Relation between choice of ethanol concentration and response rates under progressive- and fixed-ratio schedules: studies with rhesus monkeys

Sex differences in opioid reinforcement under a fentanyl vs. food choice procedure in rats

Clinical evidence suggest that men are more sensitive than women to the abuse-related effects of mu-opioid agonists. In contrast, preclinical studies suggest the opposite sex difference. The aim of the present study was to clarify this discrepancy using a fentanyl vs. diluted Ensure^® choice procedure to assess sex differences in opioid reinforcement. Sex differences in intravenous (IV) fentanyl self-administration were examined under a fixed-ratio (FR5) schedule, a multi-day progressive-ratio (PR) schedule for behavioral economic analysis, and a concurrent (choice) schedule of fentanyl and diluted Ensure^® reinforcement in Sprague-Dawley male and female rats. The fentanyl dose-effect function under the FR5 schedule was significantly shifted upward in females compared to males. Similarly, the reinforcing effectiveness of both fentanyl (3.2 and 10 µg/kg per injection, IV) and diluted Ensure^® (18 and 56%) were greater in females than in males as assessed using behavioral economic analysis, irrespective of dose or concentration. However, under a fentanyl vs. foodchoice procedure, males chose 3.2 µg/kg per injection fentanyl injections over 18%, but not 56%, diluted Ensure^® at a higher percentage compared to females. Overall, these results suggest that the expression of sex differences in opioid reinforcement depends upon the schedule of reinforcement and that preclinical opioid vs. food choice procedures provide a translationally relevant measure (i.e., behavioral allocation) consistent with the direction of sex differences reported in the clinical literature.

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.

Ethanol self-administration in rats responding under concurrent schedules for milk or ethanol plus milk

The relative reinforcing strength of drugs can be characterized by the distribution of operant behavior during the availability of other reinforcing stimuli. 'Choice' procedures are not widely used in rats, with the exception of ethanol self-administration in which there often is a choice between ethanol and water, which typically does not maintain much responding. A procedure was developed to evaluate the relative reinforcing strength of ethanol in rats when a similar appetitive reinforcer is concurrently available. Rats were trained to respond on two levers under concurrent fixed-ratio schedules of reinforcement with milk (1-50%) or ethanol+milk (4-32% ethanol+5-10% milk). Daily 60-min sessions began with a forced sample of each reinforcer, followed by the concurrent schedules. Under this schedule, rats preferentially allocated their responding to the ethanol-associated lever under conditions of ethanol+5% milk versus 5% milk, but neither preferred nor avoided ethanol when ethanol+10% milk versus 10% milk was available. When 8% ethanol+5% milk was available, 85±6% of responses were directed toward the ethanol-associated lever and the mean ethanol intake was 1.55±0.10 g/kg. The response rate decreased monotonically with the concentration of ethanol. Naltrexone injections did not affect the distribution of responding, but slightly decreased ethanol intake. It is concluded that stable behavior can be maintained under concurrent fixed-ratio schedules of ethanol and milk presentation in rats, resulting in intake of behaviorally active amounts of ethanol.

Relationship between response rates and measures of reinforcing strength using a choice procedure in monkeys

Concurrent schedules of reinforcement are increasingly being used to investigate the reinforcing strength of abused drugs. A purported advantage of concurrent schedules is that the primary dependent measure, percentage of responses emitted on the drug-associated manipulandum, is independent of the rate-altering effects of drugs. Data supporting this hypothesis are, however, rarely presented, which was one goal of this study. In addition, we tested the hypothesis that drug-induced decreases in response rates provides an additional index to characterize abuse liability of drugs. This study examined the relationship between response rate and response allocation (i.e. drug choice) when 3,4-methylenedioxymethamphetamine (MDMA, 0.03-0.3 mg/kg/inj) or cocaine (0.003-0.1 mg/kg/inj) was the alternative to food under concurrent fixed-ratio reinforcement schedules in rhesus (n=4) and cynomolgus (n=16) monkeys, respectively. Increasing doses of MDMA or cocaine resulted in increased drug choice and dose-dependent decreases in overall response rates. For both drugs, response rates on the drug-associated lever were not affected by dose and were not different from saline. Furthermore, at most doses, rates of responding on the food-associated lever were significantly higher than response rates on the drug-associated lever. Finally, MDMA but not cocaine decreased food-reinforced responding, providing evidence for potential differences between the drugs. These results demonstrate that under concurrent food-drug reinforcement schedules, response rates on the drug-associated lever are independent of measures of reinforcement, whereas disruptions in food-maintained responding may be inversely related to abuse liability.

Pharmacological activation of CB1 and D2 receptors in rats: predominant role of CB1 in the increase of alcohol relapse

The classical dopamine D2 receptor has been widely studied in alcoholism. Recently, different studies have explored the role of the CB1 receptor in alcohol-related behavior. In alcohol addiction, relapse is one of the central features. In light of this, we investigated the functional roles of and interactions between CB1 and D2 receptors in alcohol relapse. We used the learned task of alcohol operant self-administration in Wistar rats. In order to evaluate alcohol relapse, we set up a protocol essentially based on the alcohol deprivation effect. We found that subchronic activation of CB1 (WIN 55,212-2, 2 mg/kg), but not D2 receptors (quinpirole, 1 mg/kg), during a period of alcohol deprivation increased long-lasting alcohol relapse. The cannabinoid-induced potentiation of alcohol relapse was mediated by a motivational and appetitive component, and not merely by alcohol consumption. This potentiation was prevented by the pharmacological inactivation of D2 receptors (raclopride, 0.1-0.3 mg/kg). Together, these results essentially demonstrate that activation of CB1 receptors plays a key role in the increase of alcohol relapse, whereas inactivation of D2 receptors modulates this aberrant behavior. We suggest that there exists a functional and interactive relationship between both receptor systems, which controls alcohol relapse and alcohol-learned tasks.

Reinforcement magnitude modulates the rate-dependent effects of fluvoxamine and desipramine on fixed-interval responding in the pigeon

Some doses of fluvoxamine can decrease ethanol-maintained behavior more than food-maintained behavior. This might be explained by differences in reinforcement magnitude. In a previous study, the effects of fluvoxamine on fixed-ratio responding did not depend upon reinforcement magnitude. Response rates, however, differed with reinforcement magnitude. These differences in response rates might explain the failure to observe differences in the potency of fluvoxamine with changes in reinforcement magnitude. In this study, we examined whether the effects of fluvoxamine and desipramine depended on the reinforcement magnitude and response rate, by administering these drugs to pigeons responding under a multiple fixed-interval schedule, in which responding in three components was maintained by differing durations of food presentation (2, 4, and 8 s). The effects of fluvoxamine and desipramine depended jointly on control rate, reinforcement magnitude, and dose. Low fluvoxamine doses had rate-dependent effects in all three components, increasing lower rates more than higher rates: as dose increased, these rate-dependent effects became greater in the components maintained by the 2-s or 4-s food presentation; whereas, in the component maintained by the 8-s presentations, they declined. Low desipramine doses had rate-dependent effects only in the component maintained by the 2-s presentations, whereas higher doses had rate-dependent effects in components maintained by 2-s or 4-s presentations. Still higher doses had rate-dependent effects in all the three components. Although the effects of fluvoxamine and desipramine might not depend on reinforcement magnitude when studied under fixed-ratio schedules, reinforcement magnitude can modulate their effects when they are studied over a wider range of control response rates.

Genetic and Other Contributions to Alcohol Intake in Rhesus Macaques (Macaca mulatta)

BACKGROUND

The etiology of alcoholism and alcohol abuse, like many other complex diseases, is heterogeneous and multifactorial. Numerous studies demonstrate a genetic contribution to variation in the expression of alcohol-related disorders in humans. Over the past decade, nonhuman primates have emerged as a valuable model for some aspects of human alcohol abuse because of their phylogenetic proximity to humans. Long-term, longitudinal studies of rhesus macaques (Macaca mulatta) have provided much insight into environmental influences, especially early life experiences, on alcohol consumption and behavior patterns that characterize alcohol intake later in life. It is not known, however, whether there is a genetic component as well to the variation seen in alcohol consumption in rhesus macaques. A significant genetic component to variation in alcohol consumption in rhesus macaques would show for the first time that like humans, for nonhuman primates additive genetic influences are important. Moreover, their use as a model for alcohol-related disorders in humans would have even greater relevance and utility for designing experiments incorporating the expanding molecular genetics field, and allow researchers to investigate the interaction among the known environmental influences and various genotypes.

METHODS

In this study, we investigate factors contributing to variation in alcohol consumption of 156 rhesus macaques collected over 10 years when subjects were adolescent in age, belonging to a single extended pedigree, with each cohort receiving identical early rearing backgrounds and subsequent treatments. To measure alcohol consumption each animal was provided unfettered simultaneous access both to an aspartame-sweetened 8.4% (v/v) alcohol-water solution, the aspartame-sweetened vehicle, and to water for 1 hour each day during the early afternoon between 13:00 and 15:00 in their home cages for a period of 5 to 7 weeks. We use multiple regression to identify factors that significantly affect alcohol consumption among these animals and a maximum likelihood program (ASReml) that, controlling for the significant factors, estimates the genetic contribution to the variance in alcohol consumption.

RESULTS

Multiple regression analysis identified test cohort and rearing environment as contributing to 57 and 2%, respectively, of the total variance in alcohol consumption. Of the remaining 41% of the variance about half (19.8%) was attributable to additive genetic effects using a maximum likelihood program.

CONCLUSION

This study demonstrates that, as in humans, there are additive genetic factors that contribute to variation in alcohol consumption in rhesus macaques, with other nongenetic factors accounting for substantial portions of the variance in alcohol consumption, Our findings show the presence of an additive genetic component and suggest the potential utility of the nonhuman primate as a molecular genetics tool for understanding alcohol abuse and alcoholism.

Direct relation between etonitazene dose and response rate: responding under a single FI per session

Response-contingent injections of etonitazene (ETZ) have been shown to reinforce rats' lever pressing behavior. The objective of the present study was to determine the relation between response rate and ETZ dose when ETZ was administered subcutaneously once per session by the experimenter contingent upon completion of a 10-min fixed-interval (FI) schedule. When injections of the saline vehicle replaced drug injections, response rates dropped to low levels; rates subsequently increased above saline levels when drug injections were reintroduced, demonstrating that ETZ was serving as a reinforcer. A range of ETZ doses (0.01, 0.1, 1, 5.7, and 10 microg/kg) was administered subcutaneously to six rats, resulting in response rates that were directly related to drug dose. These findings are consistent with other studies that have found an increase in reinforcing effects with increases in drug dose. Thus, studies in which drug is administered once per session may be used to measure the reinforcing effects of drugs directly from rate measures, as the response rate in these studies is unaffected by satiation or direct drug effects.