GM-1020: a novel, orally bioavailable NMDA receptor antagonist with rapid and robust antidepressant-like effects at well-tolerated doses in rodents

The NMDA receptor (NMDAR) antagonist ketamine has shown great potential as a rapid-acting antidepressant; however, its use is limited by poor oral bioavailability and a side effect profile that necessitates in-clinic dosing. GM-1020 is a novel NMDAR antagonist that was developed to address these limitations of ketamine as a treatment for depression. Here, we present the preclinical characterization of GM-1020 alongside ketamine, for comparison. In vitro, we profiled GM-1020 for binding to NMDAR and functional inhibition using patch-clamp electrophysiology. In vivo, GM-1020 was assessed for antidepressant-like efficacy using the Forced Swim Test (FST) and Chronic Mild Stress (CMS), while motor side effects were assessed in spontaneous locomotor activity and on the rotarod. The pharmacokinetic properties of GM-1020 were profiled across multiple preclinical species. Electroencephalography (EEG) was performed to determine indirect target engagement and provide a potentially translational biomarker. These results demonstrate that GM-1020 is an orally bioavailable NMDAR antagonist with antidepressant-like efficacy at exposures that do not produce unwanted motor effects.

Esketamine: a glimmer of hope in treatment-resistant depression

The motive of this article is to review the pharmacological and clinical aspects of esketamine (ESK), an NMDA-receptor antagonist approved recently by the FDA for treatment-resistant depression (TRD). PubMed/Medline database was searched using keywords 'esketamine' and 'depression', 'S-ketamine' and 'depression', and 'NMDA antagonist' and 'depression'. Individual trials were searched from ClinicalTrials.gov. We included English-language articles evaluating pharmacokinetics and pharmacodynamics of intranasal (IN) esketamine, along with clinical trial data related to its efficacy and safety in patients diagnosed with TRD. Compared to placebo, IN esketamine causes significant and rapid improvement in depression. Dizziness, vertigo, headache, increase in blood pressure are some of its common adverse effects. With the growing number of patients of TRD, additional effective and safe treatment is the need of the hour. Esketamine appears to be an effective therapy when combined with oral antidepressants in patients with TRD. It is of special value due to the rapid onset of its action. Long-term clinical studies are, however, needed to ascertain its safety profile.

Acute Effects of Riluzole and Retigabine on Axonal Excitability in Patients With Amyotrophic Lateral Sclerosis: A Randomized, Double-Blind, Placebo-Controlled, Crossover Trial

Increased excitability of motor neurons in patients with amyotrophic lateral sclerosis (ALS) may be a relevant factor leading to motor neuron damage. This randomized, double-blind, three-way crossover, placebo-controlled study evaluated peripheral motor nerve excitability testing as a biomarker of hyperexcitability and assessed the effects of riluzole and retigabine in 18 patients with ALS. We performed excitability testing at baseline, and twice after participants had received a single dose of either 100 mg riluzole, 300 mg retigabine, or placebo. Between- and within-day repeatability was at least acceptable for 14 out of 18 recorded excitability variables. No effects of riluzole on excitability testing were observed, but retigabine significantly decreased strength-duration time-constant (9.2%) and refractoriness at 2 ms (10.2) compared to placebo. Excitability testing was shown to be a reliable biomarker in patients with ALS, and the acute reversal of previously abnormal variables by retigabine justifies long-term studies evaluating the impact on disease progression and survival.

Influence of the selective antagonist of the NR2B subunit of the NMDA receptor, traxoprodil, on the antidepressant-like activity of desipramine, paroxetine, milnacipran, and bupropion in mice

Pre-clinical and clinical studies indicated that a blockade of the NMDA receptor complex creates new opportunities for the treatment of affective disorders, including depression. The aim of the present study was to assess the influence of traxoprodil (10 mg/kg) on the activity of desipramine (10 mg/kg), paroxetine (0.5 mg/kg), milnacipran (1.25 mg/kg), and bupropion (10 mg/kg), each at sub-therapeutic doses. Moreover, brain levels of traxoprodil and tested agents were determined using HPLC. The obtained results were used to ascertain the nature of occurring interaction between traxoprodil and studied antidepressants. The experiment was carried out on naïve adult male Albino Swiss mice. Traxoprodil and other tested drugs were administered intraperitoneally. The influence of traxoprodil on the activity of selected antidepressants was evaluated in forced swim test (FST). Locomotor activity was estimated to exclude false positive/negative data. To assess the influence of traxoprodil on the concentration of used antidepressants, their levels were determined in murine brains using HPLC. Results indicated that traxoprodil potentiated activity of all antidepressants examined in FST and the observed effects were not due to the increase in locomotor activity. Only in the case of co-administration of traxoprodil and bupropion, increased bupropion concentrations in brain tissue were observed. All tested agents increased the traxoprodil levels in the brain. Administration of a sub-active dose of traxoprodil with antidepressants from different chemical groups, which act via enhancing monoaminergic transduction, caused the antidepressant-like effect in FST in mice. The interactions of traxoprodil with desipramine, paroxetine, milnacipran, and bupropion occur, at least partially, in the pharmacokinetic phase.

Single-dose infusion ketamine and non-ketamine N-methyl-d-aspartate receptor antagonists for unipolar and bipolar depression: a meta-analysis of efficacy, safety and time trajectories

BACKGROUND

Ketamine and non-ketamine N-methyl-d-aspartate receptor antagonists (NMDAR antagonists) recently demonstrated antidepressant efficacy for the treatment of refractory depression, but effect sizes, trajectories and possible class effects are unclear.

METHOD

We searched PubMed/PsycINFO/Web of Science/clinicaltrials.gov until 25 August 2015. Parallel-group or cross-over randomized controlled trials (RCTs) comparing single intravenous infusion of ketamine or a non-ketamine NMDAR antagonist v. placebo/pseudo-placebo in patients with major depressive disorder (MDD) and/or bipolar depression (BD) were included in the analyses. Hedges' g and risk ratios and their 95% confidence intervals (CIs) were calculated using a random-effects model. The primary outcome was depressive symptom change. Secondary outcomes included response, remission, all-cause discontinuation and adverse effects.

RESULTS

A total of 14 RCTs (nine ketamine studies: n = 234; five non-ketamine NMDAR antagonist studies: n = 354; MDD = 554, BD = 34), lasting 10.0 ± 8.8 days, were meta-analysed. Ketamine reduced depression significantly more than placebo/pseudo-placebo beginning at 40 min, peaking at day 1 (Hedges' g = -1.00, 95% CI -1.28 to -0.73, p < 0.001), and loosing superiority by days 10-12. Non-ketamine NMDAR antagonists were superior to placebo only on days 5-8 (Hedges' g = -0.37, 95% CI -0.66 to -0.09, p = 0.01). Compared with placebo/pseudo-placebo, ketamine led to significantly greater response (40 min to day 7) and remission (80 min to days 3-5). Non-ketamine NMDAR antagonists achieved greater response at day 2 and days 3-5. All-cause discontinuation was similar between ketamine (p = 0.34) or non-ketamine NMDAR antagonists (p = 0.94) and placebo. Although some adverse effects were more common with ketamine/NMDAR antagonists than placebo, these were transient and clinically insignificant.

CONCLUSIONS

A single infusion of ketamine, but less so of non-ketamine NMDAR antagonists, has ultra-rapid efficacy for MDD and BD, lasting for up to 1 week. Development of easy-to-administer, repeatedly given NMDAR antagonists without risk of brain toxicity is of critical importance.

Adolescent social defeat alters N-methyl-D-aspartic acid receptor expression and impairs fear learning in adulthood

Repeated social defeat of adolescent male rats results in adult mesocortical dopamine hypofunction, impaired working memory, and increased contextual anxiety-like behavior. Given the role of glutamate in dopamine regulation, cognition, and fear and anxiety, we investigated potential changes to N-methyl-D-aspartic acid (NMDA) receptors following adolescent social defeat. As both NMDA receptors and mesocortical dopamine are implicated in the expression and extinction of conditioned fear, a separate cohort of rats was challenged with a classical fear conditioning paradigm to investigate whether fear learning is altered by adolescent defeat. Quantitative autoradiography was used to measure 3H-MK-801 binding to NMDA receptors in regions of the medial prefrontal cortex, caudate putamen, nucleus accumbens, amygdala and hippocampus. Assessment of fear learning was achieved using an auditory fear conditioning paradigm, with freezing toward the auditory tone used as a measure of conditioned fear. Compared to controls, adolescent social defeat decreased adult NMDA receptor expression in the infralimbic region of the prefrontal cortex and central amygdala, while increasing expression in the CA3 region of the hippocampus. Previously defeated rats also displayed decreased conditioned freezing during the recall and first extinction periods, which may be related to the observed decreases and increases in NMDA receptors within the central amygdala and CA3, respectively. The alteration in NMDA receptors seen following adolescent social defeat suggests that dysfunction of glutamatergic systems, combined with mesocortical dopamine deficits, likely plays a role in the some of the long-term behavioral consequences of social stressors in adolescence seen in both preclinical and clinical studies.

In vitro pharmacological and rat pharmacokinetic characterization of LY3020371, a potent and selective mGlu2/3 receptor antagonist

Metabotropic glutamate _2/3 (mGlu_2/3) receptors are of considerable interest owing to their role in modulating glutamate transmission via presynaptic, postsynaptic and glial mechanisms. As part of our ongoing efforts to identify novel ligands for these receptors, we have discovered (1S,2R,3S,4S,5R,6R)-2-amino-3-[(3,4-difluorophenyl)sulfanylmethyl]-4-hydroxy-bicyclo[3.1.0]hexane-2,6-dicarboxylic acid; (LY3020371), a potent and selective orthosteric mGlu_2/3 receptor antagonist. In this account, we characterize the effects of LY3020371 in membranes and cells expressing human recombinant mGlu receptor subtypes as well as in native rodent and human brain tissue preparations, providing important translational information for this molecule. In membranes from cells expressing recombinant human mGlu₂ and mGlu₃ receptor subtypes, LY3020371.HCl competitively displaced binding of the mGlu_2/3 agonist ligand [³H]-459477 with high affinity (hmGlu₂ K_i = 5.26 nM; hmGlu₃ Ki = 2.50 nM). In cells expressing hmGlu₂ receptors, LY3020371.HCl potently blocked mGlu_2/3 agonist (DCG-IV)-inhibited, forskolin-stimulated cAMP formation (IC₅₀ = 16.2 nM), an effect that was similarly observed in hmGlu₃-expressing cells (IC₅₀ = 6.21 nM). Evaluation of LY3020371 in cells expressing the other human mGlu receptor subtypes revealed high mGlu_2/3 receptor selectivity. In rat native tissue assays, LY3020371 demonstrated effective displacement of [³H]-459477 from frontal cortical membranes (K_i = 33 nM), and functional antagonist activity in cortical synaptosomes measuring both the reversal of agonist-suppressed second messenger production (IC₅₀ = 29 nM) and agonist-inhibited, K⁺-evoked glutamate release (IC₅₀ = 86 nM). Antagonism was fully recapitulated in both primary cultured cortical neurons where LY3020371 blocked agonist-suppressed spontaneous Ca²⁺ oscillations (IC₅₀ = 34 nM) and in an intact hippocampal slice preparation (IC₅₀ = 46 nM). Functional antagonist activity was similarly demonstrated in synaptosomes prepared from epileptic human cortical or hippocampal tissues, suggesting a translation of the mGlu_2/3 antagonist pharmacology from rat to human. Intravenous dosing of LY3020371 in rats led to cerebrospinal fluid drug levels that are expected to effectively block mGlu_2/3 receptors in vivo. Taken together, these results establish LY3020371 as an important new pharmacological tool for studying mGlu_2/3 receptors in vitro and in vivo. This article is part of the Special Issue entitled 'Metabotropic Glutamate Receptors, 5 years on'.

Urine metabolomics in rats after administration of ketamine

In this study, we developed a urine metabonomic method, based on gas chromatography–mass spectrometry (GC-MS), to evaluate the effect of ketamine on rats. Pattern recognition analysis, including both principal component analysis and partial least squares discriminate analysis revealed that ketamine (50 mg/kg) induced metabolic perturbations. Compared with the control group, at day 7, the level of alanine, butanoic acid, glutamine, butanedioic, trimethylsiloxy, L-aspartic acid, D-glucose, cholesterol, acetamide, and oleic acid of the ketamine group was increased, while the level of 2,3,4-trihydroxybutyric acid, benzeneacetic acid, threitol, ribitol, xylitol, and glycine decreased. At day 14, the level of alanine, ethanedioic acid, L-proline, glycerol, tetradecanoic acid, l-serine, l-phenylalanine, L-aspartic acid, d-glucose, cholesterol, heptadecanoic acid, and acetamide in rat urine of the ketamine group was increased, while the 2,3,4-trihydroxybutyric acid, benzeneacetic acid, d-ribose, threitol, ribitol, glycine, pyrazine, and oleic acid levels decreased. Our results indicate that metabonomic methods based on GC-MS may be useful to elucidate ketamine abuse, through the exploration of biomarkers.

The major cholesterol metabolite cholestane-3β,5α,6β-triol functions as an endogenous neuroprotectant

Overstimulation of NMDA-type glutamate receptors is believed to be responsible for neuronal death of the CNS in various disorders, including cerebral and spinal cord ischemia. However, the intrinsic and physiological mechanisms of modulation of these receptors are essentially unknown. Here we report that cholestane-3β,5α,6β-triol (triol), a major metabolite of cholesterol, is an endogenous neuroprotectant and protects against neuronal injury both in vitro and in vivo via negative modulation of NMDA receptors. Treatment of cultured neurons with triol protects against glutamate-induced neurotoxicity, and administration of triol significantly decreases neuronal injury after spinal cord ischemia in rabbits and transient focal cerebral ischemia in rats. An inducible elevation of triol is associated with ischemic preconditioning and subsequent neuroprotection in the spinal cord of rabbits. This neuroprotection is effectively abolished by preadministration of a specific inhibitor of triol synthesis. Physiological concentrations of triol attenuate [Ca(2+)]i induced by glutamate and decrease inward NMDA-mediated currents in cultured cortical neurons and HEK-293 cells transiently transfected with NR1/NR2B NMDA receptors. Saturable binding of [(3)H]triol to cerebellar granule neurons and displacement of [(3)H]MK-801 binding to NMDA receptors by triol suggest that direct blockade of NMDA receptors may underlie the neuroprotective properties. Our findings suggest that the naturally occurring oxysterol, the major cholesterol metabolite triol, functions as an endogenous neuroprotectant in vivo, which may provide novel insights into understanding and developing potential therapeutics for disorders in the CNS.

[Effect of sub-anaesthetic doses of ketamine in the postoperative period in a patient with uncontrolled depression]

Recent studies indicate that the intravenous infusion of ketamine hydrochloride (an N-methyl-D-aspartate receptor antagonist) leads to a rapid reduction in depressive symptoms. A 42 year-old woman with breast cancer and major depression resistant to medical treatment received a 90 minute intravenous infusion of 0.3 mg/kg ketamine for 5 consecutive days. A significant reduction from 22 to 13 (-41%) was observed in the symptoms assessed using the Hamilton scale, with the effect maintained for 14 days. The possible therapeutic mechanism is discussed.

Brief mitochondrial inhibition causes lasting changes in motor behavior and corticostriatal synaptic physiology in the Fischer 344 rat

The striatum is particularly vulnerable to mitochondrial dysfunction and this problem is linked to pathology created by environmental neurotoxins, stimulants like amphetamine, and metabolic disease and ischemia. We studied the course of recovery following a single systemic injection of the mitochondrial complex II inhibitor 3-nitropropionic acid (3-NP) and found 3-NP caused lasting changes in motor behavior that were associated with altered activity-dependent plasticity at corticostriatal synapses in Fischer 344 rats. The changes in synapse behavior varied with the time after exposure to the 3-NP injection. The earliest time point studied, 24h after 3-NP, revealed 3-NP-induced an exaggeration of D1 Dopamine (DA) receptor dependent long-term potentiation (LTP) that reversed to normal by 48 h post-3-NP exposure. Thereafter, the likelihood and degree of inducing D2 DA receptor dependent long-term depression (LTD) gradually increased, relative to saline controls, peaking at 1 month after the 3-NP exposure. NMDA receptor binding did not change over the same post 3-NP time points. These data indicate even brief exposure to 3-NP can have lasting behavioral effects mediated by changes in the way DA and glutamate synapses interact.

NMDA receptors in frontal cortex and hippocampus of alcohol consumers

Specific binding of [³H]MK801 to N-methyl-D-aspartate (NMDA) receptors in the frontal cortex and hippocampus (CA1 and gyrus dentatus) was measured by receptor autoradiography in 16 Caucasian chronic alcohol consumers free of clinical manifestations of alcoholism, and compared with 16 Caucasian control subjects. Binding densities were not significantly different between heavy and moderate drinkers, neither between alcohol consumers that were abstinent or non-abstinent before death, nor between ethanol drinkers and controls. Continued alcohol consumption, in the absence of hepatic, neurologic or psychiatric disorders related to alcoholism, does not alter the binding properties of NMDA receptors in the brain areas studied.

Effects of combined treatment with cognitive enhancer memantine and antidepressant fluoxetine on CYP2D2 metabolic activity in rats

OBJECTIVES

The drug-drug interactions can result in alterations of the therapeutical responses. The present study was designed to investigate possible pharmacokinetic interactions between the cognitive agent memantine and the antidepressant fluoxetine combined often in treatments of cognitive disorders including Alzheimer disease. The attention was focused on changes of the cytochrome P450 2D2 isoenzyme activity in two animal models.

METHODS AND DESIGN

The tested drugs were administered alone or in a combination to rat males and their effects on the 2D2 isoenzyme activity was determined after in vivo administration. The levels of marker dextromethorphan, its 2D2 specific metabolite dextrorphan were analyzed in plasma of rats and using the model of isolated perfused rat liver in the perfusion medium. The dextromethorphan/dextrorphan (DEM/DEX) metabolic ratios were determined as a sign of inhibitory influences on CYP2D2.

RESULTS

The analyses showed elevation of DEM/DEX metabolic ratio after all treatments: a) memantine, b) fluoxetine and c) memantine+fluoxetine, however the results were not completely identical. The intensity of inhibitory effects on the CYP2D2 activity were: memantine < memantine + fluoxetine < fluoxetine.

CONCLUSION

The results presented suggest that the clinical pharmacotherapeutical approach to combine memantine with fluoxetine is from the point of view of pharmacokinetic drug-drug interaction on the level of CYP2D2 isoenzyme safe and even of benefit as memantine could elicit a suppression of the inhibitory influence of fluoxetine.

Pharmacodynamic profiles of ketamine (R)- and (S)- with 5-day inpatient infusion for the treatment of complex regional pain syndrome

BACKGROUND

Ketamine might be effective in blocking central sensitization of pain transmission neurons through its effect on NMDA receptors in refractory Complex Regional Pain Syndrome (CRPS) patients. At higher doses, ketamine infusions can be associated with significant risks; outpatient therapy requires return visits for a 10-day period with variable efficacy and duration.

OBJECTIVE

This study determined the efficacy of a 5-day moderate dose, continuous racemic ketamine infusion. The pharmacodynamic responses to racemic ketamine and norketamine were examined.

DESIGN

Observational study

METHODS

In this study, ketamine was titrated from 10-40 mg/hour in 16 CRPS patients, and maintained for 5 days. Pain was assessed daily. Ketamine and norketamine concentrations were obtained on Day 1 before starting the infusion; at 60 to 90 minutes, 120 to 150 minutes, 180 to 210 minutes, and 240 to 300 minutes after the initiation of the infusion on Days 2, 3, 4, and 5; and on Day 5 at 60 minutes after the conclusion of the infusion. The plasma concentrations of (R)-ketamine, (S)-ketamine, (R)-norketamine and (S)-norketamine were determined using an enantioselective liquid chromatography - mass spectrometry method.

RESULTS

Ketamine and norketamine infusion rates stabilized 5 hours after the start of the infusion. The subjects showed no evidence of significant tachycardia, arterial oxygen desaturation, or hallucinatory responses. Subjects generally experienced minimal pain relief on day one followed by significant relief by day 3. Mean pain scores decreased from the 8-9 to 3-5 ranges; however, the analgesic response to ketamine infusion was not uniform. On Day 5, there was little or no change in the pain measure assessed as the worst pain experienced over the last 24 hours in 37% of the subjects. (R)- and (S)-ketamine concentrations peaked at 240-300 min. (R)- and (S)-norketamine concentrations were lower and peaked on Day 2 of the infusion, as opposed to Day 1 for (R)- and (S)-ketamine. Significant pain relief was achieved by the second day of infusion and correlated with the maximum plasma levels of ketamine and norketamine. Pain relief continued to significantly improve over the 5-day infusion at concentrations of 200-225 ng/mL for (R)- and (S)-ketamine, and 90-120 ng/mL for (R)- and (S)-norketamine.

CONCLUSIONS

A 5-day ketamine infusion for the treatment of severe CRPS provided significant (p <0.05) pain relief by Day 3 compared to baseline. The pain relief experienced on Day 2 of the infusion continued to improve over the 5-day infusion period and correlated with the maximum plasma levels of ketamine and norketamine. We speculate that downstream metabolites of ketamine and norketamine might be playing a role in its therapeutic efficacy.

Gene expression of NMDA receptor subunits in the cerebellum of elderly patients with schizophrenia

To determine if NMDA receptor alterations are present in the cerebellum in schizophrenia, we measured NMDA receptor binding and gene expression of the NMDA receptor subunits in a post-mortem study of elderly patients with schizophrenia and non-affected subjects. Furthermore, we assessed influence of genetic variation in the candidate gene neuregulin-1 (NRG1) on the expression of the NMDA receptor in an exploratory study. Post-mortem samples from the cerebellar cortex of ten schizophrenic patients were compared with nine normal subjects. We investigated NMDA receptor binding by receptor autoradiography and gene expression of the NMDA receptor subunits NR1, NR2A, NR2B, NR2C and NR2D by in situ hybridization. For the genetic study, we genotyped the NRG1 polymorphism rs35753505 (SNP8NRG221533). Additionally, we treated rats with the antipsychotics haloperidol or clozapine and assessed cerebellar NMDA receptor binding and gene expression of subunits to examine the effects of antipsychotic treatment. Gene expression of the NR2D subunit was increased in the right cerebellum of schizophrenic patients compared to controls. Individuals carrying at least one C allele of rs35753505 (SNP8NRG221533) showed decreased expression of the NR2C subunit in the right cerebellum, compared to individuals homozygous for the T allele. Correlation with medication parameters and the animal model revealed no treatment effects. In conclusion, increased NR2D expression results in a hyperexcitable NMDA receptor suggesting an adaptive effect due to receptor hypofunction. The decreased NR2C expression in NRG1 risk variant may cause a deficit in NMDA receptor function. This supports the hypothesis of an abnormal glutamatergic neurotransmission in the right cerebellum in the pathophysiology of schizophrenia.

A comparison of the binding profiles of dextromethorphan, memantine, fluoxetine and amitriptyline: Treatment of involuntary emotional expression disorder

We compared the binding profiles of medications potentially useful in the treatment of involuntary emotional expression disorder at twenty-six binding sites in rat brain tissue membranes. Sites were chosen based on likelihood of being target sites for the mechanism of action of the agents in treating the disorder or their likelihood in producing side effects experienced by patients treated with psychoactive agents. We used radioligand binding assays employing the most selective labeled ligands available for sites of interest. Concentrations of labeled ligand were used at or below the K(i) value of the ligand for the target site. Compounds were initially screened at 1 muM. For compounds that competed for greater than 20-30% of specific binding at target sites of interest, full concentration curves were constructed. Dextromethorphan, amitriptyline and fluoxetine competed for binding to sigma(1) receptors and to serotonin transporters with high to moderate affinity. Of the target sites tested, these are the most likely to contribute to the therapeutic benefit of the various agents. In addition, all three drugs showed some activity at alpha(2) and 5-HT(1B/D) sites. Of the drugs tested, dextromethorphan bound to the fewest sites unlikely to be target sites. Although the mechanism of action of dextromethorphan or any drug that has been used in the treatment of involuntary emotional expression disorder is currently unknown, our data support that the affinity of the drug for sigma(1) receptors is consistent with its possible action through this receptor type in controlling symptoms of the disorder.

Abnormal glutamate receptor expression in the medial temporal lobe in schizophrenia and mood disorders

Pharmacological and anatomical evidence suggests that abnormal glutamate neurotransmission may be associated with the pathophysiology of schizophrenia and mood disorders. Medial temporal lobe structural alterations have been implicated in schizophrenia and to a lesser extent in mood disorders. To comprehensively examine the ionotropic glutamate receptors in these illnesses, we used in situ hybridization to determine transcript expression of N-methyl-D-aspartate (NMDA), alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionate (AMPA), and kainate receptor subunits in the medial temporal lobe of subjects with schizophrenia, bipolar disorder (BD), or major depression (MDD). We used receptor autoradiography to assess changes in glutamate receptor binding in the same subjects. Our results indicate that there are region- and disorder-specific abnormalities in the expression of ionotropic glutamate receptor subunits in schizophrenia and mood disorders. We did not find any changes in transcript expression in the hippocampus. In the entorhinal cortex, most changes in glutamate receptor expression were associated with BD, with decreased GluR2, GluR3, and GluR6 mRNA expression. In the perirhinal cortex we detected decreased expression of GluR5 in all three diagnoses, of GluR1, GluR3, NR2B in both BD and MDD, and decreased NR1 and NR2A in BD and MDD, respectively. Receptor binding showed NMDA receptor subsites particularly affected in the hippocampus, where MK801 binding was reduced in schizophrenia and BD, and MDL105,519 and CGP39653 binding were increased in BD and MDD, respectively. In the hippocampus AMPA and kainate binding were not changed. We found no changes in the entorhinal and perirhinal cortices. These data suggest that glutamate receptor expression is altered in the medial temporal lobe in schizophrenia and the mood disorders. We propose that disturbances in glutamate-mediated synaptic transmission in the medial temporal lobe are important factors in the pathophysiology of these severe psychiatric illnesses.

A brief review of the pharmacologic and therapeutic aspects of memantine in Alzheimer's disease

The past decade has seen an increase in therapeutic options for Alzheimer's disease (AD) that target neurotransmitters, such as acetylcholine, and research continues to target abnormal proteins in the AD brain. Recently, glutamate excitotoxicity has also become a target for AD treatment with the advent of memantine. Clinical trial data reviewed for memantine show good tolerability, low side-effect profiles and a positive therapeutic impact in moderate-to-severe AD, both as monotherapy and in conjunction with donepezil. However, additional data suggest variable benefits in the mild stages of AD. Furthermore, published reports support reduced dosing in patients with significant renal disease. However, the opportunity to target a second mechanism in the treatment of AD, thereby providing added symptomatic benefit, appears to be a useful consideration for clinicians who treat this devastating neurodegenerative disorder.

From pyrroles to 1-oxo-2,3,4,9-tetrahydro-1H-β-carbolines: A new class of orally bioavailable mGluR1 antagonists

Exploiting the SAR of the known pyrrole derivatives, a new class of mGluR1 antagonists was designed by replacement of the pyrrole core with an indole scaffold and consequent cyclization of the C-2 position into a tricyclic beta-carboline template. The appropriate exploration of the position C-6 with a combination of H-bond acceptor groups coupled with bulky/lipophilic moieties led to the discovery of a new series of mGluR1 antagonists. These compounds exhibited a non-competitive behavior, excellent pharmacokinetic properties, and good in vivo activity in animal models of acute and chronic pain, after oral administration.

Developmental lead (Pb) exposure reduces the ability of the NMDA antagonist MK-801 to suppress long-term potentiation (LTP) in the rat dentate gyrus, in vivo

Chronic developmental lead (Pb) exposure increases the threshold and enhances decay of long-term potentiation (LTP) in the dentate gyrus of the hippocampal formation. MK-801 and other antagonists of the N-methyl-D-aspartate (NMDA) glutamate receptor subtype impair induction of LTP. In addition, Pb exposure reduces presynaptic glutamate release and is associated with alterations in NMDA receptor expression. This study examined LTP in Pb-exposed animals challenged with a low dose of MK-801 to assess the sensitivity of this receptor to inhibition. Pregnant rats received 0.2% Pb acetate in the drinking water beginning on gestational day 16, and this regimen was continued through lactation. Adult male offspring maintained on this solution from weaning were prepared with indwelling electrodes in the perforant path and dentate gyrus. Several weeks later, input/output (I/O) functions were collected in awake animals before and after saline or MK-801 administration (0.05 mg/kg, s.c.). LTP was induced using suprathreshold train stimuli 60 min post-drug. Post-train I/O functions were reassessed 1 and 24 h after train delivery. Upon full decay of any induced LTP, drug conditions were reversed such that each animal was tested under saline and MK-801. I/O functions measured 1 and 24 h after train induction as well as immediate post-train responses revealed significant LTP of comparable magnitude that was induced in both control and Pb-exposed animals tested under saline conditions. In contrast, MK-801 reduced LTP in control but not in Pb-exposed animals. The broadening of the excitatory postsynaptic potential evident in responses evoked by train stimuli is NMDA-dependent. Pb exposure attenuated the MK-801-induced reduction in area of this NMDA component by approximately 50%. These findings are consistent with other neurochemical and behavioural observations and suggest that up-regulation of postsynaptic NMDA receptors produces subsensitivity to the inhibitory effects of MK-801 on hippocampal LTP following chronic developmental Pb exposure.

In vitro and in vivo characterization of [3H]CNS-5161—A use-dependent ligand for theN-methyl-d-aspartate receptor in rat brain

Glutamate is the major excitatory neurotransmitter in the brain. Glutamate activation of the N-methyl-D-aspartate (NMDA) receptor subtype is thought to mediate important physiological and pathological processes, including memory formation and excitotoxicity. The goal of the present work was to characterize and validate a candidate agent for noninvasive positron emission tomography (PET) imaging of this receptor. [(3)H]-labeled N-[3-(3)H]-methyl-3-(thiomethylphenyl)cyanamide (CNS-5161) was incubated with rat brain homogenates at increasing concentrations, temperatures, and times to establish the binding kinetics and affinity of the ligand in vitro. Nonspecific binding was measured with 100 microM MK-801. The compound was also injected i.v. in rats pretreated with saline, NMDA, MK801, or a combination, and organ and brain regional uptake was assessed at various times after injection by autoradiography or dissection. Blood and brain samples were assayed for metabolites by high-performance liquid chromatography. CNS-5161 binds brain membranes with high affinity (K(d) < 4 nM) and fast association and dissociation kinetics. Specific binding increased in the presence of glutamate and glycine. Intravenous administration in control rats resulted in a heterogeneous brain distribution with hippocampus and cortex > thalamus > striatum > cerebellum, and a cortex/cerebellum ratio of 1.4. Pretreatment with NMDA increased the hippocampus-to-cerebellum ratio to 1.6-1.9 while MK801 abolished this increase, resulting in ratios close to 1. Thus, CNS-5161 binds preferentially to the activated state of the NMDA receptor channel in vitro and in vivo. The high affinity and fast kinetics make it compatible with PET imaging of a carbon-11 labeled CNS-5161.

Absolute oral bioavailability of traxoprodil in cytochrome P450 2D6 extensive and poor metabolisers

BACKGROUND

Traxoprodil, a substituted 4-phenylpiperidine, is an N-methyl-D-aspartate (NMDA) receptor antagonist that is selective for receptors containing the NR2B subunit. In vivo and in vitro studies examining the disposition of traxoprodil have demonstrated that it is mainly metabolised by cytochrome P450 (CYP) 2D6, a major drug-metabolising enzyme that exhibits a genetic polymorphism.

OBJECTIVE

To assess the single-dose absolute oral bioavailability of traxoprodil in healthy male volunteers phenotyped as either CYP2D6 extensive or poor metabolisers.

METHODS

This was an open-label, three-way crossover study. Traxoprodil was administered as a single dose orally in solution of 50, 100 and 300mg and intravenously as a constant rate 2-hour infusion of 50 and 100mg. CYP2D6 phenotype was assigned following single-dose dextromethorphan administration.

RESULTS

In poor metabolisers (n = 6), oral bioavailability was approximately 80% and was consistent with a liver extraction ratio of approximately 20% (plasma clearance of approximately 4 mL/min/kg) indicating near complete absorption. Following intravenous administration, the mean volume of distribution at steady state (V(ss)) was moderate (approximately 6.5 L/kg) and the mean elimination half-life (t((1/2))) was approximately 20 hours. Following oral administration the mean maximum plasma concentration (C(max)) and area under the plasma concentration-time curve from time zero to infinity (AUC(infinity)) increased approximately proportionally with dose. In extensive metabolisers (n = 11), oral bioavailability was dose-dependent and nonlinear. At the 100mg dose, the absolute oral bioavailability was approximately 39.5%. Overall, the oral bioavailability ranged from 22.8% to 62.1% and its estimation was confounded by large differences in plasma concentrations at oral doses without equivalent intravenous doses. Following intravenous administration, plasma clearance was high (approximately 27 mL/min/kg), the V(ss) was moderate (approximately 4 L/Kg) and the t((1/2)) was approximately 2-4 hours. Following oral administration the C(max) and AUC(infinity) increased more than proportionally with dose. Apparent oral clearance decreased with increasing oral dose. However, t((1/2)) was approximately the same at all doses (approximately 4 hours).

CONCLUSION

The pharmacokinetics of traxoprodil were quite different in the two phenotypes. In extensive metabolisers, the oral bioavailability was nonlinear and dose-dependent, while in poor metabolisers, oral bioavailability appeared to be linear and dose-independent. Based on the pharmacokinetics in extensive and poor metabolisers, the nonlinear oral bioavailability in extensive metabolisers may be attributed to saturation of hepatic first-pass CYP2D6 metabolism. Thus, at a high oral dose, the impact of CYP2D6 metabolism on traxoprodil pharmacokinetics is minimal.

Pharmacokinetics and disposition of memantine in the arterially perfused bovine eye

PURPOSE

To develop an improved ((1)This is to clearly acknowledge that we have tried to improve an existing model.) arterially perfused bovine eye model and investigate the general ocular disposition of memantine.

MATERIALS AND METHODS

Fresh bovine eyes were prepared by exposing and cannulating one ciliary artery, placing the eye into a perfusion chamber and slowly increasing the rate of perfusion to 1.0 ml/min. Analysis of the arterial perfusion pressure (APP), intraocular pressure (IOP), venous perfusate for glucose consumption and lactate dehydrogenase (LDH) activity, and histopathology ensured viability. Memantine was administered with the perfusate (simulated systemic access), by an intravitreal injection and by topical infusion. At the appropriate time points, the cornea, aqueous humour, sclera, iris-ciliary body, choroid/RPE, retina and vitreous humour were harvested and analysed for memantine.

RESULTS

The preparation remained viable for at least 9 h. At this time, histopathological examination showed mild to moderate deterioration of retinal layers. However, all retinal layers remained well defined and the integrity of the inner limiting membrane and Bruch's membrane were preserved. Glucose consumption, LDH levels and constant APP and IOP showed that correct cannulation and viability was maintained. After administration, memantine accumulated in the melanin rich iris-ciliary body and choroid/RPE. Results following topical administration indicate that substantial concentrations of memantine are present in the retina and choroid/RPE.

CONCLUSIONS

The arterial perfused bovine eye system proved to be a useful system for ocular drug delivery studies. The experimental results indicate that memantine will accumulate in the posterior segment when delivered by the topical route and that melanin-binding may support sustaining significant concentrations in the retina.

Effect of δ-aminolevulinic acid and vitamin E treatments on the N-methyl-d-aspartate receptor at different ages in the striatum of rat brain

We report the effects of the chronic treatments with the oxidant agent delta-aminolevulinic acid (ALA) and with the antioxidant vitamin E on the N-methyl-D-aspartate (NMDA) receptors in the striatum of 4-, 12- and 24-month-old male Wistar rats. ALA and vitamin E were administered daily for 15 days (40 mg/kg i.p. and 20 mg/kg i.p. respectively). NMDA receptors were labeled by membrane homogenate binding, using tritiated dizocilpine ([3H]MK-801). [3H]MK-801 binding in the striatum was significantly decreased at all ages in ALA-treated rats with respect to their controls, and in contrast, was significantly increased at all ages when rats received the treatment with vitamin E. Western blot assays were performed using antibodies against the NR2A subunit, a NMDA receptor subunit widely distributed in the brain. We did not find significant differences in the amounts of NR2A in rats treated with either ALA or vitamin E with respect to those rats not treated. We conclude that the NMDA receptor densities in the rat striatum are modified by the chronic treatment with oxidants and antioxidants in an age-independent way, at least until 24 months. Also, our results support the notion that NR2A is not involved in these modifications.

Evaluation of ketamine systemic absorption from topical preparations. Short Communication

The aim of this study was to evaluate the systemic absorption of the topically administered ketamine using different vehicles and additives, in order to develop a transdermal therapeutic system (TTS) of this drug. After the application of different ketamine preparations (1% in hydrogel, o/w cream, or organogel) the ketamine appeared in the blood. The lowest level could be observed with o/w cream, while the highest concentration was achieved by means of the hydrogel system, however this difference was not significant. Further studies are going to be performed with higher drug concentrations for the characterization of the differences in the pharmacodynamics of the drug with different vehicles and to evaluate the correlation between the in vitro and in vivo absorption.

Disrupted muscarinic M1 receptor signaling correlates with loss of protein kinase C activity and glutamatergic deficit in Alzheimer's disease

There are few studies on the clinical and neurochemical correlates of postsynaptic cholinergic dysfunction in Alzheimer's disease (AD). We have previously found that attenuation of guanine nucleotide-binding (G-) protein coupling to muscarinic M(1) receptors in the neocortex was associated with dementia severity. The present study aims to study whether this loss of M(1)/G-protein coupling is related to alterations in signaling kinases and NMDA receptors. Postmortem frontal cortices of 22 AD subjects and 12 elderly controls were obtained to measure M(1) receptors, M(1)/G-protein coupling, NMDA receptors as well as protein kinase C (PKC) and Src kinase activities. We found that the extent of M(1)/G-protein coupling loss was correlated with reductions in PKC activity and NMDA receptor density. In contrast, Src kinase activity was neither altered nor associated with M(1)/G-protein coupling. Given the well established roles of neuronal PKC signaling and NMDA receptor function in cognitive processes, our results lend further insight into the mechanisms by which postsynaptic cholinergic dysfunction may underlie the cognitive features of AD, and suggest alternative therapeutic targets to cholinergic replacement.

Rationale for Combination Therapy With Galantamine and Memantine in Alzheimer's Disease

A combination of cholinergic and glutamatergic dysfunction appears to underlie the symptomatology of Alzheimer's disease. Therefore, one hypothesis is that treatment strategies should address impairments in both systems. Galantamine is an acetylcholinesterase inhibitor that, unlike other acetylcholinesterase inhibitors, has a postulated dual mode of action as a nicotinic receptor modulator. Galantamine has demonstrated long-term efficacy in improving or maintaining cognition, functionality, and behavior in patients with mild to moderate Alzheimer's disease. Memantine, a noncompetitive N-methyl-D-aspartate-receptor antagonist, reduces deterioration in cognition and function in patients with moderate to severe Alzheimer's disease. Pharmacokinetic and pharmacodynamic as well as ongoing observation studies support the concept of adjunctive therapy with memantine in patients with advanced moderate Alzheimer's disease currently treated with an established galantamine regimen. The potential to modulate both acetylcholine and glutamate pathways in Alzheimer's disease presents a novel treatment strategy for the management of mild to moderately severe Alzheimer's disease.

Synthesis, in vitro pharmacology, and structure–activity relationships of 2-aminobicyclo[3.1.0]hexane-2,6-dicarboxylic acid derivatives as mGluR2 antagonists

Chemical modification of the bicyclo[3.1.0]hexane ring C-3 position led to the discovery of 3-alkoxy-2-aminobicyclo[3.1.0]hexane-2,6-dicarboxylic acid, 3-benzylthio-, and 3-benzylamino-2-amino-6-fluorobicyclo[3.1.0]hexane-2,6-dicarboxylic acid derivatives, metabotropic glutamate receptor 2 (mGluR2) antagonists. In particular, 3-(3,4-dichlorobenzyloxy)-2-aminobicyclo[3.1.0]hexane-2,6-dicarboxylic acid (15ae), (1R,2S,5R,6R)-2-amino-3-(3,4-dichlorobenzylthio)-6-fluorobicyclo[3.1.0]hexane-2,6-carboxylic acid (15at), and (1R,2S,5R,6R)-2-amino-3-(N-(3,4-dichlorobenzylamino))-6-fluorobicyclo[3.1.0]hexane-2,6-carboxylic (15ba) showed high affinity for the mGluR2 receptor (15ae: K(i) = 2.51 nM, 15at: K(i) = 1.96 nM, and 15ba: K(i) = 3.29 nM) and potent antagonist activity for mGluR2 (15ae; IC50 = 34.21 nM, 15at; IC50 = 13.34 nM, and 15ba; IC50 = 35.96 nM). No significant agonist activity for mGluR2 was observed with 15ae, 15at, or 15ba. This paper reports on the synthesis, in vitro pharmacological profile, and structure-activity relationships (SARs) of 3-substituted-2-aminobicyclo[3.1.0]hexane-2,6-dicarboxylic acid.

Randomized, controlled trial of dextromethorphan/quinidine for pseudobulbar affect in multiple sclerosis

OBJECTIVE

To evaluate the efficacy and safety of DM/Q (capsules containing dextromethorphan [DM] and quinidine [Q]) compared with placebo, taken twice daily, for the treatment of pseudobulbar affect over a 12-week period in multiple sclerosis patients.

METHODS

A total of 150 patients were randomized in a double-blind, placebo-controlled study to assess pseudobulbar affect with the validated Center for Neurologic Study-Lability Scale. Each patient also recorded the number of episodes experienced between visits, estimated quality of life and quality of relationships on visual analog scales, and completed a pain rating scale.

RESULTS

Patients receiving DM/Q had greater reductions in Center for Neurologic Study-Lability Scale scores than those receiving placebo (p < 0.0001) at all clinic visits (days 15, 29, 57, and 85). All secondary end points also favored DM/Q, including the number of crying or laughing episodes (p <or= 0.0077), quality of life (p < 0.0001), quality of relationships (p = 0.0001), and pain intensity score (p = 0.0271). DM/Q was well tolerated; only dizziness occurred with greater frequency than with placebo.

INTERPRETATION

Results in multiple sclerosis patients were similar to those of a previous study in amyotrophic lateral sclerosis, demonstrating that DM/Q may be beneficial in treating potentially disabling pseudobulbar affect in a variety of neurological disorders.

Glutamate AMPA receptors change in motor neurons of SOD1G93A transgenic mice and their inhibition by a noncompetitive antagonist ameliorates the progression of amytrophic lateral sclerosis-like disease

Amyotrophic lateral sclerosis (ALS) is a fatal neurological disorder involving the selective degeneration of motor neurons. In a small proportion of patients, ALS is caused by mutations in copper/zinc superoxide dismutase (SOD1), and mice overexpressing SOD1(G93A) mutant develop a syndrome that closely resembles the human disease. Excitotoxicity mediated by glutamate AMPA receptors has been suggested to be implicated in the selective susceptibility of motor neurons occurring in ALS. In SOD1(G93A) mice, we found that levels of GluR2 AMPA subunit, which plays a pivotal role in the maintenance of calcium impermeability of AMPA receptors, are decreased in spinal motor neurons before symptom onset in concomitance with a modest increase of GluR3 expression, a calcium-permeable AMPA subunit. This effect can result in a higher number of calcium-permeable AMPA receptors on motor neurons of SOD1(G93A) mice, predisposing these cells to be injured by AMPA-mediated glutamate firing. In support of this, we showed that treatment with a new noncompetitive AMPA antagonist, ZK 187638, partially protected motor neurons, improved motor function, and prolonged the survival of SOD1(G93A) mice.

Behavioral effects of ketamine and toxic interactions with psychostimulants

Background

The anesthetic drug ketamine (KT) has been reported to be an abused drug and fatal cases have been observed in polydrug users. In the present study, considering the possibility of KT-enhanced toxic effects of other drugs, and KT-induced promotion of an overdose without making the subject aware of the danger due to the attenuation of several painful subjective symptoms, the intraperitoneal (i.p.) KT-induced alterations in behaviors and toxic interactions with popular co-abused drugs, the psychostimulants cocaine (COC) and methamphetamine (MA), were examined in ICR mice.

Results

A single dose of KT caused hyperlocomotion in a low (30 mg/kg, i.p.) dose group, and hypolocomotion followed by hyperlocomotion in a high (100 mg/kg, i.p.) dose group. However, no behavioral alterations derived from enhanced stress-related depression or anxiety were observed in the forced swimming or the elevated plus-maze test. A single non-fatal dose of COC (30 mg/kg, i.p.) or MA (4 mg/kg, i.p.) caused hyperlocomotion, stress-related depression in swimming behaviors in the forced swimming test, and anxiety-related behavioral changes (preference for closed arms) in the elevated plus-maze test. For the COC (30 mg/kg) or MA (4 mg/kg) groups of mice simultaneously co-treated with KT, the psychostimulant-induced hyperlocomotion was suppressed by the high dose KT, and the psychostimulant-induced behavioral alterations in the above tests were reversed by both low and high doses of KT. For the toxic dose COC (70 mg/kg, i.p.)- or MA (15 mg/kg, i.p.)-only group, mortality and severe seizures were observed in some animals. In the toxic dose psychostimulant-KT groups, KT attenuated the severity of seizures dose-dependently. Nevertheless, the mortality rate was significantly increased by co-treatment with the high dose KT.

Conclusion

Our results demonstrated that, in spite of the absence of stress-related depressive and anxiety-related behavioral alterations following a single dose of KT treatment, and in spite of the KT-induced anticonvulsant effects and attenuation of stress- and anxiety-related behaviors caused by COC or MA, the lethal effects of these psychostimulants were increased by KT.

Influence of the N-acetylation polymorphism on the metabolism of talampanel: an investigation in fasted and fed subjects genotyped for NAT2 variants

Talampanel is a 2,3-benzodiazepine-type allosteric (noncompetitive) AMPA-antagonist currently being developed as an orally active, broad-spectrum anticonvulsant. Here, a detailed study of its N-acetylation in humans is presented using plasma concentration data of both TLP and its N-acetyl metabolite obtained from healthy volunteers (n = 28) genotyped for N-acetyltansferase NAT2 isozymes. Plasma samples were obtained for up to 48 h after a single oral dose of 75 mg TLP both in fasted and in fed subjects. A perfect correspondence could be established between the phenotype inferred before the study from genotyping and that determined after the study by using plasma metabolite-to-parent molar ratios confirming that this route of metabolism is indeed mediated by NAT2. Analysis of the data has been performed using both noncompartmental analysis and a custom-built, unified parent-metabolite PK model, which incorporates three different acetylation rates according to the genotype-based classification of each subject as slow, intermediate, or fast acetylator to simultaneously fit plasma levels for both TLP and its metabolite. This suggest that for TLP in humans, (i) N-acetylation represents only a relatively small fraction of its total elimination (about one-fourth in fast acetylators and much less in slow acetylators), (ii) acetylation is about eight-twelve times faster in fast and three-six times faster in intermediate acetylators than in slow acetylators, and (iii) the N-acetyl metabolite is eliminated faster than the parent TLP.

Differential expression of NMDA and AMPA receptor subunits in rat dorsal and ventral hippocampus

Several studies have demonstrated anatomical and functional segregation along the dorsoventral axis of the hippocampus. This study examined the possible differences in the AMPA and NMDA receptor subunit composition and receptor binding parameters between dorsal and ventral hippocampus, since several evidence suggest diversification of NMDA receptor-dependent processes between the two hippocampal poles. Three sets of rat dorsal and ventral hippocampus slices were prepared: 1) transverse slices for examining a) the expression of the AMPA (GluRA, GluRB, GluRC) and NMDA (NR1, NR2A, NR2B) subunits mRNA using in situ hybridization, b) the protein expression of NR2A and NR2B subunits using Western blotting, and c) by using quantitative autoradiography, c(1)) the specific binding of the AMPA receptor agonist [(3)H]AMPA and c(2)) the specific binding of the NMDA receptor antagonist [(3)H]MK-801, 2) longitudinal slices containing only the cornus ammonis 1 (CA1) region for performing [(3)H]MK-801 saturation experiments and 3) transverse slices for electrophysiological measures of NMDA receptor-mediated excitatory postsynaptic potentials. Ventral compared with dorsal hippocampus showed for NMDA receptors: 1) lower levels of mRNA and protein expression for NR2A and NR2B subunits in CA1 with the ratio of NR2A /NR2B differing between the two poles and 2) lower levels of [(3)H]MK-801 binding in the ventral hippocampus, with the lowest value observed in CA1, apparently resulting from a decreased receptor density since the B(max) value was lower in ventral hippocampus. For the AMPA receptors CA1 our results showed in ventral hippocampus compared with dorsal hippocampus: 1) lower levels of mRNA expression for GluRA, GluRB and GluRC subunits, which were more pronounced in CA1 and in dentate gyrus region and 2) lower levels of [(3)H]AMPA binding. Intracellular recordings obtained from pyramidal neurons in CA1 showed longer NMDA receptor-mediated excitatory postsynaptic potentials in ventral hippocampus compared with dorsal hippocampus. In conclusion, the differences in the subunit mRNA and protein expression of NMDA and AMPA receptors as well as the lower density of their binding sites observed in ventral hippocampus compared with dorsal hippocampus suggest that the glutamatergic function differs between the two hippocampal poles. Consistently, the lower value of the ratio NR2A/NR2B seen in the ventral part would imply that the ventral hippocampus NMDA receptor subtype is functionally different than the dorsal hippocampus subtype, as supported by our intracellular recordings. This could be related to the lower ability of ventral hippocampus for long-term synaptic plasticity and to the higher involvement of the NMDA receptors in the epileptiform discharges, observed in ventral hippocampus compared with dorsal hippocampus.

Impaired maze performance in aged rats is accompanied by increased density of NMDA, 5-HT1A, and α-adrenoceptor binding in hippocampus

Using quantitative receptor autoradiography, we assessed binding site densities and distribution patterns of glutamate, GABA(A), acetylcholine (ACh), and monoamine receptors in the hippocampus of 32-month-old Fischer 344/Brown Norway rats. Prior to autoradiography, the rats were divided into two groups according to their retention performance in a water maze reference memory task, which was assessed 1 week after 8 days of daily maze training. The animals of the inferior group showed less long-term retention of the hidden-platform task but did not differ from superior rats in their navigation performance during place training and cued trials. The decreased retention performance in the group of inferior learners was primarily accompanied by increased alpha(1)-adrenoceptors in all hippocampal subregions under inspection (CA1-CA4 and dentate gyrus), while elevated alpha(2)-adrenoceptor binding was observed in the CA1 region and DG. Furthermore, inferior learners had higher NMDA binding in the CA2 and CA4 and increased 5-HT(1A) binding sites in the CA2, CA3, and CA4 region. No significant differences between inferior and superior learners were evident with regard to AMPA, kainate, GABA(A), muscarinergic M(1), dopamine D(1), and 5-HT(2) binding densities in any hippocampal region analyzed. These results show that increased NMDA, 5-HT(1A), and alpha-adrenoceptor binding in the hippocampus is associated with a decline in spatial memory. The increased receptor binding observed in the group of old rats with inferior maze performance might be the result of neural adaptation triggered by age-related changes in synaptic connectivity and/or synaptic activity.

IN VITRO AND IN VIVO EVALUATION OF THE METABOLISM AND BIOAVAILABILITY OF ESTER PRODRUGS OF MGS0039 (3-(3,4-DICHLOROBENZYLOXY)-2-AMINO-6-FLUOROBICYCLO[3.1.0]HEXANE-2,6-DICARBOXYLIC ACID), A POTENT METABOTROPIC GLUTAMATE RECEPTOR ANTAGONIST

MGS0039 (3-(3,4-dichlorobenzyloxy)-2-amino-6-fluorobicyclo-[3.1.0]hexane-2,6-dicarboxylic acid) has been identified as a potent and selective antagonist for metabotropic glutamate receptors. However, the oral bioavailability of MGS0039 is 10.9% in rats, due to low absorption. Several prodrugs, synthesized to improve absorption, exhibited 40 to 70% bioavailability in rats. This study investigated in vitro metabolism using liver S9 fractions from both cynomolgus monkeys and humans and oral bioavailability in cynomolgus monkeys to select the prodrug most likely to exhibit optimal pharmacokinetic profiles in humans. In monkeys, transformation to active substance was observed (5.9-72.8%) in liver S9 fractions, and n-butyl, n-pentyl, 3-methylbutyl, and 4-methylpentyl ester prodrugs exhibited high transformation ratios (>64%). Cmax levels and F values after oral dosing increased to 4.1- to 6.3-fold and 2.4- to 6.3-fold, respectively, and a close relationship between transformation ratios and Cmax and F values was observed, indicating that the hydrolysis rate in liver S9 fractions is the key factor in determining oral bioavailability in monkeys. In humans, n-hexyl, n-heptyl, n-octyl, 5-methylbutyl, and 6-methylpentyl ester prodrugs exhibited high transformation ratios (>65%) in liver S9 fractions. With these prodrugs, n-hexyl, n-heptyl, and 5-methylpentyl ester, almost complete recovery (96-99%) was obtained. Given the transformation ratio, we anticipated that the n-heptyl alkyl ester prodrug would exhibit the highest oral bioavailability of active substances in humans, if the hydrolysis rate in liver S9 fractions is indeed the key factor in determining oral bioavailability in humans. On this basis, MGS0210 (3-(3,4-dichlorobenzyloxy)-2-amino-6-fluorobicyclo[3.1.0]hexane-2,6-dicarboxylic acid n-heptyl ester) seems to be a promising candidate among MGS0039 prodrugs.

Effect of δ-aminolevulinic acid treatment on N-methyl-d-aspartate receptor at different ages in the rat brain

We report here the effects of the chronic treatment with the oxidant agent delta-aminolevulinic acid (ALA) on the N-methyl-D-aspartate (NMDA) receptors in 4-, 12- and 24-month-old male Wistar rats. ALA was administered daily for 15 days (40 mg/kg i.p). The study was performed by membrane homogenate binding and autoradiography, using tritiated 5-methyl-10, 11-dihydro-5H-dibenzo(a,d)cycloheptan-5,10-imine maleate ([3H]MK-801). [3H]MK-801 binding was significantly decreased in most areas studied (cortex and hippocampus) at all ages in treated rats with respect to their controls. Furthermore, Western blot assays were performed using antibodies against the NMDA receptor NR2A subunit, which is widely distributed in the brain, mainly in cortex and hippocampus. In cortex but not in hippocampus, the ALA treatment induced significant decreases in the amounts of NR2A subunit in 12- and 24-month-old animals. We conclude that chronic treatment with ALA is able to induce NMDA receptor decreases in an age-independent way and that NR2A subunit seems to be involved in these decreases in cerebral cortex, but not in the other structures studied.

Cyclohexenyl- and dehydropiperidinyl-alkynyl pyridines as potent metabotropic glutamate subtype 5 (mGlu5) receptor antagonists

Structure-activity relationship studies leading to the discovery of novel mGlu5 receptor antagonists are described. These compounds show high in vitro potency, have good in vivo receptor occupancy, and a reasonable intravenous pharmacokinetic profile.

Dipyridyl amines: Potent metabotropic glutamate subtype 5 receptor antagonists

Modulation of the metabotropic glutamate subtype 5 (mGlu5) receptor may be useful in the treatment of a variety of central nervous system disorders. Herein, we report on the discovery, synthesis, and biological evaluation of dipyridyl amines as small molecule mGlu5 antagonists.

Ketamine produces lasting disruptions in encoding of sensory stimuli

The current study analyzed the acute, chronic, and lasting effects of ketamine administration in four inbred mouse strains (C3H/HeHsd, C57BL/6Hsd, FVB/Hsd, and DBA/2Hsd) to evaluate vulnerability to ketamine as a drug of abuse and as a model of schizophrenia. Serum half-life of ketamine was similar between all strains (approximately 13 min). Also, the ratio of brain-to-serum ketamine levels was 3:1. Examination of multiple phases of auditory processing using auditory-evoked potentials (AEPs) following acute ketamine (0, 5, and 20 mg/kg) treatment revealed C3H/HeHsd mice to be most vulnerable to ketamine-induced alterations in AEPs, whereas FVB/Hsd mice exhibited the least electrophysiological sensitivity to ketamine. Overall, the precortical P1-evoked potential component increased in amplitude and latency, whereas the cortically generated N1 and P2 components decreased in amplitude and latency following acute ketamine across all strains. Brain catecholamine analyses indicated that ketamine decreased hippocampus epinephrine levels in C3H/HeHsd but elevated hippocampus epinephrine levels in FVB/Hsd, suggesting one potential mechanism for AEP vulnerability to ketamine. Based on results of the acute study, the immediate and lasting effects of chronic low-dose ketamine on AEPs were examined among C3H/HeHsd (sensitive) and FVB/Hsd (insensitive) mice. We observed a decrement of the N1 amplitude that persisted at least 1 week after the last exposure to ketamine across both strains. This lasting deficit in information processing occurred in the absence of acute changes among the FVB/Hsd mice. Implications for both ketamine abuse and N-methyl-D-aspartate hypofunction models of schizophrenia are discussed.

Ionotropic glutamate receptor binding in the posterior cingulate cortex in schizophrenia patients

Using quantitative autoradiography, the present study examined ionotropic glutamatergic receptor binding sites using [3H]dizocilpine, [3H]alpha-amino-3-hydroxy-5-methylisoxazole-4-propionate, and [3H]kainate in the posterior cingulate cortex of schizophrenia patients and matched controls. We found a significant increase in [3H]dizocilpine binding in the superficial layers (41%, p<0.001) and deep layers (30%, p=0.004) of the posterior cingulate cortex in the schizophrenia group compared with controls. No significant differences were observed in [3H]alpha-amino-3-hydroxy-5-methylisoxazole-4-propionate and [3H]kainate binding. In summary, the present study has for the first time demonstrated that the glutamatergic system is affected in the posterior cingulate cortex in schizophrenia patients. The fact that only the N-methyl-D-aspartate receptor densities are significantly altered suggests that this is unlikely to be caused by a simple decrease in glutamatergic transmission.

Cortical glutamatergic markers in schizophrenia

Post-mortem studies have yet to produce consistent findings on cortical glutamatergic markers in schizophrenia; therefore, it is not possible to fully understand the role of abnormal glutamatergic function in the pathology of the disorder. To better understand the changes in cortical glutamatergic markers in schizophrenia, we measured the binding of radioligands to the ionotropic glutamate receptors (N-methyl D-aspartate, [3H]CGP39653, [3H]MK-801), amino-3-hydroxy-5-methyl-4-isoxazole ([3H]AMPA), kainate ([3H]kainate), and the high-affinity glutamate uptake site ([3H]aspartate) using in situ radioligand binding with autoradiography and levels of mRNA for kainate receptors using in situ hybridization in the dorsolateral prefrontal cortex from 20 subjects with schizophrenia and 20 controls matched for age and sex. Levels of [3H]kainate binding were significantly decreased in cortical laminae I-II (p = 0.01), III-IV (p < 0.05), and V-VI (p < 0.01) from subjects with schizophrenia. By contrast, levels of [3H]MK-801, [3H]AMPA, [3H]aspartate, or [3H]CGP39653 binding did not differ between the diagnostic cohorts. Levels of mRNA for the GluR5 subunit were decreased overall (p < 0.05), with no changes in levels of mRNA for GluR6, GluR7, KA1, or KA2 in tissue from subjects with schizophrenia. These data indicate that the decreased number of kainate receptors in the dorsolateral prefrontal cortex in schizophrenia may result, in part, from reduced expression of the GluR5 receptor subunits.

Intravenous dextromethorphan to human volunteers: relationship between pharmacokinetics and anti-hyperalgesic effect

The aim of this study was to investigate the effect of dextromethorphan (DM) 0.5 mg/kg administered intravenously (i.v.) on hyperalgesia and pain after a tissue injury in human volunteers, and to describe the relationship between pharmacokinetic and pharmacodynamic data. The heat-capsaicin sensitisation model, a well-established experimental hyperalgesia model was induced in 24 healthy, male volunteers aged 21-35 years. The subjects received i.v. DM 0.5 mg/kg or isotonic saline on two separate study sessions. The primary outcome measure from 0 to 3 h was reduction in area of established secondary hyperalgesia. Secondary outcome measures were reduction in area of secondary hyperalgesia in response to brief thermal stimulation, heat pain detection thresholds and painfulness after tonic heat pain. Blood samples were collected throughout the study to describe the relationship between pharmacokinetic and pharmacodynamic data. Intravenous DM 0.5 mg/kg significantly reduced areas of established secondary hyperalgesia with an average of 39% (P<0.05). Development of secondary hyperalgesia was substantially prevented by DM (P<0.05). No significant effect was seen on either heat pain detection thresholds or after tonic heat pain. The pharmacokinetic-pharmacodynamic relationship showed a large inter-subject variation with a mean delay in effect of nearly 2 h in relation to peak serum concentration. The results strongly indicate that DM is an anti-hyperalgesic drug. The delay in effect may be explained by several mechanisms and suggests that timing of DM administration is an essential factor for using the drug in clinical settings.

In vitro and in vivo percutaneous absorption of topical dosage forms: case studies

This article evaluated the influence of vehicle compositions on topical drug availability. In vitro drug release and in vivo experiments were performed in case of the hydrophilic ketamine hydrochloride and the lipophilic piroxicam. Ketamine hydrochloride is a NMDA receptor antagonist that has been useful for anesthesia and analgesia. The study of transdermal ketamine delivery is a novelty, because nobody has investigated the hypnotic effects of ketamine after this administration route. In vitro measurements gave a good basis for screening among the developed products. The physiological changes after ketamine administration showed, that there were significant differences among the parameters tested (breathing rate, duration of sleep) from the developed products (hydrogel, lyotropic liquid crystal and o/w cream) compared to the reference product (Carbopol gel). The in vivo feedback for piroxicam was the measurement of the anti-inflammatory activity by edema inhibition percentage. Significant differences were measured in case of the developed systems compared to the reference.

Effect of vitamin E treatment on N-methyl-D-aspartate receptor at different ages in the rat brain

A comparative study using membrane homogenate binding, autoradiography, and Western blot assays was carried out to determine the age-related changes in N-methyl-D-aspartate (NMDA) receptors in 4-, 12- and 24-month-old male Wistar rats, treated or not with vitamin E. Vitamin E treatment was 20 mg/kg i.p. daily for 15 days. [(3)H] 5-methyl-10,11-dihydro-5H-dibenzo (a,d) cycloheptan-5,10-imine maleate (MK-801) binding was significantly increased in all areas studied (cortex and hippocampus) at all ages when rats received this treatment. A Western blot study in vitamin-E-treated rats and their controls did not reveal significant differences in the amounts of NR2A, an NMDA receptor subunit widely distributed in the brain mainly in cortex and hippocampus. We conclude that the effect of vitamin E on NMDA receptors is largely age independent. Previous reports and our data have described the presence of age-dependent NMDA receptor changes. The effect of vitamin E in aging is considered to be mediated by free radical scavenging, but from our data, we conclude that this mechanism is not relevant for age-dependent NMDA receptor changes. Our results also support that age or vitamin E treatment have no relevant effects on NR2A subunit, at least until 24 months in rats.

Mechanistic Pharmacokinetic and Pharmacodynamic Modeling of CHF3381 (2-[(2,3-Dihydro-1H-inden-2-yl)amino]acetamide Monohydrochloride), a Novel N-Methyl-d-aspartate Antagonist and Monoamine Oxidase-A Inhibitor in Healthy Subjects

CHF3381 (2-[(2,3-dihydro-1H-inden-2-yl)amino]acetamide monohydrochloride) is a new N-methyl-D-aspartate antagonist and reversible monoamine oxidase-A (MAO-A) inhibitor in development for the treatment of neuropathic pain. This study developed a mechanistic model to describe the pharmacokinetics of CHF3381 and of its two metabolites, the relationship with MAO-A activity and heart rate. Doses of 100, 200, and 400 mg twice daily for 2 weeks were administered orally to 36 subjects. MAO-A activity was estimated by measuring concentrations of 3,4-dihydroxyphenylglycol (DHPG), a stable metabolite of norepinephrine. A multicompartment model with time-dependent clearance was used to describe the kinetics of CHF3381 and metabolite concentrations. Estimated pharmacokinetic parameters were CL (41.2 to 27.4 l/h over the study), V (131 liters), Q (1.7 l/h), V(p) (36 liters), and k(a) (1.85 h(-1)). The relationship between CHF3381 and DHPG or heart rate was described using an indirect or a direct linear model, respectively. The production rate of DHPG (k(in)) was 2540 ng . h(-1), reduced by 63% at maximal CHF3381 concentrations. EC(50) was 1670 mug/l, not significantly different from the in vitro IC(50). The increase in heart rate due to CHF3381 was 0.0055 bpm/micro(g l-1). CHF3381 produces a concentration-dependent decrease in DHPG plasma concentrations, whose magnitude increased after multiple twice-a-day regimens for 14 days.

Synthesis, characterization, and first successful monkey imaging studies of metabotropic glutamate receptor subtype 5 (mGluR5) PET radiotracers

Three metabotropic glutamate receptor subtype 5 (mGluR5) PET tracers have been labeled with either carbon-11 or fluorine-18 and their in vitro and in vivo behavior in rhesus monkey has been characterized. Each of these tracers share the common features of high affinity for mGluR5 (0.08-0.23 nM vs. rat mGluR5) and moderate lipophilicity (log P 2.8-3.4). Compound 1b was synthesized using a Suzuki or Stille coupling reaction with [11C]MeI. Compounds 2b and 3b were synthesized by a SNAr reaction using a 3-chlorobenzonitrile precursor. Autoradiographic studies in rhesus monkey brain slices using 2b and 3b showed specific binding in cortex, caudate, putamen, amygdala, hippocampus, most thalamic nuclei, and lower binding in the cerebellum. PET imaging studies in monkey showed that all three tracers readily enter the brain and provide an mGluR5-specific signal in all gray matter regions, including the cerebellum. The specific signal observed in the cerebellum was confirmed by the autoradiographic studies and saturation binding experiments that showed tracer binding in the cerebellum of rhesus monkeys. In vitro metabolism studies using the unlabeled compounds showed that 1a, 2a, and 3a are metabolized slower by human liver microsomes than by monkey liver microsomes. In vivo metabolism studies showed 3b to be long-lived in rhesus plasma with only one other more polar metabolite observed.

4-Alkylated homoibotenic acid (HIBO) analogues: Versatile pharmacological agents with diverse selectivity profiles towards metabotropic and ionotropic glutamate receptor subtypes

4-Alkylated analogues of homoibotenic acid (HIBO) have previously shown high potency and selectivity at ionotropic and metabotropic glutamic acid receptor (iGluR and mGluR) subtypes. Compounds with different selectivity profiles are valuable pharmacological tools for neuropharmacological studies, and the series of 4-alkyl-HIBO analogues have been extended in this paper in the search for versatile agents. Pharmacological characterization of five new analogues, branched and unbranched 4-alkyl-HIBO analogues, have been carried out. The present compounds are all weak antagonists at Group I mGluRs (mGluR1 and 5) presenting only small differences in potencies (Ki values ranging from 89 to 670 microM). Affinities were studied at native and cloned iGluRs, and the compounds described show preference for the AMPA receptor subtypes GluR1 and 2 over GluR3 and 4. However, compared to previous 4-alkyl-HIBO analogues, these compounds show a remarkably high affinity for the Kain preferring subtype GluR5. The observed GluR5 affinities were either similar or higher compared to their GluR1 and 2 affinity. Isopropyl-HIBO showed the highest affinity for GluR5 (Ki=0.16 microM), and represents a unique compound with high affinity towards the three subtypes GluR1, 2 and 5. In general, these compounds represent new selectivity profiles compared to previously reported Glu receptor analogues.

Putative NMDA receptors in Hydra: a biochemical and functional study

The feeding behaviour of the freshwater polyp Hydra vulgaris (Cnidaria, Hydrozoa) is modulated by a number of molecules acting as neurotransmitters in other nervous systems. Here we present biochemical and functional evidence of the occurrence of putative NMDA receptors in Hydra tissues. Saturation experiments showed the presence of one population of binding sites with nanomolar affinity and low capacity for [3H]MK-801. Before equilibrium, [3H]MK-801 binding was increased by the agonists glutamate and glycine as well as by reduced glutathione (GSH). In vivo the glutamate receptor agonist NMDA markedly decreased the duration of the response to GSH. This effect was linearly related to ligand doses in the nanomolar concentration range and was counteracted by either the NMDAR-specific antagonist D-AP5 or by the d-serine antagonist DCKA. When NMDA concentration was increased to 10 or 100 microm, duration of the response to GSH was no longer affected unless the lectin concanavalin A, which prevents receptor desensitization in other systems, was added to the test medium. Simultaneous administration of ineffective doses of NMDA and strychnine, glycine or d-serine, an agonist at the glycine binding site of the NMDA receptor in vertebrate CNS, resulted in a strong reduction of response duration. Both D-AP5 and DCKA suppressed this effect. These results, together with the decrease in response duration produced by d-serine, support the hypothesis that NMDA-like glutamate receptors may occur in Hydra tissues where they are involved in modulation of the response to GSH with opposite actions to those of GABA and glycine.

Efficacy and safety of memantine treatment for reduction of changes associated with experimental glaucoma in monkey, I: Functional measures

PURPOSE

To determine, using electrophysiological measures of visual system function, whether oral daily dosing of memantine is both safe and effective to reduce the injury associated with experimental glaucoma in primates.

METHODS

Argon laser treatment of the anterior chamber angle was used to induce chronic ocular hypertension (COHT) in the right eye of 18 macaque monkeys. Nine animals were orally dosed daily with 4 mg/kg memantine while the other nine animals received an oral dose of vehicle only. Using both conventional and multifocal methods, recordings of the electroretinogram (ERG) were made at approximately 3, 5, and 16 months after elevation of the intraocular pressure (IOP). Recordings of the visually-evoked cortical potential (VECP) were also made at the 16-month time point.

RESULTS

Chronic ocular hypertension was associated with a reduction in the amplitude of components of the multifocal ERG response and visually-evoked cortical potential. Memantine-treated animals suffered less amplitude reduction for these measures than did vehicle-treated animals, though this treatment effect on the ERG measures was observed only at the early time points (3 and 5 months post IOP elevation). Memantine treatment was not associated with an effect on either the kinetics or amplitude of ERG or VECP response measures obtained from the normotensive eyes.

CONCLUSIONS

Systemic treatment with memantine, a compound which does not lower intraocular pressure, was both safe and effective for reduction of functional loss associated with experimental glaucoma.