Cocaine disrupts histamine H3 receptor modulation of dopamine D1 receptor signaling: σ1-D1-H3 receptor complexes as key targets for reducing cocaine's effects

ADHD and Risk of Precocious Puberty: Considering the Impact of MPH

BACKGROUND/OBJECTIVES

The complex association between attention-deficit/hyperactivity disorder (ADHD) and methylphenidate (MPH) with precocious puberty (PP) is still unclear. This study aims to investigate the association between ADHD, MPH, and PP.

METHODS

This is a nationwide cohort study including a total of 3,342,077 individuals, 186,681 with ADHD and 3,155,396 without. First, we compared the risk of PP between ADHD cases and non-ADHD cases. Second, we compared the risk of PP between MPH users and non-MPH users in patients with ADHD.

RESULTS

Patients with ADHD were at a greater risk of PP (adjusted hazard ratio [aHR], 2.01 [95% CI, 1.91-2.11]). In our moderation analyses, the female gender was a positive additive effect modifier of the association between ADHD and PP, whereas tics and intellectual disability were negative effect modifiers. In patients with ADHD, MPH users had a significantly lower risk of PP (aHR, 0.63 [95% CI 0.57-0.70]), and females had a negative effect modification on the association between MPH and PP.

CONCLUSIONS

Our study found that children with ADHD were at a greater risk of PP. Girls with ADHD were a group particularly vulnerable to PP. Comorbid tics or intellectual disability was associated with a lower risk of PP. Among patients with ADHD, MPH was protective against PP, especially in girls. However, these preliminary results need further validation due to the nature of them being from an electronic database study. Unmeasured confounding factors might affect the association between MPH and PP.

Histamine H3 Receptor Isoforms: Insights from Alternative Splicing to Functional Complexity

Alternative splicing significantly enhances the diversity of the G protein-coupled receptor (GPCR) family, including the histamine H3 receptor (H3R). This post-transcriptional modification generates multiple H3R isoforms with potentially distinct pharmacological and physiological profiles. H3R is primarily involved in the presynaptic inhibition of neurotransmitter release in the central nervous system. Despite the approval of pitolisant for narcolepsy (Wakix®) and daytime sleepiness in adults with obstructive sleep apnea (Ozawade®) and ongoing clinical trials for other H3R antagonists/inverse agonists, the functional significance of the numerous H3R isoforms remains largely enigmatic. Recent publicly available RNA sequencing data have confirmed the expression of multiple H3R isoforms in the brain, with some isoforms exhibiting unique tissue-specific distribution patterns hinting at isoform-specific functions and interactions within neural circuits. In this review, we discuss the complexity of H3R isoforms with a focus on their potential roles in central nervous system (CNS) function. Comparative analysis across species highlights evolutionary conservation and divergence in H3R splicing, suggesting species-specific regulatory mechanisms. Understanding the functionality of H3R isoforms is crucial for the development of targeted therapeutics. This knowledge will inform the design of more precise pharmacological interventions, potentially enhancing therapeutic efficacy and reducing adverse effects in the treatment of neurological and psychiatric disorders.

Discovery of AD258 as a Sigma Receptor Ligand with Potent Antiallodynic Activity

The design and synthesis of a series of 2,7-diazaspiro[4.4]nonane derivatives as potent sigma receptor (SR) ligands, associated with analgesic activity, are the focus of this work. In this study, affinities at S1R and S2R were measured, and molecular modeling studies were performed to investigate the binding pose characteristics. The most promising compounds were subjected to in vitro toxicity testing and subsequently screened for in vivo analgesic properties. Compound 9d (AD258) exhibited negligible in vitro cellular toxicity and a high binding affinity to both SRs (KiS1R = 3.5 nM, KiS2R = 2.6 nM), but not for other pain-related targets, and exerted high potency in a model of capsaicin-induced allodynia, reaching the maximum antiallodynic effect at very low doses (0.6-1.25 mg/kg). Functional activity experiments showed that S1R antagonism is needed for the effects of 9d and that it did not induce motor impairment. In addition, 9d exhibited a favorable pharmacokinetic profile.

Effect of moderate prenatal ethanol exposure on the differential expression of two histamine H3 receptor isoforms in different brain regions of adult rat offspring

We have reported that prenatal alcohol exposure (PAE) elevates histamine H3 receptor (H3R) agonist-mediated inhibition of glutamatergic neurotransmission in the dentate gyrus. Here, we hypothesized that PAE alters the expression of two prominent H3R isoforms namely, the rH3A and rH3C isoforms, which have differing intrinsic activities for H3R agonists, in a manner that may contribute to heightened H3R function in PAE rats. In contrast to our predictions, we found different effects of sex and PAE in various brain regions with significant interactions between sex and PAE in dentate gyrus and entorhinal cortex for both isoforms. Subsequently, to confirm the PAE-and sex-induced differences on H3R isoform mRNA expression, we developed a polyclonal antibody selective for the rH3A inform. Western blots of rH3A mRNA-transfected HEK-293 cells identified a  ~ 48 kDa band of binding consistent with the molecular weight of rH3A, thus confirming antibody sensitivity for rH3A protein. In parallel, we also established a pan-H3R knockout mice line to confirm antibody specificity in rodent brain membranes. Both qRT-PCR and H3R agonist-stimulated [35S]-GTPγS binding confirmed the absence of mH3A mRNA and H3 receptor-effector coupling in H3R knockout (KO) mice. Subsequent western blotting studies in both rat and mouse brain membranes were unable to detect rH3A antibody binding at ~48 kDa. Rather, the H3RA antibody bound to a  ~ 55 kDa band in both rat and mouse membranes, including H3R KO mice, suggesting H3RA binding was not specific for H3Rs in rodent membranes. Subsequent LC/MS analysis of the ~55 kDa band in frontal cortical membranes identified the highly abundant beta subunit of ATPase in both WT and KO mice. Finally, LC/MS analysis of the ~48 kDa band from rH3A mRNA-transfected HEK-293 cell membranes was able to detect rH3A protein, but its presence was below the limits of quantitative reliability. We conclude that PAE alters rH3A and rH3C mRNA expression in some of the same brain regions where we have previously reported PAE-induced alterations in H3R-effector coupling. However, interpreting the functional consequences of altered H3R isoform expression was limited given the technical challenges of measuring the relatively low abundance of rH3A protein in native membrane preparations.

The histamine H3 receptor modulates dopamine D2 receptor-dependent signaling pathways and mouse behaviors

The histamine H3 receptor (H3R) is highly enriched in the spiny projection neurons (SPNs) of the striatum, in both the D1 receptor (D1R)-expressing and D2 receptor (D2R)-expressing populations. A crossantagonistic interaction between H3R and D1R has been demonstrated in mice, both at the behavioral level and at the biochemical level. Although interactive behavioral effects have been described upon coactivation of H3R and D2R, the molecular mechanisms underlying this interaction are poorly understood. Here, we show that activation of H3R with the selective agonist R-(-)-α-methylhistamine dihydrobromide mitigates D2R agonist-induced locomotor activity and stereotypic behavior. Using biochemical approaches and the proximity ligation assay, we demonstrated the existence of an H3R-D2R complex in the mouse striatum. In addition, we examined consequences of simultaneous H3R-D2R agonism on the phosphorylation levels of several signaling molecules using immunohistochemistry. H3R agonist treatment modulated Akt (serine/threonine PKB)-glycogen synthase kinase 3 beta signaling in response to D2R activation via a β-arrestin 2-dependent mechanism in D2R-SPNs but not in D1R-SPNs. Phosphorylation of mitogen- and stress-activated protein kinase 1 and rpS6 (ribosomal protein S6) was largely unchanged under these conditions. As Akt-glycogen synthase kinase 3 beta signaling has been implicated in several neuropsychiatric disorders, this work may help clarify the role of H3R in modulating D2R function, leading to a better understanding of pathophysiology involving the interaction between histamine and dopamine systems.

The piperazine analogue para-fluorophenylpiperazine alters timing of the physiological effects of the synthetic cannabinoid receptor agonist AMB-FUBINACA, without changing its discriminative stimulus, signalling effects, or metabolism

AMB-FUBINACA is a synthetic cannabinoid receptor agonist (SCRA), which has been associated with substantial abuse and health harm since 2016 in many countries including New Zealand. A characteristic of AMB-FUBINACA use in New Zealand has included the observation that forensic samples (from autopsies) and drugs seized by police have often been found to contain para-fluorophenylpiperazine (pFPP), a relatively little-characterised piperazine analogue that has been suggested to act through 5HT1a serotonin receptors. In the current study, we aimed to characterise the interactions of these two agents in rat physiological endpoints using plethysmography and telemetry, and to examine whether pFPP altered the subjective effects of AMB-FUBINACA in mice trained to differentiate a cannabinoid (THC) from vehicle. Though pFPP did not alter the ability of AMB-FUBINACA to substitute for THC, it did appear to abate some of the physiological effects of AMB-FUBINACA in rats by delaying the onset of AMB-FUBINACA-mediated hypothermia and shortening duration of bradycardia. In HEK cells stably expressing the CB1 cannabinoid receptor, 5HT1a, or both CB1 and 5HT1a, cAMP signalling was recorded using a BRET biosensor (CAMYEL) to assess possible direct receptor interactions. Although low potency pFPP agonism at 5HT1a was confirmed, little evidence for signalling interactions was detected in these assays: additive or synergistic effects on potency or efficacy were not detected between pFPP and AMB-FUBINACA-mediated cAMP inhibition. Experiments utilising higher potency, classical 5HT1a ligands (agonist 8OH-DPAT and antagonist WAY100635) also failed to reveal evidence for mutual CB1/5HT1a interactions or cross-antagonism. Finally, the ability of pFPP to alter the metabolism of AMB-FUBINACA in rat and human liver microsomes into its primary carboxylic acid metabolite via carboxylesterase-1 was assessed by HPLC; no inhibition was detected. Overall, the effects we have observed do not suggest that increased harm/toxicity would result from the combination of pFPP and AMB-FUBINACA.

Sigma-1 Receptor Signaling: In Search of New Therapeutic Alternatives for Cardiovascular and Renal Diseases

Cardiovascular and renal diseases are among the leading causes of death worldwide, and regardless of current efforts, there is a demanding need for therapeutic alternatives to reduce their progression to advanced stages. The stress caused by diseases leads to the activation of protective mechanisms in the cell, including chaperone proteins. The Sigma-1 receptor (Sig-1R) is a ligand-operated chaperone protein that modulates signal transduction during cellular stress processes. Sig-1R interacts with various ligands and proteins to elicit distinct cellular responses, thus, making it a potential target for pharmacological modulation. Furthermore, Sig-1R ligands activate signaling pathways that promote cardioprotection, ameliorate ischemic injury, and drive myofibroblast activation and fibrosis. The role of Sig-1R in diseases has also made it a point of interest in developing clinical trials for pain, neurodegeneration, ischemic stroke, depression in patients with heart failure, and COVID-19. Sig-1R ligands in preclinical models have significantly beneficial effects associated with improved cardiac function, ventricular remodeling, hypertrophy reduction, and, in the kidney, reduced ischemic damage. These basic discoveries could inform clinical trials for heart failure (HF), myocardial hypertrophy, acute kidney injury (AKI), and chronic kidney disease (CKD). Here, we review Sig-1R signaling pathways and the evidence of Sig-1R modulation in preclinical cardiac and renal injury models to support the potential therapeutic use of Sig-1R agonists and antagonists in these diseases.

Dopamine, Immunity, and Disease

The neurotransmitter dopamine is a key factor in central nervous system (CNS) function, regulating many processes including reward, movement, and cognition. Dopamine also regulates critical functions in peripheral organs, such as blood pressure, renal activity, and intestinal motility. Beyond these functions, a growing body of evidence indicates that dopamine is an important immunoregulatory factor. Most types of immune cells express dopamine receptors and other dopaminergic proteins, and many immune cells take up, produce, store, and/or release dopamine, suggesting that dopaminergic immunomodulation is important for immune function. Targeting these pathways could be a promising avenue for the treatment of inflammation and disease, but despite increasing research in this area, data on the specific effects of dopamine on many immune cells and disease processes remain inconsistent and poorly understood. Therefore, this review integrates the current knowledge of the role of dopamine in immune cell function and inflammatory signaling across systems. We also discuss the current understanding of dopaminergic regulation of immune signaling in the CNS and peripheral tissues, highlighting the role of dopaminergic immunomodulation in diseases such as Parkinson's disease, several neuropsychiatric conditions, neurologic human immunodeficiency virus, inflammatory bowel disease, rheumatoid arthritis, and others. Careful consideration is given to the influence of experimental design on results, and we note a number of areas in need of further research. Overall, this review integrates our knowledge of dopaminergic immunology at the cellular, tissue, and disease level and prompts the development of therapeutics and strategies targeted toward ameliorating disease through dopaminergic regulation of immunity. SIGNIFICANCE STATEMENT: Canonically, dopamine is recognized as a neurotransmitter involved in the regulation of movement, cognition, and reward. However, dopamine also acts as an immune modulator in the central nervous system and periphery. This review comprehensively assesses the current knowledge of dopaminergic immunomodulation and the role of dopamine in disease pathogenesis at the cellular and tissue level. This will provide broad access to this information across fields, identify areas in need of further investigation, and drive the development of dopaminergic therapeutic strategies.

Class A and C GPCR Dimers in Neurodegenerative Diseases

Neurodegenerative diseases affect over 30 million people worldwide with an ascending trend. Most individuals suffering from these irreversible brain damages belong to the elderly population, with onset between 50 and 60 years. Although the pathophysiology of such diseases is partially known, it remains unclear upon which point a disease turns degenerative. Moreover, current therapeutics can treat some of the symptoms but often have severe side effects and become less effective in long-term treatment. For many neurodegenerative diseases, the involvement of G proteincoupled receptors (GPCRs), which are key players of neuronal transmission and plasticity, has become clearer and holds great promise in elucidating their biological mechanism. With this review, we introduce and summarize class A and class C GPCRs, known to form heterodimers or oligomers to increase their signalling repertoire. Additionally, the examples discussed here were shown to display relevant alterations in brain signalling and had already been associated with the pathophysiology of certain neurodegenerative diseases. Lastly, we classified the heterodimers into two categories of crosstalk, positive or negative, for which there is known evidence.

An open-like conformation of the sigma-1 receptor reveals its ligand entry pathway

The sigma-1 receptor (σ1R) is a non-opioid transmembrane receptor which has been implicated in many diseases, including neurodegenerative disorders and cancer. After more than forty years of research, substantial progress has been made in understanding this unique receptor, yet the molecular mechanism of its ligand entry pathway remains uncertain. Published structures of human σ1R reveal its homotrimeric organization of a cupin-fold β-barrel body that contains the ligand binding site, a carboxy-terminal V-shaped two-helix bundle, and a single amino-terminal transmembrane helix, while simulation studies have suggested a ligand entry pathway that is generated by conformational rearrangements of the cupin-fold domain. Here, we present multiple crystal structures, including an open-like conformation, of σ1R from Xenopus laevis. Together with functional binding analysis our data suggest that access to the σ1R ligand binding site is likely achieved by protein conformational changes that involve the carboxy-terminal two-helix bundle, rather than structural changes in the cupin-fold domain.

Histamine H3 and H4 receptors modulate Parkinson's disease induced brain pathology. Neuroprotective effects of nanowired BF-2649 and clobenpropit with anti-histamine-antibody therapy

Military personnel deployed in combat operations are highly prone to develop Parkinson's disease (PD) in later lives. PD largely involves dopaminergic pathways with hallmarks of increased alpha synuclein (ASNC), and phosphorylated tau (p-tau) in the cerebrospinal fluid (CSF) precipitating brain pathology. However, increased histaminergic nerve fibers in substantia nigra pars Compacta (SNpc), striatum (STr) and caudate putamen (CP) associated with upregulation of Histamine H3 receptors and downregulation of H4 receptors in human cases of PD is observed in postmortem cases. These findings indicate that modulation of histamine H3 and H4 receptors and/or histaminergic transmission may induce neuroprotection in PD induced brain pathology. In this review effects of a potent histaminergic H3 receptor inverse agonist BF-2549 or clobenpropit (CLBPT) partial histamine H4 agonist with H3 receptor antagonist, in association with monoclonal anti-histamine antibodies (AHmAb) in PD brain pathology is discussed based on our own observations. Our investigation shows that chronic administration of conventional or TiO₂ nanowired BF 2649 (1mg/kg, i.p.) or CLBPT (1mg/kg, i.p.) once daily for 1 week together with nanowired delivery of HAmAb (25μL) significantly thwarted ASNC and p-tau levels in the SNpC and STr and reduced PD induced brain pathology. These observations are the first to show the involvement of histamine receptors in PD and opens new avenues for the development of novel drug strategies in clinical strategies for PD, not reported earlier.

Discovery of a macromolecular complex mediating the hunger suppressive actions of cocaine: Structural and functional properties

Cocaine not only increases brain dopamine levels but also activates the sigma1 receptor (σ1 R) that in turn regulates orexigenic receptor function. Identification of interactions involving dopamine D1 (D1 R), ghrelin (GHS-R1a ), and σ1 receptors have been addressed by biophysical techniques and a complementation approach using interfering peptides. The effect of cocaine on receptor functionality was assayed by measuring second messenger, cAMP and Ca2+ , levels. The effect of acute or chronic cocaine administration on receptor complex expression was assayed by in situ proximity ligation assay. In silico procedures were used for molecular model building. σ1 R KO mice were used for confirming involvement of this receptor. Upon identification of protomer interaction and receptor functionality, a unique structural model for the macromolecular complex formed by σ1 R, D1 R, and GHS-R1a is proposed. The functionality of the complex, able to couple to both Gs and Gq proteins, is affected by cocaine binding to the σ1 R, as confirmed using samples from σ1 R-/- mice. The expression of the macromolecular complex was differentially affected upon acute and chronic cocaine administration to rats. The constructed 3D model is consistent with biochemical, biophysical, and available structural data. The σ1 R, D1 R, and GHS-R1a complex constitutes a functional unit that is altered upon cocaine binding to the σ1 R. Remarkably, the heteromer can simultaneously couple to two G proteins, thus allowing dopamine to signal via Ca2+ and ghrelin via cAMP. The anorexic action of cocaine is mediated by such complex whose expression is higher after acute than after chronic administration regimens.

Multi-Target Directed Ligands (MTDLs) Binding the σ1 Receptor as Promising Therapeutics: State of the Art and Perspectives

The sigma-1 (σ1) receptor is a 'pluripotent chaperone' protein mainly expressed at the mitochondria-endoplasmic reticulum membrane interfaces where it interacts with several client proteins. This feature renders the σ1 receptor an ideal target for the development of multifunctional ligands, whose benefits are now recognized because several pathologies are multifactorial. Indeed, the current therapeutic regimens are based on the administration of different classes of drugs in order to counteract the diverse unbalanced physiological pathways associated with the pathology. Thus, the multi-targeted directed ligand (MTDL) approach, with one molecule that exerts poly-pharmacological actions, may be a winning strategy that overcomes the pharmacokinetic issues linked to the administration of diverse drugs. This review aims to point out the progress in the development of MTDLs directed toward σ1 receptors for the treatment of central nervous system (CNS) and cancer diseases, with a focus on the perspectives that are proper for this strategy. The evidence that some drugs in clinical use unintentionally bind the σ1 protein (as off-target) provides a proof of concept of the potential of this strategy, and it strongly supports the promise that the σ1 receptor holds as a target to be hit in the context of MTDLs for the therapy of multifactorial pathologies.

Heteromerization of dopaminergic receptors in the brain: Pharmacological implications

Dopamine exerts its physiological effects through two subtypes of receptors, i.e. the receptors of the D₁ family (D_1R and D_5R) and the D₂ family (D_2R, D_3R, and D_4R), which differ in their pattern of distribution, affinity, and signaling. The D₁-like subfamily (D_1R and D_5R) are coupled to Gα_s/olf proteins to activate adenylyl cyclase whereas the D₂-like receptors are coupled to Gα_i/o subunits and suppress the activity of adenylyl cyclase. Dopamine receptors are capable of forming homodimers, heterodimers, and higher-order oligomeric complexes, resulting in a change in the individual protomers' recognition, signaling, and pharmacology. Heteromerization has the potential to modify the canonical pharmacological features of individual monomeric units such as ligand affinity, activation, signaling, and cellular trafficking through allosteric interactions, reviving the field and introducing a new pharmacological target. Since heteromers are expressed and formed in a tissue-specific manner, they could provide the framework to design selective and effective drug candidates, such as brain-penetrant heterobivalent drugs and interfering peptides, with limited side effects. Therefore, heteromerization could be a promising area of pharmacology research, as it could contribute to the development of novel pharmacological interventions for dopamine dysregulated brain disorders such as addiction, schizophrenia, cognition, Parkinson's disease, and other motor-related disorders. This review is articulated based on the three criteria established by the International Union of Basic and Clinical Pharmacology for GPCR heterodimers (IUPHAR): evidence of co-localization and physical interactions in native or primary tissue, presence of a new physiological and functional property than the individual protomers, and loss of interaction and functional fingerprints upon heterodimer disruption.

Histamine H3 Receptor Function Biases Excitatory Gain in the Nucleus Accumbens

BACKGROUND

Histamine (HA), a wake-promoting monoamine implicated in stress-related arousal states, is synthesized in histidine decarboxylase-expressing hypothalamic neurons of the tuberomammillary nucleus. Histidine decarboxylase-containing varicosities diffusely innervate striatal and mesolimbic networks, including the nucleus accumbens (NAc). The NAc integrates diverse monoaminergic inputs to coordinate motivated behavior. While the NAc expresses various HA receptor subtypes, mechanisms by which HA modulates NAc circuit dynamics are undefined.

METHODS

Using male D1tdTomato transgenic reporter mice, whole-cell patch-clamp electrophysiology, and input-specific optogenetics, we employed a targeted pharmacological approach to interrogate synaptic mechanisms recruited by HA signaling at glutamatergic synapses in the NAc. We incorporated an immobilization stress protocol to assess whether acute stress engages these mechanisms at glutamatergic synapses onto D1 receptor-expressing [D1(+)] medium spiny neurons (MSNs) in the NAc core.

RESULTS

HA negatively regulates excitatory gain onto D1(+)-MSNs via presynaptic H3 receptor-dependent long-term depression that requires Gβγ-directed Akt-GSK3β signaling. Furthermore, HA asymmetrically regulates glutamatergic transmission from the prefrontal cortex and mediodorsal thalamus, with inputs from the prefrontal cortex undergoing robust HA-induced long-term depression. Finally, we report that acute immobilization stress attenuates this long-term depression by recruiting endogenous H3 receptor signaling in the NAc at glutamatergic synapses onto D1(+)-MSNs.

CONCLUSIONS

Stress-evoked HA signaling in the NAc recruits H3 heteroreceptor signaling to shift thalamocortical input onto D1(+)-MSNs in the NAc. Our findings provide novel insight into an understudied neuromodulatory system within the NAc and implicate HA in stress-associated physiological states.

Dual-targeting Approach on Histamine H3 and Sigma-1 Receptor Ligands as Promising Pharmacological Tools in the Treatment of CNS-linked Disorders

With the recent market approval of Pitolisant (Wakix®), the interest in clinical application for novel multifunctional histamine H3 receptor antagonists has clearly increased. Several combinations of different H3R pharmacophores with pharmacophoric elements of other G-protein coupled receptors, transporters, or enzymes have been synthesized by numerous pharmaceutical companies and academic institutions. Since central nervous system disorders are characterized by diverse physiological dysfunctions and deregulations of a complex network of signaling pathways, optimal multipotent drugs should simultaneously and peculiarly modulate selected groups of biological targets. Interestingly, very recent studies have shown that some clinically evaluated histamine H3 receptor antagonists possess a nanomolar affinity for sigma-1 receptor binding sites, suggesting that this property might play a role in their overall efficacy. The sigma-1 receptor, unusual and yet obscure protein, is supposed to be involved in numerous CNS pathologies through neuroprotection and neuroplasticity. These two different biological structures, histamine H3 and sigma-1 receptors, combined, can represent a potential fruitful target for therapeutic developments in tackling numerous human diseases.

Small Molecules Selectively Targeting Sigma-1 Receptor for the Treatment of Neurological Diseases

The sigma-1 (σ1) receptor, an enigmatic protein originally classified as an opioid receptor subtype, is now understood to possess unique structural and functional features of its own and play critical roles to widely impact signaling transduction by interacting with receptors, ion channels, lipids, and kinases. The σ1 receptor is implicated in modulating learning, memory, emotion, sensory systems, neuronal development, and cognition and accordingly is now an actively pursued drug target for various neurological and neuropsychiatric disorders. Evaluation of the five selective σ1 receptor drug candidates (pridopidine, ANAVEX2-73, SA4503, S1RA, and T-817MA) that have entered clinical trials has shown that reaching clinical approval remains an evasive and important goal. This review provides up-to-date information on the selective targeting of σ1 receptors, including their history, function, reported crystal structures, and roles in neurological diseases, as well as a useful collation of new chemical entities as σ1 selective orthosteric ligands or allosteric modulators.

Glutathione peroxidase-1 knockout potentiates behavioral sensitization induced by cocaine in mice via σ-1 receptor-mediated ERK signaling: A comparison with the case of glutathione peroxidase-1 overexpressing transgenic mice

We demonstrated that the gene of glutathione peroxidase-1 (GPx-1), a major antioxidant enzyme, is a potential protectant against the neurotoxicity and conditioned place preference induced by cocaine. Because the sigma (σ)-1 receptor is implicated in cocaine-induced drug dependence, we investigated whether the GPx-1 gene modulates the σ-1 receptor in the behavioral sensitization induced by cocaine. Cocaine-induced behavioral sensitization was more pronounced in GPx-1 knockout (KO) than wild-type (WT) mice and was less pronounced in GPx-1 overexpressing transgenic (GPx-1 TG) than non-TG mice. Cocaine treatment significantly enhanced the oxidative burden and reduced the GSH levels in the striatum of WT, GPx-1 KO, and non-TG mice but not in that of GPx-1 TG mice. In addition, cocaine significantly increased the nuclear translocation, its DNA binding activity of nuclear factor erythroid-2-related factor 2 (Nrf2) as well as the mRNA expression of γ-glutamylcysteine (GCL). The genetic depletion of GPx-1 inhibited the Nrf2-related glutathione system, whereas the genetic overexpression of GPx-1 activated this system against behavioral sensitization. BD1047, a σ-1 receptor antagonist, and U0126, an ERK inhibitor significantly induced the Nrf2-related antioxidant potential against behavioral sensitization. Unlike BD1047, U0126 did not affect the cocaine-induced σ-1 receptor immunoreactivity, suggesting that the σ-1 receptor is an upstream molecule for ERK signaling. Importantly, BD1047 and U0126 failed to affect the σ-1 receptor immunoreactivity and ERK phosphorylation induced by cocaine in GPx-1 TG mice. Our results suggest that GPx-1 is a critical mediator for the attenuation of cocaine-induced behavioral sensitization via modulating σ-1 receptor-mediated ERK activation by the induction of the Nrf2-related system.

Histamine H3 receptor gene variants associated with drug abuse in patients with cocaine use disorder

BACKGROUND

Preclinical work revealed significant interactions between ligands of the histamine H₃ receptor and different drugs of abuse. In the case of psychostimulants, the results reported are somewhat controversial and human data are still scarce, despite the fact that an inverse agonist of the H₃ receptor (pitolisant) has reached the market after approval for the treatment of narcolepsy.

AIMS

We have studied associations between histamine H₃ receptor gene variants and cocaine use disorder to increase the knowledge of the possible involvement of histamine H₃ receptor in drug abuse.

METHODS

Seven single nucleotide polymorphisms of the histamine H3 receptor gene were genotyped by using a multiplexing assay in 248 samples of subjects with cocaine use disorder and 500 randomized samples of subjects representative of the Spanish population.

RESULTS

The study of the epidemiological information associated to the samples revealed that subjects with cocaine use disorder broadly abused alcohol, tobacco and cannabinoids. Two single nucleotide polymorphisms (rs3787430 and rs74627870) were found significantly associated with the occurrence of addiction and one more (rs13042865) was specifically related to the severity of cocaine dependence within drug abusers.

CONCLUSIONS

The associations found in this study further extend the hypothesis that histamine H₃ receptor function could be relevant in drug abuse in general and cocaine addiction in particular.

Cannabinoid Receptor Subtype 2 (CB2R) in a Multitarget Approach: Perspective of an Innovative Strategy in Cancer and Neurodegeneration

The cannabinoid receptor subtype 2 (CB2R) represents an interesting and new therapeutic target for its involvement in the first steps of neurodegeneration as well as in cancer onset and progression. Several studies, focused on different types of tumors, report a promising anticancer activity induced by CB2R agonists due to their ability to reduce inflammation and cell proliferation. Moreover, in neuroinflammation, the stimulation of CB2R, overexpressed in microglial cells, exerts beneficial effects in neurodegenerative disorders. With the aim to overcome current treatment limitations, new drugs can be developed by specifically modulating, together with CB2R, other targets involved in such multifactorial disorders. Building on successful case studies of already developed multitarget strategies involving CB2R, in this Perspective we aim at prompting the scientific community to consider new promising target associations involving HDACs (histone deacetylases) and σ receptors by employing modern approaches based on molecular hybridization, computational polypharmacology, and machine learning algorithms.

Region-specific regulation of central histaminergic H3 receptor expression in a mouse model of cow's milk allergy

Central histaminergic H3 receptor (H3R) has been extensively investigated as a potential therapeutic target for various neurological and neurodegenerative disorders. Despite promising results in preclinical rodent models, clinical trials have not provided conclusive evidence for the benefit of H3R antagonists to alleviate cognitive and behavioral symptoms of these disorders. Inconsistent pharmacological efficacies may arise from aberrant changes in H3R over time during disease development. Because H3R is involved in feedback inhibition of histamine synthesis and secretion, the expression of the autoreceptor may also be reciprocally regulated by altered histamine levels in a pathological condition. Thus, we investigated H3R expression in a mouse model of cow's milk allergy, a condition associated with increased histamine levels. Mice were sensitized to bovine whey proteins (WP) over 5 weeks and H3R protein and transcript levels were examined in the brain. Substantially increased H3R immunoreactivity was observed in various brain regions of WP-sensitized mice compared to sham mice. Elevated H3R expression was also found in the thalamic/hypothalamic region. The expression of histaminergic H1, but not H2, receptor subtype was also increased in this and the midbrain regions. Unlike the brain, all three histaminergic receptors were increased in the small intestine. These results indicated that the central histaminergic receptors were altered in WP-sensitized mice in a subtype- and region-specific manner, which likely contributed to behavioral changes we observed in these mice. Our study also suggests that altered levels of H3R could be considered during a pharmacological intervention of a neurological disease.

Modulation of dopamine D1 receptors via histamine H3 receptors is a novel therapeutic target for Huntington's disease

Early Huntington's disease (HD) include over-activation of dopamine D₁ receptors (D₁R), producing an imbalance in dopaminergic neurotransmission and cell death. To reduce D₁R over-activation, we present a strategy based on targeting complexes of D₁R and histamine H₃ receptors (H₃R). Using an HD mouse striatal cell model and HD mouse organotypic brain slices we found that D₁R-induced cell death signaling and neuronal degeneration, are mitigated by an H₃R antagonist. We demonstrate that the D₁R-H₃R heteromer is expressed in HD mice at early but not late stages of HD, correlating with HD progression. In accordance, we found this target expressed in human control subjects and low-grade HD patients. Finally, treatment of HD mice with an H₃R antagonist prevented cognitive and motor learning deficits and the loss of heteromer expression. Taken together, our results indicate that D₁R - H₃R heteromers play a pivotal role in dopamine signaling and represent novel targets for treating HD.

The Molecular Function of σ Receptors: Past, Present, and Future

The σ1 and σ2 receptors are enigmatic proteins that have attracted attention for decades due to the chemical diversity and therapeutic potential of their ligands. However, despite ongoing clinical trials with σ receptor ligands for multiple conditions, relatively little is known regarding the molecular function of these receptors. In this review, we revisit past research on σ receptors and discuss the interpretation of these data in light of recent developments. We provide a synthesis of emerging structural and genetic data on the σ1 receptor and discuss the recent cloning of the σ2 receptor. Finally, we discuss the major questions that remain in the study of σ receptors.

Neuronal Sigma-1 Receptors: Signaling Functions and Protective Roles in Neurodegenerative Diseases

Sigma-1 receptor (S1R) is a multi-functional, ligand-operated protein situated in endoplasmic reticulum (ER) membranes and changes in its function and/or expression have been associated with various neurological disorders including amyotrophic lateral sclerosis/frontotemporal dementia, Alzheimer's (AD) and Huntington's diseases (HD). S1R agonists are broadly neuroprotective and this is achieved through a diversity of S1R-mediated signaling functions that are generally pro-survival and anti-apoptotic; yet, relatively little is known regarding the exact mechanisms of receptor functioning at the molecular level. This review summarizes therapeutically relevant mechanisms by which S1R modulates neurophysiology and implements neuroprotective functions in neurodegenerative diseases. These mechanisms are diverse due to the fact that S1R can bind to and modulate a large range of client proteins, including many ion channels in both ER and plasma membranes. We summarize the effect of S1R on its interaction partners and consider some of the cell type- and disease-specific aspects of these actions. Besides direct protein interactions in the endoplasmic reticulum, S1R is likely to function at the cellular/interorganellar level by altering the activity of several plasmalemmal ion channels through control of trafficking, which may help to reduce excitotoxicity. Moreover, S1R is situated in lipid rafts where it binds cholesterol and regulates lipid and protein trafficking and calcium flux at the mitochondrial-associated membrane (MAM) domain. This may have important implications for MAM stability and function in neurodegenerative diseases as well as cellular bioenergetics. We also summarize the structural and biochemical features of S1R proposed to underlie its activity. In conclusion, S1R is incredibly versatile in its ability to foster neuronal homeostasis in the context of several neurodegenerative disorders.

The histidine decarboxylase model of tic pathophysiology: a new focus on the histamine H3 receptor

Histamine dysregulation was implicated as a rare cause of Tourette syndrome and other tic disorders a decade ago by a landmark genetic study in a high density family pedigree, which implicated a hypomorphic mutation in the histidine decarboxylase (Hdc) gene as a rare but high penetrance genetic cause. Studies in Hdc knockout (KO) mice have confirmed that this mutation causes tic-relevant behavioural and neurochemical abnormalities that parallel what is seen in patients and thus validate the KO as a potentially informative model of tic pathophysiology. Recent studies have focused on the potential role of the histamine H₃ receptor in this model, and by association in tic disorders and related neuropsychiatric conditions. The H₃ receptor is up-regulated in the striatum in Hdc KO mice. As the H₃ receptor has constitutive activity in the absence of ligand, this receptor up-regulation may have significant cellular effects despite the absence of neurotransmitter histamine in these mice. Activation in vivo of H₃ receptors in wild type mice regulates signalling in striatal medium spiny neurons (MSNs) that interacts non-linearly with dopamine receptor signalling. Baseline signalling alterations in MSNs in Hdc KO mice resemble those seen after H₃ receptor agonist treatment in wild type animals. H₃ receptor agonist treatment in the KOs further accentuates most of these signalling abnormalities and produces behavioural stereotypy. Together, these data suggest the intriguing hypothesis that constitutive signalling by up-regulated H₃ receptors explains many of the molecular and behavioural abnormalities seen in these animals. LINKED ARTICLES: This article is part of a themed section on New Uses for 21st Century. To view the other articles in this section visit http://onlinelibrary.wiley.com/doi/10.1111/bph.v177.3/issuetoc.

Allosteric Modulators of Sigma-1 Receptor: A Review

Allosteric modulators of sigma-1 receptor (Sig1R) are described as compounds that can increase the activity of some Sig1R ligands that compete with (+)-pentazocine, one of the classic prototypical ligands that binds to the orthosteric Sig1R binding site. Sig1R is an endoplasmic reticulum membrane protein that, in addition to its promiscuous high-affinity ligand binding, has been shown to have chaperone activity. Different experimental approaches have been used to describe and validate the activity of allosteric modulators of Sig1R. Sig1R-modulatory activity was first found for phenytoin, an anticonvulsant drug that primarily acts by blocking the voltage-gated sodium channels. Accumulating evidence suggests that allosteric Sig1R modulators affect processes involved in the pathophysiology of depression, memory and cognition disorders as well as convulsions. This review will focus on the description of selective and non-selective allosteric modulators of Sig1R, including molecular structure properties and pharmacological activity both in vitro and in vivo, with the aim of providing the latest overview from compound discovery approaches to eventual clinical applications. In this review, the possible mechanisms of action will be discussed, and future challenges in the development of novel compounds will be addressed.

Drug-receptor kinetics and sigma-1 receptor affinity differentiate clinically evaluated histamine H3 receptor antagonists

The histamine H3 receptor is a G protein-coupled receptor (GPCR) drug target that is highly expressed in the CNS, where it acts as both an auto- and hetero-receptor to regulate neurotransmission. As such, it has been considered as a relevant target in disorders as varied as Alzheimer's disease, schizophrenia, neuropathic pain and attention deficit hyperactivity disorder. A range of competitive antagonists/inverse agonists have progressed into clinical development, with pitolisant approved for the treatment of narcolepsy. Given the breadth of compounds developed and potential therapeutic indications, we assessed the comparative pharmacology of six investigational histamine H3 agents, including pitolisant, using native tissue and recombinant cells. Whilst all of the compounds tested displayed robust histamine H3 receptor inverse agonism and did not differentiate between the main H3 receptor splice variants, they displayed a wide range of affinities and kinetic properties, and included rapidly dissociating (pitolisant, S 38093-2, ABT-239) and slowly dissociating (GSK189254, JNJ-5207852, PF-3654746) agents. S 38093-2 had the lowest histamine H3 receptor affinity (pKB values 5.7-6.2), seemingly at odds with previously reported, potent in vivo activity in models of cognition. We show here that at pro-cognitive and anti-hyperalgesic/anti-allodynic doses, S 38093-2 preferentially occupies the mouse sigma-1 receptor in vivo, only engaging the histamine H3 receptor at doses associated with wakefulness promotion and neurotransmitter (histamine, ACh) release. Furthermore, pitolisant, ABT-239 and PF-3654746 also displayed appreciable sigma-1 receptor affinity, suggesting that this property differentiates clinically evaluated histamine H3 receptor antagonists and may play a role in their efficacy.

Differential effect of amphetamine over the corticotropin-releasing factor CRF2 receptor, the orexin OX1 receptor and the CRF2-OX1 heteroreceptor complex

Stress is one of the factors underlying drug seeking behavior that often goes in parallel with loss of appetite. We here demonstrate that orexin 1 receptors (OX₁R) may form complexes with the corticotropin releasing factor CRF₂ receptor. Two specific features of the heteromer were a cross-antagonism and a blockade by CRF₂ of OX₁R signaling. In cells expressing one of the receptors, agonist-mediated signal transduction mechanisms were potentiated by amphetamine. Sigma 1 (σ₁) and 2 (σ₂) receptors are targets of drugs of abuse and, despite sharing a similar name, the two receptors are structurally unrelated and their physiological role is not known. We here show that σ₁ receptors interact with CRF₂ receptors and that σ₂ receptors interact with OX₁R. Moreover, we show that amphetamine effect on CRF₂ receptors was mediated by σ₁R whereas the effect on OX₁ receptors was mediated by σ₂R. Amphetamine did potentiate the negative cross-talk occurring within the CRF₂-OX₁ receptor heteromer context, likely by a macromolecular complex involving the two sigma receptors and the two GPCRs. Finally, in vivo microdialysis experiments showed that amphetamine potentiated orexin A-induced dopamine and glutamate release in the ventral tegmental area (VTA). Remarkably, the in vivo orexin A effects were blocked by a selective CRF₂R antagonist. These results show that amphetamine impacts on the OX₁R-, CRF₂R- and OX₁R/CRF₂R-mediated signaling and that cross-antagonism is instrumental for in vivo detection of GPCR heteromers. This article is part of the Special Issue entitled 'Receptor heteromers and their allosteric receptor-receptor interactions'.

Striatal Signaling Regulated by the H3R Histamine Receptor in a Mouse Model of tic Pathophysiology

Histamine dysregulation has been identified as a rare genetic cause of tic disorders; mice with a knockout of the histidine decarboxylase (Hdc) gene represent a promising model of this pathophysiology. How alterations in the histamine system lead to neuropsychiatric disease, however, remains unclear. The H3R histamine receptor is elevated in the striatum of Hdc KO mice, and H3R agonists, acting in the dorsal striatum, trigger tic-like movements in the model. In wild-type mice, H3R in the dorsal striatum differentially regulates mitogen-activated protein kinase (MAPK) and protein kinase B (Akt) signaling in D1R dopamine receptor-expressing striatonigral medium spiny neurons (dMSNs) and D2R dopamine receptor-expressing striatopallidal MSNs (iMSNs), respectively. We examined the effects of H3R agonist treatment on MSN signaling in the Hdc-KO model. In dMSNs, MAPK signaling was elevated at baseline in the Hdc-KO model, resembling what is seen after H3R activation in WT animals. Similarly, in iMSNs, Akt phosphorylation was reduced at baseline in the KO model, resembling what is seen after H3R activation in WT animals. H3R activation in Hdc-KO mice further enhanced the baseline effect on Akt phosphorylation in iMSNs but attenuated the abnormality in MAPK signaling in dMSNs. These observations support the hypothesis that constitutive activity of upregulated H3R receptors in the Hdc-KO model mediates the observed alterations in baseline MSN signaling; but further activation of H3R, which produces tic-like repetitive movements in the model, has more complex effects.

G protein-coupled receptor heteromers are key players in substance use disorder

G protein-coupled receptors (GPCR) represent the largest family of membrane proteins in the human genome. Physical association between two different GPCRs is linked to functional interactions which generates a novel entity, called heteromer, with specific ligand binding and signaling properties. Heteromerization is increasingly recognized to take place in the mesocorticolimbic pathway and to contribute to various aspects related to substance use disorder. This review focuses on heteromers identified in brain areas relevant to drug addiction. We report changes at the molecular and cellular levels that establish specific functional impact and highlight behavioral outcome in preclinical models. Finally, we briefly discuss selective targeting of native heteromers as an innovative therapeutic option.

Functional histamine H3 and adenosine A2A receptor heteromers in recombinant cells and rat striatum

In the striatum, histamine H3 receptors (H3Rs) are co-expressed with adenosine A2A receptors (A2ARs) in the cortico-striatal glutamatergic afferents and the GABAergic medium-sized spiny neurons that originate the indirect pathway of the basal ganglia. This location allows H3Rs and A2ARs to regulate the striatal GABAergic and glutamatergic transmission. However, whether these receptors can physically interact has not yet been assessed. To test this hypothesis, a heteromer-selective in vitro assay was used to detect functional complementation between a chimeric A2AR302-Gαqi4 and wild-type H3Rs in transfected HEK-293T cells. H3R activation with the agonist RAMH resulted in Ca2+ mobilization (pEC50 7.31 ± 0.23; maximal stimulation, Emax 449 ± 25% of basal) indicative of receptor heterodimerization. Functional H3R-A2AR heteromers were confirmed by co-immunoprecipitation and observations of differential cAMP signaling when both receptors were co-expressed in the same cells. In membranes from rat striatal synaptosomes, H3R activation decreased A2AR affinity for the agonist CGS-21680 (pKi values 8.10 ± 0.04 and 7.70 ± 0.04). Moreover, H3Rs and A2ARs co-immunoprecipitated in protein extracts from striatal synaptosomes. These results support the existence of a H3R-A2AR heteromer with possible physiological implications for the modulation of the intra-striatal transmission.

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.

Histamine modulation of the basal ganglia circuitry in the development of pathological grooming

Aberrant histaminergic function has been proposed as a cause of tic disorders. A rare mutation in the enzyme that produces histamine (HA), histidine decarboxylase (HDC), has been identified in patients with Tourette syndrome (TS). Hdc knockout mice exhibit repetitive behavioral pathology and neurochemical characteristics of TS, establishing them as a plausible model of tic pathophysiology. Where, when, and how HA deficiency produces these effects has remained unclear: whether the contribution of HA deficiency to pathogenesis is acute or developmental, and where in the brain the relevant consequences of HA deficiency occur. Here, we address these key pathophysiological questions, using anatomically and cellularly targeted manipulations in mice. We report that specific ablation or chemogenetic silencing of histaminergic neurons in the tuberomammillary nucleus (TMN) of the hypothalamus leads to markedly elevated grooming, a form of repetitive behavioral pathology, and to elevated markers of neuronal activity in both dorsal striatum and medial prefrontal cortex. Infusion of HA directly into the striatum reverses this behavioral pathology, confirming that acute HA deficiency mediates the effect. Bidirectional chemogenetic regulation reveals that dorsal striatum neurons activated after TMN silencing are both sufficient to produce repetitive behavioral pathology and necessary for the full expression of the effect. Chemogenetic activation of TMN-regulated medial prefrontal cortex neurons, in contrast, increases locomotion and not grooming. These data confirm the centrality of striatal regulation by neurotransmitter HA in the adult in the production of pathological grooming.

An update on the physiological and therapeutic relevance of GPCR oligomers

The traditional view on GPCRs held that they function as single monomeric units composed of identical subunits. This notion was overturned by the discovery that GPCRs can form homo- and hetero-oligomers, some of which are obligatory, and can further assemble into receptor mosaics consisting of three or more protomers. Oligomerisation exerts significant impacts on receptor function and physiology, offering a platform for the diversification of receptor signalling, pharmacology, regulation, crosstalk, internalization and trafficking. Given their involvement in the modulation of crucial physiological processes, heteromers could constitute important therapeutic targets for a wide range of diseases, including schizophrenia, Parkinson's disease, substance abuse or obesity. This review aims at depicting the current developments in GPCR oligomerisation research, documenting various class A, B and C GPCR heteromers detected in vitro and in vivo using biochemical and biophysical approaches, as well as recently identified higher-order oligomeric complexes. It explores the current understanding of dimerization dynamics and the possible interaction interfaces that drive oligomerisation. Most importantly, it provides an inventory of the wide range of physiological processes and pathophysiological conditions to which GPCR oligomers contribute, surveying some of the oligomers that constitute potential drug targets. Finally, it delineates the efforts to develop novel classes of ligands that specifically target and tether to receptor oligomers instead of a single monomeric entity, thus ameliorating their ability to modulate GPCR function.

Histamine H3R receptor activation in the dorsal striatum triggers stereotypies in a mouse model of tic disorders

Tic disorders affect ~5% of the population and are frequently comorbid with obsessive-compulsive disorder, autism, and attention deficit disorder. Histamine dysregulation has been identified as a rare genetic cause of tic disorders; mice with a knockout of the histidine decarboxylase (Hdc) gene represent a promising pathophysiologically grounded model. How alterations in the histamine system lead to tics and other neuropsychiatric pathology, however, remains unclear. We found elevated expression of the histamine H3 receptor in the striatum of Hdc knockout mice. The H3 receptor has significant basal activity even in the absence of ligand and thus may modulate striatal function in this knockout model. We probed H3R function using specific agonists. The H3 agonists R-aminomethylhistamine (RAMH) and immepip produced behavioral stereotypies in KO mice, but not in controls. H3 agonist treatment elevated intra-striatal dopamine in KO mice, but not in controls. This was associated with elevations in phosphorylation of rpS6, a sensitive marker of neural activity, in the dorsal striatum. We used a novel chemogenetic strategy to demonstrate that this dorsal striatal activity is necessary and sufficient for the development of stereotypy: when RAMH-activated cells in the dorsal striatum were chemogenetically activated (in the absence of RAMH), stereotypy was recapitulated in KO animals, and when they were silenced the ability of RAMH to produce stereotypy was blocked. These results identify the H3 receptor in the dorsal striatum as a contributor to repetitive behavioral pathology.

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

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

Sigma-1 Receptor and Neuronal Excitability

The sigma-1 receptor (Sig-1R), via interaction with various proteins, including voltage-gated and ligand-gated ion channels (VGICs and LGICs), is involved in a plethora of neuronal functions. This capability to regulate a variety of ion channel targets endows the Sig-1R with a powerful capability to fine tune neuronal excitability, and thereby the transmission of information within brain circuits. This versatility may also explain why the Sig-1R is associated to numerous diseases at both peripheral and central levels. To date, how the Sig-1R chooses its targets and how the combinations of target modulations alter overall neuronal excitability is one of the challenges in the field of Sig-1R-dependent regulation of neuronal activity. Here, we will describe and discuss the latest findings on Sig-1R-dependent modulation of VGICs and LGICs, and provide hypotheses that may explain the diverse excitability outcomes that have been reported so far.

The Histamine H3 Receptor Differentially Modulates Mitogen-activated Protein Kinase (MAPK) and Akt Signaling in Striatonigral and Striatopallidal Neurons

The basal ganglia have a central role in motor patterning, habits, motivated behaviors, and cognition as well as in numerous neuropsychiatric disorders. Receptors for histamine, especially the H3 receptor (H3R), are highly expressed in the striatum, the primary input nucleus of the basal ganglia, but their effects on this circuitry have been little explored. H3R interacts with dopamine (DA) receptors ex vivo; the nature and functional importance of these interactions in vivo remain obscure. We found H3R activation with the agonist R-(-)-α-methylhistamine to produce a unique time- and cell type-dependent profile of molecular signaling events in the striatum. H3 agonist treatment did not detectably alter extracellular DA levels or signaling through the cAMP/DARPP-32 signaling pathway in either D1- or D2-expressing striatal medium spiny neurons (MSNs). In D1-MSNs, H3 agonist treatment transiently activated MAPK signaling and phosphorylation of rpS6 and led to phosphorylation of GSK3β-Ser⁹, a novel effect. Consequences of H3 activation in D2-MSNs were completely different. MAPK signaling was unchanged, and GSK3β-Ser⁹ phosphorylation was reduced. At the behavioral level, two H3 agonists had no significant effect on locomotion or stereotypy, but they dramatically attenuated the locomotor activation produced by the D1 agonist SKF82958. H3 agonist co-administration blocked the activation of MAPK signaling and the phosphorylation of rpS6 produced by D1 activation in D1-MSNs, paralleling behavioral effects. In contrast, GSK3β-Ser⁹ phosphorylation was seen only after H3 agonist treatment, with no interactive effects. H3R signaling has been neglected in models of basal ganglia function and has implications for a range of pathophysiologies.

Heteroreceptor Complexes Formed by Dopamine D1, Histamine H3, and N-Methyl-D-Aspartate Glutamate Receptors as Targets to Prevent Neuronal Death in Alzheimer's Disease

Alzheimer's disease (AD) is a neurodegenerative disorder causing progressive memory loss and cognitive dysfunction. Anti-AD strategies targeting cell receptors consider them as isolated units. However, many cell surface receptors cooperate and physically contact each other forming complexes having different biochemical properties than individual receptors. We here report the discovery of dopamine D₁, histamine H₃, and N-methyl-D-aspartate (NMDA) glutamate receptor heteromers in heterologous systems and in rodent brain cortex. Heteromers were detected by co-immunoprecipitation and in situ proximity ligation assays (PLA) in the rat cortex where H₃ receptor agonists, via negative cross-talk, and H₃ receptor antagonists, via cross-antagonism, decreased D₁ receptor agonist signaling determined by ERK1/2 or Akt phosphorylation, and counteracted D₁ receptor-mediated excitotoxic cell death. Both D₁ and H₃ receptor antagonists also counteracted NMDA toxicity suggesting a complex interaction between NMDA receptors and D₁-H₃ receptor heteromer function. Likely due to heteromerization, H₃ receptors act as allosteric regulator for D₁ and NMDA receptors. By bioluminescence resonance energy transfer (BRET), we demonstrated that D₁ or H₃ receptors form heteromers with NR1A/NR2B NMDA receptor subunits. D₁-H₃-NMDA receptor complexes were confirmed by BRET combined with fluorescence complementation. The endogenous expression of complexes in mouse cortex was determined by PLA and similar expression was observed in wild-type and APP/PS1 mice. Consistent with allosteric receptor-receptor interactions within the complex, H₃ receptor antagonists reduced NMDA or D₁ receptor-mediated excitotoxic cell death in cortical organotypic cultures. Moreover, H₃ receptor antagonists reverted the toxicity induced by ß_1-42-amyloid peptide. Thus, histamine H₃ receptors in D₁-H₃-NMDA heteroreceptor complexes arise as promising targets to prevent neurodegeneration.

The Sigma-1 Receptor as a Pluripotent Modulator in Living Systems

The sigma-1 receptor (Sig-1R) is an endoplasmic reticulum (ER) protein that resides specifically in the mitochondria-associated endoplasmic reticulum (ER) membrane (MAM), an interface between ER and mitochondria. In addition to being able to translocate to the plasma membrane (PM) to interact with ion channels and other receptors, Sig-1R also occurs at the nuclear envelope, where it recruits chromatin-remodeling factors to affect the transcription of genes. Sig-1Rs have also been reported to interact with other membranous or soluble proteins at other loci, including the cytosol, and to be involved in several central nervous system (CNS) diseases. Here, we propose that Sig-1R is a pluripotent modulator with resultant multiple functional manifestations in living systems.

Cocaine occupancy of sigma1 receptors and dopamine transporters in mice

Activation of sigma1 (σ1) receptors contributes to the behavioral and toxic effects of (-)-cocaine. We studied a key step, the ability of (-)-cocaine to occupy σ1 receptors in vivo, using CD-1(®) mice and the novel radioligand [(125) I]E-N-1-(3'-iodoallyl)-N'-4-(3",4"-dimethoxyphenethyl)-piperazine ([(125) I]E-IA-DM-PE-PIPZE). (-)-Cocaine displayed an ED50 of 68 μmol/kg for inhibition of specific radioligand binding in whole brain, with values between 73 and 80 μmol/kg for heart, lung, and spleen. For comparison, an ED50 of 26 μmol/kg for (-)-cocaine occupancy of striatal dopamine transporters (DAT) was determined by inhibition of [(125) I]3β-(4-iodophenyl)tropan-2β-carboxylic acid isopropyl ester ([(125) I]RTI-121) binding. A chief finding is the relatively small potency difference between (-)-cocaine occupancy of σ1 receptors and the DAT, although the DAT occupancy is likely underestimated. Interactions of (-)-cocaine with σ1 receptors were assessed further using [(125) I]E-IA-DM-PE-PIPZE for regional cerebral biodistribution studies and quantitative ex vivo autoradiography of brain sections. (-)-Cocaine binding to cerebral σ1 receptors proved directly proportional to the relative site densities known for the brain regions. Nonradioactive E-IA-DM-PE-PIPZE gave an ED50 of 0.23 μmol/kg for occupancy of cerebral σ1 receptors, and a 3.16 μmol/kg (i.p.) dose attenuated (-)-cocaine-induced locomotor hyperactivity by 30%. This effect did not reach statistical significance, but suggests that E-IA-DM-PE-PIPZE is a probable σ1 receptor antagonist. As groundwork for the in vivo studies, we used standard techniques in vitro to determine ligand affinities, site densities, and pharmacological profiles for the σ1 and σ2 receptors expressed in CD-1(®) mouse brain.

Protection by sigma-1 receptor agonists is synergic with donepezil, but not with memantine, in a mouse model of amyloid-induced memory impairments

Drugs activating the sigma-1 (σ1) chaperone protein are anti-amnesic and neuroprotective in neurodegenerative pathologies like Alzheimer's disease (AD). Since these so-called σ1 receptor (σ1R) agonists modulate cholinergic and glutamatergic systems in a variety of physiological responses, we addressed their putative additive/synergistic action in combination with cholinergic or glutamatergic drugs. The selective σ1 agonist PRE-084, or the non-selective σ1 drug ANAVEX2-73 was combined with the acetylcholinesterase inhibitor donepezil or the NMDA receptor antagonist memantine in the nontransgenic mouse model of AD-like memory impairments induced by intracerebroventricular injection of oligomeric Aβ25-35 peptide. Two behavioral tests, spontaneous alternation and passive avoidance response, were used in parallel and both protective and symptomatic effects were examined. After determination of the minimally active doses for each compound, the combinations were tested and the combination index (CI) calculated. Combinations between the σ1 agonists and donepezil showed a synergic protective effect, with CI<1, whereas the combinations with memantine showed an antagonist effect, with CI>1. Symptomatic effects appeared only additive for all combinations, with CI=1. A pharmacological analysis of the PRE-084+donepezil combination revealed that the synergy could be due to an inter-related mechanism involving α7 nicotinic ACh receptors and σ1R. These results demonstrated that σ1 drugs do not only offer a protective potential alone but also in combination with other therapeutic agents. The nature of neuromodulatory molecular chaperone of the σ1R could eventually lead to synergistic combinations.

Revealing G-protein-coupled receptor oligomerization at the single-molecule level through a nanoscopic lens: methods, dynamics and biological function

The introduction of super-resolution fluorescence microscopy has allowed the visualization of single proteins in their biological environment. Recently, these techniques have been applied to determine the organization of class A G-protein-coupled receptors (GPCRs), and to determine whether they exist as monomers, dimers and/or higher-order oligomers. On this subject, this review highlights recent evidence from photoactivated localization microscopy (PALM), which allows the visualization of single molecules in dense samples, and single-molecule tracking (SMT), which determines how GPCRs move and interact in living cells in the presence of different ligands. PALM has demonstrated that GPCR oligomerization depends on the receptor subtype, the cell type, the actin cytoskeleton, and other proteins. Conversely, SMT has revealed the transient dynamics of dimer formation, whereby receptors show a monomer-dimer equilibrium characterized by rapid association and dissociation. At steady state, depending on the subtype, approximately 30-50% of receptors are part of dimeric complexes. Notably, the existence of many GPCR dimers/oligomers is also supported by well-known techniques, such as resonance energy transfer methodologies, and by approaches that exploit fluorescence fluctuations, such as fluorescence correlation spectroscopy (FCS). Future research using single-molecule methods will deepen our knowledge related to the function and druggability of homo-oligomers and hetero-oligomers.

Molecular Mechanism: ERK Signaling, Drug Addiction, and Behavioral Effects

Addiction to psychostimulants has been considered as a chronic psychiatric disorder characterized by craving and compulsive drug seeking and use. Over the past two decades, accumulating evidence has demonstrated that repeated drug exposure causes long-lasting neurochemical and cellular changes that result in enduring neuroadaptation in brain circuitry and underlie compulsive drug consumption and relapse. Through intercellular signaling cascades, drugs of abuse induce remodeling in the rewarding circuitry that contributes to the neuroplasticity of learning and memory associated with addiction. Here, we review the role of the extracellular signal-regulated kinase (ERK), a member of the mitogen-activated protein kinase, and its related intracellular signaling pathways in drug-induced neuroadaptive changes that are associated with drug-mediated psychomotor activity, rewarding properties and relapse of drug seeking behaviors. We also discuss the neurobiological and behavioral effects of pharmacological and genetic interferences with ERK-associated molecular cascades in response to abused substances. Understanding the dynamic modulation of ERK signaling in response to drugs may provide novel molecular targets for therapeutic strategies to drug addiction.

Potential applications for sigma receptor ligands in cancer diagnosis and therapy

Sigma receptors (sigma-1 and sigma-2) represent two independent classes of proteins. Their endogenous ligands may include the hallucinogen N,N-dimethyltryptamine (DMT) and sphingolipid-derived amines which interact with sigma-1 receptors, besides steroid hormones (e.g., progesterone) which bind to both sigma receptor subpopulations. The sigma-1 receptor is a ligand-regulated molecular chaperone with various ion channels and G-protein-coupled membrane receptors as clients. The sigma-2 receptor was identified as the progesterone receptor membrane component 1 (PGRMC1). Although sigma receptors are over-expressed in tumors and up-regulated in rapidly dividing normal tissue, their ligands induce significant cell death only in tumor tissue. Sigma ligands may therefore be used to selectively eradicate tumors. Multiple mechanisms appear to underlie cell killing after administration of sigma ligands, and the signaling pathways are dependent both on the type of ligand and the type of tumor cell. Recent evidence suggests that the sigma-2 receptor is a potential tumor and serum biomarker for human lung cancer and an important target for inhibiting tumor invasion and cancer progression. Current radiochemical efforts are focused on the development of subtype-selective radioligands for positron emission tomography (PET) imaging. Right now, the mostpromising tracers are [18F]fluspidine and [18F]FTC-146 for sigma-1 receptors and [11C]RHM-1 and [18F]ISO-1 for the sigma-2 subtype. Nanoparticles coupled to sigma ligands have shown considerable potential for targeted delivery of antitumor drugs in animal models of cancer, but clinical studies exploring this strategy in cancer patients have not yet been reported. This article is part of a Special Issue entitled: Membrane channels and transporters in cancers.

Glyphosate Use Predicts ADHD Hospital Discharges in the Healthcare Cost and Utilization Project Net (HCUPnet): A Two-Way Fixed-Effects Analysis

There has been considerable international study on the etiology of rising mental disorders, such as attention-deficit hyperactivity disorder (ADHD), in human populations. As glyphosate is the most commonly used herbicide in the world, we sought to test the hypothesis that glyphosate use in agriculture may be a contributing environmental factor to the rise of ADHD in human populations. State estimates for glyphosate use and nitrogen fertilizer use were obtained from the U.S. Geological Survey (USGS). We queried the Healthcare Cost and Utilization Project net (HCUPNET) for state-level hospitalization discharge data in all patients for all-listed ADHD from 2007 to 2010. We used rural-urban continuum codes from the USDA-Economic Research Service when exploring the effect of urbanization on the relationship between herbicide use and ADHD. Least squares dummy variable (LSDV) method and within method using two-way fixed effects was used to elucidate the relationship between glyphosate use and all-listed ADHD hospital discharges. We show that a one kilogram increase in glyphosate use, in particular, in one year significantly positively predicts state-level all-listed ADHD discharges, expressed as a percent of total mental disorders, the following year (coefficient = 5.54E-08, p<.01). A study on the effect of urbanization on the relationship between glyphosate and ADHD indicates that the relationship is marginally significantly positive after multiple comparison correction only in urban U.S. counties (p<.025). Furthermore, total glyphosate use is strongly positively associated with total farm use of nitrogen fertilizers from 1992 to 2006 (p<.001). We present evidence from the biomedical research literature of a plausible link among glyphosate, nitrogen dysbiosis and ADHD. Glyphosate use is a significant predictor of state hospitalizations for all-listed ADHD hospital discharges, with the effect concentrated in urban U.S. counties. This effect is seen even after controlling for individual state characteristics, strong correlations over time, and other significant associations with ADHD in the literature. We draw upon the econometric results to propose unique mechanisms, borrowing principles from soil and atmospheric sciences, for how glyphosate-based herbicides may be contributing to the rise of ADHD in all populations.

Histamine and histamine receptors in Tourette syndrome and other neuropsychiatric conditions

The potential contributions of dysregulation of the brain's histaminergic modulatory system to neuropsychiatric disease, and the potential of histamine-targeting medications as therapeutic agents, are gradually coming into focus. The H3R receptor, which is expressed primarily in the central nervous system, is a promising pharmacotherapeutic target. Recent evidence for a contribution of histamine dysregulation to Tourette syndrome and tic disorders is particularly strong; although specific mutations in histamine-associated genes are rare, they have led to informative studies in animal models that may pave the way for therapeutic advances. A controlled study of an H3R antagonist in Tourette syndrome is ongoing. Preclinical studies of H3R antagonists in schizophrenia, attention deficit disorder, and narcolepsy have all shown promise. Recently reported controlled studies have been disappointing in schizophrenia and attention deficit disorder, but the H3R antagonist pitolisant shows promise in the treatment of narcolepsy and excessive daytime sleepiness and is currently under regulatory review for these conditions. This article is part of the Special Issue entitled 'Histamine Receptors'.