Discrepancies in characterization of sigma sites in the mouse central nervous system

Hazards of Using Masking Protocols When Performing Ligand Binding Assays: Lessons From the Sigma-1 and Sigma-2 Receptors

Sigma-1 and sigma-2 receptors are emerging therapeutic targets. Although the molecular identity of the sigma-2 receptor has recently been determined, receptor quantitation has used, and continues to use, the sigma-1 selective agents (+) pentazocine or dextrallorphan to mask the sigma-1 receptor in radioligand binding assays. Here, we have assessed the suitability of currently established saturation and competition binding isotherm assays that are used to quantify parameters of the sigma-2 receptor. We show that whilst the sigma-1 receptor mask (+) pentazocine has low affinity for the sigma-2 receptor (K_i 406 nM), it can effectively compete at this site with [³H] di-O-tolyl guanidine (DTG) at the concentrations frequently used to mask the sigma-1 receptor (100 nM and 1 µM). This competition influences the apparent affinity of DTG and other ligands tested in this system. A more troublesome issue is that DTG can displace (+) pentazocine from the sigma-1 receptor, rendering it partly unmasked. Indeed, commonly used concentrations of (+) pentazocine, 100 nM and 1 µM, allowed 37 and 11% respectively of sigma-1 receptors to be bound by DTG (300 nM), which could result in an overestimation of sigma-2 receptor numbers in assays where sigma-1 receptors are also present. Similarly, modelled data for 1 µM dextrallorphan show that only 71–86% of sigma-1 receptors would be masked in the presence of 300 nM DTG. Therefore, the use of dextrallorphan as a masking agent would also lead to the overestimation of sigma-2 receptors in systems where sigma-1 receptors are present. These data highlight the dangers of using masking agents in radioligand binding studies and we strongly recommend that currently used masking protocols are not used in the study of sigma-2 receptors. In order to overcome these problems, we recommend the use of a cell line apparently devoid of sigma-1 receptors [e.g., MCF7 (ATCC HTB-22)] in the absence of any masking agent when determining the affinity of agents for the sigma-2 receptor. In addition, assessing the relative levels of sigma-1 and sigma-2 receptors can be achieved using [³H] DTG saturation binding followed by two-site analysis of (+) pentazocine competition binding with [³H] DTG.

Structural basis for σ1 receptor ligand recognition

The σ₁ receptor is a poorly understood membrane protein expressed throughout the human body. Ligands targeting the σ₁ receptor are in clinical trials for treatment of Alzheimer's disease, ischemic stroke, and neuropathic pain. However, relatively little is known regarding the σ₁ receptor's molecular function. Here, we present crystal structures of human σ₁ receptor bound to the antagonists haloperidol and NE-100, and the agonist (+)-pentazocine, at crystallographic resolutions of 3.1 Å, 2.9 Å, and 3.1 Å, respectively. These structures reveal a unique binding pose for the agonist. The structures and accompanying molecular dynamics (MD) simulations identify agonist-induced structural rearrangements in the receptor. Additionally, we show that ligand binding to σ₁ is a multistep process that is rate limited by receptor conformational change. We used MD simulations to reconstruct a ligand binding pathway involving two major conformational changes. These data provide a framework for understanding the molecular basis for σ₁ agonism.

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.

Antinociception by Sigma-1 Receptor Antagonists: Central and Peripheral Effects

There is plenty of evidence supporting the modulatory role of sigma-1 receptors (σ1Rs) in nociception, mainly based on the pain-attenuated phenotype of σ1R knockout mice and on the antinociceptive effect exerted by σ1R antagonists, particularly in nonacute sensitizing conditions involving sustained afferent drive, activity-dependent plasticity/sensitization, and ultimately pain hypersensitivity, as it is the case in chronic pains of different etiology. Antinociceptive effects of σ1R antagonists both when acting alone and in combination with opioids (to enhance opioid analgesia) have been reported at both central and peripheral sites. At the central level, findings at the behavioral (animal pain models), electrophysiological (spinal wind-up recordings), neurochemical (spinal release of neurotransmitters) and molecular (NMDAR function) level supports a role for σ1R antagonists in inhibiting augmented excitability secondary to sustained afferent input. Attenuation of activity-induced plastic changes (central sensitization) following tissue injury/inflammation or nerve damage could thus underlie the central inhibitory effect of σ1R antagonists. Moreover, recent pieces of information confirm the involvement of σ1R in mechanisms regulating pain at the periphery, where σ1Rs are highly expressed, particularly in dorsal root ganglia. Indeed, local peripheral administration of σ1R antagonists reduces inflammatory hyperalgesia. Potentiation of opioid analgesia is also supported, particularly at supraspinal sites and at the periphery, where locally administered σ1R antagonists unmask opioid analgesia. Altogether, whereas σ1R activation is coupled to pain facilitation and inhibition of opioid antinociception, σ1R antagonism inhibits pain hypersensitivity and "releases the brake" enabling opioids to exert enhanced antinociceptive effects, both at the central nervous system and at the periphery.

Characterization of pulmonary sigma receptors by radioligand binding

This study establishes the expression of appreciable populations of sites on mouse lung membranes that exhibit radioligand binding properties and pharmacology consistent with assignment as sigma1 and sigma2 receptors. Specific binding of the sigma1 receptor radioligand [(3)H](+)-pentazocine reached steady state within 6h at 37°C. Saturation studies revealed high affinity binding to a single class of sites (Kd 1.36±0.04nM; Bmax 967±11fmol/mg protein). Inhibition studies showed appropriate sigma1 receptor pharmacology, including higher affinity for (+)-N-allylnormetazocine with respect to the (-)-enantiomer, and positive allosteric modulation of dextromethorphan binding by phenytoin. Using [(3)H]1,3-di(2-tolyl)guanidine in the presence of (+)-pentazocine to assess sigma2 receptor binding, steady state was achieved within 2min at 25°C. Cold saturation studies revealed one high affinity, low capacity binding site (Kd 31.8±8.3nM; Bmax 921±228fmol/mg protein) that displayed sigma2 receptor pharmacology. A very low affinity, high capacity interaction also was observed that represents saturable, but not sigma receptor specific, binding. A panel of ligands showed rank order inhibition of radioligand binding appropriate for the sigma2 receptor, with ifenprodil displaying the highest apparent affinity. In vivo, dextromethorphan inhibited the specific binding of a radioiodinated sigma1 receptor ligand in lung with an ED50 of 1.2μmol/kg, a value near the recommended dosage for the drug as a cough suppressant. Overall, the present work provides a foundation for studies of drug interactions with pulmonary sigma1 and sigma2 receptors in vitro and in vivo.

Relationship between cerebral sigma-1 receptor occupancy and attenuation of cocaine's motor stimulatory effects in mice by PD144418

Psychostimulant effects of cocaine are mediated partly by agonist actions at sigma-1 (σ1) receptors. Selective σ1 receptor antagonists attenuate these effects and provide a potential avenue for pharmacotherapy. However, the selective and high affinity σ1 antagonist PD144418 (1,2,3,6-tetrahydro-5-[3-(4-methylphenyl)-5-isoxazolyl]-1-propylpyridine) has been reported not to inhibit cocaine-induced hyperactivity. To address this apparent paradox, we evaluated aspects of PD144418 binding in vitro, investigated σ1 receptor and dopamine transporter (DAT) occupancy in vivo, and re-examined effects on locomotor activity. PD144418 displayed high affinity for σ1 sites (Ki 0.46 nM) and 3596-fold selectivity over σ2 sites (Ki 1654 nM) in guinea pig brain membranes. No appreciable affinity was noted for serotonin and norepinephrine transporters (Ki >100 μM), and the DAT interaction was weak (Ki 9.0 μM). In vivo, PD144418 bound to central and peripheral σ1 sites in mouse, with an ED50 of 0.22 μmol/kg in whole brain. No DAT occupancy by PD144418 (10.0 μmol/kg) or possible metabolites were observed. At doses that did not affect basal locomotor activity, PD144418 (1, 3.16, and 10 μmol/kg) attenuated cocaine-induced hyperactivity in a dose-dependent manner in mice. There was good correlation (r(2) = 0.88) of hyperactivity reduction with increasing cerebral σ1 receptor occupancy. The behavioral ED50 of 0.79 μmol/kg corresponded to 80% occupancy. Significant σ1 receptor occupancy and the ability to mitigate cocaine's motor stimulatory effects were observed for 16 hours after a single 10.0 μmol/kg dose of PD144418.

Synthesis, radioiodination and in vitro and in vivo sigma receptor studies of N-1-allyl-N´-4-phenethylpiperazine analogs

INTRODUCTION

Sigma-1 (σ(1)) receptor radioligands are useful for basic pharmacology studies and for imaging studies in neurology, psychiatry and oncology. We derived a hybrid structure, N-1-allyl-N´-4-phenethylpiperazine, from known ligands TPCNE and SA4503 for use as a scaffold for development of radioiodinated σ(1) receptor ligands.

METHODS

E-and Z-N-1-(3'-iodoallyl)-N´-4-(3″,4″-dimethoxyphenethyl)-piperazine (E-1 and Z-1), N-1-allyl-N´-4-(3',4'-dimethoxyphenethyl)-piperazine (2) and E-N-1-(3'-iodoallyl)-N´-4-(3″-methoxy-4'´-hydroxyphenethyl)-piperazine (3) were synthesized. Affinities for σ(1) and σ(2) receptors were determined. [(125)I]E-1 and [(125)I]Z-1 were prepared and evaluated in vivo in mice. [(125)I]E-1 was further evaluated in σ(1) receptor binding assays in vitro.

RESULTS

E-1 displayed moderately high apparent affinity (15 nM) for σ(1) sites and 84-fold selectivity against σ(2) sites. Z-1 showed similar σ(1) affinity, but only 23-fold selectivity. In contrast, 2 exhibited poor binding to both subtypes, while 3 had good affinities but poor selectivity. E-1 profiled as a probable antagonist in the phenytoin shift assay. [(125)I]E-1 and [(125)I]Z-1 were prepared in good yields and with high specific radioactivities. Log D(7.4) values (2.25 and 2.27) fall within the optimal range for in vivo studies. Both radioligands selectively labeled σ(1) receptors in mouse brain and peripheral organs in vivo. [(125)I]E-1 showed a higher level of specific binding than [(125)I]Z-1 and displayed good metabolic stability. Further, [(125)I]E-1 selectively labeled σ(1) receptors in mouse brain homogenates (K(d) 3.79 nM; B(max)=599 fmol/mg protein).

CONCLUSIONS

[(125)I]E-1 is a selective σ(1) receptor radioligand that exhibits properties amenable to in vitro and in vivo studies, with possible extension to single photon emission computed tomography using iodine-123.

Sigma receptors: potential targets for a new class of antidepressant drug

Despite the widespread and devastating impact of depression on society, our current understanding of its pathogenesis is limited. Likewise, existing treatments are inadequate, providing relief to only a subset of people suffering from depression. The search for more effective antidepressant drugs includes the investigation of new molecular targets. Among them, current data suggests that sigma receptors are involved in multiple processes effecting antidepressant-like actions in vivo and in vitro. This review summarizes accumulated evidence supporting a role for sigma receptors in antidepressant effects and provides a conceptual framework for delineating their potential roles over the course of antidepressant treatment.

Pre- versus postformalin effects of ketamine or large-dose alfentanil in the rat: discordance between pain behavior and spinal Fos-like immunoreactivity

The purpose of this animal investigation was to compare behavioral responses with spinal Fos-like immunoreactivity (FLI) after pre-versus postformalin administration of anesthetic doses of IV ketamine or alfentanil. Preformalin and postformalin injection (1.5% subcutaneously) treatment groups included IV saline control (1.5 mL/kg), ketamine (10 mg/kg), and alfentanil (170 microg/kg). In the behavioral study group, nociceptive behavior was evaluated 15-60 min after hindpaw formalin injection. In the spinal FLI study group, rats were perfused 2 h postformalin, and spinal cords were dissected, sliced at 30 microm, and processed by immunoperoxidase staining with an antibody against the Fos protein. Quantification and determination of the laminar distribution of Fos-labeled nuclei were performed at the L4-5 spinal level ipsilateral to formalin injection. Ketamine produced a selective preemptive analgesic effect in behavioral formalin experiments, yet failed to suppress spinal FLI. In contrast, alfentanil failed to demonstrate a selective preemptive analgesia in behavioral experiments, but did produce preemptive suppression of spinal FLI. Together with previous data from our laboratory, we conclude that behavioral analgesia and spinal Fos expression may be uncoupled under certain circumstances.

IMPLICATIONS

In this study, we compared pain reduction produced by IV drugs (ketamine or alfentanil) with the ability to prevent injury-induced spinal cord changes. We measured pain behavior and spinal Fos protein after rats received ketamine or alfentanil before versus after formalin injection. Fos inhibition patterns did not clearly correlate with pain reduction, providing further evidence that Fos inhibition is not always predictive of behavioral analgesia.

Up-regulation of [3H]DTG but not [3H](+)-pentazocine labeled sigma sites in mouse spinal cord by chronic morphine treatment

To monitor the possible effect of morphine on sigma sites, binding characteristics of [3H](+)-pentazocine and [3H]1,3-di-(2-tolyl)guanidine (DTG) to brain and spinal cord membranes of morphine-treated and control mice were compared. For morphine treatment, a single injection (100 mg/kg, s.c.) of morphine was followed 4 h later by pellet implantation (75 mg morphine free base). Animals were sacrificed 24, 72 h or 7 days later. The equilibrium dissociation value (Kd) and the density (Bmax) of [3H](+)-pentazocine binding remained unaffected by morphine treatment. Also, no change was found in Kd and Bmax values of [3H]DTG labeled sigma2 subtypes after any morphine treatment schedule when measured in the presence of 100 nM (+)-pentazocine. However, the Bmax of [3H]DTG binding in the spinal cord in the absence of 100 nM (+)-pentazocine, was significantly elevated 72 h after implantation of the morphine pellet and recovered by 7 days, a time when the antinociceptive effect produced by the morphine pellet had dissipated. These data suggest that one population of sigma sites, that has a high affinity for [3H]DTG, but is not equivalent with the [3H](+)-pentozocine labeled sigma1 subtype or the [3H]DTG labeled sigma2 subtype, is upregulated by morphine and, therefore, may play a role in the development of tolerance to or dependence on the effects of morphine.

Zn2+ inhibition of [3H]MK-801 binding is different in mouse brain and spinal cord: effect of glycine and glutamate

Zn2+ inhibits NMDA-type excitatory amino acid activity by a non-competitive action. Based on regional differences in the central nervous system (CNS) in binding characteristics of [3H](+)-5-methyl-10,11-dihydro-5H-dibenzo[a,d]cyclohepten-5,10-imi ne maleate ([3H]MK-801) and other non-competitive antagonists of NMDA used to label open channels in the receptor complex, we compared the inhibitory influence of Zn2+ on [3H]MK-801 binding in whole mouse brain and spinal cord membranes. Radioligand binding techniques were used in the presence and absence of maximally effective concentrations of glycine and glutamate. Using extensively washed membranes without exogenous glycine and glutamate, Zn2+ was found to be a weaker inhibitor of the [3H]MK-801-labeled site in the spinal cord than in the whole brain. In contrast, exogenous glycine and glutamate decreased the inhibitory effect of Zn2+ in the brain but dramatically increased the inhibitory effect of Zn2+ in the spinal cord. Thus the inhibitory effect of Zn2+ in the spinal cord appears to be magnified by glutamatergic and glycinergic activity while that in the brain is not. The different actions of Zn2+ may be attributable to the differential distribution of NMDA receptor subunits in the mouse brain and spinal cord.