Computer-assisted modeling of multiple dextromethorphan and sigma binding sites in guinea pig brain

Long-term Dextromethorphan Use and Acute Intoxication Results in an Episode of Mania and Autoenucleation

: The pharmacologic and neuropsychiatric sequelae of long-term dextromethorphan use and acute dextromethorphan intoxication are reviewed in this case report. Dextromethorphan ingestion at the high end of toxicity, although rare, can cause violence to oneself and others, even in those previously without any history of such behaviors. In this article, the neuropsychiatric consequences of dextromethorphan toxicity are highlighted in a case report of a 37-year-old woman who had been using dextromethorphan for 5 years. She presented to a large urban emergency department in a psychotic and manic state after attempting autoenucleation. She reported to consult liaison psychiatry staff that she had taken a total of 1400 mg of dextromethorphan over the course of 3 days with intent to experience altered state of consciousness. Toxicology screens on admission did not reveal any other substances in her system. She had no formal psychiatric history and no history of mania, psychosis, or self-harm. To our knowledge, this is the first case of autoenucleation resulting from dextromethorphan-induced mania with psychotic features.

Clinical benefits and risks of N-methyl-d-aspartate receptor antagonists to treat severe opioid use disorder: A systematic review

BACKGROUND

Demand for treatments for severe opioid use disorder is increasing worldwide. The current pharmacotherapy is mainly focused on opioid and adrenergic receptors. The N-methyl-d-aspartate receptor (NMDAR) is among other receptors that can also be targeted to treat the disease. Findings from randomized controlled trials (RTCs) on NMDAR antagonists to treat severe opioid use disorder amply varied. This study aimed to evaluate the clinical benefits and assess the potential risks for adverse events or side effects of NMDAR antagonists that were investigated for the treatment of severe opioid use disorder.

METHODS

Articles were searched in PubMed, Scopus, Google Scholar, Proquest. Cochrane Review Database, Medline Ovid, and EMBASE from their inception to March 2019. RTCs on NMDAR antagonists for the treatment of severe opioid use disorder were independently screened and assessed by two authors. The results were synthesized qualitatively.

RESULTS

Nineteen RTCs of 1459 participants met the inclusion criteria. There is moderate evidence suggesting that ketamine, memantine, amantadine, and dextromethorphan may be able to manage opioid withdrawal symptoms. There is little evidence suggesting that memantine may be able to reduce methadone maintenance dose in participants on methadone, reduce opioid use, and reduce craving. Dropout is noticeable among dextromethorphan's participants. Safety concerns are more likely associated with dextromethorphan and ketamine.

CONCLUSIONS

NMDAR antagonists have the potentiality to treat severe opioid use disorder. There is insufficient evidence to recommend them for the treatment of severe opioid use disorder due to several limitations inherent to the RCTs reviewed. Further exploration is needed.

Central inhibition of initiation of swallowing by systemic administration of diazepam and baclofen in anaesthetized rats

Dysphagia is caused not only by neurological and/or structural damage but also by medication. We hypothesized memantine, dextromethorphan, diazepam, and baclofen, all commonly used drugs with central sites of action, may regulate swallowing function. Swallows were evoked by upper airway (UA)/pharyngeal distension, punctate mechanical stimulation using a von Frey filament, capsaicin or distilled water (DW) applied topically to the vocal folds, and electrical stimulation of a superior laryngeal nerve (SLN) in anesthetized rats and were documented by recording electromyographic activation of the suprahyoid and thyrohyoid muscles and by visualizing laryngeal elevation. The effects of intraperitoneal or topical administration of each drug on swallowing function were studied. Systemic administration of diazepam and baclofen, but not memantine or dextromethorphan, inhibited swallowing evoked by mechanical, chemical, and electrical stimulation. Both benzodiazepines and GABA_A receptor antagonists diminished the inhibitory effects of diazepam, whereas a GABA_B receptor antagonist diminished the effects of baclofen. Topically applied diazepam or baclofen had no effect on swallowing. These data indicate that diazepam and baclofen act centrally to inhibit swallowing in anesthetized rats.NEW & NOTEWORTHY Systemic administration of diazepam and baclofen, but not memantine or dextromethorphan, inhibited swallowing evoked by mechanical, chemical, and electrical stimulation. Both benzodiazepines and GABA_A receptor antagonists diminished the inhibitory effects of diazepam, whereas a GABA_B receptor antagonist diminished the effects of baclofen. Topical applied diazepam or baclofen was without effect on swallowing. Diazepam and baclofen act centrally to inhibit swallowing in anesthetized rats.

Effects of dextromethorphan and oxycodone on treatment of neuropathic pain in mice

Background

Neuropathic pain is a very troublesome and difficult pain to treat. Although opioids are the best analgesics for cancer and surgical pain in clinic, only oxycodone among opioids shows better efficacy to alleviate neuropathic pain. However, many side effects associated with the use of oxycodone render the continued use of it in neuropathic pain treatment undesirable. Hence, we explored whether dextromethorphan (DM, a known N-methyl-D-aspartate receptor antagonist with neuroprotective properties) could potentiate the anti-allodynic effect of oxycodone and underlying mechanisms regarding to glial cells (astrocytes and microglia) activation and proinflammatory cytokines release in a spinal nerve injury (SNL) mice model.

Results

Oxycodone produced a dose-dependent anti-allodynic effect. Co-administration of DM at a dose of 10 mg/kg (i.p.) (DM10) which had no anti-allodynic effect by itself enhanced the acute oxycodone (1 mg/kg, s.c.) effect. When the chronic anti-allodynic effects were examined, co-administration of DM10 also significantly enhanced the oxycodone effect at 3 mg/kg. Furthermore, oxycodone decreased SNL-induced activation of glial cells (astrocytes and microglia) and plasma levels of proinflammatory cytokines (IL-6, IL-1β and TNF-α). Co-administration of DM10 potentiated these effects of oxycodone.

Conclusion

The combined use of DM with oxycodone may have therapeutic potential for decreasing the effective dose of oxycodone on the treatment of neuropathic pain. Attenuation of the glial activation and proinflammatory cytokines in the spinal cord may be important mechanisms for these effects of DM.

A double-blind, placebo-controlled trial of dextromethorphan combined with clonidine in the treatment of heroin withdrawal

Dextromethorphan has been reported to ameliorate opioid withdrawal symptoms in both animal and human subjects. In the present study, we investigated the efficacy of dextromethorphan as an add-on medication in heroin detoxification treatment in a double-blind, placebo-controlled design. Sixty-five heroin-dependent patients (male, 63; female, 2) participated in this inpatient detoxification trial after giving informed consent. Clonidine 0.075 mg 4 times a day was given as an antiwithdrawal medication at baseline. Each patient was then randomly assigned to treatment with either dextromethorphan 60 mg or placebo 4 times a day as additional medication. Flurazepam 30 mg was given before bedtime for insomnia. Other medications that were allowed included loperamide for diarrhea and lorazepam for agitation. Participants were monitored using the Objective Opioid Withdrawal Scale 3 times a day as the primary outcome to compare drug efficacy between groups. Generalized estimating equation model analysis revealed that the Objective Opioid Withdrawal Scale had no group difference between dextromethorphan and placebo group overall (P = 0.29), whereas a significant difference between groups was found during day 3 to day 6 (P = 0.04) by post hoc analysis. There was no difference in the Clinical Global Impression Scale, patient's impression of treatment, and use of ancillary medications between groups. No severe adverse effects were noticed. We suggest that dextromethorphan has some beneficial effect in attenuating the severity of opioid withdrawal symptoms and can be used as an adjunction medication in the treatment of opioid withdrawal, whereas the exact efficacy needs further investigation.

Antitussive effects of memantine in guinea pigs

BACKGROUND

The treatment of cough is a significant clinical unmet need because there is little evidence that current therapies are effective. Based on evidence supporting a role for N-methyl D-aspartate receptors (NMDARs) in cough, we hypothesized that memantine, a low-affinity, uncompetitive NMDAR channel blocker in routine use for the treatment of Alzheimer disease, could be an effective, well-tolerated, antitussive therapy. The aim of this study was to establish preclinical evidence that memantine has antitussive effects.

METHODS

We studied the influence of memantine on experimentally induced coughing in response to citric acid and bradykinin inhalation in guinea pigs. We also compared the potency and efficacy of memantine as an antitussive to other NMDAR antagonists, dextromethorphan and ketamine, and to the γ-aminobutyric acid class B receptor agonist baclofen.

RESULTS

Compared with control subjects, 10 mg/kg memantine significantly reduced the cumulative number of coughs evoked by both citric acid (median, 24.0 [interquartile range (IQR), 13.0-25.5] vs 1.5 [IQR, 0.3-10.3] coughs; P = .012) and bradykinin aerosols (median, 16.0 [IQR, 9.5-18.5] vs 0.0 [IQR, 0-0.75] coughs; P = .002). Memantine 10 mg/kg produced a similar reduction in the cumulative number of coughs to baclofen 3 mg/kg and demonstrated comparatively greater cough suppression than 30 mg/kg dextromethorphan or 30 mg/kg ketamine. This dose of memantine produced no sedative or respiratory depressive effects.

CONCLUSIONS

This study illustrates that memantine has marked antitussive effects in guinea pigs, most likely mediated through NMDAR channel blockade. Memantine, therefore, has the potential to be a safe, effective, and well-tolerated antitussive agent.

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.

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.

A magnetic resonance spectroscopy study of brain glutamate in a model of plasticity in human pharyngeal motor cortex

BACKGROUND & AIMS

Coordinated delivery of peripheral and cortical stimuli (paired associative stimulation [PAS]) has been shown to induce plasticity in limb motor cortex, however, its application in pharyngeal motor cortex and the molecular mechanisms involved in human neuroplasticity remain uncertain. Because neuroplasticity appears to form the basis for functional recovery of digestive functions such as swallowing after brain injury, the aim of this study was to characterize the induction of cortical plasticity in human pharyngeal motor cortex through PAS applied to pharyngeal musculature and investigate the potential role of glutamate in this process.

METHODS

Fifteen healthy volunteers completed a series of experiments in which cortical excitability was assessed through pharyngeal motor evoked potential amplitudes in response to transcranial magnetic stimulation. The optimal parameters and interhemispheric interactions of PAS in the bilaterally represented pharyngeal system initially were investigated. Cortical glutamate after PAS then was assessed with magnetic resonance spectroscopy.

RESULTS

The greatest increase in cortical pharyngeal excitability was seen if paired stimuli were separated by 100 ms (F[15,210] = 2.28; P < or = .05). Cortical excitability increased over 2 hours with analogous albeit lesser changes in the contralateral hemisphere. A focal and transient reduction in glutamate was found in the stimulated pharyngeal motor cortex (F[1,12] = 21.9; P = .001), without changes in any other measured brain metabolites.

CONCLUSIONS

This study shows that PAS-induced plasticity in the human pharyngeal motor system is both timing- and hemisphere-dependent and provides novel evidence for the potential role of glutamate in modulating this effect.

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.

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

In a previous study, we demonstrated that a dextromethorphan analog, dimemorfan, has neuroprotective effects. Dextromethorphan and dimemorfan are high-affinity ligands at sigma1 receptors. Dextromethorphan has moderate affinities for phencyclidine sites, while dimemorfan has very low affinities for such sites, suggesting that these sites are not essential for the anticonvulsant actions of dimemorfan. Kainate (KA) administration (10 mg kg(-1), i.p.) produced robust convulsions lasting 4-6 h in rats. Pre-treatment with dimemorfan (12 or 24 mg kg(-1)) reduced seizures in a dose-dependent manner. Dimemorfan pre-treatment also attenuated the KA-induced increases in c-fos/c-jun expression, activator protein (AP)-1 DNA-binding activity, and loss of cells in the CA1 and CA3 fields of the hippocampus. These effects of dimemorfan were comparable to those of dextromethorphan. The anticonvulsant action of dextromethorphan or dimemorfan was significantly counteracted by a selective sigma1 receptor antagonist BD 1047, suggesting that the anticonvulsant action of dextromethorphan or dimemorfan is, at least in part, related to sigma1 receptor-activated modulation of AP-1 transcription factors. We asked whether dimemorfan produces the behavioral side effects seen with dextromethorphan or dextrorphan (a phencyclidine-like metabolite of dextromethorphan). Conditioned place preference and circling behaviors were significantly increased in mice treated with phencyclidine, dextrorphan or dextromethorphan, while mice treated with dimemorfan showed no behavioral side effects. Our results suggest that dimemorfan is equipotent to dextromethorphan in preventing KA-induced seizures, while it may lack behavioral effects, such as psychotomimetic reactions.

Analgesic effect of dextromethorphan in neuropathic pain

BACKGROUND

Dextromethorphan, a clinically available N-methyl-D-aspartic acid (NMDA) receptor antagonist, has an analgesic effect in patients with diabetic neuropathy. The aim of this study was to evaluate the analgesic and adverse effects of a single high dose of dextromethorphan on spontaneous pain in patients suffering long-term neuropathic pain of traumatic origin.

METHODS

Fifteen patients with post-traumatic neuropathic pain participated in this placebo-controlled, double-blind, randomized crossover study. On two separate occasions, the participants received 270 mg of dextromethorphan hydrobromide or placebo. Pain intensity, adverse effects and serum concentrations of dextromethorphan and metabolites were registered.

RESULTS

Dextromethorphan had a statistically significant analgesic effect compared with placebo, but the effect varied markedly among the patients. Light-headedness was the most important adverse effect reported. Extensive metabolizers of dextromethorphan had an apparently better analgesic effect than poor metabolizers.

CONCLUSION

This report indicates that a single high dose of dextromethorphan has an analgesic effect in patients with neuropathic pain of traumatic origin. The main metabolite dextrorphan seems to be important for the analgesic effect. At the relatively high dose studied, the clinical usefulness of dextromethorphan is limited to that portion of the patient population experiencing analgesia without an unacceptable level of adverse effects.

Metabolism to dextrorphan is not essential for dextromethorphan's anticonvulsant activity against kainate in mice

The effects of dextromethorphan (DM), and its major metabolite dextrorphan (DX) on kainic acid-induced seizures in mice were examined. Intracerebroventricular DM or DX (5 or 10 microg/0.5 microl) pretreatment significantly attenuated seizures induced by kainic acid (0.07 microg/0.07 microl) in a dose-related manner. DM or DX pretreatment significantly attenuated kainic acid-induced increases in AP-1 DNA-binding activity and fos-related antigen-immunoreactivity as well as neuronal loss in the hippocampus. DM appears to be a more potent neuroprotectant than DX. Since the high-affinity DM binding sites are recognized as being identical to the sigma-1 site, we examined the role of the sigma-1 receptor on the pharmacological action mediated by DM or DX. Pretreatment with the sigma-1 receptor antagonist BD1047 (2.5 or 5 mg/kg, i.p.) blocked the neuroprotection by DM in a dose-related manner. This effect of BD 1047 was more pronounced in the animals treated with DM than in those treated with DX. Combined, our results suggest that metabolism of DM to DX is not essential for DM to exert its effect. They also suggest that DM provides neuroprotection from kainic acid via sigma-1 receptor modulation.

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.

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.

Modulation of amphetamine-stimulated (transporter mediated) dopamine release in vitro by sigma2 receptor agonists and antagonists

Some sigma receptor ligands have been shown to bind with low affinity to the dopamine transporter and to inhibit [3H]dopamine uptake. It has not previously been shown whether any of these compounds influence release of dopamine via facilitated exchange diffusion. To further examine the nature of the interaction between sigma receptor ligands and the dopamine transporter, the effects of sigma receptor ligands on amphetamine-stimulated [3H]dopamine release were examined in slices prepared from rat caudate putamen. In the absence of exogenous Ca2+, both (+)-pentazocine and (-)-pentazocine potentiated amphetamine-stimulated [3H]dopamine release at concentrations consistent with their affinities for sigma2 receptors. In contrast, BD737 (1S.2R-(-)-cis-N-¿2-(3,4-dichlorophenyl)ethyl¿-N-methyl-2-(1-pyrrolidiny l)cyclohexylamine), a sigma1 receptor agonist, had no effect on amphetamine-stimulated release. Neither isomer of pentazocine alone had any effect on basal [3H]dopamine release under these conditions. Three antagonists at sigma receptors, one of which is non-selective for subtypes, and two of which are sigma2-selective, all blocked the enhancement of stimulated release produced by (+)-pentazocine. Enhancement of stimulated release by (-)-pentazocine was similarly blocked by sigma2 receptor antagonists. Our data support the contention that it is possible to regulate transporter-mediated events with drugs that act at a subpopulation of sigma receptors pharmacologically identified as the sigma2 subtype.

Synthesis and characterization of [125I]3'-(-)-iodopentazocine, a selective sigma 1 receptor ligand

Pentazocine is a potent ligand at both opioid and sigma receptors, but with opposite stereoselectivities. Whereas (-)-pentazocine has high affinity for a number of opioid receptors, (+)-pentazocine labels sigma 1 receptors. Iodination of (-)-pentazocine at the 3'-position reverses its selectivity for opioid and sigma 1 receptors. 3'-(-)-Iodopentazocine competes at sigma 1 receptor binding sites with a Ki value of 8 nM, compared to approximately 40 nM for (-)-pentazocine. 3'-(-)-Iodopentazocine also has lost its affinity for opioid receptors. In contrast, iodination of (+)-pentazocine lowers its affinity at sigma 1 receptors. Synthesis of [125I]3'-(-)-iodopentazocine is readily performed with incorporations of up to 80%. Binding is of high affinity and shows the selectivity anticipated for a sigma 1 receptor-selective ligand. Exposing membranes prebound with [125I]3'-(-)-iodopentazocine to ultraviolet light can covalently couple the ligand into the membranes. Polyacrylamide gel electrophoresis reveals a major band at about 25 kDa and a minor one at about 20 kDa, indicating photolabeling of sigma 1 receptors with minor incorporation into sigma 2 sites.

An autoradiographic study of dextromethorphan high-affinity binding sites in rat brain: sodium-dependency and colocalization with paroxetine

1. The distribution and some pharmacological properties of centrally located dextromethorphan high-affinity binding sites were investigated by in vitro autoradiography. 2. Sodium chloride (50 mM) induced a 7 to 12 fold increase in dextromethorphan binding to rat brain in all areas tested. The effect of sodium was concentration-dependent with a higher dose (120 mM) exerting a smaller effect on binding. 3. [3H]-dextromethorphan binding in the presence of sodium was inhibited in the presence of the anticonvulsant phenytoin at a concentration of 100 microM, while the sigma ligand (+)-3-(-3-hydroxyphenyl)-N-(1-propyl)pipendine ((+)-PPP) had no effect on the binding, suggesting an interaction with the DM2 site. 4. The distribution of the sodium-dependent binding identified in this study correlated significantly with the distribution of the selective 5-HT uptake inhibitor [3H]-paroxetine, and paroxetine and dextromethorphan mutually displaced their binding at concentrations in the low nanomolar range. 5. These data show that dextromethorphan and paroxetine share a sodium-dependent high affinity binding site in rat brain, and suggest that dextromethorphan might interact, in the presence of sodium, with the 5-HT uptake mechanism in rat brain.

Reduction of the scopolamine-induced impairment of passive-avoidance performance by sigma receptor agonist in mice

We examined the ameliorating effects of several sigma receptor agonists on scopolamine-induced memory impairment in mice. Scopolamine was administered IP 30 min before the training session. Each sigma receptor agonist was administered 60 min before or immediately after the training session, or 60 min before the retention test in the passive-avoidance performance experiments. (+)-N-Allylnormetazocine ((+)-SKF-10,047), a prototype sigma 1 receptor agonist, showed an ameliorating effect on the scopolamine-induced memory impairment in these 3 administration schedules, and (-)-SKF-10,047, a stereoisomer with low affinity for the sigma 1 receptor subtype, failed to reduce this memory impairment in mice. In addition, 1,3-di(2-toly1)guanidine (DTG) and (+)-3-(3-hydroxyphenyl)-N-(1-propyl)piperizine ((+)-3-PPP), nonselective sigma receptor agonists, did not affect this memory impairment. Physostigmine, an acetylcholinesterase (AChE) inhibitor, alleviated the scopolamine-induced memory impairment in all these drug administration schedules. In addition, (+)-SKF-10,047-induced antiamnesic effect was antagonized by the concurrent administration of haloperidol, a sigma receptor antagonist, or N,N-dipropyl-2-(4-methoxy-3-(2-phenylethoxy) phenyl)ethylamine monohydrochloride (NE-100), a selective sigma 1 receptor antagonist. These findings indicate that the sigma 1 receptor agonist has ameliorating effects on all phases of learning and memory processes. This profile of sigma 1 receptor agonist is similar to that of an AChE inhibitor.

Differential effects of sigma ligands on the N-methyl-D-aspartate response in the CA1 and CA3 regions of the dorsal hippocampus: effect of mossy fiber lesioning

In the CA3 region of rat dorsal hippocampus, several sigma ligands, such as 1,3-di(2-tolyl)guanidine (DTG), (+)-pentazocine and (+)-N-cyclopropylmethyl-N-methyl-1, 4-diphenyl-1-ethyl-but-3-en-1-ylamine hydrochloride (JO-1784), administered intravenously at low doses, potentiate selectively the pyramidal neuron firing activity induced by microiontophoretic applications of N-methyl-D-aspartate, without affecting those induced by quisqualate, kainate or acetylcholine. A similar potentiation of the N-methyl-D-aspartate response has also been found with microiontophoretic applications of neuropeptide Y, an effect exerted via delta receptors. The present experiments were carried out to determine the effects of these sigma ligands and of neuropeptide Y; in the CA1 and CA3 regions following unilateral destruction by a local injection of colchicine of the mossy fiber system, which is a major afference to CA3 pyramidal neurons. In the CA1 region, DTG, JO-1784 and neuropeptide Y did not potentiate the activation induced by microiontophoretic applications of N-methyl-D-aspartate. However, (+)-pentazocine potentiated the N-methyl-D-aspartate response, similarly to its effect in the CA3 region on the intact side. In the CA3 region, on the intact side, (+)-pentazocine, DTG, JO-1784 and neuropeptide Y induced a selective potentiation of N-methyl-D-aspartate-induced activation, in keeping with previous reports. On the lesioned side, the effect of (+)-pentazocine on the N-methyl-D-aspartate response was still present, but those of DTG, JO-1784 and neuropeptide Y were abolished. These results suggest that (+)-pentazocine, on the one hand, and DTG, JO-1784 and neuropeptide Y, on the other, are not acting on the same subtype of sigma receptors. Since (+)-pentazocine, JO-1784 and neuropeptide Y have been suggested to act on the sigma 1 subtype of receptors, these data suggest the existence of two subtypes of sigma 1 receptors. They also suggest that the receptors on which DTG, JO-1784 and neuropeptide Y are acting are located on the mossy fiber terminals in the CA3 region and are absent in the CA1 region.

Involvement of sigma-receptors in the increase in contraction of mouse vas deferens induced by exogenous ATP

The effects of sigma-receptor ligands on the twitch contraction elicited by the exogenous application of adenosine 5'-triphosphate (ATP) in the unstimulated mouse vas deferens were studied. (-)-Pentazocine, 1,3-di(2-tolyl)guanidine(DTG) and two pairs of optical isomers of 3-(3-hydroxyphenyl)-N-(1-propyl)piperidine(3-PPP) and N-allylnormetazocine (SKF-10,047) potentiated the exogenous application of ATP-induced twitch-type contraction in a concentration-dependent manner, while (+)-pentazocine did not affect it. The order of potentiating ability was: (+)-3-PPP > (-)pentazocine > (-)-SKF-10,047> DTG > (-)-3-PPP > (+)-SKF-10,047. On the other hand, haloperidol and rimcazole, putataive sigma-receptor antagonists, suppressed this twitch contraction. In addition, these antagonists significantly blocked the (+)-3-PPP- and (-)-pentazocine-induced potentiation at concentrations which did not affect contractions per se. These findings indicate that the exogenous application of ATP-induced twitch contraction in the mouse vas deferens is regulated by sigma-receptors. In addition, the present ranking order suggests that the sigma-receptor potentiating the ATP-induced twitch contraction at post-junctional sites may differ from the sigma 1- and/or sigma 2-receptor subtypes.

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

Since unique calcium dynamics have been reported for toxic (40-80 M) and non-toxic (5-10 microM) concentrations of glutamate, we evaluated the effect of neuroprotective sigma ligands on glutamate and potassium chloride (KCl)-stimulated changes in [Ca2+]i using 12-15 day old primary rat neuronal cortical cultures. In approximately 80% of the neurons tested, 80 microM glutamate caused a sustained calcium flux previously shown to be associated with neurotoxicity. The majority of sigma ligands that were evaluated altered glutamate-induced calcium flux. For example, the primary effect of maximally neuroprotective concentrations of the sigma ligands dextromethorphan, (+)-pentazocine, (+)-cyclazocine, (+)-SKF 10047, carbetapentane and haloperidol was a shift from a sustained, to either a biphasic or a monophasic transient calcium response indicative of neuroprotection. (+)-3-PPP, previously shown not to be neuroprotective in this model system, failed to alter glutamate-induced calcium flux. In contrast to glutamate, KCl (50 mM) produced changes in [Ca2+]i which were not neurotoxic to the neurons as measured by LDH release. The primary response observed in 59% of the neurons treated with 50 mM KCl alone was an initial spike in [Ca2+]i which abruptly declined then plateaued above basal levels throughout the 12 min of analysis (modified sustained response). The highly selective sigma ligands produced a shift from the modified sustained response to a monophasic transient calcium response. Again, (+)-3-PPP had no effect on KCl-induced calcium dynamics. Of the PCP-related sigma ligands only (+)-SKF-10047 consistently attenuated the KCl-induced calcium flux. Collectively, these results indicate that modulation of [Ca2+]i through receptor and voltage-gated calcium channels contributes significantly to sigma mediated neuroprotection.

Discrepancies in characterization of sigma sites in the mouse central nervous system

The characteristics of [3H](+)-pentazocine and [3H]1,3-di(2-tolyl) guanidine (DTG) binding to mouse whole brain, cortex, cerebellum and spinal cord membranes were investigated in radioreceptor assays. [3H](+)-Pentazocine bound to a single, high affinity site (Kd = 1.2-1.6 nM) with increasing density along the neuraxis from the cortex (Bmax = 543 fmol/mg protein) to the spinal cord (Bmax = 886 fmol/mg protein). Hot saturation studies resolved the presence of one binding site for [3H]DTG showing no tissue variations in terms of density (Bmax = 1075-1264 fmol/mg protein) or affinity (Kd = 16.6-22.3 nM). Incubation with 100 nM (+)-pentazocine revealed two classes of high affinity [3H]DTG labeled binding sites corresponding to sigma 1 and sigma 2 subtypes. A preponderance of sigma 2 sites was revealed in all investigated tissues. Different pharmacological profiles were demonstrated for the sigma 2 sites in mouse whole bain compared to mouse spinal cord. However, competition studies indicated that the whole brain and spinal [3H](+)-pentazocine labeled sigma 1 binding sites exhibited similar pharmacological properties. The density of [3H](+)-pentazocine labeled sigma 1 population was found not to match that of [3H]DTG labeled sigma 1 site throughout the mouse central nervous system. The presence of low affinity [3H]DTG labeled sites was demonstrated in cold saturation experiments. Equilibrium binding data for the low affinity [3H]DTG binding site resulted in an increasing density (Bmax = 1973-11,369 fmol/mg protein) with a decreasing affinity (Kd = 242-943 nM) in mouse cortex through the spinal cord.

Acute effects of sigma ligands on the extracellular DOPAC level in rat frontal cortex and striatum

Acute administration of (+)-N-allylnormetazocine ((+)-SKF-10,047) and (+/-)-pentazocine, was found to increase the extracellular level of 3,4-dihydroxyphenylacetic acid (DOPAC), a major dopamine (DA) metabolite, in the rat frontal cortex. By contrast, these benzomorphan sigma ligands did not change the extracellular DOPAC level in the rat striatum. On the other hand, 1,3-di(2-tolyl)guanidine (DTG) increased the extracellular DOPAC level in the frontal cortex, while it decreased that level in the striatum. Another non-benzomorphan sigma ligand, (+)-3-(3-hydroxyphenyl)-N-(1-propyl)piperidine ((+)-3-PPP) decreased the extracellular DOPAC level in both frontal cortex and striatum. Moreover, the increase of the extracellular DOPAC level elicited by (+)-SKF-10,047 was significantly inhibited by rimcazole, a putative sigma antagonist, while the DTG-induced increment was not reversed by rimcazole. These findings indicated that the effects of sigma ligands on the mesocortical DA neurons differed from those on the nigrostriatal DA neurons. In addition, the effects of benzomorphan sigma ligands on the central DA neurons were different from those of non-benzomorphan sigma ligands.

Neuroprotection by N-methyl-D-aspartate antagonists in focal cerebral ischemia is dependent on continued maintenance dosing

While N-methyl-D-aspartate antagonists have been shown to attenuate neuronal damage in focal cerebral ischemia, few studies have examined whether continuous or multiple dose treatment is necessary for maximum efficacy. We studied the effect of a loading dose only or load plus maintenance infusion using several non-competitive N-methyl-D-aspartate antagonists (dextromethorphan, dextrorphan, MK-801) and the levorotatory enantiomer of dextromethorphan (levomethorphan) in a rabbit model of focal cerebral ischemia. Forty-seven anesthetized rabbits underwent occlusion of the left internal carotid, anterior cerebral and middle cerebral arteries for 2 h followed by 4 h of reperfusion. Drugs were administered 10 min after occlusion. Dextromethorphan and dextrorphan protected against ischemic edema only when given as load plus maintenance (29% and 31% reduction, respectively), while both load only and load plus maintenance of MK-801 protected against edema (26% and 31% reduction, respectively). Levomethorphan load plus maintenance also protected against ischemic edema (25% reduction). However, dextromethorphan and dextrorphan both required maintenance infusion to protect against ischemic neuronal damage (24% and 27% reduction in area of ischemic neuronal damage, respectively), while levomethorphan failed to protect against neuronal injury even when given as load plus maintenance. Administration of MK-801 as load plus maintenance reduced ischemic neuronal damage by 23%, but this difference was not quite statistically significant. These results suggest that processes of ischemic damage, such as excitotoxic injury, continue for several hours beyond the initial period of focal ischemia, and that non-competitive N-methyl-D-aspartate antagonists require more prolonged administration to achieve neuroprotection.(ABSTRACT TRUNCATED AT 250 WORDS)

Discriminative stimulus effects of dextromethorphan in the rat

This study was performed to characterize pharmacologically the discriminative stimulus effects of dextromethorphan, an antitussive that binds with high affinity to a subtype of sigma site in the brain. Dextrorphan, a metabolite of dextromethorphan, has phencyclidine (PCP)-like effects. Therefore, training was conducted with dextromethorphan injected by the SC route, which minimizes dextrorphan formation compared to the IP route. The training dose used, 30 mg/kg, by the SC route did not occasion selection of the PCP-appropriate choice lever in rats discriminating IP injections of 2.0 mg/kg PCP from saline. (In contrast, by the IP route the ED50 of dextromethorphan for PCP-appropriate lever selection was 21.7 mg/kg). In rats discriminating 30 mg/kg (SC) of dextromethorphan from distilled water, dextromethorphan was slightly more potent SC than it was IP (ED50s for dextromethorphan-appropriate lever selection: 8.5 and 14.9 mg/kg, respectively). These animals generalized dose-dependently and completely to PCP and to other PCP-receptor ligands, but selected the vehicle-appropriate choice lever when tested with sigma-site ligands, mu-opioid agonists, and naltrexone. Concurrent administration of naltrexone or sigma-site ligands with 30 mg/kg dextromethorphan did not block dextromethorphan-appropriate responding. These results show that the discriminative effects of SC dextromethorphan are PCP-like and are not mediated by the high-affinity dextromethorphan binding site or by the mu-opioid receptor. Because little dextrorphan is formed when dextromethorphan is given SC and because dextromethorphan itself has low affinity for the PCP receptor, the discriminative effects of SC dextromethorphan probably are mediated by a recognition site related closely to but different from the PCP receptor.

Ameliorating effects of sigma receptor ligands on the impairment of passive avoidance tasks in mice: involvement in the central acetylcholinergic system

Three sigma receptor ligands were examined for their ameliorating effects on p-chloroamphetamine-induced amnesia in mice. p-Chloroamphetamine was administered intraperitoneally 30 min before the training session of the passive avoidance response. Each sigma receptor ligand was administered 60 min before or immediately after the training session, or 60 min before the retention test. (+)-N-Allylnormetazocine ((+)-SKF-10,047), a prototype benzomorphan sigma receptor ligand, significantly reduced the p-chloroamphetamine-induced amnesia in these three administration schedules, as do acetylcholinesterase inhibitors. On the contrary, the significant anti-amnesic effects elicited by non-benzomorphan sigma receptor ligands, 1,3-di-(2-tolyl)guanidine (DTG) or (+)-3-(3-hydroxyphenyl)-N-(1-propyl)piperizine ((+)-3-PPP), were observed depending upon the timing of their administration. In addition, the ameliorating effect of (+)-SKF-10,047 against the p-chloroamphetamine-induced amnesia was superior to that of (-)-SKF-10,047. The (+)-SKF-10,047-induced anti-amnesic effect was significantly antagonized by the concurrent administration of either scopolamine, a muscarinic receptor antagonist, or hemicholinium-3, an inhibitor of the Na(+)-dependent high-affinity choline uptake site. These findings indicated that sigma receptor ligands had anti-amnesic effects against drug-induced memory impairment. In addition, the anti-amnesic effect of (+)-SKF-10,047 was superior to those of other sigma receptor ligands, and was mediated by both the sigma receptor and the central acetylcholinergic system.

Sigma-1 and sigma-2 sites in rat brain: comparison of regional, ontogenetic, and subcellular patterns

Radioligand binding assay conditions were established for the selective labeling of sigma-1 and sigma-2 sites in membrane homogenates of rat brain. Selective sigma-1 assays were conducted using 5 nM(+)[3H]SKF-10,047 in the presence of 300 nM dizocilpine (MK-801). Selective sigma-2 assays were conducted using 5 nM [3H]DTG in the presence of 1 microM (+)SKF-10,047. Distributions of sigma-1 and sigma-2 binding among brain regions were found to differ. While the brain stem yields the highest level of sigma-1 binding, it yields among the lowest levels of sigma-2 binding. The reverse is true in hippocampal membranes. Different ontogenetic patterns were also observed. Sigma-2 binding decreases substantially during brain development, whereas sigma-1 binding does not vary significantly. Patterns of distribution among subcellular fractions of rat brain homogenates were found to be similar. Both sigma-1 and sigma-2 sites are most enriched in microsomal fractions, and neither is enriched in synaptosomal or mitochondrial fractions. The present results suggest that sigma-1 and sigma-2 sites are distinct entities; they do not appear to be located on a common macromolecule, and they do not represent two different affinity states of a single type of binding site. While the precise subcellular locations of sigma-1 and sigma-2 sites remain to be determined, we conclude that localization of either type of binding site to synaptic regions of plasma membrane or to mitochondria is highly unlikely.

Rat liver and kidney contain high densities of sigma 1 and sigma 2 receptors: characterization by ligand binding and photoaffinity labeling

Rat liver and kidney were investigated for the presence of sigma (sigma) receptor subtypes by radioligand binding with three highly selective sigma probes and by photoaffinity labeling using [3H]azido-di-o-tolylguanidine ([3H]azido-DTG). [3H](+)-Pentazocine, a highly selective sigma 1 probe, bound to sites in liver membranes with Kd = 7.5 nM and Bmax3 = 2929 fmol/mg protein. [3H](+)-Pentazocine binding sites in kidney had Kd = 23.3 nM and Bmax = 229 fmol/mg protein. [3H]1,3-Di-o-tolylguanidine ([3H]DTG) and [3H](+)-3-(3-hydroxyphenyl)-N-(1-propyl)piperidine ([3H](+)-3-PPP) label both sigma 1 and sigma 2 receptors. Parameters for [3H]DTG in the liver were Kd = 17.9 nM and Bmax = 11,895 fmol/mg protein. Similar parameters were observed for [3H](+)-3-PPP, Kd = 51.9 nM and Bmax = 11,070 fmol/mg protein. [3H]DTG bound to rat kidney with Kd = 45.8 nM and Bmax = 1190 fmol/mg protein. The observation that either [3H]DTG or [3H](+)-3-PPP and [3H](+)-3-PPP labeled a higher number of sites relative to [3H](+)-pentazocine suggested that liver and kidney contain both subtypes of sigma receptor. This was confirmed by competition studies vs. [3H](+)-pentazocine and [3H]DTG (in the presence of dextrallorphan to mask sigma 1 sites). In both tissues, [3H](+)-pentazocine labeled sites with high affinity for haloperidol and enantioselectivity for (+)-benzomorphans over (-)-benzomorphans. [3H]DTG + dextrallorphan labeled sites in both tissues which also had high affinity for haloperidol, but which had the characteristic sigma 2 property of low affinity for (+)-benzomorphans and enantioselectivity for (-)-benzomorphans over the corresponding (+)-isomer. Similar results were obtained with [3H](+)-3-PPP + dextrallorphan. Several novel aryl diamines, such as 1S,2R-cis-N-[2-(3,4-dichlorophenylethyl]-N-methyl-2- (1-pyrrolidinyl)cyclohexylamine (BD737) and N-[2-(3,4-dichlorophenyl)ethyl]-N-methyl-2-(1-pyrrolidinyl)ethylamine (BD1008), bound to both sites with high affinity. Photoaffinity labeling with 10 nM [3H]azido-DTG resulted in specific labeling of polypeptides of 25 kDa and 21.5 kDa. Dextrallorphan (100 nM or 500 nM) completely blocked labeling of the 25 kDa polypeptide, but had no effect on labeling of the lower molecular weight protein. (+)-10,11-Dihydro-5-methyl-5H-dibenzo[a,d]cyclohepten-5,10- imine((+)-MK-801) had no effect on labeling of either polypeptide. These data are consistent with the notion that the 25 kDa and 21.5 kDa proteins represent sigma 1 and sigma 2 receptors, respectively.(ABSTRACT TRUNCATED AT 400 WORDS)

The effects of sigma ligands and of neuropeptide Y on N-methyl-D-aspartate-induced neuronal activation of CA3 dorsal hippocampus neurones are differentially affected by pertussin toxin

1. The in vivo effects of the high affinity sigma ligands 1,3-di(2-tolyl)guanidine (DTG), (+)-N-cyclopropylmethyl-N-methyl-1,4-diphenyl-1- ethyl-but-3-en-1-ylamine hydrochloride (JO-1784), (+)-pentazocine and haloperidol, as well as of those of neuropeptide Y (NPY), on N-methyl-D-aspartate (NMDA)- and quisqualate (Quis)-induced neuronal activations of CA3 pyramidal neurones were assessed, using extracellular unitary recording, in control rats and in rats pretreated with a local injection of pertussis toxin (PTX), to evaluate the possible involvement of Gi/o proteins in mediating the potentiation of the neuronal response to NMDA by the activation of sigma receptors in the dorsal hippocampus. 2. Microiontophoretic applications as well as intravenous injections of (+)-pentazocine potentiated selectively the NMDA response in control rats as well as in PTX-pretreated animals. In contrast, the PTX pretreatment abolished the potentiation of the NMDA response by DTG, JO-1784 and NPY. Moreover, microiontophoretic applications of DTG induced a reduction of NMDA-induced neuronal activation. Neither in control nor in PTX-treated rats, did the sigma ligands and NPY have any effect on Quis-induced neuronal response. 3. In PTX-treated rats, the potentiation of the NMDA response induced by (+)-pentazocine was suppressed by haloperidol, whereas the reduction of the NMDA response by DTG was not affected by haloperidol. 4. This study provides the first in vivo functional evidence that sigma ligands and NPY modulate the NMDA response by acting on distinct receptors, differentiated by their PTX sensitivity.

Interaction of histogranin and related peptides with [3H]dextromethorphan binding sites in rat brain

Histogranin (HN) and related peptides were tested for their ability to modulate the binding of the non-competitive N-methyl-D-aspartate (NMDA) receptor antagonist, [3H]dextromethorphan ([3H]DM), to rat brain membranes. HN, [Ser1]HN and the C-terminal fragment HN-(6-15) (0.1 nM-1 microM) potentiated (up to 1.6-fold) the binding of [3H]DM (5 nM) whereas the N-terminal fragment HN-(1-10) had no effect. The potentiation of [3H]DM binding by [Ser1]HN was blocked by NMDA (100 microM) and the NMDA receptor antagonist, CPP (1 microM) but not by the sigma (sigma) receptor ligand, (+)-pentazocine (0.1 microM) and the phencyclidine (PCP) receptor ligand, TCP (1 microM). Equilibrium binding experiments in presence of TCP (1 microM) to block PCP receptors indicated that [Ser1]HN (1 microM) causes a significant increase in the binding capacity (Bmax) of [3H]DM (from 2.46 to 3.46 pmol/mg protein) but no change in the apparent dissociation constant (Kd of 428 nM as compared with 487 nM). The results indicate that HN and related peptides specifically enhance the number of [3H]DM binding sites associated to the NMDA receptor complex.

Effects of haloperidol and reduced haloperidol on binding to sigma sites

The s.c. administration of a single dose of 0.1 mg/kg of reduced haloperidol to guinea pigs produced a marked inhibition of the binding of [3H]dextromethorphan and [3H]3-(3-hydroxyphenyl)-N-(n-propyl)piperidine ([3H](+)-3-PPP) to brain. The inhibition was still evident 10 days later, and it was accompanied by residual brain levels of reduced haloperidol, and much lower levels of haloperidol. Scatchard and computer-assisted analysis demonstrated that the inhibition was due to a reduction in the number of binding sites without changes in the affinity. In the rat, haloperidol and reduced haloperidol also produced a rapid inhibition of binding to sigma sites. Interestingly, the brain of the reduced haloperidol-treated rats contained both haloperidol and reduced haloperidol, but the levels of reduced haloperidol in the haloperidol-treated rats were undetectable. However, the inhibition observed was of comparable magnitude, indicating that the haloperidol remaining in the brain is also inhibitory. In vitro experiments showed that the inhibition produced by haloperidol and reduced haloperidol was apparently competitive, but when brain membranes were preincubated with either drug, the inhibition was noncompetitive. By contrast, the inhibition produced by dextromethorphan was always competitive. Moreover, the inhibition produced by haloperidol and reduced haloperidol could not be reversed by washing. This investigation strongly suggests that the inhibition observed after the administration of haloperidol or reduced haloperidol is not a classic agonist-induced receptor down-regulation. The results indicated that the inhibition produced is a complex phenomenon, and suggest the formation of a slowly reversible or irreversible complex with reduced haloperidol or haloperidol.

Characterization of the currents induced by sigma ligands in NCB20 neuroblastoma cells

Electrical and pharmacological properties of currents induced by compounds having affinities for putative sigma receptors were investigated with NCB20 cells by use of the whole-cell patch-clamp technique. Antipsychotics and naloxone induced inward currents with a decrease in membrane conductance at a holding potential of -60 mV. The rank order of potency for compounds inducing these currents was bromperidol > haloperidol > mosapramine = clocapramine > carpipramine > chlorpromazine > remoxipride > naloxone. Sulpiride, which does not have affinity for sigma receptors, induced inward currents only slightly. Haloperidol-induced currents were not affected by the pretreatments with 10 microM of sulpiride, dopamine, atropine, N-methyl-D-aspartate, 2-amino-7-phosphonoheptanoic acid, morphine or A23187, 100 nM of ICS 205-930, 100 microM of forskolin, 1 microM of phorbol-12,13-dibutyrate, or 100 ng/ml of cholera or pertussis toxins. The reversal potential of the currents induced by haloperidol, naloxone or remoxipride was dependent on the concentration of external or internal potassium. These results indicate that the currents induced by the tested compounds are due to blockade of tonic, outward potassium currents and suggest that these agents act on putative sigma receptors and that the second messenger systems within the cell are not essential for the coupling between the receptors and the channels.

Multiple [3H]DTG binding sites in guinea pig cerebellum: evidence for the presence of non-specific binding

The characteristics of the low affinity component of 1,3-di(2-[5-3H]tolyl)guanidine binding to the guinea pig cerebellum were investigated. Saturation binding assays where sigma 1 receptors were masked with dextrallorphan indicated that 1,3-di(2-[5-3H]tolyl)guanidine bound to cerebellar membranes in a fashion best described by a 1 site+non-specific binding model with a low density of specific binding sites (Bmax approximately 200 fmol/mg protein). Boiling the cerebellar membranes before addition to the saturation assay had no effect on the density of 1,3-di(2-[5-3H]tolyl)guanidine binding. In contrast, both the Kd and Bmax for 1,3-di(2-[5-3H]tolyl)guanidine binding to liver membranes was significantly reduced by boiling, as was the density of [3H](+)-pentazocine binding to cerebellum and liver. Thus, a substantial component of 1,3-di(2-[5-3H]tolyl)guanidine binding in the guinea pig cerebellum is to non-specific, proteinaceous binding sites with some of the pharmacological characteristics of the sigma 2 binding site.

Cocaine and several sigma receptor ligands inhibit dopamine uptake in rat caudate-putamen

Cocaine and several sigma receptor ligands inhibit dopamine uptake via a common site. This is evidenced by a concentration-dependent inhibition of dopamine uptake and displacement of the binding of [3H]WIN 35,428 (also called CFT), a cocaine analog with high affinity for the dopamine transporter. Since several sigma receptor ligands have been shown to block the stimulant effects of cocaine, this site may serve as a target for future drug development to treat cocaine abuse.

Differences in anticonvulsant potency and adverse effects between dextromethorphan and dextrorphan in amygdala-kindled and non-kindled rats

The anticonvulsant and adverse effects of dextromethorphan, a non-opioid antitussive, and its metabolite dextrorphan were examined in amygdala-kindled rats. Both drugs have repeatedly been proposed to be functional non-competitive N-methyl-D-aspartate (NMDA) receptor antagonists, but they also exert effects distinct from antagonism at NMDA receptors, such as blockade of voltage-gated calcium channels and sigma-site mediated actions. Since recent data have demonstrated that kindled rats are more susceptible to the adverse effects of NMDA receptor antagonists than non-kindled rats, the time course, characteristics and severity of adverse effects of dextromethorphan and dextrorphan were also determined in non-kindled animals. Dextromethorphan dose dependently increased the focal seizure threshold (i.e. the threshold for induction of afterdischarges recorded from the amygdala) in fully kindled rats. This anticonvulsant effect was found at relatively low doses (7.5-15 mg/kg i.p.) which were almost free of any adverse effects. At higher doses, dextromethorphan induced motor impairment and seizures, but no phenyclidine (PCP)-like adverse effects, such as hyperlocomotion or stereotypies. In contrast, such adverse effects were seen after dextrorphan, although only infrequently. Dextrorphan was less potent in inducing anticonvulsant but more potent in inducing motor impairing effects than dextromethorphan in kindled rats. In non-kindled rats, the motor impairment induced by dextrorphan was significantly less severe than in kindled rats, whereas no marked differences between kindled and non-kindled rats were found for dextromethorphan. The data indicate that dextromethorphan and dextrorphan differ in their mechanisms of action. Only dextrorphan exerts effects which are characteristic for NMDA receptor antagonism, whereas the potent anticonvulsant effect of dextromethorphan in presumably unrelated to the NMDA receptor complex.

Dextromethorphan blocks N-methyl-D-aspartate-induced currents and voltage-operated inward currents in cultured cortical neurons

The effect of dextromethorphan on several types of cation currents in cultured rat cortical neurons and PC12 cells was studied by using the whole-cell configuration of the patch-clamp technique. The Ba2+ current through L- and N-type Ca2+ channels was blocked with similar potencies (52-71 microM) in both types of cells. The effect was not voltage-dependent, in contrast to that of amlodipine (a dihydropyridine). Dextromethorphan was able to block the Ba2+ current completely unlike amlodipine and omega-conotoxin (an N-type channel blocker) which produced only partial inhibition. The voltage-activated Na+ and Ca2+ channels in cortical neurons were inhibited by similar concentrations of dextromethorphan (IC50 approximately 80 microM). The morphinan was at least 100 times more potent (IC50 = 0.55 microM) as a blocker of the current induced by N-methyl-D-aspartate (NMDA) in cortical neurons. Currents induced by (RS)-alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid ((RS)-AMPA) or kainic acid were not significantly affected even at 1 mM. The results suggest that the neuroprotective effect of dextromethorphan, previously found to occur in a concentration range of 10-100 microM, may be due to a complete blockade of the NMDA receptor channel and a partial inhibition of voltage-dependent Ca2+ and Na+ channels.

Multiple sigma binding sites in guinea-pig and rat brain membranes: G-protein interactions

1. Evidence is accumulating for multiple sigma (sigma) sites in the mammalian CNS. 2. We have addressed this problem and have examined sigma site - G-protein coupling in guinea-pig and rat brain membranes. 3. Ditolylorthoguanidine (DTG), (+)-3-(3-hydroxyphenyl)-N-1-(propyl)piperidine (3PPP) and dextromethorphan displaced [3H]-DTG (3.4 nM) with low Hill slopes of 0.5, 0.6 and 0.6, respectively in guinea-pig brain membranes. 4. In the presence of 5'-guanylylimidodiphosphate (Gpp(NH)p; 100 microM), the specific binding of [3H]-DTG was reduced by 36.7%, the Hill slope of 3PPP was increased to near unity, the ability of dextromethorphan to displace DTG was virtually abolished and the Hill slope for DTG remained low (0.7), indicating the presence of at least two binding sites. These data indicate that although Gpp(NH)p removes a dextromethorphan high affinity site, two DTG selective sites remain in the presence of Gpp(NH)p. 5. The present study suggests that DTG binds to at least three sites in guinea-pig brain membranes, at least one of which is G-protein linked. 6. In rat brain membranes, DTG displaced itself (3.4 nM) with a Hill slope near 1. 3PPP displacement of [3H]-DTG was comparable with the guinea-pig (Hill slope 0.5) and displaced from more than 1 site. Dextromethorphan did not displace [3H]-DTG at concentrations below 10 microM. 7. The heterogeneity of sigma sites appears to be less in rat than in guinea-pig brain membranes.

Allosteric modulation of ligand binding to [3H](+)pentazocine-defined sigma recognition sites by phenytoin

The allosteric modulation of sigma recognition sites by phenytoin (diphenylhydantoin) has been demonstrated by the ability of phenytoin to stimulate binding of various [3H] sigma ligands, as well as to slow dissociation from sigma sites and to shift sigma sites from a low- to a high-affinity state. Phenytoin stimulated the binding of the sigma 1- selective ligand [3H](+)pentazocine in a dose-dependent manner. Stimulation of binding at a final concentration of 250 microM phenytoin was associated with a decrease in the KD. The affinities of the sigma reference compounds caramiphen, dextromethorphan, dextrophan, (+)3-PPP and (+)SKF-10,047 were three- to eight-fold higher, while the affinities of benzetimide, BMY-14802, carbetapentane, DTG and haloperidol were unchanged in the presence of 250 microM phenytoin. The relative sensitivity of sigma compounds to allosteric modulation by phenytoin is not a property of all sigma ligands, and may provide an in vitro basis for distinguishing actions of sigma compounds and predicting sigma effects in vivo.