Sigma 1 receptor-related neuroactive steroids modulate cocaine-induced reward

Expanding the therapeutic potential of neuro(active)steroids: a promising strategy for hyperdopaminergic behavioral phenotypes

Imbalances in dopamine activity significantly contribute to the pathophysiology of several neuropsychiatric disorders, including addiction, ADHD, schizophrenia, impulse control disorders, and Parkinson's Disease. Neuro(active)steroids, comprising endogenous steroids that finely modulate neuronal activity, are considered crucial regulators of brain function and behavior, with implications in various physiological processes and pathological conditions. Specifically, subclasses of Neuro(active)steroids belonging to the 5α reductase pathway are prominently involved in brain disorders characterized by dopaminergic signaling imbalances. This review highlights the neuromodulatory effects of Neuro(active)steroids on the dopamine system and related aberrant behavioral phenotypes. We critically appraise the role of pregnenolone, progesterone, and allopregnanolone on dopamine signaling. Additionally, we discuss the impact of pharmacological interventions targeting 5α reductase activity in neuropsychiatric conditions characterized by excessive activation of the dopaminergic system, ranging from psychotic (endo)phenotypes and motor complications to decision-making problems and addiction.

The importance of translationally evaluating steroid hormone contributions to substance use

Clinically, women appear to be more susceptible to certain aspects of substance use disorders (SUDs). The steroid hormones 17β-estradiol (E2) and progesterone (Pg) have been linked to women-specific drug behaviors. Here, we review clinical and preclinical studies investigating how cycling ovarian hormones affect nicotine-, cocaine-, and opioid-related behaviors. We also highlight gaps in the literature regarding how synthetic steroid hormone use may influence drug-related behaviors. In addition, we explore how E2 and Pg are known to interact in brain reward pathways and provide evidence of how these interactions may influence drug-related behaviors. The synthesis of this review demonstrates the critical need to study women-specific factors that may influence aspects of SUDs, which may play important roles in addiction processes in a sex-specific fashion. It is important to understand factors that impact women's health and may be key to moving the field forward toward more efficacious and individualized treatment strategies.

The Sigma Enigma: A Narrative Review of Sigma Receptors

The sigma-1 and sigma-2 receptors were first discovered in the 1960s and were thought to be a form of opioid receptors initially. Over time, more was gradually learned about these receptors, which are actually protein chaperones, and many of their unique or unusual properties can contribute to a range of important new therapeutic applications. These sigma receptors translocate in the body and regulate calcium homeostasis and mitochondrial bioenergetics and they also have neuroprotective effects. The ligands to which these sigma receptors respond are several and dissimilar, including neurosteroids, neuroleptics, and cocaine. There is controversy as to their endogenous ligands. Sigma receptors are also involved in the complex processes of cholesterol homeostasis and protein folding. While previous work on this topic has been limited, research has been conducted in multiple disease states, such as addiction, aging. Alzheimer's disease, cancer, psychiatric disorders, pain and neuropathic pain, Parkinson's disease, and others. There is currently increasing interest in sigma-1 and sigma-2 receptors as they provide potential therapeutic targets for many disease indications.

Sigma-1 Receptor Is Critical for Mitochondrial Activity and Unfolded Protein Response in Larval Zebrafish

The sigma-1 receptor (S1R) is a highly conserved transmembrane protein highly enriched in mitochondria-associated endoplasmic reticulum (ER) membranes, where it interacts with several partners involved in ER-mitochondria Ca2+ transfer, activation of the ER stress pathways, and mitochondria function. We characterized a new S1R deficient zebrafish line and analyzed the impact of S1R deficiency on visual, auditory and locomotor functions. The s1r+25/+25 mutant line showed impairments in visual and locomotor functions compared to s1rWT. The locomotion of the s1r+25/+25 larvae, at 5 days post fertilization, was increased in the light and dark phases of the visual motor response. No deficit was observed in acoustic startle response. A critical role of S1R was shown in ER stress pathways and mitochondrial activity. Using qPCR to analyze the unfolded protein response genes, we observed that loss of S1R led to decreased levels of IRE1 and PERK-related effectors and increased over-expression of most of the effectors after a tunicamycin challenge. Finally, S1R deficiency led to alterations in mitochondria bioenergetics with decreased in basal, ATP-linked and non-mitochondrial respiration and following tunicamycin challenge. In conclusion, this new zebrafish model confirmed the importance of S1R activity on ER-mitochondria communication. It will be a useful tool to further analyze the physiopathological roles of S1R.

Physiopathological Role of Neuroactive Steroids in the Peripheral Nervous System

Peripheral neuropathy (PN) refers to many conditions involving damage to the peripheral nervous system (PNS). Usually, PN causes weakness, numbness and pain and is the result of traumatic injuries, infections, metabolic problems, inherited causes, or exposure to chemicals. Despite the high prevalence of PN, available treatments are still unsatisfactory. Neuroactive steroids (i.e., steroid hormones synthesized by peripheral glands as well as steroids directly synthesized in the nervous system) represent important physiological regulators of PNS functionality. Data obtained so far and here discussed, indeed show that in several experimental models of PN the levels of neuroactive steroids are affected by the pathology and that treatment with these molecules is able to exert protective effects on several PN features, including neuropathic pain. Of note, the observations that neuroactive steroid levels are sexually dimorphic not only in physiological status but also in PN, associated with the finding that PN show sex dimorphic manifestations, may suggest the possibility of a sex specific therapy based on neuroactive steroids.

How ovarian hormones influence the behavioral activation and inhibition system through the dopamine pathway

The behavioral activation system (BAS) and the behavioral inhibition system (BIS) have been proposed to relate to stable traits that predict inter-individual differences in motivation. Prior reports point dopamine (DA) pathways, mainly including ventral tegmental area (VTA) and substantia nigra (SN), implicate in subserving reward-related functions associated with BAS and inhibitory functions related with BIS. However, as an important factor that affects DA releasing, it remains an open question whether the ovarian hormones may also be related to BIS/BAS. Here, to investigate effects of the estradiol (E2) and progesterone (PROG) on BIS/BAS and related DA pathways, we employed a BIS/BAS scale and the resting-state functional magnetic resonance imaging (fMRI) during the late follicular phase (FP) and the mid-luteal phase (LP). On the behavioral level, when women had high PROG levels, their E2 levels were found positively correlated with BIS scores, but those women whose PROG levels were low, their E2 levels were negative correlation with BIS scores. On the neural level, we demonstrated BAS was related with the VTA pathway, included brain reward regions of nucleus accumbens (NAc) and orbitofrontal cortex (OFC). Meanwhile, the BIS was correlated with the SN-dorsolateral prefrontal cortex (dlPFC) pathway. ROI-based resting-state functional connectivity (RSFC) analyses further revealed that, RSFC between the SN and dlPFC was modulated by ovarian hormones. With higher PROG levels, increased E2 levels among women were accompanied by stronger RSFC of the SN-dlPFC, but when PROG levels were low, E2 levels were negatively correlated with the SN-dlPFC RSFC. These findings revealed a combined enhancement effect of E2 and PROG on BIS, and the SN-dlPFC pathway was mainly involved in this process.

Neuroactive steroid levels and cocaine use chronicity in men and women with cocaine use disorder receiving progesterone or placebo

BACKGROUND AND OBJECTIVES

Neuroactive steroids (NAS) may play a role in addiction, with observed increases in response to acute stress and drug use, but decreases with chronic substance use, suggesting that NAS neuroadaptations may occur with chronic substance use. However, levels of NAS in addicted individuals have not been systematically examined. Here, we evaluated a panel of NAS in men and women with cocaine use disorder (CUD) who participated in a clinical laboratory study of progesterone.

METHODS

Forty six CUD individuals were enrolled in a randomized placebo-controlled laboratory study to evaluate progesterone effects on levels of various NAS. On day 5 of a 7-day inpatient treatment regimen of 400 mg/day progesterone (15M/8F) or placebo (14M/9F), plasma levels of NAS known to be downstream of progesterone (allopregnanolone, pregnanolone), and NAS not in the progesterone synthesis pathway (androstanediol, testosterone, dehydroepiandrosterone [DHEA] and the NAS precursor, pregnenolone) were analyzed using highly sensitive gas chromatography/mass spectrometry (GC/MS). The relationship between each of the NAS and chronicity of cocaine use was also assessed.

RESULTS

Progesterone versus placebo significantly increased the GABAergic NAS allopregnanolone and pregnanolone in both CUD men and women. Levels of pregnenolone, testosterone, its GABAergic metabolite androstanediol, and the non-GABAergic DHEA were unaffected by progesterone treatment, and testosterone and androstanediol levels were significantly higher in men than women. Importantly, lower pregnenolone and androstanediol levels were associated with greater years of cocaine use.

SCIENTIFIC SIGNIFICANCE

GABAergic NAS that are upstream from the progesterone synthesis pathway appear susceptible to chronic effects of cocaine use. (Am J Addict 2019;28:16-21).

Synthesis, Receptor Affinity, and Antiallodynic Activity of Spirocyclic σ Receptor Ligands with Exocyclic Amino Moiety

In order to detect novel σ receptor ligands, the rigid spiro[[2]benzopyran-1,1'-cyclohexan]-4'-one was connected with amino moieties derived from σ₂ receptor preferring lead compounds resulting in mixtures of trans- and cis-configured amines 6, 18, and 27. In a four step synthesis the methyl acetals 6 were converted into fluoroethyl derivatives 13 and 30. The most promising σ₂ receptor ligand is the methyl acetal 6a bearing a 2,4-dimethylbenzylamino moiety. The fluoroethyl derivatives 13c and 13d reveal high σ₁ affinity but moderate selectivity over the σ₂ subtype. In mice 13c and 13d showed antiallodynic activity that is stronger than that of the reference σ₁ antagonist BD-1063 (34). Since the antiallodynic activity of 13c could only be partially reversed by the σ₁ agonist PRE-084 (35), it is postulated that a second mechanism contributes to its overall antiallodynic effect. In contrast, the antiallodynic effect of its diastereomer 13d can be totally explained by a σ₁ antagonism.

Dehydroepiandrosterone and Addiction

Drug addiction has a great negative influence on society, both social and economic burden. It was widely thought that addicts could choose to stop using drugs if only they had some self-control and principles. Nowadays, science has changed this view, defining drug addiction as a complex brain disease that affects behavior in many ways, both biological and psychological. Currently there is no ground-breaking reliable treatment for drug addiction. For more than a decade we are researching an alternative approach for intervention with drug craving and relapse to its usage, using DHEA, a well-being and antiaging food supplement. In this chapter we navigate through the significant therapeutic effect of DHEA on the brain circuits that control addiction and on behavioral performance both in animal models and addicts. We suggest that an integrative program of add-on DHEA treatment may further enable to dynamically evaluate the progress of rehabilitation of an individual patient, in a comprehensive assessment. Such a program may boost and support the detoxification and rehabilitation process, and help patients regain a normal life in a shorter amount of time.

Neuroactive Steroids: Receptor Interactions and Responses

Neuroactive steroids (NASs) are naturally occurring steroids, which are synthesized centrally as de novo from cholesterol and are classified as pregnane, androstane, and sulfated neurosteroids (NSs). NASs modulate many processes via interacting with gamma-aminobutyric acid (GABA), N-methyl-d-aspartate, serotonin, voltage-gated calcium channels, voltage-dependent anion channels, α-adrenoreceptors, X-receptors of the liver, transient receptor potential channels, microtubule-associated protein 2, neurotrophin nerve growth factor, and σ1 receptors. Among these, NSs (especially allopregnanolone) have high potency and extensive GABA-A receptors and hence demonstrate anticonvulsant, anesthetic, central cytoprotectant, and baroreflex inhibitory effects. NSs are also involved in mood and learning via serotonin and anti-nociceptive activity via T-type voltage-gated Ca2+ channels. Moreover, they are modulators of mitochondrial function, synaptic plasticity, or regulators of apoptosis, which have a role in neuroprotective via voltage-dependent anion channels receptors. For proper functioning, NASs need to be in their normal level, whereas excess and deficiency may lead to abnormalities. When they are below the normal, NSs could have a part in development of depression, neuro-inflammation, multiple sclerosis, experimental autoimmune encephalitis, epilepsy, and schizophrenia. On the other hand, stress and attention deficit disorder could occur during excessive level. Overall, NASs are very important molecules with major neuropsychiatric activity.

A Role for Sigma Receptors in Stimulant Self-Administration and Addiction

Sigma receptors (σRs) are structurally unique proteins that function intracellularly as chaperones. Historically, σRs have been implicated as modulators of psychomotor stimulant effects and have at times been proposed as potential avenues for modifying stimulant abuse. However, the influence of ligands for σRs on the effects of stimulants, such as cocaine or methamphetamine, in various preclinical procedures related to drug abuse has been varied. The present paper reviews the effects of σR agonists and antagonists in three particularly relevant procedures: stimulant discrimination, place conditioning, and self-administration. The literature to date suggests limited σR involvement in the discriminative-stimulus effects of psychomotor stimulants, either with σR agonists substituting for the stimulant or with σR antagonists blocking stimulant effects. In contrast, studies of place conditioning suggest that administration of σR antagonists or down-regulation of σR protein can block the place conditioning induced by stimulants. Despite place conditioning results, selective σR antagonists are inactive in blocking the self-administration of stimulants. However, compounds binding to the dopamine transporter and blocking σRs can selectively decrease stimulant self-administration. Further, after self-administration of stimulants, σR agonists are self-administered, an effect not seen in subjects without that specific history. These findings suggest that stimulants induce unique changes in σR activity, and once established, the changes induced create redundant, and dopamine independent reinforcement pathways. Concomitant targeting of both dopaminergic pathways and σR proteins produces a selective antagonism of those pathways, suggesting new avenues for combination chemotherapies to specifically combat stimulant abuse.

Sigma Receptors and Substance Use Disorders

Thanks to advances in neuroscience, addiction is now recognized as a chronic brain disease with genetic, developmental, and cultural components. Drugs of abuse, including alcohol, are able to produce significant neuroplastic changes responsible for the profound disturbances shown by drug addicted individuals. The current lack of efficacious pharmacological treatments for substance use disorders has encouraged the search for novel and more effective pharmacotherapies. Growing evidence strongly suggests that Sigma Receptors are involved in the addictive and neurotoxic properties of abused drugs, including cocaine , methamphetamine , and alcohol. The present chapter will review the current scientific knowledge on the role of the Sigma Receptor system in the effects of drugs and alcohol, and proposes that this receptor system may represent a novel therapeutic target for the treatment of substance use disorders and associated neurotoxicity.

Molecular Probes for Imaging the Sigma-2 Receptor: In Vitro and In Vivo Imaging Studies

The sigma-2 (σ₂) receptor has been validated as a biomarker of the proliferative status of solid tumors. Therefore, radiotracers having a high affinity and high selectivity for σ₂ receptors have the potential to assess the proliferative status of human tumors using noninvasive imaging techniques such as Positron Emission Tomography (PET). Since the σ₂ receptor has not been cloned, the current knowledge of this receptor has relied on receptor binding studies with the radiolabeled probes and investigation of the effects of the σ₂ receptor ligands on tumor cells. The development of the σ₂ selective fluorescent probes has proven to be useful for studying subcellular localization and biological functions of the σ₂ receptor, for revealing pharmacological properties of the σ₂ receptor ligands, and for imaging cell proliferation. Preliminary clinical imaging studies with [¹⁸F]ISO-1, a σ₂ receptor probe, have shown promising results in cancer patients. However, the full utility of imaging the σ₂ receptor status of solid tumors in the diagnosis and prediction of cancer therapeutic response will rely on elucidation of the functional role of this protein in normal and tumor cell biology.

A role for sigma receptors in stimulant self-administration and addiction

Sigma-1 receptors (σ1Rs) are structurally unique intracellular proteins that function as chaperones. σ1Rs translocate from the mitochondria-associated membrane to other subcellular compartments, and can influence a host of targets, including ion channels, G-protein-coupled receptors, lipids, and other signaling proteins. Drugs binding to σRs can induce or block the actions of σRs. Studies indicate that stimulant self-administration induces the reinforcing effects of σR agonists, because of dopamine transporter actions. Once established, the reinforcing effects of σR agonists are independent of dopaminergic mechanisms traditionally thought to be critical to the reinforcing effects of stimulants. Self-administered doses of σR agonists do not increase dopamine concentrations in the nucleus accumbens shell, a transmitter and brain region considered important for the reinforcing effects of abused drugs. However, self-administration of σR agonists is blocked by σR antagonists. Several effects of stimulants have been blocked by σR antagonists, including the reinforcing effects, assessed by a place-conditioning procedure. However, the self-administration of stimulants is largely unaffected by σR antagonists, indicating fundamental differences in the mechanisms underlying these two procedures used to assess the reinforcing effects. When σR antagonists are administered in combination with dopamine uptake inhibitors, an effective and specific blockade of stimulant self-administration is obtained. Actions of stimulant drugs related to their abuse induce unique changes in σR activity and the changes induced potentially create redundant and, once established, independent reinforcement pathways. Concomitant targeting of both dopaminergic pathways and σR proteins produces a selective antagonism of stimulant self-administration, suggesting new avenues for combination chemotherapies to specifically combat stimulant abuse.

The differential effects of alprazolam and oxazepam on methamphetamine self-administration in rats

BACKGROUND

Methamphetamine is the second most commonly used illicit drug in the world, and despite recent attempts by the Drug Enforcement Administration to combat this epidemic, methamphetamine use is still on the rise. As methamphetamine use increases so does polydrug use, particularly that involving methamphetamine and benzodiazepines. The present study was designed to examine the effects of two benzodiazepines on methamphetamine self-administration.

METHODS

Five doses of methamphetamine (0.0075, 0.015, 0.03, 0.09, and 0.12mg/kg/infusion) were tested, producing an inverted U-shaped dose-response curve. Rats were then pretreated with oxazepam, alprazolam, or vehicle prior to methamphetamine self-administration. To determine if the effects of these drugs were due to the GABAA receptor and/or translocator protein (TSPO), we also pretreated rats with an antagonist for the benzodiazepine-binding site on the GABAA receptor (i.e., flumazenil) and a TSPO antagonist (i.e., PK11195) prior to alprazolam or oxazepam administration.

RESULTS

Oxazepam significantly reduced methamphetamine self-administration as demonstrated by a downward shift of the dose-response curve. In contrast, alprazolam significantly enhanced methamphetamine self-administration as evidenced by a leftward shift of the dose-response curve. Flumazenil completely blocked the effects of alprazolam on methamphetamine self-administration. When administered individually, both flumazenil and PK11195 partially reversed the effects of oxazepam on methamphetamine self-administration. However, when these two antagonists were combined, the effects of oxazepam were completely reversed.

CONCLUSIONS

The GABAA receptor is responsible for the alprazolam-induced enhancement of methamphetamine self-administration, while the activation of both the GABAA receptor and TSPO are responsible for the oxazepam-induced reduction of methamphetamine self-administration.

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 neurosteroid dehydroepiandrosterone sulfate, but not androsterone, enhances the antidepressant effect of cocaine examined in the forced swim test--Possible role of serotonergic neurotransmission

One of the mechanisms of cocaine's actions in the central nervous system is its antidepressant action. This effect might be responsible for increased usage of the drug by individuals with mood disorders. Higher endogenous levels of the excitatory neurosteroid dehydroepiandrosterone sulfate (DHEAS) were reported to correlate with successful abstinence from cocaine use in addicts, but a clinical trial showed that supplementation with a high dose of DHEA increased cocaine usage instead. Such ambiguous effects of DHEA(S) could potentially be linked to its influence on the antidepressant effect of cocaine. In this study we tested DHEAS and its metabolite, androsterone, for interactions with cocaine in animal model of depression (forced swim test) and examined the effects of both steroids and cocaine on serotoninergic neurotransmission. All substances were also tested for influence on locomotor activity. A cocaine dose of 5mg/kg, which had no significant effect on locomotor activity, was chosen for the forced swim test. Neither DHEAS nor androsterone showed any antidepressant action in this test, while cocaine manifested a clear antidepressant effect. Androsterone slightly reduced the antidepressant influence of cocaine while DHEAS markedly, dose-dependently enhanced it. Such an effect might be caused by the influence of DHEAS on serotonin neurotransmission, as this steroid decreased serotonin concentration and turnover in the striatum. When DHEAS and cocaine were administered together, the levels of serotonin in the striatum and hippocampus remained unchanged. This phenomenon may explain the additive antidepressant action of DHEAS and cocaine and why co-administration of DHEAS and cocaine increases drug use.

The sigma-1 receptor agonist 4-phenyl-1-(4-phenylbutyl) piperidine (PPBP) protects against newborn excitotoxic brain injury by stabilizing the mitochondrial membrane potential in vitro and inhibiting microglial activation in vivo

Premature birth represents a clinical situation of risk for brain injury. The diversity of pathophysiological processes complicates efforts to find effective therapeutic strategies. Excitotoxicity is one important factor in the pathogenesis of preterm brain injury. The observation that sigma-1 receptor agonists possess neuroprotective potential, at least partly mediated by a variety of anti-excitotoxic mechanisms, has generated great interest in targeting those receptors to counteract brain injury. The objective of this study was to evaluate the effect of the highly specific sigma-1 receptor agonist, 4-phenyl-1-(4-phenylbutyl) piperidine (PPBP) to protect against excitotoxic developmental brain injury in vivo and in vitro. Primary hippocampal neurons were pre-treated with PPBP before glutamate was applied and subsequently analyzed for cell death (PI/calcein AM), mitochondrial activity (TMRM) and morphology of the neuronal network (WGA) using confocal microscopy. Using an established neonatal mouse model we also determined whether systemic injection of PPBP significantly attenuates excitotoxic brain injury. PPBP significantly reduced neuronal cell death in primary hippocampal neurons exposed to glutamate. Neurons treated with PPBP showed a less pronounced loss of mitochondrial membrane potential and fewer morphological changes after glutamate exposure. A single intraperitoneal injection of PPBP given one hour after the excitotoxic insult significantly reduced microglial cell activation and lesion size in cortical gray and white matter. The present study provides strong support for the consideration of sigma-1 receptor agonists as a candidate therapy for the reduction of neonatal excitotoxic brain lesions and might offer a novel target to counteract developmental brain injury.

Future pharmacological treatments for substance use disorders

Substance use disorders represent a serious public health and social issue worldwide. Recent advances in our understanding of the neurobiological basis of the addictive processes have led to the development of a growing number of pharmacological agents to treat addictions. Despite this progress, there are no approved pharmacological treatments for cocaine, methamphetamine and cannabis addiction. Moving treatment development to the next stage will require novel ways of approaching substance use disorders. One such novel approach is to target individual vulnerabilities, such as cognitive function, sex differences and psychiatric comorbidities. This review provides a summary of promising pharmacotherapies for alcohol, opiate, stimulant and nicotine addictions. Many medications that target positive and negative reinforcement of drugs, as well as individual vulnerabilities to addiction, are in different phases of development. Clinical trials testing the efficacy of these medications for substance use disorder are warranted.

Effects of the neuroactive steroid allopregnanolone on intracranial self-stimulation in C57BL/6J mice

RATIONALE

The neuroactive steroid (3α,5α)-3-hydroxy-pregnan-20-one (3α,5α-THP, allopregnanolone) has effects on reward-related behaviors in mice and rats that suggest that it may activate brain reward circuits. Intracranial self-stimulation (ICSS) is an operant behavioral technique that detects changes in the sensitivity of brain reward circuitry following drug administration.

OBJECTIVE

To examine the effects of the neuroactive steroid allopregnanolone on ICSS and to compare these effects to those of cocaine.

METHODS

Male C57BL/6J mice implanted with stimulating electrodes implanted into the medial forebrain bundle responded for reinforcement by electrical stimulation (brain stimulation reward (BSR)). Mice received cocaine (n = 11, 3.0-30.0 mg/kg, intraperitoneal (i.p.)) or the neuroactive steroid allopregnanolone (n = 11, 3.0-17.0 mg/kg, i.p.). BSR thresholds (θ 0) and maximum (MAX) operant response rates after drug treatments were compared to those after vehicle injections.

RESULTS

Cocaine and allopregnanolone dose dependently lowered BSR thresholds relative to vehicle injections. Cocaine was maximally effective (80 % reduction) in the second 15 min following the 30 mg/kg dose, while allopregnanolone was maximally effective (30 % reduction) 15-45 min after the 17 mg/kg dose. Neither drug had significant effects on MAX response rates.

CONCLUSIONS

The effects of allopregnanolone on BSR thresholds are consistent with the previously reported effects of benzodiazepines and alcohol, suggesting that positive modulation of GABAA receptors can facilitate reward-related behaviors in C57BL/6J mice.

Effects of combined exercise and progesterone treatments on cocaine seeking in male and female rats

BACKGROUND

Individually, both treatment with progesterone and concurrent access to an exercise wheel reduce cocaine self-administration under long-access conditions and suppress cocaine-primed reinstatement in female rats. In the present study, wheel running and progesterone (alone and combined) were assessed for their effects on reinstatement of cocaine-seeking primed by yohimbine, cocaine, and cocaine-paired cues.

METHODS

Male and female rats were implanted with an intravenous catheter and allowed to self-administer cocaine (0.4 mg/kg/inf, iv) during 6-h sessions for 10 days. Subsequently, the groups of male and female rats were each divided into two groups that were given concurrent access to either a locked or unlocked running wheel under extinction conditions for 14 days. Next, all four groups were tested in a within-subjects design for reinstatement of cocaine-seeking precipitated by separate administration of cocaine-paired stimuli, yohimbine, or cocaine or the combination of yohimbine + cocaine-paired stimuli or cocaine + cocaine-paired stimuli. These priming conditions were tested in the presence of concurrent wheel access (W), pretreatment with progesterone (P), or both (W + P).

RESULTS

In agreement with previous results, females responded more for cocaine than males during maintenance. Additionally, concurrent wheel running attenuated extinction responses and cocaine-primed reinstatement in females but not in males. Across all priming conditions, W + P reduced reinstatement compared to control conditions, and for cocaine-primed reinstatement in male rats, the combined W + P treatment was more effective than W or P alone.

CONCLUSION

Under certain conditions, combined behavioral (exercise) and pharmacological (progesterone) interventions were more successful at reducing cocaine-seeking behavior than either intervention alone.

Effects of inhibitory GABA-active neurosteroids on cocaine seeking and cocaine taking in rats

RATIONALE

Several compounds that potentiate GABA-induced inhibitory currents also decrease stress, anxiety and addiction-related behaviors. Because of the well-established connection between stress and addiction, compounds that reduce stress-induced responses might be efficacious in treating addiction. Since endogenous neurosteroids such as allopregnanolone may function in a manner similar to benzodiazepines to reduce HPA axis activation and anxiety following stressful stimuli, we hypothesized that exogenously applied neurosteroids would reduce cocaine reinforcement in two animal models.

METHODS

Male Wistar rats were trained to self-administer cocaine and food under a concurrent alternating operant schedule of reinforcement. Two separate groups of rats were trained to self-administer cocaine or food pellets and were then exposed to similar cue-induced reinstatement paradigms. Both groups of rats were pretreated with various doses of neurosteroids.

RESULTS

Allopregnanolone and 3α-hydroxy-3β-methyl-17β-nitro-5α-androstane (R6305-7, a synthetic neurosteroid) were ineffective in selectively decreasing cocaine relative to food self-administration. On the other hand, both allopregnanolone and R6305-7 significantly decreased the cue-induced reinstatement of extinguished cocaine seeking, confirmed by one-way ANOVA.

CONCLUSIONS

These results suggest that neurosteroids may be effective in reducing the relapse to cocaine use without affecting ongoing cocaine self-administration.

Role of GABA-active neurosteroids in the efficacy of metyrapone against cocaine addiction

Previous research has demonstrated a complicated role for stress and HPA axis activation in potentiating various cocaine-related behaviors in preclinical models of drug dependence. However, the investigation of several antiglucocorticoid therapies has yielded equivocal results in reducing cocaine-related behaviors, possibly because of varying mechanisms of actions. Specifically, research suggests that metyrapone (a corticosterone synthesis inhibitor) may reduce cocaine self-administration in rats via a nongenomic, extra-adrenal mechanism without altering plasma corticosterone. In the current experiments, male rats were trained to self-administer cocaine infusions and food pellets in a multiple, alternating schedule of reinforcement. Metyrapone pretreatment dose-dependently decreased cocaine self-administration as demonstrated previously. Pharmacological inhibition of neurosteroid production by finasteride had significant effects on cocaine self-administration, regardless of metyrapone pretreatment. However, metyrapone's effects on cocaine self-administration were significantly attenuated with bicuculline pretreatment, suggesting a role for GABA-active neurosteroids in cocaine-reinforced behaviors. In vitro binding data also confirmed that metyrapone does not selectively bind to GABA-related proteins. The results of these experiments support the hypothesis that metyrapone may increase neurosteroidogenesis to produce effects on cocaine-related behaviors.

Sigma (σ) receptors as potential therapeutic targets to mitigate psychostimulant effects

Many psychostimulants, including cocaine and methamphetamine, interact with sigma (σ) receptors at physiologically relevant concentrations. The potential therapeutic relevance of this interaction is underscored by the ability to selectively target σ receptors to mitigate many behavioral and physiological effects of psychostimulants in animal and cell-based model systems. This chapter begins with an overview of these enigmatic proteins. Provocative preclinical data showing that σ ligands modulate an array of cocaine and methamphetamine effects are summarized, along with emerging areas of research. Together, the literature suggests targeting of σ receptors as an innovative option for combating undesired actions of psychostimulants through both neuronal and glial mechanisms.

Effects of BD1047, a σ1 receptor antagonist, on the expression of mTOR, Camk2γ and GSK-3β in fluvoxamine-treated N2a cells

Fluvoxamine, a common antidepressant agent, is designed to exert its pharmacological effect by inhibiting synaptic serotonin reuptake. However, increasing evidence has demonstrated that σ₁ receptors are likely to be involved in the mechanism of action of fluvoxamine. The present study aimed to observe the effects of fluvoxamine on the expression levels of mammalian target of rapamycin (mTOR), Ca²⁺/calmodulin-dependent protein kinase 2γ (Camk2γ) and glycogen synthase kinase-3β (GSK-3β) in fluvoxamine-treated N2a cells and attempted to elucidate whether σ₁ receptors mediate the pharmacological effects of fluvoxamine. The N2a cells were randomly divided into three groups (each n=6): DMEM group (D group), 0.5 μmol/l fluvoxamine group (F group) and 0.2 μmol/l BD1047 (a σ₁ receptor antagonist) + 0.5 μmol/l fluvoxamine group (BF group). Western blotting was used to determine the expression levels of mTOR, Camk2γ and GSK-3β in the cultured N2a cells after two days of incubation. The F group exhibited significant increases in the expression levels of mTOR and Camk2γ and a significant reduction in the expression levels of GSK-3β compared with those in the D group (P<0.01). By contrast, the BF group demonstrated significant reductions in the expression levels of mTOR and Camk2γ and a significant increase in the expression levels of GSK-3β, compared with those in the F group (P<0.01). These results suggest that σ₁ receptors mediate fluvoxamine-elicited changes in the expression levels of mTOR, Camk2γ and GSK-3β in N2a cells, which indicates that σ₁ receptors are likely to be involved in the pharmacological effects of fluvoxamine.

The σ2 receptor: a novel protein for the imaging and treatment of cancer

The σ2 receptor is an important target for the development of molecular probes in oncology because of its 10-fold higher density in proliferating tumor cells compared with that in quiescent tumor cells and because of the observation that σ2 receptor agonists are able to kill tumor cells via apoptotic and nonapoptotic mechanisms. Although recent evidence indicates that the σ2 receptor binding site is localized within the progesterone receptor membrane component 1 (PGRMC1), most information regarding this protein has been obtained using either radiolabeled or fluorescent receptor-based probes and from biochemical analysis of the effect of σ2 selective ligands on cells grown in culture. This article reviews the development of σ2 receptor ligands and presents an overview of how they have been used in vitro and in vivo to increase our understanding of the role of the σ2 receptor in cancer and proliferation.

Hyperammonemia alters the modulation by different neurosteroids of the glutamate-nitric oxide-cyclic GMP pathway through NMDA- GABAA - or sigma receptors in cerebellum in vivo

Several neurosteroids modulate the glutamate-nitric oxide (NO)-cGMP pathway in cerebellum through modulation of NMDA- GABAA - or sigma receptors. Hyperammonemia alters the concentration of several neurosteroids and impairs the glutamate-NO-cGMP pathway, leading to impaired learning ability. This work aimed to assess whether chronic hyperammonemia alters the modulation by different neurosteroids of GABAA, NMDA, and/or sigma receptors and of the glutamate-NO-cGMP pathway in cerebellum. Neurosteroids were administered through microdialysis probes, and extracellular cGMP and citrulline were measured. Then NMDA was administered to assess the effects on the glutamate-NO-cGMP pathway activation. Hyperammonemia completely modifies the effects of pregnanolone and pregnenolone. Pregnanolone acts as a GABAA receptor agonist in controls, but as an NMDA receptor antagonist in hyperammonemic rats. Pregnenolone does not induce any effect in controls, but acts as a sigma receptor agonist in hyperammonemic rats. Hyperammonemia potentiates the actions of tetrahydrodeoxy-corticosterone (THDOC) as a GABAA receptor agonist, allopregnanolone as an NMDA receptor antagonist, and pregnenolone sulfate as an NMDA receptor activation enhancer. Neurosteroids that reduce the pathway (pregnanolone, THDOC, allopregnanolone, DHEAS) may contribute to cognitive impairment in hyperammonemia and hepatic encephalopathy. Pregnenolone would impair cognitive function in hyperammonemia. Neurosteroids that restore the pathway in hyperammonemia (pregnenolone sulfate) could restore cognitive function in hyperammonemia and encephalopathy.

Repeated in vivo exposure of cocaine induces long-lasting synaptic plasticity in hypocretin/orexin-producing neurons in the lateral hypothalamus in mice

Hypocretin (orexin), a neuropeptide synthesized exclusively in the perifornical/lateral hypothalamus, is critical for drug seeking and relapse, but it is not clear how the circuitry centred on hypocretin-producing neurons (hypocretin neurons) is modified by drugs of abuse and how changes in this circuit might alter behaviours related to drug addiction. In this study, we show that repeated, but not single, in vivo cocaine administration leads to a long-lasting, experience-dependent potentiation of glutamatergic synapses on hypocretin neurons in mice following a cocaine-conditioned place preference (CPP) protocol. The synaptic potentiation occurs postsynaptically and probably involves up-regulation of AMPA-type glutamate receptors on hypocretin neurons. Phosphorylation of cAMP response element-binding protein (CREB) is also significantly increased in hypocretin neurons in cocaine-treated animals, suggesting that CREB-mediated pathways may contribute to synaptic potentiation in these cells. Furthermore, the potentiation of synaptic efficacy in hypocretin neurons persists during cocaine withdrawal, but reverses to baseline levels after prolonged abstinence. Finally, the induction of long-term potentiation (LTP) triggered by a high-frequency stimulation is facilitated in hypocretin neurons in cocaine-treated mice, suggesting that long-lasting changes in synapses onto hypocretin neurons would probably be further potentiated by other stimuli (such as concurrent environmental cues) paired with the drug. In summary, we show here that hypocretin neurons undergo experience-dependent synaptic potentiation that is distinct from that reported in other reward systems, such as the ventral tegmental area, following exposure to cocaine. These findings support the idea that the hypocretin system is important for behavioural changes associated with cocaine administration in animals and humans.

The sigma-1 receptor: roles in neuronal plasticity and disease

Sigma-1 receptors (Sig-1Rs) have been implicated in many neurological and psychiatric conditions. Sig-1Rs are intracellular chaperones that reside specifically at the endoplasmic reticulum (ER)-mitochondrion interface, referred to as the mitochondrion-associated ER membrane (MAM). Here, Sig-1Rs regulate ER-mitochondrion Ca(2+) signaling. In this review, we discuss the current understanding of Sig-1R functions. Based on this, we suggest that the key cellular mechanisms linking Sig-1Rs to neurological disorders involve the translocation of Sig-1Rs from the MAM to other parts of the cell, whereby Sig-1Rs bind and modulate the activities of various ion channels, receptors, or kinases. Thus, Sig-1Rs and their associated ligands may represent new avenues for treating aspects of neurological and psychiatric diseases.

Targeting sigma receptors: novel medication development for drug abuse and addiction

Psychostimulant abuse is a serious health and societal problem in industrialized and developing countries. However, the identification of an effective pharmacotherapy to treat it has remained elusive. It has long been known that many psychostimulant drugs, including cocaine and methamphetamine, interact with sigma receptors in the brain and heart, offering a logical target for medication development efforts. However, selective pharmacological agents and molecular biological tools have only recently become available to rigorously evaluate these receptors as viable medication development targets. The current review will summarize provocative preclinical data, demonstrating the ability of sigma receptor antagonists and antisense oligonucleotides to ameliorate cocaine-induced convulsions, lethality, locomotor activity and sensitization, and conditioned place-preference in rodents. Recent studies suggest that the protective effects of sigma receptor antagonists also extend to actions produced by methamphetamine, 3,4-methylenedioxymethamphetamine, ethanol and other abused substances. Together, the data indicate that targeting sigma receptors, particularly the σ(1)-subtype, may offer an innovative approach for combating the effects of cocaine, and perhaps other abused substances.

How reward and emotional stimuli induce different reactions across the menstrual cycle

Despite widespread belief that moods are affected by the menstrual cycle, researchers on emotion and reward have not paid much attention to the menstrual cycle until recently. However, recent research has revealed different reactions to emotional stimuli and to rewarding stimuli across the different phases of the menstrual cycle. The current paper reviews the emerging literature on how ovarian hormone fluctuation during the menstrual cycle modulates reactions to emotional stimuli and to reward. Behavioral and neuroimaging studies in humans suggest that estrogen and progesterone have opposing influences. That is, it appears that estrogen enhances reactions to reward, but progesterone counters the facilitative effects of estrogen and decreases reactions to rewards. In contrast, reactions to emotionally arousing stimuli (particularly negative stimuli) appear to be decreased by estrogen but enhanced by progesterone. Potential factors that can modulate the effects of the ovarian hormones (e.g., an inverse quadratic function of hormones' effects; the structural changes of the hippocampus across the menstrual cycle) are also discussed.

Insights into the Sigma-1 receptor chaperone's cellular functions: a microarray report

We previously demonstrated that Sig-1Rs are critical regulators in neuronal morphogenesis and development via the regulation of oxidative stress and mitochondrial functions. In the present study, we sought to identify pathways and genes that are affected by Sig-1R. Gene expression profiles were examined in rat hippocampal neurons that had been cultured for 18 days in vitro (DIV). The cells were transduced with AAV siRNA targeting Sig-1R on DIV 10 for 7 days, followed by gene expression analysis using a rat genome cDNA array. The gene array results indicated that Sig-1R knockdown hampered cellular functions including steroid biogenesis, protein ubiquitination, actin cytoskeleton network, and Nrf-2 mediated oxidative stress. Many of the cellular components important for actin polymerization and synapse plasticity, including F-actin capping protein and neurofilaments, were significantly changed in AAV-siSig-1R neurons. Further, cytochrome c was reduced in AAV-Sig-1R neurons whereas free-radical generating enzymes including cytochrome p450 and cytochrome b-245 were increased. The microarray results also suggest that Sig-1Rs may regulate genes that are involved in the pathogenesis of many CNS diseases including Alzheimer's disease and Parkinson's disease. These data further confirmed that Sig-1Rs play critical roles in the CNS and thus these findings may aid in future development of therapeutic treatments targeting neurodegenerative disorders.

CM156, a sigma receptor ligand, reverses cocaine-induced place conditioning and transcriptional responses in the brain

Repeated exposure to cocaine induces neuroadaptations which contribute to the rewarding properties of cocaine. Using cocaine-induced conditioned place preference (CPP) as an animal model of reward, earlier studies have shown that sigma (σ) receptor ligands can attenuate the acquisition, expression and reactivation of CPP. However, the underlying molecular mechanisms that are associated with these changes are not yet understood. In the present study, CM156, a novel antagonist with high selectivity and affinity for σ receptors was used to attenuate the expression of cocaine-induced CPP in mice. Immediately following the behavioral evaluations, mouse brain tissues were collected and alterations in gene expression in half brain samples were profiled by cDNA microarray analysis. Microarray data was analyzed by three distinct normalization methods and four genes were consistently found to be upregulated by cocaine when compared to saline controls. Each of these gene changes were found by more than one normalization method to be reversed by at least one dose of CM156. Quantitative real time PCR confirmed that a single administration of CM156 was able to reverse the cocaine-induced increases in three of these four genes: metastasis associated lung adenocarcinoma transcript 1 (malat1), tyrosine 3-monooxygenase/tryptophan 5-monooxygenase activation protein (ywhaz), and transthyretin (ttr). These genes are involved in processes related to neuroplasticity and RNA editing. The data presented herein provides evidence that pharmacological intervention with a putative σ receptor antagonist reverses alterations in gene expression that are associated with cocaine-induced reward.

Effects of the selective sigma receptor ligand, 1-(2-phenethyl)piperidine oxalate (AC927), on the behavioral and toxic effects of cocaine

BACKGROUND

Sigma receptors represent a unique structural class of proteins and they have become increasingly studied as viable medication development targets for neurological and psychiatric disorders, including drug abuse. Earlier studies have shown that cocaine and many other abused substances interact with sigma receptors and that antagonism of these proteins can mitigate their actions.

METHODS

In the present study, AC927 (1-(2-phenethyl)piperidine oxalate), a selective sigma receptor ligand, was tested against the behavioral and toxic effects of cocaine in laboratory animals.

RESULTS

Acute administration of AC927 in male, Swiss Webster mice significantly attenuated cocaine-induced convulsions, lethality, and locomotor activity, at doses that alone had no significant effects on behavior. Subchronic administration of AC927 also attenuated cocaine-induced conditioned place preference in mice, at doses that alone had no effects on place conditioning. In drug discrimination studies in male, Sprague-Dawley rats, AC927 partially substituted for the discriminative stimulus effects of cocaine. When it was administered with cocaine, AC927 shifted the cocaine dose-response curve to the left, suggesting an enhancement of the discriminative stimulus effects of cocaine. In non-human primates, AC927 was self-administered, maintaining responding that was intermediate between contingent saline and a maintenance dose of cocaine.

CONCLUSION

The ability of AC927 to elicit some cocaine-like appetitive properties and to also reduce many cocaine-induced behaviors suggests that it is a promising lead for the development of a medication to treat cocaine abuse.

A Role for Sigma Receptors in Stimulant Self Administration and Addiction

Sigma₁ receptors (σ₁Rs) represent a structurally unique class of intracellular proteins that function as chaperones. σ₁Rs translocate from the mitochondria-associated membrane to the cell nucleus or cell membrane, and through protein-protein interactions influence several targets, including ion channels, G-protein-coupled receptors, lipids, and other signaling proteins. Several studies have demonstrated that σR antagonists block stimulant-induced behavioral effects, including ambulatory activity, sensitization, and acute toxicities. Curiously, the effects of stimulants have been blocked by σR antagonists tested under place-conditioning but not self-administration procedures, indicating fundamental differences in the mechanisms underlying these two effects. The self administration of σR agonists has been found in subjects previously trained to self administer cocaine. The reinforcing effects of the σR agonists were blocked by σR antagonists. Additionally, σR agonists were found to increase dopamine concentrations in the nucleus accumbens shell, a brain region considered important for the reinforcing effects of abused drugs. Although the effects of the σR agonist, DTG, on dopamine were obtained at doses that approximated those that maintained self administration behavior those of another agonist, PRE-084 required higher doses. The effects of DTG were antagonized by non-selective or a preferential σ₂R antagonist but not by a preferential σ₁R antagonist. The effects of PRE-084 on dopamine were insensitive to σR antagonists. The data suggest that the self administration of σR agonists is independent of dopamine and the findings are discussed in light of a hypothesis that cocaine has both intracellular actions mediated by σRs, as well as extracellular actions mediated through conventionally studied mechanisms. The co-activation and potential interactions among these mechanisms, in particular those involving the intracellular chaperone σRs, may lead to the pernicious addictive effects of stimulant drugs.

Progesterone improves cognitive performance and attenuates smoking urges in abstinent smokers

BACKGROUND

Progesterone, a steroid hormone, has been implicated in many CNS functions including reward, cognition, and neuroprotection. The goal of this study was to examine the dose-dependent effects of progesterone on cognitive performance, smoking urges, and smoking behavior in smokers.

METHODS

Thirty female and thirty-four male smokers participated in a double-blind, placebo-controlled study. Female smokers were in the early follicular phase of their menstrual cycle during study participation. Smokers were randomly assigned to either 200 or 400mg/day of progesterone or placebo, given in two separate doses, during clinic visit. The first 3 days of the treatment period, smokers abstained from smoking, which was verified with breath CO levels. Smokers attended an experimental session on day 4 where the number of cigarettes smoked were recorded starting 2h after the medication treatment.

RESULTS

Progesterone treatment, 200mg/day, significantly improved cognitive performance in the Stroop and the Digit Symbol Substitution Test. Progesterone at 400mg/day was associated with reduced urges for smoking but did not change ad lib smoking behavior.

CONCLUSIONS

These findings suggest a potential therapeutic value of progesterone for smoking cessation.

Sex differences and ovarian hormones in animal models of drug dependence

Increasing evidence indicates the presence of sex differences in many aspects of drug abuse. Most studies reveal that females exceed males during the initiation, escalation, extinction, and reinstatement (relapse) of drug-seeking behavior, but males are more sensitive than females to the aversive effects of drugs such as drug withdrawal. Findings from human and animal research indicate that circulating levels of ovarian steroid hormones account for these sex differences. Estrogen (E) facilitates drug-seeking behavior, while progesterone (P) and its metabolite, allopregnanalone (ALLO), counteract the effects of E and reduce drug seeking. Estrogen and P influence other behaviors that are affiliated with drug abuse such as drug-induced locomotor sensitization and conditioned place preference. The enhanced vulnerability to drug seeking in females vs. males is also additive with the other risk factors for drug abuse (e.g., adolescence, sweet preference, novelty reactivity, and impulsivity). Finally, treatment studies using behavioral or pharmacological interventions, including P and ALLO, also indicate that females show greater treatment effectiveness during several phases of the addiction process. The neurobiological basis of sex differences in drug abuse appears to be genetic and involves the influence of ovarian hormones and their metabolites, the hypothalamic pituitary adrenal (HPA) axis, dopamine (DA), and gamma-hydroxy-butyric acid (GABA). Overall, sex and hormonal status along with other biological risk factors account for a continuum of addiction-prone and -resistant animal models that are valuable for studying drug abuse prevention and treatment strategies.

Progesterone attenuates cocaine-induced responses

In this review, we summarize literature focused on how progesterone alters cocaine-induced psychomotor, reinforcement, and physiological responses. Clinical studies suggest that progesterone attenuates the subjective effects of cocaine. Similarly, preclinical studies have demonstrated that cocaine-induced reward and psychomotor responses are attenuated after progesterone administration. In rats progesterone also reduces the reinforcement effects of cocaine attenuates acquisition, escalation, reinstatement of cocaine self-administration, and cocaine-seeking behaviors. Progesterone also counteracts the facilitatory effects of estrogen on cocaine self-administration and psychomotor activation. These findings suggest that progesterone has a potential in clinical applications as a treatment for cocaine addiction. Constantly changing progesterone serum levels in female humans and rats affect the female's reinforcement responses to cocaine and may in part contribute to the known sex differences in cocaine responses.

Sigma-1 ligands: Tic-hydantoin as a key pharmacophore

Sigma-1 receptors are involved in numerous pathological dysfunctions and the synthesis of selective ligands is of interest. We identified a fused tetrahydroisoquinoline-hydantoin (Tic-hydantoin) structure with high affinity and selectivity for these receptors. We report here our efforts towards the pharmacomodulation of this substructure, the synthesis of 9 analogs with stereochemistry inversion, opening of isoquinoline ring, removal of isoquinoline nitrogen, replacement of isoquinoline by pyridine, of Tic-hydantoin moiety by quinazolinedione heterocycle. All these analogs provided a loss in the affinity for the sigma-1 receptor. The present work underlines the real importance of the Tic-hydantoin moiety for the obtainment of high affinity ligands.

The pharmacology of sigma-1 receptors

Originally considered an enigmatic protein, the sigma-1 receptor has recently been identified as a unique ligand-regulated molecular chaperone in the endoplasmic reticulum of cells. This discovery causes us to look back at the many proposed roles of this receptor, even before its molecular function was identified, in many diseases such as methamphetamine or cocaine addiction, amnesia, pain, depression, Alzheimer's disease, stroke, retinal neuroprotection, HIV infection, and cancer. In this review, we examine the reports that have clearly shown an agonist-antagonist relationship regarding sigma-1 receptors in models of those diseases and also review the relatively known mechanisms of action of sigma-1 receptors in an attempt to spur the speculation of readers on how the sigma-1 receptor at the endoplasmic reticulum might relate to so many diseases. We found that the most prominent action of sigma-1 receptors in biological systems including cell lines, primary cultures, and animals is the regulation and modulation of voltage-regulated and ligand-gated ion channels, including Ca(2+)-, K(+)-, Na(+), Cl(-), and SK channels, and NMDA and IP3 receptors. We found that the final output of the action of sigma-1 receptor agonists is to inhibit all above-mentioned voltage-gated ion channels, while they potentiate ligand-gated channels. The inhibition or potentiation induced by agonists is blocked by sigma-1 receptor antagonists. Other mechanisms of action of sigma-1 receptors, and to some extent those of sigma-2 receptors, were also considered. We conclude that the sigma-1 and sigma-2 receptors represent potential fruitful targets for therapeutic developments in combating many human diseases.

Tic hydantoin sigma-1 agonist: pharmacological characterization on cocaine-induced stimulant and appetitive effects

Activation of the newly identified sigma(1) chaperone protein is involved in several aspects of the psychostimulant and addictive effects of cocaine. The development of ligands that selectively target the sigma(1) protein may lead to putative potent anti-cocaine agents. Here, we synthetized substituted hydantoins with high affinity for the sigma(1) protein and evaluated their behavioral efficacy. Two pure enantiomers were designed and synthesized: tetrahydroisoquinoleine-hydantoin fused compounds 3 and 4. They increased cocaine-induced locomotor stimulation or sensitization. The most active enantiomer 4, facilitated CPP acquisition but failed to substitute for cocaine. When CPP was acquired with cocaine and then extinguished, 4 provoked reactivation of CPP. These observations showed that compound 4 shows a typical profile of sigma(1) protein activator, facilitating cocaine-induced behavioral effects. Preliminary ADME properties are in favour of an optimal therapeutic development. Such Tic-hydantoin compound may serve as a new effective agonist therapy in cocaine addiction.

The sigma-1 antagonist BMY-14802 inhibits L-DOPA-induced abnormal involuntary movements by a WAY-100635-sensitive mechanism

RATIONALE

Levodopa (L-DOPA), the gold standard treatment for Parkinson's disease (PD), eventually causes L-DOPA-induced dyskinesia (LID) in up to 80% of patients. In the 6-hydroxydopamine (6-OHDA) rat model of PD, L-DOPA induces a similar phenomenon, which has been termed abnormal involuntary movement (AIM). We previously demonstrated that BMY-14802 suppresses AIM expression in this model.

OBJECTIVES

Although BMY-14802 is widely used as a sigma-1 antagonist, it is also an agonist at serotonin (5-HT) 1A and adrenergic alpha-1 receptors. The current study was conducted to determine which of these mechanisms underlies BMY-14802's AIM-suppressing effect. This characterization included testing the 5-HT1A agonist buspirone and multiple sigma agents. When these studies implicated a 5-HT1A mechanism, we subsequently undertook a pharmacological reversal study, evaluating whether the 5-HT1A antagonist WAY-100635 counteracted BMY-14802's AIM-suppressing effects.

RESULTS

Buspirone dose-dependently suppressed AIM, supporting past findings. However, no AIM-suppressing effects were produced by drugs with effects at sigma receptors, including BD-1047, finasteride, SM-21, DTG, trans-dehydroandrosterone (DHEA), carbetapentane, and opipramol. Finally, we show for the first time that the AIM-suppressing effect of BMY-14802 was dose-dependently prevented by WAY-100635 but not by the alpha-1 antagonist prazosin.

CONCLUSIONS

BMY-14802 exerts its AIM-suppressing effects via a 5-HT1A agonist mechanism, similar to buspirone. Other 5-HT1A agonists have failed clinical trials, possibly due to submicromolar affinity at other receptors, including D2, which may exacerbate PD symptoms. BMY-14802 is a promising candidate for clinical trials due to its extremely low affinity for the D2 receptor and lack of extrapyramidal effects during prior clinical trials for schizophrenia.

Antipsychotic-like properties of 5-alpha-reductase inhibitors

Recent evidence indicates that neuroactive steroids may participate in the pathogenesis of schizophrenia spectrum disorders, yet the mechanisms of this involvement are elusive. As 5-alpha-reductase (5AR) is the rate-limiting enzyme of one of the two major metabolic pathways in brain steroidogenesis, we investigated the effects of its blockade in several rat models of psychotic-like behavior. The 5AR inhibitor finasteride (FIN, 60 or 100 mg/kg, intraperitoneal, i.p.) dose- and time-dependently antagonized prepulse inhibition (PPI) deficits induced by apomorphine (APO, 0.25 mg/kg, subcutaneous, s.c.) and d-amphetamine (AMPH, 5 mg/kg, s.c.), in a manner analogous to haloperidol (HAL, 0.1 mg/kg, i.p.) and clozapine (CLO, 5 mg/kg, i.p.). Similar results were observed with the other 5AR inhibitors dutasteride (DUT, 40 or 80 mg/kg, i.p.) and SKF 105111 (30 mg/kg, i.p.). FIN (60 or 100 mg/kg, i.p.) also reduced hyperlocomotion induced by AMPH (1 or 3 mg/kg, s.c.) and attenuated stereotyped behaviors induced by APO (0.25 mg/kg, s.c.). Nevertheless, FIN (100 mg/kg, i.p.) did not reverse the PPI disruption induced by the N-methyl-d-aspartate receptor antagonist dizocilpine (0.1 mg/kg, s.c.). FIN (60-300 mg/kg, i.p.) induced no catalepsy in either the bar test or the paw test. Our results suggest that 5AR inhibitors elicit antipsychotic-like effects in animals and may be proposed as a putative novel target in the management of psychotic disorders.

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.

Differential effects of sigma1 receptor blockade on self-administration and conditioned reinstatement motivated by cocaine vs natural reward

Growing evidence suggests a role for sigma(1) (sigma(1)) receptors in cognitive function, anxiety, depression, regulation of stress responses, and, recently, the appetitive effects of cocaine as measured by conditioned place preference. This study was designed to extend understanding of the role of sigma(1) receptors in addiction-relevant conditioned effects of cocaine by testing the effects of a potent and selective sigma(1) receptor antagonist, BD1047, on conditioned reinstatement of cocaine-seeking. To determine whether modification of conditioned reinstatement by BD1047 is selective for drug-directed behavior or reflects general suppressant effects on motivated behavior, BD1047 was tested also on reinstatement induced by stimuli conditioned to a natural reward, sweetened condensed milk (SCM). Additionally, because sigma(1) receptors have been implicated also in processes linked to the acute reinforcing actions of cocaine, tests of the effects of BD1047 on cocaine self-administration-including a comparison with the sigma(1) antagonist effects on SCM self-administration-were conducted as well. Cocaine self-administering male Wistar rats were trained to associate a discriminative stimulus (S(D)) with the availability of cocaine or SCM, and then subjected to reinstatement tests following extinction of cocaine or SCM-reinforced behavior. BD1047 (1-30 mg/kg) reversed response reinstatement induced by the cocaine S(D) at 20 and 30 mg/kg but did not modify SCM S(D)-induced responding at all but the highest 30 mg dose, at which responding was reversed to extinction levels. BD1047 did not modify responding reinforced directly by SCM or cocaine. The findings support a role for sigma(1) receptors in regulating conditioned responses to cocaine-related contextual stimuli and identify this receptor as a potential treatment target for the prevention of craving and relapse.

Measuring reward with the conditioned place preference (CPP) paradigm: update of the last decade

Conditioned place preference (CPP) continues to be one of the most popular models to study the motivational effects of drugs and non-drug treatments in experimental animals. This is obvious from a steady year-to-year increase in the number of publications reporting the use this model. Since the compilation of the preceding review in 1998, more than 1000 new studies using place conditioning have been published, and the aim of the present review is to provide an overview of these recent publications. There are a number of trends and developments that are obvious in the literature of the last decade. First, as more and more knockout and transgenic animals become available, place conditioning is increasingly used to assess the motivational effects of drugs or non-drug rewards in genetically modified animals. Second, there is a still small but growing literature on the use of place conditioning to study the motivational aspects of pain, a field of pre-clinical research that has so far received little attention, because of the lack of appropriate animal models. Third, place conditioning continues to be widely used to study tolerance and sensitization to the rewarding effects of drugs induced by pre-treatment regimens. Fourth, extinction/reinstatement procedures in place conditioning are becoming increasingly popular. This interesting approach is thought to model certain aspects of relapse to addictive behavior and has previously almost exclusively been studied in drug self-administration paradigms. It has now also become established in the place conditioning literature and provides an additional and technically easy approach to this important phenomenon. The enormous number of studies to be covered in this review prevented in-depth discussion of many methodological, pharmacological or neurobiological aspects; to a large extent, the presentation of data had to be limited to a short and condensed summary of the most relevant findings.

In vitro and in vivo binding of neuroactive steroids to the sigma-1 receptor as measured with the positron emission tomography radioligand [18F]FPS

Sigma-1 receptors are widely expressed in the mammalian brain and also in organs of the immune, endocrine and reproductive systems. Based on behavioral and pharmacological assessments, sigma-1 receptors are important in memory and cognitive processes, and are thought to be involved in specific psychiatric illnesses, including schizophrenia, depression, and drug addiction. It is thought that specific neuroactive steroids are endogenous ligands for these sites. In addition, several sigma-1 receptor binding steroids including progesterone, dihydroepiandrosterone (DHEA), and testosterone are being examined clinically for specific therapeutic purposes; however, their mechanisms of action have not been clearly defined. We previously described the high affinity sigma-1 receptor selective PET tracer [(18)F]FPS. This study examines the effect of neuroactive steroids on [(18)F]FPS binding in vitro and in vivo. Inhibition constants were determined in vitro for progesterone, testosterone, DHEA, estradiol, and estriol binding to the [(18)F]FPS labeled receptor. The affinity order (K(i) values) for these steroids ranged from 36 nM for progesterone to >10,000 nM for estrodiol and estriol. Biodistribution studies revealed that i.v. coadministration of progesterone (10 mg/kg), testosterone (20 mg/kg), or DHEA (20 mg/kg) significantly decreased [(18)F]FPS uptake (%ID/g) by up to 50% in nearly all of eight brain regions examined. [(18)F]FPS uptake in several peripheral organs that express sigma-1 receptors (heart, spleen, muscle, lung) was also reduced (54-85%). These studies clearly demonstrate that exogenously administered steroids can occupy sigma-1 receptors in vivo, and that [(18)F]FPS may provide an effective tool for monitoring sigma-1 receptor occupancy of specific therapeutic steroids during clinical trials.

dextro-Morphine attenuates the morphine-produced conditioned place preference via the sigma(1) receptor activation in the rat

An unbiased conditioned place preference paradigm was used to evaluate the effect of dextro-morphine on the morphine-produced reward in male CD rats. Morphine sulfate (1-10 mg/kg) given intraperitoneally dose-dependently produced the conditioned place preference. Pretreatment with dextro-morphine at a dose from 0.1 to 3 microg/kg given subcutaneously dose-dependently attenuated the morphine-produced conditioned place preference. However, dextro-morphine at a higher dose 100 microg/kg did not affect the morphine-produced conditioned place preference. Thus, dextro-morphine pretreatment induces a U-shaped dose-response curve for attenuating the morphine-produced conditioned place preference. The attenuation of the morphine-produced conditioned place preference was reversed by the pretreatment with the sigma(1) receptor antagonist BD1047 (N-[2-(3,4-Dichlorophenyl)ethyl]-N-methyl-2-(dimethylamino)ethylamine dihydrobromide. dextro-Morphine or BD1047 given alone did not affect the baseline place conditioning. It is concluded that dextro-morphine attenuated the morphine-produced conditioned place preference via the sigma(1) receptor activation.