A. D. Lê

Effects of Naltrexone and Fluoxetine on Alcohol Self-Administration and Reinstatement of Alcohol Seeking Induced by Priming Injections of Alcohol and Exposure to Stress

We have recently shown that priming injections of alcohol and footshock stress reinstate alcohol seeking in drug-free rats. Here we tested whether naltrexone and fluoxetine, two drugs used in the treatment of alcohol dependence, would affect reinstatement of alcohol seeking induced by these events. We also determined the effects of these drugs on alcohol self-administration during the maintenance phase. Rats were trained to press a lever for a 12% w/v alcohol solution. After stable drug-taking behavior was obtained, lever pressing for alcohol was extinguished. Reinstatement of drug seeking was then determined after priming injections of alcohol (0.24–0.96 g/kg) or exposure to intermittent footshock (5 and 15 min). Rats were pretreated with naltrexone (0.2–0.4 mg/kg) or fluoxetine (2.5–5 mg/kg) during maintenance or during tests for reinstatement. Both naltrexone and fluoxetine decreased lever presses for alcohol during the maintenance phase. Naltrexone blocked alcohol-induced, but not stress-induced reinstatement. In contrast, fluoxetine blocked stress-induced reinstatement, while its effect on alcohol-induced reinstatement was less consistent. The implications of these data to the understanding of relapse to alcohol are discussed.

3-(4-Chloro-2-Morpholin-4-yl-Thiazol-5-yl)-8-(1-Ethylpropyl)-2,6-Dimethyl-Imidazo[1,2-b]Pyridazine: A Novel Brain-Penetrant, Orally Available Corticotropin-Releasing Factor Receptor 1 Antagonist with Efficacy in Animal Models of Alcoholism

We describe a novel corticotropin-releasing factor receptor 1 (CRF1) antagonist with advantageous properties for clinical development, and its in vivo activity in preclinical alcoholism models. 3-(4-Chloro-2-morpholin-4-yl-thiazol-5-yl)-8-(1-ethylpropyl)-2,6-dimethyl-imidazo[1,2-b]pyridazine (MTIP) inhibited 125I-sauvagine binding to rat pituitary membranes and cloned human CRF1 with subnanomolar affinities, with no detectable activity at the CRF2 receptor or other common drug targets. After oral administration to rats, MTIP inhibited 125I-sauvagine binding to rat cerebellar membranes ex vivo with an ED50 of ∼1.3 mg/kg and an oral bioavailability of 91.1%. Compared with R121919 (2,5-dimethyl-3-(6-dimethyl-4-methylpyridin-3-yl)-7-dipropylamino-pyrazolo[1,5-a]pyrimidine) and CP154526 (N-butyl-N-ethyl-4,9-dimethyl-7-(2,4,6-trimethylphenyl)-3,5,7-triazabicyclo[4.3.0]nona-2,4,8,10-tetraen-2-amine), MTIP had a markedly reduced volume of distribution and clearance. Neither open-field activity nor baseline exploration of an elevated plus-maze was affected by MTIP (1–10 mg/kg). In contrast, MTIP dose-dependently reversed anxiogenic effects of withdrawal from a 3 g/kg alcohol dose. Similarly, MTIP blocked excessive alcohol self-administration in Wistar rats with a history of dependence, and in a genetic model of high alcohol preference, the msP rat, at doses that had no effect in nondependent Wistar rats. Also, MTIP blocked reinstatement of stress-induced alcohol seeking both in postdependent and in genetically selected msP animals, again at doses that were ineffective in nondependent Wistar rats. Based on these findings, MTIP is a promising candidate for treatment of alcohol dependence.

Impulsivity predicts individual susceptibility to high levels of alcohol self-administration

Clinical studies indicate large individual differences in susceptibility to alcohol abuse. Poor behavioral self regulation has been proposed to reflect a predisposing factor. Like humans, only some rats regularly consume large and intoxicating amounts of alcohol. We hypothesized that clinical indications of impaired behavioral self-regulation should be reflected in an animal model of impulse control, and in this study we assessed impulsivity with a delay-of-reward paradigm. We found that three groups representing three levels of impulsivity predicted augmenting levels of alcohol self-administration. Also, overall impulsivity scores were found to be significantly correlated with magnitude of alcohol self-administration. The finding that high impulsivity is linked to elevated consumption represents an animal model that may mirror clinical depictions of an alcohol abuse syndrome. This animal model may help elucidate the neurobiological basis of individual susceptibility to alcohol addiction.

Stress reinstates nicotine seeking but not sucrose solution seeking in rats.

Abstract  Rationale: Intermittent footshock stress effectively reinstates extinguished heroin-, cocaine- and alcohol-taking behaviors, but not behaviors previously maintained by food reinforcers. Here we tested further the generality of the phenomenon of stress-induced reinstatement by determining the effect of footshock on reinstatement of operant responding previously maintained by nicotine or palatable sucrose solutions. Methods: Groups of rats were trained to self-administer either nicotine (0.03 mg/kg per infusion, 14 days) or sucrose (10 or 30% w/v, 14–20 days). After extinction of the nicotine- or the sucrose-reinforced behaviors for 5–15 days, the rats were exposed to intermittent footshock stress (5 and 15 min, 0.8 mA) during tests for reinstatement. Results: Footshock reliably reinstated nicotine seeking after extinction of the drug-reinforced behavior. In contrast, the same parameters of footshock stress did not consistently reinstate operant responding previously maintained by sucrose solutions. Conclusions: These and previous data suggest that stressors may be more effective stimuli for reinstatement of behaviors previously maintained by drug reinforcers as compared with non-drug reinforcers.

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.

Effects of environmental and pharmacological stressors on c-fos and corticotropin-releasing factor mRNA in rat brain: Relationship to the reinstatement of alcohol seeking

We have observed marked heterogeneity among different stressors in their ability to reinstate alcohol seeking in rats. Of the stressors we have tested, only the environmental stressor footshock and the pharmacological stressor yohimbine induce reinstatement. The reasons for such differences among stressors are not known. The purpose of the experiments presented here is to determine the neuroanatomical substrates that underlie these behavioral differences. To this end, we assessed whether stressors effective in inducing reinstatement of alcohol seeking activate a different set of neuronal pathways than do those that are ineffective, using the technique of in situ hybridization of the mRNAs for c-fos, a marker of neuronal activation, and corticotropin-releasing factor (CRF), a stress-related peptide we have shown to be critical to footshock-induced reinstatement of alcohol seeking. Exposure of rats to the environmental stressors footshock, restraint or social defeat, or the pharmacological stressors yohimbine or FG-7142 increased levels of the mRNAs for c-fos and CRF in the brain in a number of areas previously shown to be responsive to stressors. We found regionally specific effects of the stressors on c-fos and CRF mRNA in brain regions associated with the rewarding effects of alcohol and other abused drugs. The two stressors we have previously shown to be effective in inducing reinstatement of alcohol seeking, footshock and yohimbine, induced c-fos mRNA in the shell of the nucleus accumbens, and the basolateral and central amygdalar nuclei. These two stressors also induced CRF mRNA in the dorsal region of the bed nucleus of the stria terminalis. Taken together, these results provide evidence that activity in these regions may be involved in the reinstatement of alcohol seeking induced by these stressors. These results are also in keeping with the previously demonstrated role of CRF neurons in the dorsal bed nucleus of the stria terminalis in the reinstatement of alcohol seeking induced by stress.

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.

Effect of the 5-HT3 antagonist ondansetron on voluntary ethanol intake in rats and mice maintained on a limited access procedure

The effect of the 5-HT3 antagonist ondansetron on ethanol self-administration was examined in a limited access paradigm. Acute administration of ondansetron (0.01 and 0.1 mg/kg) reduced ethanol intake in male Wistar rats by 35%, whilst water intake was unaffected. Both a lower (0.001 mg/kg) and higher dose (1 mg/kg) of ondansetron failed to modify ethanol consumption. Ondansetron did not, however, alter the pharamacokinetic profile of an orally administered dose of ethanol (1 g/kg) over the same dose range. To examine the generality of these findings and to determine if tolerance would develop to the suppressant effects of ondansetron on ethanol intake, male C57BL/6 mice were treated with ondansetron (0.001, 0.01 and 0.1 mg/kg) over 22 days, 30 min prior to scheduled access to ethanol. Both 0.01 and 0.1 mg/kg doses reduced ethanol intake; however, water intake was not altered by either dose. This finding confirms and extends the generality of the effects of 5-HT3 receptor antagonists on ethanol intake across different species and different paradigms of ethanol consumption. More importantly, the present study shows that the reduction in ethanol intake induced by ondansetron was maintained even after a prolonged period of treatment and is not due to an alteration in the absorption or metabolism of ethanol.

Tolerance to and cross-tolerance among ethanol, pentobarbital and chlordiazepoxide

The acute administration of ethanol, pentobarbital and chlordiazepoxide impaired, in a dose-dependent manner, the performance of rats on the moving-belt and two-way shuttle-box avoidance tests. Administration of these drugs for three weeks resulted in tolerance to their motor-impairing effects. Tolerance to ethanol or pentobarbital was characterized by a parallel shift of the dose-response curve to the right. Tolerance to chlordiazepoxide, however, was of greater extent and was accompanied by an apparent flattening of the dose-response curve. Symmetrical cross-tolerance developed between ethanol and pentobarbital. On the other hand, while chlordiazepoxide treatment conferred full cross-tolerance to ethanol and pentobarbital, only partial cross-tolerance to chlordiazepoxide was observed following treatment with ethanol or pentobarbital. These results suggest that at least part of the tolerance to chlordiazepoxide depends on changes in specific benzodiazepine receptors and is independent of the tolerance associated with non-specific changes in the cell membrane.