Phencyclidine and glycine modulate NMDA-activated high conductance cationic channels by acting at different sites

The FGIN period: electrophysiological studies

This historical review of the electrophysiology laboratory complemented the activity of the various research teams at the Fidia Georgetown Institute for the Neurosciences and it was the fulfillment of Dr. Erminio Costa's dream to be able to study the inhibitory and excitatory synapse in the central nervous system. These studies were facilitated by the development of the patch clamp technique that allows the functional testing of several of the biochemical and pharmacological hypotheses. The studies described here were the results of the hard work of all the collaborators involved in the projects that will never forget the passionate and stimulating discussion with Dr Costa during and after the development of these projects.

Glycine and neuroprotective effect of hypothermia in hypoxic-ischemic brain damage

The beneficial effects of hypothermia have long been known in non-traditional medicine but it is only in the past few decades that studies on the neuroprotective effects of hypothermia in hypoxic-ischemic brain injury have begun. Different mechanisms have been put forward to explain hypothermic neuroprotection including reduction of the excessive release of the excitatory amino acid neurotransmitter, glutamate. Recent experiments have questioned the key role of this neurotoxin in hypoxic-ischemic neuropathogenesis. In contrast, a mediatory role for another neurotransmitter, glycine in the neuroprotective effects of hypothermia has become more attractive, along with an indication of its role in the pathogenesis of ischemic neuronal damage. Thus, on the basis of reviewing relevant literature the hypothesis of a glycine-related mechanism of hypothermic neuroprotection in ischemia-induced neuronal injury has been put forward.

Electrophysiological demonstration of N-methyl-D-aspartate receptors at the afferent synapse of catfish electroreceptor organs

An excitatory amino acid, most probably L-glutamate, acts as a neurotransmitter at the receptor cell--afferent fibre synapses in the ampullary electroreceptor organs of the freshwater catfish Ictalurus nebulosus. In the present study, we have used an electrophysiological approach to investigate the presence of N-methyl-D-aspartate receptors at this level. N-Methyl-D-aspartate, dissolved in an Mg(2+)-containing (normal) solution, had no effect on afferent activity, not even at 5 mM. However, addition of 5 mM N-methyl-D-aspartate to an Mg(2+)-free solution evoked an enduring increase in firing rate. The application of N-methyl-D-aspartate combined with electrical sine wave stimulation produced a firing increase in the primary afferents, even in the presence of Mg2+ (1.5 mM). Glycine (0.01-0.001 mM) significantly potentiated the N-methyl-D-aspartate responses. Addition of antagonists of the actions of N-methyl-D-aspartate, 7-chlorokynurenic acid, DL-2-amino-5-phosphonovaleric acid and ketamine in concentrations of 0.5-2.0 mM led to a decrease in resting and stimulus-evoked activity. 7-Chlorokynurenic acid also blocked the responses to application of N-methyl-D-aspartate. The glycine agonist D-serine (0.01 mM) prevented the 7-chlorokynurenic inhibitory effect. These results suggest the involvement of N-methyl-D-aspartate receptors in mediating the actions of L-glutamate at the afferent synapses of the electroreceptor organs of the catfish.

Effects of vinconate on age-related alterations in [3H]MK-801, [3H]glycine, sodium-dependent D-[3H]aspartate, [3H]FK-506 and [3H]PN200-110 binding in rats

We investigated the effects of age and (+/-)-methyl-3-ethyl-2,3,3a,4-tetrahydro-1 H-in-dolo[3,2,1-de] [1,5] naphthyridine-6-carboxylate hydrochloride (vinconate), an indolonaphthyridine derivative, on calcium channels, neurotransmitter receptor systems and immunophilin in Fischer rat brain using quantitative receptor autoradiography. [3H]MK-801, [3H]glycine, sodium-dependent D-[3H]aspartate, [3H]FK-506 and [3H]PN200-110 were used to label N-methyl-D-aspartate (NMDA) receptors, glycine receptors, excitatory amino acid transport sites, FK-506 binding proteins (FKBP) and voltage-dependent L-type calcium channels, respectively. [3H]Glycine and sodium-dependent D-[3H]aspartate binding significantly decreased in the frontal cortex, parietal cortex, striatum, nucleus accumbens, hippocampus, thalamus, substantia nigra and cerebellum of 24 month old rats in comparison with 6 month old animals. In contrast, [3H]MK-801, [3H]FK-506 and [3H]PN200-110 binding showed no significant changes in the brain of 24 month old rats. Intraperitoneal chronic treatment with vinconate (10 and 30 mg/kg, once a day for 4 weeks) dose-dependently ameliorated the significant reduction in [3H]glycine and sodium-dependent D-[3H]aspartate binding in the brain of 24 month old rats. These results demonstrate that glycine receptors and excitatory amino acid transport sites are more susceptible to aging processes than NMDA receptors, immunophilin and voltage-dependent L-type calcium channels. Furthermore, our findings suggest that vinconate may have a beneficial effect on age-related changes in glycine receptors and excitatory amino acid transport sites.

Different age-related changes in NMDA and glycine receptors in the rat brain

We investigated the age-related changes of N-methyl-d-aspartate (NMDA) and strychnine-insensitive glycine receptors in the brain from Fischer rats aged 3 weeks (immature), 6 months (adult), 12 months (mature), 18 months (middle-aged) and 24 months (aged) using receptor autoradiography. [(3)H]MK-801 and [(3)H]glycine were used to label the NMDA receptor and the glycine receptor, respectively. In immature rats, [(3)H]MK-801 binding showed a significant decline only in the nucleus accumbens, whereas [(3)H]glycine binding exhibited a significant increase in the frontal cortex, parietal cortex, striatum and thalamus as compared with young rats. In mature, middle-aged and aged rats, [(3)H]MK-801 binding showed no significant change in the brain. In contrast, [(3)H]glycine binding showed a conspicuous reduction in the striatum and hippocampal CA3 sector and thalamus from mature rats. Thereafter, the age-related reduction in [(3)H]glycine binding was observed in all brain areas of middle-aged and aged rats. These results demonstrate that the glycine receptor in the rat brain is far more susceptible to aging processes than the NMDA receptor. Furthermore, they suggest the conspicuous differences in the developmental pattern between NMDA and glycine receptors in the rat brain after birth. These findings suggest that glycine receptor in the brain is primarily and severely affected in aging processes and this may lead to age-related neurological deficits.

The effects of (-)- and (+)-beta-cyclazocine on NMDA-evoked responses and NMDA-mediated cell damage in cultured rat hippocampal neurons

Microspectrofluorimetric measurements of excitatory amino acid-evoked rises in intracellular free calcium concentration ([Ca2+]i), electrophysiological measurements of currents through single NMDA receptor-operated ion channels and estimates of cellular viability following NMDA challenge were employed to examine the interactions of (-)- and (+)-beta-cyclazocine with the NMDA receptor-channel complex in cultured rat hippocampal neurons. Rises in [Ca2+]i evoked by NMDA, but not those evoked by kainate, AMPA or 50 mM K+, were reduced by (-)-beta-cyclazocine in a concentration- and use-dependent manner with an estimated IC50 value of 272 nM. In outside-out patches, (-)-beta-cyclazocine did not change the magnitudes of unitary NMDA-evoked currents but diminished both the frequency of channel openings and their mean open time. The IC50 for (-)-beta-cyclazocine against NMDA channel open state probability was estimated at 84 nM. The actions of (-)-beta-cyclazocine were consistent with a voltage-dependent open channel block of the NMDA channel with a blocking rate constant of 7.03.10(7) M-1.s-1 at -40 mV. Neurons exposed to a high concentration of NMDA in vitro were protected from death by 1 and 10 microM (-)-beta-cyclazocine. In all of the above assays, (+)-beta-cyclazocine was considerably less potent an NMDA antagonist and neuroprotective agent than (-)-beta-cyclazocine; the IC50 for (+)-beta-cyclazocine against channel open state probability was estimated at 14 microM. The results demonstrate that (-)-beta-cyclazocine is a potent and selective inhibitor of NMDA-evoked responses in cultured rat hippocampal neurons and an effective neuroprotective agent in vitro.

Antagonism by NG-nitro-L-arginine of L-glutamate-induced neurotoxicity in cultured neonatal rat cortical neurons. Prolonged application enhances neuroprotective efficacy

The effects of NG-nitro-L-arginine on L-glutamate-induced neurotoxicity have been evaluated on primary cultures of neonatal rat cortical neurons. Treatment of cultures with increasing concentrations of L-glutamate during 5 min produced a delayed neuronal death, as measured by lactate dehydrogenase release in the medium 24 h later. Maximal toxicity was obtained with 500 microM of L-glutamate. Substantial nitric oxide synthase activity was detected in these cortical cultures. Nitric oxide synthase activity and cellular L-glutamate-induced cyclic guanosine 3',5'-monophosphate accumulation were totally inhibited by 100 microM NG-nitro-L-arginine. Addition of NG-nitro-L-arginine (100 microM) to the medium either 5 min prior to and during L-glutamate exposure (500 microM, 5 min) or for 24 h after L-glutamate exposure decreased the amino acid-induced neurotoxicity by 23% (not significant) and 43%, respectively. When added 5 min before L-glutamate and just after L-glutamate removal and kept in contact with neurons for the following 24 h, NG-nitro-L-arginine (100 microM) antagonized by 74% the L-glutamate-induced neurotoxicity. This effect was not reversed by a co-application of L-arginine (1 mM). The neuroprotective effect of NG-nitro-L-arginine was concentration-dependent, a half-maximal inhibition of L-glutamate-induced neurotoxicity being observed with the addition (before and after L-glutamate) of 4 microM of the drug. These results suggest that the neuroprotective effect of NG-nitro-L-arginine previously observed in vivo is exerted at the neuronal level.(ABSTRACT TRUNCATED AT 250 WORDS)

Ontogeny of glycine-enhanced [3H]MK-801 binding to N-methyl-D-aspartate receptor-coupled ion channels

The N-methyl-D-aspartate (NMDA) subtype of glutamate receptor is thought to play a critical role in neuronal development, differentiation and plasticity. A number of studies have shown an enhanced sensitivity to NMDA receptor ligands in neonatal animals. This study examined the ontogenetic changes in the glycinergic modulation of NMDA-coupled cation channels in the developing central nervous system of rat pups. The nonequilibrium binding of the specific channel ligand [3H]MK-801 was used as a measure of NMDA channel access. Glycine (10(-5) M) enhancement of [3H]MK-801 binding at 2 h in forebrain membranes from adult rats was significantly greater than that observed in tissues from 8- to 28-day-old rat pups. This difference was due to changes in the efficacy, but not potency of glycine. The observed ontogenetic changes in the efficacy of glycine-enhanced [3H]MK-801 binding were attributable to developmental changes in receptor site density, as determined by equilibrium [3H]MK-801 saturation isotherms. Kinetic studies revealed that glycine increased the association rate constants of [3H]MK-801 in 8-day and adult membranes by a similar magnitude (0.111 +/- 0.021 vs 0.094 +/- 0.009 nM-1 h-1, respectively). Similarly, the fractional amount of [3H]MK-801 bound (i.e., amount bound at time t normalized to amount bound at equilibrium) in the presence of glycine was relatively constant throughout neonatal development. These findings suggest that the allosteric modulation of the NMDA ionophore by glycine is similar in postnatal and adult rats.(ABSTRACT TRUNCATED AT 250 WORDS)

Differential induction of immediate early genes by excitatory amino acid receptor types in primary cultures of cortical and striatal neurons

In primary cultures of neurons from cerebral cortex and striatum, 30 s stimulation with the excitatory amino acid glutamate elicited a 5 to 9-fold increase in immediate early gene (IEG) mRNAs. Glutamate increased c-fos, c-jun, jun-B, and NGFI-A (zif/268) mRNAs by binding to both alpha-amino-3-hydroxy-5-methylisoxazolepropionic acid (AMPA) and N-methyl-D-aspartate (NMDA) receptor types, and increased c-fos, jun-B, and NGFI-A mRNAs by binding to the metabotropic receptor. NMDA receptor activation elicited IEG expression by a transmembrane calcium influx; AMPA receptor-induced depolarization played a permissive role for the opening of the NMDA receptor channel. The protein kinase C (PKC) inhibitor H-7 (but not inhibitors of cyclic nucleotide-dependent and calcium/calmodulin-dependent protein kinases) partially blocked IEG expression induced by glutamate.

Ca2+ Mobilization in Cultured Rat Cerebellar Cells: Astrocytes are Activated by t-ACPD

Using primary rat cerebellar cell cultures we observed that trans-1-amino-cyclopentyl-1,3-dicarboxylic acid (t-ACPD) was able to induce an increase in intracellular [Ca2+] in different cell types. This response was not abolished by external Ca2+ withdrawal, indicating that t-ACPD triggered the release of intracellularly stored Ca2+. In neurons the t-ACPD response was monophasic and inhibited by l-2-amino-4-phosphonobutyrate (APB). In astrocytes, characterized by their immunoreactivity to antisera to glial fibrillary acidic protein and S-100 protein, the response was oscillatory and resistant to APB application. These results suggest the presence of glutamate metabotropic receptor subtypes in the mammalian brain.

Kinetic mechanisms of glycine requirement for N-methyl-D-aspartate channel activation

Glycine potentiates N-methyl-D-aspartate (NMDA) receptor-mediated responses via its interaction with a strychnine-insensitive glycine recognition site. We have previously shown that the potent glycine receptor antagonist 7-chlorokynurenic acid (7Cl-KYN) dose-dependently inhibits [3H]MK-801 binding to the PCP receptor and that this effect is reversed by glycine. [3H]MK-801 binding to the PCP receptor within the NMDA receptor-gated ion channel is a measure of channel activation. Association of PCP receptor ligands is biexponential with the fast component of binding serving as a marker of activated NMDA channels. In the present study we utilize 7Cl-KYN as a probe of the kinetic mechanism of the glycine effect upon NMDA receptor functioning. In the presence of L-glutamate, incubation with 7Cl-KYN completely abolished the fast component of [3H]MK-801 association in 4 out of 5 experiments. In the fifth experiment where the fast component was detected, it accounted for less than half of that seen in the presence of L-glutamate alone. 7Cl-KYN-induced inhibition of the fast component of [3H]MK-801 association was reversed by the addition of glycine. Since the fast component represents ligand binding to the PCP receptor via the open NMDA channel, selective reduction of this component by 7Cl-KYN indicates that glycine receptor antagonists reduce the probability of channel opening, and also that the selective reduction in the component of [3H]MK-801 binding that manifests fast kinetics can serve as a marker for glycine antagonists.

Modulation of protein kinase C translocation by excitatory and inhibitory amino acids in primary cultures of neurons

In primary cultures of neurons from rat cerebral cortex and neostriatum, excitatory amino acids stimulate the translocation of protein kinase C (PKC) from the cytoplasm to the membrane. In the presence of a physiological concentration of Mg2+ in the extracellular medium, glutamate induces PKC translocation by binding to both N-methyl-D-aspartate (NMDA) and alpha-amino-3-hydroxy-5-methylisoxazolepropionic acid (AMPA) excitatory amino acid receptors. Quisqualate translocates the enzyme by stimulating primarily AMPA receptors and possibly metabotropic receptors. NMDA receptor-induced PKC translocation is sodium independent, whereas quisqualate receptor-induced PKC translocation is sodium dependent; none of the agonists is active in the absence of calcium from the extracellular medium. Muscimol does not modify excitatory amino acid stimulation; however, blockade of gamma-aminobutyric acid(A) receptors by bicuculline greatly enhances glutamate-induced PKC translocation. This enhancement is blocked by the NMDA receptor antagonist (+)-5-methyl-10,11-dihydro-5H-dibenzo[a,d]cyclohepten-5,10-imine hydrogen maleate (MK-801) and by tetrodotoxin.

Electrophysiological evidence for the presence of NMDA receptors in the guinea pig cochlea

An excitatory amino acid, possibly L-glutamate, which probably acts as a neurotransmitter at the inner hair cell-afferent fiber synapses in the cochlea. In the present study, we have used an electrophysiological approach to investigate at this level the presence of a major type of excitatory amino acid receptor, namely the glutamatergic receptor for which N-methyl-D-aspartate is a selective agonist. Our results show that, when N-methyl-D-aspartate and the antagonist 2-amino-5-phosphonovalerate are perfused through the perilymphatic scalae, they induced, by different mechanisms, a significant reduction of the amplitude of the compound action potential and an increase of the N1 latency, both predominant at high intensity tone burst stimulations. No significant difference was found in the presence or absence of Mg2+ in the artificial perilymph used as a vehicle. A further slight N-methyl-D-aspartate-induced decrease of the amplitude of the compound action potential, although non significant, was observed when the Mg2(+)-free perilymph contained 100 or 1000 microM glycine. In all the experimental conditions, no effect was observed on the cochlear microphonic potential. This observation is consistent with an action of N-methyl-D-aspartate and 2-amino-5-phosphonovalerate at receptors located on the auditory nerve dendrites contacting the inner hair cells. In conclusion, our results suggest the presence of N-methyl-D-aspartate receptors in the cochlea.

Amiloride blocks glutamate-operated cationic channels and protects neurons in culture from glutamate-induced death

The diuretic amiloride has been suggested as a specific inhibitor of T-type neuronal Ca2+ channels. The effects of amiloride on glutamate receptor-gated cationic channels and glutamate-induced. Ca2(+)-dependent neuronal death were investigated in primary neuronal cultures from neonatal rats. In primary cultures of cerebellar granule neurons of the rat, receiving 50 microM glutamate for 15 min, at 22 degrees C, in the absence of Mg2+, about 80% of neurons were killed in about 24 hr. Exposure of neurons to such a pulse of glutamate, in the presence of various concentrations of amiloride, resulted in a dose-dependent protection from neurotoxicity (EC50 300 microM, complete protection 1 mM). In voltage-clamped cortical and cerebellar neurons of neonatal rats in primary culture, 100 microM amiloride diminished (by about 25%) glutamate- and/or NMDA-evoked cationic currents, recorded in the whole-cell mode. About 80% of the NMDA-(20 microM) stimulated current was inhibited by 700 microM amiloride. The inhibitory effect of amiloride was not voltage-dependent. In outside-out membrane patches, excised from granule cells and held at -50 mV, 100 microM amiloride changed the NMDA-elicited single channel activity into a fast flickering between the open and closed states. The noise analysis of the data revealed that, although resembling the Mg2(+)-induced flickering, the amiloride-induced channel block was more similar to the effects described for the action of local anaesthetics on the nicotinic cholinergic channel. The pharmacological relevance of this action of amiloride requires further characterization; the data point out the necessity of a cautious use of amiloride in studying neuronal function.

Interaction of cholinergic and glutamatergic transmission in the hippocampus: an in vitro autoradiographic receptor analysis

Quantitative in vitro receptor autoradiography was used to examine the effect of acute scopolamine administration on specific binding to components of the N-methyl-D-aspartate (NMDA) receptor complex in four regions of mouse hippocampus. The binding of [3H]glycine to the strychnine-insensitive site was increased 1 h after administration of scopolamine hydrobromide (10 mg/kg) in the ventral dentate gyrus. The study suggests that rapid alterations in strychnine-insensitive glycine binding can occur in response to cholinergic perturbations. Moreover, these data suggest a delicate interaction between cholinergic and glutamatergic projections in the hippocampus.

Primary cultures of corticostriatal cells from newborn rats: a model to study muscarinic receptor subtypes regulation and function

In the present work we characterized both the presynaptic and postsynaptic components of cholinergic transmission in a primary culture of corticostriatal neurons prepared from newborn rat brain. This culture preparation contains a small population of choline acetyltransferase (ChAT) immunoreactive neurons, corresponding to approximately 3% of the total cell number, and synthesizes increasing amounts of acetylcholine (ACh) from the third day in vitro (DIV), which reaches a plateau around the 10 day of culture. Muscarinic cholinergic receptors (mAChR), measured by the binding of the muscarinic antagonist [3H]quinuclidinyl benzilate ([3H]QNB), are detectable from the fifth DIV and increase linearly during the time of culture. At the twelfth DIV, the density of mAChRs (approximately 600 fmol/mg protein) is comparable to the density of mAChR in adult rat cortex. These receptors are coupled to second messenger systems, since muscarinic agonists inhibit adenylate cyclase activity and stimulate phosphoinositide breakdown with efficacies and potencies similar to those found in adult rat cortex. Moreover, by using the reverse transcriptase-polymerase chain reaction (RT-PCR) technique, we were able to demonstrate the presence of the m1, m3, and m4 mAChR subtype mRNAs in this neuronal culture at 12 DIV. Our data suggest that corticostriatal neuronal cultures develop in vitro ACh-synthesizing neurons and functionally active cholinergic receptors. This therefore makes them ideally suited to study the development and properties of brain mAChR subtypes.

Is picolinic acid a glycine agonist at strychnine-sensitive receptors?

By means of unit recording and electrophoretic application, the effect of picolinic acid on feline spinal interneurons in situ was studied in comparison with glycine. Picolinic acid inhibited neuronal firing in 60% of neurons and in some cases the inhibitory actions were antagonized by strychnine. Inhibition of firing by glycine, which was also strychnine-sensitive, was reduced in case of concomitant administration of picolinic acid. These results suggest that picolinic acid might act as a glycine agonist at strychnine-sensitive receptors.

Role of glycine in the N-methyl-D-aspartate-mediated neuronal cytotoxicity

Current evidence indicates that glutamate acting via the N-methyl-D-aspartate (NMDA) receptor/ion channel complex plays a major role in the neuronal degeneration associated with a variety of neurological disorders. In this report the role of glycine in NMDA neurotoxicity was examined. We demonstrate that NMDA-mediated neurotoxicity is markedly potentiated by glycine and other amino acids, e.g., D-serine. Putative glycine antagonists HA-966 and 7-chlorokynurenic acid were highly effective in preventing NMDA neurotoxicity, even in the absence of added glycine. The neuroprotective action of HA-966 and 7-chlorokynurenic acid, but not that of NMDA antagonists 3-(2-carboxypiperazine-4-yl)propylphosphonate and MK-801, could be reversed by glycine. These results indicate that glycine, operating through a strychinine-insensitive glycine site, plays a central permissive role in NMDA-mediated neurotoxicity.

Coupling of glutamatergic receptors to changes in intracellular Ca2+ in rat cerebellar granule cells in primary culture

Changes in cytosolic free Ca2+ concentrations, [Ca2+]i, in response to glutamate and glutamate receptor agonists were measured in rat cerebellar granule cells grown on coverslips. The intracellular Ca2+ as measured with fura-2 increased by applying kainate, N-methyl-D-aspartate (NMDA), quisqualate, and (RS)-d-amino-3-hydroxy-5-methyl-4-isoxazole-propionic (AMPA). When the extracellular Mg2+ was removed, the effects of NMDA and the NMDA receptor agonist cis-(+-)-1-amino-1,3-cyclopentanedicarboxylic acid (cis-ACPD) on intracellular Ca2+ were augmented. Glycine potentiated the effects of NMDA and cis-ACPD if the membrane was depolarized by increasing the extracellular K+ concentration. The NMDA receptor antagonist DL-2-amino-5-phosphonopentanic acid (AP5) abolished and the antagonist 3-([+-]-2-carboxypiperazin-4-yl)propyl-1-phosphonic acid (CPP) greatly reduced the effect of NMDA in both the normal and the Mg-free media. The dose-response curves of NMDA and, to a lesser extent, of kainate were shifted to the left, and that of quisqualate became biphasic in the Mg-free medium. The increase in [Ca2+]i produced by high quisqualate concentrations in the Mg-free medium was totally abolished by AP5. The results suggest that Ca2+ influx in cerebellar granule cells occurs through both NMDA- and non-NMDA-coupled ion channels. A part of the quisqualate-induced rise in cytosolic Ca2+ seems to be linked to the activation of NMDA receptors.

Age-related changes of strychnine-insensitive glycine receptors in rat brain as studied by in vitro autoradiography

Age-related changes of strychnine-insensitive glycine receptors in the rat brain were studied through quantitative in vitro autoradiography with 3H-glycine. 3H-glycine binding sites were most concentrated in the hippocampus, cerebral cortex, and olfactory tubercle, and moderate densities of binding sites were located in the striatum, nucleus accumbens, amygdala, and certain thalamic nuclei. Low densities of 3H-glycine binding sites were observed in the lateral septal nucleus, midbrain nuclei such as the superior colliculus and central gray matter, and granule cell layer of the cerebellum. In aged animals, severe decline of 3H-glycine binding sites was observed in the telencephalic regions including the hippocampus and cerebral cortex. On the other hand, decrease of binding sites in the midbrain nuclei was of lesser degree, and there were no changes in the cerebellum. These results suggest that the decrease of glycine receptors in particular brain regions has some relation with changes of neuronal functions associated with aging process in these areas. The glutamatergic neuronal system, particularly the N-methyl-D-aspartate (NMDA) subtype, has been considered to play an important role in learning and memory. Taking into consideration that strychnine-insensitive glycine receptors are contained in the NMDA receptor complex, the present study implies that the decrease of glycine receptors may be involved in impairments of learning and memory occurring in aged brains.

Desensitization of NMDA receptors does not proceed in the presence of kynurenate

The agonist-induced steady-state desensitization of the N-methyl-D-aspartate (NMDA) receptor was investigated by means of whole cell patch and concentration clamp in isolated pyramidal neurons from rat hippocampus. When administered against a background of previously applied agonist, glycine (Gly) produced a response that was smaller than the response elicited by simultaneous application of agonist and Gly. This feature could be explained by an agonist-induced suppression of NMDA receptor sensitivity to the facilitatory action of Gly. This type of 'steady-state desensitization' did not develop when the preincubating solutions contained kynurenate (250 microM). It is proposed that NMDA receptors at all concentrations of agonist may acquire two distinct and interconverting conformations that are sensitive or insensitive to the facilitatory action of Gly. Kynurenate shifts the equilibrium between these states keeping the receptor in a Gly-sensitive conformation.

The PCP site of the NMDA receptor complex

Evidence from electropharmacological experimentation favors the hypothesis that the PCP site is intimately associated with the channel domain of the NMDA receptor. But it is too early to state that this site lies deep within the NMDA channel pore. Determining the molecular details of the PCP site will require a complete and detailed kinetic analysis of NMDA single channel behavior. Furthermore, it is likely that hydrophobic receptor site(s) are responsible for some aspects of the blockade by at least some members of the dissociative anaesthetic family.

Characterization and regional distribution of strychnine-insensitive [3H]glycine binding sites in rat brain by quantitative receptor autoradiography

Recent evidence suggests that a strychnine-insensitive glycine modulatory site is associated with the N-methyl-D-aspartate receptor-channel complex. A quantitative autoradiographic method was used to characterize the pharmacological specificity and anatomical distribution of strychnine-insensitive [3H]glycine binding sites in rat brain. [3H]Glycine binding was specific, saturable, reversible, pH and temperature-sensitive and of high affinity. [3H]Glycine interacted with a single population of sites having a KD of approximately 200 nM and a maximum density of 6.2 pmol/mg protein (stratum radiatum, CA1). Binding exhibited a pharmacological profile similar to the physiologically defined strychnine-insensitive glycine modulatory site. Binding was stereoselective; the rank order of potency of simple amino acids as displacers of binding was: glycine greater than D-serine greater than D-alanine greater than L-serine greater than L-alanine greater than L-valine greater than D-valine. Binding was not altered by the inhibitory glycine receptor ligand, strychnine, by the glutamate agonists, quisqualate and kainate, or by GABA receptor selective ligands. Most competitive agonists or antagonists of the N-methyl-D-aspartate recognition site were ineffective displacers of glycine binding. The exceptions were the aminophosphono series of antagonists, D-alpha-aminoadipate, gamma-D-glutamyglycine and beta-D-aspartylaminomethylphosphonic acid. However, the inhibition of [3H]glycine binding produced by the aminophosphono compounds could be accounted for by the level of glycine contamination present in these compounds. The non-competitive NMDA receptor-channel blockers, phencyclidine, its thienyl derivative, and MK-801 did not alter glycine binding. Kynurenate, glycine methylester, L-serine-O-sulfate, L-homocysteic acid, and several glycine-containing dipeptides were effective displacers of glycine binding. Structure-activity relations of agonists and antagonists of the strychinine-insensitive glycine binding site are discussed. The distribution of strychnine-insensitive [3H]glycine binding was heterogeneous with the following rank order of binding densities: hippocampus greater than cerebral cortex greater than caudate-putamen greater than or equal to thalamus greater than cerebellum greater than brain stem. This distribution of binding was correlated with N-methyl-D-aspartate-sensitive [3H]glutamate binding (r2 = 0.77; P less than 0.001; Pearson product-moment) and [3H]thienylcyclohexylpiperidine binding (r2 = 0.72; P less than 0.001). These observations are consistent with the hypothesis that the strychnine-insensitive glycine binding site is closely associated with the N-methyl-D-aspartate receptor-channel complex.(ABSTRACT TRUNCATED AT 400 WORDS)

Glycine reverses 7-chlorokynurenic acid-induced inhibition of [3H]MK-801 binding

7-Chlorokynurenic acid (7-Cl KYNA) has been reported to attenuate N-methyl-D-aspartate (NMDA) receptor functioning by a potent and selective inhibitory action mediated at the strychnine-insensitive glycine recognition site of the NMDA complex. Here we report that 7-Cl KYNA dose-dependently inhibits [3H]MK-801 binding to the PCP receptor, and that this effect is reversed by addition of glycine. Since [3H]MK-801 binding is a measure of channel activation, our results are consistent with the hypotheses that 7-Cl KYNA exerts its NMDA receptor antagonism by acting at the glycine site, and that activation of the glycine site is required for NMDA channel activity to occur.

Uptake and release of glycine in cerebellar granule cells and astrocytes in primary culture: potassium-stimulated release from granule cells is calcium-dependent

The properties of [3H]glycine uptake and release were studied with cerebellar granule cells, 7-9 days in vitro, (DIV) and astrocytes, 14-15 DIV, in primary cultures. The uptake of glycine in both cell types consisted of a saturable high-affinity transport and nonsaturable diffusion. The transport constant (Km) and maximal velocity (V) were significantly higher in granule cells than in astrocytes. Uptake was strictly Na+-dependent and also markedly diminished in low-Cl medium. The specificity of the uptake was similar in both cell types. The spontaneous release of glycine from granule cells and astrocytes was fast. Homoexchange with extracellularly added glycine in granule cells suggests that the efflux is at least partly mediated via membrane transport sites in these cells. Kainate stimulated the release more effectively in neurons than in glial cells, the effect apparently being mediated by specific kainate-sensitive receptors in both cell types. The release was enhanced by veratridine and by depolarization of cell membranes by high K (50 mM) in both neurons and astrocytes. The potassium-stimulated release was partially Ca-dependent in neurons but Ca-independent in glial cells. The results suggest a functional role for glycine in both cerebellar astrocytes and glutamatergic granule cells.

Cycloleucine blocks NMDA responses in cultured hippocampal neurones under voltage clamp: antagonism at the strychnine-insensitive glycine receptor

1. Radioligand binding studies have demonstrated that the neutral amino acid cycloleucine may act as a competitive antagonist at the glycine modulatory site on the N-methyl-D-aspartate (NMDA) receptor complex. In the present study, we examined the effects of cycloleucine on NMDA-evoked inward current responses in dissociated hippocampal neuronal cultures using the whole cell voltage-clamp technique. 2. In the presence of 1 microM glycine, cycloleucine caused a reversible, dose-dependent inhibition of NMDA responses with an IC50 of 24 microM. An increase in glycine to 100 microM resulted in a shift to the right of the cycloleucine concentration-effect curve (IC50, 1.4 mM). However, with cycloleucine concentrations less than or equal to 100 microM, a fraction of the block could not be overcome by glycine even at concentrations as high as 1 mM. 3. The cycloleucine block was unaffected by shifts in the holding potential (-60 to +60 mV), and there was no effect of cycloleucine on the reversal potential of the NMDA-evoked current. 4. Cycloleucine failed to effect kainic acid- and quisqualic acid-evoked currents at concentrations which inhibited NMDA responses. 5. We conclude that cycloleucine is a potent and selective antagonist of NMDA-receptor mediated responses. Although this effect occurs in part via competitive antagonism at the glycine modulatory site, the cycloleucine block cannot be completely reversed by glycine indicating an interaction with an additional site on the receptor-channel complex.

Glycine potentiates the stimulation of inositol phospholipid hydrolysis by excitatory amino acids in primary cultures of cerebellar neurons

Glycine potentiates stimulation of inositol phospholipid hydrolysis by glutamate and N-methyl-D-aspartate, but not by quisqualate or carbamylcholine, in primary cultures of cerebellar granule cells. This potentiation occurs in the absence of extracellular Mg2+, but is more evident when stimulation of inositol phospholipid hydrolysis by N-methyl-D-aspartate is measured in the presence of 1 mM Mg2+. The action of glycine is not antagonized by strychnine. These results suggest that glycine acts as a positive modulator of signal transduction at a specific class of N-methyl-D-aspartate-sensitive glutamate receptors coupled to inositol phospholipid hydrolysis in cerebellar granule cells.

On the multiple-conductance single channels activated by excitatory amino acids in large cerebellar neurones of the rat

1. Single-channel currents evoked by excitatory amino acids have been examined in outside-out patches from large cerebellar neurones (including Purkinje cells) in tissue culture. L-Glutamate (3-10 microM), L-aspartate (3-10 microM), NMDA (N-methyl-D-aspartate, 10-50 microM), ibotenate (50 microM), quisqualate (3-50 microM), and kainate (3-50 microM) all produced single-channel currents with multiple amplitudes. 2. Single-channel currents recorded over a range of patch potentials had a mean interpolated reversal potential of -3.8 +/- 0.5 mV. The directly resolvable multiple conductance levels could be classified into five main groups, with mean values (averaged for all agonists) of: 47.9 +/- 0.7, 38.5 +/- 0.8, 27.8 +/- 1.4, 18.2 +/- 0.5 and 8.3 +/- 0.6 pS. 3. From the relative areas under current amplitude histograms it was estimated that the percentage of openings with conductances greater than 30 pS was about 83% with NMDA, 79% with glutamate and 78% with aspartate. In some patches, the majority of greater than 30 pS events evoked by these agonists were to the maximum conductance of 48 pS, whereas in other patches there were more 38 pS openings than 48 pS openings. Only 27% of quisqualate openings, and about 10% of kainate openings, were greater than 30 pS. 4. Of the small amplitude (less than 20 pS) events, 93% of quisqualate openings were to the 8 pS level whereas approximately 87% of less than 20 pS currents produced by NMDA, glutamate and aspartate were to the 18 pS level (the remainder being 8 pS). Direct transitions could occur between certain levels (including events above and below 30 pS) suggesting that these are sublevels of multiple-conductance channels. The most frequently occurring transitions were between the 48 and 38 pS levels, and the 38 and 18 pS levels. 5. Channel openings occurred in bursts, within which individual openings were separated either by brief closures (gaps), or by direct transitions between the multiple conductance levels. The briefest of these gaps (less than 200-400 microseconds) could represent a mixture of transitions to lower conductance levels as well as partially resolved complete shuttings. The mean duration of the longer gaps within bursts, thought to represent complete but partially resolved shuttings was 1.05 +/- 0.25 ms (pooled for all agonists). 6. Burst-length distributions could be fitted with the sum of three exponentials. The briefest component may have arisen from brief single openings. The two slower components probably reflect the existence of two kinetically distinct open states.(ABSTRACT TRUNCATED AT 400 WORDS)

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

Glycine is the principal inhibitory neurotransmitter in posterior regions of the brain. In addition, glycine serves as an allosteric regulator of excitatory neurotransmission mediated by the N-methyl-D-aspartate (NMDA) acidic amino acid receptor subtype. The studies presented here characterize [3H]glycine binding to washed membranes prepared from rat spinal cord and cortex, areas enriched in glycine inhibitory and NMDA receptors, respectively, in an attempt to define the glycine recognition sites on the two classes of receptors. Specific binding for [3H]glycine was seen in both cortex and spinal cord. Saturation analyses in cortex were best fitted by a two-site model with respective equilibrium dissociation constants (KD values) of 0.24 and 5.6 microM and respective maximal binding constants (Bmax values) of 3.4 and 26.7 pmol/mg of protein. Similar analyses in spinal cord were best fitted by a one-site model with a KD of 5.8 microM and Bmax of 20.2 pmol/mg of protein. Na+ had no effect on [3H]glycine binding to cortical membranes but increased the binding to spinal cord membranes by greater than 15-fold. This Na+-dependent binding may reflect glycine binding to the recognition site of the high-affinity, Na+-dependent glycine uptake system. Several short-chain, neutral amino acids displaced [3H]glycine binding from both cortical and spinal cord membranes. The most potent displacers of [3H]glycine binding to cortical membranes were D-serine and D-alanine, followed by the L-isomers of serine and alanine and beta-alanine. In contrast, D-serine and D-alanine were similar in potency to L-serine in spinal cord membranes. Compounds active at receptors for the acidic amino acids had disparate effects on the binding of [3H]glycine. At 10 microM, NMDA resulted in a 25% increase, whereas D- and L-2-amino-5-phosphonovaleric acid at 100 microM resulted in a 30% decrease, in [3H]glycine binding to cortical membranes. Kynurenic acid was the most potent of the acidic amino acid-related compounds at displacing [3H]glycine binding. In cortical membranes, kynurenic acid displacement was resolved into a high- and a low-affinity component; the high-affinity component displaced the high-affinity component of [3H]glycine binding.(ABSTRACT TRUNCATED AT 400 WORDS)

Glycine and D-serine act as positive modulators of signal transduction at N-methyl-D-aspartate sensitive glutamate receptors in cultured cerebellar granule cells

In cultures of rat neonatal cerebellar granule cells the signal transduction at ionotropic NMDA-sensitive glutamate receptors (GC1) was measured as an increase of influx of 45Ca2+. This transmitter-mediated influx of Ca2+ was enhanced by glycine and D-serine in a dose-dependent manner. D-Alanine was less active than glycine and D-serine, while L-alanine and L-serine were inactive. These amino acids failed to activate basal influx of Ca2+. Activation of calcium influx at GC2 receptors by kainate was unchanged by the amino acids mentioned above. Glycine and D-serine increased the potency but failed to change the efficacy of GC1 agonists. This action was not changed by strychnine. The enhancement of aspartate signal transduction by glycine and D-serine was inhibited by the noncompetitive GC1 receptor antagonist, phencyclidine, but was even more evident in presence of Mg2+ ions. Hence, glycine and D-serine may function as positive allosteric modulators of signal transduction at NMDA-sensitive (GC1) glutamate receptors.

Kynurenic acid inhibits the activation of kainic and N-methyl-D-aspartic acid-sensitive ionotropic receptors by a different mechanism

The action of kynurenic acid on currents elicited by the activation of amino acid receptors was investigated in primary cultures of cortical neurons prepared from neonatal rats. Kynurenic acid was tested on currents elicited by both N-methyl-D-aspartic acid (NMDA) and kainate, using patch-clamp recording techniques in "outside-out" and "whole-cell" configurations. The inhibition by kynurenic acid was compared with that elicited by amino-phosphono-valeric acid (APV). Whole-cell currents, elicited by increasing doses of NMDA, were antagonized competitively by APV and non-competitively by kynurenic acid (ID50 70 microM); in contrast, kynurenic acid inhibited competitively the whole-cell currents elicited by kainic acid (ID50 500 microM). The non-competitive inhibition by kynurenic acid of the whole cell currents elicited by NMDA was antagonized competitively by glycine, a specific positive allosteric modulator of NMDA receptors; on the other hand glycine failed to change the inhibition by APV of the NMDA-elicited responses. Thus, kynurenic acid inhibits NMDA receptors allosterically (non-competitively) and kainic acid receptors isosterically (competitively).

A glycine site associated with N-methyl-D-aspartic acid receptors: characterization and identification of a new class of antagonists

Membranes from rat telencephalon contain a single class of strychnine-insensitive glycine sites. That these sites are associated with N-methyl-D-aspartic acid (NMDA) receptors is indicated by the observations that [3H]glycine binding is selectively modulated by NMDA receptor ligands and, conversely, that several amino acids interacting with the glycine sites increase [3H]N-[1-(2-thienyl)cyclohexyl]piperidine ([3H]TCP) binding to the phencyclidine site of the NMDA receptor. The endogenous compound kynurenate and several related quinoline and quinoxaline derivatives inhibit glycine binding with affinities that are much higher than their affinities for glutamate binding sites. In contrast to glycine, kynurenate-type compounds inhibit [3H]TCP binding and thus are suggested to form a novel class of antagonists of the NMDA receptor acting through the glycine site. These results suggest the existence of a dual and opposite modulation of NMDA receptors by endogenous ligands.

[3H]glycine binding sites, NMDA and PCP receptors have similar distributions in the human hippocampus: an autoradiographic study

The distribution of [3H]glycine binding sites was compared with that of N-methyl-D-aspartate (NMDA) receptors labelled with L-[3H]glutamate, and with that of phencyclidine (PCP) receptors labelled with [3H]1-(1-(2-thienyl)-cyclohexyl)piperidine ([3H]TCP) in sections from 7 normal human hippocampi. The results indicate that strychnine-insensitive glycine binding sites are present in high concentrations in CA1 and the molecular layer of the dentate gyrus. This distribution is very similar to the distributions of NMDA and PCP receptors in the human hippocampus.

mRNA from NCB-20 cells encodes the N-methyl-D-aspartate/phencyclidine receptor: a Xenopus oocyte expression study

The mouse neuroblastoma--Chinese hamster brain hybrid cell line NCB-20 is the only clonal cell line in which binding studies indicate the presence of phencyclidine (PCP) receptors. We report here that Xenopus oocytes injected with NCB-20 cell poly(A)+ RNA express N-methyl-D-aspartate (NMDA)-activated channels and that these channels include the PCP receptor site. In injected oocytes, NMDA application evoked a partially desensitizing inward current that was potentiated by glycine, blocked by the competitive antagonist D-2-amino-5-phosphonovaleric acid, blocked by Mg2+ and by Zn2+, and blocked in a use-dependent manner by the PCP receptor ligands PCP and MK-801. There was little or no response to kainate or quisqualate (agonists of the other excitatory amino acid receptors), to gamma-aminobutyric acid (an inhibitory transmitter), or to glycine (an inhibitory transmitter as well as an allosteric potentiator of NMDA channels). Thus, NMDA/PCP receptors expressed from NCB-20 cell mRNA exhibit properties similar to those of the neuronal receptors. The absence of expression of other excitatory amino acid receptors in this system makes it particularly useful for study of NMDA-evoked responses without interference from responses mediated by other receptors. Moreover, NCB-20 mRNA may be an appropriate starting material for cloning the cDNA(s) encoding the NMDA/PCP-receptor complex.

Interaction of [3H]MK-801 with multiple states of the N-methyl-D-aspartate receptor complex of rat brain

N-Methyl-D-aspartate (N-Me-D-Asp) and phencyclidine receptors interactively mediate central nervous system processes including psychotomimetic effects of drugs as well as neurodegenerative, cognitive, and developmental events. To elucidate the mechanism of this interaction, effects of N-Me-D-Asp agonists and antagonists and of glycine-like agents upon binding of the radiolabeled phencyclidine receptor ligand [3H]MK-801 were determined in rat brain. Scatchard analysis revealed two discrete components of [3H]MK-801 binding after 4 hr of incubation. Incubation in the presence of L-glutamate led to an increase in apparent densities but not in affinities of both components of [3H]MK-801 binding as well as conversion of sites from apparent low to high affinity. Incubation in the presence of combined D-serine and L-glutamate led to an increase in the apparent density of high-affinity [3H]MK-801 binding compared with incubation in the presence of either L-glutamate or D-serine alone. These data support a model in which phencyclidine receptor ligands bind differentially to closed as well as open conformations of the N-Me-D-Asp receptor complex and in which glycine-like agents permit or factilitate agonist-induced conversion of N-Me-D-Asp receptors from closed to open conformations.

Glycine and D-serine increase the affinity of N-methyl-D-aspartate sensitive glutamate binding sites in rat brain synaptic membranes

In previously frozen and extensively washed brain membranes [3H]glutamate binds to a single population of sites characteristic of the NMDA-sensitive glutamate receptor subtype. This binding cannot be displaced by glycine and D-serine, but actually is enhanced by these amino acids in a dose-dependent manner. Glycine and D-serine increase the affinity of glutamate binding without changing the density of binding sites. These results delineate glycine as an allosteric modulator of the recognition site for the NMDA-sensitive glutamate receptor.

Gangliosides prevent glutamate and kainate neurotoxicity in primary neuronal cultures of neonatal rat cerebellum and cortex

Using a sensitive histofluorescence staining method that allows for a quantitation of neuronal death, we compared the protective effects of gangliosides (a group of naturally occurring glycosphingolipids), phencyclidine (PCP), and MK-801 (dibenzocyclohepteneimine) on glutamate- and kainate-induced neuronal death in primary cultures of cortical and cerebellar neurons prepared from neonatal rats. PCP and MK-801 block neurotoxicity induced by glutamate doses 50 times higher than the LD50 (LD50 in Mg2+-free medium, 10 microM) but only partially block the kainate neurotoxicity (LD50 in presence of Mg2+, 100 microM). In contrast, pretreatment with gangliosides (GT1b greater than GD1b greater than GM1) results in complete and insurmountable protection against the neurotoxicity elicited by glutamate or kainate. In primary cultures of cerebellar granule cells gangliosides, unlike PCP and MK-801, fail to block glutamate-gated cationic currents and the glutamate-evoked increase of (i) inositol phospholipid hydrolysis (ii) c-fos mRNA content, and (iii) nuclear accumulation of c-fos protein. Protection of glutamate neurotoxicity by gangliosides does not require their presence in the incubation medium; however, it is proportional to the amount of glycosphingolipid accumulated in the neuronal membranes. The ganglioside concentration (30-60 microM) that blocks glutamate-elicited neuronal death also prevents glutamate- and kainate-induced protein kinase C translocation from cytosol to neuronal membranes.