggest that granule cells express a heterogeneous population of AMPA receptors, a subset of which are segregated to postsynaptic sites after synaptogenesis.

AMPA receptor subunit mRNAs and intracellular [Ca(2+)] in cultured mouse and rat cerebellar granule cells

Cultured mouse cerebellar granule cells differ from their rat counterparts in that they survive well when grown in non-depolarising medium (5 mM K(+)). However, when chronically stimulated by added glutamate agonists, including (RS)alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA), rat cerebellar granule cells also survive well in non-depolarising medium. We hypothesised that the relatively good survival of mouse cerebellar granule cells in the absence of added glutamate agonists might reflect AMPA receptors resistant to desensitisation. These receptors might be stimulated by endogenous glutamate. We tested this hypothesis by comparing cultured mouse and rat cerebellar granule cells grown in depolarising (25 mM K(+)) and non-depolarising (5 mM K(+)) medium. We studied the AMPA-induced increase in intracellular Ca(2+) concentration ([Ca(2+)](i)), using the fluorescent Ca(2+) chelator, Fluo-3, and the relative concentrations of mRNAs for the four AMPA receptor subunits, GluR1-4. GluR1-4 mRNAs were measured by restriction enzyme analysis of a PCR product containing cDNA with a composition proportional to the four subunit mRNAs. We found that the [Ca(2+)](i)-response to AMPA receptor activation in cultured cerebellar granule cells is determined mainly by the desensitisation properties of the AMPA receptors rather than by their ion permeability. We also found that mouse cerebellar granule cells express AMPA receptors which are more resistant to desensitisation than the corresponding rat AMPA receptors. Thus, relatively slow AMPA receptor desensitisation kinetics may contribute to the survival of mouse cerebellar granule cells in non-depolarising medium.

Pharmacology of AMPA/kainate receptor ligands and their therapeutic potential in neurological and psychiatric disorders

It has been postulated, consistent with the ubiquitous presence of glutamatergic neurons in the brain, that defects in glutamatergic neurotransmission are associated with many human neurological and psychiatric disorders. This review evaluates the possible application of ligands acting on glutamate alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionate (AMPA) and kainate (KA) receptors to minimise the pathology and/or symptoms of various diseases. Glutamate activation of AMPA receptors is thought to mediate most fast synaptic neurotransmission in the brain, while transmission via KA receptors contributes only a minor component. Variants of the protein subunits forming these receptors greatly extend the pharmacological and electrophysiological properties of AMPA/KA receptors. Disease and drug use can differentially affect the expression of the subunits and their variants. Ligands bind to AMPA receptors by competing with glutamate at the glutamate binding site, or non-competitively at other sites on the proteins (allosteric modulators). Ligands showing selective competitive antagonist actions at the AMPA/ KA class of glutamate receptors were first reported in 1988, and the systemically active antagonist 2,3-dihydroxy-6-nitro-7-sulphamoyl-benzo(F)quinoxaline (NBQX) was first shown to have useful therapeutic effects on animal models of neurological diseases in 1990. Since then, newer antagonists with increased potency, higher specificity, increased water solubility, and a longer duration of action in vivo have been developed. Negative allosteric modulators such as the prototype GYKI-52466 also block AMPA receptors but have little action at KA receptors. Positive allosteric modulators enhance glutamatergic neurotransmission at AMPA receptors. Polyamines and adamantane derivatives bind within the ion channel of calcium-permeable AMPA receptors. The latest developments include ligands selective for KA receptors containing Glu-R5 subunits. Evidence for advantages of AMPA receptor antagonists over N-methyl-D-aspartate (NMDA) receptor antagonists for symptomatic treatment of neurological and psychiatric conditions, and for minimising neuronal loss occurring after acute neurological diseases, such as physical trauma, ischaemia or status epilepticus, have been shown in animal models. However, as yet AMPA receptor antagonists have not been shown to be effective in clinical trials. On the other hand, a limited number of clinical trials have been reported for AMPA receptor ligands that enhance glutamatergic neurotransmission by extending the ion channel opening time (positive allosteric modulators). These acute studies demonstrate enhanced memory capability in both young and aged humans, without any apparent serious adverse effects. The use of these allosteric modulators as antipsychotic drugs is also possible. However, the long term use of both direct agonists and positive allosteric modulators must be approached with considerable caution because of potential adverse effects.

Terfenadine induces toxicity in cultured cerebellar neurons: a role for glutamate receptors

Exposure of cultured cerebellar neurons to the histamine H1 receptor antagonist terfenadine resulted in neuronal degeneration and death. Terfenadine neurotoxicity was dependent upon concentration and time of exposure. After 2 h exposure, 20 microM terfenadine reduced the number of surviving neurons by 75%, and as low as 10 nM terfenadine induced significant neurotoxicity after 5 days of exposure. Neuronal sensitivity to terfenadine changed with age in culture, and at 25 days in culture neurons appeared to be much less sensitive than at 5 or 9-17 days in culture. Neurotoxicity by terfenadine could not be prevented by high concentrations of histamine (5 mM), but it was significantly delayed by blocking NMDA or non-NMDA glutamate receptors with MK-801 or CNQX respectively, suggesting the involvement of excitatory transmission mediated by glutamate in the neurotoxicity induced by terfenadine in these neurons. We also found that the presence of terfenadine (5 microM) unveiled the potential excitotoxity of the non-NMDA receptor agonist AMPA (100 microM), and reduced the concentration of glutamate necessary to induce excitotoxicity, compared to untreated cultures. These results suggest a role for terfenadine in the modulation of the excitotoxic response mediated in cerebellar neurons through ionotropic glutamate receptors.

Turnover analysis of glutamate receptors identifies a rapidly degraded pool of the N-methyl-D-aspartate receptor subunit, NR1, in cultured cerebellar granule cells

The number, composition, and location of receptors in neurons are critically important factors in determining the neuron's response to neurotransmitters. The functional expression of receptors appears to be regulated both generally, at the level of transcription or translation, and locally, at the level of the individual synapse. A key component in the regulation of any protein is its turnover rate, which, measured in half-lives, ranges from a few minutes to several days. In the present study, we measured the turnover rates of subunits of N-methyl-D-aspartate (NMDA) and alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid (AMPA) receptors, the two major ionotropic glutamate receptors, using cultured cerebellar granule cells. Turnover rates for NR1, NR2A/B, GluR2/3, and GluR4 subunits were determined by pulse-chase labeling of cells with [35S]methionine. Half-lives were found to be 18 +/- 5 h and 23 +/- 8 h for the AMPA receptor subunits GluR2/3 and GluR4, respectively, and 16 +/- 5 h for NR2A. The NR1 subunit showed a biphasic decay with half-lives of 2 and 34 h for the rapidly and slowly degraded populations, respectively. Splice variants of the NR1 subunit with different carboxyl-terminal cassettes, C2 and C2', showed similar biphasic degradation patterns. To further characterize the rapidly degraded pool of NR1, surface receptors were labeled by biotinylation, and half-lives of the biotinylated proteins were determined. All surface NR1 was slowly degraded with a pattern similar to that of NR2A, GluR2/3, and GluR4, suggesting that the rapidly degraded pool is confined to the cytoplasm and not assembled with NR2 subunits. A significant amount of NR1 was not immunoprecipitated by NR2 subunit-specific antibodies after solubilization with deoxycholate. This unassembled pool, but not the assembled one, was greatly diminished following treatment of cycloheximide for 5 h, indicating that the rapidly degraded pool of NR1 is not assembled with NR2. These results show that NMDA and AMPA receptors have similar turnover rates, but NMDA receptors have a separate pool of NR1 subunits that is rapidly degraded and accounts for most of the intracellular pool.

Surface expression and metabolic half-life of AMPA receptors in cultured rat cerebellar granule cells

The surface expression and metabolic turnover time of alpha-amino-3-hydroxy-5-methylisoxazole-4-propionate (AMPA) receptors are important factors in determining the involvement of these proteins in synaptic function. We used the membrane-impermeant chemical crosslinking reagent BS3 and subsequent immunoblot analysis to show that a large proportion of each of the AMPA receptor subunits GluR1-4 is surface expressed in cultured rat cerebellar granule cells. The apparent molecular mass of the individual subunits was 105 kDa and the surface expressed crosslinked complex was 530 kDa for each of the antibodies tested. These results are consistent with functional ionotropic glutamate receptors being pentameric subunit assemblies. We investigated the time course of AMPA receptor surface expression using the membrane-impermeant biotinylating agent NHS-SS-biotin. The half-life of surface expressed AMPA receptors was found to be 30 h. To estimate the mean degradation rate of AMPA receptors and total trichloracetic acid (TCA)-precipitable protein we used [35S]methionine/cysteine pulse-chase labelling. The half-life of AMPA receptors immunoprecipitated with anti-GluR1 antibody was approximately 48 h and the half-life of total TCA-precipitable protein from the same samples was 37 h.

Investigations of non-NMDA receptor-induced toxicity in serum-free antioxidant-rich primary cultures of murine cerebellar granule cells

A culture system was developed whereby murine cerebellar granule cells were grown under serum-free conditions in chemically defined B27-supplemented neurobasal medium plus depolarizing K+ levels, to allow the investigation of the role of agonists at the kainate and alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionate (AMPA) receptors in glutamate-mediated neurotoxicity. Neurones were killed in a concentration-dependent manner by L-glutamate, kainate and its analogues, domoate and 4-(2-methoxyphenyl)-2-carboxy-3-pyrrolidineacetic acid, but not by (S)-AMPA or (S)-5-fluorowillardiine. Kainate (60% maximal cell death at 1mM) was markedly more toxic than NMDA (40% maximal cell death at 1mM) and was shown to be the predominant cause of excitatory amino acid-induced toxicity in these cells as the neuronal death induced by KA was attenuated by the non-NMDA antagonist CNQX, but not the AMPA antagonist LY293558. This study suggests that serum-free cultures of cerebellar granule cells in B27-supplemented neurobasal medium provide a valuable model system for investigations of the role of the kainate receptor in excitatory amino acid-induced neurodegeneration.

AMPA receptor mediated excitotoxicity in neocortical neurons is developmentally regulated and dependent upon receptor desensitization

AMPA (alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid) excitotoxicity was examined in cultured neocortical neurons using the reduction of 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) to measure cell viability. Neurons were exposed to AMPA at different culture periods during development of the neurons. In order to describe the pharmacology of AMPA-mediated toxicity, several glutamate receptor antagonists were used: MK-801, NS 394, NBQX, GYKI 52466, GYKI 53405 and GYKI 53655. Increased excitotoxicity was observed when cortical neurons cultured for 5, 8 and 12 days in vitro (DIV) were exposed to a high concentration of AMPA (500 microM) for 6 h. However, only at DIV 12 was part of the toxicity mediated directly through AMPA receptors since 10 microM MK-801 blocked all AMPA toxicity at DIV 5 and 8, but only some of the AMPA response at DIV 12. This indicated that NMDA receptors were being activated, causing some of the observed toxicity. The high dose of AMPA was not sufficient to damage all neurons since 59% remained viable after exposure to AMPA even for neurons that were cultured for 12 DIV. Since it is known that both glutamate and AMPA activate AMPA receptors with a fast and rapidly desensitizing response, this could explain the relatively low toxicity produced by 500 microM AMPA. This was investigated by blocking AMPA receptor desensitization with cyclothiazide. Using a lower concentration (25 microM) of AMPA, addition of 50 microM cyclothiazide increased the AMPA induced excitotoxicity in cultured cortical neurons at all DIV except for DIV 2. This combination of AMPA + cyclothiazide yielded 77% cell death for DIV 12 cultures. In contrast to the results observed with 500 microM AMPA, the neurotoxicity mediated directly by AMPA receptors when desensitization was blocked was seen as early as 5 DIV since 10 microM MK-801 did not completely block the response whereas 10 microM NBQX did. The 2,3-benzodiazepine GYKI compounds, which have been reported to be selective non-competitive AMPA receptor antagonists, were here observed to block the AMPA toxicity with the following rank order: GYKI 53655 > GYKI 52466 > or = GYKI 53405, which is in agreement with their published potencies.

Mouse cerebellar granule cell differentiation: electrical activity regulates the GABAA receptor alpha 6 subunit gene

GABAA receptor alpha6 subunit gene expression marks cerebellar granule cell maturation. To study this process, we used the Deltaalpha6lacZ mouse line, which has a lacZ reporter inserted into the alpha6 gene. At early stages of postnatal cerebellar development, alpha6-lacZ expression is mosaic; expression starts at postnatal day 5 in lobules 9 and 10, and alpha6-lacZ is switched on inside-out, appearing first in the deepest postmigratory granule cells. We looked for factors regulating this expression in cell culture. Membrane depolarization correlates inversely with alpha6-lacZ expression: granule cells grown in 25 mM [K+]o for 11-15 d do not express the alpha6 gene, whereas cultures grown for the same period in 5 mM [K+]o do. This is influenced by a critical early period: culturing for >/=3 d in 25 mM [K+]o curtails the ability to induce the alpha6 gene on transfer to 5 mM [K+]o. If the cells start in 5 mM [K+]o, however, they still express the alpha6-lacZ gene in 25 mM [K+]o. In contrast to granule cells grown in 5 mM [K+]o, cells cultured in 25 mM [K+]o exhibit no action potentials, mEPSCs, or mIPSCs. In chronic 5 mM [K+]o, factors may therefore be released that induce alpha6. Blockade of ionotropic and metabotropic GABA and glutamate receptors or L-, N-, and P/Q-type Ca2+ channels did not prevent alpha6-lacZ expression, but inhibition of action potentials with tetrodotoxin blocked expression in a subpopulation of cells.

Expression and heteromeric interactions of non-N-methyl-D-aspartate glutamate receptor subunits in the developing and adult cerebellum

The localization and expression of ionotropic non-N-methyl-D-aspartate glutamate receptors (GluR) were investigated in the developing and adult rat cerebellum using subunit-specific polyclonal antibodies for immunocytochemical, immunoblot and immunoprecipitation studies. In P7 animals, alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionate receptor immunoreactivity was detected in all layers of the cerebellar cortex with the exception of the external granule cell layer. Antibodies against the alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionate receptor subunits GluR1 and GluR4 gave strong immunoreactive staining of Bergmann glia in both young and adult animals, and both antibodies showed prominent staining of the molecular layer in the adult cerebellum. Dense immunoreactive staining of Purkinje cell somata and dendrites was obtained with anti-GluR2/3/4c in both the developing and adult cerebellum. Whereas each of the three alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionate receptor antibodies stained the internal, but not the external, granule cell layer, immunostaining for the kainate-type subunits GluR6/7 and KA2 was detected in both the external and internal granule cell layers. as well as in the molecular layer in both P7 and adult cerebellum. Immunoblot analysis of total cerebellar protein indicated that the level of GluR4 expression increased 15-fold from P1 to P18, whereas the expression of the KA2 subunit protein was nine-fold lower in adult cerebellum than it was at P1. The expression of GluR1 increased moderately (two-fold) from P1 to adult. Subunit interactions between GluR1 and GluR4, as well as between GluR6/7 and KA2, were demonstrated in immunoprecipitation experiments; and the GluR4 and KA2 subunits appear to be present exclusively in heteromeric assemblies with GluR1 and GluR6/7, respectively. The results show that the various alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionate- and kainate-type subunits are differentially expressed during cerebellar development and further define the possible subunit composition of non-N-methyl-D-aspartate receptors in the major cerebellar cell types.

Developmental changes in the inhibitory actions of ethanol on glutamate-induced translocation of protein kinase C in cerebellar granule neurons

The effects of increasing concentrations of ethanol (25-200 mM) on the enhancement of [3H]phorbol-12,13-dibutyrate ([3H]PDBu) binding produced by different glutamate receptor agonists, indicative of a translocation of the intracellular enzyme protein kinase C (PKC), were studied in rat cerebellar granule cells at 2, 4, 8, and 12 days in vitro (DIV). Glutamate-produced stimulation of [3H]PDBu binding was inhibited by 50 mM ethanol at 2 DIV, whereas higher ethanol concentrations (> 100 mM) were needed to reduce the increase of [3H]PDBu binding in cells grown for 4, 8, and 12 DIV. Ethanol significantly inhibited NMDA-stimulated [3H]PDBu binding in a concentration-dependent fashion in cells maintained in culture for 4 and 8 days, respectively, with a slightly less pronounced inhibition by ethanol (50 mM) seen in cells kept for 2 and 12 DIV. Application of higher ethanol concentrations (> 100 mM), inhibited the NMDA-induced stimulation in all cell preparations. Following kainic acid-induced enhancement of [3H]PDBu binding, ethanol (100 mM) reduced the binding only in cells maintained for 2 DIV. Even higher ethanol concentrations (200 mM) inhibited the effects of kainic acid only in cells maintained for 2 and 4 DIV, respectively. Our data suggest that various subclasses of glutamate receptors display a developmentally determined differential sensitivity to ethanol at least in cerebellar granule cells in vitro.

7-Chloro-3-methyl-3,4-dihydro-2H-1,2,4-benzothiadiazine S,S-dioxide: a partial modulator of AMPA receptor desensitization devoid of neurotoxicity

In cerebellar granule neurons of neonatal rats micromolar concentrations of 7-chloro-3-methyl-3,4-dihydro-2H-1,2, 4-benzothiadiazine S,S-dioxide (IDRA-21) and cyclothiazide, two negative modulators of the spontaneous agonist-dependent rapid desensitization of alpha-amino-3-hydroxy-5-methylisoxazolepropionic acid (AMPA)-gated ion channels, facilitate AMPA receptor function by increasing the content of free cytosolic Ca2+ as measured by single-cell fura-2 acetoxymethyl ester (Fura-2) Ca2+-dependent fluorescence and intracellular Na+ measured with the sodium-binding bezofuran isophthalate acetoxymethyl ester fluorescence indicator. IDRA-21 increases intracellular Na+ transient with a threshold (5 microM) that is approximately 10 times higher and has an intrinsic activity significantly lower than that of cyclothiazide. By virtue of its low intrinsic activity, IDRA-21 elicits a free cytosolic Ca2+ transient increase that is shorter lasting than that elicited by cyclothiazide even when the drug is left in contact with cultured granule cells for several minutes. Additionally, while dose dependently, 5-25 microM cyclothiazide in the presence of AMPA is highly neurotoxic, IDRA-21 (up to 100 microM) is devoid of neurotoxicity. The neurotoxicity elicited by cyclothiazide persists in the presence of dizocilpine (an antagonist of N-methyl-D-aspartate-selective glutamate receptors) but is blocked by 2,3-dihydroxy-6-nitrosulfamoylbenzo[f]quinoxaline (a competitive AMPA receptor antagonist) and the 1-(aminophenyl)-4-methyl-7, 8-methylendioxy-5H-2,3-benzodiazepine (GYKI 52466; a noncompetitive AMPA receptor antagonist). Since the doses of IDRA-21 that enhance cognitive processes in rats and monkeys are several orders of magnitude lower than those required to elicit marginal neurotoxicity in cultured neurons, it can be surmised that IDRA-21 is a potent cognition-enhancing drug virtually devoid of neurotoxic liability because it acts as a partial negative allosteric modulator of AMPA receptor desensitization.

Distribution of Ca(2+)-permeable AMPA receptors among cultured rat cerebellar granule cells

Some AMPA receptors are permeable to Ca2+. It has been suggested that cultured rat cerebellar granule cells express Ca(2+)-permeable AMPA receptors, but their distribution at a single cell level is unknown. We report that AMPA (in the presence of cyclothiazide) induced Ca2+ entry (measured by Mn2+ quench of fura-2 fluorescence) and intracellular Ca2+ increases in cerebellar granule cells in the absence of extracellular Na+, supporting the presence of Ca(2+)-permeable AMPA receptors. Analysis of intracellular Ca2+ signals in single cells demonstrated a heterogeneous distribution of Ca(2+)-permeable AMPA receptors.