Binding of phencyclidine to rat brain membranes: technical aspect

Tris inhibits (+)-[3H]SKF-10,047 binding to sigma receptors

Tris-HCl is the most commonly used buffer in studies of radioligand binding to sigma receptors, with concentrations as high as 50 or 100 mM often used. We report here that these concentrations of Tris substantially inhibit (+)-[3H]SKF-10,047 binding to sigma receptors. The well-established inhibitory effect of Tris-HCl on ligand binding to PCP receptors did not contribute to the presently reported inhibition of (+)-[3H]SKF-10,047 binding. The IC50 of Tris, determined in the presence of 10 mM potassium phosphate buffer, was 15.4 +/- 1.2 mM (n = 3, pH 8.0, 25 degrees C, 1 nM radioligand). Equilibrium saturation studies revealed an apparent competitive inhibition of binding.

Phencyclidine/SKF-10,047 binding sites: evaluation of function

Results of correlation analyses comparing rank-order affinities with rank-order potencies of (+)SKF-10,047, phencyclidine (PCP), and several PCP analogs support the involvement of [3H]-1-[1-(2-thienyl)cyclohexyl]piperidine binding sites (TCP sites) in mediating both the discriminative stimulus properties of PCP and production of 180 degrees perseveration in a 4-arm radial maze. For the same group of drugs, no significant relationship was found to exist between affinities at haloperidol-sensitive (+)[3H]SKF-10,047 binding sites (H-S-SKF sites) and potencies. Also, H-S-SKF sites were found to lack pharmacological selectivity and to be localized in the microsomal fraction of cells. It is concluded that TCP sites may represent receptors which mediate effects not only of PCP, but also of (+)SKF-10,047. In addition, the possibility that H-S-SKF sites may represent a type of membrane-bound enzyme is discussed.

Whole-body autoradiographic localization of [3H]phencyclidine and its metabolites in mice

When evaluated by whole-body autoradiography (WBAR) and quantitative densitometry, [3H]phencyclidine (PCP) equivalents were found to be removed rapidly from blood, after a single iv dose in mice, and avidly taken up as early as 1 min after dosage by glandular tissues including thyroid, salivary glands, pancreas, pituitary and, most prominently, by stomach mucosa. Stomach:blood [3H]PCP concentration ratios showed that rapid secretion of [3H]PCP from mucosa to the stomach contents occurred within 2 min after dosing. During early intervals, chromatographic analysis of tissue sections demonstrated that PCP was present in brain, liver, and gut primarily in its unaltered chemical form. Mice killed at 60 and 120 min showed persistently high levels of [3H]PCP equivalents within the stomach and intestines, these levels being the highest of all other tissues densitometrically measured. The early time course and magnitude of [3H]PCP uptake by stomach glandular mucosa strongly suggests that cycling of PCP occurs principally through gastroenteric recirculation. Very striking was the high concentration of [3H]PCP radioactivity observed within the adrenal as early as 5 min. The concentration of [3H]PCP equivalents in pituitary, choroid plexus, cortex, hippocampus, and thalamus was highest at 1-20 min following injection. Application of high-resolution quantitative WBAR was found to be a useful tool in the study of the biodistribution of labeled PCP, especially during very early post-treatment time points where alternative tissue counting techniques would not be feasible.

Gestational exposure to phencyclidine (PCP) in rats decreases PCP binding sites in term fetal brain

Pregnant Sprague-Dawley rats were treated with 5 mg/kg body weight of phencyclidine (PCP) injected at 1 ml/kg subcutaneously on three consecutive days at four different stages of gestation. Within 10-30 min after treatment, dams showed some lack of motor coordination and became lethargic. On gestational day 21, all rats were killed by decapitation and brains were dissected and stored from mother and fetus for neurochemical analysis. PCP, dopamine and muscarinic cholinergic receptor binding was measured in membranes prepared from maternal and fetal whole brain. Neurotransmitter concentrations were also measured in the fetal brain homogenates. There was a significant decrease in PCP binding sites in fetal but not maternal brains after maternal PCP injection at gestational days 12-14, 15-17 and 18-20, but not at 9-11 days. Dopamine and muscarinic cholinergic receptor binding was not significantly altered in fetal or maternal brain when compared with vehicle control animals. The whole brain dopamine, 3,4-dihydroxyphenylacetic acid, serotonin, and 5-hydroxyindoleacetic acid concentrations did not show significant change in any group studied. These data indicate that gestational exposure to PCP decreases high affinity binding of PCP in term fetal brain at doses which do not alter maternal PCP receptor binding.

An endogenous ligand of the brain sigma/PCP receptor antagonizes NMDA-induced neurotransmitter release

The present study provides evidence for the presence of an endogenous ligand for the phencyclidine (PCP) receptor of mammalian brain. Partially purified bovine hippocampal extracts potently and dose dependently inhibit binding to PCP receptors of [3H]N-(1-[2-thienyl]-cyclohexyl)piperidine (TCP), a highly potent and specific ligand of PCP receptors. In addition to demonstrating PCP-like binding properties, the partially purified extract mimics biological actions of PCP upon neurotransmitter release. HPLC fractions active in the [3]TCP binding assay, by contrast to fractions inactive in the binding assay, potently elicited stimulation of spontaneous acetylcholine and dopamine efflux and inhibited NMDA-stimulated release of acetylcholine and dopamine. The transmitter release assay provides validation of a PCP-like physiological activity exerted by bovine hippocampal extracts partially purified by HPLC.

(+)- and (-)-N-allylnormetazocine binding sites in mouse brain: in vitro and in vivo characterization and regional distribution

In vivo and in vitro binding studies, both in whole brain and in selected areas, indicate that non-identical (+)- and (-)-NANM sites exist in the mouse brain, and each exhibits a different regional distribution. The in vivo binding of (+)-3H-NANM was found to be saturable at pharmacologically relevant doses, and represents a relatively small (10-22%) portion of total brain (+)-3H-NANM concentrations. The in vivo binding of (+)-3H-NANM was selectively displaced by (+)-NANM and PCP, and more sensitive to haloperidol and (+)-ketocyclazocine than the (-)-3H-NANM site. The in vivo binding of (-)-3H-NANM was selectively displaced by (-)-NANM, and more sensitive to naloxone and (-) ketocyclazocine than the (+)-3H-NANM site, and insensitive to PCP. This study indicates that the investigation of NANM binding sites is possible using in vivo binding techniques, and that each isomer apparently binds, in the mouse brain, to a single class of distinct sites.

[3H]thienyl-phencyclidine ([3H]TCP) binds to two different sites in rat brain. Localization by autoradiographic and biochemical techniques

A high affinity [3H]thienyl-phencyclidine ([3H]TCP) binding and its similarity to that of [3H]phencyclidine ([3H]PCP) have been demonstrated on whole rat brain homogenates. We now describe the regional distribution of the [3H]TCP binding sites in the rat brain with fixed sections and frozen slide-mounted sections visualized by autoradiography and with homogenates of 12 regions by direct binding experiments. The 3 techniques give a similar pattern for the [3H]TCP binding distribution and the biochemical study reveals that two distinct binding sites for [3H]TCP exist: one of high affinity (5-10 nM) in the forebrain, which should be responsible for the psychotropic effects and a second one of lower affinity (50-80 nM) in the hindbrain and the spinal cord, which should be involved in the extrapyramidal behavior induced by PCP and congeneers. Competition experiments have shown that muscarinic compounds interact only with the hindbrain receptor possibly in two different sites, although morphine interacts with a very low affinity with the forebrain's high affinity receptor. Results obtained with SKF-10,047 (N-allylnormetazocine) seem to indicate that TCP and sigma-receptors are different.

Phencyclidine-induced stereotyped behaviors after injection of morphine and N-allylnormetazocine (SKF 10,047) in rats

We investigated the effects of N-allylnormetazocine (SKF 10,047) and morphine on the stereotyped behaviors induced by the intraperitoneal injection of phencyclidine (PCP). PCP-induced turning and backpedalling were significantly potentiated by pretreatment with SKF 10,047 (10 mg/kg) but sniffing and head weaving were not. On the other hand, pretreatment with morphine dose-dependently attenuated PCP-induced sniffing and head weaving, but not turning and backpedalling. These results suggest that PCP-induced stereotypy may be mediated by not only a sigma opioid receptor but also some other receptors. In addition, each component of PCP-induced stereotypy may be controlled by different opioid systems and/or neuronal systems.

Behavioral and receptor binding studies of phencyclidine (PCP) and lithium interaction in the rat

Three groups of female Sprague-Dawley rats (n = 4) were conditioned to drink water during a daily 2 hr session. The water was then changed to a solution of 1.0 mg/ml lithium chloride producing average doses between 62.9 and 72.1 mg/kg/day for Groups I and II. These rats were challenged with 4 mg/kg PCP i.p. before and during lithium treatment. Group I was tested for spontaneous locomotor activity in the open field apparatus. Lithium alone did not affect activity. After 1, 2, and 3 weeks of chronic lithium, PCP-induced activity increased 2.1, 1.7, and 2.8 fold, respectively, relative to PCP-induced activity during limited access to water only. Whole brain homogenates from Group II, after one week of chronic lithium, were used for receptor binding experiments using [3H] PCP; Group III served as water controls. The Kd (nM +/- S.E.M.) was not different in untreated (146.39 +/- 18.95) and lithium-treated (181.22 +/- 14.35) rats. The Bmax (pmole/mg protein +/- S.E.M.), however, was increased 48% (p less than 0.01) from 1.50 +/- 0.08 to 2.22 +/- 0.10 after lithium. These preliminary results suggest that chronic administration of lithium modifies the behavioral effects of PCP possibly via alterations at the receptor level.

Effects of phencyclidine and analog drugs on acetylcholine receptor of cultured muscle cells

Myotubes grown in culture provided a convenient experimental system for the study of the effects of phencyclidine (PCP) and analog drugs on both acetylcholine receptor (AChR) function and on its binding properties. The extent of PCP retention by these cells was studied on the same preparations. PCP, N-ethyl-l-phenylcyclohexylamine (PCE), PCP methiodide (PCPMeI), 1-[1-(3-aminophenyl)-cyclohexyl] piperidine (NH2PCP) and 1-[1-(2-thienyl)cyclohexyl] piperidine (TCP) were found to inhibit carbamylcholine (CbCh)-induced 22Na and 45Ca ion fluxes with 50% inhibition (I50) at 2-6 microM drug concentration. The I50 for CbCh-induced 42K+ efflux was 8-20 microM. Ketamine was less efficient with an I50 of 100 microM. Binding of [125I] alpha-bungarotoxin [( 125I]alpha-BGT) was not affected at drug concentrations that cause 100% inhibition of ion fluxes. Retention of [3H]PCP by the myotubes was a saturable process with half-maximal saturation at approximately 20 microM PCP. It was inhibited by PCP and several tertiary analogs, with and I50 of approximately 20 microM. PCPMeI was much less effective, with an I50 of 1 mM. PCPMeI was, however, as potent as PCP in its inhibition of the AChR function although the amount retained by the cells was 50-fold lower than that of PCP. These results are consistent with the theory that PCP and analog drugs affect AChR at a site other than the alpha-BGT binding site, possibly at the ionic channel of the nicotinic receptor.

Phencyclidine receptors in rat brain cortex

The binding of [3H]phencyclidine (PCP) to receptors in rat brain cortex has been studied. Two receptors have been detected, a high affinity receptor site with a KD of 23.5 +/- 7.4 nM and a low affinity site with a KD of 7.6 +/- 1.8 microM. The binding of [3H]PCP to its receptors was pH and temperature dependent and was destroyed by heat-denaturation. The binding of [3H]PCP was inhibited by compounds which produce PCP-like behavioral effects including dexoxadrol, etoxadrol and ketamine as well as a novel series of benz(f)isoquinolines. The low affinity site was blocked by PCP, etoxadrol and (+)-SKF-10,047 but not morphine or leu-enkephalin, suggesting that it also represents a specific PCP site. Stereoselective displacement of PCP at the high affinity receptor was observed with the isomers of cyclazocine, cyclorphan, SKF-10,047 and dioxadrol (dexoxadrol and levoxadrol). Naloxone, 4,5,6,7-tetrahydroisoxazolo(S,4-C)pyridin-3-ol (THIP) hydrate and haloperidol inhibited binding poorly (Ki greater than 1 microM), suggesting that these compounds do not interact significantly with the high affinity PCP receptor in vivo. The affinity of ligands for the phencyclidine receptor was highly correlated (r = 0.714, P less than 0.01) with their potency to produce catalepsy in pigeons.

Interaction of dextrorotatory opioids with phencyclidine recognition sites in rat brain membranes

The potencies of several dextrorotatory opioids, including four pairs of enantiomers, as inhibitors of specific [3H]PCP binding to rat brain synaptic membranes has been determined. Of the compounds tested unlabeled phencyclidine (PCP) was the most potent followed by (-)-cyclazocine greater than dextrorphan greater than (+) ketamine greater than (+) cyclazocine greater than (+)- SKF10 ,047 greater than levorphanol greater than dextromethorphan greater than (-) SKF10 ,047 greater than (-)-ketamine greater than (+/-) pentazocine and greater than (+/-) ethylketocyclazocine. The opiate mu receptor ligands, morphine, naloxone and naltrexone were virtually inactive as competitors of specific [3H]PCP binding. Unlike the stereostructural requirements for opiate mu receptors where activity resides predominantly in the levorotatory enantiomers, the present results support the contention that binding to the [3H]PCP labeled recognition site may reside in either the levorotatory or the dextrorotatory enantiomer. The specific binding of [3H]PCP which was defined as total binding minus that occurring in the presence of 10 microM dextrorphan was found to be of a high affinity, saturable, reversible and sensitive to thermal degradation. These results suggest that certain dextrorotatory morphinan derivatives may prove to be useful probes in further investigations of the molecular characteristics of the [3H]PCP binding site in brain membrane preparations.

[3H]TCP: a new tool with high affinity for the PCP receptor in rat brain

PCP binding sites have previously been demonstrated in the central nervous system with [3H]PCP. We now describe the binding properties to rat brain membranes of [3H]TCP, a PCP derivative. It is very advantageous to use [3H]TCP instead of [3H]PCP for the 3 following reasons: (i) it has a better affinity (Kd = 7.4 nM) for PCP binding sites than PCP itself; (ii) it dissociates slowly from its binding sites (t 1/2 = 20 min); (iii) the non-specific binding component obtained with [3H]TCP is much lower than that found with [3H]PCP.

Phencyclidine in nanomolar concentrations binds to synaptosomes and blocks certain potassium channels

Phencyclidine [1-(phenylcyclohexyl)piperidine; PCP], in low dose (approximately equal to 0.1-0.2 mg/kg of body weight), induces a schizophrenia-like behavioral syndrome in man; this effect has been attributed to block of neuronal K channels. We used a K-stimulated 86Rb efflux assay to demonstrate that low concentrations of PCP (10-50 nM) block a class of depolarization-activated K channels in rat brain synaptosomes--pinched-off presynaptic nerve terminals. The dose-response curve is biphasic, and much higher PCP concentrations (greater than 10 microM) are required to block the remainder of the K-stimulated 86Rb efflux. The [3H]PCP binding curve for synaptosomes is also biphasic: PCP binds to some components with high affinity (Kd approximately equal to 6.0 X 10(-8) M), and to other components with much lower affinity (Kd approximately equal to 1.15 X 10(4) M). PCP can be photoactivated with UV light to form covalent bonds: after UV irradiation, previously-bound [3H]PCP is no longer displaceable by a large excess of unlabeled PCP. Preliminary data from NaDodSO4/polyacrylamide gel electrophoresis studies after covalent binding of [3H]PCP to synaptosomes, suggest that the high-affinity binding site may be on a large protein (Mr approximately equal to 220,000). We conclude that the high-affinity PCP binding protein is associated with the K channels that are blocked by nanomolar concentrations of PCP. Block of these channels could, by prolonging action-potential duration in presynaptic nerve terminals, enhance calcium entry and neurotransmitter release, thereby altering transmission at central synapses involved in behavioral expression.

Specific binding of [3H]phencyclidine in rat central nervous tissue: further characterization and technical considerations

The interaction of phencyclidine (PCP) with its specific receptor sites in the central nervous system has been further characterized. Kinetic association and dissociation rate constants of 2.9 X 10(6) M-1 and 4.8 X 10(-1) min-1 were determined, yielding a kinetic KD of 1.6 X 10(-7) M, in agreement with the KD previously determined at equilibrium. Permissible separation time of 13 s was calculated from the kinetic data, well above the actual separation time of less than 10 s in the rapid filtration assay. Presoaking of filters in 0.01% poly-L-lysine eliminated displacable [3H]PCP adsorption to filter material. Binding data obtained via centrifugation assays was identical to that obtained with the rapid filtration method. Stereospecificity of the PCP receptor was demonstrated by the finding that (+)-ketamine is four-fold more potent than (-)-ketamine in displacing specifically bound [3H]PCP. Several proteolytic enzymes including trypsin, papain and thermolysin potently inactivated PCP receptors. Detailed regional distribution studies showed highest density of PCP receptors in subicular cortex and hippocampus, intermediate levels in hypothalamus, striatum, frontal cortex and cerebellum, lower levels in brainstem and spinal cord, and negligible levels in corpus callosum, a white-matter control area. Benzomorphan opiates with PCP-like behavioral effects interact with the PCP receptor. These data support the pharmacological relevance of the PCP receptor site as demonstrated by the rapid filtration method.

Biochemical properties of the brain phencyclidine receptor

This paper gives a detailed account of techniques which can be used to measure [3H]phencyclidine binding to its receptor. The main properties of the binding component are the following: (i) It is rapidly heat-inactivated at temperatures over 50 degrees C. (ii) It is destroyed by proteases like trypsin, pronase or papain suggesting that it is of a protein nature. The receptor structure is resistant to chymotrypsin. (iii) A good correlation was found between the pharmacological activity of 30 different analogs as measured by the rotarod assay and the affinity of these different molecules for the phencyclidine receptor. (iv) Monovalent and divalent cations antagonize [3H]phencyclidine binding to its receptor. The dissociation constant is 15 mM, the same for Na+, Li+, K+, cholinium or Tris. Na+ (and other monovalent cations) and phencyclidines bind to distinct sites. The saturation of the Na+ site by Na+ modulates the affinity of phencyclidine for its receptor. Divalent cations antagonize [3H]phencyclidine binding in the absence of Na+. This antagonism is of the non-competitive type. (v) [3H]phencyclidine binding is also antagonized by histrionicotoxin and by local anaesthetics.

Stereospecific binding of 3H-phencyclidine in brain membranes

Phencyclidine (PCP) displaceable binding of 3H-PCP to glass-fiber filters was eliminated and total binding markedly reduced by initial treatment of the discs with 0.05% polyethyleneimine. Assessed with treated filters, unlabeled PCP displaced 3H-PCP in both rat and pigeon brain membranes with an EC50 of 1 microM. Of similar high inhibitory potency were dextrorphan, levorphanol, SKF 10047 and ketamine, while morphine, naloxone and etorphine had EC50 values higher then 1 mM. Using the dissociative anesthetic dexoxadrol and its inactive isomer levoxadrol as displacing agents, stereospecific binding of 3H-PCP was obtained in rat and pigeon brain membranes. The markedly higher potency of dexoxadrol, relative to levoxadrol, in displacing bound 3H-PCP is compatible with behavioral data for these enantiomers. However, they were equipotent in displacing 3H-PCP bound to glass-fiber filters in the absence of tissue. Heat denaturation, but not freezing, abolished stereospecific binding of 3H-PCP, which was also absent in rat liver membranes. The stereospecific binding component in brain displayed biphasic saturability at 60-70 nM and 300-400 nM, respectively.

Effects of phencyclidine and its derivatives on enteric neurones

The effects of N-(1-phenylcyclohexyl) piperidine (PCP) and related drugs on isolated intact segments of the guinea-pig ileum were determined. 1-1-(2-Thienyl) cyclohexylpiperidine (TCP), PCP and ketamine decreased the height of electrically induced contractions (0.1 Hz) of intact segments of isolated guinea-pig ileum. Thirty to forty percent of the inhibition of contraction height (0.1 Hz) was reversed by pretreatment with the pure narcotic antagonist, naloxone. This naloxone-reversible component showed cross-tolerance with morphine. PCP pretreatment caused a shift to the right in the dose-response curve to acetylcholine (ACh) that was not parallel with the control dose-response curve. Thus PCP does not interact with the muscarinic cholinoceptor in a strictly atropine-like competitive fashion. Binding sites for [3H]-PCP were detected in homogenates of the guinea-pig longitudinal muscle-myenteric plexus preparation. The affinity constants and the rank order of potencies of various PCP derivatives competing with [3H]-PCP for binding suggest that these binding sites are very similar to those found in the central nervous system. These data suggest that the guinea-pig isolated ileum may be used as an in vitro system for studying the mechanism of action of phencyclidines.

Phencyclidine (angel dust)/sigma "opiate" receptor: visualization by tritium-sensitive film

[3H]Phencyclidine ([3H]PCP) binds specifically to an apparently single class of binding sites on slide-mounted sections of rat olfactory bulb (Kd = 46 nM; Bmax = 10.5 fmol per slice). Bound [3H]PCP can be displaced by nonradioactive PCP and a series of its analogs with relative potencies that correlate closely (P less than 0.001) with values determined in a rat discrimination test that utilized PCP as a cue. Although morphine, naloxone, and opiate peptides do not displace bound [3H]PCP, psychotomimetic benzomorphans, classed as "sigma opiates," are quite potent displacers in vitro and have PCP-like behavioral properties in vivo. These results suggest that phencyclidine and the sigma opiates act at the same sites. [3H]PCP binding sites were visualized by using tritium-sensitive LKB film analyzed by computerized densitometry and color coding. The [3H]PCP binds most densely to cortical areas, diffusely in neocortex, and somewhat heterogeneously in the laminae of the hippocampal formation and dentate gyrus. Most of the brainstem and spinal cord show low specific [3H]PCP binding, with gray matter generally showing more binding than white.

Compared properties of central and peripheral binding sites for phencyclidine

Phencyclidine and its derivatives bind specifically and reversibly to rat brain and peripheral organs. Binding characteristics are different in brain, lung, kidney, heart and liver. Affinities of phencyclidines for the brain receptor but not those for peripheral organs are correlated with the pharmacological activities of phencyclidines as measured in the rotarod test.