Effects of feeding condition on orally delivered ethanol as a reinforcer for rhesus monkeys

Concurrent nonindependent fixed-ratio schedules of alcohol self-administration: Effects of schedule size on choice

Choice behavior was studied under concurrent nonindependent fixed‐ratio fixed‐ratio (nFR) schedules of reinforcement, as these schedules result in frequent changeover responses. With these schedules, responses on either operandum count toward the completion of the ratio requirements of both schedules. Five monkeys were subjects, and two pairs of liquid reinforcers were concurrently available: 16% (w/v) and 0% ethanol or 16% and 8% ethanol. For each pair of reinforcers, the nFR sizes were systematically altered across sessions while keeping the schedule size equal for both liquids. Responding varied as a function of reinforcer pair and nFR size. With the 16% and 0% pair, higher response rates were maintained by 16% and were an inverted U‐shape function of nFR size. With 16% and 8%, a greater number of responses initially occurred on the schedule that delivered 8% ethanol. However, as nFR size increased, preference reversed such that responses that delivered 16% ethanol were greater. When the nFR size was subsequently decreased, preference reverted back to 8%. Number of responses emitted per delivery was a dependent variable and, in behavioral economic terms, was the price paid for each liquid delivery. With 16% and 0%, changeover responses initially increased and then decreased as schedule size became larger. In contrast, with the 16% and 8% pair, changeover responses increased directly with schedule size. Responding under nFR schedules is sensitive to differences in reinforcer magnitude and demonstrates that relative reinforcing effects can change as a function of schedule size.

Comparison of dehydroepiandrosterone (DHEA) and pregnanolone with existing pharmacotherapies for alcohol abuse on ethanol- and food-maintained responding in male rats

The present study compared two putative pharmacotherapies for alcohol abuse and dependence, dehydroepiandrosterone (DHEA) and pregnanolone, with two Food and Drug Administration (FDA)-approved pharmacotherapies, naltrexone and acamprosate. Experiment 1 assessed the effects of different doses of DHEA, pregnanolone, naltrexone, and acamprosate on both ethanol- and food-maintained responding under a multiple fixed-ratio (FR)-10 FR-20 schedule, respectively. Experiment 2 assessed the effects of different mean intervals of food presentation on responding for ethanol under a FR-10 variable-interval (VI) schedule, whereas Experiment 3 assessed the effects of a single dose of each drug under a FR-10 VI-80 schedule. In Experiment 1, all four drugs dose-dependently decreased response rate for both food and ethanol, although differences in the rate-decreasing effects were apparent among the drugs. DHEA and pregnanolone decreased ethanol-maintained responding more potently than food-maintained responding, whereas the reverse was true for naltrexone. Acamprosate decreased responding for both reinforcers with equal potency. In Experiment 2, different mean intervals of food presentation significantly affected the number of food reinforcers obtained per session; however, changes in the number of food reinforcements did not significantly affect responding for ethanol. Under the FR-10 VI-80 schedule in Experiment 3, only naltrexone significantly decreased both the dose of alcohol presented and blood ethanol concentration (BEC). Acamprosate and pregnanolone had no significant effects on any of the dependent measures, whereas DHEA significantly decreased BEC, but did not significantly decrease response rate or the dose presented. In summary, DHEA and pregnanolone decreased ethanol-maintained responding more potently than food-maintained responding under a multiple FR-10 FR-20 schedule, and were more selective for decreasing ethanol self-administration than either naltrexone or acamprosate under that schedule. Experiment 2 showed that ethanol intake was relatively independent of the interval of reinforcement in the food-maintained component, and Experiment 3 showed that naltrexone was the most effective drug at the doses tested when the interval for food reinforcement was low and maintained under a variable-interval schedule.

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.

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.

Advances in nonhuman primate alcohol abuse and alcoholism research

Advances in our understanding of the biological basis of alcohol abuse and alcoholism and the development of prevention and therapeutic intervention require appropriate animal models. Nonhuman primates are important to the study of complex biomedical disease processes. Genetic, anatomical, physiological, and behavioral similarities to humans offer unique opportunities for translational research along with the advantage of a degree of experimental control that is not possible in human studies. The purpose of this review is to outline the approaches taken with nonhuman primates as subjects in alcohol research and to highlight our current understanding of data on organismal variables that can be uniquely studied in these complex organisms. We review literature on alcohol self-administration to provide an integrative framework for discussion of progress in 2 important areas of research. Designs that incorporate self-administration provide a context for studying excessive alcohol consumption, including the organismal and environmental factors that influence risk for heavy drinking. We then review the use of monkeys to identify aspects of adverse biomedical consequences that follow excessive alcohol consumption. One of the primary conclusions to be drawn from this review is that nonhuman primates are a central part of the translational bridge in alcohol research, providing powerful and unique opportunities for experimental work that can address the biomedical complexities of alcohol abuse and alcoholism.

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.

Ethanol and food pellet self-administration by baboons

During daily 23-h sessions, baboons had concurrent access to food pellets and an oral ethanol/dextrose solution. The effect of increasing the fixed-ratio or "cost" for pellets on pellet and fluid intake was examined when baboons had access to 2%, 4%, or 8% (w/v) ethanol. Increasing the response requirement for a pellet decreased pellet intake. The rate of decrease in pellet intake with increasing pellet cost was unaffected by the availability of ethanol solutions, which were either self-administered or given in investigator-planned doses. Increasing the response cost for pellets significantly increased self-administration of 4% ethanol. The effect of increasing the cost for fluid on fluid and pellet intake was examined when baboons had access to vehicle, 4% or 8% (w/v) ethanol. Although the total daily number of fluid deliveries was significantly greater when 4% ethanol was available, compared to vehicle, increasing the cost for a fluid delivery to 32 responses and above decreased intake of all three fluids similarly. Increasing the cost of ethanol did not affect food intake.

Ethanol as an oral reinforcer in normal weight rhesus monkeys: dose-response functions

Responding maintained by orally delivered ethanol was studied in three adult male rhesus monkeys with no previous drug-taking histories. The subjects had continuous access to water throughout the study and were fed sufficient food to maintain a positive caloric balance. During daily 3-h experimental sessions, ethanol solution and water were concurrently available under fixed-ratio reinforcement schedules from two liquid delivery systems that were mounted symmetrically on the side of the monkeys' cages. The positions of the ethanol and water alternated daily. A range of ethanol doses (1%, 2%, 4%, 5.7%, 8%, 11.3%, 16%, 22.7, and 32% w/v) was tested in ascending and descending order. Changes in concentration were made after six consecutive stable sessions. Generally, the number of ethanol deliveries increased and then decreased as the ethanol concentration was raised (an inverted U-shaped function). For two of the monkeys, ethanol deliveries exceeded water deliveries at concentrations from 1.0% to 5.7% w/v, whereas for a third monkey ethanol solution was preferred to water at concentrations as high as 16% w/v. Ethanol intake (g/kg) tended to increase linearly and then flatten as the concentration was raised. Blood ethanol levels, measured immediately following sessions in which 8% w/v ethanol was available, ranged from 45 to 146 mg/dl. Although no special training or acquisition procedures were used to establish responding, ethanol functioned as a reinforcer for each of the monkeys tested.

Effects of continuous versus limited access to ethanol on ethanol self-administration

Eight male, experimentally naive Long-Evans rats were housed in operant chambers 23 h per day following initiation to self-administer ethanol. While housed in the chambers, the animals had continuous access to food pellets according to a fixed ratio 1 schedule of reinforcement, 10% ethanol (v/v) according to a fixed ratio 4 schedule of reinforcement and water in a drinking tube with licks recorded via a drinkometer. Over a series of experimental phases, daily availability of the ethanol solution was limited to 16, 6, 4, 2, or 1 30-min period per day. The 1 30-min period access was examined during the 12th hour or the second hour of the daily sessions. Over the course of the experiment, total responses on the lever that operated the dipper, g/kg per day and number of ethanol drinking bouts per day decreased significantly as the number of daily access periods decreased. On the other hand, the number of dippers presented per ethanol bout, g/kg per ethanol bout and ethanol bout duration increased, with significant increases in dippers per bout occurring when one 30-min access period per day was provided. These data indicate that the size of a single ethanol drinking bout can be increased somewhat by limiting the opportunity to obtain ethanol reinforcement and agrees with earlier research that has shown similar effects.

Ethanol-reinforced responding of naive rhesus monkeys: acquisition without induction procedures

Rodents will typically consume greater amounts of low concentration ethanol (1-6%) than water. However, few primate oral self-administration studies have examined low ethanol concentrations. Additionally, there is a scarcity of data showing ethanol-maintained behavior without using induction procedures in either rats or primates. In this study, 14, free-feeding, naive, adult, male rhesus monkeys were given access to a 2% (w/v) ethanol solution and vehicle (tap water) during daily 3-hour sessions. Water was freely available between sessions. Liquids were available under a concurrent fixed-ratio four reinforcement schedule; thus, four responses (mouth-spout contacts) on either spout were immediately followed by the delivery of approximately 0.65 ml of liquid. In phase 1, tap water was available from both spouts. In phase 2, vehicle and a 2% ethanol solution were concurrently available. The 2% ethanol solution maintained considerably higher response rates than vehicle for 12 of 14 monkeys. In phase 3, food intake was limited. During this phase, the 2% ethanol solution maintained significantly higher response rates than vehicle for all monkeys. Additionally, during phase 3, ethanol intakes were greater than those in phase 2 for all monkeys. Central drug effects and taste factors are discussed. These results demonstrate that ethanol will serve as a reinforcer for naive rhesus monkeys in the absence of induction procedures.