Role of calcium in sigma-mediated neuroprotection in rat primary cortical neurons

Evolving therapeutic interventions for the management and treatment of Alzheimer's disease

Alzheimer's Disease (AD) patients experience diverse symptoms, including memory loss, cognitive impairment, behavioral abnormalities, mood changes, and mental issues. The fundamental objective of this review is to discuss novel therapeutic approaches, with special emphasis on recently approved marketed formulations for the treatment of AD, especially Aducanumab, the first FDA approved moiety that surpasses the blood-brain barrier (BBB) and reduces amyloid plaques in the brain, thereby reducing associated cognitive decline. However, it is still in the phase IV trial and is to be completed by 2030. Other drugs such as lecanemab are also under clinical trial and has recently been approved by the FDA and is also discussed here. In this review, we also focus on active and passive immunotherapy for AD as well as several vaccines, such as amyloid-beta epitope-based vaccines, amyloid-beta DNA vaccines, and stem cell therapy for AD, which are in clinical trials. Furthermore, ongoing pre-clinical trials associated with AD and other novel strategies such as curcumin-loaded nanoparticles, Crispr/ cas9, precision medicine, as well as some emerging therapies like anti-sense therapy are also highlighted. Additionally, we discuss some off-labeled drugs like non-steroidal anti-inflammatory drugs (NSAID), anti-diabetic drugs, and lithium, which can manage symptoms of AD and different non-pharmacological approaches are also covered which can help to manage AD. In summary, we have tried to cover all the therapeutic interventions which are available for the treatment and management of AD under sections approved, clinical phase, pre-clinical phase or futuristic interventions, off-labelled drugs, and non-pharmacological interventions for AD, offering positive findings and well as challenges that remain.

Brain-derived neurotrophic factor for high-throughput evaluation of selective Sigma-1 receptor ligands

The Sigma-1 receptor (S1R) is an endoplasmic reticulum (ER) chaperone protein that has been implicated in attenuating inflammatory stress-mediated brain injuries. Selective S1R agonists represent a new class of therapeutic agent for treating neuropsychiatric and neurodegenerative disorders, however, to date, no S1R ligand has been approved for therapeutic purposes. We used three potential methods on known and potential S1R ligands to develop an unambiguous high-throughput cell screen for S1R activity. We screened known and potential S1R ligands using radioligand binding and previously reported markers of S1R activity including BDNF release, modulation of IP3 mediated calcium release, and modulation of NGF-induced neurite sprouting. Here, we present results several prototypical S1R compounds and some compounds with the potential for drug repurposing. Using an in-situ ELISA approach we demonstrated that these compounds could stimulate S1R-mediated BDNF release, which is a valuable therapeutic property since BDNF plays a critical role in neuronal support. These compounds were classified as S1R agonists because the BDNF response was comparable to the prototypical agonist 4-PPBP and because it could be reversed by a S1R selective concentration of the antagonist BD1063. When modulation of IP3 mediated calcium response and NGF-induced neurite sprouting were used as a measure of S1R activation, we were unable to reproduce the published results and determined that they are not reliable measures for evaluating functional properties of S1R ligands.

The molecular chaperone sigma 1 receptor mediates rescue of retinal cone photoreceptor cells via modulation of NRF2

Sigma 1 receptor (Sig1R), a putative molecular chaperone, has emerged as a novel therapeutic target for retinal degenerative disease. Earlier studies showed that activation of Sig1R via the high-affinity ligand (+)-pentazocine ((+)-PTZ) induced profound rescue of cone photoreceptor cells in the rd10 mouse model of retinitis pigmentosa; however the mechanism of rescue is unknown. Improved cone function in (+)-PTZ-treated mice was accompanied by reduced oxidative stress and normalization of levels of NRF2, a transcription factor that activates antioxidant response elements (AREs) of hundreds of cytoprotective genes. Here, we tested the hypothesis that modulation of NRF2 is central to Sig1R-mediated cone rescue. Activation of Sig1R in 661W cone cells using (+)-PTZ induced dose-dependent increases in NRF2-ARE binding activity and NRF2 gene/protein expression, whereas silencing Sig1R significantly decreased NRF2 protein levels and increased oxidative stress, although (+)-PTZ did not disrupt NRF2-KEAP1 binding. In vivo studies were conducted to investigate whether, in the absence of NRF2, activation of Sig1R rescues cones. (+)-PTZ was administered systemically for several weeks to rd10/nrf2+/+ and rd10/nrf2-/- mice. Through post-natal day 42, cone function was significant in rd10/nrf2+/+, but minimal in rd10/nrf2-/- mice as indicated by electroretinographic recordings using natural noise stimuli, optical coherence tomography and retinal histological analyses. Immunodetection of cones was limited in (+)-PTZ-treated rd10/nrf2-/-, though considerable in (+)-PTZ-treated rd10/nrf2+/+mice. The data suggest that Sig1R-mediated cone rescue requires NRF2 and provide evidence for a previously-unrecognized relationship between these proteins.

Imaging sigma receptors in the brain: New opportunities for diagnosis of Alzheimer's disease and therapeutic development

The sigma-1 (σ₁) receptor is a chaperone protein located on the mitochondria-associated membrane of the endoplasmic reticulum, while the sigma-2 receptor (σ₂) is an endoplasmic reticulum-resident membrane protein. Recent evidence indicates that both of these receptors figure prominently in the pathophysiology of Alzheimer's disease (AD) and thus are targets for the development of novel, disease-modifying therapeutic strategies. Radioligand-based molecular imaging technique such as positron emission tomography (PET) imaging is a powerful tool for the investigation of protein target expression and function in living subjects. In this review, we survey the development of PET radioligands for the σ₁ or σ₂ receptors and assess their potential for human imaging applications. The availability of PET imaging with σ₁ or σ₂ receptor-specific radioligands in humans will allow the investigation of these receptors in vivo and lead to further understanding of their respective roles in AD pathogenesis and progression. Moreover, PET imaging can be used in target occupancy studies to assess target engagement and correlate receptor occupancy and therapeutic response of σ₁ receptor agonists and σ₂ receptor antagonists currently in clinical trials. It is expected that neuroimaging of σ₁ and σ₂ receptors in the brain will shed new light on AD pathophysiology and may provide us with new biomarkers for diagnosis of AD and efficacy monitoring of emerging AD therapeutic strategies.

The Role of Sigma-1 Receptor, an Intracellular Chaperone in Neurodegenerative Diseases

BACKGROUND

Widespread protein aggregation occurs in the living system under stress or during aging, owing to disturbance of endoplasmic reticulum (ER) proteostasis. Many neurodegenerative diseases may have a common mechanism: the failure of protein homeostasis. Perturbation of ER results in unfolded protein response (UPR). Prolonged chronical UPR may activate apoptotic pathways and cause cell death.

METHODS

Research articles on Sigma-1 receptor were reviewed.

RESULTS

ER is associated to mitochondria by the mitochondria-associated ER-membrane, MAM. The sigma-1 receptor (Sig-1R), a well-known ER-chaperone localizes in the MAM. It serves for Ca2+-signaling between the ER and mitochondria, involved in ion channel activities and especially important during neuronal differentiation. Sig-1R acts as central modulator in inter-organelle signaling. Sig-1R helps cell survival by attenuating ER-stress. According to sequence based predictions Sig-1R is a 223 amino acid protein with two transmembrane (2TM) domains. The X-ray structure of the Sig-1R [1] showed a membrane-bound trimeric assembly with one transmembrane (1TM) region. Despite the in vitro determined assembly, the results of in vivo studies are rather consistent with the 2TM structure. The receptor has unique and versatile pharmacological profile. Dimethyl tryptamine (DMT) and neuroactive steroids are endogenous ligands that activate Sig-1R. The receptor has a plethora of interacting client proteins. Sig-1R exists in oligomeric structures (dimer-trimer-octamer-multimer) and this fact may explain interaction with diverse proteins.

CONCLUSION

Sig-1R agonists have been used in the treatment of different neurodegenerative diseases, e.g. Alzheimer's and Parkinson's diseases (AD and PD) and amyotrophic lateral sclerosis. Utilization of Sig-1R agents early in AD and similar other diseases has remained an overlooked therapeutic opportunity.

Activation of the sigma-1 receptor by haloperidol metabolites facilitates brain-derived neurotrophic factor secretion from human astroglia

Glial cells play a critical role in neuronal support which includes the production and release of the neurotrophin brain-derived neurotrophic factor (BDNF). Activation of the sigma-1 receptor (S1R) has been shown to attenuate inflammatory stress-mediated brain injuries, and there is emerging evidence that this may involve a BDNF-dependent mechanism. In this report we studied S1R-mediated BDNF release from human astrocytic glial cells. Astrocytes express the S1R, which mediates BDNF release when stimulated with the prototypical S1R agonists 4-PPBP and (+)-SKF10047. This effect could be antagonized by a selective concentration of the S1R antagonist BD1063. Haloperidol is known to have high affinity interactions with the S1R, yet it was unable to facilitate BDNF release. Remarkably, however, two metabolites of haloperidol, haloperidol I and haloperidol II (reduced haloperidol), were discovered to facilitate BDNF secretion and this effect was antagonized by BD1063. Neither 4-PPBP, nor either of the haloperidol metabolites affected the level of BDNF mRNA as assessed by qPCR. These results demonstrate for the first time that haloperidol metabolites I and II facilitate the secretion of BDNF from astrocytes by acting as functionally selective S1R agonists.

Recent Advances in Neurogenic Small Molecules as Innovative Treatments for Neurodegenerative Diseases

The central nervous system of adult mammals has long been considered as a complex static structure unable to undergo any regenerative process to refurbish its dead nodes. This dogma was challenged by Altman in the 1960s and neuron self-renewal has been demonstrated ever since in many species, including humans. Aging, neurodegenerative, and some mental diseases are associated with an exponential decrease in brain neurogenesis. Therefore, the controlled pharmacological stimulation of the endogenous neural stem cells (NSCs) niches might counteract the neuronal loss in Alzheimer’s disease (AD) and other pathologies, opening an exciting new therapeutic avenue. In the last years, druggable molecular targets and signalling pathways involved in neurogenic processes have been identified, and as a consequence, different drug types have been developed and tested in neuronal plasticity. This review focuses on recent advances in neurogenic agents acting at serotonin and/or melatonin systems, Wnt/β-catenin pathway, sigma receptors, nicotinamide phosphoribosyltransferase (NAMPT) and nuclear erythroid 2-related factor (Nrf2).

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.

Role of sigma-1 receptors in neurodegenerative diseases

Neurodegenerative diseases with distinct genetic etiologies and pathological phenotypes appear to share common mechanisms of neuronal cellular dysfunction, including excitotoxicity, calcium dysregulation, oxidative damage, ER stress and mitochondrial dysfunction. Glial cells, including microglia and astrocytes, play an increasingly recognized role in both the promotion and prevention of neurodegeneration. Sigma receptors, particularly the sigma-1 receptor subtype, which are expressed in both neurons and glia of multiple regions within the central nervous system, are a unique class of intracellular proteins that can modulate many biological mechanisms associated with neurodegeneration. These receptors therefore represent compelling putative targets for pharmacologically treating neurodegenerative disorders. In this review, we provide an overview of the biological mechanisms frequently associated with neurodegeneration, and discuss how sigma-1 receptors may alter these mechanisms to preserve or restore neuronal function. In addition, we speculate on their therapeutic potential in the treatment of various neurodegenerative disorders.

Neuroprotective effects of the sigma-1 receptor ligand PRE-084 against excitotoxic perinatal brain injury in newborn mice

Excessive glutamate release followed by N-methyl-D-aspartate receptor (NMDAR) activation plays a crucial role in perinatal brain injury. We have previously shown that dextromethorphan, a low-affinity NMDAR antagonist with anti-inflammatory properties, is neuroprotective against neonatal excitotoxic brain injury. Of interest, dextromethorphan is also a sigma-1 receptor (σ1R) agonist. The pharmacologic class of σ1R agonists has yielded propitious results in various animal models of adult central nervous system pathology. In an established neonatal mouse model of excitotoxic brain injury, we evaluated the effect of the selective σ1R agonist 2-(4-morpholinethyl) 1-phenylcyclohexanecarboxylate (PRE-084). A single intraperitoneal injection of 0.1 μg/g (low dose) or 10 μg/g (high dose) bodyweight (bw) PRE-084, given 1h after the excitotoxic insult, significantly reduced lesion size in cortical gray matter 24 h and 120 h after the insult. Repetitive injections of 0.1 μg/g PRE-084 proved to be equally effective. PRE-084 treatment resulted in a decrease in cell death indicated by reduced TUNEL positivity and caspase-3 activation. Furthermore, it lowered the number of isolectin B4-positive, activated microglial cells. PRE-084 had no effect on developmental apoptosis in the undamaged brain. In vitro findings in primary hippocampal neurons suggest that PRE-084 treatment provides partial protection against glutamate induced morphological and functional changes. For excitotoxicity as playing a crucial role in the pathogenesis of perinatal brain injury, we demonstrate for the first time that systemic treatment with the highly selective σ1R agonist PRE-084 protects against NMDAR-mediated excitotoxic brain damage.

Sigma-1 receptors do not regulate calcium influx through voltage-dependent calcium channels in mouse brain synaptosomes

Several lines of evidence suggest that σ(1) receptors regulate intracellular calcium concentration [Ca(2+)](i). However, no previous studies have demonstrated a consistent role for these receptors in the modulation of extracellular calcium entry through plasmalemmal voltage-dependent calcium channels (VDCCs). To search for evidence of such a role we compared [Ca(2+)](i) under basal conditions and after depolarization with KCl in fura-2-loaded synaptosomes from wild-type and σ(1) receptor knockout (σ(1)R-KO) mice. We also tested the effects of the selective σ(1) receptor agonists PRE-084 and (+)-pentazocine and antagonists BD-1047 and NE-100 on the increase in [Ca(2+)](i) induced by depolarization with 60mM KCl. Mibefradil, a nonselective blocker of VDCCs, was used as a positive control. Basal [Ca(2+)](i) and the increase in [Ca(2+)](i) caused by KCl-induced depolarization were similar in brain synaptosomes from both wild-type and σ(1)R-KO mice. Mibefradil (1-30 μM) and all σ(1) receptor ligands studied (3-100 μM) inhibited the KCl-induced increase in [Ca(2+)](i) in a concentration-dependent way. The order of maximum inhibition for the ligands compared here was NE-100>BD-1047=PRE 084>(+)-pentazocine. There were no appreciable differences in their effects between wild-type and σ(1)R-KO mice. These findings indicate that σ(1) receptors are not involved in calcium influx through VDCCs or in the inhibitory effects of these σ(1) ligands on Ca(2+) channels.

Sigma-1 receptor ligand PRE-084 reduced infarct volume, neurological deficits, pro-inflammatory cytokines and enhanced anti-inflammatory cytokines after embolic stroke in rats

Sigma receptor agonists have been found to provide potent neuroprotection in rats and mice. This neuroprotection is thought to be mediated through anti-excitotoxic mechanisms. Neuroprotective and immune modulatory effects of sigma ligands have not been investigated in embolic stroke. In the present study, rats were subjected to embolic stroke or sham stroke and were treated with the sigma-1 receptor agonist PRE-084 (5mg/kg i.p.) or saline vehicle 3 and 24h after stroke. Infarct volume and behavioural tests were conducted, and cytokine levels (ILs-1α and β, IL-2, IL-4, IL-6, IL-10, GM-CSF and TNF-α) were determined in ischemic and non-ischemic cortices. Axonal damage was determined using the pNF-H ELISA assay. Treatment with PRE-084 afforded neuroprotection following embolic stroke as evidenced by significantly reduced infarct volume and improved behavioural outcome. Remarkably, treatment with PRE-084 reduced levels of pro-inflammatory cytokines and enhanced anti-inflammatory cytokines. Levels of pNF-H were lower in rats treated with PRE-084 suggesting reduced axonal damage but this finding did not reach statistical significance. The findings of the present study suggest that part of the neuroprotective effects of sigma-1 receptor agonists may be mediated through a dual effect on cytokine release following stroke.

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.

Sigma receptor ligand 4-phenyl-1-(4-phenylbutyl)-piperidine modulates neuronal nitric oxide synthase/postsynaptic density-95 coupling mechanisms and protects against neonatal ischemic degeneration of striatal neurons

In adult stroke models, 4-phenyl-1-(4-phenylbutyl) piperidine (PPBP), a sigma receptor agonist, attenuates activity of neuronal nitric oxide synthase (nNOS), blunts ischemia-induced nitric oxide production, and provides neuroprotection. Here, we tested the hypothesis that PPBP attenuates neuronal damage in a model of global hypoxia-ischemia (H-I) in newborn piglets. Piglets subjected to hypoxia followed by asphyxic cardiac arrest were treated with saline or two dosing regimens of PPBP after resuscitation. Sigma-1 receptors were found in striatal neurons. PPBP dose-dependently protected neurons in putamen at 4 days of recovery from H-I. Immunoblots of putamen extracts at 3 h of recovery showed that PPBP decreased H-I-induced recruitment of nNOS in the membrane fraction and reduced the association of nNOS with NMDA receptor NR2 subunit. The latter effect was associated with changes in the coupling of nNOS to postsynaptic density-95 (PSD-95), but not NR2-PSD-95 interactions. Moreover, PPBP suppressed NOS activity in the membrane fraction and reduced H-I-induced nitrative and oxidative damage to proteins and nucleic acids. These findings indicate that PPBP protects striatal neurons in a large animal model of neonatal H-I and that the protection is associated with decreased coupling of nNOS to PSD-95.

Neuroprotective profile of dextromethorphan in an experimental model of penetrating ballistic-like brain injury

Dextromethorphan (DM) has been well-characterized as a neuroprotective agent in experimental models of CNS injury. The goal of this study was to determine the neuroprotective profile of DM in a military-relevant model of penetrating ballistic-like brain injury (PBBI). In an acute (3 day) dose-response study, anesthetized male Sprague-Dawley rats were exposed to a unilateral frontal PBBI with DM (0.156-10 mg/kg) or vehicle delivered as an i.v. bolus from 30 min to 48 h post-injury. In a follow-up (7 day) experiment, the 10-mg/kg bolus injections of DM were administered in conjunction with a 6-h infusion (5 mg/kg/h). DM bolus injections alone produced a dose-dependent improvement in motor recovery on a balance beam task at 3 days post-injury. However, more rapid recovery (24 h) was observed on this task when the bolus injections were combined with the 6-h infusion. Moreover, the DM bolus/infusion treatment regimen resulted in a significant (76%) improvement in cognitive performance in a novel object recognition (NOR) task at 7 days post-injury. Although post-injury administration of DM (all doses) failed to reduce core lesion size, the maximum dose of DM (10 mg/kg) was effective in reducing silver-stained axonal fiber degeneration in the cortical regions adjacent to the injury.

sigma-1 receptor modulation of acid-sensing ion channel a (ASIC1a) and ASIC1a-induced Ca2+ influx in rat cortical neurons

Acid-sensing ion channels (ASICs) are proton-gated cation channels found in peripheral and central nervous system neurons. The ASIC1a subtype, which has high Ca2+ permeability, is activated by ischemia-induced acidosis and contributes to the neuronal loss that accompanies ischemic stroke. Our laboratory has shown that activation of sigma receptors depresses ion channel activity and [Ca2+](i) dysregulation during ischemia, which enhances neuronal survival. Whole-cell patch-clamp electrophysiology and fluorometric Ca2+ imaging were used to determine whether sigma receptors regulate the function of ASIC in cultured rat cortical neurons. Bath application of the selective ASIC1a blocker, psalmotoxin1, decreased proton-evoked [Ca2+](i) transients and peak membrane currents, suggesting the presence of homomeric ASIC1a channels. The pan-selective sigma-1/sigma-2 receptor agonists, 1,3-di-o-tolyl-guanidine (100 microM) and opipramol (10 microM), reversibly decreased acid-induced elevations in [Ca2+](i) and membrane currents. Pharmacological experiments using sigma receptor-subtype-specific agonists demonstrated that sigma-1, but not sigma-2, receptors inhibit ASIC1a-induced Ca2+ elevations. These results were confirmed using the irreversible sigma receptor antagonist metaphit (50 microM) and the selective sigma-1 antagonist BD1063 (10 nM), which obtunded the inhibitory effects of the sigma-1 agonist, carbetapentane. Activation of ASIC1a was shown to stimulate downstream Ca2+ influx pathways, specifically N-methyl-D-aspartate and (+/-)-alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid/kainate receptors and voltage-gated Ca2+ channels. These subsequent Ca2+ influxes were also inhibited upon activation of sigma-1 receptors. These findings demonstrate that sigma-1 receptor stimulation inhibits ASIC1a-mediated membrane currents and consequent intracellular Ca2+ accumulation. The ability to control ionic imbalances and Ca2+ dysregulation evoked by ASIC1a activation makes sigma receptors an attractive target for ischemic stroke therapy.

Prevention of excitotoxicity in primary retinal ganglion cells by (+)-pentazocine, a sigma receptor-1 specific ligand

PURPOSE

Sigma receptors (sigmaRs) are nonopioid, nonphencyclidine binding sites with robust neuroprotective properties. Previously, the authors induced death in the RGC-5 cell line using very high concentrations (1 mM) of the excitatory amino acids glutamate (Glu) and homocysteine (Hcy) and demonstrated that the sigmaR1 ligand (+)-pentazocine ((+)-PTZ) could protect against cell death. The purpose of the present study was to establish a physiologically relevant paradigm for testing the neuroprotective effect of (+)-PTZ in retinal ganglion cells (RGCs).

METHODS

Primary ganglion cells (GCs) were isolated by immunopanning from retinas of 1-day-old mice, maintained in culture for 3 days, and exposed to 10, 20, 25, or 50 microM Glu or 10, 25, 50, or 100 microM Hcy for 6 or 18 hours in the presence or absence of (+)-PTZ (0.5, 1, 3 microM). Cell viability was measured using the viability and apoptosis detection fluorescein in situ assays. Expression of sigmaR1 was assessed by immunocytochemistry, RT-PCR, and Western blotting. Morphologic appearance of live ganglion cells and their processes was examined over time (0, 3, 6, 18 hours) by differential interference contrast (DIC) microscopy after exposure to excitotoxins in the presence or absence of (+)-PTZ.

RESULTS

Primary GCs showed robust sigmaR1 expression. The cells were exquisitely sensitive to Glu or Hcy toxicity (6-hour treatment with 25 or 50 microM Glu or 50 or 100 microM Hcy induced marked cell death). Primary GCs pretreated for 1 hour with (+)-PTZ followed by 18-hour cotreatment with 25 microM Glu and (+)-PTZ showed a marked decrease in cell death: 25 microM Glu alone, 50%; 25 microM Glu/0.5 microM (+)-PTZ, 38%; 25 microM Glu/1 microM (+)-PTZ, 20%; 25 microM Glu/3 microM (+)-PTZ, 18%. Similar results were obtained with Hcy. sigmaR1 mRNA and protein levels did not change in the presence of the excitotoxins. DIC examination of cells exposed to excitotoxins revealed substantial disruption of neuronal processes; cotreatment with (+)-PTZ revealed marked preservation of these processes. The stereoselective effect of (+)-PTZ for sigmaR1 was established in experiments in which (-)-PTZ, the levo-isomer form of pentazocine, had no neuroprotective effect on excitotoxin-induced ganglion cell death.

CONCLUSIONS

Primary GCs express sigmaR1; their marked sensitivity to Glu and Hcy toxicity mimics the sensitivity observed in vivo, making them a highly relevant model for testing neuroprotection. Pretreatment of cells with 1 to 3 microM (+)-PTZ, but not (-)-PTZ, affords significant protection against Glu- and Hcy-induced cell death. sigmaR1 ligands may be useful therapeutic agents in retinal diseases in which ganglion cells die.

Sigma receptor agonists provide neuroprotection in vitro by preserving bcl-2

BACKGROUND

Sigma (sigma)-receptor agonists attenuate brain injury after experimental focal cerebral ischemia in several species. We tested the hypothesis that the potent, prototypical sigma(1)-receptor agonist, 4-phenyl-1-(4-phenylbutyl) piperidine (PPBP), protects neurons by a mechanism involving the antiapoptotic protein bcl-2.

METHODS

Primary cortical neuronal cultures were exposed to either 2 h of oxygen-glucose deprivation (OGD) or glutamate (100 microM). PPBP treatment was initiated either 15 min prior to the insult or at 15 min postinsult then continued for 24 h. In another set of experiments, cultured neurons were preincubated for 2 h prior to PPBP treatment with sigma1-receptor antagonist, rimcazole, in a dose-dependent manner. Alive and dead cells were detected with calcein-AM and propidium iodide respectively. Bcl-2 and bax expression were determined by quantitative real time reverse transcription polymerase chain reaction and western blotting, and DNA damage was detected by TUNEL staining.

RESULTS

PPBP pretreatment attenuated neuronal injury induced by OGD or glutamate (50 or 100 microM). This protection was reversed with rimcazole (cell death: OGD 48 +/- 2%, OGD plus PPBP 31 +/- 3%, OGD plus PPBP with rimcazole 46 +/- 2%). PPBP treatment increased bcl-2 but not bax mRNA levels. PPBP's ability to preserve bcl-2 protein after OGD by PPBP was fully abolished by rimcazole. Lastly, PPBP reduced the number of TUNEL-positive cells after OGD, suggesting fewer cells with overt DNA damage.

CONCLUSIONS

These data demonstrate that PPBP reduces cell death in vitro by a mechanism involving receptor-dependent preservation of protective genes such as bcl-2.

Sigma-1 receptor activation prevents intracellular calcium dysregulation in cortical neurons during in vitro ischemia

Sigma receptors are putative targets for neuroprotection following ischemia; however, little is known on their mechanism of action. One of the key components in the demise of neurons following ischemic injury is the disruption of intracellular calcium homeostasis. Fluorometric calcium imaging was used to examine the effects of sigma receptor activation on changes in intracellular calcium concentrations ([Ca(2+)](i)) evoked by in vitro ischemia in cultured cortical neurons from embryonic rats. The sigma receptor agonist, 1,3-di-o-tolyl-guanidine (DTG), was shown to depress [Ca(2+)](i) elevations observed in response to ischemia induced by sodium azide and glucose deprivation. Two sigma receptor antagonists, metaphit [1-(1-(3-isothiocyanatophenyl)-cyclohexyl)-piperidine] and BD-1047 (N-[2-3,4-dichlorophenyl)-ethyl]-N-methyl-2-(dimethylamino)ethylamine), were shown to blunt the ability of DTG to inhibit ischemia-evoked increases in [Ca(2+)](i), revealing that the effects are mediated by activation of sigma receptors and not via the actions of DTG on nonspecific targets such as N-methyl-d-aspartate receptors. DTG inhibition of ischemia-induced increases in [Ca(2+)](i) was mimicked by the sigma-1 receptor-selective agonists, carbetapentane, (+)-pentazocine and PRE-084 [2-(4-morpholinethyl) 1-phenylcyclohexanecarboxylate hydrochloride], but not by the sigma-2-selective agonist, ibogaine, showing that activation of sigma-1 receptors is responsible for the effects. In contrast, DTG, carbetapentane, and ibogaine blocked spontaneous, synchronous calcium transients observed in our preparation at concentrations consistent with sigma receptor-mediated effects, indicating that both sigma-1 and sigma-2 receptors regulate events that affect [Ca(2+)](i) in cortical neurons. Our studies show that activation of sigma receptors can ameliorate [Ca(2+)](i) dysregulation associated with ischemia in cortical neurons and, thus, identify one of the mechanisms by which these receptors may exert their neuroprotective properties.

Expression, subcellular localization, and regulation of sigma receptor in retinal muller cells

PURPOSE

Sigma receptors (sigmaRs) are nonopioid, nonphencyclidine binding sites with robust neuroprotective properties. Type 1 sigmaR1 (sigmaR1) is expressed in brain oligodendrocytes, but its expression and binding capacity have not been analyzed in retinal glial cells. This study examined the expression, subcellular localization, binding activity, and regulation of sigmaR1 in retinal Müller cells.

METHODS

Primary mouse Müller cells (MCs) were analyzed by RT-PCR, immunoblotting, and immunocytochemistry for the expression of sigmaR1, and data were compared with those of the rat Müller cell line (rMC-1) and the rat ganglion cell line (RGC-5). Confocal microscopy was used to determine the subcellular sigmaR1 location in primary mouse MCs. Membranes prepared from these cells were used for binding assays with [3H]-pentazocine (PTZ). The kinetics of binding, the ability of various sigmaR1 ligands to compete with sigmaR1 binding, and the effects of donated nitric oxide (NO) and reactive oxygen species (ROS) on binding were examined.

RESULTS

sigmaR1 is expressed in primary mouse MCs and is localized to the nuclear and endoplasmic reticulum membranes. Binding assays showed that in primary mouse MCs, rMC-1, and RGC-5, the binding of PTZ was saturable. [3H]-PTZ bound with high affinity in RGC-5 and rMC-1 cells, and the binding was similarly robust in primary mouse MCs. Competition studies showed marked inhibition of [3H]-PTZ binding in the presence of sigmaR1-specific ligands. Incubation of cells with NO and ROS donors markedly increased sigmaR1 binding activity.

CONCLUSIONS

MCs express sigmaR1 and demonstrate robust sigmaR1 binding activity, which is inhibited by sigmaR1 ligands and is stimulated during oxidative stress. The potential of Müller cells to bind sigmaR1 ligands may prove beneficial in retinal degenerative diseases such as diabetic retinopathy.

Sigma 1 receptor agonists act as neuroprotective drugs through inhibition of inducible nitric oxide synthase

Postischemic administration of the sigma-1 agonists reduces ischemic brain injury; however, the mechanism is unclear. We hypothesized that the sigma-1 agonist (+)isoform of pentazocine (P(+)) reduces damage in part by ameliorating cell death mediated via inducible nitric oxide synthase (iNOS) and that the (-)isoform (P(-)) lacks this effect. We compared treatment with P(+) with or without the iNOS inhibitor aminoguanidine (AG) and also the effects of P(+) in iNOS deficient (iNOSKO) mice. A possible mechanism of neuroprotection is inhibition of iNOS expression. Male C57/Bl6 mice were subjected to transient middle cerebral artery occlusion (90 min) and drugs were administered with reperfusion: 1) P(+) with AG (P+/AG), 2) P(+), 3) P(-), 4) AG, or 5) placebo. iNOSKOs were treated with either P(+) or placebo. Infarction (triphenyltetrazolium chloride histology, 72 h) was reduced by P(+) treatment in striatum by 44% and in neocortex by 23% versus placebo (P < 0.05), a reduction comparable to AG effect. P(-) did not attenuate brain injury. There was no difference in P(+)/AG treatment compared with showed the same level of neuroprotection as P(+) alone. P(+) also did not provide further neuroprotection for iNOSKOs. We conclude that postischemic administration of P(+) reduces infarct volume in mice. Because AG provides no additional benefit to P(+) treatment and iNOSKOs do not benefit from P(+), we speculate that P(+) acts by suppressing cell death resulting from iNOS toxicity.

Neuro(active)steroids actions at the neuromodulatory sigma1 (sigma1) receptor: biochemical and physiological evidences, consequences in neuroprotection

Steroids from peripheral sources or synthesized in the brain, i.e. neurosteroids, exert rapid modulations of neurotransmitter responses through specific interactions with membrane receptors, mainly the gamma-aminobutyric acid type A (GABA(A)) receptor and N-methyl-d-aspartate (NMDA) type of glutamate receptor. Progesterone and 3alpha-hydroxy-5alpha-pregnan-20-one (allopregnanolone) act as inhibitory steroids while pregnenolone sulfate or dehydroepiandrosterone sulfate act as excitatory steroids. Some steroids also interact with an atypical protein, the sigma(1) (sigma(1)) receptor. This receptor has been cloned in several species and is centrally expressed in neurons and oligodendrocytes. Activation of the sigma(1) receptor modulates cellular Ca(2+) mobilization, particularly from endoplasmic reticulum pools, and contributes to the formation of lipid droplets, translocating towards the plasma membrane and contributing to the recomposition of lipid microdomains. The present review details the evidences showing that the sigma(1) receptor is a target for neurosteroids in physiological conditions. Analysis of the sigma(1) protein sequence confirmed homologies with the ERG2/emopamil binding protein family but also with the steroidogenic enzymes isopentenyl diphosphate isomerase and 17beta-estradiol dehydrogenase. Biochemical and physiological arguments for an interaction of neuro(active)steroids with the sigma(1) receptor are analyzed and the impact on physiopathological outcomes in neuroprotection is illustrated.

Plasma profile and pharmacokinetics of dextromethorphan after intravenous and oral administration in healthy dogs

Dextromethorphan is an N-methyl-D-aspartate (NMDA) noncompetitive antagonist which has been used as an antitussive, analgesic adjunct, probe drug, experimentally to attenuate acute opiate and ethanol withdrawal, and as an anticonvulsant. A metabolite of dextromethorphan, dextrorphan, has been shown to behave pharmacodynamically in a similar manner to dextromethorphan. The pharmacokinetics of dextromethorphan were examined in six healthy dogs following intravenous (2.2 mg/kg) and oral (5 mg/kg) administration in a randomized crossover design. Dextromethorphan behaved in a similar manner to other NMDA antagonists upon injection causing muscle rigidity, ataxia to recumbency, sedation, urination, and ptyalism which resolved within 90 min. One dog repeatedly vomited upon oral administration and was excluded from oral analysis. Mean +/- SD values for half-life, apparent volume of distribution, and clearance after i.v. administration were 2.0 +/-0.6 h, 5.1 +/- 2.6 L/kg, and 33.8 +/- 16.5 mL/min/kg. Oral bioavailability was 11% as calculated from naive pooled data. Free dextrorphan was not detected in any plasma sample, however enzymatic treatment of plasma with glucuronidase released both dextromethorphan and dextrorphan indicating that conjugation is a metabolic route. The short half-life, rapid clearance, and poor bioavailability of dextromethorphan limit its potential use as a chronic orally administered therapeutic.

The sigma receptor ligand (+)-pentazocine prevents apoptotic retinal ganglion cell death induced in vitro by homocysteine and glutamate

Recent studies demonstrated that the excitotoxic amino acid homocysteine induces apoptotic death of retinal ganglion cells in vivo. In the present study, an in vitro rat retinal ganglion cell (RGC-5), culture system was used to analyze the toxicity of acute exposure to high levels of homocysteine, the mechanism of homocysteine-induced toxicity, and the usefulness of type 1 sigma receptor (sigmaR1) ligands as neuroprotectants. When cultured RGC-5 cells were subjected to treatment with 1 mM D,L-homocysteine, a significant increase in cell death was detected by terminal dUTP nick end labeling (TUNEL) analysis and analysis of activated caspase. When cells were treated with homocysteine- or glutamate in the presence of MK-801, an antagonist of the N-methyl-D-aspartate (NMDA) receptor, the cell death was inhibited significantly. In contrast, NBQX, an antagonist of the AMPA/Kainate receptor, and nifedipine, a calcium channel blocker, did not prevent the homocysteine- or glutamate-induced cell death. Semiquantitative RT-PCR and immunocytochemical analysis demonstrated that RGC-5 cells were exposed to homocysteine or glutamate express type 1 sigma receptor at levels similar to control cells. Treatment of RGC-5 cells with 3 or 10 microM concentrations of the sigmaR1-specific ligand (+)-pentazocine inhibited significantly the apoptotic cell death induced by homocysteine or glutamate. The results suggest that homocysteine is toxic to ganglion cells in vitro, that the toxicity is mediated via NMDA receptor activation, and that the sigmaR1-specific ligand (+)-pentazocine can block the RGC-5 cell death induced by homocysteine and glutamate.

Potent sigma 1-receptor ligand 4-phenyl-1-(4-phenylbutyl) piperidine provides ischemic neuroprotection without altering dopamine accumulation in vivo in rats

The in vivo signaling of ischemic neuroprotection provided by sigma-receptor ligands remains unclear. Catecholamines have been implicated in the propagation of ischemic neuronal injury, and previous in vitro studies suggest that sigma ligands modulate dopaminergic neurotransmission. In this study, we tested the hypothesis that the potent sigma(1)-receptor ligand 4-phenyl-1-(4-phenylbutyl) piperidine (PPBP) attenuates the increase of extracellular dopamine in ischemic striatum. Under controlled physiological conditions, a microdialysis probe was implanted in right caudoputamen (CP) complex of adult male Wistar rats. Rats were subjected to 2 h of transient middle cerebral artery occlusion (MCAO) by the intraluminal suture technique. In a blinded, randomized fashion, rats were divided into five treatment groups: Group 1 (n = 8; saline-saline) continuous i.v. infusion of saline vehicle 30 min before MCAO followed by saline at reperfusion until the end of the experiment; Group 2 (n = 8; PPBP-PPBP) i.v. PPBP 30 min before MCAO followed by 1 micromol x kg(-1) x h(-1) of PPBP; Group 3 (n = 8; saline-PPBP) i.v. saline before MCAO followed by PPBP; Group 4 (n = 4) surgical shams (saline-saline); and Group 5 (n = 4) surgical shams (PPBP-PPBP). Infarction volume at 22 h of reperfusion in the CP complex (percentage of ipsilateral structure) was significantly attenuated in rats treated with PPBP-PPBP (27.3% +/- 9.1%) and saline-PPBP (27.8% +/- 12.7%) compared with saline-saline (59.3% +/- 7.3%) treatment. There was a three- to fourfold increase in dopamine concentrations in the microdialysates within 40 min of the onset of MCAO. Dopamine and its metabolites dihydroxy phenylacetic acid and homovallinic acid levels were similar among the three groups subjected to MCAO. Therefore, PPBP provides significant ischemic neuroprotection in the CP complex without altering the acute accumulation of dopamine in vivo during transient focal ischemia in the rat.

Cardiovascular effects of intravenous pentazocine and cyclazocine in conscious, curarized-conscious, and anesthetized dogs

The cardiovascular effects of intravenous pentazocine and cyclazocine in dogs were studied under conscious, curarized-conscious (paralyzed by gallamine), and anesthetized states. In the conscious state, blood pressure and heart rate were dose-dependently increased by pentazocine (1, 2, 3 mg/kg) and to a lesser extent by cyclazocine (0.3 mg/kg). In all subsequent experiments on dogs, the results were obtained using 3 mg/kg pentazocine and 0.3 mg/kg cyclazocine. Pentazocine accelerated breathing, peaking at about 10 min, whereas cyclazocine reduced breathing to a minimum in 1 min, followed by a gradual recovery thereafter. In the curarized-conscious state, the blood pressure response to pentazocine was biphasic, namely an initial decrease followed by an increase; chronotrophic activity was stimulated. Pretreatment with either ganglionic or alpha andrenergic blocking agents not only significantly antagonized the pressory responses to the drug but also potentiated the initial decreases in blood pressure and unmasked a bradycardic component, but these parameters were not altered by 0.3 mg/kg naxalone. In open-chest anesthetized dogs, blood pressure, heart rate, contractility, and mean peripheral vascular resistance were simultaneously decreased by both pentazocine and cyclazocine, initially accompanied by increases in aortic blood flow. During the later stages of drug action, only the blood pressure and contractility were increased above control levels (biphasic effect). A comparison of blood pressure and heart rate responses to pentazocine in dogs kept under differing experimental conditions revealed that conscious dogs were more sensitive than curarized conscious and anesthetized animals to pentazocine action. In isolated guinea pig atria, the effect of adrenaline (0.1, 0.3, or 1 mg/mL) on the spontaneous breathing rate was significantly augmented by 10 mg/mL pentazocine (p < 0.02 for 0.3 g/mL; p < 0.01 for 0.1 g/mL adrenaline). In dogs, however, adrenaline (1 mg/kg)-induced increases in heart contractility, aortic blood flow, and blood pressure remained almost unaltered in the presence of pentazocine. We concluded that the abovementioned cardiovascular responses to pentazocine and cyclazocine are a consequence of the sum of the two following opposing effects: (i) an indirect reflex activation of sympathetic neuromediation in the periphery, and (ii) a direct membrane effect on the heart leading to bradycardia and a depression in myocardial contractility.

Analysis of sigma receptor (sigmaR1) expression in retinal ganglion cells cultured under hyperglycemic conditions and in diabetic mice

The type 1 sigma receptor (sigmaR1) is a nonopiate and nonphencyclidine binding site that has numerous pharmacological and physiological functions. In some studies, agonists for sigmaR1 have been shown to afford neuroprotection against overstimulation of the NMDA receptor. sigmaR1 expression has been demonstrated recently in retinal ganglion cells (RGC). RGCs undergo apoptosis early in diabetic retinopathy via NMDA receptor overstimulation. In the present study we asked whether RGCs cultured under hyperglycemic conditions and RGCs of diabetic mice continue to express sigmaR1. RGCs were cultured 48 h in RPMI medium containing either 45 mM glucose or 11 mM glucose plus 34 mM mannitol (osmolar control). C57BL/6 mice were made diabetic using streptozotocin. The retina was dissected from normal and streptozotocin-induced diabetic mice 3, 6 and 12 weeks post-onset of diabetes. sigmaR1 was analyzed in cells using semiquantitative RT-PCR and in tissues by semiquantitative RT-PCR, in situ hybridization, Western blot analysis and immunolocalization. The RT-PCR analysis of cultured RGCs showed that sigmaR1 mRNA is expressed under hyperglycemic conditions at levels similar to control cells. Similarly, analysis of retinas of diabetic mice showed no difference in levels of mRNA encoding sigmaR1 compared to retinas of control mice. In situ hybridization analysis showed that expression patterns of sigmaR1 mRNA in the ganglion cell layer were similar between diabetic and control mice. Western blot analysis suggested that levels of sigmaR1 in retina were similar between diabetic and control retinas. Immunohistochemical analysis of sigmaR1 showed a similar pattern of sigmaR1 protein expression between control and diabetic retina. These studies demonstrate that sigmaR1 is expressed under hyperglycemic conditions in vitro and in vivo.

Spreading depression: imaging and blockade in the rat neocortical brain slice

Spreading depression (SD) is a profound but transient depolarization of neurons and glia that migrates across the cortical and subcortical gray at 2-5 mm/min. Under normoxic conditions, SD occurs during migraine aura where it precedes migraine pain but does not damage tissue. During stroke and head trauma, however, SD can arise repeatedly near the site of injury and may promote neuronal damage. We developed a superfused brain slice preparation that can repeatedly support robust SD during imaging and electrophysiological recording to test drugs that may block SD. Submerged rat neocortical slices were briefly exposed to artificial cerebrospinal fluid (ACSF) with KCl elevated to 26 mM. SD was evoked within 2 min, recorded in layers II/III both as a negative DC shift and as a propagating front of elevated light transmittance (LT) representing transient cell swelling in all cortical layers. An SD episode was initiated focally and could be repeatedly evoked and imaged with no damage to slices. As reported in vivo, pretreatment with one of several N-methyl-D-aspartate (NMDA) receptor antagonists blocked SD, but a non-NMDA glutamate receptor antagonist (CNQX) had no effect. NMDA receptor (NMDAR) activation does not initiate SD nor are NMDAR antagonists tolerated therapeutically so we searched for more efficacious drugs to block SD generation. Pretreatment with the sigma-one receptor (sigma(1)R) agonists dextromethorphan (10-100 microM), carbetapentane (100 microM), or 4-IBP (30 microM) blocked SD, even when KCl exposure was extended beyond 5 min. The block was independent of NMDA receptor antagonism. Two sigma(1)R antagonists [(+)-3PPP and BD-1063] removed this block but had no effect upon SD alone. Remarkably, the sigma(1)R agonists also substantially reduced general cell swelling evoked by bath application of 26 mM KCl. More potent sigma(1)R ligands that are therapeutically tolerated could prove useful in reducing SD associated with migraine and be of potential use in stroke or head trauma.

Neuroprotective effect of sigma(1)-receptor ligand 4-phenyl-1-(4-phenylbutyl) piperidine (PPBP) is linked to reduced neuronal nitric oxide production

BACKGROUND AND PURPOSE

The potent final sigma(1)-receptor ligand 4-phenyl-1-(4-phenylbutyl) piperidine (PPBP) provides neuroprotection in experimental stroke. We tested the hypothesis that PPBP attenuates striatal tissue damage after middle cerebral artery occlusion (MCAO) by a mechanism involving reduction of ischemia-evoked nitric oxide (NO) production. Furthermore, we determined whether the agent fails to protect ischemic brain when neuronal nitric oxide synthase (nNOS) is genetically deleted or pharmacologically inhibited (selective nNOS inhibitor, 7-nitroindazole [7-NI]).

METHODS

Halothane-anesthetized adult male Wistar rats were subjected to 2 hours of MCAO by the intraluminal filament occlusion technique. All physiological variables were controlled during the ischemic insult. In vivo striatal NO production was estimated via microdialysis by quantification of local, labeled citrulline recovery after labeled arginine infusion. In a second series of experiments, nNOS null mutants (nNOSKOs) and the genetically matched wild-type (WT) strain were treated with 90 minutes of MCAO. Brains were harvested at 22 hours of reperfusion for measurement of infarction volume by triphenyltetrazolium chloride histology.

RESULTS

PPBP attenuated infarction volume at 22 hours of reperfusion in cerebral cortex and striatum and markedly attenuated NO production in ischemic and nonischemic striatum during occlusion and early reperfusion. Treatment with 7-NI mimicked the effects of PPBP. In WT mice, infarction volume was robustly decreased by both PPBP and 7-NI, but the efficacy of PPBP was not altered by pharmacological nNOS inhibition in combined therapy. In contrast, PPBP did not decrease infarction volume in nNOSKO mice.

CONCLUSIONS

These data suggest that the mechanism of neuroprotection of PPBP in vivo is through attenuation of nNOS activity and ischemia-evoked NO production. Neuroprotective effects of PPBP are lost when nNOS is not present or is inhibited; therefore, PPBP likely acts upstream from NO generation and its subsequent neurotoxicity.

Neuroprotection from glutamate toxicity with ultra-low dose glutamate

The protective effects of ultra-low doses (ULD) of glutamate against glutamate toxicity was studied in primary rat spinal, cortical and cerebellar neurons. Neurons were exposed to four subtoxic, ultra-low concentrations of glutamate (10(-18) M, 10(-20)M, 10(-22) M and 10(-30) M) for 72 h and then subsequently challenged with toxic concentrations (25 microM) of glutamate. Neuron viability was consistently 10% higher in spinal and cortical neurons pre-exposed to glutamate concentrations of 10(-18) M and 10(-22) M, and in cerebellar neurons pre-exposed to 10(-20) M and 10(-30) M. Using laser scanning confocal microscopy and the fluorescent calcium probe fluo-3, we found no alterations in intracellular calcium dynamics in the protected cells. This protective effect is consistent with a growing body of evidence for tolerance induced by low-dose toxin exposure but is the first time that such tolerance has been demonstrated with ultra-low glutamate exposure. Our data show that pre-exposure of neuronal cells to ULD glutamate can protect against subsequent exposure to toxic levels of glutamate.

sigma receptor ligands attenuate N-methyl-D-aspartate cytotoxicity in dopaminergic neurons of mesencephalic slice cultures

We investigated the potential neuroprotective effects of several sigma receptor ligands in organotypic midbrain slice cultures as an excitotoxicity model system. When challenged with 100-microM N-methyl-D-aspartate (NMDA) for 24 h, dopaminergic neurons in midbrain slice cultures degenerated, and this was prevented by (5R, 10S)-(+)-5-methyl-10,11-dihydro-5H-dibenzo[a,b]-cyclohepten-5, 10-imine (MK-801; 1-10 microM). Concomitant application of ifenprodil (1-10 microM) or haloperidol (1-10 microM), both of which are high-affinity sigma receptor ligands, significantly attenuated the neurotoxicity of 100 microM NMDA. The sigma(1) receptor-selective ligand (+)-N-allylnormetazocine ((+)-SKF 10047; 1-10 microM) was also effective in attenuating the toxicity of NMDA. The effect of R(-)-N-(3-phenyl-1-propyl)-1-phenyl-2-aminopropane hydrochloride ((-)-PPAP), a sigma receptor ligand with negligible affinity for the phencyclidine site of NMDA receptors, was also examined. (-)-PPAP (3-100 microM) caused a concentration-dependent reduction of NMDA cytotoxicity, with significant protection at concentrations of 30 and 100 microM. In contrast, (+)-SKF 10047 (10 microM) and (-)-PPAP (100 microM) showed no protective effects against cell death induced by the Ca(2+) ionophore ionomycin (1-3 microM). These results indicate that sigma receptor ligands attenuate the cytotoxic effects of NMDA on midbrain dopaminergic neurons, possibly via inhibition of NMDA receptor functions.

Relationship between modulation of the cerebellorubrospinal system in the in vitro turtle brain and changes in motor behavior in rats: effects of novel sigma ligands

Saturation and competition binding studies showed that the turtle brain contains sigma sites labeled by both [3H]di-o-tolylguanidine (DTG) and [3H](+)-pentazocine. There was a significant correlation between the IC50 values of sigma ligands for [3H]DTG sites in the turtle vs. rat brain, suggesting that the sites are comparable in the two species. In contrast, [3H](+)-pentazocine, which primarily labels sigma1 sites in the rodent brain, labels a heterogeneity of sites in the turtle brain. In extracellular recordings from the in vitro turtle brainstem, some sigma ligands enhanced the burst responses of red nucleus (RN) neurons (DTG, haloperidol, BD1031, BD1052, BD1069) while other sigma ligands decreased the burst responses (BD1047, BD1063). Control compounds (turtle Ringer vehicle control, opiate antagonist naloxone, atypical neuroleptic sulpiride) had no significant effects on the RN burst responses recorded from the in vitro turtle brain. The ED50s of the ligands for altering the burst responses in RN neurons from the turtle brain were correlated with their IC50s for turtle brain sites labeled with [3H]DTG, but not [3H](+)-pentazocine; this pattern is identical to that previously reported in rats, where there is a correlation between the potencies of sigma ligands for producing dystonic postures after microinjection into the rat RN and their binding to rat brain sites labeled with [3H]DTG, but not [3H](+)-pentazocine. When the novel sigma ligands were microinjected into the rat RN, dystonic postures were produced by ligands that increased the burst duration of RN neurons in the turtle brain. Novel sigma ligands that reduced the burst responses in the in vitro turtle brain have previously been reported to have no effects on their own when microinjected into the rat RN, but to block the dystonic postures produced by other sigma ligands. Taken together, the data suggest that the opposite effects of the novel ligands in the turtle electrophysiological studies represent the actions of agonists vs. antagonists, and that the directionality of the effects has predictive value for the expected motor effects of the drugs.

Beneficial effects of acute and repeated administrations of sigma receptor agonists on behavioral despair in mice exposed to tail suspension

In an attempt to examine whether sigma receptor agonists alleviate behavioral despair, we investigated the effects of sigma receptor agonists on the tail suspension-induced immobility in mice. The acute and repeated (14 days) administrations of sigma1 receptor agonists, such as 1-(3,4-dimethoxyphenethyl)-4-(3-phenylpropyl)piperazine dihydrochloride (SA4503) (1 and/or 3 mg/kg) and (+)-pentazocine (5.6 mg/kg), sigma1/2 receptor agonists, such as 1,3-di(2-tolyl)guanidine (DTG) (3 and/or 5.6 mg/kg), desipramine (7.5 and/or 15 mg/kg), and fluoxetine (10 and/or 20 mg/kg), reduced immobility in mice exposed to tail suspension. N,N-Dipropyl-2-[4-methoxy-3-(2-phenylethoxy)phenyl] ethylamine monohydrochloride (NE-100), a sigma1 receptor antagonist, significantly antagonized the decrease in immobility induced by acute administrations of SA4503 (1 mg/kg) and (+)-pentazocine (5.6 mg/kg). Although not significant, NE-100 showed a tendency to inhibit the DTG (5.6 mg/kg)-induced decrease in immobility. In contrast, repeated administrations of SA4503 (1 and 3 mg/kg), (+)-pentazocine (5.6 mg/kg) or DTG (5.6 mg/kg) failed to affect the increase in body weight. These results suggest that acute and repeated stimulations of sigma, possibly a sigma1 receptor subtype, alleviate behavioral despair, unaccompanied with changes in body weight.

Dextromethorphan and dextrorphan in rats: common antitussives--different behavioural profiles

Dextromethorphan (DM), a widely used and well-tolerated centrally acting antitussive, has been tested in several clinical trials for its antiepileptic and neuroprotective properties. However, the use of DM in these new clinical indications requires higher doses than antitussive doses, which may therefore induce phencyclidine (PCP)-like side-effects (memory and psychotomimetic disturbances) through its metabolic conversion to the active metabolite dextrorphan (DX), a more potent PCP-like non-competitive antagonist at the N-methyl-D-aspartate (NMDA) receptor than DM. Thus, we compared the behavioural effects in rats of intraperitoneal administration of DM and DX on motor activity in an open field and on learning and memory in the Morris water maze. DM (20, 30, 40 mg/kg) produced a dose-dependent decrease in both locomotion and stereotyped behaviour with a slight ataxia for the highest dose. DX (20, 30, 40 mg/kg) induced a dose-dependent increase in locomotion and stereotypies (swaying, turning) with moderate ataxia. Assessments of learning and memory were performed with lower doses of DM (10, 20, 30 mg/kg) and DX (5, 10, 15 mg/kg) because of motivational deficits (40 mg/kg of DM, 20-40 mg/kg of DX) and motor disorders (30, 40 mg/kg of DX) in the cue learning procedure. DX (10, 15 mg/kg) impaired spatial learning with a long-lasting effect for the highest dose whereas 5 mg/kg of DX and DM (10-30 mg/kg) did not. Only 15 mg/kg of DX appeared to slightly impair working memory. DM (10-30 mg/kg) and DX (5-15 mg/kg) did not impair reference memory. Thus, the two antitussives DM and DX induced different behavioural effects suggesting sedative effects for DM and PCP-like effects for DX. However, PCP-like side-effects with DM remain possible through its metabolic conversion to DX, with very high doses and/or in extensive metabolizers and/or in aged subjects prone to cognitive dysfunction. Therefore, the identification of DM metabolism phenotype, an adapted prescription and a pharmacological modulation of the DM metabolism may avoid adverse effects.

Biphasic modulatory action of the selective sigma receptor ligand SR 31742A on N-methyl-D-aspartate-induced neuronal responses in the frontal cortex

The technique of intracellular recording was used to assess the effect of SR 31742A, a selective sigma receptor ligand, on N-methyl-d-aspartate (NMDA) and (+/-)-alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid (AMPA) receptor-mediated responses in pyramidal cells of the rat medial prefrontal cortex in vitro brain slice preparations. Bath application of SR 31742A produced a biphasic effect on NMDA responses: SR 31742A facilitated and inhibited NMDA-induced inward current at low (0.01, 0.05 and 0.1 microM) and higher (0.5, 1 and 10 microM) concentrations, respectively. The potentiating effect reached the peak (366%) at 0.1 microM, with an estimated EC50 value of 23 nM. Correspondingly, SR 31742A also produced a similar biphasic modulatory action on excitatory postsynaptic potentials or currents (EPSPs/EPSCs) evoked by electrical stimulation of the forceps minor. In contrast, SR 31742A produced a modest potentiation of AMPA responses at the concentrations from 0.01 to 1 microM. The potentiating action of SR 31742A on NMDA-receptor mediated neurotransmission may account for, at least partially, its antipsychotic and cognitive-enhancing potential, whereas the inhibitory action on NMDA responses at higher concentrations may be related to the purported neuroprotective action of sigma receptor ligands.

Neuroprotective sigma ligands attenuate NMDA and trans-ACPD-induced calcium signaling in rat primary neurons

The effect of neuroprotective sigma ligands possessing a range of relative selectivity for sigma and phencyclidine (PCP) binding sites on N-methyl-D-aspartate (NMDA) and (+/-)-1-aminocyclopentane-trans-1,3-dicarboxylic acid (trans-ACPD)-stimulated calcium flux was studied in 12-15-day-old primary cultures of rat cortical neurons. In approximately 80% of the neurons tested, NMDA (80 microM) caused a sustained increase in intracellular calcium ([Ca2+]i). With the exception of R-(+)-3-(3-hydroxyphenyl)-N-propylpiperidine hydrochloride ((+)-3-PPP) (previously shown not to be neuroprotective) all of the sigma ligands studied significantly altered NMDA-induced calcium dynamics. The primary effect of dextromethorphan, (+)-pentazocine, (+)-cyclazocine, (+)-SKF10047, carbetapentane, 1,3-di(2-tolyl) guanidine (DTG), and haloperidol was to shift the NMDA response from a sustained, to either a biphasic or a transient, calcium event. In contrast to NMDA, the primary response observed in 62% of the neurons treated with trans-ACPD (100 microM) was a transient elevation in [Ca2+]i. Here, however, only the highly selective neuroprotective sigma ligands (i.e., those lacking substantial PCP binding affinity) significantly decreased the number of transient responses elicited by trans-ACPD whereas the PCP-related sigma ligands such as dextromethorphan, (+)-SKF10047 and (+)-cyclazocine were ineffective. Unexpectedly, (+)-3-PPP potentiated trans-ACPD activity. These results demonstrating attenuating effects of sigma ligands on NMDA-stimulated neuronal calcium responses agree with earlier studies using glutamate and KCl and identify a sigma receptor modulation of functional NMDA responsiveness. Furthermore, the ability of sigma ligands to attenuate NMDA-, trans-ACPD- and KCl-evoked neuronal calcium dynamics indicates that the receptor mechanisms mediating sigma neuroprotection comprise complex interactions involving ionotropic, metabotropic, and even voltage-gated calcium signaling processes.

Dextromethorphan protects against cerebral injury following transient, but not permanent, focal ischemia in rats

Dextromethorphan (DM) has been observed to afford neuroprotection in a variety of in vitro and in vivo experimental models of CNS injury. We have evaluated the neuroprotective activity of DM following both transient (2 h) and permanent focal cerebral ischemia in the rat. Middle cerebral artery occlusion (MCAO) was produced in male Sprague-Dawley rats using the intraluminal filament technique. Animals were dosed s.c with 20 mg/kg DM at 0.5, 1, 2, 4, and 6 hours post occlusion. Analysis of brain injury was performed 24 hours after permanent occlusion or reperfusion. Following transient MCAO, vehicle treated rats exhibited a total infarct volume of 203 +/- 33 mm3. DM produced a 61% reduction in infarct volume to 79 +/- 13 mm3. Permanent MCAO produced a larger infarct volume (406 +/- 44 mm3) which was not significantly reduced in size by treatment with DM (313 +/- 58 mm3). Infarcted hemispheric oedema was not different in vehicle treated rats following transient or permanent MCAO and was not reduced by DM in either group. Following transient MCAO, rectal temperature was elevated 1,2 and 5 hours post occlusion. While not inducing hypothermia or altering physiological parameters such as blood pressure and blood gases, DM attenuated this injury-related increase in temperature, an effect which appeared to correlate with its ability to protect neurons in temperature regulating hypothalamic centres. The DM-induced reduction in infarction demonstrated in our model of transient focal cerebral ischemia provides further support for the in vivo neuroprotective activity of this compound. Importantly, these data demonstrate the limited neuroprotective efficacy of DM when attempting to combat more severe focal ischemic injuries and imply that drug-induced hypothermia is not ultimately responsible for its protective action.

Neuroprotective and anti-amnesic potentials of sigma (sigma) receptor ligands

1. Although the physical nature of sigma (sigma) receptors have not yet been fully defined, several classes of selective ligands have been characterised, demonstrating a plethora of physiological actions. In the present review, the authors have set out to highlight two important aspects of the biological activities of sigma ligands, their neuroprotective and anti-amnesic effects. 2. The sigma ligands present a therapeutic potential as neuroprotective agents in brain ischemia. The neuroprotective activity of many non-selective sigma ligands is primarily a result of their affinity for the NMDA receptor complex. However, selective sigma ligands are also neuroprotective, possibly by inhibition of the ischemic-induced presynaptic release of excitotoxic amino acids. 3. The sigma 1 ligands prevent the experimental amnesia induced by muscarinic cholinergic antagonists at either the learning, consolidation or retention phase of the mnesic process. This effect involves a potentation of acetylcholine release induced by sigma 1 ligands selectively in the hippocampal formation and cortex. 4. The sigma 1 receptor ligands also attenuate the learning impairment induced by dizocilpine, a non-competitive antagonist of the NMDA receptor, and may relate to the potentiating effect of sigma 1 ligands on several NMDA receptor-mediated responses previously described in vitro and in vivo in the hippocampus. This effect is shared by NPY- and CGRP-related peptides and by neuroactive steroids, confirming the in vitro evidences of functional interactions between the sigma 1 receptors and these different systems. 5. Additional amnesia models also seem to be alleviated by sigma 1 ligands, such as phencyclidine-induced cognitive dysfunctions, and amnesia induced by the calcium channel blocker nimodipine, or by exposure to carbon monoxide. Furthermore, a preliminary study in an animal model of age-related memory deficits, the senescence-accelerated mouse, strengthened the therapeutic potentials of selective sigma 1 receptor ligands in aging-related pathologies.

Synergy by secretory phospholipase A2 and glutamate on inducing cell death and sustained arachidonic acid metabolic changes in primary cortical neuronal cultures

Secretory and cytosolic phospholipases A2 (sPLA2 and cPLA2) may contribute to the release of arachidonic acid and other bioactive lipids, which are modulators of synaptic function. In primary cortical neuron cultures, neurotoxic cell death and [3H]arachidonate metabolism was studied after adding glutamate and sPLA2 from bee venom. sPLA2, at concentrations eliciting low neurotoxicity (</=100 ng/ml), induced a decrease of [3H]arachidonate-phospholipids and preferential reesterification of the fatty acid into triacylglycerols. Free [3H]arachidonic acid accumulated at higher enzyme concentrations, below those exerting highest toxicity. Synergy in neurotoxicity and [3H]arachidonate release was observed when low, nontoxic (10 ng/ml, 0.71 nM), or mildly toxic (25 ng/ml, 1. 78 nM) concentrations of sPLA2 were added together with glutamate (80 microM). A similar synergy was observed with the sPLA2 OS2, from Taipan snake venom. The NMDA receptor antagonist MK-801 blocked glutamate effects and partially inhibited sPLA2 OS2 but not sPLA2 from bee venom-induced arachidonic acid release. Thus, the synergy with glutamate and very low concentrations of exogenously added sPLA2 suggests a potential role for this enzyme in the modulation of glutamatergic synaptic function and of excitotoxicity.

Emopamil-binding protein, a mammalian protein that binds a series of structurally diverse neuroprotective agents, exhibits delta8-delta7 sterol isomerase activity in yeast

Delta8-delta7 sterol isomerase is an essential enzyme on the sterol biosynthesis pathway in eukaryotes. This endoplasmic reticulum-resident membrane protein catalyzes the conversion of delta8-sterols to their corresponding delta7-isomers. No sequence data for high eukaryote sterol isomerase being available so far, we have cloned a murine sterol isomerase-encoding cDNA by functional complementation of the corresponding deficiency in the yeast Saccharomyces cerevisiae. The amino acid sequence deduced from the cDNA open reading frame is highly similar to human emopamil-binding protein (EBP), a protein of unknown function that constitutes a molecular target for neuroprotective drugs. A yeast strain in which the sterol isomerase coding sequence has been replaced by that of human EBP or its murine homologue recovers the ability to convert delta8-sterol into delta7-sterol, both in vivo and in vitro. In these recombinant strains, both cell proliferation and the sterol isomerization reaction are inhibited by the high affinity EBP ligand trifluoperazine, as is the case in mammalian cells but not in wild type yeast cell. In contrast, the recombinant strains are much less susceptible to the sterol inhibition effect of haloperidol and fenpropimorph, as compared with wild type yeast strains. Our results strongly suggest that EBP and delta8-delta7 sterol isomerase are identical proteins in mammals.

Dextromethorphan analogs are neuroprotective in vitro and block glutamate-induced excitotoxic calcium signals in neurons

Consistent with the neuroprotective effects of the non-opioid antitussive dextromethorphan (DM) described in several models of CNS injury, micromolar concentrations of three novel analogs of DM markedly attenuated the injury produced by glutamate in cultured rat cortical neurons. Furthermore, the neuroprotective actions of the DM analogs correlated with their effects to block glutamate-induced excitotoxic calcium signals and were unrelated to metabolism to the phencyclidine (PCP)-like drug dextrorphan (DX). These observations establish a new class of compounds related to DM which, by virtue of their efficacy to protect neurons against a severe glutamate insult, may possess therapeutic potential as treatment modalities for a number of neurodegenerative diseases.