Molecular diversity of glutamate receptors and implications for brain function

Assembly of the inhibitory glycine receptor: identification of amino acid sequence motifs governing subunit stoichiometry

The inhibitory glycine receptor (GlyR) is a pentameric protein composed of two types (alpha and beta) of membrane-spanning subunits. Coexpression in Xenopus oocytes of a low affinity mutant of the alpha 2 subunit with the alpha 1 and beta subunits indicated that GlyRs assembled from alpha 1 and alpha 2 polypeptides contain variable subunit ratios, whereas alpha/beta hetero-oligomers have an invariant (3:2) stoichiometry. Analysis of different alpha/beta chimeric constructs revealed that this difference in assembly behavior is mediated by the N-terminal extracellular regions of the receptor subunits. Substitution of residues diverging between the alpha and beta subunits identified combinations of sequence motifs determining subunit stoichiometry.

(1S,3R)-1-aminocyclopentane-1,3-dicarboxylic acid attenuates N-methyl-D-aspartate-induced neuronal cell death in cortical cultures via a reduction in delayed Ca2+ accumulation

The effects of (1S,3R)-ACPD, a selective metabotropic glutamate receptor agonist, on NMDA-induced 45Ca2+ accumulation and delayed neuronal cell death were determined using primary cerebrocortical cultures. Exposure to (1S,3R)-ACPD alone, although causing small increases in 45Ca2+ accumulation, was not neurotoxic. The presence of (1S,3R)-ACPD during exposure to NMDA attenuated the resulting sustained accumulation of 45Ca2+ and delayed neuronal cell death. Reductions in sustained Ca2+ accumulation were associated both with Ca2+ efflux, in the absence of cell death, and inhibition of delayed intracellular Ca2+ accumulation. The protective effects of (1S,3R)-ACPD on NMDA-induced cell death were inhibited by pretreatment of cultures with pertussis toxin. These results suggest that activation of metabotropic glutamate receptors may stimulate intracellular processes capable of limiting sustained elevations in intracellular calcium and the resulting excitotoxic neuronal damage.

Metabotropic glutamate receptors: an original family of G protein-coupled receptors

In 1985, we discovered a new glutamate receptor which was coupled to phospholipase C via a G protein and which was later termed metabotropic glutamate receptor (mGluR). In this review, both the diversity of mGluRs and the cellular events they control are discussed, as well as their roles in physiological regulation and brain function.

Multiphasic responses of cerebellar Purkinje cells to 1S,3R-ACPD: an in vivo study

The effects of iontophoretically applied (1S,3R)-1-aminocyclopentane-1,3-dicarboxylic acid (1S,3R-ACPD), an agonist of metabotropic glutamate receptors, were examined in rat cerebellar Purkinje cells in vivo. Multibarrel electrodes were used for extracellular recordings of spontaneous single unit discharges and iontophoretic ejection of 1S,3R-ACPD. The effect of 1S,3R-ACPD depended on both the strength and the duration of the iontophoretic current. Application of the agonist with ejection currents at or slightly above the response threshold for up to 60 s resulted in an increased rate of action potential firing. With larger ejection currents of the same duration the initial increase in activity was followed by a depression and eventually a cessation of activity. In the transition phase between low frequency firing and firing arrest, Purkinje cells generated almost exclusively complex spikes. When the drug application was continued for longer durations (1-10 min) the initial response was followed by a characteristic cyclic firing pattern. These cycles consisted of alternating phases of mainly simple spike activity, predominantly complex spike activity and silent intervals. At the end of drug applications using large ejection currents, a prolonged period (on average 66 s) with almost no spiking activity was observed. This period ended with an abrupt onset of simple spike firing. These findings point to an important function of cerebellar metabotropic glutamate receptors in the regulation of Purkinje cell activity.

Immunohistochemical localization of a metabotropic glutamate receptor, mGluR5, in the rat brain

A trpE-fusion protein containing a C-terminal sequence of a rat metabotropic glutamate receptor, mGluR5, was used to produce an antibody. On immunoblot, the antibody specifically reacted with mGluR5 expressed in mammalian cells and rat brain. Immunohistochemical analysis revealed intense mGluR5-like immunoreactivity (LI) in the olfactory bulb, anterior olfactory nuclei, olfactory tubercle, cerebral cortex, hippocampus, lateral septum, striatum, nucleus accumbens, inferior colliculus, and spinal trigeminal nuclei. The distribution pattern of mGluR5-LI corresponds very well with that of mGluR5 mRNA. Electron microscope analysis of the striatum revealed dense accumulation of immunoreaction products in dendrites which were often provided with asymmetrical synapses. These results suggest that mGluR5 is predominantly located in postsynaptic elements.

Conantokin-G antagonism of the NMDA receptor subtype expressed in cultured cerebellar granule cells

Conantokins are peptide antagonists of the N-methyl-D-aspartate (NMDA) subclass of excitatory amino acid receptors. We compared conantokin-G and AP5 antagonism of NMDA receptor activity expressed in cultures of neonatal rat cerebellar granule cells, using the fluorescent calcium indicator dye fura-2. The results were consistent with the binding of two molecules of agonist (NMDA) for channel activation and one antagonist molecule (AP5) for inhibition. However, conantokin-G antagonism was more complex: the peptide inhibited only approximately 70% of the elevation of intracellular free calcium produced by NMDA. These results, when combined with previous ones [8], suggest that conantokin-G may have different affinities for, and functional effects on, different subtypes of NMDA receptor complexes expressed in the mammalian CNS.

Inhibition of a slow synaptic response by a metabotropic glutamate receptor antagonist in hippocampal CA3 pyramidal cells

The effects of a novel antagonist of metabotropic glutamate receptors were investigated in CA3 pyramidal cells in hippocampal slice cultures of the rat. Earlier experiments showed that selective activation of metabotropic glutamate receptors with low concentrations of an agonist, 1S, 3R-1-amino-cyclopentane-1,3-dicarboxylic acid (ACPD), induced an inward current associated with a decrease in membrane conductance and inhibition of the slow calcium-dependent potassium current. These responses were strongly and reversibly reduced by the antagonist, (RS)-alpha-methyl-4-carboxyphenylglycine (MCPG, 0.5-1 mM). In the presence of antagonists of ionotropic glutamate receptors, stimulation of the afferent mossy fibres evoked postsynaptic responses in CA3 pyramidal cells which paralleled those observed with exogenously applied metabotropic glutamate receptor agonists, i.e. a slow inward current and a reduction of calcium-dependent potassium current. Both responses were greatly reduced by bath-applied MCPG (1 mM). These results show that MCPG acts as an effective antagonist at metabotropic glutamate receptors coupled to potassium conductances in the hippocampus. Furthermore, they confirm that glutamate release from presynaptic terminals can modulate postsynaptic properties by activation of metabotropic glutamate receptors.

Polyamine-enhanced NMDA receptor activity: effect of ethanol

The effect of ethanol on spermidine-enhanced, N-methyl-D-aspartate (NMDA)-induced seizures and c-fos expression was investigated in the rat brain. The latency of tonic-clonic convulsions induced by i.p. administration of NMDA (50 mg/kg) was decreased by prior i.c.v. injection of spermidine (0.1-2.5 mumol) in a dose-dependent manner. Neither NMDA (50 mg/kg) nor spermidine (up to 2.5 mumol) alone induced c-fos mRNA expression in the brain. When both agents were administered, significant induction of c-fos expression occurred 30 min after the convulsion. Prior treatment with ethanol did not alter the curve of spermidine dose-dependency over most of the range. The c-fos expression induced by a combination of NMDA (50 mg/kg) and spermidine (1.0 mumol) was unaffected by ethanol. Only at a high dose of ethanol (2.0 g/kg) and at minimal spermidine enhancement was NMDA-induced seizure and c-fos expression inhibited. These results suggest that polyamines may have an important role in modulating NMDA receptor function in vivo and that polyamine enhancement of NMDA receptor function is relatively insensitive to the inhibitory effects of ethanol.

The non-psychotropic cannabinoid (+)-(3S,4S)-7-hydroxy-delta 6- tetrahydrocannabinol 1,1-dimethylheptyl (HU-211) attenuates N-methyl-D-aspartate receptor-mediated neurotoxicity in primary cultures of rat forebrain

The non-psychotropic cannabinoid (+)-(3S,4S)-7-hydroxy-delta 6- tetrahydrocannabinol 1,1-dimethylheptyl (HU-211), a stereoselective inhibitor of the N-methyl-D-aspartate (NMDA) receptor, protects primary cultures of rat forebrain against NMDA receptor-mediated neurotoxicity. Cell mortality produced by exposure for 10 min to NMDA or glutamate was reduced to approximately 18 or 27%, respectively, by application of 50 microM HU-211 for 10-15 min during or after exposure of cultures to excitatory amino acid. This effect of HU-211 was dependent on its concentration (EC50 = 8.7 +/- 4 microM). HU-211 also reduced the toxicity induced by brief exposure (10 min) to kinase or quisqualate, though less effectively. HU-211 may therefore prove useful as a non-psychoactive drug that protects against neurotoxicity mediated by the NMDA receptor.

Intracerebral 1S,3R-1-aminocyclopentane-1,3-dicarboxylic acid (1S,3R-ACPD) produces limbic seizures that are not blocked by ionotropic glutamate receptor antagonists

The functional role of metabotropic glutamate receptor (mGluR) activation was investigated following intracerebral administration of 1S,3R-ACPD in mice. Injections of 1S,3R-ACPD (50-800 nmol in 5 microliters) into the thalamus produced a dose-dependent increase in limbic seizures. These effects were stereoselective since 1R,3S-ACPD, did not elicit seizure activity. Pharmacologically, limbic seizures were attenuated by the mGluR partial agonist/antagonist L-2-amino-3-phosphonopropionate (L-AP3) and dantrolene, an inhibitor of intracellular calcium mobilization, but not by D-AP3 or ionotropic glutamate receptor antagonists (MK-801 or GYKI-52466). Thus, activation of mGluRs by 1S,3R-ACPD in mice, induces limbic seizures that may involve the mobilization of intracellular calcium stores.

Characterization of adenylyl cyclase in goldfish brain

The rate of production of cAMP by the adenylyl cyclase enzyme from goldfish brain was linear with time and with protein concentration. In agreement with mammalian adenylyl cyclase systems the enzyme is divalent cation dependent, being activated in the presence of either Mg2+ or Mn2+. Forskolin also stimulated the rate of reaction in a dose-dependent manner with a half-maximal effect of 1 microM. The activated enzyme was inhibited by high concentrations of Ca2+ but was independent of Na+ concentration. The presence of guanine nucleotide binding proteins (G-proteins) was demonstrated by the fact that both NaF and guanosine 5'-[beta gamma-imido]triphosphate (p[NH]ppG) stimulated the basal rate. In addition, the p[NH]ppG dose-response curve of the forskolin-stimulated enzyme was biphasic, similar to that observed for other systems. At low concentrations of p[NH]ppG a small inhibition was observed while higher concentrations produced a stimulation. These data suggest that the goldfish brain adenylyl cyclase enzyme complex includes both stimulatory and inhibitory G-proteins in addition to the catalytic unit. A series of known and putative goldfish neurotransmitter substances failed to either stimulate or inhibit the adenylyl cyclase activity. The endogenous neurotransmitters which interact with this second messenger system remain to be determined.

Glutamate- and AMPA-mediated calcium influx through glutamate receptor channels in medial septal neurons

The Ca(2+)-fraction of the ion current flowing through glutamate receptor channels activated either by glutamate or by AMPA was determined in forebrain neurons of the rat medial septum. By combining whole-cell patch-clamp and fura-2 fluorometric measurements we found that, at negative membrane potentials and at an extracellular free Ca(2+)-concentration of 1.6 mM, the Ca(2+)-fraction of the current activated by glutamate is 5.7%. A pharmacological analysis of responses produced by ionophoretically-released glutamate demonstrated a large contribution of NMDA-receptors but a small contribution of AMPA/kainate receptors to these responses. Interestingly, also AMPA-mediated currents were associated with significant changes in Ca(2+)-sensitive fluorescence. The fractional Ca2+ current of AMPA-induced responses was 1.2 +/- 0.4% (n = 5).

Differential regulation of N-methyl-D-aspartate receptor subunit messenger RNAs in kindling-induced epileptogenesis

N-methyl-D-aspartate-receptors are implicated in several neuropathological conditions including epilepsy. As a model of complex partial seizures, rapid hippocampal kindling was chosen to investigate changes in the expression of messenger RNAs encoding the N-methyl-D-aspartate-receptor subunits NR1, NR2A and NR2B both during and in the period immediately following the induction of the kindled state. The study demonstrates a cell-specific, time-dependent modulation of the N-methyl-D-aspartate-receptor subunit messenger RNAs almost entirely restricted to the granule cells of the dentate gyrus. In partially kindled animals (10 stimulations), while the NR1 subunit messenger RNA remained unaltered after a period of 2 h, the NR2A and NR2B subunit messenger RNAs were bilaterally reduced in dentate gyrus granule cells by around 50% below control values. In fully kindled animals (40 stimulations), a progressive reduction in NR1 subunit messenger RNA levels in the dentate gyrus was observed, being maximal after 4 h (-67%). At the same time point, NR2A and NR2B transcript levels were transiently increased by 102% and 46% above control values, respectively. These data point to a differential regulation of N-methyl-D-aspartate-receptor subunit messenger RNAs. No alterations were detected in pyramidal cells. Long-term maintenance of the kindled state was not associated with alterations in N-methyl-D-aspartate-receptor subunit messenger RNAs since control levels of messenger RNA were attained by 12 h and persisted for at least five days. The early changes in messenger RNAs described in this study indicate that the expression of N-methyl-D-aspartate-receptor subunits is under independent regulatory control. This phenomenon may contribute to epileptogenesis and to kindling-associated plasticity by mediating a structural reorganization of N-methyl-D-aspartate-receptors, leading to an altered excitability of dentate gyrus granule cells.

Multiple structural determinants of voltage-dependent magnesium block in recombinant NMDA receptors

The voltage-dependent block of NMDA channels by Mg2+ is an important functional element of NMDA receptors, since relief of block by depolarization plays a key role in some forms of ischemic neurodegeneration and synaptic plasticity. To identify the relevant structural domains responsible for block by Mg2+ and TCP, we used site-directed mutagenesis to change individual amino acids of the rat NR1A subunit in a transmembrane region (599-DALTLSSAMWFSWGVLLNSGIGE-621, mutated residues underlined) previously shown to donate residues that influence ionic selectivity. Ten mutant NR1A subunits were co-expressed in Xenopus oocytes with either the epsilon 1 or NR2A subunits, and receptor properties were analyzed under two-electrode voltage clamp. The mutation N616R virtually abolished voltage-dependent Mg2+ block, reduced Zn2+ block 5-fold and greatly reduced Ba2+ permeability in confirmation of previous reports. This mutation also reduced the potency of TCP as a use-dependent blocker by 200-fold. The remaining low-affinity TCP block did not appear to be use-dependent, suggesting two blocking sites for TCP. None of the other mutations differed significantly from NR1A itself except S617N, which displayed a 6-fold reduction in Mg2+ block. A well-barrier model of permeation through the NMDA receptor channel is presented that quantitatively reproduces voltage-dependent Mg2+ block. This model demonstrates that only minimum changes energy profiles experienced by permeating ions, equivalent to the energy of a single hydrogen or ionic bond, are required to abolish Mg2+ block. These findings indicate that only small structural changes are needed to convert a Mg(2+)-insensitive ion channel to a channel with pronounced voltage-dependent Mg2+ block.

The NMDA receptor complex

The synaptic responses elicited by glutamate and aspartate in the CNS are mediated by distinct groups of receptors which include the ionotropic NMDA receptor. The NMDA receptor is activated by high-strength synaptic input and produces relatively sustained depolarization which can lead to repetitive burst firing. These characteristics allow it to be involved in the maintainance of rhythmic neuronal activity and in the modulation of synaptic efficacy and plasticity. Overstimulation of the NMDA receptor appears to play a pivotal role in the physiopathology of ischemic brain injury. The NMDA receptor contains an integral cationic channel which is highly permeable to Ca2+ as well as to Na+ and K+. This receptor has several domains in addition to the NMDA recognition site: i) a divalent cation binding site within the channel pore, at which Mg2+ ions bind, ii) a binding site recognized by dissociative anesthetics and MK-801 within the channel; and iii) modulatory sites sensitive to glycine, Zn2+ and polyamines. The NMDA receptor is strictly controlled by Mg2+ ions in a voltage-dependent manner. Moreover, it is modulated by protons, by changes in the redox state and by endogenous physiological substances, eg NO and arachidonic acid. Selective antagonists now exist for the NMDA recognition site and glycine and polyamine modulatory sites. Molecular cloning of the NMDA receptor has identified a subunit termed NMDA-R1 and four additional subunits (NMDA-R2A through NMDA-R2D). Functionally distinct NMDA receptor subtypes are formed by heteromeric assembly of NMDA-R1 with NMDA-R2 subunits. NMDA receptor subunits contain consensus phosphorylation sites for protein kinases at the cytoplasmic domain. The high Ca2+ permeability and sensitivity to channel block by Mg2+ are imparted by asparagine residues in a putative channel-forming segment of the protein transmembrane 2. The knowledge of the molecular structure of the NMDA receptor will help to understand the molecular mechanisms responsible for its regulatory features and the molecular bases of neurotoxicity.

Characterization of a presynaptic glutamate receptor

Glutamate receptors mediate excitatory neurotransmission in the brain and are important in the formation of memory and in some neurodegenerative disorders. A complementary DNA clone that encoded a 33-kilodalton protein (GR33) was obtained by screening a library with an antibody generated against glutamate binding proteins. The sequence of GR33 is identical to that of the recently reported presynaptic protein syntaxin. When GR33 was expressed in Xenopus oocytes, it formed glutamate-activated ion channels that are pharmacologically similar to those of N-methyl-D-aspartate receptors but with different electrophysiological properties. Mutation of the leucine 278 residue in the single putative transmembrane segment of GR33 affects the properties of the channel. Thus, in vivo GR33 may be a presynaptic glutamate receptor.

Heterogeneity of metabotropic glutamate receptors in the striatum: electrophysiological evidence

In order to investigate the functional role of metabotropic glutamate receptors (mGluRs) in the striatum we performed extracellular and intracellular recordings from a corticostriatal brain slice preparation. The effects of L-2-amino-3-phosphopropionic acid (L-AP3), an antagonist of mGluRs, were studied both on long-term synaptic depression (LTD) and on presynaptic inhibition of excitatory postsynaptic potentials (EPSPs) induced by different agonists of mGluRs. L-AP3 produced a dose-dependent (3-30 microM) reduction of the LTD evoked in the striatum by the tetanic stimulation of the corticostriatal pathway. In contrast to this action, L-AP3 (10-100 microM) did not significantly affect the presynaptic inhibitory effect of 1-amino-cyclopentyl-trans-dicarboxylic acid (t-ACPD), an agonist of mGluRs, on corticostriatal transmission. Higher concentrations of L-AP3 (0.3-1 mM) reduced by themselves the EPSP amplitude. The inhibitory effect of t-ACPD on the cortically evoked EPSPs was mimicked either by the active stereoisomer 1S,3R-ACPD or by amino-4-phosphonobutyric acid (L-AP4), a glutamate autoreceptor agonist. In some neurons, these inhibitory actions were coupled with membrane depolarizations. The depression of synaptic transmission caused by t-ACPD, 1S,3R-ACPD and L-AP4 was not altered following the induction of LTD. Chronic lithium treatment of the animals (60-120 mg/kg i.p. for 10 days) blocked striatal LTD but not presynaptic inhibition mediated by mGluR agonists. The present findings show that the mechanisms underlying LTD and the presynaptic inhibition induced by different agonists of mGluRs exhibit functional and pharmacological differences. These data suggest heterogeneity of mGluRs in the striatum.

A new metabotropic glutamate receptor agonist: developmental change of its sensitivity to receptors in the newborn rat spinal cord

A novel metabotropic glutamate receptor agonist, (2S,1'R,2'R,3'R)-2-(2,3-dicarboxycyclopropyl)glycine (DCG-IV), reduced the monosynaptic excitation in newborn rat spinal cord rather than polysynaptic discharges at the nanomolar range without causing postsynaptic depolarization of motoneurones. Its inhibitory action on the monosynaptic excitation reduced in due course of time after birth. On the contrary, the inhibitory action of a metabotropic GABAB receptor agonist, baclofen, did not show marked developmental change. DCG-IV should be expected to have the potential to provide further useful information on the physiological function of metabotropic glutamate receptors.

Toward an understanding of the role of glutamate in experimental parkinsonism: agonist-sensitive sites in the basal ganglia

Increased glutamatergic transmission in the basal ganglia is implicated in the pathophysiology of Parkinson's disease. However, the mechanisms by which activation of glutamate receptors produce parkinsonism are unknown. Therefore, we examined whether the glutamate agonists N-methyl-D-aspartate (NMDA), alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionate (AMPA), kainate, and trans-(+/-)-1-amino-1,3-cyclopentanedicarboxylate produce parkinsonism in rats after microapplication into different subregions of the basal ganglia. Electromyographic activity was used as a measure of parkinsonian rigidity. We found that in the rostral striatum, excitation mediated by NMDA but not by non-NMDA receptors led to parkinsonism. In the substantia nigra pars reticulata, internal pallidal segment/entopeduncular nucleus, and subthalamic nucleus, activation of AMPA/kainate and metabotropic receptors but not of NMDA receptors led to parkinsonian rigidity. Rigidity occurred also in animals bearing ibotenate-induced lesions of the posterior part of the striatum and of the external pallidal segment, but not in animals with lesions of the anterior striatum, subthalamic nucleus, internal pallidal segment/entopeduncular nucleus, or substantia nigra pars reticulata. These observations suggest that the activation of glutamate receptor subtypes in the basal ganglia may be differentially involved in the expression of parkinsonian symptoms.

The metabotropic glutamate receptor (mGluR1 alpha) is concentrated at perisynaptic membrane of neuronal subpopulations as detected by immunogold reaction

An antiserum to mGluR1 alpha labeled a 160 kd protein in immunoblots of membranes derived from rat brain or cells transfected with mGluR1 alpha. Immunoreactivity for mGluR1 alpha was present in discrete subpopulations of neurons. The GABAergic neurons of the cerebellar cortex were strongly immunoreactive; only some Golgi cells were immunonegative. Somatostatin/GABA-immunopositive cells in the neocortex and hippocampus were enriched in mGluR1 alpha. The hippocampal cells had spiny dendrites that were precisely codistributed with the local axon collaterals of pyramidal and granule cells. Electron microscopic immunometal detection of mGluR1 alpha showed a preferential localization at the periphery of the extensive postsynaptic densities of type 1 synapses in both the cerebellum and the hippocampus. The receptor was also present at sites in the dendritic and somatic membrane where synapses were not located.

Definition of a pharmacophore for the metabotropic glutamate receptors negatively linked to adenylyl cyclase

(2S,3S,4S)-alpha-Carboxycyclopropylglycine (L-CCG I) and trans-1-amino-(1S,3R)-cyclopentanedicarboxylic acid ((1S,3R)-ACPD), partially constrained L-glutamate analogs known to be agonists at the metabotropic glutamate receptors (mGluRs) adenylyl cyclase coupled, have been submitted to conformational analysis and the data obtained utilized to define a pharmacophore which takes into account the location of hydrogen bonding donating sites of the receptor. This pharmacophore has been utilized to define the agonist mGluRs decreases cAMP bioactive conformation of L-Glu.

Differential usage of multiple brain-derived neurotrophic factor promoters in the rat brain following neuronal activation

The rat brain-derived neurotropic factor (BDNF) gene consists of four 5' exons linked to separate promoters and one 3' exon encoding the prepro-BDNF protein. To gain insights into the regulation of BDNF mRNA expression, probes specific for the different 5' exons were used to study the expression of BDNF mRNA in the brain. Following a systemic injection of the glutamate analog kainic acid, exon I, II, and III mRNAs increased transiently in hippocampus and cerebral cortex. A modest increase was seen for exon IV, where a new transcription initiation site was induced by this treatment. Pretreatments with the N-methyl-D-aspartate (NMDA) receptor antagonist MK801 or the alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptor antagonist 2,3-dihydroxy-6-nitrosulfanoylbenzo(f)quinoxaline revealed two region-specific patterns of glutamate receptor-mediated regulation. The first pattern found in neocortex, piriform cortex, and amygdala involves regulation of BDNF exon I, II, and III mRNAs through NMDA and AMPA/kainate receptors. The second pattern found in the hippocampus involves regulation of BDNF exon I, II, and III mRNAs by high-affinity kainate or metabotropic receptors. Treatment with the gamma-aminobutyric acid subtype A (GABAA) receptor antagonist bicuculline increased exon I and III mRNAs in the denate gyrus, and the muscarinic receptor agonist pilocarpine increased exon I mRNA mainly in the neocortex. These data show that the four BDNF promoters allow multiple points of BDNF mRNA regulation and suggest that the activation of different subtypes of glutamate receptors differentially regulates the expression of BDNF exon-specific mRNAs in the brain.

Characterization of metabotropic glutamate receptors negatively linked to adenylyl cyclase in brain slices

We have characterized the pharmacological profile of activation of metabotropic glutamate receptors negatively linked to adenylyl cyclase (mGluR decreases cAMP) in brain slices. Among the putative mGluR agonists, (2S,1'R,2'R,3'R)-2-(2,3-dicarboxycyclopropyl)glycine (DCG-IV) and (1S,3R)-1-aminocyclopentane-1,3-dicarboxylic acid (ACPD), were the most potent inhibitors of forskolin-stimulated cAMP formation in hippocampal slices, followed by ibotenate, L-2-amino-3-phosphonopropionate (AP3), quisqualate, L-glutamate and beta-N-methylamino-L-alanine (BMAA). Inhibition of forskolin-stimulated cAMP formation by DL-2-amino-4-phosphonobutanoate (AP4) was biphasic, suggesting that the drug interacts with more than one mGluR decreases cAMP subtype. Both L-AP4 and L-serine-O-phosphate (a restricted analogue of AP4) were much more effective in inhibiting forskolin-stimulated cAMP formation than their D-isomers, indicating that interaction of these drugs with the mGluR decreases cAMP is stereoselective. Despite the fact that DCG-IV and ibotenate behave as NMDA receptor agonists, their effect was insensitive to MK-801. The regional pattern of expression of mGluR decreases cAMPS, as estimated by using 1S,3R-ACPD as an agonist, did not correlate with the steady-state levels of mGluR2 mRNA. Thus, 1S,3R-ACPD inhibited forskolin-stimulated cAMP in slices from hippocampus, cerebral cortex, corpus striatum, olfactory tubercle or hypothalamus, but not in slices from olfactory bulb or cerebellum; in contrast, mGluR2 mRNA levels were high in the olfactory bulb and very low in the corpus striatum. 1S,3R-ACPD also inhibited forskolin-stimulated cAMP formation in cortical membranes, excluding the involvement of trans-synaptic mechanisms in the activity of mGluR decreases cAMPS.(ABSTRACT TRUNCATED AT 250 WORDS)

Cloning and sequence analysis of cDNAs encoding human hippocampus N-methyl-D-aspartate receptor subunits: evidence for alternative RNA splicing

Several cDNA clones encoding human N-methyl-D-aspartate receptor (hNR1) subunit polypeptides were isolated from a human hippocampus library. Degenerate oligodeoxyribonucleotide (oligo) primers based on the published rat NR1 (rNR1) amino acid (aa) sequence [K. Moriyoshi et al. Nature 354 (1991) 31-37] amplified a 0.7-kb fragment from a human hippocampus cDNA library, via the polymerase chain reaction (PCR). This fragment was used as a probe for subsequent hybridization screening. DNA sequence analysis of 28 plaque-purified clones indicated three distinct classes, designated hNR1-1, hNR1-2 and hNR1-3, presumably generated by alternative RNA splicing. One of these clones, hNR1-1(5A), was isolated as a full-length cDNA. The hNR1-2 and hNR1-3 cDNAs represented 66.8 and 98.9%, respectively, of the total aa coding information predicted for the polypeptides. The hNR1 cDNAs demonstrated an 84-90.8% nucleotide (nt) identity with the corresponding rodent cDNAs. The nt sequences of hNR1-1, hNR1-2 and hNR1-3 would encode 885-, 901- and 938-aa proteins, respectively, that have 99.1-99.8% identity with the corresponding rodent NR1 (roNR1) subunits. The changes between the predicted aa sequences of hNR1 and the corresponding roNR1 subunits are confined to the extracellular N-terminal regions. We have also identified two possible allelic variations of the hNR1-3 cDNA that result in aa substitutions in the extracellular N- and C-terminal regions. One of these naturally occurring aa variations is situated within a potential glutamate-binding site.

Distribution of the mRNA for a metabotropic glutamate receptor (mGluR3) in the rat brain: an in situ hybridization study

Distribution of the mRNA for a metabotropic glutamate receptor, mGluR3, which is coupled to the inhibitory cAMP cascade, was examined in the central nervous system of the adult albino rat by in situ hybridization. The hybridization signals of mGluR3 were detected not only on neuronal cells but also on many glial cells throughout the brain and spinal cord. In the neuronal cells, prominent expression of mGluR3 mRNA was seen in the thalamic reticular nucleus. Moderately labeled neurons were seen in the anterior olfactory nucleus, cerebral neo- and mesocortical regions, lateral amygdaloid nucleus, ventral part of the basolateral amygdaloid nucleus, dorsal endopiriform nucleus, supraoptic nucleus, superficial layers of the superior colliculus, inferior colliculus, interpeduncular nucleus, superior olivary nuclei, and Golgi cells in the cerebellar cortex. Weakly labeled neurons were observed in the striatum, nucleus accumbens, ventral pallidum, globus pallidus, entopeduncular nucleus, lateral hypothalamic area, hypothalamic paraventricular nucleus, medial habenular nucleus, anterior pretectal nucleus, Barrington's nucleus, Nucleus O, paragenual nucleus, trigeminal sensory complex, cochlear nuclei, dorsal motor nucleus of the trigeminal nerve, dorsal cap of the inferior olive, spinal dorsal horn, and lamina X of the spinal cord. The stellate cells in the cerebellar cortex, and neurons in the deep cerebellar nuclei were also labeled weakly. The granule cell layer of the dentate gyrus, as a whole, appeared to be labeled intensely, but each of the granule cells was labeled only weakly. No significant labeling was detected in the mitral and tufted cells in the olfactory bulb, hippocampal pyramidal cells, Purkinje and granule cells in the cerebellar cortex, or somatic motoneurons. The distribution of mGluR3 mRNA in particular neurons and glial cells indicates specific roles of mGluR3 in the glutamatergic system of the central nervous system.

Expression of the metabotropic glutamate receptor mGluR1 alpha and the ionotropic glutamate receptor GluR1 in the brain during the postnatal development of normal mouse and in the cerebellum from mutant mice

Expression of the metabotropic glutamate receptor type 1 alpha (mGluR1 alpha) and the non-N-methyl-D-aspartate (NMDA) ionotropic glutamate receptor type 1 (GluR1) in mouse brain was investigated using the antibodies raised against the synthetic peptides corresponding to their C-terminal amino acid sequences. Both receptor proteins are glycosylated predominantly in an asparagine-linked manner, and are abundant in post-synaptic membranes. We showed that mGluR1 alpha and GluR1 expression within the first 3 postnatal weeks undergoes dramatic changes in time and space, i.e., in the hippocampus and cerebellum. These spatio-temporal expression patterns appear to be correlated with the postnatal ontogenesis and establishment of the glutamatergic neurotransmission system in the hippocampus and cerebellum, cell migration, dendritic and axonal growth, spine formation, and synaptogenesis. In the adult cerebellum, mGluR1 alpha is intensely expressed in Purkinje neurons and GluR1 in Bergmann glial cells. Both receptors are expressed to a fair degree in weaver mutant cerebellum despite granule cell degeneration. However, the intrinsic expression levels of both mGluR1 alpha and GluR1 are markedly reduced in the cerebellum of the Purkinje cell-deficient and underdeveloped mutant mice, Purkinje-cell-degeneration, Lurcher, and staggerer, suggesting that GluR1 expression in Bergmann glia cells may be correlated with the sustained interaction with adjacent Purkinje neurons.

N-acetylaspartylglutamate inhibits forskolin-stimulated cyclic AMP levels via a metabotropic glutamate receptor in cultured cerebellar granule cells

The neuronal dipeptide N-acetylaspartylglutamate (NAAG) fulfills several of the criteria for classification as a neurotransmitter including localization in synaptic vesicles, calcium-dependent release after neuronal depolarization, and low potency activation of N-methyl-D-aspartate receptors. In the present study, the influence of NAAG on metabotropic receptor activation in cerebellar granule cells was examined in cell culture. Stimulation of granule cell adenylate cyclase with forskolin increased cyclic AMP (cAMP) several hundredfold above basal levels within 10 min in a concentration-dependent manner. Although glutamate, NAAG, and the metabotropic receptor agonist trans-1-amino-1,3-cyclopentanedicarboxylic acid did not alter the low basal cAMP levels, the application of 300 microM glutamate or NAAG or trans-1-amino-1,3-cyclopentanedicarboxylic acid reduced forskolin-stimulated cAMP in granule cells by 30-50% in the absence or presence of inhibitors of ionotropic acidic amino acid receptors, as well as 2-amino-4-phosphonobutyrate. No additivity in the inhibition of cAMP was found when 300 microM NAAG and trans-1-amino-1,3-cyclopentanedicarboxylic acid were coapplied. The beta-analogue of NAAG failed to reduce cAMP levels. Similar effects of NAAG and glutamate were obtained under conditions of inhibition of phosphodiesterase activity and were prevented by pretreatment of the cells with pertussis toxin. These data are consistent with the activation by NAAG of a metabotropic acidic amino acid receptor coupled to an inhibitory G protein. In contrast, the metabotropic acidic amino acid receptor coupled to phosphoinositol turnover in these cells was not activated by NAAG.(ABSTRACT TRUNCATED AT 250 WORDS)

Glutamate receptor-mediated currents and toxicity in embryonal carcinoma cells

While primary neuronal cell cultures have been used to investigate excitotoxicity, development of cell lines exhibiting glutamate receptor-mediated death is desirable. P19 mouse embryonal carcinoma cells, exposed to retinoic acid and plated onto a layer of cultured mouse cortical glial cells, differentiated into neuron-like elements immunoreactive for neurofilaments and neuron-specific enolase. Whole-cell recordings revealed inward currents in response to extracellular application of either NMDA or kainate. The NMDA-induced currents exhibited a voltage-dependent blockade by magnesium, required glycine for maximal activation, and were blocked by the NMDA antagonist dizocilpine. Kainate-induced currents were blocked by the AMPA/kainate receptor antagonist CNQX. Exposure to 500 microM NMDA for 24 h destroyed most P19 cells (EC50 approximately 70 microM); death was prevented by dizocilpine or D-APV. Exposure to 500 microM kainate also resulted in widespread death reduced by CNQX. Thus differentiated P19 cells exhibited both excitatory amino acid responses and vulnerability to excitotoxicity, characteristic of CNS neurons. These cells may provide a genetically open system useful for studying glutamate receptor-mediated phenomena at a molecular level.

The TINS/TiPS Lecture. The molecular biology of mammalian glutamate receptor channels

In native brain membranes the principal excitatory neurotransmitter L-glutamate activates cation-conducting channels with distinct biophysical and pharmacological properties. Molecular cloning has revealed the existence of 16 channel subunits that can assemble in homomeric or heteromeric configurations in vitro to form receptor channels with disparate functional properties. This review describes the different channel types obtained by recombinant means and the genetic mechanisms controlling the expression of functionally important channel structures.

Molecular size of the kainate binding protein in goldfish brain determined by radiation inactivation

Radiation inactivation analysis was used to estimate the target size of a putative glutamate receptor subtype in goldfish brain. A simple, linear inactivation curve was obtained. The calculated molecular size of the [3H]kainate binding site was 33.8 kDa. The results presented here are comparable to the molecular masses determined for putative glutamate receptors in other lower vertebrates but are markedly different from the sizes of the corresponding glutamate receptor subtypes in mammalian central nervous system.

Excitatory amino acid response in cultured rat striatal neurons results in a developmentally regulated cGMP formation

Glutamate and its analogues play a central role in excitatory neurotransmission throughout the brain. Their signal in the postsynaptic cells can be transduced by several second messengers. Here we show that in primary cultures of embryonic rat striatum, excitatory amino acid receptor stimulation increases cyclic GMP intracellular concentration and the magnitude of this response depends upon the time in culture. Formation of cyclic GMP appears to be mediated by both N-methyl-D-aspartate (NMDA) and non-NMDA type excitatory amino acid receptors, it is blocked by specific excitatory amino acid antagonists and requires extracellular Ca++. The effect mediated via the NMDA receptor is also regulated by extracellular Mg++. These results show that excitatory amino acids make use of cyclic GMP for signal transduction in striatal neurons in vitro. We suggest that cyclic GMP may be an independent second messenger possibly important in the development of a defined population of striatal neurons.

Attenuation of excitatory amino acid toxicity by metabotropic glutamate receptor agonists and aniracetam in primary cultures of cerebellar granule cells

Activation of glutamate ionotropic receptors represents the primary event in the neurotoxicity process triggered by excitatory amino acids. We demonstrate here that the concentration-dependent stimulation of metabotropic glutamate receptor (mGluR) by the selective agonist trans-1-aminocyclopentane-1,3-dicarboxylate or by quisqualate counteracts both glutamate- and kainate-induced neurotoxicity in primary cultures of rat cerebellar granule cells. The mGluR-evoked responses are potentiated by aniracetam, which per se also elicits neuroprotection. Aniracetam concentration-dependently counteracted glutamate-, kainate-, or alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid-induced cell death and greatly facilitated neuroprotective response achieved by different concentrations of both quisqualate and trans-1-aminocyclopentane-1,3-dicarboxylate. In addition, aniracetam potentiated the mGluR-coupled stimulation of phospholipase C, as revealed by the measurement of 3H-inositol phosphate formation. Thus, mGluRs could be a suitable target for novel pharmacological strategies pointing to the treatment of neurodegenerative diseases.

Glutamate and GABA in retinal circuitry

Glutamate and GABA have been identified as the major neurotransmitters in the radial and lateral synaptic pathways, respectively, of the vertebrate retina. Over the past year or so new information has appeared that has significantly increased the knowledge of how these compounds can elicit a range of responses. Key features of this new information are the identification and localization of many receptor subtypes within the retina, the recognition that glutamate can modulate membrane potential through cGMP-gated ion channels, and the finding that GABA can be released through non-vesicular pathways.

Reduced Mg2+ block of N-methyl-D-aspartate receptor-mediated synaptic potentials in developing visual cortex

Molecular cloning has demonstrated a diversity of artificially expressed N-methyl-D-aspartate (NMDA) receptors, implying a similar diversity of naturally occurring NMDA receptors in situ. Particularly significant was the success in expression of NMDA receptor classes exhibiting various sensitivities to Mg2+ block, a voltage-dependent channel blockade by Mg2+ that is essential to NMDA receptor functioning. Release from Mg2+ block often allows or facilitates the occurrence of long-term potentiation, a form of synaptic plasticity. Here we show that in the immature visual cortex, which is more susceptible to long-term potentiation than adult visual cortex, synaptically activated NMDA receptors, unlike those in the adult, have varying but clearly reduced sensitivities to Mg2+ block. We propose that the initially expressed, later-eliminated NMDA receptors exhibiting a reduced Mg2+ block may underlie the greater susceptibility to plasticity in the immature neocortex.

Mutations at two distinct sites within the channel domain M2 alter calcium permeability of neuronal alpha 7 nicotinic receptor

The relative permeability for sodium, potassium, and calcium of chicken alpha 7 neuronal nicotinic receptor was investigated by mutagenesis of the channel domain M2. Mutations in the "intermediate ring" of negatively charged residues, located at the cytoplasmic end of M2 (site 1), reduce calcium permeability without significantly modifying other functional properties (activation and desensitization) of the receptor; a similar change of ion selectivity is also noticed when mutations at site 1 are done in the context of a receptor mutant that conducts ions in a desensitized state. Moreover, mutations of two adjacent rings of leucines at the synaptic end of M2 (site 2) have multiple effects. They abolish calcium permeability, increase the apparent affinity for acetylcholine by 10- to 100-fold, augment Hill numbers (up to 4.6-5.0) of acetylcholine dose-response relationships, slow rates of ionic response onset, and lower the extent of desensitization. Mutations at these two topographically distinct sites within M2 selectively alter calcium transport without affecting the relative permeabilities for sodium and potassium.

A novel metabotropic glutamate receptor agonist: marked depression of monosynaptic excitation in the newborn rat isolated spinal cord

1. Neuropharmacological actions of a novel metabotropic glutamate receptor agonist, (2S,1'R,2'R,3'R)-2(2,3-dicarboxycyclopropyl)glycine (DCG-IV), were examined in the isolated spinal cord of the newborn rat, and compared with those of the established agonists of (2S,1'S,2'S)-2-(carboxycyclopropyl)glycine (L-CCG-I) or (1S,3R)-1-aminocyclopentane-1,3-dicarboxylic acid ((1S,3R)-ACPD). 2. At concentrations higher than 10 microM, DCG-IV caused a depolarization which was completely blocked by selective N-methyl-D-aspartate (NMDA) antagonists. The depolarization was pharmacologically quite different from that caused by L-CCG-I and (1S,3R)-ACPD. 3. DCG-IV reduced the monosynaptic excitation of motoneurones rather than polysynaptic discharges in the nanomolar range without causing postsynaptic depolarization of motoneurones. DCG-IV was more effective than L-CCG-I, (1S,3R)-ACPD or L-2-amino-4-phosphonobutanoic acid (L-AP4) in reducing the monosynaptic excitation of motoneurones. 4. DCG-IV (30 nM-1 microM) did not depress the depolarization induced by known excitatory amino acids in the newborn rat motoneurones, but depressed the baseline fluctuation of the potential derived from ventral roots. Therefore, DCG-IV seems to reduce preferentially transmitter release from primary afferent nerve terminals. 5. Depression of monosynaptic excitation caused by DCG-IV was not affected by any known pharmacological agents, including 2-amino-3-phosphonopropanoic acid (AP3), diazepam, 2-hydroxysaclofen, picrotoxin and strychnine. 6. DCG-IV has the potential of providing further useful information on the physiological function of metabotropic glutamate receptors.

Estimated conductance of glutamate receptor channels activated during EPSCs at the cerebellar mossy fiber-granule cell synapse

We have analyzed the variance associated with the decay of the non-NMDA receptor component of synaptic currents, recorded from mossy fiber-granule cell synapses in cerebellar slices, to obtain a conductance estimate for the synaptic channel. Current fluctuations arising from the random channel gating properties were separated from those arising from the fluctuations in the population of channels by subtracting the mean excitatory postsynaptic current (EPSC) waveform scaled to the EPSC peak amplitude. A weighted mean single-channel conductance of approximately 20 pS was determined from the relationship between the mean current and the variance around the mean during the decay of evoked and spontaneous synaptic currents. This result suggests that high conductance non-NMDA channels, such as the 10-30 pS glutamate receptor channel previously characterized in granule cells, carry the majority of the fast component of the EPSC at this synapse. In addition, our data are consistent with the activation of surprisingly few (approximately 10) non-NMDA channels by a single packet of transmitter.

The TiPS/TINS lecture: the molecular biology of mammalian glutamate receptor channels

In native brain membranes the principal excitatory neurotransmitter L-glutamate activates cation-conducting channels with distinct biophysical and pharmacological properties. Molecular cloning has revealed the existence of 16 channel subunits that can assemble in homomeric or heteromeric configurations in vitro to form receptor channels with disparate functional properties. This review describes the different channel types obtained by recombinant means and the genetic mechanisms controlling the expression of functionally important channel structures.

Heterogeneous distribution and developmental change of metabotropic glutamate receptors in rat hippocampal CA1 pyramidal neurons

The metabotropic glutamate (mGlu) response in dissociated rat hippocampal CA1 pyramidal neurons was recorded, using the nystatin-perforated patch-clamp technique. The mGlu response was localized in the ventral site of the hippocampal CA1 region. In both the ventral and dorsal sites of the CA1 region, the current amplitude of the mGlu responses as well as their induction probability (responsiveness) were reduced with development of the brain.

Regional patterns of cholinergic and glutamate activity in the developing and aging human brain

The levels of choline acetyltransferase (ChAT) and the binding activity of N-methyl-D-aspartate (NMDA) and non-NMDA receptors have been measured in the hippocampus, entorhinal cortex, frontal cortex and cerebellum, in a series of human brains from 24 weeks gestation to 100 years. The patterns of ChAT and glutamate receptor activity during aging and development were strikingly different in the different brain areas. In the hippocampus and associated cortex, ChAT activity did not reach a peak until middle age, when it almost immediately started to decline by 50-60% to the 10th decade, whereas in the frontal cortex ChAT peaked transiently in the infant and then stayed constant during aging. In the cerebellum ChAT activity was very high in the foetus and fell in the neonate to maintain a constant level more in line with the concentrations found in the other brain areas through the rest of life. The high levels of ChAT in the foetal cerebellum were not associated with high acetylcholinesterase (AChE) content, which tended to increase during development, and was present initially in Purkinje cells (foetus and neonate) and the molecular layer in the adult. In the hippocampus and entorhinal cortex, autoradiographic [3H]MK-801 binding was relatively constant throughout life, however, [3H]CNQX binding rose from the perinatal period up to a peak in the 1st or 2nd decade and then tended to fall with age. In the cerebellum, autoradiographic binding of both ligands rose from the foetal period to reach a plateau by the age of 10 years and there was no apparent further change during aging. These data on cholinergic and glutamatergic phenotypic changes during development and senescence reflect marked variations in regional plasticity and aging within and between the two transmitter systems and are likely to contribute to our understanding of their role in the different brain areas investigated.

Kainate elicits elevated nuclear calcium signals in retinal neurons via calcium-induced calcium release

Intracellular Ca2+ was imaged in cultured neonatal rat retinal neurons using the Ca(2+)-sensitive dye fluo-3 and confocal scanning laser microscopy. Depolarization via elevation of bath K+ concentration resulted in large cytoplasmic and nuclear Ca2+ signals; responses in the nucleus exceeded those of the cytoplasm. Glutamate or kainate application elicited the same intracellular pattern of elevated Ca2+ signals. Kainate stimulation was blocked by the non-NMDA receptor antagonist, 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX), and greatly reduced by removing Ca2+ from the bath and adding ethylene glycol-bis (beta-amino-ethyl ether) N,N,N',N'-tetraacetic acid (EGTA). Kainate was equally effective in eliciting Ca2+ signals when bath Na+ was replaced with equimolar concentrations of choline, or in the presence of the NMDA receptor antagonist, 2-amino-5-phosphonovaleric acid (APV). Caffeine treatment significantly reduced the kainate-induced intracellular Ca2+ response. These results suggest that Ca2+ can enter through the kainate receptor of retinal neurons and amplify the Ca2+ signals in the cytoplasm and nucleus by releasing Ca2+ from intracellular stores.

Stimulatory effects of protein kinase C and calmodulin kinase II on N-methyl-D-aspartate receptor/channels in the postsynaptic density of rat brain

To clarify the regulatory mechanism of the N-methyl-D-aspartate (NMDA) receptor/channel by several protein kinases, we examined the effects of purified type II of protein kinase C (PKC-II), endogenous Ca2+/calmodulin-dependent protein kinase II (CaMK-II), and purified cyclic AMP-dependent protein kinase on NMDA receptor/channel activity in the postsynaptic density (PSD) of rat brain. Purified PKC-II and endogenous CaMK-II catalyzed the phosphorylation of 80-200-kDa proteins in the PSD and L-glutamate- (or NMDA)-induced increase of (+)-5-[3H]methyl-10,11-dihydro-5H-dibenzo[a,d]cyclohepten-5,10-imi ne maleate ([3H]MK-801; open channel blocker for NMDA receptor/channel) binding activity was significantly enhanced. However, the pretreatment of PKC-II- and CaMK-II-catalyzed phosphorylation did not change the binding activity of L-[3H]glutamate, cis-4-[3H](phosphonomethyl)piperidine-2-carboxylate ([3H]CGS-19755; competitive NMDA receptor antagonist), [3H]glycine, alpha-[3H]-amino-3-hydroxy-5-methyl-isoxazole-4-propionate, or [3H]-kainate in the PSD. Pretreatment with PKC-II- and CaMK-II-catalyzed phosphorylation enhanced L-glutamate-induced increase of [3H]MK-801 binding additionally, although purified cyclic AMP-dependent protein kinase did not change L-glutamate-induced [3H]MK-801 binding. From these results, it is suggested that PKC-II and/or CaMK-II appears to induce the phosphorylation of the channel domain of the NMDA receptor/channel in the PSD and then cause an enhancement of Ca2+ influx through the channel.

Regulation of NMDA receptor phosphorylation by alternative splicing of the C-terminal domain

The NMDA (N-methyl D-aspartate) receptors in the brain play a critical role in synaptic plasticity, synaptogenesis and excitotoxicity. Molecular cloning has demonstrated that NMDA receptors consist of several homologous subunits (NMDAR1, 2A-2D). A variety of studies have suggested that protein phosphorylation of NMDA receptors may regulate their function and play a role in many forms of synaptic plasticity such as long-term potentiation. We have examined the phosphorylation of the NMDA receptor subunit NMDAR1 (NR1) by protein kinase C (PKC) in cells transiently expressing recombinant NR1 and in primary cultures of cortical neurons. PKC phosphorylation occurs on several distinct sites on the NR1 subunit. Most of these sites are contained within a single alternatively spliced exon in the C-terminal domain, which has previously been proposed to be on the extracellular side of the membrane. These results demonstrate that alternative splicing of the NR1 messenger RNA regulates its phosphorylation by PKC, and that mRNA splicing is a novel mechanism for regulating the sensitivity of glutamate receptors to protein phosphorylation. These results also provide evidence that the C-terminal domain of the NR1 protein is located intracellularly, suggesting that the proposed transmembrane topology model for glutamate receptors may be incorrect.

The ligand-binding domain in metabotropic glutamate receptors is related to bacterial periplasmic binding proteins

Receptors for the major excitatory neurotransmitter glutamate include metabotropic (G protein-coupled) and ionotropic (glutamate-gated ion channel) types. These receptors have large, presumably extracellular, amino-terminal domains. Sensitive sequence analysis techniques indicate that the metabotropic receptor extracellular domain is similar to bacterial periplasmic amino acid binding proteins. A structural model built using the observed similarity predicts a ligand-binding site, and mutants with conservative amino acid substitutions at this site are shown to have reduced ligand affinity. The metabotropic receptor extracellular domain is a member of a family of structural domains linked to a variety of receptor types, including ionotropic glutamate receptors.

Glutamate receptors of Drosophila melanogaster. Primary structure of a putative NMDA receptor protein expressed in the head of the adult fly

The NMDA subtype of ionotropic glutamate receptors has been implicated in the activity-dependent modification of synaptic efficacy in the mammalian brain. Here we describe a cDNA isolated from Drosophila melanogaster which encodes a putative invertebrate NMDA receptor protein (DNMDAR-I). The deduced amino acid sequence of DNMDAR-I displays 46% amino acid identity to the rat NMDAR1 polypeptide and shows significant homology (16-23%) to other vertebrate and invertebrate glutamate receptor proteins. The DNMDAR-I gene maps to position 83AB of chromosome 3R and is highly expressed in the head of adult flies. Our data indicate that the NMDA subtype of glutamate receptors evolved early during phylogeny and suggest the existence of activity-dependent synaptic plasticity in the insect brain.