Increase of extracellular acetylcholine level in rat frontal cortex induced by (+)N-allylnormetazocine as measured by brain microdialysis

Revisiting the sigma-1 receptor as a biological target to treat affective and cognitive disorders

Depression and cognitive disorders are diseases with complex and not-fully understood etiology. Unfortunately, the COVID-19 pandemic dramatically increased the prevalence of both conditions. Since the current treatments are inadequate in many patients, there is a constant need for discovering new compounds, which will be more effective in ameliorating depressive symptoms and treating cognitive decline. Proteins attracting much attention as potential targets for drugs treating these conditions are sigma-1 receptors. Sigma-1 receptors are multi-functional proteins localized in endoplasmic reticulum membranes, which play a crucial role in cellular signal transduction by interacting with receptors, ion channels, lipids, and kinases. Changes in their functions and expression may lead to various diseases, including depression or memory impairments. Thus, sigma-1 receptor modulation might be useful in treating these central nervous system diseases. Importantly, two sigma-1 receptor ligands entered clinical trials, showing that this compound group possesses therapeutic potential. Therefore, based on preclinical studies, this review discusses whether the sigma-1 receptor could be a promising target for drugs treating affective and cognitive disorders.

Intranasal pregnenolone increases acetylcholine in frontal cortex, hippocampus, and amygdala-Preferentially in the hemisphere ipsilateral to the injected nostril

This study determined the effects of intranasal pregnenolone (IN-PREG) on acetylcholine (ACh) levels in selected areas of the rat brain, using in vivo microdialysis. Previous studies showed that PREG rapidly reaches the rodent brain after intranasal administration and that direct infusion of PREG and PREG-S into the basal forebrain modulates ACh release in frontal cortex, amygdala, and hippocampus. In the present study, we investigated the effects of IN-PREG on the cholinergic system in the rat brain. In the first experiment, IN-PREG (5.6 and 11.2 mg/ml) or vehicle was applied bilaterally, and we hypothesized that IN-PREG would increase ACh levels in amygdala, hippocampus, and frontal cortex, relative to baseline and vehicle. Dialysate was collected for 100 min, based on pilot data of duration of effect. Bilateral IN-PREG (5.6 and 11.2 mg/ml) increased frontal cortex and hippocampal ACh relative to both baseline and vehicle. Moreover, 11.2 mg/ml PREG increased ACh in the amygdala relative to baseline, the lower dose, and vehicle. Therefore, in the second experiment, IN-PREG (11.2 mg/ml) was applied only into one nostril, with vehicle applied into the other nostril, in order to determine whether ACh is predominantly increased in the ipsilateral relative to the contralateral amygdala. Unilateral application of IN-PREG increased ACh in the ipsilateral amygdala, whereas no effect was observed on the contralateral side, suggesting that PREG was transported from the nostrils to the brain via the olfactory epithelial pathway, but not by circulation. The present data provide additional information on IN-PREG action in the cholinergic system of frontal cortex, amygdala, and hippocampus. This may be relevant for therapeutic IN application of PREG in neurogenerative and neuropsychiatric disorders.

Role of σ1 Receptors in Learning and Memory and Alzheimer's Disease-Type Dementia

The present chapter will review the role of σ1 receptor in learning and memory and neuroprotection , against Alzheimer's type dementia. σ1 Receptor agonists have been tested in a variety of pharmacological and pathological models of learning impairments in rodents these last past 20 years. Their anti-amnesic effects have been explained by the wide-range modulatory role of σ1 receptors on Ca2+ mobilizations, neurotransmitter responses, and particularly glutamate and acetylcholine systems, and neurotrophic factors. Recent observations from genetic and pharmacological studies have shown that σ1 receptor can also be targeted in neurodegenerative diseases, and particularly Alzheimer's disease . Several compounds, acting partly through the σ1 receptor, have showed effective neuroprotection in transgenic mouse models of Alzheimer's disease . We will review the data and discuss the possible mechanisms of action, particularly focusing on oxidative stress and mitochondrial integrity, trophic factors and a novel hypothesis suggesting a functional interaction between the σ1 receptor and α7 nicotinic acetylcholine receptor. Finally, we will discuss the pharmacological peculiarities of non-selective σ1 receptor ligands, now developed as neuroprotectants in Alzheimer's disease , and positive modulators, recently described and that showed efficacy against learning and memory deficits.

Sigma-1 Agonist Binding in the Aging Rat Brain: a MicroPET Study with [(11)C]SA4503

PURPOSE

Sigma-1 receptor ligands modulate the release of several neurotransmitters and intracellular calcium signaling. We examined the binding of a radiolabeled sigma-1 agonist in the aging rat brain with positron emission tomography (PET).

PROCEDURES

Time-dependent uptake of [(11)C]SA4503 was measured in the brain of young (1.5 to 3 months) and aged (18 to 32 months) Wistar Hannover rats, and tracer-kinetic models were fitted to this data, using metabolite-corrected plasma radioactivity as input function.

RESULTS

In aged animals, the injected probe was less rapidly metabolized and cleared. Logan graphical analysis and a 2-tissue compartment model (2-TCM) fit indicated changes of total distribution volume (V T) and binding potential (BP ND) of the tracer. BP ND was reduced particularly in the (hypo)thalamus, pons, and medulla.

CONCLUSIONS

Some areas showed reductions of ligand binding with aging whereas binding in other areas (cortex) was not significantly affected.

Sigma receptors [σRs]: biology in normal and diseased states

This review compares the biological and physiological function of Sigma receptors [σRs] and their potential therapeutic roles. Sigma receptors are widespread in the central nervous system and across multiple peripheral tissues. σRs consist of sigma receptor one (σ₁R) and sigma receptor two (σ₂R) and are expressed in numerous regions of the brain. The sigma receptor was originally proposed as a subtype of opioid receptors and was suggested to contribute to the delusions and psychoses induced by benzomorphans such as SKF-10047 and pentazocine. Later studies confirmed that σRs are non-opioid receptors (not an µ opioid receptor) and play a more diverse role in intracellular signaling, apoptosis and metabolic regulation. σ₁Rs are intracellular receptors acting as chaperone proteins that modulate Ca²⁺ signaling through the IP₃ receptor. They dynamically translocate inside cells, hence are transmembrane proteins. The σ₁R receptor, at the mitochondrial-associated endoplasmic reticulum membrane, is responsible for mitochondrial metabolic regulation and promotes mitochondrial energy depletion and apoptosis. Studies have demonstrated that they play a role as a modulator of ion channels (K⁺ channels; N-methyl-d-aspartate receptors [NMDAR]; inositol 1,3,5 triphosphate receptors) and regulate lipid transport and metabolism, neuritogenesis, cellular differentiation and myelination in the brain. σ₁R modulation of Ca²⁺ release, modulation of cardiac myocyte contractility and may have links to G-proteins. It has been proposed that σ₁Rs are intracellular signal transduction amplifiers. This review of the literature examines the mechanism of action of the σRs, their interaction with neurotransmitters, pharmacology, location and adverse effects mediated through them.

The effects of sigma (σ1) receptor-selective ligands on muscarinic receptor antagonist-induced cognitive deficits in mice

BACKGROUND AND PURPOSE

Cognitive deficits in patients with Alzheimer's disease, Parkinson's disease, traumatic brain injury and stroke often involve alterations in cholinergic signalling. Currently available therapeutic drugs provide only symptomatic relief. Therefore, novel therapeutic strategies are needed to retard and/or arrest the progressive loss of memory.

EXPERIMENTAL APPROACH

Scopolamine-induced memory impairment provides a rapid and reversible phenotypic screening paradigm for cognition enhancement drug discovery. Male C57BL/6J mice given scopolamine (1 mg·kg(-1) ) were used to evaluate the ability of LS-1-137, a novel sigma (σ1) receptor-selective agonist, to improve the cognitive deficits associated with muscarinic antagonist administration.

KEY RESULTS

LS-1-137 is a high-affinity (Ki = 3.2 nM) σ1 receptor agonist that is 80-fold selective for σ1, compared with σ2 receptors. LS-1-137 binds with low affinity at D2-like (D2, D3 and D4) dopamine and muscarinic receptors. LS-1-137 was found to partially reverse the learning deficits associated with scopolamine administration using a water maze test and an active avoidance task. LS-1-137 treatment was also found to trigger the release of brain-derived neurotrophic factor from rat astrocytes.

CONCLUSIONS AND IMPLICATIONS

The σ1 receptor-selective compound LS-1-137 may represent a novel candidate cognitive enhancer for the treatment of muscarinic receptor-dependent cognitive deficits.

The cognition-enhancing activity of E1R, a novel positive allosteric modulator of sigma-1 receptors

BACKGROUND AND PURPOSE

Here, we describe the in vitro and in vivo effects of (4R,5S)-2-(5-methyl-2-oxo-4-phenyl-pyrrolidin-1-yl)-acetamide (E1R), a novel positive allosteric modulator of sigma-1 receptors.

EXPERIMENTAL APPROACH

E1R was tested for sigma receptor binding activity in a [³H](+)-pentazocine assay, in bradykinin (BK)-induced intracellular Ca²⁺ concentration ([Ca²⁺](i)) assays and in an electrically stimulated rat vas deferens model. E1R's effects on cognitive function were tested using passive avoidance (PA) and Y-maze tests in mice. A selective sigma-1 receptor antagonist (NE-100), was used to study the involvement of the sigma-1 receptor in the effects of E1R. The open-field test was used to detect the effects of E1R on locomotion.

KEY RESULTS

Pretreatment with E1R enhanced the selective sigma-1 receptor agonist PRE-084's stimulating effect during a model study employing electrically stimulated rat vasa deferentia and an assay measuring the BK-induced [Ca²⁺](i) increase. Pretreatment with E1R facilitated PA retention in a dose-related manner. Furthermore, E1R alleviated the scopolamine-induced cognitive impairment during the PA and Y-maze tests in mice. The in vivo and in vitro effects of E1R were blocked by treatment with the selective sigma-1 receptor antagonist NE-100. E1R did not affect locomotor activity.

CONCLUSION AND IMPLICATIONS

E1R is a novel 4,5-disubstituted derivative of piracetam that enhances cognition and demonstrates efficacy against scopolamine-induced cholinergic dysfunction in mice. These effects are attributed to its positive modulatory action on the sigma-1 receptor and this activity may be relevant when developing new drugs for treating cognitive symptoms related to neurodegenerative diseases.

The cholinergic system, sigma-1 receptors and cognition

This article provides an overview of present knowledge regarding the relationship between the cholinergic system and sigma-1 receptors, and discusses potential applications of sigma-1 receptor agonists in the treatment of memory deficits and cognitive disorders. Sigma-1 receptors, initially considered as a subtype of the opioid family, are unique ligand-regulated molecular chaperones in the endoplasmatic reticulum playing a modulatory role in intracellular calcium signaling and in the activity of several neurotransmitter systems, particularly the cholinergic and glutamatergic pathways. Several central nervous system (CNS) drugs show high to moderate affinities for sigma-1 receptors, including acetylcholinesterase inhibitors (donepezil), antipsychotics (haloperidol, rimcazole), selective serotonin reuptake inhibitors (fluvoxamine, sertraline) and monoamine oxidase inhibitors (clorgyline). These compounds can influence cognitive functions both via their primary targets and by activating sigma-1 receptors in the CNS. Sigma-1 agonists show powerful anti-amnesic and neuroprotective effects in a large variety of animal models of cognitive dysfunction involving, among others (i) pharmacologic target blockade (with muscarinic or NMDA receptor antagonists or p-chloroamphetamine); (ii) selective lesioning of cholinergic neurons; (iii) CNS administration of β-amyloid peptides; (iv) aging-induced memory loss, both in normal and senescent-accelerated rodents; (v) neurodegeneration induced by toxic compounds (CO, trimethyltin, cocaine), and (vi) prenatal restraint stress.

Anti-amnesic and neuroprotective potentials of the mixed muscarinic receptor/sigma 1 (σ1) ligand ANAVEX2-73, a novel aminotetrahydrofuran derivative

Tetrahydro-N, N-dimethyl-2, 2-diphenyl-3-furanmethanamine hydrochloride (ANAVEX2-73) binds to muscarinic acetylcholine and sigma(1) (σ(1)) receptors with affinities in the low micromolar range. We characterized its anti-amnesic and neuroprotective potentials in pharmacological and pathological amnesia models. Spatial working memory was evaluated using spontaneous alternation in the Y-maze and non-spatial memory using passive avoidance procedures. ANAVEX2-73 (0.01-3.0 mg/kg i.p.) alleviated the scopolamine- and dizocilpine-induced learning impairments. ANAVEX2-73 (300 µg/kg) also reversed the learning deficits in mice injected with Aβ(25-35) peptide, a non-transgenic Alzheimer's disease model. When the drug was injected simultaneously with Aβ(25-35), 7 days before the tests, it blocked the appearance of learning impairments. This protective activity was confirmed since ANAVEX2-73 blocked the Aβ(25-35)-induced oxidative stress in the hippocampus. This effect was differentially sensitive to the muscarinic receptor antagonist scopolamine or the σ(1) protein antagonist BD1047, confirming the mixed muscarinic/σ(1) pharmacological action. Finally, its unique demethyl metabolite, ANAVEX19-144, was also effective and ANAVEX2-73 presented a longer duration of action, effective 12 h before Aβ(25-35), than its related compound ANAVEX1-41. The neuroprotective activity of ANAVEX2-73, its mixed cholinergic/σ(1) activity, its low active dose range and its long duration of action together reinforce its therapeutic potential in Alzheimer's disease.

Improvement of memory impairment by (+)- and (-)-pentazocine via sigma, but not kappa opioid receptors

(+/-)-Pentazocine is widely used clinically to treat mild to moderate pain as a racemic compound. Although it is known that (-)-pentazocine acts as a kappa opioid receptor agonist to exhibit analgesic actions and (+)-pentazocine acts as a sigma receptor agonist without analgesic effects, their combined effect on memory has not been investigated in detail. In this study, the effect of (+)- and/or (-)-pentazocine on scopolamine-induced memory impairment in mice was investigated using spontaneous alternation performance in a Y-maze. (+)-Pentazocine (0.35 micromol/kg, s.c.) administered 30 min before behavioral testing significantly improved the impairment of spontaneous alternation induced by scopolamine. A higher dose of (-)-pentazocine (3.50 micromol/kg, s.c.) also reversed the scopolamine-induced impairment of alternation performance. Interestingly, the ameliorating effects of not only (+)-pentazocine, but also (-)-pentazocine were antagonized by a selective sigma receptor antagonist, N,N-dipropyl-2-[4-methoxy-3-(2-phenylenoxy)-phenyl]-ethylamine monohydrochloride (NE-100) (2.6 micromol/kg, i.p.). However, those effects were not antagonized by a selective kappa opioid receptor antagonist, nor-binaltorphimine (4.9 nmol/mouse, i.c.v.). Coadministration of (+)- and (-)-pentazocine (0.35 or 3.50 micromol/kg each) did not have any additive or antagonizing effects on the percent alternation. An antinociceptive effect was observed only with (-)-pentazocine (3.50 micromol/kg, s.c.), and was antagonized by nor-binaltorphimine (4.9 nmol/mouse, i.c.v.), but not by NE-100 (2.6 micromol/kg, i.p.). These results suggest that although the analgesic effect of pentazocine was mediated via kappa opioid receptors, the ameliorating effect on scopolamine-induced impairment of spontaneous alternation was mediated via sigma receptors, not via kappa opioid receptors.

The Sigma1 Protein as a Target for the Non-genomic Effects of Neuro(active)steroids: Molecular, Physiological, and Behavioral Aspects

Steroids synthesized in the periphery or de novo in the brain, so called 'neurosteroids', exert both genomic and nongenomic actions on neurotransmission systems. Through rapid modulatory effects on neurotransmitter receptors, they influence inhibitory and excitatory neurotransmission. In particular, progesterone derivatives like 3alpha-hydroxy-5alpha-pregnan-20-one (allopregnanolone) are positive allosteric modulators of the gamma-aminobutyric acid type A (GABA(A)) receptor and therefore act as inhibitory steroids, while pregnenolone sulphate (PREGS) and dehydroepiandrosterone sulphate (DHEAS) are negative modulators of the GABA(A) receptor and positive modulators of the N-methyl-D-aspartate (NMDA) receptor, therefore acting as excitatory neurosteroids. Some steroids also interact with atypical proteins, the sigma (sigma) receptors. Recent studies particularly demonstrated that the sigma1 receptor contributes effectively to their pharmacological actions. The present article will review the data demonstrating that the sigma1 receptor binds neurosteroids in physiological conditions. The physiological relevance of this interaction will be analyzed and the impact on physiopathological outcomes in memory and drug addiction will be illustrated. We will particularly highlight, first, the importance of the sigma1-receptor activation by PREGS and DHEAS which may contribute to their modulatory effect on calcium homeostasis and, second, the importance of the steroid tonus in the pharmacological development of selective sigma1 drugs.

Involvement of kappa-opioid receptors and sigma receptors in memory function demonstrated using an antisense strategy

Although antinociceptive effects of U-50,488H (trans-(+/-)-3,4-dichloro-N-methyl-N-(2-[1-pyrrolidinyl] cyclohexyl) benzeneacetamide methanesulfonate and (-)-pentazocine have been reported to influence kappa-opioid receptors, the involvement of kappa-opioid receptors in learning and/or memory is still controversial. We have recently reported that the memory improving effect of (-)-pentazocine was antagonized by sigma1 receptor antagonist. In this study, we examined the effects of several antisense oligodeoxynucleotides (antisenses) to kappa1-opioid receptors and sigma1 receptor on memory and nociceptive function. Male ddY mice were treated subcutaneously (s.c.) with scopolamine (1.65 mumol/kg) and/or test drugs 30 min before a Y-maze test. U-50,488H significantly improved the scopolamine-induced impairment of spontaneous alternation behavior. Twenty micrograms of antisense targeting exons 2 and 3 of the kappa1-opioid receptor significantly reversed the effects of U-50,488H, but antisense targeting exon 1 and mismatch sense did not. The antisense targeting exon 3 was most effective. These antisenses themselves did not affect normal mice, indicating that kappa1-opioid receptors do not tonically regulate memory function. All three antisenses equally prevented U-50,488H-induced antinociceptive effects in the acetic-acid-induced writhing test. Pretreatment with antisense targeting sigma1 receptors (AS-sigma1) completely prevented the memory-improving effects of (-)- and (+)-pentazocine, although U-50,488H ameliorated the scopolamine-induced impairment of spontaneous alternation behavior in AS-sigma1-treated mice. These results suggest that kappa1-opioid receptors containing different exons have a distinct function in memory and nociceptive functions. Furthermore, kappa-opioid receptors agonist showing analgesic effects act on kappa-opioid receptors or sigma receptors and play important roles only when memory function is impaired, but the two neuronal systems regulate memory function independently.

Dimemorfan enhances acetylcholine release from rat hippocampal slices

Our previous study reported that an antitussive drug, dimemorfan, attenuates cholinergic dysfunction-induced amnesia in mice and acts like a sigma1 receptor agonist. This study further showed that dimemorfan (30 microM), like the putative sigma1 receptor agonist (+)-SKF-10047 (10 microM), significantly enhanced 25 mM KCl-evoked [3H]acetylcholine release from rat hippocampal but not striatal slices, which was antagonized by a sigma1 receptor antagonist haloperidol (0.3 microM).

Anti-amnesic effect of dimemorfan in mice

(1) Dimemorfan, an antitussive for more than 25 years, has previously been reported to be a relative high-affinity ligand at sigma-1 (sigma(1)) receptor with the K(i) value of 151 nM. (2) To test whether dimemorfan has anti-amnesic effects similar to a sigma(1) receptor agonist, this study examined its effects on scopolamine- and beta-amyloid peptide-(25-35)-induced amnesia in mice. (3) Dimemorfan (10-40 mg kg(-1), i.p.) administered 30 min before the training trial, immediately after the training trial, or 30 min before the retention test significantly improved scopolamine (1 mg kg(-1), i.p.)- or beta-amyloid peptide-(25-35) (3 nmol mouse(-1), i.c.v.)-induced amnesia in a step-through passive avoidance test. Dimemorfan (5-40 mg kg(-1), i.p.) pretreatment also attenuated scopolamine (8 mg kg(-1), i.p.)-induced amnesia in a water-maze test. And, these anti-amnesic effects of dimemorfan, like the putative sigma(1) receptor agonist (+)-N-allylnormetazocine ((+)-SKF-10047), were antagonized by a sigma receptor antagonist haloperidol (0.25 mg kg(-1), i.p.). (4) These results indicated that dimemorfan has anti-amnesic effects and acts like a sigma(1) receptor agonist.

Involvement of kappa-opioid and sigma receptors in short-term memory in mice

Kappa-opioid receptor agonists, trans-(+/-)-3,4-dichloro-N-methyl-N-(2-[1-pyrrolidinyl] cyclohexyl) benzeneacetamide methanesulfonate (U-50,488H) and dynorphin A-(1-13), improve impairments of learning and memory in mice and rats. sigma Receptor agonists, (+)-N-allylnormetazocine ((+)-SKF10,047) and 1-(3,4-dimethoxyphenethyl)-4-(3-phenylpropyl) piperazine dihydrochloride (SA4503), also reverse learning and memory impairment in various animal models. However, the mechanisms underlying these effects are not well understood. In the present study, the effect of coadministration of U-50,488H and (+)-SKF10,047 on scopolamine-induced memory impairment was investigated in mice using spontaneous alternation performance in a Y-maze. U-50,488H (0.21-2.15 micromol/kg, subcutaneously (s.c.)) and (+)-SKF10,047 (0.10-1.02 micromol/kg, s.c.) 25 min before the Y-maze test improved the impairment of spontaneous alternation induced by scopolamine (1.65 micromol/kg, s.c.). When U-50,488H and (+)-SKF10,047 were coadministered, no additive effect was observed. Furthermore, the ameliorating effects of U-50,488H and (+)-SKF10,047 were not antagonized by a selective sigma receptor antagonist, N,N-dipropyl-2-[4-methoxy-3-(2-phenylenoxy)-phenyl]-ethylamine monohydrochloride (NE-100), and a selective kappa-opioid receptor antagonist, nor-binaltorphimine, respectively. These results suggest that the mechanisms underlying the ameliorating effects on memory impairment are independent and no direct modulation exists in kappa-opioid and sigma receptors-mediated mechanisms.

Sigma(1) (sigma(1)) receptor antagonists represent a new strategy against cocaine addiction and toxicity

Cocaine is a highly addictive substance abused worldwide. Its mechanism of action involves initially inhibition of neuronal monoamine transporters in precise brain structures and primarily the dopamine reuptake system located on mesolimbic neurons. Cocaine rapidly increases the dopaminergic neurotransmission and triggers adaptive changes in numerous neuronal circuits underlying reinforcement, reward, sensitization and the high addictive potential of cocaine. Current therapeutic strategies focus on counteracting the cocaine effects directly on the dopamine transporter, through post-synaptic D(1), D(2) or D(3) receptors or through the glutamatergic, serotoninergic, opioid or corticotropin-releasing hormone systems. However, cocaine administration also results in the activation of numerous particular targets. Among them, the sigma(1) (sigma(1)) receptor is involved in several acute or chronic effects of cocaine. The present review will first bring concise overviews of the present strategies followed to alleviate cocaine addiction and animal models developed to analyze the pharmacology of cocaine addiction. Evidence involving activation of the sigma(1) receptor in the different aspects of cocaine abuse, will then be detailed, following acute, repeated, or overdose administration. The therapeutic potentials and neuropharmacological perspectives opened by the use of selective sigma(1) receptor antagonists in cocaine addiction will finally be discussed.

Differential involvement of the sigma(1) (sigma(1)) receptor in the anti-amnesic effect of neuroactive steroids, as demonstrated using an in vivo antisense strategy in the mouse

1. The sigma(1) (sigma(1)) receptor cDNA was cloned in several animal species. Molecular tools are now available to identify its endogenous effectors, such as neuroactive steroids, and to establish its precise physiological role. In particular, the sigma(1) receptor is involved in memory processes, as observed in pharmacological and pathological rodent models of amnesia. 2. In order to establish the involvement of sigma(1) receptors in memory, a 16-mer oligodeoxynucleotide antisense to the sigma(1) receptor cDNA (aODN), and its mismatched control (mODN) were prepared and centrally administered into the mouse brain. The anti-amnesic effects induced by the selective sigma(1) agonist PRE-084 and the steroid dehydroepiandrosterone (DHEA) sulphate or pregnenolone sulphate were examined in ODN-treated animals. 3. The aODN treatment failed to affect the dissociation constant (K(d)) but significantly decreased the number of sigma(1) sites (B(max)) labelled with [(3)H]-(+)-SKF-10,047 in the hippocampus and cortex. In these structures, the in vivo binding levels were also diminished, according to the dose and number of injections, as compared with control animals injected with saline or mODN. 4. Cannulation and injections failed to affect the open-field behaviour of the animals. However, the anti-amnesic effects of PRE-084 and DHEA sulphate against the dizocilpine-induced impairments were blocked after aODN treatment in the short- and long-term memory tests. The anti-amnesic effects of pregnenolone sulphate remained unchanged. 5. These observations bring a molecular basis to the modulatory role of sigma(1) receptors in memory, and reveal that the anti-amnesic action of neuroactive steroids may not similarly involve an interaction with sigma(1) receptors.

The anti-amnesic effects of sigma1 (sigma1) receptor agonists confirmed by in vivo antisense strategy in the mouse

The sigma1 (sigma1) receptor cDNA was recently cloned in several animal species, including the mouse. In order to firmly establish the implication of sigma1 receptors in memory, a phosphorothioate-modified antisense oligodeoxynucleotide (aODN) targeting the sigma1 receptor mRNA and a mismatched analog (mODN) were administered intracerebroventricularly for 3 days in mice. Scatchard analyses of in vitro (+)-[3H]SKF-10,047 binding to sigma1 sites showed that Bmax values were significantly decreased in the hippocampus (-58.5%) and cortex (-38.1%), but not in the cerebellum, of aODN treated mice, as compared to saline- or mODN-treated animals. In vivo binding levels were also significantly decreased after aODN treatment in the hippocampus and cortex but not in the cerebellum. The anti-amnesic effects of the selective sigma1 agonists PRE-084 or SA4503 were evaluated against the learning impairments induced by dizocilpine or scopolamine, respectively, using spontaneous alternation behavior and passive avoidance task. The anti-amnesic effects of PRE-084 or SA4503, observed after saline- or mODN-treatment, were blocked after aODN administration. These observations bring a molecular basis to the modulatory role of sigma1 receptors in memory processes.

Effects of physostigmine and calcium on acetylcholine efflux from the hippocampus of freely moving rats as determined by in vivo microdialysis and a radioimmunoassay

The effects varying the concentration of Ca2+ in perfused artificial cerebrospinal fluid ([Ca2+]csf) on basal acetylcholine (ACh) efflux from the hippocampus of freely moving rats, in the presence and absence of the cholinesterase (ChE) inhibitor physostigmine, were investigated using in vivo microdialysis and a highly specific radioimmunoassay for ACh. In the absence of physostigmine, basal ACh efflux was 3.4+/-0.7 pg/30 min (mean +/- SEM) at [Ca2+]csf = 1.26 mM. Stepwise increases in [Ca2+]csf elicited a gradual increase in ACh efflux that was significant at [Ca2+]csf = 5.04 mM. Inhibition of ChE by addition of 10 microM physostigmine to the perfusate increased the efflux of ACh to 103.2+/-21.1 pg/30 min ([Ca2+]csf = 1.26 mM), and the efflux was augmented still further by increasing [Ca2+]csf, a change that became significant at [Ca2+]csf = 3.78. These results illustrate the sensitivity of basal ACh efflux from the hippocampus to changes in the extracellular Ca2+ concentration, and suggest that a more accurate picture of hippocampal cholinergic activity is obtained by microdialysis using normal artificial cerebrospinal fluid, under physiological conditions, rather than in the presence of a ChE inhibitor.

Immunocytochemical localization of the sigma(1) receptor in the adult rat central nervous system

In order to characterize the localization of the sigma(1) receptor in the adult rat central nervous system, a polyclonal antibody was raised against a 20 amino acid peptide, corresponding to the fragment 143-162 of the cloned sigma(1) receptor protein. Throughout the rostrocaudal regions of the central nervous system extending from the olfactory bulb to the spinal cord, intense to moderate immunostaining was found to be associated with: (i) ependymocytes bordering the entire ventricular system, and (ii) neuron-like structures located within the parenchyma. Double fluorescence studies confirmed that, throughout the parenchyma, sigma(1) receptor-immunostaining was essentially associated with neuronal structures immunostained for the neuronal marker betaIII-tubulin. In all rats examined, high levels of immunostaining were always associated with neurons located within specific regions including the granular layer of the olfactory bulb, various hypothalamic nuclei, the septum, the central gray, motor nuclei of the hindbrain and the dorsal horn of the spinal cord. In contrast, only faint immunostaining was associated with neurons located in the caudate-putamen and the cerebellum. Electron microscope studies indicated that sigma(1) receptor immunostaining was mostly associated with neuronal perikarya and dendrites, where it was localized to the limiting plasma membrane, the membrane of mitochondria and of some cisternae of the endoplasmic reticulum. At the level of synaptic contacts, intense immunostaining was associated with postsynaptic structures including the postsynaptic thickening and some polymorphous vesicles, whereas the presynaptic axons were devoid of immunostaining. These data indicate that the sigma(1) receptor antibody prepared here, represents a promising tool for further investigating the role of sigma(1) receptors.

Neuroactive neurosteroids as endogenous effectors for the sigma1 (sigma1) receptor: pharmacological evidence and therapeutic opportunities

Neuroactive neurosteroids, including progesterone, allopregnanolone, pregnenolone and dehydroepiandrosterone, represent steroid hormones synthesized de novo in the brain and acting locally on nervous cells. Neurosteroids modulate several neurotransmitter systems such as gamma-aminobutyric acid type A (GABA(A)), N-methyl-D-aspartate (NMDA) and acetylcholine receptors. As physiologic consequences, they are involved in neuronal plasticity, learning and memory processes, aggression and epilepsy, and they modulate the responses to stress, anxiety and depression. The sigma1-receptor protein was recently purified and its cDNA was cloned in several species. The amino-acid sequences are structurally unrelated to known mammalian proteins, but shared homology with a fungal sterol C8-C7 isomerase. The sigma1-receptor ligands exert a potent neuromodulation on excitatory neurotransmitter systems, including the glutamate and cholinergic systems. Consequently, selective sigma1 agonists show neuroprotective properties and beneficial effects in memory processes, stress and depression. The evidence of a direct interaction between neurosteroids and sigma1 receptors was first suggested by the ability of several steroids to inhibit the binding of sigma1-receptor radioligands in vitro and in vivo. A crossed pharmacology between neurosteroids and sigma1-receptor ligands was described in several physiological tests and behavioral responses. This review will detail the recent evidence for a common mechanism of action between neurosteroids and sigma1-receptor ligands and focus on the potential therapeutic interests of such interaction in the physiopathology of learning and memory impairments, stress, depression and neuroprotection.

Modulation of steroidal levels by adrenalectomy/castration and inhibition of neurosteroid synthesis enzymes affect sigma1 receptor-mediated behaviour in mice

The interaction between neurosteroids and sigma1 (sigma1) receptors may be of therapeutic interest during physiological or pathological ageing, particularly concerning their neuromodulatory role on cognitive functions. Neurosteroids modulate memory processes through a mechanism involving interactions with GABAA, N-methyl-D-aspartate and/or sigma1 receptors. To measure the contribution of endogenous neurosteroid levels to the antiamnesic effects of sigma1 agonists, we investigated the effects of inhibitors of key enzymes involved in neurosteroid synthesis, in adrenalectomized/castrated (AdX/CX) mice to avoid the effect of circulating steroids. Trilostane, a 3beta-hydroxysteroid-deshydrogenase inhibitor, blocks the pregnenolone to progesterone conversion and leads to a decrease of progesterone. Finasteride, a 5alpha-reductase inhibitor, blocks the progesterone to 5alpha-pregnane-3,20-dione conversion and leads to an accumulation of progesterone. The in vivo binding of (+)-[3H]SKF-10 047 to sigma1 sites was measured in the mouse hippocampus and cortex. The attenuating effect of the selective sigma1 agonist PRE-084 (0.1-3 mg/kg) against dizocilpine (0.15 mg/kg)-induced learning impairment was examined using spontaneous alternation behaviour, step-down passive avoidance and place learning in the elevated plus-maze. The in vivo (+)-[3H]SKF-10 047 binding appeared significantly increased in AdX/CX mice and after trilostane treatment (10 mg/kg twice a day, 7 days), compared with sham-operated animals. The finasteride treatment (25 mg/kg, 7 days) significantly decreased binding levels. The learning deficits induced by dizocilpine were not affected by the treatments. The antiamnesic effect of PRE-084 was facilitated in AdX/CX mice and even more after trilostane treatment, as several parameters for animals treated with both PRE-084 and dizocilpine returned to control values. The PRE-084 effect was blocked after finasteride. These results confirmed that endogenous neurosteroidal levels modulate sigma1 receptor-mediated behaviour directly, and revealed that, among neurosteroids, progesterone may be the main modulator of sigma1 receptors.

The attenuation of learning impairments induced after exposure to CO or trimethyltin in mice by sigma (sigma) receptor ligands involves both sigma1 and sigma2 sites

1. Sigma (sigma) receptor ligands were previously reported to alleviate learning and memory impairments on several pharmacological and pathological rodent models of amnesia. Such effect was demonstrated as involving the sigma1 subtype of sigma receptor. 2. In this study, we characterized the pharmacological effect mediated by sigma ligands on two lesional models of amnesia in mice: (1) the hypoxia-related learning and memory impairment model induced by repeated exposure to carbon monoxide (CO) gas; and (2) the intoxication with trimethyltin (1 mg kg(-1)). 3. The selective sigma1 ligand PRE-084 (1 mg kg(-1)) or the non-selective sigma1/sigma2 compounds DTG (0.1 mg kg(-1)), BD1008 (3 mg kg(-1)), and haloperidol (0.1 mg kg(-1)) reversed significantly the spontaneous alternation deficits observed 7 days after exposure to CO or 14 days after intoxication with trimethyltin. 4. The selective sigma1 receptor antagonist NE-100 (1 mg kg(-1)) was ineffective by itself, but blocked completely the PRE-084 effects, partially the DTG effects, and did not affect the effects induced by BD1008 or haloperidol. 5. A similar pharmacological profile was observed in the step-down type passive avoidance test performed 8 days after exposure to CO. 6. These results show that, in contrast to the previously reported amnesia models, the impairments induced after exposure to CO or intoxication with trimethyltin could be alleviated not only by sigma1 receptor agonists but also by sigma2 agonists. The particular pattern of neurodegeneration observed in these lesional models may explain these differences.

Possible involvement of a sigma receptor subtype in the neck dystonia in rats

To clarify which subtype of sigma receptors is involved in the sigma receptor-mediated neck dystonia in rats, we examined whether 1-(3,4-dimethoxyphenethyl)-4-(3-phenylpropyl)piperazine dihydrochloride (SA4503), a selective sigma1 receptor agonist, and 1,3-di-(2-tolyl)guanidine (DTG), a sigma1 and sigma2 receptor agonist, induce neck dystonia in rats. Microinjection of SA4503 into the red nucleus of rat brain scarcely produced neck dystonia at the concentration of 10 nmol/0.5 microl. On the contrary, DTG produced significant dystonia at a concentrations of more than 5 nmol/0.5 microl. These results indicate that the sigma2 receptor subtype, but not sigma1 receptor subtype, may play an important role in the sigma receptor-mediated neck dystonia in rats.

The modulation by neurosteroids of the scopolamine-induced learning impairment in mice involves an interaction with sigma1 (sigma1) receptors

Neurosteroids have been reported to modulate learning and memory processes in aged animals and in pharmacological models of amnesia. We report here the effects of dehydroepiandrosterone sulfate (DHEAS), pregnenolone sulfate (PREGS), and progesterone (PROG) on the learning impairment induced in mice by the muscarinic acetylcholine receptor antagonist, scopolamine. Spatial working memory was examined using the spontaneous alternation behavior in a Y-maze and long-term memory using place learning in a rectangular water-maze adapted for mice. Both DHEAS and PREGS (5-20 mg/kg, s.c.) prevented dose-dependently and significantly the scopolamine (2 mg/kg, s.c.)-induced alternation deficits. PROG (2-20 mg/kg, s.c.) failed to affect the scopolamine-induced deficits, but blocked, at 20 mg/kg, the beneficial effects induced by DHEAS or PREGS. In the water-maze, DHEAS (20 mg/kg) attenuated significantly the scopolamine-induced deficits, as observed during the acquisition sessions or the retention test. PROG (2, 20 mg/kg) did not affect the control or scopolamine-treated group performances, but blocked the ameliorating effect of DHEAS. Furthermore, in both tests, the selective sigma1 (sigma1) receptor antagonist NE-100 (1 mg/kg, i.p.) failed to affect the behaviors showed by the control or scopolamine-treated groups, but it blocked the ameliorating effects induced by DHEAS or PREGS. These results confirm the modulating role of neurosteroids in learning and memory processes and demonstrate that their modulation of the cholinergic systems involves an interaction with sigma1 receptors.

Sigma1 (sigma 1) receptor agonists and neurosteroids attenuate B25-35-amyloid peptide-induced amnesia in mice through a common mechanism

The sigma1 (sigma 1) receptor agonists exert potent anti-amnesic effects, as they apparently block the learning impairments either induced by the muscarinic receptor antagonist scopolamine, the N-methyl-D-aspartate receptor antagonist dizocilpine or inherently due to the age-related deficits in senescence-accelerated mice. We recently described the amnesia induced by the beta-amyloid-related peptide beta 25-35, administered centrally in an aggregated form, in mice. The deficits were sensitive to cholinomimetics or to N-methyl-D-aspartate/glycine modulatory site agonists. Herein, we examined the effects of sigma 1 receptor ligands on the beta 25-35 peptide-induced amnesia. The effects of neuro(active) steroids, which interact in vitro and in vivo with sigma 1 receptors were examined in parallel. Mnesic capacity was evaluated seven days after administration of aggregated beta 25-35 peptide (3 nmol), using spontaneous alternation in the Y-maze for spatial short-term memory, or after 14 days, using the step-down type passive avoidance test for long-term memory. The sigma 1 receptor agonists (+)-pentazocine, PRE-084, or SA4503 attenuated, in a dose-dependent and bell-shaped manner, the beta 25-35 peptide-induced deficits on both tests. These effects were antagonized by haloperidol or BMY-14802, confirming the sigma 1 receptor pharmacology. Pregnenolone, dehydroepiandrosterone, and their sulphate esters, but not progesterone, also dose-dependently attenuated the beta 25-35 peptide-induced deficits. Progesterone blocked the beneficial effects of each other neurosteroid, behaving as an antagonist. Furthermore, haloperidol blocked the effects induced by neurosteroids, whereas progesterone antagonized the effects of the non-steroidal sigma 1 receptor agonists, showing a clear crossed pharmacology of different drug classes. These results demonstrate that: (i) the anti-amnesic effect of sigma 1 receptor agonists may be of therapeutic relevance in pathological states affecting the cholinergic and/or glutamatergic systems, such as in pathological aging; (ii) neurosteroids play an important role in learning processes and may collectively constitute a therapeutic target; (iii) the interaction between sigma 1 systems and neurosteroids appears indeed of behavioural relevance.

Sigma ligands stimulate GTPase activity in mouse prefrontal membranes: evidence for the existence of metabotropic sigma receptor

We studied effects of various sigma ligands on GTPase activity in mouse prefrontal membranes. Some representative sigma agonists, such as (+)-pentazocine, SA4503 and (+)-3-PPP, stimulated the GTPase activity in a concentration-dependent manner in ranges of 10 nM to 10 microM. Maximal effect was almost 10% increase to the control without treatment of drugs. However, another representative agonist, (+)-SKF10,047 showed only a partial activity with maximal effect 5% at 1 microM. NE-100, a representative antagonist, showed no effect at concentrations not more than 100 nM, while it did stimulate GTPase activity at 1 and 10 microM. Furthermore, these stimulative effects of both (+)-pentazocine and SA4503 on GTPase activity were significantly antagonized by NE-100 at 100 nM, suggesting that NE-100 possesses agonist-antagonist property. These findings suggest the possibility that there exist metabotropic sigma receptors.

SA4503, a novel cognitive enhancer with sigma1 receptor agonist properties, facilitates NMDA receptor-dependent learning in mice

The selective sigma1 receptor agonist 1-(3,4-dimethoxyphenethyl)-4-(3-phenyl propyl)piperazine dihydrochloride (SA4503) was reported to reverse the amnesia induced by the muscarinic receptor antagonist scopolamine at sub-mg/kg doses. We examined its effect on the learning impairment induced in mice by the non-competitive NMDA receptor antagonist dizocilpine. Learning capacities were evaluated using spontaneous alternation in the Y-maze for spatial working memory, and step-down type passive avoidance. SA4503 (0.03-1 mg/kg s.c.) attenuated the dizocilpine (0.15 mg/kg i.p.)-induced memory deficits following a bell-shaped curve in both tests. These effects of SA4503 were blocked by haloperidol (0.05 mg/kg i.p.), implicating sigma1 receptors. SA4503 also reversed the alternation deficit induced by N(omega)-nitro-L-arginine methyl ester (L-NAME, 100 mg/kg i.p.) at the same dosage, indicating that it acted on working memory through the nitric oxide (NO)-mediated signalling pathway. Furthermore, progesterone (2 mg/kg s.c.) blocked the SA4503 effects in the dizocilpine- and L-NAME-amnesia models, in accordance with the purported neurosteroids/sigma1 receptors interaction. These results demonstrate a promising neurobehavioural profile of SA4503, a ligand equally efficient to reverse the deficit in the glutamatergic as well as in the cholinergic amnesia model. Pertinent informations on the potential mechanism of the anti-amnesic effects of sigma1 receptor ligands were also obtained.

SA4503, a novel cognitive enhancer, with sigma 1 receptor agonistic properties

We found a potent and selective sigma 1 (sigma 1) receptor ligand, SA4503 (1-(3,4-dimethoxyphenethyl)-4-(3-phenylpropyl)piperazine dihydrochloride). This compound had a high affinity for sigma 1 receptor subtype (IC50 = 17 +/- 1.9 nM), but a low affinity for sigma 2 receptor subtype (IC50 = 1800 +/- 310 nM). The present study examines the effect of this compound on the central cholinergic functions, since sigma receptor has been reported to interact with the central cholinergic neurons. SA4503 elicited the increase in extracellular acetylcholine level in rat frontal cortex, while it did not affect the striatal acetylcholine level. On the other hand, tetrahydroaminoacridine (THA), an acetylcholinesterase (AChE) inhibitor, increased the extracellular acetylcholine level in both regions. Although both compounds had anti-amnesic effect against scopolamine-induced memory impairment, THA also induced catalepsy in rats. These results suggest that SA4503 may be a novel cognitive enhancer, with sigma 1 receptor agonistic properties. In addition, SA4503 does not cause striatal cholinomimetic side-effects, which is different from THA.

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.

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.

Ameliorating effect of SA4503, a novel sigma 1 receptor agonist, on memory impairments induced by cholinergic dysfunction in rats

We found a potent and selective sigma 1 receptor agonist, SA4503 (1-(3,4-dimethoxyphenethyl)-4-(3-phenylpropyl)piperazine dihydro-chloride), with high affinity for the sigma 1 receptor subtype (IC50 = 17 nM), but low affinity for the sigma 2 receptor subtype (IC50 = 1800 nM). The binding activity and selectivity of SA4503 resembled those of (+)-pentazocine, a prototype sigma 1 receptor agonist. We have previously shown that the sigma 1 receptor agonist activated central cholinergic functions. Therefore, we examined the effects of SA4503 on the cholinergic dysfunction-induced memory impairments in a passive avoidance task. Scopolamine, a muscarinic acetylcholine receptor antagonist, produced memory impairment, when it was administered 30 min before the training session of the passive avoidance task in rats. Single administration of SA4503 significantly reduced the scopolamine-induced memory impairment. In addition, the lesioning by injection of alpha-amino-3-hydroxy-5-isoxazole acetic acid (ibotenic acid) into the basal forebrain area produced memory impairment in rats. Repeated administration of SA4503 after lesioning of the basal forebrain area ameliorated the basal forebrain lesion-induced memory impairment. Moreover, the ameliorating effect of SA4503 against the scopolamine-induced memory impairment was antagonized by both 4-[4-(4-chlorophenyl)-4-hydroxy-1-piperidinyl]-1-(4-fluorophenyl)-1-buta none (haloperidol), a sigma receptor antagonist, and N,N-dipropyl-2- [4-methoxy-3-(2-phenylethoxy)phenyl]-ethylamine monohydrochloride (NE-100), a putative sigma 1 receptor antagonist. These results suggest that SA4503 has an anti-amnesic effect against cholinergic dysfunction-induced memory impairment, and that the effect of SA4503 is mediated by the sigma 1 receptor subtype.

Regional differences of the effect of sigma receptor ligands on the acetylcholine release in the rat brain

We found that sigma receptor ligands differentially regulated the acetylcholine (ACh) neurotransmission in the rat brain. Acute administration of (+)-N-allylnormetazocine [(+)-SKF-10,047], a prototype sigma 1 receptor ligand, and 1,3-di(2-tolyl)guanidine (DTG), a non-specific sigma receptor ligand, increased the extracellular ACh level in the rat hippocampus. This increase of hippocampal extracellular ACh level elicited by (+)-SKF-10,047 was more potent than that elicited by DTG. On the other hand, the striatal extracellular ACh level was slightly affected by (+)-SKF-10,047. In addition, DTG did not affect the striatal extracellular ACh level. Our previous studies have shown that both (+)-SKF-10,047 and DTG increased the extracellular ACh level in the rat frontal cortex. Taking all these data into consideration, the regulation of ACh neurotransmission by sigma receptor ligands are different depending upon the brain region.

Involvement of sigma 1 receptor in (+)-N-allylnormetazocine-stimulated hippocampal cholinergic functions in rats

The effects of the stereoisomers of N-allylnormetazocine (SKF-10,047) on the hippocampal cholinergic functions were compared in rats. A putative sigma 1 receptor agonist, (+)-SKF-10,047, elicited an increase of hippocampal extracellular acetylcholine level and anti-amnesic effect against scopolamine-induced memory dysfunctions in rats. These phenomena were not produced by (-)-SKF-10,047, and were reversed by haloperidol, a putative sigma 1 receptor antagonist. Such stereoselectivity and antagonism imply an involvement of sigma 1 receptors in these (+)-SKF-10,047-stimulated hippocampal cholinergic functions.

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.

Prevention of nimodipine-induced impairment of learning by the selective sigma ligand PRE-084

The high selectivity of the phencyclidine derivative PRE-084 for sigma (sigma) sites is demonstrated. We previously reported that this compound is able to markedly attenuate the impairment of learning induced in mice by the non-competitive NMDA antagonist MK-801, and the cholinergic nicotinic antagonist mecamylamine. In this study, we examined the effect of PRE-084 on the impairment of learning induced by acute administration of the calcium channel antagonist nimodipine. Nimodipine (0.3 mg/kg i.p.) impaired the spontaneous alternation behaviour in a Y-maze, decreased the step-down latency (SDL) in a passive avoidance task, and altered place learning and retention in a water-maze paradigm, with no marked effect on the motility observed using an open-field test. Preadministration of PRE-084 resulted in an attenuation of the impairment of alternation, in the 0.3-1 mg/kg s.c. range, in a marked increase in SDL, at 1-3 mg/kg, and improved place learning and retention in the water-maze, at 1 mg/kg. The effects on alternation behaviour and passive avoidance were completely prevented by co-administration of the purported sigma antagonist BMY-14802 (10 mg/kg i.p.), implicating the sigma sites. These results confirm the beneficial effect of the sigma ligand PRE-084 on pharmacological models of learning impairments, and indicate that sigma sites may modulate Ca2+ fluxes through VDCC, which may in turn bear some as yet unknown relationship to the previously described interaction with neurotransmitter systems.

PRE-084, a sigma selective PCP derivative, attenuates MK-801-induced impairment of learning in mice

We investigated the effect of the sigma selective PCP derivative PRE-084 on the impairment of learning induced in mice by the noncompetitive NMDA antagonist MK-801. Learning capacities were evaluated using the spontaneous alternation in a Y-maze test for spatial working memory, the step-down passive avoidance and the elevated plus-maze test for long-term memory. At doses about 1 mg/kg IP, PRE-084 significantly attenuated MK-801 (0.2 mg/kg IP) induced impairment of learning. The dose-response curve was bell-shaped which is typical for cognition enhancers. The effect of PRE-084 was antagonized by BMY-14802 (10 mg/kg IP) and suppressed by a chronic treatment with haloperidol (4 mg/kg/day SC for 7 days). Furthermore, PRE-084 did not affect scopolamine (1 mg/kg SC) induced amnesia but significantly attenuated mecamylamine (10 mg/kg IP) induced amnesia. These results show that sigma sites mediate a modulation of the NMDA receptor complex-dependent learning processes and may similarly affect the cholinergic nicotinic memory processes. Moreover, the PCP derivative PRE-084, acting selectively at sigma sites, reverses the amnesia induced by a drug acting at the PCP site.

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.

Behavioral evidence for a modulating role of sigma ligands in memory processes. I. Attenuation of dizocilpine (MK-801)-induced amnesia

The potentiating effect of low doses of sigma ligands on the N-methyl-D-aspartate (NMDA)-induced excitation of pyramidal CA3 dorsal hippocampal neurons has recently been reported. In the present study, we investigated behavioral effects relevant to these findings in the experimental amnesia induced by the non-competitive NMDA antagonist, dizocilpine (MK-801), in mice. At doses below 1 mg/kg s.c., the sigma ligands, 1,3-di-(2-tolyl)guanidine (DTG), (+)-SKF 10,047, and (+)-pentazocine, but not their (-)-isomers, significantly decreased MK-801 (100 microgram/kg s.c.)-induced impairment of spontaneous alternation performances in 8-min sessions of a Y-maze exploration, an index of spatial working memory, without affecting the concomitant hyperlocomotion. The effect of DTG (100 micrograms/kg s.c.) was completely antagonized by the simultaneous administration of BMY 14802 (10 mg/kg i.p.) and NE-100 (1 mg/kg i.p.), two putative sigma antagonists, which had no effect by themselves. In long-term memory tests (step-down and step-through types of passive avoidance, elevated plus-maze), DTG exhibited a significant attenuation of MK-801-induced amnesia, at doses of 10 and 100 micrograms/kg s.c. In all tests of short- and long-term memory, the effects exhibited by the sigma ligands tested had a bell-shaped curve; no effect was seen at 1 mg/kg. DTG did not affect the impairment of alternation induced by CPP (5 mg/kg i.p.): the modulation may selectively target the blockade of NMDA receptor-associated ion channels. Moreover, DTG (1-1000 micrograms/kg) did not affect the impairment induced by scopolamine (1 mg/kg i.p.) or diazepam (4 mg/kg i.p.), but significantly prevented the impairment induced by mecamylamine (10 mg/kg i.p.). These results suggest that the potentiating effect of sigma ligands on NMDA receptor-mediated glutamatergic neurotransmission, already demonstrated electrophysiologically, may have some relevance to learning and memory processes in the hippocampus. A similar modulation may also affect cholinergic nicotinic systems.

Behavioral evidence for a modulating role of sigma ligands in memory processes. II. Reversion of carbon monoxide-induced amnesia

This study examined the effect of low doses of sigma ligands on amnesia induced in mice by successive carbon monoxide (CO) exposure. Mice were exposed three consecutive times to CO (10 ml/min, 30-50 s) at 38 degrees C. Spatial working memory impairment was investigated 5 days later by monitoring spontaneous alternation behavior in a Y-maze. Delayed amnesia was examined 7 days after CO exposure by using a step-down passive avoidance test. The preadministration of the sigma ligand 1,3-di-(2-tolyl)guanidine (DTG), at doses of 1 to 1000 microgram/kg, s.c., 30 min before CO exposure did not affect the resulting amnesia in either test. However, when administered 30 min before the test, i.e., 5 or 7 days after CO exposure, this agent completely reversed the CO-induced decrease in alternation performance, at doses of 10 to 100 micrograms/kg. The same effect was observed with (+)-N-allylnormetazocine ((+)-SKF 10,047), at doses of 100 to 300 micrograms/kg, but not with (-)-SKF 10,047. DTG, at the same dose range that reversed the decrease in alternation, also totally reversed the CO-induced decrease in step-down latency in the passive avoidance test. The curve for these effects was bell-shaped; the effects were not observed at the dose of 1 mg/kg. Moreover, alpha-(4-fluorophenyl-2-pyrimidinyl)-1-piperazine butanol (BMY 14802), a putative sigma antagonist (1-10 mg/kg i.p.), did not affect CO-induced amnesia, but when simultaneously administered with DTG, it completely prevented its effect in both tests.(ABSTRACT TRUNCATED AT 250 WORDS)

Similar ameliorating effects of benzomorphans and 5-HT2 antagonists on drug-induced impairment of passive avoidance response in mice: comparison with acetylcholinesterase inhibitors

Mice were trained to avoid electric shocks by means of step-down type passive avoidance learning tasks, and memory retention was measured 24 h after the training session. Memory impairment (amnesia) was produced by administering either p-chloroamphetamine (PCA), a serotonin (5-HT) releaser or scopolamine (SCOP), a muscarinic cholinoceptor antagonist, 30 min prior to the training session. Benzomorphans, 5-HT2 antagonists and acetylcholinesterase (AChE) inhibitors were administered immediately after the training session. PCA- but not SCOP-induced amnesia was attenuated by the post-training administration of two benzomorphans, (+)N-allylnormetazocine ((+)SKF-10,047) and (+/- )pentazocine ((+/- )PTZ). Similarly, PCA-induced amnesia was reversed by the post-training administration of 5-HT2 antagonists, ritanserin (RIT) and mianserin (MIA), but SCOP-induced amnesia was not. However, the AChE inhibitors, tetrahydroaminoacridine (THA) and physostigmine (PHY) attenuated both PCA- and SCOP-induced amnesia when administered immediately after the training session. These results indicated that benzomorphans and 5-HT2 antagonists have antiamnestic effects in mice, as do AChE inhibitors. In addition, it is interesting that the patterns of ameliorating effect of benzomorphans were similar to those of 5-HT2 antagonists, which differ from those of AChE inhibitors.