Acute glucagon-like peptide-1 receptor agonist liraglutide prevents cue-, stress-, and drug-induced heroin-seeking in rats.

Substance use disorder is challenging to treat due to its relapsing nature. In the last decade, opioid use disorder has been a threat to public health, being declared an epidemic by the Centers for Disease Control and Prevention. This is a tragic situation, considering there currently are only three effective, yet not ideal, treatments to prevent relapse to opioids. Recent research has shown that hormones that modulate hunger and satiety also can modulate motivated behavior for drugs of abuse. For example, the short-acting analog of glucagon-like peptide-1 (GLP-1), an incretin hormone that regulates homeostatic feeding, has been shown to reduce responding for rewarding stimuli such as food, cocaine, heroin, and nicotine when administered over several days or weeks. This may serve as an effective adjuvant during treatment; however, whether it would be effective when used acutely to bridge a patient between cessation of use and onset of medication for the treatment of an opioid addiction is unknown. Here, we tested the acute effects of the longer acting GLP-1 analog, liraglutide, on heroin-seeking. In rats with heroin self-administration experience, we found that subcutaneous administration of an acute dose of 0.3-mg/kg liraglutide was effective in preventing drug-seeking after exposure to three major precipitators: drug-associated cues, stress (yohimbine-induced), and the drug itself. Finally, we confirmed that the reduction in drug-seeking is not due to a locomotor impairment, as liraglutide did not significantly alter performance in a rotarod test. As such, acute use of GLP-1 analogs may serve as a new and effective nonopioid bridge to treatment.

Glucagon-like peptide-1 receptor agonist, liraglutide, reduces heroin self-administration and drug-induced reinstatement of heroin-seeking behaviour in rats.

Drug addiction is a chronic brain disease characterized by the uncontrolled use of a substance. Due to its relapsing nature, addiction is difficult to treat, as individuals can relapse following even long periods of abstinence and, it is during this time, that they are most vulnerable to overdose. In America, opioid overdose has been increasing for decades, making finding new treatments to help patients remain abstinent and prevent overdose deaths imperative. Recently, glucagon-like peptide-1 (GLP-1) receptor agonists have shown promise in reducing motivated behaviours for drugs of abuse. In this study, we test the effectiveness of the GLP-1 analogue, liraglutide (LIR), in reducing heroin addiction-like behaviour, and the potential side effects associated with the treatment. We show that daily treatment with LIR (0.1 mg/kg sc) increases the latency to take heroin, reduces heroin self-administration, prevents escalation of heroin self-administration and reduces drug-induced reinstatement of heroin-seeking behaviour in rats. A 1-h pretreatment time, however, was too short to reduce cue-induced seeking in our study. Moreover, we showed that, while LIR (0.1, 0.3, 0.6 and 1.0 mg/kg sc) supported conditioned taste avoidance of a LIR-paired saccharin cue, it did not elicit intake of the antiemetic kaolin in heroin-naïve or heroin-experienced rats. Further, 0.1 mg/kg LIR did not produce great disruptions in food intake or body weight. Overall, the data show that LIR is effective in reducing heroin taking and heroin seeking at doses that do not cause malaise and have a modest effect on food intake and body weight gain.

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.

Weak ventral striatal responses to monetary outcomes predict an unwillingness to resist cigarette smoking.

As a group, cigarette smokers exhibit blunted subjective, behavioral, and neurobiological responses to nondrug incentives and rewards, relative to nonsmokers. Findings from recent studies suggest, however, that there are large individual differences in the devaluation of nondrug rewards among smokers. Moreover, this variability appears to have significant clinical implications, since reduced sensitivity to nondrug rewards is associated with poorer smoking cessation outcomes. Currently, little is known about the neurobiological mechanisms that underlie these individual differences in the responsiveness to nondrug rewards. Here, we tested the hypothesis that individual variability in reward devaluation among smokers is linked to the functioning of the striatum. Specifically, functional magnetic resonance imaging was used to examine variability in the neural response to monetary outcomes in nicotine-deprived smokers anticipating an opportunity to smoke-circumstances found to heighten the devaluation of nondrug rewards by smokers in prior work. We also investigated whether individual differences in reward-related brain activity in those expecting to have access to cigarettes were associated with the degree to which the same individuals subsequently were willing to resist smoking in order to earn additional money. Our key finding was that deprived smokers who exhibited the weakest response to rewards (i.e., monetary gains) in the ventral striatum were least willing to refrain from smoking for monetary reinforcement. These results provide evidence that outcome-related signals in the ventral striatum serve as a marker for clinically meaningful individual differences in reward-motivated behavior among nicotine-deprived smokers.

Sleep is increased by liraglutide, a glucagon-like peptide-1 receptor agonist, in rats.

INTRODUCTION: Sleep disturbances are prominent in drug use disorders, including those involving opioids in both humans and animals. Recent studies have shown that administration of liraglutide, a glucagon-like peptide-1 agonist, significantly reduces heroin taking and seeking in rats. In an effort to further understand the action of this substance on physiological functions and to evaluate safety issues for its potential clinical use, the aim of the present study was to determine whether the dose of liraglutide found effective in reducing responding for an opioid also could improve sleep in drug-naïve rats. METHODS: Using a within-subjects design, adult male rats chronically implanted with EEG and EMG electrodes received subcutaneous injection of saline or 0.06, 0.10, 0.30 or 0.60 mg/kg liraglutide. The 0.10 and 0.30 mg/kg doses are known to be most effective in reducing responding for heroin in rats at light or dark onset during a 12:12 h light-dark cycle (0.10 mg/kg for taking and seeking, 0.30 mg/kg for seeking). EEG and EMG were recorded across the 24 h period following each injection. RESULTS: After both dark and light onset injections, liraglutide dose-dependently decreased wakefulness and increased non-rapid eye movement (NREM) sleep except at the lowest dose. The bout length of wakefulness and NREM sleep were decreased and increased, respectively. Whether administered at light or dark onset, the above alterations occurred primarily during the dark period (i.e., during the active period). The animals' body weight was decreased after liraglutide treatments as expected since it is clinically used for the treatment of obesity. CONCLUSION: These data indicate that liraglutide, at doses known to reduce responding for heroin and fentanyl, also increases NREM sleep, suggesting that the increase in sleep may contribute to the protective effects of liraglutide and may promote overall general health.

Environmental enrichment protects against the acquisition of cocaine self-administration in adult male rats, but does not eliminate avoidance of a drug-associated saccharin cue.

One of the most menacing consequences of drug addiction is the devaluation of natural rewards (e.g. food, sex, work, money, caring for one's offspring). However, evidence also suggests that natural rewards, such as an enriched environment, can devalue drugs of abuse. Thus, this study used a rodent model to test whether exposure to an enriched environment could protect adult rats from acquiring cocaine self-administration and from the resultant drug-induced devaluation of a natural saccharin reward cue. Adult male Sprague-Dawley rats were implanted with intravenous jugular catheters. Rats were then separated into two housing conditions: an enriched condition, including social companions(four/cage) and novel objects (e.g. balls, polyethylene tubes, paper, etc.), and a nonenriched condition where the rats were singly housed with no novel objects. During testing, the rats were given 5-min access to 0.15% saccharin, followed by 1 h to self-administer saline or cocaine (0.167 mg/infusion) on fixed ratio and progressive ratio schedules of reinforcement. The results showed that rats that were singly housed in the nonenriched environment fell into two groups: low drug-takers (n=34) and high drug-takers (n=12). In comparison, only one out of the 22 rats housed in the enriched environment was a high drug-taker. Thus, all rats in the enriched environment, except one, behaved like low drug-takers under the nonenriched condition. As such, these rats self-administered almost no drug on either the fixed ratio or the progressive ratio schedule of reinforcement and were extremely slow to self-administer their first cocaine infusion. Interestingly, despite their very low levels of drug self-administration, low-drug-taking rats housed in the enriched environment continued to avoid intake of the drug-associated saccharin cue. Taken together, these data suggest that the enriched environment itself served as a salient natural reward that reduced cocaine seeking and cocaine taking, but had little impact on avoidance of the cocaine-paired taste cue. The protective effects of the enriched environment were robust and, as such, have important implications for the methods used in the study of drug addiction in animal models and for the prevention, and possibly the treatment, of the disease in adult humans.

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 number of 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-administered 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 (0.3 mg/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.

Greater avoidance of a saccharin cue paired with passive delivery of heroin is associated with a select increase in expression of CRFR2 and CRFbp in the hippocampus in rats.

As a drug of abuse tightens its hold on addicted individuals, aspects of life that once brought pleasure lose their appeal while attention and motivation are turned toward acquiring drug. In a rodent model of self-administration and reward devaluation, we previously showed that animals that suppress intake of a drug-paired saccharin cue show greater addiction-like behaviors, as well as increased gene-expression of elements of the corticotropin releasing factor (CRF) pathway in the prefrontal cortex (mPFC), hippocampus (Hipp), and ventral tegmental area (VTA). In the present study, we explored whether the observed differences in components of the CRF signaling pathway were a function of self-administration or devaluation of the cue. Moreover, as an increasing body of work illustrates, functional and molecular hemispheric differences in reward pathway components, we examined whether these CRF pathway components exhibited hemispheric differences in response to heroin administration. Over a period of 7 trials, 30 male rats received brief access to saccharin followed by passive (IP) injection of heroin (n = 20) or saline (n = 10). Saccharin intakes between large saccharin suppressors (LS; 12 animals) and small suppressors (SS; 8 animals) were statistically different after trial 1 and separated further with ensuing trials. We then assessed gene expression for components of the CRF pathway in the mPFC, Hipp, VTA, Amygdala, and nucleus accumbens (NAc). Within the Hipp, LS showed greater expression of CRF binding protein (CRFbp). No differences were observed in the mPFC, VTA, NAc or Amygdala. Several hemisphere differences in CRF signaling pathway genes were detected. These findings indicate that avoidance of the experimenter delivered heroin-paired saccharin cue, do not recapitulate findings observed for avoidance of the iv self-administered heroin-paired saccharin cue, at least in terms of the expression of genes within the CRF pathway, and provide further evidence that consideration should be given to hemisphere differences when exploring molecular phenomena.

An RNA-seq study of the mPFC of rats with different addiction phenotypes.

Addiction is a disease of brain-reward circuitry whereby attention, motivation, memory and emotional systems become enslaved to the goal of seeking and acquiring drug, instead of responding to the natural rewards for which these systems evolved. At the intersection of reward/limbic structures, the medial prefrontal cortex (mPFC) receives and consolidates signals regarding environment and orchestrates the most appropriate response (i.e., decision-making and attention). As such, mPFC function plays a critical role in the vulnerability or resilience to drug addiction. In our model of drug-induced reward devaluation, an outbred group of Sprague-Dawley rats parsed into two distinct drug-taking phenotypes: those, referred to as small suppressors (SS) that readily ingest a heroin-paired sweet cue and then take little drug, and those, referred to large suppressors (LS), that avoid the heroin-paired cue, but then respond greatly for the drug of abuse. In the present study, we analyzed the mPFC transcriptome of rats from these divergent groups to discover differences in gene expression that underlie these distinct phenotypes. Genes found to be differentially expressed were those associated with schizophrenia and dopamine signaling, signal transduction, development and synaptic plasticity. These genes may underlie the circumstance whereby some individuals succumb to addiction, while others do not, and may reveal new pharmacological targets for the treatment of drug addiction.

Dose titration with the glucagon-like peptide-1 agonist, liraglutide, reduces cue- and drug-induced heroin seeking in high drug-taking rats.

Opioid use disorder (OUD), like other substance use disorders (SUDs), is widely understood to be a disorder of persistent relapse. Despite the use of three FDA-approved medications for OUD, typically in conjunction with behavioral treatments, relapse rates remain unacceptably high. Whereas medication assisted therapy (MAT) reduces the risk of opioid overdose mortality, the benefits of MAT are negated when people discontinue the medications. Currently approved medications present barriers to efficient use, including daily visits to a treatment center or work restrictions. With spiking increases in opioid relapse and death, it is imperative to identify new treatments that can reduce the risk of relapse. Recent evidence suggests that glucagon-like peptide-1 receptor agonists (GLP-1RAs), currently FDA-approved to treat obesity and type two diabetes, may be promising candidates to reduce relapse. GLP-1RAs have been shown to reduce relapse in rats, whether elicited by cues, drug, and/or stress. However, GLP-1RAs also can cause gastrointestinal malaise, and therefore, in humans, the medication typically is titrated up to full dose when initiating treatment. Here, we used a rodent model to test whether cue- and drug-induced heroin seeking can be reduced by the GLP-1RA, liraglutide, when the dose is titrated across the abstinence period and prior to test. The results show this titration regimen is effective in reducing both cue-induced heroin seeking and drug-induced reinstatement of heroin seeking, particularly in rats with a history of high drug-taking. Importantly, this treatment regimen had no effect on either circulating glucose or insulin. GLP-1RAs, then, appear strong candidates for the non-opioid prevention of relapse to opioids.

A novel method to study reward-context associations and drug-seeking behaviors.

Animal models have significantly contributed to the understanding of reward-related behaviors, such as in Substance Use Disorder research. One of the most heavily utilized paradigms to date is conditioned place preference (CPP). However, CPP is limited by non-contingent exposure. Our new method advances this classic method by utilizing its benefits and simultaneously diminishing its limitations. We used a traditional 3-compartment CPP apparatus, where each chamber differs by both visual and tactile contexts. We restructured the apparatus allowing for insertion of bottles so that mice could orally self-administer sucrose or morphine-containing solutions in a specific context. Our results show that mice who self-administer sucrose or morphine show a place preference for the sucrose- or morphine-paired chamber. This place preference lasts for 21 d in sucrose-treated, but not morphine-treated mice. Additionally, we found that that mice will drink more water in the morphine-paired context during extinction tests. This model combines the distinct contextual cues associated with conditioned place preference and combines them with voluntary self-administration, thus enabling researchers to measure behavior using a model that incorporates spatial memory involved in affective states, while also providing a quantifiable readout of context/environment-specific drug seeking. In conclusion, we combined CPP and voluntary intake to establish a novel technique to assess not only preference for a context associated with rewarding stimuli (natural or drug), but also seeking, retention, and locomotor activity. This model can be further utilized to examine other drugs of abuse, extinction training, other learning models, or to allow for the assessment of neurobiological manipulations.

Gene expression changes following extinction testing in a heroin behavioral incubation model.

BACKGROUND: A number of gene expression studies have investigated changes induced by drug exposure, but few reports describe changes that persist following relapse. In this study, genome-wide analysis of gene expression was conducted following an extinction session (90 min) in rats that expressed behavioral incubation of heroin-seeking and goal-directed behavior. As an important modulator of goal-directed behavior, the medial prefrontal cortex (mPFC) was the target of genomic analysis. Rats were trained to self-administer heroin during 3 h daily sessions for 14 d. Following the self-administration period, rats were reintroduced to the self-administration chambers for a 90-minute extinction session in which they could seek heroin, but received none. Extinction sessions were conducted on groups after either 1 d or 14 d of drug-free enforced abstinence to demonstrate behavioral incubation. RESULTS: Behavioral data demonstrated incubation (increased expression) of heroin-seeking and goal-directed behavior after the 14 d abstinent period. That is, following 14 d of enforced abstinence, animals displayed heightened drug-seeking behavior when returned to the environment where they had previously received heroin. This increased drug-seeking took place despite the fact that they received no drug during this extinction session. Whole genome gene expression analysis was performed and results were confirmed by quantitative real-time PCR (RT-qPCR). Microarrays identified 66 genes whose expression was identified as changed by at least 1.4 fold (p < 0.02) following 14 d of abstinence and the 90-minute extinction session compared to the saline treated controls. Orthogonal confirmation by RT-qPCR demonstrated significant alterations in bdnf, calb1, dusp5, dusp6, egr1, npy, rgs2. CONCLUSION: Ontological analysis indicates that several of the genes confirmed to be changed are important for neuroplasticity, and through that role may impact learning and behavior. The importance of drug-seeking behavior and memory of previous drug-taking sessions suggest that such genes may be important for relapse. The global gene expression analysis adds to the knowledge of heroin-induced changes and further highlights similarities between heroin and other drugs of abuse.

Symposium overview--Food addiction: fact or fiction?

Food addiction is a pervasive, yet controversial, topic that has gained recent attention in both lay media and the scientific literature. The goal of this series of articles is to use a combination of preclinical and clinical data to determine whether foods, like drugs of abuse, can be addictive, the conditions under which the addiction develops, and the underlying neurophysiological substrates. Operational definitions of addiction that have been used in the treatment of human disorders and to guide research in both humans and animals are presented, and an overview of the symposium articles is provided. We propose that specific foods, especially those that are rich in fat and/or sugar, are capable of promoting "addiction"-like behavior and neuronal change under certain conditions. That is, these foods, although highly palatable, are not addictive per se but become so following a restriction/binge pattern of consumption. Such consummatory patterns have been associated with increased risk for comorbid conditions such as obesity, early weight gain, depression, anxiety, and substance abuse as well as with relapse and treatment challenges. The topic of food addiction bears study, therefore, to develop fresh approaches to clinical intervention and to advance our understanding of basic mechanisms involved in loss of control.

Overexpression of DeltaFosB is associated with attenuated cocaine-induced suppression of saccharin intake in mice.

Rodents suppress intake of saccharin when it is paired with a drug of abuse (Goudie, Dickins, & Thornton, 1978; Risinger & Boyce, 2002). By the authors' account, this phenomenon, referred to as reward comparison, is thought to be mediated by anticipation of the rewarding properties of the drug (P. S. Grigson, 1997; P. S. Grigson & C. S. Freet, 2000). Although a great deal has yet to be discovered regarding the neural basis of reward and addiction, it is known that overexpression of DeltaFosB is associated with an increase in drug sensitization and incentive. Given this, the authors reasoned that overexpression of DeltaFosB should also support greater drug-induced devaluation of a natural reward. To test this hypothesis, NSE-tTA x TetOp-DeltaFosB mice (Chen et al., 1998) with normal or overexpressed DeltaFosB in the striatum were given access to a saccharin cue and then injected with saline, 10 mg/kg cocaine, or 20 mg/kg cocaine. Contrary to the original prediction, overexpression of DeltaFosB was associated with attenuated cocaine-induced suppression of saccharin intake. It is hypothesized that elevation of DeltaFosB not only increases the reward value of drug, but the reward value of the saccharin cue as well.

Gustatory insular cortex lesions disrupt drug-induced, but not lithium chloride-induced, suppression of conditioned stimulus intake.

Rats suppress intake of a normally preferred 0.15% saccharin conditioned stimulus (CS) when it is paired with an aversive agent like lithium chloride (LiCl) or a preferred substance such as sucrose or a drug of abuse. The reward comparison hypothesis suggests that rats avoid intake of a saccharin cue following pairings with a drug of abuse because the rats are anticipating the availability of the rewarding properties of the drug. The present study used bilateral ibotenic acid lesions to examine the role of the gustatory cortex in the suppression of CS intake induced by cocaine, morphine, and LiCl. The results show that bilateral lesions of the insular gustatory cortex (1) fully prevent the suppressive effects of both a 15 and a 30 mg/kg dose of morphine, (2) attenuate the suppressive effect of a 10 mg/kg dose of cocaine, but (3) are overridden by a 20 mg/kg dose of the drug. Finally, these same cortical lesions had no impact on LiCl-induced conditioned taste aversion. The current data show that the insular taste cortex plays an integral role in drug-induced avoidance of a gustatory CS.

Heroin self-administration: II. CNS gene expression following withdrawal and cue-induced drug-seeking behavior.

In the accompanying paper, we described incubation of heroin-seeking behavior in rats following 14 days of abstinence. To gain an understanding of genomic changes that accompany this behavioral observation, we measured the expression of genes previously reported to respond to drugs of abuse. Specifically, after 1 or 14 days of abstinence, mRNA expression was measured for 11 genes in the medial prefrontal cortex (mPFC) and nucleus accumbens (NAc) immediately following a single 90 min extinction session. Additionally, the role of contingency was examined in control rats that received yoked, response-independent heroin administration. Gene expression was quantified by real-time quantitative PCR. Expression of five genes (Arc, EGR1, EGR2, Fos, and Homer1b/c) was changed in the mPFC. EGR1 and EGR2 expression was increased following the 90 min extinction session in a contingency-specific manner and this increase persisted through the 14 days of abstinence. Fos expression was also increased after 1 and 14 days of abstinence, but at 14 days this increase was response-independent (i.e., it occurred in both the rats with a history of heroin self-administration and in the yoked controls). Arc expression increased following the extinction session only in rats with a history of heroin self-administration and only when tested following 1, but not 14, days of abstinence. Homer 1 b/c decreased after 14 days of enforced abstinence in rats that received non-contingent heroin. Expression of only a single gene (EGR2) was increased in the NAc. These data demonstrate that behavioral incubation is coincident with altered levels of specific transcripts and that this response is contingently-specific. Moreover, EGR1 and EGR2 are specifically upregulated in self-administering rats following extinction and this finding persists through 14 days of abstinence, suggesting that these genes are particularly associated with the incubation phenomenon. These latter observations of persistent changes in gene expression following abstinence may reflect molecular correlates of relapse liability.

Heroin self-administration: I. Incubation of goal-directed behavior in rats.

This study used heroin self-administration to investigate incubation of goal-directed heroin-seeking behavior following abstinence. Male Sprague-Dawley rats self-administered heroin on a fixed ratio 10 (FR10) schedule of reinforcement with licking of an empty spout serving as the operant behavior during 14 daily 3 h sessions. After this acquisition period, all rats received a 90 min extinction session following either 1 day or 14 days of home cage abstinence. When the extinction session occurred after only 1 day of home cage abstinence, rats with a history of heroin self-administration divided their responses equally between the previously "active" and "inactive" spouts. However, when the extinction session occurred following 14 days of home cage abstinence, the rats exhibited marked goal-directed heroin-seeking behavior by licking more on the previously "active" than "inactive" spout. These findings demonstrate that heroin-seeking behavior incubates over time, resulting in goal-directed heroin-seeking behavior in rats following 14 days but not 1 day of abstinence. Moreover, this facilitatory effect occurred in response to a different training schedule, lower total drug intake, and shorter periods of daily access than previously reported with heroin.

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.

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.

Heroin-induced suppression of saccharin intake in OPRM1 A118G mice.

The single nucleotide polymorphism of the µ-opioid receptor, OPRM1 A118G, has been associated with greater drug and alcohol use, increased sensitivity to pain, and reduced sensitivity to the antinociceptive effects of opiates. In the present studies, we employed a 'humanized' mouse model containing the wild-type (118AA) or variant (118GG) allele to examine behavior in a model of heroin-induced devaluation of an otherwise palatable saccharin cue when repeated saccharin-heroin pairings occurred every 24h (Experiment 1) or every 48h (Experiment 2). The results showed that, while both the 118AA and 118GG mice demonstrated robust avoidance of the heroin-paired saccharin cue following daily taste-drug pairings, only the 118AA mice suppressed intake of the heroin-paired saccharin cue when 48h elapsed between each taste-drug pairing. Humanized 118GG mice, then, defend their intake of the sweet cue despite saccharin-heroin pairings and this effect is illuminated by the use of spaced, rather than massed, trials. Given that this pattern of strain difference is not evident with saccharin-cocaine pairings (Freet et al., 2015), reduced avoidance of the heroin-paired saccharin cue by the 118GG mice may be due to an interaction between the opiate and the subjects' drive for the sweet or, alternatively, to differential downstream sensitivity to the aversive kappa mediated properties of the drug. These alternative hypotheses are addressed.