Greater avoidance of a heroin-paired taste cue is associated with greater escalation of heroin self-administration in rats

Ketamine retards recovery from reward downshift and supports conditioned taste aversion

Ketamine is a noncompetitive N-methyl-d-aspartate (NMDA) receptor antagonist with antidepressant, anxiolytic, and memory effects in clinical and preclinical studies. The present studies investigated the behavioral effects of ketamine in animals exposed to a consummatory successive negative contrast (cSNC) task involving unexpected reward downshift, negative emotion (frustration), and aversive memory. Food-restricted male rats had 5-min access to 32 % sucrose in each of 10 preshift sessions followed by 4 % sucrose in 4 postshift sessions. Unshifted controls had access to 4 % sucrose during all 14 sessions. Ketamine (10 mg/kg, ip) was injected 30 min before sessions 11 and 12 (Experiment 1) or immediately after session 11 (Experiment 3). The results showed that both pre- and postdownshift session injection of ketamine increased consummatory suppression, as Group 32/Ket exhibited lower sucrose intake than Groups 32/Sal, 4/Ket, and 4/Sal. These effects extended beyond the day(s) of injection. Experiments 2 and 4 showed that the same dose, route of administration, and time of injection induced significant conditioned taste aversion to 4 % sucrose, in the absence of reward downshift. These data suggest that ketamine induces an aversive state that may summate with frustration induced by reward downshift in the cSNC task and also support a conditioned taste aversion to 4 % sucrose in the absence of reward downshift. Implications for these and other experiments involving pre- and postsession administration of ketamine are discussed.

Paternal morphine exposure enhances morphine self-administration and induces region-specific neural adaptations in reward-related brain regions of male offspring

Background

A growing body of preclinical studies report that preconceptional experiences can have a profound and long-lasting impact on adult offspring behavior and physiology. However, less is known about paternal drug exposure and its effects on reward sensitivity in the next generation.

Methods

Adult male rats self-administered morphine for 65 days; controls received saline. Sires were bred to drug-naïve dams to produce first-generation (F1) offspring. Morphine, cocaine, and nicotine self-administration were measured in adult F1 progeny. Molecular correlates of addiction-like behaviors were measured in reward-related brain regions of drug naïve F1 offspring.

Results

Male, but not female offspring produced by morphine-exposed sires exhibited dose-dependent increased morphine self-administration and increased motivation to earn morphine infusions under a progressive ratio schedule of reinforcement. This phenotype was drug-specific as self-administration of cocaine, nicotine, and sucrose were not altered by paternal morphine history. The male offspring of morphine-exposed sires also had increased expression of mu-opioid receptors in the ventral tegmental area but not in the nucleus accumbens.

Conclusions

Paternal morphine exposure increased morphine addiction-like behavioral vulnerability in male but not female progeny. This phenotype is likely driven by long-lasting neural adaptations within the reward neural brain pathways.

Acute treatment with the glucagon-like peptide-1 receptor agonist, liraglutide, reduces cue- and drug-induced fentanyl seeking in rats

Opioid Use Disorder (OUD) is a chronic relapsing disorder that has severe negative impacts on the individual, the family, and the community at large. In 2021, opioids contributed to nearly 70% of all drug overdose deaths in the United States. This rise in opioid related deaths coincides with a significant rise in the use of fentanyl, a synthetic opioid that is 150 times more potent than morphine. Furthermore, this overdose trend has spared no demographic and costs the nation an estimated $51.2 billion annually. Thus, it is imperative to better understand the underlying mechanisms of OUD in an effort to identify new treatment targets. Using animal models, studies have shown that rats readily self-administer heroin and increase seeking following exposure to cues for drug, the drug itself, or stress. We have shown that treatment with the glucagon-like peptide-1 receptor (GLP-1R) agonist, liraglutide, can reduce heroin taking and seeking behavior in rats. Therefore, using our rodent model, we established a fentanyl self-administration paradigm to test whether acute treatment with the GLP-1R agonist also can reduce fentanyl seeking in fentanyl experienced rats. The results showed that rats readily self-administer fentanyl (2.5 ug/kg) intravenously, with marked individual differences in drug taking behavior. As with other drugs of abuse tested, rats exhibited high seeking behavior when challenged with a drug-related cue or, after a period of extinction, the drug itself. Here, acute treatment with the GLP-1R agonist, liraglutide (0.3mg/kg s.c.), was found to attenuate both cue-induced fentanyl seeking and drug-induced reinstatement of fentanyl seeking with the same efficacy as the currently approved partial opioid agonist, buprenorphine. Taken together, these data suggest that a known satiety signal, GLP-1, may serve as an effective non-opioid alternative for the treatment of OUD.

Glucagon-like peptide-1 receptor agonist, exendin-4, reduces reinstatement of heroin-seeking behavior in rats

Opioid use disorder (OUD) causes the death of nearly 130 Americans daily. It is evident that new avenues for treatment are needed. To this end, studies have reported that 'satiety' agents such as the glucagon-like peptide-1 receptor (GLP-1R) agonist, exendin-4 (Ex-4), decreases responding for addictive drugs such as cocaine, nicotine, alcohol, and oxycodone, but no work has been done with heroin. In this study, we used a reward devaluation model in which rats avoid ingesting a saccharin solution that predicts drug availability to test the effects of 2.4 μg/kg Ex-4 on responding for a natural reward cue (i.e., saccharin) and on cue- and drug-induced heroin seeking. The results showed that treatment with Ex-4 during the 16-day abstinence period and on the test day decreased cue-induced heroin seeking. Drug-induced heroin seeking also was reduced by Ex-4, but only when using a 1 h, but not a 6 h, pretreatment time. Treatment with Ex-4 did not alter intake of the saccharin cue when the drug was on board, but a history of treatment with Ex-4 increased acceptance of the saccharin cue in later extinction trials. Finally, treatment with Ex-4 did not alter body weight, but was associated with increased Orexin 1 receptor (OX1) mRNA expression in the nucleus accumbens shell. Taken together, these findings are the first to show that treatment with a GLP-1R agonist can reduce both cue-induced seeking and drug-induced reinstatement of heroin seeking. As such, a GLP-1R agonist may serve as an effective treatment for OUD in humans.

Nicotine pre-treatment reduces sensitivity to the interoceptive stimulus effects of commonly abused drugs as assessed with taste conditioning paradigms

BACKGROUND

Drug pre-exposure attenuates sensitivity to the interoceptive stimulus properties of additional subsequently administered drugs in drug-induced conditioned taste avoidance (CTA) and conditioned place preference (CPP) paradigms. Specifically, nicotine, commonly used in conjunction with other addictive substances, attenuates acquisition of ethanol and caffeine CTAs and morphine-induced CPP.

METHODS

Because nicotine use is comorbid with a number of substance use disorders, we systematically examined the effects of nicotine pre-exposure on two different conditioning paradigms involving integration of the interoceptive stimulus properties of multiple commonly abused drugs, in male and female rats, designed to examine both the aversive and reinforcing properties of these drugs.

RESULTS

Nicotine dose-dependently interfered with acquisition of CTA to passively administered morphine, ethanol, and cocaine, but not lithium chloride, demonstrating that the effects of nicotine are not simply a matter of reduced orosensory processing or an inability to learn such associations. Moreover, nicotine-treated rats required higher doses of drug in order to develop CTA and did not show increased acceptance of the taste of self-administered ethanol compared with saline-treated rats.

CONCLUSIONS

These data demonstrate that nicotine pre-exposure attenuates sensitivity to the stimulus effects of multiple drugs in two conditioning paradigms, in a manner which is consistent with a reduced ability to integrate the interoceptive properties of abused drugs. Through reducing these stimulus properties of drugs of abuse, concomitant nicotine use may result in a need to increase either the frequency or strength of doses during drug-taking, thus likely contributing to enhanced addiction liability in smokers.

Heroin self-administration as a function of time of day in rats

RATIONALE

Drug addiction is a complex disease that is impacted by numerous factors. One such factor, time of day, influences drug intake, but there have been no investigations of how time of day affects the amount of drug taken and the development of addiction-like behavior. Previous data from our group show circadian disruption in rats given access to heroin during the light phase, which is important because circadian disruption, itself, can increase drug intake. Thus, the goal of this experiment was to determine how time of day of access affects heroin self-administration and the development of addiction-like behaviors including escalation of heroin intake, willingness to work for heroin on a progressive ratio schedule of reinforcement, seeking during extinction, incubation of seeking, and reinstatement of heroin-seeking behavior.

MATERIALS AND METHODS

Male Sprague Dawley rats were given the opportunity to self-administer heroin for 6 h per trial during the second half of either the light or dark phase for 18 trials, including one progressive ratio challenge. Rats then underwent 14 days of abstinence, with a 5-h extinction test occurring on both the first and the 14th days of abstinence. The second extinction test was followed by a heroin prime and 1 h of reinstatement testing. On the following day, a subset of rats were tested in an additional extinction test where rats were tested either at the same time of the day as their previous self-administration sessions or during the opposite light/dark phase.

RESULTS

Relative to Light Access rats, Dark Access rats took more heroin, exhibited more goal-directed behavior, exhibited more seeking during the dark phase, failed to extinguish seeking during the 5-h extinction test in the dark phase, and exhibited greater incubation of heroin seeking following abstinence. However, Dark Access rats did not escalate drug taking over trials, work harder for drug, or seek more during drug-induced reinstatement than Light Access rats.

CONCLUSIONS

These results show that time of access to heroin affects overall heroin intake and seeking in extinction, but does not affect other addiction-like behaviors in rats.

Exposure to environmental enrichment attenuates addiction-like behavior and alters molecular effects of heroin self-administration in rats

Environmental factors profoundly affect the addictive potential of drugs of abuse and may also modulate the neuro-anatomical/neuro-chemical impacts of uncontrolled drug use and relapse propensity. This study examined the impact of environmental enrichment on heroin self-administration, addiction-related behaviors, and molecular processes proposed to underlie these behaviors. Male Sprague-Dawley rats in standard and enriched housing conditions intravenously self-administered similar amounts of heroin over 14 days. However, environmental enrichment attenuated progressive ratio, extinction, and reinstatement session responding after 14 days of enforced abstinence. Molecular mechanisms, namely DNA methylation and gene expression, are proposed to underlie abstinence-persistent behaviors. A global reduction in methylation is reported to coincide with addiction, but no differences in total genomic methylation or repeat element methylation were observed in CpG or non-CpG (CH) contexts across the mesolimbic circuitry as assessed by multiple methods including whole genome bisulfite sequencing. Immediate early gene expression associated with drug seeking, taking, and abstinence also were examined. EGR1 and EGR2 were suppressed in mesolimbic regions with heroin-taking and environmental enrichment. Site-specific methylation analysis of EGR1 and EGR2 promoter regions using bisulfite amplicon sequencing (BSAS) revealed hypo-methylation in the EGR2 promoter region and EGR1 intragenic CpG sites with heroin-taking and environmental enrichment that was associated with decreased mRNA expression. Taken together, these findings illuminate the impact of drug taking and environment on the epigenome in a locus and gene-specific manner and highlight the need for positive, alternative rewards in the treatment and prevention of drug addiction.

Female rats exhibit less avoidance than male rats of a cocaine-, but not a morphine-paired, saccharin cue

Rats avoid intake of an otherwise palatable taste cue when paired with drugs of abuse (Grigson and Twining, 2002). In male rats, avoidance of drug-paired taste cues is associated with conditioned blunting of dopamine in the nucleus accumbens (Grigson and Hajnal, 2007), conditioned elevation in circulating corticosterone (Gomez et al., 2000), and greater avoidance of the drug-paired cue predicts greater drug-taking (Grigson and Twining, 2002). While female rats generally are more responsive to drug than male rats, in this self-administration model, female rats consume more of a cocaine-paired saccharin cue and take less drug than males (Cason and Grigson, 2013). What is not known, however, is whether the same is true when a saccharin cue predicts availability of an opiate, particularly when the amount of drug experienced is held constant via passive administration by the experimenter. Here, avoidance of a saccharin cue was evaluated following pairings with experimenter delivered cocaine or morphine in male and female rats. Results showed that males and females avoided intake of a taste cue when paired with experimenter administered morphine or cocaine, and individual differences emerged whereby some male and female rats exhibited greater avoidance of the drug-paired cue than others. Female rats did not drink more of the saccharin cue than males when paired with morphine in Experiment 1, however, they did drink more of the saccharin cue than male rats when paired with cocaine in Experiment 2. While no pattern with estrous cycle emerged, avoidance of the cocaine-paired cue, like avoidance of a morphine-paired cue (Gomez et al., 2000), was associated with a conditioned elevation in corticosterone in both male and female rats.

Once is too much: Early development of the opponent process in taste reactivity behavior is associated with later escalation of cocaine self-administration in rats

Evidence suggests that the addiction process may begin immediately in some vulnerable subjects. Specifically, some rats have been shown to exhibit aversive taste reactivity (gapes) following the intraoral delivery of a cocaine-predictive taste cue after as few as 1-2 taste-drug pairings. After only 3-4 trials, the number of gapes becomes a reliable predictor of later cocaine self-administration. Given that escalation of drug-taking behavior over time is recognized as a key feature of substance use disorder (SUD) and addiction, the present study examined the relationship between early aversion to the cocaine-predictive flavor cue and later escalation of cocaine self-administration in an extended-access paradigm. The data show that rats who exhibit the greatest conditioned aversion early in training to the intraorally delivered cocaine-paired cue exhibit the greatest escalation of cocaine self-administration over 15 extended-access trials. This finding suggests that early onset of the conditioned opponent process (i.e., the near immediate shift from ingestion to rejection of the drug-paired cue) is a reliable predictor of future vulnerability and resilience to cocaine addiction-like behavior. Future studies must determine the underlying neural mechanisms associated with this early transition and, hence, with early vulnerability to the later development of SUD and addiction. In so doing, we shall be in position to discover novel diagnostics and novel avenues of prevention and treatment.

Modeling the development of drug addiction in male and female animals

An increasing emphasis has been placed on the development and use of animal models of addiction that capture defining features of human drug addiction, including escalation/binge drug use, enhanced motivation for the drug, preference for the drug over other reward options, use despite negative consequences, and enhanced drug-seeking/relapse vulnerability. The need to examine behavior in both males and females has also become apparent given evidence demonstrating that the addiction process occurs differently in males and females. This review discusses the procedures that are used to model features of addiction in animals, as well as factors that influence their development. Individual differences are also discussed, with a particular focus on sex differences. While no one procedure consistently produces all characteristics, different models have been developed to focus on certain characteristics. A history of escalating/binge patterns of use appears to be critical for producing other features characteristic of addiction, including an enhanced motivation for the drug, enhanced drug seeking, and use despite negative consequences. These characteristics tend to emerge over abstinence, and appear to increase rather than decrease in magnitude over time. In females, these characteristics develop sooner during abstinence and/or following less drug exposure as compared to males, and for psychostimulant addiction, may require estradiol. Although preference for the drug over other reward options has been demonstrated in non-human primates, it has been more difficult to establish in rats. Future research is needed to define the parameters that optimally induce each of these features of addiction in the majority of animals. Such models are essential for advancing our understanding of human drug addiction and its treatment in men and women.

The Affective Properties of Synthetic Cathinones: Role of Reward and Aversion in Their Abuse

The drug class known as synthetic cathinones has gained significant attention in the last few years as a result of increased use and abuse. These compounds have been shown to possess reinforcing efficacy in that they are abused in human populations and are self-administered in animal models. The present chapter outlines the affective properties of synthetic cathinones that are thought to impact drug self-administration in general and presents research confirming that these drugs have both rewarding and aversive effects in standalone and concurrent assessments. The implications of these affective properties for the overall abuse potential of these compounds are discussed along with directions for future research.

The role of dose and restriction state on morphine-, cocaine-, and LiCl-induced suppression of saccharin intake: A comprehensive analysis

Rats avoid intake of a taste cue when paired with a drug of abuse or with the illness-inducing agent, lithium chloride (LiCl). Although progress has been made, it is difficult to compare the suppressive effects of abused agents and LiCl on intake of a gustatory conditioned stimulus (CS) because of the cross-laboratory use of different CSs, different unconditioned stimuli (USs), and different doses of the drugs, different conditioning regimens, and different restriction states. Here we have attempted to unify these variables by comparing the suppressive effects of a range of doses of morphine, cocaine, and LiCl on intake of a saccharin CS using a common regimen in non-restricted, food restricted, or water restricted male Sprague-Dawley rats. The results showed that, while the putatively aversive agent, LiCl, was effective in suppressing intake of the taste cue across nearly all doses, regardless of restriction state, the suppressive effects of both morphine and cocaine were greatly reduced when evaluated in either food or water restricted rats. Greater sensitivity to drug was revealed, at very low doses, when testing occurred in the absence of need (i.e., when the rats were non-restricted). Together, these results provide the first uniform and comprehensive analysis of the suppressive effects of morphine, cocaine, and LiCl as a function of dose and restriction state. In the present case, the suppressive effects of morphine and cocaine are found to differ from those of LiCl and, in some respects, from one another as well.

Assessment of individual differences in the rat nucleus accumbens transcriptome following taste-heroin extended access

Heroin addiction is a disease of chronic relapse that harms the individual through devaluation of personal responsibilities in favor of finding and using drugs. Only some recreational heroin users devolve into addiction but the basis of these individual differences is not known. We have shown in rats that avoidance of a heroin-paired taste cue reliably identifies individual animals with greater addiction-like behavior for heroin. Here rats received 5min access to a 0.15% saccharin solution followed by the opportunity to self-administer either saline or heroin for 6h. Large Suppressors of the heroin-paired taste cue displayed increased drug escalation, motivation for drug, and drug loading behavior compared with Small Suppressors. Little is known about the molecular mechanisms of these individual differences in addiction-like behavior. We examined the individual differences in mRNA expression in the nucleus accumbens (NAc) of rats that were behaviorally stratified by addiction-like behavior using next-generation sequencing. We hypothesized that based on the avoidance of the drug-paired cue there will be a unique mRNA profile in the NAc. Analysis of strand-specific whole genome RNA-Seq data revealed a number of genes differentially regulated in NAc based on the suppression of the natural saccharine reward. Large Suppressors exhibited a unique mRNA prolife compared to Saline controls and Small Suppressors. Genes related to immunity, neuronal activity, and behavior were differentially expressed among the 3 groups. In total, individual differences in avoidance of a heroin-paired taste cue are associated with addiction-like behavior along with differential NAc gene expression.

Drug-motivated behavior in rats with lesions of the thalamic orosensory area

Rats suppress intake of a palatable taste cue when paired with a rewarding or an aversive stimulus in appetitive or aversive conditioning, respectively. A similar phenomenon occurs with drugs of abuse, but the nature of this conditioning has been subject for debate. While relatively little is known about the underlying neural circuitry, we recently reported bilateral lesions of the thalamic trigeminal orosensory area isolate drug-induced suppression of intake of a taste cue. The lesion blocks avoidance of the taste cue when paired with experimenter delivered drugs of abuse, yet has no effect on avoidance of the same cue when paired with an aversive agent or when it predicts access to a highly palatable sucrose solution. We hypothesize the lesion may blunt the rewarding properties of the drug. To test this, we used a runway apparatus, as running speed has been shown to increase with increasing reward value. Our hypothesis was supported by failure of the lesioned rats to increase running speed for morphine. Interestingly, lesioned rats did avoid intake of the drug-paired cue when presented in the runway apparatus and displayed naloxone-precipitated withdrawal. Using a partial crossover design, the lesion prevented avoidance of a cocaine-paired cue when presented in the home cage. We conclude that the lesion disrupts avoidance of a taste cue in anticipation of the rewarding properties of a drug but, at least in the presence of contextual cues, allows for avoidance of a taste cue as it elicits the onset of an aversive conditioned state of withdrawal.

Reward devaluation and heroin escalation is associated with differential expression of CRF signaling genes

One of the most damaging aspects of drug addiction is the degree to which natural rewards (family, friends, employment) are devalued in favor of seeking, obtaining and taking drugs. We have utilized an animal model of reward devaluation and heroin self-administration to explore the role of the coricotropin releasing factor (CRF) pathway. Given access to a saccharin cue followed by the opportunity to self-administer heroin, animals will parse into distinct phenotypes that suppress their saccharin intake (in favor of escalating heroin self-administration) or vice versa. We find that large saccharin suppressors (large heroin takers) demonstrate increased mRNA expression for elements of the CRF signaling pathway (CRF, CRF receptors and CRF binding protein) within the hippocampus, medial prefrontal cortex and the ventral tegmental area. Moreover, there were no gene expression changes of these components in the nucleus accumbens. Use of bisulfite conversion sequencing suggests that changes in CRF binding protein and CRF receptor gene expression may be mediated by differential promoter methylation.

Low expression of D2R and Wntless correlates with high motivation for heroin

Drug overdose now exceeds car accidents as the leading cause of accidental death in the United States. Of those drug overdoses, a large percentage of the deaths are due to heroin and/or pharmaceutical overdose, specifically misuse of prescription opioid analgesics. It is imperative, then, that we understand the mechanisms that lead to opioid abuse and addiction. The rewarding actions of opioids are mediated largely by the mu-opioid receptor (MOR), and signaling by this receptor is modulated by various interacting proteins. The neurotransmitter dopamine also contributes to opioid reward, and opioid addiction has been linked to reduced expression of dopamine D2 receptors (D2R) in the brain. That said, it is not known if alterations in the expression of these proteins relate to drug exposure and/or to the "addiction-like" behavior exhibited for the drug. Here, we held total drug self-administration constant across acquisition and showed that reduced expression of the D2R and the MOR interacting protein, Wntless, in the medial prefrontal cortex was associated with greater addiction-like behavior for heroin in general and with a greater willingness to work for the drug in particular. In contrast, reduced expression of the D2R in the nucleus accumbens and hippocampus was correlated with greater seeking during signaled nonavailability of the drug. Taken together, these data link reduced expression of both the D2R and Wntless to the explicit motivation for the drug rather than to differences in total drug intake per se.

Early avoidance of a heroin-paired taste-cue and subsequent addiction-like behavior in rats

The ability to predict individual vulnerability to substance abuse would allow for a better understanding of the progression of the disease and development of better methods for prevention and/or early intervention. Here we use drug-induced devaluation of a saccharin cue in an effort to predict later addiction-like behavior in a model akin to that used by Deroche-Gamonet et al. (2004) and seek to link such vulnerability to changes in expression of various mu opioid receptor and D2 receptor-interacting proteins in brain. The results show that the greatest heroin-induced suppression of intake of a saccharin cue is associated with the greatest vulnerability to later addiction-like behavior and to differences in the expression of WLS, β-catenin, and NCS-1 in brain compared to rats that exhibited the least suppression of intake of the heroin-paired cue and/or saline controls. Finally, because the self-administration model employed produced no significant differences in drug intake between groups, overall, the resultant changes in protein expression can be more closely linked to individual differences in motivation for drug.

Reversal of the sleep-wake cycle by heroin self-administration in rats

The goal of this study was to examine how heroin self-administration, abstinence, and extinction/reinstatement affect circadian sleep-wake cycles and the associated sleep architecture. We used electroencephalography (EEG) and electromyography (EMG) to measure sleep patterns in male Sprague-Dawley rats over 16 trials of heroin self-administration (acquisition), 14 days of abstinence, and a single day of extinction and drug-induced reinstatement. Rats self-administering heroin showed evidence of reversed (diurnal) patterns of wakefulness, non-rapid eye movement (NREM) sleep, and rapid eye movement (REM) sleep throughout acquisition. During abstinence, their wake and NREM sleep patterns were immediately restored to the normal nocturnal distribution. REM patterns remained inverted for the first 3-6 days of abstinence in heroin self-administering rats. The single extinction/reinstatement test was without effect. These data suggest that heroin may have the ability to affect circadian distribution of sleep and wakefulness, either indirectly, where animals shift their sleep-wake cycle to allow for drug taking, or directly, through wake-promoting actions or actions at circadian oscillators in the brain.