Anxiolytic-like and antinociceptive effects of MK-801 accompanied by sedation and ataxia in primates

Time to re-engage psychiatric drug discovery by strengthening confidence in preclinical psychopharmacology

BACKGROUND

There is urgent need for new medications for psychiatric disorders. Mental illness is expected to become the leading cause of disability worldwide by 2030. Yet, the last two decades have seen the pharmaceutical industry withdraw from psychiatric drug discovery after costly late-stage trial failures in which clinical efficacy predicted pre-clinically has not materialised, leading to a crisis in confidence in preclinical psychopharmacology.

METHODS

Based on a review of the relevant literature, we formulated some principles for improving investment in translational neuroscience aimed at psychiatric drug discovery.

RESULTS

We propose the following 8 principles that could be used, in various combinations, to enhance CNS drug discovery: (1) consider incorporating the NIMH Research Domain Criteria (RDoC) approach; (2) engage the power of translational and systems neuroscience approaches; (3) use disease-relevant experimental perturbations; (4) identify molecular targets via genomic analysis and patient-derived pluripotent stem cells; (5) embrace holistic neuroscience: a partnership with psychoneuroimmunology; (6) use translational measures of neuronal activation; (7) validate the reproducibility of findings by independent collaboration; and (8) learn and reflect. We provide recent examples of promising animal-to-human translation of drug discovery projects and highlight some that present re-purposing opportunities.

CONCLUSIONS

We hope that this review will re-awaken the pharma industry and mental health advocates to the opportunities for improving psychiatric pharmacotherapy and so restore confidence and justify re-investment in the field.

Systematic review of the pharmacological agents that have been tested against spreading depolarizations

Spreading depolarization (SD) occurs alongside brain injuries and it can lead to neuronal damage. Therefore, pharmacological modulation of SD can constitute a therapeutic approach to reduce its detrimental effects and to improve the clinical outcome of patients. The major objective of this article was to produce a systematic review of all the drugs that have been tested against SD. Of the substances that have been examined, most have been shown to modulate certain SD characteristics. Only a few have succeeded in significantly inhibiting SD. We present a variety of strategies that have been proposed to overcome the notorious harmfulness and pharmacoresistance of SD. Information on clinically used anesthetic, sedative, hypnotic agents, anti-migraine drugs, anticonvulsants and various other substances have been compiled and reviewed with respect to the efficacy against SD, in order to answer the question of whether a drug at safe doses could be of therapeutic use against SD in humans.

Modulation of NMDA receptors in the cerebellum. II. Signaling pathways and physiological modulators regulating NMDA receptor function

NMDA receptors in cerebellum have specific characteristics that make their function and modulation different from those of NMDA receptors in other brain areas. The properties of the NMDA receptor that modulate its function: Subunit composition, post-translational modifications and synaptic localization are summarized in an accompanying article. In this review we summarize how different signaling molecules modulate the function of NMDA receptors. The function of the receptors is modulated by the co-agonists glycine and serine and this modulation is different in cerebellum than in other areas. The NMDA receptor also has binding sites for polyamines that regulate its function. Other signaling molecules that modulate NMDA receptors function are: cAMP, neurotrophic factors such as BDNF, FGF-2 or neuregulins. These and other molecules allow an interplay between NMDA receptors and other receptors for neurotransmitters that may in this way modulate NMDA receptor function. This has been reported, for example, for metabotropic glutamate receptors. The expression and function of NMDA receptor is also modulated by synaptic activity, allowing an adaptation of the receptors function to the external inputs. NMDA receptors modulate important cerebral processes. NMDA receptors in different brain areas seem to modulate different processes. Cerebellar NMDA receptors play a special role in the modulation of motor learning and coordination. This is also briefly reviewed.

Attenuation of behavioral effects of cocaine by the Metabotropic Glutamate Receptor 5 Antagonist 2-Methyl-6-(phenylethynyl)-pyridine in squirrel monkeys: comparison with dizocilpine

Growing evidence suggests a role for metabotropic glutamate receptors (mGluRs) in the behavioral effects of cocaine related to its abuse. The mGluR5 subtype, in particular, has come under scrutiny due to its distribution in brain regions associated with drug addiction. This study investigated interactions between the selective mGluR5 antagonist 2-methyl-6-(phenylethynyl)-pyridine (MPEP) and cocaine in squirrel monkeys whose lever-pressing behavior was 1) maintained under a second-order schedule of cocaine self-administration, 2) extinguished and then reinstated by cocaine priming, and 3) controlled by the discriminative stimulus (DS) effects of cocaine. Additional studies determined the effects of MPEP on unconditioned behaviors, coordination, and muscle resistance. In each experiment, the effects of MPEP were compared with those of the N-methyl-d-aspartate antagonist dizocilpine. MPEP attenuated cocaine self-administration, cocaine-induced reinstatement of drug seeking, and the DS effects of cocaine at doses that did not markedly impair motor function or operant behavior in the context of drug discrimination. Dizocilpine also attenuated cocaine self-administration, but it did not significantly alter cocaine-induced reinstatement of drug seeking, and it enhanced rather than attenuated the DS effects of cocaine. The findings point to a significant contribution of mGluR5 mechanisms in the behavioral effects of cocaine related to its abuse and suggest that MPEP has properties of a functional cocaine antagonist, which are not secondary to antagonism at NMDA receptors. The contrasting interactions of MPEP and dizocilpine with cocaine imply that glutamate acting through different metabotropic and ionotropic receptors may modulate the behavioral effects of cocaine in qualitatively different ways.

The competitive N-methyl-D-aspartate receptor antagonist (-)-6-phosphonomethyl-deca-hydroisoquinoline-3-carboxylic acid (LY235959) potentiates the antinociceptive effects of opioids that vary in efficacy at the mu-opioid receptor

(-)-6-Phosphonomethyl-deca-hydroisoquinoline-3-carboxylic acid (LY235959) is a competitive N-methyl-D-aspartate receptor antagonist shown to prevent the development of tolerance to the antinociceptive effects of morphine in rodents. Although administration of LY235959 alone generally does not produce antinociception, LY235959 potentiates the antinociceptive effects of morphine in squirrel monkeys. The present study was designed to determine whether LY235959 would potentiate the acute antinociceptive effects of morphine as well those of the opioid receptor agonists l-methadone, levorphanol, butorphanol, and buprenorphine. A squirrel monkey titration procedure was used in which shock (delivered to the tail) increased in intensity every 15 s (0.01-2.0 mA) in 30 increments. Five lever presses during any given 15-s shock period (fixed ratio 5) produced a 15-s shock-free period after which shock resumed at the next lower intensity. Morphine (0.3-3.0 mg/kg i.m.), l-methadone (0.1-0.56 mg/kg i.m.), levorphanol (0.1-1.0 mg/kg i.m.), butorphanol (1.0-10 mg/kg i.m.), and buprenorphine (0.01-0.03 mg/kg i.m.), but not LY235959 (0.1-1.0 mg/kg i.m.), dose and time dependently increased the intensity below which monkeys maintained shock 50% of the time (median shock level, MSL). LY235959 dose dependently potentiated the effect of each opioid agonist on MSL when concurrently administered to monkeys. Although LY235959 potentiated the antinociceptive effect of each opioid examined in a statistically significant manner, LY235959 seemed more potent and effective when combined with higher efficacy opioids. The present data suggest that the N-methyl-D-aspartate antagonist, LY235959, can potentiate the antinociceptive effects of a range of opioid receptor agonists independently of nonspecific motor effects.

NMDA receptors mediate lasting increases in anxiety-like behavior produced by the stress of predator exposure--implications for anxiety associated with posttraumatic stress disorder

It has been proposed that NMDA-dependent long-term potentiation (LTP) of limbic system circuits controlling defensive behavior underlies stressor-induced lasting increases in anxiety-like behavior (ALB). Findings in cats given the stress-inducing beta-carboline, FG-7142, support this hypothesis. An animal model of lasting affective change following traumatic stress has recently been developed. In this model, lasting increases in anxiety-like behavior (ALB) assessed in the elevated plus maze are produced by a single 5-min exposure of a rat to a cat. Rats become more anxious in the plus maze for up to 3 weeks after the exposure. The present study demonstrates that blockade of NMDA receptors in rats with MK-801, AP7, or CPP, given 30 min prior to exposure to a cat, prevents the increase in ALB assessed 1 week later. MK-801 or AP7, given 30 min after exposure to a cat, do not prevent the increase in ALB seen 1 week later, however. MK-801, but not CPP or AP7, promotes approaches to cats during exposure. This "fearlessness" may reflect some anxiolytic action of MK-801. Approach to cats following injection of MK-801 was eliminated by prior injection of Prazosin. Prazosin did not interfere with the block of increases in ALB following cat exposure, however. These findings are consistent with the view that NMDA receptors are involved in initiation, but not maintenance of neural changes mediating lasting increases in anxiety following severe stress. The significance of these findings for PTSD are discussed.

The stereo-specific effect of D-serine ethylester and the D-cycloserine in ataxic mutant mice

Spinocerebellar ataxia is one of the most common neurological disorders. However, few therapeutics are effective for the treatment of this disorder. In the present study, we investigated the efficacy of d-serine ethylester and a related substance, d-cycloserine, as therapeutic agents for ataxia in a murine model. Both compounds are known to stereospecific modulate N-methyl-d-aspartate type glutamate receptors, and impaired glutamate-mediated signaling has been implicated in spinocerebellar ataxia. Using a microdialysis method, we found that intraperitoneal administration of d-serine ethylester increases the extracellular content of endogenous d-serine in the mouse cerebellum for at least 3 h. Maximum elevation of the extracellular d-serine was observed at 40 min after injection. An open-field study was used to assay the effect of the d-serine derivatives on movement and ataxia. In mice exhibiting cytosine arabinoside-induced ataxia, d-serine ethylester reduced the falling index in a dose-dependent manner. The effect of d-serine ethylester was stereo-specific in that l-serine ethylester had no effect on the falling index at the maximum doses tested, and was partially inhibited by 5,7-dichlorokynurenate, an antagonist that binds to the glycine-binding site. Locomotor activity was not changed by the d-serine ethylester treatment. d-cycloserine also significantly reduced the falling index of the mice. Both d-serine ethylester and d-cycloserine had longer lasting effects than other potential therapeutic reagents for ataxia. Growing evidence suggests the essential involvement of endogenous d-serine in mammalian brain function, and our results suggest that d-serine derivatives may represent an effective new therapeutic for the treatment of spinocerebellar ataxia.

The putative AMPA receptor antagonist, LY326325, produces anxiolytic-like effects without altering locomotor activity in rats

Anxiolytic-like effects produced by the novel, water-soluble AMPA/kainate receptor antagonist, LY326325 (3RS,4aRS,6RS,8aRS)-6-[2-(1(2)H-tetrazole-5-yl)e thyl]decahydro-isoquinoline-3-carboxylic acid), were examined in the elevated plus-maze and in a conflict-suppressed drinking situation. Administration of low doses (0.5, 1.2, and 5 mg/kg; i.p., -30 min) of LY326325 to Sprague-Dawley rats did not alter the percentage of entries into the open arms of the plus-maze, whereas only one dose of LY326325 (1 mg/kg) produced a slight, but significant, increase of the time spent in the open arms of the plus maze. In the conflict-suppressed drinking test, similar doses of LY326325 (2.5 and 5 mg/kg; i.p., -30 min) caused a dose-dependent and significant increase of punished drinking behavior without having any significant effects on unpunished drinking. The anxiolytic-like effects of LY326325 in the plus-maze and in the anticonflict tests were observed at doses, which, by themselves, had no influence on various measures of locomotor activity, i.e., horizontal activity, forward locomotion, and corner time. Our data suggest that the putative AMPA/glutamate receptor antagonist, LY326325, produces anxiolytic-like effects similar to those of diazepam in the conflict-suppressed drinking test, but displays considerably weaker anxiety-reducing properties compared to diazepam in the elevated plus-maze.

The involvement of adenosine receptors in the effect of dizocilpine on mice in the elevated plus-maze

It has been claimed that blockade of receptors for N-methyl-D-aspartate (NMDA) can enhance adenosine receptor function on single neurones. Previous work has also indicated that the NMDA channel blocker dizocilpine, and the A1 selective agonist N6-cyclopentyladenosine (CPA) both had anxiolytic profiles in the elevated plus-maze. The anxiolytic effect of dizocilpine was accompanied by an increase in locomotor activity. In the present study, the elevated plus-maze has been used to determine whether dizocilpine's effects on behaviour are mediated through activation of adenosine receptors. When co-administered with dizocilpine (0.05 mg/kg), CPA (0.05 mg/kg) reduced the anxiolytic and locomotor effects of dizocilpine. The A1 selective antagonist 1,3-dipropyl-8-cyclopentylxanthine (CPX, 0.05 mg/kg) had no effect when administered alone. When co-administered with dizocilpine, CPX reversed the anxiolytic and increased locomotor effects induced by dizocilpine. The A2 receptor selective agonist N6-[2-(3,5-dimethoxyphenyl)-2(2-methylphenyl)ethyladenosine (DPMA) (1 mg/kg) reversed both the anxiolytic effect and the increased locomotion induced by dizocilpine, while the A2 selective antagonist 3,7-dimethyl-1-propargylxanthine (DMPX) (1 mg/kg) did not. It is concluded that at least part of the anxiolytic and locomotor stimulant properties of dizocilpine may be explained by the release of endogenous adenosine acting at A1, but not A2 receptors.

Reversal of parkinsonian symptoms in primates by antagonism of excitatory amino acid transmission: potential mechanisms of action

Parkinsonism is characterised by overactive glutamatergic transmission in the cortico-striatal and subthalamo-medial pallidal pathways. Local blockade of glutamatergic transmission in these pathways can alleviate parkinsonian symptoms. The effectiveness of the treatment, however, is often limited by the simultaneous appearance of unwanted side-effects. These side-effects, including ataxia and dissociative anaesthesia, are particularly problematic when N-methyl-D-aspartate (NMDA) antagonists are used. In an attempt to overcome these problems we have attempted to manipulate excitatory amino acid (EAA)-mediated neurotransmission indirectly by targeting the NMDA receptor associated modulatory sites. We review evidence which demonstrates that antagonists for both the NMDA associated glycine and polyamine sites can reverse parkinsonian symptoms when injected intra-cerebrally in both MPTP-treated and bilateral 6-OHDA lesioned marmosets without eliciting unwanted side-effects. We further review preliminary data which suggest that ifenprodil, a polyamine site antagonist, has striking anti-parkinsonian actions in the marmoset. Potential mechanisms of action underlying these effects are discussed in terms of NMDA receptor subtypes and the neuroanatomical locus of action. The anti-parkinsonian efficacy of intra-striatally administered EAA antagonists leads us to question the view of dopamine acting in the striatum as a simple neuromodulator.

Differential effects of agents acting at various sites of the NMDA receptor complex in a place preference conditioning model

A conditioned place preference paradigm was used to assess the potential rewarding properties of the uncompetitive NMDA receptor antagonist, MK-801 (dizolcipine), the two competitive NMDA receptor antagonists, CGP 37849 (DL-(E)-2-amino-4-methyl-5-phosphono-3-pentonoic acid) and its (R)-enantiomer CGP 40116, as well as the partial agonist at strychnine-insensitive glycine receptors, ACPC (1-aminocyclopropanecarboxylic acid). MK-801 (0.3 mg/kg), CGP 37849 (1.25-10 mg/kg) and CGP 40116 (1.25-10 mg/kg), administered in association with either the initially non-preferred or initially preferred side of the two-arm chamber, caused a significant increase in the time spent on that side in a post-conditioning test. In contrast, ACPC did not support the conditioned place preference. Thus, the time spent on the drug-associated side following conditioning with ACPC (50-400 mg/kg) did not significantly differ from that measured in the pre-conditioning test, irrespective of whether it was associated with the initially non-preferred black side or the initially preferred white side. These results are consistent with both clinical and pre-clinical data demonstrating differences in psychopharmacological properties among compounds acting at the multiple, allosteric regulatory sites on the NMDA receptor complex. Moreover, these results indicate that the abuse potential of ACPC, which acts as a functional NMDA receptor antagonist, may be lower than that of either uncompetitive or competitive NMDA receptor antagonists.

Effects of decahydroisoquinoline-3-carboxylic acid monohydrate, a novel AMPA receptor antagonist, on glutamate-induced CA2+ responses and neurotoxicity in rat cortical and cerebellar granule neurons

In this study, we examined the effects of a novel water-soluble, putative AMPA receptor antagonist, (-)(3S,4aR,6R,8aR)-6-[2-(1(2)H-tetrazole-5-yl)ethyl]-1,2,3, 4,4a,5,6,7,8,8a-decahydroisoquinoline-3-carboxylic acid monohydrate (LY326325), on glutamate-, N-methyl-D-aspartic acid (NMDA), alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA)-, and kainic acid (KA)-induced elevations of intracellular Ca2+ concentrations ([Ca2+]i) and 45Ca2+ uptake, as well as glutamate agonist-induced neurotoxicity in primary cultures of intact rat cortical and cerebellar granule neurons. In some experiments, the actions of LY326325 were tested in the presence of cyclothiazide, a compound that is known to block glutamate-induced desensitization of AMPA-preferring subtypes of glutamate receptors, thereby largely potentiating the functional effects of AMPA. LY326325 fully blocked the elevations of [Ca2+]i induced by NMDA and non-NMDA glutamate receptor agonists in both cortical and cerebellar granule neurons. The application of increasing concentrations of cyclothiazide was not able to reverse the LY326325-induced blockade of glutamate receptors in cortical neurons. In contrast, the same cyclothiazide treatment fully reversed the blockade produced by the noncompetitive AMPA/KA receptor antagonist 1-(4-aminophenyl)-4-methyl-7,8-methylenedioxy-5H-2, 3-benzodiazepine HCl (GYKI 52466). In 45Ca2+ uptake studies. LY325325 inhibited the NMDA-, AMPA-, and KA-induced enhancement of 45Ca2+ uptake in a concentration-dependent fashion in both cortical and cerebellar granule cells. In analogy to the results obtained with [Ca2+]i recordings, cyclothiazide failed to counteract the LY326325-induced blockade of KA-stimulated 45Ca2+ uptake in cerebellar granule neurons, whereas the blockade induced by the noncompetitive AMPA/KA receptor blocking agent GYKI 52466 was fully reversed by cyclothiazide. Because a similar, although not identical pattern of actions was seen following the application of the competitive AMPA/KA receptor antagonist 6-nitro-7-sulphamoyl-benzo(f)quinoxaline-2-3-dione (NBQX), it is suggested that the inhibitory actions of LY326325 are similar to those produced by NBQX but clearly differ from those caused by the noncompetitive AMPA/KA receptor antagonist GYKI 52466. Finally, when the neuroprotective actions of LY326325 on glutamate agonist-induced neurotoxicity were examined in cerebellar granule neurons, we found that LY326325 almost completely blocked the neurotoxic actions of NMDA, AMPA, and KA, respectively, whereas it produced only a partial blockade of glutamate-induced neurotoxicity. Taken together, our current results suggest that although LY326325 blocked both nonNMDA and NMDA-induced Ca2+ responses, it still displayed a preferential affinity of nonNMDA receptors as compared to NMDA receptors. However, LY326325 appears to be a less selective AMPA/KA receptor antagonist than NBQX and GYKI52466, respectively.

Evidence for an anxiogenic action of AMPA receptor antagonists in the plus-maze test

The effects of the non-NMDA receptor antagonists, the new alpha-amino-3-hydroxy-5-methyl-4-isoxazole-propionic acid (AMPA)-selective receptor antagonist, LY326325, and the AMPA/kainate-selective receptor antagonist, NBQX (6-nitro-7-sulfamoylbenzo(f)quinoxaline-2,3-(1H,4H)dione), on plus-maze behavior and locomotor activity were examined. LY326325 induced a dose-dependent decrease in the per cent time spent in open arms as well as in the per cent entries into the open arms. NBQX caused a dose-dependent reduction in the per cent time spent in open arms but had no effect on the per cent entries into the open arms. The behavioral actions of the AMPA receptor antagonists were observed at doses which had no influence on the locomotor activity of the animals. Based upon the current findings it is suggested that AMPA receptor antagonists produce a dose-dependent increase of anxiogenic behavior in the plus-maze test situation.

Effects of antagonists at the NMDA receptor complex in two models of anxiety

The effects of an antagonist at the strychnine insensitive glycine site (5,7-dichlorokynurenic acid, i.c.v.), and of noncompetitive (MK-801, i.p.) and competitive (CGP 37849, i.p.; CGP 39551, i.p.; AP-7, i.c.v.) NMDA antagonists were compared with diazepam (i.p.) in two animal models of anxiety (the open field exploratory behavior of non-habituated rats, and the Vogel conflict test). All drugs when applied in appropriate doses increased punished drinking in the Vogel test, without producing any significant changes in free drinking and the stimulus threshold at their lowest anticonflict doses. The effective doses were as follows: diazepam 1.5 and 2.5 mg/kg; MK-801 0.005 and 0.01 mg/kg; CGP 39551 5.0 and 20.0 mg/kg; CGP 37849 1.0 and 2.5 mg/kg; 5,7-dichlorokynurenic acid 5.0 microgram (i.c.v.); AP-7 0.5 microgram (i.c.v.). In the open field diazepam (0.05 mg/kg), MK-801 (0.1 mg/kg), CGP 37849 (0.01, 0.1, 1.0 mg/kg), and AP-7 2.5 micrograms (i.c.v.) significantly increased exploratory activity in the central sectors of the open field (anti-neophobic reaction), without changing motor activity of the rat. MK-801 at the highest tested dose of 0.2 mg/kg significantly stimulated animal locomotor activity. CGP 37849 in the largest dose examined (10 mg/kg) significantly depressed the motor behavior of rats. Overall, it appeared that different NMDA antagonists showed an anxiolytic-like profile, similar to that of the benzodiazepine diazepam. Among different NMDA receptor complex antagonists studied, CGP 37849 was characterized by the largest distinction between the doses showing an anxiolytic-like action in the open field test, and changing rat motor behavior.

Interaction of the NMDA receptor noncompetitive antagonist MK-801 with model and native membranes

MK-801, a noncompetitive antagonist of the NMDA (N-methyl-D-aspartate) receptor, has protective effects against excitotoxicity and ethanol withdrawal seizures. We have determined membrane/buffer partition coefficients (Kp[mem]) of MK-801 and its rates of association with and dissociation from membranes. Kp[mem] (+/- SD) = 1137 (+/- 320) in DOPC membranes and 485 (+/- 99) in synaptoneurosomal (SNM) lipid membranes from rat cerebral cortex (unilamellar vesicles). In multilamellar vesicles, Kp[mem] was higher: 3374 (+/- 253) in DOPC and 6879 (+/- 947) in SNM. In cholesterol/DOPC membranes, Kp[mem] decreased as the cholesterol content increased. MK-801 associated with and dissociated from membranes rapidly. Addition of ethanol to SNM did not affect Kp[mem]. MK-801 decreased the cooperative unit size of DMPC membranes. The decrease was smaller than that caused by 1,4-dihydropyridine drugs, indicating a weaker interaction with the hydrocarbon core. Small angle x-ray diffraction, with multilayer autocorrelation difference function modeling, indicated that MK-801 in a cholesterol/DOPC membrane (mole ratio = 0.6) causes a perturbation at approximately 16.0 A from the bilayer center. In bilayers of cholesterol/DOPC = 0.15 (mole ratio) or pure DOPC, the perturbation caused by MK-801 was more complex. The physical chemical interactions of MK-801 with membranes in vitro are consistent with a fast onset and short duration of action in vivo.