Structure-activity studies on amphetamine analogs using drug discrimination methodology

Enantioseparation and a comprehensive spectroscopic analysis of novel synthetic cathinones laterally substituted with a trifluoromethyl group

Recently, the number of structural modifications of synthetic cathinones has been growing making them the second largest group of new psychoactive substances in Europe. Although they are abused because of their various psychoactive effects, some compounds from this group also serve as pharmaceuticals. Since synthetic cathinones are chiral molecules with one chiral center, their biological, toxicological, and pharmacological properties may significantly differ according to their absolute configuration and enantiomeric excess. In this study, we have synthesized two substances bearing a pharmacologically interesting trifluoromethyl group and developed a chiral liquid chromatography method using a polysaccharide chiral stationary phase to separate the corresponding enantiomers of both these drugs. Subsequently, we utilized molecular spectroscopic methods including chiroptical (electronic circular dichroism and vibrational circular dichroism) and non-polarizable (infrared and ultraviolet absorption) spectroscopies. In combination with density functional theory calculations, we have obtained stable conformers of selected enantiomers in solution and their relative abundances, which we used to simulate their spectra. The experimental and calculated data have been used to elucidate the 3D structure of the enantiomerically pure compounds and assign the absolute configuration of all prepared compounds.

2-Aminoindan and its ring-substituted derivatives interact with plasma membrane monoamine transporters and α2-adrenergic receptors

RATIONALE

Over the last decade, many new psychostimulant analogues have appeared on the recreational drug market and most are derivatives of amphetamine or cathinone. Another class of designer drugs is derived from the 2-aminoindan structural template. Several members of this class, including the parent compound 2-aminoindan (2-AI), have been sold as designer drugs. Another aminoindan derivative, 5-methoxy-2-aminoindan (5-MeO-AI or MEAI), is the active ingredient in a product marketed online as an alcohol substitute.

METHODS

Here, we tested 2-AI and its ring-substituted derivatives 5-MeO-AI, 5-methoxy-6-methyl-2-aminoindan (MMAI), and 5,6-methylenedioxy-2-aminoindan (MDAI) for their abilities to interact with plasma membrane monoamine transporters for dopamine (DAT), norepinephrine (NET) and serotonin (SERT). We also compared the binding affinities of the aminoindans at 29 receptor and transporter binding sites.

RESULTS

2-AI was a selective substrate for NET and DAT. Ring substitution increased potency at SERT while reducing potency at DAT and NET. MDAI was moderately selective for SERT and NET, with tenfold weaker effects on DAT. 5-MeO-AI exhibited some selectivity for SERT, having sixfold lower potency at NET and 20-fold lower potency at DAT. MMAI was highly selective for SERT, with 100-fold lower potency at NET and DAT. The aminoindans had relatively high affinity for α₂-adrenoceptor subtypes. 2-AI had particularly high affinity for α_2C receptors (K_i = 41 nM) and slightly lower affinity for the α_2A (K_i = 134 nM) and α_2B (K_i = 211 nM) subtypes. 5-MeO-AI and MMAI also had moderate affinity for the 5-HT_2B receptor.

CONCLUSIONS

2-AI is predicted to have (+)-amphetamine-like effects and abuse potential whereas the ring-substituted derivatives may produce 3,4-methylenedioxymethamphetamine (MDMA)-like effects but with less abuse liability.

Stereoselective effects of the second-generation synthetic cathinone α-pyrrolidinopentiophenone (α-PVP): assessments of conditioned taste avoidance in rats

RATIONALE

Work with α-pyrrolidinopentiophenone (α-PVP), a second-generation synthetic cathinone, has been generally limited to the racemate. Given that with other synthetic cathinones, there are behavioral and neurochemical differences between their enantiomers, differences may also be seen with α-PVP.

OBJECTIVES

The present study assessed the relative contribution of each enantiomer to the aversive effects of racemic-α-PVP by comparing their ability to induce a conditioned taste avoidance.

METHODS

Adult male Sprague-Dawley rats were exposed every other day for four exposures to a novel saccharin solution followed immediately by an injection of 0 (saline vehicle) or 1.5, 3, or 6 mg/kg of S-, R-, or racemic-α-PVP (IP). On alternating days, all subjects were given access to water to assess any unconditioned effects of α-PVP on general fluid consumption.

RESULTS

Rats injected with the racemate and S-isomer of α-PVP displayed avoidance of the drug-associated saccharin solution, although this avoidance was dose-dependent only for the subjects injected with the racemate. There was no evidence of taste avoidance in animals injected with the R-enantiomer at any dose tested. Animals injected with 3 mg/kg racemic-α-PVP did not differ in avoidance from those treated with 1.5 mg/kg of the S-enantiomer, but subjects treated with 6 mg/kg racemic-α-PVP displayed a significantly stronger avoidance than those treated with 3 mg/kg S-α-PVP.

CONCLUSIONS

The present work suggests that the aversive effects of racemic α-PVP are mediated primarily by its S-isomer. The fact that at the highest dose tested (6 mg/kg), the racemate induces an avoidance greater than the simple additive effects of the S- and R-isomers (at 3 mg/kg) suggests that while the R-isomer may not induce taste avoidance at this dose, it may interact synergistically with the S-isomer in mediating the effects of the racemic mixture. These results were discussed in terms of similar effects with other behavioral and physiological endpoints reported with a number of psychostimulants and suggest that the enantiomers of α-PVP are an important variable in characterizing its behavioral effects.

Pharmacology of MDMA- and Amphetamine-Like New Psychoactive Substances

New psychoactive substances (NPS) with amphetamine-, aminoindan-, and benzofuran basic chemical structures have recently emerged for recreational drug use. Detailed information about their psychotropic effects and health risks is often limited. At the same time, it emerged that the pharmacological profiles of these NPS resemble those of amphetamine or 3,4-methylenedioxymethamphetamine (MDMA). Amphetamine-like NPS induce psychostimulation and euphoria mediated predominantly by norepinephrine (NE) and dopamine (DA) transporter (NET and DAT) inhibition and transporter-mediated release of NE and DA, thus showing a more catecholamine-selective profile. MDMA-like NPS frequently induce well-being, empathy, and prosocial effects and have only moderate psychostimulant properties. These MDMA-like substances primarily act by inhibiting the serotonin (5-HT) transporter (SERT) and NET, also inducing 5-HT and NE release. Monoamine receptor interactions vary considerably among amphetamine- and MDMA-like NPS. Clinically, amphetamine- and MDMA-like NPS can induce sympathomimetic toxicity. The aim of this chapter is to review the state of knowledge regarding these substances with a focus on the description of the in vitro pharmacology of selected amphetamine- and MDMA-like NPS. In addition, it is aimed to provide links between pharmacological profiles and in vivo effects and toxicity, which leads to the conclusion that abuse liability for amphetamine-like NPS may be higher than for MDMA-like NPS, but that the risk for developing the life-threatening serotonin syndrome may be increased for MDMA-like NPS.

Serotonergic Hallucinogen-Induced Visual Perceptual Alterations

Serotonergic hallucinogens, such as lysergic acid diethylamide (LSD), psilocybin, and N,N-dimethyltryptamine (DMT), are famous for their capacity to temporally and profoundly alter an individual's visual experiences. These visual alterations show consistent attributes despite large inter- and intra-individual variances. Many reports document a common perception of colors as more saturated, with increased brightness and contrast in the environment ("Visual Intensifications"). Environmental objects might be altered in size ("Visual illusions") or take on a modified and special meaning for the subject ("Altered self-reference"). Subjects may perceive light flashes or geometrical figures containing recurrent patterns ("Elementary imagery and hallucinations") influenced by auditory stimuli ("Audiovisual synesthesia"), or they may envision images of people, animals, or landscapes ("Complex imagery and hallucinations") without any physical stimuli supporting their percepts. This wide assortment of visual phenomena suggests that one single neuropsychopharmacological mechanism is unlikely to explain such vast phenomenological diversity. Starting with mechanisms that act at the cellular level, the key role of 5-HT2A receptor activation and the subsequent increased cortical excitation will be considered. Next, it will be shown that area specific anatomical and dynamical features link increased excitation to the specific visual contents of hallucinations. The decrease of alpha oscillations by hallucinogens will then be introduced as a systemic mechanism for amplifying internal-driven excitation that overwhelms stimulus-induced excitations. Finally, the hallucinogen-induced parallel decrease of the N170 visual evoked potential and increased medial P1 potential will be discussed as key mechanisms for inducing a dysbalance between global integration and early visual gain that may explain several hallucinogen-induced visual experiences, including visual hallucinations, illusions, and intensifications.

Structure-Activity Relationships of Synthetic Cathinones

Until recently, there was rather little interest in the structure-activity relationships (SARs) of cathinone analogs because so few agents were available and because they represented a relatively minor drug abuse problem. Most of the early SAR was formulated on the basis of behavioral (e.g., locomotor and drug discrimination) studies using rodents. With the emergence on the clandestine market in the last few years of a large number of new cathinone analogs, termed "synthetic cathinones", and the realization that they likely act at dopamine, norepinephrine, and/or serotonin transporters as releasing agents (i.e., as substrates) or reuptake inhibitors (i.e., as transport blockers), it has now become possible to better examine their SAR and even their quantitative SAR (QSAR), in a more effective and systematic manner. An SAR picture is beginning to emerge, and key structural features, such as the nature of the terminal amine, the size of the α-substituent, stereochemistry, and the presence and position of aromatic substituents, are being found to impact action (i.e., as releasing agents or reuptake inhibitors) and transporter selectivity.

Decoding the Structure of Abuse Potential for New Psychoactive Substances: Structure-Activity Relationships for Abuse-Related Effects of 4-Substituted Methcathinone Analogs

Many cathinone analogs act as substrates or inhibitors at dopamine, norepinephrine, and serotonin transporters (DAT, NET, SERT, respectively). Drug selectivity at DAT vs. SERT is a key determinant of abuse potential for monoamine transporter substrates and inhibitors, such that potency at DAT > SERT is associated with high abuse potential, whereas potency at DAT < SERT is associated with low abuse potential. Quantitative structure-activity relationship (QSAR) studies with a series of 4-substituted methcathinone analogs identified volume of the 4-position substituent on the methcathinone phenyl ring as one structural determinant of both DAT vs. SERT selectivity and abuse-related behavioral effects in an intracranial self-stimulation procedure in rats. Subsequent modeling studies implicated specific amino acids in DAT and SERT that might interact with 4-substituent volume to determine effects produced by this series of cathinone analogs. These studies illustrate use of QSAR analysis to investigate pharmacology of cathinones and function of monoamine transporters.

Cocaine-like discriminative stimulus effects of amphetamine, cathinone, methamphetamine, and their 3,4-methylenedioxy analogs in male rhesus monkeys

RATIONALE

Synthetic cathinones have emerged as the newest class of abused monoamine transporter substrates. Structurally, these compounds are all beta-ketone amphetamine (cathinone) analogs. Whether synthetic cathinone analogs produce differential behavioral effects from their amphetamine analog counterparts has not been systematically examined. Preclinical drug discrimination procedures have been useful for determining the structure activity relationships (SARs) of abused drugs; however, direct comparisons between amphetamine and cathinone analogs are lacking and, in particular, in non-human primate models.

OBJECTIVES

The study aim was to determine the potency and time course of (±)-amphetamine, (±)-cathinone, and (±)-methamphetamine and their 3,4-methylenedioxy analogs (±)-MDA, (±)-MDC, and (±)-MDMA, respectively, to produce cocaine-like discriminative stimulus effects. If cathinone analogs have similar behavioral pharmacological properties to their amphetamine counterparts, then we would predict similar potencies and efficacies to produce cocaine-like discriminative stimulus effects.

METHODS

Male rhesus monkeys (n = 4) were trained to discriminate intramuscular cocaine (0.32 mg/kg) from saline in a two-key food-reinforced discrimination procedure.

RESULTS

Racemic amphetamine, cathinone, and methamphetamine produced dose-dependent and full substitution, ≥90 % cocaine-appropriate responding, in all monkeys. Addition of 3,4-methylenedioxy moiety attenuated both the potency and efficacy of amphetamine (MDA), cathinone (MDC), and methamphetamine (MDMA) to produce full cocaine-like effects. Moreover, the cocaine-like effects of amphetamine and cathinone were attenuated to a greater extent than those of methamphetamine or previously published methcathinone (Smith et al. 2016).

CONCLUSION

The presence of an N-methyl group blunted both the potency and the efficacy shift of the 3,4-methylenedioxy addition for both amphetamine and cathinone analogs.

Abuse-related neurochemical and behavioral effects of cathinone and 4-methylcathinone stereoisomers in rats

Cathinone and many of its analogs produce behavioral effects by promoting transporter-mediated release of the monoamine neurotransmitters dopamine, norepinephrine and/or serotonin. Stereoselectivity is one determinant of neurochemical and behavioral effects of cathinone analogs. This study compared effectiveness of the S(-) and R(+) enantiomers of cathinone and 4-methylcathinone to produce in vitro monoamine release and in vivo abuse-related behavioral effects in rats. For neurochemical studies, drug effects were evaluated on monoamine release through dopamine, norepinephrine, and serotonin transporters (DAT, NET and SERT, respectively) in rat brain synaptosomes. For behavioral studies, drug effects were evaluated on responding for electrical brain stimulation in an intracranial self-stimulation (ICSS) procedure. The cathinone enantiomers differed in potency [S(-)>R(+)], but both enantiomers were >50-fold selective at promoting monoamine release through DAT vs. SERT, and both enantiomers produced ICSS facilitation. The 4-methylcathinone enantiomers also differed in potency [S(-)>R(+)]; however, in neurochemical studies, the decrease in potency from S(-) to R(+)4-methylcathinone was less for DAT than for SERT, and as a result, DAT vs. SERT selectivity was greater for R(+) than for S(-)4-methylcathinone (4.1- vs. 1.2-fold). Moreover, in behavioral studies, S(-)4-methylcathinone produced only ICSS depression, whereas R(+)4-methylcathinone produced ICSS facilitation. This study provides further evidence for stereoselectivity in neurochemical and behavioral actions of cathinone analogs. More importantly, stereoselective 4-methylcathinone effects on ICSS illustrate the potential for diametrically opposite effects of enantiomers in a preclinical behavioral assay of abuse potential.

Stereoselective Effects of Abused "Bath Salt" Constituent 3,4-Methylenedioxypyrovalerone in Mice: Drug Discrimination, Locomotor Activity, and Thermoregulation

3,4-Methylenedioxypyrovalerone (MDPV) is a common constituent of illicit "bath salts" products. MDPV is a chiral molecule, but the contribution of each enantiomer to in vivo effects in mice has not been determined. To address this, mice were trained to discriminate 10 mg/kg cocaine from saline, and substitutions with racemic MDPV, S(+)-MDPV, and R(-)-MDPV were performed. Other mice were implanted with telemetry probes to monitor core temperature and locomotor responses elicited by racemic MDPV, S(+)-MDPV, and R(-)-MDPV under a warm (28°C) or cool (20°C) ambient temperature. Mice reliably discriminated the cocaine training dose from saline, and each form of MDPV fully substituted for cocaine, although marked potency differences were observed such that S(+)-MDPV was most potent, racemic MDPV was less potent than the S(+) enantiomer, and R(-)-MDPV was least potent. At both ambient temperatures, locomotor stimulant effects were observed after doses of S(+)-MDPV and racemic MDPV, but R(-)-MDPV did not elicit locomotor stimulant effects at any tested dose. Interestingly, significant increases in maximum core body temperature were only observed after administration of racemic MDPV in the warm ambient environment; neither MDPV enantiomer altered core temperature at any dose tested, at either ambient temperature. These studies suggest that all three forms of MDPV induce biologic effects, but R(-)-MDPV is less potent than S(+)-MDPV and racemic MDPV. Taken together, these data suggest that the S(+)-MDPV enantiomer is likely responsible for the majority of the biologic effects of the racemate and should be targeted in therapeutic efforts against MDPV overdose and abuse.

Alpha-ethyltryptamines as dual dopamine-serotonin releasers

The dopamine (DA), serotonin (5-HT), and norepinephrine (NE) transporter releasing activity and serotonin-2A (5-HT2A) receptor agonist activity of a series of substituted tryptamines are reported. Three compounds, 7b, (+)-7d and 7f, were found to be potent dual DA/5-HT releasers and were >10-fold less potent as NE releasers. Additionally, these compounds had different activity profiles at the 5-HT2A receptor. The unique combination of dual DA/5-HT releasing activity and 5-HT2A receptor activity suggests that these compounds could represent a new class of neurotransmitter releasers with therapeutic potential.

Bath salts, mephedrone, and methylenedioxypyrovalerone as emerging illicit drugs that will need targeted therapeutic intervention

The term "synthetic cathinones" is fairly new, but, although the abuse of synthetic cathinones is a recent problem, research on cathinone analogs dates back >100 years. One structural element cathinone analogs have in common is an α-aminophenone moiety. Introduction of amine and/or aryl substituents affords a large number of agents. Today, >40 synthetic cathinones have been identified on the clandestine market and many have multiple "street names." Many cathinone analogs, although not referred to as such until the late 1970s, were initially prepared as intermediates in the synthesis of ephedrine analogs. The cathinones do not represent a pharmacologically or mechanistically homogeneous class of agents. Currently abused synthetic cathinones are derived from earlier agents and seem to produce their actions primarily via the dopamine, norepinephrine, and/or serotonin transporter; that is, they either release and/or inhibit the reuptake of one or more of these neurotransmitters. The actions of these agents can resemble those of central stimulants such as methamphetamine, cocaine, and/or empathogens such as 1-(3,4-methylenedioxyphenyl)-2-aminopropane (Ecstasy) and/or produce other effects. Side effects are primarily of a neurological and/or cardiovascular nature. The use of the "and/or" term is emphasized because synthetic cathinones represent a broad class of agents that produce a variety of actions; the agents cannot be viewed as being pharmacologically equivalent. Until valid structure-activity relationships are formulated for each behavioral/mechanistic action, individual synthetic cathinones remain to be evaluated on a case-by-case basis. Treatment of synthetic cathinone intoxication requires more "basic science" research. At this time, treatment is mostly palliative.

Synthetic cathinones: chemical phylogeny, physiology, and neuropharmacology

This mini-review summarizes the history of cathinone and its synthesized derivatives from early records to the present day, including the appearance of synthetic cathinones in the drug combination known as bath salts. Bath salts may consist of one compound (MDPV) or combinations of MDPV and one or more other synthetic cathinones, which may also appear alone without MDPV. We briefly review recent in vitro studies of bath salts components alone or in combination, focusing on pharmacological and biophysical studies. Finally we summarize new data from in vivo procedures that characterize the abuse-related neurochemical and behavioral effects of synthetic cathinones in rats.

Comparison of the behavioral and cardiovascular effects of mephedrone with other drugs of abuse in rats

RATIONALE

Exceedingly little experimental research exists on the popular recreational drug mephedrone (4-methylmethcathinone) despite clinical reports concerning its behavioral and cardiovascular toxicity.

OBJECTIVES

To characterize mephedrone preclinically by examining its capacity to (1) serve as a discriminative stimulus, (2) disrupt the acquisition of response sequences, and (3) disrupt mean arterial pressure (MAP) and heart rate (HR).

METHODS AND RESULTS

In one group of subjects that reliably discriminated 3.2 mg/kg of mephedrone from saline (n = 9), substitution tests indicated that stimulants (cocaine, MDMA, and methamphetamine) more closely approximated the mephedrone discriminative stimulus than non-stimulants (fenfluramine, morphine, and phencyclidine), although none fully substituted. In a second group (n = 6), mephedrone (0.56-10 mg/kg, i.p.) dose-dependently decreased response rate and increased errors in both components of a procedure in which subjects either acquired a new response sequence each session (repeated acquisition) or completed the same response sequence each session (performance). Finally, in a third group (n = 12), radio telemetry probes were used to measure the changes in MAP and HR elicited by mephedrone and then compared them to a known stimulant, methamphetamine. In these studies, mephedrone (0.01-9 mg/kg, i.v.) elicited increases in MAP and HR that were very similar to those elicited by methamphetamine (0.01-9 mg/kg, i.v.). The tachycardia and pressor responses to mephedrone (3 mg/kg) were blocked by the β-blocker atenolol (1 mg/kg, i.v.) and the α1, α2-blocker phentolamine (3 mg/kg, i.v.), respectively.

CONCLUSIONS

Mephedrone produces behavioral and cardiovascular responses that are similar to other stimulants; however, differences from the classical stimulants were also apparent.

Metabolic transformation plays a primary role in the psychostimulant-like discriminative-stimulus effects of selegiline [(R)-(-)-deprenyl]

l-Deprenyl [selegiline, (R)-(-)-deprenyl] is a selective inhibitor of monoamine oxidase B (MAO-B) used in the treatment of Parkinson's disease and proposed as an antidepressant and an aid for cigarette-smoking cessation and treatment of psychostimulant abuse. Beneficial therapeutic effects of (R)-(-)-deprenyl may also result from indirect actions. Brain levels of dopamine and beta-phenylethylamine (beta-PEA), a behaviorally active endogenous trace amine, increase after (R)-(-)-deprenyl treatment due to MAO-B blockade and (R)-(-)-deprenyl is metabolized to (R)-(-)-methamphetamine and (R)-(-)-amphetamine, suggesting that (R)-(-)-deprenyl may have psychostimulant-like behavioral effects. Indeed, (R)-(-)-deprenyl produces psychostimulant-like discriminative-stimulus effects in experimental animals. Here, we tested the hypothesis that psychostimulant-like behavioral effects of (R)-(-)-deprenyl are mainly mediated by its metabolites. Male Fisher F344 rats were trained to discriminate i.p. injection of 1.0 mg/kg (S)-(+)-methamphetamine or 10.0 mg/kg cocaine from injection of saline using two-lever choice schedules of food delivery or stimulus shock termination. When (R)-(-)-deprenyl was tested by substitution, it had (S)-(+)-methamphetamine- and cocaine-like discriminative-stimulus effects, but only at doses of 10 to 30 mg/kg, doses 10 to 20 times higher than those selective for MAO-B inhibition. Ro 16-6491 [N-(2-aminoethyl)-4-chlorobenzamide hydrochloride], a selective inhibitor of MAO-B enzyme activity without psychoactive metabolites, had no psychostimulant-like discriminative effects. In addition, blockade of (R)-(-)-deprenyl's metabolism with SKF 525A (beta-DEAE-diphenylpropylacetate hydrochloride; 50 mg/kg i.p.) reduced or eliminated (R)-(-)-deprenyl's psychostimulant-like discriminative effects. When beta-PEA synthesis was blocked by NSD 1015 (m-hydroxy-benzyl-hydrazine; 30 mg/kg i.p.), there was a modest reversal of (R)-(-)-deprenyl's psychostimulant-like discriminative effects under some conditions, indicating a facilitatory modulation of the psychostimulant-like discriminative effects of (R)-(-)-deprenyl metabolites by elevated levels of beta-PEA under certain conditions.

Hallucinogens

Hallucinogens (psychedelics) are psychoactive substances that powerfully alter perception, mood, and a host of cognitive processes. They are considered physiologically safe and do not produce dependence or addiction. Their origin predates written history, and they were employed by early cultures in a variety of sociocultural and ritual contexts. In the 1950s, after the virtually contemporaneous discovery of both serotonin (5-HT) and lysergic acid diethylamide (LSD-25), early brain research focused intensely on the possibility that LSD or other hallucinogens had a serotonergic basis of action and reinforced the idea that 5-HT was an important neurotransmitter in brain. These ideas were eventually proven, and today it is believed that hallucinogens stimulate 5-HT(2A) receptors, especially those expressed on neocortical pyramidal cells. Activation of 5-HT(2A) receptors also leads to increased cortical glutamate levels presumably by a presynaptic receptor-mediated release from thalamic afferents. These findings have led to comparisons of the effects of classical hallucinogens with certain aspects of acute psychosis and to a focus on thalamocortical interactions as key to understanding both the action of these substances and the neuroanatomical sites involved in altered states of consciousness (ASC). In vivo brain imaging in humans using [(18)F]fluorodeoxyglucose has shown that hallucinogens increase prefrontal cortical metabolism, and correlations have been developed between activity in specific brain areas and psychological elements of the ASC produced by hallucinogens. The 5-HT(2A) receptor clearly plays an essential role in cognitive processing, including working memory, and ligands for this receptor may be extremely useful tools for future cognitive neuroscience research. In addition, it appears entirely possible that utility may still emerge for the use of hallucinogens in treating alcoholism, substance abuse, and certain psychiatric disorders.

Dopamine transporter: basic science and human variation of a key molecule for dopaminergic function, locomotion, and parkinsonism

We review the basic science of the dopamine transporter (DAT), a key neurotransmitter for locomotor control and reward systems, including those lost or deranged in Parkinson's disease (PD). Physiology, pharmaceutical features, expression, cDNA, protein structure/function relationships, and phosphorylation and regulation are discussed. The localization of DAT provides the best marker for the integrity of just the pre-synaptic dopaminergic systems that are most affected in PD. Its function is key for the actions of several toxins that provide some of the best current models for idiopathic parkinsonism, and its variation can clearly alter movement. The wealth of information about this interesting molecule that has been developed over the last 12 years has led to increased interest in DAT among workers interested in both normal and abnormal movement.

Drug discrimination in neurobiology

Areas of neurobiological interest are identified towards which drug discrimination (DD) studies have made important contributions. DD allows ligand actions to be analyzed at the whole organism level, with a neurobiological specificity that is exquisite and often unrivalled. DD analyses have thus been made of a vast array of CNS agents acting on receptors, enzymes, or ion channels, including most drugs of abuse. DD uniquely offers access to the study of subjective drug effects in animals, using a methodology that also is transposable to humans and has generated unprecedented models of pathology (e.g., chronic pain, opiate addiction). Parametric studies of such independent variables as training dose and reinforcement provide refined insights into the dynamic psychophysiological mechanisms of both drug effects and behavior. Three different mechanisms have been identified by which discriminative, and perhaps other behaviors, can come about. DD also is superbly sensitive to small, partial activation of molecular substrates; this has enabled DD analyses to pioneer the unravelling of molecular mechanisms of drug action (attributing, f.ex., LSD's particular subjective effects to an unusual, partial activation of 5-HT, and perhaps other receptors). DD has both oriented and served as a tool to conduct drug discovery research (e.g., pirenperone-risperidone, loperamide). The DD response arguably constitutes a quantal, rather than graded, variable, and as such allows a comprehension of molecular, pharmacological, and behavioral mechanisms that would have been otherwise inaccessible. Perhaps most important are the following further contributions. One is the notion that particular, different levels of receptor activation are associated with qualities of neurobiological actions that also differ and are unique, this notion arguably constituting the most significant addition to affinity and intrinsic activity since the earliest theoretical conceptions of molecular pharmacology. Another contribution consists of studies that render redundant the notion of tolerance and identify fundamental mechanisms of signal transduction; these mechanisms account for apparent tolerance, dependence, addiction, and sensitization, and appear to operate ubiquitously in a bewildering array of biological systems.

Serotonergic basis of antipsychotic drug effects in schizophrenia

Recent attention has been focused on the involvement of serotonin (5-HT) in the pathophysiology of schizophrenia and its role in mediating antipsychotic drug effects. There are two reasons for the new emphasis: the tremendous success of the so-called "atypical" antipsychotic drugs (a common feature of which is their high affinity for specific 5-HT receptor subtypes); and the elucidation of a complex family of 5-HT receptors whose function and pharmacology is only beginning to be understood. This paper will review the evidence that pertains to the role of 5-HT in mediating antipsychotic drug effects. The interaction of dopamine and 5-HT systems will be reviewed, and the mechanisms of action of atypical antipsychotic drugs will be evaluated in this context. The impact of serotonin on neurodevelopment, and the involvement of serotonin in the psychotomimetic and psychotogenic properties of hallucinogens, will be discussed. Together, these facts will be placed into the context of changes in serotonergic function in schizophrenia.

Indan analogs of fenfluramine and norfenfluramine have reduced neurotoxic potential

N-Ethyl-5-trifluoromethyl-2-aminoindan (ETAI) and 5-trifluoromethyl-2-aminoindan (TAI) were synthesized to examine the effects of side-chain cyclization on the pharmacology of the anorectic drugs fenfluramine (FEN) and norfenfluramine (norFEN), respectively. ETAI and TAI inhibited synaptosomal accumulation of 5-HT but were less effective at inhibiting catecholamine uptake than FEN or norFEN, respectively. In vivo, ETAI and TAI were less neurotoxic than FEN or norFEN; decreases in the number of [3H]paroxetine-labeled 5-HT uptake sites were 50% less than the decreases produced by FEN or norFEN. Rats treated with ETAI. TAI, FEN, and norFEN lost 10-15% of their pretreatment body weight over a 4-day period, while saline-treated control animals gained 8%. In two-lever drug discrimination (DD) assays in rats, TAI fully substituted for the 5-HT releaser/uptake inhibitor, (+)-MBDB [(+)-N-methyl-1-(1,3-benzodioxol-5-yl)-2-aminobutane]. ETAI produced only partial substitution in this test. Neither TAI nor ETAI mimicked (+)-amphetamine in the DD assay. These studies demonstrate that incorporation of the side-chain of phenylisopropylamines into the five-membered ring of a 2-aminoindan changes both the molecular pharmacology and the neurotoxic profile of FEN and norFEN, but does not diminish the drugs' ability to reduce body weight.

5-HT2A/2C receptor agonists potentiate the discriminative cue of (+)-amphetamine in the rat

The possible effect of 5-HT2A/2C receptor agonists on an amphetamine-induced behavioral response was examined using the two-lever drug discrimination paradigm. The experiments were designed to investigate an interaction of the hallucinogenic 5-HT2A/2C agonists lysergic acid diethylamide (LSD) and 2,5-dimethoxy-4-iodoamphetamine (DOI), with the discriminative stimulus elicited by a relatively low dose of (+)-amphetamine (1.35 micromol/kg, 0.25 mg/kg, which produced approximately 50% selection of the drug lever). DOI and LSD did not produce amphetamine-like responding at any dose tested or time of administration. However, LSD alone was able to induce a drug-appropriate response in two of nine amphetamine-trained rats. Simultaneous administration of DOI or LSD with amphetamine was not significantly different from the response produced by amphetamine alone. Pre-administration of DOI (3 hr) or of LSD (2 hr) before amphetamine, however, evoked significant enhancement of the amphetamine cue. The results suggest that the enhanced behavioral response to amphetamine may be due either to an increased sensitivity of dopaminergic neurons in the mesolimbic area, or to an enhanced release of dopamine by amphetamine.

Psychostimulant-like effects of p-fluoroamphetamine in the rat

The present study was undertaken to compare the pharmacological properties of p-fluoroamphetamine with those of amphetamine and of other halogenated amphetamines, using several in vivo and in vitro tests. These included substitution testing in (+)-amphetamine (1 mg/kg, 5.4 mu mol/kg, i.p.)-, (+)-N-methyl-1-(1,3-benzodioxol-5-yl)-2-butanamine [(+)-MBDB] (1.75 mg/kg, 7.8 mu mol/kg, i.p.)-, and 5-methoxy-6-methyl-2-aminoindan (MMAI) (1.71 mg/kg, 8 mu mol/kg, i.p.)-trained rats, [3H]5-HT and [3H]dopamine uptake inhibition in whole brain synaptosomes, and changes in striatal extracellular levels of dopamine, 3,4-dihydroxyphenylacetic acid (DOPAC), and homovanillic acid (HVA) as measured by in vivo microdialysis in freely moving rats. In drug discrimination substitution tests, p-fluoroamphetamine fully mimicked (+)-amphetamine (ED50 0.43 mg/kg, 2.11 mu mol/kg), whereas 'no substitution' was observed in rats trained to discriminate the serotonin (5-hydroxytryptamine, 5-HT)-releasing agents (+)-MBDB or MMAI from saline. p-Chloroamphetamine did not substitute for amphetamine but fully substituted for the (+)-MBDB and MMAI cues (ED50 0.17 mg/kg, 0.82 mu mol/kg, and 0.14 mg/kg, 0.69 mu mol/kg, respectively). p-Fluoroamphetamine, in comparison with p-chloroamphetamine and p-iodoamphetamine, showed much stronger inhibition of [3H]dopamine than [3H]5-HT uptake into rat brain synaptosomes but was less selective than amphetamine. p-Fluoroamphetamine (7.0 mg/kg, i.p.), 1 h after administration, strongly elevated (849% of baseline) extracellular dopamine in rat striatum measured using in vivo microdialysis. Amphetamine (2 mg/kg, i.p.) increased extracellular dopamine in rat striatum with a maximum at the same time as did p-fluoroamphetamine, but the latter gave a smaller increase. The data presented suggest that p-fluoroamphetamine resembles amphetamie more than it does the 5-HT-releasing type amphetamines.

Direct microinjection of cathinone into the rat brain produces discriminative stimuli

Rats were trained to discriminate IP administration of 800 micrograms/kg cathinone using a food-motivated, two-lever discrimination procedure. Following training, 800 micrograms/kg cathinone discrimination was produced (generalized) by lower cathinone doses in a dose-responsive manner after IP administration; an ED50 value of 330 micrograms/kg was calculated. Subsequently, guide cannulae were implanted into the lateral ventricle and bilaterally into the nucleus accumbens. After recovery, injections were made via cannulae that extended 0.5 mm past the tip of the guide cannulae. ICV administration of 256 micrograms cathinone/rat produced discriminative responding on the cathinone-appropriate lever to the same degree as did the peripherally administered training dose of cathinone. Decreasing ICV doses produced decreased discriminative performance and allowed the calculation of an ED50 value of 90.5 micrograms. Likewise, administration of 64 micrograms cathinone/nucleus accumbens (for a total of 128 micrograms/rat) substituted for the IP training dose of cathinone. These results evidence the central mediation of the cathinone-induced discriminative stimulus cue and show that administration of cathinone into the nucleus accumbens is sufficient to produce these stimuli. Thus, these data suggest that receptors in the nucleus accumbens are important for the discrimination of this psychostimulant.

Cathinone, a natural amphetamine

Cathinone is an alkaloid that has been discovered some fifteen years ago in the leaves of the khat bush. This plant grows in East Africa and in southern Arabia, and the inhabitants of these regions frequently chew khat because of its stimulating properties. Cathinone, which is S(-)-alpha-aminopropiophenone, was soon found to have a pharmacological profile closely resembling that of amphetamine; indeed, in a wide variety of in vitro and in vivo experiments it was demonstrated that cathinone shares the action of amphetamine on CNS as well as its sympathomimetic effects; thus, for example, drug-conditioned animals will not distinguish between cathinone and amphetamine. These various observations were confirmed by a clinical experiment showing that cathinone also in humans produces amphetamine-like objective and subjective effects. Finally, it was demonstrated that cathinone operates through the same mechanism as amphetamine, i.e. it acts by releasing catecholamines from presynaptic storage sites. Thus, much experimental evidence indicates that cathinone is the main psychoactive constituent of the khat leaf and that, in fact, this alkaloid is a natural amphetamine.

Structural variation and (+)-amphetamine-like discriminative stimulus properties

Rats were trained to discriminate (+)-amphetamine sulfate (5.43 mumol/kg, 1 mg/kg) from saline in a food-reinforced, two-lever drug discrimination paradigm. Side chain variations of the amphetamine molecular structure were analyzed for their effects on the discriminative stimulus properties of this prototype central nervous system stimulant. Partial generalization was observed for the alpha-ethyl homologue of (+)-amphetamine, (+)-AEPEA, and for 2-aminoindan (AI), while 5,6-methylenedioxy-2-aminoindan (MDAI) elicited only saline-appropriate responding. By contrast, 2-amino-1,2-dihydronaphthalene (ADN) and 2-aminotetralin (AT) completely substituted for (+)-amphetamine. Relative to the training drug, ADN was 1/4 as potent and AT was 1/8 as potent. The S-(-)-isomer of ADN was found to be responsible for the (+)-amphetamine-like discriminative properties of the racemate. The results suggest that constraining or extending the alpha-alkyl substituent of (+)-amphetamine has a deleterious effect on the ability of the resulting analogue to adopt the active conformation of (+)-amphetamine, thereby diminishing its characteristic discriminative stimulus properties.

Rats become acutely tolerant to cathine after amphetamine or cathinone administration

The drug discrimination paradigm was used to evaluate in rats the ability of the discriminate response to either 0.8 mg/kg d-amphetamine or 0.8 mg/kg l-cathinone to generalize to 2.4-6.0 mg/kg of the active cathinone metabolite d-norpseudoephedrine, also known as cathine. When tested 24 h after vehicle administration, cathine generalized in a dose-related fashion in rats (n = 6) trained with cathinone (ED50 = 3.03 mg/kg) and in rats (n = 8) trained with amphetamine (ED50 = 2.93 mg/kg). In contrast, when cathine was tested 24 h after the administration of either amphetamine or cathinone, it produced significantly decreased discriminative performance. The possibility that this acute tolerance may have been produced by release, and subsequent depletion, of brain dopamine was tested by pretreating rats with the dopamine release inhibitor CGS 10746B. When CGS 10746B was administered prior to cathinone it significantly decreased cathinone discrimination. In addition, acute tolerance to cathine at 24 h after vehicle-cathinone co-administration was reversed when cathine was tested 24 h after CGS 10746B-cathinone co-administration. The results suggest that cathinone-produced discriminative stimulus, as well as the acute tolerance to cathine, may be dopaminergically mediated.

Pharmacological properties of the stimulant khat

The chewing of the stimulant leaf khat is a habit that is widespread in certain countries of East Africa and the Arabian peninsula. During the last decade, important progress has been made in understanding the pharmacological basis for the effects of khat. It is now known that the CNS action of this drug is due to the presence of the alkaloid cathinone, and the results of various in vitro and in vivo experiments indicate that this substance must be considered a natural amphetamine. It is the purpose of the present review to describe briefly the khat habit and to summarize the pharmacology of khat and of its active constituents.

Temporal parameters of cathinone, amphetamine and cocaine

Rats were trained to discriminate intraperitoneally administered 0.8 mg/kg 1-cathinone from its vehicle in a two-lever operant procedure. The normal injection-to-session interval was fifteen minutes. When tested in session at 2-180 min postadministration, cathinone discrimination was seen to have a rapid onset (5 minutes) and offset (60 minutes). When the same rats were tested with either 0.8 mg/kg d-amphetamine or 10.0 mg/kg cocaine at the same postinjection time periods, the peak discriminative generalization to each of these other psychostimulants was observed to be later, i.e., an onset of action at 15-30 minutes with a slightly longer duration of action. The results indicate that cathinone exerts discriminative response control within five minutes of intraperitoneal injection and that it has a shorter duration than amphetamine and cocaine.

Potentiation of cathinone by caffeine and nikethamide

The drug discrimination paradigm was employed to evaluate the effect of coadministration of both caffeine and nikethamide upon the discrimination of a low dose of cathinone. In rats trained to discriminate between 0.8 mg/kg l-cathinone and its vehicle in a two-lever food-motivated operant task, 0.2 mg/kg cathinone produced 29.2% of responses on the cathinone-appropriate lever. This lever was chosen in 0 and 50% of trials with 25 mg/kg nikethamide and 20 mg/kg caffeine, respectively. Coadministration of caffeine, nikethamide, or caffeine plus nikethamide with low-dose cathinone produced strong cathinone-like discriminative performance. This potentiattion of cathinone by caffeine and nikethamide is reflective of noncontrolled drugs of abuse containing similar combinations especially for that of antiadipositum X-112, a drug containing all three agents and widely abused in Europe.

Central nervous system stimulants: neuropharmacological mechanisms

The mechanisms underlying CNS-stimulant drug discrimination are discussed. Although different doses of CNS stimulants may produce qualitatively different cues, it appears that a relatively low dose of d-amphetamine (e.g., 1 mg/kg) elicits a "general" CNS-stimulant cue. Presynaptically, this cue may primarily depend on release of endogenous dopamine whereas inhibition of dopamine reuptake, per se, is insufficient to elicit the cue. Postsynaptically, the involvement of both dopamine D-1 and D-2 receptors is implicated. Furthermore, in the drug discrimination situation, D-1/D-2 receptors may be coupled differently than in dopamine-dependent locomotor activation. Anatomically, CNS-stimulant drug discrimination may depend primarily on mesolimbic dopamine systems.

Further evidence for an amphetamine-like mechanism of action of the alkaloid cathinone

The alkaloid (-)cathinone is a potent stimulant with pharmacological properties closely resembling those of (+)amphetamine. Since (-)cathinone is capable of inducing release at physiological catecholamine storage sites, it has been suggested that (-)cathinone and (+)amphetamine have the same mechanism of action. In the present study, the potency of (-)cathinone in inducing the release of radioactivity from 3H-dopamine prelabelled tissue of the rat caudate nucleus was compared to that of several structural analogs, i.e. to that of four other aminophenones. (-)Cathinone was found to be the most potent of the compounds under investigation, and among these only demethylcathinone had an effect that was within the same order of magnitude as that of (-)cathinone. Furthermore, (-)cathinone and two of its analogs were evaluated in behavioral experiments with regard to their ability to substitute for (+)amphetamine in rats trained to discriminate between (+)amphetamine and saline. It was found that, unlike the other aminophenones, (-)cathinone is capable of producing (+)amphetamine-like stimulus effects, and these can be antagonized by haloperidol in a dose-related manner. Taken together, these findings support the hypothesis that (+)amphetamine and (-)cathinone produce their central stimulant effect via the same dopaminergic mechanism.

Discriminative stimulus properties of beta-phenylethylamine, deuterated beta-phenylethylamine, phenylethanolamine and some metabolites of phenylethylamine in rodents

The discriminative stimulus (cue) properties of phenylethylamine (PEA) were analysed in rodents in a conventional two lever FR10 operant drug discrimination task. Rats trained to discriminate phenylethylamine at 30 mg/kg showed complete dose-related generalization to PEA and to two potential PEA metabolites: phenylethanolamine (PEOH) and N-Methyl PEA (NMPEA). Only partial (50%) generalization was seen with N-Methylphenylethanolamine (NMPEOH), another potential PEA metabolite. The specificity of PEA's action as a discriminative stimulus was demonstrated by the finding that fenfluramine, a substituted phenylethylamine, failed to generalize to PEA even at high doses with marked behavioural effects which are known to have discriminative stimulus properties themselves. These data suggest that NMPEA and PEOH may be functionally important active metabolites of PEA, particularly if the major pathway of PEA metabolism to phenylacetic acid under the influence of MAO Type B is for any reason impaired. A long acting deuterium substituted form of PEA (alpha, alpha, d2 PEA), which is resistant to metabolism by MAO, produced complete dose-related generalization to the PEA cue but was more potent than PEA, due presumably to its resistance to metabolism by MAO. Deuterated PEA may therefore be a useful agent to use in future studies of the PEA cue, because the discriminability of PEA itself appears to be low due to its very rapid metabolism in vivo.

Discriminative stimulus properties of phenylisopropylamine derivatives

The phenylisopropylamine unit is a common structural fragment amongst many centrally-acting agents. However, these agents do not necessarily produce similar behavioral effects in test subjects. For example, the phenylisopropylamine derivative amphetamine is a central nervous system (CNS) stimulant whereas its 2,5-dimethoxy-4-methyl analog, i.e. DOM, is considered to be a hallucinogen. Employing animals trained to discriminate either (+)-amphetamine or (+/-)-DOM from saline in a two-lever operant procedure, stimulus generalization studies were conducted to evaluate members of a series of methoxy-substituted, and related, phenylisopropylamines. In this manner, it was possible to classify these agents as to which produced amphetamine-like effects, and which produced DOM-like effects.

Structure-activity studies on methoxy-substituted phenylisopropylamines using drug discrimination methodology

Eighteen rats were trained to discriminate 1.0 mg/kg of (+)-amphetamine sulfate from saline in a two-lever operant procedure. Once responding was stable, these animals were administered various doses of sixteen different methoxy-substituted phenylisopropylamines in tests of stimulus generalization. Of three possible mono-methoxyphenylisopropylamines, all three produced amphetamine-appropriate responding, but none was as potent as racemic amphetamine. The amphetamine-stimulus did not completely generalize to any of the di- or tri-methoxyphenylisopropylamines.