Discriminative Stimulus Effects Research Articles

Characterization of 17-Cyclopropylmethyl-3,14β-dihydroxy-4,5α-epoxy-6α-(indole-7-carboxamido)morphinan (NAN) as a Novel Opioid Receptor Modulator for Opioid Use Disorder Treatment.

The opioid crisis is a significant public health issue with more than 115 people dying from opioid overdose per day in the United States. The aim of the present study was to characterize the in vitro and in vivo pharmacological effects of 17-cyclopropylmethyl-3,14β-dihydroxy-4,5α-epoxy-6α-(indole-7-carboxamido)morphinan (NAN), a μ opioid receptor (MOR) ligand that may be a potential candidate for opioid use disorder treatment that produces less withdrawal signs than naltrexone. The efficacy of NAN was compared to varying efficacy ligands at the MOR, and determined at the δ opioid receptor (DOR) and κ opioid receptor (KOR). NAN was identified as a low efficacy partial agonist for G-protein activation at the MOR and DOR, but had relatively high efficacy at the KOR. In contrast to high efficacy MOR agonists, NAN did not induce MOR internalization, downregulation, or desensitization, but it antagonized agonist-induced MOR internalization and stimulation of intracellular Ca2+ release. Opioid withdrawal studies conducted using morphine-pelleted mice demonstrated that NAN precipitated significantly less withdrawal signs than naltrexone at similar doses. Furthermore, NAN failed to produce fentanyl-like discriminative stimulus effects in rats up to doses that produced dose- and time-dependent antagonism of fentanyl. Overall, these results provide converging lines of evidence that NAN functions mainly as a MOR antagonist and support further consideration of NAN as a candidate medication for opioid use disorder treatment.

Discriminative stimulus effects of mecamylamine and nicotine in rhesus monkeys: Central and peripheral mechanisms

Mecamylamine is a non-competitive nicotinic acetylcholine receptor (nAChR) antagonist that has been prescribed for hypertension and as an off-label smoking cessation aid. Here, we examined pharmacological mechanisms underlying the interoceptive effects (i.e., discriminative stimulus effects) of mecamylamine (5.6 mg/kg s.c.) and compared the effects of nAChR antagonists in this discrimination assay to their capacity to block a nicotine discriminative stimulus (1.78 mg/kg s.c.) in rhesus monkeys. Central (pempidine) and peripherally restricted nAChR antagonists (pentolinium and chlorisondamine) dose-dependently substituted for the mecamylamine discriminative stimulus in the following rank order potency (pentolinium > pempidine > chlorisondamine > mecamylamine). In contrast, at equi-effective doses based on substitution for mecamylamine, only mecamylamine antagonized the discriminative stimulus effects of nicotine, i.e., pentolinium, chlorisondamine, and pempidine did not. NMDA receptor antagonists produced dose-dependent substitution for mecamylamine with the following rank order potency (MK-801 > phencyclidine > ketamine). In contrast, behaviorally active doses of smoking cessation aids including nAChR agonists (nicotine, varenicline, and cytisine), the smoking cessation aid and antidepressant bupropion, and the benzodiazepine midazolam did not substitute for the discriminative stimulus effects of mecamylamine. These data suggest that peripheral nAChRs and NMDA receptors may contribute to the interoceptive stimulus effects produced by mecamylamine. Based on the current results, the therapeutic use of mecamylamine (i.e., for smoking or to alleviate green tobacco sickness) should be weighed against the potential for mecamylamine to produce interoceptive effects that overlap with another class of abused drugs (i.e., NMDA receptor agonists).

Oxycodone-like discriminative stimulus effects of fentanyl-related emerging drugs of abuse in mice

BackgroundFentanyl and its structurally related compounds have emerged as the most significant contributors to opioid overdose fatalities in recent years. While there is abundant information about the pharmacological effects of fentanyl, far less is known of its more recently abused analogs. The objective of this study was to determine whether fentanyl and several fentanyl-related substances would engender oxycodone-like responding in a mouse model of oxycodone discrimination. Oxycodone was selected as the training drug due to its high selectivity for mu opioid receptors. Compounds that elicited oxycodone-like responding in this procedure would likely evoke overlapping subjective experiences. MethodsAdult male C57BL/6 mice were trained to discriminate 1.3 mg/kg oxycodone from vehicle in a food-reinforced, two-lever choice procedure. Generalization tests were conducted with fentanyl and the following fentanyl-related compounds: ocfentanil, 3-furanyl fentanyl, crotonylfentanyl, and valerylfentanyl. ResultsFentanyl and each of its analogs completely generalized to the 1.3 mg/kg oxycodone discriminative stimulus and naltrexone pretreatment significantly decreased oxycodone-like responding for each compound. Rank order potency for engendering oxycodone-appropriate responding was ocfentanil > fentanyl > 3-furanyl fentanyl ≈ crotonylfentanyl > oxycodone > valerylfentanyl. Drug doses that evoked full substitution also significantly suppressed response rates compared to vehicle. ConclusionsThese results indicate that the discriminative stimulus, and by extension, the interoceptive and subjective effects of the tested fentanyl analogs, overlap with those of oxycodone. These observations consequentially support the prediction that they would also engender the likelihood for abuse similar to oxycodone.This article is part of the Special Issue entitled ‘New Vistas in Opioid Pharmacology’.

Effectiveness comparisons of G-protein biased and unbiased mu opioid receptor ligands in warm water tail-withdrawal and drug discrimination in male and female rats

One emerging strategy to address the opioid crisis is the development of mu opioid receptor (MOR) ligands that preferentially signal the G-protein vs. β-arrestin pathway. The present study compared the relative potency and effectiveness of two G-protein biased (GPB)-MOR ligands TRV130 and SR-14968 to five unbiased MOR ligands (NAQ, nalbuphine, buprenorphine, morphine, and methadone) on therapeutic-related (e.g. antinociception) and abuse-related (e.g. discriminative stimulus effects) endpoints. Male and female rats were tested in a warm water tail-withdrawal procedure (50 °C) or trained to discriminate fentanyl (0.04 mg/kg, SC) from saline in a two-lever food-reinforced discrimination procedure. TRV130 and SR-14968 were approximately two-fold more potent to produce fentanyl stimulus effects vs. antinociception. Morphine, fentanyl, and methadone were significantly more potent in the fentanyl discrimination vs. tail withdrawal procedure. In addition, maximum antinociceptive and discriminative stimulus effects of fixed-proportion fentanyl/naltrexone mixtures (1:0.018, 1:0.054, 1:0.18, 1:0.3, and 1:0.54) were used to quantify 1) the relative in vivo efficacy of the two GPB-MOR agonists and five unbiased MOR ligands, and 2) potential species differences in MOR ligand effects between rats and monkeys. The efficacy-effect function generated from the fentanyl/naltrexone mixtures stratified the five unbiased ligands consistent with agonist-stimulated GTPγS binding (NAQ = nalbuphine < buprenorphine < morphine < methadone). However, TRV130 and SR-14968 produced greater antinociception and less fentanyl-like stimulus effects than was predicted. Furthermore, there was a significant positive correlation between rat and monkey antinociceptive effects. Overall, these results demonstrate GPB-MOR agonists produce undesirable abuse-related effects, albeit with slightly better potency and efficacy ratios compared to unbiased agonists.This article is part of the Special Issue entitled ‘New Vistas in Opioid Pharmacology’.

Contribution of monoaminergic mechanisms to the discriminative stimulus effects of 3,4-methylenedioxypyrovalerone (MDPV) in Sprague-Dawley rats.

3,4-Methylenedioxypyrovalerone (MDPV) is a popular synthetic cathinone reported to have a high abuse potential. Recent preclinical research indicates the psychopharmacology of MDPV is comparable to cocaine. Despite a recent influx of research on the psychopharmacology of MDPV, few studies have employed preclinical drug discrimination methods to discern the neurochemical mechanisms involved in its interoceptive stimulus effects. The aim of this study was to evaluate a variety of monoaminergic agents for substitution, potentiation, or antagonism in rats trained to discriminate MDPV. Male Sprague-Dawley rats were trained to discriminate 0.5 (experiment 1) or 1mg/kg MDPV (experiment 2) from saline under an FR 20 schedule of food reinforcement. In experiment 1, MDMA, MDA, and their respective optical isomers (0.75-3mg/kg), cocaine (2.5-20mg/kg), GBR 12909 (5-40mg/kg), and desipramine (3.2-10mg/kg) were assessed for substitution. GBR 12909 (40mg/kg) and desipramine (3.2mg/kg) were subsequently assessed for potentiation of the MDPV cue. In experiment 2, stimulus antagonism tests were conducted with dopamine antagonists (Sch 23390, haloperidol) and serotonin antagonists (pirenperone, MDL100907, WAY 100635). The MDMA and MDA enantiomers produced divergent results, with virtually no substitution by (-)-MDMA or (-)-MDA, partial substitution with (+)-MDA, and full substitution with (+)-MDMA, as well as full substitution by the racemates, (±)-MDMA and (±)-MDA. Consistent with previous findings, cocaine fully substituted for MDPV. Although no dose of GBR 12909 or desipramine substituted for MDPV, these reuptake inhibitors enhanced the discriminative stimulus effects of lower MDPV doses. Both D1 (Sch 23390) and D2 (haloperidol) DA antagonists attenuated 1mg/kg MDPV discrimination, whereas none of the 5-HT antagonists assessed altered MDPV discrimination. These findings indicate MDPV's interoceptive stimulus effects are mediated predominantly by dopaminergic actions, although serotonergic and/or noradrenergic modulation of these effects cannot be ruled out. Further investigations into the neurochemical actions involved in the discriminative stimulus effects of MDPV may serve to inform medication discovery and development for the treatment of MDPV abuse.

Rapid nicotine tolerance and cross-tolerance to varenicline in rhesus monkeys: Drug discrimination.

Acute tolerance to effects of nicotine plays an important role in nicotine dependence, but the mechanism underlying these effects is unclear. Drug discrimination was used in the current study to examine the impact of nicotine pretreatment on sensitivity to the discriminative stimulus effects of nicotine and the FDA-approved smoking cessation pharmacotherapy varenicline. Rhesus monkeys (n = 4) discriminated 0.032 mg/kg nicotine base iv from saline under an FR5 schedule of stimulus-shock termination. Both nicotine and varenicline increased drug-appropriate responding; ED50 values (95% confidence limits) were 0.0087 [0.0025, 0.030] and 0.028 [0.0096, 0.082] mg/kg, respectively. Additional pretreatment injections of the training dose of nicotine (0.032 mg/kg, iv) produced tolerance to its discriminative stimulus effects and the magnitude of this effect was related to the number of pretreatment injections administered. Two pretreatment injections of the training dose of nicotine (0.032 mg/kg, iv) produced a 5.4-fold rightward shift in the nicotine dose-response function and a sevenfold rightward shift in the varenicline dose-response function. The duration of tolerance under these conditions was less than 60 min. These results demonstrate that tolerance to the discriminative stimulus effects of nicotine can be produced by acute nicotine exposure. Acute cross-tolerance from nicotine to varenicline is consistent with similar actions at nAChRs, and suggests that conditions resulting in acute nicotine tolerance could impact sensitivity to other nAChR agonists. (PsycINFO Database Record (c) 2018 APA, all rights reserved).

Effects of cocaine on the discriminative stimulus and reinforcing effects of mephedrone in male rats.

Abuse of cathinones has been a worldwide health concern for some time. Their chemical structures and wide variation in pharmacodynamic effects have led to clinical and preclinical effects that can be both similar to and different from other psychoactive substances such as methylenedioxymethamphetamine (MDMA), methamphetamine, and cocaine. The present study examined the discriminative stimulus and reinforcing effects of mephedrone to further characterize the behavioral and pharmacological profile of this first-generation substituted methcathinone. Rats were trained to discriminate mephedrone (3.2mg/kg) from saline under a fixed-ratio 20 (FR-20) schedule of food presentation. After establishing dose-effect curves for increasing cumulative doses of mephedrone, substitution tests were conducted with bupropion (5.6-32mg/kg), cocaine (1.8-18mg/kg), morphine (0.56-10mg/kg), and amitriptyline (3.2-32mg/kg). In addition, cocaine (3.2-18mg/kg) and the serotonin type-2 (5-HT2) receptor antagonist ritanserin (1, 3.2, and 10mg/kg) were administered prior to the cumulative doses of mephedrone. Lastly, varying infusion doses of cocaine were substituted for mephedrone in subjects trained to self-administer mephedrone, and varying infusion doses of mephedrone were substituted for cocaine in subjects trained to self-administer cocaine to assess the importance of drug history on the reinforcing effects of mephedrone. Of the drugs tested, cocaine had the highest level of mephedrone-lever responding when administered alone (73.5%). In combination with mephedrone, cocaine shifted the mephedrone dose-effect curve upwards in an infra-additive manner. Ritanserin had a small, but non-significant, effect on mephedrone's discriminative stimulus effects. An extensive history (baseline) of cocaine self-administration increased mephedrone self-administration compared to that obtained in mephedrone-trained subjects, whereas a baseline of mephedrone self-administration decreased cocaine self-administration compared to that obtained in cocaine-trained subjects. The similarity between the discriminative stimulus effects of cocaine and mephedrone in male rats suggests an important overlap and the relative importance of the dopamine (DAT) and serotonin (SERT) transporters. The self-administration data suggest that mephedrone is less reinforcing than cocaine, but that a history of responding for cocaine can increase the reinforcing effects of mephedrone.

Can the discriminative stimulus effects of nicotine function concurrently as modulatory opponents in operant and pavlovian occasion setting paradigms in rats?

Nicotine promotes interoceptive changes in the nervous system. Such interoceptive stimuli play important roles in modulating addictive behavior. Operant and Pavlovian stimulus control modulate responsiveness to environmental stimuli related to drug-seeking and self-administration. Nicotine functions as a discriminative stimulus in modulating operant behavior as well as Pavlovian feature stimuli in modulating the conditional responding (CR) to exteroceptive CS→US contingencies. Elucidation of the interaction of these interoceptive stimulus control functions is vital for a comprehensive understanding of nicotine use/abuse, which might lead to better behavioral treatment strategies. This experiment evaluated the interaction among Pavlovian feature positive (FP) and feature negative (FN) effects of nicotine on concurrently occurring operant SD and SΔ effects. Sixteen rats were trained in a Pavlovian and operant bidirectional contingency paradigm, using nicotine (0.3 mg/kg) and non-drug (saline) states as interoceptive cues for operant discriminative stimulus conditions (SD and SΔ) as well as Pavlovian FP and FN for a light-CS, either leading to a shared food pellet outcome or non-outcome. Nicotine and saline sessions were intermixed. For one group of rats (n = 8), nicotine served as an SD for lever pressing (variable interval 60 s) and simultaneously functioned as an FN for CS-light→noUS relation on the same sessions. On intermixed sessions, saline served as the SΔ for lever pressing (non-reinforced) and FP, during which the 8-sec light preceded delivery of the food pellet (variable time ITI = 60 s). For the other group (n = 8) nicotine served as the SΔ (lever pressing non-reinforced) and FP for the CS, with saline serving in the reverse roles. Consecutive brief non-reinforcement tests revealed that: A) rates of lever pressing were significantly greater in SD than SΔ with nicotine and saline suggesting strong operant discriminative stimulus control; B) FP responding to the light CS with nicotine and saline was evident; and C) FN suppression of the CR with nicotine was not evident but weak under saline. These data suggest that nicotine can function as an interoceptive context that hierarchically can enter into concurrently opposing modulatory relations in Pavlovian and operant drug discrimination procedures.

Cannabinoid-like effects of five novel carboxamide synthetic cannabinoids

A new generation of novel cannabinoid compounds have been developed as marijuana substitutes to avoid drug control laws and cannabinoid blood tests. 5F-MDMB-PINACA (also known as 5F-ADB, 5F-ADB-PINACA), MDMB-CHIMICA, MDMB-FUBINACA, ADB-FUBINACA, and AMB-FUBINACA (also known as FUB-AMB, MMB-FUBINACA) were tested for in vivo cannabinoid-like effects to assess their abuse liability. Locomotor activity in mice was tested to screen for locomotor depressant effects and to identify behaviorally-active dose ranges and times of peak effect. Discriminative stimulus effects were tested in rats trained to discriminate Δ9-tetrahydrocannabinol (3 mg/kg, 30-min pretreatment). 5F-MDMB-PINACA (ED50 = 1.1 mg/kg) and MDMB-CHIMICA (ED50 = 0.024 mg/kg) produced short-acting (30 min) depression of locomotor activity. ADB-FUBINACA (ED50 = 0.19 mg/kg), and AMB- FUBINACA (ED50 = 0.19 mg/kg) depressed locomotor activity for 60–90 min; whereas MDMB-FUBINACA (ED50 = 0.04 mg/kg) depressed locomotor activity for 150 min. AMB-FUBINACA produced tremors at the highest dose tested. 5F-MDMB-PINACA (ED50 = 0.07), MDMB-CHIMICA (ED50 = 0.01 mg/kg), MDMB-FUBINACA (ED50 = 0.051 mg/kg), ADB-FUBINACA (ED50 = 0.075 mg/kg) and AMB-FUBINACA (ED50 = 0.029) fully substituted for the discriminative stimulus effects of Δ9-THC following 15-min pretreatment. All 5 compounds decreased locomotor activity and produced discriminative stimulus effects similar to those of Δ9-THC, which suggests they may have abuse liability similar to that of Δ9-THC. AMB-FUBINACA may have an increased risk of toxicities in recreational users.

Ethanol→Nicotine & Nicotine→Ethanol drug-sequence discriminations: Conditional stimulus control with two interoceptivedrug elements in rats.

Self-administration of ethanol (E) and nicotine (N) occurs frequently in tandem orders (i.e., N→E vs. E→N) and thereby produces differing interoceptive profiles of subjective effects in humans. If the interoceptive stimulus characteristics of N→E differ from E→N, it is possible that such differences contribute to their co-dependence. The rationale for the present investigation was to determine whether ethanol, when preceded or followed by nicotine, produces different discriminative stimulus effects in rats. In two experiments, using a one-manipulandum operant drug discrimination procedure, rats were trained to discriminate temporal sequential administrations of ethanol (1.0g/kg) that was followed or preceded by nicotine (0.3mg/kg). Sessions alternated between food-reinforcement sessions on a variable interval 30-sec schedule (i.e., SD) and non-reinforcement sessions (i.e., SΔ). In Experiment 1, administrations of ethanol were followed or preceded by a 10-min interval of nicotine. Training sessions took place 10min following the second drug injection. Four groups of rats were trained to discriminate only one sequence from sequential administrations of saline, and each drug sequence was counterbalanced across groups for their roles as SD or SΔ. There was robust stimulus control. N→E and E→N functioned equally well as SD or SΔ. Experiment 2 used two groups of rats. For one group, the E→N sequence functioned as the SD and the N→E sequence functioned as the SΔ. The drug sequences were counterbalanced for the other group. Brief non-reinforcement tests revealed significantly greater responding during the SD sequence compared to the SΔ sequence for both groups. These results suggest that different drug sequences of ethanol followed or preceded by nicotine established reliable discriminative stimulus control over operant responding, potentially because of characteristic differences in the overlapping pharmacokinetic profiles of the NE compound. The results are discussed in terms of: 1) conditional stimulus control among two interoceptive drug states; and 2) the clinical modulation of human alcohol consumption and tobacco smoking.

Limited modulation of the abuse-related behavioral effects of d-methamphetamine by disulfiram.

Disulfiram (Antabuse), an acetaldehyde dehydrogenase and dopamine-beta hydroxylase inhibitor, has shown promise in preclinical and clinical studies as a pharmacotherapy for cocaine addiction. However, the extent to which disulfiram may alter the abuse-related behavioral effects of related psychostimulants, such as methamphetamine, is unknown. Here, the therapeutic potential of disulfiram was evaluated by examining its impact on the reinforcing and discriminative stimulus effects of d-methamphetamine in adult rhesus monkeys (N = 4 per group). In subjects trained to respond for injections of methamphetamine or food delivery, i.v. methamphetamine (.001-.032 mg/kg) maintained dose-related and stable levels of self-administration in all subjects. Pretreatment with disulfiram (5.6 mg/kg) produced a significant downward shift in the d-methamphetamine dose-response function; surprisingly, lower and higher pretreatment doses (3.0 mg/kg; 10 mg/kg) were ineffective. Also, disulfiram (3-10 mg/kg) did not significantly alter food-maintained responding or, in subjects trained to discriminate the effects of cocaine from vehicle, the ability of d-methamphetamine (.032-.32 mg/kg) to substitute for cocaine. Taken together, the present data reveal dose-dependent effects of disulfiram in modifying some of the abuse-related effects of d-methamphetamine and provides support for future investigations examining the capacity of disulfiram as a treatment for d-methamphetamine abuse. (PsycINFO Database Record (c) 2018 APA, all rights reserved).

The discriminative stimulus effects of nicotine & ethanol with two distinct olfactory contexts in male and female rats

Odor cues and interoceptive cues can combine in promoting drug seeking behavior. Drug discrimination methodology was combined with odor-context conditioning in 8 male and 8 female rats. One drug (nicotine or EtOH) plus odorant (peppermint or anise) compound functioned as setting the occasion for sessions of food-reinforced nose poke responses (i.e., the SD) that were maintained on a variable interval 30 s schedule (VI-30), whereas the opposite drug (EtOH or nicotine) plus odorant (anise or peppermint) compound predicted intermixed sessions of non-reinforcement of nose poking (i.e., the SΔ). During brief non-reinforcement tests conducted with each condition there was significantly greater responding under the SD drug plus odor compound compared to the SΔ drug plus odor compound. Discriminative control was evident and there was a sex by stimulus role interaction with greater SΔ responding in females. The odor contexts and drug contexts alone also sustained strong stimulus control but to a lesser extent compared to the full drug-odor compounds. These data suggest configural learning among drug and odor cues.

Involvement of Nicotinic Receptor Subtypes in the Behavioral Effects of Nicotinic Drugs in Squirrel Monkeys.

Evidence suggests that the α4β2, but not the α7, subtype of the nicotinic acetylcholine receptor (nAChR) plays a key role in mediating the behavioral effects of nicotine and related drugs. However, the importance of other nAChR subtypes remains unclear. The present studies were conducted to examine the involvement of nAChR subtypes by determining the effects of selected nicotinic agonists and antagonists in squirrel monkeys either 1) responding for food reinforcement or 2) discriminating the nicotinic agonist (+)-epibatidine (0.001 mg/kg) from vehicle. In food-reinforcement studies, nicotine, (+)-epibatidine, varenicline and cytisine all produced dose-dependent decreases in rates of food-maintained responding. The rate-decreasing effects of nicotine were antagonized by mecamylamine (nonselective), not appreciably altered by dihydro-β-erythroidine (α4β2 selective), and exacerbated by the nicotinic partial agonists, varenicline and cytisine. Results from discrimination studies show that non-nicotinic drugs did not substitute for (+)-epibatidine, and that except for lobeline, the nicotinic agonists produced either full [(+)-epibatidine, (-)-epibatidine, and nicotine] or partial (varenicline, cytisine, anabaseine, and isoarecolone) substitution for (+)-epibatidine. In interaction studies with antagonists differing in selectivity, (+)-epibatidine discrimination was substantively antagonized by mecamylamine, slightly attenuated by hexamethonium (peripherally restricted) or dihydro-β-erythroidine, and not altered by methyllycaconitine (α7 selective). Varenicline and cytisine enhanced (+)-epibatidine's discriminative-stimulus effects. Correlational analysis revealed a close correspondence between relative behavioral potencies of nicotinic agonists in both studies and their published relative binding affinities at α4β2 and α3β4, but not α7 nAChR, subtypes. Collectively, these results are consistent with the idea that the α4β2 and α3β4, but not α7 nAChR subtypes play a role in the behavioral effects of nicotinic agonists.

Nociceptin/orphanin FQ receptors modulate the discriminative stimulus effects of oxycodone in C57BL/6 mice

BackgroundNociceptin/orphanin FQ (NOP) receptor ligands have shown efficacy as putative analgesics and can modulate the abuse-related effects of opioids, suggesting therapeutic applications. The discriminative stimulus effects of a drug are related to their subjective effects, a predictor of abuse potential. To determine whether activation of NOP receptors could alter the subjective effects of an abused opioid analgesic, a novel oxycodone discrimination was established in mice, characterized with positive and negative controls, and its expression evaluated with a NOP receptor agonist. MethodsAdult male C57BL/6 mice were trained to discriminate 1.3 mg/kg oxycodone from vehicle in a two-lever operant procedure. The discrimination was characterized with naloxone challenge, and generalization tests with the μ-opioid receptor agonists, heroin and morphine, and the κ-opioid receptor selective agonist, U50488. Subsequently, effects of the NOP agonist Ro64-6198 were evaluated with and without oxycodone. ResultsOxycodone generalization occurred in a dose-dependent manner and was reversed by naloxone pretreatment. Heroin and morphine, but not U50488, substituted for oxycodone. Co-treatment of 1 mg/kg Ro64-6198 with the oxycodone training dose reduced % oxycodone lever responding (%OLR) and restored response rates to vehicle control levels. J-113397, a NOP antagonist, reversed these effects. Co-administration of 1 mg/kg Ro64-6198 with a range of oxycodone doses resulted in rightward dose-effect curve shifts in %OLR and response rates compared to oxycodone alone. ConclusionsThese results provide additional evidence that NOP receptor activation can modulate the subjective effects of opioid analgesics and represent the first characterization of oxycodone’s discriminative stimulus effects in mice.

Discriminative Stimulus Effects of Gabapentin

Gabapentin (Neurontin) is synthetic analog of GABA that functions as a voltage‐gated calcium channel (VDCC) blocker and is FDA approved for the management of neuropathic pain and seizure disorders. While not a controlled substance, gabapentin misuse is associated with illicit opioid use, suggesting, along with anecdotal reports, that gabapentin may elicit positive subjective effects. The present study was conducted to examine the subjective effects of gabapentin using rats (N=10) trained to discriminate a 30.0 mg/kg dose of gabapentin (60 min pretreatment time) versus saline in a two‐choice drug discrimination task. The discrimination was acquired in all 10 animals (M = 67.4 sessions, +/− 7.2 SEM). A time course assessment using the gabapentin training dose revealed full substitution (&gt; 80% Gabapentin‐appropriate responding) at 30 and 60 minutes post‐injection. In addition to the training dose, full substitution also occurred for a 60 and 120 mg/kg dose of Gabapentin. Full substitution was shown for the barbiturate and GABAA receptor positive modulator pentobarbital and for the gabapentin analog and VDCC blocker pregabalin. Partial substitution was shown with the psychostimulant d‐amphetamine and the anxiolytic drug and 5‐HT1A partial agonist buspirone. The present findings demonstrate gabapentin's discriminative stimulus effects appear to be primarily mediated by GABAA receptors and VDCC blockade and partially mediated by dopamine and 5‐HT receptors. Full substitution by pentobarbital and partial substitution by amphetamine suggests the potential for positive subjective effects produced by gabapentin.This abstract is from the Experimental Biology 2018 Meeting. There is no full text article associated with this abstract published in The FASEB Journal.

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.

Effects of Chronic, Intermittent Sucrose Consumption on the Discriminative Stimulus Effects of Dopamine or Opioid Antagonists

Chronic, intermittent sucrose access has been shown to induce neurochemical changes in the brain, including increasing the levels of dopamine and D3‐receptor mRNA, decreasing the levels of D2‐receptor mRNA, and increasing μ‐opioid receptor binding. We wondered if chronic, intermittent sucrose consumption would alter the ability of rats to discriminate drugs that affect endorphin and dopamine functioning. We successfully trained rats with chronic, intermittent sucrose access to discriminate various naltrexone training doses (ranging from 0.32 – 3.2 mg/kg) from saline. In the present series of studies, we sought to determine if chronic, intermittent sucrose consumption alters the ability of haloperidol to serve as a discriminative stimulus. Sprague‐Dawley rats were trained to discriminate between haloperidol [0.018–0.56 mg/kg, 30 min pretreatment (PT)] and vehicle in a two‐lever, operant choice procedure. Most subjects reached the discrimination criteria (80% or greater condition‐appropriate responding for 8 of 10 consecutive sessions). Among subjects that acquired the discrimination, there were no significant differences in acquisition between subjects with chronic, intermittent sucrose access and subjects with 24‐hr water access. Haloperidol (0.032 – 0.056 mg/kg) produced significant rate suppression. Increasing the PT to 60 minutes increased rates of lever pressing and reinforcers earned, but did not alter haloperidol generalization functions. Taken together, chronic sucrose consumption enhanced naltrexone's ability to function as a discriminative stimulus, but did not affect the discriminative stimulus effects of haloperidol.Support or Funding InformationUniversity of Wisconsin‐Eau Claire Office of Research and Sponsored ProgramsThis abstract is from the Experimental Biology 2018 Meeting. There is no full text article associated with this abstract published in The FASEB Journal.

Abuse‐Related Discriminative Stimulus Effects of Synthetic Cathinones

The use of synthetic cathinones, or “bath salts,” has become more widespread in recent years. “Bath salts” share many chemical properties with psychostimulants like methamphetamine or with entactogens like 3,4‐methylenedioxymethamphetamine (MDMA or “Ecstasy”), presumably reflecting structural differences that lead to differing neurochemical actions. Yet, currently available synthetic cathinones have not been well‐characterized in primate species, and it is unclear which pharmacological activity mediates their abuse‐related behavioral effects. The present study was initiated to address this question, using drug discrimination procedures to evaluate several synthetic cathinones in two groups of squirrel monkeys. Each subject was trained to respond on either of two levers under a FR10 schedule of reinforcement and, during training sessions, responding on one lever was reinforced after pretreatment with vehicle whereas responding on the second lever was reinforced after pretreatment with the training drug. Under terminal conditions, one group of monkeys discriminated 0.1 mg/kg methamphetamine from saline; the other group discriminated 0.6 mg/kg MDMA from saline. Drug or saline lever assignments were counter balanced across the left and right levers within each group. Training sessions comprised multiple components, and a component consisted of a ten‐minute timeout period followed by ten presentations of the FR10 schedule, each followed by a brief 50‐second timeout. After criterion performance was met (≥ 90% correct), test days interspersed training days, with at least two training days between each test day. The effects of five different synthetic cathinones ‐ α‐pyrrolidinovalerophenone (α‐PVP), methylenedioxypyrovalerone (MDPV), mephedrone, methylone, and methcathinone) were tested in both groups. A full range of doses of each drug was administered using cumulative dosing procedures. Results show that doses of 0.1 mg/kg – 0.32 mg/kg of MDPV, α‐PVP, and methcathinone were identified as methamphetamine in subjects trained to discriminate 0.1 mg/kg methamphetamine from vehicle, whereas doses mephedrone and methylone up to those that decreased overall response rates did not generalize to MA. Preliminary results suggest an opposite profile of substitution in MDMA‐trained animals. Thus, mephedrone and methylone, but not MDPV, α‐PVP, and methcathinone did appear to generalize to MDMA. These results demonstrate that synthetic cathinones have differing substitution profiles in animals trained to discriminate methamphetamine or MDMA, which may reflect differing subjective effects and, perhaps, differing abuse liability in man.Support or Funding InformationSupported by NIH Grants: DA002519. GM113117, DA039306, GM111385This abstract is from the Experimental Biology 2018 Meeting. There is no full text article associated with this abstract published in The FASEB Journal.

Cocaine Enhances the Discriminative Stimulus Effects of Mephedrone

Mephedrone (4‐methylmethcathinone) is one of the major constituents of “bath salts.” It is a drug with discriminative effects similar to CNS stimulants and it can produce auditory and visual hallucinations, as well as problematic cardiovascular effects. This study compared the discriminative stimulus effects of mephedrone (0.32–10 mg/kg) with other prototypical drugs with CNS effects, such as ketamine (1.8–18 mg/kg), bupropion (5.6–56 mg/kg), and 3,4‐methylenedioxyamphetamine (MDA) (0.32–5.6 mg/kg). In addition, mephedrone was administered in combination with cocaine (5.6–32 mg/kg), which inhibits the reuptake of catecholamines and serotonin. Rats (n=6) were trained to discriminate an intraperitoneal injection of 3.2 mg/kg mephedrone from saline under a fixed‐ratio 20 schedule. Following training, increasing cumulative doses of mephedrone produced dose‐dependent increases in mephedrone‐lever responding, with full substitution considered to be greater than 80% responding and partial substitution to be greater than 50% responding. During substitution tests, ketamine never produced more than 17% drug‐lever responding up to doses that significantly decreased overall response rate. In addition, bupropion and MDA produced 36% and 77% drug‐lever responding, respectively, without significantly decreasing response rate. When 3.2 mg/kg of cocaine was administered prior to increasing doses of mephedrone, there was no substitution for cocaine alone; however, this dose produced a rightward shift in the dose‐effect curve for mephedrone‐lever responding. Unlike this dose, 5.6 mg/kg of cocaine alone did not substitute for mephedrone or shift the mephedrone dose‐effect curve. When the dose of cocaine was increased to 10 mg/kg, there was still no mephedrone substitution, but this dose increased the mephedrone‐lever responding of the low doses of mephedrone. Finally, 18 and 32 mg/kg of cocaine produced 38.1% and 77.7% substitution, respectively, and dose‐dependently increased the substitution of the low doses of mephedrone (0.32 – 1.8 mg/kg). This data suggest that mephedrone may be similar to cocaine in having a higher affinity for the serotonin transorter (SERT) than the dopamine transporter (DAT), but more efficacy at DAT than SERT. Certainly, due to the capacity of cocaine to only enhance the substitution of the low mephedrone doses, the discriminative stimulus effects of the training dose of mephedrone are mediated predominately by dopamine rather than serotonin.Support or Funding InformationThis research was supported by the Louisiana Board of Regents Fellowship.This abstract is from the Experimental Biology 2018 Meeting. There is no full text article associated with this abstract published in The FASEB Journal.

Multiple Discriminative Stimulus Effects of the α4β2*‐Selective Agonists Ispronicline and Metanicotine

Ispronicline and metanicotine are selective ligands at central nicotinic acetylcholine α4β2* receptors (Gao et al. Pain 2010: 149, 33; Grady et al. Neuropharmacol 2010: 58, 1054). In our study, a group of rats was trained to discriminate the stimulus properties of the nicotinic acetylcholine receptor (nAChR) agonist nicotine, and a separate group of rats was trained to discriminate the stimulus properties of the muscarinic acetylcholine receptor agonist arecoline. Ispronicline and metanicotine produced full dose‐related selection of the drug‐appropriate response option in both the nicotine‐trained and the arecoline‐trained rats. The nAChR antagonist mecamylamine blocked the discriminative stimulus effects of ispronicline and metanicotine in both groups of rats, as did the α4β2*‐selective antagonist, dihydro‐beta‐erythroidine. Neither antagonist blocked the discriminative stimulus effects of arecoline in these experiments. These data raise the possibility that ispronicline, metanicotine and similar drugs may have novel effects and require a more extensive characterization of receptor activities, through both agonists and antagonistsSupport or Funding InformationResearch supported by NIMH Grant 107499This abstract is from the Experimental Biology 2018 Meeting. There is no full text article associated with this abstract published in The FASEB Journal.