Role of the aromatic group in the inhibition of phencyclidine binding and dopamine uptake by PCP analogs

Spectroscopic, theoretical and computational investigations of novel benzo[b]thiophene based ligand and its M(II) complexes: As high portentous antimicrobial and antioxidant agents

The reaction of 3-chlorobenzo[b]thiophene-2-carbohydrazide with 4-(diethylamino) salicylaldehyde gave the new ligand; 3-chloro-N'-(4-(diethylamino)-2-hydroxybenzylidene)-benzo[b]thiophene-2-carbohydrazide. The Cu(II), Co(II), Ni(II), and Zn(II) complexes have been successfully prepared. The ligand and the complexes were characterized by analytical, FT-IR, 1H NMR, mass, UV-visible spectroscopy, molar conductivity, and magnetic susceptibility measurements. The FT-IR spectral data showed that the ligand adopted a tridentate fashion when binding with the metal ions via the nitrogen atoms of the imine (C = N), carboxyl (C = O), and phenolic oxygen (O-H) donor atoms. Density Functional Theory (DFT) estimations for the ligand at the DFT/B3LYP level via 6-31G++ (d, p) replicate the structure and geometry. Finally, HOMO and LUMO analyses were used for the charge transfer interface of the structure. Furthermore, molecular docking and ADME calculations were also performed to correlate and interpret the experimental results. The antimicrobial activity study illustrated enhancement in the activity of the free ligand upon complex formation, and the Cu(II) complex (MIC 25 µg mL-1) may be considered a promising antibacterial agent, and the Ni(II) and Zn(II) complexes (MIC 25 µg mL-1) as promising antifungal agents. Also, synthesized Cu(II) and Zn(II) metal complexes (MIC 3.125 µg mL-1) showed promising anti-TB activity against M. tuberculosis. Further, benzo[b]thiophene-based ligand and its metal complexes were evaluated for in vitro antioxidant activity, and in silico docking studies were carried out against Cytochrome c Peroxidase (PDB ID: 2X08).

Pharmacological and behavioral divergence of ketamine enantiomers: implications for abuse liability

Ketamine, a racemic mixture of (S)-ketamine and (R)-ketamine enantiomers, has been used as an anesthetic, analgesic and more recently, as an antidepressant. However, ketamine has known abuse liability (the tendency of a drug to be used in non-medical situations due to its psychoactive effects), which raises concerns for its therapeutic use. (S)-ketamine was recently approved by the United States' FDA for treatment-resistant depression. Recent studies showed that (R)-ketamine has greater efficacy than (S)-ketamine in preclinical models of depression, but its clinical antidepressant efficacy has not been established. The behavioral effects of racemic ketamine have been studied extensively in preclinical models predictive of abuse liability in humans (self-administration and conditioned place preference [CPP]). In contrast, the behavioral effects of each enantiomer in these models are unknown. We show here that in the intravenous drug self-administration model, the gold standard procedure to assess potential abuse liability of drugs in humans, rats self-administered (S)-ketamine but not (R)-ketamine. Subanesthetic, antidepressant-like doses of (S)-ketamine, but not of (R)-ketamine, induced locomotor activity (in an opioid receptor-dependent manner), induced psychomotor sensitization, induced CPP in mice, and selectively increased metabolic activity and dopamine tone in medial prefrontal cortex (mPFC) of rats. Pharmacological screening across thousands of human proteins and at biological targets known to interact with ketamine yielded divergent binding and functional enantiomer profiles, including selective mu and kappa opioid receptor activation by (S)-ketamine in mPFC. Our results demonstrate divergence in the pharmacological, functional, and behavioral effects of ketamine enantiomers, and suggest that racemic ketamine's abuse liability in humans is primarily due to the pharmacological effects of its (S)-enantiomer.

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.

DARK Classics in Chemical Neuroscience: Phencyclidine (PCP)

Phencyclidine (PCP, "angel dust", an arylcyclohexylamine) was the first non-natural, man-made illicit drug of abuse, and was coined 'the most dangerous drug in America" in the late 1970s (amidst sensational horror stories of the drug's effects); however, few other illicit drugs have had such a significant and broad impact on society-both good and bad. Originally developed as a new class of anesthetic, PCP-derived psychosis gave way to the PCP hypothesis of schizophrenia (later coined the NMDA receptor hypofunction hypothesis or the glutamate hypothesis of schizophrenia), which continues to drive therapeutic discovery for schizophrenia today. PCP also led to the discovery of ketamine (and a new paradigm for the treatment of major depression), as well as other illicit, designer drugs, such as methoxetamine (MXE) and a new wave of Internet commerce for illicit drugs (sold as research chemicals, or RCs). Furthermore, PCP is a significant contaminant/additive of many illegal drugs sold today, due to its ease of preparation by clandestine chemists. Here, we will review the history, importance, synthesis (both legal and clandestine), pharmacology, drug metabolism, and folklore of PCP, a true DARK classic in chemical neuroscience.

Phencyclidine-Based New Psychoactive Substances

The serendipitous discovery of phencyclidine (PCP) in 1956 sets the stage for significant research efforts that resulted in a plethora of analogs and derivatives designed to explore the biological effects of this class. PCP soon became the prototypical dissociative agent that eventually sneaked through the doors of clinical laboratories and became an established street drug. Estimations suggest that around 14 PCP analogs were identified as "street drugs" in the period between the 1960s and 1990s. Fast forward to the 2000s, and largely facilitated by advancements in electronic forms of communication made possible through the Internet, a variety of new PCP analogs began to attract the attention of communities interested in the collaborative exploration of these substances. Traditionally, as was the case with the first-generation analogs identified in previous decades, the substances explored represented compounds already known in the scientific literature. As the decade of the noughties unfolded, a number of new PCP-derived substances appeared on the scene, which included some analogs that have not been previously recorded in the published literature. The aim of this chapter is to present a brief introductory overview of substances that have materialized as PCP-derived new psychoactive substances (NPS) in recent years and their known pharmacology. Since N-methyl-D-aspartate receptor (NMDAR) antagonism is implicated in mediating the subjective and mind-altering effects of many dissociative drugs, additional data are included from other analogs not presently identified as NPS.

The novel ketamine analog methoxetamine produces dissociative-like behavioral effects in rodents

RATIONALE

Methoxetamine (MXE) is a ketamine analog sold online that has been subject to widespread abuse for its dissociative and hallucinogenic effects. Previous studies have shown that MXE has high affinity for the phencyclidine (PCP) binding site located within the channel pore of the NMDA receptor (NMDAR), but little is known about its behavioral effects. Dissociative anesthetics such as ketamine and PCP produce a characteristic behavioral profile in rats that includes locomotor hyperactivity and disruption of prepulse inhibition (PPI) of acoustic startle.

METHODS

The goal of the present investigation was to determine whether MXE produces PCP-like effects in Sprague-Dawley rats using the PPI paradigm and the behavioral pattern monitor (BPM), which enables analyses of patterns of locomotor activity and investigatory behavior. PPI studies were conducted with several other uncompetitive NMDAR antagonists that produce dissociative effects in humans, including PCP, the S-(+) and R-(-) isomers of ketamine, and N-allylnormetazocine (NANM; SKF-10,047).

RESULTS

MXE disrupted PPI when administered at 3 and 10 mg/kg SC. The rank order of potency of MXE and the other test compounds in the PPI paradigm (PCP > MXE > S-(+)-ketamine > NANM > R-(-)-ketamine) parallels their affinities for the PCP binding site reported in the literature. When tested in the BPM, 10 mg/kg MXE induced locomotor hyperactivity, reduced the number of rearings, increased the roughness of locomotor paths, and produced perseverative patterns of locomotion. Administration of PCP (2.25 and 6.75 mg/kg, SC) produced a similar profile of effects in the BPM.

CONCLUSIONS

These results indicate that MXE produces a behavioral profile similar to that of other psychotomimetic uncompetitive NMDAR antagonists. Our findings support the classification of MXE as a dissociative drug and suggest that it likely has effects and abuse potential similar to that of PCP and ketamine.

The subchronic phencyclidine rat model: relevance for the assessment of novel therapeutics for cognitive impairment associated with schizophrenia

RATIONALE

Current treatments for schizophrenia have modest, if any, efficacy on cognitive dysfunction, creating a need for novel therapies. Their development requires predictive animal models. The N-methyl-D-aspartate (NMDA) hypothesis of schizophrenia indicates the use of NMDA antagonists, like subchronic phencyclidine (scPCP) to model cognitive dysfunction in adult animals.

OBJECTIVES

The objective of this study was to assess the scPCP model by (1) reviewing published findings of scPCP-induced neurochemical changes and effects on cognitive tasks in adult rats and (2) comparing findings from a multi-site study to determine scPCP effects on standard and touchscreen cognitive tasks.

METHODS

Across four research sites, the effects of scPCP (typically 5 mg/kg twice daily for 7 days, followed by at least 7-day washout) in adult male Lister Hooded rats were studied on novel object recognition (NOR) with 1-h delay, acquisition and reversal learning in Morris water maze and touchscreen-based visual discrimination.

RESULTS

Literature findings showed that scPCP impaired attentional set-shifting (ASST) and NOR in several labs and induced a variety of neurochemical changes across different labs. In the multi-site study, scPCP impaired NOR, but not acquisition or reversal learning in touchscreen or water maze. Yet, this treatment regimen induced locomotor hypersensitivity to acute PCP until 13-week post-cessation.

CONCLUSIONS

The multi-site study confirmed that scPCP impaired NOR and ASST only and demonstrated the reproducibility and usefulness of the touchscreen approach. Our recommendation, prior to testing novel therapeutics in the scPCP model, is to be aware that further work is required to understand the neurochemical changes and specificity of the cognitive deficits.

The ketamine analogue methoxetamine and 3- and 4-methoxy analogues of phencyclidine are high affinity and selective ligands for the glutamate NMDA receptor

In this paper we determined the pharmacological profiles of novel ketamine and phencyclidine analogues currently used as ‘designer drugs’ and compared them to the parent substances via the resources of the National Institute of Mental Health Psychoactive Drug Screening Program. The ketamine analogues methoxetamine ((RS)-2-(ethylamino)-2-(3-methoxyphenyl)cyclohexanone) and 3-MeO-PCE (N-ethyl-1-(3-methoxyphenyl)cyclohexanamine) and the 3- and 4-methoxy analogues of phencyclidine, (1-[1-(3-methoxyphenyl)cyclohexyl]piperidine and 1-[1-(4-methoxyphenyl)cyclohexyl]piperidine), were all high affinity ligands for the PCP-site on the glutamate NMDA receptor. In addition methoxetamine and PCP and its analogues displayed appreciable affinities for the serotonin transporter, whilst the PCP analogues exhibited high affinities for sigma receptors. Antagonism of the NMDA receptor is thought to be the key pharmacological feature underlying the actions of dissociative anaesthetics. The novel ketamine and PCP analogues had significant affinities for the NMDA receptor in radioligand binding assays, which may explain their psychotomimetic effects in human users. Additional actions on other targets could be important for delineating side-effects.

Dopamine D2High receptors stimulated by phencyclidines, lysergic acid diethylamide, salvinorin A, and modafinil

Although it is commonly stated that phencyclidine is an antagonist at ionotropic glutamate receptors, there has been little measure of its potency on other receptors in brain tissue. Although we previously reported that phencyclidine stimulated cloned-dopamine D2Long and D2Short receptors, others reported that phencyclidine did not stimulate D2 receptors in homogenates of rat brain striatum. This study, therefore, examined whether phencyclidine and other hallucinogens and psychostimulants could stimulate the incorporation of [(35)S]GTP-gamma-S into D2 receptors in homogenates of rat brain striatum, using the same conditions as previously used to study the cloned D2 receptors. Using 10 microM dopamine to define 100% stimulation, phencyclidine elicited a maximum incorporation of 46% in rat striata, with a half-maximum concentration of 70 nM for phencyclidine, when compared with 80 nM for dopamine, 89 nM for salvinorin A (48 nM for D2Long), 105 nM for lysergic acid diethylamide (LSD), 120 nM for R-modafinil, 710 nM for dizocilpine, 1030 nM for ketamine, and >10,000 nM for S-modafinil. These compounds also inhibited the binding of the D2-selective ligand [(3)H]domperidone. The incorporation was inhibited by the presence of 200 microM guanylylimidodiphosphate and also by D2 blockade, using 10 microM S-sulpiride, but not by D1 blockade with 10 microM SCH23390. Hypertonic buffer containing 150 mM NaCl inhibited the stimulation by phencyclidine, which may explain negative results by others. It is concluded that phencyclidine and other psychostimulants and hallucinogens can stimulate dopamine D2 receptors at concentrations related to their behavioral actions.

Synthesis and evaluation of 1-(1-(Benzo[b]thiophen-2-yl)cyclohexyl)piperidine (BTCP) analogues as inhibitors of trypanothione reductase

Thirty two analogues of phencyclidine were synthesised and tested as inhibitors of trypanothione reductase (TryR), a potential drug target in trypanosome and leishmania parasites. The lead compound BTCP (1, 1-(1-benzo[b]thiophen-2-yl-cyclohexyl) piperidine) was found to be a competitive inhibitor of the enzyme (K(i)=1 microM) and biologically active against bloodstream T. brucei (EC(50)=10 microM), but with poor selectivity against mammalian MRC5 cells (EC(50)=29 microM). Analogues with improved enzymatic and biological activity were obtained. The structure-activity relationships of this novel series are discussed.

Dopamine transporter is essential for the maintenance of spontaneous activity of auditory nerve neurones and their responsiveness to sound stimulation

Dopamine, a neurotransmitter released by the lateral olivocochlear efferents, has been shown tonically to inhibit the spontaneous and sound-evoked activity of auditory nerve fibres. This permanent inhibition probably requires the presence of an efficient transporter to remove dopamine from the synaptic cleft. Here, we report that the dopamine transporter is located in the lateral efferent fibres both below the inner hair cells and in the inner spiral bundle. Perilymphatic perfusion of the dopamine transporter inhibitors nomifensine and N-[1-(2-benzo[b]thiophenyl)cyclohexyl]piperidine into the cochlea reduced the spontaneous neural noise and the sound-evoked compound action potential of the auditory nerve in a dose-dependent manner, leading to both neural responses being completely abolished. We observed no significant change in cochlear responses generated by sensory hair cells (cochlear microphonic, summating potential, distortion products otoacoustic emissions) or in the endocochlear potential reflecting the functional state of the stria vascularis. This is consistent with a selective action of dopamine transporter inhibitors on auditory nerve activity. Capillary electrophoresis with laser-induced fluorescence (EC-LIF) measurements showed that nomifensine-induced inhibition of auditory nerve responses was due to increased extracellular dopamine levels in the cochlea. Altogether, these results show that the dopamine transporter is essential for maintaining the spontaneous activity of auditory nerve neurones and their responsiveness to sound stimulation.

Forebrain substrates of reward and motivation

Electrical stimulation of the medial forebrain bundle can reward arbitrary acts or motivate biologically primitive, species-typical behaviors like feeding or copulation. The subsystems involved in these behaviors are only partially characterized, but they appear to transsynaptically activate the mesocorticolimbic dopamine system. Basal function of the dopamine system is essential for arousal and motor function; phasic activation of this system is rewarding and can potentiate the effectiveness of reward-predictors that guide learned behaviors. This system is phasically activated by most drugs of abuse and such activation contributes to the habit-forming actions of these drugs.

The role of antagonism of NMDA receptor-mediated neurotransmission and inhibition of the dopamine reuptake in the neuroendocrine effects of phencyclidine

Phencyclidine (PCP) activates the hypothalamo-pituitary-adrenal (HPA) axis and decreases plasma prolactin levels in the rat. PCP is a noncompetitive N-methyl-d-aspartate (NMDA) receptor antagonist, but it also inhibits the reuptake of dopamine, serotonin and norepinephrine. The purpose of the present study was to utilize the PCP analogue N-[1-(2-thienyl)cyclohexyl]piperidine; (TCP), the potent dopamine reuptake inhibitor N-[1-(2-benzo(b)thiophenyl) cyclohexyl]piperidine; (BTCP) and the nonselective monoamine reuptake inhibitor cocaine as pharmacologic probes in order to determine the roles of noncompetitive NMDA receptor blockade and inhibition of dopamine reuptake in the neuroendocrine effects of PCP. PCP, TCP and cocaine increased plasma levels of adrenocorticotropin and corticosterone, but BTCP had no effect. In contrast, PCP, BTCP and cocaine decreased plasma prolactin, but TCP produced no such effect. The data suggest that mechanisms besides inhibition of dopamine reuptake are involved in the effects of PCP on the HPA axis, and the PCP-induced decrease in plasma prolactin is not a consequence of inhibition of NMDA receptor-mediated neurotransmission.

Priming with BTCP, a dopamine reuptake blocker, reinstates cocaine-seeking and enhances cocaine cue-induced reinstatement

N-[1-(2-benzo[b]thiophenyl)cyclohexyl]piperidine (BTCP), a potent dopamine reuptake inhibitor, substitutes for the reinforcing effects of cocaine and meets other criteria for possible agonist pharmacotherapeutic potential. The purpose of this study was to determine (1) whether BTCP modifies reinstatement of cocaine-seeking elicited by cocaine-related environmental stimuli and (2) whether this compound produces priming effects. Male Wistar rats were trained to associate discriminative stimuli (S(D)) with cocaine availability (0.25 mg/infusion) versus non-reward and then were subjected to repeated extinction sessions during which the reinforcer and S(D) were withheld. Subsequent presentation of the cocaine S(D) produced recovery of cocaine-seeking. BTCP (2.5-30 mg/kg; i.p.) did not attenuate the conditioned reinstatement induced by the cocaine S(D) but, rather, potentiated this effect at 10 mg/kg. To test whether BTCP, by itself, exerts priming effects, different groups of rats were trained to self-administer cocaine (0.25 mg/infusion) for 2 weeks. After a 2-week extinction period, BTCP (5, 10 and 20 mg/kg, i.p.) reinstated cocaine-seeking, showing that BTCP not only increases cocaine-seeking induced by cocaine-related stimuli but also produces priming effects following abstinence. The results suggest that, in cocaine abstinent rats, BTCP produces cocaine-like effects.

3-OH-pip-BTCP, a metabolite of the potent DA uptake blocker BTCP, exerts cocaine-like action in rats

A previous study showed that N-[1-(2-benzo[b]thiophenyl)cyclohexyl]piperidin-3-ol (3-OH-pip-BTCP), an active metabolite of the potent dopamine reuptake inhibitor N-[1-(2-benzo[b]thiophenyl)cyclohexyl]piperidine (BTCP), exhibits stimulant effects in rats (i.e. increases dopamine levels in the nucleus accumbens and increases locomotion). The present study sought to further characterize the behavioral profile of 3-OH-pip-BTCP, specifically, its interaction with cocaine-maintained behavior. Cocaine self-administration was examined in 3-OH-pip-BTCP pretreated rats. 3-OH-pip-BTCP (0-20 mg/kg, i.p.) dose-dependently decreased cocaine self-administration. Additionally, 3-OH-pip-BTCP (10 mg/kg) shifted the cocaine dose-response to the left. Together, these results suggest that 3-OH-pip-BTCP shares similar reinforcing effects with cocaine and therefore may serve as a substitutive drug and possible agonist therapy for cocaine addiction.

Dopamine transporter as target for drug development of cocaine dependence medications

Because much evidence implicates the dopamine transporter in the reinforcing effects of cocaine, development of potential medications for cocaine dependence has included the dopamine transporter as a target. The present overview covers progress in the drug development area regarding several classes of dopamine uptake inhibitors, with an emphasis on structure-activity relationships that enhance potency and selectivity at transporters for dopamine compared with those for serotonin or norepinephrine. The following categories of compounds are covered: tropane, benztropine, 1-[2-[bis(4-fluorophenyl)methoxy]ethyl]-4-(3-phenylpropyl)piperazine (GBR), methylphenidate, mazindol, and phencyclidine analogs. Activity at transporters as well as on behavior is highlighted.

A single dose of monoclonal anti-phencyclidine IgG offers long-term reductions in phencyclidine behavioral effects in rats

These studies tested the hypothesis that a single dose of high-affinity anti-phencyclidine monoclonal antibody (anti-PCP mAb) provides long-term protection against behavioral effects of repeated PCP administration in rats. Rats were treated with saline, nonspecific bovine IgG (NS-IgG), or anti-PCP mAb (1.0 g/kg). The next morning, the rats were challenged with escalating i.v. doses of PCP (0.32, 0.56, and 1.0 mg/kg) at 90-min intervals. This regimen was repeated every 3 days for 2 weeks. In the saline and NS-IgG control groups, PCP yielded reproducible and linear dose-dependent effects that remained constant during the experiment. In contrast, the anti-PCP mAb treatment blocked PCP effects on day 1, and sustained significant (P < 0.05) reductions in drug effects for the entire 2-week experiment. Brain PCP concentrations (determined at study termination) were reduced by ~55%, whereas serum concentrations were increased over 4000% compared with controls. Thus, a single dose of antibody medication provided long-term reductions in drug effects and brain concentrations, beyond the expected capacity of the drug-antibody interaction. These data challenge current concepts about in vivo dose dependence and unimolecular interaction between antibody binding sites and small molecules and establish that neuroprotection by mAbs may have an unique mechanism of action.

A simple procedure for assessing ataxia in rats: effects of phencyclidine

The present study describes an objective, cost- and time-efficient procedure for characterizing the ataxic effects of psychoactive drugs. Male Sprague-Dawley rats were administered an intraperitoneal injection of either saline or one of three doses (1, 5 or 10 mg/kg) of phencyclidine (PCP) 15 min prior to being placed into an empty standard operant conditioning chamber (all manipulanda were removed). The floor of the test apparatus consisted of parallel rows of metal rods spaced approximately 1.5 cm apart. During a 5-min test, a single observer counted the frequency with which each animal's paws (front or back) slipped between the rows of bars that constituted the cage floor. The data demonstrated that while saline animals exhibited no instability in their ambulation, PCP-treated animals demonstrated a highly reliable dose-dependent increase in the number of "paw slips" in a single trial. Since animals are known to develop tolerance to the ataxic response to PCP, the validity of the test as a measure of drug-induced ataxia was examined in a separate group of animals treated with the middle (5 mg/kg) dose every other day over the course of a 9-day period (i.e., resulting in five injection trials). In this experiment, each subsequent test produced a reliable reduction in the magnitude of the ataxic response, and by the fifth drug challenge, the PCP animals were performing at near-control levels. These results suggest that the "paw slip test" can serve as a simple, reliable, objective and valid measure of drug-induced ataxia. The relevance of the ataxia data for interpreting the locomotor response of animals treated with PCP is also discussed.

Involvement of dopaminergic system in the nucleus accumbens in the discriminative stimulus effects of phencyclidine

The effects of microinjection of phencyclidine (PCP) and dizocilpine, non-competitive NMDA receptor antagonists, and dopamine into the nucleus accumbens were examined in rats trained to discriminate PCP (1.5 mg/kg i.p.) from saline under a two-lever fixed ratio 20 schedule of food reinforcement. Microinjection of PCP (2-40 microg) and dizocilpine (2-12 microg) into the bilateral nucleus accumbens produced a dose-dependent increase in PCP-appropriate responding and fully substituted for systemically administered PCP, whereas microinjection of dopamine (1-4 microg) did not produce PCP-like discriminative stimulus effects. The performance of PCP discrimination was assessed after bilateral destruction of the dopaminergic nerve neurons in the nucleus accumbens with dopaminergic neurotoxin, 6-hydroxydopamine (6-OHDA, 4 microg/1 microl/side). The destruction of dopaminergic nerve neurons in the nucleus accumbens failed to prevent the performance of PCP discrimination. There was no difference in the average percentages of PCP-appropriate responding between vehicle and 6-OHDA-treated rats in the dose-response tests. These results suggest that the dopaminergic system in the nucleus accumbens does not play a critical role in the discriminative stimulus effects of PCP.

BTCP is a potent reinforcer in rats: comparison of behavior maintained on fixed- and progressive-ratio schedules

N-[1-(2-benzo[b]thiophenyl)cyclohexyl]piperidine (BTCP) is a phencyclidine (PCP) derivative that acts as a potent dopamine (DA) reuptake inhibitor. Earlier studies have shown that BTCP can substitute for the reinforcing effects of cocaine. Therefore, the aim of the study was to further characterize the reinforcing effects of BTCP. The reinforcing actions of BTCP were compared to those of cocaine at equimolar concentrations in drug-naïve rats. Two groups of animals were implanted with jugular catheters and trained to intravenously self-administer BTCP or cocaine (0.25 mg/infusion) on a fixed-ratio five schedule (FR 5) of reinforcement. Both BTCP and cocaine produced comparable inverted U-shaped dose-effect curves on this schedule over doses of 0.03, 0.06, 0.125, and 0.25 mg/infusion. Two doses (0.125 and 0.25 mg/infusion) that produced reliable self-administration in all the animals for cocaine and BTCP were then tested on a progressive-ratio schedule. At each dose, BTCP supported higher breaking points (BPs) than cocaine. The results demonstrate that rats readily acquire responding maintained by BTCP and suggest that BTCP may have greater reinforcing effects than cocaine at equimolar concentrations.

Behavioral and neurochemical effects of 3-OH-pip-BTCP, an active metabolite of BTCP in rats

The effects of N-[1-(2-benzo[b]thiophenyl)cyclohexyl]piperidin-3-ol (3-OH-pip-BTCP), an active metabolite of N-[1-(2-benzo[b]thiophenyl)cyclohexyl]piperidine (BTCP) was examined on locomotor activity and dopamine (DA) levels in the nucleus accumbens (NAcc) in rats. To study the stimulant action of 3-OH-pip-BTCP, rats were placed into activity chambers and injected with the compound (0-40 mg/kg, i.p.). To measure the effects of 3-OH-pip-BTCP on DA levels, rats were implanted with microdialysis probes into the NAcc and the same doses as used in the locomotor activity experiment were administered i.p. 3-OH-pip-BTCP dose-dependently increased locomotor activity and DA levels in the NAcc which lasted 4-5 h at 20 and 40 mg/kg. The results suggest that 3-OH-pip-BTCP exerts long lasting stimulating effects on locomotion and extracellular DA levels in the NAcc, suggesting that 3-OH-pip-BTCP contributes importantly to the pharmacological effects of its parent compound, BTCP.

Four-choice drug discrimination in pigeons

(+)Amphetamine was added as a training stimulus for pigeons previously trained to discriminate among pentobarbital, morphine and saline using a three-choice procedure. Pigeons quickly learned the four-choice drug discrimination. Generalization from the training drugs was similar to that established with simpler drug discriminations; pentobarbital generalized to chlordiazepoxide, morphine generalized to methadone, and (+)amphetamine generalized to cocaine and methamphetamine. Low doses of phencyclidine generalized to saline, while higher doses partially generalized to pentobarbital and (+)amphetamine. When dose-response curves were redetermined with a cumulative-dosing procedure, the same pattern of generalization occurred as under single-dose procedures. Dose-response curves were quantal under both the single-dose and the cumulative-dosing procedures. The four-choice procedure offers some important advantages for studying the discriminative stimulus effects of drugs that interact with multiple receptor subtypes and for studying drug mixtures.

Gacyclidine: a new neuroprotective agent acting at the N-methyl-D-aspartate receptor

Gacyclidine is a new phencyclidine derivative with neuroprotective properties. Tritiated gacyclidine and its enantiomers bind to NMDA receptors with binding parameters similar to those of other non-competitive NMDA receptor antagonists. The (-)enantiomer, (-)GK11, exhibits an affinity (2.5 nM) similar to that of dizocilpine (MK-801), while the (+)enantiomer, (+)GK11, has a 10 times lower affinity. When its interaction with NMDA receptors is prevented, gacyclidine binds also to "non-NMDA" binding sites which are mainly located in the molecular layer of the cerebellum on the dendritic tree of Purkinje cells. These binding sites do not appear to be related to any known neurotransmitters. In primary cortical cultures, gacyclidine and its enantiomers, at 0.1 to 5.0 microM, prevent glutamate-induced neuronal death. In rats, in vivo neurotoxicity of gacyclidine is far low than that of MK-801. No necrotic neurons were detected in animals sacrificed at 18 or 96 h after treatment with gacyclidine (1, 5, 10 or 20 mg/kg i.v.). At the highest (20 mg/kg) but not the lower doses (1-100 mg/kg) electron microscopy revealed the presence of few cytoplasmic or intramitochondrial vacuoles. In soman-treated monkeys gacyclidine enhanced neuroprotective activity of "three drugs cocktail" (atropine + diazepam + pralidoxime). Moreover, in rats, gacyclidine exerts a dose- and time-dependent neuroprotection in three models of spinal cord lesion. Beneficial effects of gacyclidine include reduction of lesion size and improvement of functional parameters after injury. In traumatic brain injury models gacyclidine improves also behavioral parameters and neuronal survival. Optimal protection is obtained when gacyclidine is administered at 0 to 30 min after injury. It is, therefore, concluded that gacyclidine exhibits neuroprotective effects similar to those of other NMDA receptor antagonists, with the advantage of being substantially less neurotoxic maybe due to its interaction with "non-NMDA" binding sites.

Gacyclidine: a new neuroprotective agent acting at the N-methyl-D-aspartate receptor

Gacyclidine is a new phencyclidine derivative with neuroprotective properties. Tritiated gacyclidine and its enantiomers bind to NMDA receptors with binding parameters similar to those of other non-competitive NMDA receptor antagonists. The (-)enantiomer, (-)GK11, exhibits an affinity (2.5 nM) similar to that of dizocilpine (MK-801), while the (+)enantiomer, (+)GK11, has a 10 times lower affinity. When its interaction with NMDA receptors is prevented, gacyclidine binds also to "non-NMDA" binding sites which are mainly located in the molecular layer of the cerebellum on the dendritic tree of Purkinje cells. These binding sites do not appear to be related to any known neurotransmitters. In primary cortical cultures, gacyclidine and its enantiomers, at 0.1 to 5.0 microM, prevent glutamate-induced neuronal death. In rats, in vivo neurotoxicity of gacyclidine is far low than that of MK-801. No necrotic neurons were detected in animals sacrificed at 18 or 96 h after treatment with gacyclidine (1, 5, 10 or 20 mg/kg i.v.). At the highest (20 mg/kg) but not the lower doses (1-100 mg/kg) electron microscopy revealed the presence of few cytoplasmic or intramitochondrial vacuoles. In soman-treated monkeys gacyclidine enhanced neuroprotective activity of "three drugs cocktail" (atropine + diazepam + pralidoxime). Moreover, in rats, gacyclidine exerts a dose- and time-dependent neuroprotection in three models of spinal cord lesion. Beneficial effects of gacyclidine include reduction of lesion size and improvement of functional parameters after injury. In traumatic brain injury models gacyclidine improves also behavioral parameters and neuronal survival. Optimal protection is obtained when gacyclidine is administered at 0 to 30 min after injury. It is, therefore, concluded that gacyclidine exhibits neuroprotective effects similar to those of other NMDA receptor antagonists, with the advantage of being substantially less neurotoxic maybe due to its interaction with "non-NMDA" binding sites.

Non reciprocal cross-sensitization between cocaine and BTCP on locomotor activity in the rat

Measurement of locomotor sensitization was employed to characterize the effect of intermittent treatment with N-[1-(2-benzo[b]thiophenyl)cyclohexyl]piperidine (BTCP) and cocaine in the rat. Like cocaine, BTCP possesses high affinity for the dopamine transporter and inhibits dopamine reuptake. Although both drugs exhibit similar behavioral and neurochemical profiles with acute administration, there is tentative evidence to suggest that following chronic treatment BTCP does not induce neurochemical sensitization, and can attenuate cocaine-induced neurochemical sensitization in the striatum. Male Wistar rats were randomly divided into five groups after determining baseline locomotor activity. Three groups were treated with either saline (saline/saline), cocaine (20 mg/kg; cocaine/cocaine), or BTCP (10 mg/kg; BTCP/BTCP) for 10 days. The remaining two groups were treated with cocaine (20 mg/kg) or BTCP (10 mg/kg) for 3 days, followed by administration of BTCP (10 mg/kg; cocaine/BTCP) or cocaine (20 mg/kg; BTCP/cocaine) for 7 days. Locomotor sensitization was observed in all groups. However, although cross-sensitization on the day of substitution (day 4) was found in the BTCP/cocaine group, cross-sensitization was not observed in the cocaine/BTCP group. These results suggest that although the locomotor-activating effects of BTCP and cocaine are similar, the two drugs do not act identically, and different neural mechanisms may underlie BTCP and cocaineinduced sensitization.

Conformational preferences of the potent dopamine reuptake blocker BTCP and its analogs and their incorporation into a pharmacophore model

Molecular mechanics calculations using MM3-92 and ab initio quantum mechanical calculations using SPARTAN 5.0 were performed on the structurally similar PCP and BTCP, in which only the latter has a cocaine-like pharmacological profile as a dopamine reuptake blocker. Calculations were also performed on BTCP analogs with a methyl group in various positions of the cyclohexane ring. The results for the cis-2-methyl compound, which retains good pharmacological activity, allowed us to determine that an aryl-axial conformer is the biologically active form for at least some of the compounds in this series. However, an aryl-equatorial conformer presents the identical pharmacophore, as shown by superposition of the two conformers. X-ray crystallographic structures were also obtained for BTCP and related compounds with a 2-methyl group on the cyclohexane ring, with reasonable agreement between the computational and experimental results. Superposition studies were performed with two rigid analogs of cocaine which illustrate the optimal orientations of the ammonium hydrogen for monoamine transporters. There is excellent agreement between a 'back-bridged' cocaine analog that is optimal as a dopamine reuptake blocker and the previously proposed biologically active conformer of methylphenidate. However, BTCP is found to be a better fit to the 'front-bridged' cocaine analog that is optimal for a serotonin reuptake blocker.

1-[1-(2-Benzo[b]thiopheneyl)cyclohexyl]piperidine hydrochloride (BTCP) yields two active primary metabolites in vivo. Identification and quantification of BTCP primary metabolites in mice plasma, urine, and brain and their affinity for the neuronal dopamine transporter

1-[1-(2-Benzo[b]thiopheneyl)cyclohexyl]piperidine hydrochloride (BTCP) and cocaine bind to the neuronal dopamine transporter (DAT) to strongly inhibit dopamine (DA) reuptake. Although similar to acute administration, cocaine and BTCP produce sensitization and tolerance, respectively, on chronic administration. We previously found that liver microsomes produced two primary metabolites from BTCP with a high affinity for DAT. Because such metabolites, if produced in vivo, could account for the pharmacological difference with cocaine, it was important to compare BTCP biotransformations in vitro and in vivo. Therefore, we identified and quantified BTCP and primary metabolites in mice urine, plasma, and brain after acute i.p. administration. The low recovery yield suggest that BTCP might behave like its close analogue, phencyclidine, with long-term storage of metabolites. Two active metabolites found in vitro were found in mice brain with estimated half-life values similar to that of BTCP ( approximately 0.3 h). Although respective brain concentrations were 20 and 40 times lower than that of BTCP, their potency to displace in vivo [3H]BTCP bound to the DAT was 50 and 10 times higher, respectively, than that of BTCP. They could, therefore, contribute to the inhibition of DA transport and play an important role in BTCP pharmacology. They could also explain the differences between BTCP and cocaine on repeated administration.

N-phenylalkyl-substituted tropane analogs of boat conformation with high selectivity for the dopamine versus serotonin transporter

A series of N-phenylalkyl-substituted tropane analogs of boat conformation was synthesized, and these tropanes were evaluated for their ability to inhibit high affinity uptake of dopamine (DA) and serotonin (5-HT) into striatal nerve endings (synaptosomes). Some of these compounds exhibit high affinity for the DA transporter with a 5-HT/DA transporter selectivity ratio of >50.

Crystal structure of monoclonal 6B5 Fab complexed with phencyclidine

The crystal structure of monoclonal antibody (mAb) 6B5 Fab fragment complexed with 1-(1-phenylcyclohexyl)piperidine (PCP or phencyclidine) was determined at 2.2-A resolution. 6B5 was originally produced from a mouse immunized with a phencyclidine analogue hapten 5-[N-(1'phenylcyclohexyl)amino]pentanoic acid conjugated to bovine serum albumin. This mAb was selected for further study because of its high affinity (Kd = 2 x 10(-9) M/liter) for PCP and usefulness in reversing PCP-induced central nervous system toxicity in laboratory animals. The dominant feature of the 6B5 Fab.PCP complex is the deep binding site and hydrophobic nature of the interaction. The ligand binding pocket of 6B5 Fab has numerous aromatic side chains, as compared with other known Fab structures. The most notable feature of the binding site is a Trp at position 97H (H-chain), and the side chain of this residue appears to act as a hydrophobic umbrella on the ligand in the antigen binding pocket. There are only two other known Fabs found with a Trp at the 97H position in complementarity determining region (CDR) H3, but they do not play a major role in the interaction with their respective antigens; in both Fab TE33 and R6.5 the Trp 97H side chain is positioned away from the bound antigen. Comparison of the CDR residues of 6B5 with other Fab structures with similar CDR sizes and amino acid compositions reveals a number of important patterns of residue substitutions that appear to be critical for specific PCP ligand interactions.

Potent and selective ligands for the dopamine transporter (DAT): structure-activity relationship studies of novel 4-[2-(diphenylmethoxy)ethyl]-1-(3-phenylpropyl)piperidine analogues

Molecular structural modifications of 4-[2-(diphenylmethoxy)ethyl]-1-(3-phenylpropyl)piperidine (1a), a dopamine transporter (DAT)-specific ligand, generated several novel analogues. Biological activities of these new molecules for their binding to the DAT and serotonin transporter (SERT) were evaluated in rat striatal membranes. Some of these new analogues were more potent and selective than GBR 12909 when their binding to the DAT relative to SERT was compared. Thus compounds 9 and 19a were among the most potent (IC50 = 6.6 and 6.0 nM, respectively) and selective (DAT/SERT = 33.8 and 30.0, respectively) compounds in this series, and they were more active than GBR 12909 (IC50 = 14 nM, DAT/SERT = 6.1). Introduction of a double bond in the N-propyl side chain of these molecules did not influence their activities to a great extent. Bioisosteric replacement of the aromatic phenyl group by a thiophene moiety produced some of the most potent compounds in this series.

Brain microsomal metabolism of phencyclidine in male and female rats

These studies examined the microsomal brain metabolism of phencyclidine (PCP) in male and female Sprague-Dawley rats. Several monohydroxylated metabolites of PCP were detected including cis- and trans-1-(1-phenyl-4-hydroxycyclohexyl)piperidine (c-PPC and t-PPC) and 1-(1-phenylcyclohexyl)-4-hydroxypiperidine (PCHP). The in vitro formation of these metabolites required NADPH and was inhibited by carbon monoxide. c-PPC was formed in the male and female brain microsomes at rates of 7.1 +/- 1.3 and 5.7 +/- 1.1 fmol/min per mg, respectively, while t-PPC was formed at rates of 16.2 +/- 3.3 and 16.5 +/- 4.2 fmol/min per mg. PCHP had the highest formation rate at 50.7 +/- 8.9 and 48.2 +/- 8.8 fmol/min per mg, respectively. Although previous studies with rat liver microsomes find higher levels of PCP metabolism in male rats and the formation of an irreversibly bound metabolite in male rats, the present study of brain metabolism found no sex differences in brain metabolism. The formation of PCP metabolites in male rat livers is at least partially mediated by the male-specific isozyme CYP2C11, and possibly CYP2D1. Nevertheless, the formation of the major brain metabolite, PCHP, was not inhibited by an anti-CYP2C11 or an anti-CYP2D6 antibody. However, PCHP formation was inhibited by drug inhibitors of CYP2D1-mediated metabolism, suggesting the involvement of a CYP2D isoform. These data indicate brain metabolism of PCP is significant, but unlike the liver it is not sexually dimorphic.

Long-term monitoring of extracellular dopamine concentration in the rat striatum by a repeated microdialysis procedure

This study examined a protocol for repeated measurement of the extracellular dopamine (DA) concentration in the rat striatum by microdialysis. Rats were implanted with a guide cannula in the striatum and the probe was inserted on each dialysing day, i.e. ten times over a 23 day period. During this period the animals were submitted to a control saline treatment. DA concentration was measured using the no-net-flux method. In these conditions, DA concentration remained remarkably constant over the 23 day period. The histological analysis using glial fibrillary acidic protein (GFAP), dopamine (DA) and tyrosine hydroxylase (TH) immunocytochemistry showed a moderate gliosis and a discrete increase of immunoreactivity of catecholaminergic fibres around the probe implantation site. This increase is probably related to a plasticity of the dopaminergic system in response to the lesion due to the probe implantations. This study shows that such a paradigm makes possible to measure the whole time course of the DA concentration in the rat striatum during chronic treatments with psychoactive drugs such cocaine or other compounds acting in the nigrostriatal system.

Rewarding actions of phencyclidine and related drugs in nucleus accumbens shell and frontal cortex

Rats learned to lever-press when such behavior was reinforced by microinjections of phencyclidine (PCP) directly into the ventromedial (shell) region of nucleus accumbens, indicating that the drug has direct rewarding actions in that region. Separate groups of rats learned to lever-press when reinforced with microinjections of dizoclipine (MK-801) or 3-((+/-)2-carboxypiperazin-4yl)propyl-1-phosphate (CPP), drugs known to block NMDA receptor function but not dopamine uptake, into the same region. Each drug was ineffective or markedly less effective when injected at a slightly more dorsal and lateral site in the core of nucleus accumbens. Self-administration of PCP, MK-801, or CPP directly into nucleus accumbens was not altered by co-infusion of a dose of the dopamine antagonist sulpiride that effectively blocked intracranial self-administration of the dopamine uptake inhibitor nomifensine, suggesting that the rewarding actions of the NMDA receptor antagonists are not dopamine-dependent. Rats also developed lever-pressing habits when PCP, MK-801, and CPP were each microinjected directly into frontal cortex, a region previously associated with the rewarding actions of cocaine but not nomifensine. Thus nucleus accumbens and frontal cortex are each potential substrates for the rewarding properties of PCP and related drugs, and the ability of these drugs to disrupt NMDA receptor function seems sufficient to account for their rewarding actions. When considered with independent evidence, the present results suggest a model of drug reward within which the critical event is inhibition of medium spiny neurons in nucleus accumbens.

Structure-activity relationship studies of novel 4-[2-[bis(4-fluorophenyl)methoxy]ethyl]-1-(3-phenylpropyl)piperidine analogs: synthesis and biological evaluation at the dopamine and serotonin transporter sites

Several analogs of the potent dopamine (DA) transporter ligand 4-[2-[bis(4-fluorophenyl)-methoxy]ethyl]-1-(3-phenylpropyl)piperidine, 1b, were made and biologically evaluated for their binding at the DA and serotonin (5HT) transporters in rat striatal membranes. Different alkyl chain lengths and substitutions were introduced in these molecules to generate an optimum activity and selectivity for the DA transporter. In general, unsubstituted and fluoro-substituted compounds were the most active and selective for the DA transporter. The compound 4-[2-(diphenylmethoxy)ethyl]-1-benzylpiperidine, 9a, showed high potency and was the most selective for the DA transporter (5HT/DA = 49) in this series of compounds. Some of these novel analogs were found to be more selective in binding at the DA transporter than the original GBR 12909 molecule, 1-[2-]bis(4-fluorophenyl)methoxy]ethyl]-4-(3- phenylpropyl)piperidine.

Effects of the major metabolite of phencyclidine, the trans isomer of 4-phenyl-4-(1-piperidinyl)cyclohexanol, on [3H]N-(1-[2-thienyl] cyclohexyl)-3,4-piperidine ([3H]TCP) binding and [3H]dopamine uptake in the rat brain

The major metabolite of phencyclidine (PCP), the trans isomer of 4-phenyl-4-(1-piperidinyl)cyclohexanol [(trans)-4-PPC], inhibited [3H]N-(1-(2-thienyl)cyclohexyl)-3,4-piperidine ([3H]TCP) binding to well-washed rat cortical membranes with much less activity than PCP itself. In contrast, it inhibited [3H]dopamine ([3H]DA) uptake in rat striatal synaptosomes to a similar extent as PCP. Considering our previous observations that intraperitoneally administered (trans)-4-PPC elicits dose-related increases in locomotor activity and rearing in mice, (trans)-4-PPC as well as PCP may be involved in psychotomimetic effects of PCP due to its inhibitory effect on DA uptake.

Synthesis and evaluation of 1-(1-[5-(2'-[18F]fluoroethyl)-2-thienyl]-cyclohexyl)piperidine as a potential in vivo radioligand for the NMDA receptor-channel complex

1-(1-[5-(2'-[18F]Fluoroethyl)-2-thienyl]cyclohexyl)piperidine (18FE-TCP) was prepared as a fluorine-substituted analogue of the potent NMDA receptor channel blocker, 1-[1-(2-thienyl)cyclohexyl]piperidine (TCP), by the mesylate displacement with [18F]fluoride ion with isolated radiochemical yields of 6-12%, and the synthesis time including a two step HPLC purification was 120 min. The regional distribution in rat brain after i.v. injection of 18FE-TCP was heterogeneous and similar to the known distribution of phencyclidine recognition sites, with hippocampus-cerebellum, striatum-cerebellum and cerebral cortex-cerebellum concentration ratios of 2.08, 1.7 and 1.54, respectively, 15 min post-injection. Furthermore, this localized regional cerebral distribution was blocked by co-injection with the unlabelled FE-TCP or pretreatment with cis-2-hydroxymethyl-r-1-(N-piperidyl)-1-(2-thienyl)cyclohexane, with the greatest reductions seen in the hippocampus followed by the striatum and cerebral cortex. However, relatively low receptor binding affinity and high non-specific binding due to its high lipophilicity suggest that 18FE-TCP may not be a suitable radioligand for in vivo PET investigations of the NMDA receptor-channel complex.

Endogenous dopamine differently affects N-[1-(2-benzo(b)thiophenyl)cyclohexyl]piperidine and cocaine binding to the dopamine uptake complex

1. [3H]N-[1-(2-Benzo(b)thiophenyl)cyclohexyl]piperidine ([3H]BTCP) labels in vivo the dopamine uptake complex in the mouse striatum. 2. In mice treated with gamma-butyrolactone (GBL, 400 mg/kg), [3H]BTCP specific binding was increased and ID50 values of BTCP and cocaine for the prevention of [3H]BTCP binding were significantly lowered. 3. In GBL-treated mice, cocaine (5 mg/kg, s.c.) had no effect on the BTCP ID50 value, whereas GBR 12783 (2 mg/kg, s.c.) increased it significantly. 4. Thus in vivo, endogenous dopamine and cocaine are competitive and non-competitive inhibitors, respectively, of the binding of [3H]BTCP.