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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.

Analgesics

Phencyclidine suppresses hippocampal long-term potentiation through stereospecific activation of phencyclidine receptors.

The effects of phencyclidine and the dioxolane enantiomers, dexoxadrol and levoxadrol, on long-term potentiation in the hippocampus were compared. Field potentials were evoked by stimulation of Schaffer collaterals and recorded from the CA1 region. Long-term potentiation was induced by stimulation with a single train of 25 pulses at 50 Hz. The drugs were delivered by pressure, 1 min before tetanization. Phencyclidine and its receptors ligand, dexoxadrol, abolished the induction of long-term potentiation. Levoxadrol which has very low affinity for the phencyclidine receptor was devoid of this action although it reduced the magnitude of long-term potentiation. These results indicate that phencyclidine blocks long-term potentiation by stereospecific activation of phencyclidine receptors.

Analgesics

Multiple mode of binding of phencyclidines: high affinity association between phencyclidine receptors in rat brain and a monovalent ion-sensitive polypeptide.

Two populations of phencyclidine (PCP) binding sites are shown to exist in the rat brain: a high-affinity monovalent ion-sensitive site (Kd of 10-14 nM for [3H]TCP, [3H]N-[1-(2-thienyl)cyclohexyl]piperidine), which exists in both the frontal cortex and the hippocampus, and a lower affinity site (Kd of 80-130 nM for [3H]TCP) which is found in the hippocampus but not in the frontal cortex. The nature of the interactions between the ion-binding sites and the high affinity PCP receptors depend on both ligand structure (PCP or TCP) and the ion involved (K' or Na'). The high-affinity sites are associated with an Mr 90,000 polypeptide whose labeling by [3H]azido phencyclidine is selectively inhibited by monovalent ions.

Animals

Genetic polymorphism of cytochrome P-450-dependent phencyclidine hydroxylation in mice. Comparison of phencyclidine hydroxylation in humans.

A mouse model of P-450 phencyclidine 3-cyclohydroxylase (P-450 PCP 3-cyclohydroxylase) genetic polymorphism is described. Up to a 3-fold difference was observed in the constitutive liver microsomal activity of P-450 PCP 3-cyclohydroxylase between "slow" (A/J and DBA/2J) and "rapid" (C57BL/6J and BALB/CJ) phencyclidine (PCP) metabolizers. The segregation of slow and rapid hydroxylator phenotypes between 17 recombinant inbred mouse strains derived from A/J and C57BL/2J mice suggests control of the activity by a single gene located on the X-chromosome or, less likely, on chromosome 17. A liver deficiency of P-450 PCP 3-cyclohydroxylase was observed in the New Zealand rabbit and Wistar rat, as well as in some human subjects. Any relationship of P-450 PCP 3-cyclohydroxylase polymorphism to other well characterized P-450 polymorphisms in mice (aryl hydrocarbon hydroxylase and coumarin hydroxylase) was excluded on the basis of differences in inducibility and activity distribution among the inbred mouse strains. Lack of relationship to the P-450 debrisoquine hydroxylase was confirmed by direct comparison of both activities in the same mouse and human liver microsomes. The pharmacological consequence of the observed polymorphism in mice appears to be that the rapid PCP metabolizers are more resistant to the effects of PCP compared to the slow metabolizers as based upon its ED50 and duration of action in A/J and C57BL/6J mice. The relevance of this data to humans remains to be determined, but clearly the latter show marked differences in PCP 3-cyclohydroxylase activity, which separate into low, intermediate, and high groups.

Animals

Phencyclidine and related compounds evoked [3H]dopamine release from rat mesencephalic cell cultures by a mechanism independent of the phencyclidine receptor, sigma binding site, or dopamine uptake site.

At concentrations greater than or equal to 100 microM, phencyclidine (PCP), N-(1-(2-thienyl)-cyclohexyl)piperidine (TCP), and MK-801 induced [3H]dopamine release from dissociated cell cultures of rat mesencephalon. This release was Ca2+ independent and tetrodotoxin insensitive. Tetrodotoxin (2 microM) itself had no effect on spontaneous release of [3H]dopamine. [3H]Dopamine release was induced by 1,3-di(2-tolyl)guanidine, a sigma ligand, and by 4-aminopyridine (1-3 mM), a K+ channel blocker. No stereoselectivity was observed for [3H]dopamine release evoked by the dioxadrol enantiomers, dexoxadrol, and levoxadrol, or by enantiomers of N-allylnormetazocine (SKF 10,047). The selective dopamine uptake inhibitor 1-(2-[bis(4-fluorophenyl)methoxy]ethyl)-4-(3-phenylpropyl)piperazine dihydrochloride (GBR 12909) did not affect spontaneous or TCP-evoked [3H]dopamine release. Together, these data suggest that the dopamine-releasing effects of PCP-like compounds on the mesencephalic cells were not mediated by actions at the PCP receptor or sigma binding site, Ca2+, or Na+ channels, or at the high affinity dopamine uptake site. It remains conceivable that blocking actions of PCP-like compounds at voltage-regulated K+ channels may at least partly explain the response. These results are discussed in comparison with findings in intact brain.

2-Amino-5-phosphonovalerate

The psychotomimetic drug phencyclidine labels two high affinity binding sites in guinea pig brain: evidence for N-methyl-D-aspartate-coupled and dopamine reuptake carrier-associated phencyclidine binding sites.

Numerous studies have now demonstrated that a binding site for the psychotomimetic drug phencyclidine (PCP) exists within the receptor channel complex for the excitatory amino acid neurotransmitter glutamate, specifically the glutamate receptor selectively activated by N-methyl-D-aspartate (NMDA). Several lines of evidence support the hypothesis that all PCP receptors in rat brain are associated with the NMDA receptor complex. In the present study, we reexamine this hypothesis. We report that the PCP analog [3H]1-[1-(2-thienyl)cyclohexyl]piperidine [( 3H]TCP) labels two high affinity binding sites in membranes prepared from guinea pig brain site 1 (Kd = 14.1 nM, Bmax = 631 fmol/mg of protein) and site 2 (Kd = 46.5 nM, Bmax = 829 fmol/mg of protein). (+)-5-Methyl-10 11-dihydro-5H-dibenzo[a,d]cyclohepten-5,10-imine maleate bound to site 1 with high affinity (Kl = 3.2 nM) and to site 2 with low affinity (Kl = 5208 nM). The order of potency of drugs for inhibiting [3H]TCP binding to site 1 correlated with their ED50 values for inhibition of NMDA-mediated responses reported in the literature, whereas the order of potency of drugs for inhibiting [3H]TCP binding to site 2 correlated with their ED50 values for inhibition of [3H]dopamine reuptake reported in the literature. Kinetic experiments demonstrated that glutamate, 2-amino-7-phosphonoheptanoic acid, and Mg2+ modulated [3H]TCP binding to site 1 but not site 2. Preincubation of guinea pig striatal membranes with varying concentrations of the high affinity dopamine reuptake inhibitors N-[1-(2-benzo(b)thiophenyl)cyclohexyl]piperidine and 1-[2-[bis(4-fluorophenyl)methoxy]ethyl]-4-[3- phenylpropyl]piperazine caused a wash-resistant inhibition of [3H]TCP binding to site 2 but not site 1. Taken collectively, these data demonstrate the existence of a high affinity PCP binding site associated with the dopamine reuptake carrier and raise the possibility that the therapeutic and psychotomimetic effects of PCP in humans are separable and mediated via different binding sites.

Acetylcholine

Structural determinants of affinity for the phencyclidine binding site of the N-methyl-D-aspartate receptor complex: discovery of a rigid phencyclidine analogue of high binding affinity.

To learn more about the binding conformation of phencyclidine (PCP) and to arrive at analogues of higher affinity, which may serve as noncompetitive N-methyl-D-aspartate receptor antagonists, eight optically pure PCP analogues were designed with the aid of computer. These compounds represent conformationally constrained versions of PCP in which the motion of the phenyl ring is frozen, thus allowing a determination of the orientation of the phenyl ring relevant to binding. The analogues were synthesized by a Diels-Alder strategy and tested in a radioligand binding assay to evaluate their affinity for the PCP binding site of the N-methyl-D-aspartate receptor complex. One of the analogues was found to bind with nanomolar affinity (IC50 = 19 nM) and to be 73-fold more potent in binding than its enantiomer. These results, which further elucidate the structural determinants of high affinity binding, should aid both in the design of higher affinity molecular probes of the PCP binding site and in the discovery of potential neuroprotective agents.

Animals

Inhibition of carbachol-induced inositol phosphate accumulation by phencyclidine, phencyclidine-like ligands and sigma agonists involves blockade of the muscarinic cholinergic receptor: a novel dioxadrol-preferring interaction.

The effect of phencyclidine (PCP) on carbachol-induced phosphoinositol hydrolysis was examined in rat brain slices taken from cortex, caudate-putamen and hippocampus. In all three regions studied, PCP significantly inhibited carbachol-induced [3H]inositol phosphate accumulation working as low as 10(-6) M in the cerebral cortex. Because PCP has been shown to act at two sites, a PCP-site and a sigma site, various PCP-like agonists [levoxadrol (Lev), dexoxadrol (Dex) and MK-801 [(+)-5-methyl-10,11-dihydro- 5H-dibenzo(a,b)cyclo-hepaten-5, 10-imine maleate]] as well as sigma agonists [(+)-SKF10047 and 1,3-di(2-toly)guanidine (DTG) were examined for their effects on carbachol-induced phosphoinositol hydrolysis. All but MK-801 significantly inhibited the carbachol action; however, their order of potencies, Lev greater than or equal to Dex much greater than PCP greater than or equal to DTG greater than or equal to (+)-SKF10047 differed from those of other known PCP interactions at PCP and sigma sites. Inasmuch as it is known that PCP competes for binding at muscarinic sites, we examined the effects of PCP, Lev, Dex, DTG and MK-801 on the binding of L-[3H]-3-quinuclidinyl benzilate to its muscarinic site. All blocked L-[3H]-3-quinuclidinyl benzilate binding and exhibited a rank order of potency almost identical to that obtained in the inositol studies with Lev greater than Dex much much greater than DTG much greater than PCP MK-801. In addition, the IC50 values obtained from both studies were very similar. It is concluded that PCP, PCP-like compounds and sigma agonists block carbachol-induced inositol-phosphate accumulation by blockade of muscarinic receptors.(ABSTRACT TRUNCATED AT 250 WORDS)

Animals

Effect of dopaminergic drugs on enhancement of the binding of quinuclidinyl benzilate by phencyclidine in vivo.

Phencyclidine has been shown to enhance the specific binding of [3H]quinuclidinyl benzilate (QNB) in the brain of the mouse when both compounds are given in vivo. Since a variety of studies indicate that dopaminergic (DA) systems are involved in the action of phencyclidine, it seemed that DA systems might mediate the enhancement of the binding of QNB by phencyclidine. The results of the present studies demonstrate that the neurotoxin 6-hydroxydopamine, the DA D1 antagonist (R)-8-[chloro]-2,3,4,5-tetrahydro-3-methyl-5-phenyl-1-H-3-benzazepine-7- ol) (SCH23390), the DA D1 agonist p-dibenzylamino benzonitrile (SKF38393) and the DA D2 antagonist spiperone, all failed to modify the ability of phencyclidine to enhance the accumulation of QNB in brain. Quinpirole, a DA D2 agonist diminished accumulation of QNB after phencyclidine, however, this effect was confounded by quinpirole decreasing the non-specific binding of QNB, as well as increasing concentrations of QNB in plasma, when given with phencyclidine. In contrast, haloperidol, a DA antagonist, decreased the enhanced specific binding of QNB after phencyclidine without any apparent confounding influences. Since haloperidol is also known to strongly inhibit the binding of ligands to the sigma receptors, it was hypothesized that the actions of phencyclidine and haloperidol on the binding of QNB in vivo may be through sigma receptors rather than DA systems.

2,3,4,5-Tetrahydro-7,8-dihydroxy-1-phenyl-1H-3-ben

Spatial and temporal patterning distinguishes the locomotor activating effects of dizocilpine and phencyclidine in rats.

A behavioral pattern monitor was used to assess the effects of dizocilpine (MK-801) and phencyclidine on the spatial and temporal patterns of locomotion and investigatory behavior in rats. The monitor provided both quantitative measures of crossovers, rearings and holepokes and qualitative measurement of the spatial and temporal patterns of locomotion. Dizocilpine (0.004-0.5 mg/kg) and phencyclidine (0.25-5.0 mg/kg) produced similar, dose-dependent increases in locomotor activity. At small doses, dizocilpine and phencyclidine increased investigatory holepokes, while at larger doses, both drugs significantly decreased the number of holepokes. Rearings were reduced similarly by the larger doses of each drug. Both dizocilpine and phencyclidine produced perseverative spatial patterns of locomotion, especially at larger doses. However, the locomotor patterns produced by these drugs were found to be dissimilar in spatial quality. After phencyclidine, animals frequently circled the perimeter of the monitor chamber or moved repetitively in horseshoe or figure-8 patterns. By contrast, rats given dizocilpine completed small rotations about either end of the chamber. Pretreatment with a small dose (0.02 mg/kg) of haloperidol, prior to either dizocilpine (0.5 mg/kg) or phencyclidine (5.0 mg/kg) had no effect on the increase in locomotor activity or the decreases in investigatory holepokes produced by the drugs. However, haloperidol altered the effects of phencyclidine on the spatial and temporal patterns of locomotion, suggesting that sigma receptors or other haloperidol-sensitive binding sites, may influence the quality but not the quantity of phencyclidine-induced hyperactivity.

Animals

Cardiac actions of phencyclidine in isolated guinea pig and rat heart: possible involvement of slow channels.

The mechanisms responsible for the positive inotropic effect of phencyclidine were studied in isolated preparations of guinea pig and rat heart. In electrically paced left atrial muscle preparations, phencyclidine increased the force of contraction; rat heart muscle preparations were more sensitive than guinea pig heart muscle preparations. The positive inotropic effect of phencyclidine was not significantly reduced by a combination of phentolamine and nadolol; however, the effect was competitively blocked by verapamil in the presence of phentolamine and nadolol. Inhibition of the outward K+ current by tetraethylammonium chloride also produced a positive inotropic effect; however, the effect of tetraethylammonium was reduced by phentolamine and nadolol, and was almost insensitive to verapamil. The inotropic effect of phencyclidine was associated with a marked prolongation of the action potential duration and a decrease in maximal upstroke velocity of the action potential, with no change in the resting membrane potential. In partially depolarized atrial muscle preparations, which were reactivated by elevation of the extracellular Ca2+ level and stimulation at a high voltage, phencyclidine markedly increased the amplitude and the duration of the slow action potentials. Rat heart muscle preparations had higher sensitivity to these effects of phencyclidine. The specific [3H]phencyclidine binding observed with membrane preparations from guinea pig ventricular muscle was saturable with a single class of high-affinity binding site. This binding was inhibited by verapamil, diltiazem, or nitrendipine, but not by ryanodine or tetrodotoxin. These results suggest that the positive inotropic effect of phencyclidine results from enhanced Ca2+ influx via slow channels, either by stimulation of the channels or secondary to inhibition of outward K+ currents.

Animals

Some structure activity relationships of phencyclidine derivatives as anticholinergic agents in vitro and in vivo.

Phencylidine derivatives exhibit multiple interactions with cholinergic systems: they block nicotinic and muscarinic receptors,and inhibit both acetyl and butyrylcholinesterase. In peripheral tissue, the net pharmacological effects of the phencyclidines is antiacetylcholine activity. The dissociation constants measured in isolated smooth muscle and from competition experiments for the muscarinic high-affinity binding sites in brain homogenates (Kd = 10(-5) - 10(-6) M) are 3--4 orders of magnitude lower than those of anticholinergic glycolate esters. However, phencyclidines have comparable potency to that of d-tubocurarine in blocking the nicotinic receptor in the isolated frog rectus abdominis (Kd = 10(-6) M). Brain uptake experiments of (3H) labeled phencyclidine showed that during the time period in which central effects are observed with these drugs their concentration in brain reaches values close to the Kd (10(-5) - 10(-6) M). This finding, and the cross tolerance observed in vivo between phencyclidine and other centrally acting cholinergic drugs supports the possible involvement of cholinergic interactions in the psychotropic action of phenyclidine derivatives. Quantum chemical calculations of the interaction pharmacophores of drugs in the phencyclidine series have indicated the molecular determinants for the interaction of these drugs with the muscarinic receptor. The calculations revealed that these drugs can match the reactivity characteristics of ACh and the semi-rigid muscarinic agonist 3-acetoxyquinuclidine, but their rigid molecular frame will be conductive to antagonistic rather than agonistic activity when the drug-receptor complex is formed. The identification of a "cholinergic interaction pharmacophore" for these drugs by quantum mechanical calculations made possible the suggestion of other active phencyclidine derivatives, e.g. p-NH2 and p-OH analogs which proved to be equipotent to phencyclidine. The inactivity of the p-NO2 derivative was also predicted on this basis and served as an additional confirmation of the theoretical criterion for activity; the difference between the activities of the ethynyl and cyano derivatives was explained by the modification of the cholinergic interaction pharmacophore. On the basis of these theoretical predictions, electrophysiological studies were carried out by the others and the results prompted the suggestion that "physostigmine is of potential value in the treatment of post-operative patients emerging from ketamine anesthesia and in the treatment of phencyclidine overdosed patients".

Animals

A comparison of the psychopharmacological profiles of phencyclidine, ketamine and (+) SKF 10,047 in the trimethyltin rat model.

The potential neuroprotective effects of phencyclidine, ketamine and (+) SKF 10,047 were investigated in the trimethyltin (TMT)-treated rat. Of the three drugs used in this study, only phencyclidine (5 mg/kg i.p.) reversed the behavioral hyperactivity and deficits in spatial localization of TMT-treated rats. Neurochemically, phencyclidine and (+) SKF 10,047 were without effect on the neurotransmitters (e.g. noradrenaline, dopamine, serotonin and 5-hydroxindole 3-acetic acid), examined in the amygdaloid cortex and hippocampal regions, while ketamine increased the steady state concentrations of 5-HIAA in the amygdaloid cortex. These results suggest the involvement of the phencyclidine receptor in reversal of the behavioural impairments produced by TMT in rats. The significance of these results with respect to phencyclidine and sigma receptors is discussed. The lack of effect of (+) SKF 10,047 in this model may reflect behavioural differences between phencyclidine and sigma ligands. It may be concluded that the TMT model can be exploited for studying the mechanism of action of molecules liable to have an effect at the phencyclidine receptor site, as opposed to the sigma receptor.

Animals

Plasma phencyclidine pharmacokinetics in dog and monkey using a gas chromatography selected ion monitoring assay.

Phencyclidine was determined by gas chromatography selected ion monitoring in six dogs and seven monkeys. Aliquots of venous blood were taken over 4 h in the monkey after 1.1 mg kg-1 and over 24 h in the dog after 1.0 mg kg-1 of phencyclidine i.v. Pentadeuterated phencyclidine was used as the internal standard. In the electron impact mode the most abundant fragments in the mass spectrum of phencyclidine were m/e 91 and 200, and 96 and 205 in the [2H5]phencyclidine spectrum. These fragments were used to quantitate the amount of phencyclidine present. In both species, a complex exponential decline of plasma phencyclidine was found in most animals that fit a two compartment open model. In monkeys, the mean half-life (beta phase) was 2.36 h and in the dog it was 2.86 h. Compared with the monkey, the dog considerable emergence delirium. The two species had rather different pharmacokinetics which may be relevant to the observed differences in degree of anesthesia and recovery.

Animals

Phencyclidine and ketamine: comparison with the effect of cocaine on the noradrenergic neurones of the rat brain cortex.

In slices of rat occipital cortex, the influence of phencyclidine and ketamine on the accumulation of 3H-noradrenaline and the subsequent outflow of tritium was investigated, and was compared with the effect of cocaine.--All three drugs inhibited the accumulation of tritium during incubation of the slices with 3H-noradrenaline. Phencyclidine was slightly, whereas ketamine was much less effective than cocaine.--All three drugs accelerated the spontaneous outflow of tritium from slices preincubated with 3H-noradrenaline. The acceleration caused by low concentrations probably reflects an inhibition of the re-uptake of spontaneously released 3H-noradrenaline; in addition, high concentrations (10(-4) M phencyclidine, 3 X 10(-4)-10(-3) M cocaine and 10(-3)-3 X 10(-3) M ketamine) appear to release tritiated compounds from the neurones. The distance between uptake-inhibiting and releasing concentrations was much greater for cocaine than for phencyclidine and ketamine.--All three drugs enhanced the overflow of tritium evoked by electrical field stimulation. The increase probably reflects an inhibition of the re-uptake of released 3H-noradrenaline; in addition, phencyclidine appears to enhance the release of noradrenaline per pulse.--The actions of phencyclidine and ketamine on central noradrenergic neurones may contribute to the characteristic psychotropic side-effects of these general anaesthetics.

Animals

Phencyclidine-induced head-weaving and head-twitch through interaction with 5-HT1 and 5-HT2 receptors in reserpinized rats.

Phencyclidine mainly produced head-weaving and head-twitches at doses of 5-7.5 mg/kg and of 7.5-12.5 mg/kg, respectively. Phencyclidine-induced head-twitches and head-weaving were blocked by pretreatment with ritanserin (1 mg/kg), a selective serotonin (5-HT)2 receptor antagonist and with pindolol (20 mg/kg, s.c.), a 5-HT1 receptor antagonist, respectively. In reserpine-pretreated rats, the degree of utilization of 5-HT and the number of 5-HT1 ([3H]5-HT) and 5-HT2 ([3H]ketanserin) binding sites were significantly increased compared with the figures for the vehicle-pretreated rats. The intensity of phencyclidine-induced head-weaving (at the dose of 2.5 mg/kg) and head-twitch (at the doses of 2.5 and 5 mg/kg) was significantly increased in reserpine-pretreated rats compared with that of vehicle-pretreated rats. Furthermore, in the reserpine-pretreated rats, the intensity of phencyclidine (1.25 mg/kg)-induced head-weaving and head-twitches was increased in combination with imipramine, while the intensity of phencyclidine (2.5 mg/kg)-induced head-weaving and head-twitch was decreased by pretreatment with mianserin, a non-selective 5-HT receptor antagonist. These results indicate that phencyclidine induced head-weaving by interacting with 5-HT1 receptors, indirectly after the release of 5-HT and/or with some other mechanisms and induced head-twitch by interacting with 5-HT2 receptors directly and/or indirectly.

Animals

Interaction of phencyclidine ("angel dust") with a specific receptor in rat brain membranes.

[3H]Phencyclidine binds to synaptic membranes from rat brain in a saturable, reversible, and selective fashion, with a dissociation constant Kd of 0.25 microM and a maximal binding capacity of 2.4 pmol/mg of membrane protein--i.e., 250 pmol/g of brain. The binding activity is concentrated in synaptosomal fractions, is higher in cerebral cortex and corpus striatum than in other parts of the rat brain, and is not detectable in the spinal cord. Only molecules of the phencyclidine series and ketamine are able to bind to the phencyclidine receptor. [3H]Phencyclidine bound to its receptor is not displaced by the classical neurotransmitters or neuromodulators. There is a good correlation between the apparent affinities of a series of phencyclidine analogs for the phencyclidine receptor and the pharacological activities of these analogs as measured by the rotarod assay.

Animals