Drug Discrimination Assay Research Articles

Phencyclidine-Like Abuse Liability and Psychosis-Like Neurocognitive Effects of Novel Arylcyclohexylamine Drugs of Abuse in Rodents.

Abuse of novel arylcyclohexylamines (ACX) poses risks for toxicities, including adverse neurocognitive effects. In vivo effects of ring-substituted analogs of phencyclidine (PCP), eticyclidine (PCE), and ketamine are understudied. Adult male National Institutes of Health Swiss mice were used to assess locomotor effects of PCP and its 3-OH, 3-MeO, 3-Cl, and 4-MeO analogs, PCE and its 3-OH and 3-MeO analogs, and ketamine and its deschloro and 2F-deschloro analogs, in comparison with those of methamphetamine (METH), 3,4-methylenedioxymethamphetamine (MDMA), and two benzofuran analogs of MDMA. PCP-like interoceptive effects for all of these ACXs were determined using a food-reinforced drug discrimination procedure in adult male Sprague Dawley rats. A novel operant assay of rule-governed behavior incorporating aspects of attentional set-shifting was used to profile psychosis-like neurocognitive effects of PCP and 3-Cl-PCP in rats, in comparison with cocaine and morphine. PCP-like ACXs were more effective locomotor stimulants than the amphetamines, PCE-like ACXs were as effective as the amphetamines, and ketamine-like ACXs were less effective than the amphetamines. Addition of -Cl, -OH, or -OMe at the 3-position on the aromatic ring did not impact locomotor effectiveness, but addition of -OMe at the 4-position reduced locomotor effectiveness. Lethal effects were induced by drugs with -OH at the 3-position or -OMe at the 3- or 4-position. All novel ACXs substituted at least partially for PCP, and PCP and 3-Cl-PCP elicited dose-dependent psychosis-like neurocognitive deficits in the rule-governed behavior task not observed with cocaine or morphine. Novel ACXs exhibit substantial abuse liability and toxicities not necessarily observed with their parent drugs. SIGNIFICANCE STATEMENT: Novel arylcyclohexylamine analogs of PCP, PCE, and ketamine are appearing on the illicit market, and abuse of these drugs poses risks for toxicities, including adverse neurocognitive effects. These studies demonstrate that the novel ACXs exhibit PCP-like abuse liability in the drug discrimination assay, elicit varied locomotor stimulant and lethal effects in mice, and induce psychosis-like neurocognitive effects in rats.

Evaluation of novel epibatidine analogs in the rat nicotine drug discrimination assay and in the rat chronic constriction injury neuropathic pain model.

Nicotine is the primary psychoactive component responsible for maintaining tobacco dependence in humans. Chronic pain is often a consequence of tobacco-related pathologies, and the development of a dual therapeutic that could treat chronic pain and tobacco dependence would be advantageous. Epibatidine reliably substitutes for nicotine in the drug discrimination assay, and is a potent analgesic, but has a side-effect profile that limits its therapeutic potential. Thus, considerable efforts to produce epibatidine derivatives are underway. Here we tested three epibatidine derivatives, 2'-fluoro-3'-(4-nitrophenyl)deschloroepibatidine (RTI-7527-102; i.e., RTI-102), 2'-fluorodeschloroepibatidine (RTI-7527-36; i.e., RTI-36), and 3'-(3″-dimethylaminophenyl)-epibatidine (RTI-7527-76; i.e., RTI-76) in both the rat nicotine drug discrimination assay as well as in the rat chronic constriction injury (CCI) of the sciatic nerve neuropathic pain model. Male and female Sprague-Dawley rats were trained on a fixed-ratio 10 schedule to discriminate nicotine (0.32mg/kg base) from vehicle. All compounds dose-dependently substituted for nicotine, without significant decreases in response rates. In the discrimination assay the rank order potency was RTI-36 > nicotine > RTI-102 > RTI-76. Evidence suggests the α4β2* subtype is particularly important to nicotine-related abuse potential. Thus, here we utilized the antagonist dihydro-β-erythroidine (DHβE) to examine relative β2 subunit contribution. DHβE (3.2mg/kg, s.c.) antagonized the discriminative stimulus effects of nicotine. However, relative to antagonism of nicotine, DHβE produced less antagonism of RTI-102 and RTI-76 and greater antagonism of RTI-36. It is likely that at nicotinic receptor subunits RTI-102, RTI-76 and RTI-36 possess differing activity. To confirm that the full discriminative stimulus of these compounds was due to nAChR activity beyond the β2 subunit, we examined these compounds in the presence of the non-selective nicotinic receptor antagonist mecamylamine. Mecamylamine (0.56mg/kg, s.c.) pretreatment abolished nicotine-paired lever responding for all compounds. In a separate cohort, male and female Sprague-Dawley rats underwent CCI surgery and tested for CCI-induced mechanical allodynia via the von Frey assay. Each compound produced CCI-induced mechanical allodynia reversal. RTI-36 displayed higher potency than either RTI-102 or RTI-76. These novel epibatidine analogs may prove to be useful tools in the fight against nicotine dependence as well as novel neuropathic pain analgesics.

Comparison of locomotor stimulant and drug discrimination effects of four synthetic cathinones to commonly abused psychostimulants.

The underground market is constantly flooded with newer synthetic as alternatives to the older cathinones. Drug Enforcement Administration (DEA) has identified four cathinone compounds of particular concern: 3,4-methylenedioxy-alpha-pyrrolidinohexanophenone (3,4-MD-α-PHP), 4-chloro-α-pyrrolidinopropiophenone (4-Cl-α-PPP), alpha-pyrrolidinoisohexiophenone (α-PiHP) and 4-chloro-pentedrone (4-Cl-pentedrone). The current study aimed to evaluate the behavioral pharmacology of four synthetic cathinones. 3,4-MD-α-PHP, 4-Cl-α-PPP, α-PiHP, and 4-CPD were tested for locomotor activity in mice and in a drug discrimination assay with rats trained to discriminate either methamphetamine or cocaine. Locomotor stimulant effects of 3,4-MD-α-PHP ((effective dose) ED50 = 1.98 mg/kg), α-PiHP (ED50 = 2.46 mg/kg), and 4-Cl-α-PPP (ED50 = 7.18 mg/kg) were observed within 10 min following injection and lasted from 2 to 3.5 h. The stimulant action of 4-CPD (ED50 = 17.24 mg/kg) was delayed, occurring 40-70 min following injection. The maximal motor stimulant actions of 3,4-MD-α-PHP and α-PiHP 1 were equivalent to that of cocaine and methamphetamine, whereas 4-CPD (50% of cocaine) and 4-Cl-α-PPP (73% of cocaine) were less efficacious. All of the test compounds fully substituted for the discriminative stimulus effects of cocaine, 3,4-MD-α-PHP (ED50 = 2.28 mg/kg), α-PiHP (ED50 = 3.84 mg/kg), and 4-Cl-α-PPP (ED50 = 15.56 mg/kg). Only 3,4-MD-α-PHP (ED50 = 1.65 mg/kg), α-PiHP (ED50 = 1.87 mg/kg), and 4-Cl-α-PPP (ED50 = 9.79 mg/kg) fully substituted for the discriminative stimulus effects of methamphetamine. 4-Cl-pentedrone caused 55-70% methamphetamine-appropriate responding at doses that also suppressed responding and produced convulsions. These data indicate that 3,4-MD-α-PHP, α-PiHP, and 4-Cl-α-PPP have a potential for abuse similar to that of methamphetamine and cocaine. In contrast, 4-Cl-pentedrone may not be popular for recreational use due to its convulsant effects.

Study design criteria for regulatory-based drug control action: Drug discrimination

This “methods paper” focusses on one specific and limited aspect of drug safety evaluations required for all new drug entities that affect the central nervous system – the drug discrimination (DD) assay. We focus on three critical factors involved in experimental design and protocol development for the conduct of DD studies for abuse liability risk assessment that comply with the Good Laboratory Practice Guidelines (GLPs). The selection of 1) the reference drug(s) choice, 2) training dose selection, and 3) the selected route-of-administration will determine the applicability of the data to meet the regulatory expectations of the 8-factors determinative of schedule control recommendations. The study conduct and resulting data submission to the FDA are intended for drug scheduling review by the Controlled Substances Staff in the Center for Drug Evaluation and Research (CDER) at the US Food & Drug Administration (FDA). These animal studies are required to meet the statutory requirements of the Controlled Substances Act of 1970. The abuse liability study is conducted during Phase II and III of human clinical trials. Procedural or method-based errors this late in drug development can result in a significant economic and business threat to the program.

Discovery of 4-Phenylpiperidine-2-Carboxamide Analogues as Serotonin 5-HT2C Receptor-Positive Allosteric Modulators with Enhanced Drug-like Properties.

Targeting the serotonin (5-HT) 5-HT2C receptor (5-HT2CR) allosteric site to potentiate endogenous 5-HT tone may provide novel therapeutics to alleviate the impact of costly, chronic diseases such as obesity and substance use disorders. Expanding upon our recently described 5-HT2CR-positive allosteric modulators (PAMs) based on the 4-alkylpiperidine-2-carboxamide scaffold, we optimized the undecyl moiety at the 4-position with variations of cyclohexyl- or phenyl-containing fragments to reduce rotatable bonds and lipophilicity. Compound 12 (CTW0415) was discovered as a 5-HT2CR PAM with improved pharmacokinetics and reduced off-target interactions relative to our previous series of molecules. The in vivo efficacy of compound 12 to potentiate the effects of a selective 5-HT2CR agonist was established in a drug discrimination assay. Thus, 12 is reported as a 5-HT2CR PAM with characteristics suitable for in vivo pharmacological studies to further probe the biological and behavioral mechanisms of allosteric modulation of a receptor important in several chronic diseases.

Investigation of the Optical Isomers of Methcathinone, and Two Achiral Analogs, at Monoamine Transporters and in Intracranial Self-Stimulation Studies in Rats.

Methcathinone (MCAT; 1), the progenitor of numerous and widely abused "synthetic cathinone" central stimulants, exists as a pair of optical isomers. Although S(-)MCAT is several-fold more potent than R(+)MCAT in rodent locomotor stimulation and in stimulus generalization studies in rat drug discrimination assays, the individual optical isomers of MCAT have never been directly compared for their actions at monoamine transporters that seem to underlie their actions and have never been examined for their relative abuse potential. Here, we found that the isomers of MCAT are nearly equieffective at dopamine and norepinephrine transporters (DAT and NET, respectively) as transporter substrates (i.e., as releasing agents) and are ≥63-fold less potent at the serotonin transporter (SERT). In intracranial self-stimulation (ICSS) studies to evaluate abuse-related drug effects in rats, S(-)MCAT was approximately twice as potent as its R-enantiomer. Achiral analogs, α-methyl MCAT (3) and α-des-methyl MCAT (4), also were DAT/NET substrates and also produced abuse-related ICSS effects, indicating that they retain abuse potential and that they might be useful for the further study of the stereochemistry of synthetic cathinone analogs with chiral β- (or other) substituents.

Evaluation of Novel Epibatidine Analogs in Rats Discriminating Nicotine

Tobacco use is a persistent public health issue. One of every three cancer deaths is linked to tobacco use, including nearly 90% of all deaths related to lung cancer. Nicotine is the primary psychoactive component in tobacco, and nicotine has been identified as the primary compound responsible for maintaining tobacco dependence in humans. Drug discrimination is a pharmacologically selective bioassay, utilized in the past for elucidating the receptor pharmacology of nicotine. If a test compound shares discriminative stimulus effects with nicotine, then it might serve as an effective substitution pharmacotherapy. Epibatidine reliably substitutes for nicotine in the drug discrimination assay, but has a side‐effect profile that limits its therapeutic potential. Thus, considerable efforts are underway to produce derivatives of epibatidine. Here we tested three epibatidine derivatives, 2′‐fluoro‐3′‐(4‐nitrophenyl)deschloroepibatidine (RTI‐7527‐102; i.e., RTI‐102), 2′‐fluorodeschloroepibatidine (RTI‐7527‐36; i.e., RTI‐36), and 3′‐(3″‐dimethylaminophenyl)‐epibatidine (RTI‐7527‐76; i.e., RTI‐76). A total of 4 male and 4 female Sprague‐Dawley rats were trained on a fixed‐ratio 10 schedule to discriminate nicotine (0.32 mg/kg base) from vehicle. All compounds dose‐dependently substituted for nicotine, without significant decreases in response rates. The rank‐order potency in the discrimination assay was RTI‐36 > nicotine > RTI‐102 > RTI‐76. There is abundant evidence that the α4β2* subtype is of particular importance to the abuse potential of nicotine. Thus, we examined the relative contribution of the β2 subunit with the antagonist by dihydro‐β‐erythroidine (DHβE). DHβE antagonized the discriminative stimulus effects of nicotine. However, relative to antagonism of nicotine, DHβE produced less antagonism of RTI‐102, RTI‐36 and RTI‐76. Thus, it is likely that RTI‐102, RTI‐36 and RTI‐76 possess differing activity at nicotinic receptor subunits.Support or Funding InformationThis work was supported by the National Institutes of Health National Institute on Drug Abuse [DA25267] and [DA48353].

In vivo evaluation of the CB1 allosteric modulator LDK1258 reveals CB1-receptor independent behavioral effects

In the present study, we examined whether LDK1258, which produces strong CB1 receptor allosteric effects in in vitro assays, would elicit in vivo effects consistent with allosteric activity. In initial studies, LDK1258 reduced food consumption and elicited delayed antinociceptive effects in the chronic constrictive injury of the sciatic nerve (CCI) model of neuropathic pain, which unexpectedly emerged 4 h post-injection. UPLC-MS/MS analysis quantified significant levels of LDK1258 in both blood and brain tissue at 30 min post-administration that remained stable up to 4 h. The observation that LDK1258 also produced respective antinociceptive and anorectic effects in rimonabant-treated wild type mice and CB1 (−/−) mice suggests an off-target mechanism of action. Likewise, LDK1258 produced a partial array of common cannabimimetic effects in the tetrad assay, which were not CB1 receptor mediated. Additionally, LDK1258 did not substitute for the CB1 receptor orthosteric agonists CP55,940 or anandamide in the drug discrimination paradigm. In other in vivo assays sensitive to CB1 receptor allosteric modulators, LDK1258 failed to shift the dose-response curves of either CP55,940 or anandamide in producing thermal antinociception, catalepsy, or hypothermia, and did not alter the generalization curve of either drug in the drug discrimination assay. Thus, this battery of tests yielded results demonstrating that LDK1258 produces antinociceptive effects in the CCI model of neuropathic pain, anorectic effects, and other in vivo pharmacological effects in a manner inconsistent with CB1 receptor allosterism. More generally, this study offers a straightforward screening assay to determine whether newly synthesized CB1 receptor allosteric modulators translate to the whole animal.

Discriminative stimulus properties of the typical antipsychotic haloperidol compared to other antipsychotic drugs in C57BL/6 mice.

Haloperidol (HAL) was developed in 1958 for the treatment of schizophrenia and is classified as a typical antipsychotic drug (APD). Effective in treating positive symptoms of schizophrenia, it does not treat negative symptoms and produces extrapyramidal motor side-effects. Atypical APDs like clozapine treat both positive and negative symptoms of schizophrenia, have reduced extrapyramidal motor side-effects and possess other clinical advantages. This study used a drug discrimination assay to allow a direct comparison between the subjective effects of HAL and other APDs. Eleven C57BL/6 mice were trained to discriminate 0.05 mg/kg HAL from the vehicle in a two-lever drug discrimination task. The HAL generalization curve (0.001563-0.2 mg/kg) yielded an ED50=0.0024 mg/kg (95% confidence interval: 0.0012-0.0048 mg/kg). The typical APD chlorpromazine produced full substitution at 4.0 mg/kg with 82.7% drug-lever responding (%DLR) with significant rate suppression and partial substitution (73.9% DLR) at 1.0 mg/kg with no rate suppression. The atypical APD clozapine produced partial substitution at 2.5 mg/kg (64.8% DLR) with significant rate suppression. The atypical APD amisulpride failed to substitute for HAL with a maximum %DLR of 57.9% at 40 mg/kg with no rate suppression. The atypical APD aripiprazole partially substituted with a maximum of 75.9% DLR at 1.25 mg/kg with significant rate suppression. These results demonstrate that HAL can be trained as a discriminative stimulus in C57BL/6 mice, and its discriminative cue appears to be unique and distinct from that of atypical APDs.

Evaluation of samidorphan, a μ-opioid antagonist, in a drug discrimination assay in rats.

Escalating doses of samidorphan were substituted for morphine and rats were allowed to respond on two levers for food reward. Doses of samidorphan were chosen based on other pharmacodynamic assays in which samidorphan blocked the effects of morphine (such as blocking analgesia in a hot plate test; data not shown). In addition, a pharmacokinetic study was conducted to determine if these doses would reflect predicted exposure levels that translate to human equivalent doses. Rats discriminating morphine from vehicle responded predominantly on the vehicle lever after receiving samidorphan. In addition, samidorphan was rapidly absorbed, and plasma concentrations of the doses tested in this study bracket therapeutically relevant concentrations. In summary, samidorphan produced saline-like behavioral responses over a wide dose range.

Characterization of the discriminative stimulus effects of the delta‐opioid receptor agonist, SNC80, in rats

Delta opioid receptor (DOR) agonists produce antihyperalgesia and antidepressant‐like effects, but they also induce convulsions limiting their clinical development. Recently, some work has shown that not all DOR agonists produce convulsions in rodents. Therefore, the current study aimed to further evaluate the pharmacological properties of these DOR ligands using a drug discrimination paradigm in male Sprague‐Dawley rats. The drug discrimination assay is useful for identifying compounds with similar interoceptive properties and evaluating the pharmacological activity of ligands in vivo. For this study, rats were trained to discriminate 3.2 mg/kg SNC80 from saline on a fixed ratio (FR) 10 schedule of reinforcement in a food‐reinforced, drug discrimination paradigm. Substitution patterns with purported DOR agonists AZD2327, DPI287, KNT127, and MMP2200 were evaluated as well as profiles of antagonism following pretreatment with naltrindole. Agonists that are structurally similar to SNC80, AZD2327 and DPI287, generalized to SNC80 while structurally unrelated agonists, KNT127 and MMP2200, did not. Morphine, ketamine, desipramine, cocaine, and nicotine failed to generalize to SNC80 up to doses that suppressed responding; however, amphetamine produced partial generalization to the discriminative stimulus effects of SNC80. The DOR antagonist naltrindole produced parallel, rightward shifts in the SNC80 dose effect curve. Overall, these data suggest that interoceptive effects differ among DOR agonists, which may contribute to their distinct profiles of behavioral activity and potentially diverse mechanisms of action. Future studies will further probe the pharmacological and/or signaling mechanisms that differentiate the behavioral effects of DOR agonists.Support or Funding InformationThis work is supported in part by R01 DA042092 (EMJ) and the IRPs of NIDA and NIAAA (KCR).This abstract is from the Experimental Biology 2018 Meeting. There is no full text article associated with this abstract published in The FASEB Journal.

Molecular and Behavioral Pharmacological Characterization of Abused Synthetic Cannabinoids MMB- and MDMB-FUBINACA, MN-18, NNEI, CUMYL-PICA, and 5-Fluoro-CUMYL-PICA.

Synthetic cannabinoids are a class of novel psychoactive substances that exhibit high affinity at the cannabinoid type-1 (CB1) receptor and produce effects similar to those of Δ-9-tetrahydrocannabinol (THC), the primary psychoactive constituent of cannabis. Illicit drug manufacturers are continually circumventing laws banning the sale of synthetic cannabinoids by synthesizing novel structures and doing so with little regard for the potential impact on pharmacological and toxicological effects. Synthetic cannabinoids produce a wide range of effects that include cardiotoxicity, seizure activity, and kidney damage, and they can cause death. Six synthetic cannabinoids, recently detected in illicit preparations, MMB-FUBINACA, MDMB-FUBINACA, CUMYL-PICA, 5F-CUMYL-PICA, NNEI, and MN-18 were assessed for: 1) receptor binding affinity at the human CB1 and human CB2 receptors, 2) function in [35S]GTPγS and cAMP signaling, and 3) THC-like effects in a mouse drug discrimination assay. All six synthetic cannabinoids exhibited high affinity for human cannabinoid receptors type-1 and type-2 and produced greater maximal effects than THC in [35S]GTPγS and cAMP signaling. Additionally, all six synthetic cannabinoids substituted for THC in drug discrimination, suggesting they probably possess subjective effects similar to those of cannabis. Notably, MDMB-FUBINACA, a methylated analog of MMB-FUBINACA, had higher affinity for CB1 than the parent, showing that minor structural modifications being introduced can have a large impact on the pharmacological properties of these drugs. This study demonstrates that novel structures being sold and used illicitly as substitutes for cannabis are retaining high affinity at the CB1 receptor, exhibiting greater efficacy than THC, and producing THC-like effects in models relevant to subjective effects in humans.

The drug discrimination assay: Interpretative value of partial generalization for drug schedule control actions

The U.S. Food & Drug Administration (FDA) has issued a final guidance document on the preclinical determination of abuse potential that must be conducted in animals for all new molecular entities (NMEs) submitted for a new drug application (NDA). Under statutory restrictions government guidance documents serve only as a guide or an expression of the agency's current thinking on the topic. Guidelines do not legally bind the agency or its registrants to any content in the guidance. There are no statutory (legal) descriptions of what study designs or methodology must be submitted to the Drug Enforcement Administration with respect to drug scheduling review. This paper describes the utility of an alternate method used, worldwide, to assess the internal subjective effects of drugs to predict the abuse liability that provides additional information to address the relative aspects of that liability.

Preclinical abuse liability assessment of ABT-126, an agonist at the α7 nicotinic acetylcholine receptor (nAChR)

ABT-126 is a nicotinic acetylcholine receptor (nAChR) agonist that is selective for the α7 subtype of the receptor. nAChRs are thought to play a role in a variety of neurocognitive processes and have been a pharmacologic target for disorders with cognitive impairment, including schizophrenia and Alzheimer's disease. As part of the preclinical safety package for ABT-126, its potential for abuse was assessed. While the involvement of the α4β2 subtype of the nicotinic receptor in the addictive properties of nicotine has been demonstrated, the role of the α7 receptor has been studied much less extensively. A number of preclinical assays of abuse potential including open-field, drug discrimination and self-administration were employed in male rats. ABT-126 had modest effects on locomotor activity in the open-field assay. In nicotine and d-amphetamine drug discrimination assays, ABT-126 administration failed to produce appreciable d-amphetamine-like or nicotine-like responding, suggesting that its interoceptive effects are distinct from those of these drugs of abuse. In rats trained to self-administer cocaine, substitution with ABT-126 was similar to substitution with saline, indicating that it lacks reinforcing effects. No evidence of physical dependence was noted following subchronic administration. Overall, these data suggest that ABT-126 has a low potential for abuse. Together with other literature on this drug class, it appears that drugs that selectively activate α7 nAChRs are not likely to result in abuse or dependence.

Muscarinic receptor M4 positive allosteric modulators attenuate central effects of cocaine

BackgroundCocaine addiction is a chronic brain disease affecting neurotransmission. Muscarinic cholinergic receptors modulate dopaminergic signaling in the reward system, and muscarinic receptor stimulation can block direct reinforcing effects of cocaine. Here, we tested the hypothesis that specific muscarinic M4 receptor stimulation can attenuate the discriminative stimulus effects and conditioned rewarding effects of cocaine, measures believed to predict the ability of cocaine and cocaine-associated cues to elicit relapse to drug taking. MethodsWe tested the M4-selective positive allosteric modulators VU0152100 and VU0467154 in a drug discrimination assay and a conditioned place preference assay, including extinction and reinstatement of place preference. Specificity of the cocaine discrimination effect was verified using knockout mice lacking either M1 or M4 receptors (M1−/−, M4−/−). We also replicated previous findings in cocaine-induced locomotor hyperactivity and striatal dopamine microdialysis assays. ResultsVU0152100 attenuated the discriminative stimulus effect of cocaine in wild-type mice and M1−/− mice, but not in M4−/− mice, without affecting rates of responding. As previously shown with VU0152100, VU0467154 almost eliminated cocaine-induced hyperactivity and striatal dopamine efflux. VU0467154 failed to attenuate acquisition of cocaine-conditioned place preference, but facilitated extinction and prevented reinstatement of the conditioned place preference. ConclusionsThese findings further support the notion that M4 receptors are promising targets for the treatment of cocaine addiction, by showing that results can be replicated using distinct ligands, and that in addition to blocking reinforcing effects of cocaine relevant to ongoing drug taking, M4 positive allosteric modulators can also attenuate subjective and conditioned effects relevant to relapse.

Discriminative Stimulus Effects of Psychostimulants.

Numerous drugs elicit locomotor stimulant effects at appropriate doses; however, we typically reserve the term psychostimulant to refer to drugs with affinity for monoamine reuptake transporters. This chapter comprises select experiments that have characterized the discriminative stimulus effects of psychostimulants using drug discrimination procedures. The substitution profiles of psychostimulants in laboratory rodents are generally consistent with those observed in human and nonhuman primate drug discrimination experiments. Notably, two major classes of psychostimulants can be distinguished as those that function as passive monoamine reuptake inhibitors (such as cocaine) and those that function as substrates for monoamine transporters and stimulate monoamine release (such as the amphetamines). Nevertheless, the discriminative stimulus effects of both classes of psychostimulant are quite similar, and drugs from different classes will substitute for one another. Most importantly, for both the cocaine-like and amphetamine-like psychostimulants, dopaminergic mechanisms most saliently determine discriminative stimulus effects, but these effects can be modulated by alterations in noradrenergic and serotonergic neurotransmission as well. Thusly, the drug discrimination assay is useful for characterizing the interoceptive effects of psychostimulants and determining the mechanisms that contribute to their subjective effects in humans.

The Selective Monoacylglycerol Lipase Inhibitor MJN110 Produces Opioid-Sparing Effects in a Mouse Neuropathic Pain Model.

Serious clinical liabilities associated with the prescription of opiates for pain control include constipation, respiratory depression, pruritus, tolerance, abuse, and addiction. A recognized strategy to circumvent these side effects is to combine opioids with other antinociceptive agents. The combination of opiates with the primary active constituent of cannabis (Δ(9)-tetrahydrocannabinol) produces enhanced antinociceptive actions, suggesting that cannabinoid receptor agonists can be opioid sparing. Here, we tested whether elevating the endogenous cannabinoid 2-arachidonoylglycerol through the inhibition of its primary hydrolytic enzyme monoacylglycerol lipase (MAGL), will produce opioid-sparing effects in the mouse chronic constriction injury (CCI) of the sciatic nerve model of neuropathic pain. The dose-response relationships of i.p. administration of morphine and the selective MAGL inhibitor 2,5-dioxopyrrolidin-1-yl 4-(bis(4-chlorophenyl)methyl)piperazine-1-carboxylate (MJN110) were tested alone and in combination at equieffective doses for reversal of CCI-induced mechanical allodynia and thermal hyperalgesia. The respective ED50 doses (95% confidence interval) of morphine and MJN110 were 2.4 (1.9-3.0) mg/kg and 0.43 (0.23-0.79) mg/kg. Isobolographic analysis of these drugs in combination revealed synergistic antiallodynic effects. Acute antinociceptive effects of the combination of morphine and MJN110 required μ-opioid, CB1, and CB2 receptors. This combination did not reduce gastric motility or produce subjective cannabimimetic effects in the drug discrimination assay. Importantly, combinations of MJN110 and morphine given repeatedly (i.e., twice a day for 6 days) continued to produce antiallodynic effects with no evidence of tolerance. Taken together, these findings suggest that MAGL inhibition produces opiate-sparing events with diminished tolerance, constipation, and cannabimimetic side effects.

Discriminative Stimulus Properties of Opioid Ligands: Progress and Future Directions.

Opioid receptors (MOP-r, KOP-r, DOP-r, as well as NOP-r) and their endogenous neuropeptide agonist systems are involved in diverse neurobiological and behavioral functions, in health and disease. These functions include pain and analgesia, addictions, and psychiatric diseases (e.g., depression-, anxiety-like, and stress-related disorders). Drug discrimination assays have been used to characterize the behavioral pharmacology of ligands with affinity at MOP-r, KOP-r, or DOP-r (and to a lesser extent NOP-r). Therefore, drug discrimination studies with opioid ligands have an important continuing role in translational investigations of diseases that are affected by these neurobiological targets and their pharmacotherapy.

Differential effects of 3,4-methylenedioxypyrovalerone (MDPV) and 4-methylmethcathinone (mephedrone) in rats trained to discriminate MDMA or a d-amphetamine + MDMA mixture.

Recent reports on the abuse of novel synthetic cathinone derivatives call attention to serious public health risks of these substances. In response to this concern, a growing body of preclinical research has characterized the psychopharmacology of these substances, particularly mephedrone (MEPH) or methylenedioxypyrovalerone (MDPV), noting their similarities to 3,4-methylenedioxymethamphetamine (MDMA) and cocaine. Few studies have utilized drug discrimination methodology to characterize the psychopharmacological properties of these substances. The present study employed a rodent drug discrimination assay to further characterize the stimulus effects of MEPH and MDPV in comparison to MDMA and to a drug mixture comprised of d-amphetamine and MDMA. Eight male Sprague-Dawley rats were trained to discriminate 1.5mg/kg MDMA, and eight rats were trained to discriminate a mixture of 1.5mg/kg MDMA and 0.5mg/kg d-amphetamine (MDMA + AMPH) from vehicle. Substitution tests were conducted with MDMA, d-amphetamine, MDPV, MEPH, and cocaine. Dose-response curves generated with MDMA and MEPH were comparable between training groups. In contrast, AMPH, MDPV, and cocaine produced only partial substitution in animals trained to discriminate MDMA but produced full substitution in animals trained to discriminate the MDMA + AMPH mixture. These findings indicate that MDPV's effects may be more similar to those of traditional psychostimulants, whereas MEPH exerts stimulus effects more similar to those of MDMA. Additional experiments with selective DA and 5-hydroxytryptamine (5-HT) receptor antagonists are required to further elucidate specific receptor mechanisms mediating the discriminative stimulus effects of MDPV and mephedrone.

Locomotor depression and THC-like discriminative stimulus effects of three novel synthetic cannabinoids: ADBICA, THJ-2201, and ADB-PINACA

Aims: Synthetic cannabinoids have become increasingly popular as quasi-legal alternatives to marijuana in recent years. As is often the case with designer drugs, when one group of synthetic cannabinoids is scheduled, novel analogs are synthesized to replace them. Numerous adverse effects and incidences of overdose have been associated with newer synthetic cannabinoid analogs. The goal of the current studywas to characterize the locomotor anddiscriminative stimulus effects of three novel synthetic cannabinoid compounds: ADBICA, THJ-2201, and ADB-PINACA. Methods: The effects of these compounds on locomotor activity were tested in male Swiss-Webster (ND4) mice over an 8-hour period in an open field assay of locomotor activity (n=8 per dose). The discriminative stimulus effects of the compounds were tested in male Sprague-Dawley rats trained to discriminate 3mg/kg 9THC from vehicle in a drug-discrimination assay (n=6 per drug). Results: 9-THC (ID50 =13.2mg/kg), ADBICA (ID50 =1.0mg/kg), THJ-2201 (ID50 =0.8mg/kg), and ADB-PINACA (ID50 =0.28mg/kg) produced significant depression of locomotor activity compared to vehicle. Locomotor depression induced by the test compoundswas shorter acting (30–80min) compared to 9-THC (120–210min). ADBICA (ED50 =0.11mg/kg), THJ-2201 (ED50 =0.15), and ADBPINACA (ED50 =0.38) fully substituted for the discriminative stimulus effects of 9-THC. ADBICA and ADB-PINACA significantly attenuated response rate at the doses tested. Conclusions: Our results indicate that these novel synthetic cannabinoid compounds produce locomotor depression and discriminative stimulus effects similar to 9-THC, and may have potential for abuse as alternatives to 9-THC. The stimulus effects of each of the compounds were shorter acting and at least tenfold more potent than 9-THC. Future studies on the reinforcing properties of these compounds are necessary to fully elucidate their abuse liability. Financial support: NIH N01DA-13-8908.