The behavioral effects of phencyclidines may be due to their blockade of potassium channels

Application and assessment of deep learning for the generation of potential NMDA receptor antagonists

Uncompetitive antagonists of the N-methyl d-aspartate receptor (NMDAR) have demonstrated therapeutic benefit in the treatment of neurological diseases such as Parkinson's and Alzheimer's, but some also cause dissociative effects that have led to the synthesis of illicit drugs. The ability to generate NMDAR antagonists in silico is therefore desirable for both new medication development and preempting and identifying new designer drugs. Recently, generative deep learning models have been applied to de novo drug design as a means to expand the amount of chemical space that can be explored for potential drug-like compounds. In this study, we assess the application of a generative model to the NMDAR to achieve two primary objectives: (i) the creation and release of a comprehensive library of experimentally validated NMDAR phencyclidine (PCP) site antagonists to assist the drug discovery community and (ii) an analysis of both the advantages conferred by applying such generative artificial intelligence models to drug design and the current limitations of the approach. We apply, and provide source code for, a variety of ligand- and structure-based assessment techniques used in standard drug discovery analyses to the deep learning-generated compounds. We present twelve candidate antagonists that are not available in existing chemical databases to provide an example of what this type of workflow can achieve, though synthesis and experimental validation of these compounds are still required.

In Vitro and In Vivo Inhibition of Rat Brain Nitric Oxide Synthase Activity by Phencyclidine

Phencyclidine (PCP) is a widely abused psychoactive drug that perturbs many neurotransmitter systems studied to date. Nitric oxide (NO) has been established as a neuronal messenger and its rapid diffusibility across cell membranes makes NO an extensive and versatile messenger in brain development and functioning. The present study was initiated to investigate the effect of PCP on rat brain nitric oxide synthase (NOS) activity both in vitro and in vivo. Brain cytosolic fractions from normal rats were used for in vitro and in vivo studies. The rats were treated with a single dose of PCP (10 mg/kg, intraperitoneally); the brains were removed at 0, 1, 2, 6, and 12 hours after PCP treatment and the cytosolic fractions were prepared by homogenization and centrifugation. NOS activity was assessed by quantifying the release of [3H]-citrulline from [3H]-arginine. PCP significantly inhibited rat brain NOS in vitro in a concentration (0.05–2 mM)-dependent manner. The kinetic evaluation of arginine, NADPH, and Ca2+ activation of NOS revealed that PCP (0.5 mM) inhibited NOS activity competitively with respect to arginine and NADPH and noncompetitively inhibited with respect to Ca2+. PCP also caused a time-dependent reduction of brain NOS activity in vivo as early as 1 hour after treatment. Even after 12 hours of PCP treatment, NOS activity did not reverse to its normal level as compared to the control group, suggesting sequestration and persistence of the drug in the central nervous system. These results suggest that inhibition of brain NOS by PCP might be one of the mechanisms through which PCP causes neurotoxicity.

Ketamine and phencyclidine: the good, the bad and the unexpected

The history of ketamine and phencyclidine from their development as potential clinical anaesthetics through drugs of abuse and animal models of schizophrenia to potential rapidly acting antidepressants is reviewed. The discovery in 1983 of the NMDA receptor antagonist property of ketamine and phencyclidine was a key step to understanding their pharmacology, including their psychotomimetic effects in man. This review describes the historical context and the course of that discovery and its expansion into other hallucinatory drugs. The relevance of these findings to modern hypotheses of schizophrenia and the implications for drug discovery are reviewed. The findings of the rapidly acting antidepressant effects of ketamine in man are discussed in relation to other glutamatergic mechanisms.

Non-specific immunostaining by a rabbit antibody against gustducin α subunit in mouse brain

Gustducin is a guanosine nucleotide-binding protein functionally coupled with taste receptors and thus originally identified in taste cells of the tongue. Recently, bitter taste receptors and gustducin have been detected in the airways, digestive tracts and brain. The existing studies showing taste receptors and gustducin in the brain were carried out exclusively on frozen sections. In order to avoid the technical shortcomings associated with frozen sectioning, we performed immunofluorescence staining using vibratome-cut sections from mouse brains. Using a rabbit gustducin antibody, we could not detect neurons or astrocytes as reported previously. Rather, we found dense fibers in the nucleus accumbens and periventricular areas. We assumed these staining patterns to be specific after confirmation with conventional negative control staining. For the verification of this finding, we stained gustducin knockout mouse brain and tongue sections with the same rabbit gustducin antibody. Whereas negative staining was confirmed in the tongue, intensive fibers were constantly stained in the brain. Moreover, immunostaining with a goat gustducin antibody could not demonstrate the fibers in the brain tissue. The present study implies a cross immunoreaction that occurs with the rabbit gustducin antibody in mouse brain samples, suggesting that the conventional negative controls may not be sufficient when an immunostaining pattern is to be verified.

Inhibition of calcium ATPase by phencyclidine in rat brain

Phencyclidine (PCP) is a potent psychotomimetic drug of abuse and has profound effect on the functioning of the central nervous system (CNS). Many of the CNS functions are known to be mediated by calcium (Ca2+). In the present study we have investigated the effects of PCP on Ca2+ ATPase activity in rat brain both in vitro and in vivo. For in vitro studies, synaptic membrane fractions prepared from normal rat brain were incubated with PCP at different concentrations (25-100 microM) before the addition of substrate. For in vivo studies, rats were treated with a single moderate dose of PCP (10 mg/kg, i.p.) and animals were sacrificed at 1,2, 6 and 12 h after treatment. Ca2+ ATPase activity in synaptic membrane fractions was assayed by estimation of inorganic phosphate. PCP inhibited the Ca2+ ATPase in vitro in a concentration dependent manner with significant effect at 50 and 100 microM. A significant time-dependent reduction of the Ca2+ ATPase activity was evident in vivo. As early as 2 h after the treatment of rats with PCP the ATPase activity was significantly reduced. The reduction of Ca2+ ATPase observed even at 12 h after treatment suggesting a prolonged presence of the drug in the brain tissue. Further, kinetic studies in vitro indicated PCP to be a competitive inhibitor of Ca2+ ATPase with respect to the substrate, ATP. The present findings indicate that PCP inhibits synaptic membrane Ca2+ ATPase thus altering cellular Ca2+ homeostasis in CNS which may partially explain the pharmacological effects of the drug and/or its neurotoxicity.

Phencyclidine block of Ca2+ ATPase in rat heart sarcoplasmic reticulum

Phencyclidine hydrochloride (PCP) also known as Angel Dust is a very potent psychotomimetic drug of abuse. Besides its central nervous system (CNS) effects PCP produces a number of adverse effects in a variety of tissues including the cardiovascular system. Since PCP is known to alter the cellular calcium homeostasis the present studies were initiated to determine the changes in cardiac Ca2+ ATPase activity in rats treated with PCP. For in vitro studies the cardiac sarcoplasmic reticulum (SR) fractions prepared from normal rats were incubated with 25, 50 and 100 microM PCP and the enzyme activities were estimated. Whereas, for in vivo studies the cardiac SR fractions prepared from rats treated with PCP (10 mg/kg body wt. single dose, intra-peritoneally (i.p.)) and sacrificed at different time intervals were used. PCP reduced the Ca2+ ATPase activity significantly both in vitro and in vivo. A 50% inhibition of the enzyme activity was obtained with 100 microM PCP in vitro. A significant reduction of SR Ca2+ ATPase was also evident as early as 1 h after treatment of rats with PCP. The reduction of Ca2+ ATPase activity in SR was irreversible even at 12 h after treatment. The in vitro kinetic studies revealed that PCP was found to be a competitive inhibitor of Ca2+ ATPase with respect to the substrate, ATP, and non-competitive with respect to Ca2+ activation. These results indicate that PCP alters the myocardial Ca2+ homeostasis by inhibiting the Ca2+ ATPase in cardiac SR in rats. Inhibition of SR Ca2+ ATPase may result in the impairment of contraction and relaxation coupling processes in the myocardium.

Phencyclidine binds to blood platelets with high affinity and specifically inhibits their activation by adrenaline

The ion channel probe phencyclidine [1-(1-phenylcyclohexyl)piperidine; PCP] selectively inhibited aggregation, secretion and ultrastructural changes in platelets induced by adrenaline, but did not affect activation induced by other common platelet agonists such as alpha-thrombin, ADP, collagen or ionophore A23187. [3H]PCP bound to platelets with high affinity (Kd 134 +/- 33 nM; 3600 +/- 1020 sites/platelet), as did the thienyl analogue [3H]TCP (1-[1-(2-thienyl)cyclohexyl]piperidine). PCP binding to platelets was increased 3-4-fold in N-methylglucamine buffer in the absence of Na+ ions. Binding was unaffected by haloperidol and was only weakly inhibited (EC50 10-20 microM), without significant stereoselectivity by the two sets of stereoselective ligands, dexoxadrol/levoxadrol and (+)MK801/(-)MK801. Binding of PCP was not competed for by adrenaline or yohimbine. Only the high-affinity binding of [3H]PCP to platelets was blocked by prior treatment of the platelets with the covalent affinity probe Metaphit, and these platelets no longer aggregated in response to adrenaline although they responded normally to alpha-thrombin, ADP and collagen. These results suggest that platelets contain high-affinity receptors for PCP that can modulate adrenaline-induced platelet activation.

Functional properties of the nicotinic and glutamatergic receptors

Several important physiological processes such as plasticity, memory, cell death, and rhythmic firing involve the N-methyl-D-aspartate (NMDA)-type of glutamatergic receptor. Nicotinic acetylcholine receptors (AChR), recently demonstrated in the central nervous system (CNS), are also of great interest. We have used several ligands to study the physiology and pharmacology of the agonist recognition sites of these receptors and kinetic properties of associated ion channels using whole-cell, cell-attached or outside-out variants of the patch-clamp technique. Enzymatically dissociated frog interosseal muscles were used to study peripheral AChRs, and tissue cultured or acutely dissociated hippocampal neurons and retinal ganglion cells (RGCs) for CNS receptors. For reproducible and fast solution changes when recording in the whole-cell configuration, we modified the "U"-shaped tube system to obtain different outputs from the same outflow port. We used fluorescent rhodamine-labeled latex microspheres to identify RGCs. Our studies provide important information regarding the molecular mechanisms of several clinically used agents. Additionally, similar actions of noncompetitive agents on the ion channels of the nicotinic ACh and NMDA receptors support the concept of a receptor ion channel superfamily.

Excitatory amino acid binding sites in the rat hippocampus after transient forebrain ischemia

The influence of transient forebrain ischemia on the temporal alteration of glutamate receptors in the hippocampal formation was analyzed by means of in vitro quantitative receptor autoradiography. We compared the binding of N-methyl-D-aspartate (NMDA) receptors using [3H]3-[+/-)2-carboxypiperazin-4-yl)-propyl-1-phosphonic acid (CPP), noncompetitive NMDA antagonist binding sites using [3H]N-(1-(2-thienyl)-cyclohexyl)-3,4-piperidine (TCP), and kainate (KA) receptors. In the CA1 subfield of the hippocampus, the number of NMDA receptors and noncompetitive NMDA antagonist binding sites remained constant during the early stage of recirculation when the CA1 pyramidal cells remained histologically intact. A significant reduction of these receptor densities was observed 7 days following ischemia, when NMDA receptors and noncompetitive NMDA antagonist binding sites lost 64 and 29% of their binding sites in the stratum radiatum of the CA1, respectively. The KA receptor density in the CA1 subfield decreased by 44% 7 days after ischemia. Marked loss of the above-mentioned receptors in the CA1 after selective depletion of the CA1 pyramidal cells indicated that NMDA receptors, noncompetitive NMDA antagonist binding sites, and KA receptors in the CA1 are predominantly localized on the CA1 pyramidal cells. NMDA receptor density in the CA3 gradually decreased during the recirculation period. The stratum moleculare of the dentate gyrus, whose structure was histologically intact after ischemic insult, also showed a reduction of NMDA receptors 7 days following ischemia. [3H]KA receptor density in the stratum lucidum of the CA3 and in the hilus also decreased during recirculation. These

Glutathione metabolism in the lung: inhibition of its synthesis leads to lamellar body and mitochondrial defects

Mice treated with buthionine sulfoximine, an inhibitor of glutathione synthesis, showed striking alterations of morphology of lung type 2 cell lamellar bodies (swelling and disintegration) and mitochondria (degeneration) and of lung capillary endothelial cells (mitochondrial swelling). These effects probably may be ascribed to glutathione deficiency; administration of glutathione monoester protects against them. Measurements of arteriovenous plasma glutathione levels across the lung indicate that the net uptake of glutathione by this organ is substantial. Thus, glutathione exported from the liver to the blood plasma is utilized by the lung which, like the liver, kidney, and lymphocytes (and unlike skeletal muscle), exhibits a high overall rate of glutathione turnover. Intraperitoneal injection of glutathione into buthionine sulfoximine-treated mice leads to very high levels of plasma glutathione without significant increase in the glutathione levels of liver, lung, and lymphocytes; on the other hand, administration of glutathione monoester leads to markedly increased tissue and mitochondrial levels of glutathione. Administration of glutathione monoester (in contrast to glutathione) to control mice also increases mitochondrial glutathione levels. The findings indicate that glutathione is required for mitochondrial integrity and that it probably also functions in the processing and storage of surfactant in lamellar bodies. The morphological changes observed after treatment with buthionine sulfoximine and their prevention by glutathione monoester as well as findings on glutathione metabolism indicate that this tripeptide plays an important role in the lung. The previously observed failure of buthionine sulfoximine-treated mice to gain weight is mainly due to glutathione deficiency in the intestinal mucosa.

Psychotomimetic sigma-ligands, dexoxadrol and phencyclidine block the same presynaptic potassium channel in rat brain

1. Efflux of 86Rb from synaptosomes prepared from rat forebrain was used to assess voltage-gated changes in K+ permeability in mammalian central nerve terminals. 2. Although they are structurally unrelated to phencyclidine (PCP), the sigma-ligands, N-allyl-normetazocine (NANM; SKF 10,047) and cyclazocine, generalize to PCP in behavioral assays, displace [3H]PCP from a high-affinity binding site in brain, and potently block the same voltage-gated K+ channel as PCP itself. 3. The block of the voltage-gated K+ channel in nerve terminals by NANM and cyclazocine was stereoselective and was unaffected by the opioid antagonist naloxone. Moreover, in our experiments the relative activity of the stereoisomers of NANM and cyclazocine compared favourably with their relative activity in behavioural paradigms and binding assays. 4. Dexoxadrol, the D-isomer of dioxodrol, which produces PCP-like behavioural effects and displaces bound [3H]PCP, was a potent blocker of the PCP-sensitive, voltage-gated K+ channel. The corresponding L-isomer, levoxadrol, which produces morphine-like antinociception and sedation, but does not produce PCP-like behaviour nor displace bound [3H]PCP, was a very weak blocker of the voltage-gated K+ channel. 5. Levoxadrol, but not dexoxadrol, activated a separate K+ channel, as manifested by an increase in 86Rb efflux. This effect was blocked by naloxone. 6. We conclude that one of the PCP-sigma-ligand binding sites in the brain may be associated with the voltage-gated, non-inactivating K+ channel we observe in nerve terminals. Our findings are also consistent with the view that some of the behavioural manifestations of PCP intoxication are mediated by block of presynaptic K+ channels.

Phencyclidine and some of its analogues have distinct effects on NMDA receptors of rat hippocampal neurons

Phencyclidine (PCP) is a dissociative anesthetic agent which blocks the excitatory effect of N-methyl-D-aspartate (NMDA) in the central nervous system. To investigate the role of the PCP reactive site in the control of NMDA activation of hippocampal pyramidal cells, we have examined the action of PCP and some of its analogues on the response properties of single NMDA receptors. Application of NMDA (5-15 microM) to outside-out patches of membrane elicited bursts of ion channel openings which were greatly reduced in frequency and duration in the presence of PCP (2.5-10 microM) or m-amino-PCP (2.5-10 microM), a behaviorally active derivative of PCP. These effects of PCP were reversed when the membrane potential was shifted from negative to positive values. Application of the behaviorally inactive agent 1-piperidino-cyclohexanecarbonitrile (greater than or equal to 220 microM) left NMDA-activated currents relatively unaltered. Treatment with another analogue, m-nitro-PCP (5-20 microM), resulted in an unexpected increase in frequency of openings. At a higher concentration (100-300 microM), however, m-nitro-PCP acted like PCP in reducing frequency of opening and channel life-time. Like PCP, these effects of m-nitro-PCP were reversed at positive potentials. Taken together, these results suggest that PCP and its derivatives block the open state of the NMDA channel. Moreover, the dual effect of m-nitro-PCP shows that excitability is not necessarily decreased by PCP analogues but may instead be enhanced depending on modifications of the PCP molecule.

Effects of the facilitatory compounds catechol, guanidine, noradrenaline and phencyclidine on presynaptic currents of mouse motor nerve terminals

Catechol, guanidine, noradrenaline, and phencyclidine can increase acetylcholine release at neuromuscular junctions. To determine if they act by affecting nerve terminal action potentials, the electrical activity of the terminal regions of motor nerves was recorded with an extracellular electrode inserted in the perineural sheaths of nerves in the mouse triangularis sterni preparation. Catechol (from 10 microM) and guanidine (from 1 mM) produced a selective reduction in the component of the perineural waveform associated with voltage-dependent K+ currents, without significant effects on Na+, Ca2+, or Ca2+-activated K+ currents. A selective block of K+ channels in nerve terminals would cause a prolonged depolarization and hence a large influx of Ca2+ to trigger acetylcholine release; this could explain the facilitatory effects of guanidine and catechol. Noradrenaline produced a slight increase in the amplitude of the perineural waveform. This is consistent with hyperpolarization of the resting membrane potential of the nerve, which could lead to facilitation of acetylcholine release. Phencyclidine blocked Na+- and K+-related portions of the signal.

Macromolecular sites for specific neurotoxins and drugs on chemosensitive synapses and electrical excitation in biological membranes

The present review deals with the molecular mechanisms and elementary phenomena underlying the activation of the voltage- and chemo-sensitive membrane macromolecules: sodium- and potassium-ion channels and nicotinic ACh receptors and their associated ion channel. To achieve an understanding of their various kinetics and conformational states, a number of novel alkaloids, BTX, HTXs, gephyrotoxins, and certain psychotomimetic drugs such as phencyclidine, and many other pharmacologically active agents have been used. Biochemical assays and various electrophysiological techniques have been used in a number of biological preparations--e.g., Torpedo membranes, brain synaptosomes, amphibian and mammalian neuromuscular preparations--to describe the action of such agents. The availability of BTX and scorpion toxins together with aconitine and veratridine as activators and TTX and STX as antagonists of the voltage-sensitive sodium channels, made possible the identification and the physiological and pharmacological characterization of these channels. These studies provided the basis for understanding the mechanisms underlying electrical excitability and culminated, more recently, in the purification and reconstitution of sodium channels from rat brain and in the successful cloning of these channels with the elucidation of their primary structure. We now know that the sodium channel has a molecular mass of 316,000 daltons, consists of five subunits, and has multiple sites for various ligands. In contrast to sodium channels, various classes of potassium channels (inward and outward rectifier potassium channels and Ca(2+)-activated potassium channels) have been described. Unlike the sodium channels, there are no known specific activators for potassium channels. However, a number of potassium channel blockers such as 4-aminopyridine, HTX, histamine, and norepinephrine have been identified which complement the varying types of potassium channels in different neurons. One class of potassium channel blockers with profound medical and social implications comprises PCP and its analogues. The blockade of the potassium-induced 86Rb+ efflux from brain cells, the resulting prolongation of muscle and nerve action potentials, and the increase in transmitter release observed with PCP and some analogues are all highly suggestive of a role for the potassium channel in the behavioral effects of these drugs and its potential involvement in schizophrenia. A number of toxic principles of both plant and animal origin played a significant role in the development of our knowledge about the nAChR.(ABSTRACT TRUNCATED AT 400 WORDS)

Binding studies and photoaffinity labeling identify two classes of phencyclidine receptors in rat brain

Binding and photoaffinity labeling experiments were employed in order to differentiate 1-(1-phenylcyclohexyl)piperidine (PCP) receptor sites in rat brain. Two classes of PCP receptors were characterized and localized: one class binds [3H]-N-[1-(2-thienyl)cyclohexyl]piperidine [( 3H]TCP) with high affinity (Kd = 10-15 nM) and the other binds the ligand with a relatively low affinity (Kd = 80-100 nM). The two classes of sites have different patterns of distribution. Forebrain regions are characterized by high-affinity sites (hippocampus greater than frontal cortex greater than thalamus greater than olfactory bulb greater than hypothalamus), but some parts (e.g., hippocampus, hypothalamus) contain low-affinity sites as well. In the cerebellum only low-affinity sites were detected. Binding sites for [3H]PCP and for its photolabile analogue [3H]azido-PCP showed a regional distribution similar to that of the [3H]TCP sites. The neuroleptic drug haloperidol did not block binding to either the high- or the low-affinity [3H]TCP sites, whereas Ca2+ inhibited binding to both. Photoaffinity labeling of the PCP receptors with [3H]AZ-PCP indicated that five specifically labeled polypeptides of these receptors (Mr 90,000, 62,000, 49,000, 40,000, and 33,000) are unevenly distributed in the rat brain. Two of the stereoselectively labeled polypeptides (Mr 90,000 and 33,000) appear to be associated with the high- and low-affinity [3H]TCP-binding sites; the density of the Mr 90,000 polypeptide in various brain regions correlates well with the localization of the high-affinity sites, whereas the density of the Mr 33,000 polypeptide correlates best with the distribution of the low-affinity sites.(ABSTRACT TRUNCATED AT 250 WORDS)

Phencyclidine. Physiological actions, interactions with excitatory amino acids and endogenous ligands

Phenycyclidine (PCP) produces many profound effects in the central nervous system. PCP has numerous behavioral and neurochemical effects such as inhibiting the uptake and facilitating the release of dopamine, serotonin, and norepinephrine. PCP also interacts with sigma, mu opioid, muscarinic, and nicotinic receptors. However, the psychotomimetic effects induced by PCP are believed to be mediated by specific PCP receptors, where PCP binds with greater potency than sigma compounds. Electrophysiological, behavioral, and neuro-chemical evidence strongly suggests that at least some of the many PCP actions result from antagonism of excitatory amino acid-induced responses via PCP receptors. The recent isolation and partial characterization of the alpha and beta endopsychosins and the identification of other endogenous ligands for the PCP and sigma receptors, is another promising area of research in the elucidation of the physiological role of an endogenous PCP and sigma system.

Histrionicotoxins: effects on binding of radioligands for sodium, potassium, and calcium channels in brain membranes

A series of eight histrionicotoxins and two synthetic analogs inhibit binding of [3H]batrachotoxinin B to sites on voltage dependent sodium channels in brain membranes. Perhydrohistrionicotoxin (IC50 0.33 microM) and octahydrohistrionicotoxin (IC50 1.2 microM) are comparable in activities to potent local anesthetics. Histrionicotoxin (IC50 17 microM) and the other histrionicotoxins are much less potent. The histrionicotoxins also inhibit binding of [3H]phencyclidine to putative potassium channels in brain membranes. Histrionicotoxin (IC50 15 microM) and the other histrionicotoxins are much more potent than perhydrohistrionicotoxin (IC50 200 microM), but are at least 200-fold less potent than phencyclidine. The histrionicotoxins enhance binding of [3H]nitrendipine to sites on calcium channels in brain membranes, with the exception of perhydrohistrionicotoxin, which inhibits binding. Structure activity relationships at these channel sites and at the sites for noncompetitive blockers on the nicotinic acetylcholine receptor channel (AChR) complex differ. The histrionicotoxins are more potent at the sites on the AChR complex than at sites on other channels with the exception of perhydrohistrionicotoxin, which has comparable potency at the AChR complex and sodium channels.

Phencyclidine in low doses selectively blocks a presynaptic voltage-regulated potassium channel in rat brain

Phencylidine (PCP) is a major drug of abuse in the United States. It produces a toxic confusional psychosis in man. We show here that nanomolar to micromolar concentrations of PCP and behaviorally active congeners selectively block voltage-regulated noninactivating (or very slowly inactivating) presynaptic K channels in the brain. The rank order of potency for blockage of these K channels parallels both the relative ability of these agents to produce characteristic behavioral deficits in rats and their ability to displace [3H]PCP from its high-affinity binding sites in brain. In view of the enhanced voltage-gated Ca influx that would be expected to accompany blockage of presynaptic K channels, this mechanism could explain the excessive neurotransmitter release that is characteristic of PCP intoxication.

Phencyclidine (PCP) blocks glutamate-activated postsynaptic currents

Phencyclidine (PCP) was tested on the metathoracic tibialis muscles of Locusta migratoria. In physiological solution, the peak amplitude of the excitatory postsynaptic currents (EPSCs) evoked by nerve stimulation was linearly related to membrane potential between -50 and -150 mV. The decay time constant of the EPSC (tau EPSC) was exponentially dependent on voltage and decreased with hyperpolarization. The membrane potential change required to produce an e-fold change in tau EPSC was 315 mV. PCP (5-40 microM) produced a concentration-dependent depression of both EPSC peak amplitude and tau EPSC. A slight nonlinearity in the current-voltage relationship could be discerned at high concentrations of PCP. The shortening of the decay time constant of EPSC (tau EPSC) occurred without significant change in the voltage sensitivity observed under control conditions. Under all experimental conditions, the decay of the EPSCs remained a single exponential of time. Fluctuation analysis indicated that 5 microM PCP shortens the lifetime of the glutamate-activated channels by 25.7 +/- 3%. PCP (10-80 microM) did not induced desensitization of the glutamate receptors. These results suggest that PCP interacts with the open conformation of ion channels activated by the glutamate receptor.

Phencyclidine increases the affinity of dihydropyridine calcium channel antagonist binding in rat brain

Phencyclidine (PCP) significantly reduces the apparent dissociation constant (KD) of the dihydropyridine (DHP) calcium channel antagonist, [3H]nitrendipine, in synaptosomal membranes of rat and mouse brain without significantly effecting the maximum binding capacity (Bmax). At an optimum concentration of PCP (10 microM) the apparent KD of [3H]nitrendipine was reduced from 178 +/- 9 pM to 112 +/- 9 pM in rat forebrain, a 58% increase in affinity. The structural derivatives of PCP, P-Br-PCP [1-[1-(4-bromo-phenyl-cyclohexyl)piperidine]], m-NH2-PCP [1-[1-(3-anilo)-cyclohexyl]piperidine], (+/-)-PCMP [1-(1-phenyl)-cyclo-hexyl-3-methylpiperidine] also increased the apparent affinity of [3H]nitrendipine in the following order, p-Br-PCP much greater than PCMP greater than PCP greater than m-NH2-PCP. Local anesthetics either reduced the apparent affinity of [3H]nitrendipine or had no effect. Kinetic analysis revealed that PCP both increased the microassociation rate constant and decreased the microdissociation rate constant of [3H]nitrendipine. The magnitude of this enhanced binding varied with the brain region studied; the greatest increase in apparent affinity of [3H]nitrendipine was observed in striatum, while no significant increase in affinity was observed in brainstem. In some brain areas, PCP was more effective in reducing the KD in crude homogenates than in washed tissue. PCP (10 microM) did not alter the KD of [3H]nitrendipine to rat cardiac tissue. Both Ca2+ and Mg2+ inhibited the effect of PCP, while monovalent ions were ineffective in this regard.(ABSTRACT TRUNCATED AT 250 WORDS)

Possible allosteric interaction of 4-aminopyridine with rat brain muscarinic acetylcholine receptors

The interaction of the potassium channel blocker 4-aminopyridine (4-AP) and its analogs with muscarinic acetylcholine receptors was studied in rat brain homogenate. 4-AP displaced specific [3H]quinuclidinyl benzilate [( 3H]QNB) binding in a concentration-dependent fashion. Hill coefficient values decreased with increasing the concentration of [3H]QNB and different analogs of 4-AP demonstrated varying potencies. Scatchard analysis of saturation isotherms of specific [3H]QNB binding showed that low concentrations of 4-AP slightly reduced maximum binding without affecting the equilibrium dissociation constant, whereas higher concentrations reduced maximum binding further and significantly increased the equilibrium dissociation constant. Schild plots of these data resulted in curvilinear functions. The results are discussed in terms of possible allosteric interactions between potassium channels and muscarinic receptor binding sites.

Potassium channels in isolated presynaptic nerve terminals from rat brain

86Rb efflux from pinched-off rat brain presynaptic nerve terminals (synaptosomes) was used to measure the K permeability of the terminals. Synaptosomes were pre-loaded with 86Rb and the suspensions were then filtered on glass fibre filters. The terminals trapped on the filters were superfused with 'efflux solutions', and the effluent and filters were then counted. 86Rb efflux into physiological saline (PSS) containing 5 mM-K and 145 mM-Na was about 0.4% of the 86Rb load per second (component 'R'). Increasing extracellular K concentration [( K]o), or adding veratridine and sea anemone toxin, stimulated efflux; presumably by depolarizing the nerve terminals. The K-stimulated 86Rb efflux was a graded function of [K]o. High [K]o evoked at least three components of efflux: a 'fast phase' (T) that apparently inactivated in less than 1 s, a 'slower phase' (S) that was linear for 3-5 s, and a Ca-dependent phase (C). Some, but not all, of the slow phase 86Rb efflux (component S) may be attributable to increased efflux mediated by the 'resting' K permeability mechanism when the driving force is increased by depolarization. K efflux was also studied and was found to be qualitatively similar to 86Rb efflux. 86Rb: 42K permeability ratios were 0.6-0.8 for most components of the efflux. Raising the Mg concentration in the efflux solution shifted the 86Rb efflux versus [K]o curve in the direction of increased [K]o. This shift may be the result of screening of surface charges by Mg. Several agents that block various K channels in other preparations inhibited K-stimulated 86Rb efflux in synaptosomes: tetraethylammonium (TEA), tetrabutylammonium (TBA), and 4-aminopyridine (4-AP). The fast component (T) of high [K]o-stimulated 86Rb efflux was selectively blocked by low concentrations of 4-AP (apparent half-maximal inhibition, KI = 0.1-0.2 mM); it was also blocked by TEA (KI = 0.6 mM) and TBA (KI = 0.8-1.0 mM). Dose-response curves for inhibition of component T by all three agents were monophasic. the slow component (S) of the K-stimulated 86Rb efflux was much less sensitive to all three agents, than was component T; the broad dose-response curves were consistent with the view that two (or more) different K conductances may contribute to component S.(ABSTRACT TRUNCATED AT 400 WORDS)

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.

Effect of phencyclidine on inhibition in the hippocampal slice

The effects of phencyclidine (PCP) on synaptic transmission were studied in the hippocampal slice. Population spikes evoked by orthodromic or antidromic stimulation were recorded from CAl pyramidal cells. Bath applied PCP (10(-4) M) reduced moderately both the orthodromic and antidromic population spikes. Lower concentrations, 5 X 10(-6) to 5 X 10(-5) M of PCP, which did not depress the population spikes, reduced inhibition of the orthodromically evoked spike in a dose dependent reversible manner. Diazepam (10(-6) to 10(-5) M) restored the inhibition despite the continued presence of PCP. It is suggested that PCP-induced seizures and other signs of hyperexcitability could be a result of reduced inhibition.