Douglas Funk

Increased Vulnerability to Nicotine Self-Administration and Relapse in Alcohol-Naive Offspring of Rats Selectively Bred for High Alcohol Intake

The prevalence of smoking in human alcoholics is substantially higher than in the general population, and results from twin studies suggest that a shared genetic vulnerability underlies alcohol and nicotine addiction. Here, we directly tested this hypothesis by examining nicotine-taking behavior in alcohol-naive offspring of alcohol-preferring (P) rats and alcohol-nonpreferring (NP) rats that had been selectively bred for high and low alcohol intake. The self-administration of intravenous nicotine (0.015–0.060 mg/kg per infusion) in P rats was more than twice than that of NP rats. Nicotine seeking induced by reexposure to nicotine cues in extinction tests was also substantially greater in P rats than in NP rats. In a subsequent relapse test, priming nicotine injections reinstated drug seeking in P rats but not NP rats. P rats also self-administered higher amounts of oral sucrose (1–20%) than NP rats, a finding consistent with previous reports. In contrast, self-administration of intravenous cocaine (0.1875–1.125 mg/kg per infusion) was remarkably similar in the P and NP rats; however, P–NP differences in cocaine seeking emerged in subsequent extinction and cocaine priming-induced reinstatement tests. In both cases, lever responding was higher in P rats than in NP rats. Thus, alcohol-naive offspring of rats genetically selected for high alcohol intake are highly susceptible to nicotine self-administration and relapse, and this susceptibility is not likely caused by general reward deficits in NP rats. The present findings provide experimental evidence for the hypothesis that a shared genetic determinant accounts for the co-abuse of nicotine and alcohol.

Stress-Induced Reinstatement of Drug Seeking: 20 Years of Progress.

In human addicts, drug relapse and craving are often provoked by stress. Since 1995, this clinical scenario has been studied using a rat model of stress-induced reinstatement of drug seeking. Here, we first discuss the generality of stress-induced reinstatement to different drugs of abuse, different stressors, and different behavioral procedures. We also discuss neuropharmacological mechanisms, and brain areas and circuits controlling stress-induced reinstatement of drug seeking. We conclude by discussing results from translational human laboratory studies and clinical trials that were inspired by results from rat studies on stress-induced reinstatement. Our main conclusions are (1) The phenomenon of stress-induced reinstatement, first shown with an intermittent footshock stressor in rats trained to self-administer heroin, generalizes to other abused drugs, including cocaine, methamphetamine, nicotine, and alcohol, and is also observed in the conditioned place preference model in rats and mice. This phenomenon, however, is stressor specific and not all stressors induce reinstatement of drug seeking. (2) Neuropharmacological studies indicate the involvement of corticotropin-releasing factor (CRF), noradrenaline, dopamine, glutamate, kappa/dynorphin, and several other peptide and neurotransmitter systems in stress-induced reinstatement. Neuropharmacology and circuitry studies indicate the involvement of CRF and noradrenaline transmission in bed nucleus of stria terminalis and central amygdala, and dopamine, CRF, kappa/dynorphin, and glutamate transmission in other components of the mesocorticolimbic dopamine system (ventral tegmental area, medial prefrontal cortex, orbitofrontal cortex, and nucleus accumbens). (3) Translational human laboratory studies and a recent clinical trial study show the efficacy of alpha-2 adrenoceptor agonists in decreasing stress-induced drug craving and stress-induced initial heroin lapse.

Nicotine Self-Administration, Extinction Responding and Reinstatement in Adolescent and Adult Male Rats: Evidence Against a Biological Vulnerability to Nicotine Addiction during Adolescence

Initiation of smoking behavior typically occurs during adolescence and rarely occurs during adulthood. Despite this epidemiological evidence, relatively little is known about possible neurobiological differences in the response to nicotine in adolescents that might make them more vulnerable to nicotine addiction. In the current study, we assessed nicotine self-administration under fixed ratio (FR) and progressive ratio (PR) reinforcement schedules in adolescent (postnatal day (P) 33–35) and adult (P91–94) rats. We then assessed extinction and reinstatement of nicotine seeking in adulthood in rats that initiated nicotine self-administration during either adolescence or adulthood. Nicotine self-administration (0.03 mg/kg/infusion, i.v.) was higher in adult rats than in adolescent rats under FR5 and PR reinforcement schedules; no age differences in nicotine self-administration were observed under FR1 or FR2 reinforcement schedules. In contrast, saccharin self-administration under FR5 and PR reinforcement schedules was similar in both age groups, potentially ruling out age differences in general performance. Rats that initiated nicotine self-administration as adults demonstrated a greater resistance to extinction of nicotine taking behavior when saline was substituted for nicotine than rats that initiated self-administration as adolescents. Reinstatement of nicotine seeking following nicotine priming injections (0.075, 0.15, 0.3 mg/kg, s.c.) was independent of the age of onset of nicotine self-administration. The present data from established rat models of drug self-administration and drug relapse suggest that nicotine is less reinforcing in adolescent compared with adult rats and that processes other than the reinforcing effects of nicotine may be involved in the greater susceptibility to smoking during the adolescent developmental stage.

Effects of unconditioned and conditioned social defeat on alcohol self-administration and reinstatement of alcohol seeking in rats

Rationale and objectivesWe and others have shown that a stressor commonly used in laboratory studies, intermittent footshock, reinstates alcohol seeking in a rat relapse model. The effects of more ethologically relevant stressors on reinstatement have not been examined. Here, we characterized the effects of social defeat (a naturalistic stressor) or a cue associated with the defeat experience on reinstatement of alcohol seeking. We also examined the effect of unconditioned and conditioned social defeat on alcohol self-administration.MethodsRats were trained to self-administer alcohol (12% w/v, 1 h day−1), and after stable responding, one group of animals received five exposures to social defeat paired with peppermint odor prior to daily self-administration sessions. After three more self-administration sessions, these rats were tested for the effects of the peppermint odor cue on self-administration. In another group of rats, the effects of three daily exposures to social defeat paired with peppermint odor on extinction of responding were examined. After further extinction sessions, the effect of the odor cue on reinstatement was tested in these animals. The acute effect of social defeat on reinstatement was examined in another group of animals.ResultsAcute exposure to social defeat decreased alcohol self-administration, reduced rates of responding during extinction, and did not reinstate alcohol seeking. Exposure to a discrete odor cue previously paired with social defeat decreased alcohol self-administration but induced modest reinstatement of alcohol seeking.ConclusionsResults provide the first demonstration of reinstatement of alcohol seeking by a cue paired with social defeat and are also in agreement with previous findings on the suppressive effect of social defeat stress on alcohol self-administration.

Effects of opioid receptor blockade on the renewal of alcohol seeking induced by context : relationship to c-fos mRNA expression

Contextual stimuli associated with alcohol availability can induce alcohol seeking during abstinence. Using a renewal procedure, we tested the effect of opioid receptor blockade on context‐induced alcohol seeking, and its relationship to the activity of brain sites involved in learning and reward. Thirty‐six male Wistar rats were trained to lever press for a 12% (w/v) alcohol solution before undergoing extinction sessions (no alcohol delivery). Half of the rats underwent training, extinction and testing in a single context with a distinct set of olfactory, visual, auditory and tactile properties [training, extinction and test in Context A (AAA)]. The other half were trained and extinguished in different contexts and returned to the training context on the test day [training, extinction and test in Contexts A, B and A, respectively (ABA)]. On the test day, the rats from each condition were pre‐treated with either saline or 1 mg/kg naltrexone (s.c.) and tested for alcohol seeking. Immediately following the test session, rats were killed and their brains were analysed for c‐fos mRNA expression using in‐situ hybridization. Re‐exposure to the alcohol‐associated context (ABA) significantly increased operant behaviour on the previously active lever relative to the AAA groups and this increased responding was associated with increased c‐fos mRNA expression in the basal and lateral amygdala and the CA3 subregion of the hippocampus. Naltrexone pre‐treatment attenuated context‐induced alcohol seeking and inhibited c‐fos mRNA expression in the lateral amygdala and CA3. Our findings point to a critical role for the basolateral amygdala and dorsal hippocampus in mediating context‐induced renewal of alcohol seeking and suggest that opioidergic mechanisms mediate this effect.

ROLES OF OPIOID RECEPTOR SUBTYPES IN MEDIATING ALCOHOL SEEKING INDUCED BY DISCRETE CUES AND CONTEXT

The aim of this study was to assess the effects of selective blockade of the delta (DOP) or mu (MOP) opioid receptors on alcohol‐seeking induced by discrete cues and context. In Experiment 1, rats were trained to self‐administer alcohol in an environment with distinct sensory properties. After extinction in a different context with separate sensory properties, rats were tested for context‐induced renewal in the original context following treatment with the DOP receptor antagonist naltrindole (0–15 mg/kg, i.p.) or the MOP receptor antagonist D‐Phe‐Cys‐Tyr‐D‐Trp‐Orn‐Thr‐Pen‐Thr‐NH2 (CTOP) (0–3 μg/4 μL, i.c.v.). In Experiment 2, reinstatement was tested with the presentation of a discrete light + tone cue previously associated with alcohol delivery, following extinction without the cue. The effects of naltrindole (0–5 mg/kg, i.p.) or CTOP (0–3 μg/4 μL, i.c.v.) were assessed. For context‐induced renewal, 7.5 mg/kg naltrindole reduced responding without affecting locomotor activity. Both doses of CTOP attenuated responding in the first 15 min of the renewal test session; however, total responses did not differ at the end of the session. For discrete‐cue‐induced reinstatement, 1 and 5 mg/kg naltrindole attenuated responding but CTOP had no effect. We conclude that whereas DOP receptors mediate alcohol‐seeking induced by discrete cues and context, MOP receptors may play a modest role only in context‐induced renewal. These findings point to a differential involvement of opioid receptor subtypes in the effects of different kinds of conditioned stimuli on alcohol‐seeking and support a more prominent role for DOP receptors.

Acute nicotine enhances c-fos mRNA expression differentially in reward-related substrates of adolescent and adult rat brain

A number of studies have demonstrated that adolescent rodents are more sensitive to the rewarding effects of nicotine compared to adults. To help determine the potential brain circuitry involved, we investigated the effect of acute nicotine administration (0.4 or 0.8mg/kg, s.c.) on the expression of c-fos mRNA in the brains of adolescent (P35) and adult (P67-70) male Wistar rats using in situ hybridization. Nicotine administration increased c-fos mRNA expression in several brain regions, including the central amygdala, locus coeruleus, nucleus accumbens core, paraventricular nucleus of the hypothalamus and lateral septum of adolescent and adult rats. Nicotine increased c-fos mRNA expression more robustly in the bed nucleus of the stria terminalis, nucleus accumbens shell and ventral tegmental area in adolescent rats. The current results suggest that nicotine may have greater activational effects in brain regions associated with reward in adolescent rats and may help to explain the differences between adolescents and adults in behavioral responses to nicotine.

The role of kappa opioid receptors in stress-induced reinstatement of alcohol seeking in rats.

Stress is related to heavy alcohol use and relapse in alcoholics. Using the reinstatement model, we have shown that corticotropin‐releasing factor (CRF) underlies stress‐induced relapse to alcohol seeking in laboratory rodents. Little is known about how other neurotransmitters interact with CRF in these effects. Dynorphin and its receptor (kappa opioid receptor, KOR) are involved in stress responses and in alcohol seeking. KOR and CRF receptors (CRF R) may interact in the production of stress‐related behaviors but it is not known whether this interaction is involved in reinstatement of alcohol seeking.

Role of corticotropin-releasing factor in the median raphe nucleus in yohimbine-induced reinstatement of alcohol seeking in rats

The pharmacological stressor yohimbine increases ongoing alcohol self‐administration and reinstates alcohol seeking in rats. This effect is attenuated by systemic injections of a corticotropin‐releasing factor (CRF) antagonist. The brain sites involved in CRFs role in yohimbine‐induced alcohol taking and seeking are unknown. We report that injections of the CRF receptor antagonist d‐Phe CRF into the median raphe nucleus (MRN) attenuated yohimbine‐induced reinstatement of alcohol seeking but had no effect on yohimbine‐induced increases in alcohol intake during ongoing self‐administration. Results indicate an important role of MRN CRF receptors in yohimbine‐induced reinstatement of alcohol seeking but not yohimbine‐induced increases in alcohol intake.

Role of the kappa-opioid receptor system in stress-induced reinstatement of nicotine seeking in rats.

RATIONALE The correlation between stress and smoking is well established. The mechanisms that underlie this relationship are, however, unclear. Recent data suggest that the kappa-opioid system is involved in the mediation of negative affective states associated with stress thereby promoting drug addiction and relapse. Pharmacological treatments targeting the kappa-opioid system and this mechanism may prove to be useful therapeutics for nicotine addiction in the future. OBJECTIVES We sought to determine whether there was a stress-specific role of the kappa-opioid system in nicotine seeking behavior. METHOD Groups of male Long Evans rats were trained to self-administer nicotine intravenously; their operant responding for nicotine was extinguished prior to tests of reinstatement. Pretreatment with systemic injections of the kappa-opioid receptor (KOR) antagonist nor-binaltorphimine (nor-BNI) was given prior to tests of stress (systemic injections of yohimbine (YOH)) or cue-induced reinstatement of nicotine seeking. Systemic injections of the KOR agonist U50,488 were also given in a test for reinstatement of nicotine seeking. RESULTS Nor-BNI pretreatment at 1h and 24h prior to testing was able to block YOH-induced, but not cue-induced reinstatement of nicotine seeking. U50,488 reinstated nicotine seeking behavior in a dose-dependent manner. CONCLUSIONS These findings support the hypothesis that the kappa-opioid system is involved in relapse to nicotine seeking induced by stress, but not by conditioned cues. KOR antagonists such as nor-BNI may therefore be useful novel therapeutic agents for decreasing the risk of stress-induced drug relapse.