Escalation patterns of varying periods of heroin access.

The prevalence of opioid abuse and dependence has been on the rise in just the past few years. Animal studies indicate that extended access to heroin produces escalation of intake over time, whereas stable intake is observed under limited-access conditions. Escalation of drug intake has been suggested to model the transition from controlled drug use to compulsive drug seeking and taking. Here, we directly compared the pattern of heroin intake in animals with varying periods of heroin access. Food intake was also monitored over the course of escalation. Rats were allowed to lever press on a fixed-ratio 1 schedule of reinforcement to receive intravenous infusions of heroin for 1, 6, 12, or 23h per day for 14 sessions. The results showed that heroin intake in the 12 and 23h groups markedly increased over time, whereas heroin intake in the 1h group was stable. The 6h group showed a significant but modest escalation of intake. Total heroin intake was similar in the 12 and 23h groups, but the rate of heroin self-administration was two-fold higher in the 12h group compared with the 23h group. Food intake decreased over sessions only in the 12h group. The 12 and 23h groups showed marked physical signs of naloxone-precipitated withdrawal. These findings suggest that 12h heroin access per day may be the optimal access time for producing escalation of heroin intake. The advantages of this model and the potential relevance for studying drug addiction are discussed.

Exposure to nicotine during periadolescence or early adulthood alters aversive and physiological effects induced by ethanol.

The majority of smokers begin their habit during adolescence, which often precedes experimentation with alcohol. Interestingly, very little preclinical work has been done examining how exposure to nicotine during periadolescence impacts the affective properties of alcohol in adulthood. Understanding how periadolescent nicotine exposure influences the aversive effects of alcohol might help to explain why it becomes more acceptable to this preexposed population. Thus, Experiment 1 exposed male Sprague Dawley rats to either saline or nicotine (0.4mg/kg, IP) from postnatal days 34 to 43 (periadolescence) and then examined changes in the aversive effects of alcohol (0, 0.56, 1.0 and 1.8g/kg, IP) in adulthood using the conditioned taste aversion (CTA) design. Changes in blood alcohol concentration (BAC) as well as alcohol-induced hypothermia and locomotor suppression were also assessed. To determine if changes seen were specific to nicotine exposure during periadolescence, the procedures were replicated in adults (Experiment 2). Preexposure to nicotine during periadolescence attenuated the acquisition of the alcohol-induced CTAs (at 1.0g/kg) and the hypothermic effects of alcohol (1.0g/kg). Adult nicotine preexposure produced similar attenuation in alcohol's aversive (at 1.8g/kg) and hypothermic (1.8g/kg) effects. Neither adolescent nor adult nicotine preexposure altered BACs or alcohol-induced locomotor suppression. These results suggest that nicotine may alter the aversive and physiological effects of alcohol, regardless of the age at which exposure occurs, possibly increasing its overall reinforcing value and making it more likely to be consumed.

Suppression of alcohol preference by naltrexone in the rhesus macaque: a critical role of genetic variation at the micro-opioid receptor gene locus.

BACKGROUND: The role of a nonsynonymous A118G polymorphism of the human micro-opioid receptor gene (OPRM1) for alcohol reward and therapeutic efficacy of naltrexone remains controversial. A functionally equivalent OPRM1 C77G polymorphism in rhesus macaques allows this to be addressed under controlled experimental conditions. METHODS: Twenty-one rhesus macaques (13 female rhesus macaques, 8 male rhesus macaques) were genotyped for OPRM1 C77G and studied during 1-hour sessions for preference between an aspartame-sweetened alcohol solution (8.4% vol/vol) and a nonalcoholic control fluid in a baseline session followed by naltrexone (1 mg/kg) and vehicle treatment in a counterbalanced within-subject design. RESULTS: Mixed-model analysis of variance controlling for baseline and sex showed a highly significant (p = .003) interaction between genotype and treatment. Post hoc analysis showed that vehicle-treated 77G carriers had markedly higher alcohol preference than 77C homozygous subjects (p = .001). Following naltrexone administration, 77G carriers decreased their preference (p = .002) and no longer differed from 77C homozygous subjects. In contrast, the latter group was unaffected by treatment and, in fact, showed a trend-level increase of preference following naltrexone. CONCLUSIONS: These results support a critical pharmacogenetic role of OPRM1 variation for therapeutic efficacy of naltrexone.

Protracted withdrawal from alcohol and drugs of abuse impairs long-term potentiation of intrinsic excitability in the juxtacapsular bed nucleus of the stria terminalis.

The juxtacapsular bed nucleus of the stria terminalis (jcBNST) is activated in response to basolateral amygdala (BLA) inputs through the stria terminalis and projects back to the anterior BLA and to the central nucleus of the amygdala. Here we show a form of long-term potentiation of the intrinsic excitability (LTP-IE) of jcBNST neurons in response to high-frequency stimulation of the stria terminalis. This LTP-IE, which was characterized by a decrease in the firing threshold and increased temporal fidelity of firing, was impaired during protracted withdrawal from self-administration of alcohol, cocaine, and heroin. Such impairment was graded and was more pronounced in rats that self-administered amounts of the drugs sufficient to maintain dependence. Dysregulation of the corticotropin-releasing factor (CRF) system has been implicated in manifestation of protracted withdrawal from dependent drug use. Administration of the selective corticotropin-releasing factor receptor 1 (CRF(1)) antagonist R121919 [2,5-dimethyl-3-(6-dimethyl-4-methylpyridin-3-yl)-7-dipropylamino-pyrazolo[1,5-a]pyrimidine)], but not of the CRF(2) antagonist astressin(2)-B, normalized jcBNST LTP-IE in animals with a history of alcohol dependence; repeated, but not acute, administration of CRF itself produced a decreased jcBNST LTP-IE. Thus, changes in the intrinsic properties of jcBNST neurons mediated by chronic activation of the CRF system may contribute to the persistent emotional dysregulation associated with protracted withdrawal.

Corticotropin-releasing factor-1 receptor antagonists decrease heroin self-administration in long- but not short-access rats.

Dysregulation of the stress-related corticotropin-releasing factor (CRF) system has been implicated in the development of drug dependence. The present study examined the effects of administering CRF type 1 (CRF(1)) receptor antagonists on heroin self-administration in animals allowed short (1 hour) or long (8-12 hours) access to intravenous heroin self-administration sessions. The nonpeptide CRF(1) antagonists MJL-1-109-2 (1 hour versus 8 hours access) or R121919 (1 hour versus 12 hours access) were systemically injected in both short- and long-access rats. MJL-1-109-2 (10 mg/kg) and R121919 (10 and 20 mg/kg) reduced heroin self-administration in long-access animals without altering heroin intake in short-access animals. Both MJL-1-109-2 and R121919 decreased first-hour intravenous heroin self-administration selectively in long-access rats, with R121919 decreasing cumulative heroin intake across the 12-hour session. The results demonstrate that blockade of the CRF-CRF(1) receptor system attenuates the increased heroin intake of rats with extended access to the drug.

Genetic and early environmental contributions to alcohol's aversive and physiological effects.

Genetic and early environmental factors interact to influence ethanol's motivational effects. To explore these issues, a reciprocal cross-fostering paradigm was applied to Fischer and Lewis rats. The adult female offspring received vehicle or the kappa opioid antagonist nor-BNI (1 mg/kg) followed by assessments of conditioned taste aversion (CTA), blood alcohol concentrations (BACs) and hypothermia induced by 1.25 g/kg intraperitoneal ethanol. CTA acquisition in the in-fostered Fischer and Lewis animals did not differ; however, the Fischer maternal environment produced stronger acquisition in the cross-fostered Lewis rats versus their in-fostered counterparts. CTAs in the Fischer rats were not affected by cross-fostering. In extinction, the in-fostered Lewis animals displayed stronger aversions than the Fischer groups on two trials (of 12) whereas the cross-fostered Lewis differed from the Fischer groups on nine trials. Despite these CTA effects, Lewis rats exhibited higher BACs and stronger hypothermic responses than Fischer with no cross-fostering effects in either strain. No phenotypes were affected by nor-BNI. These data extend previous findings dissociating the aversive and peripheral physiological effects of ethanol in female Fischer and Lewis rats, and highlight the importance of genetic and early environmental factors in shaping subsequent responses to alcohol's motivational effects in adulthood.

The effects of nicotine on ethanol-induced conditioned taste aversions in Long-Evans rats.

RATIONALE: Overall drug acceptability is thought to be a function of the balance between its rewarding and aversive effects, the latter of which is reportedly affected by polydrug use. OBJECTIVES: Given that nicotine and alcohol are commonly co-used, the present experiments sought to assess nicotine's impact on ethanol's aversive effects within a conditioned taste aversion design. MATERIALS AND METHODS: Experiment 1 examined various doses of nicotine (0, 0.4, 0.8, 1.2 mg/kg) to determine a behaviorally active dose, and experiment 2 examined various doses of ethanol (0, 0.5, 1.0, 1.5 g/kg) to determine a dose that produced intermediate aversions. Experiment 3 then examined the aversive effects of nicotine (0.8 mg/kg) and ethanol (1.0 g/kg) alone and in combination. Additionally, nicotine's effects on blood alcohol concentrations (BAC) and ethanol-induced hypothermia were examined. RESULTS: Nicotine and ethanol combined produced aversions significantly greater than those produced by either drug alone or the summed aversive effects of the individual compounds. These effects were unrelated to changes in BAC, but nicotine and ethanol combined produced a prolonged hypothermic effect which may contribute to the increased aversions induced by the combination. CONCLUSIONS: These data demonstrate that nicotine may interact with ethanol, increasing ethanol's aversive effects. Although the rewarding effects of concurrently administered nicotine and ethanol were not assessed, these data do indicate that the reported high incidence of nicotine and ethanol co-use is unlikely due to reductions in the aversiveness of ethanol with concurrently administered nicotine. It is more likely attributable to nicotine-related changes in ethanol's rewarding effects.

Dissociation between the aversive and pharmacokinetic effects of ethanol in female Fischer and Lewis rats.

In humans and laboratory animal models, vulnerability to alcohol abuse is influenced by endogenous factors such as genotype. Using the inbred Fischer and Lewis rat strains, we previously reported stronger conditioned taste aversions (CTA) in male Fischer rats that could not be predicted by genotypic differences in alcohol absorption [Roma PG, Flint WW, Higley JD, Riley AL. Assessment of the aversive and rewarding effects of alcohol in Fischer and Lewis rats. Psychopharmacology (Berl) 2006;189:187-99]. The present study made similar assessments in Fischer and Lewis females via four-trial CTA induced by 1 or 1.5 g/kg intraperitoneal (IP) ethanol (n=10-12/strain/dose) as well as measures of blood alcohol concentrations (BAC) at 15, 60 and 180 min post-injection with 1.5 g/kg IP ethanol or saline (n=7-8/strain/dose). Dose-dependent CTAs were produced, but the strains did not differ from each other in these measures; however, BACs in the Lewis females were significantly higher than Fischer at all three time points. As with males of the Fischer and Lewis genotypes, a dissociation between BACs and the aversive effects of alcohol was observed. These data are the first assessments of these particular phenotypes in Fischer and Lewis females, and when considered with the historical data, suggest a Genotype x Sex interaction in the centrally mediated sensitivity to alcohol's aversive effects.

Viral vector-induced amygdala NPY overexpression reverses increased alcohol intake caused by repeated deprivations in Wistar rats.

Acute administration of neuropeptide Y (NPY) modulates alcohol intake in genetic and chemical models of high intake, while leaving intake unaffected during 'normal' or baseline conditions. In non-selected, normal rat lines, alcohol consumption can be increased by prolonged exposure to alcohol, and it is unclear what effect a constitutive increase in NPY function will have on alcohol intake. The purpose of the present study was to examine the effects on alcohol intake of an inducible, constitutive overexpression of NPY, one of the most abundant neuropeptides in the central nervous system. A liquid diet was used in combination with repeated alcohol deprivation sessions to increase alcohol intake in normal Wistar rats. We then examined the effect of NPY overexpression in the amygdala on excessive alcohol intake produced by prolonged exposure to alcohol and alcohol deprivation. Repeated withdrawal increased alcohol consumption in a 24-h continuous access two-bottle choice model. Both the number of withdrawals as well as the length of the withdrawal periods affected alcohol consumption with an increased intake resulting from multiple withdrawals and the alcohol deprivation effect being enhanced by longer periods of abstinence. The increase in intake following repeated abstinence was blunted by intra-amygdala administration of a Sindbis viral vector containing NPY cDNA. Amygdala NPY overexpression also was demonstrated to be anxiolytic in the open field test. Repeated withdrawal in combination with a history of alcohol consumption significantly elevated alcohol intake, and the amygdala may mediate the transition to high-drinking states in this model.

Association of a functional polymorphism in the mu-opioid receptor gene with alcohol response and consumption in male rhesus macaques.

CONTEXT: Innate differences in opioid neurotransmission are hypothesized to influence abuse liability of alcohol. In humans, a variant of the mu-opioid receptor gene (OPRM1A118G) increases receptor affinity, alcohol-induced euphoria, and risk for alcohol use disorders. OBJECTIVE: To determine whether a variant in the mu-opioid receptor gene (OPRM1C77G) that increases affinity of the receptor is associated with alcohol response and consumption in macaques. DESIGN: Young adult rhesus macaques (Macaca mulatta) were intravenously administered 2.0 to 2.1 g of ethanol per kilogram of body weight and assessed for alcohol response. Animals were later given simultaneous access to an aspartame-sweetened 8.4% (vol/vol) ethanol solution and a vehicle for 1 hour per day, 5 days a week, for a period of 6 weeks. Animals (N = 82) were genotyped for the OPRM1C77G polymorphism; the effects of the genotype on alcohol response and consumption were determined by analysis of variance, with sex included as a nominal independent variable. MAIN OUTCOME MEASURES: Alcohol response (ataxia, stimulation, and sedation), average alcohol consumption, the percentage of days during which an animal consumed alcohol at a level sufficient to produce intoxication (> or =0.67 g of alcohol per kilogram of body weight), and alcohol preference (calculated as 100 x {alcoholic solution/[alcoholic solution + nonalcoholic solution]}). RESULTS: Increased alcohol-induced stimulation was observed among male macaques carrying the OPRM1C77G allele. OPRM1C77G allele carriers consumed more ethanol and exhibited increased ethanol preference. Male carriers of the OPRM1C77G allele exhibited higher alcohol preference and consumption, and drank to intoxication more frequently than did C/C males. CONCLUSIONS: These findings demonstrate that the rhesus macaques' equivalent of the OPRM1A118G variant is associated with increased alcohol response, consumption, and preference. Our results reveal effects of the OPRM1C77G genotype to be male-restricted or more marked among male macaques. This is of interest, given the fact that early-onset type II alcoholism is more common among men and that, among addicted individuals, men are more responsive to mu-opioid receptor blockade.

Extended access to nicotine self-administration leads to dependence: Circadian measures, withdrawal measures, and extinction behavior in rats.

The present study characterized nicotine intake, circadian patterns of food and water intake, precipitated somatic signs of withdrawal, and extinction of nicotine-seeking behavior in rats with 23-h access to intravenous self-administration (IVSA). Separate groups of animals were allowed access to nicotine IVSA (0.015, n = 9; 0.03, n = 14; 0.06, n = 16; mg/kg/0.1 ml infusion/s; fixed ratio 1) and trained to nosepoke for food and water 23 h/day for 40 consecutive days. Somatic signs of nicotine withdrawal were examined following saline or mecamylamine administration (1.5 mg/kg i.p.), and extinction of nicotine-seeking behavior was assessed. A dose-dependent decrease in lever responding and an increase in nicotine intake were observed, with the highest nicotine dose producing the lowest amount of lever responding and the highest amount of nicotine intake. Nicotine acutely reduced diurnal and nocturnal food intake, producing smaller and fewer meals, and an increased rate of eating. Differences in rate of nicotine intake between the light and dark phase decreased significantly, especially in rats receiving higher unit nicotine doses (0.03 and 0.06 mg/kg), along with long-term decreases in the circadian profile and amplitude of feeding. Mecamylamine precipitated robust withdrawal signs, the magnitude of which was positively correlated with the total amount of self-administered nicotine. Extinction of nicotine-seeking behavior was observed and was facilitated by removal of nicotine-associated cues. The results demonstrate that rats will self-administer nicotine to the point of producing dependence, as measured by somatic signs, resistance to extinction, and measures of food intake.

Conditioned withdrawal drives heroin consumption and decreases reward sensitivity.

Aspects of drug withdrawal may become conditioned to previously neutral environmental stimuli via classical conditioning processes. Nevertheless, the significance of conditioned withdrawal effects in motivating drug intake remains largely unexplored. Here, we investigated the effects of conditioned withdrawal in modulating heroin consumption and brain reward sensitivity in rats. Rats intravenously self-administered heroin (20 microg/infusion) during 0 h (control), 1 h (nondependent), or 23 h (dependent) sessions and had daily intracranial self-stimulation (ICSS) thresholds assessed. ICSS thresholds remained stable and unaltered in control rats. In nondependent rats, heroin self-administration induced a transient activation of reward systems, reflected in lowering of ICSS thresholds. In dependent rats, heroin intake escalated across sessions and was associated with a gradual decrease in reward sensitivity, reflected in progressively elevated ICSS thresholds. Thus, as dependence develops, heroin may be consumed not only for its acute reward-facilitating effects, but also to counter persistent deficits in reward sensitivity. In nondependent rats, the opioid receptor antagonist naloxone (30 microg/kg) increased heroin consumption and reversed heroin-induced lowering of ICSS thresholds, effects resistant to classical conditioning. In contrast, in dependent rats naloxone (30 microg/kg) increased heroin consumption and also elevated ICSS thresholds above their already elevated baseline levels (i.e., precipitated withdrawal). Most importantly, stimuli repeatedly paired with naloxone-precipitated withdrawal provoked heroin consumption and elevated ICSS thresholds in dependent rats. Thus, conditioned stimuli predicting the onset of heroin withdrawal, and hence the reward deficits coupled with this state, may play a critical role in provoking craving and relapse in human opiate addicts.

Unlimited access to heroin self-administration: independent motivational markers of opiate dependence.

The goal of the present study was to develop and validate an animal model of unlimited access to intravenous heroin self-administration combined with responding for food and water to characterize the transition to drug dependence. Male Wistar rats were allowed to lever press for heroin (60 microg/kg/0.1 ml infusion/s; fixed ratio 1; 20-s time out) and nosepoke for food and water in consecutive, daily 23-h sessions. Daily heroin intake increased over days, reaching significance by Day 14. Drug-taking increased across the circadian cycle, reflected as increases in both the nocturnal peak and diurnal nadir of heroin intake. Changes in the circadian pattern of food intake and meal patterning preceded and paralleled the changes in heroin intake. By Day 7, the circadian amplitude of feeding was blunted. Nocturnal intake decreased because rats consumed smaller and briefer meals. Diurnal intake increased due to increased meal frequency, whereas total daily food intake decreased. To control for time or experience in the self-administration boxes as a possible confound, rats with saline (no drug) tethers were tested and did not display significant changes in food intake pattern. Body weight gain slowed slightly in heroin rats relative to saline controls. Separate groups of rats revealed that significant physical dependence as measured by physical signs of opiate withdrawal following a naloxone injection (1.0 mg/kg, subcutaneous (s.c.)) was reached by Day 14. Significant increases in heroin intake could be produced using low doses of naloxone (0.003-0.03 mg/kg, s.c.) on days 28-31 of heroin access. After 6 weeks of heroin self-administration, rats injected with buprenorphine (0, 0.01, 0.04, and 0.2 mg/kg, s.c.) showed a dose-dependent reduction in heroin intake. Changes in the pattern of drug and food intake in the present unlimited heroin access model may serve as independent motivational markers for the transition to a drug-dependent state.

Neurobiological mechanisms in the transition from drug use to drug dependence.

Drug addiction is a chronic relapsing disorder characterized by compulsive drug intake, loss of control over intake, and impairment in social and occupational function. Animal models have been developed for various stages of the addiction cycle with a focus in our work on the motivational effects of drug dependence. A conceptual framework focused on allostatic changes in reward function that lead to excessive drug intake provides a heuristic framework with which to identify the neurobiologic mechanisms involved in the development of drug addiction. Neuropharmacologic studies in animal models have provided evidence for the dysregulation of specific neurochemical mechanisms in specific brain reward and stress circuits that provide the negative motivational state that drives addiction. The allostatic model integrates molecular, cellular and circuitry neuroadaptations in brain motivational systems produced by chronic drug ingestion with genetic vulnerability, and provides a new opportunity to translate advances in animal studies to the human condition.

Chronic opioid exposure produces increased heroin self-administration in rats.

The purpose of this study was to determine the significance of chronic opioid exposure on the level of heroin self-administration in the rat. Rats were divided into morphine (M, subcutaneous morphine pellets) and placebo (P, subcutaneous placebo pellets) groups and self-administered several different doses of heroin during daily limited access 1-h sessions and prolonged access 8-h sessions. No effects on heroin self-administration occurred when the rats were implanted with morphine pellets and allowed to self-administer heroin in a limited access paradigm (1-h group). However, rats with morphine pellet implantation showed a rapid escalation (Days 0-3 post-pellet) in heroin self-administration in the more prolonged access group (8 h group) compared to placebo-pelleted animals also with 8-h access. Ultimately, placebo-pelleted 8-h exposed animals showed an escalation in heroin self-administration but this effect was delayed until Days 16-18 post-pellet. These results suggest that passive administration of morphine sufficient to produce and maintain dependence facilitates escalation in heroin intake.