Putative sigma(3) sites in mammalian brain have histamine H(1) receptor properties: evidence from ligand binding and distribution studies with the novel H(1) radioligand [(3)H]-(-)-trans-1-phenyl-3-aminotetralin

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 expression and functional characterization of sigma (sigma) 1 receptors in breast cancer cell lines

Sigma (sigma) receptors have been implicated in cancer. However, to date there is little molecular data demonstrating the role of sigma1 receptors in cancer. Expression of sigma1 receptors in various human cancer cell lines in comparison to non-cancerous cell lines was investigated, using real time RT-PCR and by western blotting with a sigma1 receptor specific antibody. Our results indicate that cancer cells express higher levels of sigma1 receptors than corresponding non-cancerous cells. Localization of the sigma1 receptor was investigated in MDA-MB-231 cells by immunocytochemistry and confocal microscopy, expression was visualized predominantly at the cell periphery. We have tested the effect of sigma1 and sigma2 drugs and a sigma1 receptor silencing construct on various aspects of the metastatic process on two breast cell lines of different metastatic potential and a normal breast cell line. Both sigma1 and sigma2 drugs and the sigma1 receptor silencing construct had effects on proliferation and adhesion for breast cancer cell lines, compared to a non-cancerous breast cell line. This data suggests sigma1 receptor plays a role in proliferation and adhesion of breast cancer cells. Therefore, it is likely to be a potential target for the diagnosis and therapy of breast cancer.

Novel ligands for the human histamine H1 receptor: Synthesis, pharmacology, and comparative molecular field analysis studies of 2-dimethylamino-5-(6)-phenyl-1,2,3,4-tetrahydronaphthalenes

This paper reports the synthesis of a novel series of (+/-)-2-dimethylamino- 5- and 6-phenyl-1,2,3,4-tetrahydronaphthalene derivatives (5- and 6-APTs), and, corresponding affinity, functional activity, and, molecular modeling studies with regard to drug design targeting the human histamine H1 receptor. The 5-APTs have 2- to 4-fold higher H1 receptor affinity than the endogenous agonist histamine. The chemical nature of a meta-substituent on the 5-APT pendant phenyl moiety does not significantly affect H1 affinity. In contrast, analogous meta-substitution for the 6-APTs increases H1 affinity up to 100-fold. The new APTs do not activate H1 receptor-linked intracellular signaling and apparently are competitive H1 antagonists. A new model that establishes structural parameters for binding to the human H1 receptor by APTs and other ligands was developed using 3-D QSAR (CoMFA). The model predicts H1 ligand binding with a higher degree of external predictability compared to a previously reported model. The APTs also were examined for activity at human serotonin 5-HT2A and 5-HT2C receptors, which are phylogenetically closely related to the H1 receptor. 5-APT and m-Cl-6-APT were identified as novel agonists that selectively activate 5-HT2C receptors. It is concluded that the lipophilic (brain-penetrating) APT molecular scaffold may have pharmacotherapeutic potential in neuropsychiatric diseases.

Role of PKA and PKC in histamine H1 receptor-mediated activation of catecholamine neurotransmitter synthesis

Activation of the histamine H1 receptor stimulates tyrosine hydroxylase (TH) to increase catecholamine neurotransmitter synthesis in mammalian brain and adrenal tissues. Histamine non-selectively activates both H1-linked phospholipase (PL) C/inositol phosphates (IP)/diacylglycerol (DAG) signaling and adenylyl cyclase (AC)/adenosine 3',5'-cyclic monophosphate (cAMP) signaling, confounding determination of signaling events involved in H(1)-mediated TH activation. This research uses two new functionally-selective H1 agonists, cis-PAB and trans-PAT, that selectively activate H1/PLC/IP/DAG and H1/AC/cAMP signaling, respectively, to characterize H(1)-mediated activation of TH in rat striatum and bovine adrenal chromaffin (BAC) cells. Histamine, cis-PAB, and trans-PAT produced a two-fold maximal TH activation by an H1 receptor mechanism in rat striatum and BAC cells. Histamine is more potent and efficacious in BAC cells (EC50 approximately 0.2 microM, Emax approximately 200% basal) versus rat striatum (EC50 approximately 0.4 microM; Emax approximately 150%). Cis-PAB and trans-PAT are more potent in rat striatum (EC50 approximately 0.1 microM for both agonists) versus BAC cells (EC50 approximately 1.0 microM for both), with similar efficacy in both preparations (Emax approximately 160% for both agonists). Signaling studies in BAC cells revealed that protein kinase (PK) A but not PKC is involved in H1 -mediated TH activation by trans-PAT and histamine, while, both PKA and PKC are involved for cis-PAB. Results for cis-PAB suggest H1/PLC/IP/DAG/PKC signaling activates PKA, downstream of cAMP formation, indicating apparent direct activation of PKA by PKC.

Sigma Receptors and Cancer

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

Ligand-directed functional heterogeneity of histamine H1 receptors: novel dual-function ligands selectively activate and block H1-mediated phospholipase C and adenylyl cyclase signaling

The autacoid and neurotransmitter histamine activates the H(1) G protein-coupled receptor (GPCR) to stimulate predominantly phospholipase C (PLC)/inositol phosphate (IP) signaling and, to a lesser extent, adenylyl cyclase (AC)/cAMP signaling in a variety of mammalian cells and tissues, as well as H(1)-transfected clonal cell lines. This study reports that two novel H(1) receptor ligands developed in our laboratory, (-)-trans-1-phenyl-3-dimethylamino-1,2,3,4-tetrahydronaphthalene (trans-PAT) and (+/-)-cis-5-phenyl-7-dimethylamino-5,6,7,8-tetrahydro-9H-benzocycloheptane (cis-PAB), activate H(1) receptors to selectively stimulate AC/cAMP formation and PLC/IP formation, respectively, in Chinese hamster ovary cells transfected with guinea pig H(1) receptor cDNA. trans-PAT and cis-PAB also are shown to be functionally selective antagonists of H(1)-linked PLC/IP and AC/cAMP signaling, respectively. Whereas cis-PAB H(1) receptor activity is shown to be typically competitive, trans-PAT displays a complex interaction with the H(1) receptor that is not competitive regarding antagonism of saturation binding by the standard H(1) antagonist radioligand [(3)H]mepyramine or H(1)/PLC/IP functional activation by histamine. trans-PAT, however, does competitively block H(1)/PLC/IP functional activation by cis-PAB. Molecular determinants for trans-PAT versus cis-PAB differential binding to H(1) receptors, which presumably leads to differential activation of AC/cAMP versus PLC/IP signaling, likely involves stereochemical factors as well as more subtle steric influences. Results suggest the trans-PAT and cis-PAB probes will be useful to study molecular mechanisms of ligand-directed GPCR multifunctional signaling. Moreover, because most untoward cardiovascular-, respiratory-, and gastrointestinal H(1) receptor-mediated effects proceed via the PLC/IP pathway, PAT-type agonists that selectively enhance H(1)-mediated AC/cAMP signaling provide a mechanistic basis for exploiting H(1) receptor activation for drug design purposes.

Domain swapping in the human histamine H1 receptor

G-protein-coupled receptors (GPCRs) represent the largest family of receptors involved in transmembrane signaling. Although these receptors were generally believed to be monomeric entities, accumulating evidence supports the presence of GPCRs in multimeric forms. Here, using immunoprecipitation as well as time-resolved fluorescence resonance energy transfer to assess protein-protein interactions in living cells, we unambiguously demonstrate the occurrence of dimerization of the human histamine H(1) receptor. We also show the presence of domain-swapped H(1) receptor dimers in which there is the reciprocal exchange of transmembrane domain TM domains 6 and 7 between the receptors present in the dimer. Mutation of aspartate(107) in transmembrane (TM) 3 or phenylalanine(432) in TM6 to alanine results in two radioligand-binding-deficient mutant H(1) receptors. Coexpression of H(1)D(107) A and H(1)F(432)A, however, results in a reconstituted radioligand binding site that exhibits a pharmacological profile that corresponds to the wild-type H(1) receptor. Interestingly, the H(1) receptor radioligands [(3)H]mepyramine and [(3)H]-(-)-trans-1-phenyl-3-N,N-dimethylamino-1,2,3,4-tetrahydronaphthalene show differential saturation binding values (B(max)) for wild-type H(1) receptors but not for the radioligand binding site that is formed upon coexpression of H(1) D(107)A and H(1) F(432)A receptors, suggesting the presence of different H(1) receptor populations.

Evidence of histamine receptors in fish brain using an in vivo [14C]2-deoxyglucose autoradiographic method and an in vitro receptor-binding autoradiographic method

It was hypothesized that fish possess functioning H1 histamine receptors that have the ability to bind agonists and antagonists specific to the H1 histamine receptor subtype. For these experiments, a combination of a novel, in vivo 2-deoxyglucose method and a standard in vitro autoradiography procedure was utilized. A regional, statistically significant dose response in neurological functioning was observed when fish were exposed to histaminergic agents (i.e., H1 agonists and antagonists), which created the first neurological profile for the H1 histamine receptor in fish brain. The H1 histamine receptor was chosen as a characterization receptor in fish because histamine has been linked to a variety of neurological functions such as the control of arousal, attention, sensory processing, and cognition. Histamine also plays a role in pituitary hormone secretion, appetite control, and, potentially, regulation of vestigular reactivity. In addition, the fish brain is well characterized structurally, and the existence of an H3-like receptor has been documented recently in zebrafish. However, to date there is little detailed information about specific localization and functioning of the H1 histamine receptor in fish.

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

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

A novel phenylaminotetralin radioligand reveals a subpopulation of histamine H(1) receptors

Previously, (-)-trans-1-phenyl-3-N,N-dimethylamino-1,2,3,4-tetrahydronaphthalene ([-]-trans-H(2)-PAT) was shown to activate stereospecifically histamine H(1) receptors coupled to modulation of tyrosine hydroxylase activity in guinea pig and rat forebrain in vitro and in vivo. Furthermore, the novel radioligand [(3)H](-)-trans-H(2)-PAT was shown to label selectively H(1) receptors in guinea pig and rat brain with high affinity (K(D), ~0.1 and 0.5 nM, respectively) and a B(max) about 50 and 15%, respectively, of that observed for the H(1) antagonist radioligand [(3)H]mepyramine. In the current study, [(3)H](-)-trans-H(2)-PAT-labeled cloned guinea pig and human H(1) receptors in Chinese hamster ovary (CHO) cell membranes with high affinity (K(D), ~0.08 and 0.23 nM, respectively) and a B(max) about 15% of that observed for [(3)H]mepyramine. The binding of H(2)-PAT to H(1) receptors in both CHO-H(1) cell lines was stereoselective with the (-)-trans-isomer having affinity (K(i), ~1.5 nM) about 4-, 20-, and 50-times higher than the (-)-cis-, (+)-trans-, and (+)-cis-isomers, respectively; the affinity of (-)-trans-H(2)-PAT was unaffected by excess GTP. In functional assays, (-)-trans-H(2)-PAT was a full antagonist of histamine H(1)-mediated stimulation of phospholipase C (PLC) and [(3)H]inositol phosphates (IP) formation in CHO-H(1) cells, a full inverse agonist of constitutively active H(1) receptors in COS-7-H(1) cells, and a full competitive antagonist (pA(2) = 9.2) of histamine H(1)-mediated contraction of guinea pig ileum. It is concluded that (-)-trans-H(2)-PAT is an antagonist at H(1) receptors coupled to PLC/IP formation and smooth muscle contraction. Meanwhile, the observation that [(3)H](-)-trans-H(2)-PAT labels only a subpopulation of H(1) receptors and that (-)-trans-H(2)-PAT activates H(1) receptors coupled to modulation of tyrosine hydroxylase suggests that there may be post-translational H(1) receptor heterogeneity.

MDMA ('ecstasy') enhances basal acetylcholine release in brain slices of the rat striatum

The pharmacological basis of acute (+/-)-MDMA (3, 4-methylenedioxymethamphetamine) intoxication still awaits full characterization. According to present knowledge, MDMA enhances the release of serotonin and dopamine in striatal slices and interacts with different types of receptors such as 5-HT2 (5-hydroxytryptamine or serotonin), M1 and M2 muscarinic acetylcholine (ACh), and histamine H1 receptors. Currently, no information is available about the influence of (+/-)-MDMA on striatal cholinergic neurotransmission. In the present study, we used the in vitro perfusion technique to investigate the effect of (+/-)-MDMA on ACh release in rat striatal slices. Perfusions with (+/-)-MDMA (10-300 microM) resulted in a dose-dependent increase of spontaneous ACh release (EC50 approximately 30 microM). The effect was reversible and Ca++- and tetrodotoxin-sensitive. To determine the neurochemical pathways underlying this response, we perfused with (+/-)-MDMA in the presence of various inhibitors of neurotransmitter receptors. Blockade of glutamate or muscarinic ACh receptors as well as 5-HT1, 5-HT2, 5-HT3C or dopamine D2 receptors did not modulate (+/-)-MDMA-induced ACh release. However, the presence of histamine H1 receptor antagonists in the perfusion medium abolished (+/-)-MDMA-induced ACh release. The present data clearly demonstrate that (+/-)-MDMA enhances the activity of striatal cholinergic neurons and suggest an involvement of histamine H1 receptors. The effect is not mediated by glutamate and does not involve the activation of receptors of dopamine D2, 5-HT1, 5-HT2, 5-HT3C or muscarinic ACh. Considering the relatively high affinity of (+/-)-MDMA for the H1 histamine receptor (Ki 6 microM), a direct activation of this type of receptor might represent a plausible mechanism for (+/-)-MDMA-induced ACh release.

A novel phenylaminotetralin (PAT) recognizes histamine H1 receptors and stimulates dopamine synthesis in vivo in rat brain

A series of novel phenylaminotetralins (PATs) previously was shown to recognize discrete binding sites that are stereoselectively labeled by [3H]-(-)-trans-1-phenyl-3-N,N-dimethylamino-1,2,3,4-tetrahydronaphthalen e (H2-PAT) and highly localized in catecholaminergic nerve terminal regions in guinea pig forebrain. Furthermore, certain PATs stimulate tyrosine hydroxylase and dopamine synthesis in guinea pig and rat brain in vitro. In the current studies, we characterized sites labeled by [3H]-(-)-trans-H2-PAT and measured effects of PATs on dopamine synthesis in vivo in rat brain. [3H]-(-)-Trans-H2-PAT binds saturably (Bmax approximately 13 fmol/mg protein) and with high affinity (K(D) approximately 0.5 nM) to a single population of sites in rat brain. The ligand binding profile of [3H]-(-)-trans-H2-PAT labeled sites is very similar to histamine H1 receptors labeled with [3H]-mepyramine. After i.c.v. injection to rats, (+/-)-trans H2-PAT (4-40 nmoles/kg) stimulates dopamine synthesis (to about 180% of control levels) selectively in the limbic brain region nucleus accumbens vs. the extrapyramidal region striatum; this effect is fully blocked by (+/-)-cis-H2-PAT and the H1 antagonist triprolidine. At higher doses (> 40 nmoles/kg), the observed stimulation of dopamine synthesis is attenuated to control levels, likely due to activation of feedback mechanisms resulting from non-receptor mediated displacement of intraneuronal dopamine. We propose that PATs represent a novel class of ligands for H1 receptors that can modulate tyrosine hydroxylase activity and dopamine synthesis in the limbic region of mammalian forebrain.