Pharmacological characteristics and distributions of sigma- and phencyclidine receptors in the animal kingdom

An Overview of Stroke: Mechanism, In vivo Experimental Models Thereof, and Neuroprotective Agents

BACKGROUND

Stroke is one of the causes of death and disability globally. Brain attack is because of the acute presentation of stroke, which highlights the requirement for decisive action to treat it.

OBJECTIVE

The mechanism and in-vivo experimental models of stroke with various neuroprotective agents are highlighted in this review.

METHOD

The damaging mechanisms may proceed by rapid, nonspecific cell lysis (necrosis) or by the active form of cell death (apoptosis or necroptosis), depending upon the duration and severity and of the ischemic insult.

RESULTS

Identification of injury mediators and pathways in a variety of experimental animal models of global cerebral ischemia has directed to explore the target-specific cytoprotective strategies, which are critical to clinical brain injury outcomes.

CONCLUSION

The injury mechanism, available encouraging medicaments thereof, and outcomes of natural and modern medicines for ischemia have been summarized. In spite of available therapeutic agents (thrombolytics, calcium channel blockers, NMDA receptor antagonists and antioxidants), there is a need for an ideal drug for strokes.

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.

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

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

Mechanisms of Brain Injury after Global Cerebral Ischemia

Cerebral ischemia results in a rapid depletion of energy stores that triggers a complex cascade of cellular events such as cellular depolarization and Ca2+ influx, resulting in excitotoxic cell death. The critical determinant of severity of brain injury is the duration and severity of the ischemic insult and early restoration of CBF. Induced therapeutic hypothermia following CA is the only strategy that has demonstrated improvement in outcomes in prospective, randomized clinical trials. Although pharmacologic neuro-protection has been disappointing thus far in a variety of experimental animal models, further research efforts are directed at using some agents that demonstrate marginal or moderate efficacy in combination with hypothermia. Although the signal transduction pathways and intracellular molecular events during cerebral ischemia and reperfusion are complex, potential therapeutic neuroprotective strategies hold promise for the future.

Characterization of the non-competitive antagonist binding site of the NMDA receptor in dark Agouti rats

The ability of non-competitive NMDA antagonists and other selected compounds to inhibit [3H]MK-801 binding to the NMDA receptor in brain membranes was evaluated in female, dark Agouti rats. In homologous competition binding studies the average apparent affinity (KD) of [3H]MK-801 for its binding site was 5.5 nM and the binding site density (Bmax) was 1.83 pmol/mg protein. Inhibition of [3H]MK-801 binding by non-competitive NMDA antagonists was best described with a one-site competition model and the average Hill coefficients were -1. A series of eight non-competitive NMDA antagonists inhibited [3H]MK-801 binding with the following rank order of affinity (K(i), nM): MK-801 (5.5) > dexoxadrol (21.5) > or = TCP (24.2) > phencyclidine (100.8) > (+)-SKF 10,047 (357.7) > dextrorphan (405.2) > ketamine (922.2) > dextromethorphan (2913). These inhibition binding constants determined in dark Agouti rat brain membranes were significantly correlated (P = 0.0002; r2 = 0.95) with previously reported values determined in Sprague-Dawley rats [Wong et al., 1988, J. Neurochem. 50, 274-281]. Despite significant differences in metabolic capability between these strains, the central nervous system NMDA receptor ion channel shares similar characteristics.

Mechanisms of ischemic brain damage

Neurologic complications from cerebral ischemia occur frequently following cardiac arrest, as well as in the perioperative period in cardiac surgery. The cellular and molecular mechanisms of cerebral ischemia are complex. This article discusses several important cell death and salvage pathways that are important in experimental cerebral ischemia that may be critical to outcome in clinical brain injury.

Rimcazole analogs attenuate the convulsive effects of cocaine: correlation with binding to sigma receptors rather than dopamine transporters

Cocaine interacts with dopamine transporters and sigma receptors at concentrations that are achievable in vivo, suggesting that they may both be viable targets for the development of anti-cocaine agents. Rimcazole binds to both of these targets and also attenuates cocaine-induced locomotor activity and sensitization. To further characterize the mechanism(s) underlying the attenuation of cocaine-induced convulsions and lethality, rimcazole and three analogs (SH3/24, SH2/21, SH1/57), with a range of affinities for dopamine transporters and sigma receptors, were evaluated. The highly selective and potent sigma receptor ligand LR176 was used as a reference. Competition binding studies confirmed that the rank order of the compounds at dopamine transporters vs. sigma receptors differed, thus enabling a correlation between the relative anti-cocaine activities of the compounds in behavioral studies and their affinities for dopamine transporters vs. sigma receptors. In behavioral studies, male Swiss Webster mice were pre-treated with one of the compounds (0-60 mg/kg, i.p.), then challenged 15 min later with either a convulsive (60 mg/kg, i.p.) or lethal (125 mg/kg, i.p.) dose of cocaine. When the compounds were ranked according to their protective effect, there was a significant correlation between their anticonvulsant actions and their affinities for sigma receptors, but not dopamine transporters. Although the rimcazole analogs were ineffective against the lethal effects of cocaine, the selective sigma receptor ligand LR176 provided significant protection. These data thus suggest that sigma receptors may mediate some of the toxic effects associated with cocaine and that sigma receptor antagonists may be developed as pharmacotherapeutic agents for this application.

Involvement of the sigma receptor in passive-avoidance learning in the day-old chick during the second wave of neuronal activity

The specific sigma-receptor agonist (+)-SKF 10047 and antagonist BD 1047 were used to investigate whether this receptor was involved in passive-avoidance training in the day-old chick. We found 300 microM (+)-SKF 10047 to be amnesic when injected into the lobus parolfactorius 5 h after training (p < .01). Higher or lower concentrations of (+)-SKF 10047 did not disrupt memory formation. The amnesia produced by the efficacious dose of (+)-SKF 10047 was reversed by the specific antagonist, BD 1047. It is suggested that the sigma-receptor may exert its effect on passive-avoidance memory consolidation during the later stages of long-term memory formation by modulation of memory-related neurotransmission.

Pharmacological identification of SM-21, the novel sigma(2) antagonist

SM-21 is a tropane analogue with high affinity and selectivity for sigma(2) receptor subtype. In the absence of highly selective sigma(2) antagonists, the aim of the present study was to determine whether SM-21 is endowed with antagonistic activity. The experiments were conducted in rats by inducing neck dystonia, which is reported to be subsequent to activation of sigma(2) receptors. SM-21 (10 nmol/0.5 microl) was able to prevent torsion of the neck obtained by administration of the sigma(1)-sigma(2) agonist 1,3-di-(2-tolyl)guanidine (DTG, 5 nmol/0.5 microl) in the red nucleus. These data indicate that SM-21 is a potent and selective sigma(2) antagonist.

Correlation between neuroleptic binding to sigma(1) and sigma(2) receptors and acute dystonic reactions

Acute dystonic reactions are motor side effects that occur soon after the initiation of neuroleptic treatment. Although earlier studies indicate that these abnormal movements can be induced in animals and humans via activation of sigma receptors, the relative contribution of the different sigma receptor subtypes is unknown. Since sigma(1) and sigma(2) receptor are differentially represented in motor regions of the brain, the affinities of 17 neuroleptics for these sigma receptor subtypes were determined using competition binding studies. The results revealed that most neuroleptics do not exhibit selectivity for either of the sigma receptor subtypes, as reflected by a significant correlation between the affinities of the neuroleptics for sigma(1) vs. sigma(2) receptors. Moreover, when the sigma binding affinities of the neuroleptics were correlated with the tendency of the drugs to produce acute dystonic reactions in humans, there was a significant correlation for both subtypes. Together with earlier studies in animals, the data suggest that neuroleptic-induced motor side effects can be mediated through both sigma(1) and sigma(2) receptors.

Relationship between modulation of the cerebellorubrospinal system in the in vitro turtle brain and changes in motor behavior in rats: effects of novel sigma ligands

Saturation and competition binding studies showed that the turtle brain contains sigma sites labeled by both [3H]di-o-tolylguanidine (DTG) and [3H](+)-pentazocine. There was a significant correlation between the IC50 values of sigma ligands for [3H]DTG sites in the turtle vs. rat brain, suggesting that the sites are comparable in the two species. In contrast, [3H](+)-pentazocine, which primarily labels sigma1 sites in the rodent brain, labels a heterogeneity of sites in the turtle brain. In extracellular recordings from the in vitro turtle brainstem, some sigma ligands enhanced the burst responses of red nucleus (RN) neurons (DTG, haloperidol, BD1031, BD1052, BD1069) while other sigma ligands decreased the burst responses (BD1047, BD1063). Control compounds (turtle Ringer vehicle control, opiate antagonist naloxone, atypical neuroleptic sulpiride) had no significant effects on the RN burst responses recorded from the in vitro turtle brain. The ED50s of the ligands for altering the burst responses in RN neurons from the turtle brain were correlated with their IC50s for turtle brain sites labeled with [3H]DTG, but not [3H](+)-pentazocine; this pattern is identical to that previously reported in rats, where there is a correlation between the potencies of sigma ligands for producing dystonic postures after microinjection into the rat RN and their binding to rat brain sites labeled with [3H]DTG, but not [3H](+)-pentazocine. When the novel sigma ligands were microinjected into the rat RN, dystonic postures were produced by ligands that increased the burst duration of RN neurons in the turtle brain. Novel sigma ligands that reduced the burst responses in the in vitro turtle brain have previously been reported to have no effects on their own when microinjected into the rat RN, but to block the dystonic postures produced by other sigma ligands. Taken together, the data suggest that the opposite effects of the novel ligands in the turtle electrophysiological studies represent the actions of agonists vs. antagonists, and that the directionality of the effects has predictive value for the expected motor effects of the drugs.

Characterisation of [3H]MK-801 binding and its cooperative modulation by pig brain membranes

Cooperative modulation of [3H]MK-801 binding to extensively washed pig cortical brain membranes in the presence of various concentrations of L-glutamate, glycine, spermine, CPP and DCKA was evaluated in association experiments. In saturation experiments [3H]MK-801 labelled a homogeneous population of binding sites with a Kd-value of 1.26 +/- 0.18 nmol 1(-1) and a Bmax-value of 2130 +/- 200 fmol/mg protein. The pharmacological profile of this site was further evaluated in competition experiments with known NMDA receptor channel blockers. In nonequilibrium binding experiments EC50-values of reference compounds acting at the L-glutamate, at the glycine, and at the polyamine site, were determined by increasing or decreasing [3H]MK-801 binding. Ifenprodil reduced [3H]MK-801 binding in a biphasic manner. All the data obtained are in agreement with results from [3H]MK-801 binding to rodent as well as human brain membranes. This study therefore strongly suggests, that pig cortical membranes are a suitable alternative to rodent brain membranes, and an acceptable substitute for human brain membranes in [3H]MK-801 binding experiments.

sigma2 Site-mediated inhibition of electrically evoked guinea pig ileum longitudinal muscle/myenteric plexus contractions

Functional and binding studies were performed in order to characterize the relative efficacy and affinity of a number of compounds that bind to sigma sites. The ability of sigma site ligands to inhibit electrically evoked contraction of the guinea pig ileum longitudinal muscle/myenteric plexus preparation was compared to the affinities of these compounds for sigma1 sites (assessed by displacement of [3H](+)-pentazocine) and sigma2 sites (assessed by displacement of [3H]1,3-di-o-tolylguanidine (DTG) in the presence of 5 microM dextromethorphan). It was shown that the rank order of potencies for suppression of electrically evoked contractions of guinea pig ileum perfectly matched the rank order of affinities of these compounds for the sigma2 binding site, while correlating poorly with the sigma1 binding site. In addition, no significant correlations were found between the efficacy of the tested compounds to inhibit contraction of the guinea pig ileum preparation and previously reported affinities for muscarinic, dopamine D2 or MK-801 binding sites. Thus, the present study represents the first functional bioassay selectively sensitive to agents interacting with the sigma2 receptor subtype binding site, and provides a means with which to further elucidate the functional role of sigma2 sites.

Strain differences in rat brain and liver sigma binding: lack of cytochrome P450-2D1 involvement

Substrates for cytochrome P450-2D1 exhibit a high affinity for sigma binding sites suggesting that sigma sites may be associated with the cytochrome P450-2D1 isozyme. In contrast to Sprague-Dawley, Dark Agouti rat liver does not express the P450-2D1 gene product. Therefore, if a subpopulation of sigma sites is associated with the P450-2D1 enzyme, then the number (Bmax) of sigma sites is predicted to be decreased in Dark Agouti brain and liver compared to Sprague-Dawley tissues. In the present study, binding of [3H](+)-3-(3-hydroxyphenyl)-N-(1-propyl)piperidine ([3H](+)3-PPP) in brain and liver from Dark Agouti, Sprague-Dawley, Long Evans and Wistar rat strains was examined. Results demonstrate marked variation in Bmax among the strains, with a consistently lower value for Dark Agouti tissues. However, the absolute difference in sigma binding between brain and liver for each strain was not consistent with reported differences in the activity or levels of P450-2D1. Additionally, the percentage decrease in Bmax for Dark Agouti liver was found to be similar to that for Dark Agouti brain. Taken together these results suggest that P450-2D1 does not account for the strain-related difference in sigma binding; but rather, other genetic factor(s) may be responsible for the decrease in the number of sigma sites in the Dark Agouti strain compared to the other rat strains examined.

Discriminative stimulus effects of the PCP/sigma-ligand (+)-N-allylnormetazocine in monkeys

(+)-N-Allylnormetazocine [(+)-NANM] binds to both the phencyclidine (PCP) receptor and the sigma-site in brain, with some selectivity for the latter. In rats, the discriminative stimulus effects of (+)-NANM are primarily PCP like. The present study was performed to determine if the discriminative effects of (+)-NANM in a primate species might reflect the actions of this drug at the sigma-site. Six squirrel monkeys were trained to discriminate between IM injections of saline and 1.0 mg/kg (+)-NANM in a two-choice discrete-trial avoidance procedure. In tests of stimulus generalization, dose-dependent increases in trials completed on the (+)-NANM choice lever were produced by (+)- and (-)-NANM, by PCP and the PCP-like drugs MK-801 and thienylcyclohexyl-piperidine, and by the opioids (+)- and (-)-cyclazocine and dextrorphan; order of potency correlated with reported affinities for the PCP receptor. High-affinity sigma-ligands, (+)-pentazocine, 1,3-di-ortho-tolylguanidine (DTG), haloperidol, and BMY 14802, as well as agonists at mu- and kappa-opioid receptors, occasioned selection of the saline-appropriate choice lever. Selection of the (+)-NANM choice lever was reduced by up to 35-50% when 1.0 mg/kg (+)-NANM was given concurrently with haloperidol or BMY 14802, but was not affected substantially by (-)-butaclamol, another sigma-ligand, or by naltrexone, an opioid antagonist. The discriminative effects of (+)-NANM in squirrel monkeys appear to be mediated largely by the PCP receptor and not by the sigma-site or opioid receptors.

Sigma receptors are associated with cortical limbic areas in the primate brain

Putative sigma receptors are a current target for antipsychotic drug development. Novel antipsychotic agents which possess selective and high affinity for sigma binding sites may serve as an alternative to the principal neuroleptic drugs currently in clinical use which mediate extrapyramidal side effects and dyskinesias through their blockade of dopamine receptors. We have used in vitro autoradiography to localize putative sigma receptors labelled with (+)-[3H]-3-(3-hydroxyphenyl)-N-(1-propyl)piperidine [(+)-[3H]-3-PPP] in the brain of the rhesus macaque. The binding characteristics of (+)-[3H]-3-PPP in the primate brain were comparable to those previously described in the rodent. Saturation analysis demonstrated a single class of sites in cerebellar and hippocampal membranes with a Kd value of 28 nM. Sigma receptors labeled with (+)-[3H]-3-PPP in the primate brain displayed the appropriate rank order of potency and stereoselectivity in competition binding assays. Haloperidol displaced (+)-[3H]-3-PPP binding in the low nanomolar range, and the (+) isomer of pentazocine was 50-fold more potent than (-) pentazocine. Computerized densitometric analysis of the autoradiograms demonstrated a striking enrichment of sigma binding sites over the paralimbic belt cortices, including the orbitofrontal, cingulate, insular, parahippocampal, and temporopolar gyri. Peak densities of sigma receptors were seen over the medial and central nuclei of the amygdala and were widely distributed within the hippocampal formation. Sigma binding sites densities were elevated over the suprachiasmatic and supraoptic nuclei of the hypothalamus. Moderate sigma receptor densities were observed over the ventromedial sectors of the caudate and the putamen. Sigma receptors were also elevated over autonomic relay nuclei of the brainstem, including the nucleus of the solitary tract and the dorsal motor nucleus of the vagus. The distribution of sigma receptors in the primate brain suggests that the paralimbic belt cortices, amygdala, hippocampus, hypothalamus, and autonomic relay nuclei of the brainstem may be interrelated by a topographic chemical linkage. The autoradiographic visualization of sigma receptor distributions in the primate brain provides further support for a role of sigma receptor mechanisms in the functions of the limbic system.

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.

Selective loss of cerebral cortical sigma, but not PCP binding sites in schizophrenia

Drugs such as phencyclidine (PCP) that interact with PCP and sigma binding sites can produce psychotomimetic effects that resemble some symptoms of schizophrenia. Therefore, it has been suggested that PCP and sigma receptors may be important in the clinical manifestations of schizophrenia. Assays of these two binding sites in human postmortem brains showed consistent significant reductions in the density of sigma, but not PCP sites, in schizophrenics as compared with age-matched and postmortem interval-matched normal and suicide controls. Reductions in the density of sigma binding sites in schizophrenia were most prominent in temporal cerebral cortex, and were accompanied by a small increase in affinity for the ligand [3H]haloperidol. These data provide the first evidence for alterations in sigma binding sites in schizophrenia, and suggest that selective sigma ligands may be useful in the treatment of the disorder.

Similar behavioural effects of sigma agonists and PCP-like non-competitive NMDA antagonists in guinea-pigs

The present study examined the behavioural effects of sigma agonists and PCP-like non-competitive N-methyl-D-aspartate (NMDA) antagonists in guinea-pigs. Subcutaneous (SC) injection of the putative sigma agonist (+)NANM (1 and 10 mg/kg SC) and (-)NANM (1 and 10 mg/kg SC) produced a behavioural response in guinea-pigs which was characterized by sedation and exophthalmos, with locomotor depression, flattened posture and flaccidity, whereas the sigma ligand pentazocine induced sedation but no flattened posture. Ketamine (20 mg/kg SC) and (+)dizocilpine (0.025, 0.1 and 1 mg/kg SC) produced similar effects to those of (+) and (-)NANM. However, the putative sigma receptor ligand DTG (1 and 10 mg/kg SC) had no observable effect on behaviours in guinea-pigs, similar to results for other species. The behavioural effects produced by (+) and (-)NANM were not reversed by injection 1 h later of naloxone hydrochloride (15 mg/kg SC), haloperidol (10 mg/kg SC) or DTG (10 and 30 mg/kg SC), but the effects of all drugs were reversed by the selective dopamine D-2 agonist quinpirole (3 mg/kg IP). Moreover, injection of naloxone (15 mg/kg SC), DTG (10 and 30 mg/kg SC) or haloperidol (1 and 10 mg/kg SC) 10 min before, did not reverse the behaviour induced by (+)NANM (10 mg/kg SC). These data indicate that sigma and PCP-like drugs have a similar gross behavioural effect in guinea-pigs, possibly mediated by non-competitive antagonism of the NMDA subtype of glutamate receptors. The results demonstrating behavioural depression were in contrast to the stimulatory effects of these drugs at similar doses in other rodent species.(ABSTRACT TRUNCATED AT 250 WORDS)

Interaction of sigma-compounds with receptor-stimulated phosphoinositide metabolism in the rat brain

Sigma-receptors are nonopioid, nondopaminergic receptors that bind with high affinity several antipsychotic drugs and appear to be involved in regulation of posture and movement. The second messenger system coupled to these receptors is still unknown. Recently, an inhibitory effect of various sigma-compounds on carbachol-stimulated phosphoinositide metabolism has been reported. We have investigated the effect of six sigma-compounds on carbachol- and norepinephrine-stimulated 3H-inositol phosphate accumulation in rat cerebral cortex slices. All compounds tested had a dose-dependent inhibitory effect on both systems, although their order of potency differed between neurotransmitters. Pentazocine and 1,3-di-o-tolylguanidine were the most potent inhibitors of carbachol-stimulated phosphoinositide turnover (IC50 = 31.5 and 45.7 microM, respectively), while haloperidol showed the greatest potency on the norepinephrine-coupled system (IC50 = 3.5 microM). In the presence of IC50 concentrations of these inhibitors, the dose-response curves for the agonists were shifted to the right and the EC50 values were significantly increased. Sigma-compounds also inhibited the binding of [3H]quinuclidinyl benzilate to muscarinic receptors and of [3H]prazosin to alpha 1-adrenoceptors in cortical membranes. In the presence of IC50 concentration (11 microM) of 1,3-di-o-tolylguanidine, no differences were found in the maximal number of muscarinic binding sites, whereas the dissociation constant increased approximately fivefold, indicating a decrease of the radioligand's affinity for the receptor. These results indicate that sigma-compounds, at micromolar concentrations, inhibit muscarinic and alpha 1-adrenergic receptor-coupled phosphoinositide metabolism, probably through an interaction with the neurotransmitter recognition sites.