Effects of neurotransplants and BDNF on the survival and regeneration of injured adult spinal motoneurons

We compared the effects of peripheral nerve grafts, embryonic spinal cord transplants and brain-derived neurotrophic factor (BDNF) on the survival and axon regeneration of adult rat spinal motor neurons undergoing retrograde degeneration after ventral root avulsion. Following implantation into the dorsolateral funiculus of the injured spinal cord segment, neither a peripheral nerve graft nor a combination of peripheral nerve graft with embryonic spinal cord transplant could prevent the retrograde motor neuron degeneration induced by ventral root avulsion. However, intrathecal infusion of BDNF promoted long-term survival of the lesioned motor neurons and induced abundant motor axon regeneration from the avulsion zone along the spinal cord surface towards the BDNF source. A combination of ventral root reconstitution and BDNF treatment might therefore be a promising means for the support of both motor neuron survival and guided motor axon regeneration after ventral root lesions.     

High levels of extracellular glutamate are present in retina during neonatal development

The three major classes of neurons which comprise the primary visual pathway in retina are glutamatergic. These cells are generated in two separate developmental stages, with one subclass of photoreceptors (cones) and ganglion cells generated before birth; and the other subclass of photoreceptors (rods) and bipolar cells generated during the first week after birth. Gas chromatography/mass spectroscopy analysis coupled with a new method for collecting small samples of extracellular fluids from retina were used to determine the levels of endogenous glutamate present during differentiation and synaptogenesis of these different cell types. As expected the total retinal content of glutamate increased during the postnatal period in synchrony with the generation and maturation of glutamatergic cells. However, a significant proportion of the endogenous pool was found extracellularly at birth. Intracellular glutamate is localized within cell bodies and growing processes of cones and ganglion cells at this time but few glutamatergic synapses are present. The extracellular concentration of glutamate actually declined during the most active period of synaptogenesis, reaching very low levels in the adult. The high concentrations of extracellular glutamate in neonatal retina could play an important role in a variety of developmental events such as dendritic pruning, programmed cell death and neurite sprouting.     

Regulation of intrinsic and synaptic properties of neonatal rat trigeminal motoneurons by metabotropic glutamate receptors

We studied how metabotropic glutamate receptor (mGluR) activation modifies the synaptic and intrinsic membrane properties of neonatal rat trigeminal motoneurons using the broad-spectrum mGluR agonist (1S,3R)-1-amino-1,3-cyclopentane-dicarboxylic acid [(1S,3R)-ACPD], group I/II antagonist (+/-)-alpha-methyl-4-carboxy-phenylglycine (MCPG), and group III agonist L-2-amino-4-phosphonobutanoate (L-AP4). (1S,3R)-ACPD depressed excitatory transmission to trigeminal motoneurons presynaptically and postsynaptically via presynaptic inhibition and by reducing the currents carried by ionotropic glutamate receptors selective for AMPA. (1S,3R)-ACPD also depolarized trigeminal motoneurons and increased input resistance by suppressing a Ba2+-sensitive leakage K+ current. These effects were not mimicked by L-AP4 (100-200 microM). High-threshold Ca2+ currents were also suppressed by (1S,3R)-ACPD. Repetitive stimulation of excitatory premotoneurons mimicked the postsynaptic effects of (1S, 3R)-ACPD. The postsynaptic effects of (1S,3R)-ACPD and repetitive stimulation were both antagonized by MCPG, suggesting that mGluRs were similarly activated in both experiments. We conclude that mGluRs can be recruited endogenously by glutamatergic premotoneurons and that mGluR-mediated depression of excitatory transmission, combined with increased postsynaptic excitability, enhances the signal-to-noise ratio of oral-related synaptic input to trigeminal motoneurons during rhythmical jaw movements.     

An improved method for the detection of changes in brain extracellular glutamate levels

We developed a method for in vivo real-time monitoring of the concentration of extracellular glutamate ([Glu]e) in the brain under anoxic conditions. A dialysis electrode (Sycopel Int., UK) was employed as a sensing device to measure the concentration of glutamate by enzyme amperometry, and an electron mediator, ferrocene, was introduced into the electrode together with glutamate oxidase. The ferrocene was covalently conjugated with a high molecular weight molecule, bovine serum albumin, to avoid outward diffusion through the dialysis membrane. With this set-up, the amperometric response was independent of the pO2 around the electrode in vitro up to 400 microM glutamate. Using this method, we investigated the dynamics of [Glu]e in the rat striatum during anoxia. [Glu]e increased rapidly at 102+/-5.4s (n = 6) after the start of nitrogen inhalation. The increase continued for about 30 s, and then [Glu]e decreased. The peak value of delta[Glu]e was 141+/-37 micro M. [Glu]e subsequently underwent another gradual increase, reaching 213+/-69 microM at 15 min after the start of nitrogen inhalation. This distinct biphasic profile was reproducible. We conclude that this method is very useful for monitoring [Glu]e in the brain under low pO2 conditions.     

Craniofacial and TMJ effects after glutamate and TRH microsphere implantation in proximity to trigeminal motoneurons of growing rats

The sequelae of sustained, in vivo delivery of two important neurotransmitter substances, glutamate and thyrotropin-releasing hormone (TRH), upon craniofacial growth and development have previously not been investigated. Our purpose was to document and compare the relative effects of glutamate and TRH microspheres stereotactically placed in proximity to trigeminal motoneurons within the trigeminal motor nucleus. The following null hypotheses were tested: (1) TRH microspheres in proximity to trigeminal motoneurons have no significant effect upon the craniofacial skeleton, and (2) there are no significant differences between the relative effects of chronic, long-term delivery of glutamate and TRH upon the neuromusculoskeletal system of growing rats. Forty male Sprague-Dawley rats were divided into 4 experimental groups (glutamate microspheres, TRH microspheres, blank microspheres, sham surgeries) and underwent stereotactic neurosurgery at 35 days; 5 rats of each group were killed at 14 and 21 days for data collection. Histology revealed that implants were clustered in the pontine reticular formation, close to the ventrolateral tegmental nucleus. Both glutamate and TRH rats had implant-side deviation of their facial skeleton and snout regions; 4 x 2 ANOVA and post hoc t-tests revealed significant (P < or = 0.05, 0.01) differences between groups and sides for motoneuron count, muscle weight, and osteometric data. TRH rats also demonstrated larger implant-side TMJ discs and mandibular fossae in comparison with the other groups. The stated null hypotheses were therefore rejected.     

Origins of the extracellular glutamate released during total metabolic blockade in the immature retina

Previous studies have shown that complete blockade of metabolism in embryonic chick retina causes a time-dependent increase in the release of glutamate into the extracellular space. The present study examined the cellular source of this glutamate, i.e., neuronal and/or glial. Pure cultures of retinal neurons or glia were labeled for 10 min at 37 degrees C with [3H]acetate. Retinal glia, but not retinal neurons, were found to selectively and preferentially metabolize acetate, thus producing 3H-labeled amino acids in the glial compartment. This finding provides direct evidence to substantiate findings from several other laboratories that have indirectly determined the preferential metabolism of acetate by glia by using mixed neuronal/glial populations. To study the cellular source of glutamate released during total metabolic blockade, whole retina were prelabeled with [3H]acetate plus [U-14C]glucose (to label the neuronal compartment). Total metabolic blockade was instituted with a combination of iodoacetate (IOA) plus KCN, and the release of glutamate into the medium was followed at 5, 15, and 30 min. During total energy blockade, net extracellular glutamate was not elevated at 5 min [0.17 +/- 0.02 vs. 0.12 +/- 0.01 microM for treated vs. control retina (means +/- SEM), respectively], but was increased significantly at 15 (1.2 +/- 0.26 microM) and 30 min (2.6 +/- 0.22 microM). Total [3H]glutamate in the medium during IOA/KCN treatment was unchanged at 5 min, but was increased 1.5- and threefold above basal levels at 15 and 30 min, respectively. During the time when extracellular glutamate increased, the specific activity of [3H]glutamate remained fairly constant, 731 +/- 134 and 517 +/- 82 dpm/nmol (means +/- SEM) at 15 and 30 min, respectively. In contrast, 14C-labeled glutamate in the medium did not increase during IOA/KCN treatment and paralleled basal levels. Thus, the specific activity of 14C-labeled extracellular glutamate decreased from 309 +/- 87 dpm/nmol at 15 min to 42 +/- 8 dpm/nmol at 30 min. Prior loading of the tissue with 0.5 mM trans-pyrrolidine-2,4-dicarboxylate (t-PDC), a glutamate transport inhibitor, blocked 57% of the glutamate released at 30 min of IOA/KCN exposure, suggesting that reversal of an Na+-dependent glutamate transporter was a key contributor to the appearance of extracellular glutamate during energy deprivation. The increase in extracellular [3H]glutamate, constancy of the specific activity of extracellular [3H]glutamate, decrease in the specific activity of extracellular [14C]glutamate, and attenuation of release by prior loading with t-PDC indicate that glial pools of glutamate released via reversal of the transporter contribute significantly to the rise in extracellular glutamate after metabolic inhibition in this preparation.     

Consumption of a high dietary dose of monosodium glutamate fails to affect extracellular glutamate levels in the hypothalamic arcuate nucleus of adult rats

We examined the effects of systemic or oral ad libitum monosodium glutamate (MSG) administration on glutamate levels in plasma, and on glutamate release from the arcuate nucleus of the hypothalamus (estimated using brain microdialysis). Systemic MSG administration (0.25, 0.5, 1 or 2 g/kg, i.p.) to adult rats caused dose-dependent increases in glutamate levels within arcuate nucleus dialysates. These levels increased during the initial 20 min after systemic MSG administration, and peaked during the second 20-min interval (maximally to 116 +/- 7%, 146 +/- 15%, 790 +/- 191% and 1230 +/- 676% of basal values, respectively). Plasma glutamate levels, measured simultaneously, were increased maximally during the initial 20 min after MSG administration. These increases were 10-, 13-, 76- and 163-fold after doses of 0.25, 0.5, 1 and 2 g/kg, i.p., respectively. In feeding experiments, consumption of 2.3 g/kg of MSG by previously-trained rats during an 1-h period increased plasma glutamate levels to 352 +/- 61% of basal values 140 min after the start of the feeding period. No changes were observed in glutamate levels of arcuate nucleus dialysates. These findings may explain why ad libitum dietary consumption of MSG apparently lacks neurotoxic potential.     

Enhancement of vascular action of arginine vasopressin by diminished extracellular sodium concentration

The present study was undertaken to examine the effects of diminished extracellular sodium concentration on the vascular action of arginine vasopressin (AVP) in cultured rat vascular smooth muscle cells (VSMC). The preincubation of cells with the 110 mM extracellular Na+ ([Na+]e) solution supplemented with 30 mM choline chloride for 60 minutes enhanced the effect of AVP- (1 x 10(-8) M) induced VSMC contraction. The treatment of 110 mM [Na+]e solution also enhanced the cellular contractile response to the protein kinase C (PKC) activators, phorbol 12-myristate 13-acetate and 1-oleoyl-2-acetyl-glycerol. Furthermore, preincubation with the 110 mM [Na+]e solution also potentiated the effect of 1 x 10(-8) M AVP, but not 1 x 10(-6) M, to increase the cytosolic-free Ca2+ ([Ca2+]i) concentration. The 110 mM [Na+]e media decreased the basal intracellular Na+ concentration and increased intracellular 45Ca2+ accumulation, basal [Ca2+]i and AVP-produced 45Ca2+ efflux. These effects of 110 mM [Na+]e solution to enhance the vascular action of AVP were abolished by using Ca(2+)-free 110 mM [Na+]e solution during the preincubation period. The preincubation with the 110 mM [Na+]e solution did not change either the Kd and Bmax of AVP V1 receptor of VSMC or the AVP-induced production of inositol 1,4,5-trisphosphate. The present in vitro results therefore indicate that the diminished extracellular fluid sodium concentration within a range observed in clinical hyponatremic states enhances the vascular action of AVP.(ABSTRACT TRUNCATED AT 250 WORDS)     

Net glutamate release from astrocytes is not induced by extracellular potassium concentrations attainable in brain

Elevated extracellular potassium concentration ([K+]e) has been shown to induce reversal of glial Na+-dependent glutamate uptake in whole-cell patch clamp preparations. It is uncertain, however, whether elevated [K+]e similarly induces a net glutamate efflux from intact cells with a physiological intracellular milieu. To answer this question, astrocyte cultures prepared from rat and mouse cortices were incubated in medium with elevated [K+]e (by equimolar substitution of K+ for Na+), and glutamate accumulation was measured by HPLC. With [K+]e elevations to 60 mM, medium glutamate concentrations did not increase during incubation periods of 5-120 min. By contrast, 45 min of combined inhibition of glycolytic and oxidative ATP production increased medium glutamate concentrations 50-100-fold. Similar results were obtained in both rat and mouse cultures. Studies were also performed using astrocytes loaded with the nonmetabolized glutamate tracer D-aspartate, and parallel results were obtained; no increase in medium D-aspartate content resulted from [K+]e elevation up to 90 mM, whereas a large increase occurred during inhibition of energy metabolism. These results suggest that a net efflux of glutamate from intact astrocytes is not induced by any [K+]e attainable in brain.     

Expression and purification of the extracellular ligand binding region of metabotropic glutamate receptor subtype 1

Each metabotropic glutamate receptor possesses a large extracellular domain that consists of a sequence homologous to the bacterial periplasmic binding proteins and a cysteine-rich region. Previous experiments have proposed that the extracellular domain is responsible for ligand binding. However, it is currently unknown whether the extracellular ligand binding site can bind ligands without other domains of the receptor. We began by obtaining a sufficient amount of receptor protein on a baculovirus expression system. In addition to the transfer vector that encodes the entire coding region, transfer vectors that encode portions of the extracellular domain were designed. Here, we report a soluble metabotropic glutamate receptor that encodes only the extracellular domain and retains a ligand binding characteristic similar to that of the full-length receptor. The soluble receptor secreted into culture medium showed a dimerized form. Furthermore, we have succeeded in purifying it to homogeneity. Dose-response curves of agonists for the purified soluble receptor were examined. The effective concentration for half-maximal inhibition (IC50) of quisqualate for the soluble receptor was 3.8 x 10(-8) M, which was comparable to that for the full-length receptor. The rank order of inhibition of the agonists was quisqualate > ibotenate >/= L-glutamate approximately (1S,3R)-1-aminocyclopentane-1, 3-dicarboxylic acid. These data demonstrate that a ligand binding event in metabotropic glutamate receptors can be dissociated from the membrane domain.     

Taste dimensions of monosodium glutamate (MSG) in a food system: role of glutamate in young American subjects

Fibrosing cholestatic hepatitis is a histological variant of hepatitis B virus infection with a high rate of mortality. We describe a patient who acquired acute hepatitis B virus infection 8 months after renal transplantation. Clinical features of rapidly progressive liver failure, indicated by prolonged prothrombin time (57 seconds) and increased bilirubin (40.4 mg/dL) and ammonia (129 mumol/L) concentrations, were accompanied by an extremely high serum HBV DNA level (2.153 x 10(6) pg/mL). Liver biopsy specimen showed fibrosing cholestatic hepatitis with widespread balloon degeneration of hepatocytes, focal hepatocyte loss, bile stasis, periportal fibrosis, mild lymphocytic infiltration, and strongly positive immunohistochemical staining for hepatitis B surface antigen (HBsAg) and hepatitis B core antigen. Lamivudine therapy suppressed HBV DNA to < 10 pg/mL within 4 weeks, which was followed by gradual recovery of liver function from a state of hepatic precoma. Twenty-four months after the onset of hepatitis, the patient had normal prothrombin time and bilirubin, transaminase, and albumin levels. She remained HBsAg positive and hepatitis B e antigen negative. Renal allograft function was stable, with a creatinine level of 1.52 mg/dL. HBV DNA remained suppressed after 22 months of lamivudine therapy. Our experience shows that fibrosing cholestatic hepatitis and liver failure caused by HBV infection can be successfully treated with lamivudine.     

Amphetamine increases extracellular concentrations of glutamate in the prefrontal cortex of the awake rat: a microdialysis study

OBJECTIVE: To determine if motion automated perimetry can identify early glaucomatous visual field defects in patients with suspected glaucoma (by disc), those with ocular hypertension, and those with primary open-angle glaucoma. METHODS: Motion automated perimetry, a foveally centered motion test, and standard visual field tests were conducted on one randomly selected eye of normal patients (n = 38), patients with suspected glaucoma (by disc) (n = 28), patients with ocular hypertension (n = 18), and patients with primary open-angle glaucoma (n = 21). Subjects' performance on both motion tests were compared with their performance on standard perimetry. RESULTS: Perimetric motion thresholds significantly distinguished the groups (P< or =.001), while the foveally centered motion test was unable to separate them (P< or =.32). Of the total patients, 90.5% of those with glaucoma, 39.3% of those with suspected glaucoma, 27.8% of those with ocular hypertension, and 5.3% of the normal subjects had abnormal results on motion automated perimetry testing. Perimetric motion thresholds were significantly correlated with standard visual field thresholds (P< or =.001). CONCLUSION: Motion automated perimetry identifies visual field defects in patients who already show standard visual field loss as well as in a moderate percentage of those with suspected glaucoma and ocular hypertension, indicating that the testing of discrete locations might be necessary for increased diagnostic utility.     

Neuronal high-affinity glutamate transport in the rat central nervous system

EAAC1 is a neuronal and epithelial high affinity glutamate transporter previously cloned from rabbit intestine. Here we report the isolation of EAAC 1 from rat brain* and its expression in the central nervous system based on in situ hybridization. Strong signals were detected in brain, spinal cord and retina. Expression of EAAC1 was particularly strong in pyramidal cells of the cerebral cortex, pyramidal cells of the hippocampus, mitral cells of the olfactory bulb, various thalamic nuclei and cells of certain retinal layers. EAAC1 was also expressed in non-glutamatergic neurons such as GABAergic cerebellar Purkinje cells and alpha-motor neurons of the spinal cord. We propose that EAAC1 is not only involved in the sequestration of glutamate at glutamatergic synapses and in protecting neurons from glutamate excitotoxicity, but also in the cellular metabolism involving glutamate.     

Carbon monoxide, a novel neural messenger, does not modulate extracellular glutamate concentration in forebrain ischemia

We investigated the role of carbon monoxide as a neural modulator of extracellular glutamate concentration in rat hippocampus CA1 in transient forebrain ischemia by using metalloporphyrins, which block the production of carbon monoxide through the inhibition of heme oxygenase (HO) activity. Infusion of 10 and 100 microM zinc protoporphyrin IX, which inhibits nitric oxide synthase activity as well as HO activity, significantly increased glutamate concentration compared with that on the vehicle-treated side. However, infusion of 100 microM tin mesoporphyrin IX, which inhibits only HO activity, did not affect glutamate concentration in ischemia. Our results therefore do not support the hypothesis that carbon monoxide acts as a neural messenger through the modulation of extracellular glutamate concentration in ischemia.     

A modulatory effect of extracellular Ca2+ on type 1alpha metabotropic glutamate receptor-mediated signalling

DiGeorge syndrome or anomaly consists of a developmental field defect which is characterized by congenital absence or hypoplasia of the thymus and parathyroids, as well as facial dysmorphism and congenital heart defects. Other congenital malformations may coexist, in particular, thyroid abnormalities. A case of congenital hypothyroidism and DiGeorge syndrome is reviewed. Necropsy, clinical, and experimental studies also show that thyroid abnormalities may be a feature of DiGeorge syndrome. Although this could be purely coincidental, our case suggests that thyroid gland dysgenesis may be more common than previously thought. Thus, children with the DiGeorge syndrome may be at higher risk for hypothyroidism. Because of this potential association, patients who are considered to have this anomaly should have early newborn thyroid screening.     

Dose-dependent effects of acute in vivo ethanol exposure on extracellular glutamate concentration in the cerebral cortex of the near-term fetal sheep

The cerebral cortex is a target site of ethanol teratogenesis. L-Glutamate is a major excitatory neurotransmitter that plays an important neurotrophic role in brain development. It has been proposed that optimal function of the glutamate neuronal system is required for normal brain development; overactivation could lead to excitotoxic-induced neuronal injury, whereas underactivation could delay/restrict brain development. The objective of this study was to test the hypothesis that acute in vivo ethanol exposure alters basal glutamate release in the fetal cerebral cortex. The experimental approach involved measuring fetal cortical extracellular glutamate concentration using the technique of in vivo microdialysis. Near-term fetal sheep were chronically instrumented with a microdialysis probe placed in the parasagittal cortex. At 124 +/- 3 days of gestation, the effects of maternal intravenous infusion of 2 g or 4 g ethanol/kg maternal body weight or an equivalent volume of saline, given as four equally divided doses over 5 hr, on fetal cerebral cortical extracellular glutamate concentration were determined. None of the three treatment regimens produced fetal or maternal demise during the time course of the study. There was an ethanol dose-dependent increase, p = 0.005, in extracellular glutamate concentration in the fetal cerebral cortex. This increase was paroxysmal in nature and was not directly related to the fetal blood ethanol concentration. In view of the proposed role for glutamate in neuronal development, this apparent ethanol-induced increase in glutamate release may be important in the pathogenesis of ethanol teratogenesis involving the cerebral cortex.     

Perineuronal nets--a specialized form of extracellular matrix in the adult nervous system

One century ago, Camillo Golgi described 'perineuronal nets' enwrapping the cell bodies and proximal dendrites of certain neurons in the adult mammalian central nervous system and suggested that they represent a supportive and protective scaffolding. Although other neuroanatomists validated the existence of these nets on selected neurons in the adult brain, there was a lack of agreement on their origins, composition and function. The application of modern molecular and ultrastructural methods has brought new insights and a renewed interest in these classic observations. Recent data suggest that perineuronal nets result from the visualization of extracellular matrix molecules that are confined to the space interposed between glial processes and the nerve cells that they outline. The material confined to these spaces can be visualized selectively by antibodies directed to glycoproteins (e.g., tenascin and restrictin/janusin), proteoglycans (e.g., chondroitin sulfates), markers for hyaluronan as well as by lectins recognizing N-acetylgalactosamine and by monoclonal antibodies directed to epitopes on unknown molecules (e.g., HNK-1, VC1.1 and Cat 301). This review examines the emerging clarification of classical observations of perineuronal nets and the functional implications suggested by their molecular composition. Also discussed are studies that further extend observations on the time of development and of the specificity in the occurrence of perineuronal nets. In the adult brain the molecules constituting the 'perineuronal nets of matrix' could serve as recognition molecules between certain neurons and their surrounding cells and participate in the selection and consolidation of their relationship.     

Agonist-induced changes in substituted cysteine accessibility reveal dynamic extracellular structure of M3-M4 loop of glutamate receptor GluR6

Recent evidence suggests that the transmembrane topology of ionotropic glutamate receptors differs from other members of the ligand-gated ion channel superfamily. However, the structure of the segment linking membrane domains M3 and M4 (the M3-M4 loop) remains controversial. Although various data indicate that this loop is extracellular, other results suggest that serine residues in this segment are sites of phosphorylation and channel modulation by intracellular protein kinases. To reconcile these data, we hypothesized that the M3-M4 loop structure is dynamic and, more specifically, that the portion containing putative phosphorylation sites may be translocated across the membrane to the cytoplasmic side during agonist binding. To test this hypothesis, we mutated Ser 684, a putative cAMP-dependent protein kinase site in the kainate-type glutamate receptor GluR6, to Cys. Results of biochemical and electrophysiological experiments are consistent with Cys 684 being accessible, in the unliganded state, from the extracellular side to modification by a Cys-specific biotinylating reagent followed by streptavidin (SA). Interestingly, our data suggest that this residue becomes inaccessible to the extracellular biotinylating reagent during agonist binding. However, we find it unlikely that Cys 684 undergoes membrane translocation, because the addition of SA to Cys-biotinylated GluR6(S684C) has no effect on peak glutamate-evoked current and only a small effect on macroscopic desensitization. We conclude that residue 684 in GluR6 is extracellular in the receptor-channel's closed, unliganded state and does not cross the membrane after agonist binding. However, an agonist-induced conformational change in the receptor substantially alters accessibility of position 684 to the extracellular environment.     

Metabotropic glutamate receptor antagonists, like GABA(B) antagonists, potentiate dorsal root-evoked excitatory synaptic transmission at neonatal rat spinal motoneurons in vitro

Recordings of whole-cell synaptic current responses elicited by electrical stimulation of dorsal roots were made from motoneurons, identified by antidromic invasion, in isolated spinal cord preparations from five- to eight-day-old Wistar rats. Supramaximal electrical stimulation of the dorsal root evoked complex excitatory postsynaptic currents with mean latencies (+/- S.E.M.) of 6.1 +/- 0.26 ms, peak amplitude of -650 +/- 47 pA and duration of 4.30 +/- 0.46 s (n=34). All phases of excitatory postsynaptic currents were potentiated to approximately 20% above control levels in the presence of the metabotropic glutamate receptor antagonists S-2-amino-2-methyl-4-phosphonobutanoate (MAP4; 200 microM; n=15) and 2S, 1'S,2'S-2-methyl-2-(carboxycyclopropyl)glycine (MCCG; 200 microM; n=9). A similar level of potentiation was produced by the GABA(B) receptor antagonist 3-N[1-(S)-(3,4-dichlorophenyl)ethyl]amino-2-(S)-hydroxypropyl-P-benzyl-p hosphinic acid (CGP55845; 200 nM; n=5). MAP4 (200 microM) produced a six-fold rightward shift in the concentration-effect plot for the depressant action of the metabotropic glutamate receptor agonist S-2-amino-4-phosphonobutanoate (L-AP4), whereas CGP55845 produced no significant change in the potency of L-AP4. MAP4 did not antagonize the depressant actions of baclofen (n=8), 1S,3S-1-aminocyclopentane-1,3-dicarboxylate (n=4) or 2-S,1'S,2'S-2-(carboxycyclopropyl)glycine (n=4). The metabotropic glutamate receptor antagonists produced no change in the holding current of any of the neurons, indicating that they had no significant postsynaptic excitatory actions. These results are the first to indicate a possible physiological role for metabotropic glutamate receptors in the spinal cord. Like GABA(B) receptors, they control glutamatergic synaptic transmission in the segmental spinal pathway to motoneurons. This is likely to be a presynaptic control mechanism.