Drug Discrimination Procedure Research Articles

Sex differences in extinction and reinstatement of nicotine discrimination in rats: The effects of reinforcer devaluation.

Nicotine functions effectively as an interoceptive operant discriminative stimulus (SD) that sets the occasion for voluntarily emitted behavior to be reinforced by biologically relevant outcomes (e.g., food). This has been demonstrated primarily with male rats. Far less is known about nicotine's operant SD functions in female rats. There are no reports of sex differences in extinction and recovery of the SD functions of nicotine, which may elucidate smoking cessation and relapse. In view of this, eight male and eight female rats were trained to nose poke differentially among quasirandomly intermixed sessions of food reinforcement variable interval (VI-30 s) and nonreinforcement in a go/no-go across session one-manipulanda operant drug discrimination procedure. For half the rats, presession administration of nicotine (0.30 mg/kg, subcutaneous) occasioned reinforcement sessions of nose pokes (i.e., SD); for the remaining rats, it occasioned nonreinforcement (SΔ). Saline sessions occasioned the opposite contingencies. Training was conducted first under feeding restriction and then under free feeding, which was then followed by extinction sessions that were also conducted with free feeding. During discrimination training, response rates for females did not differ from males when conducted under restricted feeding but did so during training and later extinction conducted under free feeding. Females also exhibited greater reinstatement of responding under the nicotine SD but not the SΔ. These data provide additional evidence for sex differences in rats with the discriminative stimulus functions of nicotine under low, but not high, food-drive states-and may have implications for sex/gender differences in smoking cessation and relapse. (PsycInfo Database Record (c) 2024 APA, all rights reserved).

MGlu2 and mGlu3 receptor negative allosteric modulators attenuate the interoceptive effects of alcohol in male and female rats

RationaleThe subjective effects of alcohol are associated with alcohol use disorder (AUD) vulnerability and treatment outcomes. The interoceptive effects of alcohol are part of these subjective effects and can be measured in animal models using drug discrimination procedures. The newly developed mGlu2 and mGlu3 negative allosteric modulators (NAMs) are potential therapeutics for AUD and may alter interoceptive sensitivity to alcohol. ObjectivesTo determine the effects of mGlu2 and mGlu3 NAMs on the interoceptive effects of alcohol in rats. MethodsLong-Evans rats were trained to discriminate the interoceptive stimulus effects of alcohol (2.0 g/kg, i.g.) from water using both operant (males only) and Pavlovian (male and female) drug discrimination techniques. Following acquisition training, an alcohol dose-response (0, 0.5, 1.0, 2.0 g/kg) experiment was conducted to confirm stimulus control over behavior. Next, to test the involvement of mGlu2 and mGlu3, rats were pretreated with the mGlu2-NAM (VU6001966; 0, 3, 6, 12 mg/kg, i.p.) or the mGlu3-NAM (VU6010572; 0, 3, 6, 12 mg/kg, i.p.) before alcohol administration (2.0 g/kg, i.g.). ResultsIn Pavlovian discrimination, male rats showed greater interoceptive sensitivity to 1.0 and 2.0 g/kg alcohol compared to female rats. Both mGlu2-NAM and mGlu3-NAM attenuated the interoceptive effects of alcohol in male and female rats using Pavlovian and operant discrimination. There may be a potential sex difference in response to the mGlu2-NAM at the highest dose tested. ConclusionsMale rats may be more sensitive to the interoceptive effects of the 2.0 g/kg alcohol training dose compared to female rats. Both mGlu2-and mGlu3-NAM attenuate the interoceptive effects of alcohol in male and female rats. These drugs may have potential for treatment of AUD in part by blunting the subjective effects of alcohol.

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.

Δ9-tetrahydrocannabinol discrimination: Effects of route of administration in mice

BackgroundRoute of administration is an important pharmacokinetic variable in development of translationally relevant preclinical models. Humans primarily administer cannabis through smoking, vaping, and edibles. In contrast, preclinical research has historically utilized injected Δ9-tetrahydrocannabinol (THC). The present study sought to examine how route of administration affected the potency and time course of THC's discriminative stimulus properties. MethodsAdult female and male C57BL/6 mice were trained to discriminate intraperitoneal (i.p.) THC from vehicle in a drug discrimination procedure. After discrimination was acquired, a dose-effect curve was determined for i.p., oral (p.o.), subcutaneous (s.c.), and aerosolized THC. Subsequently, the time course of effects of each route of administration was determined. ResultsTHC administered i.p., p.o., s.c., or via aerosolization fully substituted for i.p. THC. The potency of THC's psychoactive effects was similar for i.p., p.o., and s.c., except that THC was more potent when administered s.c. vs p.o. in females. All routes of administration had a similar potency in both sexes. The duration of THC's psychoactive effects was similar across i.p., s.c., and p.o. routes of administration, whereas aerosolized THC produced a faster onset and shorter duration of effects compared to the other routes. ConclusionTHC administered via multiple routes of administration, including those commonly used in preclinical research (i.p. and s.c.) and more translationally relevant routes (aerosol and p.o.), produced THC-like discriminative stimulus effects in mice trained to discriminate i.p. THC. More precise predictions of THC's effects in humans may result from use of these translationally relevant routes of administration.

Predator odor (TMT) exposure potentiates interoceptive sensitivity to alcohol and increases GABAergic gene expression in the anterior insular cortex and nucleus accumbens in male rats.

Post-traumatic stress disorder (PTSD) confers enhanced vulnerability to developing comorbid alcohol use disorder (AUD). Exposure to the scent of a predator, such as the fox odor TMT, has been used to model a traumatic stressor with relevance to PTSD symptomatology. Alcohol produces distinct interoceptive (subjective) effects that may influence vulnerability to problem drinking and AUD. As such, understanding the lasting impact of stressors on sensitivity to the interoceptive effects of alcohol is clinically relevant. The present study used a 2-lever, operant drug discrimination procedure to train male Long-Evans rats to discriminate the interoceptive effects of alcohol (2g/kg, i.g. [intragastrically]) from water. Upon stable performance, rats underwent a 15-min exposure to TMT. Two weeks later, an alcohol dose-response curve was conducted to evaluate the lasting effects of the TMT stressor on the interoceptive effects of alcohol. The TMT group showed a leftward shift in the effective dose (ED50) of the dose-response curve compared to controls, reflecting potentiated interoceptive sensitivity to alcohol. TMT exposure did not affect response rate. GABAergic signaling in both the anterior insular cortex (aIC) and the nucleus accumbens (Acb) is involved in the interoceptive effects of alcohol and stressor-induced adaptations. As such, follow-up experiments in alcohol-naïve rats examined neuronal activation (as measured by c-Fos immunoreactivity) following TMT and showed that TMT exposure increased c-Fos expression in the aIC and the nucleus accumbens core (AcbC). Two weeks after TMT exposure, Gad-1 gene expression was elevated in the aIC and Gat-1 was increased in the Acb, compared to controls. Lastly, the alcohol discrimination and alcohol-naïve groups displayed dramatic differences in stress reactive behaviors during the TMT exposure, suggesting that alcohol exposure may alter the behavioral response to predator odor. Together, these data suggest that predator odor stressor results in potentiated sensitivity to alcohol, possibly through GABAergic adaptations in the aIC and Acb, which may be relevant to understanding PTSD-AUD comorbidity.

Preclinical Assessment of the Abuse Potential of Purified Botanical Cannabidiol: Self-Administration, Drug Discrimination, and Physical Dependence

Cannabidiol (CBD) is a constituent of the cannabis plant with a diverse array of pharmacological activities as well as potential therapeutic uses. An oral formulation of CBD (Epidiolex® in the US; Epidyolex® in Europe) is approved for treating seizures associated with rare and severe forms of epilepsy. These studies, which supported the approval of the medication, investigated abuse-related effects of CBD in rats and nonhuman primates (NHPs) using drug self-administration, drug discrimination, and physical dependence procedures and characterized its pharmacokinetics. In NHPs (n=5) that self-administered midazolam (0.01 or 0.032 mg/kg/infusion), CBD (0.1-3.2 mg/kg/infusion) failed to maintain responding above vehicle levels. CBD maintained very modest levels of self-administration in rats (n=7-8) that self-administered heroin (0.015 mg/kg/infusion) and did not increase drug-lever responding, up to a dose of 150 mg/kg (p.o.), in rats (n=6) trained to discriminate 0.5 mg/kg (i.p.) midazolam. In juvenile (5-6 weeks old) and adult (10-11 weeks old) male and female rats, discontinuation of chronic treatment (twice daily for 20 days) with an oral formulation of CBD (20 or 100 mg/kg, p.o.) did not reliably produce signs of withdrawal. Pharmacokinetic studies confirmed that the dosing regimens used in these studies resulted in therapeutically relevant plasma levels. Taken together, the lack of reliable self-administration, the failure to increase drug-lever responding in rats trained to discriminate midazolam, and the absence of withdrawal signs upon discontinuation of chronic treatment indicate that CBD has very low abuse potential and is unlikely to produce physical dependence. Significance Statement Legalization of cannabis across the United States and elsewhere has led to intense investigation into the safety and therapeutic potential of cannabis and its constituent materials, including cannabidiol (CBD). Results of these preclinical abuse potential studies on CBD indicate no rewarding properties, physical dependence potential, or similarity to a benzodiazepine or THC. Together with data from in vitro pharmacology and human abuse potential studies, the abuse potential of Epidiolex in humans is likely to be negligible.

Ulotaront, a novel TAAR1 agonist with 5-HT1A agonist activity, lacks abuse liability and attenuates cocaine cue-induced relapse in rats

BackgroundUlotaront (SEP-363856) is a trace amine-associated receptor 1 (TAAR1) agonist with 5-hydroxytryptamine type 1A (5-HT1A) agonist activity that is currently in Phase 3 clinical development for the treatment of schizophrenia. Unlike available antipsychotics, the efficacy of ulotaront is not mediated by blockade of dopamine D2 or serotonin 5-HT2A receptors. In a short-term randomized clinical trial, ulotaront has demonstrated significant efficacy in the treatment of adults with an acute exacerbation of schizophrenia. Given ulotaront’s novel mechanism of action a series of preclinical studies were performed to evaluate its potential abuse liability. MethodsA battery of studies were conducted in male and female rats to evaluate whether ulotaront produces behavioral changes suggestive of human abuse potential. In addition, studies were undertaken to probe the potential for ulotaront to block reinstatement of cocaine-seeking behavior in male rats. ResultsUlotaront was not self-administered by rats trained to self-administer amphetamine, cocaine, or heroin. The subjective qualities of ulotaront were distinct from those produced by amphetamine in a drug discrimination procedure. Ulotaront, and buspirone, a non-scheduled anxiolytic with 5-HT1A agonism, partially generalized to the interoceptive cue elicited by 3, 4-methylenedioxymethamphetamine (MDMA). In addition, ulotaront demonstrated a trend to reduce cocaine-primed induced reinstatement, and dose-dependently reduced cue-reinstated responding. ConclusionThe current results suggest that the TAAR1/5-HT1A agonist ulotaront is not likely to pose a risk for recreational abuse in humans and may have potential therapeutic utility as a treatment of substance use disorders.

Characterization of DREADD receptor expression and function in rhesus macaques trained to discriminate ethanol.

Circuit manipulation has been a staple technique in neuroscience to identify how the brain functions to control complex behaviors. Chemogenetics, including designer receptors exclusively activated by designer drugs (DREADDs), have proven to be a powerful tool for the reversible modulation of discrete brain circuitry without the need for implantable devices, thereby making them especially useful in awake and unrestrained animals. This study used a DREADD approach to query the role of the nucleus accumbens (NAc) in mediating the interoceptive effects of 1.0 g/kg ethanol (i.g.) in rhesus monkeys (n = 7) using a drug discrimination procedure. After training, stereotaxic surgery was performed to introduce an AAV carrying the human muscarinic 4 receptor DREADD (hM4Di) bilaterally into the NAc. The hypothesis was that decreasing the output of the NAc by activation of hM4Di with the DREADD actuator, clozapine-n-oxide (CNO), would potentiate the discriminative stimulus effect of ethanol (i.e., a leftward shift the ethanol dose discrimination curve). The results showed individual variability shifts of the ethanol dose-response determination under DREADD activation. Characterization of the expression and function of hM4Di with MRI, immunohistochemical, and electrophysiological techniques found the selectivity of NAc transduction was proportional to behavioral effect. Specifically, the proportion of hM4Di expression restricted to the NAc was associated with the potency of the discriminative stimulus effects of ethanol. Together, these experiments highlight the NAc in mediating the interoceptive effects of ethanol, provide a framework for validation of chemogenetic tools in primates, and underscore the importance of robust within-subjects examination of DREADD expression for interpretation of behavioral findings.

Δ9-Tetrahydrocannabinol discrimination: Effects of route of administration in rats

Cannabis users typically smoke or vape cannabis or ingest it in edibles, whereas cannabinoids are typically administered via injection in rodent research. The present study examined the effects of route of administration (ROA) of Δ9-tetrahydrocannabinol (THC), the primary psychoactive constituent of cannabis. Adult female and male Long Evans rats were trained to discriminate intraperitoneal (i.p.) THC from vehicle in a drug discrimination procedure. Following acquisition, dose-effect curves were determined with THC using i.p., oral (p.o.), and subcutaneous (s.c.) injection in both sexes and aerosol exposure in males only, followed by a time course with one dose for each ROA. Both sexes acquired THC discrimination in a similar number of sessions, although baseline response rates were significantly lower in females than males. THC fully substituted for the 3 mg/kg i.p. training dose across all ROA. While potencies were similar for ROA involving first-pass metabolism (i.p. and p.o.), THC potency was lower with s.c. administration. During the time course analysis, aerosol administration had the shortest latency to onset of discriminative stimulus effects and the shortest duration of effect, whereas s.c. administration had the longest duration. The results of this examination of the effects of ROA on an abuse-related effect of THC provide an empirical foundation to facilitate choice of ROA for mechanistic investigation of THC’s pharmacology. Further, animal models using translationally relevant ROA may facilitate more accurate predictions of their effects in humans.

Gabapentin attenuates somatic signs of precipitated THC withdrawal in mice

Cannabis is the most frequently used federally illicit substance in the United States. However, there are currently no FDA-approved pharmacotherapies to mitigate the withdrawal symptoms associated with cessation in heavy users. A promising, readily available, non-cannabinoid therapy are the gabapentinoids. Although currently approved for epilepsy and neuropathic pain, gabapentinoids are increasingly used for their “off-label” efficacy in treating various psychiatric conditions and substance abuse. Gabapentin (GBP) synergizes with cannabinoid agonism in neuropathic pain models, substitutes for Δ9-tetrahydrocannabinol (THC) in drug discrimination procedures, and reduced withdrawal symptoms in an outpatient clinical trial. However, there are limited data on the biological plausibility of the therapeutic action of gabapentinoids in cannabinoid withdrawal in preclinical models. The purpose of the current study was to determine the efficacy of GBP on attenuating THC withdrawal in mice, using an array of tests targeting withdrawal-induced and withdrawal-suppressed behaviors. Separate cohorts of male and female mice were administered THC (10 mg/kg, s.c.) or vehicle for 5.5 days, and withdrawal was precipitated by the CB1 antagonist rimonabant (2 or 3 mg/kg, i.p.) on the sixth day. GBP (≥10 mg/kg) reduced somatic signs of withdrawal (i.e., paw tremors and head twitches), but had no effect in locomotor activity or conditioned place preference. GBP (50 mg/kg) also restored withdrawal-suppressed responding on a progressive ratio reinforcement schedule. However, GBP (50 mg/kg) had no effect in withdrawal-suppressed marble burying or tail suspension struggling and did not normalize the stress response induced by THC withdrawal, as indicated by plasma corticosterone. These data suggest gabapentin may be effective at treating cannabinoid withdrawal symptoms including somatic and affective symptoms but may act independently of endocrine stress activation.

Enhancement of the rewarding effects of 3,4-methylenedioxymethamphetamine in orexin knockout mice

Orexinergic neurons, which are closely associated with narcolepsy, regulate arousal and reward circuits through the activation of monoaminergic neurons. Psychostimulants as well as 5-HT-related compounds have potential in the treatment of human narcolepsy. Previous studies have demonstrated that orexin receptor antagonists as well as orexin deficiencies affect the pharmacological effects of psychostimulants. However, little information is available on the consequences of psychostimulant use under orexin deficiency. Therefore, the present study was designed to investigate the abuse liability of psychostimulants in orexin knockout (KO) mice. In the present study, conditioned place preferences induced by methamphetamine and methylphenidate were not altered in orexin KO mice. Interestingly, we found that MDMA induced a conditioned place preference in orexin KO mice, but not in wild type (WT) mice. In addition, MDMA produced methylphenidate/methamphetamine-like discriminative stimulus effects in orexin KO mice, but not WT mice. Increases in 5-HT and dopamine release in the nucleus accumbens induced by MDMA were not altered by knockout of orexin; the steady-state level of G protein activation was higher in the limbic forebrain of orexin KO mice. In substitution tests using a drug discrimination procedure, substitution of 5-HT1A receptor agonist for the discriminative stimulus effects of methylphenidate was enhanced in orexin KO mice. These findings indicate that the orexinergic system is involved the rewarding effects of psychostimulants. However, there is a risk of establishing rewarding effects of psychostimulants even under orexin deficiency. On the other hand, deficiencies in orexin may enhance the abuse liability of MDMA by changing a postsynaptic signal transduction accompanied by changes in discriminative stimulus effects themselves.

Administration of low doses of the 5-HT1A receptor agonist 8-OH-DPAT attenuates the discriminative signal of amphetamine in the conditioned taste aversion procedure

Several studies have reported that low doses of the 5-HT1A receptor agonist 8-OH-DPAT reduce cocaine-induced locomotor activity. However, it has also been reported that high doses of 8-OH-DPAT do not substitute for or alter the discriminative signal of cocaine (COC) or amphetamine (AMPH). This study aimed to evaluate the effects of low and high doses of the 5-HT1A agonist 8-OH-DPAT on the discriminative signal of AMPH using conditioned taste aversion as a drug discrimination procedure. Additionally, to establish a correlation between the behavioral effects in drug discrimination and changes in dopamine (DA) and gamma-aminobutyric acid (GABA) concentrations, we evaluated the effect of systemic administration of low or high doses of the 5-HT1A receptor agonist 8-OH-DPAT and of the 5-HT1A receptor antagonist WAY100135 on DA and GABA extracellular concentrations in the nucleus accumbens (nAcc) and ventral tegmental area (VTA), respectively, using cerebral microdialysis. The behavioral results showed that low but not high doses of 8-OH-DPAT produced a reduction in the AMPH-induced discriminative signal, while WAY100135 administration prevented such effects. The microdialysis results showed that a low dose of 8-OH-DPAT decreased extracellular DA concentrations in the nAcc and increased GABA concentrations in the VTA. Pretreatment with WAY100135 prevented these effects. These data support the hypothesis that 5-HT1A receptors modulate the behavioral effects of psychostimulant drugs, such as AMPH, through somatodendritic 5-HT1A autoreceptors in the raphe nucleus indicating that 5-HT1A receptors may be an important target for the development of pharmacological treatments for psychostimulant addiction.

In vivo effects of 3,4-methylenedioxymethamphetamine (MDMA) and its deuterated form in rodents: Drug discrimination and thermoregulation

BackgroundRecent clinical studies support the use of 3,4-methylenedioxymethamphetamine (MDMA) as an adjunct treatment for posttraumatic stress disorder (PTSD). Despite these promising findings, MDMA administration in controlled settings can increase blood pressure, heart rate, and body temperature. Previous studies indicate thatO-demethylated metabolites of MDMA contribute to its adverse effects. As such, limiting the conversion of MDMA to reactive metabolites may mitigate some of its adverse effects and potentially improve its safety profile for therapeutic use. MethodsWe compared the interoceptive and hyperthermic effects of a deuterium-substituted form of MDMA (d2-MDMA) to MDMA using rodent drug discrimination and biotelemetry procedures, respectively. ResultsCompared to MDMA, d2-MDMA produced full substitution for a 1.5 mg/kg MDMA training stimulus with equal potency and effectiveness in the drug discrimination experiment. In addition, d2-MDMA produced increases in body temperature that were shorter-lasting and of lower magnitude compared to equivalent doses of MDMA. Last, d2-MDMA and MDMA were equally effective in reversing the hypothermic effects of the selective 5-HT2A/2C antagonist ketanserin. ConclusionThese findings indicate that deuterium substitution of hydrogen at the methylenedioxy ring moiety does not impact MDMA’s interoceptive effects, and compared to MDMA, d2-MDMA has less potential for producing hyperthermic effects and likely has similar pharmacodynamic properties. Given that d2-MDMA produces less adverse effects than MDMA, but retains similar desirable effects that are thought to relate to the effective treatment of PTSD, additional investigations into its effects on cardiovascular functioning and pharmacokinetic properties are warranted.

Discriminative Stimulus and Low Abuse Liability Effects of Novel Endomorphin Analogs Suggest a Potential Treatment Indication for Opioid Use Disorder.

Opioid dependence can be difficult to manage using existing pharmacotherapies. A long-acting opioid with low abuse liability that substitutes for a shorter-acting opioid may improve treatment of opioid use disorders (OUDs). We recently characterized an endomorphin (EM) analog (ZH853) that produced a longer duration of antinociception compared with morphine, but did not produce self-administration or several other adverse effects preclinically. Here, we further characterized ZH853 in tests of antinociception, abuse liability, and drug discrimination. A conditioned place preference (CPP) procedure, that included a locomotor activity assessment, was used to test abuse liability in rats. Subsequently, dopamine (DA) cell-somas located in the ventral tegmental area (VTA) from these rats were assessed by size using immunohistochemistry and Stereo Investigator software. A hot-plate antinociception test in male and female mice confirmed central penetration. Morphine-substitution effects of several EM analogs (ZH850, ZH831, and ZH853) were tested in a drug discrimination (DD) procedure in rats. Morphine produced dose-dependent CPP and locomotor sensitization and reduced the size of DA cell somas in VTA, whereas ZH853 did not produce any of these effects relative to control. The antinociceptive effects of ZH853 were μ-receptor selective since β-funaltrexamine antagonized these effects. Rats responded on a morphine-trained lever when injected with ZH831 and ZH853 during DD experiments. The favorable morphine-substitution effects of these EM analogs relative to their low abuse liability indicate promising novel compounds that may improve treatment of OUD. SIGNIFICANCE STATEMENT: In this experiment, we investigated the preclinical effects of novel endomorphin analogs for use as substitution therapies for opioid use disorder, a problem that has contributed to an opioid overdose epidemic. Several endomorphin analogs substituted for morphine without producing adverse effects, including reward behaviors associated with abuse liability. These compounds have the potential to become important additional tools to treat opioid use disorders.

Role of mPFC and nucleus accumbens circuitry in modulation of a nicotine plus alcohol compound drug state.

Combined use of nicotine and alcohol constitute a significant public health risk. An important aspect of drug use and dependence are the various cues, both external (contextual) and internal (interoceptive) that influence drug-seeking and drug-taking behavior. The present experiments employed the use of Designer Receptors Exclusively Activated by Designer Drugs (DREADDs) and complementary Pavlovian drug discrimination procedures (feature-positive and feature-negative training conditions) in order to examine whether medial prefrontal cortex (prelimbic; mPFC-PL) projections to the nucleus accumbens core (AcbC) modulate sensitivity to a nicotine+alcohol (N+A) interoceptive cue. First, we show neuronal activation in mPFC-PL and AcbC following treatment with N+A. Next, we demonstrate that chemogenetic silencing of projections from mPFC-PL to nucleus accumbens core decrease sensitivity to the N+A interoceptive cue, while enhancing sensitivity to the individual components, suggesting an important role for this specific projection. Furthermore, we demonstrate that clozapine-N-oxide (CNO), the ligand used to activate the DREADDs, had no effect in parallel mCherry controls. These findings contribute important information regarding our understanding of the cortical-striatal circuitry that regulates sensitivity to the interoceptive effects of a compound N+A cue.

Dopamine and serotonin antagonists fail to alter the discriminative stimulus properties of ±methylenedioxymethamphetamine

Most studies on discriminative stimulus effects of 3,4-methylenedioxymethamphetamine (MDMA) have been conducted using a relatively low dose (1.5 mg/kg), and those studies have invariably implicated serotonergic mechanisms. In contrast, dopaminergic mechanisms mediate the discriminative stimulus effects of amphetamine (AMPH). Some studies have suggested that the discriminative stimulus effects of a higher (3.0 mg/kg) dose of MDMA might rely on both serotonergic and dopaminergic mechanisms. This study aimed to determine effects of selective dopamine (DA) and serotonin (5HT) antagonists on the discriminative stimulus properties of AMPH (0.5 mg/kg) and MDMA (3.0 mg/kg). Separate groups of rats were trained to discriminate AMPH (0.5 mg/kg) or MDMA (3.0 mg/kg) from saline using a food-reinforced drug-discrimination procedure. Effects of DA (SCH 23390: 0.003-0.03 mg/kg and eticlopride: 0.03-0.3 mg/kg) or 5HT (ritanserin: 1.0-10.0 mg/kg, WAY-100635: 0.3-1.0 mg/kg and GR129375: 1.0-3.0 mg/kg) antagonists on the discriminative stimulus effects of both drugs were determined. Both DA antagonists dose-dependently decreased the AMPH but not the MDMA discrimination. None of the 5HT antagonists altered the discriminative stimulus effects of either drug. The MDMA (3.0 mg/kg) stimulus comprises both a DAergic and 5HTergic response, and the results suggest that either one is sufficient, but not required, to maintain the stimulus effects.

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.

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.

Discriminative stimulus effects of cocaine-levamisole combinations in Sprague-Dawley rats.

Levamisole (LVM) is a common adulterant in clandestinely manufactured cocaine (COC), with a range of well-documented deleterious health effects. Although the prevalence of LVM in COC has been widely noted, the subjective effects related to concomitant COC-LVM administration are poorly understood. The present study sought to compare the subjective effects of LVM alone and in combination with COC in male and female Sprague-Dawley rats trained to discriminate COC from vehicle injections. Male and female Sprague-Dawley rats were trained to discriminate COC from vehicle injections using a two-lever, food-reinforced drug discrimination procedure. Subsequent dose-effect curves were generated for COC, LVM, and a variety of COC-LVM combinations. No significant difference in males and females was observed on any measure of responding. LVM alone dose-dependently decreased response rates and failed to produce substitution for COC. When LVM was administered with COC, previously ineffective doses of COC engendered COC-appropriate responding. LVM potentiates the subjective effects of COC when administered concomitantly. These findings are consistent with the popular notion that LVM is added as an adulterant to COC to amplify the subjective effects of COC administration.

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.