Differential effects of the novel kappa opioid receptor antagonist, JDTic, on reinstatement of cocaine-seeking induced by footshock stressors vs cocaine primes and its antidepressant-like effects in rats

Rationale Stress and depression have been linked to relapse of cocaine abuse. Antagonism of the kappa opioid receptor (KOR) has been reported to attenuate some effects of stressors, and antagonism of the KOR has been reported to have antidepressant-like properties.Objectives Our objective was to determine whether the potent and selective KOR antagonist, (3R)-7-hydroxy-N-{(1S)-1-{[(3R,4R)-4-(3-hydroxyphenyl)-3,4-dimethyl-1-piperidinyl]methyl}-2-methylpropyl}-1,2,3,4-tetrahydro-3-isoquinoline-carboxamide (JDTic), can reduce the ability of a stressor (intermittent footshock) to reinstate cocaine-seeking behavior and to have antidepressant-like effects in the forced swim test (FST).Methods Male Long–Evans hooded rats were trained to lever-press, reinforced with 0.5 mg/kg i.v. infusion of cocaine, according to fixed ratio 1 reinforcement schedules during daily 2-h experimental sessions. After performance had stabilized, lever pressing was extinguished for 12 consecutive sessions, and doses of 0 (vehicle), 3, 10, and 30 mg/kg JDTic were then administered i.g. to separate groups of 12 rats. Twenty four hours later, the rats were given 15 min of intermittent footshock (0.87 mA, 0.5 s activation time, average interactivation interval of 40 s) or a 17-mg/kg i.p. administration of cocaine prime followed by a 2-h reinstatement test session. JDTic was also evaluated for its ability to block diuresis induced by the KOR agonist, U50,488H (10 mg/kg, s.c.), during 5-h test sessions beginning 1 h after footshock reinstatement tests to verify its KOR antagonist activity. In the FST, male Sprague–Dawley rats were treated with either nor-binaltorphimine (nor-BNI) or JDTic (both at 0.3, 1, 3, or 10 mg/kg, injected s.c. 23 h before), or desipramine (5.6, 10, or 17 mg/kg, injected i.p. 23, 5, and 1 h before) and placed in a cylinder of water, during which the predominance of immobility, swimming, and climbing were scored during 5-s intervals for 5 min.Results The 10- and 30-mg/kg doses of JDTic significantly reduced footshock-induced reinstatement of responding previously reinforced by cocaine and significantly attenuated U50,488H-induced diuresis. In contrast, JDTic did not affect cocaine-prime-induced reinstatement. Both nor-BNI and JDTic decreased immobility and increased swimming time in the FST, similar to the antidepressant desipramine.Conclusions Depression and stress are two states during cocaine abstinence which users identify as precipitating relapse, and JDTic may have properties which attenuate both.     

Effects of the kappa opioid agonist U50,488 and the kappa opioid antagonist nor-binaltorphimine on choice between cocaine and food in rhesus monkeys

Rationale Selective kappa opioid receptor agonists usually decrease cocaine self-administration in procedures that use rate-based measures of reinforcement; however, the rate-altering effects of kappa agonists complicate interpretation of these findings.Objectives To evaluate the effects of the selective kappa agonist U50,488 and the selective kappa antagonist nor-binaltorphimine (nor-BNI) on concurrent choice between cocaine and food in rhesus monkeys. The concurrent-choice procedure provides a rate-independent measure of the relative reinforcing effects of cocaine in comparison with food.Methods Four rhesus monkeys were trained to respond under a concurrent-choice schedule for food (1-g pellets) or cocaine (0–0.1 mg/kg per injection). Saline and increasing doses of U50,488 (0.0032–0.1 mg/kg per h) were administered by pseudo-continuous i.v. infusion (one infusion every 20 min) during sequential 3-day blocks. In a separate experiment, monkeys were treated with nor-BNI (3.2 mg/kg, i.v.), and cocaine choice was re-determined during pseudo-continuous infusion with saline or U50,488 (0.1 mg/kg per h).Results During saline treatment, cocaine maintained a dose-dependent and monotonic increase in cocaine choice. Monkeys responded primarily for food when low cocaine doses were available (0–0.01 mg/kg per injection) and primarily for cocaine when higher cocaine doses were available (0.032–0.1 mg/kg per injection). U50,488 produced a dose-dependent increase in cocaine choice, manifested as leftward shifts in the cocaine-choice, dose–effect curve. U50,488 also dose-dependently decreased overall response rates. Nor-BNI did not alter cocaine choice, but it attenuated the effects of U50,488.Conclusions These results suggest that continuous treatment with U50,488 produces a kappa receptor-mediated increase in the relative reinforcing effects of cocaine in comparison with food.     

Inhibition of kappa opioid receptors attenuated increased cocaine intake in rats with extended access to cocaine

Objective  Previous studies demonstrated that the dynorphin/κ opioid system was up-regulated upon repeated cocaine self-administration. In the present study, we tested the hypothesis that increased cocaine self-administration with extended access was associated with increased activity of the κ opioid system in rats. Materials and methods  Rats self-administered 0.5 mg/kg per injection of cocaine on a fixed-ratio (FR) schedule in either 1-h (short access, ShA) or 6-h (long access, LgA) sessions. After cocaine intake in the LgA rats increased to a maximum, the effects of κ opioid receptor antagonists and a partial agonist were tested on cocaine intake in ShA and LgA rats. Results  Cocaine self-administration increased under FR and progressive-ratio (PR) schedules in LgA rats. Nor-BNI (15–30 mg/kg), a κ receptor antagonist, decreased cocaine intake in LgA rats under a PR schedule (ShA, +1.7%; LgA, −27.4% from baseline), whereas naltrexone (0.3–10 mg/kg) and SG-II-49 (0.025–0.1 mg/kg), a nonspecific opioid receptor antagonist and a partial agonist, respectively, decreased cocaine intake in both groups (PR data: SG-II-49, ShA −28.6%, LgA −19.8%; naltrexone, ShA −34.6%, LgA −11.8% compared with vehicle data). Conclusions  The present study demonstrated that the antagonism of κ opioid receptors attenuated only the increased cocaine intake in LgA rats under a PR schedule, whereas the antagonism of μ and κ receptors decreased cocaine intake in both ShA and LgA groups. The data suggest that increased motivation for cocaine in rats with extended access may be related to increased κ opioid activity and may contribute to compulsive use. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Deep brain stimulation of the nucleus accumbens reduces ethanol consumption in rats

Recent studies have shown that deep brain stimulation (DBS) of the nucleus accumbens (NAcc) has an inhibitory effect on drug-seeking behaviors including reinstatement responding for cocaine. The objective of the present study was to expand on these findings by assessing the effects of DBS on behaviors related to alcohol consumption. The specific aim of this study was to determine whether DBS delivered to either the shell or core of the NAcc would reduce ETOH intake in rats using a two-bottle choice limited access procedure. Long Evans rats were induced to drink a 10% ethanol solution using a saccharin fading procedure. Bipolar electrodes were implanted bilaterally into either the core or shell of the NAcc. During testing animals received DBS 5 min prior to and during a 30-minute test session in which both ETOH and water bottles were accessible. Current was delivered at amplitudes ranging from 0 to 150 µA. ETOH consumption was significantly reduced from baseline levels at the 150 µA current for both shell and core electrode placements. A significant current effect was not found for water consumption for either site. These results provide evidence that DBS delivered either to the nucleus accumbens core or shell subregions can significantly reduce ethanol intake in the rat.     

Kappa opioid antagonist effects of the novel kappa antagonist 5'-guanidinonaltrindole (GNTI) in an assay of schedule-controlled behavior in rhesus monkeys

Rationale: Opioid receptors are divided into three types: kappa, mu, and delta receptors. Receptor-selective antagonists are useful experimental tools for evaluation of opioid receptor-mediated processes. 5′-Guanidinonaltrindole (GNTI) was recently developed as a novel kappa-selective antagonist. Objectives: To evaluate the potency, time course, and selectivity of GNTI's opioid antagonist effects in rhesus monkeys in an assay of schedule-controlled responding. Methods: Five rhesus monkeys were trained to respond under a fixed ratio 30 schedule of food reinforcement. The rate-decreasing effects of the kappa agonists U50,488 and U69,593, the mu agonist morphine, and the delta agonist SNC80 were examined alone and after pretreatment with GNTI (0.1 and 1.0 mg/kg i.m.; 1 h to 14 days). Results: U50,488, U69,593, morphine, and SNC80 dose-dependently decreased response rates in this procedure. GNTI produced a dose- and time-dependent antagonism of the rate-decreasing effects of U50,488. The kappa antagonist effects of GNTI had a slow onset and a long duration of action, and peak antagonist effects were observed after 24 h. A higher dose of 3.2 mg/kg GNTI eliminated responding in one monkey and was not studied further. The antagonist effects of GNTI were kappa selective, because 1.0 mg/kg GNTI also antagonized the effects of U69,593, but not those of morphine or SNC80. Conclusions: These results suggest that GNTI is a potent and selective kappa antagonist with a slow onset and long duration of action in rhesus monkeys. Relative to the prototype kappa antagonist nor-binaltorphimine, GNTI may have some advantages as a tool for the study of kappa receptor-mediated processes. Electronic Publication     

Voluntary consumption of ethanol in 15 inbred mouse strains

To determine genetic differences in ethanol consumption, 15 commonly used inbred strains of mice were given ad libitum two-bottle choice between ethanol, 0.2% saccharin, or ethanol plus saccharin in one bottle versus tap water in the other bottle. Three different concentrations of ethanol were used: 3%, 6% and 10% (v/v). Of the 15 strains, the C57BL/6J, C57BR/cdJ and C57L/J strains showed the most consistent higher intake of ethanol either with or without 0.2% saccharin. In marked contrast, the DBA/1J and DBA/2J strains consistently showed the lowest intake. Consumption of 3% ethanol without saccharin was highly genetically correlated with saccharin consumption (r=0.77), suggesting that low concentrations of ethanol may have a sweet taste that affects voluntary consumption. Most strains showed very different patterns of response to ethanol with or without saccharin. Three patterns of strain responses were identified. Some strains avoided higher concentrations of ethanol whether in water or saccharin; some appeared to be sensitive to the ability of saccharin to mask the odor of ethanol; and some may have reduced consumption only when ethanol concentrations were high enough to produce aversive postingestional effects. Whereas earlier studies generally attempted to explain strain differences in consumption by invoking a single mechanism, our results demonstrate that more than one mechanism is necessary to explain the preferential ethanol intake of all strains studied.     

Effects of early maternal separation on ethanol intake,GABA receptors and metabolizing enzymes in adult rats

Rationale Maternal separation (MS) in neonatal rats affects ethanol self-administration (SA) in adulthood; however, the conditions and mechanisms need to be clarified. Objectives The goal of this study was to determine the effect of MS on ethanol SA in adulthood in different groups of rats, which control for time of separation, handling, and rearing conditions and, for mechanistic assessment, to examine GABA-A receptors in the central nucleus of the amygdala (CeA) and levels of liver metabolizing enzymes. Methods Newborn, male Long–Evans rats were randomly assigned to different groups and treated over postnatal days 2–14. The rats were picked up by their tails and put back down with no separation (MS0), separated from their mother for 15 min/day (MS15), separated from their mother for 180 min/day (MS180), handled once for a bedding change (NH), or were animal facility reared (AFR). In adulthood, these rats were allowed 5-day continuous access to ethanol, and GABA-A receptors and liver enzymes were measured. Results The MS15 group consumed and preferred significantly less ethanol (about one third) than the MS180 group; however, neither group was different from the MS0 or the AFR group. The NH group consumed and preferred significantly more ethanol than all other groups, at least twice that of the MS180s. GABA-A receptors were increased in the CeA in MS15s, which could help explain the effects. Alcohol dehydrogenase may have been altered in the AFRs. Conclusions Various treatments in neonates affect ethanol intake and GABA-A receptors, and possibly ethanol metabolism, in adulthood. These changes were not simply related to time of separation but were also due to the degree of handling.     

Chronic THC during adolescence increases the vulnerability to stress-induced relapse to heroin seeking in adult rats

Cannabis derivatives are among the most widely used illicit substances among young people. The addictive potential of delta-9-tetrahydrocannabinol (THC), the major active ingredient of cannabis is well documented in scientific literature. However, the consequence of THC exposure during adolescence on occurrence of addiction for other drugs of abuse later in life is still controversial. To explore this aspect of THC pharmacology, in the present study, we treated adolescent rats from postnatal day (PND) 35 to PND-46 with increasing daily doses of THC (2.5–10 mg/kg). One week after intoxication, the rats were tested for anxiety-like behavior in the elevated plus maze (EPM) test. One month later (starting from PND 75), rats were trained to operantly self-administer heroin intravenously. Finally, following extinction phase, reinstatement of lever pressing elicited by the pharmacological stressor, yohimbine (1.25 mg/kg) was evaluated. Data revealed that in comparison to controls, animals treated with chronic THC during adolescence showed a higher level of anxiety-like behavior. When tested for heroin (20 μg per infusion) self-administration, no significant differences were observed in both the acquisition of operant responding and heroin intake at baseline. Noteworthy, following the extinction phase, administration of yohimbine elicited a significantly higher level of heroin seeking in rats previously exposed to THC. Altogether these findings demonstrate that chronic exposure to THC during adolescence is responsible for heightened anxiety and increased vulnerability to drug relapse in adulthood.     

The caloric and intoxicating properties of fluid intake as components of stress-induced ethanol consumption in rats

Rats housed continuously in a test environment for 25 days were offered water, 5% v/v ethanol, 10% v/v ethanol, and propylene glycol at 7.5% w/v. The propylene glycol concentration represented a caloric midpoint between the 5 and 10% ethanol. After 10 baseline sessions, during which preference for the four solutions was shown to be statistically equal, shock schedules were introduced. The consumption of ethanol at both concentrations showed significant peaks for the interval immediately following 12 min of shock each hour. Intake peaks were not observed for the water or propylene glycol. Baseline blood alcohol levels were negligible, but blood levels under shock averaged 143 mg/dl and ranged from 45.0 mg/dl to 295.0 mg/dl. After the shock sessions were terminated, baseline drinking indicated no significant change in preference relative to pre-shock baselines, but there was an elevation in preference for 5% ethanol relative to the other fluids. The preference for propylene glycol or water did not change.     

Effects of the kappa opioid receptor antagonist, norbinaltorphimine, on stress and drug-induced reinstatement of nicotine-conditioned place preference in mice

Rationale

Several studies implicate stress as a risk factor for the development and maintenance of drug addictive behaviors and drug relapse. Kappa opioid receptor (KOR) antagonists have been shown to attenuate behavioral responses to stress and stress-induced reinstatement of cocaine and ethanol seeking and preference.

Objectives

In the current study, we determined whether the selective KOR antagonist, norbinaltorphimine (nor-BNI), would block stress-induced reinstatement of nicotine preference.

Methods

Adult Institute of Cancer Research mice were conditioned with 0.5 mg/kg nicotine, injected subcutaneously (s.c.) for 3 days and tested in the nicotine-conditioned place preference (CPP) model. After 3 days extinction, nor-BNI (10 mg/kg, s.c.) was administered 16 h prior to a priming dose of nicotine (0.1 mg/kg, s.c.), and mice were tested in the CPP model for nicotine-induced reinstatement of CPP. A separate group of mice was subjected to a 2-day modified forced swim test (FST) paradigm to induce stress after 3 days extinction from CPP. Mice were given vehicle or nor-BNI (10 mg/kg, s.c.) 16 h prior to each FST session.

Results

Nor-BNI pretreatment significantly attenuated stress-induced reinstatement of nicotine-CPP, but had no effect on nicotine-primed reinstatement.

Conclusions

Blockade of KORs by selective antagonists attenuates stress-induced reinstatement of nicotine-CPP. Overall, the kappa opioid system may serve as a therapeutic target for suppressing multiple signaling processes which contribute to maintenance of smoking, smoking relapse, and drug abuse in general.     

Effects of ondansetron administration on opioid withdrawal syndrome observed in rats

This study tested whether a 5-HT₃ receptor antagonist could reverse the signs of precipitated opioid withdrawal. Rats were treated with either saline or morphine for 4 days. After the four days, half of the rats in each group received naloxone and half received saline. Each animal also received one of four doses of ondansetron (0, 1, 2 and 4 mg/kg i.p.). Administration of ondansetron to rats receiving naloxone after chronic morphine decreased the intensity of withdrawal signs such as increased defecation, jumping and wet-dog shakes, elevated the nociceptive threshold values which were decreased by precipitated withdrawal, but produced no change in urination, rectal temperature or salivation. The effects exhibited by ondansetron administration may be explained through interference of its 5-HT₃ receptor antagonist activity with serotoninergic mechanisms involved in the regulation of these withdrawal symptoms. The use of this drug is thus suggested as a possible treatment of opioid withdrawal signs in heroin addicts.     

A single injection of a novel kappa opioid receptor agonist salvinorin A attenuates the expression of cocaine-induced behavioral sensitization in rats

Kappa opioid receptor (KOPr) activation antagonizes many cocaine-related behaviors but adverse side-effects such as sedation, dysphoria, and depression limit their therapeutic use. Recently, salvinorin A (Sal A), a naturally occurring KOPr agonist, has been shown to attenuate cocaine-induced drug seeking in a model of relapse in rats. The present study evaluated the effects of acute Sal A exposure on cocaine-induced hyperactivity and cocaine sensitization in rats. Acute treatment with a dose of Sal A that decreased drug seeking in a previous study (0.3 mg/kg) significantly attenuated the expression of cocaine sensitization. This dose of Sal A failed to affect spontaneous locomotion or to produce a conditioned taste aversion to a novel-tasting saccharin solution. However, Sal A decreased climbing and swimming time and increased time spent immobile in the forced swim test. These findings indicate that Sal A, just like traditional KOPr agonists, attenuates cocaine-induced behavioral sensitization but does not produce the adverse effect of conditioned aversion, suggesting improved potential compliance. However, prodepressive effects were also produced and these effects may limit the therapeutic potential.     

Effects of training and withdrawal periods on heroin seeking induced by conditioned cue in an animal of model of relapse

Rationale  A high incidence of relapse can be triggered by exposure to conditioned cues previously associated with heroin. Extended access to drug and withdrawal are thought to affect the motivation for drug seeking. Objectives  The present study evaluated how different periods of training to self-administer heroin and different periods of withdrawal affected drug seeking. Materials and methods  Following 1 to 14 days of heroin self-administration, rats were left in the home environment for 1 or 14 days. Subsequently, rats were evaluated for extinction of nose poke during the first hour after being returned to the training apparatus. One hour later, a conditioned stimulus was presented to initiate cue-induced reinstatement. Results  Extending the training period from 1 to 14 days caused an escalation of reinstatement of drug seeking induced by conditioned cues. Increasing the withdrawal period from 1 to 14 days produced a similar increase in reinstatement of drug seeking induced by cues. Reinstatement of drug seeking induced by cues was augmented by pretreatment with naltrexone (1, 5 mg/kg) 24 h prior to reinstatement on day 1, but not at 14 days of withdrawal from heroin self-administration. Conclusions  These experiments demonstrate that increasing the duration of either heroin self-administration or the withdrawal periods from heroin self-administration augments the reinstatement induced by cues that were associated previously with heroin reinforcement. Additionally, we provide one of the first demonstrations that opiate withdrawal induces heroin seeking, as assessed in the reinstatement model.     

Modulation of the endogenous opioid system after morphine self-administration and during its extinction: a study in Lewis and Fischer 344 rats

Lewis (LEW) and Fischer 344 (F344) rats show differential morphine self-administration rates. In this study, after animals of both strains self-administered morphine (1mg/kg) or extinguished this behaviour for 3, 7 or 15days, we measured the binding to, and functional state of mu opioid receptors (MORs) as well as proenkephalin (PENK) mRNA content in several brain regions. The results showed that in most brain areas: 1) LEW rats had less binding to MORs in basal conditions than F344 rats; 2) after morphine self-administration, either one of the strains or both (depending on the brain area) showed increased levels of binding to MORs as compared to basal groups; and 3) these binding levels in morphine self-administration animals came down in each extinction group. Moreover, F344 rats exhibited, in general, an increased functionality of MORs after morphine self-administration, as compared to basal groups, which also went down during extinction. Finally, the basal content of PENK mRNA was lower in LEW rats than in F344 rats and it decreased more after self-administration; during extinction, the levels of PENK mRNA got normalized in this strain. This differential modulation of the endogenous opioid system might be related to the different rates of morphine self-administration behavior exhibited by both inbred rat strains.     

The tripping point: The potential role of psychedelic-assisted therapy in the response to the opioid crisis

The increasing contamination of the drug supply with illicitly manufactured fentanyl and related analogs in North America has resulted in the most severe drug-overdose crisis in history. Available pharmacotherapy options for the treatment of opioid use disorder have had limited success in curbing the current crisis, and a growing body of evidence highlights the need for innovative interventions that target underlying social-structural drivers of opioid use disorder. Re-emerging clinical research suggests that psychedelic-assisted therapy has potential as an alternative treatment for refractory substance use disorders and related comorbidities. Based on the available evidence, our viewpoint supports advancing research on the potential role of psychedelic-assisted therapy within a multifaceted response to the opioid crisis.     

The nicotinic acetylcholine receptor antagonist mecamylamine prevents escalation of cocaine self-administration in rats with extended daily access

Rationale Escalation from moderate to excessive drug intake is a hallmark of human addiction that can be modeled in rats by giving them longer daily access time to self-administer cocaine. Nicotine and cocaine are commonly coabused drugs in humans and recent work in animals suggests that activation of nicotinic acetylcholine receptors (nAChR) can increase cocaine self-administration. Objectives Determine the role of nAChR in the escalation of cocaine self-administration. Methods Control rats self-administered cocaine (0.75 mg/kg/infusion) for either 1 or 6 h per day. Experimental groups had the nAChR antagonist mecamylamine (MEC) added to the cocaine solution for 5 days after the transition from short (1 h per day) to long access (6 h per day) for cocaine self-administration. After 5 days, MEC was removed from the cocaine solution. Results Control rats and rats that received a low dose of MEC (7 μg/infusion) with cocaine increased their average hourly intake over 5 days of 6 h per day cocaine access. Rats that received a higher dose of MEC (70 μg/infusion) did not increase their intake of cocaine during 6 h access but continued to self-administer cocaine. When MEC was removed, this group showed an escalation in cocaine self-administration. MEC did not alter cocaine intake in a group that had continuous 1 h access. Conclusions Antagonism of nAChRs during the initial exposure to extended cocaine self-administration access time prevented escalation of, but did not eliminate, drug intake. These findings indicate that MEC-sensitive nAChRs are critical for determining cocaine intake as a function of longer access time.     

Extended-release naltrexone for alcohol and opioid dependence: A meta-analysis of healthcare utilization studies

Through improved adherence, once-monthly injectable extended-release naltrexone (XR-NTX) may provide an advantage over other oral agents approved for alcohol and opioid dependence treatment. The objective of this study was to evaluate cost and utilization outcomes between XR-NTX and other pharmacotherapies for treatment of alcohol and opioid dependence. Published studies were identified through comprehensive search of two electronic databases. Studies were included if they compared XR-NTX to other approved medicines and reported economic and healthcare utilization outcomes in patients with opioid or alcohol dependence. We identified five observational studies comparing 1,565 patients using XR-NTX to other therapies over 6 months. Alcohol dependent XR-NTX patients had longer medication refill persistence versus acamprosate and oral naltrexone. Healthcare utilization and costs was generally lower or as low for XR-NTX-treated patients relative to other alcohol dependence agents. Opioid dependent XR-NTX patients had lower inpatient substance abuse-related utilization versus other agents and $8170 lower total cost versus methadone.     

Shock induced ethanol consumption in rats

Two experiments are presented which describe the temporal and volumetric changes in ethanol consumption by rats exposed to recurring schedules of inescapable random shock. The animals in Experiment 1, which had a choice between ethanol and water, increased their voluntary ethanol consumption immediately after the shock schedule. The postshock changes occurred with both 5% and 10% V/V ethanol, were specific to the presence of shock and were not reflected by measures of total daily ethanol intake. Experiment 2 exposed rats to extended 22 hr stress sessions, during which each animal had four simulataneous fluid choices available: water, saccharin 0.1% W/V, ethanol 5% V/V, and ethanol 10% V/V. Temporal intake patterns for both 5% and 10% ethanol showed pronounced peaks for the interval immediately following the shock schedule. A shift of intake from 5% to 10% ethanol was also demonstrated with increasing time under shock, while saccharin and water intake decreased. The results are interpreted as a relationship between voluntary ethanol intake and escape from the consequences of stress.