The effect of prototypic sigma ligands on the binding of [3H]dextromethorphan to guinea pig brain

[Recent Findings on the Mechanism of Cough Hypersensitivity as a Cause of Chronic Cough]

An increasing number of patients complain to medical institutions about a cough that persists for more than 8 weeks, namely chronic cough. The cough observed in patients with chronic cough is not responsive to conventional antitussive agents such as dihydrocodeine and dextromethorphan, and this is a major clinical problem. The most common pathology of chronic cough in Japan is dry cough. Two causes of dry cough are increased sensitivity of cough receptors (cough hypersensitivity) and increased contraction of bronchial smooth muscle. Among these, the mechanisms of cough hypersensitivity are diverse, and understanding these mechanisms is important for the diagnosis and treatment of chronic cough. In this paper I will review the regulatory mechanisms of cough hypersensitivity, especially the regulation of Aδ fiber excitability by C fibers. Furthermore, the central mechanisms involved cough reflex are discussed in relation to central acting antitussives.

Effects of rimcazole, a specific antagonist of sigma sites, on the antitussive effects of non-narcotic antitussive drugs

We examined the effects of rimcazole, a specific antagonist of sigma sites, on the antitussive effects of dextromethorphan and noscapine in mice. Intraperitoneal injection of rimcazole, in doses from 1 to 10 mg/kg, significantly and dose dependently antagonized the cough depressant effect of N,N'-di(orthotolyl)guanidine (DTG), a sigma ligand. The cough depressant effects of dextromethorphan (3 mg/kg i.p.) and noscapine (10 mg/kg i.p.) were also significantly and dose dependently reduced by pretreatment with rimcazole. However, rimcazole (10 mg/kg i.p.) did not have a significant effect on the antitussive effect of morphine (3 mg/kg i.p.). Furthermore, rimcazole by itself (10 and 30 mg/kg i.p.) had no significant effect on the number of coughs. These results suggest that sigma sites may be involved in the antitussive mechanism of centrally acting non-narcotic antitussive drugs.

Lacrimal secretion stimulants: sigma receptors and drug implications

3H-DTG (1.3-di(2-[5-3H]tolyl)guanidine) or 3H-haloperidol was added to sigma-receptors (25 nM) in the presence of 25 nM spiperone and incubated with increasing concentrations of bromhexine derivatives (phenylalkylamines; 10(-9) to 10(-2)M) in membrane homogenate suspensions. IC50 values for two derivatives ranged from 3.2 to 8.8 nM for both radioligands. A preferred derivative, 7A (N,N'-dimethyl-2-phenyl-ethylamine), yielded an IC50 of 7.8 nM for 3H-haloperidol but showed much less affinity in displacing 3H-DTG (IC50 = 900 nM). Applying the technic of Bromberg [Exp. Eye Res., 40:313-320, 1985], in vitro protein secretion rates were measured following stimulation of either lacrimal gland slices or isolated, intact lacrimocytes with the compounds. In vitro protein secretion rates exhibit a dose-response relationship with increases in protein release up to a concentration of 10(-8) to 10(-4) M for various derivatives of bromhexine and 10(-4) M for carbachol. By means of Schirmer strips, tear fluid was collected over a five minute period at 10 and 60 minutes post-dosing following the topical application (50 microliters) to the right eye of New Zealand white rabbits (n = 20-24) of 7A at various concentrations. Incubation of lacrimocytes with 7A alone (10(-4) M), with haloperidol (10(-4) M) alone or in combination show that 7A is acting as an agonist to stimulate protein release, whereas haloperidol is acting as an antagonist to inhibit release. In vivo protein secretion rates also show a dose-response curve (at both 10 and 60 minutes post-dosing) for 7A that reach a statistically significant maximum in the dosed eye at a concentration of 0.15% w/v. Analysis of protein extracts using size exclusion HPLC shows an increase in secretory proteins, particularly tear-specific prealbumin.

The presence of sigma-receptors in the lacrimal gland

Applying the technic of S.A. Wolfe et al. [Endocrinology, 124, 1160-1172, 1989], we have established the presence of sigma receptors in isolated, but intact, lacrimocytes excised from main lacrimal gland tissue of the New Zealand white rabbit. 3H-Haloperidol was used as the ligand (0.5-2500 nM), in the presence of spiperone. From a Scatchard plot, a single binding site was statistically chosen over two sites for a majority of the data associated with the intact lacrimocytes. Kd (71.0 +/- 46.4 nM) and Bmax (588.2 +/- 166.7 fmol/mg of protein) values showed lower binding affinity but a similar density of sigma sites in rabbit lacrimocytes when compared to published results obtained for rat exocrine glands and brain tissue. Using the technic of McCann and Su [J. Pharm. Exp. Therap., 257, 547, 1991], membrane suspensions of the sigma receptor were also prepared and tested for binding to radioligands, 3H-DTG, as well as 3H-haloperidol. A Scatchard plot revealed two binding sites for 3H-DTG and one binding site for 3H-haloperidol. The high affinity site for 3H-DTG yielded a Kd of 1.04 +/- 0.64 nM, whereas, Bmax was 135.9 +/- 11.62 fmol/mg of protein. The low affinity site gave a Kd = 75.3 +/- 26.8 nM and Bmax = 344.0 +/- 222.0 fmol/mg of protein. The weaker site is suspected to be intracellular. IC50 values were determined for N,N-disubstituted arylphenylalkylamines (Kd approximately low nM).

Involvement of haloperidol-sensitive sigma-sites in antitussive effects

The effects of selective sigma-ligands on the capsaicin-induced cough reflex in rats were studied. Intraperitoneal injection of (+)-N-allylnormetazocine ((+)-SKF-10,047) and N,N'-di(ortho-tolyl)guanidine (DTG) in doses that ranged from 0.3 to 3.0 mg/kg decreased the number of coughs dose dependently. The antitussive effects of these sigma-ligands were significantly attenuated by pretreatment with haloperidol. Pretreatment with haloperidol also markedly reduced the antitussive effects of (+/-)-pentazocine and dextromethorphan. These results suggest that haloperidol-sensitive sigma-sites may be involved in the regulation of coughs.

Pharmacological comparison of the sigma recognition site labelled by [3H]haloperidol in human and rat cerebellum

The radioligand binding characteristics of [3H]haloperidol (in the presence of spiperone, 25 nmolL-1) were investigated in rat and human cerebellar membranes. In both rat and human cerebellar membrane preparations saturation studies with [3H]haloperidol (non-specific binding defined by pentazocine, 10 mumolsL-1) demonstrated high affinity saturable specific binding to a homogenous population of binding sites (rat, Bmax 6693 +/- 1242 fmol mg-1 protein, pKD 8.33 +/- 0.08; human, Bmax 2550 +/- 437 fmol mg-1 protein, pKD 8.59 +/- 0.11; mean +/- SEM, n = 3-6). Competition studies employing a wide range of structurally diverse competing compounds displayed that the [3H]haloperidol binding site was pharmacologically similar in both preparations and comparable to sigma recognition sites previously identified in various tissues originating from different species. In addition, with reference to the potential subtypes of sigma recognition sites, the labelling of these sites by low nanomolar concentrations of [3H]haloperidol provides evidence that they belong to the sigma-1 recognition site subtype. The present findings suggest that the pharmacology of the rat and human cerebellar sigma recognition site are directly comparable and provides further supporting evidence towards the use of [3H]haloperidol radioligand binding studies in the rat to detect sigma receptor ligands with potential therapeutic activity.

Computer-assisted modeling of multiple dextromethorphan and sigma binding sites in guinea pig brain

Computer-assisted, simultaneous analysis of self- and cross-displacement experiments demonstrated the existence of several binding sites in guinea pig brain for dextromethorphan, (+)-3-(3-hydroxyphenyl)-N-(1-propyl)piperidine ((+)-3-PPP), and 1,3-di-o-tolyl guanidine (DTG). Dextromethorphan binds with high affinity to two sites (R1 Kd 50-83 and R2 Kd 8-19 nM) and with low affinity to two additional sites (R3 and R4). (+)-3-PPP binds to one high-affinity (R1 Kd 24-36 nM), to one intermediate-affinity (R3 Kd 210-320 nM), and to two (R2 and R4) low-affinity sites. DTG binds with almost identical high affinity to two different sites (R1 Kd 22-24 and R3 Kd 13-16 nM). These results confirm that dextromethorphan, (+)-3-PPP, and DTG bind to the common DM1/sigma 1 site (R1). The binding of DTG to two different sites with identical affinities precludes the use of this compound as a specific marker for sigma receptors. Besides, haloperidol displaces labeled ligands from both high-affinity DTG sites (R1 and R3) with high affinity. Thus, haloperidol sensitivity should not be used as the single criterion to identify a putative receptor. The resolution of these novel sites also may provide new insights into the multiple effects of antipsychotic drugs. In addition, this investigation has important implications regarding the methods that must be applied to characterize multiple binding sites and their relations with putative receptors.

[3H]DTG and [3H](+)-3-PPP label pharmacologically distinct sigma binding sites in guinea pig brain membranes

The interaction of various compounds with sigma binding sites was examined in membranes prepared from whole guinea pig brain. Whereas [3H](+)-3-(3-hydroxyphenyl)-N-(1-propyl)piperidine labeled a single population of binding sites exhibiting a Kd of 43 nM, [3H]1,3-di-o-tolylguanidine bound to two sites having Kds of 35 and 212 nM, and to a greater maximum number of sites than [3H](+)-3-(3-hydroxyphenyl)-N-(1-propyl)piperidine. Haloperidol, 1,3-di-o-tolylguanidine, BMY 14802, and (-)-pentazocine each displayed nearly equal affinity for binding sites labeled by [3H](+)-3-(3-hydroxyphenyl)-N-(1-propyl)piperidine and [3H]1,3-di-o-tolylguanidine, whereas (+)-3-(3-hydroxyphenyl)-N-(1-propyl)piperidine was 3 times more potent in inhibiting [3H](+)-3-(3-hydroxyphenyl)-N-(1-propyl)piperidine than [3H]1,3-di-o-tolylguanidine binding. In contrast, (+)-SKF 10,047, (+)-cyclazocine and (+)-pentazocine exhibited more than 9-fold higher affinity for [3H](+)-3-(3-hydroxyphenyl)-N-(1-propyl)piperidine than [3H]1,3-di-o-tolylguanidine binding sites. Dextromethorphan was 15-fold more potent against [3H](+)-3-(3-hydroxyphenyl)-N-(1-propyl)piperidine than [3H]1,3-di-o-tolylguanidine, inhibited [3H](+)-3-(3-hydroxyphenyl)-N-(1-propyl)piperidine binding in a biphasic manner, and inhibited [3H]haloperidol and [3H](+)-SKF 10,047 binding with potencies similar to those obtained against [3H]1,3-di-o-tolylguanidine and [3H](+)-3-(3-hydroxyphenyl)-N-(1-propyl)piperidine, respectively. Phenytoin increased [3H](+)-3-(3-hydroxyphenyl)-N-(1-propyl)piperidine and [3H](+)-SKF 10,047 binding, but did not enhance [3H]1,3-di-o-tolylguanidine or [3H]haloperidol binding. However, the potency of dextromethorphan to inhibit [3H]1,3-di-o-tolylguanidine binding was increased in the presence of phenytoin.(ABSTRACT TRUNCATED AT 250 WORDS)

Ropizine concurrently enhances and inhibits [3H]dextromethorphan binding to different structures of the guinea pig brain: autoradiographic evidence for multiple binding sites

Ropizine (10 microM) produces a simultaneous enhancement and inhibition of [3H]dextromethorphan (DM) high-affinity binding to different areas of the guinea pig brain. These results imply that there are two distinct types of high-affinity [3H]DM binding sites, which are present in variable proportions in different brain structures. The ropizine-enhanced [3H]DM binding type was preferentially inhibited by (+)-pentazocine. This is consistent with the presumption that the (+)-pentazocine-sensitive site is identical with the common site for DM and 3-(-3-Hydroxyphenyl)-N-(1-propyl)piperidine ((+)-3-PPP). The second binding type, which is inhibited by ropizine and is not so sensitive to (+)-pentazocine, has not been fully characterized. This study demonstrates that the biphasic effects of ropizine are due, at least in part, to the effects of ropizine on two different types of [3H]DM binding sites. However, this study does not rule out that common DM/(+)-3-PPP site also might be inhibited by higher concentrations of ropizine.

Sigma and opioid receptors in human brain tumors

Human brain tumors (obtained as surgical specimens) and nude mouse-borne human neuroblastomas and gliomas were analyzed for sigma and opioid receptor content. Sigma binding was assessed using [3H]1,3-di-o-tolylguanidine (DTG), whereas opoid receptor subtypes were measured with tritiated forms of the following: mu, [D-ala2,mePhe4,gly-ol5]enkephalin (DAMGE); kappa, ethylketocyclazocine (EKC) or U69,593; delta, [D-pen2,D-pen5]enkephalin (DPDPE) or [D-ala2,D-leu5]enkephalin (DADLE) with mu suppressor present. Binding parameters were estimated by homologous displacement assays followed by analysis using the LIGAND program. Sigma binding was detected in 15 of 16 tumors examined with very high levels (pmol/mg protein) found in a brain metastasis from an adenocarcinoma of lung and a human neuroblastoma (SK-N-MC) passaged in nude mice. kappa opioid receptor binding was detected in 4 of 4 glioblastoma multiforme specimens and 2 of 2 human astrocytoma cell lines tested but not in the other brain tumors analyzed.

Dextromethorphan and neuromodulation: old drug coughs up new activities

Dextromethorphan is one of the most widely used non-opioid cough suppressants, representing the active ingredient in several over-the-counter antitussive formulations. It does not possess the CNS pharmacology of other opiates in humans (i.e. analgesia, respiratory depression, abuse liability or psychotomimetic properties), but since the discovery in 1981 of high affinity recognition sites in brain for dextromethorphan a unique neuropharmacological profile has emerged for this relatively innocuous drug. Anticonvulsant and neuroprotective properties have been demonstrated, and treatment with dextromethorphan has been shown to improve the cerebrovascular and functional consequences of global cerebral ischemia. Frank Tortella and colleagues review the CNS pharmacology of dextromethorphan, its possible involvement with NMDA or sigma-receptors, and the potential clinical importance of this old 'new' drug.

Calcium-dependent displacement of haloperidol-sensitive sigma receptor binding in rat hippocampal slices following tissue depolarization

To evaluate the possible existence of an endogenous ligand for the haloperidol-sensitive sigma receptor, we developed an in vitro competition assay to measure endogenous ligand release. Depolarization of in vitro hippocampal slices by either veratridine or potassium reduced [3H]ditolylguanidine binding in a calcium-dependent and transient manner. None of the drugs or iron substitutions directly affected [3H]ditolylguanidine binding to rat brain membranes. Veratridine-induced depolarization also reduced the binding of [3H](+)3-(3-hydroxyphenyl)-N-(1-propyl)piperidine, another sigma radioligand, in a calcium-dependent manner. Radioligand displacement was not associated with alteration in sigma receptor dissociation kinetics or receptor degradation in the hippocampal slice. In contrast, KC1 depolarization had no effect on [3H]ditolyguanidine binding to sigma receptors in liver slices. The results suggest that a calcium-dependent, depolarization-induced reduction in sigma receptor binding may have been caused by the release of an endogenous sigma ligand in rat hippocampal tissue.

Autoradiographic localization of [3H]dextromethorphan in guinea pig brain: allosteric enhancement by ropizine

Dextromethorphan (DM) is an antitussive with anticonvulsant activity that binds to high- and low-affinity sites in guinea pig brain homogenates. We examined the autoradiographic localization of [3H]DM using the anticonvulsant ropizine, an allosteric modifier that decreases the dissociation rate of [3H]DM. Competition studies demonstrated that the binding to brain sections was identical to that of brain homogenates [Craviso and Musacchio: Mol Pharmacol 23:629-640, 1983b]. Computer-assisted quantitative analysis of the autoradiographic images demonstrated that [3H]DM binds to discrete structures throughout the brain, but with higher density in the midbrain, pons, and medulla oblongata. The most intense labeling was observed in the rhabdoid, dorsal raphe, median raphe, caudal linear raphe nuclei, and cranial motor nerve nuclei. The central gray showed moderate to high-density labeling throughout its entire rostro-caudal extent, with very high binding in the dorsal tegmental nucleus and the locus coeruleus. Moderate and high binding was also seen in several hypothalamic structures. Distinct bands of moderate binding were seen in the pyramidal cell layer of the piriform cortex, the retrosplenial cortex, the granular cell layer of the dentate gyrus, the pyramidal cell layer of the hippocampus, and the Purkinje cell layer of the cerebellum. The striking similarity between the binding distribution of [3H]DM and sigma ligands, plus competition studies in brain homogenate, support the hypothesis that DM and sigma ligands share a common high-affinity binding site [Musacchio et al: Mol Pharmacol 35:1-5, 1989]. The distribution of [3H]DM binding provides possible anatomical substrates for both the antitussive and anticonvulsant actions of DM.