Dextromethorphan and sigma ligands: common sites but diverse effects

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.

Acute effects of sigma ligands on the electrophysiological activity of rat nigrostriatal and mesoaccumbal dopaminergic neurons

The effects of acute i.v. administration of several sigma ligands on the single-unit activity of nigrostriatal and mesoaccumbal dopaminergic (DA) neurons were evaluated in chloral hydrate-anesthetized rats. DTG (1,3-di(o-tolyl)guanidine) did not alter DA neuronal activity at nontoxic doses and JO 1784 [(+)-N-cyclopropylmethyl-N-methyl-1,4-diphenyl-1-ethylbut-3-en-1-+ ++ylamine] was inactive. (+)-Pentazocine was more effective in increasing mesoaccumbal vs. nigrostriatal DA cell firing rates. BMY 14802(alpha-(4-fluorophenyl)-4-(5-fluoro-2-pyrimidinyl)-1-piperazine-but anol) dose-dependently increased DA cell firing rate in both populations. The inhibition of nigrostriatal DA cell firing rate by (+)-3-(3-hydroxyphenyl)-N-(1-propyl)piperidine [(+)-3-PPP] was reversed by (-)-eticlopride and (+)-but not (-)-butaclamol, which supports previous evidence that (+)-3-PPP-induced inhibition is due to the DA agonist properties of the drug. From what is known of the pharmacological properties of these compounds, it is concluded that acute sigma receptor occupation does not markedly alter the firing rate of DA neurons. The dose-response curve for inhibition of nigrostriatal DA neuronal activity by the D2 DA agonist, quinpirole, was shifted to the right tenfold by BMY 14802 pretreatment (8 mg/kg, i.v.) and twofold by (+)-pentazocine (8 mg/kg, i.v.), but was not changed by DTG (2 mg/kg, i.v.). It is concluded that the marked effects of certain sigma ligands on DA cell electrophysiology are likely due to their non-sigma properties.

Neuroprotective effects of SKF 10,047 in cultured rat cerebellar neurons and in gerbil global brain ischemia

BACKGROUND AND PURPOSE

Excitatory amino acids and their receptors are involved in mediating ischemic neuronal damage. The sigma-agonists are believed to interact with the N-methyl-D-aspartate receptor. Therefore, we studied the neuroprotective, hypothermic, and motor deficit effects of the sigma-agonist SKF 10,047 and the N-methyl-D-aspartate antagonist MK-801.

METHODS

Neuroprotective effects were compared using an in vitro ischemia model of cultured rat cerebellar granule cells and the gerbil model of global brain ischemia induced by 5 minutes of bilateral carotid artery occlusion followed by 7 days of reperfusion.

RESULTS

In vitro, (+)MK-801 protected against 100 microM glutamate with a 50% protective concentration of 30 nM, followed by (-)MK-801 (150 nM), cyclazocine (0.5 microM), (+)SKF 10,047 (3.3 microM), pentazocine (5 microM), and (-)SKF 10,047 (10 microM). In vivo, (+)SKF 10,047 pretreatment (60 mg/kg) or multiple postischemic treatments provided neuroprotection comparable with MK-801 pretreatment (10 mg/kg). When ischemic animals were administered the multiple dosing regimen of (+)SKF 10,047, no hypothermic effect was noted in the temporalis muscle over 4 hours' postischemia. Motor deficits monitored by a swing grid test showed that 50% recovery from (+)SKF 10,047 was 5.5 times faster than recovery from MK-801.

CONCLUSIONS

These results are the first to report a hypothermia-free, in vivo neuroprotective effect of (+)SKF 10,047, a prototypical drug of the sigma-agonist class.

(125I)iodoazidococaine, a photoaffinity label for the haloperidol-sensitive sigma receptor

A carrier-free radioiodinated cocaine photo-affinity label, (-)-3-(125I)iodo-4-azidococaine [(125I)IACoc], has been synthesized and used as a probe for cocaine-binding proteins. Photoaffinity labeling with 0.5 nM (125I)IACoc resulted in selective derivatization of a 26-kDa polypeptide with the pharmacology of a sigma receptor in membranes derived from whole rat brain, rat liver, and human placenta. Covalent labeling of the 26-kDa polypeptide was inhibited by 1 microM haloperidol, di(2-tolyl)guanidine (DTG), 3-(3-hydroxyphenyl)-N-(1-propyl)piperidine (3-PPP), dextromethorphan, and carbetapentane. Stereoselective protection of (125I)IACoc photolabeling by 3-PPP [(+)-3-PPP more potent than (-)-3-PPP] was observed. (125I)IACoc labeling of the 26-kDa polypeptide was also inhibited by 10 microM imipramine, amitriptyline, fluoxetine, benztropine, and tetrabenazine. The size of the (125I)I-ACoc-labeled proteins is consistent with the size of proteins photolabeled in guinea pig brain and liver membranes by using the sigma photolabel azido-[3H]DTG. Kinetic analysis of (125I)IACoc binding to rat liver microsomes revealed two sites with Kd values of 19 and 126 pM, respectively. The presence or absence of proteolytic inhibitors during membrane preparation did not alter the size of the photolabeled sigma receptor, indicating that the 26-kDa polypeptide was not derived from a larger protein. In summary, (125I)IACoc is a potent and highly specific photoaffinity label for the haloperidol-sensitive sigma receptor and will be useful for its biochemical and molecular characterization.

Subcellular distribution of (+)-[3H]SKF 10,047 binding sites in rat liver

The distribution of (+)-[3H]SKF 10,047 binding sites and the distribution of the established plasma membrane, nuclear, mitochondrial and endoplasmic reticulum markers in subcellular fractions of rat liver have been studied. The distribution profile of (+)-[3H]SKF 10,047 binding sites coincided with that of NADPH-cytochrome c reductase, the endoplasmic reticulum marker. (+)-[3H]SKF 10,047 binding sites in rat liver are therefore suggested to be located on the endoplasmic reticulum membrane and to represent a membrane-bound enzyme.

Electrophysiological effects of phencyclidine and the sigma agonist ditolylguanidine in the cerebellum of the rat

The electrophysiological actions of phencyclidine (PCP) and the sigma agonist 1,3-di(2tolyl)guanidine (DTG) were examined in the cerebellum of urethane-anesthetized rats. The object of the study was to determine if PCP and sigma agonists shared a common mechanism of action. The cerebellar Purkinje neuron was chosen because it has sigma receptors but not N-methyl-D-aspartate receptors, where PCP has additional effects. Both DTG and PCP decreased the spontaneous discharge rate of cerebellar Purkinje neurons after parenteral administration. When the drugs were applied locally to single Purkinje neurons, using pressure ejection through multibarrel micropipettes, both compounds decreased the spontaneous activity of the neurons with equal potency. Previous studies have shown that the actions of PCP in the cerebellum are dependent upon an interaction with noradrenergic terminals from the nucleus locus coeruleus. A similar finding was made in this study for DTG. Elimination of the noradrenergic input by lesion with the neurotoxin, 6-hydroxydopamine, diminished equally the effects of PCP and DTG. Treatment of the animals with haloperidol had similar effects. It is concluded that PCP and the sigma agonist DTG both act as indirect noradrenergic agonists in the cerebellum.

Regulation of sigma receptors and responsiveness to guanine nucleotides following repeated exposure of rats to haloperidol: further evidence for multiple sigma binding sites

The sigma binding sites are postulated to be involved in various central nervous system (CNS) disorders. The neuroleptic drug, haloperidol, displays high affinity for these receptor sites in the CNS. In the present study the effect of repeated exposure of rats to haloperidol (4 mg/kg/day for 14 days) on sigma binding sites labeled with (+)-3-(3-hydroxyphenyl)-N-1-(propyl)piperidine [+)-3-PPP) and 1,3-di-o-tolyl-guanidine (DTG) was investigated. In addition, the regulatory effect of guanine nucleotides on the binding of these two ligands to brain membranes derived from saline and haloperidol-treated rats was examined. Repeated administration of haloperidol induced down-regulation of (+)[3H]-3-PPP binding sites (75% decrease in the number of binding sites compared to control) which persisted for at least 7 days after termination of the haloperidol-treatment. The down-regulation of (+)-3-PPP binding sites was accompanied by reduced responsiveness to guanine nucleotides (i.e. 5-guanylylimidodiphosphate (Gpp(NH)p) compared to the sensitivity of (+)-3-PPP binding sites to the nucleotides tested in control membranes. However, at the 28th day after termination of the haloperidol-treatment, a complete recovery in the total number of (+)[3H]-3-PPP binding sites was observed, and the sensitivity to guanine nucleotides was regained. These findings suggest a marked plasticity in (+)-3-PPP/sigma receptor binding activity. In contrast, [3H]DTG binding sites expressed neither sensitivity to the repeated exposure to haloperidol nor to guanine nucleotides, suggesting a distinction between DTG and (+)-3-PPP binding sites in rat brain.

Non-opioid antitussives inhibit endogenous glutamate release from rabbit hippocampal slices

The non-opioid antitussive drugs, dextromethorphan, caramiphen and carbetapentane, are also anticonvulsant. The effects of these antitussives on potassium-stimulated release of endogenous amino acids from rabbit hippocampal slices was tested. All 3 drugs significantly reduced the release of glutamate, with carbetapentane being the most potent (IC50 approximately 40 microM). We suggest that the anticonvulsant action may be due to their ability to decrease glutamate release.

Behavioural and biochemical evidence of the interaction of the putative antipsychotic agent, BMY 14802 with the 5-HT1A receptor

The behavioural and biochemical profile of the sigma ligand and putative antipsychotic agent, BMY 14802 (alpha-(4-fluorophenyl)-4-(5-fluoro-2- pyrimidinyl)-1-piperazine butanol) has been determined in the mouse and rat. In mice, pretreatment with BMY 14802 attenuated both amphetamine-induced hyperactivity and conditioned avoidance responding, consistent with its previously reported antipsychotic potential. In common with 5-HT1A receptor agonists or partial agonists, BMY 14802 induced (a) a dose-dependent hypothermia in mice; (b) aspects of the 5-HT behavioural syndrome in rats, (c) antagonised mescaline-induced head twitches in mice and (d) generalised to the 8-hydroxy-2-(di-n-propylamino)tetralin discriminative stimulus over the dose range of 3-15 mg/kg. BMY 14802 had appreciable affinity for the 5-HT1A receptor (pIC50 = 6.7 compared to 7.3 for sigma binding) and antagonised forskolin-stimulated adenylate cyclase activity with a pEC50 of 6.2, consistent with an agonist action at this receptor. The results support the involvement of 5-HT1A receptors, but not the sigma binding site, in the behavioural profile of BMY 14802.

The human sigma site, which resembles that in NCB20 cells, may correspond to a low-affinity site in guinea pig brain

1,3-di(2-[5-3H]tolyl)Guanidine ([3H]DTG) was found to bind to a single saturable population of binding sites in human cerebral cortex and NCB20 cells, a second low-affinity site was apparent in guinea pig brain. Displacement studies were performed to determine the pharmacology of the [3H]DTG binding site in these 3 membrane preparations. In human cortical tissue and NCB20 cell membranes the (+)-stereoisomers of benzomorphans displaced binding with Hill coefficients close to one, displayed similar affinity and did not give the biphasic displacement curve characteristic of guinea pig membranes. The pIC50 of the low-affinity component of the sigma binding site in guinea pig brain correlates best with the affinity of drugs for the binding site in human cortex.

Two subtypes of enteric non-opioid sigma receptors in guinea-pig cholinergic motor neurons

In the longitudinal muscle-myenteric plexus preparation (LMMP) of the guinea-pig ileum, the non-opioid sigma receptors agonists, 1,3-di-ortho-tolylguanidine (DTG) and (+)N-allyl-N-normetazocine [(+)SKF 10,047], had opposite effects on nerve-mediated cholinergic contractions caused by electrical field stimulation. DTG (0.1-10 microM) inhibited and (+)SKF 10,047 (0.1-10 microM) markedly enhanced these contractile responses. Both effects were evaluated in the presence (0.5 or 1 microM) of the putative antagonists at central sigma sites: haloperidol, rimcazole, BMY 14802 and dextromethorphan. Haloperidol and dextromethorphan were ineffective. Rimcazole antagonized the effect of both DTG and (+)SKF 10.047. BMY 14802 antagonized the (+)SKF 10.047-mediated excitatory response only. These results suggest that two sigma receptor subtypes are present in enteric cholinergic motor neurons innervating the longitudinal coat. Rimcazole and BMY 14802 may provide useful tools for the characterization of peripheral non-opioid sigma receptors.

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.

Effects of NMDA receptor antagonists and sigma ligands on the acquisition of conditioned fear in mice

Recent studies have shown that several compounds known to act as competitive or non-competitive antagonists of NMDA receptors can disrupt learning in rodents. The present study was carried out to investigate the effects of a range of NMDA antagonists, acting at several sites in the NMDA receptor complex, on the acquisition of learned fear in mice. Dose-related disruptions of learning were produced by the non-competitive antagonists phencyclidine, dizocilpine, dextromethorphan and (+) and (-)N-allylnormetazocine. The (+) enantiomer of N-allylnormetazocine was approximately twice as potent as the (-) enantiomer. The competitive NMDA receptor antagonist, CGS 19755, also blocked the acquisition of learned fear as did the non-specific glutamate antagonist riluzole. In contrast, the anti-ischaemic drugs ifenprodil and SL 82.0715, which probably act as NMDA antagonists through an effect on the polyamine site, had no effect on learning up to doses which substantially reduced locomotion. The sigma receptor ligand DTG was also inactive. These results confirm that both competitive and non-competitive NMDA antagonists disrupt learning but indicate that the extent to which such an effect is observed may depend on the site at which the compounds act within the receptor complex. Activity at sigma receptors is unrelated to the effect on learning.

SKF 525-A and cytochrome P-450 ligands inhibit with high affinity the binding of [3H]dextromethorphan and sigma ligands to guinea pig brain

The DM1/sigma 1 site binds dextromethorphan (DM) and sigma receptor ligands. The broad binding specificity of this site and its peculiar subcellular distribution prompted us to explore the possibility that this site is a member of the cytochrome P-450 superfamily of enzymes. We tested the effects of the liver microsomal monooxygenase inhibitor SKF 525-A (Proadifen), and other P-450 substrates on the binding of [3H]dextromethorphan, [3H]3-(-3-Hydroxyphenyl)-N-(1-propyl)piperidine and (+)-[3H]1,3-Di-o-tolyl-guanidine ([3H]DTG) to the guinea pig brain. SKF 525-A, l-lobeline and GBR-12909 inhibited the binding of the three labeled ligands with nM affinity. Each drug has identical nM Ki values for the high-affinity site labeled by the three ligands. This indicated that they displaced the labeled ligands from the common DM1/sigma 1 site. Debrisoquine and sparteine, prototypical substrates for liver debrisoquine 4-hydroxylase, displayed Ki values of 9-13 and 3-4 microM respectively against the three labeled ligands. These results, the broad specificity of the DM1/sigma 1 binding site, and its peculiar subcellular distribution, raises the possibility that this binding site is a member of the cytochrome P-450 superfamily of isozymes, rather than a neurotransmitter receptor. These findings may have important implications for the understanding of the therapeutic, side effects and toxicity of several neurotropic drugs.

Evidence for peripheral mechanisms mediating the antitussive actions of opioids in the guinea pig

1. Comparisons were made between the doses required of aerosol and intraperitoneally administered morphine, dextromethorphan, codeine and the specific peripherally acting mu-receptor agonist DALDA (H-Tyr-D-Arg-Phe-Lys-NH2) to suppress citric acid-induced coughing in conscious guinea pigs. 2. Estimated ID50s for inhibition of numbers of coughs induced by an aerosol of 5% citric acid were 1.0 and 2.4 mg/kg for intraperitoneally administered morphine and dextromethorphan, respectively. 3. The estimated ID50s after inhalation of morphine and dextromethorphan as aerosols were approximately 2.2 and approximately 12 micrograms/kg, respectively. 4. Aerosilized codeine (approximately 72 micrograms/kg, n = 5) significantly inhibited coughing by 62 +/- 23% whereas 3 mg/kg, i.p. was required to significantly reduce coughing by a similar degree (60 +/- 6%, n = 7). 5. Inhalation of DALDA (approximately 7.2 micrograms/kg, n = 7) also significantly inhibited coughing. 6. The antitussive effect of inhaled morphine (approximately 7.2 micrograms/kg, n = 11) was inhibited after administration of 3 mg/kg of either naloxone hydrochloride or naloxone methylbromide intraperitoneally. 7. The results support the hypothesis that effects at a peripheral site can make a major contribution to the antitussive actions of these drugs.

Wash-resistant inhibition of phencyclidine- and haloperidol-sensitive sigma receptor sites in guinea pig brain by putative affinity ligands: determination of selectivity

Several putative affinity ligands, based on the structures of phencyclidine etoxadrol, 5-methyl-10,11-dihydro-5H-dibenzo[a,d] cycloheptene-5,10-imine (MK801) and 1,3-di-(2-methylphenyl)guanidine (DTG) were evaluated in vitro for their ability to produce a wash-resistant inhibition of phencyclidine and sigma receptor sites in homogenates of the brain of the guinea pig. All the phencyclidine-based ligands, including 1-[1-(3-isothiocyanatophenyl)cyclohexyl]piperidine (Metaphit) and (+/-)-N-(2-isothiocyanatoethyl) MK801 [(+/-)-MK801-NCS], produced a wash-resistant inhibition of binding sites for phencyclidine, labelled by [3H]-1-[1-(2-thienyl)cyclohexyl]piperidine ([3H]TCP) and sigma binding sites, labelled by [3H]DTG. The DTG-based ligands, 1-(4-isothiocyanato-2-methylphenyl)-3-(2-methylphenyl)guanidine (DIGIT) and 1-(4-[2-(2-isothiocyanatoethoxy)ethoxy]-2-methyl-phenyl)-3-(2- methylphenyl)guanidine (DIGIE), produced a wash-resistant inhibition of sigma sites, at concentrations as small as 1 microM and also inhibited binding sites for phencyclidine at larger concentrations (100 microM). Both 1-(3-isothiocyanatophenyl)-1-ethyl-4-(2-piperidyl)-1,3-dioxolane (ETOX-NCS) and 1-[1-(3-bromoacetyloxyphenyl)cyclohexyl]-1,2,3,6-tetrahydropyri din e (Bromoacetyl-PCP) were the most potent and selective inhibitors of the binding of [3H]TCP, while DIGIT was the most selective inhibitor of the binding of [3H]DTG. Future studies will examine the selectivity of these agents in vivo after intracerebroventricular administration.

Specificity of phencyclidine-like drugs and benzomorphan opiates for two high affinity phencyclidine binding sites in guinea pig brain

Recently, the presence of two high affinity binding sites for phencyclidine were described in guinea pig brain, with one site coupled to the glutamate excitatory amino acid receptor, specifically activated by N-methyl-D-aspartate (NMDA) (site 1) and the other site associated with the dopamine (DA) reuptake carrier (site 2). Phencyclidine and its analogs, as well as the benzomorphan opiates, are known to interact with binding sites for phencyclidine. In this study, the equilibrium dissociation constants (Kd) of these compounds for the two binding sites for phencyclidine were determined. Phencyclidine and 1-[1-(2-thienyl)cyclohexyl]piperidine (TCP), an analog of PCP, were essentially non-selective between the two sites and also were the two drugs of the group observed to have the highest affinity for site 2. (+)-5-Methyl-10,11-dihydro-5H-dibenzo[a,d]cycloheptene-5,10-imine [(+)MK801] was the most selective agent for site 1, while none of the drugs tested showed selectivity for site 2. In humans, phencyclidine produces psychotomimetic effects, while (+)MK801 has been reported to produce minimal, if any, psychotomimetic effects, at doses sufficient to reduce seizures. These clinical observations, in conjunction with the present biochemical binding data, suggest that (+)MK801 may serve as a "marker" for site 1 and that the psychotomimetic effects of phencyclidine might be mediated by site 2.

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.

Computer-assisted analysis of dextromethorphan and (+)-3-(-3-hydroxyphenyl)-N-(1-propyl)piperidine binding sites in rat brain. Allosteric effects of ropizine

Computer-assisted analysis of self- and cross-displacement studies between dextromethorphan (DM) and (+)-3-(3-hydroxyphenyl)-N-(1-propyl) piperidine ((+)-3-PPP) demonstrated in the rat brain the existence of two high-affinity and one low-affinity binding site for each ligand. One high-affinity site is the common DM1/sigma 1 site, the affinity of which is allosterically increased 4 to 5-fold by 10 microM ropizine. The Kd values of the DM1/sigma 1 for DM and (+)-3-PPP are 17 and 11 nM respectively. DM binds to the second high-affinity site (R2) with a Kd of 15 nM; this site has low affinity for (+)-3-PPP. Conversely, (+)-3-PPP binds with high affinity (Kd 53 nM) to another site (R3), that has low-affinity for DM. The Bmax of the common DM1/sigma 1 site in the rat is about ten times smaller than that in the guinea pig. Thus, extreme caution should be exercised in extrapolating from one species to another. Since DM and most sigma ligands bind to more than one site, not all of which are shared, it is important not to attribute the complex pharmacological effects of these ligands to a single hypothetical receptor.

Sigma binding sites in the brain; an emerging concept for multiple sites and their relevance for psychiatric disorders

An increasing amount of evidence suggests the existence of specific binding sites for psychotomimetic drugs from the opiate-benzomorphan and arylcyclohexylamine series. The sigma binding sites have preferential affinity for the dextrorotatory isomers of certain opiate benzomorphans, such as (+)SKF 10047, (+)cyclazocine and (+)pentazocine and also for some neuroleptics (e.g., haloperidol). The PCP receptor has preferential affinity for phencyclidine (PCP) analogs and other non-competitive N-methyl-D-aspartate (NMDA) receptor antagonists. The physiological significance of the PCP receptor is associated with the blockade of the NMDA type of the glutamate receptor, implying a neuroprotective role of the PCP receptor. However, the significance of the sigma binding sites is less conspicuous. It is not only that drugs from distinct pharmacological classes display a certain degree of affinity for the "sigma/haloperidol" binding sites, but also that drugs which do not induce or block psychotomimetic activity, i.e., (+)3-(3-hydroxyphenyl)-N-(1-propyl) piperidine [(+)3-PPP] and 1,3-di-o-tolyl-guanidine (DTG), display relatively high affinity for the sigma binding sites. The diversity of the compounds which are proposed to interact with the sigma receptors and the variety of the responses elicited by these drugs suggest the existence of sigma receptor subtypes. The finding that the type A of monoamine oxidase (MAO) inhibitors, which are used in treatment of affective disorders, display high affinity for the sigma binding sites suggests their involvement in affective or schizoaffective disorders. Revealing the existence of sigma receptor subtypes may help to elucidate their association with various psychiatric disorders.