Principles of drug abuse liability assessment in laboratory animals

Nonsedating anxiolytics

Anxiety disorders are the most prevalent psychiatric pathology with substantial cost to society, but the existing treatments are often inadequate. This has rekindled the interest in the GABAA-receptor (GABAAR) positive allosteric modulator (PAM) compounds, which have a long history in treatment of anxiety beginning with diazepam, chlordiazepoxide, and alprazolam. While the GABAAR PAMs possess remarkable anxiolytic efficacy, they have fallen out of favor due to a host of adverse effects including sedation, motor impairment, addictive potential and tolerance development. A substantial effort was thus devoted to the design of GABAAR PAMs as anxiolytics with reduced sedative liabilities. Several non-benzodiazepine (BZD) GABAAPAMs progressed to clinical trials (bretazenil, abecarnil, alpidem, and ocinaplon) with alpidem obtaining regulatory approval as anxiolytic, but later withdrawn from market due to hepatotoxicity. Advances in molecular biology gave birth to a host of subtype selective GABAAR-PAMs which suffered from signs of sedation and motor impairment and only three compounds progressed to proof-of-concept studies (TPA-023, AZD7325 and PF-06372865). TPA-023 was terminated due to toxicity in preclinical species while AZD7325 and PF-06372865 did not achieve efficacy endpoints in patients. We highlight a new compound, KRM-II-81, that is an imidazodiazepine selective for GABAAR containing α2/3 and β3 proteins. In preclinical studies KRM-II-81 produced anxiolytic-like effects but with minimal sedation, respiratory depression, and abuse liability. Thus, KRM-II-81 is a newly discovered, non- BZD anxiolytic compound, which targets a selective population of GABAAR for improved therapeutic gain and reduced side effects.

Drug discrimination learning: Interoceptive stimulus control of behavior and its implications for regulated and dysregulated drug intake

Drug discrimination research has generated rich evidence for the capacity of interoceptive drug stimuli to control behavior by serving as discriminative cues. Owing to its neuropharmacological specificity, drug discrimination learning has been widely used to characterize the stimulus effects and neuropharmacological underpinning of drugs. Apart from such utility, discriminative drug stimuli may help regulate drug use by disambiguating conditioned associations and post-intake outcomes. First, this review summarizes the evidence supporting interoceptive regulation of drug intake from the literature of exteroceptive discriminative control of drug-related behavior, effects of drug priming, and self-titration of drug intake. Second, an overview of interoceptive control of reward-seeking and the animal model of discriminated goal-tracking is provided to illustrate interoceptive stimulus control of the initiation and patterning of drug intake. Third, we highlight the importance of interoceptive control of aversion-avoidance in the termination of drug-use episodes and describe the animal model of discriminated taste avoidance that supports such a position. In bridging these discriminative functions of drug stimuli, we propose that interoceptive drug stimuli help regulate intake by disambiguating whether intake will be rewarding, nonrewarding, or aversive. The reflection and discussion on current theoretical formulations of interoceptive control of drug intake may further scientific advances to improve animal models to study the mechanisms by which interoceptive stimuli regulate drug intake, as well as how alterations of interoceptive processes may contribute to the transition to dysregulated drug use.

Stimulus mediation, specificity and impact of menthol in rats trained to discriminate puffs of nicotine e-cigarette aerosol from nicotine-free aerosol

RATIONALE

It is unclear if e-cigarettes have reduced abuse liability relative to traditional cigarettes, especially when considering advanced devices which deliver nicotine more efficiently. Translatable and predictive animal models are needed to addresses this question.

OBJECTIVES

Our goal was to explore the subjective stimulus effects of e-cigarettes by training rats to discriminate puffs of nicotine aerosol from vehicle aerosol using an aerosol delivery system designed to model e-cigarette use patterns in humans.

METHODS

Rats were trained to discriminate between ten, 10 s puffs of aerosol generated from 3 mg/ml nicotine e-liquid and nicotine-free e-liquid using a food-reinforced operant procedure. Following acquisition, tests were conducted to determine the specificity of the nicotine aerosol stimulus as well as the impact to the stimulus effects of nicotine resulting from the addition of menthol to e-liquid.

RESULTS

Rats learned the nicotine aerosol puff vs vehicle puff discrimination in a mean of 25 training sessions. Injected nicotine fully substituted for the stimulus effects of nicotine aerosol. The stimulus effects of nicotine aerosol were blocked by the nicotinic receptor antagonist mecamylamine. The nicotinic receptor partial agonist, varenicline as well as the stimulant d-amphetamine substituted more robustly for nicotine aerosol puffs than did the NMDA antagonist, ketamine. Menthol enhanced the stimulus effects of nicotine aerosol without altering nicotine blood plasma levels.

CONCLUSIONS

Nicotine aerosol puffs can function as a training stimulus in rats. The stimulus effects were CNS-mediated and receptor specific. Menthol appears to enhance the stimulus effects of nicotine aerosol through a pharmacodynamic rather than pharmacokinetic mechanism.

The role of β-Nicotyrine in E-Cigarette abuse liability I: Drug Discrimination

Background

β-Nicotyrine (β-Nic) is a unique minor alkaloid constituent in electronic nicotine delivery systems (ENDS) that is derived from nicotine (Nic) degradation and can reach 25% of Nic concentrations in ENDS aerosol. β-Nic slows Nic metabolism and prolongs systemic Nic exposure, which may alter the discriminability of Nic. The present study sought to examine β-Nic has interoceptive effects itself, and if it alters the subjective effects ENDS products within a drug-discrimination paradigm.

Methods

The pharmacodynamics of β-Nic were examined in vitro, and a nicotine discrimination paradigm was used to determine if β-Nic (0 - 5.0 mg/kg) shares discriminative stimulus properties with Nic (0.2 mg/kg) in male (n = 13) and female (n = 14) rats after 10- & 60-min β-Nic pretreatment delays. A second group of rats was trained to discriminate β-Nic and Nornicotine (Nornic) from saline to determine if β-Nic alone has interoceptive properties and whether they are similar to Nornic.

Results

β-Nic had similar binding affinity and efficacy at the α4β2 nicotinic receptor subtype as Nornic, ~50% of Nic efficacy. However, β-Nic only weakly substituted for Nic during substitution testing in female rats, but not males, whereas Nornic fully substituted for Nic. Combination testing at the 10 and 60-min pretreatment intervals showed that β-Nic dose-dependently increased the duration of nicotine's discriminative stimulus effects, especially at the 60-min delay. Drug naïve rats could reliably discriminate Nornic, but not β-Nic, from Sal.

Conclusion

β-Nic increased and prolonged the interoceptive stimulus properties of Nic, suggesting it may alter to the abuse liability of ENDS through its ability to slow Nic metabolism.

A critical assessment of the abuse, dependence and associated safety risks of naturally occurring and synthetic cannabinoids

Various countries and US States have legalized cannabis, and the use of the psychoactive1 and non-psychoactive cannabinoids is steadily increasing. In this review, we have collated evidence from published non-clinical and clinical sources to evaluate the abuse, dependence and associated safety risks of the individual cannabinoids present in cannabis. As context, we also evaluated various synthetic cannabinoids. The evidence shows that delta-9 tetrahydrocannabinol (Δ9-THC) and other psychoactive cannabinoids in cannabis have moderate reinforcing effects. Although they rapidly induce pharmacological tolerance, the withdrawal syndrome produced by the psychoactive cannabinoids in cannabis is of moderate severity and lasts from 2 to 6 days. The evidence overwhelmingly shows that non-psychoactive cannabinoids do not produce intoxicating, cognitive or rewarding properties in humans. There has been much speculation whether cannabidiol (CBD) influences the psychoactive and potentially harmful effects of Δ9-THC. Although most non-clinical and clinical investigations have shown that CBD does not attenuate the CNS effects of Δ9-THC or synthetic psychoactive cannabinoids, there is sufficient uncertainty to warrant further research. Based on the analysis, our assessment is cannabis has moderate levels of abuse and dependence risk. While the risks and harms are substantially lower than those posed by many illegal and legal substances of abuse, including tobacco and alcohol, they are far from negligible. In contrast, potent synthetic cannabinoid (CB1/CB2) receptor agonists are more reinforcing and highly intoxicating and pose a substantial risk for abuse and harm. 1 "Psychoactive" is defined as a substance that when taken or administered affects mental processes, e.g., perception, consciousness, cognition or mood and emotions.

The psychomotor, reinforcing, and discriminative stimulus effects of synthetic cathinone mexedrone in male mice and rats

The chronic use of the novel synthetic cathinone mexedrone, like other psychoactive drugs, can be considered addictive, with a high potential for abuse and the ability to cause psychological dependence in certain users. However, little is known about the neurobehavioral effects of mexedrone in association with its potential for abuse. We investigated the abuse potential for mexedrone abuse through multiple behavioral tests. In addition, serotonin transporter (SERT) levels were measured in the synaptosome of the dorsal striatum, and serotonin (5-HT) levels were measured in the dorsal striatum of acute mexedreone (50 mg/kg)-treated mice. To clarify the neuropharmacological mechanisms underlying the locomotor response of mexedrone, the 5-HT2A receptor antagonist M100907 (0.5 or 1.0 mg/kg) was administered prior to the acute injection of mexedrone in the locomotor activity experiment in mice. Mexedrone (10-50 mg/kg) produced a significant place preference in mice and mexedrone (0.1-0.5 mg/kg/infusion) maintained self-administration behavior in rats in a dose-dependent manner. In the drug discrimination experiment, mexedrone (5.6-32 mg/kg) was fully substituted for the discriminative stimulus effects of cocaine in rats. Mexedrone increased locomotor activity, and these effects were reversed by pretreatment with M100907. Acute mexedrone significantly increased c-Fos expression in the dorsal striatum and decreased SERT levels in the synaptosome of the dorsal striatum of mice, resulting in an elevation of 5-HT levels. Taken together, our results provide the possibility that mexedrone has abuse potential, which might be mediated, at least in part, by the activation of the serotonergic system in the dorsal striatum.

Rate of onset of dopamine transporter inhibitors assessed with intracranial self-stimulation and in vivo dopamine photometry in rats

Drug self-administration and intracranial self-stimulation (ICSS) are two preclinical behavioral procedures used to predict abuse potential of drugs, and abuse-related drug effects in both procedures are thought to depend on increased mesolimbic dopamine (DA) signaling. Drug self-administration and ICSS yield concordant metrics of abuse potential across a diverse range of drug mechanisms of action. The "rate of onset," defined as the velocity with which a drug produces its effect once administered, has also been implicated as a determinant of abuse-related drug effects in self-administration procedures, but this variable has not been systematically examined in ICSS. Accordingly, this study compared ICSS effects produced in rats by three DA transporter inhibitors that have different rates of onset (fastest to slowest: cocaine, WIN-35428, RTI-31) and that produced progressively weaker metrics of abuse potential in a drug self-administration procedure in rhesus monkeys. Additionally, in vivo photometry using the fluorescent DA sensor dLight1.1 targeted to the nucleus accumbens (NAc) was used to assess the time course of extracellular DA levels as a neurochemical correlate of behavioral effects. All three compounds produced ICSS facilitation and increased DA levels assessed by dLight. In both procedures, the rank order of onset rate was cocaine > WIN-35428 > RTI-31; however, in contrast to monkey drug self-administration results, maximum effects did not differ across compounds. These results provide additional evidence that drug-induced increases in DA drive ICSS facilitation in rats and illustrate the utility of both ICSS and photometry to evaluate the time course and magnitude of abuse-related drug effects in rats.

Psychedelic drug abuse potential assessment for new drug applications and controlled substance scheduling: A United States perspective

BACKGROUND

Psychedelics are an increasingly active area of research and pharmaceutical development. This includes abuse potential assessment to better understand their pharmacological mechanisms and effects and guide controlled substance regulation. Psychedelics pose challenges to abuse assessments to ensure valid, reliable, and generalizable outcomes and safe study conduct.

FINDINGS

Key nonclinical techniques, for example, receptor binding and functional assays in vitro, and nonclinical physical dependence determinations, are easily adaptable to psychedelics. However, the entactogens (weak reinforcers) and hallucinogens (non-reinforcers) require more flexible approaches than typically recommended by regulatory agencies. Phase 1 pharmacokinetic/pharmacodynamic safety studies and Phases 2/3 efficacy/safety trials with systematic monitoring of abuse-related adverse events are readily applicable to psychedelics. Human abuse trials require modification because supratherapeutic doses may not be safe and procedures, for example, personal monitors to manage serious adverse events, might bias outcomes.

RECOMMENDATIONS

Abuse-related studies for psychedelics requiring approval by Food and Drug Administration and other agencies should take into consideration existing knowledge that will vary from extensive, for example, psilocybin, to zero for novel hallucinogens and entactogens. Many abuse assessments can be reasonably applied to animals and humans without compromising scientific integrity. Modification of existing techniques and incorporating a broader range of nonclinical tests should ensure generalizable outcomes. Human abuse studies merit reconsideration and possible modification to ensure safety and validity for psychedelic drug evaluation. Other nonclinical and clinical methods can provide evaluations of the pharmacological equivalence of test drugs to known drugs of abuse to provide context to the abuse assessment and guide drug scheduling.

PPL-138 (BU10038): A bifunctional NOP/mu partial agonist that reduces cocaine self-administration in rats

The search for new and effective treatments for cocaine use disorder (CUD) is a priority. We determined whether PPL-138 (BU10038), a compound with partial agonist activity at both nociceptin opioid peptide (NOP) and mu-opioid receptors, reduces cocaine consumption, reinstatement, and whether the compound itself produces reinforcing effects in rats. Using an intermittent access (IntA) cocaine self-administration procedure, we found that PPL-138 (0.1 and 0.3 mg/kg) effectively decreased the total number of cocaine infusions and burst-like cocaine intake in both male and female rats. Responses for food in an IntA model of food self-administration were not altered for either sex, although locomotor activity was increased in female but not male rats. Blockade of NOP receptors with the selective antagonist J-113397 (5 mg/kg) did not prevent the PPL-138-induced suppression of cocaine self-administration, whereas blockade of mu-opioid receptors by naltrexone (1 mg/kg) reversed such effect. Consistently, treatment with morphine (1, 3, and 10 mg/kg) dose-dependently reduced IntA cocaine self-administration measures. PPL-138 also reduced reinstatement of cocaine seeking at all doses examined. Although an initial treatment with PPL-138 (2.5, 10, and 40 μg/kg/infusion) appeared rewarding, the compound did not maintain self-administration behavior. Animals treated with PPL-138 showed initial suppression of cocaine self-administration, which was eliminated following repeated daily dosing. However, suppression of cocaine self-administration was retained when subsequent PPL-138 treatments were administered 48 h apart. These findings demonstrate that the approach of combining partial NOP/mu-opioid activation successfully reduces cocaine use, but properties of PPL-138 seem to depend on the timing of drug administration.

Appetitive Pavlovian conditioning of the stimulus effects of nicotine enhances later nicotine self-administration

Nicotine produces robust stimulus effects that can be conditioned to exert stimulus control over behavior through associative learning. Additionally, nicotine has weak reinforcing effects that are inconsistent with its prevalence of use and the tenacity of nicotine dependence. The present study investigated whether conditioned associations to the nicotine drug stimulus may confer additional reinforcing strength to nicotine that thereby increase its use liability, and presents a new methodological approach to investigating the interaction between the stimulus effects and reinforcing effects of drugs. Male and female Sprague-Dawley rats were divided into groups receiving intravenous infusions of either 0.03 mg/kg nicotine or 0.9% saline that were either Paired (30 s delayed) or explicitly Unpaired (4 to 6 min delayed) with sucrose deliveries over 24 daily conditioning sessions. Thereafter, recessed nosepoke response devices were installed in the chambers and infusions of their assigned drug solutions were contingently available according to a progressive ratio schedule. Rats in the Paired Nicotine condition acquired the nosepoke response, expressed active nosepoke discrimination, and self-administered significantly more infusions than rats in any of the other groups. These results demonstrate that the interoceptive stimulus effects of nicotine can form Pavlovian associations with reinforcing events that alter its reinforcement efficacy.

The imidazodiazepine, KRM-II-81: An example of a newly emerging generation of GABAkines for neurological and psychiatric disorders

GABAkines, or positive allosteric modulators of γ-aminobutyric acid-A (GABA_A) receptors, are used for the treatment of anxiety, epilepsy, sleep, and other disorders. The search for improved GABAkines, with reduced safety liabilities (e.g., dependence) or side-effect profiles (e.g., sedation) constituted multiple discovery and development campaigns that involved a multitude of strategies over the past century. Due to the general lack of success in the development of new GABAkines, there had been a decades-long draught in bringing new GABAkines to market. Recently, however, there has been a resurgence of efforts to bring GABAkines to patients, the FDA approval of the neuroactive steroid brexanolone for post-partum depression in 2019 being the first. Other neuroactive steroids are in various stages of clinical development (ganaxolone, zuranolone, LYT-300, Sage-324, PRAX 114, and ETX-155). These GABAkines and non-steroid compounds (GRX-917, a TSPO binding site ligand), darigabat (CVL-865), an α2/3/5-preferring GABAkine, SAN711, an α3-preferring GABAkine, and the α2/3-preferring GABAkine, KRM-II-81, bring new therapeutic promise to this highly utilized medicinal target in neurology and psychiatry. Herein, we also discuss possible conditions that have enabled the transition to a new age of GABAkines. We highlight the pharmacology of KRM-II-81 that has the most preclinical data reported. KRM-II-81 is the lead compound in a new series of orally bioavailable imidazodiazepines entering IND-enabling safety studies. KRM-II-81 has a preclinical profile predicting efficacy against pharmacoresistant epilepsies, traumatic brain injury, and neuropathic pain. KRM-II-81 also produces anxiolytic- and antidepressant-like effects in rodent models. Other key features of the pharmacology of this compound are its low sedation rate, lack of tolerance development, and the ability to prevent the development of seizure sensitization.

Examining the effects of psychoactive drugs on complex behavioral processes in laboratory animals

Behavioral pharmacology has been aided significantly by the development of innovative cognitive tasks designed to examine complex behavioral processes in laboratory animals. Performance outcomes under these conditions have provided key metrics of drug action which serve to supplement traditional in vivo assays of physiologic and behavioral effects of psychoactive drugs. This chapter provides a primer of cognitive tasks designed to assay different aspects of complex behavior, including learning, cognitive flexibility, memory, attention, motivation, and impulsivity. Both capstone studies and recent publications are highlighted throughout to illustrate task value for two distinct but often interconnected translational strategies. First, task performance in laboratory animals can be utilized to elucidate how drugs of abuse affect complex behavioral processes. Here, the expectation is that adverse effects on such processes will have predictive relevance to consequences that will be experienced by humans. Second, these same task outcomes can be used to evaluate candidate therapeutics. In this case, the extent to which drug doses with medicinal value perturb task performance can contribute critical information for a more complete safety profile appraisal and advance the process of medications development. Methodological and theoretical considerations are discussed and include an emphasis on determining selectivity in drug action on complex behavioral processes.

Functional Profile of Systemic and Intrathecal Cebranopadol in Nonhuman Primates

BACKGROUND

Cebranopadol, a mixed nociceptin/opioid receptor full agonist, can effectively relieve pain in rodents and humans. However, it is unclear to what degree different opioid receptor subtypes contribute to its antinociception and whether cebranopadol lacks acute opioid-associated side effects in primates. The authors hypothesized that coactivation of nociceptin receptors and μ receptors produces analgesia with reduced side effects in nonhuman primates.

METHODS

The antinociceptive, reinforcing, respiratory-depressant, and pruritic effects of cebranopadol in adult rhesus monkeys (n = 22) were compared with μ receptor agonists fentanyl and morphine using assays, including acute thermal nociception, IV drug self-administration, telemetric measurement of respiratory function, and itch-scratching responses.

RESULTS

Subcutaneous cebranopadol (ED50, 2.9 [95% CI, 1.8 to 4.6] μg/kg) potently produced antinociception compared to fentanyl (15.8 [14.6 to 17.1] μg/kg). Pretreatment with antagonists selective for nociceptin and μ receptors, but not δ and κ receptor antagonists, caused rightward shifts of the antinociceptive dose-response curve of cebranopadol with dose ratios of 2 and 9, respectively. Cebranopadol produced reinforcing effects comparable to fentanyl, but with decreased reinforcing strength, i.e., cebranopadol (mean ± SD, 7 ± 3 injections) versus fentanyl (12 ± 3 injections) determined by a progressive-ratio schedule of reinforcement. Unlike fentanyl (8 ± 2 breaths/min), systemic cebranopadol at higher doses did not decrease the respiratory rate (17 ± 2 breaths/min). Intrathecal cebranopadol (1 μg) exerted full antinociception with minimal scratching responses (231 ± 137 scratches) in contrast to intrathecal morphine (30 μg; 3,009 ± 1,474 scratches).

CONCLUSIONS

In nonhuman primates, the μ receptor mainly contributed to cebranopadol-induced antinociception. Similar to nociceptin/μ receptor partial agonists, cebranopadol displayed reduced side effects, such as a lack of respiratory depression and pruritus. Although cebranopadol showed reduced reinforcing strength, its detectable reinforcing effects and strength warrant caution, which is critical for the development and clinical use of cebranopadol.

EDITOR’S PERSPECTIVE

Safety evaluation of oliceridine for the management of postoperative moderate-to-severe acute pain

INTRODUCTION

Opioids for managing postoperative pain are associated with side effects including opioid-induced respiratory depression (OIRD) and gastrointestinal complications. Opioids induce analgesia via G-protein signaling, while adverse effects are mediated by the β-arrestin pathway. Oliceridine is a biased ligand that preferentially activates G-protein signaling over β-arrestin, theoretically reducing adverse effects. Oliceridine has been approved by the Food and Drug Administration to treat acute pain severe enough to require intravenous opioid analgesics.

AREAS COVERED

Preclinical and clinical trials demonstrate the analgesic efficacy of oliceridine. Available evidence suggests that oliceridine may have a lower risk of OIRD and gastrointestinal complications compared to conventional opioids.

EXPERT OPINION

The analgesic efficacy of oliceridine has been evaluated in several clinical trials. However, safety data were obtained from an open-label observational study and studies assessing adverse effects as secondary outcomes, as post-hoc analyses, or from retrospective studies. These may be affected by gaps in detecting adverse events, heterogeneity in the original studies, and the limitations of retrospective studies. Prospective trials examining the safety of oliceridine versus conventional opioids are needed. Studies are also needed to assess the safety and efficacy of oliceridine in obstetric and pediatric populations, and in the context of multimodal analgesia and Enhanced Recovery after Surgery protocols.

Selective Inhibition of Phosphodiesterase 7 Enzymes Reduces Motivation for Nicotine Use through Modulation of Mesolimbic Dopaminergic Transmission

Approximately 5 million people die from diseases related to nicotine addiction and tobacco use each year. The nicotine-induced increase of corticomesolimbic dopaminergic (DAergic) transmission and hypodopaminergic conditions occurring during abstinence are important for maintaining drug-use habits. We examined the notion of reequilibrating DAergic transmission by inhibiting phosphodiesterase 7 (PDE7), an intracellular enzyme highly expressed in the corticomesolimbic circuitry and responsible for the degradation of cyclic adenosine monophosphate (cAMP), the main second messenger modulated by DA receptor activation. Using selective PDE7 inhibitors, we demonstrated in male rats that systemic PDE7 enzyme inhibition reduced nicotine self-administration and prevented reinstatement to nicotine seeking evoked by cues or by the pharmacological stressor yohimbine. The effect was also observed by direct application of the PDE7 inhibitors into the nucleus accumbens (NAc) shell but not into the core. Inhibition of PDE7 resulted in increased DA- and cAMP-regulated neuronal phosphoprotein and cAMP response element-binding protein and their phosphorylated forms in the NAc. It also enhanced the DA D1 receptor agonism-mediated effects, indicating potentiation of protein kinase A-dependent transmission downstream of D1 receptor activation. In electrophysiological recordings from DA neurons in the lateral posterior ventral tegmental area, the PDE7 inhibitors attenuated the spontaneous activity of DA neurons. This effect was exerted through the potentiation of D1 receptor signaling and the subsequent facilitation of γ-aminobutyric acid transmission. The PDE7 inhibitors did not elicit conditioned place preference and did not induce intravenous self-administration, indicating lack of reinforcing properties. Thus, PDE7 inhibitors have the potential to treat nicotine abuse.SIGNIFICANCE STATEMENT The World Health Organization estimates that there are 1.25 billion smokers worldwide, representing one-third of the global population over the age of 15. Nicotine-induced increase of corticomesolimbic DAergic transmission and hypodopaminergic conditions occurring during abstinence are critical for maintaining drug-use habits. Here, we demonstrate that nicotine consumption and relapse to nicotine seeking are attenuated by reequilibrating DAergic transmission through inhibition of PDE7, an intracellular enzyme responsible for the degradation of cAMP, the main second messenger modulated by DA receptor activation. PDE7 inhibition may represent a novel treatment approach to aid smoking cessation.

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.

Psychostimulant Use Disorder, an Unmet Therapeutic Goal: Can Modafinil Narrow the Gap?

The number of individuals affected by psychostimulant use disorder (PSUD) has increased rapidly over the last few decades resulting in economic, emotional, and physical burdens on our society. Further compounding this issue is the current lack of clinically approved medications to treat this disorder. The dopamine transporter (DAT) is a common target of psychostimulant actions related to their use and dependence, and the recent availability of atypical DAT inhibitors as a potential therapeutic option has garnered popularity in this research field. Modafinil (MOD), which is approved for clinical use for the treatment of narcolepsy and sleep disorders, blocks DAT just like commonly abused psychostimulants. However, preclinical and clinical studies have shown that it lacks the addictive properties (in both behavioral and neurochemical studies) associated with other abused DAT inhibitors. Clinical availability of MOD has facilitated its off-label use for several psychiatric disorders related to alteration of brain dopamine (DA) systems, including PSUD. In this review, we highlight clinical and preclinical research on MOD and its R-enantiomer, R-MOD, as potential medications for PSUD. Given the complexity of PSUD, we have also reported the effects of MOD on psychostimulant-induced appearance of several symptoms that could intensify the severity of the disease (i.e., sleep disorders and impairment of cognitive functions), besides the potential therapeutic effects of MOD on PSUD.

The self-serving benefits of being a good host: A role for our micro-inhabitants in shaping opioids' function

Opioids are highly efficacious in their ability to relieve pain, but they are liable for abuse, dependence, and addiction. Risk factors to develop opioid use disorders (OUD) include chronic stress, socio-environment, and preexisting major depressive disorders (MDD) and posttraumatic stress disorders (PTSD). Additionally, opioids reduce gut motility, induce loss of gut barrier function, and alter the composition of the trillions of microbes hosted in the gastrointestinal tract, known as the gut microbiota. The microbiota are significant contributors to the reciprocal communication between the central nervous system (CNS) and the gut, termed the gut-brain axis. They have strong influences on their host behaviors, including the ability to cope with stress, sociability, affect, mood, and anxiety. Thus, they are implicated in the etiology of MDD and PTSD. Here we review the latest studies demonstrating that intestinal flora can, directly and indirectly, by affecting sociability levels, responses to stress, and mental state, alter the responses to opioids. It suggests that microbiota can potentially be used to increase the resilience to develop analgesic tolerance and OUD.

Oliceridine: A Novel Drug for the Management of Moderate to Severe Acute Pain - A Review of Current Evidence

Optimal pain relief requires a balance between adequate analgesia and risk of adverse effects. Opioids remain the cornerstone for managing moderate to severe pain, but are associated with opioid-induced respiratory depression (OIRD) and gastrointestinal complications. Opioids exert their analgesic effects predominantly via G-protein signaling, however, adverse effects including OIRD are mediated by the β-arrestin pathway. Oliceridine is the first of a new class of biased opioid agonists that preferentially activate G-protein signaling over β-arrestin, which would theoretically improve analgesia and reduce the risk of adverse effects. Oliceridine is approved by the Food and Drug Administration (FDA) for the treatment of moderate to severe acute pain. The efficacy of Oliceridine was mainly established in two randomized controlled Phase III clinical trials of patients experiencing moderate to severe pain after bunionectomy (APOLLO-1) and abdominoplasty (APOLLO-2). The results of the APOLLO studies demonstrate that Oliceridine, when administered via patient-controlled analgesia (PCA) demand boluses of 0.35mg and 0.5mg, provides superior analgesia compared to placebo, and is equianalgesic to PCA morphine 1mg demand boluses, without significant difference in the incidence of respiratory complications. In a more pragmatic trial of surgical and non-surgical patients, the ATHENA observational cohort study reported rapid onset of analgesia with Oliceridine given with or without multimodal analgesia. However, these studies were designed to evaluate analgesic efficacy, and it is still uncertain if Oliceridine has a better safety profile than conventional opioids. Although several post hoc analyses of pooled data from the APOLLO and ATHENA trials reported that Oliceridine was associated with lower OIRD and gastrointestinal complications compared to morphine, prospective studies are needed to elucidate if biased agonists such as Oliceridine reduce the risk of adverse effects compared to conventional opioids.

Factors mediating pain-related risk for opioid use disorder

Pain is a complex experience with far-reaching organismal influences ranging from biological factors to those that are psychological and social. Such influences can serve as pain-related risk factors that represent susceptibilities to opioid use disorder. This review evaluates various pain-related risk factors to form a consensus on those that facilitate opioid abuse. Epidemiological findings represent a high degree of co-occurrence between chronic pain and opioid use disorder that is, in part, driven by an increase in the availability of opioid analgesics and the diversion of their use in a non-medical context. Brain imaging studies in individuals with chronic pain that use/abuse opioids suggest abuse-related mechanisms that are rooted within mesocorticolimbic processing. Preclinical studies suggest that pain states have a limited impact on increasing the rewarding effects of opioids. Indeed, many findings indicate a reduction in the rewarding and reinforcing effects of opioids during pain states. An increase in opioid use may be facilitated by an increase in the availability of opioids and a decrease in access to non-opioid reinforcers that require mobility or social interaction. Moreover, chronic pain and substance abuse conditions are known to impair cognitive function, resulting in deficits in attention and decision making that may promote opioid abuse. A better understanding of pain-related risk factors can improve our knowledge in the development of OUD in persons with pain conditions and can help identify appropriate treatment strategies. This article is part of the special issue on 'Vulnerabilities to Substance Abuse.'.

The reinforcement threshold and elasticity of demand for nicotine in an adolescent rat model of depression

BACKGROUND

The Food and Drug Administration (FDA) is considering setting a nicotine standard for tobacco products to reduce their addictiveness. Such a standard should account for the apparent greater vulnerability to nicotine addiction in some subpopulations, such as adolescents with depression. The present study examined whether the reinforcement threshold and elasticity of demand (i.e., reinforcing efficacy) for nicotine in a genetic inbred rat model of depression (Flinders Sensitive Line [FSL]) differs from an outbred control strain.

METHODS

Acquisition of nicotine self-administration (NSA) across a wide range of nicotine doses was measured in both FSL and Sprague-Dawley (SD) control adolescent rats. At the highest dose, elasticity of demand was also measured. Nicotine pharmacokinetics was examined to determine whether it might modulate NSA, as it does smoking in humans.

RESULTS

FSL rats acquired self-administration quicker and showed more inelastic demand (greater reinforcing efficacy) than SDs at the highest unit dose. However, there was no strain difference in the reinforcement threshold of nicotine. FSL rats exhibited faster nicotine clearance, larger volume of distribution, and lower plasma and brain nicotine concentrations. However, these differences were not consistently related to strain differences in NSA measures.

CONCLUSION

These findings are consistent with studies showing greater dependence and reinforcing efficacy of cigarettes in smokers with depression and those with relatively fast nicotine metabolism. However, these findings also suggest that a nicotine standard to reduce initiation of tobacco use should be similarly effective in both the general adolescent population and those with depression.

Translational value of non-human primates in opioid research

Preclinical opioid research using animal models not only provides mechanistic insights into the modulation of opioid analgesia and its associated side effects, but also validates drug candidates for improved treatment options for opioid use disorder. Non-human primates (NHPs) have served as a surrogate species for humans in opioid research for more than five decades. The translational value of NHP models is supported by the documented species differences between rodents and primates regarding their behavioral and physiological responses to opioid-related ligands and that NHP studies have provided more concordant results with human studies. This review highlights the utilization of NHP models in five aspects of opioid research, i.e., analgesia, abuse liability, respiratory depression, physical dependence, and pruritus. Recent NHP studies have found that (1) mixed mu opioid and nociceptin/orphanin FQ peptide receptor partial agonists appear to be safe, non-addictive analgesics and (2) mu opioid receptor- and mixed opioid receptor subtype-based medications remain the only two classes of drugs that are effective in alleviating opioid-induced adverse effects. Given the recent advances in pharmaceutical sciences and discoveries of novel targets, NHP studies are posed to identify the translational gap and validate therapeutic targets for the treatment of opioid use disorder. Pharmacological studies using NHPs along with multiple outcome measures (e.g., behavior, physiologic function, and neuroimaging) will continue to facilitate the research and development of improved medications to curb the opioid epidemic.

Medications Development for Treatment of Opioid Use Disorder

This review describes methods for preclinical evaluation of candidate medications to treat opioid use disorder (OUD). The review is founded on the propositions that (1) drug self-administration procedures provide the most direct method for assessment of medication effectiveness, (2) procedures that assess choice between opioid and nondrug reinforcers are especially useful, and (3) states of opioid dependence and withdrawal profoundly influence both opioid reinforcement and effects of candidate medications. Effects of opioid medications and vaccines on opioid choice in nondependent and opioid-dependent subjects are reviewed. Various nonopioid medications have also been examined, but none yet have been identified that safely and reliably reduce opioid choice. Future research will focus on (1) strategies for increasing safety and/or effectiveness of opioid medications (e.g., G-protein-biased μ-opioid agonists), and (2) continued development of nonopioid medications (e.g., clonidine) that might serve as adjunctive agents to current opioid medications.

Effects of early life stress on cocaine intake in male and female rhesus macaques

RATIONALE

It is critical to identify potential risk factors, such as a history of early life stress (ELS), that may confer specific vulnerabilities to increased drug intake.

OBJECTIVE

In this study, we examined whether male and female rhesus monkeys with a history of ELS (infant maltreatment; MALT) demonstrated significantly greater cocaine intake compared with controls.

METHODS

Monkeys were trained to self-administer cocaine during 4-h sessions at a peak dose (0.003-0.1 mg/kg/infusion; extended access, "EA peak") and a dose of 0.1 mg/kg/infusion (EA 0.1) of cocaine. These data were compared with data obtained previously in monkeys trained during 1-h limited access (LA) sessions at the same peak dose of cocaine used here (Wakeford et al. Psychopharmacology, 236:2785-2796, 2019).

RESULTS

Monkeys significantly increased total number of infusions earned in EA compared with LA, but total session response rates significantly decreased in EA compared with LA. There was no evidence of escalation in drug intake when we compared response rates to obtain the first 20 cocaine infusions between LA and EA peak conditions. Moreover, there was no evidence of escalation in drug intake during an additional 7 weeks of self-administration at 0.1 mg/kg/injection.

CONCLUSIONS

The current study expands on previous reports demonstrating that rhesus macaques did not escalate cocaine intake under the experimental conditions employed and extended these findings by using a unique population of nonhuman primates with a history of infant MALT to test the hypothesis that ELS is a risk factor for escalation of cocaine intake in nonhuman primates. There was no clear evidence of escalation in cocaine intake as a consequence of ELS.

Antinociceptive, reinforcing, and pruritic effects of the G-protein signalling-biased mu opioid receptor agonist PZM21 in non-human primates

BACKGROUND

A novel G-protein signalling-biased mu opioid peptide (MOP) receptor agonist, PZM21, was recently developed with a distinct chemical structure. It is a potent G_i/o activator with minimal β-arrestin-2 recruitment. Despite intriguing activity in rodent models, PZM21 function in non-human primates is unknown. The aim of this study was to investigate PZM21 actions after systemic or intrathecal administration in primates.

METHODS

Antinociceptive, reinforcing, and pruritic effects of PZM21 were compared with those of the clinically used MOP receptor agonists oxycodone and morphine in assays of acute thermal nociception, capsaicin-induced thermal allodynia, itch scratching responses, and drug self-administration in gonadally intact, adult rhesus macaques (10 males, six females).

RESULTS

After subcutaneous administration, PZM21 (1.0-6.0 mg kg^-1) and oxycodone (0.1-0.6 mg kg^-1) induced dose-dependent thermal antinociceptive effects (P<0.05); PZM21 was 10 times less potent than oxycodone. PZM21 exerted oxycodone-like reinforcing effects and strength as determined by two operant schedules of reinforcement in the intravenous drug self-administration assay. After intrathecal administration, PZM21 (0.03-0.3 mg) dose-dependently attenuated capsaicin-induced thermal allodynia (P<0.05). Although intrathecal PZM21 and morphine induced MOP receptor-mediated antiallodynic effects, both compounds induced robust, long-lasting itch scratching.

CONCLUSIONS

PZM21 induced antinociceptive, reinforcing, and pruritic effects similar to clinically used MOP receptor agonists in primates. Although structure-based discovery of PZM21 identified a novel avenue for studying G-protein signalling-biased ligands, biasing an agonist towards G-protein signalling pathways did not determine or alter reinforcing (i.e. abuse potential) or pruritic effects of MOP receptor agonists in a translationally relevant non-human primate model.

Pre-exposure of adolescent mice to morphine results in stronger sensitization and reinstatement of conditioned place preference than pre-exposure to hydrocodone

BACKGROUND

Opioids are commonly prescribed to treat moderate-to-severe pain. However, their use can trigger the development of opioid use disorder. A major problem in treating opioid use disorder remains the high rate of relapse.

AIM

The purpose of this study was to determine whether there are differences among opioids in their ability to trigger relapse after pre-exposure during adolescence.

METHODS

On postnatal day 33, mice were examined for the acute locomotor response to saline, morphine, or hydrocodone (5 mg/kg). They were administered with the corresponding opioid or saline during postnatal days 34-38 (20 mg/kg) and 40-44 (40 mg/kg). On postnatal day 45, they were recorded for the development of locomotor sensitization (5 mg/kg). Starting on postnatal day 55, mice were examined for the acquisition (1, 5, 10, 20, and 40 mg/kg), extinction, and drug-induced reinstatement (1, 2.5, and 5 mg/kg) of conditioned place preference.

RESULTS

There were no significant differences in the acute locomotor response to morphine and hydrocodone. Morphine induced significantly stronger locomotor sensitization as compared to hydrocodone. Pre-exposure to morphine, but not hydrocodone, sensitized the acquisition of conditioned place preference. There were no significant differences in extinction rates. Mice pre-exposed to morphine reinstate conditioned place preference after priming with a 1 mg/kg dose. In contrast, higher priming doses were required for reinstatement in all other experimental groups.

CONCLUSIONS

Adolescent mice administered with morphine develop greater sensitization to its effects and subsequently reinstate conditioned place preference more readily than mice administered with hydrocodone. This suggests higher risk for relapse after pre-exposure to morphine during adolescence as compared to hydrocodone.

Nicotine Self-administration Is Not Increased in the Methylazoxymethanol Acetate Rodent Model of Schizophrenia

INTRODUCTION

Patients with schizophrenia (SCZ) smoke at a rate of 4-5 times higher than the general population, contributing to negative health consequences in this group. One possible explanation for this increased smoking is that individuals with SCZ find nicotine (NIC) more reinforcing. However, data supporting this possibility are limited.

METHODS

The present experiments examined self-administration of NIC, alone or in combination with other reinforcers, across a range of doses in the methylazoxymethanol acetate (MAM) rodent model of SCZ.

RESULTS

MAM and control animals did not differ in NIC self-administration across a range of doses and schedules of reinforcement, in both standard 1-hour self-administration sessions and 23-hour extended access sessions. However, MAM animals responded less for sucrose or reinforcing visual stimuli alone or when paired with NIC.

CONCLUSIONS

To the extent that MAM-treated rats are a valid model of SCZ, these results suggest that increased NIC reinforcement does not account for increased smoking in SCZ patients.

IMPLICATIONS

This study is the first to utilize nicotine self-administration, the gold standard for studying nicotine reinforcement, in the methylazoxymethanol acetate model of schizophrenia, which is arguably the most comprehensive animal model of the disease currently available. Our assessment found no evidence of increased nicotine reinforcement in methylazoxymethanol acetate animals, suggesting that increased reinforcement may not perpetuate increased smoking in schizophrenia patients.

Serotonin 2A receptors are a stress response system: implications for post-traumatic stress disorder

Serotonin, one of the first neurotransmitters to be identified, is an evolutionarily old molecule that is highly conserved across the animal kingdom, and widely used throughout the brain. Despite this, ascribing a specific set of functions to brain serotonin and its receptors has been difficult and controversial. The 2A subtype of serotonin receptors (5-HT2A receptor) is the major excitatory serotonin receptor in the brain and has been linked to the effects of drugs that produce profound sensory and cognitive changes. Numerous studies have shown that this receptor is upregulated by a broad variety of stressors, and have related 5-HT2A receptor function to associative learning. This review proposes that stress, particularly stress related to danger and existential threats, increases the expression and function of 5-HT2A receptors. It is argued that this is a neurobiological adaptation to promote learning and avoidance of danger in the future. Upregulation of 5-HT2A receptors during stressful events forms associations that tune the brain to environmental cues that signal danger. It is speculated that life-threatening situations may activate this system and contribute to the symptoms associated with post-traumatic stress disorder (PTSD). 3,4-Methylenedioxymethamphetamine, which activates 5-HT2A receptors, has been successful in the treatment of PTSD and has recently achieved status as a breakthrough therapy. An argument is presented that 3,4-methylenedioxymethamphetamine may paradoxically act through these same 5-HT2A receptors to ameliorate the symptoms of PTSD. The central thematic contention is that a key role of serotonin may be to function as a stress detection and response system.

The Rise and Fall of Kappa-Opioid Receptors in Drug Abuse Research

Substance use disorders represent a global public health issue. This mental health disorder is hypothesized to result from neurobiological changes as a result of chronic drug exposure and clinically manifests as inappropriate behavioral allocation toward the procurement and use of the abused substance and away from other behaviors maintained by more adaptive nondrug reinforcers (e.g., social relationships, work). The dynorphin/kappa-opioid receptor (KOR) is one receptor system that has been altered following chronic exposure to drugs of abuse (e.g., cocaine, opioids, alcohol) in both laboratory animals and humans, implicating the dynorphin/KOR system in the expression, mechanisms, and treatment of substance use disorders. KOR antagonists have reduced drug self-administration in laboratory animals under certain experimental conditions, but not others. Recently, several human laboratory and clinical trials have evaluated the effectiveness of KOR antagonists as candidate pharmacotherapies for cocaine or tobacco use disorder to test hypotheses generated from preclinical studies. KOR antagonists failed to significantly alter drug use metrics in humans suggesting translational discordance between some preclinical drug self-administration studies and consistent with other preclinical drug self-administration studies that provide concurrent access to an alternative nondrug reinforcer (e.g., food). The implications of this translational discordance and future directions for examining the therapeutic potential of KOR agonists or antagonists as candidate substance use disorder pharmacotherapies are discussed.

Effects of early life stress on cocaine self-administration in post-pubertal male and female rhesus macaques

RATIONALE

Early life stress (ELS), including childhood maltreatment, is a predictive factor for the emergence of cocaine use disorders (CUDs) in adolescence.

OBJECTIVE

Accordingly, we examined whether post-pubertal male and female rhesus macaques that experienced infant maltreatment (maltreated, n = 7) showed greater vulnerability to cocaine self-administration in comparison with controls (controls, n = 7).

METHODS

Infant emotional reactivity was measured to assess differences in behavioral distress between maltreated and control animals as a result of early life caregiving. Animals were then surgically implanted with indwelling intravenous catheters and trained to self-administer cocaine (0.001-0.3 mg/kg/infusion) under fixed-ratio schedules of reinforcement. Days to acquisition, and sensitivity to (measured by the ED_Max dose of cocaine) and magnitude (measured by response rates) of the reinforcing effects of cocaine were examined in both groups.

RESULTS

Maltreated animals demonstrated significantly higher rates of distress (e.g., screams) in comparison with control animals. When given access to cocaine, control males required significantly more days to progress through terminal performance criteria compared with females and acquired cocaine self-administration slower than the other three experimental groups. The dose that resulted in peak response rates did not differ between groups or sex. Under 5-week, limited-access conditions, males from both groups had significantly higher rates of responding compared with females.

CONCLUSIONS

In control monkeys, these data support sex differences in cocaine self-administration, with females being more sensitive than males. These findings also suggest that ELS may confer enhanced sensitivity to the reinforcing effects of cocaine, especially in males.

Core Outcome Measures in Preclinical Assessment of Candidate Analgesics

All preclinical procedures for analgesic drug discovery involve two components: 1) a "pain stimulus" (the principal independent variable), which is delivered to an experimental subject with the intention of producing a pain state; and 2) a "pain behavior" (the principal dependent variable), which is measured as evidence of that pain state. Candidate analgesics are then evaluated for their effectiveness to reduce the pain behavior, and results are used to prioritize drugs for advancement to clinical testing. This review describes a taxonomy of preclinical procedures organized into an "antinociception matrix" by reference to their types of pain stimulus (noxious, inflammatory, neuropathic, disease related) and pain behavior (unconditioned, classically conditioned, operant conditioned). Particular emphasis is devoted to pain behaviors and the behavioral principals that govern their expression, pharmacological modulation, and preclinical-to-clinical translation. Strengths and weaknesses are compared and contrasted for procedures using each type of behavioral outcome measure, and the following four recommendations are offered to promote strategic use of these procedures for preclinical-to-clinical analgesic drug testing. First, attend to the degree of homology between preclinical and clinical outcome measures, and use preclinical procedures with behavioral outcome measures homologous to clinically relevant outcomes in humans. Second, use combinations of preclinical procedures with complementary strengths and weaknesses to optimize both sensitivity and selectivity of preclinical testing. Third, take advantage of failed clinical translation to identify drugs that can be back-translated preclinically as active negative controls. Finally, increase precision of procedure labels by indicating both the pain stimulus and the pain behavior in naming preclinical procedures.

BU10038 as a safe opioid analgesic with fewer side-effects after systemic and intrathecal administration in primates

BACKGROUND

The marked increase in mis-use of prescription opioids has greatly affected our society. One potential solution is to develop improved analgesics which have agonist action at both mu opioid peptide (MOP) and nociceptin/orphanin FQ peptide (NOP) receptors. BU10038 is a recently identified bifunctional MOP/NOP partial agonist. The aim of this study was to determine the functional profile of systemic or spinal delivery of BU10038 in primates after acute and chronic administration.

METHODS

A series of behavioural and physiological assays have been established specifically to reflect the therapeutic (analgesia) and side-effects (abuse potential, respiratory depression, itch, physical dependence, and tolerance) of opioid analgesics in rhesus monkeys.

RESULTS

After systemic administration, BU10038 (0.001-0.01 mg kg^-1) dose-dependently produced long-lasting antinociceptive and antihypersensitive effects. Unlike the MOP agonist oxycodone, BU10038 lacked reinforcing effects (i.e. little or no abuse liability), and BU10038 did not compromise the physiological functions of primates including respiration, cardiovascular activities, and body temperature at antinociceptive doses and a 10-30-fold higher dose (0.01-0.1 mg kg^-1). After intrathecal administration, BU10038 (3 μg) exerted morphine-comparable antinociception and antihypersensitivity without itch scratching responses. Unlike morphine, BU10038 did not cause the development of physical dependence and tolerance after repeated and chronic administration.

CONCLUSIONS

These in vivo findings demonstrate the translational potential of bifunctional MOP/NOP receptor agonists such as BU10038 as a safe, non-addictive analgesic with fewer side-effects in primates. This study strongly supports that bifunctional MOP/NOP agonists may provide improved analgesics and an alternative solution for the ongoing prescription opioid crisis.

Enrichment-induced differences in methamphetamine drug discrimination in male rats

Rats raised in an enriched environment show a decrease in sensitivity to the subjective effects of the psychostimulant d-amphetamine. The purpose of the present study was to determine if environmental enrichment during development alters the subjective effects of the more commonly abused drug methamphetamine. Male Sprague-Dawley rats were raised in either an enriched (EC) or an isolated condition (IC). EC and IC rats were trained on a two-lever operant procedure to discriminate 1.0 mg/kg (i.p.) methamphetamine from saline. Following acquisition of the discrimination a methamphetamine generalization curve (0.1-1.0 mg/kg) was determined. The antagonistic effects of dopamine D₁ receptor antagonist SCH23390 (0.0075-0.06 mg/kg) and the dopamine D₂ receptor antagonist eticlopride (0.01-0.3 mg/kg) were also tested. Finally, the ability of nicotine (0.05-0.5 mg/kg) to generalize and the ability of the nicotinic receptor antagonist mecamylamine (0.125-0.5 mg/kg) to antagonize the discriminative stimulus effects of methamphetamine were determined. EC rats were less sensitive to discriminative stimulus effects of methamphetamine compared to IC rats at a low 0.3 mg/kg dose and showed full antagonism of methamphetamine discrimination following SCH23390 compared to IC rats. There were no environmentally-induced differences in the effects of eticlopride. Nicotine only partially generalized to the effects of methamphetamine in both EC and IC rats. While mecamylamine failed to antagonize the effects of methamphetamine in either EC or IC rats. These results suggest that environmental enrichment decreases sensitivity to the discriminative effects of methamphetamine and the differences may be mediated through changes in the D₁ dopamine receptor.

Establishing performance characteristics for positive control article selection in drug self-administration studies

The selection of a controlled substance (CS) for use as the positive control article in a nonclinical drug abuse liability (DAL) assessment study should be contemplated carefully and with full understanding of the stated intent of the study design. Any CS that can maintain day-to-day stable baseline responding of voluntary intravenous intakes in animals may be selected under the current guidelines. Schedule I - IV CNS stimulants, depressants, and sedative/hypnotics can serve as maintenance drugs in these protocols, but not all of these compounds will provide comparatively efficient, robust, and stable daily intakes. Each Sponsor is directed to select a positive control article and training dose that will provide the most balanced, predictive, and scientifically-sound comparison consistent with the mechanism of action or therapeutic target of the test article. The SA study design is not a "one-size-fits-all" assay. This is a discussion of the critical design factors to be considered in selecting the most appropriate positive control article to use for a SA study.

Drug Discrimination: Historical Origins, Important Concepts, and Principles

Research on the stimulus properties of drugs began with studies on state dependent learning during the first half of the twentieth century. From that research, an entirely new approach evolved called drug discrimination. Animals (including humans) could discriminate the presence or absence of a drug; once learned, the drug could serve as a discriminative stimulus, signaling the availability or nonavailability of reinforcement. Early drug discrimination research involved the use of a T-maze task, which evolved in the 1970s into a two-lever operant drug discrimination task that is still used today. A number of important concepts and principles of drug discrimination are discussed. (1) The discriminative stimulus properties of drugs are believed in large part to reflect the subjective effects of drugs. While it has been impossible to directly measure subjective effects in nonhuman animals, drug discrimination studies in human subjects have generally supported the belief that discriminative stimulus properties of drugs in nonhuman animals correlate highly with subjective effects of drugs in humans. In addition to the ability of the drug discrimination procedure to measure the subjective effects of drugs, it has a number of other strengths that help make it a valuable preclinical assay. (2) Drug discrimination can be used for classification of drugs based on shared discriminative stimulus properties. (3) The phenomena of tolerance and cross-tolerance can be studied with drug discrimination. (4) Discriminative stimulus properties of drugs typically have been found to be stereospecific, if a drug is comprised of enantiomers. (5) Discriminative stimulus properties of drugs reflect specific CNS activity at neurotransmitter receptors. (6) Both human and nonhuman subjects display individual differences in their sensitivity to discriminative stimuli and drugs. (7) The drug discrimination procedure has been used extensively as a preclinical assay in drug development. This chapter is the first in the volume The Behavioural Neuroscience of Drug Discrimination, which includes chapters concerning the discriminative stimulus properties of various classes of psychoactive drugs as well as sections on the applications and approaches for using this procedure.

Effects of NOP-Related Ligands in Nonhuman Primates

The nociceptin/orphanin FQ peptide (NOP) receptor-related ligands have been demonstrated in preclinical studies for several therapeutic applications. This article highlights (1) how nonhuman primates (NHP) were used to facilitate the development and application of positron emission tomography tracers in humans; (2) effects of an endogenous NOP ligand, nociceptin/orphanin FQ, and its interaction with mu opioid peptide (MOP) receptor agonists; and (3) promising functional profiles of NOP-related agonists in NHP as analgesics and treatment for substance use disorders. NHP models offer the most phylogenetically appropriate evaluation of opioid and non-opioid receptor functions and drug effects. Based on preclinical and clinical data of ligands with mixed NOP/MOP receptor agonist activity, several factors including their intrinsic efficacies for activating NOP versus MOP receptors and different study endpoints in NHP could contribute to different pharmacological profiles. Ample evidence from NHP studies indicates that bifunctional NOP/MOP receptor agonists have opened an exciting avenue for developing safe, effective medications with fewer side effects for treating pain and drug addiction. In particular, bifunctional NOP/MOP partial agonists hold a great potential as (1) effective spinal analgesics without itch side effects; (2) safe, nonaddictive analgesics without opioid side effects such as respiratory depression; and (3) effective medications for substance use disorders.

Propylene glycol, a major electronic cigarette constituent, attenuates the adverse effects of high-dose nicotine as measured by intracranial self-stimulation in rats

BACKGROUND

Non-nicotine tobacco constituents may contribute to the abuse liability of tobacco products. We previously reported that electronic cigarette (EC) refill liquids containing nicotine and a range of non-nicotine constituents attenuated the anhedonic/aversive effects of nicotine in an intracranial self-stimulation (ICSS) model. The alcohol propylene glycol (PG) is a primary ingredient in these and other EC liquids, yet its abuse potential has not been established. The goal of this study was to evaluate the effects of parenteral administration of PG alone and PG combined with nicotine on ICSS in rats.

METHODS AND RESULTS

PG alone did not affect ICSS at concentrations up to 100%. PG (25% or 60%) did not affect nicotine's reinforcement-enhancing (ICSS threshold-decreasing) effects at low to moderate nicotine doses, but attenuated nicotine's reinforcement-attenuating/aversive (ICSS threshold-increasing) effects at a high nicotine dose. PG concentrations similar to those in EC liquid doses used in our previous studies (1% or 3%) modestly attenuated the ICSS threshold-elevating effects of a high nicotine dose.

CONCLUSIONS

PG attenuated elevations in ICSS thresholds induced by high-dose nicotine, which may reflect an attenuation of nicotine's acute aversive/anhedonic and/or toxic effects. PG may have contributed to the attenuated ICSS threshold-elevating effects of EC liquids reported previously. Further examination of PG in models of addiction and toxicity is needed to understand the consequences of EC use and to inform the development of EC product standards by the FDA.

The G-protein biased mu-opioid agonist, TRV130, produces reinforcing and antinociceptive effects that are comparable to oxycodone in rats

Mu-opioid agonists (e.g., oxycodone) are highly effective therapeutics for pain. However, they also produce reinforcing effects that increase their likelihood of abuse. Recent strategies in drug development have focused on opioids with biased receptor-signaling profiles that favor activation of specific intracellular pathways over others with the aim of increasing therapeutic selectivity. TRV130, a mu agonist biased towards G-protein signaling, produces antinociceptive effects comparable to the mu agonist, morphine, but exhibits reduced side effects. However, in terms of abuse potential, we know of no published preclinical data investigating the effects of TRV130 as a reinforcer. In the present study, we assessed the relative reinforcing effects of TRV130 and oxycodone, a commonly-prescribed mu agonist, in rats self-administering the drugs under a progressive-ratio (PR) schedule of reinforcement. In addition, we assessed the relative potency and efficacy of TRV130 and oxycodone in rats in a test of thermal antinociception (Hot Plate). For self-administration, male Sprague-Dawley rats (n = 7) self-administered intravenous infusions of TRV130 or oxycodone (0.01-0.32 mg/kg/inj) under a PR schedule of reinforcement. For the Hot-Plate test, male rats (n = 7) received subcutaneous injections of TRV130 (0.1-3.2 mg/kg/inj) or oxycodone (0.1-5.6 mg/kg/inj), and nociceptive response latencies were measured. TRV130 and oxycodone were equi-potent and equi-effective in self-administration and thermal antinociception. This study demonstrates that TRV130 produces reinforcing and antinociceptive effects that are quantitatively similar to oxycodone, and that a biased-signaling profile does not necessarily reduce abuse potential.

Consortium on Methods Evaluating Tobacco: Research Tools to Inform US Food and Drug Administration Regulation of Snus

Introduction

The US Food and Drug Administration (FDA) has purview over tobacco products. To set policy, the FDA must rely on sound science, yet most existing tobacco research methods have not been designed to specifically inform regulation. The NCI and FDA-funded Consortium on Methods Evaluating Tobacco (COMET) was established to develop and assess valid and reliable methods for tobacco product evaluation. The goal of this article is to describe these assessment methods using a US manufactured "snus" as the test product.

Methods

In designing studies that could inform FDA regulation, COMET has taken a multidisciplinary approach that includes experimental animal models and a range of human studies that examine tobacco product appeal, addictiveness, and toxicity. This article integrates COMET's findings over the last 4 years.

Results

Consistency in results was observed across the various studies, lending validity to our methods. Studies showed low abuse liability for snus and low levels of consumer demand. Toxicity was less than cigarettes on some biomarkers but higher than medicinal nicotine.

Conclusions

Using our study methods and the convergence of results, the snus that we tested as a potential modified risk tobacco product is likely to neither result in substantial public health harm nor benefit.

Implications

This review describes methods that were used to assess the appeal, abuse liability, and toxicity of snus. These methods included animal, behavioral economics, consumer perception studies, and clinical trials. Across these varied methods, study results showed low abuse-liability and appeal of the snus product we tested. In several studies, demand for snus was lower than for less toxic nicotine gum. The consistency and convergence of results across a range of multi-disciplinary studies lends validity to our methods and suggests that promotion of snus as a modified risk tobacco products is unlikely to produce substantial public health benefit or harm.

A review of nonhuman primate models of early life stress and adolescent drug abuse

Adolescence represents a developmental stage in which initiation of drug use typically occurs and is marked by dynamic neurobiological changes. These changes present a sensitive window during which perturbations to normative development lead to alterations in brain circuits critical for stress and emotional regulation as well as reward processing, potentially resulting in an increased susceptibility to psychopathologies. The occurrence of early life stress (ELS) is related to a greater risk for the development of substance use disorders (SUD) during adolescence. Studies using nonhuman primates (NHP) are ideally suited to examine how ELS may alter the development of neurobiological systems modulating the reinforcing effects of drugs, given their remarkable neurobiological, behavioral, and developmental homologies to humans. This review examines NHP models of ELS that have been used to characterize its effects on sensitivity to drug reinforcement, and proposes future directions using NHP models of ELS and drug abuse in an effort to develop more targeted intervention and prevention strategies for at risk clinical populations.

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ELS has long-lasting neurobiological and behavioral consequences.

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ELS is a major risk factor for the initiation of adolescent drug use.

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Sex differences are apparent in the consequences of ELS, including drug use.

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Nonhuman primate models of ELS are critical for understanding ELS effects on neurobiology and risk for adolescent drug use.

A bifunctional nociceptin and mu opioid receptor agonist is analgesic without opioid side effects in nonhuman primates

Misuse of prescription opioids, opioid addiction, and overdose underscore the urgent need for developing addiction-free effective medications for treating severe pain. Mu opioid peptide (MOP) receptor agonists provide very effective pain relief. However, severe side effects limit their use in the clinical setting. Agonists of the nociceptin/orphanin FQ peptide (NOP) receptor have been shown to modulate the antinociceptive and reinforcing effects of MOP agonists. We report the discovery and development of a bifunctional NOP/MOP receptor agonist, AT-121, which has partial agonist activity at both NOP and MOP receptors. AT-121 suppressed oxycodone's reinforcing effects and exerted morphine-like analgesic effects in nonhuman primates. AT-121 treatment did not induce side effects commonly associated with opioids, such as respiratory depression, abuse potential, opioid-induced hyperalgesia, and physical dependence. Our results in nonhuman primates suggest that bifunctional NOP/MOP agonists with the appropriate balance of NOP and MOP agonist activity may provide a dual therapeutic action for safe and effective pain relief and treating prescription opioid abuse.

Touchscreen technology in the study of cognition-related behavior

There is a growing need for new translational animal models designed to capture complex behavioral phenotypes implicated in addiction and other neuropsychiatric conditions. For example, a complete understanding of the effects of commonly abused drugs, as well as candidate medications, requires assessments of their effects on learning, memory, attention, and other cognition-related behavior. Modern touch-sensitive technology provides an extremely flexible means to expose an experimental subject to a variety of complex behavioral tasks designed to assay dimensions of cognitive function before, during, and after drug administration. In addition to tailored variants of gold-standard cognitive assessments, touchscreen chambers offer the ability to develop novel tasks based upon the researcher's needs. This methods perspective presents (i) a brief review of previous touchscreen-based animal studies, (ii) a primer on the construction of a touch-sensitive experimental chamber, and (iii) data from a proof-of-concept study examining cross-species continuity in performance across a diverse assortment of animal subjects (rats, marmosets, squirrel monkeys, and rhesus macaques) using the repeated acquisition task - a modern variant of a traditional animal model of learning. Taken together, the procedures and data discussed in this review illustrate the point that contemporary touchscreen methodology can be tailored to desired experimental goals and adapted to provide formal similarity in cognition-related tasks across experimental species. Moreover, touchscreen methodology allows for the development of new translational models that emerge through laboratory and clinical discovery to capture important dimensions of complex behavior and cognitive function.

Status and Future Directions of Preclinical Behavioral Pharmacology in Tobacco Regulatory Science

Behavioral pharmacology is a branch of the experimental analysis of behavior that has had great influence in drug addiction research and policy. This paper provides an overview of recent behavioral pharmacology research in the field of tobacco regulatory science, which provides the scientific foundation for the Food and Drug Administration Center for Tobacco Products (FDA CTP) to set tobacco control policies. The rationale and aims of tobacco regulatory science are provided, including the types of preclinical operant behavioral models it deems important for assessing the abuse liability of tobacco products and their constituents. We then review literature relevant to key regulatory actions being considered by the FDA CTP, including regulations over nicotine and menthol content of cigarettes, and conclude with suggesting some directions for future research. The current era of tobacco regulatory science provides great opportunities for behavioral pharmacologists to address the leading cause of preventable death and disease worldwide.

Substitutability of nicotine alone and an electronic cigarette liquid using a concurrent choice assay in rats: A behavioral economic analysis

BACKGROUND

For the Food and Drug Administration to effectively regulate tobacco products, the contribution of non-nicotine tobacco constituents to the abuse liability of tobacco must be well understood. Our previous work compared the abuse liability of electronic cigarette refill liquids (EC liquids) and nicotine (Nic) alone when each was available in isolation and found no difference in abuse liability (i.e., demand elasticity). Another, and potentially more sensitive measure, would be to examine abuse liability in a choice context, which also provides a better model of the tobacco marketplace.

METHODS

Demand elasticity for Nic alone and an EC liquid were measured when only one formulation was available (alone-price demand) and when both formulations were concurrently available (own-price demand), allowing an assessment of the degree to which each formulation served as a substitute (cross-price demand) when available at a low fixed-price.

RESULTS

Own-price demand for both formulations were more elastic compared to alone-price demand, indicating that availability of a substitute increased demand elasticity. During concurrent access, consumption of the fixed-price formulation increased as the unit-price of the other formulation increased. The rate of increase was similar between formulations, indicating that they served as symmetrical substitutes.

CONCLUSION

The cross-price model reliably quantified the substitutability of both nicotine formulations and indicated that the direct CNS effects of non-nicotine constituents in EC liquid did not alter its abuse liability compared to Nic. These data highlight the sensitivity of this model and its potential utility for examining the relative abuse liability and substitutability of tobacco products.

Evaluating the abuse potential of psychedelic drugs as part of the safety pharmacology assessment for medical use in humans

Psychedelics comprise drugs come from various pharmacological classes including 5-HT_2A agonists, indirect 5-HT agonists, e.g., MDMA, NMDA antagonists and κ-opioid receptor agonists. There is resurgence in developing psychedelics to treat psychiatric disorders with high unmet clinical need. Many, but not all, psychedelics are schedule 1 controlled drugs (CDs), i.e., no approved medical use. For existing psychedelics in development, regulatory approval will require a move from schedule 1 to a CD schedule for drugs with medical use, i.e., schedules 2-5. Although abuse of the psychedelics is well documented, a systematic preclinical and clinical evaluation of the risks they pose in a medical-use setting does not exist. We describe the non-clinical tests required for a regulatory evaluation of abuse/dependence risks, i.e., drug-discrimination, intravenous self-administration and physical dependence liability. A synopsis of the existing data for the various types of psychedelics is provided and we describe our findings with psychedelic drugs in these models. FDA recently issued its guidance on abuse/dependence evaluation of drug-candidates (CDER/FDA, 2017). We critically review the guidance, discuss the impact this document will have on non-clinical abuse/dependence testing, and offer advice on how non-clinical abuse/dependence experiments can be designed to meet not only the expectations of FDA, but also other regulatory agencies. Finally, we offer views on how these non-clinical tests can be refined to provide more meaningful information to aid the assessment of the risks posed by CNS drug-candidates for abuse and physical dependence. This article is part of the Special Issue entitled 'Psychedelics: New Doors, Altered Perceptions'.