J. David Leander

The behavioral pharmacology of NMDA receptor antagonists

There is considerable interest in the development of NMDA antagonists as potential therapeutic agents in the treatment of convulsant, neurodegenerative and anxiety disorders. Because the clinical use of phencyclidine (PCP) has been precluded by its psychotomimetic effects and abuse potential, there has been concern that other NMDA antagonists including those acting competitively might produce similar untoward effects. However, the studies in animals, reviewed here by Joyce Willetts, Robert Balster and David Leander, suggest that while there are certain similarities in the behavioral effects of PCP-like and competitive antagonists, there are also differences. These differences have implications for the development of NMDA antagonists with less likelihood for producing PCP-like side-effects.

GLYX-13, a NMDA Receptor Glycine-Site Functional Partial Agonist, Induces Antidepressant-Like Effects Without Ketamine-Like Side Effects

Recent human clinical studies with the NMDA receptor (NMDAR) antagonist ketamine have revealed profound and long-lasting antidepressant effects with rapid onset in several clinical trials, but antidepressant effects were preceded by dissociative side effects. Here we show that GLYX-13, a novel NMDAR glycine-site functional partial agonist, produces an antidepressant-like effect in the Porsolt, novelty induced hypophagia, and learned helplessness tests in rats without exhibiting substance abuse-related, gating, and sedative side effects of ketamine in the drug discrimination, conditioned place preference, pre-pulse inhibition and open-field tests. Like ketamine, the GLYX-13-induced antidepressant-like effects required AMPA/kainate receptor activation, as evidenced by the ability of NBQX to abolish the antidepressant-like effect. Both GLYX-13 and ketamine persistently (24 h) enhanced the induction of long-term potentiation of synaptic transmission and the magnitude of NMDAR-NR2B conductance at rat Schaffer collateral-CA1 synapses in vitro. Cell surface biotinylation studies showed that both GLYX-13 and ketamine led to increases in both NR2B and GluR1 protein levels, as measured by Western analysis, whereas no changes were seen in mRNA expression (microarray and qRT-PCR). GLYX-13, unlike ketamine, produced its antidepressant-like effect when injected directly into the medial prefrontal cortex (MPFC). These results suggest that GLYX-13 produces an antidepressant-like effect without the side effects seen with ketamine at least in part by directly modulating NR2B-containing NMDARs in the MPFC. Furthermore, the enhancement of ‘metaplasticity’ by both GLYX-13 and ketamine may help explain the long-lasting antidepressant effects of these NMDAR modulators. GLYX-13 is currently in a Phase II clinical development program for treatment-resistant depression.

Anticonvulsant effects of phencyclidine-like drugs: relation to N-methyl-d-aspartic acid antagonism

Various compounds that have been identified in the literature as binding to the [3H]phencyclidine receptor site and as producing behavioral effects similar to phencyclidine (phencyclidine-like) protected mice from maximal electric shock-induced tonic-extensor seizures. These anticonvulsant effects appear to be due to blockade of the N-methyl-D-aspartic acid receptor, as recently reported for phencyclidine-like compounds. Phencyclidine-like compounds produced their anticonvulsant effects at doses that were also neurologically impairing.

N-Methyl-d-aspartic acid-induced lethality in mice: selective antagonism by phencylidine-like drugs

Abstract N- Methyl- d -aspartic acid (NMDA) produced a dose-related increase in lethality in mice, with 200 mg/kg (i.p.) effecting 100% lethality. Upon daily dosing, acutely sublethal doses of NMDA produced deaths. This NMDA-induced lethality was stereoselective: N- methyl- l -aspartic acid had no effects at doses as high as 400 mg/kg. Moderate doses of phencyclidine (PCP) and drugs having PCP-like behavioral effects blocked the NMDA-induced lethality. Other classes of psychoactive drugs, including opioids, anticonvulsants and antipsychotics, were ineffective in preventing NMDA-induced lethality. The potency of PCP-like drugs to block the NMDA-induced lethality correlates highly with the dose necessary to produce PCP-like catalepsy and PCP-like discrimination in pigeons. These data support the hypothesis that PCP-like drugs produce many of their effects by impairing the normal functioning of the NMDA-defined excitatory neurotransmitter receptor in the central nervous system.

Fluoxetine, a Selective Serotonin‐Uptake Inhibitor, Enhances the Anticonvulsant Effects of Phenytoin, Carbamazepine, and Ameltolide (LY 201116)

Summary: Dose‐response curves for the prototypical anticonvulsants phenytoin (PHT) and carbamazepine (CBZ), and a novel anticonvulsant, Ameltolide (LY201116), were determined with and without pretreatment with the selective serotonin‐uptake inhibitor fluoxetine by maximal electroshock seizure (MES) test in mice. Fluoxetine (2.5, 5, and 10 mg/kg intraperitoneally, i.p.) produced a dose‐related decrease in the ED50, values for the anticonvulsants (i.p. administration) to protect against MES‐induced tonic‐extensor seizures. Fluoxetine (10 mg/kg i.p.) also decreased the intravenous (i.v.) ED50 doses of the three anticonvulsants by a factor of ∼2. These data suggest that fluoxetine, through its selective inhibition of serotonergic reuptake, may have beneficial advantages as compared with common antidepressant drugs in treatment of depressed patients with epilepsy and may also enhance the seizure control of prototypical anticonvulsants in treatment of epilepsy.

Phencyclidine receptors in rat brain cortex

The binding of [3H]phencyclidine (PCP) to receptors in rat brain cortex has been studied. Two receptors have been detected, a high affinity receptor site with a KD of 23.5 +/- 7.4 nM and a low affinity site with a KD of 7.6 +/- 1.8 microM. The binding of [3H]PCP to its receptors was pH and temperature dependent and was destroyed by heat-denaturation. The binding of [3H]PCP was inhibited by compounds which produce PCP-like behavioral effects including dexoxadrol, etoxadrol and ketamine as well as a novel series of benz(f)isoquinolines. The low affinity site was blocked by PCP, etoxadrol and (+)-SKF-10,047 but not morphine or leu-enkephalin, suggesting that it also represents a specific PCP site. Stereoselective displacement of PCP at the high affinity receptor was observed with the isomers of cyclazocine, cyclorphan, SKF-10,047 and dioxadrol (dexoxadrol and levoxadrol). Naloxone, 4,5,6,7-tetrahydroisoxazolo(S,4-C)pyridin-3-ol (THIP) hydrate and haloperidol inhibited binding poorly (Ki greater than 1 microM), suggesting that these compounds do not interact significantly with the high affinity PCP receptor in vivo. The affinity of ligands for the phencyclidine receptor was highly correlated (r = 0.714, P less than 0.01) with their potency to produce catalepsy in pigeons.

Buprenorphine is a potent κ-opioid receptor antagonist in pigeons and mice

Abstract Buprenorphine was studied for its antagonist activity against the specific κ-opioid agonist U-50,488H in pigeons responding under a multiple schedule of grain presentation and in mice in an antinociception test. U-50,488H decreased rates of responding of pigeons over the dose range (2.5–20 mg/kg i.m.). In the presence of 0.32 mg/kg of buprenorphine, the U-50,488H dose-effect curve was shifted to the right approximately two-fold. Buprenorphine alone (0.01–0.08 mg/kg s.c.) inhibited in mice the abdominal stretching induced by i.p. acetic acid. β-Funaltrexamine pretreatment blocked the μ-like agonist analgesic effect of buprenorphine and revealed an antagonist action of buprenorphine against 2.5 mg/kg of U-50,488H over the same dose range that it produced antinociception at the μ-receptor. Thus, buprenorphine is a potent κ-opioid receptor antagonist, producing the κ-antagonist activity over the same dose range that it produces its μ-mediated partial agonist activity.

D,L-(Tetrazol-5-yl) glycine: a novel and highly potent NMDA receptor agonist

This paper describes the pharmacological activity of D,L-(tetrazol-5-yl)glycine, a structurally novel and highly potent agonist at the N-methyl-D-aspartate (NMDA) subtype of excitatory amino acid receptor. D,L-(Tetrazol-5-yl)glycine potently displaced NMDA receptor binding to rat brain membranes as measured using [3H]CGS19755 (IC50 = 98 +/- 7 nM) and [3H]glutamate (IC50 = 36 +/- 18 nM) as ligands. D,L-(Tetrazol-5-yl)glycine did not appreciably inhibit the binding of D,L-alpha-[5-methyl-3H] amino-3-hydroxy-5-methylisoxazole-4-propionic acid (AMPA), [3H]kainate, or [3H]glycine (IC50s greater than 30,000 nM). D,L-(Tetrazol-5-yl)glycine was more potent than NMDA or cis-methanoglutamate as a depolarizing agent in the rat cortical slice, and unlike these other agents induced rapid receptor-mediated neurotoxicity. Depolarization by D,L-(tetrazol-5-yl)glycine was antagonized by LY233053, a selective NMDA receptor antagonist. D,L-(Tetrazol-5-yl)glycine was a highly potent convulsant when given to neonatal rats (ED50 = 0.071 mg/kg i.p.). Convulsions in neonatal rats or lethality in mice induced by D,L-(tetrazol-5-yl)glycine were selectively antagonized by competitive and non-competitive NMDA receptor antagonists. D,L-(Tetrazol-5-yl)glycine is a structurally novel (tetrazole-substituted) compound that is a highly potent and selective NMDA receptor agonist. D,L-(Tetrazol-5-yl)glycine could be used to probe further NMDA receptor function in vitro and in vivo.

Evidence that nalorphine, butorphanol and oxilorphan are partial agonists at a κ-opioid receptor

Nalorphine, butorphanol and oxilorphan were compared in their ability to increase urinary output in the normally hydrated rat and to antagonize the increased urinary output produced by the full kappa-agonist, bremazocine. Nalorphine, butorphanol and oxilorphan increased 5-h cumulative urine output compared to controls, but the maximal effect was less than half the amount excreted after injection with 0.08 mg/kg of bremazocine. The effects of bremazocine were antagonized by nalorphine, butorphanol and oxilorphan. These effects of nalorphine, butorphanol and oxilorphan satisfy the criteria for classifying these agents as partial agonists at the kappa-opioid receptor responsible for mediating the effect of increased urinary output.

GLYX-13, an NMDA receptor glycine site functional partial agonist enhances cognition and produces antidepressant effects without the psychotomimetic side effects of NMDA receptor antagonists

Introduction: The N-methyl-d-aspartate receptor-ionophore complex plays a key role in learning and memory and has efficacy in animals and humans with affective disorders. GLYX-13 is an N-methyl-d-aspartate receptor (NMDAR) glycine-site functional partial agonist and cognitive enhancer that also shows rapid antidepressant activity without psychotomimetic side effects. Areas covered: The authors review the mechanism of action of GLYX-13 that was investigated in preclinical studies and evaluated in clinical studies. Specifically, the authors review its pharmacology, pharmacokinetics, and drug safety that were demonstrated in clinical studies. Expert opinion: NMDAR full antagonists can produce rapid antidepressant effects in treatment-resistant subjects; however, they are often accompanied by psychotomimetic effects that make chronic use outside of a clinical trial inpatient setting problematic. GLYX-13 appears to exert its antidepressant effects in the frontal cortex via NMDAR-triggered synaptic plasticity. Understanding the mechanistic underpinning of GLYX-13s antidepressant action should provide both novel insights into the role of the glutamatergic system in depression and identify new targets for therapeutic development.