Pharmacological characteristics of cyclic homologues of glycine at the N-methyl-D-aspartate receptor-associated glycine site

Photoredox-Catalyzed Generation of Tertiary Anions from Primary Amines via a Radical Polar Crossover

A method for the generation of tertiary carbanions via a deaminative radical-polar crossover is reported using redox active imines from α-tertiary primary amines. A variety of benzylic amines and amino esters can be used in this approach, with the latter engaging in a novel "aza-Reformatsky" reaction. Electronic trends correlate the stability of the resulting carbanion with reaction efficiency. The anions can be trapped with different electrophiles including aldehydes, ketones, imines, Michael acceptors, and H2 O/D2 O. Selective anion formation can be achieved in the presence of another equivalent or more acidic C-H bond in both an inter- and intramolecular fashion. Mechanistic studies suggest the intermediacy of a discrete carbanion intermediate.

Structure, Function, and Pharmacology of Glutamate Receptor Ion Channels

Many physiologic effects of l-glutamate, the major excitatory neurotransmitter in the mammalian central nervous system, are mediated via signaling by ionotropic glutamate receptors (iGluRs). These ligand-gated ion channels are critical to brain function and are centrally implicated in numerous psychiatric and neurologic disorders. There are different classes of iGluRs with a variety of receptor subtypes in each class that play distinct roles in neuronal functions. The diversity in iGluR subtypes, with their unique functional properties and physiologic roles, has motivated a large number of studies. Our understanding of receptor subtypes has advanced considerably since the first iGluR subunit gene was cloned in 1989, and the research focus has expanded to encompass facets of biology that have been recently discovered and to exploit experimental paradigms made possible by technological advances. Here, we review insights from more than 3 decades of iGluR studies with an emphasis on the progress that has occurred in the past decade. We cover structure, function, pharmacology, roles in neurophysiology, and therapeutic implications for all classes of receptors assembled from the subunits encoded by the 18 ionotropic glutamate receptor genes. SIGNIFICANCE STATEMENT: Glutamate receptors play important roles in virtually all aspects of brain function and are either involved in mediating some clinical features of neurological disease or represent a therapeutic target for treatment. Therefore, understanding the structure, function, and pharmacology of this class of receptors will advance our understanding of many aspects of brain function at molecular, cellular, and system levels and provide new opportunities to treat patients.

Effects of Cycloleucine in the Nucleus Accumbens Septi on the Elevated plus Maze Test in Rats

INTRODUCTION

In recent years, an important number of studies have emphasized the psychopharmacological actions of cycloleucine (1-aminocyclopentanecarboxylic acid) acting on the NR1 subunit (glycine allosteric site) of NMDA (N-methyl-D-aspartic acid) receptor. We studied the effects of its injection in an anxiety test.

METHODS

The elevated plus maze test was used. Male rats bilaterally cannulated into the nucleus accumbens septi (NAS) were employed. Rats were divided into 5 groups that received either 1 µL injections of saline or cycloleucine (0.5, 1, 2, or 4 µg) 15 min before testing.

RESULTS

Time spent in the open arm was significantly increased by cycloleucine treatment with all doses (1 and 2 µg, p < 0.05; 0.5 and 4 µg, p < 0.01), like number of extreme arrivals (0.5 and 1 µg, p < 0.05; 2 µg, p < 0.01; and 4 µg, p < 0.001). Open arm entries were increased by the highest dose only (4 µg, p < 0.01).

DISCUSSION/CONCLUSION

Present results show no difference between all doses in the time spent in the open arm, suggesting an indirect, noncompetitive action of the drug. The increase in extreme arrivals and open arm entries suggests a dose influence in these parameters. We conclude that cycloleucine influence on the NMDA receptors within NAS leads to anxiolytic-like effects and behavioral disinhibition, which once more confirms the involvement of NAS in anxiety processing.

The Role of Dopamine D1 and D3 Receptors in N-Methyl-D-Aspartate (NMDA)/GlycineB Site-Regulated Complex Cognitive Behaviors following Repeated Morphine Administration

BACKGROUND

Opiate addiction is associated with complex cognitive impairment, which contributes to the development of compulsive drug use and relapses. Dopamine and N-methyl-D-aspartate receptors play critical roles in opiate-induced cognitive deficits. However, the roles of D1 and D3 receptors in the N-methyl-D-aspartate/glycineB receptor-regulated cognitive behaviors induced by morphine remain unknown.

METHODS

The 5-choice serial reaction time task was used to investigate the cognitive profiles associated with repeated morphine administration in D1 (D1-/-)- and D3 (D3-/-)-receptor knockout mice. The expression of phosphorylated NR1, Ca2+/calmodulin-dependent protein kinase II (CaMKII), and cAMP response element-binding protein (CREB) in the brain was examined by western blotting. D1-/- and D3-/- mice were treated with the N-methyl-D-aspartate/glycineB site agonist l-aminocyclopropanecarboxylic acid and the antagonist L-701,324 to chronically disrupt N-methyl-D-aspartate receptor function and investigate their effects on morphine-induced cognitive changes.

RESULTS

Repeated morphine administration impaired attentional function and caused impulsive and compulsive behaviors. D1-/- mice exhibited hardly any premature nosepokes. D3-/- mice showed robustly increased morphine-induced impulsive behavior. The numbers of premature responses were decreased by L-701,324 administration and increased by ACPC administration; these effects were completely abolished in D1-/- mice due to their inability to perform reward-based tasks. In contrast, the inhibitory effects of L-701,324 on impulsive behavior were significantly augmented in D3-/- mice.

CONCLUSIONS

N-methyl-D-aspartate/glycineB site functions may contribute to morphine-induced cognitive deficits, especially those related to impulsive behavior. D1 and D3 receptors may have contrasting effects with respect to modulating impulsive behavior. D3 receptors have inhibitory effects on impulsive behaviors, and these effects are clearly mediated by N-methyl-D-aspartate/glycineB receptor and μ-opioid receptor interactions.

NMDA receptor structures reveal subunit arrangement and pore architecture

N-methyl-d-aspartate (NMDA) receptors are Hebbian-like coincidence detectors, requiring binding of glycine and glutamate in combination with the relief of voltage-dependent magnesium block to open an ion conductive pore across the membrane bilayer. Despite the importance of the NMDA receptor in the development and function of the brain, a molecular structure of an intact receptor has remained elusive. Here we present x-ray crystal structures of the GluN1/GluN2B NMDA receptor with the allosteric inhibitor, Ro25-6981, partial agonists and the ion channel blocker, MK-801. Receptor subunits are arranged in a 1-2-1-2 fashion, demonstrating extensive interactions between the amino terminal and ligand binding domains. The transmembrane domains harbor a closed-blocked ion channel, a pyramidal central vestibule lined by residues implicated in binding ion channel blockers and magnesium, and a ~2-fold symmetric arrangement of ion channel pore loops. These structures provide new insights into the architecture, allosteric coupling and ion channel function of NMDA receptors.

Drug design, in vitro pharmacology, and structure-activity relationships of 3-acylamino-2-aminopropionic acid derivatives, a novel class of partial agonists at the glycine site on the N-methyl-D-aspartate (NMDA) receptor complex

Retaining agonistic activity at the glycine coagonist site of the NMDA receptor in molecules derived from glycine or d-serine has proven to be difficult because in the vicinity of the alpha-amino acid group little substitution is tolerated. We have solved this problem by replacing the hydroxy group of d-serine with an amido group, thus keeping the hydrogen donor function and allowing for further substitution and exploration of the adjacent space. Heterocyclic substitutions resulted in a series of 3-acylamino-2-aminopropionic acid derivatives, with high affinities in a binding assay for the glycine site. In a functional assay assessing the activation of the glycine site, these compounds displayed a wide range of intrinsic efficacies, from antagonism to a high degree of partial agonism. Structure-activity relationships reveal that lipophilic substituents, presumably filling an additional hydrophobic pocket, are accepted by the glycine site, provided that they are separated from the alpha-amino acid group by a short linker.

Mechanism of Partial Agonist Action at the NR1 Subunit of NMDA Receptors

Partial agonists produce submaximal activation of ligand-gated ion channels. To address the question of partial agonist action at the NR1 subunit of the NMDA receptor, we performed crystallographic and electrophysiological studies with 1-aminocyclopropane-1-carboxylic acid (ACPC), 1-aminocyclobutane-1-carboxylic acid (ACBC), and 1-aminocyclopentane-1-carboxylic acid (cycloleucine), three compounds with incrementally larger carbocyclic rings. Whereas ACPC and ACBC partially activate the NMDA receptor by 80% and 42%, respectively, their cocrystal structures of the NR1 ligand binding core show the same degree of domain closure as found in the complex with glycine, a full agonist, illustrating that the NR1 subunit provides a new paradigm for partial agonist action that is distinct from that of the evolutionarily related GluR2, AMPA-sensitive receptor. Cycloleucine behaves as an antagonist and stabilizes an open-cleft conformation. The NR1-cycloleucine complex forms a dimer that is similar to the GluR2 dimer, thereby suggesting a conserved mode of subunit-subunit interaction in AMPA and NMDA receptors.

N-methyl-D-aspartate antagonists for stroke and head trauma

The N-methyl-D-aspartate (NMDA) receptor is a ligand-gated ion channel which is widely distributed in the central nervous system (CNS), and which mediates most of the fast excitatory neuronal transmission in the CNS. As with other ligand-gated ion channels, the NMDA receptor is a macromolecular complex which possesses a number of intricate regulatory sites within and around a central ion channel. The key regulatory components for which prototypic antagonists have been developed are the competitive NMDA antagonist binding site, the non-competitive NMDA antagonist binding site within the ion channel, and the NMDA receptor-associated glycine antagonist site. The binding domains for each of these binding sites possess discrete and non-overlapping SAR with regard to the chemical series developed to date. The potential utility of NMDA antagonists in the treatment of stroke and traumatic brain injury was investigated soon after the synthesis of the first bioavailable NMDA antagonists. Efficacy in preclinical models was demonstrated with both competitive and non-competitive NMDA antagonists. However, preclinical testing also revealed potentially clinically-limiting side-effects which included phencyclidine (PCP)-like actions indicative of possible psychotomimetic activity, cerebral vacuolisation of limbic cortical neurones, low therapeutic indices relative to incapacitating motor side-effects and, in the case of non-competitive antagonists, hypertension. These limitations have led to the design of clinical trials that should define the therapeutic index for this type of compound in humans. Currently, the first competitive antagonist to enter clinical trials, selfotel, is on hold, while D-CPPene is still in development. The non-competitive antagonist, aptiganel, is currently in Phase III clinical trials and its therapeutic efficacy and index should be defined in 1997 and 1998. The well-defined limitations of competitive and non-competitive NMDA antagonists have been a key impetus in the investigation of alternative approaches to modulating the NMDA receptor complex. In the case of glycine site antagonists, these compounds have been shown in preclinical studies to be devoid of PCP-like actions and the neuronal vacuolisation associated with the competitive and non-competitive NMDA antagonists. This has induced the development of a number of chemical series with at least three compounds currently in Phase I and II clinical trials. These include ACEA 1021, GV150526A and ZD9379. Clinical efficacies and therapeutic indices of these compounds should be defined in 1998 and 1999. An alternative approach using a partial agonist of the glycine site (1-aminocyclopropane-carboxylic acid, ACPC) has been halted in Phase I. Another approach which has led to the development of NMDA receptor antagonists, selective for the NMDA receptor subunits 1A/2B (NR1A/2B subtype), was the discovery in early studies of the neuroprotective actions of ifenprodil. Structural analogues include eliprodil, CP-101,606 and lubeluzole. In the cases of eliprodil and lubeluzole, these compounds have demonstrated neuroprotection in preclinical models, but they possess the extremely dangerous side-effect of increasing cardiac repolarisation time (i.e., increased QTc interval). The therapeutic index for these compounds is low. This has led to the termination of eliprodil's development and has limited the current dosing strategy with lubeluzole. It has not been disclosed if CP-101,606 possesses this dose-limiting side-effect. In summary, strategies for drug design and development based on our knowledge of the NMDA receptor complex have led to the development of a new generation of compounds for the treatment of stroke and traumatic brain injury, which remain to be evaluated in the clinic. The success of this approach will be defined in the next two to three years.

Differential Neuroprotective Effects of the NMDA Receptor-Associated Glycine Site Partial Agonists 1-Aminocyclopropanecarboxylic Acid (ACPC) and d-Cycloserine in Lithium-Pilocarpine Status Epilepticus

The status epilepticus (SE) induced in rats by lithium-pilocarpine (Li-pilo) shares many common features with soman-induced SE including a glutamatergic phase that is inhibited by NMDA antagonists. The present study determined whether 1-aminocyclopropanecarboxylic acid (ACPC) or D-cycloserine (DCS), both partial agonists of the strychnine-insensitive glycine site on the NMDA receptor ionophore complex, exerted anticonvulsant or neuroprotectant activity in Li-pilo SE. ACPC or DCS were administered either immediately following pilocarpine (exposure treatment) or 5 min after the onset of SE as determined by ECoG activity. SE was allowed to proceed for 3 h before termination with propofol. The rats were sacrificed 24 h following pilocarpine administration. Neither drug had an effect on the latency to seizure onset or the duration of seizure activity. ACPC administered 5 min after SE onset produced significant neuroprotection in cortical regions, amygdala and CA1 of the hippocampus. In contrast, when administered as exposure treatment ACPC enhanced the neural damage in the thalamus and CA3 of the hippocampus suggesting the neuropathology in those regions is mediated by a different subset of NMDA receptors. DCS had no neuroprotectant activity in Li-pilo SE but exacerbated neuronal damage in the thalamus. Neither drug affected the cholinergic convulsions but both had differential effects on neural damage. This suggests that the SE-induced seizure activity and subsequent neuronal damage involve independent mechanisms.

Mechanisms of activation, inhibition and specificity: crystal structures of the NMDA receptor NR1 ligand-binding core

Excitatory neurotransmission mediated by the N-methyl-D-aspartate subtype of ionotropic glutamate receptors is fundamental to the development and function of the mammalian central nervous system. NMDA receptors require both glycine and glutamate for activation with NR1 and NR2 forming glycine and glutamate sites, respectively. Mechanisms to describe agonist and antagonist binding, and activation and desensitization of NMDA receptors have been hampered by the lack of high-resolution structures. Here, we describe the cocrystal structures of the NR1 S1S2 ligand-binding core with the agonists glycine and D-serine (DS), the partial agonist D-cycloserine (DCS) and the antagonist 5,7-dichlorokynurenic acid (DCKA). The cleft of the S1S2 'clamshell' is open in the presence of the antagonist DCKA and closed in the glycine, DS and DCS complexes. In addition, the NR1 S1S2 structure reveals the fold and interactions of loop 1, a cysteine-rich region implicated in intersubunit allostery.

Specificity of putative partial agonist, 1-aminocyclopropanecarboxylic acid, for rat N-methyl-D-aspartate receptor subunits

The neuroprotective compound, 1-aminocyclopropanecarboxylic acid (ACPC), has been reported to act on the N-methyl-D-aspartate (NMDA) receptors simultaneously as a glycine binding site agonist and a glutamate binding site competitive antagonist. The complex kinetics of NMDA current changes measured by a whole-cell voltage clamp in rat hippocampal neurons resulting from application and removal of 1 mM ACPC in the continual presence of 15 microM NMDA confirm this hypothesis. Two-electrode voltage clamp on Xenopus oocytes expressing NR1-1a and either NR2A, NR2B or NR2C subunits yielded biphasic ACPC dose response curves with 15 microM NMDA. NR1-1a/NR2B and NR1-1a/NR2C subunit combinations yielded overlapping dose response curves with a maximal efficacy of approximately 80%; the maximal efficacy of ACPC for the NR1-1a/NR2A subunit combination was significantly lower at approximately 60%.

Synthesis and preliminary biological evaluation at the glycineB site of (+)- and (-)-3-oxetanylglycine, novel non-proteinogenic amino acids

Two novel non-proteinogenic amino acids, (+)- and (-)-3-oxetanylglycine were synthesized and evaluated for their ability to diplace [3H]-glycine from the glycine site of the NMDA receptor complex. The lack of activity of these compounds at concentrations up to 100 microM may help in understanding the topological requirements of the glycine site of the NMDA receptor complex.

Biologic effects of and clinical disorders caused by nonprotein amino acids

Despite the fact that nonprotein amino acids are present in many commonly eaten foods, the biologic and clinical significance of this class of molecules has been largely overlooked. This is owing in part to their relatively low concentrations and their negligible nutritive value. Many of these compounds have the ability to interfere with a wide range of fundamental biochemical processes and cause disease. It is likely that the clinical effects of the ingestion of some nonprotein amino acids are yet to be described. Serious disorders in humans have followed the ingestion of these compounds as the result of food faddism, prodded by the commercial promotion of inadequately tested products. In view of the current popularity of herbal remedies and alternative medicine, these facts serve as another reminder to health care providers and the public at large about the need for critical analysis of the alleged benefits and the risks of exotic remedies and nutritional supplements. Beyond the public health issues they raise, non-protein amino acids take on significance because their misincorporation into proteins can trigger vigorous autoimmune attacks. To what extent this mechanism is responsible for highly prevalent diseases of autoimmunity remains to be determined.

IV glycine and oral D-cycloserine effects on plasma and CSF amino acids in healthy humans

BACKGROUND

The amino acid glycine, modulates neurotransmission via actions at GLY-A receptor and GLY-B receptor. The latter are coagonist sites associated with N-Methyl-D-Aspartate (NMDA) glutamate receptors. The central bioavailability of peripherally administered glycine has not been adequately characterized in humans.

METHODS

Healthy human subjects were administered either oral D-cycloserine (50 mg or placebo) and intravenous glycine (saline, 100 mg/kg or 200 mg/kg) in random order over 4 test days under double-blind conditions. Cerebrospinal fluid was collected by lumbar puncture performed on the first test day was analyzed to determine amino acid levels. The acoustic startle response was measured on the second test day.

RESULTS

Intravenous glycine dose-dependently increased both serum and CSF glycine and serine levels. Neither glycine nor DCS produced any significant effects on behavior, cognition or the acoustic startle response. Neither IV glycine nor DCS were associated with any toxicity.

CONCLUSIONS

Thus, peripheral glycine administration raised CSF glycine levels without producing any clear central nervous system effects. Glycine and D-cycloserine did not worsen cognitive test performance and did not induce behavioral symptoms on their own. The possibility that glycine and D-cycloserine enhanced cognitive test performance cannot be excluded given the psychometric limitations of the test battery.

The NMDA receptor partial agonist, 1-aminocyclopropanecarboxylic acid (ACPC), reduces ethanol consumption in the rat

The present studies assessed the effects of both systemic and intraaccumbens injections of 1-aminocyclopropanecarboxylic acid (ACPC), and NMDA partial agonist, on ethanol consumption in a limited access procedure in Wistar rats. Systemically administered ACPC reduced ethanol consumption in a dose-dependent manner, while a single dose of ACPC administered bilaterally into the nucleus accumbens also reversibly reduced ethanol consumption. Indirect measures of general appetitive behavior showed no effect of ACPC on weight or water intake, which suggests that this effect of ACPC may be specific to ethanol. These data are compatible with the role of NMDA receptors in modulating ethanol consumption and provide the first data showing that ACPC can reduce ethanol consumption. ACPC has neuroprotective effects and does not show the psychotomimetic effects observed with NMDA receptor agents. Thus, ACPC may be helpful in future clinical studies designed to reduce alcohol use.

Nitric oxide synthase inhibitors have antidepressant-like properties in mice. 1. Acute treatments are active in the forced swim test

Previous studies have demonstrated that antagonists at the NMDA receptor are as efficacious as tricyclic antidepressants in pre-clinical antidepressant screening procedures and in blocking or reversing the behavioral deficits associated with animal analogs of major depressive symptomatology. The NMDA receptor complex gates Ca2+, which interacts with calmodulin to subsequently activate nitric oxide (NO) synthase. We hypothesized that NO synthase antagonists might display antidepressant-like properties, similar to NMDA receptor antagonists. We examined the effects of N(G)-nitro-L-arginine (L-NNA), its dextrorotatory enantiomer, D-NNA, N(G)-nitro-L-arginine methyl ester (L-NAME) and N(G)-monomethyl-L-arginine (L-NMMA) at doses from 1 to 30 mg/kg in the forced swim test in mice. We now report that NO synthase antagonists are as efficacious as imipramine (15 mg/kg) in reducing the duration of immobility in the mouse forced swim test. The effects of NO synthase antagonists, as well as those of imipramine were blocked by pre-treatment with L-arginine (L-Arg) (500 mg/kg). In contrast to imipramine, the NO synthase antagonists were without effect on locomotor activity over the dose range active in the forced swim test (3-10 mg/kg). Likewise, L-Arg was without effect on locomotor activity. These data support the hypothesis that NO synthase antagonists possess antidepressant properties and may represent a novel class of therapeutics for major depressive disorders.

Sustained exposure to a glycine receptor partial agonist differentially alters NMDA receptor agonist and antagonist potencies in cultured spinal cord neurons

Sustained (20 h) exposure to the glycine partial agonist 1-aminocyclopropanecarboxylic acid (ACPC) significantly reduced N-methyl-D-aspartate (NMDA)-induced neurotoxicity in cultured spinal cord neurons when the NMDA (25 and 100 microM) was added to the cultures 30 min after removal of the ACPC (1 mM). In contrast, ACPC preexposure failed to protect against kainate-induced neuronal injury. The magnitude of neuronal protection against NMDA (100 microM) was further enhanced if the neurons pretreated with ACPC were reexposed to this drug during the NMDA challenge. In addition, the potencies of both the competitive NMDA antagonist AP5 and the noncompetitive antagonist dizocilpine to block NMDA toxicity were significantly increased following ACPC preexposure, while the potency of the competitive glycine receptor antagonist 7-chlorokynurenate (7-CK) was unchanged. Analysis of Northern blots suggest that ACPC-induced changes in NMDA receptor function were not associated with alterations in the levels of the mRNAs encoding the NMDAR-1, -2A, -2B, or -2C subunits. These results indicate that sustained exposure to ACPC modifies NMDA receptors in a manner that diminishes NMDA receptor-mediated neurotoxicity while selectively enhancing the potencies of several NMDA receptor antagonists. These effects do not appear to be related to changes in expression of specific NMDA receptor subunits, and may instead involve a post-translational modification of one or more subunit proteins.

Strychnine-insensitive [3H] glycine binding to synaptosomal membranes from the chick retina

The pharmacology and kinetics of strychnine-insensitive [3H] glycine binding to synaptic membranes from the outer (P1) and the inner (P2) plexiform layers of chick retina was studied. Inhibition curves for glycine, D-serine, 1-amincyclopropanecarboxylic acid (ACPC) and strychnine were analyzed by non-linear regression. Hill slopes for glycine and D-serine were not different from unity, whereas those for ACPC were < 1 in both fractions, revealing heterogeneity of binding sites in these membranes. Non-linear regression analysis of time course and saturation experiments strengthen the idea that [3H] glycine binds to more than one class of sites, with similar affinities at equilibrium. Antagonists of strychnine-insensitive glycine receptors in the CNS did not inhibit [3H] glycine binding to these membranes, which demonstrates that NMDA receptors in the retina have different structural requirements for ligand interaction at these sites. pH affected the specific binding, in agreement with the participation of specific amino acid residues at glycine binding sites on NMDA receptors, and also with functional studies in which the modulation of affinity at this site by protons has been observed. These results support previous studies regarding CPP and MK-801 binding, and provide evidence which indicates that the pharmacophore for glycine and other NMDA-related ligands is distinct for the retina, compared to the CNS, mainly regarding the effects of glycine-site antagonists.

Chronic administration of NMDA glycine partial agonists induces tolerance in the Porsolt swim test

The Porsolt swim test (PST) was used to assess behavioral effects following acute or chronic treatment with two N-methyl-D-aspartate (NMDA) glycine partial agonists, 1-aminocyclopropanecarboxylic acid (ACPC), and D-cycloserine (DCS). Consistent with previous findings in mice, single intravenous doses of ACPC in rats produced a significant, dose-dependent reduction in immobility in the PST compared to saline. Single dose DCS also elicited significant dose-dependent reductions in PST immobility times. Single-dose ACPC or DCS (200 mg/kg) reduced immobility (p < 0.05) by 26 or 30%, respectively, compared to saline. However, multiple dosing with either ACPC or DCS (6 daily doses, 200 mg/kg) produced an apparent behavioral adaptation, as the immobility data were indistinguishable from chronic saline administration. Moreover, pretreatment with a 5-day course of ACPC or DCS promoted the development of a behavioral cross-tolerance following a sixth dose of DCS or ACPC, respectively. The development of a behavioral tolerance in the PST following chronic therapy of these drugs appears to be a general feature of glycine partial agonists. In toto, these findings support the hypothesis that chronic administration of NMDA glycine partial agonists produces a behavioral tolerance putatively through an adaptation of the NMDA receptor complex.

Chronic administration of a partial agonist at strychnine-insensitive glycine receptors: a novel experimental approach to the treatment of ischemias

During the past decade, converging lines of evidence have linked the abnormal release or leak of excitatory amino acids to the neurodegeneration associated with a wide range of pathologies including cerebral ischemias, Huntington's disease, and AIDS dementia (Coyle and Robinson, 1987; Lipton, 1994; Meldrum, 1994). Pharmacological studies indicate that activation of both ionotropic and metabotropic glutamate receptors can substantially contribute to excitotoxic cell damage (Choi, 1992; Pizzi et al., 1993; Sheardown et al., 1993; Xue et al., 1994). Based on these findings, therapeutic strategies based on blunting or blocking glutamatergic transmission may be useful in treating a variety of neurodegenerative disorders.

Motor activity and quantitative autoradiographic analysis of muscarinic receptors in the brain of rats subjected to the forced swimming test

A cholinergic dysfunction has been involved in the neurobiological mechanisms of stress and depression. In the present study, we determined the autoradiographic distribution of muscarinic cholinergic receptors in the brain of rats subjected to the forced swimming test for 15 days. Motor activity was automatically analyzed daily before swimming. In the forced swimming test group, both total horizontal activity and ambulatory movements exhibited a significant decrease, when the data from 1st and 15th day were compared. Neither the affinity of [3H]-quinuclidinyl benzilate nor the maximal number of receptors were affected by the forced swimming test in the caudate-putamen, cortex, and hippocampus. The distribution of [3H]-quinuclidinyl benzilate binding sites did not show significant differences in the 30 analyzed areas. Further analysis of muscarinic receptor subtypes after forced swimming test would be necessary to discard any cholinergic involvement.

Antidepressant-like effects of a partial agonist at strychnine-insensitive glycine receptors and a competitive NMDA receptor antagonist

Using the forced swimming test in rats (Porsolt test), we examined the antidepressant-like activity of 1-aminocyclopropanecarboxylic acid (ACPC), a partial agonist at strychnine-insensitive glycine receptors associated with the N-methyl-D-aspartate (NMDA) receptor complex, and of DL-(E)-2-amino-4-methyl-5-phosphono-3-pentenoic acid (CGP 37849), a competitive NMDA receptor antagonist, after their intraperitoneal (i.p.) and intrahippocampal (i.hp.) administration. ACPC (200-400 mg/kg) and CGP 37849 (0.625-5 mg/kg), administered i.p., produced a dose-dependent and significant reduction of the immobility time in the forced swimming test. A similar effect was also observed in i.hp. administration of ACPC (10 and 30 micrograms) and CGP 37849 (0.03 and 0.1 micrograms). Imipramine, used as a reference drug, significantly shortened the immobility time after both i.p. (30 and 40 mg/kg) and i.hp (0.1 and 0.3 micrograms) administration. The compounds studied, administered in doses effective in the forced swimming test, did not change the exploratory activity of the rats, evaluated by an open field test. The present results indicate that, like imipramine, ACPC and CGP 37849 exhibit an antidepressant-like activity in the forced swimming test in rats; moreover, they seem to show the hippocampus may be one of the neuroanatomical sites involved in this effect.

Acute or prolonged exposure to 1-aminocyclopropanecarboxylic acid protects spinal neurons against NMDA toxicity

1-Aminocyclopropanecarboxylic acid (ACPC) is a high affinity partial agonist for the glycine binding site within the NMDA receptor complex. Chronic treatment with ACPC in vivo appears to reversibly desensitize the NMDA receptor complex, prompting suggestions that it might provide an effective means of ameliorating degenerative mechanisms mediated through this ligand-gated ion channel. In the present experiments, cultured rat spinal cord neurons were used to further examine the effects of acute and sustained ACPC exposures on N-methyl-D-aspartate (NMDA)-induced neurotoxicity. Cell damage was quantitatively assessed using a tetrazolium salt colorimetric assay. With coincubation, 1 mM ACPC significantly reduced the neuronal cell damage caused by 30 min exposure to 25 or 50 microM concentrations of NMDA, but, in contrast to other competitive and non-competitive NMDA receptor antagonists (D-(-)-2-amino-5-phosphonovaleric acid (APV), dizocilpine maleate (MK-801) and 7-chlorokynurenic acid (7-CK)), it failed to alter the cell injury induced by 100 microM NMDA. The protective effect of ACPC was competitively abolished by coaddition of glycine, verifying that it was mediated through glycine binding sites. Sustained 20 h exposure to 1 mM ACPC (which was removed 30 min before addition of 25 microM NMDA) also caused cells to be significantly less responsive to the neurotoxic effects of NMDA. Pre-exposure to ACPC for shorter intervals ( < 1 h) failed to alter subsequent NMDA toxicity. Acute or sustained exposures to ACPC alone did not affect cell viability. These results support earlier indications that: (1) ACPC provides an effective means of antagonizing excitotoxic phenomena, and (2) sustained exposure to ACPC desensitizes the NMDA receptor complex.

Antidepressant-like effects of 1-aminocyclopropanecarboxylic acid and D-cycloserine in an animal model of depression

Antidepressant activity of partial agonists at strychnine-insensitive glycine receptors, 1-aminocyclopropanecarboxylic acid (ACPC) and D-cycloserine, was studied in a chronic mild stress model of depression. In this model, a substantial decrease in consumption of a palatable sucrose solution is observed over time in rats subjected to a variety of mild stressors. This decrement can be reversed by chronic administration of antidepressant drugs. Chronic (5 weeks) treatment with ACPC gradually reversed chronic mild stress-induced reductions in sucrose consumption, and the magnitude of this effect was comparable to that observed following similar administration of imipramine (10 mg/kg). The time-course for reversal of chronic mild stress-induced deficits in sucrose consumption by ACPC was dose-dependent. Thus, the first statistically significant effect of the low dose of ACPC (100 mg/kg) was observed after four weeks of treatment (comparable to the 3-5 weeks required for imipramine), while only two weeks of treatment was required in the group receiving a higher dose (200 mg/kg) of ACPC. Like imipramine, reversal of chronic mild stress-induced deficits in sucrose consumption by ACPC persisted for at least one week following cessation of treatment. The effects of chronic D-cycloserine were variable, and apparently not dose-related in the chronic mild stress model. D-cycloserine (10 mg/kg) increased sucrose intake in stressed animals, but the magnitude of this effect was smaller than in either imipramine or ACPC treated animals. Lower (2.5 mg/kg) and higher (40, 100 mg/kg) doses of D-cycloserine were ineffective. These results suggest that ACPC may have antidepressant properties comparable to conventional drugs, but with a faster onset of action.

Effect of 1-aminocyclopropanecarboxylic acid on N-methyl-D-aspartate-stimulated [3H]-noradrenaline release in rat hippocampal synaptosomes

1. The effect of 1-aminocyclopropanecarboxylic acid (ACPC), a partial agonist at the glycine site of the N-methyl-D-aspartate (NMDA) receptor complex that exhibits neuroprotective, anxiolytic and antidepressant-like actions, was investigated in a functional assay for presynaptic NMDA receptors. 2. NMDA (100 microM) produced a 36% increase of tritium efflux above basal efflux in rat hippocampal synaptosomes preincubated with [3H]-noradrenaline ([3H]-NA), reflecting a release of tritiated noradrenaline. This effect was prevented by 10 microM 7-chlorokynurenic acid, an antagonist of the glycine site of the NMDA receptor. 3. Glycine enhanced the effect of NMDA with Emax and EC50 values of 84 +/- 11% and 1.82 +/- 0.04 microM, respectively. ACPC potentiated the effect of NMDA on tritium overflow with a lower EC50 (43 +/- 6 nM) and a lower maximal effect (Emax = 40 +/- 9%) than glycine. Furthermore, ACPC (0.1 microM) shifted the EC50 of glycine from 1.82 microM to > or = 3 mM. 4. These results show that ACPC can reduce the potentiation by glycine of NMDA-evoked [3H]-NA release and hence, may act as an antagonist at the glycine site of presynaptic hippocampal NMDA receptors when the concentration of glycine is high.

Anticonflict effects of a competitive NMDA receptor antagonist and a partial agonist at strychnine-insensitive glycine receptors

Using the conflict drinking Vogel test in rats as a model, in the present study we examined the anxiolytic-like activity of DL-(E)-2-amino-4-methyl-5-phosphono-3-pentenoic acid (CGP 37849), a competitive N-methyl-D-aspartate (NMDA) receptor antagonist and 1-aminocyclopropanecarboxylic acid (ACPC), a partial agonist at strychnine-insensitive glycine receptors associated with the NMDA receptor complex, after their intraperitoneal (i.p.) and intrahippocampal (IHP) administration. CGP 37849, administered in doses of 1.25-5 mg/kg i.p., produced an anticonflict effect in a dose-dependent manner, but was inactive when injected in doses of 0.01-0.1 micrograms IHP. At the same time, when administered in higher doses (10 mg/kg i.p. or 0.3 micrograms IHP), that drug induced motor impairment. On the other hand, ACPC exhibited an anxiolytic-like activity after both i.p. (100-200 mg/kg) and IHP (3-30 micrograms) administration. These results, as well as the literature data on the lack of motor-impairing effects of ACPC, indicate that the latter drug seems to be more advantageous than CGP 37849 as a potential therapeutic agent in the treatment of anxiety disorders. Furthermore, they also show that the hippocampus may be one of the neuroanatomical sites of the anxiolytic-like effect of ACPC, but not of CGP 37849.

[3H]1-aminocyclopropanecarboxylic acid, a novel probe for strychnine- insensitive glycine receptors

[3H]1-Aminocyclopropanecarboxylic acid (ACPC) exhibits high affinity, specific binding to strychnine-insensitive glycine receptors. In extensively washed rat forebrain membranes, the specific binding of [3H]ACPC was optimal at 25 degrees C in the presence of 10 mM MgCl2. Comparable levels of specific [3H]ACPC binding were obtained using centrifugation and filtration for separation of bound from free radioligand. [3H]ACPC labels two sites with Kdl and Bmax1 values of 129 +/- 34 nM and 2.30 +/- 0.37 pmol/mg protein and Kd2 and Bmax2 values of 7.26 +/- 1.69 microM and 20.6 +/- 2.2 pmol/mg protein for the high and low affinity sites, respectively. The Kd of [3H]ACPC (66 nM) estimated under non-equilibrium conditions (koff = 8.91 +/- 0.78 x 10(-3) s-1; kon = 1.35 x 10(-4) nM-1 s-1) was similar to the value obtained for the high affinity site obtained by equilibrium binding. The Kd1 of[3H]ACPC is in good agreement with the previously reported Ki values of ACPC to inhibit the binding of other glycinergic ligands including [3H]glycine, [3H]5,7-dichlorokynurenic acid (5,7-DCKA) and [3H]L-689,560 ((+/-)-4-(trans)-2-carboxy-5,7-dichloro-4- phenylaminocarbonylamino-1,2,3,4-tetrahydroquinoline). Moreover, the potencies of a series of glycine site ligands, including glycine. ACPC, 1-aminocyclobutanecarboxylic acid (ACBC), 5,7-DCKA, 7-chlorokynurenic acid (7-CKA), R(+)-3-amino-1-hydroxy-2- pyrrolidine (HA-966) and D-serine, to inhibit [3H]ACPC binding were highly correlated with their potencies to inhibit [3H]glycine and [3H]5,7-DCKA binding (r2 = 0.98-0.51). These results demonstrate that [3H]ACPC is a useful tool for examining the neurochemical and pharmacological properties of strychnine-insensitive glycine receptors.

Characterization of the effects of polyamines on the modulation of the N-methyl-D-aspartate receptor by glycine

We have investigated the effects of polyamine agonists and antagonists on the modulation of the N-methyl-D- aspartate receptor by glycine using a [3H]dizocilpine ([3H]MK801) binding assay. We monitored the non-equilibrium binding of [3H]dizocilipine in the presence of 5,7-dichlorokynurenate to preclude occupation of the glycine site by endogenous glycine. Using this assay, spermine and spermidine increased both the affinity and the maximum effect of glycine. Similar effects are produced by other polyamine agonists including 1,5-diethylaminopiperidine, neomycin and Ca2+. These actions are reversed by the polyamine glycine produced by polyamine agonists appears to be due to an increase in the equilibrium affinity of [3H]dizocilpine, and cannot therefore be attributed solely to modulation of glycine binding. Interestingly, 1,5-diethylaminopiperidine increases the maximum effect of glycine to a greater extent than it alters glycine affinity, suggesting that the two effects may be mediated by different sites or mechanisms. These studies will help to define the role of the glycine site in the modulation of the N-methyl-D-aspartate receptor by polyamines.

Gas chromatographic-mass spectrometric determination of urinary 1-aminocyclopropanecarboxylic acid in mice using a deuterated internal standard

A gas chromatographic-mass spectrometric method is described for the determination of 1-aminocyclopropanecarboxylic acid in mouse urine using the 2,2,3,3-[2H4] isotopolog as an internal standard. Samples (0.1 ml) were extracted using an exchange resin, then derivatized with pentafluoropropanol and pentafluoropropionic anhydride at 100 degrees C for 25 min. Gas chromatography was performed on a (5% phenyl)methylpolysiloxane column and detection was by selected-ion monitoring of M-CO2CH2CF2CF3 fragment ions. The method provided high response linearity (mean r = 0.999) and precision (< 5% coefficient of variation). After orally dosing mice with 1-aminocyclopropanecarboxylic acid (300 mg/kg), 46 and 10% of the dose was excreted unchanged in the 0-24 h and 24-48 h urines, respectively.

Glycine-induced CA1 excitotoxicity in the rat hippocampal slice

We evaluated the effects of glycine exposure upon CA1-evoked response in the rat hippocampal slice. Exposure to 10 mM glycine for 16 min produced rapid neuronal firing and increased orthodromic population spike (PS), followed by loss of CA1 neural transmission. Upon recovery, CA1 orthodromic and antidromic PS regained a mean of only 12 +/- 6% and 8 +/- 5%, of initial amplitude. The electrophysiological pattern of glycine injury was similar to the excitotoxic damage produced by 8 min exposure to sodium glutamate (9 mM). L-Histidine, an inhibitor of glycine transport, exacerbated glycine-induced injury, just as dihydrokainic acid, a glutamate transport inhibitor, exacerbated glutamate-induced injury. The anticonvulsant felbamate (1.3 mM), as well as 100 microM zinc chloride, provided excellent protection from glycine-induced injury: 7-clorokynurenic acid appeared to be toxic. Blockers of the NMDA-associated ionic channel and methyl arginine prevented loss of neural transmission, but did not prevent accompanying hyper-excitability. Only 10 mM magnesium sulfate provided full protection against 9 mM glutamate exposure. Perfusion with low calcium ACSF protected against both glycine- and glutamate-induced injury. Thus, exposure to glycine resembled the excitotoxic effects of glutamate, but showed a different profile of protection. These results suggest that glycine elevations, as occur under physiologic and pathologic conditions, may modulate neuronal activity.

Low doses of the glycine/NMDA receptor antagonist R-(+)-HA-966 but not D-cycloserine induce paroxysmal activity in limbic brain regions of kindled rats

(+)-HA-966 [R-(+)-3-amino-1-hydroxypyrrolid-2-one], a functional antagonist at the glycine modulatory site on the N-methyl-D-aspartate (NMDA) receptor/ion channel complex, was evaluated in amygdala-kindled rats, a model of epilepsy recently shown to exhibit enhanced susceptibility to the adverse effects of competitive and non-competitive NMDA receptor antagonists. Since (+)-HA-966 displays weak partial agonistic effects at the glycine site (approximately 10% efficacy of glycine), D-cycloserine, a glycine ligand with much higher intrinsic activity, was evaluated in kindled rats for comparison. Following drug administration, electrographic activity was recorded from the basolateral amygdala (i.e. the focal site) as well as the ipsilateral piriform cortex, ventral hippocampus and nucleus accumbens. In addition to the evaluation of original recordings, power spectrum analysis was used to delineate drug effects. (+)-HA-966 (20-40 mg/kg i.p.) induced marked alterations in electrographic recordings, including increases in amplitude and isolated spiking, i.e. signs of paroxysmal activity. The severity or duration of fully kindled seizures was not changed by (+)-HA-966, but the drug dramatically increased the duration of immobilization and limbic seizure activity following a kindled motor seizure. In contrast to (+)-HA-966, D-cycloserine did not induce any electrographic changes, even when administered in much higher doses than (+)-HA-966. The changes in electrographic recordings seen after administration of (+)-HA-966 in kindled rats were almost absent in non-kindled rats, indicating that kindling had increased the sensitivity to the paroxysmal effects of the glycine/NMDA receptor ligand. The data indicate that functional glycine/NMDA antagonists with low intrinsic efficacy may bear the risk of proconvulsant activity.

1-Aminocyclopropanecarboxylic acid protects against dynorphin A-induced spinal injury

Lumbar subarachnoid injection of dynorphin A causes an ischemia-induced neuronal degeneration and persistent hindlimb paralysis. The protective effects of a variety of competitive and non-competitive N-methyl-D-aspartate (NMDA) receptor antagonists indicate that activation of the NMDA receptor complex is essential for dynorphin A-induced spinal cord injury. 1-Aminocyclopropanecarboxylic acid (ACPC) is a high affinity, partial agonist at strychnine-insensitive glycine receptors associated with the NMDA receptor complex. Pretreatment of rats with ACPC (100 and 200 mg/kg, i.p., 30 min prior to dynorphin A) significantly eliminated the persistent hindlimb motor deficits and neuropathological changes produced by 20 nmol of this peptide. The neuroprotective effects of ACPC (100 mg/kg, i.p.) were abolished by parenteral administration of glycine (800 mg/kg, 30 min prior to ACPC), consistent with other in vivo and in vitro studies indicating that the pharmacological actions of ACPC are effected through strychnine-insensitive glycine receptors. When given instead as six daily injections (200 mg/kg, i.p.) followed by an injection-free day, ACPC also significantly improved neurological recovery following dynorphin-A injection. These results support earlier indications that: (1) activation of the NMDA receptor complex plays a critical role in mediating dynorphin A-induced rat spinal cord injury; (2) ACPC provides an effective means of antagonizing excitotoxic phenomena; and (3) chronic administration of ACPC can elicit a persistent change in the NMDA receptor complex.

1-Aminocyclopropane carboxylic acid (ACPC) prevents mu and delta opioid tolerance

1-Aminocyclopropane carboxylic acid (ACPC), a partial agonist of the glycine site on the NMDA receptor, prevents tolerance to the mu opioid morphine and the delta ligand [D-Pen2,D-Pen5]enkephalin (DPDPE) when co-administered with the opioid. In contrast, ACPC does not significantly influence tolerance to the kappa1 opioid U50,488H or the kappa3 ligand naloxone benzoylhydrazone (NalBzoH). The actions of ACPC are restricted to tolerance. When given alone, ACPC has no analgesic actions in the tailflick assay and it does not change morphine's ED50 in naive mice. Chronic administration of ACPC alone for 5 days does not affect the sensitivity of mice to morphine. ACPC also reverses preexisting tolerance. When mice are made tolerant to morphine over 5 days and then receive ACPC along with their morphine, analgesia returns to naive levels within 3 days despite the continued administration of morphine. The actions of ACPC on opioid tolerance correspond closely with those previously described with both competitive and non-competitive NMDA antagonists.

MK-801, but not drugs acting at strychnine-insensitive glycine receptors, attenuate methamphetamine nigrostriatal toxicity

Repeated administration of methamphetamine (METH) results in damage to nigrostriatal dopaminergic neurons. Both competitive N-methyl-D-aspartate (NMDA) receptor antagonists and use-dependent cation channel blockers attenuate METH-induced damage. The objectives of the present study were to examine whether comparable reductions in METH-induced damage could be obtained by compounds acting at strychnine-insensitive glycine receptors on the NMDA receptor complex. Four injections of METH (5 mg/kg i.p.) resulted in a approximately 70.9% depletion of striatal dopamine (DA) and approximately 62.7% depletion of dihydroxyphenylacetic acid (DOPAC) content, respectively. A significant protection against METH-induced DA and DOPAC depletion was afforded by the use-dependent channel blocker, MK-801. The competitive glycine antagonist 7-chlorokynurenic acid (7-Cl-KA), the low efficacy glycine partial agonist (+)-3-amino-1-hydroxy-2-pyrrolidone ((+)-HA-966), and the high efficacy partial glycine agonist 1-aminocyclopropane-carboxylic acid (ACPC) were ineffective against METH-induced toxicity despite their abilities to attenuate glutamate-induced neurotoxicity under both in vivo and in vitro conditions. These results indicate that glycinergic ligands do not possess the same broad neuroprotective spectrum as other classes of NMDA antagonists.

Desensitization of the NMDA receptor complex by glycinergic ligands in cerebellar granule cell cultures

Glutamate neurotoxicity was examined in cultured cerebellar granule neurons following both prolonged (20-24 h) and brief (45 min) exposure to compounds acting at strychnine-insensitive glycine receptors. Glutamate neurotoxicity was reduced in a concentration-dependent fashion by brief exposure to the glycine partial agonists 1-aminocyclopropanecarboxylic acid (ACPC) and (+-)-3-amino-1-hydroxy-2-pyrrolidone (HA-966) and the competitive antagonist, 7-chlorokynurenic acid (7-CK) with a rank order efficacy: 7-CK > HA-966 > ACPC. Neither D-cycloserine (D-CS) nor glycine affected neurotoxicity produced by maximum glutamate concentrations, while glycine but not D-CS augmented the effects of submaximum glutamate concentrations. Prolonged exposure of cultures to either full (glycine) or partial agonists (ACPC, D-CS, HA-966) abolished the neuroprotective effects of ACPC and significantly diminished the neuroprotective effects of HA-966. In contrast, the neuroprotective effects of 7-CK were only marginally reduced by prolonged exposure to glycinergic ligands, while the neuroprotection afforded by compounds acting at other loci on the NMDA receptor complex (e.g. 2-amino-5-phosphonopentanoate (APV) and dizocilpine (MK-801)) were unaltered. These effects may represent homologous desensitization of the NMDA receptor complex at its strychnine-insensitive glycine receptor induced by prolonged exposure to glycinergic agonists and partial agonists. Nonetheless, levels of the NMDA receptor subunit zeta 1 mRNA were unaffected by prolonged exposure to ACPC, indicating the apparent desensitization could involve a post-translational modification of the NMDA receptor complex.

A novel treatment of global cerebral ischaemia with a glycine partial agonist

Chronic treatment of gerbils with 1-aminocyclopropanecarboxylic acid (a high affinity, partial agonist at strychnine-insensitive glycine receptors) resulted in a 3-fold increase in survival, a significant improvement in neurological status, and an extensive protection of vulnerable brain regions following severe forebrain ischaemia. A bolus of 1-aminocyclopropanecarboxylic acid 30 min prior to ischaemia did not further improve outcome compared to gerbils receiving their last injection 24 h prior to ischaemia. These findings are consistent with the hypothesis that chronic treatment with a glycine partial agonist desensitizes the N-methyl-D-aspartate receptor complex. Pharmacological intervention at the strychnine-insensitive glycine receptor may be an effective means of ameliorating the consequences of neuronal degeneration caused by excitotoxic phenomena.

High-performance liquid chromatographic assay for 1-aminocyclopropanecarboxylic acid from plasma and brain

A reversed-phase high-performance liquid chromatographic method for the analysis of 1-aminocyclopropanecarboxylic acid (ACPC) from plasma or brain tissue is described. Samples were deproteinized with perchloric acid, centrifuged, alkalinized with potassium hydroxide and recentrifuged. The supernatants were derivatized with o-phthaldialdehyde and injected onto a C18 3-microns column (100 mm x 4 mm I.D.) pumped with 1 ml/min methanol-acetonitrile-0.1 M sodium phosphate buffer pH 6.0 (28:5:67, v/v). The retention times for ACPC and the internal standard were 15 and 31 min, respectively. The minimum detectable amount of ACPC was 0.08 nmol. The extraction recovery of ACPC (2.7-270 nmol) from spiked plasma or brain tissue ranged from 88 to 109%. The intra- and inter-day coefficients of variation for 27 nmol ACPC were 3.9 and 4.9%, respectively. This method was utilized to obtain preliminary pharmacokinetic parameters following ACPC administration to mice.