Food restriction and sex differences on concurrent, oral ethanol and water reinforcers in juvenile rhesus monkeys

Aggressive temperament predicts ethanol self-administration in late adolescent male and female rhesus macaques

RATIONALE

Anxiety and aggression are associated with ethanol self-administration, but these behaviors can serve as either risk factors for or consequences of heavy drinking in rodents and humans. Baseline levels of aggressive-like and anxious-like behavior in non-human primates have not yet been characterized in relation to future or prior ethanol intake.

OBJECTIVE

The objective of the study was to test the association between temperament at baseline with future ethanol self-administration in late adolescent male (n = 21) and female (n = 11) rhesus monkeys.

METHODS

Shortly after entering the laboratory and before exposure to ethanol, the Human Intruder Test (HIT) and the Novel Object Test (NOT) were used to determine baseline anxious-like and aggressive-like behavior in age-matched male and female rhesus monkeys (Macaca mulatta). The monkeys were induced to drink ethanol 4 % (w/v) using a schedule-induced polydipsia procedure, followed by "open-access" ethanol self-administration in which the monkeys were allowed a choice of water or 4 % ethanol (w/v) for 22 h/day for 52 weeks.

RESULTS

Aggressive monkeys self-administered more ethanol and attained higher blood ethanol concentrations (BECs). No significant differences in ethanol intakes or BECs were found between anxious and non-anxious monkeys or between behaviorally inhibited and non-inhibited monkeys. Baseline aggressive behavior positively correlated with ethanol intake and intoxication.

CONCLUSIONS

Baseline reactive aggression was associated with higher future ethanol intake and intoxication. While significant sex differences in HIT reactivity were observed, the relationship between aggression and ethanol drinking was observed across sex and is not sex-specific.

Emergence of sex differences in the development of substance use and abuse during adolescence

Substance use and abuse begin during adolescence. Male and female adolescent humans initiate use at comparable rates, but males increase use faster. In adulthood, more men than women use and abuse addictive drugs. However, some women progress more rapidly from initiation of use to entry into treatment. In animal models, adolescent males and females consume addictive drugs similarly. However, reproductively mature females acquire self-administration faster, and in some models, escalate use more. Sex/gender differences exist in neurobiologic factors mediating both reinforcement (dopamine, opioids) and aversiveness (CRF, dynorphin), as well as intrinsic factors (personality, psychiatric co-morbidities) and extrinsic factors (history of abuse, environment especially peers and family) which influence the progression from initial use to abuse. Many of these important differences emerge during adolescence, and are moderated by sexual differentiation of the brain. Estradiol effects which enhance both dopaminergic and CRF-mediated processes contribute to the female vulnerability to substance use and abuse. Testosterone enhances impulsivity and sensation seeking in both males and females. Several protective factors in females also influence initiation and progression of substance use including hormonal changes of pregnancy as well as greater capacity for self-regulation and lower peak levels of impulsivity/sensation seeking. Same sex peers represent a risk factor more for males than females during adolescence, while romantic partners increase risk for women during this developmental epoch. In summary, biologic factors, psychiatric co-morbidities as well as personality and environment present sex/gender-specific risks as adolescents begin to initiate substance use.

Sex differences and ovarian hormones in animal models of drug dependence

Increasing evidence indicates the presence of sex differences in many aspects of drug abuse. Most studies reveal that females exceed males during the initiation, escalation, extinction, and reinstatement (relapse) of drug-seeking behavior, but males are more sensitive than females to the aversive effects of drugs such as drug withdrawal. Findings from human and animal research indicate that circulating levels of ovarian steroid hormones account for these sex differences. Estrogen (E) facilitates drug-seeking behavior, while progesterone (P) and its metabolite, allopregnanalone (ALLO), counteract the effects of E and reduce drug seeking. Estrogen and P influence other behaviors that are affiliated with drug abuse such as drug-induced locomotor sensitization and conditioned place preference. The enhanced vulnerability to drug seeking in females vs. males is also additive with the other risk factors for drug abuse (e.g., adolescence, sweet preference, novelty reactivity, and impulsivity). Finally, treatment studies using behavioral or pharmacological interventions, including P and ALLO, also indicate that females show greater treatment effectiveness during several phases of the addiction process. The neurobiological basis of sex differences in drug abuse appears to be genetic and involves the influence of ovarian hormones and their metabolites, the hypothalamic pituitary adrenal (HPA) axis, dopamine (DA), and gamma-hydroxy-butyric acid (GABA). Overall, sex and hormonal status along with other biological risk factors account for a continuum of addiction-prone and -resistant animal models that are valuable for studying drug abuse prevention and treatment strategies.

Sex differences in the self-administration of cannabinoids and other drugs of abuse

Many studies have provided evidence for important sex-dependent differences in the origins, outcomes and treatment of drug abuse and dependence. Preclinical studies typically have employed animal models of addiction, such as oral or intravenous self-administration, to untangle the environmental, neurobiological and genetic factors that contribute to the shift from occasional, recreational use to compulsive, uncontrolled intake of drugs. Craving and relapse of drug seeking in abstinent individuals have also been found to differ between men and women. Identification of the neurobiological basis of craving and drug dependence continues to pose a challenge to addiction research. Significant sex differences are emerging in substance-abuse-related behavior, which has increased the demand for research on how drug consumption may have different causes, progression and consequences in men and women. In keeping with epidemiological data in humans, differences between the two sexes in drug seeking and intake have been well-documented in animal studies, with most recent findings related to abuse of cannabinoids. Clinical and preclinical findings indicate that sex and gonadal hormones may account for individual differences in susceptibility to the reinforcing effects of addictive substances, and that differences in vulnerability to drug abuse may be mediated by the same biological mechanisms. This review focuses on the differences between males and females in relation to drug self-administration and how such behavior may be affected by hormonal status.

Early malnutrition favours heavy ethanol intake in weaning rats without long-lasting effects

INTRODUCTION

Although food restriction is well known to increase ethanol intake, the subject has not been extensively studied in developing animals which could be more vulnerable to long-lasting effects. Therefore, the aim of this study was to show some findings concerning this subject.

MATERIALS AND METHODS

Food restriction was used to produce malnutrition either during lactation (lactating dams food restricted 60%) or after weaning (pups food restricted 60%). At weaning, day 25, males were assigned to one of the following groups: C, food ad libitum throughout the experiment - control group; MW, malnourished only after weaning; ML, malnourished only during lactation period; and MLW, malnourished throughout the experiment, during lactation and after weaning. All rats were kept isolated in cages in which they could choose to drink either a 10% ethanol solution or tap water (from days 25 to 45). Re-exposure to this model was performed on day 49. Between exposure and re-exposure, rats drank tap water for 4 days.

RESULTS

There was a significant effect of malnutrition during lactation, up to day 35, with heavy drinking patterns (ethanol intake day 2: C, 8 g/kg; MW, 9 g/kg; ML, 19 g/kg; and MLW, 22 g/kg). This heavy drinking pattern lasted until the recovery of body weight. Food restriction after weaning had significant effects after 14 days, when a statistically significant decrease in body weight occurred (body weight day 39: C, 147.8 g; MW, 98.5 g). Only rats which were persistently malnourished (MLW and MW) drank ethanol more significantly than their ad libitum-fed counterparts during the re-exposure period (ethanol intake: malnourished, 5 g/kg; and well-nourished, 2.5 g/kg). Adulteration of the ethanol solution with quinine (0.1 g/l) precluded the effect of malnutrition.

CONCLUSIONS

Malnutrition during early development had no long-lasting effects on ethanol consumption. In addition, malnutrition increased ethanol consumption as long as it kept body weight low, which was apparently more significant in young animals.

Consistent, high-level ethanol consumption in pig-tailed macaques via a multiple-session, limited-intake, oral self-dosing procedure

BACKGROUND

Alcohol abuse is a major public health burden that can lead to many adverse health effects such as impaired hepatic, gastrointestinal, central nervous system and immune system function. Preclinical animal models of alcohol abuse allow for experimental control over variables often difficult to control in human clinical studies (e.g., ethanol exposure before or during the study, history of other drug use, access to medical care, nutritional status, etc). Nonhuman primate models in particular provide increased genetic, anatomic and physiologic similarity to humans, relative to rodent models. A small percentage of macaques will spontaneously consume large quantities of ethanol; however, most nonhuman primate models of "voluntary" ethanol intake produce relatively low daily ethanol intake in the majority of monkeys.

METHODS

To facilitate study of chronic exposure to high levels of ethanol intake, a macaque model has been developed that induces consistent, daily high-level ethanol consumption. This multiple-session procedure employed 4 drinking sessions per day, with sessions occurring once every 6 hours.

RESULTS

The group average alcohol consumption was 4.6 g/kg/d (SEM 0.4), roughly twice the group average consumption of previous reports. Ethanol drinking sessions produced group mean blood ethanol levels of 95 mg/dl after 60 minutes, and fine motor control was impaired up to 90 minutes after a drinking session.

CONCLUSION

This model of multiple-session, limited access, oral ethanol self-dosing produced consistent, high-level ethanol consumption with each session qualifying as a "binge" drinking session using the definition of "binge" provided by the NIAAA (>80 mg/dl/session). This model of ethanol drinking in macaques will be of great utility in the study of immunological, physiological and behavioral effects of ethanol in nonhuman primates.

Consumption of alcohol by sows in a choice test

The domestic pig (Sus scrofa domesticus) has been proposed as an animal model for human alcoholism because pigs have been observed to consume alcohol voluntarily to a state of intoxication and to exhibit tolerance and physical dependence. However, it has not been established whether pigs can develop psychological dependence on alcohol. We hypothesised that feed-restricted, stall-housed pregnant sows fed alcohol non-voluntarily for 5 weeks would develop a preference for alcohol and retain this preference after removal of alcohol from the diet. We fed crossbred commercial sows (n=10) 280 ml of 95% ethanol mixed with 0.91 kg of feed and 720 ml of water twice daily for 5 weeks during the first trimester of pregnancy. Control sows (n=7) received dextrose in their feed as a caloric control, and water was added to give the feed a consistency similar to that of the alcohol-treated feed. Immediately before and after 5 weeks of alcohol or dextrose treatment and 3 weeks later, after termination of alcohol or dextrose treatment, we evaluated sow diet preference by comparing the amount of alcohol-supplemented, dextrose-supplemented and plain feed consumed during a 5-min choice test. Contrary to our hypothesis, there was no treatment effect on sow diet preference. Both alcohol-treated and control sows ate less of the alcohol diet than the other two diets in all choice tests. They did not discriminate between the plain and dextrose diets. We conclude that 5 weeks of non-voluntary consumption of alcohol in feed did not produce a preference for alcohol in pregnant sows, either during treatment or after withdrawal, thus providing no evidence for the development of psychological dependence on alcohol under these conditions.

Controlled and behaviorally relevant levels of oral ethanol intake in rhesus macaques using a flavorant-fade procedure

BACKGROUND

Flavorant-fading procedures can initiate and maintain oral ethanol intake in rodents. The present study developed a similar procedure to achieve controlled and behaviorally relevant levels of ethanol intake in monkeys.

METHODS

Male rhesus macaques (N = 13) were initially given the opportunity to consume 0.5 g/kg of a 1% (w/v) ethanol plus 4% (w/v) Tang solution in 1-hr limited-access sessions without the requirement of an operant response. Once consumption was stable at a particular concentration (%) and/or amount (g/kg), animals were given access to higher concentrations and/or amounts of ethanol. Animals were tested on a bimanual motor skill (BMS) task 20 and 90 min after consumption to assess behavioral impairment. Blood alcohol levels (BALs) were assessed after a session in which animals had the opportunity to consume up to 3.0 g/kg of 6% (w/v) ethanol.

RESULTS

The gradual fading up of higher concentrations and amounts of ethanol resulted in controlled and robust levels (>2.0 g/kg) of ethanol intake in half of the subjects. Increasing the concentration of the sweetener from 4 to 6% (w/v) was effective in initiating consumption in three animals. Two monkeys required the additional step of presenting the increased-sweetener solutions after a meal (postprandial consumption) to initiate significant ethanol intake. Animals were significantly impaired on the BMS task after consumption of 2.0, 2.5, and 3.0 g/kg of ethanol. Individual consumption ranging from 0.8 to 3.0 g/kg of ethanol produced BALs of 18 to 269 mg/dl.

CONCLUSIONS

The flavorant-fading procedure was effective in producing behaviorally relevant levels of ethanol consumption in rhesus macaques. This model facilitated a randomized-dose procedure to determine the behavioral effects of 0.5 to 3.0 g/kg of ethanol. This procedure therefore is of significant utility in determining behavioral or physiologic effects of specific doses of consumed ethanol in monkeys.

Oral drug self-administration: an overview of laboratory animal studies

Many abused drugs can be established as orally delivered reinforcers for rhesus monkeys and other animals. Benzodiazepines, barbiturates, opioids, psychomotor stimulants, dissociative anesthetics, and ethanol can come to serve as reinforcers when taken by mouth. The principal problems in establishing drugs as reinforcers by the oral route of administration are (1) aversive taste, (2) delay in onset of central nervous system effects, and (3) consumption of low volumes of drug solution. Strategies have been devised to successfully overcome these problems, and orally delivered drugs can be established as effective reinforcers. Reinforcing actions are demonstrated by consumption of greater volumes of drug solution than volumes of the water vehicle, and supporting evidence for reinforcing effects consists of the maintenance of behavior under intermittent schedules of reinforcement and the generation of orderly dose-response functions. This article presents an overview of studies of behavior reinforced by oral drug reinforcement. Factors that control oral drug intake include dose, schedule of reinforcement, food restriction, and alternative reinforcers. Many drugs, administered by the experimenter, can alter oral drug reinforcement. Relative reinforcing effects can be assessed by choice procedures and by persistence of behavior across increases in schedule size. In general, reinforcing effects increase directly with dose. Rhesus monkeys prefer combinations of reinforcing drugs to the component drugs. The taste of drug solutions may act as a conditioned reinforcer and a discriminative stimulus. Consequences of drug intake include tolerance and physiological dependence. Findings with orally self-administered drugs are similar to many findings with other positive reinforcers, including intravenously self-administered drugs.