Sigma(2)-receptor regulation of dopamine transporter via activation of protein kinase C

Sigma-1 receptor ligand PD144418 and sigma-2 receptor ligand YUN-252 attenuate the stimulant effects of methamphetamine in mice

RATIONALE

Previous research indicates that the selective sigma-1 receptor ligand PD144418 and the selective sigma-2 ligands YUN-252 can inhibit cocaine-induced hyperactivity. The effects of these ligands on other stimulants, such as methamphetamine, have not been reported.

OBJECTIVES

The present study examined the effects of PD144418 and YUN-252 pretreatment on methamphetamine-induced hyperactivity after acute treatment.

METHODS

Mice (n = 8-14/group) were injected with PD144418 (3.16, 10, or 31.6 μmol/kg), YUN-252 (0.316, 3.16, 31.6 μmol/kg), or saline. After 15 min, mice injected with 2.69 μmol/kg methamphetamine or saline vehicle, where distance traveled during a 60-min period was recorded. Additionally, the effect of PD144418 on the initiation and expression of methamphetamine sensitization was determined by treating mice (n = 8-14/group) with PD144418, methamphetamine or saline repeatedly over a 5-day period, and testing said mice with a challenge dose after a 7-day withdrawal period.

RESULTS

Results indicate that both PD144418 and YUN-252, in a dose-dependent manner, attenuated hyperactivity induced by an acute methamphetamine injection. Specifically, 10 μmol/kg or 31.6 μmol/kg of PD144418 and 31 μmol/kg of YUN-252 suppressed methamphetamine-induced hyperactivity. In regard to methamphetamine sensitization, while 10 μmol/kg PD144418 prevented the initiation of methamphetamine sensitization, it did not have an effect on the expression.

CONCLUSIONS

Overall, the current results suggest an intriguing potential for this novel sigma receptor ligand as a treatment for the addictive properties of methamphetamine. Future analysis of this novel sigma receptor ligand in assays directly measuring reinforcement properties will be critical.

Significance of protein kinase C in the neuropsychotoxicity induced by methamphetamine-like psychostimulants

The abuse of methamphetamine (MA), an amphetamine (AMPH)-type stimulant, has been demonstrated to be associated with various neuropsychotoxicity, including memory impairment, psychiatric morbidity, and dopaminergic toxicity. Compelling evidence from preclinical studies has indicated that protein kinase C (PKC), a large family of serine/threonine protein kinases, plays an important role in MA-induced neuropsychotoxicity. PKC-mediated N-terminal phosphorylation of dopamine transporter has been identified as one of the prerequisites for MA-induced synaptic dopamine release. Consistently, it has been shown that PKC is involved in MA (or AMPH)-induced memory impairment and mania-like behaviors as well as MA drug dependence. Direct or indirect regulation of factors related to neuronal plasticity seemed to be critical for these actions of PKC. In addition, PKC-mediated mitochondrial dysfunction, oxidative stress or impaired antioxidant defense system has been suggested to play a role in psychiatric and cognitive disturbance induced by MA (or AMPH). In MA-induced dopaminergic toxicity, particularly PKCδ has been shown to trigger oxidative stress, mitochondrial dysfunction, pro-apoptotic changes, and neuroinflammation. Importantly, PKCδ may be a key mediator in the positive feedback loop composed of these detrimental events to potentiate MA-induced dopaminergic toxicity. This review outlines the role of PKC and its individual isozymes in MA-induced neuropsychotoxicity. Better understanding on the molecular mechanism of PKCs might provide a great insight for the development of potential therapeutic or preventive candidates for MA (or AMPH)-associated neuropsychotoxicity.

The sigma-1 receptor as a regulator of dopamine neurotransmission: A potential therapeutic target for methamphetamine addiction

Methamphetamine (METH) abuse is a major public health issue around the world, yet there are currently no effective pharmacotherapies for the treatment of METH addiction. METH is a potent psychostimulant that increases extracellular dopamine levels by targeting the dopamine transporter (DAT) and alters neuronal activity in the reward centers of the brain. One promising therapeutic target for the treatment of METH addiction is the sigma-1 receptor (σ1R). The σ1R is an endoplasmic reticulum-localized chaperone protein that is activated by cellular stress, and, unique to this chaperone, its function can also be induced or inhibited by different ligands. Upon activation of this unique "chaperone receptor", the σ1R regulates a variety of cellular functions and possesses neuroprotective activity in the brain. Interestingly, a variety of σ1R ligands modulate dopamine neurotransmission and reduce the behavioral effects of METH in animal models of addictive behavior, suggesting that the σ1R may be a viable therapeutic target for the treatment of METH addiction. In this review, we provide background on METH and the σ1R as well as a literature review regarding the role of σ1Rs in modulating both dopamine neurotransmission and the effects of METH. We aim to highlight the complexities of σ1R pharmacology and function as well as the therapeutic potential of the σ1R as a target for the treatment of METH addiction.

Cocaine Effects on Dopaminergic Transmission Depend on a Balance between Sigma-1 and Sigma-2 Receptor Expression

Sigma σ₁ and σ₂ receptors are targets of cocaine. Despite sharing a similar name, the two receptors are structurally unrelated and their physiological role is unknown. Cocaine increases the level of dopamine, a key neurotransmitter in CNS motor control and reward areas. While the drug also affects dopaminergic signaling by allosteric modulations exerted by σ₁R interacting with dopamine D₁ and D₂ receptors, the potential regulation of dopaminergic transmission by σ₂R is also unknown. We here demonstrate that σ₂R may form heteroreceptor complexes with D₁ but not with D₂ receptors. Remarkably σ₁, σ₂, and D₁ receptors may form heterotrimers with particular signaling properties. Determination of cAMP levels, MAP kinase activation and label-free assays demonstrate allosteric interactions within the trimer. Importantly, the presence of σ₂R induces bias in signal transduction as σ₂R ligands increase cAMP signaling whereas reduce MAP kinase activation. These effects, which are opposite to those exerted via σ₁R, suggest that the D₁ receptor-mediated signaling depends on the degree of trimer formation and the differential balance of sigma receptor and heteroreceptor expression in acute versus chronic cocaine consumption. Although the physiological role is unknown, the heteroreceptor complex formed by σ₁, σ₂, and D₁ receptors arise as relevant to convey the cocaine actions on motor control and reward circuits and as a key factor in acquisition of the addictive habit.

Kinase-dependent Regulation of Monoamine Neurotransmitter Transporters

Modulation of neurotransmission by the monoamines dopamine (DA), norepinephrine (NE), and serotonin (5-HT) is critical for normal nervous system function. Precise temporal and spatial control of this signaling in mediated in large part by the actions of monoamine transporters (DAT, NET, and SERT, respectively). These transporters act to recapture their respective neurotransmitters after release, and disruption of clearance and reuptake has significant effects on physiology and behavior and has been linked to a number of neuropsychiatric disorders. To ensure adequate and dynamic control of these transporters, multiple modes of control have evolved to regulate their activity and trafficking. Central to many of these modes of control are the actions of protein kinases, whose actions can be direct or indirectly mediated by kinase-modulated protein interactions. Here, we summarize the current state of our understanding of how protein kinases regulate monoamine transporters through changes in activity, trafficking, phosphorylation state, and interacting partners. We highlight genetic, biochemical, and pharmacological evidence for kinase-linked control of DAT, NET, and SERT and, where applicable, provide evidence for endogenous activators of these pathways. We hope our discussion can lead to a more nuanced and integrated understanding of how neurotransmitter transporters are controlled and may contribute to disorders that feature perturbed monoamine signaling, with an ultimate goal of developing better therapeutic strategies.

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.

Synthesis and pharmacological characterization of a novel sigma receptor ligand with improved metabolic stability and antagonistic effects against methamphetamine

Methamphetamine interacts with sigma receptors at physiologically relevant concentrations suggesting a potential site for pharmacologic intervention. In the present study, a previous sigma receptor ligand, CM156, was optimized for metabolic stability, and the lead analog was evaluated against the behavioral effects of methamphetamine. Radioligand binding studies demonstrated that the lead analog, AZ66, displayed high nanomolar affinity for both sigma-1 and sigma-2 receptors (2.4 ± 0.63 and 0.51 ± 0.15, respectively). In addition, AZ66 had preferential affinity for sigma receptors compared to seven other sites and a significantly longer half-life than its predecessor, CM156, in vitro and in vivo. Pretreatment of male, Swiss Webster mice with intraperitoneal (10-20 mg/kg) or oral (20-30 mg/kg) dosing of AZ66 significantly attenuated the acute locomotor stimulatory effects of methamphetamine. Additionally, AZ66 (10-20 mg/kg, i.p.) significantly reduced the expression and development of behavioral sensitization induced by repeated methamphetamine administration. Taken together, these data indicate that sigma receptors can be targeted to mitigate the acute and subchronic behavioral effects of methamphetamine and AZ66 represents a viable lead compound in the development of novel therapeutics against methamphetamine-induced behaviors.

AC927, a σ receptor ligand, blocks methamphetamine-induced release of dopamine and generation of reactive oxygen species in NG108-15 cells

Methamphetamine is a highly addictive psychostimulant drug of abuse that causes neurotoxicity with high or repeated dosing. Earlier studies demonstrated the ability of the selective σ receptor ligand N-phenethylpiperidine oxalate (AC927) to attenuate the neurotoxic effects of methamphetamine in vivo. However, the precise mechanisms through which AC927 conveys its protective effects remain to be determined. With the use of differentiated NG108-15 cells as a model system, the effects of methamphetamine on neurotoxic endpoints and mediators such as apoptosis, necrosis, generation of reactive oxygen species (ROS) and reactive nitrogen species (RNS), and dopamine release were examined in the absence and presence of AC927. Methamphetamine at physiologically relevant micromolar concentrations caused apoptosis in NG108-15 cells. At higher concentrations of methamphetamine, necrotic cell death was observed. At earlier time points, methamphetamine caused ROS/RNS generation, which was detected with the fluorigenic substrate 5-(and-6)-chloromethyl-2',7'-dichlorodihydrofluorescin diacetate, acetyl ester, in a concentration- and time-dependent manner. N-Acetylcysteine, catalase, and l-N(G)-monomethyl arginine citrate inhibited the ROS/RNS fluorescence signal induced by methamphetamine, which suggests the formation of hydrogen peroxide and RNS. Exposure to methamphetamine also stimulated the release of dopamine from NG108-15 cells into the culture medium. AC927 attenuated methamphetamine-induced apoptosis, necrosis, ROS/RNS generation, and dopamine release in NG108-15 cells. Together, the data suggest that modulation of σ receptors can mitigate methamphetamine-induced cytotoxicity, ROS/RNS generation, and dopamine release in cultured cells.

Role of sigma receptors in methamphetamine-induced neurotoxicity

Methamphetamine (METH) is a widely abused substance world over. Currently, there is no effective pharmacotherapy to treat its effects. This necessitates identification of potential novel therapeutic targets. METH interacts with sigma (σ) receptors at physiologically relevant micromolar concentrations. In addition, σ receptors are present in organs like the brain, heart, and lungs at which METH acts. Additionally, σ receptors have been implicated in various acute and subchronic effects like locomotor stimulation, development of sensitization and neurotoxicity, where σ receptor antagonists attenuate these effects. σ Receptors may also have a role in METH-induced psychiatric complications such as depression, psychosis, cognitive and motor deficits. The neurotoxic effects of METH, which are cause for concern, can be prevented by σ receptor antagonists in mice. Mechanistically, METH-induced neurotoxicity involves factors like dopamine release, oxidative stress, endoplasmic reticulum stress, activation of mitochondrial death cascades, glutamate release, apoptosis, microglial activation, and hyperthermia. This review compiles studies from the literature that suggests an important role for σ receptors in many of the mechanisms of METH-induced neurotoxicity.

CM156, a high affinity sigma ligand, attenuates the stimulant and neurotoxic effects of methamphetamine in mice

Methamphetamine (METH) is a highly addictive psychostimulant drug of abuse. Low and high dose administration of METH leads to locomotor stimulation, and dopaminergic and serotonergic neurotoxicity, respectively. The behavioral stimulant and neurotoxic effects of METH can contribute to addiction and other neuropsychiatric disorders, thus necessitating the identification of potential pharmacotherapeutics against these effects produced by METH. METH binds to σ receptors at physiologically relevant concentrations. Also, σ receptors are present on and can modulate dopaminergic and serotonergic neurons. Therefore, σ receptors provide a viable target for the development of pharmacotherapeutics against the adverse effects of METH. In the present study, CM156, a σ receptor ligand with high affinity and selectivity for σ receptors over 80 other non-σ binding sites, was evaluated against METH-induced stimulant, hyperthermic, and neurotoxic effects. Pretreatment of male, Swiss Webster mice with CM156 dose dependently attenuated the locomotor stimulation, hyperthermia, striatal dopamine and serotonin depletions, and striatal dopamine and serotonin transporter reductions produced by METH, without significant effects of CM156 on its own. These results demonstrate the ability of a highly selective σ ligand to mitigate the effects of METH.

Synthesis and pharmacological evaluation of 6-acetyl-3-(4-(4-(4-fluorophenyl)piperazin-1-yl)butyl)benzo[d]oxazol-2(3H)-one (SN79), a cocaine antagonist, in rodents

Cocaine interacts with monoamine transporters and sigma (σ) receptors, providing logical targets for medication development. In the present study, in vitro and in vivo pharmacological studies were conducted to characterize SN79, a novel compound which was evaluated for cocaine antagonist actions. Radioligand binding studies showed that SN79 had a nanomolar affinity for σ receptors and a notable affinity for 5-HT(2) receptors, and monoamine transporters. It did not inhibit major cytochrome P450 enzymes, including CYP1A2, CYP2A6, CYP2C19, CYP2C9*1, CYP2D6, and CYP3A4, suggesting a low propensity for potential drug-drug interactions. Oral administration of SN79 reached peak in vivo concentrations after 1.5 h and exhibited a half-life of just over 7.5 h in male, Sprague-Dawley rats. Behavioral studies conducted in male, Swiss Webster mice, intraperitoneal or oral dosing with SN79 prior to a convulsive or locomotor stimulant dose of cocaine led to a significant attenuation of cocaine-induced convulsions and locomotor activity. However, SN79 produced sedation and motor incoordination on its own at higher doses, to which animals became tolerant with repeated administration. SN79 also significantly attenuated the development and expression of the sensitized response to repeated cocaine exposures. The ability of SN79 to significantly attenuate the acute and subchronic effects of cocaine provides a promising compound lead to the development of an effective pharmacotherapy against cocaine.

Sigma (σ) receptor ligand, AC927 (N-phenethylpiperidine oxalate), attenuates methamphetamine-induced hyperthermia and serotonin damage in mice

Methamphetamine interacts with sigma (σ) receptors and AC927, a selective σ receptor ligand, protects against methamphetamine-induced dopaminergic neurotoxicity. In the present study, the effects of AC927 on methamphetamine-induced hyperthermia and striatal serotonergic neurotoxicity were evaluated. Male, Swiss Webster mice were injected (i.p.) every 2 h, for a total of four times, with one of the following treatments: Saline+Saline; Saline+Methamphetamine (5 mg/kg); AC927 (5, 10, 20 mg/kg)+Methamphetamine (5 mg/kg); or AC927 (5, 10, 20 mg/kg)+Saline. Pretreatment with AC927 (10 mg/kg) significantly attenuated methamphetamine-induced striatal serotonin depletions, striatal serotonin transporter reductions, and hyperthermia. At the doses tested, AC927 itself had no significant effects on serotonin levels, serotonin transporter expression, or body temperature. To evaluate the effects of higher ambient temperature on methamphetamine-induced neurotoxicity, groups of mice were treated at 37 °C. Overall, there was an inverse correlation between the body temperature of the animals and striatal serotonin levels. Together, the data suggest that AC927 (10 mg/kg) protects against methamphetamine-induced neurotoxicity. The reduction of methamphetamine-induced hyperthermia by AC927 may contribute to the observed neuroprotection in vivo.

Sigma1 receptor antagonists determine the behavioral pattern of the methamphetamine-induced stereotypy in mice

OBJECTIVE

The effects of sigma receptor antagonists on methamphetamine (METH)-induced stereotypy have not been examined. We examined the effects of sigma antagonists on METH-induced stereotypy in mice.

RESULTS

The administration of METH (10 mg/kg) to male ddY mice induced stereotyped behavior consisting of biting (90.1%), sniffing (4.2%), head bobbing (4.1%), and circling (1.7%) during an observation period of 1 h. Pretreatment of the mice with BMY 14802 (alpha-(4-fluorophenyl)-4-(5-fluoro-2-pyrimidinyl)-1-piperazinebutanol; 1, 5, and 10 mg/kg), a non-specific sigma receptor antagonist, significantly increased METH-induced sniffing (19.2%, 30.5%, and 43.8% of total stereotypical behavior) but decreased biting (76.6%, 66.9%, and 49.3% of total stereotypical behavior) in a dose-dependent manner. This response was completely abolished by (+)-SKF 10,047 ([2S-(2alpha,6alpha,11R)]-1,2,3,4,5,6-hexahydro-6,11-dimethyl-3-(2-propenyl)-2,6-methano-3-benzazocin-8-ol; 4 and 10 mg/kg), a putative sigma(1) receptor agonist, and partially by PB 28 (1-cyclohexyl-4-[3-(1,2,3,4-tetrahydro-5-methoxy-1-naphthalen-1-yl)-n-propyl]piperazine; 1 and 10 mg/kg), a putative sigma(2) receptor agonist. The BMY 14802 action on METH-induced stereotypy was mimicked by BD 1047 (N-[2-(3,4-dichlorophenyl)ethyl]-N-methyl-2-(dimethylamino)ethylamine; 10 mg/kg), a putative sigma(1) receptor antagonist, but not by SM-21 ((+/-)-tropanyl 2-(4-chlorophenoxy)butanoate; 1 mg/kg), a putative sigma(2) receptor antagonist. The BD 1047 effect on METH-induced stereotypy was also abolished completely by (+)-SKF 10,047 and partially by PB 28. The overall frequency of METH-induced stereotypical behavior was unchanged with these sigma receptor ligands, despite the alteration in particular behavioral patterns. The BMY 14802 action on METH-induced stereotypy was unaffected by pretreatment with centrally acting histamine H(1) receptor antagonists (pyrilamine or ketotifen, 10 mg/kg), suggesting that these effects are independent of histamine H(1) receptor signaling systems.

CONCLUSION

In summary, modulation of central sigma(1) receptors alters the pattern of METH-induced stereotypy, producing a shift from stereotypical biting to stereotypical sniffing, without affecting the overall frequency of stereotypical behavior.

Attenuation of methamphetamine-induced effects through the antagonism of sigma (sigma) receptors: Evidence from in vivo and in vitro studies

Methamphetamine (METH) and many other abused substances interact with sigma receptors. sigma receptors are found on dopaminergic neurons and can modulate dopaminergic neurotransmission. Antisense knock down of sigma receptors also mitigates METH-induced stimulant effects, suggesting that these proteins are viable medication development targets for treating psychostimulant abuse. In the present study, AC927, a sigma receptor antagonist, was evaluated for its ability to attenuate METH-induced effects in vivo and in vitro. Radioligand binding studies showed that AC927 had preferential affinity for sigma receptors compared to 29 other receptors, transporters and ion channels. Pretreatment of male, Swiss Webster mice with AC927 significantly attenuated METH-induced locomotor stimulation, striatal dopamine depletions, striatal dopamine transporter reductions, and hyperthermia. When the neurotoxicity of METH was further examined in vitro under temperature-controlled conditions, co-incubation with AC927 mitigated METH-induced cytotoxicity. Together, the results demonstrate that AC927 protects against METH-induced effects, and suggests a new strategy for treating psychostimulant abuse.

Modulation of neurotransmitter systems by dehydroepiandrosterone and dehydroepiandrosterone sulfate: mechanism of action and relevance to psychiatric disorders

Dehydroepiandrosterone (DHEA) is synthesized in the brain and several studies have shown that this steroid is a modulator of synaptic transmission. The effect of DHEA, and its sulfate ester DHEAS, on glutamate and GABA neurotransmission has been extensively studied but some effects on other neurotransmitter systems, such as dopamine, serotonin and nitric oxide, have also been reported. This review summarizes studies showing the effect of DHEA and DHEAS on neurotransmitter systems at different levels (metabolism, release, reuptake, receptor activation), as well as the activation of voltage-gated ion channels and calcium homeostasis, showing the variety of effects that these steroids exert on those systems, allowing the discussion of its mechanisms of action and its relevance to psychiatric disorders.

The sigma-2 receptor agonist PB28 inhibits calcium release from the endoplasmic reticulum of SK-N-SH neuroblastoma cells

In this paper we demonstrate that PB28 abolishes the Ca2+ release through the inositol 1,4,5-trisphosphate (InsP3) receptors and ryanodine receptors in SK-N-SH cells. Sigma receptors are divided into the subtypes sigma-1 and sigma-2, which are expressed in tumor cell lines and characterized by distinct pharmacological profiles. The sigma-1 receptor has been recently cloned, whereas the sigma-2 receptor is less well characterized. The endogenous ligand(s) of both subtypes remain unclear. In isolated guinea pig ileum, PB28 inhibits the contraction induced by carbachol dose dependently and in a non-competitive manner. In SK-N-SH cells PB28 challenge does not affect the intracellular Ca2+ concentration but incubation with PB28 for 45 min abolishes the cytosolic Ca2+ increases evoked by carbachol or histamine. This effect, not sensitive to cycloheximide, is caused by direct inhibition of the InsP3 receptors, since PB28 abolishes the response elicited by InsP3 administration in permeabilized SK-N-SH cells. Finally, incubation for 45 min with PB28 also abolishes the cytosolic Ca2+ increase evoked by caffeine.

Sigma2 (sigma2) receptors as a target for cocaine action in the rat striatum

Studies from our laboratory have shown that agonists at sigma1 and sigma2 receptors inhibit N-methyl-D-aspartate (NMDA)-stimulated dopamine release from motor and limbic areas of rat brain. In the current study, we examined the effects of cocaine on N-methyl-D-aspartate (NMDA)-stimulated [3H]dopamine release in rat striatal slices. Cocaine inhibited N-methyl-D-aspartate-stimulated [3H]dopamine release in a concentration-dependent manner with a Ki of approximately 10 microM, under conditions in which the dopamine transporter (DAT) was blocked by 10 microM nomifensine. The inhibition seen by cocaine was reversed by the selective sigma2 antagonist 1'-[4-[1-(4-fluorophenyl)-1H-indol-3-yl]-1-butyl]-spiro[isobenzofuran-1(3H), 4'piperidine] (Lu28-179). Inhibition of release by cocaine and (+)pentazocine, under conditions in which sigma1 receptors were blocked, was also reversed by the conventional PKC inhibitor 3-[1-[3-(dimethylamino)propyl-1H-indole-3-yl]-1-H-pyrpole-2-5'-dione. These results suggest that cocaine or other agonists, acting through the sigma2 receptor, require an intact conventional PKC (cPKC), most likely PKCalpha or PKCgamma in order to inhibit dopamine release.

A Medicinal-Chemistry-Guided Approach to Selective and Druglike Sigma 1 Ligands

Based on a medicinal-chemistry-guided approach, three novel series of druglike cycloalkyl-annelated pyrazoles were synthesized and display high affinity (pKi>8) for the sigma1 receptor. Structure-affinity relationships were established, and the different scaffolds were optimized with respect to sigma1 binding and selectivity versus the sigma2 receptor and the hERG channel, resulting in selective compounds that have Ki values (for sigma1) in the subnanomolar range. Selected compounds were screened for cytochrome P450 inhibition (CYP1A2, CYP2A6, CYP2B6, CYP2C8, CYP2C9, CYP2C19, CYP2D6, CYP2E1, CYP3A4), metabolic stability (rat and human liver microsomes), and cell-membrane permeability (Caco-2). They showed favorable in vitro ADME properties as well as favorable calculated druglike and experimental physicochemical properties. Furthermore, compounds 7 f and 17 a, for example, displayed high selectivity (affinity) for the sigma1 receptor against a wide range of other receptors (>60). With these valuable tool compounds in hand, we are further exploring the role of the sigma1 receptor in relevant animal models corresponding to such medicinal indications as drug abuse, pain, depression, anxiety, and psychosis.

Sigma Receptors and Cancer

The sigma (sigma) receptor and its agonists have been implicated in a myriad of cellular functions, biological processes and diseases. Whereas the precise molecular mechanism(s) of sigma receptors and their involvement in cancer cell biology have not been elucidated, recent work has started to shed some light on these issues. A molecular model has been proposed for the cloned sigma1 receptor; the precise molecular nature of the sigma2 receptor remains unknown. sigma receptors have been found to be frequently up-regulated in human cancer cells and tissues. sigma2 receptor drugs particularly have been shown to have antiproliferative effects. An interesting possibility is that sigma and/or sigma1 drugs could produce anticancerous effects by modulating ion channels. As well as proliferation, a variety of other metastatic cellular behaviors such as adhesion, motility, and secretion may also be affected. Other mechanisms of sigma receptor action may involve interaction with ankyrin and modulation of intracellular Ca(2+) and sphingolipid levels. Although more research is needed to further define the molecular physiology of sigma receptors, their involvement in the cellular pathophysiology of cancer raises the possibility that sigma drugs could be useful as novel therapeutic agents.

Dynamic regulation of the dopamine transporter

In the mammalian central nervous system the dopamine transporter (DAT) is the primary mechanism for clearance of dopamine from the extracellular space. Presynaptic receptors for dopamine and other neurotransmitters (auto-receptors and hetero-receptors) present on dopaminergic neurons are poised to regulate the activity of the dopamine transporter acutely through their actions on intracellular signaling systems. The mechanisms proposed for acute presynaptic regulation of dopamine transport include direct effects of phosphorylation on enzymatic rate, indirect effects through the alteration of the electrical and chemical gradients that drive transport and/or the modulation of transporter number through the trafficking of carriers to and from the cell surface. This review focuses on recent evidence for several distinct mechanisms which dynamically regulate dopamine transporter activity and thus have an important role in shaping the duration and amplitude of dopamine signals in the brain.

Amphetamine regulation of dopamine transport. Combined measurements of transporter currents and transporter imaging support the endocytosis of an active carrier

Dopaminergic neurotransmission is fine-tuned by the rate of removal of dopamine (DA) from the extracellular space via the Na(+)/Cl(-)-dependent DA transporter (DAT). DAT is a target of psychostimulants such as amphetamine (AMPH) and cocaine. Previously, we reported that AMPH redistributes the human DAT away from the cell surface. This process was associated with a reduction in transport capacity. This loss of transport capacity may result either from a modification of the function of DAT that is independent of its cell surface redistribution and/or from a reduction in the number of active transporters at the plasma membrane that results from DAT trafficking. To discriminate between these possibilities, we stably transfected HEK-293 cells with a yellow fluorescent protein (YFP)-tagged human DAT (hDAT cells). In hDAT cells, acute exposure to AMPH induced a time-dependent loss of hDAT activity. By coupling confocal imaging with patch-clamp whole-cell recordings, we have demonstrated for the first time that the loss of AMPH-induced hDAT activity temporally parallels the accumulation of intracellular hDAT. In addition, presteady-state current analysis revealed a cocaine-sensitive, voltage-dependent capacitance current that correlated with the level of transporter membrane expression and in turn served to monitor the AMPH-induced trafficking of hDAT. We found that the decrease in hDAT cell surface expression induced by AMPH was not paralleled by changes in the ability of the single transporter to carry charges. Quasi-stationary noise analysis of the AMPH-induced hDAT currents revealed that the unitary transporter current remained unaltered during the loss of hDAT membrane expression. Taken together, these data strongly suggest that the AMPH-induced reduction of hDAT transport capacity results from the removal of active hDAT from the plasma membrane.

Repeated amphetamine treatment induces neurite outgrowth and enhanced amphetamine-stimulated dopamine release in rat pheochromocytoma cells (PC12 cells) via a protein kinase C- and mitogen activated protein kinase-dependent mechanism

Repeated intermittent treatment with amphetamine (AMPH) induces both neurite outgrowth and enhanced AMPH-stimulated dopamine (DA) release in PC12 cells. We investigated the role of protein kinases in the induction of these AMPH-mediated events by using inhibitors of protein kinase C (PKC), mitogen activated protein kinase (MAP kinase) or protein kinase A (PKA). PKC inhibitors chelerythrine (100 nm and 300 nm), Ro31-8220 (300 nm) and the MAP kinase kinase inhibitor, PD98059 (30 micro m) inhibited the ability of AMPH to elicit both neurite outgrowth and the enhanced AMPH-stimulated DA release. The direct-acting PKC activator, 12-O-tetradecanoyl phorbol 13-acetate (TPA, 250 nm) mimicked the ability of AMPH to elicit neurite outgrowth and enhanced DA release. On the contrary, a selective PKA inhibitor, 100 micro m Rp-8-Br-cAMPS, blocked only the development of AMPH-stimulated DA release but not the neurite outgrowth. Treatment of the cells with acute AMPH elicited an increase in the activity of PKC and MAP kinase but not PKA. These results demonstrated that AMPH-induced increases in MAP kinase and PKC are important for induction of both the enhancement in transporter-mediated DA release and neurite outgrowth but PKA was only required for the enhancement in AMPH-stimulated DA release. Therefore the mechanisms by which AMPH induces neurite outgrowth and the enhancement in AMPH-stimulated DA release can be differentiated.

The effect of phosphorylation on amphetamine-mediated outward transport

Amphetamine elicits its locomotor-activating and drug-reinforcing effects by releasing the catecholamines dopamine and norepinephrine into the synapse. Amphetamine is a substrate of the plasmalemmal transporters for both dopamine and norepinephrine. As such, it binds to the transporters in conjunction with Na+ and Cl-, facilitating a conformational change leading the transporter to face inward. The subsequent binding of intracellular catecholamine results in an outward transport and release of the catecholamine into the synapse. Both inward and outward transport through the catecholamine transporters are regulated by protein kinases, particularly protein kinase C, but the effect of the enzyme on the two processes appears to be asymmetric. The purpose of this review is to discuss the evidence showing that protein kinase C activation facilitates outward transport through the catecholamine plasmalemmal transporters which may mediate amphetamine action in intact tissue.

Rapid regulation of dopamine transporter function by substrates, blockers and presynaptic receptor ligands

The extracellular actions of dopamine are terminated primarily through its binding to dopamine transporters and translocation back into dopamine neurons. The transporter thereby serves as an optimal target to regulate dopamine neurotransmission. Although acute pharmacological blockade of dopamine transporters is known to reversibly inhibit transporter function by preventing the binding of its endogenous substrate dopamine, it recently has become clear that dopamine transporter substrates, such as amphetamines, and blockers, such as cocaine, also have the ability to rapidly and persistently regulate transporter function after their direct pharmacological effect has subsided. Presynaptic receptor ligands can also regulate dopamine transporter function. This has been investigated most extensively for dopamine D2 receptors, but there is also evidence for regulation by gamma-aminobutyric acid (GABA) GABAB receptors, metabotropic glutamate, nicotinic acetylcholine, serotonin, sigma2- and kappa-opioid receptors. The focus of this review is the rapid, typically reversible, regulation of dopamine transporter velocity by substrates, blockers and presynaptic receptor ligands. The research discussed here suggests that a common mechanism through which these different classes of compounds regulate transporter activity is by altering the cell surface expression of dopamine transporters.

Generation and phenotypic analysis of sigma receptor type I (sigma 1) knockout mice

Sigma (sigma) sites are a type of nonopiate receptor whose role has been associated with several behaviours, including anxiety, depression, analgesia, learning processes and psychosis. Although there are several known sigma receptor types, only the type I receptor (sigma 1) has been cloned. To uncover the in vivo relevance of sigma-receptors, we have generated knockout mice for sigma 1. Despite the broad expression pattern found for the sigma 1-gene, homozygous mutant mice are viable, fertile and do not display any overt phenotype, compared with their wild-type litter-mates, in mixed genetic backgrounds. However, a significant decrease in the hypermotility response has been measured in knockout mice upon challenge with (+)SKF-10 047, in agreement with the involvement of sigma 1-receptors in the induction of psychostimulant actions. The activity of sigma 2-receptors seems to be unaffected in sigma 1-mutant mice. These knockout mice could contribute to better understand the in vivo role of sigma-receptors.

The Caenorhabditis elegans dopaminergic system: opportunities for insights into dopamine transport and neurodegeneration

The neurotransmitter dopamine (DA) plays a central role in the coordination of movement, attention, and the recognition of reward. Loss of DA from the basal ganglia, as a consequence of degeneration of neurons in the substantia nigra, triggers postural instability and Parkinson's disease (PD). DA transporters (DATs) regulate synaptic DA availability and provide a conduit for the uptake of DA mimetic neurotoxins, which can be used to evoke neuronal death and Parkinson-like syndrome. Recently, we have explored the sensitivity of DA neurons in the nematode Caenorhabditis elegans to the Parkinsonian-inducing neurotoxin 6-hydroxydopamine (6-OHDA) and found striking similarities, including DAT dependence, to neurodegeneration observed in mammalian models. In this review, we present our findings in the context of molecular and behavioral dimensions of DA signaling in C. elegans with an eye toward opportunities for uncovering DAT mutants, DAT regulators, and components of toxin-mediated cell death.

Steroids modulate N-methyl-D-aspartate-stimulated [3H] dopamine release from rat striatum via sigma receptors

Steroids have been proposed as endogenous ligands at sigma receptors. In the current study, we examined the ability of steroids to regulate N-methyl-d-aspartate (NMDA)-stimulated [3H]dopamine release from slices of rat striatal tissue. We found that both progesterone and pregnenolone inhibit [3H]dopamine release in a concentration-dependent manner similarly to prototypical agonists, such as (+)-pentazocine. The inhibition seen by both progesterone and pregnenolone exhibits IC50 values consistent with reported Ki values for these steroids obtained in binding studies, and was fully reversed by both the sigma1 antagonist 1-(cyclopropylmethyl)-4-2'-4"flurophenyl)-2'oxoethyl)piperidine HBr (DuP734) and the sigma2 antagonist 1'-[4-[1-(4-fluorophenyl)-1-H-indol-3-yl]-1-butyl]spiro[iso-benzofuran-1(3H), 4'piperidine] (Lu28-179). Lastly, to determine whether a protein kinase C (PKC) signaling system might be involved in the inhibition of NMDA-stimulated [3H]dopamine release, we tested the PKCbeta-selective inhibitor 5,21:12,17-dimetheno-18H-dibenzo[i,o]pyrrolo[3,4 - 1][1,8]diacyclohexadecine-18,20(19H)-dione,8-[(dimethylamino)methyl]-6,7,8,9,10,11-hexahydro-monomethanesulfonate (9Cl) (LY379196) against both progesterone and pregnenolone. We found that LY379196 at 30 nM reversed the inhibition of release by both progesterone and pregnenolone. These findings support steroids as candidates for endogenous ligands at sigma receptors.

Sigma receptors: potential medications development target for anti-cocaine agents

The ability of cocaine to interact with sigma receptors suggests a viable target for medications development. Recently, numerous novel compounds and antisense oligodeoxynucleotides targeting sigma receptors have been synthesized and shown to prevent the behavioral toxicity and psychomotor stimulant effects of cocaine in animals. Protective doses of sigma receptor antagonists have also been shown to prevent changes in gene expression that are induced by cocaine. Together, the studies provide insight and promising future directions for the development of potential medications for the treatment of cocaine addiction and overdose.

Sigma1 receptor agonist-mediated regulation of N-methyl-D-aspartate-stimulated [3H]dopamine release is dependent upon protein kinase C

We have previously shown that sigma1 receptor agonists inhibit N-methyl-D-aspartate (NMDA)-stimulated [3H]dopamine from slices of rat striatum in a concentration-related manner and that the inhibition is reversed by sigma1 receptor-selective and nonsubtype-selective sigma receptor antagonists. Based on previous evidence from our laboratory as well as other laboratories, we hypothesized that sigma1 receptors might use a protein kinase C (PKC) signaling pathway to modulate stimulated dopamine release. We tested several inhibitors of PKC isozymes, as well as a phospholipase C inhibitor for their effects on sigma1 receptor agonist-mediated regulation of [3H]dopamine release. Although none of the inhibitors tested affected the ability of NMDA to stimulate [3H]dopamine release, they all abolished regulation by the sigma1 receptor agonist (+)-pentazocine in a concentration-related manner. We also found that prior exposure to 1 microM phorbol 2-myristate 13-acetate for 30 min abolished regulation by (+)-pentazocine. We concluded that an intact PKC system was required for sigma1 agonist-mediated regulation of NMDA-stimulated [3H]dopamine release from rat striatal slices. Based on the pharmacological profile of the PKC inhibitors tested, as well as reports in the literature on PKC

Birth insult interacts with stress at adulthood to alter dopaminergic function in animal models: possible implications for schizophrenia and other disorders

Altered subcortical dopaminergic activity is thought to be involved in the pathophysiology of several disorders including schizophrenia, substance abuse and attention deficit hyperactivity disorder. Epidemiological studies have implicated perinatal insults, particularly obstetric complications involving fetal or neonatal hypoxia, as etiological risk factors for schizophrenia. This suggests the possibility that perinatal hypoxia might have lasting effects on dopaminergic function. In animal models, dopaminergic systems appears to be particularly vulnerable to a wide range of perinatal insults, resulting in persistent alterations in function of mesolimbic and mesostriatal pathways. This review summarizes recent work characterizing long-term changes in dopaminergic function and biochemistry in models of Caesarean section (C-section) birth and of C-section birth with added global anoxia in the rat and guinea pig. C-section birth and C-section with anoxia appear to be two distinct hypoxic birth insults, with somewhat differing patterns of lasting effects on dopamine systems. In addition, birth insult alters the manner in which dopaminergic function is regulated by stress at adulthood. The possible relevance of these finding to effects of human birth procedures is discussed.