Administration Of D-serine Research Articles

Phencyclidine-induced cognitive deficits in mice are improved by subsequent subchronic administration of the glycine transporter-1 inhibitor NFPS and d-serine

Accumulating evidence suggests that the glycine modulatory site on the NMDA receptor could be potential therapeutic target for cognitive deficits in schizophrenia. The present study was undertaken to examine the effects of the glycine transporter-1 (GlyT-1) inhibitor, (R)-( N-[3-(4′-fluorophenyl)-3-(4′-phenylphenoxy)propyl])sarcosine (NFPS), on cognitive deficits in mice after repeated administration of the NMDA receptor antagonist phencyclidine (PCP). PCP (10 mg/kg/day for 10 days)-induced cognitive deficits were significantly improved by subsequent subchronic (2-week) administration of NFPS (1.0 and 3.0 mg/kg/day) or d-serine (600 mg/kg/day). However, PCP-induced cognitive deficits were not improved by a single administration of NFPS (3.0 mg/kg). Furthermore, Western blot analysis revealed that levels of GlyT-1 in the hippocampus, but not frontal cortex, of the PCP (10 mg/kg/day for 10 days)-treated mice were significantly higher than those of saline-treated mice. An in vivo microdialysis study revealed that repeated PCP administration significantly decreased the extracellular levels of glycine in the hippocampus, but not frontal cortex, of mice. These findings suggest that repeated PCP administration increased the density of GlyT-1 in the hippocampus of mouse brain, and that the GlyT-1 inhibitor NFPS could ameliorate cognitive deficits in mice after repeated administration of PCP.

Role for D-Serine within the Ventral Tegmental Area in the Development of Cocaine's Sensitization

Repeated exposure to cocaine results in motor sensitization that, in the ventral tegmental area (VTA), is associated to enhanced glutamate release, which in turn leads to enhanced calcium levels in dopaminergic neurons. Calcium influx activates calcium-calmodulin-dependent protein kinases such as CaMKII. D-Serine could participate on these effects, and the objective was to discern the role of VTA D-serine after a sensitizing regimen of cocaine (10 mg/kg daily), and to discern consequent expression changes in CaMKII and its activated form. For this purpose, D-serine, sodium benzoate (inhibitor of D-amino acid oxidase, the degradating enzyme of D-serine), and 7-chlorokynurenate (inhibitor of the glycine site of NMDA receptors) were injected into the VTA (in either the induction or expression phase of sensitization), and activation state of CaMKII was assessed through blotting. The findings indicated that intra-VTA administration of D-serine (5 mM) and sodium benzoate (100 and 200 microg/microl) during the induction phase (not expression) reliably augmented the expression of behavioral sensitization to cocaine, providing evidence that D-serine in the VTA participates in the initiation of motor sensitization to this psychostimulant drug. Intra-VTA infusions of D-serine, sodium benzoate and 7-chlorokynurenate did not elicit a motor effect of their own. Confirming the important role of NMDA receptors and their activation at the glycine site, the employment of 7-chlorokynurenate (2 and 5 microg/microl) led to blocking of the development of sensitization to cocaine. CaMKII within the VTA was found to participate in D-serine's effects because this kinase, that is activated after repeated cocaine, was further activated after co-treatment with D-serine or sodium benzoate. Besides CaMKII activity was otherwise reduced by 7-chlorokynurenate.

D-serine relieves chronic lead exposure-impaired long-term potentiation in the CA1 region of the rat hippocampus in vitro

Chronic lead-exposure produces long-lasting astroglial morphological and functional changes, which disturb the neuronal functions in the hippocampus. It has been shown that glia-derived d-serine is an essential signal for N-methyl- d-aspartate receptor (NMDAR)-dependent synaptic plasticity in the hippocampal CA1 region. However, the relationship between d-serine and the chronic lead exposure-induced deficit of synaptic plasticity is not clear. In the present study, the properties of d-serine on the chronic lead exposure-impaired synaptic plasticity in the rat hippocampal CA1 region were investigated with electrophysiological recording techniques in vitro. We found that 50 μM d-serine rescued the chronic lead exposure-induced deficit of long-term potentiation (LTP). However, this effect could be abolished by 7-chlorokynurenic acid (7-ClKY), which is a specific antagonist of the glycine-binding site of NMDARs. In contrast, d-serine had no effect on the NMDAR-independent LTP, which was induced in the mossy-CA3 synapses. In addition, we found that d-serine rescued the acute Pb 2+-impaired NMDAR-mediated excitatory postsynaptic currents (EPSCs) partially. These findings demonstrate that d-serine relieves the chronic lead exposure-induced deficit of synaptic plasticity via NMDAR activation suggesting that administration of d-serine may be a potential therapeutic intervention to treat chronic lead exposure-impaired cognitive functions or affective disorders.

Activation of supraspinal NMDA receptors by both D-serine alone or in combination with morphine leads to the potentiation of antinociception in tail-flick test of rats

Although there is a variety of information concerning the effects of the N-methyl-D-aspartate (NMDA) receptor on opioid-induced antinociception at the spinal level, little is known about the effects at the supraspinal level. To clarify the role of the NMDA receptor on the morphine-induced antinociception at the supraspinal level, we investigated the effects of the intracerebroventricular (i.c.v.) administration of D-serine, a selective agonist for the glycine site of the NMDA receptors, alone or in combination with morphine using the tail-flick test. The i.c.v. administration of D-serine, but not L-serine, alone produced a dose-dependent antinociception in the tail-flick response. D-Serine also dose-dependently potentiated the antinociceptive effect induced by the i.c.v. administration of morphine and the simultaneous administration produced an additive effect. The potentiation of the antinociception produced by both D-serine alone or in combination with morphine was dose-dependently attenuated by the i.c.v. administration of L-701,324, a selective antagonist for the glycine site of the NMDA receptors. In addition, the potentiation of the D-serine-induced antinociception was antagonized by the i.c.v. administration of naloxone, a nonselective opioid receptor antagonist. These observations, together with the fact that D-serine is an endogenous and selective co-agonist for the glycine site of the NMDA receptors, strongly suggested that the activation of the supraspinal NMDA receptors by D-serine leads to the potentiation of the antinociception in the tail-flick test and that endogenous D-serine could modulate the μ-opioid receptor mediated antinociception via the glycine site of the NMDA receptors at the supraspinal level.

A CSF and postmortem brain study of d-serine metabolic parameters in schizophrenia

Clinical trials demonstrated that d-serine administration improves schizophrenia symptoms, raising the possibility that altered levels of endogenous d-serine may contribute to the N-methyl d-aspartate receptor hypofunction thought to play a role in the disease. We hypothesized that cerebro-spinal fluid (CSF) d-serine levels are decreased in the patients due to reduced synthesis and/or increased degradation in brain. We now monitored amino acid levels in CSF from 12 schizophrenia patients vs. 12 controls and in postmortem parietal-cortex from 15 control subjects and 15 each of schizophrenia, major-depression and bipolar patients. In addition, we monitored postmortem brain serine racemase and d-amino acid oxidase protein levels by Western-blot analysis. We found a 25% decrease in d-serine levels and d/ l-serine ratio in CSF of schizophrenia patients, while parietal-cortex d-serine was unaltered. Levels of l-serine, l-glutamine and l-glutamate were unaffected. Frontal-cortex (39%) and hippocampal (21%) serine racemase protein levels and hippocampal serine racemase/ d-amino acid oxidase ratio (34%) were reduced. Hippocampal d-amino-acid-oxidase protein levels significantly correlated with duration of illness ( r = 0.6, p = 0.019) but not age. d-amino acid oxidase levels in patients with DOI > 20 years were 77% significantly higher than in the other patients and controls. Our results suggest that reduced brain serine racemase and elevated d-amino acid oxidase protein levels may contribute to the lower CSF d-serine levels in schizophrenia.

The pharmacological stimulation of NMDA receptors via co-agonist site: an fMRI study in the rat brain

d-Serine has been proposed as an endogenous modulator at the co-agonist glycine-binding site of N-methyl- d-aspartate (NMDA) receptors. There is still some debate as to whether this site is saturated in vivo, but it seems likely that this depends on regional differences in local glycine or d-serine concentrations. In order to identify areas where the co-agonist site was not fully activated in vivo, we studied the effect of intraperitoneal d-serine administration in the rat brain using functional magnetic resonance imaging (fMRI). Using contrast agent injection, the variations in the relative cerebral blood volume (CBV rel) in several regions of interest were evaluated. d-Serine (50 mg/kg) elicited a significant statistical increase in the CBV rel in the hippocampus. This effect was inhibited by the specific full antagonist of the co-agonist glycine site L-701,324 indicating that the hippocampal activation occurred through the binding of the agonist d-serine to the glycine-binding site of NMDA receptors. This result demonstrates that in the hippocampus, the co-agonist sites of NMDA receptors are not endogenously saturated under our experimental conditions, suggesting an important role of d-serine in the modulation of receptor function in the hippocampus.

Sodium benzoate attenuates d-serine induced nephrotoxicity in the rat

D-Serine causes selective necrosis to the straight portion of the rat renal proximal tubules. The onset is rapid, occurring within 3-4 h and accompanied by proteinuria, glucosuria and aminoaciduria. The metabolism of D-serine by D-amino acid oxidase (D-AAO) may be involved in the mechanism of toxicity. D-AAO is localized within the peroxisomes of renal tubular epithelial cells, which is also the location of D-serine reabsorption. To address the role of D-AAO in D-serine-induced nephrotoxicity, we have examined the effect of sodium benzoate (SB) on the renal injury. SB has been shown to be a potent, competitive inhibitor of kidney D-AAO in vitro. Male Alderley Park rats were exposed to D-serine (500 mg/kg i.p.) 1 h after exposure to SB (125, 250, 500 or 750 mg/kg i.p.). Urine was collected for 0-6 h, then terminal plasma samples and kidneys were taken at 6.5 h. A second group of animals was given SB (500 mg/kg) followed by D-serine (500 mg/kg i.p.; 1 h later) and urine was collected after 0-6, 6-24 and 24-48 h. Terminal plasma samples and kidneys were taken at 48 h. 1H NMR spectroscopic analysis of urine, combined with principal component analysis, demonstrated that SB was able to prevent D-serine-induced perturbations to the urinary profile in a dose dependent manner. This was confirmed by measurement of plasma creatinine and urinary glucose and protein and histopathological examination of the kidneys. Assessment 48 h after D-serine administration revealed that nephrotoxicity was observed in animals pre-treated with SB (500 mg/kg) although the extent of injury was less pronounced than following D-serine alone. These results demonstrate that whilst prior exposure to SB prevents the initial onset of D-serine-induced nephrotoxicity, renal injury is still apparent at later time points. D-AAO activity in the kidney was decreased by 50% 1 h after dosing with SB suggesting that inhibition of this enzyme may be responsible for the observed protection.

Simultaneous determination of D‐ and L‐serine in rat brain microdialysis sample using a column‐switching HPLC with fluorimetric detection

Both D- and L-serine in rat brain microdialysis sample were simultaneously determined by pre-column fluorescence derivatization with 4-fluoro-7-nitro-2,1,3-benzoxadiazole (NBD-F), separation of the derivatives on ODS column, TSKgel ODS-80TsQA, followed by Pirkle type chiral columns, Sumichiral OA-2500 (S), which gave a sufficient enantiomeric separation of NBD-D-serine and NBD-L-serine, and fluorimetric detection at a wavelength of 540 nm with an excitation wavelength of 470 nm. The peaks of NBD-D-serine and NBD-L-serine in the rat brain microdialysis sample were clearly found, and the validation study showed satisfactory results; the precision and accuracy were within 5.14 and 109%, respectively. Using the proposed HPLC method, the time-course profile of D-serine concentration in rat prefrontal cortex following intraperitoneal administration of D-serine was investigated. As a consequence, D-serine appeared to be rapidly distributed in the brain, and then decreased gradually with time in the extracellular fluid of the rat prefrontal cortex. The proposed HPLC method will be useful for in vivo studies on D-serine, which acts as a coagonist for N-methyl-D-aspartate receptor, to the extracellular fluid of rat brain.

Effect of systemic administration of d-serine on the levels of d- and l-serine in several brain areas and periphery of rat

To obtain further insight into the distribution and metabolism of exogenous d-serine, we have investigated the effect of the intraperitoneal administration of d-serine (10 mmol/kg) on the concentrations of d- and l-serine in several brain areas and periphery of infant and adult rats. The administration produced a significant augmentation of the d-serine levels not only in the cortex but also in the hippocampus, striatum, cerebellum and periphery. The rapid decline in the enhanced d-serine levels was observed in the periphery and cerebellum, whereas the injection caused a prolonged elevation of the d-serine levels in the cortex and hippocampus. The application caused a slight increase in the l-serine levels in several brain areas and periphery 3 or 6 h after the injection, whereas a significant decrease in the l-serine concentration was observed in the periphery, diencephalon and cerebellum 3 or 7 days after the injection. Because a structural abnormality and N-methyl- d-aspartate (NMDA) receptor hypofunction has been demonstrated in the cortex and hippocampus of schizophrenic subjects, d-serine treatment may offer a new therapeutic approach to diseases related to the hypofunction of NMDA receptors such as schizophrenia.

Effect of the glycine modulatory site of the N-methyl- d-aspartate receptor on synaptic responses in kitten visual cortex

In visual cortical slices taken from kittens, the administration of d-serine, an agonist of the N-methyl- d-aspartate receptor complex, significantly enhanced synaptically evoked responses using field potential recordings in lower layer II/III. Expression of normally appearing long-term potentiation (LTP) took place in the presence of the agonist during high-frequency stimulation (HFS). The administration of an antagonist of this receptor, 7-chloro-kynurenic acid (7-Cl KY) alone had no appreciable effect on low-frequency synaptic transmission while HFS failed to induce LTP. Combined administration of d-serine and 7-Cl KY resulted in no alteration of low-frequency synaptic transmission, although expression of LTP was normally obtained. These results suggest that in visual cortex of kittens, the availability of this glycine binding site of the NMDA receptor is necessary for the expression of LTP.

Cloning of a [formula omitted]-Serine-Regulated Transcript dsr-1 from the Rat Cerebral Cortex

To obtain insight into the molecular mechanisms underlying the metabolism and functions of endogenous d-serine, we have explored d-serine-regulated transcripts in the neocortex of the infant rat treated with acute d-serine administration by using an RNA fingerprinting technique. Cloning and sequence analysis of the corresponding cDNAs to the identified transcripts have revealed that the dsr-1 (d-serine responsive transcript-1) mRNA is presumed to contain a novel sequence at the 5′-region, while the 631-base nucleotide sequence of its 3′-end is identical with that of rat M9.2 mRNA encoding a subunit of vacuolar type proton-ATPase. The predicted two open reading frames and their deduced amino acid sequences suggest that the dsr-1 product has a membrane spanning domain. The dsr-1 transcript was detected as a single band around 2.1 kb on the Northern blot. RT-PCR analyses have indicated that the dsr-1 transcript is expressed predominantly in the brain, lung, and testis, and that acute intraperitoneal injection of d-serine significantly upregulates dsr-1 expression in the neocortex 3 and 15 h later without affecting the levels of the M9.2 gene transcript. These results suggest that dsr-1 products may be involved in the d-serine-related metabolic or signaling pathways in mammalian brains.

In vivo evidence for the link between L- and D-serine metabolism in rat cerebral cortex.

To obtain an insight into the metabolic pathways of endogenous D-serine in mammalian brains, we have investigated in the infant rat the effects of systemic administration of L-serine, D-serine, and related amino acids, including glycine and threonine, on the amino acid contents in the cerebral cortex. Intraperitoneal injection of L-serine induced a rapid and transient elevation of the levels of L-serine itself in the neocortex, with its peak at 3 h post injection, and a delayed and prolonged increase in D-serine contents from 1.5 h to at least 24 h thereafter. Similarly, a significant augmentation in cerebral D-serine contents was observed 6 h after intraperitoneal administration of glycine, which also elevated the cortical L-serine levels. In contrast, L-threonine injection affected the concentrations of neither D- nor L-serine in the cortex of the pups. D-Serine given systemically, in turn, increased the neocortical contents of L-serine as well as D-serine itself, but failed to alter those of glycine and L-threonine. These in vivo data suggest the possible link between metabolic pathways of D- and L-serine in the cerebral cortex of the rat.

Thalamic NMDA transmission in a genetic model of absence epilepsy in rats.

In the selected strain of GAERS Wistar rats (Genétic Absence Epilepsy Rats from Strasbourg), all animals present spontaneously recurrent absence seizures characterized by bilateral and synchronous generalized spike-and-wave discharges (SWD) accompanied by behavioural arrest. SWD depend on a thalamo-cortical network connecting the reticular and relay nuclei of the thalamus and their cortical projection areas. This loop involves both GABAergic and glutamatergic synapses. In the present study, we investigated the implication of NMDA transmission in the genesis of absence seizures in GAERS. Intra-peritoneal or intra-cerebroventricular injections of NMDA, the competitive NMDA antagonist CGP 40116, the non-competitive NMDA antagonist (+)-MK 801 and the antagonist of the glycine modulatory site 5,7-dichlorokynurenic acid dose-dependently suppressed SWD. Bilateral infusions of the same drugs in the lateral relay nuclei of the thalamus had similar suppressive effects. Intra-cerebroventricular or intrathalamic administration of D-serine, an agonist of the glycine modulatory site, had no effect on SWD. These data show that NMDA neurotransmission, especially within the thalamus, plays a major role in the control of absence seizures in GAERs. Disregulation of NMDA-mediated transmission by NMDA or antagonists, interacting with various sites of the receptor complex, may suppress the thalamo-cortical oscillatory activity which underlies SWD.

Intrathecally administered NMDA receptor antagonists reduce the MAC of isoflurane in rats.

We studied the effects of intrathecal administration of an N-methyl-D-aspartate (NMDA) receptor antagonist and an antagonist of the glycine site of the NMDA receptor on the minimum alveolar anaesthetic concentration (MAC) of isoflurane in rats, and on locomotor activity in conscious rats. In Wistar rats fitted with indwelling intrathecal catheters, we determined the MAC of isoflurane after the administration of saline (control group); the competitive NMDA receptor antagonist 3-(2-carboxypiperazin-4-yl)propyl-1-phosponic acid(CPP) at 0.01, 0.1, and 1.0 nM; and the selective antagonist of the glycine site on the NMDA receptor complex 7-chlorokynurenic acid (7CKA) at 0.1, 1.0, and 10 nM. After measurement of MAC following administration of the antagonist, the equipotent reversal dose of NMDA or D-serine was administered. The rats were examined for the presence of locomotor dysfunction by intrathecal administration of NMDA receptor antagonists, NMDA and D-serine in conscious rats. All of the experiments were performed using randomization and masking of drugs. CPP at 0.1 and 1.0 nM decreased the MAC of isoflurane by 9.9-17.6% (P < 0.05). 7CKA at 1.0 and 10 nM reduced MAC from 10.5-15.5% (P < 0.05). Intrathecal administration of NMDA or D-serine reversed the decreases in MAC to control values. Locomotor activity was not changed. We believe that NMDA receptor plays an important role in determining the MAC of isoflurane in the spinal cord.

Felbamate modulates the strychnine-insensitive glycine receptor

Felbamate (2-phenyl-1,3-propanediol dicarbamate) is a novel anticonvulsant substance whose mechanism of action is not clearly understood. The present investigation examined its ability to modulate the strychnine-insensitive glycine receptor associated with the N-methyl-D-aspartate (NMDA) receptor. Felbamate decreased the magnitude of glycine (100 microM)-enhanced NMDA (100 microM)-induced intracellular calcium ([Ca2+]i) transients in mouse cerebellar granule cells which had been loaded with the Ca(2+)-sensitive fluorescent probe indo-1 acetoxymethyl ester (indo-1/AM). This effect of felbamate was concentration dependent, with a maximal effect observed at 300 microM (65 +/- 4% of control). In the Frings audiogenic seizure-susceptible mouse model of reflex epilepsy, the glycine agonist D-serine (150 nmol, i.c.v.) completely blocked the anticonvulsant activity of a maximally effective dose of felbamate (19 mg/kg, i.p.). This effect of D-serine could be reversed by increasing the administered dose of felbamate to 29 mg/kg. Furthermore, administration of D-serine (300 nmol, i.c.v.) to felbamate-treated Frings mice produced a parallel right shift in felbamate's anticonvulsant dose-response curve (ED50s: 9.4 mg/kg for felbamate vs. 17.7 mg/kg for felbamate + D-serine). The results obtained in this investigation suggest that the ability of felbamate to modulate the strychnine-insensitive glycine receptor may be physiologically and behaviorally relevant to its anticonvulsant mechanism of action.

N-methyl- d-aspartate receptor-mediated changes in thermal nociception: Allosteric modulation at glycine and polyamine recognition sites

The effects of allosteric modulators of the N-methyl- d-aspartic acid receptor ion-channel complex on the nociceptive tail-flick reflex were studied in awake rats. Intrathecal administration of d-serine (100 fmol−1 μmol) but not l-serine or glycine to the lumbar spinal cord produced a facilitation of the tail-flick reflex at doses ⩾ 1 pmol (maximum at 0.5−1 min). Intrathecal pretreatment with the glycine modulatory site antagonist 7-chlorokynurenic acid (3 pmol) blocked both d-serine-produced and N-methyl- d-aspartate-produced facilitation of the tail-flick reflex. d-serine-produced facilitation was also blocked by intrathecal pretreatment with a N-methyl- d-aspartate receptor ion-channel blocker, MK 801 (100 fmol), or with an alternate substrate for nitric oxide synthase, N G-nitro- l-arginine-methyl ester (100 nmol). Intrathecal administration of spermine (0.01 nmol−3 μmol) produced biphasic effects on tail-flick latency accompanied by mechanical hyperesthesia and vocalization at greater doses. Spermine-produced facilitation (maximum with 0.01 nmol to 1 nmol at 1 min) was blocked by intrathecal pretreatment with MK 801 (100 fmol), N G-nitro- l-arginine-methyl ester (100 nmol) or the polyamine modulatory site antagonist, arcaine (10 nmol). Spermine-produced inhibition (maximum with 300 nmol at 2 min) was blocked by intrathecal administration of MK 801 (1 nmol). Intrathecal administration of the N-methyl- d-aspartate receptor antagonist, d-2-amino-5-phosphonopentanoic acid (1 nmol), blocked inhibition and uncovered a facilitation produced by 1 μmol spermine. In addition, spermine produced multi-stage motor effects (immediate- and late-onset). Intrathecal pretreatment with MK 801 (1 nmol) blocked only the immediate-onset motor effects while the late-onset motor effects were selectively blocked by pretreatment with the kappa opioid receptor antagonist, nor-binaltorphamine (200 nmol). Taken together, these data suggest that d-serine and spermine facilitate nociceptive transmission by positive allosteric modulation of the N-methyl- d-aspartate receptor ion-channel. Furthermore, activation of the N-methyl- d-aspartate receptor is also necessary to elicit the immediate-onset motor effects and inhibition of the tail-flick reflex produced by greater doses of spermine. Because kappa opioid receptors appear to be involved, the spermine-produced late-onset motor effects may involve endogenous dynorphin release.

Indole-2-carboxylates, novel antagonists of the [formula omitted] (NMDA)-associated glycine recognition sites: In vivo characterization

Recent in vitro receptor binding studies have indicated that indole-2-carboxylates with halogen substitutions at the position 5 or 6 are potent competitive antagonists of the NMDA (N- methyl- D-aspartate ) -associated strychnine-insensitive glycine receptor (Gray N.M., Dappen M.S., Cheng B.K., Cordi A. A., Biesterfeldt J. P., Hood W. F. and Monahan J. B. (1992) J. med. Chem. 34: 1283–1292; Hood W. F., Gray N. M., Dappen M. S., Watson G. B., Compton R. P., Cordi A. A., Lanthorn T. H. and Monahon J. B. (1992) J. Pharmac. exp. Ther. 262: 654–660). In the present investigation, a series of indole-2-carboxylates and two putative antagonists of glycine receptor HA-966 (3-amino-1-hydroxypyrrolidin-2-one) and 7-chlorokynurenic acid were examined for their effects on cGMP responses, mediated by the NMDA receptor complex, in vivo. Both SC-49648 (6-chloro-2-carboxyindole-3-acetic acid, intracerebellar injection, i.c.b.) and HA-966 (i.c.b. or intraperitoneal, i.p.) antagonized increases in levels of cyclic GMP in the cerebellum of the mouse, induced by the intracerebellar administration of NMDA and D-serine, agonists of the NMDA and the NMDA-associated glycine recognition sites, respectively. The drugs SC-49648 and 7-chlorokynurenic acid (i.p.) did not affect cGMP responses, suggesting poor bioavailability in brain. Following direct intracerebellar injection, SC-49648 was eliminated with a half-life of 12 min from the brain. Following intraperitoneal administration, SC-50132, the 3-ethylester analog of SC-49648, was eliminated from the brain with a half-life of 35 min and was found to be metabolized to SC-49648, in vivo. Some lipophilic analogs of SC-49648, designed as its prodrugs, were minimally active as glycine antagonists, in vitro. Following intraperitoneal administration, some of the prodrugs were active at reversing both harmaline- and D-serine-induced increases in cerebellar levels of cGMP in the mouse, indicating biotransformation to and/or improved bioavailability over SC-49648. These studies indicate that indole-2-carboxylates, examined in the present investigation, represent novel, potent and parenterally available competitive glycine receptor antagonists.

In vivo modulation of N-methyl- d- aspartate receptor-dependent long-term potentiation by the glycine modulatory site

The role of the glycine modulatory site in N-methyl- d-aspartic receptor function was examined by determining the effect of the glycine site antagonist, 7-chlorokynurenic acid, on the induction of long-term potentiation at the commissural-CA1 synapse in anesthetized rats. Robust long-term potentiation of population excitatory postsynaptic potentials and population spike responses recorded extracellularly in the stratum pyramidale and in stratum radiatum of CA1 developed after high frequency stimulation (100 Hz for 1 s) of commissural fibers during continuous intrahippocampal administration of vehicle solution (0.15 M NaCl). In contrast, infusion of either 7-chlorokynurenic acid (400 μM) or of the N-methyl- d-aspartic receptor antagonist, d-2-amino-5-phosphonovaleric acid (100 μM), significantly attenuated or completely blocked the development of long-term potentiation. When 7-chlorokynurenic acid was infused together with the glycine analog, d-serine (1 mM), long-term potentiation developed that was comparable to that observed in control animals. Intrahippocampal administration of d-serine alone was associated with slightly greater magnitude of long-term potentiation than observed in control animals. Collectively, these findings establish that in intact hippocampus, activity at the glycine modulatory site is necessary for activation of the N-methyl- d-aspartic receptor complex. Furthermore, these results suggest that the glycine modulatory site may not be fully saturated in vivo, and thus can serve to regulate N-methyl- d-aspartate receptor function.

Opipramol, a potent sigma ligand, is an anti-ischemic agent: Neurochemical evidence for an interaction with the [formula omitted] receptor complex in vivo by cerebellar cGMP measurements

Opipramol, a potent sigma ligand and a tricyclic antidepressant compound, provided significant neuronal protection ( P < 0.0001) against ischemia-induced neuronal cell loss in the hippocampus in Mongolian gerbils, at a dose of 50 mg/kg (30 min pretreatment). However, opipramol did not oner protection when given 60 min after the ischemic insult. Opipramol decreased basal levels of cGMP in the cerebellum of the mouse and harmaline-induced increases in levels of cGMP, with approximate ED 50 values of 4 and 27 mg/kg. Opipramol antagonized methamphetamine- and pentylenetetrazol-induced increases in levels of cGMP. Parenteral administration of opipramol also antagonized the increases in levels of cGMP in the cerebellum of the mouse after the local administration of D-serine, an agonist at the N- methyl- D-aspartate (NMDA)-associated, strychnine-insensitive glycine receptor. These results indicate that opipramol attenuates responses mediated through the NMDA receptor complex. These results further support the functional modulation of the NMDA receptor complex by sigma ligands and provide a neurochemical correlate for the observed anti-ischemic properties of opipramol.

Glycine, glycinamide and d-serine act as positive modulators of signal transduction at the N-Methyl-D-Aspartate (NMDA) receptor [formula omitted]: Differential effects on mouse cerebellar cyclic guanosine monophosphate levels

Direct intracerebellar (icb) administration of glycine, glycinamide and D-serine produced time- and dose-dependent changes in mouse cerebellar cGMP levels, indicating a modulation of ongoing neuronal activity through the NMDA receptor complex. Intracerebro-ventricular administration of glycinamide also produced a timedependent change in cGMP levels, indicating a central mechanism of action. The icb dose-response data indicated a unimolecular interaction for these compounds. D-serine-, glycine-, and glycinamide-mediated increases in cGMP levels were reversed by the competitive NMDA antagonist, CPP and the NMDA-associated glycine receptor antagonist, HA-966, indicating mediation via the NMDA receptor complex. Glycine and D-serine were less effective than glycinamide at increasing cerebellar cGMP levels. In contrast, L-and D-serinamide did not affect cGMP levels. These results indicate that glycine receptor is not saturated under physiological conditions and also suggest possible existence of multiple glycine pools.