The dextromethorphan analog dimemorfan attenuates kainate-induced seizures via sigma1 receptor activation: comparison with the effects of dextromethorphan

Sigma-1 receptor and seizures

Over the last decade, sigma-1 receptor (Sig1R) has been recognized as a valid target for the treatment of seizure disorders and seizure-related comorbidities. Clinical trials with Sig1R ligands are underway testing therapies for the treatment of drug-resistant seizures, developmental and epileptic encephalopathies, and photosensitive epilepsy. However, the direct molecular mechanism by which Sig1R modulates seizures and the balance between excitatory and inhibitory pathways has not been fully elucidated. This review article aims to summarize existing knowledge of Sig1R and its involvement in seizures by focusing on the evidence obtained from Sig1R knockout animals and the anti-seizure effects of Sig1R ligands. In addition, this review article includes a discussion of the advantages and disadvantages of the use of existing compounds and describes the challenges and future perspectives on the use of Sig1R as a target for the treatment of seizure disorders.

Sigma-1 receptor attenuates osteoclastogenesis by promoting ER-associated degradation of SERCA2

Sigma-1 receptor (Sigmar1) is a specific chaperone located in the mitochondria-associated endoplasmic reticulum membrane (MAM) and plays a role in several physiological processes. However, the role of Sigmar1 in bone homeostasis remains unknown. Here, we show that mice lacking Sigmar1 exhibited severe osteoporosis in an ovariectomized model. In contrast, overexpression of Sigmar1 locally alleviated the osteoporosis phenotype. Treatment with Sigmar1 agonists impaired both human and mice osteoclast formation in vitro. Mechanistically, SERCA2 was identified to interact with Sigmar1 based on the immunoprecipitation-mass spectrum (IP-MS) and co-immunoprecipitation (co-IP) assays, and Q615 of SERCA2 was confirmed to be the critical residue for their binding. Furthermore, Sigmar1 promoted SERCA2 degradation through Hrd1/Sel1L-dependent ER-associated degradation (ERAD). Ubiquitination of SERCA2 at K460 and K541 was responsible for its proteasomal degradation. Consequently, inhibition of SERCA2 impeded Sigmar1 deficiency enhanced osteoclastogenesis. Moreover, we found that dimemorfan, an FDA-approved Sigmar1 agonist, effectively rescued bone mass in various established bone-loss models. In conclusion, Sigmar1 is a negative regulator of osteoclastogenesis, and activation of Sigmar1 by dimemorfan may be a potential treatment for osteoporosis in clinical practice.

Iminyl radical-triggered relay annulation for the construction of bridged aza-tetracycles bearing four contiguous stereogenic centers

Bridged tetracyclic nitrogen scaffolds are found in numerous biologically active molecules and medicinally relevant structures. Traditional methods usually require tedious reaction steps, and/or the use of structurally specific starting materials. We report an unprecedented, iminyl radical-triggered relay annulation from oxime-derived peresters and azadienes, which shows good substrate scope and functional group compatibility, and can deliver various bridged aza-tetracyclic compounds with complex molecular topology and four contiguous stereogenic centers (dr > 19 : 1) in a single operation. This transformation represents the first example of trifunctionalization of iminyl radicals through simultaneous formation of one C–N and two C–C bonds. DFT calculation studies were conducted to obtain an in-depth insight into the reaction pathways, which revealed that the reactions involved an interesting 1,6-hydrogen atom transfer process.

A novel radical relay annulation is realized for the construction of various bridged aza-tetracyclic compounds with complex molecular topology and four contiguous stereogenic centers (dr > 19 : 1) in a single operation.

Beating Pain with Psychedelics: Matter over Mind?

Basic pain research has shed light on key cellular and molecular mechanisms underlying nociceptive and phenomenological aspects of pain. Despite these advances, [[we still yearn for] the discovery of novel therapeutic strategies to address the unmet needs of about 70% of chronic neuropathic pain patients whose pain fails to respond to opioids as well as to other conventional analgesic agents. Importantly, a substantial body of clinical observations over the past decade cumulatively suggests that the psychedelic class of drugs may possess heuristic value for understanding and treating chronic pain conditions. The present review presents a theoretical framework for hitherto insufficiently understood neuroscience-based mechanisms of psychedelics' potential analgesic effects. To that end, searches of PubMed-indexed journals were performed using the following Medical Subject Headings' terms: pain, analgesia, inflammatory, brain connectivity, ketamine, psilocybin, functional imaging, and dendrites. Recursive sets of scientific and clinical evidence extracted from this literature review were summarized within the following key areas: (1) studies employing psychedelics for alleviation of physical and emotional pain; (2) potential neuro-restorative effects of psychedelics to remediate the impaired connectivity underlying the dissociation between pain-related conscious states/cognitions and the subcortical activity/function leading to the eventual chronicity through immediate and long-term effects on dentritic plasticity; (3) anti-neuroinflammatory and pro-immunomodulatory actions of psychedelics as the may pertain to the role of these factors in the pathogenesis of neuropathic pain; (4) safety, legal, and ethical consideration inherent in psychedelics' pharmacotherapy. In addition to direct beneficial effects in terms of reduction of pain and suffering, psychedelics' inclusion in the analgesic armamentarium will contribute to deeper and more sophisticated insights not only into pain syndromes but also into frequently comorbid psychiatric condition associated with emotional pain, e.g., depressive and anxiety disorders. Further inquiry is clearly warranted into the above areas that have potential to evolve into further elucidate the mechanisms of chronic pain and affective disorders, and lead to the development of innovative, safe, and more efficacious neurobiologically-based therapeutic approaches.

Proceedings of the 2020 Epilepsy Foundation Pipeline Conference: Emerging Drugs and Devices

From August 27-28, 2020 the Epilepsy Foundation hosted the Pipeline Conference, exploring emerging issues related to antiepileptic drug and device development. The conference featured epilepsy therapeutic companies and academic laboratories developing drugs for focal epilepsies, innovations for rare and ultra-rare diseases, and devices both in clinical trials and approved for use. In this paper, we outline the virtual presentations by the authors, including novel data from their development pipeline.

Peripherally active dextromethorphan derivatives lower blood glucose levels by targeting pancreatic islets

Dextromethorphan (DXM) acts as cough suppressant via its central action. Cell-protective effects of this drug have been reported in peripheral tissues, making DXM potentially useful for treatment of several common human diseases, such as type 2 diabetes mellitus (T2DM). Pancreatic islets are among the peripheral tissues that positively respond to DXM, and anti-diabetic effects of DXM were observed in two placebo-controlled, randomized clinical trials in humans with T2DM. Since these effects were associated with central side effects, we here developed chemical derivatives of DXM that pass the blood-brain barrier to a significantly lower extent than the original drug. We show that basic nitrogen-containing residues block central adverse events of DXM without reducing its anti-diabetic effects, including the protection of human pancreatic islets from cell death. These results show how to chemically modify DXM, and possibly other morphinans, as to exclude central side effects, while targeting peripheral tissues, such as pancreatic islets.

Sigma receptors and neurological disorders

Sigma receptors were identified relatively recently, and their presence has been confirmed in the central nervous system and peripheral organs. Changes in sigma receptor function or expression may be involved in neurological diseases, and thus sigma receptors represent a potential target for treating central nervous system disorders. Many substances that are ligands for sigma receptors are widely used in therapies for neurological disorders. In the present review, we discuss the roles of sigma receptors, especially in the central nervous system disorders, and related therapies.

Dextromethorphan Dampens Neonatal Astrocyte Activation and Endoplasmic Reticulum Stress Induced by Prenatal Exposure to Buprenorphine

Prenatal exposure to buprenorphine renders offspring vulnerable to cerebral impairments. In this study, our data demonstrate, for the first time, that prenatal exposure to buprenorphine escalates astrocyte activation concurrent with indications of endoplasmic reticulum (ER) stress in the hippocampi of neonates, and this can be prevented by the coadministration of dextromethorphan with buprenorphine. Furthermore, dextromethorphan can inhibit the accumulation of GPR37 in the hippocampus of newborns caused by buprenorphine and is accompanied by the proapoptotic ER stress response that involves the procaspase-3/CHOP pathway. Primary astrocyte cultures derived from the neonates of the buprenorphine group also displayed aberrant ER calcium mobilization and elevated basal levels of cyclooxygenase-2 (COX-2) at 14 days in vitro while showing sensitivity to lipopolysaccharide-activated expression of COX-2. Similarly, these long-lasting defects in the hippocampus and astrocytes were abolished by dextromethorphan. Our findings suggest that prenatal exposure to buprenorphine might instigate long-lasting effects on hippocampal and astrocytic functions. The beneficial effects of prenatal coadministration of dextromethorphan might be, at least in part, attributed to its properties in attenuating astrocyte activation and hippocampal ER stress in neonates.

Efficacy of dextromethorphan for the treatment of depression: a systematic review of preclinical and clinical trials

INTRODUCTION

The large percentage of adults with major depressive disorder (MDD) insufficiently responding and/or tolerating conventional monoamine-based antidepressants invites the need for mechanistically novel treatments. Convergent evidence implicates glutamatergic signaling as a potential therapeutic target in MDD.

AREAS COVERED

The synthesis herein of preclinical and clinical studies indicates that dextromethorphan (DXM) is well tolerated and exhibits clinically significant antidepressant effects; DXM combined with bupropion has demonstrated replicated and relatively rapid onset efficacy in adults with MDD. DXM efficacy has been preliminarily reported in adults with bipolar depression. The combination of DXM and bupropion represents a pharmacokinetic and pharmacodynamic synergy which may account for the rapidity of action in MDD.

EXPERT OPINION

The combination of DXM and bupropion is a safe, well tolerated and efficacious treatment option in adults with MDD. Priority questions are whether DXM/bupropion is uniquely effective across discrete domains of psychopathology (e.g. anhedonia, reward processing, general cognitive systems) and/or whether it is able to significantly improve patient-reported outcomes (e.g. quality of life, psychosocial functioning). The availability of ketamine/esketamine and DXM/bupropion instantiates the relevance of glutamate as a treatment target in MDD. Studies in bipolar depression with DXM/bupropion are warranted as well as in MDD with suicidality.

AVP-786 as a promising treatment option for Alzheimer's Disease including agitation

INTRODUCTION

To date, there is no FDA-approved treatment for agitation in Alzheimer's disease (AD). Medications currently used off-label have modest clinical efficacy and serious side effects.

AREAS COVERED

The authors review the pharmacology, mechanism of action, pharmacokinetics, efficacy, safety and tolerability data of AVP-786, for the treatment of agitation in AD.

EXPERT OPINION

AVP-786, the deuterated form of dextromethorphan/quinidine (AVP-923) which is an approved treatment for Pseudo-Bulbar Affect, emerges as a promising and safe treatment for agitation in AD. Deuteration is an innovative technology that accelerates drug development by conducting faster and less costly clinical trials. No phase II trial was conducted with AVP-786 for the treatment of agitation in AD; the decision to expedite the development of this drug was based on a successful phase II study with AVP-923. Phase III trials with AVP-786 (TRIAD-1 and TRIAD-2) showed mixed findings probably due to the difference in study design. Future phase III studies should use innovative study designs such as the Sequential Parallel Comparison Design to mitigate high placebo response, and the Cohen-Mansfield Agitation Inventory for agitation assessment. They should also include positron emission tomography studies to assess occupancy of various receptors in the brain after AVP-786 is administered.

State-of-the-art pharmacotherapy for diabetic neuropathy

INTRODUCTION

The global epidemic of diabetes has led to an epidemic of diabetes complications. Diabetic neuropathy is the most common microvascular complication, of which diabetic peripheral neuropathy (DPN) and autonomic neuropathy (AN) are the most prevalent, affecting ~50% of patients. DPN results in pain with a poor quality of life and a loss of sensation with an increased risk of foot ulceration. Autonomic neuropathy can cause significant morbidity in a minority and is associated with increased mortality. The cornerstone of treatment to prevent or limit the progression of DPN/AN is multifactorial risk factor modification including treatment of glycemia, lipids and blood pressure. Whilst, there are no FDA-approved disease-modifying therapies, there are a number of therapies to relieve symptoms in DPN and AN.

AREAS COVERED

The authors discuss current approved therapies for painful diabetic neuropathy and autonomic neuropathy. They also address the potential role of improving risk factors to limit the development and progression of diabetic neuropathy and new pathogenetic and pain-relieving treatments.

EXPERT OPINION

The FDA-approved Pregabalin and Duloxetine over 25 years ago and Tapentadol, 6 years ago for painful diabetic neuropathy. There are currently no FDA-approved disease-modifying treatments for diabetic neuropathy which has been attributed to inappropriate models of the disease with limited translational capacity and major limitations of trial designs and endpoints in clinical trials.

Effect of dextromethorphan/quinidine on pentylenetetrazole- induced clonic and tonic seizure thresholds in mice

INTRODUCTION

This study aimed to investigate the effects of dextromethorphan (DM) or dextromethorphan/quinidine (DM/Q) against pentylenetetrazole (PTZ)- induced seizure threshold in mice and the probable involvement of N-methyl d-aspartate (NMDA), sigma-1 and serotonin 1A (5-HT_1A) receptors.

MATERIAL AND METHODS

NMRI male mice (25-30 g) received quinidine (10, 20, and 30 mg/kg), DM (5, 10, 25, and 50 mg/kg) or DM/Q (10/20, 25/20, and 50/20 mg/kg), 30 min before the infusion of PTZ. ketamine (1 and 5 mg/kg), BD-1047 (2.5 and 5 mg/kg) or WAY-100635 (0.5 and 1 mg/kg) were administrated as pre-treatment 30 min before the selected dose of DM/Q. Seizures were induced by intravenous PTZ infusion. All data were presented as means ± S.E.M. One-way ANOVA test was used to determine statistical significance (p < 0.05).

RESULTS

DM (25 and 50 mg/kg) significantly increased PTZ- induced seizure threshold. DM/Q at doses of 10/20 and 25/20 mg/kg had anticonvulsant effect, while at a dose of 50/20 mg/kg attenuated anticonvulsant effect of DM 50 mg/kg. Ketamine (5 mg/kg) or WAY-100635 (1 mg/kg) potentiated, while BD-1047 (2.5 and 5 mg/kg) attenuated the anticonvulsant effect of DM/Q 10/20 mg/kg.

CONCLUSION

The results of present study demonstrate that combination with quinidine potentiates the anticonvulsant effect of DM at lower doses, while attenuates it at higher dose. Meanwhile, the effects of DM/Q on seizure activity likely involve an interaction with NMDA, the sigma-1 or the 5-HT_1A receptor which may be secondary to the elevation of DM levels.

Pharmacophore and docking-based sequential virtual screening for the identification of novel Sigma 1 receptor ligands

Sigma 1 receptor (σ1), a small transmembrane protein expressed in most human cells participates in modulating the function of other membrane proteins such as G protein coupled receptors and ion channels. Several ligands targeting this receptor are currently in clinical trials for the treatment of Alzheimer's disease, ischemic stroke and neuro-pathic pain. Hence, this receptor has emerged as an attractive target for the treatment of neuro-pathological diseases with unmet medical needs. It is of interest to identify and characterise novelσ1 receptor ligands with different chemical scaffolds using computer-aided drug designing approach. In this work, a GPCR-focused chemical library consisting of 8543 compounds was screened by pharmacophore and docking-based virtual screening methods using LigandScout 4.3 and Autodock Vina 1.1.2 in PyRx 0.8, respectively. The pharmacophore model was constructed based on the interactions of a selective agonist and another antagonist ligand with high binding affinity to the human σ1receptors. Candidate compounds were filtered sequentially by pharmacophore-fit scores, docking energy scores, drug-likeness filters and ADMET properties. The binding mode and pharmacophore mapping of candidate compounds were analysed by Autodock Vina 1.1.2 and LigandScout 4.3 programs, respectively. A pharmacophore model composed of three hydrophobic and positive ionizable features with recognized geometry was built and used as a 3D query for screening a GPCR-focused chemical library by LigandScout 4.3 program. Among the screened 8543 compounds, 159 candidate compounds were obtained from pharmacophore-based screening. 45 compounds among them bound to σ 1receptor with high binding-affinity scores in comparison to the co-crystallized ligand. Amongst these, top five candidate compounds with excellent druglikeness and ADMET properties were selected. These five candidate compounds may act as potential σ1 receptor ligands.

Interaction of sigma-1 receptor modulators with seizure development in pentylenetetrazole-induced kindled mice

This study aimed to investigate the effects of sigma receptor modulators, opipramol and BD-1063, on epileptogenesis in pentylenetetrazole (PTZ)-kindling model of epilepsy. Mice (n = 6/group) were received PTZ (30 mg/kg), PTZ plus opipramol (5 or 10 mg/kg), PTZ plus opipramol (5 mg/kg) plus BD-1063 (5 mg/kg, a selective sigma-1 receptor antagonist), and PTZ plus BD-1063 on alternate days for 15 days. Opipramol (5 and 10 mg/kg) + PTZ groups became fully kindled and had higher seizure scores compared to the PTZ group. In contrast, the PTZ plus BD-1063 and the PTZ plus opipramol (5 mg/kg) plus BD-1063 group did not show full kindling. These findings indicate that opipramol has a pro-convulsant effect, which is possibly mediated through activation of sigma-1 receptors.

Identification of novel analgesics through a drug repurposing strategy

The identification of new indications for approved or failed drugs is a process called drug repositioning or drug repurposing. The motivation includes overcoming the productivity gap that exists in drug development, which is a high-cost–high-risk process. Repositioning also includes rescuing drugs that have safely entered the market but have failed to demonstrate sufficient efficiency for the initial clinical indication. Considering the high prevalence of chronic pain, the lack of sufficient efficacy and the safety issues of current analgesics, repositioning seems to be an attractive approach. This review presents example of drugs that already have been repositioned and highlights new technologies that are available for the identification of additional compounds to stimulate the curiosity of readers for further exploration.

Dextromethorphan/quinidine pharmacotherapy in patients with treatment resistant depression: A proof of concept clinical trial

BACKGROUND

At least one-third of patients with major depressive disorder (MDD) have treatment-resistant depression (TRD), defined as lack of response to two or more adequate antidepressant trials. For these patients, novel antidepressant treatments are urgently needed.

METHODS

The current study is a phase IIa open label clinical trial examining the efficacy and tolerability of a combination of dextromethorphan (DM) and the CYP2D6 enzyme inhibitor quinidine (Q) in patients with TRD. Dextromethorphan acts as an antagonist at the glutamate N-methyl-d-aspartate (NMDA) receptor, in addition to other pharmacodynamics properties that include activity at sigma-1 receptors. Twenty patients with unipolar TRD who completed informed consent and met all eligibility criteria we enrolled in an open-label study of DM/Q up to 45/10mg by mouth administered every 12h over the course of a 10-week period, and constitute the intention to treat (ITT) sample. Six patients discontinued prior to study completion.

RESULTS

There was no treatment-emergent suicidal ideation, psychotomimetic or dissociative symptoms. Montgomery-Asberg Depression Rating Scale (MADRS) score was reduced from baseline to the 10-week primary outcome (mean change: -13.0±11.5, t₁₉=5.0, p<0.001), as was QIDS-SR score (mean change: -5.9±6.6, t₁₉=4.0, p<0.001). The response and remission rates in the ITT sample were 45% and 35%, respectively.

LIMITATIONS

Open-label, proof-of-concept design.

CONCLUSIONS

Herein we report acceptable tolerability and preliminary efficacy of DM/Q up to 45/10mg administered every 12h in patients with TRD. Future larger placebo controlled randomized trials in this population are warranted.

The activity of selective sigma-1 receptor ligands in seizure models in vivo

Sigma-1 receptor (Sig1R) is a ligand-regulated protein which, since its discovery, has been widely studied as a novel target to treat neurological disorders, including seizures. However, the roles and mechanisms of Sig1R in the regulation of seizures are not fully understood. The aim of the present study was to test and compare effects of often used selective Sig1R ligands in models of experimentally induced seizures. The anti-seizure activities and interactions of selective Sig1R agonist PRE-084, selective Sig1R antagonist NE-100 and novel positive allosteric Sig1R modulator E1R were evaluated in pentylenetetrazol (PTZ) and (+)-bicuculline (BIC)-induced seizure models in mice. Sig1R antagonist NE-100 at a dose of 25mg/kg demonstrated pro-convulsive activity on PTZ-induced seizures. Agonist PRE-084 did not change the thresholds of chemoconvulsant-induced seizures. Positive allosteric modulator E1R at a dose of 50mg/kg showed anti-convulsive effects on PTZ- and BIC-induced clonic and tonic seizures. The anti-seizure activity of E1R was blocked by NE-100. Surprisingly, NE-100 at a dose of 50mg/kg induced convulsions, but E1R significantly alleviated the convulsive behaviour induced by NE-100. In conclusion, the selective Sig1R antagonist NE-100 induced seizures that could be partially attenuated by positive allosteric Sig1R modulator. Our results confirm that Sig1R could be a novel molecular target for new anti-convulsive drugs.

A Role for Sigma Receptors in Stimulant Self-Administration and Addiction

Sigma receptors (σRs) are structurally unique proteins that function intracellularly as chaperones. Historically, σRs have been implicated as modulators of psychomotor stimulant effects and have at times been proposed as potential avenues for modifying stimulant abuse. However, the influence of ligands for σRs on the effects of stimulants, such as cocaine or methamphetamine, in various preclinical procedures related to drug abuse has been varied. The present paper reviews the effects of σR agonists and antagonists in three particularly relevant procedures: stimulant discrimination, place conditioning, and self-administration. The literature to date suggests limited σR involvement in the discriminative-stimulus effects of psychomotor stimulants, either with σR agonists substituting for the stimulant or with σR antagonists blocking stimulant effects. In contrast, studies of place conditioning suggest that administration of σR antagonists or down-regulation of σR protein can block the place conditioning induced by stimulants. Despite place conditioning results, selective σR antagonists are inactive in blocking the self-administration of stimulants. However, compounds binding to the dopamine transporter and blocking σRs can selectively decrease stimulant self-administration. Further, after self-administration of stimulants, σR agonists are self-administered, an effect not seen in subjects without that specific history. These findings suggest that stimulants induce unique changes in σR activity, and once established, the changes induced create redundant, and dopamine independent reinforcement pathways. Concomitant targeting of both dopaminergic pathways and σR proteins produces a selective antagonism of those pathways, suggesting new avenues for combination chemotherapies to specifically combat stimulant abuse.

Sigma-1 Receptors and Neurodegenerative Diseases: Towards a Hypothesis of Sigma-1 Receptors as Amplifiers of Neurodegeneration and Neuroprotection

Sigma-1 receptors are molecular chaperones that may act as pathological mediators and targets for novel therapeutic applications in neurodegenerative diseases. Accumulating evidence indicates that sigma-1 ligands can either directly or indirectly modulate multiple neurodegenerative processes, including excitotoxicity, calcium dysregulation, mitochondrial and endoplasmic reticulum dysfunction, inflammation, and astrogliosis. In addition, sigma-1 ligands may act as disease-modifying agents in the treatment for central nervous system (CNS) diseases by promoting the activity of neurotrophic factors and neural plasticity. Here, we summarize their neuroprotective and neurorestorative effects in different animal models of acute brain injury and chronic neurodegenerative diseases, and highlight their potential role in mitigating disease. Notably, current data suggest that sigma-1 receptor dysfunction worsens disease progression, whereas enhancement amplifies pre-existing functional mechanisms of neuroprotection and/or restoration to slow disease progression. Collectively, the data support a model of the sigma-1 receptor as an amplifier of intracellular signaling, and suggest future clinical applications of sigma-1 ligands as part of multi-therapy approaches to treat neurodegenerative diseases.

AVP-786 for the treatment of agitation in dementia of the Alzheimer's type

INTRODUCTION

Agitation is common and distressing in patients with Alzheimer-type dementia, but safe, effective treatments remain elusive. Psychological treatments are first-line options, but they have limited efficacy. Off-label psychotropic medications are frequently used, but they also have limited effectiveness, and their use may have harmful side effects, including death. Areas covered: This review discusses the history leading to the conception of AVP-786 (deuterated (d6)-dextromethorphan/quinidine), its pharmacokinetic and pharmacodynamic profiles and safety issues, together with an overview of recent clinical trials. Data were found in the medical literature, in US and EU clinical trial registries and in information provided by the manufacturer. Expert opinion: AVP-786 is one of six investigational compounds in recent phase III clinical development for agitation in Alzheimer disease (AD). Quinidine and deuteration appear to prolong dextromethorphan's plasma half-life and facilitate brain penetration. The FDA granted fast-track designation to AVP-786 and allowed use of data generated on dextromethorphan-quinidine (AVP-923, Nuedexta®) for regulatory filings. AVP-923 reduced agitation in AD and was well tolerated in a phase II RCT that included more than 200 patients. A phase III clinical development program of AVP-786 for AD agitation was recently initiated. This program is expected to start generating results in July 2018.

4-aroylpiperidines and 4-(α-hydroxyphenyl)piperidines as selective sigma-1 receptor ligands: synthesis, preliminary pharmacological evaluation and computational studies

Background

Sigma (σ) receptors are membrane-bound proteins characterised by an unusual promiscuous ability to bind a wide variety of drugs and their high affinity for typical neuroleptic drugs, such as haloperidol, and their potential as alternative targets for antipsychotic agents. Sigma receptors display diverse biological activities and represent potential fruitful targets for therapeutic development in combating many human diseases. Therefore, they present an interesting avenue for further exploration. It was our goal to evaluate the potential of ring opened spipethiane (1) analogues as functional ligands (agonists) for σ receptors by chemical modification.

Results

Chemical modification of the core structure of the lead compound, (1), by replacement of the sulphur atom with a carbonyl group, hydroxyl group and 3-bromobenzylamine with the simultaneous presence of 4-fluorobenzoyl replacing the spirofusion afforded novel potent sigma-1 receptor ligands 7a–f, 8a–f and 9d–e. The sigma-1 receptor affinities of 7e, 8a and 8f were slightly lower than that of 1 and their selectivities for this receptor two to threefold greater than that of 1.

Conclusions

It was found that these compounds have higher selectivities for sigma-1 receptors compared to 1. Quantitatitive structure–activity relationship studies revealed that sigma-1 binding is driven by hydrophobic interactions.Graphical abstract

Pharmacology of dextromethorphan: Relevance to dextromethorphan/quinidine (Nuedexta®) clinical use

Dextromethorphan (DM) has been used for more than 50years as an over-the-counter antitussive. Studies have revealed a complex pharmacology of DM with mechanisms beyond blockade of N-methyl-d-aspartate (NMDA) receptors and inhibition of glutamate excitotoxicity, likely contributing to its pharmacological activity and clinical potential. DM is rapidly metabolized to dextrorphan, which has hampered the exploration of DM therapy separate from its metabolites. Coadministration of DM with a low dose of quinidine inhibits DM metabolism, yields greater bioavailability and enables more specific testing of the therapeutic properties of DM apart from its metabolites. The development of the drug combination DM hydrobromide and quinidine sulfate (DM/Q), with subsequent approval by the US Food and Drug Administration for pseudobulbar affect, led to renewed interest in understanding DM pharmacology. This review summarizes the interactions of DM with brain receptors and transporters and also considers its metabolic and pharmacokinetic properties. To assess the potential clinical relevance of these interactions, we provide an analysis comparing DM activity from in vitro functional assays with the estimated free drug DM concentrations in the brain following oral DM/Q administration. The findings suggest that DM/Q likely inhibits serotonin and norepinephrine reuptake and also blocks NMDA receptors with rapid kinetics. Use of DM/Q may also antagonize nicotinic acetylcholine receptors, particularly those composed of α3β4 subunits, and cause agonist activity at sigma-1 receptors.

Dextromethorphan: An update on its utility for neurological and neuropsychiatric disorders

Dextromethorphan (DM) is a commonly used antitussive and is currently the only FDA-approved pharmaceutical treatment for pseudobulbar affect. Its safety profile and diverse pharmacologic actions in the central nervous system have stimulated new interest for repurposing it. Numerous preclinical investigations and many open-label or blinded clinical studies have demonstrated its beneficial effects across a variety of neurological and psychiatric disorders. However, the optimal dose and safety of chronic dosing are not fully known. This review summarizes the preclinical and clinical effects of DM and its putative mechanisms of action, focusing on depression, stroke, traumatic brain injury, seizure, pain, methotrexate neurotoxicity, Parkinson's disease and autism. Moreover, we offer suggestions for future research with DM to advance the treatment for these and other neurological and psychiatric disorders.

A Nonrewarding NMDA Receptor Antagonist Impairs the Acquisition, Consolidation, and Expression of Morphine Conditioned Place Preference in Mice

N-methyl-D-aspartate (NMDA) receptor antagonists block morphine-induced conditioned place preference (CPP). Although polyamines are endogenous modulators of the NMDA receptor, it is not known whether polyaminergic agents induce CPP or modulate morphine-induced CPP. Here, we examined whether polyamine ligands modify morphine CPP acquisition, consolidation, and expression. Adult male albino Swiss mice received saline (0.9 % NaCl, intraperitoneally (i.p.)) or morphine (5 mg/kg, i.p.) and were respectively confined to a black or a white compartment for 30 min for four consecutive days for CPP induction. The effect of arcaine (3 mg/kg, i.p.) or spermidine (30 mg/kg, i.p.), respectively, an antagonist and an agonist of the polyamine-binding site at the NMDA receptor, on the acquisition, consolidation, and expression of morphine CPP was studied. In those experiments designed to investigate whether spermidine prevented or reversed the effect of arcaine, spermidine (30 mg/kg, i.p.) was administered 15 min before or 15 min after arcaine, respectively. Arcaine and spermidine did not induce CPP or aversion per se. Arcaine (3 mg/kg, i.p.) impaired the acquisition, consolidation, and expression of morphine CPP. Spermidine prevented the impairing effect of arcaine on the acquisition of morphine CPP but not the impairing effect of arcaine on consolidation or expression of morphine CPP. These results suggest that arcaine may impair morphine CPP acquisition by modulating the polyamine-binding site at the NMDA receptor. However, the arcaine-induced impairment of consolidation and expression of morphine CPP seems to involve other mechanisms.

Dual Therapeutic Effects of C-10068, a Dextromethorphan Derivative, Against Post-Traumatic Nonconvulsive Seizures and Neuroinflammation in a Rat Model of Penetrating Ballistic-Like Brain Injury

Post-traumatic seizures can exacerbate injurious outcomes of severe brain trauma, yet effective treatments are limited owing to the complexity of the pathology underlying the concomitant occurrence of both events. In this study, we tested C-10068, a novel deuterium-containing analog of (+)-N-methyl-3-ethoxymorphinan, in a rat model of penetrating ballistic-like brain injury (PBBI) and evaluated the effects of C-10068 on PBBI-induced nonconvulsive seizures (NCS), acute neuroinflammation, and neurofunctional outcomes. NCS were detected by electroencephalographic monitoring. Neuroinflammation was evaluated by immunohistochemical markers, for example, glial fibrillary acidic protein and major histocompatibility complex class I, for activation of astrocytes and microglia, respectively. Neurofunction was tested using rotarod and Morris water maze tasks. Three infusion doses of C-10068 (1.0, 2.5, and 5.0 mg/kg/h × 72 h) were tested in the antiseizure study. Neuroinflammation and neurofunction were evaluated in animals treated with 5.0 mg/kg/h × 72 h C-10068. Compared to vehicle treatment, C-10068 dose dependently reduced PBBI-induced NCS incidence (40-50%), frequency (20-70%), and duration (30-82%). The most effective antiseizure dose of C-10068 (5.0 mg/kg/h × 72 h) also significantly attenuated hippocampal astrocyte activation and perilesional microglial reactivity post-PBBI. Within C-10068-treated animals, a positive correlation was observed in reduction in NCS frequency and reduction in hippocampal astrocyte activation. Further, C-10068 treatment significantly attenuated astrocyte activation in seizure-free animals. However, C-10068 failed to improve PBBI-induced motor and cognitive functions with the dosing regimen used in this study. Overall, the results indicating that C-10068 exerts both potent antiseizure and antiinflammatory effects are promising and warrant further investigation.

Recent Advances in the Development of Sigma-1 Receptor Ligands

The existence of two distinct sigma (σ) receptor subtypes was established in the early 1990s. Sigma-1 and sigma-2 receptors (S1Rs and S2Rs, respectively) were shown to possess distinct molecular size, anatomical distribution, and ligand discrimination. S2R is overexpressed in numerous human cancers, and has therapeutic potential for the imaging and treatment of certain tumours. In contrast, S1R is more broadly involved in a wide variety of central nervous system (CNS) diseases including motor disorders, memory deficits, depression, schizophrenia, anxiety, pain, drug addiction, and many more. Since the human S1R was cloned in 1996, numerous high affinity ligands with excellent selectivity for S1R have been developed. This review focuses on recent developments in the generation of structurally diverse S1R-selective ligands and novel therapeutic candidates targeting S1Rs.

Simple and sensitive LC-MS/MS-based assay for the quantification of dimemorfan in human plasma for use in a pharmacokinetic study

Dimemorfan phosphate has been widely used for 40 years throughout the world for the treatment of coughs. This is the first report on the use of an LC-MS/MS-based assay for the determination of dimemorfan in human plasma using estazolam as an internal standard after one-step protein precipitation with acetonitrile. Chromatographic separation was achieved on a Phenomenex Luna C18 column (3 µm, 50 × 2.0 mm) using a fast gradient method, which involves water and methanol as the mobile phase (both containing 0.1% formic acid). Dimemorfan and estazolam were detected with proton adducts at m/z values of 255.8 → 155.1 and 295.0 → 267.0, respectively, in the selected reaction monitoring positive mode. The linear dynamic range of the assay was 0.04-5.00 ng/mL. The chromatographic run time for each plasma sample was <5 min. The method was proven to be accurate, precise, and repeatable. Finally, the developed method was successfully applied for the determination of dimemorfan in a pharmacokinetic study using healthy Chinese subjects.

Drug repositioning: playing dirty to kill pain

The number of approved new molecular entity drugs has been decreasing as the pharmaceutical company investment in research and development is increasing. As we face this painful crisis, called an innovation gap, there is increasing awareness that development of new uses of existing drugs may be a powerful tool to help overcome this obstacle because it takes too long, costs too much and can be risky to release drugs developed de novo. Consequently, drug repositioning is emerging in different therapeutic areas, including the pain research area. Worldwide, pain is the main reason for seeking healthcare, and pain relief represents an unmet global clinical need. Therefore, development of analgesics with better efficacy, safety and cost effectiveness is of paramount importance. Despite the remarkable advancement in research on cellular and molecular mechanisms underlying pain pathophysiology over the past three decades, target-based therapeutic opportunities have not been pursued to the same extent. Phenotypic screening remains a more powerful tool for drug development than target-based screening so far. It sounds somewhat heretical, but some multi-action drugs, rather than very selective ones, have been developed intentionally. In the present review, we first critically discuss the utility of drug repositioning for analgesic drug development and then show examples of 'old' drugs that have been successfully repositioned or that are under investigation for their analgesic actions. We conclude that drug repositioning should be more strongly encouraged to help build a bridge between basic research and pain relief worldwide.

Alteration in glutathione homeostasis and oxidative stress during the sequelae of trimethyltin syndrome in rat brain

Trimethyltin (TMT), a by-product of tin, is used in a wide variety of industrial and agricultural purposes which serves as a model neurotoxicant in hippocampal neurodegeneration, and this could, in turn, be exploited for various therapeutic compounds essential for hippocampal neurodegeneration. Therefore, the present investigation explores the sequential changes in behavior, oxidative burden, and apoptosis following TMT administration in rat hippocampus. Male SD rats weighing 250 g were given single dose of 8.5 mg/kg TMT (i.p.) that resulted in "TMT syndrome" which begins at the third post-TMT exposure and continued till 21 days posttreatment. This resulted in behavioral alteration (aggression and spontaneous seizures), cognitive impairment as assessed by plus maze, and passive avoidance resulting in short-term memory deficits. These behavioral alterations were associated with an increase in oxidative stress. The levels of malondialdehyde, reactive oxygen species, and protein carbonyl were significantly increased (p < 0.001) in the TMT-treated rats after the third day of exposure and were maximum at day 14 postexposure. The glutathione system was not able to adapt rapidly in response to oxidative stress which resulted in imbalance in redox status. The imbalance in the redox state resulted in the death of neurons as seen by a significant increase in caspase activation at gene as well as protein level after TMT exposure on day 14, quoting an extent of changes. Therefore, it is proposed that behavioral deficits could be accounted by the impairment of endogenous glutathione homeostasis which resulted in death of neurons in the hippocampal region.

Neuroprotective strategies in hippocampal neurodegeneration induced by the neurotoxicant trimethyltin

The selective vulnerability of specific neuronal subpopulations to trimethyltin (TMT), an organotin compound with neurotoxicant effects selectively involving the limbic system and especially marked in the hippocampus, makes it useful to obtain in vivo models of neurodegeneration associated with behavioural alterations, such as hyperactivity and aggression, cognitive impairment as well as temporal lobe epilepsy. TMT has been widely used to study neuronal and glial factors involved in selective neuronal death, as well as the molecular mechanisms leading to hippocampal neurodegeneration (including neuroinflammation, excitotoxicity, intracellular calcium overload, mitochondrial dysfunction and oxidative stress). It also offers a valuable instrument to study the cell-cell interactions and signalling pathways that modulate injury-induced neurogenesis, including the involvement of newly generated neurons in the possible repair processes. Since TMT appears to be a useful tool to damage the brain and study the various responses to damage, this review summarises current data from in vivo and in vitro studies on neuroprotective strategies to counteract TMT-induced neuronal death, that may be useful to elucidate the role of putative candidates for translational medical research on neurodegenerative diseases.

Dextromethorphan-induced psychotoxic behaviors cause sexual dysfunction in male mice via stimulation of σ-1 receptors

Dextromethorphan (DM) is a well-known antitussive dextrorotatory morphinan. We and others have demonstrated that sigma (σ) receptors may be important for DM-mediated neuromodulation. Because an earlier report suggested that DM might affect sexual function and that σ receptor ligands affect signaling pathways in the periphery, we examined whether DM-induced psychotoxic burden affected male reproductive function. We observed that DM had a high affinity at σ-1 receptors in the brain and testis but relatively low affinity at σ-2 receptors. Prolonged treatment with DM resulted in conditioned place preference and hyperlocomotion, followed by an increase in Fos-related antigen expression in the nucleus accumbens in male mice. Simultaneously, DM induced significant reductions in gonadotropin-releasing-hormone immunoreactivity in the hypothalamus. Moreover, we observed that DM induced increased sperm abnormalities and decreased sperm viability and sexual behavior. These phenomena were significantly attenuated by combined treatment with BD1047, a σ-1 receptor antagonist, but not by SM-21, a σ-2 receptor antagonist. Thus, these results suggest that DM psychotoxicity might lead to reproductive stress in male mice by activating σ-1 receptors.

Neuropsychopharmacological understanding for therapeutic application of morphinans

Morphinans are a class of compounds containing the basic structure of morphine. It is well-known that morphinans possess diverse pharmacological effects on the central nervous system. This review will demonstrate novel neuroprotective effects of several morphinans such as, dextromethorphan, its analogs and naloxone on the models of multiple neurodegenerative disease by modulating glial activation associated with the production of a host of proinflammatory and neurotoxic factors, although dextromethorphan possesses neuropsychotoxic potentials. The neuroprotective effects and the therapeutic potential for the treatment of excitotoxic and inflammatory neurodegenerative diseases, and underlying mechanism of morphinans are discussed.

Synthesis and characterization of N,N-dialkyl and N-alkyl-N-aralkyl fenpropimorph-derived compounds as high affinity ligands for sigma receptors

The sigma-1 receptor is a unique non-opioid, non-PCP binding site that has been implicated in many different pathophysiological conditions including psychosis, drug addiction, retinal degeneration and cancer. Based on the structure of fenpropimorph, a high affinity (K(i)=0.005 nM)(1) sigma-1 receptor ligand and strong inhibitor of the yeast sterol isomerase (ERG2), we previously deduced a basic sigma-1 receptor pharmacophore or chemical backbone composed of a phenyl ring attached to a di-substituted nitrogen atom via an alkyl chain.(2) Here, we report the design and synthesis of various N,N-dialkyl or N-alkyl-N-aralkyl derivatives based on this pharmacophore as well as their binding affinities to the sigma-1 receptor. We introduce three high affinity sigma-1 receptor compounds, N,N-dibutyl-3-(4-fluorophenyl)propylamine (9), N,N-dibutyl-3-(4-nitrophenyl)propylamine (3), and N-propyl-N'-4-aminophenylethyl-3-(4-nitrophenyl)propylamine (20) with K(i) values of 17.7 nM, 0.36 nM, and 6 nM, respectively. In addition to sigma receptor affinity, we show through cytotoxicity assays that growth inhibition of various tumor cell lines occurs with our high affinity N,N-dialkyl or N-alkyl-N-aralkyl derivatives.

Pharmacology and therapeutic potential of sigma(1) receptor ligands

Sigma (σ) receptors, initially described as a subtype of opioid receptors, are now considered unique receptors. Pharmacological studies have distinguished two types of σ receptors, termed σ₁ and σ₂. Of these two subtypes, the σ₁ receptor has been cloned in humans and rodents, and its amino acid sequence shows no homology with other mammalian proteins. Several psychoactive drugs show high to moderate affinity for σ₁ receptors, including the antipsychotic haloperidol, the antidepressant drugs fluvoxamine and sertraline, and the psychostimulants cocaine and methamphetamine; in addition, the anticonvulsant drug phenytoin allosterically modulates σ₁ receptors. Certain neurosteroids are known to interact with σ₁ receptors, and have been proposed to be their endogenous ligands. These receptors are located in the plasma membrane and in subcellular membranes, particularly in the endoplasmic reticulum, where they play a modulatory role in intracellular Ca²⁺ signaling. Sigma₁ receptors also play a modulatory role in the activity of some ion channels and in several neurotransmitter systems, mainly in glutamatergic neurotransmission. In accordance with their widespread modulatory role, σ₁ receptor ligands have been proposed to be useful in several therapeutic fields such as amnesic and cognitive deficits, depression and anxiety, schizophrenia, analgesia, and against some effects of drugs of abuse (such as cocaine and methamphetamine). In this review we provide an overview of the present knowledge of σ₁ receptors, focussing on σ₁ ligand neuropharmacology and the role of σ₁ receptors in behavioral animal studies, which have contributed greatly to the potential therapeutic applications of σ₁ ligands.

Protection against kainate neurotoxicity by ginsenosides: attenuation of convulsive behavior, mitochondrial dysfunction, and oxidative stress

We previously demonstrated that kainic acid (KA)-mediated mitochondrial oxidative stress contributed to hippocampal degeneration and that ginsenosides attenuated KA-induced neurotoxicity and neuronal degeneration. Here, we examined whether ginsenosides affected KA-induced mitochondrial dysfunction and oxidative stress in the rat hippocampus. Treatment with ginsenosides attenuated KA-induced convulsive behavior dose-dependently. KA treatment increased lipid peroxidation and protein oxidation and decreased the reduced glutathione/oxidized glutathione (GSH/GSSG) ratio to a greater degree in the mitochondrial fraction than in the hippocampal homogenate. KA treatment resulted in decreased Mn-superoxide dismutase expression and diminished the mitochondrial membrane potential. Furthermore, KA treatment increased intramitochondrial Ca(2+) and promoted ultrastructural degeneration in hippocampal mitochondria. Treatment with ginsenosides dose-dependently attenuated convulsive behavior and the KA-induced mitochondrial effects. Protection appeared to be more evident in mitochondria than in tissue homogenates. Collectively, the results suggest that ginsenosides prevent KA-induced neurotoxicity by attenuating mitochondrial oxidative stress and mitochondrial dysfunction.

Anti-inflammatory effects of dimemorfan on inflammatory cells and LPS-induced endotoxin shock in mice

BACKGROUND AND PURPOSE

Dimemorfan (a sigma1 receptor agonist) showed neuroprotective properties in animal models of inflammation-mediated neurodegenerative conditions, but its effects on inflammatory cells and systemic inflammation remain unclear.

EXPERIMENTAL APPROACH

The effects of dimemorfan on phorbol-12-myristate-13-acetate (PMA)- and N-formyl-methionyl-leucyl-phenylalanine (fMLP)- induced neutrophils and lipopolysaccharide (LPS)-activated microglial cells, as well as LPS-induced endotoxin shock in mice were elucidated.

KEY RESULTS

Dimemorfan decreased PMA- and fMLP-induced production of reactive oxygen species (ROS) and CD11b expression in neutrophils, through mechanisms independent of sigma1 receptors, possibly by blocking ROS production and G-protein-mediated intracellular calcium increase. Dimemorfan also inhibited LPS-induced ROS and nitric oxide (NO) production, as well as that of monocyte chemoattractant protein-1 and tumour necrosis factor-alpha (TNF-alpha), by inhibition of NADPH oxidase (NOX) activity and suppression of iNOS up-regulation through interfering with nuclear factor kappa-B (NF-kappaB) signalling in microglial cells. Treatment in vivo with dimemorfan (1 and 5 mg kg(-1), i.p., at three successive times after LPS) decreased plasma TNF-alpha, and neutrophil infiltration and oxidative stress in the lung and liver.

CONCLUSIONS AND IMPLICATIONS

Our results suggest that dimemorfan acts via sigma1 receptor-independent mechanisms to modulate intracellular calcium increase, NOX activity, and NF-kappaB signalling, resulting in inhibition of iNOS expression and NO production, and production of pro-inflammatory cytokines. These effects may contribute its anti-inflammatory action and protective effects against endotoxin shock in mice.

Dimemorfan protects rats against ischemic stroke through activation of sigma-1 receptor-mediated mechanisms by decreasing glutamate accumulation

Dimemorfan, an antitussive and a sigma-1 (sigma(1)) receptor agonist, has been reported to display neuroprotective properties. We set up an animal model of ischemic stroke injury by inducing cerebral ischemia (for 1 h) followed by reperfusion (for 24 h) (CI/R) in rats to examine the protective effects and action mechanisms of dimemorfan against stroke-induced damage. Treatment with dimemorfan (1.0 microg/kg and 10 microg/kg, i.v.) either 15 min before ischemia or at the time of reperfusion, like the putative sigma(1) receptor agonist, PRE084 (10 microg/kg, i.v.), ameliorated the size of the infarct zone by 67-72% or 51-52%, respectively, which was reversed by pre-treatment with the selective sigma(1) receptor antagonist, BD1047 (20 microg/kg, i.v.). Major pathological mechanisms leading to CI/R injury including excitotoxicity, oxidative/nitrosative stress, inflammation, and apoptosis are all downstream events initiated by excessive accumulation of extracellular glutamate. Dimemorfan treatment (10 microg/kg, i.v., at the time of reperfusion) inhibited the expressions of monocyte chemoattractant protein-1 and interleukin-1beta, which occurred in parallel with decreases in neutrophil infiltration, activation of inflammation-related signals (p38 mitogen-activated protein kinase, nuclear factor-kappaB, and signal transducer and activator of transcription-1), expression of neuronal and inducible nitric oxide synthase, oxidative/nitrosative tissue damage (lipid peroxidation, protein nitrosylation, and 8-hydroxy-guanine formation), and apoptosis in the ipsilateral cortex after CI/R injury. Dimemorfan treatment at the time of reperfusion, although did not prevent an early rise of glutamate level, significantly prevented subsequent glutamate accumulation after reperfusion. This inhibitory effect was lasted for more than 4 h and was reversed by pre-treatment with BD1047. These results suggest that dimemorfan activates the sigma(1) receptor to reduce glutamate accumulation and then suppresses initiation of inflammation-related events and signals as well as induction of oxidative and nitrosative stresses, leading to reductions in tissue damage and cell death. In conclusion, our results demonstrate for the first time that dimemorfan exhibits protective effects against ischemic stroke in CI/R rats probably through modulation of sigma(1) receptor-dependent signals to prevent subsequent glutamate accumulation and its downstream pathologic events.

Kainate-induced mitochondrial oxidative stress contributes to hippocampal degeneration in senescence-accelerated mice

We have demonstrated that kainate (KA) induces a reduction in mitochondrial Mn-superoxide dismutase (Mn-SOD) expression in the rat hippocampus and that KA-induced oxidative damage is more prominent in senile-prone (SAM-P8) than senile-resistant (SAM-R1) mice. To extend this, we examined whether KA seizure sensitivity contributed to mitochondrial degeneration in these mouse strains. KA-induced seizure susceptibility in SAM-P8 mice paralleled prominent increases in lipid peroxidation and protein oxidation and was accompanied by significant impairment in glutathione homeostasis in the hippocampus. These findings were more pronounced in the mitochondrial fraction than in the hippocampal homogenate. Consistently, KA-induced decreases in Mn-SOD protein expression, mitochondrial transmembrane potential, and uncoupling protein (UCP)-2 expression were more prominent in SAM-P8 than SAM-R1 mice. Marked release of cytochrome c from mitochondria into the cytosol and a higher level of caspase-3 cleavage were observed in KA-treated SAM-P8 mice. Additionally, electron microscopic evaluation indicated that KA-induced increases in mitochondrial damage and lipofuscin-like substances were more pronounced in SAM-P8 than SAM-R1 animals. These results suggest that KA-mediated mitochondrial oxidative stress contributed to hippocampal degeneration in the senile-prone mouse.

Sigma 1 receptor-mediated increase in hippocampal extracellular dopamine contributes to the mechanism of the anticonvulsant action of neuropeptide Y

The potent anticonvulsant properties of neuropeptide Y (NPY) are generally attributed to a Y2 receptor-mediated inhibition of glutamatergic synaptic transmission. Independent studies have shown that NPY increases brain dopamine content, possibly via interaction with sigma 1 receptors. Recently, we showed that increased extracellular hippocampal dopamine attenuates pilocarpine-induced limbic seizures via activation of hippocampal D2 receptors. Our aim in this study was to elucidate the role of increased hippocampal dopamine in the mechanism of the anticonvulsant action of NPY and to investigate the involvement of Y2 and sigma 1 receptors in this process. Limbic seizures were evoked in freely moving rats by intrahippocampal administration of pilocarpine via a microdialysis probe. NPY was administered intracerebroventricularly, intrahippocampally via the microdialysis probe, or coadministered intrahippocampally with the D2 receptor antagonist remoxipride, the Y2 receptor antagonist BIIE0246 or the sigma 1 receptor antagonist BD1047. Changes in hippocampal extracellular dopamine were monitored, and behavioural changes indicative of seizure activity were scored. Intracerebroventricular (10 nmol/3 microL) and intrahippocampal (20-50 microm) NPY administration increased hippocampal dopamine and attenuated pilocarpine-induced seizures. Hippocampal D2 receptor blockade (4 microm remoxipride) reversed the anticonvulsant effect of NPY. Y2 receptor blockade (1 microm BIIE0246) reversed the anticonvulsant effect of NPY but did not prevent NPY-induced increases in hippocampal dopamine. Sigma 1 receptor blockade (10 microm BD1047) abolished NPY-induced increases in hippocampal dopamine and reversed the anticonvulsant effect of NPY. Our results indicate that NPY-induced increases in hippocampal dopamine are mediated via sigma 1 receptors and contribute to the anticonvulsant effect of NPY via increased activation of hippocampal D2 receptors. This novel mechanism of anticonvulsant action of NPY is separate from, and may be complementary to, the well established Y2 receptor-mediated inhibition of hippocampal excitability.

Dextromethorphan as a Potential Neuroprotective Agent With Unique Mechanisms of Action

BACKGROUND

Dextromethorphan (DM) is a widely-used antitussive. DM's complex central nervous system (CNS) pharmacology became of interest when it was discovered to be neuroprotective due to its low-affinity, uncompetitive N-methyl-D-aspartate (NMDA) receptor antagonism.

REVIEW SUMMARY

Mounting preclinical evidence has proven that DM has important neuroprotective properties in various CNS injury models, including focal and global ischemia, seizure, and traumatic brain injury paradigms. Many of these protective actions seem functionally related to its inhibitory effects on glutamate-induced neurotoxicity via NMDA receptor antagonist, sigma-1 receptor agonist, and voltage-gated calcium channel antagonist actions. DM's protection of dopamine neurons in parkinsonian models may be due to inhibition of neurodegenerative inflammatory responses. Clinical findings are limited, with preliminary evidence indicating that DM protects against neuronal damage. Negative findings seem to relate to attainment of inadequate DM brain concentrations. Small studies have shown some promise for treatment of perioperative brain injury, amyotrophic lateral sclerosis, and symptoms of methotrexate neurotoxicity. DM safety/tolerability trials in stroke, neurosurgery, and amyotrophic lateral sclerosis patients demonstrated a favorable safety profile. DM's limited clinical benefit is proposed to be associated with its rapid metabolism to dextrorphan, which restricts its central bioavailability and therapeutic utility. Systemic concentrations of DM can be increased via coadministration of low-dose quinidine (Q), which reversibly inhibits its first-pass elimination. Potential drug interactions with DM/Q are discussed.

CONCLUSIONS

Given the compelling preclinical evidence for neuroprotective properties of DM, initial clinical neuroprotective findings, and clinical demonstrations that the DM/Q combination is well tolerated, this strategy may hold promise for the treatment of various acute and degenerative neurologic disorders.