Ethanol consumption in rats selectively bred for differential saccharin intake

Selective pressure on a saccharin intake phenotype and its correlates: a replication study

The Occidental High- and Low-Saccharin rats (respectively, HiS and LoS lines) were selectively bred for decades to examine mechanisms and correlates of a saccharin intake phenotype. Observed line differences ranged from taste and eating to drug self-administration and defensive behavior, paralleling human research on relationships between gustation, personality, and psychopathology. The original lines were terminated in 2019, and replicate lines (HiS-R and LoS-R) were selectively bred for 5 generations to test for reproducible, rapid selection for the phenotype and its correlates. The line differences chosen for replication included intake of tastants (saccharin, sugars, quinine-adulterated sucrose, sodium chloride, and ethanol) and foods (cheese, peas, Spam, and chocolate) and several noningestive behaviors (deprivation-induced hyperactivity, acoustic startle, and open field behavior). The HiS-R and LoS-R lines diverged on intake of saccharin, disaccharides, quinine-adulterated sucrose, sodium chloride, and complex foods, and open field behavior. Differences from the original lines also were observed. Reasons for and implications of the pattern of replication and lack thereof in 5 generations are discussed.

Increased alcohol preference and intake in nicotine-preferring rats

BACKGROUND

Alcohol and tobacco are among the leading substances that are misused together and shared genetic vulnerability is likely. Increased susceptibility to nicotine self-administration has been shown in alcohol-preferring rat-lines. However, a nicotine-preferring (nP) rat-line has not been studied for alcohol preference.

OBJECTIVES

To evaluate alcohol preference and intake in male and female nP rats. We hypothesized that nP rats and females would drink more ethanol than control rats and males, respectively.

METHODS

nP rats are being selectively outbred for high oral nicotine intake at Ege University. Seventeen nP (18^th generation) and 20 naïve female and male SD rats, not previously exposed to alcohol or nicotine, were used. Twelve-week-old rats were given intermittent access to 20% ethanol in a 2-bottle-choice-procedure for six weeks. After one week withdrawal, six weeks of oral nicotine self-administration was applied.

RESULTS

nP rats drank significantly more ethanol than controls and their preference for ethanol over water was higher. Female rats' ethanol intake was higher than males'. The nP rats' nicotine preference and intake were higher than controls, and they gained less weight.

CONCLUSION

We have shown for the first time that nP rats also have high alcohol intake. Our results support the hypothesis that shared genetic factors may underlie concurrent addiction to nicotine and alcohol and have translational value in understanding their misuse. Considering the increased vulnerability for alcohol use disorder in smokers and sex differences observed, early preventive measures in families with a history of tobacco addiction, specifically targeting female members, could have public health benefits.

"Us vs. Them" Pair Housing: Effects on Body Weight, Open Field Behavior, and Gut Microbiota in Rats Selectively Bred on a Taste Phenotype

Taste is increasingly recognized as being related to reward, risk, and social processes beyond the ingestive domain. Occidental High (HiS) and Low (LoS) Saccharin Consuming rats have been selectively bred for more than 25 years to study those relationships. The present study examined LoS and HiS rats' sensitivity to a social partner's lineage. The role of gut microbiome transfer between lines was also explored as a possible mediating mechanism. Rats were pair-housed with a rat from either their own line (same-line condition) or the other line (other-line condition); weight gain, saccharin intake, acoustic startle, and open field behavior were measured. Results show for the first time that the lines express different behavioral strategies in a novel open field. In addition, weight gain and open field measures indicate that other-line housing was stressful. Saccharin intake, however, was unaffected by housing condition. A previous finding that the lines possess different gut microbiota was replicated. Although microbial transfer occurred between social partners, no clear evidence was obtained that housing-condition effects on weight gain or behavior were mediated by microbial transfer. Overall, these findings add to the characterization of non-gustatory correlates of a taste phenotype and suggest that rats differing strikingly on the taste phenotype and/or its correlates may be socially incompatible.

Parametric Characterization of a Taste Phenotype in Rats Selectively Bred for High Versus Low Saccharin Intake

Taste signals food quality and reflects energy status and associated processes. Occidental high- and low-saccharin consuming rats (HiS, LoS) have been selectively bred for nearly 60 generations on intake of 0.1% saccharin in a 23-h two-bottle test, as a tool for studying individual differences in taste and its correlates in the domains of feeding, defensive, and social behavior. The saccharin phenotype itself has not been well characterized until now. The present series of parametric studies examined suprathreshold saccharin concentration-intake functions (Experiment 1), saccharin preference threshold (Experiments 2A and 2B), and intra- and inter-sweetener carryforward effects (Experiments 2B, 3A–3D). Results indicate high stability in line differences in behavior toward saccharin and also line-specific mutability of intake of saccharin and certain other sweeteners. Methodological and conceptual implications are discussed.

Family history of alcoholism and the human brain response to oral sucrose

A heightened hedonic response to sweet tastes has been associated with increased alcohol preference and alcohol consumption in both humans and animals. The principal goal of this study was to examine blood oxygenation level dependent (BOLD) activation to high- and low-concentration sweet solutions in subjects who are either positive (FHP) or negative (FHN) for a family history of alcoholism. Seventy-four non-treatment seeking, community-recruited, healthy volunteers (22.8 ± 1.6 SD years; 43% men) rated a range of sucrose concentrations in a taste test and underwent functional magnetic resonance imaging (fMRI) during oral delivery of water, 0.83 M, and 0.10 M sucrose. Sucrose compared to water produced robust activation in primary gustatory cortex, ventral insula, amygdala, and ventral striatum. FHP subjects displayed greater bilateral amygdala activation than FHN subjects in the low sucrose concentration (0.10 M). In secondary analyses, the right amygdala response to the 0.10 M sucrose was greatest in FHP women. When accounting for group differences in drinks per week, the family history groups remained significantly different in their right amygdala response to 0.10 M sucrose. Our findings suggest that the brain response to oral sucrose differs with a family history of alcoholism, and that this response to a mildly reinforcing primary reward might be an endophenotypic marker of alcoholism risk.

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Studies in humans and animals have suggested sweet tastes as a trait correlate of alcohol risk.

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Oral sucrose resulted in robust BOLD activation of gustatory and limbic areas.

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Amygdala responses to 0.10 M sucrose were greatest in drinkers with family histories of alcoholism.

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This study is first to suggest endophenotypic brain responses to sucrose in familial alcoholism.

Sweet taste liking is associated with subjective response to amphetamine in women but not men

RATIONALE AND OBJECTIVE

Preference for sweet taste rewards has been linked to the propensity for drug use in both animals and humans. Here, we tested the association between sweet taste liking and sensitivity to amphetamine reward in healthy adults. We hypothesized that sweet likers would report greater euphoria and stimulation following D-amphetamine (20 mg) compared to sweet dislikers.

METHODS

Men (n = 36) and women (n = 34) completed a sweet taste test in which they rated their liking of various concentrations of sucrose and filtered water (0.05, 0.10, 0.21, 0.42, and 0.83 M). Participants who preferred the highest concentration were classified as "sweet likers." All others were classified as "sweet dislikers." They then completed four sessions in which they received D-amphetamine (20 mg) and placebo in alternating order, providing self-report measures of euphoria and stimulation on the Addiction Research Center Inventory (ARCI) at regular intervals. We conducted linear mixed effects models to examine relationships between sweet liking and drug-induced euphoria and stimulation.

RESULTS

Sweet likers reported significantly greater amphetamine-induced euphoria than did sweet dislikers among women. By contrast, sweet liking was not associated with amphetamine response in men. No associations with stimulation were observed.

CONCLUSION

The association between sweet preference and amphetamine response in women is consistent with animal studies linking sweet taste preference and drug reward and also fits with observations that individuals who use drugs show a preference for sweet tastes. Whether the sex difference is related to circulating hormones, or other variables, remains to be determined.

The Role of Orexins/Hypocretins in Alcohol Use and Abuse

Addiction is a chronic relapsing disorder characterized by compulsive drug seeking and drug taking despite negative consequences. Alcohol abuse and addiction have major social and economic consequences and cause significant morbidity and mortality worldwide. Currently available therapeutics are inadequate, outlining the need for alternative treatments. Detailed knowledge of the neurocircuitry and brain chemistry responsible for aberrant behavior patterns should enable the development of novel pharmacotherapies to treat addiction. Therefore it is important to expand our knowledge and understanding of the neural pathways and mechanisms involved in alcohol seeking and abuse. The orexin (hypocretin) neuropeptide system is an attractive target, given the recent FDA and PMDA approval of suvorexant for the treatment of insomnia. Orexin is synthesized exclusively in neurons located in the lateral (LH), perifornical (PEF), and dorsal medial (DMH) hypothalamus. These neurons project widely throughout the neuraxis with regulatory roles in a wide range of behavioral and physiological responses, including sleep-wake cycle neuroendocrine regulation, anxiety, feeding behavior, and reward seeking. Here we summarize the literature to date, which have evaluated the interplay between alcohol and the orexin system.

Gut Microbiota and a Selectively Bred Taste Phenotype: A Novel Model of Microbiome-Behavior Relationships

OBJECTIVES

The microbiota-gut-brain axis is increasingly implicated in obesity, anxiety, stress, and other health-related processes. Researchers have proposed that gut microbiota may influence dietary habits, and pathways through the microbiota-gut-brain axis make such a relationship feasible; however, few data bear on the hypothesis. As a first step in the development of a model system, the gut microbiome was examined in rat lines selectively outbred on a taste phenotype with biobehavioral profiles that have diverged with respect to energy regulation, anxiety, and stress.

METHODS

Occidental low and high-saccharin-consuming rats were assessed for body mass and chow, water, and saccharin intake; littermate controls had shared cages with rats in the experimental group but were not assessed. Cecum and colon microbial communities were profiled using Illumina 16S rRNA sequencing and multivariate analysis of microbial diversity and composition.

RESULTS

The saccharin phenotype was confirmed (low-saccharin-consuming rats, 0.7Δ% [0.9Δ%]; high-saccharin-consuming rats, 28.1Δ% [3.6Δ%]). Regardless of saccharin exposure, gut microbiota differed between lines in terms of overall community similarity and taxa at lower phylogenetic levels. Specifically, 16 genera in three phyla distinguished the lines at a 10% false discovery rate.

DISCUSSION

The study demonstrates for the first time that rodent lines created through selective pressure on taste and differing on functionally related correlates host different microbial communities. Whether the microbiota are causally related to the taste phenotype or its correlates remains to be determined. These findings encourage further inquiry on the relationship of the microbiome to taste, dietary habits, emotion, and health.

Effects of neonatal allopregnanolone manipulations and early maternal separation on adult alcohol intake and monoamine levels in ventral striatum of male rats

Changes in endogenous neonatal levels of the neurosteroid allopregnanolone (AlloP) as well as a single 24h period of early maternal separation (EMS) on postnatal day (PND) 9 affect the development of the central nervous system (CNS), causing adolescent/adult alterations including systems and behavioural traits that could be related to vulnerability to drug abuse. In rats, some behavioural alterations caused by EMS can be neutralised by previous administration of AlloP. Thus, the aim of the present work is to analyse if manipulations of neonatal AlloP could increase adult alcohol consumption, and if EMS could change these effects. We administered AlloP or finasteride, a 5α-reductase inhibitor, from PND5 to PND9, followed by 24h of EMS at PND9. At PND70 we measured alcohol consumption using a two-bottle free-choice model (ethanol 10% (v/v)+glucose 3% (w/v), and glucose 3% (w/v)) for 15days. Ventral striatum samples were obtained to determine monoamine levels. Results revealed that neonatal finasteride increased both ethanol and glucose consumption, and AlloP increased alcohol intake compared with neonatal vehicle-injected animals. The differences between neonatal groups in alcohol consumption were not found in EMS animals. In accordance, both finasteride and AlloP animals that did not suffer EMS showed lower levels of dopamine and serotonin in ventral striatum. Taken together, these results reveal that neonatal neurosteroids alterations affect alcohol intake; an effect which can be modified by subsequent EMS. Thus, these data corroborate the importance of the relationship between neonatal neurosteroids and neonatal stress for the correct CNS development.

Removal of high-fat diet after chronic exposure drives binge behavior and dopaminergic dysregulation in female mice

A significant contributor to the obesity epidemic is the overconsumption of highly palatable, energy dense foods. Chronic intake of palatable foods is associated with neuroadaptations within the mesocorticolimbic dopamine system adaptations which may lead to behavioral changes, such as overconsumption or bingeing. We examined behavioral and molecular outcomes in mice that were given chronic exposure to a high-fat diet (HFD; 12weeks), with the onset of the diet either in adolescence or adulthood. To examine whether observed effects could be reversed upon removal of the HFD, animals were also studied 4weeks after a return to chow feeding. Most notably, female mice, particularly those exposed to HFD starting in adolescence, demonstrated the emergence of binge-like behavior when given restricted access to a palatable food. Further, changes in dopamine-related gene expression and dopamine content in the prefrontal cortex were observed. Some of these HFD-driven phenotypes reversed upon removal of the diet, whereas others were initiated by removal of the diet. These findings have implications for obesity management and interventions, as both pharmacological and behavioral therapies are often combined with dietary interventions (e.g., reduction in calorie dense foods).

Sex Differences in Behavioral Dyscontrol: Role in Drug Addiction and Novel Treatments

The purpose of this review is to discuss recent findings related to sex differences in behavioral dyscontrol that lead to drug addiction, and clinical implications for humans are discussed. This review includes research conducted in animals and humans that reveals fundamental aspects of behavioral dyscontrol. The importance of sex differences in aspects of behavioral dyscontrol, such as impulsivity and compulsivity, is discussed as major determinants of drug addiction. Behavioral dyscontrol during adolescence is also an important consideration, as this is the time of onset for drug addiction. These vulnerability factors additively increase drug-abuse vulnerability, and they are integral aspects of addiction that covary and interact with sex differences. Sex differences in treatments for drug addiction are also reviewed in terms of their ability to modify the behavioral dyscontrol that underlies addictive behavior. Customized treatments to reduce behavioral dyscontrol are discussed, such as (1) using natural consequences such as non-drug rewards (e.g., exercise) to maintain abstinence, or using punishment as a consequence for drug use, (2) targeting factors that underlie behavioral dyscontrol, such as impulsivity or anxiety, by repurposing medications to relieve these underlying conditions, and (3) combining two or more novel behavioral or pharmacological treatments to produce additive reductions in drug seeking. Recent published work has indicated that factors contributing to behavioral dyscontrol are an important target for advancing our knowledge on the etiology of drug abuse, intervening with the drug addiction process and developing novel treatments.

An animal model of differential genetic risk for methamphetamine intake

The question of whether genetic factors contribute to risk for methamphetamine (MA) use and dependence has not been intensively investigated. Compared to human populations, genetic animal models offer the advantages of control over genetic family history and drug exposure. Using selective breeding, we created lines of mice that differ in genetic risk for voluntary MA intake and identified the chromosomal addresses of contributory genes. A quantitative trait locus was identified on chromosome 10 that accounts for more than 50% of the genetic variance in MA intake in the selected mouse lines. In addition, behavioral and physiological screening identified differences corresponding with risk for MA intake that have generated hypotheses that are testable in humans. Heightened sensitivity to aversive and certain physiological effects of MA, such as MA-induced reduction in body temperature, are hallmarks of mice bred for low MA intake. Furthermore, unlike MA-avoiding mice, MA-preferring mice are sensitive to rewarding and reinforcing MA effects, and to MA-induced increases in brain extracellular dopamine levels. Gene expression analyses implicate the importance of a network enriched in transcription factor genes, some of which regulate the mu opioid receptor gene, Oprm1, in risk for MA use. Neuroimmune factors appear to play a role in differential response to MA between the mice bred for high and low intake. In addition, chromosome 10 candidate gene studies provide strong support for a trace amine-associated receptor 1 gene, Taar1, polymorphism in risk for MA intake. MA is a trace amine-associated receptor 1 (TAAR1) agonist, and a non-functional Taar1 allele segregates with high MA consumption. Thus, reduced TAAR1 function has the potential to increase risk for MA use. Overall, existing findings support the MA drinking lines as a powerful model for identifying genetic factors involved in determining risk for harmful MA use. Future directions include the development of a binge model of MA intake, examining the effect of withdrawal from chronic MA on MA intake, and studying potential Taar1 gene × gene and gene × environment interactions. These and other studies are intended to improve our genetic model with regard to its translational value to human addiction.

Impulsive choice predicts anxiety-like behavior, but not alcohol or sucrose consumption, in male Long-Evans rats

BACKGROUND

Prior human research indicates robust, positive relations between impulsive choice (i.e., preference for smaller, immediate over larger, delayed rewards) and alcohol use disorders. However, varied findings in the nonhuman literature reveal a relatively ambiguous relation between impulsive choice and alcohol consumption in rodents. In addition, few rodent studies have investigated potential relations between impulsive choice and common covariates of alcohol consumption (e.g., avidity for sweet substances or anxiety-like behavior).

METHODS

Ninety-two male Long-Evans rats completed an impulsive-choice task. From this larger sample, extreme high- and low-impulsive groups (n = 30 each) were retained for further testing. In separate tests, subsequent open-field behavior and consumption of oral alcohol (12% w/v) and isocaloric sucrose were examined. Impulsive choice was then retested to examine whether behavior remained stable over the course of the experiment.

RESULTS

No significant relations emerged between impulsive choice and either alcohol or sucrose consumption. However, impulsive choice predicted greater anxiety-like behavior (avoidance of the center field, defecation) in the open-field test. In turn, greater anxiety predicted lower alcohol and sucrose consumption. Finally, choice remained generally stable across the experiment, although high-impulsive rats tended toward less impulsive choice in the retest.

CONCLUSIONS

Although impulsive choice and alcohol consumption appear to share some variance with anxiety-like behavior, the present data offer no support for a relation between impulsive choice and alcohol consumption in Long-Evans rats. Together with mixed rodent data from prior reports, these findings attenuate cross-species comparisons to human relations between impulsive choice and alcohol use disorders.

Intracranial self-stimulation reward thresholds during morphine withdrawal in rats bred for high (HiS) and low (LoS) saccharin intake

RATIONALE

Sweet preference is a marker of vulnerability to substance use disorders, and rats selectively bred for high (HiS) vs. low saccharin (LoS) intake display potentiated drug-seeking behaviors. Recent work indicated that LoS rats were more responsive to the negative effects of drugs in several assays.

OBJECTIVE

The current study used the intracranial self-stimulation (ICSS) procedure to investigate the anhedonic component of morphine withdrawal in male HiS and LoS rats.

METHODS

Rats were administered morphine (10mg/kg) or saline for 8 days. To evaluate withdrawal effects, reward thresholds were measured 24 and 28h following the 8th morphine injection (spontaneous withdrawal) and again for 4 days following daily acute morphine and naloxone (1mg/kg) administration (precipitated withdrawal).

RESULTS

24h following the final morphine injection, reward thresholds in LoS rats were significantly elevated compared to reward thresholds in LoS controls, indicating spontaneous withdrawal. This effect was not observed in HiS rats. LoS rats also showed greater elevations of reward thresholds on several days during naloxone-precipitated withdrawal compared to their HiS counterparts.

CONCLUSIONS

LoS rats were more sensitive to morphine withdrawal-mediated elevations in ICSS thresholds than HiS rats. While these differences were generally modest, our data suggest that severity of the negative affective component of opiate withdrawal may be influenced by genotypes related to addiction vulnerability.

Alcohol sensory processing and its relevance for ingestion

Alcohol possesses complex sensory attributes that are first detected by the body via sensory receptors and afferent fibers that promptly transmit signals to brain areas involved in mediating ingestive motivation, reinforcement, and addictive behavior. Given that the chemosensory cues accompanying alcohol consumption are among the most intimate, consistent, and immediate predictors of alcohol's postabsorptive effects, with experience these stimuli also gain powerful associative incentive value to elicit craving and related physiologic changes, maintenance of ongoing alcohol use, and reinstatement of drug seeking after periods of abstinence. Despite the above, preclinical research has traditionally dichotomized alcohol's taste and postingestive influences as independent regulators of motivation to drink. The present review summarizes current evidence regarding alcohol's ability to directly activate peripheral and central oral chemosensory circuits, relevance for intake of the drug, and provides a framework for moving beyond a dissociation between the sensory and postabsorptive effects of alcohol to understand their neurobiological integration and significance for alcohol addiction.

Cocaine-, caffeine-, and stress-evoked cocaine reinstatement in high vs. low impulsive rats: treatment with allopregnanolone

BACKGROUND

Previous research indicates that individual differences in traits such as impulsivity, avidity for sweets, and novelty reactivity are predictors of several aspects of drug addiction. Specifically, rats that rank high on these behavioral measures are more likely than their low drug-seeking counterparts to exhibit several characteristics of drug-seeking behavior. In contrast, initial work suggests that the low drug-seeking animals are more reactive to negative events (e.g., punishment and anxiogenic stimuli). The goal of this study was to compare high and low impulsive rats on reinstatement of cocaine-seeking behavior elicited by cocaine (COC) and by negative stimuli such as the stress-inducing agent yohimbine (YOH) or a high dose of caffeine (CAFF). An additional goal was to determine whether treatment with allopregnanolone (ALLO) would reduce reinstatement (or relapse) of cocaine-seeking behavior under these priming conditions.

METHODS

Female rats were selected as high (HiI) or low (LoI) impulsive using a delay-discounting task. After selection, they were allowed to self-administer cocaine for 12 days. Cocaine was then replaced with saline, and rats extinguished lever responding over 16 days. Subsequently, rats were pretreated with either vehicle control or ALLO, and cocaine seeking was reinstated by injections of COC, CAFF, or YOH.

RESULTS

While there were no phenotype differences in maintenance and extinction of cocaine self-administration or reinstatement under control treatment conditions, ALLO attenuated COC- and CAFF-primed reinstatement in LoI but not HiI rats.

CONCLUSIONS

Overall, the present findings suggest that individual differences in impulsive behavior may influence efficacy of interventions aimed to reduce drug-seeking behavior.

"Jello® shots" and cocktails as ethanol vehicles: parametric studies with high- and low-saccharin-consuming rats

Naïve humans and rats voluntarily consume little ethanol at concentrations above ~6% due to its aversive flavor. Developing procedures that boost intake of ethanol or ethanol-paired flavors facilitates research on neural mechanisms of ethanol-associated behaviors and helps identify variables that modulate ethanol intake outside of the lab. The present study explored the impact on consumption of ethanol and ethanol-paired flavors of nutritionally significant parametric variations: ethanol vehicle (gelatin or solution, with or without polycose); ethanol concentration (4% or 10%); and feeding status (chow deprived or ad lib.) during flavor conditioning and flavor preference testing. Individual differences were modeled by testing rats of lines selectively bred for high (HiS) or low (LoS) saccharin intake. A previously reported preference for ethanol-paired flavors was replicated when ethanol had been drunk during conditioning. However, indifference or aversion to ethanol-paired flavors generally obtained when ethanol had been eaten in gelatin during conditioning, regardless of ethanol concentration, feeding status, or caloric value of the vehicle. Modest sex and line variations occurred. Engaging different behavioral systems when eating gelatin, rather than drinking solution, may account for these findings. Implications for parameter selection in future neurobiological research and for understanding conditions that influence ethanol intake outside of the lab are discussed.

A preliminary study of the human brain response to oral sucrose and its association with recent drinking

BACKGROUND

A preference for sweet tastes has been repeatedly shown to be associated with alcohol preference in both animals and humans. In this study, we tested the extent to which recent drinking is related to blood oxygen level-dependent (BOLD) activation from an intensely sweet solution in orbitofrontal areas known to respond to primary rewards.

METHODS

Sixteen right-handed, non-treatment-seeking, healthy volunteers (mean age: 26 years; 75% male) were recruited from the community. All underwent a taste test using a range of sucrose concentrations, as well as functional magnetic resonance imaging (fMRI) during pseudorandom, event-driven stimulation with water and a 0.83 M concentration of sucrose in water.

RESULTS

[Sucrose > water] provoked a significant BOLD activation in primary gustatory cortex and amygdala, as well as in the right ventral striatum and in bilateral orbitofrontal cortex. Drinks/drinking day correlated significantly with the activation as extracted from the left orbital area (r = 0.52, p = 0.04 after correcting for a bilateral comparison). Using stepwise multiple regression, the addition of rated sucrose liking accounted for significantly more variance in drinks/drinking day than did left orbital activation alone (multiple R = 0.79, p = 0.002).

CONCLUSIONS

Both the orbitofrontal response to an intensely sweet taste and rated liking of that taste accounted for significant variance in drinking behavior. The brain response to sweet tastes may be an important phenotype of alcoholism risk.

Reduced emotional signs of opiate withdrawal in rats selectively bred for low (LoS) versus high (HiS) saccharin intake

RATIONALE

Rats bred for high (HiS) and low (LoS) saccharin intake exhibit divergent behavioral responses to multiple drugs of abuse, with HiS rats displaying greater vulnerability to drug taking. Previous research indicates that this effect may be due to increased sensitivity to reward in HiS rats and to the aversive effects of acute drug administration in LoS rats.

OBJECTIVE

The current study investigated whether HiS and LoS rats also exhibit different behavioral signs of withdrawal following one or repeated opiate exposures.

METHODS

Emotional signs of opiate withdrawal were assessed with potentiation of the acoustic startle reflex and conditioned place aversion (CPA) in male and female HiS and LoS rats. Startle was measured before and 4 h after a 10-mg/kg injection of morphine on days 1, 2, and 7 of opiate exposure. CPA was induced with a 2-day, naloxone-precipitated conditioning paradigm. Somatic signs of withdrawal and weight loss were also measured.

RESULTS

Male and female LoS rats exhibited lower startle potentiation than HiS rats on the seventh day of morphine exposure. LoS male rats also failed to develop a CPA to morphine withdrawal. No differences in physical withdrawal signs were observed between HiS and LoS rats, but males of both lines had more physical signs of withdrawal than females.

CONCLUSIONS

These results suggest that LoS rats are less vulnerable to the negative emotional effects of morphine withdrawal than HiS rats. A less severe withdrawal syndrome may contribute to decreased levels of drug taking in the LoS line.

Escalation of i.v. cocaine intake in peri-adolescent vs. adult rats selectively bred for high (HiS) vs. low (LoS) saccharin intake

RATIONALE

Adolescence marks a period of increased vulnerability to the development of substance use disorders. High sweet preference is a genetically mediated behavioral trait that also predicts vulnerability to substances of abuse. Previous research has shown that while adolescent rats selectively bred for high (HiS) saccharin intake acquire cocaine self-administration at the same rate as adult HiS rats, adolescent rats bred for low saccharin intake (LoS) acquire cocaine self-administration faster than adult LoS rats.

OBJECTIVES

This study was conducted to investigate the interaction of the addiction vulnerability factors of peri-adolescence and saccharin preference on cocaine intake using an animal model of escalation of cocaine consumption over 6-h/day sessions.

METHODS

Peri-adolescent and adult HiS and LoS female rats self-administered i.v. cocaine (0.4 mg/kg/inf) during short-access (2-h/day) sessions for 2 days. Next, a long-access (6-h/day) period (LgA) commenced and lasted 16 days. Following LgA, session length was returned to 2-h/day for a second short access phase.

RESULTS

LoS peri-adolescent rats escalated cocaine intake over the LgA period and consumed more drug than LoS adult rats; however, peri-adolescent and adult HiS rats consumed similar amounts of cocaine during this period. Additionally, adult HiS rats self-administered more cocaine than adult LoS rats during the LgA period, while there was no phenotypic difference between the rat lines during peri-adolescence for the LgA period. During the first short-access phase, peri-adolescent rats self-administered more cocaine than adult rats.

CONCLUSIONS

These results emphasize the importance of adolescent drug abuse prevention by illustrating that phenotypic protection from addiction may not be expressed until adulthood.

The role of orexins/hypocretins in alcohol use and abuse: an appetitive-reward relationship

Orexins (hypocretins) are neuropeptides synthesized in neurons located in the lateral (LH), perifornical, and dorsomedial (DMH) hypothalamus. These neurons innervate many regions in the brain and modulate multiple other neurotransmitter systems. As a result of these extensive projections and interactions orexins are involved in numerous functions, such as feeding behavior, neuroendocrine regulation, the sleep-wake cycle, and reward-seeking. This review will summarize the literature to date which has evaluated a role of orexins in the behavioral effects of alcohol, with a focus on understanding the importance of this peptide and its potential as a clinical therapeutic target for alcohol use disorders.

Differential orexin/hypocretin expression in addiction-prone and -resistant rats selectively bred for high (HiS) and low (LoS) saccharin intake

Rats that have been selectively bred for high (HiS) saccharin intake demonstrate elevated drug-seeking behavior in several phases of addiction compared to those bred for low (LoS) saccharin intake. HiS rats also consume greater amounts of highly palatable substances compared to LoS rats; however, little is known about the neurobiological substrates moderating the divergent behaviors found between the HiS and LoS lines. Orexins are neuropeptides that have been implicated in the conditioned cue aspects of drug abuse and overconsumption of palatable substances, and differential orexin activity in the HiS and LoS phenotypes may enhance our understanding of the close relationship between food and drug reward, and ultimately food and drug addiction. The lateral hypothalamus (LH) and perifornical area (PFA) are brain regions that have been implicated in regulating feeding behavior and addiction processes, and they contain orexinergic neurons that project broadly throughout the brain. Thus, we investigated orexin and c-Fos expression in the LH and PFA using immunohistochemistry in HiS and LoS rats following either control or cocaine (15 mg/kg) injections. Results indicated that HiS rats have higher orexin-positive cell counts compared to LoS rats in both the LH and PFA, regardless of cocaine (vs. saline) treatment. In contrast, neuronal activity indicated by c-Fos expression did not differ in either of these brain areas in HiS vs. LoS rats. These results suggest that the orexin system may be involved in aspects of genetically-mediated differences in vulnerability to compulsive, reward-driven behaviors.

Effects of progesterone on escalation of intravenous cocaine self-administration in rats selectively bred for high or low saccharin intake

Progesterone decreases cocaine self-administration in women and in female rats. In a previous study using rats selectively bred for high (HiS) or low (LoS) saccharin intake, HiS rats escalated their cocaine intake compared with LoS rats. Our goal was to examine the effects of progesterone on the escalation of cocaine self-administration in HiS and LoS rats. Four groups of female rats were compared: HiS P (progesterone treated), LoS P, HiS VEH (vehicle treated), and LoS VEH. Rats were trained to self-administer 0.8 mg/kg cocaine intravenously under a fixed-ratio 1 schedule during daily short-access (ShA) 2-h sessions. Rats then self-administered three randomly-presented doses of cocaine (0.2, 0.4, and 1.6 mg/kg), and then had daily 6-h long-access (LgA) sessions with 0.4 mg/kg of cocaine for 21 days. Cocaine intake was then reassessed with the four doses under the ShA condition. Throughout the experiment, rats were treated with daily subcutaneous injections of progesterone (0.5 mg/kg) or an equal volume of vehicle 30 min before each session. During the initial ShA condition, HiS rats earned more cocaine infusions than LoS rats at all doses, and during the subsequent LgA condition, HiS rats escalated cocaine intake, whereas the LoS rats maintained a steady rate. Progesterone treatment potentiated escalation of cocaine intake in the HiS rats but had an opposite effect on LoS rats, attenuating their cocaine self-administration. Results from the post-LgA dose-response ShA condition indicated that both LoS and HiS vehicle-treated and progesterone-treated rats earned more infusions than pre-LgA, but mainly at low doses. These results suggest that genetic differences in drug abuse vulnerability contribute differentially to treatment outcomes during escalation, a critical phase of the drug abuse process.

Cocaine-induced c-Fos expression in rats selectively bred for high or low saccharin intake and in rats selected for high or low impulsivity

Sweet preference and impulsivity are predictors of cocaine self-administration; however, no research has been conducted to investigate neuronal activation in key brain reward areas after first time exposure to cocaine in rats that differ in their propensity for cocaine-seeking and -taking behavior. In this study we used rats that had been selectively bred for high vs. low saccharin intake and rats selected for high vs. low impulsivity for food. The goal of this study was to investigate whether there are differences of c-Fos reactivity between high and low phenotypes and determine whether these differences are similar between the two animal models. A group of rats was bred for high or low saccharin intake. Another group of rats was selected as high or low impulsive based on performance in a delay-discounting task. Subsequently, rats were given an acute injection of cocaine or saline and then c-Fos expression was observed and analyzed in several brain regions. The low reward-seeking phenotypes showed higher cocaine-induced c-Fos expression in several of these regions. Low saccharin preferring rats showed higher cocaine-induced c-Fos expression in the nucleus accumbens shell, and low impulsive rats showed higher cocaine-induced c-Fos expression in the orbitofrontal cortex and cingulate gyrus 1 area. In addition, both low impulsive and low saccharin rats had higher cocaine-induced c-Fos in the dorsal medial and dorsal lateral caudate putamen. The results indicate that individual differences in neuronal reactivity exist prior to chronic exposure to drugs of abuse. Furthermore, similar differences between the two animal models may be indicative of a common mechanism underlying vulnerability to drugs of abuse.

Preference for sucralose predicts behavioral responses to sweet and bittersweet tastants

Rats can be classified as either sucralose avoiders (SA) or sucralose preferrers (SP) based on their behavioral responses in 2-bottle preference, 1-bottle intake, and brief-access licking tests. The present study demonstrates that this robust phenotypic variation in the preference for sucralose predicts acceptance of saccharin, an artificial sweetener with a purported concentration-dependent "bitter" side taste and a 0.25 M sucrose solution adulterated with increasing concentrations of quinine hydrochloride (QHCl). Specifically, SA displayed decreased preference for and intakes of saccharin (≥41.5 mM) and sucrose-QHCl (>0.5 mM QHCl) solutions, relative to SP. In a second experiment involving brief-access (30-s) tests, SP and SA did not differ in their unconditioned licking responses across a range of sodium chloride or QHCl solutions (0.03-1 mM). However, the acceptability threshold for sucrose was lower in SA, relative to SP (0.06 and 0.13 M, respectively). Our findings suggest that phenotypic differences in sucralose preference are indicative of a more general difference in the hedonic processing of stimuli containing "bittersweet" or "sweet" taste qualities.

Adolescent nicotine exposure sensitizes cue-induced reinstatement of cocaine seeking in rats bred for high and low saccharin intake

BACKGROUND

Environmental factors such as early drug exposure influence drug abuse vulnerability, and evidence also suggests that drug abuse is highly heritable. The purpose of the present study was to determine whether environmental and genetic factors interact to produce additive drug abuse vulnerability.

METHODS

An animal model of relapse was used to examine the effects of adolescent nicotine exposure on adult cocaine seeking in rats bred for high (HiS) and low (LoS) saccharin intake. Rats from HiS and LoS progenitor lines received s.c. injections of nicotine for 10 days (postnatal days 22-31). Rats were then allowed to reach adulthood and were trained to lever press for cocaine infusions. During each self-administration session, the house light (HL) was illuminated and each lever press activated a set of lights adjacent to the lever (LL). Following cocaine self-administration, the HL and LL were deactivated, cocaine solutions were replaced with saline, and rats extinguished lever pressing. Subsequently, rats were tested under a multi-component reinstatement procedure consisting of: (1) cue-induced reinstatement with LL alone and the HL presented alone, (2) cocaine-induced reinstatement without LL and HL present, (3) and cocaine-induced reinstatement with LL present.

RESULTS

The results indicated that adolescent nicotine exposure sensitized the reinstatement of cocaine seeking during adulthood in HiS (but not LoS) rats when lever pressing resulted in LL cue presentations. In addition, following administration of the cocaine priming injection, rats exposed to nicotine (vs. saline) during adolescence (LoS and HiS) engaged in more cocaine seeking under the cocaine-primed reinstatement condition when lever pressing illuminated the LL.

CONCLUSION

These results suggest that drug abuse vulnerability may be a function of early life exposure to drugs of abuse in addition to genetic influences.

Baclofen has opposite effects on escalation of cocaine self-administration: increased intake in rats selectively bred for high (HiS) saccharin intake and decreased intake in those selected for low (LoS) saccharin intake

Rats selectively bred for high saccharin intake (HiS) self-administer more cocaine, escalate their cocaine intake during long access, and reinstate cocaine seeking at higher levels than those bred for low saccharin intake (LoS). The present study was conducted to determine if baclofen, an agonist at the GABA(b) receptor, has differential effects on the escalation of i.v. cocaine intake and reinstatement of cocaine-seeking in HiS and LoS rats. HiS and LoS rats self-administered cocaine during a 2-h daily short-access (ShA) phase for 3 days and then long-access (LgA) sessions for 21 days followed by a second ShA phase. One group of HiS and LoS rats received i.p. injections of 2.5 mg/kg baclofen (HiS+B and LoS+B, respectively), and other groups of HiS and LoS rats received saline (HiS+Sal and LoS+Sal) before each daily session. In a second experiment, HiS and LoS rats self-administered i.v. cocaine during 2-h sessions for 14 days followed by a 21-day extinction period. Baclofen (2.5 mg/kg, i.p.) or saline was administered before saline- or cocaine-primed reinstatement sessions. The HiS+B group escalated their cocaine self-administration and had increased cocaine infusions in the post-LgA ShA phase. The LoS+B group self-administered less cocaine throughout the entire LgA period compared to the LoS+Sal or HiS groups. Baclofen attenuated reinstatement of cocaine seeking in both the HiS and LoS rats with no phenotype differences. Thus, baclofen had opposite effects on cocaine intake in HiS and LoS rats during escalation; but similar effects during reinstatement. These results suggest that treatment effects might vary with individual differences (HiS vs. LoS) and the phase of drug-motivated behavior that is modeled.

Genetics of sweet taste preferences

Sweet taste is a powerful factor influencing food acceptance. There is considerable variation in sweet taste perception and preferences within and among species. Although learning and homeostatic mechanisms contribute to this variation in sweet taste, much of it is genetically determined. Recent studies have shown that variation in the T1R genes contributes to within- and between-species differences in sweet taste. In addition, our ongoing studies using the mouse model demonstrate that a significant portion of variation in sweetener preferences depends on genes that are not involved in peripheral taste processing. These genes are likely involved in central mechanisms of sweet taste processing, reward and/or motivation. Genetic variation in sweet taste not only influences food choice and intake, but is also associated with proclivity to drink alcohol. Both peripheral and central mechanisms of sweet taste underlie correlation between sweet-liking and alcohol consumption in animal models and humans. All these data illustrate complex genetics of sweet taste preferences and its impact on human nutrition and health. Identification of genes responsible for within- and between-species variation in sweet taste can provide tools to better control food acceptance in humans and other animals.

Food dependence in rats selectively bred for low versus high saccharin intake. Implications for "food addiction"

The "food addiction" concept implies that proneness to drug dependence and to food dependence should covary. The latter was studied in low- (LoS) and high- (HiS) saccharin-consuming rats, who differ in drug self-administration (HiS>LoS) and withdrawal (LoS>HiS). Sugary food intake in the first 1-2 h was higher in HiS than LoS rats. Sugar intake predicted startle during abstinence only among LoS rats. These results may suggest bingeing-proneness in HiS rats and withdrawal-proneness among LoS rats. However, intake escalation and somatic withdrawal did not differ between lines. Further study with selectively bred rats, with attention to definitions and measures, is warranted.

Rats display a robust bimodal preference profile for sucralose

Female Sprague-Dawley rats display considerable variability in their preference for the artificial sweetener sucralose over water. While some rats can be classified as sucralose preferrers (SP), as they prefer sucralose across a broad range of concentrations, others can be classified as sucralose avoiders (SA), as they avoid sucralose at concentrations above 0.1 g/L. Here, we expand on a previous report of this phenomenon by demonstrating, in a series of 2-bottle 24-h preference tests involving water and an ascending series of sucralose concentrations, that this variability in sucralose preference is robust across sex, stage of the estrous cycle, and 2 rat strains (Long-Evans and Sprague-Dawley). In a second experiment involving a large sample of rats (n = 50), we established that the ratio of SP to SA is approximately 35-65%. This bimodal behavioral response to sucralose appears to be driven by taste because rats display a similar bimodal licking response to a range of sucralose solutions presented during brief-access tests. Finally, we have shown that sucralose avoidance is extremely robust as 23-h water-deprived SA continue to avoid sucralose in 1-h single-bottle intake tests. Based on their reduced licking responses to sucralose during brief-access (taste driven) tests, and the fact that their distaste for sucralose cannot be overcome by the motivation to rehydrate, we conclude that SA detect a negative taste quality of sucralose that SP are relatively insensitive to.

Effects of the serotonin transporter (5-HTTLPR) and α2A-adrenoceptor (C-1291G) genotypes on substance use in children and adolescents: a longitudinal study

RATIONALE AND OBJECTIVE

Twin studies suggest that substance use initiation in children and adolescents is determined primarily by environmental influences, whereas the establishment of use patterns is strongly controlled by genetic factors. The present study analysed the effects of the serotonin transporter promoter polymorphism [5-HT transporter gene-linked polymorphic region (5-HTTLPR)] and the α(2A)-adrenoceptor C-1291G genotype (ADRA2A C-1291G) as well as their interaction effects on alcohol, tobacco and drug use from preadolescence to the late adolescence.

METHODS

Initial sample of 9-year-old children of Estonian Children Personality Behaviour and Health Study (n = 583) was recalled at ages 15 and 18. Participants reported in all waves how frequently they smoked and used alcohol and illicit drugs.

RESULTS

5-HTTLPR had age-dependent effects on alcohol, tobacco and drug use: substance use did not differ by genotype at age 9, but at age 15, the participants with the short (s)/s genotype had higher tobacco use, and at age 18, they were more active alcohol, drug and tobacco users. Effects of ADRA2A C-1291G on drug use were dependent on gender, age and 5-HTTLPR. Males (age 18) with ADRA2A CG genotype, when compared to other participants, tended to have higher drug use especially when they had s/s genotype of 5-HTTLPR.

CONCLUSIONS

Our results reveal that expression of genetic vulnerability for substance use in children and adolescents may depend on age, gender, interaction of genes, and type of substance.

Derivation and characterization of replicate high- and low-alcohol preferring lines of mice and a high-drinking crossed HAP line

Selectively breeding lines of mice and rats to differ in alcohol intake has proven useful for defining which traits correlate with high alcohol drinking behavior, as well as for creating animal models of alcoholism. This study reports the derivation of two novel sets of selected lines, High Alcohol Preferring (HAP) and Low Alcohol Preferring (LAP) replicate 2 and 3 lines. Mice were mass-selected using the same procedure as in the replicate 1 lines: using HS/Ibg as a progenitor, mice were selected for differences in 2-bottle choice intake of 10% alcohol during a 4-week testing period. In addition, another high-drinking line, the crossed HAP (cHAP) line was selectively bred from a progenitors that were a cross of replicate 1 (S27) × replicate 2 (S21) HAP lines. All lines were characterized for saccharin intake. Overall, the response to selection of the HAP and LAP replicate 2 and 3 lines was quite similar. As anticipated, following selection, the cHAP line drank more than either parent HAP line (consuming 26.0 g/kg per day of alcohol by S11), suggesting that this method of crossing replicate lines and selecting from that cross captures more alleles than any single selected line, as well as producing a line with exceptionally high voluntary alcohol intake. As expected, saccharin consumption was highly associated with alcohol consumption; data from 7 lines (HAP 1, 2, and 3, LAP 1, 2, and 3, and cHAP) indicated a genetic correlation between 10% alcohol and 0.32% saccharin intake of 0.91. Overall, these findings show the practicality of developing replicate lines divergent in alcohol preference, and validate a novel procedure for generating very high-drinking mouse populations.

Operant responding for sucrose by rats bred for high or low saccharin consumption

The use of rats differing in the intake of sweet substances has highlighted some interesting parallels between taste preferences and drug self-administration. For example, rats selectively bred to consume high (HiS) or low (LoS) amounts of a 0.1% saccharin solution (when compared to water consumption), show corresponding differences across several measures of cocaine self-administration (HiS>LoS). In this study, we measured whether the two strains also differ when response requirements are imposed for obtaining a sucrose reinforcer. Male HiS and LoS rats were measured for operant responding for sucrose pellets under fixed-ratio (FR) schedules of 1, 3, 5 and 10 and under a progressive-ratio (PR) schedule, during which the response requirement for each successive pellet increased exponentially. The effect of systemic naltrexone (0.3, 1 and 3mg/kg) on PR responding for sucrose pellets was also tested. Under all FR and PR schedules, the number of pellets obtained by the LoS rats were significantly lower than those obtained by the HiS rats. Although the LoS weighed more than the HiS rats, this difference does not appear to explain differences in operant behavior. No strain differences in the effect of naltrexone were observed; the 3mg/kg dose reduced the number of pellets obtained in both strains. Measures of locomotor activity taken prior to operant trials suggest that the differences in responding were not due to differences in general activity levels. These studies provide further characterization of the HiS and LoS rat lines by demonstrating that motivation to consume sucrose is greater in HiS than in LoS rats.

Hybrid mice as genetic models of high alcohol consumption

We showed that F1 hybrid genotypes may provide a broader variety of ethanol drinking phenotypes than the inbred progenitor strains used to create the hybrids (Blednov et al. in Alcohol Clin Exp Res 29:1949-1958, 2005). To extend this work, we characterized alcohol consumption as well as intake of other tastants (saccharin, quinine and sodium chloride) in five inbred strains of mice (FVB, SJL, B6, BUB, NZB) and in their reciprocal F1 hybrids with B6 (FVBxB6; B6xFVB; NZBxB6; B6xNZB; BUBxB6; B6xBUB; SJLxB6; B6xSJL). We also compared ethanol intake in these mice for several concentrations before and after two periods of abstinence. F1 hybrid mice derived from the crosses of B6 and FVB and also B6 and SJL drank higher levels of ethanol than their progenitor strains, demonstrating overdominance for two-bottle choice drinking test. The B6 and NZB hybrid showed additivity in two-bottle choice drinking, whereas the hybrid of B6 and BUB demonstrated full or complete dominance. Genealogical origin, as well as non-alcohol taste preferences (sodium chloride), predicted ethanol consumption. Mice derived from the crosses of B6 and FVB showed high sustained alcohol preference and the B6 and NZB hybrids showed reduced alcohol preference after periods of abstinence. These new genetic models offer some advantages over inbred strains because they provide high, sustained, alcohol intake, and should allow mapping of loci important for the genetic architecture of these traits.

Milk consumption during adolescence decreases alcohol drinking in adulthood

Early onset of alcohol consumption increases the risk for the development of dependence. Whether adolescent consumption of other highly palatable solutions may also affect alcohol drinking in adulthood is not known. The purpose of this study was to determine the effects of adolescent consumption of four solutions: water, sucrose, sucrose-milk and milk on ethanol drinking in adult rats. Rats had limited access to one of the four solutions from day PND 29 to PND 51 and were subsequently trained to consume ethanol (E) using a sucrose (S) fade-out procedure. Adolescent consumption of sucrose and sucrose-milk solutions increased intake of 2.5% E when it was combined with 10% S but it had no effect on the drinking of 10% E alone. Adolescent consumption of milk and sucrose-milk significantly decreased the intake of 10% E when it was combined with 10% S, and milk significantly reduced 10% E consumption alone and when it was combined with 5% S. Adolescent exposure to the sucrose-milk and sucrose solutions was also found to increase sucrose and sucrose-milk consumption. Our findings suggest adolescent exposure to sucrose increases, whereas, exposure to milk reduces ethanol consumption in adult rats. Our results may provide a new theoretical approach to the early prevention of alcoholism.

High-alcohol preferring mice are more impulsive than low-alcohol preferring mice as measured in the delay discounting task

BACKGROUND

Repeated studies have shown that high impulsivity, when defined as the tendency to choose small immediate rewards over larger delayed rewards, is more prevalent in drug addicts and alcoholics when compared with nonaddicts. Assessing whether impulsivity precedes and potentially causes addiction disorders is difficult in humans because they all share a history of drug use. In this study, we address this question by testing alcohol-naïve mice from lines showing heritable differences in alcohol intake.

METHODS

Replicated selected lines of outbred high-alcohol preferring (HAP) mice were compared to a low-alcohol preferring (LAP) line as well as the low-drinking progenitor line (HS/Ibg) on an adjusting amount delay discounting (DD) task. The DD task employs 2 levers to present subjects with a choice between a small, immediate and a large, delayed saccharin reward. By adjusting the quantity of the immediate reward up and down based on choice behavior, the task allows an estimate of how the subjective value of the delayed reinforcer decreases as delays increase. Latency to respond was also measured for each trial.

RESULTS

Both HAP2 and HAP1 lines of mice were more impulsive than the LAP2 and HS/Ibg lines, respectively. Hyperbolic curve-fitting confirmed steeper discounting in the high-alcohol drinking lines. In addition, the high-alcohol drinking lines demonstrated greater within-session increases in reaction times relative to the low-alcohol drinking lines. No other differences (consumption of saccharin, total trials completed) consistently mapped onto genetic differences in alcohol drinking.

CONCLUSIONS

Alcohol-naïve outbred mice selected for high-alcohol drinking were more impulsive with saccharin reinforcers than low-alcohol drinkers. These data are consistent with results seen using inbred strain descendents of high-alcohol drinking and low-alcohol drinking rat lines, and suggest that impulsivity is a heritable difference that precedes alcoholism.

Consumption of SC45647 and sucralose by rats selectively bred for high and low saccharin intake

Mammals' affinity for sweet tastes exists alongside dramatic variation among species and individuals in responses to sweeteners. The present paper focused on consumption by Occidental High- (HiS) and Low-Saccharin (LoS)-consuming rats in 23-h 2-bottle tests of 2 sweeteners for which few data from rats are available: SC45647 and sucralose. Every HiS and LoS rat preferred SC45647 to water at every concentration, with HiS rats consuming it more avidly. Most HiS rats preferred sucralose to water at one or more concentrations; some HiS rats and most LoS rats avoided sucralose at every concentration. However, both HiS and LoS rats preferred a sucralose-maltodextrin mixture (Splenda) to water; thus, Splenda's "bulking" ingredient maltodextrin transforms highly variable responses to sucralose into a relatively homogeneous preference for the product. Implications for the study of variation in sweet taste are discussed.

Impulsive choice and environmental enrichment: effects of d-amphetamine and methylphenidate

Individual differences in impulsive choice and rearing in differential environments are factors that predict vulnerability to drug abuse. The present study determined if rearing influences impulsive choice, and if d-amphetamine or methylphenidate alters impulsive choice in differentially reared rats. Male Sprague-Dawley rats were raised from 21 days of age in either an enriched condition (EC) or an isolated condition (IC) and were tested as young adults on an adjusting delay task. In this task, two levers were available and a response on one lever yielded one 45mg food pellet immediately, whereas a response on the other yielded three pellets after an adjusting delay. The delay was initially set at 6s, and it decreased or increased by 1s following responses on the immediate or delayed levers, respectively. A mean adjusted delay (MAD) was calculated upon completion of each daily session, and it served as the quantitative measure of impulsivity. Once MADs stabilized, rats were injected with saline, d-amphetamine (0.5, 1.0, or 2.0mg/kg, s.c.), or methylphenidate (2.5, 5.0, or 10.0mg/kg, s.c.) 15min prior to adjusting delay sessions. EC rats had higher baseline MADs (were less impulsive) than IC rats. Additionally, administration of d-amphetamine, but not methylphenidate, dose-dependently increased impulsive choice (decreased MADs) in EC rats. In IC rats, d-amphetamine and methylphenidate dose-dependently decreased impulsivity (increased MADs). These results indicate that rearing environment influences impulsive choice and moderates the effect of psychostimulants on impulsive choice. Specifically, psychostimulants may decrease environment-dependent impulsive choice in individuals with high levels of impulsivity (e.g., those with ADHD), whereas they may increase impulsive choice in individuals with low levels of impulsivity.

Multivariate analyses reveal common and drug-specific genetic influences on responses to four drugs of abuse

Vulnerability to abused drugs is influenced by multiple genes unique to each drug and to risk genes for polydrug abuse. If several inbred mouse strains respond to different drugs similarly, this implies the action of a common group of genes. Simultaneous analysis of multiple responses to multiple drugs has been attempted infrequently. We performed multivariate analyses of published strain responses to four drugs. Genetic similarity in responses did not simply track pharmacological class. Withdrawal severity and preference for ethanol and diazepam were affected by many genes in common, although inversely. We focused on behavioral responses, but there is a growing archival database of physiological, pharmacological and biochemical strain traits. The genomics community is increasingly focusing on single-nucleotide polymorphism and haplotype-based gene mapping approaches, for which inbred strain data are also useful. Thus, similar analyses should be applicable to other laboratories, traits and genotypes.

Preferences of 14 rat strains for 17 taste compounds

Two-bottle choice tests were used to assess the taste preferences of 8 male and 8 female rats from 3 outbred strains (SD, LE, WI) and 11 inbred strains (BN, BUF, COP, DA, Dahl-S, F344, FHH, LEW, Noble, PVG, SHR). Each rat received a series of 109 48-h tests with a choice between water and a "taste solution". Four to eight concentrations of the following compounds were tested: NaCl, CaCl2, NH4Cl, KCl, MgCl2, saccharin, sucrose, ethanol, HCl, citric acid, quinine hydrochloride (QHCl), caffeine, denatonium, monosodium glutamate (MSG), Polycose, corn oil, and capsaicin. Strain differences (p<0.001) were observed in preferences for at least one concentration of all compounds tested except denatonium (p=0.0015). There were also strain differences in the following ancillary measures: fungiform papillae number, water intake, food intake, and body weight. There were sex differences in food intake and body weight but no concerted sex differences in any of the other measures, including preferences for any taste solution. This comprehensive source of information can be used to guide the choice of appropriate rat strains and taste solution concentrations for future genetic studies.

The role of impulsive behavior in drug abuse

BACKGROUND

Impulsivity is a multifaceted construct that has recently been recognized as a factor contributing to enhanced vulnerability to drug abuse.

OBJECTIVES

In the present review, we focus on two facets of impulsivity (and tasks that measure them): (1) impulsive choice (delay discounting task) and (2) inhibitory failure (go/no-go, stop signal reaction time, and five-choice serial reaction time tasks). We also describe how performance on each of these tasks is associated with drug-related behavior during phases of drug abuse that capture the essential features of addiction (acquisition, escalation, and reinstatement of drug-seeking after drug access has terminated). Three hypotheses (H) regarding the relationship between impulsivity and drug abuse are discussed: (1) increased levels of impulsivity lead to drug abuse (H1), (2) drugs of abuse increase impulsivity (H2), and (3) impulsivity and drug abuse are associated through a common third factor (H3).

CONCLUSION

Impulsivity expressed as impulsive choice or inhibitory failure plays a role in several key transition phases of drug abuse. There is evidence to support all three nonexclusive hypotheses. Increased levels of impulsivity lead to acquisition of drug abuse (H1) and subsequent escalation or dysregulation of drug intake. Drugs of abuse may increase impulsivity (H2), which is an additional contributor to escalation/dysregulation. Abstinence, relapse, and treatment may be influenced by both H1 and H2. In addition, there is a relationship between impulsivity and other drug abuse vulnerability factors, such as sex, hormonal status, reactivity to nondrug rewards, and early environmental experiences that may impact drug intake during all phases of addiction (H3). Relating drug abuse and impulsivity in phases of addiction via these three hypotheses provides a heuristic model from which future experimental questions can be addressed.

Effects of naltrexone, duloxetine, and a corticotropin-releasing factor type 1 receptor antagonist on binge-like alcohol drinking in rats

A 'binge' is defined by National Institute on Alcohol Abuse and Alcoholism as an excessive pattern of alcohol drinking that produces blood-alcohol levels (BALs) greater than 0.08 g% within a 2-h period and may or may not be associated with dependence. The purpose of this investigation was to explore the effects of several neuropharmacological agents in an animal model in which outbred rats voluntarily and orally self-administer pharmacologically meaningful alcohol doses that produce BALs >or=0.08 g% in daily limited access two-bottle choice and operant drinking sessions. Rats were trained to self-administer either 10% (w/v) alcohol solution sweetened with 'supersac' (3% glucose+0.125% saccharin) or supersac alone versus water in a two-bottle choice or operant situation during 30-min daily sessions. Rats were then injected systemically with multiple doses of duloxetine, naltrexone, and the corticotropin-releasing factor antagonist, MPZP, in Latin-square designs. Alcohol binge drinkers reliably consumed amounts of alcohol sufficient to produce BALs >or=0.08 g%. Duloxetine dose-dependently suppressed two-bottle choice alcohol binge drinking and operant alcohol responding as well as operant supersac responding, but did not affect two-bottle choice supersac drinking. Naltrexone-suppressed alcohol binge drinking at very low doses and suppressed supersac drinking at moderate-to-high doses. MPZP did not affect alcohol or supersac consumption. Different profiles for drugs that suppress binge-like alcohol drinking compared with dependence-induced drinking provide a heuristic foundation for future medications development.

Perception of sweet taste is important for voluntary alcohol consumption in mice

To directly evaluate the association between taste perception and alcohol intake, we used three different mutant mice, each lacking a gene expressed in taste buds and critical to taste transduction: alpha-gustducin (Gnat3), Tas1r3 or Trpm5. Null mutant mice lacking any of these three genes showed lower preference score for alcohol and consumed less alcohol in a two-bottle choice test, as compared with wild-type littermates. These null mice also showed lower preference score for saccharin solutions than did wild-type littermates. In contrast, avoidance of quinine solutions was less in Gnat3 or Trpm5 knockout mice than in wild-type mice, whereas Tas1r3 null mice were not different from wild type in their response to quinine solutions. There were no differences in null vs. wild-type mice in their consumption of sodium chloride solutions. To determine the cause for reduction of ethanol intake, we studied other ethanol-induced behaviors known to be related to alcohol consumption. There were no differences between null and wild-type mice in ethanol-induced loss of righting reflex, severity of acute ethanol withdrawal or conditioned place preference for ethanol. Weaker conditioned taste aversion (CTA) to alcohol in null mice may have been caused by weaker rewarding value of the conditioned stimulus (saccharin). When saccharin was replaced by sodium chloride, no differences in CTA to alcohol between knockout and wild-type mice were seen. Thus, deletion of any one of three different genes involved in detection of sweet taste leads to a substantial reduction of alcohol intake without any changes in pharmacological actions of ethanol.

Higher locomotor response to cocaine in female (vs. male) rats selectively bred for high (HiS) and low (LoS) saccharin intake

Rats selectively bred for high saccharin consumption (HiS) self-administer more oral ethanol and i.v. cocaine than those selectively bred for low saccharin consumption (LoS). Male and female drug-seeking-prone (HiS) and -resistant (LoS) rats were used in the present experiment to test the prediction that cocaine-induced locomotor activity and sensitization varied with sex and their selective breeding status (HiS and LoS). All rats were intermittently exposed over 2 weeks to pairs of sequential saline and cocaine injections, separated by 45 min. The first 5 pairs of injections, each separated by 2-3 days (10-12 days total), were given to examine the development of cocaine-induced locomotor activity and the development of locomotor sensitization, which was determined by comparing the effects of cocaine injection 1 with injection 6 (given 2 weeks after the 5 pairs of intermittent injections). Results indicated that after the first injection pair (saline, cocaine) the HiS and LoS groups did not differ (saline vs. cocaine) in locomotor activity; however, after cocaine injection pairs 1, 5, and 6, HiS females were more active than HiS males and LoS females. There were also significant phenotype differences (HiS>LoS) in locomotor activity after cocaine injections 5 and 6. There was a weak sensitization effect in cocaine-induced locomotor activity in HiS females after cocaine injection 5 (compared to 1); however it was not present after injection 6 or in other groups. The lack of a strong sensitization effect under these temporal and dose conditions was inconsistent with previous reports. However, the results showing HiS>LoS and females>males on cocaine-induced activity measures are consistent with several measures of cocaine-seeking behavior (acquisition, maintenance, escalation, extinction, and reinstatement), and they suggest that cocaine-induced locomotor activity and sensitization are behavioral markers of drug-seeking phenotypes.

Behavioral genetics and taste

This review focuses on behavioral genetic studies of sweet, umami, bitter and salt taste responses in mammals. Studies involving mouse inbred strain comparisons and genetic analyses, and their impact on elucidation of taste receptors and transduction mechanisms are discussed. Finally, the effect of genetic variation in taste responsiveness on complex traits such as drug intake is considered. Recent advances in development of genomic resources make behavioral genetics a powerful approach for understanding mechanisms of taste.