Cocaine and amphetamine attenuate the discriminative stimulus effects of naltrexone in opioid-dependent rhesus monkeys

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.

Modulation of drug choice by extended drug access and withdrawal in rhesus monkeys: Implications for negative reinforcement as a driver of addiction and target for medications development

Chronic drug exposure is hypothesized to recruit negative reinforcement processes that increase the magnitude and alter the mechanisms of drug reinforcement. Candidate substrates of negative reinforcement include increased signaling via stress-related neurotransmitters such as corticotropin releasing factor (CRF, acting at CRF receptors) or dynorphin (acting at kappa opioid receptors) and/or decreased signaling via reward-related neurotransmitters such as dopamine. Determinants of drug reinforcement can be examined with choice procedures, in which subjects choose between a drug of interest (e.g. heroin or cocaine) and a non-drug alternative reinforcer (e.g. food). This review summarizes evidence collected from studies of drug choice in rhesus monkeys to address the negative reinforcement hypothesis. In monkeys choosing between heroin and food, chronic heroin exposure and subsequent withdrawal produces a robust increase in heroin choice. This withdrawal-associated increase in heroin choice is blocked by morphine and by other mu opioid agonists used to treat opioid use disorder (methadone, buprenorphine); however, withdrawal-associated increases in heroin choice are not reliably blocked by antagonists of CRF or kappa opioid receptors or by an indirect dopamine agonist. In monkeys choosing between cocaine and food, chronic cocaine exposure and withdrawal fail to increase cocaine choice or alter sensitivity of cocaine choice to treatment with candidate therapeutics including an indirect dopamine agonist and a kappa opioid receptor antagonist. These results support a role for negative reinforcement in self-administration of heroin but not cocaine. The constellation of neurobiological changes that constitutes the negative reinforcing stimulus in opioid-dependent rhesus monkeys remains to be determined.

Medications development for opioid abuse

Here we describe methods for preclinical evaluation of candidate medications to treat opioid abuse and dependence. Our perspective is founded on the propositions that (1) drug self-administration procedures provide the most direct method for assessment of medication effects, (2) procedures that assess choice between opioid and nondrug reinforcers are especially useful, and (3) the states of opioid dependence and withdrawal profoundly influence both opioid reinforcement and the effects of candidate medications. Effects of opioid medications 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, and (2) continued development of nonopioids such as inhibitors of endocannabinoid catabolic enzymes or inhibitors of opioid-induced glial activation.

Some effects of dopamine transporter and receptor ligands on discriminative stimulus, physiologic, and directly observable indices of opioid withdrawal in rhesus monkeys

RATIONALE

Some monoamine uptake inhibitors (e.g., cocaine) attenuate the subjective and discriminative stimulus effects of opioid withdrawal.

OBJECTIVE

This study examined a role for dopamine transporters and receptors as targets for drugs to modify the discriminative stimulus effects of opioid withdrawal and further examined a subset of these drugs for their capacity to modify some directly observable and physiologic indices of withdrawal.

MATERIALS AND METHODS

Rhesus monkeys receiving 2 mg/kg/day of L: -alpha-acetylmethadol discriminated the opioid antagonist naltrexone (0.0178 mg/kg s.c.).

RESULTS

The naltrexone discriminative stimulus was attenuated not only by the mu agonist morphine but also by the dopamine D(2)-like receptor agonists bromocryptine and quinpirole. In contrast, the naltrexone discriminative stimulus was not consistently modified by the non-selective, D(1)- and D(2)-like agonist apomorphine or by uptake inhibitors with high selectivity for dopamine transporters (GBR 12909, RTI 113, and RTI 177). In the same monkeys, naltrexone dose dependently decreased body temperature, increased breathing frequency, and induced directly observable signs (grimacing, salivation, and unusual posture). Hypothermia, hyperventilation, and signs of withdrawal were significantly attenuated by morphine and not by quinpirole.

CONCLUSIONS

Attenuation of opioid withdrawal by D(2)-like receptor agonists that have lower efficacy than dopamine, and not by uptake inhibitors with selectivity for dopamine transporters, suggests that magnitude of receptor stimulation (e.g., efficacy) and selectivity at dopamine receptors are important factors in the modulation of opioid withdrawal. Attenuation of the naltrexone discriminative stimulus by drugs that inhibit both dopamine and serotonin uptake (e.g., cocaine) could result from an inhibitory effect of serotonin on dopamine. The role of dopamine in opioid withdrawal appears to be restricted to subjective (i.e., not somatic) indices [corrected]

Mechanisms of withdrawal-associated increases in heroin self-administration: pharmacologic modulation of heroin vs food choice in heroin-dependent rhesus monkeys

Opioid withdrawal can produce a constellation of physiological and behavioral signs, including an increase in opioid self-administration. Different mechanisms mediate different withdrawal signs, and the present study used pharmacologic tools to assess mechanisms underlying withdrawal-associated increases in opioid reinforcement. Five rhesus monkeys were rendered heroin dependent via daily 21-h heroin self-administration sessions. One hour after each heroin self-administration session, monkeys chose between heroin (0-0.1 mg/kg per injection) and food (1 g pellets) during 2-h choice sessions. Under these conditions, heroin maintained a dose-dependent increase in heroin choice, such that monkeys responded primarily for food when low heroin doses were available (0-0.01 mg/kg per injection) and primarily for heroin when higher heroin doses were available (0.032-0.1 mg/kg per injection). Periods of spontaneous withdrawal were intermittently introduced by omitting one 21-h heroin self-administration session, and test drugs were administered during these withdrawal periods. Untreated withdrawal robustly increased heroin choice during choice sessions. Withdrawal-associated increases in heroin choice were completely suppressed by the mu opioid agonist morphine (0.032-0.32 mg/kg/h, i.v.), but not by the alpha-2 noradrenergic agonist clonidine (0.01-0.1 mg/kg/h, i.v.), the dopamine/norepinephrine releaser amphetamine (0.032-0.1 mg/kg/h, i.v.), or the kappa-opioid antagonist 5'-guanidinonaltrindole (1.0 mg/kg, i.m.). The corticotropin-releasing factor 1 antagonist antalarmin (1.0-10 mg/kg per day, i.m.) produced a morphine-like suppression of withdrawal-associated increases in heroin choice in one of three monkeys. These results suggest that mechanisms of withdrawal-associated increases in the relative reinforcing efficacy of opioid agonists may be different from mechanisms of many other somatic, mood-related, and motivational signs of opioid withdrawal.

Antagonist-precipitated and discontinuation-induced withdrawal in morphine-dependent rhesus monkeys

RATIONALE

Upon discontinuation of chronic opioid treatment, withdrawal typically peaks in 1-3 days and decreases markedly within 1 week; however, persistent physiological changes have been reported long after other signs have waned.

OBJECTIVE

The goal of this study was to compare the discriminative stimulus, directly observable signs, and physiological effects of withdrawal in morphine-treated monkeys.

MATERIALS AND METHODS

Monkeys received 5.6 mg/kg/12 h morphine and discriminated 0.0178 mg/kg naltrexone while responding under a fixed-ratio 5 schedule of stimulus-shock termination. Drug discrimination, behavioral observation, and telemetry were used to monitor the emergence of withdrawal, as well as any persistent changes, following discontinuation of morphine treatment.

RESULTS

Naltrexone dose (0.001-0.032 mg/kg, s.c.) was positively related with indices of withdrawal. In the discrimination study, monkeys responded on the naltrexone lever 1-5 days following discontinuation of treatment; thereafter, they responded exclusively on the saline lever. After discontinuation of morphine, the frequency of observable signs peaked within 2-3 days and most were not significantly increased after 5 days. In contrast, increased heart rate and body temperature persisted for 14 days, returning to values obtained prior to discontinuation by 21 days.

CONCLUSIONS

To the extent that discriminative stimulus effects of withdrawal in nonhumans are predictive of subjective reports of withdrawal in humans, these data indicate that effective treatments for opioid dependence must address not only the short-term subjective components of withdrawal but also, and perhaps more importantly, lingering behavioral and physiological effects that might contribute to relapse long after chronic drug use is discontinued.

Monoaminergic drugs and directly observable signs of LAAM withdrawal in rhesus monkeys

Monoaminergic ligands modified a naltrexone discriminative stimulus in rhesus monkeys dependent on 2 mg/kg per day of the mu opioid L-alpha-acetylmethadol (LAAM). This study examined a role for monoamines in the directly observable and physiologic manifestations of LAAM withdrawal induced by naltrexone in the same monkeys. The effects of saline, clonidine (0.032 mg/kg), haloperidol (0.032 mg/kg), cocaine (1.0 mg/kg), amphetamine (1.0 mg/kg) and imipramine (10.0 mg/kg) were examined in LAAM-dependent monkeys that subsequently received saline or naltrexone (0.0001-1.0 mg/kg). Naltrexone dose-dependently increased respiration, abdominal rigidity and salivation. Clonidine attenuated each of these withdrawal signs, whereas haloperidol increased some (i.e. respiration) and decreased others (i.e. salivation). When administered alone, cocaine and amphetamine increased respiration and also increased the respiratory stimulant effects of naltrexone; cocaine and amphetamine did not attenuate any measure of withdrawal. With the exception of a decrease in naltrexone-induced salivation, imipramine was without effect. These results are strikingly different from results in these same LAAM-dependent monkeys showing that cocaine and amphetamine, but not clonidine, markedly attenuated a naltrexone discriminative stimulus. That monoaminergic ligands differentially alter the directly observable and discriminative stimulus effects of naltrexone in LAAM-dependent monkeys supports the view that monoamines differentially mediate the physical manifestations (norepinephrine) and subjective experience (dopamine) of opioid withdrawal.

Effects of chronic methadone treatment on cocaine- and food-maintained responding under second-order, progressive-ratio and concurrent-choice schedules in rhesus monkeys

The effects of chronic infusion with saline or methadone (0.032-1.0 mg/kg/h) were examined on cocaine- and food-maintained responding in rhesus monkeys using three procedures. In one procedure, cocaine injections (0.0032-0.032 mg/kg per injection) and food pellets were available under a second-order schedule during alternating daily sessions. During saline treatment, cocaine maintained a dose-dependent increase in the number of cocaine injections per day, and monkeys usually responded for the maximum number of pellets. Methadone dose-dependently decreased cocaine self-administration, and methadone doses that decreased cocaine self-administration had variable effects on food-maintained responding. In the second procedure, 0.032 mg/kg per injection cocaine or food pellets were available under a progressive-ratio schedule. During saline treatment, cocaine and food maintained similar break points. Methadone produced a dose-dependent and non-selective decrease in break points maintained by both cocaine and food. In the third procedure, cocaine injections (0-0.1 mg/kg per injection) and food pellets were available under a concurrent-choice schedule. During saline treatment, increasing unit doses of cocaine produced a dose-dependent increase in cocaine choice. Methadone had little effect on the cocaine choice dose-effect curve up to doses that eliminated responding. These results provide little evidence to suggest that chronic methadone altered the reinforcing effects of cocaine; rather methadone appeared to non-selectively decrease rates of operant responding.

Discriminative stimulus effects of the cannabinoid antagonist, SR 141716A, in delta -sup-9-tetrahydrocannabinol-treated rhesus monkeys

This study examined whether the cannabinoid antagonist, SR 141716A, could be established as a discriminative stimulus in rhesus monkeys treated with delta -sup-9-tetrahydrocannabinol (delta -sup-9-THC). Stimulus control was established with SR 141716A (1.0 mg/kg) in 3 delta -sup-9-THC-treated monkeys (1.12 mg/kg/day) in 113-124 sessions. The SR 141716A discriminative stimulus was dose related, attenuated by an acute injection of delta -sup-9-THC, and not mimicked by cocaine or ketamine. SR 141716A-appropriate responding occasioned by temporary discontinuation of delta -sup-9-THC treatment was attenuated by delta -sup-9-THC and not ketamine. The SR 141716A discriminative stimulus in delta -sup-9-THC-treated monkeys appears to be mediated by cannabinoid receptors and could be related to delta -sup-9-THC withdrawal.

Cocaine and Other Indirect-Acting Monoamine Agonists Differentially Attenuate a Naltrexone Discriminative Stimulus in Morphine-Treated Rhesus Monkeys

Monoaminergic drugs can modify opioid withdrawal in nonhumans, and cocaine is reported to attenuate opioid withdrawal in humans. Drug discrimination was used to examine whether s.c. cocaine or other indirect-acting monoamine agonists attenuate morphine (3.2 mg/kg/day) withdrawal induced by naltrexone and by 27 h of morphine deprivation. Naltrexone-precipitated withdrawal was attenuated not only by morphine but also by cocaine, amphetamine, and imipramine. However, reversal of naltrexone-precipitated withdrawal was greater for morphine than for any of the indirect-acting monoamine agonists. Attenuation of the naltrexone discriminative stimulus by indirect-acting monoamine agonists was pharmacologically selective insofar as drugs lacking affinity for monoamine transporters (ketamine and triazolam) were without effect. Twenty-seven hours of morphine deprivation occasioned naltrexone-lever responding and decreased response rate, and both effects were reversed by morphine, cocaine, and amphetamine and not by imipramine, desipramine, ketamine, and triazolam. Thus, indirect-acting monoamine agonists attenuate some (e.g., discriminative) aspects of naltrexone-precipitated withdrawal, whereas only indirect-acting agonists with high affinity for dopamine transporters attenuate deprivation-induced withdrawal. These results suggest that dopamine is differentially involved in naltrexone- and deprivation-induced withdrawal and support the notion that opioid-dependent individuals use stimulants, in part, to attenuate withdrawal.

Relative efficacy of buprenorphine, nalbuphine and morphine in opioid-treated rhesus monkeys discriminating naltrexone

Efficacy is one determinant of whether a drug is an agonist or an antagonist under a particular set of conditions. Relative efficacy among the micro opioid receptor (MOR) ligands buprenorphine, nalbuphine, and morphine was examined in monkeys dependent on morphine (3.2 mg/kg/day) or l-alpha-acetylmethadol (LAAM) (1.0 mg/kg twice daily) and that discriminated naltrexone (0.0178 mg/kg) from saline. In morphine-treated monkeys, buprenorphine and not nalbuphine substituted for naltrexone. When administered before naltrexone in morphine-treated monkeys, morphine and nalbuphine shifted the naltrexone dose-effect curve to the right, while buprenorphine shifted the naltrexone dose-effect curve to the left. Under conditions of acute morphine deprivation, naltrexone-lever responding was slightly attenuated by buprenorphine and markedly attenuated by nalbuphine and morphine. In LAAM-treated monkeys, buprenorphine substituted completely for naltrexone in only one monkey, while nalbuphine and morphine failed to substitute in any monkey. When administered before naltrexone in LAAM-treated monkeys, buprenorphine, nalbuphine, and morphine dose dependently shifted the naltrexone dose-effect curve to the right, with the exception of one monkey in which buprenorphine shifted the naltrexone dose-effect curve to the left. These results demonstrate that a low efficacy MOR ligand can exert agonist or antagonist actions in the same animal depending on immediate pharmacologic history. The qualitatively different effects of buprenorphine in morphine- and LAAM-treated monkeys might be related to magnitude of dependence insofar as dependence can determine the efficacy required for agonist activity. Thus, buprenorphine has markedly different effects across different levels of opioid dependence.