Subcellular distribution of (+)-[3H]SKF 10,047 binding sites in rat liver

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.

Investigation of the role of sigma1-receptors in inositol 1,4,5-trisphosphate dependent calcium signaling in hepatocytes

In hepatocytes, as in other cell types, Ca(2+) signaling is subject to complex regulations, which result largely from the intrinsic characteristics of the different inositol 1,4,5-trisphosphate receptor (InsP(3)R) isoforms and from their interactions with other proteins. Although sigma1 receptors (Sig-1Rs) are widely expressed in the liver, their involvement in hepatic Ca(2+) signaling remains unknown. We here report that in this cell type Sig-1R interact with type 1 isoforms of the InsP(3) receptors (InsP(3)R-1). These results obtained by immunoprecipitation experiments are confirmed by the observation that Sig-1R proteins and InsP(3)R-1 colocalize in hepatocytes. However, Sig-1R ligands have no effect on InsP(3)-induced Ca(2+) release in hepatocytes. This can be explained by the rather low expression level expression of InsP(3)R-1. In contrast, we find that Sig-1R ligands can inhibit agonist-induced Ca(2+) signaling via an inhibitory effect on InsP(3) synthesis. We show that this inhibition is due to the stimulation of PKC activity by Sig-1R, resulting in the well-known down-regulation of the signaling pathway responsible for the transduction of the extracellular stimulus into InsP(3) synthesis. The PKC sensitive to Sig-1R activity belongs to the family of conventional PKC, but the precise molecular mechanism of this regulation remains to be elucidated.

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.

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.

Chronic treatment with the monoamine oxidase inhibitors clorgyline and pargyline down-regulates non-adrenoceptor [3H]-idazoxan binding sites in the rat brain

1. The binding of [3H]-idazoxan in the presence of 10(-6) M (-)-adrenaline was used to quantitate non-adrenoceptor idazoxan binding sites (NAIBS) in the rat brain after treatment with various psychotropic drugs. 2. Chronic treatment (14 days) with the monoamine oxidase (MAO) inhibitors clorgyline (0.3-10 mg kg-1, i.p.) and pargyline (10 mg kg-1, i.p.), but not with Ro 41-1049 (1 mg kg-1, i.p.), markedly decreased (30-50%) the density of NAIBS in the cerebral cortex without any apparent change in the affinity of the radioligand. 3. Acute (1 day) and/or chronic treatments (14 days) with other psychotropic drugs such as desipramine (3 mg kg-1, i.p.), cocaine (10 mg kg-1, i.p.), reserpine (0.12 mg kg-1, s.c.), haloperidol (1 mg kg-1, i.p.) and diazepam (10 mg kg-1, i.p.) did not alter the density of NAIBS in the cerebral cortex. 4. In vitro, the propargylamines clorgyline, pargyline and deprenyl displaced the binding of [3H]-idazoxan to NAIBS from two distinct sites, but only clorgyline displayed an apparent very high affinity for a relevant population of NAIBS (KiH = 40 pM; KiL = 10.6 microM). The structurally diverse MAO inhibitors Ro 16-6491 (selective for MAO-B) and Ro 41-1049 (selective for MAO-A), as well as the other psychotropic drugs (desipramine, cocaine, reserpine and haloperidol) displaced the binding of [3H]-idazoxan to NAIBS monophasically and with very low potencies. As expected, the MAO inhibitors clorgyline and Ro 41-1049 displaced the binding of [3H]-Ro 41-1049 to MAO-A monophasically and with high potencies (Ki values: 0.18 nM and 22 nM, respectively). In contrast, idazoxan displayed very low affinity (Ki =40 microM) against the binding of pH]-Ro 41-1049 to MAO-A. These results disprove a direct interaction between [3H]-idazoxan and the enzyme MAO.5. Preincubation of cortical membranes with clorgyline (10-9M or 10-6 M for 30 min) or pargyline(10-6 M or 10-5M for 30 min), reduced by 30-50% and by 17-30%, respectively, the total density of NAIBS without any apparent change in the affinity of the radioligand. Preincubation with 10-6M clorgyline did not alter the affinity of cirazoline for the two populations of NAIBS, but reduced by 60%the binding of [3H]-idazoxan to the high affinity site without affecting the binding of the radioligand to the low affinity site. These results indicate that the two MAO inhibitors irreversibly block the binding of[3H]-idazoxan to NAIBS.6. In vivo, however, various acute treatments with clorgyline (1-20 mg kg-1, i.p.) for different time intervals (6-48 h) did not alter the density of NAIBS. In vivo, only very high doses of clorgyline (40 and 80 mg kg-1, i.p.) induced modest decreases (21-28%) in the density of NAIBS in the cerebral cortex.7. Together the results indicate that the irreversible binding of clorgyline and pargyline to NAIBSfound in vitro does not fully explain the marked decreases in the density of NAIBS found in vivo after the chronic treatments. It is suggested that the down-regulation of NAIBS induced in vivo by clorgyline and pargyline, through a direct or indirect mechanism, may have functional implications.