Neurotransmitter Specific Roles in the Basolateral Amygdala and Their Effect on Ethanol-Seeking and Intake

poster abstractRelapse is a major problem in alcoholism treatment. Environmental cues can act as triggers that can reinstate alcohol use. By understanding specific neurochemical processes in the brain we can develop new treatments which will be focused on relapse prevention. Specifically the basolateral amygdala (BLA) which is involved in motivated responding and cue-induced reinstatement is of key interest. The aim of this study was to dissect drinking behaviors in an animal model (Long Evans rats) into two parts: appetitive (related to cue-induced reinstatement) and consummatory (related to primary reinforcement). Using operant chambers, lever pressing was a measure of an appetitive response and intake measured consummatory response. We looked at involvement of specific neurotransmitters in the BLA via microinjections of a dopamine and a glutamate antagonist. After initial lever press training, the rats received weekly microinjections of the two drugs as well as artificial cerebrospinal fluid in a randomized order to study their effects on ethanol (n = 5-8/group) and sucrose (n = 6-11/group) responding. Preliminary findings suggest both neurotransmitter- and behavior- specific effects. That is, manipulations of the BLA do not affect the intake of either sucrose or ethanol. This is consistent with findings suggesting that this area is not involved in processing primary reinforcement. However, the administration of the glutamate antagonist (but not the dopamine antagonist) in the BLA had a tendency to decrease reinforcer-seeking at the highest dose (p<0.09). This effect was not reinforcer specific, suggesting that the BLA glutamate activity may be involved in reinforcer-seeking rather than specifically in ethanol-seeking. Overall, the findings of this study will provide new insight into neurotransmitter function in the BLA, its relationship to alcohol intake, and will hopefully drive future research into development of new drugs that will reduce alcohol cravings and chance of relapse

The Effects of Long-Term Varenicline Administration on Ethanol and Sucrose Seeking and Self-Administration in Male P Rats

Background Varenicline, a partial agonist at α4β2 and full agonist at α7 nicotinic cholinergic receptors, is FDA-approved for treatment of smoking cessation and has been found to reduce alcohol craving in clinical populations. In rodents, varenicline decreases free-choice ethanol (EtOH) intake with somewhat mixed findings in operant paradigms that utilize a combined appetitive/consummatory response. Methods The present experiment utilized an operant paradigm that procedurally separates appetitive from consummatory responding and a “reward-blocking” approach (i.e., rats were able to consume EtOH during treatment) to better understand the efficacy of varenicline as a treatment for EtOH self-administration and subsequent EtOH seeking. Separate groups of EtOH- and sucrose-reinforced alcohol-preferring, male P rats experienced alternating cycles of vehicle (2-week cycles) and varenicline (0.3, 1.0, and 2.0 mg/kg self-administered in a gelatin preparation) treatment (3-week cycles) prior to daily sessions where a single lever press resulted in 20 minutes of reinforcer access. At the end of each cycle, a single extinction session assessed the seeking response in the absence of drug pretreatment. Results Varenicline dose dependently decreased EtOH intake. Sucrose intake was largely unaffected, with no overall treatment effects and only sporadic days where the medium and high dose differed from vehicle. Neither sucrose nor EtOH seeking was significantly decreased by varenicline, and there were no treatment effects on either lick or lever-press latency. Overall effect sizes were much greater for both drinking and seeking in the EtOH group as compared to the sucrose group. Conclusions Varenicline effectively attenuates EtOH self-administration during treatment, but the experience with EtOH consumption while varenicline is “on board” is not sufficient to alter subsequent EtOH seeking. The overall pattern of findings indicates that varenicline blocks the rewarding properties of EtOH while not substituting for EtOH, that the nonspecific effects on an alternate reinforcer are negligible, and that blood levels of varenicline need to be maintained in order for treatment to remain effective

Tolcapone suppresses ethanol intake in alcohol-preferring rats performing a novel cued access protocol

BACKGROUND: Dopamine (DA) has been shown to play a central role in regulating motivated behavior and encoding reward. Chronic drug abuse elicits a state of hypodopaminergia in the mesocorticolimbic (MCL) system in both humans and preclinical rodent models of addiction, including those modeling alcohol use disorders (AUD). METHODS: Working under the hypothesis that reductions in the bioavailability of DA play an integral role in the expression of the excessive drinking phenotype, the catechol-O-methyltransferase (COMT) inhibitor tolcapone was used as a means to amplify cortical DA concentration and drinking behaviors were then assessed. Sucrose and ethanol (EtOH) consumption were measured in P and Wistar rats in both a free choice drinking protocol and a novel cued access protocol. RESULTS: Tolcapone attenuated the consumption of EtOH, and to a lesser extent sucrose, in P rats in the cued access protocol, while no effect was observed in the free choice drinking protocol. Tolcapone also decreased EtOH consumption in high drinking Wistar rats. A follow-up experiment using the indirect DA agonist d-amphetamine showed no change in EtOH consumption. CONCLUSIONS: Collectively, these data suggest that COMT inhibitors may be capable of alleviating the extremely motivating or salient nature of stimuli associated with alcohol. The hypothesis is put forth that the relative specificity of tolcapone for cortical DA systems may mediate the suppression of the high seeking/drinking phenotype

Intravenous Alcohol Self-Administration in the P Rat

Alcohol consumption produces a complex array of effects that can be divided into two types: the explicit pharmacological effects of ethanol (which can be temporally separate from time of intake) and the more temporally “relevant” effects (primarily olfactory and taste) that bridge the time from intake to onset of the pharmacological effects. Intravenous (IV) self-administration of ethanol limits the confounding “non-pharmacological” effects associated with oral consumption, allows for controlled and precise dosing, and bypasses first order absorption kinetics, allowing for more direct and better-controlled assessment of alcohol’s effect on the brain. IV ethanol self-administration has been reliably demonstrated in mouse and human experimental models; however, models of IV self-administration have been historically problematic in the rat. An operant multiple-schedule study design was used to elucidate the role of each component of a compound IV-ethanol plus oral-sucrose reinforcer. Male alcohol-preferring P rats had free access to both food and water during all IV self-administration sessions. Animals were trained to press a lever for orally delivered 1% sucrose (1S) on a fixed ratio 4 schedule, and then surgically implanted with an indwelling jugular catheter. Animals were then trained to respond on a multiple FR4-FR4 schedule composed of alternating 2.5-min components across 30-min sessions. For the multiple schedule, two components were used: an oral 1S only and an oral 1S plus IV 20% ethanol (25 mg/kg/injection). Average total ethanol intake was 0.47 ± 0.04 g/kg. We found significantly higher earning of sucrose-only reinforcers and greater sucrose-lever error responding relative to the compound oral-sucrose plus IV-ethanol reinforcer. These response patterns suggest that sucrose, not ethanol, was responsible for driving overall responding. The work with a compound IV ethanol-oral sucrose reinforcer presented here suggests that the existing intravenous ethanol self-administration methodology cannot overcome the aversive properties of ethanol via this route in the rat

Excessive Ethanol-Seeking as Related to Impulsive Behavior as Measured by Delay Discounting

poster abstractThe discounting of delayed rewards, a specific type of impulsive behavior, has been associated with alcohol use disorders. However, the way in which this characteristic is related to the genetic and behavioral paths which lead to high-drinking is a newly emerging area of alcohol research. Rodents selectively bred for extreme high vs. low alcohol preference have shown parallel patterns of delay discounting. This study investigated whether or not delayed discounting is preferentially related to ethanol-seeking vs. consumption. Alcohol preferring rats (P; n=5), High Alcohol Drinking rats (HAD2; n=15), and Long Evans rats (LE; n=11) were used and have previously been identified as high seeking/high drinking, moderate seeking/high drinking, and moderate seeking/moderate drinking, respectively. Six levels of delay (0, 2, 4, 8, 12 and 16 seconds) were assessed using a sucrose reinforcer. The average indifference points for each delay were then fitted to hyperbolic equations to yield a single parameter (k). An ANOVA for those values, along with post hoc testing, revealed Ps to have larger k values than both HAD2s and LEs. However, the HAD2s and LEs were not different from each other. A mixed ANOVA for indifference points showed a main effect of Delay (p<.01), Group (p<.01), and no group/delay interaction (p=.08). The main effect of group revealed the same pattern of findings for the indifference points as for the k values. Both the higher k values and the lower indifference points of the Ps indicate their steeper discounting in comparison to both HAD2s and LE. These results suggest that this measure of impulsivity could be associated with the quantity of ethanol-seeking, and not just with the inclination to consume ethanol. These results extend previous findings, and as all animals were ethanol naïve, these results support the idea that increased impulsivity is a characteristic that precedes addictive disorders

Differential COMT expression and behavioral effects of COMT inhibition in male and female Wistar and alcohol preferring rats

Polymorphisms of the catechol-O-methyl transferase (COMT) gene have been associated with alcoholism, suggesting that alterations in the metabolism of catecholamines may be a critical component of the neuropathology of alcoholism. In the current experiments, the COMT inhibitor tolcapone was utilized in an operant behavioral model of reinforcer-seeking and drinking to determine if this compound was capable of remediating the excessive seeking and drinking phenotype of the alcohol-preferring P rat. Tolcapone was administered to male and female alcohol-reinforced P and Wistar rats. Additionally, tolcapone was administered to male sucrose-reinforced P and Wistar rats to determine if its effects also extended to a natural reinforcer. Animals were trained to make an operant response that resulted in 20 min uninterrupted access to the reinforcer solutions. Tolcapone had no effect in female rats on either seeking or consumption of ethanol. However, reductions of both reinforcer seeking and consumption were observed in male P rats, but only of seeking in Wistars. In separate experiments, using reinforcer naïve male and female animals, COMT expression was assessed via Western Blot analysis. Sex differences in COMT expression were also observed, where male P rats exhibited a marked reduction in protein expression relative to females in the PFC. Sex differences were not observed for Wistars or in the striatum and hippocampus. These data complement our previous findings in which tolcapone reduced cue-evoked responses in P rats and further suggest clinical utility of COMT inhibitors in the treatment of addiction disorders, specifically in male high drinkers

Tolcapone suppresses ethanol intake in alcohol-preferring rats performing a novel cued access protocol

BACKGROUND: Dopamine (DA) has been shown to play a central role in regulating motivated behavior and encoding reward. Chronic drug abuse elicits a state of hypodopaminergia in the mesocorticolimbic (MCL) system in both humans and preclinical rodent models of addiction, including those modeling alcohol use disorders (AUD). METHODS: Working under the hypothesis that reductions in the bioavailability of DA play an integral role in the expression of the excessive drinking phenotype, the catechol-O-methyltransferase (COMT) inhibitor tolcapone was used as a means to amplify cortical DA concentration and drinking behaviors were then assessed. Sucrose and ethanol (EtOH) consumption were measured in P and Wistar rats in both a free choice drinking protocol and a novel cued access protocol. RESULTS: Tolcapone attenuated the consumption of EtOH, and to a lesser extent sucrose, in P rats in the cued access protocol, while no effect was observed in the free choice drinking protocol. Tolcapone also decreased EtOH consumption in high drinking Wistar rats. A follow-up experiment using the indirect DA agonist d-amphetamine showed no change in EtOH consumption. CONCLUSIONS: Collectively, these data suggest that COMT inhibitors may be capable of alleviating the extremely motivating or salient nature of stimuli associated with alcohol. The hypothesis is put forth that the relative specificity of tolcapone for cortical DA systems may mediate the suppression of the high seeking/drinking phenotype

A Critical Review of Front-loading: A Maladaptive Drinking Pattern Driven by Alcohol's Rewarding Effects

This paper is now published open-access at Alcoholism: Clinical and Experimental Research: https://onlinelibrary.wiley.com/doi/10.1111/acer.14924. Front-loading is a drinking pattern where intake is skewed toward the onset of reward access. This phenomenon has been reported across several different alcohol self-administration protocols using a wide variety of species, including humans. The hypothesis of the current review is that front-loading emerges in response to the rewarding effects of alcohol and can be used as a measure of motivation to consume alcohol. Alternative or additional hypotheses for what front-loading may represent are considered and contrasted with the main hypothesis that: 1) front-loading is directed at overcoming chronic and/or metabolic tolerance and 2) front-loading is driven by negative reinforcement. Evidence for each of these explanations is reviewed. We also consider how front-loading has been evaluated statistically in previous research and make recommendations for defining this intake pattern in future studies. Because front-loading may predict long-term maladaptive alcohol drinking patterns leading to the development of alcohol use disorder (AUD), several future directions are proposed to elucidate the relationship between front-loading and AUD

A critical review of front-loading: A maladaptive drinking pattern driven by alcohol's rewarding effects

Front‐loading is a drinking pattern in which alcohol intake is skewed toward the onset of reward access. This phenomenon has been reported across several different alcohol self‐administration protocols in a wide variety of species, including humans. The hypothesis of the current review is that front‐loading emerges in response to the rewarding effects of alcohol and can be used to measure the motivation to consume alcohol. Alternative or additional hypotheses that we consider and contrast with the main hypothesis are that: (1) front‐loading is directed at overcoming behavioral and/or metabolic tolerance and (2) front‐loading is driven by negative reinforcement. Evidence for each of these explanations is reviewed. We also consider how front‐loading has been evaluated statistically in previous research and make recommendations for defining this intake pattern in future studies. Because front‐loading may predict long‐term maladaptive alcohol drinking patterns leading to the development of alcohol use disorder (AUD), several future directions are proposed to elucidate the relationship between front‐loading and AUD

Differential effects of quinine adulteration of alcohol on seeking and drinking

Alcohol dependence is characterized by compulsive alcohol use. Alcohol-paired stimuli can drive compulsive alcohol use, induce craving, and lead to relapse. Alcohol dependence is highly heritable and individuals with a family history are at elevated risk to develop an alcohol use disorder. Understanding the association between genetic vulnerability to alcohol dependence and neural alterations which promote an addiction phenotype are critical to the prevention and treatment of alcohol dependence. Here we use selectively bred alcohol-preferring P rats and their progenitor strain, Wistar rats, to investigate the relationship between genetic liability and alcohol-seeking and drinking behaviors in a discriminative stimuli paradigm. To further investigate strain differences in motivated responding, alcohol was adulterated with quinine and intake and responding were assessed. While both strains learn to discriminate between stimuli which predict alcohol availability, P rats learn faster and consume more alcohol. Quinine adulteration reduced ethanol intake in both strains with no effect on ethanol seeking measures. These data suggest genetic vulnerability to alcohol dependence is associated with increased motivated behaviors and highlight the utility of P rats in teasing apart the neural mechanisms associated with this phenotype. Additionally, these data suggest a dissociation between the neural systems which engage ethanol drinking versus compulsive ethanol seeking