Glycine Partial Agonist Research Articles

Novel Glutamatergic Treatments for Severe Mood Disorders.

All currently approved antidepressant medications for major depressive disorder (MDD) and bipolar disorder act primarily on the monoaminergic system and have varying affinities for serotonergic, norepinephrine-ergic, and/or dopaminergic receptors. Unfortunately, these drugs are only effective in approximately two-thirds of patients. Glutamate is the major excitatory neurotransmitter in the central nervous system, and the glutamatergic system has been implicated in the pathophysiology of MDD. Here, we review the putative involvement of the glutamate receptor subtypes-N-methyl-D-aspartate (NMDA), α-amino-3-hydroxyl-5-methyl-4-isoxazoleproprionic acid (AMPA), kainate, and the group I, II, and III metabotropic glutamate receptors (mGluRs)-in the development of novel and more effective treatments for MDD as well as preclinical and clinical trials of drugs targeting these receptors. The rapid and robust antidepressant effects of ketamine-an NMDA receptor antagonist-have been consistently replicated in multiple trials. Other glutamatergic drugs have been investigated with varying success. Here, we highlight some of the most interesting results, including: 1) repeated oral, intramuscular, and sublingual ketamine appears to be less robustly effective than intravenous ketamine, but also causes fewer significant adverse effects; 2) the glycine partial agonist GLYX-13 appears to be effective both as monotherapy and adjunctive treatment in the treatment of MDD. An oral analogue, NRX-1074, is currently under investigation; and 3) mGluR modulators targeting mGluR5 have demonstrated convincing preclinical results.

Effect of NMDAR antagonists in the tetrabenazine test for antidepressants: comparison with the tail suspension test.

The N-methyl-d-aspartate receptor (NMDAR) antagonist ketamine, produces rapid and enduring antidepressant effect in patients with treatment-resistant depression. Similar dramatic effects have not been observed in clinical trials with other NMDAR antagonists indicating ketamine may possess unique pharmacological properties. Tetrabenazine induces ptosis (a drooping of the eyelids), and the reversal of this effect, attributed to a sympathomimetic action, has been used to detect first-generation antidepressants, as well as ketamine. Because the actions of other NMDAR antagonists have not been reported in this measure, we examined whether reversal of tetrabenazine-induced ptosis was unique to ketamine, or a class effect of NMDAR antagonists. The effects of ketamine and other NMDAR antagonists to reverse tetrabenazine-induced ptosis were examined and compared with their antidepressant-like effects in the tail suspension test (TST) in mice. All the NMDAR antagonists tested produced a partial reversal of tetrabenazine-induced ptosis and, as expected, reduced immobility in the TST. Ketamine, memantine, MK-801 and AZD6765 were all about half as potent in reversing tetrabenazine-induced ptosis compared to reducing immobility in the TST, while an NR2B antagonist (Ro 25-6981) and a glycine partial agonist (ACPC) were equipotent in both tests. The ability to reverse tetrabenazine-induced ptosis is a class effect of NMDAR antagonists. These findings are consistent with the hypothesis that the inability of memantine, AZD6765 (lanicemine) and MK-0657 to reproduce the rapid and robust antidepressant effects of ketamine in the clinic result from insufficient dosing rather than a difference in mechanism of action among these NMDAR antagonists.

A trial of d-cycloserine to treat the social deficit in older adolescents and young adults with autism spectrum disorders.

Autism spectrum disorders are difficult for older adolescents and young adults as impaired social communication affects the transition to adult life. d-Cycloserine, a partial glycine agonist at the N-methyl-d-aspartic acid receptor, was tested in a double-blind randomized trial in 20 older adolescents and young adults with autism spectrum disorders using two dosing strategies (50 mg daily versus 50 mg weekly) for 8 weeks with a 2-week follow-up after discontinuation. d-Cycloserine caused statistically and clinically significant improvement with no differentiation between dosing strategies on the Social Responsiveness Scale and the Aberrant Behavior Checklist before and after d-cycloserine administration.

Probing the Modulation of Acute Ethanol Intoxication by Pharmacological Manipulation of the NMDAR Glycine Co‐Agonist Site

Stimulating the glycine(B) binding site on the N-methyl-d-aspartate ionotropic glutamate receptor (NMDAR) has been proposed as a novel mechanism for modulating behavioral effects of ethanol (EtOH) that are mediated via the NMDAR, including acute intoxication. Here, we pharmacologically interrogated this hypothesis in mice. Effects of systemic injection of the glycine(B) agonist, d-serine, the GlyT-1 glycine transporter inhibitor, ALX-5407, and the glycine(B) antagonist, L-701,324, were tested for the effects on EtOH-induced ataxia, hypothermia, and loss of righting reflex (LORR) duration in C57BL/6J (B6) and 129S1/SvImJ (S1) inbred mice. Effects of the glycine(B) partial agonist, d-cycloserine (DCS), the GlyT-1 inhibitor, N-[3-(4'-fluorophenyl)-3-(4'-phenylphenoxy)propyl]sarcosine (NFPS), and the glycine(B) antagonist, 5,7-dichlorokynurenic (DCKA), on EtOH-induced LORR duration were also tested. Interaction effects on EtOH-induced LORR duration were examined via combined treatment with d-serine and ALX-5407, d-serine and MK-801, d-serine and L-701,324, as well as L-701,324 and ALX-5407, in B6 mice, and d-serine in GluN2A and PSD-95 knockout mice. The effect of dietary depletion of magnesium (Mg), an element that interacts with the glycine(B) site, was also tested. Neither d-serine, DCS, ALX-5407, nor NFPS significantly affected EtOH intoxication on any of the measures or strains studied. L-701,324, but not DCKA, dose-dependently potentiated the ataxia-inducing effects of EtOH and increased EtOH-induced (but not pentobarbital-induced) LORR duration. d-serine did not have interactive effects on EtOH-induced LORR duration when combined with ALX-5407. The EtOH-potentiating effects of L-701,324, but not MK-801, on LORR duration were prevented by d-serine, but not ALX-5407. Mg depletion potentiated LORR duration in B6 mice and was lethal in a large proportion of S1 mice. Glycine(B) site activation failed to produce the hypothesized reduction in EtOH intoxication across a range of measures and genetic strains, but blockade of the glycine(B) site potentiated EtOH intoxication. These data suggest endogenous activity at the glycine(B) opposes EtOH intoxication, but it may be difficult to pharmacologically augment this action, at least in nondependent subjects, perhaps because of physiological saturation of the glycine(B) site.

Characterization of the interactive effects of glycine and D-cycloserine in men: further evidence for enhanced NMDA receptor function associated with human alcohol dependence.

Reduced responses to N-methyl-D-aspartate (NMDA) glutamate receptor antagonists in alcohol-dependent animals and humans provided evidence that chronic alcohol consumption increased NMDA receptor function. To further probe alterations in NMDA glutamate receptor function associated with human alcohol dependence, this study examined the interactive effects of agents acting at the glycine(B) coagonist site of the NMDA receptor. In doing so, it tested the hypothesis that raising brain glycine concentrations would accentuate the antagonist-like effects of the glycine(B) partial agonist, D-cycloserine (DCS). Twenty-two alcohol-dependent men and 22 healthy individuals completed 4 test days, during which glycine 0.3 g/kg or saline were administered intravenously and DCS 1000 mg or placebo were administered orally. The study was conducted under double-blind conditions with randomized test day assignment. In this study, DCS produced alcohol-like effects in healthy subjects that were deemed similar to a single standard alcohol drink. The alcohol-like effects of DCS were blunted in alcohol-dependent patients, providing additional evidence of increased NMDA receptor function in this group. Although glycine administration reduced DCS plasma levels, glycine accentuated DCS effects previously associated with the NMDA receptor antagonists, ketamine and ethanol. Thus, this study provided evidence that raising glycine levels accentuated the NMDA receptor antagonist-like effects of DCS and that alcohol-dependent patients showed tolerance to these DCS effects.

D-Cycloserine improves the impaired sociability of the Balb/c mouse

The genetically inbred Balb/c mouse strain shows evidence of impaired sociability in a standard paradigm. For example, relative to 8-week-old male outbred Swiss-Webster mice, 8 week-old male Balb/c mice spend less time sniffing and in the vicinity of an enclosed 4 week-old male ICR stimulus mouse and, when allowed to interact freely with the stimulus mouse for five minutes, make fewer discrete episodes of social approach and show suppression of locomotor activity. We explored the effect of d-cycloserine (320 mg/kg, intraperitoneally), a partial glycine agonist that binds to the obligatory co-agonist glycine binding site on the NMDA receptor, on the sociability of the Balb/c and Swiss-Webster mouse strains in a standard paradigm. The results show that treatment with d-cycloserine increased the locomotor activity of the Balb/c mouse strain in the presence of an enclosed social stimulus mouse and when these mice were allowed to interact freely with each other. Also, d-cycloserine increased the number of discrete episodes of social approach when Balb/c mice were allowed to interact freely with social stimulus mice. However, d-cycloserine had similar effects on measures of sociability in the Swiss-Webster mouse, raising the possibility that the positive effects on the sociability of the Balb/c mouse strain may be mediated by indirect effects on locomotion, arousal, and anxiety.

D-cycloserine Deters Reacquisition of Cocaine Self-Administration by Augmenting Extinction Learning

Augmentation of cue exposure (extinction) therapy with cognitive-enhancing pharmacotherapy may offer an effective strategy to combat cocaine relapse. To investigate this possibility at the preclinical level, rats and squirrel monkeys were trained to self-administer cocaine paired with a brief visual cue. Lever pressing was subsequently extinguished by withholding cocaine injections while maintaining response-contingent presentations of the cue. The glycine partial agonist D-cycloserine (DCS; 15 and 30 mg/kg in rats, 3 and 10 mg/kg in monkeys) was evaluated for its effects on the rate of extinction and subsequent reacquisition of cocaine self-administration. Compared with vehicle, pretreatment with 30 mg/kg DCS 0.5 h before extinction training reduced the number of responses and latency to reach the extinction criterion in rats, but neither dose of DCS altered these measures in monkeys. In both species, pretreatment with the higher dose of DCS before extinction training significantly attenuated reacquisition of cocaine self-administration compared with either extinction training in the absence of DCS or DCS in the absence of explicit extinction. Furthermore, treatment with 30 mg/kg DCS accompanied by brief handling (a stress induction) immediately after but not 6 h after extinction training attenuated reacquisition of cocaine self-administration in rats. No adverse effects of 10 mg/kg DCS were evident in quantitative observational studies in monkeys. The results suggest that DCS augmented consolidation of extinction learning to deter reacquisition of cocaine self-administration in rats and monkeys. The results suggest that DCS combined with exposure therapy may constitute a rational strategy for the clinical management of cocaine relapse.

D -Cycloserine Augmentation for Behavioral Therapy

Back to table of contents Previous article Next article Letters to the EditorFull Accessd -Cycloserine Augmentation for Behavioral TherapySEONG S. SHIM M.D, Ph.DMICHAEL D. HAMMONDS Ph.D.MATTHEW M. VRABEL M.D.,SEONG S. SHIM M.D, Ph.DSearch for more papers by this authorMICHAEL D. HAMMONDS Ph.D.Search for more papers by this authorMATTHEW M. VRABEL M.D.Search for more papers by this author,Published Online:1 Aug 2008https://doi.org/10.1176/appi.ajp.2008.08030455AboutSectionsPDF/EPUB ToolsAdd to favoritesDownload CitationsTrack Citations ShareShare onFacebookTwitterLinked InEmail To the Editor: In the March 2008 issue of the Journal , Sabine Wilhelm, Ph.D., et al. (1) reported that d -cycloserine augmentation enhanced the efficacy of behavioral therapy for treating obsessive-compulsive disorder (OCD). d -Cycloserine is a partial agonist of the glycine site at the N -methyl- d -aspartate (NMDA) glutamatergic receptor. Occupation of the glycine site at the NMDA receptor is required in order for NMDA agonists to fully activate the receptor. Since the NMDA receptor is crucial in the domains of learning and memory, the Wilhelm et al. study sought to enhance the learning that occurs during behavioral therapy sessions by potentiating the NMDA receptor via stimulation of the glycine site using d -cycloserine. d -Cycloserine is effective in stimulating the glycine site only within the narrow range of low doses (50–125 mg daily [1, 2]), acting as a partial glycine agonist. At higher doses, d -cycloserine does not show efficacy (3) and can exacerbate schizophrenia symptoms (4) , since d -cycloserine blocks full activation of the glycine site at these doses, acting as a full glycine antagonist. In contrast, full agonists such as glycine and d -serine stimulate the glycine site more potently and improve learning and memory more effectively within the high-dose range, as shown in their efficacy in the treatment of cognitive symptoms in schizophrenia (4) . Thus, full glycine agonists are likely to be more effective and safer relative to d -cycloserine in augmenting behavioral therapy for the treatment of OCD. Dr. Wilhelm et al. stated that the efficacy of d -cycloserine augmentation of behavioral therapy was more powerful early in the treatment of OCD, and this efficacy was decreased with prolonged treatment. The authors generalized this conclusion to d -cycloserine augmentation trials in the treatment of Alzheimer’s disease and schizophrenia patients, stating that long-term use of d -cycloserine “has yielded generally disappointing results” ( 1 , pp. 338–339). However, in these trials, the lack of efficacy of d -cycloserine augmentation was likely to have been associated with the nature of d -cycloserine as a partial glycine agonist (4) . Additionally, these trials concluded that glycine agonists began to show efficacy 2 weeks from the time of drug treatment, and their efficacy was measured biweekly for 6- to 12-week trials (4) . This time frame is clearly different from the time period for d -cycloserine augmentation in the treatment of OCD, in which the efficacy of d -cycloserine was measured per an hourly basis. This discrepancy indicates that neuronal mechanisms underlying d -cycloserine augmentation of behavioral therapy for OCD and the augmentation of antipsychotic drugs with glycine agonists for treating schizophrenia and Alzheimer’s disease may be different. Cleveland, OhioThe authors report no competing interests.This letter (doi: 10.1176/appi.ajp.2008.08030455) was accepted for publication in April 2008.

Absence of Significant Interactive Effects of High‐Dose d‐Cycloserine and Ethanol in Healthy Human Subjects: Preliminary Insights Into Ethanol Actions at the GlycineB Site of NMDA Glutamate Receptors

Ethanol reduces N-methyl-d-aspartate (NMDA) glutamate receptor function via multiple cellular targets. It is not yet clear whether direct ethanol antagonism of the glycine(B) co-agonist site of NMDA receptors is relevant to this effect. The purpose of this study was to evaluate whether ethanol effects at the glycine(B) co-agonist site was clinically relevant by evaluating some aspects of the psychopharmacologic interactions between the glycine(B) partial agonist, D-cycloserine (DCS), and ethanol in healthy human subjects. All subjects completed 4 test days under double-blind conditions in which DCS or placebo was administered orally prior to ethanol or an ethanol-tainted placebo drink. Two groups of healthy subjects were studied. A first group of subjects (n = 25) were pretreated orally with DCS 500 mg or placebo 4 hours prior to ethanol (0.8 g/kg, p.o. or placebo) administration. A second group of subjects (n = 20) were pretreated with DCS 1000 mg or placebo prior to ethanol administration. Outcomes included subjective and cognitive responses to the experimental interventions. Predictable ethanol responses were observed in both groups of subjects, although the response to ethanol and the breath alcohol levels, but not plasma alcohol levels, were slightly but significantly lower in the group that received the higher DCS dose. DCS produced mild sedative effects that were greater for the lower than the higher dose. It also produced a mild impairment of verbal fluency without impairing attention. No statistically significant interactions between ethanol and DCS emerged in analyses. However, the combination of ethanol and DCS produced significantly greater impairment than both ethanol or DCS administered alone on a test of verbal fluency and aspects of memory function. DCS and ethanol both produced sedative and cognitive effects, consistent with their ability to reduce NMDA receptor function. However, the absence of interactive effects observed in this study raises questions about the clinical significance of the glycine(B) site as a target for ethanol in the brain at levels of ethanol intoxication associated with social drinking. However, it should be noted that this conclusion is limited to the dependent measures evaluated and the doses of ethanol and DCS studied.

Ethanol Inhibition ofN-Methyl-d-aspartate Receptors Is Reduced by Site-directed Mutagenesis of a Transmembrane Domain Phenylalanine Residue

N-Methyl-D-aspartate (NMDA) receptors (NRs) are ionotropic receptors activated by glutamate and the co-agonist glycine. Ethanol inhibits NMDA receptor function, although its site of action is undefined. We hypothesized that ethanol acts at specific amino acids contained within the transmembrane (TM) domains of the receptor. In this study, NR1 and NR2A subunits were altered by mutagenesis and tested for sensitivity to ethanol. Three NR1 mutants (W636A, F817A, and L819A) and one NR2A mutant (F637A) failed to generate functional receptors. Pre-TM1 (I546A, L551A, F554A, and F558A), TM1 (W563A), and TM2 (W611A) NR1 mutations did not affect ethanol sensitivity of heteromeric receptors. In contrast, altering a TM3 phenylalanine to alanine (F639A) reduced the ethanol inhibition of NMDA receptors expressed in oocytes and human embryonic kidney 293 cells. Mutation of the nearby methionine (M641) to alanine did not affect ethanol sensitivity, whereas changing Phe(639) to tryptophan slightly enhanced ethanol inhibition. NR1(F639A) did not alter the agonist potency of glutamate but did produce a leftward shift in the glycine concentration response for receptors containing NR2A and NR2B subunits. NR1(F639A) also reduced the potency of the competitive glycine antagonist 5,7-dichlorokynurenic acid and increased the efficacy of the glycine partial agonist 3-amino-1-hydroxy-2-pyrrolidinone ((+)-HA-966). These results suggest that ethanol may interact with amino acids contained in the TM3 domain of NMDA subunits that are involved in transducing agonist binding to channel opening.

GlycineB Receptor Antagonists and Partial Agonists Prevent Memory Deficits in Inhibitory Avoidance Learning

Activation of N-methyl-d-aspartate (NMDA) receptors has been hypothesized to mediate certain forms of learning and memory. This hypothesis is based on the ability of competitive and uncompetitive NMDA receptor antagonists to disrupt learning. We investigated the effects of glycine site antagonists and partial agonists on deficits of acquisition (learning) and consolidation (memory) in a single trial inhibitory avoidance learning paradigm. Posttraining administration of either hypoxia (exposure to 7% oxygen) or the convulsant drug pentylenetetrazole (PTZ) (45 mg/kg) to mice impaired consolidation without producing neuronal cell death. Pretreatment with the competitive glycine antagonist 7-chlorokynurenic acid (7KYN) and the glycine partial agonists 1-aminocyclopropanecarboxylic acid (ACPC) and (+)HA-966 prevented memory deficits induced by hypoxia and PTZ, but did not affect scopolamine-induced learning impairment. In addition, ACPC prevented consolidation deficits evoked by a nonexcitotoxic concentration of l-trans-pyrrolidine-2, 4-dicarboxylate, a competitive inhibitor of glutamate transport that increases extracellular levels of glutamate. Moreover, (+)HA-966, 7KYN, and ACPC facilitated both acquisition and consolidation of inhibitory avoidance training, an effect that was dose-dependent and reversed by glycine. These results indicate that memory deficits induced by both hypoxia and PTZ involve NMDA receptor activation. Furthermore, the present findings demonstrate that glycine site antagonists and partial agonists prevent memory deficits of inhibitory avoidance learning by affecting consolidation, but not acquisition processes.

Effects of the partial glycine agonist d-cycloserine on cognitive functioning in chronic low dose MPTP-treated monkeys

d-Cycloserine, a partial agonist at the glycine recognition site of the N-methyl- d-aspartate (NMDA) receptor complex, has been shown to facilitate certain forms of memory formation and to improve visual recognition memory in normal monkeys. In the present study, the effects of d-cycloserine on spatial short-term memory deficits in monkeys induced by chronic low-dose 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) administration were examined. Chronic low-dose MPTP administration resulted in deficits in the performance of a variable delayed-response task (VDR). Single administration of d-cycloserine (320 or 1000 μg/kg) significantly improved the performance on this task. High-dose d-cycloserine (8000 μg/kg) or MK-801 (10-32 μg/kg) administration had no effects on delayed-response performance but impaired performance on a visual discrimination (VD) task that was not adversely affected by MPTP administration. These results show that at low doses, d-cycloserine has cognition-enhancing properties in this model of early Parkinsonism.

Stress and a Glycinergic Intervention Interact in the Modulation of MK-801–Elicited Mouse Popping Behavior

The ability of d-cycloserine, a partial glycine agonist, to modulate mouse popping behavior elicited by MK-801, a noncompetitive NMDA receptor antagonist, was studied in unstressed and stressed mice. In unstressed animals, d-cycloserine (5.6 and 10 mg/kg) attenuated the ability of MK-801 (1.0 mg/kg) to elicit this behavior. However, the ability of d-cycloserine to attenuate MK-801–elicited mouse-popping behavior was not evident in stressed mice, 24 h after they were forced to swim for up to 10 min in cold water. Thus, the therapeutic value of glycinergic interventions may be limited by environmental factors, such as stress.

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 μM) 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 μM) 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.

Chronic dosing with 1-aminocyclopropanecarboxylic acid, a glycine partial agonist, modulates NMDA inhibition of muscarinic-coupled PI hydrolysis in rat cortical slices.

Chronic dosing with the glycine partial NMDA agonist, 1-aminocyclopropanecarboxylic acid (ACPC) elicited an altered allosteric regulation of cortical NMDA receptor binding. The present study hypothesized that these allosteric receptor binding changes would be manifest as pharmacologically functional reductions in NMDA receptor activity following chronic ACPC dosing. NMDA inhibition of carbachol-induced phosphoinositide (PI) hydrolysis was used as a functional assay to assess NMDA receptor function in rat cerebral cortex. NMDA inhibition of stimulated PI turnover was similar in naive (46% +/- 4.5%; mean +/- SE; n = 34) and ACPC dosed rats (39% +/- 2.3%; n = 34). While ACPC reversed NMDA's inhibitory effects in naive rats (80% +/- 13%; n = 9), it was ineffective (P < 0.05) in ACPC pretreated rats (48% +/- 9.8%; n = 9). In contrast, the NMDA antagonists, MK-801 (ion channel), 7-chlorokynurenic acid (glycine site) and AP-7 (glutamate site), effectively reversed NMDA's inhibitory effects in naive and ACPC treated rats. The potency of these antagonists were unaltered by prior ACPC dosing. Thus, chronic ACPC therapy does not alter the functioning of the NMDA ion channel or glutamate receptor sites, but elicits functional tolerance to ACPC's actions in the cortical NMDA complex.

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.

Failure of glycine site NMDA receptor antagonists to protect against l-2-chloropropionic acid-induced neurotoxicity highlights the uniqueness of cerebellar NMDA receptors

Cultured cerebellar granule cells and cerebellar slices from neonatal rats have been widely used to examine the biochemistry of excitatory amino acid-induced cell death mediated in part by the activation of NMDA receptors. However, the NMDA subunit stoichiometry, producing functional NMDA receptors is different in cultured granule cells, neonatal and adult rat cerebellum as compared to the NMDA receptors in forebrain regions. We have used the l-2-chloropropionic acid ( l-CPA) (750 mg/kg) model of NMDA-medialed selective cerebellar granule cell necrosis in vivo to examine the role of the glycine binding site and possible effect of the NR2C subunit (which is largely expressed only in the cerebellum) on granule cell necrosis. The abilities of various NMDA receptor antagonists were examined in vivo to determine the relative contribution of both glutamate and glycine sites involved in the l-CPA-induced neurotoxicity. The potent neuroprotective, non-competitive NMDA receptor antagonist dizocilpine (MK-801) was compared with glutamate and glycine site NMDA antagonists. We have examined a number of markers for the l-CPA-induced granule cell necrosis. The l-CPA-induced reduction in cerebellar aspartate and glutamate concentrations were used as markers of granule cell necrosis. We also measured the cerebellar water content and sodium concentrations as measures of the l-CPA-induced cerebellar edema that accompanies the granule cell necrosis. Finally the ability of the NMDA antagonists to attenuate the l-CPA-induced reductions in body weight gain and the prevention of the loss in hindlimb function using a behavioral measure of hindlimb retraction were examined. The potent glutamate antagonists, CPP and CGP40116 and dizocilpine prevented the l-CPA-induced locomotor dysfunction and granule cell necrosis as measured by their ability to prevent l-CPA-induced reductions in aspartate and glutamate concentrations. CPR CGP40116 and dizocilpine also prevented the appearance of cerebellar edema following l-CPA administration. In addition, dizocilpine, CPP and CGP40116 were able to partially prevent the l-CPA-induced loss in body weight over the 48 h experimental period. In contrast, none of the glycine partial agonists or antagonists, namely (±)HA-966, d-cycloserine, MDL-29951, DPCQ, MNQX or L-701 252 were able to prevent the l-CPA-induced loss in body weight, l-CPA-induced granule cell necrosis and behavioral disturbances when administered to rats. None of the NMDA antagonists had any effect on the cerebellar neurochemistry when injected alone or had any effect on animal behavior except for dizocilpine, CPP, CGP401 l6 and (±)HA-966 which resulted in a transient sedation for between three and five hours immediately following their administration. In conclusion, we demonstrate that NMDA open channel blockade and glutamate antagonists can provide full neuroprotection against the l-CPA-induced granule cell necrosis. The failure of the glycine partial agonists and antagonists to provide any neuroprotection against l-CPA-induced neurotoxicity in the cerebellum contrasts with their neuroprotective efficacy in other animal models of excitatory amino acid-induced cell death in forebrain regions in vivo. We therefore suggest that the glycine site plays a lesser role in modulating NMDA receptor function in the cerebellum and may explain why cells expressing NMDA receptors composed of NR1/NR2C subunits are particularly resistant to excitatory amino acid-induced neurotoxicity.

Infusion of NMDA antagonists into the nucleus reticularis pontis oralis inhibits the maximal electroshock seizure response

The nucleus reticularis pontis oralis (RPO) is necessary for the expression of tonic hindlimb extension (THE) in maximal electroshock (MES) seizures of rats. Previous work in this laboratory has demonstrated that both systemic administration and focal RPO microinfusion of d-cycloserine inhibits THE. The purpose of the present study was to characterize specific components of the NMDA receptor/ionophore complex that regulate the anticonvulsant activity mediated by the RPO. Bilateral RPO microinfusion of the competitive NMDA antagonists (−)AP7 and D-CPP as well as the uncompetitive antagonist dizocilpine ((+)MK-801) inhibited THE in a dose-related fashion. Bilateral RPO microinfusion of NMDA did not affect the THE response to MES but did induce convulsions resembling audiogenic seizures in genetically epilepsy prone rats. Bilateral RPO microinfusion of the strychnine-insensitive glycine site partial agonist d-cycloserine and the antagonist 5,7-dichlorokynurenic acid inhibited THE. The strychnine-insensitive glycine partial agonists (+)HA-966 and ACPC, as well as the agonists glycine and d-serine, did not significantly affect the THE response. Strychnine microinfusions in the RPO had no effect on THE. The results support a hypothesis that the RPO is a site of anticonvulsant drug action in MES and indicate that either competitive or uncompetitive NMDA antagonist action regulates the anticonvulsant activity mediated by the RPO. The role of the strychnine-insensitive glycine site in the regulation of the anticonvulsant activity medicated by the RPO is uncertain.

Antidepressant-like actions of the polyamine site NMDA antagonist, eliprodil (SL-82.0715)

Functional N- methyl- d- aspartate (NMDA) antagonists including competitive antagonists, glycine partial agonists, and use-dependent channel blockers exhibit antidepressant-like actions in preclinical models. The present study examined the effects of eliprodil (SL-82.0715), an NMDA antagonist acting at polyamine sites, in behavioral and neurochemical tests predictive of antidepressant activity. In mice, eliprodil produced a dose-dependent reduction in immobility in the forced swim test, but was inactive in the tail suspension test. Chronic treatment with eliprodil produced both a significant downregulation of β-adrenoceptors and a reduction in the potency of glycine to inhibit [ 3H]5,7-dichlorokynurenic acid binding to strychnine-insensitive glycine receptors in neocortical membranes. In toto, these data indicate that like other NMDA antagonists, eliprodil possesses antidepressant-like actions in preclinical tests predictive of clinical efficacy.

Behavioral studies with the glycine partial agonist (+)-HA966 on cocaine-induced locomotor activity and reinforcement

Recent evidence suggests that excitatory amino acids may play a critical role in the mediation of the behavioral effects of cocaine. The present experiments were designed to examine the effects of the glycine-site partial agonist (+)-HA966, a compound modulating NMDA receptor function, on the development of sensitization the locomotor activating effects of cocaine and on intravenous cocaine self-administration. After chronic cocaine pretreatment (20mg/kg i.p. daily for 3 days), Sprague-Dawley rats showed much greater increases in activity after a cocaine challenge (20mg/kg i.p.) than did saline-pretreated controls. This sensitized response was diminished by (+)-HA966 (30, 100 and 200µg), when administered intraventricularly (i.c.v.) 5min before each of the three cocaine pretreatment injections. (+)-HA966 when given alone for 3 days did not significantly diminish subsequent cocaine-induced locomotor activity. However, a small dose of (+)-HA966 (30µg) potentiated the acute stimulatory effects of cocaine on locomotor activity; higher doses were without effect. With limited daily access to intravenous cocaine (0.33mg/kg/infusion), rats showed reliable patterns of self-administration under a fixed-ratio 3 (FR3) schedule of reinforcement. Pretreatment with small doses of (+)-HA966 administered 5min prior to cocaine self-administration sessions had little effect. However, larger doses (100 and 200µg i.c.v.) significantly decreased responding. This effect was selective, since similar doses produced significantly less suppression of operant responding for food. The non-competitive NMDA receptor antagonist, dizocilpine (0.03-0.3mg/kg i.p.) administered 30min prior to sessions, significantly decreased cocaine self-administration without modifying behavior maintained by food. The present findings demonstrate that modulation of NMDA systems can significantly modify the behavioral effects of cocaine.