Glycine binding to rat cortex and spinal cord: binding characteristics and pharmacology reveal distinct populations of sites

Bergmann glial GlyT1 mediates glycine uptake and release in mouse cerebellar slices

Glycine is an inhibitory neurotransmitter and is critical for NMDA receptor activation. These roles are dependent on extracellular glycine levels, which are regulated by Na(+)/Cl(-)-dependent glycine transporters (GlyTs) in neurones and glia. The glial GlyT subtype GlyT1 is well located to activate NMDA receptors. However, glial GlyTs have not been studied in an intact system thus far. Whole-cell patch-clamp recordings were obtained from Bergmann glia in mice cerebellar slices to determine whether these glia express functional GlyT1 that can mediate both glycine uptake and efflux. In the presence of a glycine receptor blocker, glycine and a substrate agonist for GlyT1, sarcosine, induced voltage-dependent inward currents that were abolished by removing external Na(+), identifying them as transport currents. Inhibitors of glycine transport through GlyT1 (sarcosine and (N-[3-(4'-fluorophenyl)-3-(4'-phenylphenoxy)propyl]sarcosine (NFPS)) reduced glycine currents by approximately 85%, consistent with positive immunostaining for GlyT1 in Bergmann glia while inhibitors of glycine transport through GlyT2 (4-benzyloxy-3,5-dimethoxy-N-[1-(dimethylaminocyclopently)methyl]benzamide (ORG 25543) and amoxapine) or through systems A and ASC did not affect glycine transport currents. Following internal glycine perfusion during the recording, outward currents progressively developed at -50 mV and external glycine-induced uptake currents were reduced. Using paired recordings of a Bergmann glial cell and a granule cell in the whole cell and outside-out modes, respectively, depolarizations of Bergmann glia to +20 mV induced a 73% increase in the open probability of glycine receptor channels in membrane patches of granule cells. This increase was prevented when NFPS was included in the bath solution. Overall, these results demonstrate for the first time that Bergmann glia express functional GlyT1 that can work in reverse at near-physiological ionic and internal glycine conditions in brain slices. These glial GlyTs can probably mediate glycine efflux under conditions of metabolic impairments like ischaemia.

Pressor response to chemoreflex activation before and after microinjection of glycine into the NTS of awake rats

Microinjection of glycine into the rostral (bilateral) and caudal (midline) commissural nucleus of the solitary tract (NTS) using three guide cannulas implanted in the direction of these sites produced an increase in mean arterial pressure (MAP) and abolished the pressor response to chemoreflex activation [potassium cyanide (n = 7)]. Strychnine, a glycine receptor antagonist, attenuated the increase in MAP, and in this new experimental condition (n = 5) the pressor response to chemoreflex activation was not altered. Considering that the effect of glycine on the attenuation of the pressor response to chemoreflex activation could be secondary to the increase in baseline MAP, in a third group of rats (n = 5) sodium nitroprusside infusion (intravenous) after microinjections of glycine into the NTS normalizes MAP. In this case, the pressor response to chemoreflex activation was similar to the control. These data show that glycine when microinjected bilaterally into the lateral commissural NTS as well as into the medial commissural NTS plays no major inhibitory role in the processing of the neurotransmission of the sympathoexcitatory component of the chemoreflex.

Glycine-immunoreactive synapses in the central nucleus of the inferior colliculus. An electron microscopic study in the cat

It has been known that the inferior colliculus contains many glycinergic fibers of both intrinsic and extrinsic nature. In the present study, glycine-immunoreactive (Gly-ir) synapses were examined in the central nucleus of the inferior colliculus in the cat. About half of 891 axondendritic synapses that were identified in the present study were Gly-ir. In 75% of these Gly-ir synapses, synaptic axon terminals contained pleomorphic or flattened synaptic vesicles and made symmetric synapses, while in 25% of Gly-ir synapses, synaptic axon terminals were filled with spherical synaptic vesicles and formed asymmetric synapses. Thus, Gly-immunoreactivity was detected in axodendritic synapses that formed both Gray's type I and Gray's type II synapses.

Glycine and D-serine decrease MK-801-induced hyperactivity in mice

It is well known that the un-competitive N-methyl-D-aspartate (NMDA) receptor antagonist phencyclidine can induce a syndrome in humans that mimics both positive and negative symptoms of schizophrenia. In the light of this observation, it has been hypothesised that schizophrenia might be due to a hypofunction of central glutamate systems. A glycine agonist, by strengthening glutamatergic transmission, has been suggested to be useful as treatment. A crucial issue is the uncertainty regarding the degree of saturation of the glycine site associated with the NMDA receptor. The purpose of this study was to investigate if it is possible to strengthen NMDA receptor-mediated neurotransmission by modulating the associated glycine site. The effects of systemic and intraventricular administration of glycine. D-serine and L-serine on the hyperactivity induced in mice by the uncompetitive NMDA receptor antagonist MK-801 were tested. Systemically administered glycine and D-serine were found to decrease MK-801-induced hyperactivity. Intraventricularly administered D-serine in doses of 50 or 100 micrograms/side was found to decrease MK-801-induced hyperactivity during the second half hour of registration; L-serine given in the same doses did not affect the MK-801-induced hyperactivity during this period. These data may suggest that the NMDA receptor-associated glycine site is not saturated in vivo.

The co-agonist concept: is the NMDA-associated glycine receptor saturated in vivo?

Our current knowledge of the structure and function of NMDA receptors is expanding at a rapid pace; however, advances regarding regulation of the supply of glutamate and its co-agonist, glycine, have been slower. While the anatomical sources and metabolic compartmentation of glutamate have been studied, limited efforts have been dedicated to defining the dynamics and compartmentation of the co-agonist, glycine. In fact, most investigators have made the assumption that glycine is freely available, via diffusion, for synaptic transmission at NMDA-type synaptic clefts. This assumption ignores the intricate inactivation mechanisms potentially involved in regulating synaptic levels of this amino acid and the recent descriptions of high levels of endogenous D-serine, another potential agonist of the NMDA-associated glycine receptor, in the brain. In this review, the relevance of these data and pharmacological experiments pertinent to the question of whether the NMDA-associated glycine receptor is saturated in vivo or not, is presented.

Free D-serine in post-mortem brains and spinal cords of individuals with and without neuropsychiatric diseases

We have measured the concentrations of free D-serine post-mortem in the prefrontal cortex, parietal cortex, cerebellum and spinal cord from individuals with and without (controls) neuropsychiatric diseases using high-performance liquid chromatography with fluorometric detection. The levels of D-serine were found to be high in the prefrontal and parietal cortex (around 100 nmol/g wet weight) and very low in the cerebellum and spinal cord (below 10 nmol/g wet weight). The uneven distribution of the D-amino acid in the human central nervous system (CNS) resembles that observed in rodents, suggesting that, as shown in the rat CNS, the regional variation of D-serine content in the human brain might also be closely correlated with those of the N-methyl-D-aspartate (NMDA) type excitatory amino acid receptor. In the prefrontal cortex, the gray and white matter had a similar concentration of D-serine. These findings, together with the selective action of D-serine at the NMDA-related glycine site and the non-neurogenic nature of extracellular D-serine release, add further support to the view that D-serine could be an intrinsic modulator of the NMDA receptor liberated from certain glial cells in the mammalian brain. Despite the anti-psychotogen activity of D-serine in the rat, there were no statistically significant differences between the D-serine contents in the prefrontal or parietal cortex of controls and those of patients with schizophrenia or dementia of the Alzheimer type.

Anticonvulsant activity of antagonists for the NMDA-associated glycine binding site

Coupled to the N-methyl-D-aspartate (NMDA) receptor-channel complex is a strychnine-insensitive binding site for glycine. Pharmacological antagonism of glycine binding at this site can produce anticonvulsant activity. Derivatives of the glycine antagonists kynurenic acid and 2-carboxy-indole were synthesized and evaluated for anticonvulsant effects. Compounds were tested in mice against seizures induced by electroshock and pentylenetetrazole, and in the rotorod assay for neurological deficit. The derivatives were also assayed for binding at the NMDA-associated glycine site. The most potent anticonvulsant was ethyl 4-methylamino-5,7-dichloro-2-quinoline carboxylate. This compound provided protection against maximal electroshock (MES) induced seizures at a dose level including 5-fluoro-2-indole carboxylic acid and the diethyl ester of 2,6-pyridine dicarboxylic acid.

Regional variation of excitatory and inhibitory amino acid-induced responses in rat dissociated CNS neurons

Regional differences in glutamate (Glu), aspartate (Asp), gamma-aminobutyric acid (GABA) and glycine (Gly) responses in CNS neurons were investigated by means of the whole-cell mode of the patch-clamp technique. The neurons were freshly dissociated from rat cortex, limbic system (hippocampal CA1 region), diencephalon (ventromedial hypothalamus), medulla (nucleus paragigantocellularis lateralis) and spinal cord (spinal dorsal horn). The current amplitudes induced by Glu and GABA did not show any regional differences whereas those of Asp- and Gly-induced responses were significantly different among CNS regions. The enhancement of Asp response by Gly was observed in all regions, and the facilitatory ratio did not differ among these regions. Even though the NMDA response in cortical neurons was significantly greater than that in spinal neurons, the ratios of NMDA response facilitation by Gly were also the same in both regions. When the current amplitudes induced by individual amino acids were estimated for the unit surface area of respective neurons (current density), the Glu, Asp and Gly responses showed regional heterogeneity whereas the GABA response did not.

Two types of steady-state desensitization of N-methyl-D-aspartate receptor in isolated hippocampal neurones of rat

1. Whole-cell voltage-clamp recordings were made from rat isolated hippocampal neurones. Aspartate (Asp) and/or glycine (Gly) were applied by a method in which the external solution could be changed within 30 ms and thereafter held constant. 2. Asp and Gly applied together at maximal concentrations (5 mM and 10 microM, respectively) evoked an inward current due to activation of N-methyl-D-aspartate (NMDA) receptors. The current peaked and then declined to a steady state during the application. The time constant of desensitization (tau) was about 1 s when the agonists were applied soon after the onset of whole-cell recording. The desensitization became more rapid (tau = 0.3 s) and more complete during the first 15 min of recording, and thereafter remained stable; the amplitude of the peak response did not change throughout. In solutions containing 10 microM-Gly, Asp had an apparent Kd of 51 microM at the peak of response and 20 microM measured at the steady state. The steady-state current was 14% of the peak current. 3. Asp was applied after a conditioning exposure of the cell of Gly (from 1 to 50 microM), together with the same Gly concentration. The maximum current evoked by the application of Asp was increased while increasing Gly in the conditioning solution, with no change in the apparent Kd for Asp at the peak of Asp-activated response. 4. Various concentrations of Asp (plus 10 microM-Gly) were applied after a conditioning exposure to Asp (which alone was without effect). The maximum current induced by Asp applications was only 28% of that observed without conditioning Asp application, but the apparent Kd was unchanged (about 57 microM). 5. Test solution containing maximal concentrations of Asp and Gly was applied after conditioning exposure to both Asp (varying concentrations) and Gly (10 microM). Complete desensitization was caused by 200 microM-Asp. The apparent Kd for Asp to induce desensitization (8.7 microM) was less than the Kd as an agonist (51 microM). 6. Test solution containing maximal concentrations of Asp and Gly was applied after conditioning exposure to both Gly (varying concentrations) and Asp (5 mM). Complete desensitization was caused by 1 microM-Gly. The apparent Kd for Gly to induce desensitization (120 nM) was less than the Kd as a co-agonist (about 1 microM).(ABSTRACT TRUNCATED AT 400 WORDS)

Immunocytochemical evidence for the involvement of glycine in sensory centers of the rat brain

Glycine-like immunoreactivity was localized to a number of sites in the rat brain which are involved in processing sensory information. In the auditory and vestibular systems, glycine immunoreactivity was seen in dorsal and ventral cochlear nuclei, superior olive, trapezoid body, medial and lateral vestibular nuclei, and inferior colliculus. Staining in the visual system was seen in retina, dorsal lateral geniculate nucleus, and superior colliculus. The olfactory system exhibited staining in the olfactory bulb and accessory olfactory formation. Somatosensory centers with glycine immunoreactivity included the dorsal column nuclei, spinal trigeminal nucleus, principal sensory nucleus of V, reticular formation, and periaqueductal gray. Glycine-immunoreactive neurons were also seen in cerebellar cortex, deep cerebellar nuclei, hippocampus, cerebral cortex, and striatum. The distribution of staining indicates that glycine plays a major role in sensory centers with actions at both strychnine-sensitive and strychnine-insensitive receptors.

Stereoselective inhibition by D- and L-alanine of phencyclidine-induced locomotor stimulation in the rat

Bilateral injection of D-alanine, but not L-alanine, (10-100 micrograms per side for each amino acid) into the lateral ventricle reduced the increasing effect of phencyclidine (PCP, 10 mg/kg, given intraperitoneally) on locomotor activity in the rat in a dose dependent manner. This stereoselectivity agrees with the potency of these agents as agonists for the strychnine-insensitive glycine receptor in the N-methyl-D-aspartate receptor complex, suggesting that stimulation of the allosteric regulation site may antagonize the ability of PCP to produce hyperactivity.

Neurochemical aspects of the N-methyl-D-aspartate receptor complex

The N-methyl-D-aspartic acid (NMDA)-sensitive subclass of brain excitatory amino acid receptors is supposed to be a receptor-ionophore complex consisting of at least 3 different major domains including an NMDA recognition site, glycine (Gly) recognition site and ion channel site. Biochemical labeling of the NMDA domain using [3H]L-glutamic acid (Glu) as a radioactive ligand often meets with several critical methodological pitfalls and artifacts that cause a serious misinterpretation of the results. Treatment of brain synaptic membranes with a low concentration of Triton X-100 induces a marked disclosure of [3H]Glu binding sensitive to displacement by NMDA with a concomitant removal of other several membranous constituents with relatively high affinity for the neuroactive amino acid. The NMDA site is also radiolabeled by the competitive antagonist (+/-)-3-(2-carboxypiperazin-4-yl)propyl-1-phosphonic acid that reveals possible heterogeneity of the site. The Gly domain is sensitive to D-serine and D-alanine but insensitive to strychnine, and this domain seems to be absolutely required for an opening of the NMDA channels by agonists. The ionophore domain is radiolabeled by a non-competitive type of NMDA antagonist that is only able to bind to the open but not closed channels. The binding of these allosteric antagonists is markedly potentiated by NMDA agonists in a manner sensitive to antagonism by isosteric antagonists in brain synaptic membranes and additionally enhanced by further inclusion of Gly agonists through the Gly domain. Furthermore, physiological and biochemical responses mediated by the NMDA receptor complex are invariably potentiated by several endogenous polyamines, suggesting a novel polyamine site within the complex. At any rate, activation of the NMDA receptor complex results in a marked influx of Ca2+ as well as Na+ ions, which subsequently induces numerous intracellular metabolic alterations that could be associated with neuronal plasticity or excitotoxicity. Therefore, any isosteric and allosteric antagonists would be of great benefit for the therapy and treatment of neurodegenerative disorders with a risk of impairing the acquisition and formation process of memories.

Characterisation of the glycine modulatory site of the N-methyl-D-aspartate receptor-ionophore complex in human brain

[3H]Glycine binding and glycine modulation of [3H]MK-801 binding have been used to study the glycine allosteric site associated with the N-methyl-D-aspartate receptor complex in postmortem human brain. The effect of glycine on [3H]MK-801 binding appeared sensitive to duration of terminal coma, and possibly postmortem delay. Thirty percent of the binding occurred in a subfraction of brain tissue and did not show enhancement by glycine and glutamic acid. [3H]Glycine binding to a subfraction free from this component was studied and showed high specific binding. KD and Bmax values showed considerable intersubject variability which did not appear to be due to demographic features or to tissue content of amino acids with an affinity for this site. The pharmacological characteristics of binding in this subfraction and a correlation between Bmax values and the maximal enhancement of [3H]MK-801 binding by glycine are consistent with [3H]glycine binding occurring to an N-methyl-D-aspartate receptor complex associated site. Further support for this is provided by a significantly lower Bmax value for [3H]glycine binding in subjects with Alzheimer's disease and reduced glycine enhancement of [3H]MK-801 binding. However, the effect of perimortem factors makes it difficult to confidently attribute this solely to a disease-related change in the receptor. The possible role of the glycine allosteric site in the treatment of neuropsychiatric disorders is discussed.

Identification of a novel structural class of positive modulators of the N-methyl-D-aspartate receptor, with actions mediated through the glycine recognition site

This study describes a new structural class of compounds which interact at the N-methyl-D-aspartate (NMDA) receptor-associated glycine recognition site. These E-gamma-substituted vinylglycine derivatives were active in displacing [3H]glycine binding from the NMDA receptor-associated recognition site in rat forebrain synaptic plasma membranes, with Ki values in the range of 0.24-8.7 microM. Functional analyses of these compounds indicate that they positively modulate basal [3H](+)-5-methyl-10,11-dihydro-5H- [a,d]cyclohepaten-5,10-imine ([3H]MK-801) binding, consistent with their having agonist characteristics. Little stereospecificity is observed with the gamma-substituted methyl and propyl derivatives while the L-isomer of the hexyl analog is significantly more potent than the D-isomer. The D- and L-hydroxyethyl gamma-substituted vinylglycines were the most potent inhibitors of [3H]glycine binding with Ki values of 0.75 +/- 0.06 microM and 0.24 +/- 0.02 microM, respectively. The 3,4-double bond was necessary for activity in that the saturated hexyl derivative (2-aminodecanoate) was inactive. Based on the results reported herein, the hypothesis that there is a distinct size restriction for functional agonists which interact with the glycine recognition site, should be altered to include these larger extensions of vinylglycine.