Pentobarbital self-administration in rhesus monkeys: drug concentration and fixed-ratio size interactions

Addiction as a BAD, a Behavioral Allocation Disorder

Addiction is continued drug use despite its harm. As one always has alternatives, addiction can be construed as a decision to allocate behavior to drug use. While decision making is commonly discussed and studied as if it resulted from deliberative, evaluative processes, such processes are actually only rarely involved in behavior allocation. These deliberative processes are too slow, effortful and inefficient to guide behavior other than when necessary. Rather, most actions are guided by faster, more automatic processes, often labeled habits. Habits are mostly adaptive, and result from repeated reinforcement leading to over-learned behavior. Habitual behavior occurs rapidly in response to particular contexts, and the behavior occurring first is that which occurs, i.e., the behavior that is decided upon. Thus, as drug use becomes habitual, drug use is likely to be chosen over other available activities in that particular context. However, while drug use becoming habitual is necessary for addiction to develop, it is not sufficient. Typically, constraints limit even habitual drug use to safer levels. These constraints might include limiting occasions for use; and, almost always, constraints on amount consumed. However, in a minority of individuals, drug use is not sufficiently constrained and addiction develops. This review discusses the nature of these constraints, and how they might fail. These failures do not result from abnormal learning processes, but rather unfortunate interactions between a person and their environment over time. These accumulate in the maladaptive allocation of behavior to drug use. This Behavior Allocation Disorder (BAD) can be reversed; occasionally easily when the environment significantly changes, but more often by the arduous application of deliberative processes generally absent from decision making. These deliberative processes must continue until new more adaptive habits become the most probable behavior in the contexts encountered. As alternatives to drug use become the most probable behavior, relapse risk diminishes.

The behavioral economics of drug dependence: towards the consilience of economics and behavioral neuroscience

In this chapter, we review the research in this growing field by first discussing the concepts related to price and consumption (demand), its applications to the study of drug consumption and drug seeking, and the impact of other commodities on drug consumption. We then review the discounting of future commodities and events among the addicted, review the most recent research examining the neural correlates of discounting, and describe and review the new theory of addiction that results from that research. We conclude by addressing the next research steps that these advances engender.

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

RATIONALE

A fundamental problem in the study of drugs as reinforcers is the evaluation of a drug's relative reinforcing effects and changes in such effects. Relative reinforcing effects can be measured by determining the preference for one drug dose relative to another drug dose. However, in IV drug self-administration studies technical limitations make direct comparisons between drug doses difficult. An alternative procedure is to measure the relative persistence of behavior across increases in schedule size.

OBJECTIVE

To develop a more rapid method to measure the relative persistence of behavior. Instead of increasing the schedule size across sessions, schedule size was increased within sessions by use of a progressive-ratio schedule (PR).

METHODS

Male rhesus monkeys orally self-administered ethanol during daily 3-h sessions. At each concentration responding was measured with fixed-ratio (FR) 8 schedules to obtain baseline values. Subsequently behavior was studied with a PR schedule. Relative persistence of behavior was calculated by dividing the mean response rate under the PR schedule by the mean response rate under the FR8 schedules. To compare these findings with results of choice between concentrations, monkeys were given concurrent access to pairs of ethanol concentrations.

RESULTS

The relative persistence of behavior increased with increases in drug concentration. When two concentrations were concurrently available, the higher concentration maintained higher response rates.

CONCLUSIONS

The relative persistence of behavior can be efficiently measured by dividing the response rate under the PR schedule by the response rate under the FR schedule. Measures of relative persistence corresponded well with measures of choice and show that relative reinforcing effects increase as dose increases.

Principles of drug abuse liability assessment in laboratory animals

This paper describes the rationale for use of preclinical assessments of abuse liability in laboratory animals, and then discusses "cross-cutting" methodological issues that apply to behavioral evaluations intended to contribute to an abuse liability evaluation package. Issues include use of: (1) positive and negative control conditions; (2) full dose-effect evaluations, (3) multiple dependent measures, (4) pharmacokinetic evaluations to guide choice of dose ranges, (5) a species for which good methodological and comparative data are available to aid interpretation of results, and (6) appropriate methods for the group or single-subject experimental design selected. The remainder of the paper describes basic methodology by which three core pieces of behavioral data required by the Food and Drug Administration for its use in the overall abuse liability analysis can be obtained preclinically. Reinforcing effects are assessed in study of drug self-administration; drug discrimination assesses degree of overlap of interoceptive stimulus effects with relevant comparison drugs; physical dependence potential is determined by assessing whether a withdrawal syndrome occurs after chronic drug administration. Background and methodological issues specific to each procedure are discussed. A key consideration for cross-cutting and specific methodological issues is that choices made enable confident interpretation of both positive and negative results.

Relative reinforcing effects of different oral ethanol doses in rhesus monkeys

The relative reinforcing effects of different doses of orally delivered ethanol were evaluated. Mouth-contact responding by rhesus monkeys was measured under concurrent fixed-ratio fixed-ratio schedules of liquid delivery (0.67 ml/delivery) from each of two spouts during daily 3-hr sessions. Experiment 1 examined persistence of responding with ethanol (2%, 8%, and 32% wt/vol) and water available. When fixed-ratio values from 8 to 128 were tested, the number of ethanol deliveries obtained per session decreased as the response requirement increased. The decrease in deliveries was less at higher than at lower ethanol concentrations, however. Experiment 2 examined choice between two ethanol concentrations under concurrent fixed-ratio 16 schedules (4% vs. 8%, 4% vs. 16%, 8% vs. 16%, 2% vs. 8%, 2% vs. 32%, 8% vs. 32%). Higher concentrations (16%, 32%) generally maintained more responding than concurrently available concentrations of 8% or less. An exception was the observation of a preference for 8% over 32% ethanol. When the fixed-ratio value was increased, however, the relative preference for these two doses was reversed so that 32% ethanol maintained more responding than 8% ethanol. Thus, the direction of the preference depended on the size of the response requirement. These results indicate that the reinforcing effects of ethanol increase with dose.

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.

Effects of reinforcement magnitude and ratio values on behaviour maintained by a cyclic ratio schedule of reinforcement

In Experiments 1 and 2, lever pressing by rats was reinforced on a cyclic ratio schedule of food reinforcement, comprising a repeated sequence of fixed-ratio component schedules. Reinforcement magnitude was varied, on occasional sessions in Experiment 1 and across blocks of sessions in Experiment 2, from one to two or three 45-mg food pellets. In the one-pellet condition, post-reinforcement pauses increased with component schedule value. At higher magnitudes, post-reinforcement pauses increased, and overall response rates declined. Response rate on component schedules was a decreasing linear function of the obtained rate of reinforcement in all conditions. Plotted against component schedule value, response rate increased exponentially to an asymptote that decreased when reinforcement magnitude increased. These findings are consistent with regulatory accounts of food-reinforced behaviour. In Experiment 3, rats were trained under a cyclic ratio schedule comprising fixed-ratio components including higher values, and some inverted U-shaped response functions were obtained. Those rats that did not show this relationship were trained on cyclic ratios with even higher values, and all showed inverted U-shaped response functions. This suggests that behaviour on cyclic ratio schedules can reflect activating of reinforcement as well as the satiating effects seen in Experiments 1 and 2.

Reinforcing effects of a combination of ethanol and methadone relative to each drug alone

Studies report a high incidence of alcohol abuse in methadone maintenance patients. There is, however, little data on the reinforcing effects of combinations of ethanol and methadone. In the present study, oral self-administration of a combination of 1% (w/v) ethanol and 0.2 mg/ml methadone was compared to each drug alone in three rhesus monkeys in which methadone alone was not a reinforcer. In Experiment 1, ethanol and the combination, but not methadone alone, served as reinforcers. In Experiment 2, there was no preference for ethanol or the combination at fixed ratio (FR) 8 or 16. When the FR size was doubled (FR 16 or 32), all three animals preferred the combination to 1% ethanol. Experiment 3 further examined the effect of work requirement on preference for ethanol or the combination by varying FR values [1, 2, 4, 8, 16, or 32]. At lower FRs, ethanol was significantly preferred to the combination. As FR was increased, there was a significant reduction in preference for ethanol over the combination. The results show that an ethanol + methadone combination will be orally self-administered by monkeys and suggest that work requirement differentially modifies preference for the combination and ethanol alone.

Ratio size and cocaine concentration effects on oral cocaine-reinforced behavior

Monkeys were given a choice between cocaine solutions and water under concurrent fixed-ratio reinforcement schedules. The operant response was spout contact. Six rhesus monkeys served as subjects. The cocaine concentration was varied from 0.0125 to 0.8 mg/ml, and the fixed-ratio value was varied from 8 to 128. Cocaine maintained higher response rates than did water over a wide range of conditions. Response rate and number of cocaine deliveries per session were inverted U-shaped functions of concentration. These functions were shifted to the right as the fixed ratio was increased. The number of cocaine deliveries was more persistent as fixed-ratio value was increased when the unit dose was larger rather than smaller. Cocaine consumption was analyzed as a function of unit price (fixed-ratio value divided by cocaine concentration), and unit price accounted for between 77% and 92% of the variance in cocaine consumption for individual monkeys. The current data support the claim that a drug's reinforcing effects increase directly with dose and underscore the need to gather parametric data when examining the effects of experimental manipulations on a drug-reinforced baseline.

Matching under nonindependent variable-ratio schedules of drug reinforcement

Response-contingent deliveries of oral pentobarbital maintained responding of 3 rhesus monkeys during daily 3-hr sessions. Deliveries of pentobarbital were arranged under nonindependent concurrent variable-ratio variable-ratio schedules. Responses to either schedule counted toward completion of both variable-ratio schedule requirements. This schedule is similar in some respects to conventional concurrent variable-interval variable-interval schedules, in which passage of time counts toward completion of the interval value on both schedules. Restricted nonindependent concurrent variable-ratio variable-ratio schedules were also studied. On that schedule, when a drug delivery was assigned to one spout, it had to be collected before responses on the opposite spout again counted toward completion of the schedule requirements. Relative reinforcer magnitude was varied by changing the drug concentration on one schedule while keeping the drug concentration constant on the other variable-ratio schedule. Under both types of concurrent variable-ratio schedules, the relative rate of responding corresponded to the relative drug intake. Unlike earlier studies of concurrent variable-interval variable-interval intravenous cocaine reinforcement, preference was proportionate to concentration, and exclusive preferences did not develop. The relationship between relative rate of responding and relative drug intake was well described by the generalized matching law.

Orally delivered methadone as a reinforcer in rhesus monkeys

Methadone usually is taken orally for drug abuse treatment in humans but oral methadone self-administration by laboratory animals has not been investigated extensively. The present study examines acquisition and maintenance of oral methadone maintained responding in four adult male rhesus monkeys. Drug solution was available from one liquid delivery system and water from a second system during daily 3-h sessions. Locations of liquids were reversed each session, and liquid (0.65 ml per delivery) was delivered according to a fixed-ratio reinforcement schedule. Initially a test for the reinforcing effects of 0.00625-0.4 mg/ml methadone solutions was carried out but a consistent preference for drug over water was not seen. To establish methadone as a reinforcer, a fading procedure was used in which responding was first maintained by solutions of methadone (0.00625-0.4 mg/ml) combined with ethanol (0.0325-2.0% w/v). Subsequently, the concentration of the ethanol in the combination was gradually reduced to zero. Methadone-maintained responding (0.4 mg/ml) persisted when ethanol was no longer present. To confirm that the drug was serving as a reinforcer, the dose was varied: (a) by changing the volume delivered while the concentration was held constant and (b) by changing the concentration of the methadone while the volume per delivery was held constant. Over a wide range of doses, deliveries of methadone solution usually exceeded deliveries of concurrently available water. Orderly relationships were observed among methadone dose, response rate, and drug intake. The study of oral self-administration of opioid drugs by nonhuman primates may be a useful strategy for the development and evaluation of new drug substitution or replacement therapies.

Relative reinforcing effects of different doses of orally delivered cocaine

The relative reinforcing effects of different doses of oral cocaine were investigated in two adult male rhesus monkeys. In the first experiment, a range of cocaine doses (0.1-0.8 mg/ml) was studied with drug and water concurrently available for 3 h each day under identical and independent fixed-ratio schedules. The side positions of the drug and vehicle were alternated from session to session. Drug deliveries always exceeded vehicle deliveries, i.e., orally delivered cocaine functioned as a reinforcer. The highest rates of responding occurred at either the lowest or next to lowest dose (0.1 or 0.2 mg/ml). In the second experiment, pairs of different cocaine doses were systematically presented under identical and independent fixed-ratio schedules. The higher of two concurrently available doses usually maintained the higher response rate. These findings suggest that the relative reinforcing effects of orally delivered cocaine increase with dose. Absolute response rates obtained with single cocaine doses and water concurrently available do not always reflect the magnitude of the reinforcing effects indicated when pairs of cocaine doses are studied together. The results of this study are in agreement with earlier investigations in which the relative reinforcing effects of pairs of intravenous cocaine doses or oral pentobarbital doses were studied. Taken together these findings indicate that, over a range of doses and across pharmacological classes and routes of administration, relative reinforcing effects of a drug increase directly as a function of increases in dose.

The effects of delay of reinforcement and dose on the self-administration of cocaine and procaine in rhesus monkeys

Delays of reinforcement were imposed while three rhesus monkeys intravenously self-administered either cocaine or procaine to determine whether the magnitude of delay-induced reductions in infusion frequency were dose- or drug-dependent. Drug-maintained baselines demonstrating the greatest resistance to change under delay conditions might be conceptualized as involving the greatest strength of responding and include drug reinforcers with the greatest efficacy. Under baseline conditions, cocaine or procaine was infused contingent upon lever pressing according to fixed ratio 10 schedules during daily, 1-h experimental sessions. Under delay conditions, a 120-s delay of reinforcement was inserted between the completion of each fixed ratio contingency and infusion delivery. Tests under baseline and delay conditions alternated until several doses of cocaine (3, 10, 33, 100, and 300 micrograms/kg), and subsequently, of procaine (33, 100, 300, 1000, and 3000 micrograms/kg) were tested. The results indicated that during both baseline and delay conditions increases in dose of either drug resulted in increases followed by decreases in the number of infusions obtained. Introducing delays of reinforcement resulted in decreases in the number of infusions obtained at a given dose. Generally, however, when reinforcement delays were imposed the decreases in infusion number, as a percentage of baseline number, were less at the higher doses of both drugs. The doses of procaine which were maximally effective as reinforcers (i.e. during which infusion number was minimally reduced by delay) were as effective as the maximally effective doses of cocaine.(ABSTRACT TRUNCATED AT 250 WORDS)

Persistence of ethanol self-administration as a function of interreinforcer interval and concentration

Dipper cups filled with an ethanol solution were presented to Long-Evans hooded rats according to multiple extinction x s fixed-ratio 1 (mult EXT x s FR1) schedules of reinforcement. The scheduled duration of the EXT component was varied to manipulate minimum interreinforcer interval. Increasing the minimum interreinforcer interval by increasing the EXT component was used to challenge responding maintained by ethanol for purposes of evaluating the persistence of ethanol-maintained responding. EXT durations of 0 s (baseline conditions of continuous reinforcement) to 480 s were examined across ethanol concentrations of 0 (water, vehicle), 2, 4, 8, 16, and 32% (w/v). Increasing the EXT component duration resulted in reductions in the number of dipper presentations obtained at each concentration. Reductions in dipper presentations were less, relative to 0-s baseline conditions, the higher the concentration of the ethanol solution available. It was concluded that increasing the ethanol concentration that is self-administered increases the strength of responding that is subsequently maintained in that drug-maintained behavior becomes more resistant to modulation by a procedure (scheduling minimum interreinforcer intervals) which generally reduces numbers of drug deliveries.

Unit price as a useful metric in analyzing effects of reinforcer magnitude

In this paper, we applied the behavioral-economic concept of unit price to the study of reinforcer magnitude in an attempt to provide a consistent account of the effects of reinforcer magnitude on behavior. Recent research in the experimental analysis of behavior and in behavioral pharmacology suggests that reinforcer magnitude interacts with the schedule of reinforcement to determine response rate and total consumption. The utility of the unit-price concept thus stems from its ability to quantify this interaction as a cost-benefit ratio (i.e., unit price = characteristics of the schedule of reinforcement divided by magnitude of reinforcement). Research employing the unit-price concept has shown that as unit price increases, a positively decelerating function exists for consumption (i.e., a function with an increasingly negative slope, when plotted on log coordinates) and a bitonic function exists for response rate. Based on these findings, the present analysis applied the unit-price concept to those studies of reinforcer magnitude and drug self-administration that examined the effects of reinforcer magnitude on response rate using simple schedules of reinforcement (e.g., fixed-ratio schedule). This resulted in three findings: (a) Reinforcer-magnitude manipulations and schedule manipulations interact in a manner that can be quantified in terms of unit price as benefit and cost factors, respectively; (b) different reinforcer-magnitude manipulations are functionally interchangeable as benefit factors in the unit-price ratio; and (c) these conclusions appear warranted despite the differences in reinforcers (food or drug), species (dogs, monkeys, or rats), and schedules (interval or ratio), and despite the fact that these studies were not designed for a unit-price analysis. In methodological terms, these results provide further evidence that employing the unit-price concept is a parsimonious method for examining the effects of reinforcer magnitude. In theoretical terms, these results suggest that a single process may underlie the effect of combined reinforcer-magnitude and schedule manipulations.

Behavioral economics: a novel experimental approach to the study of drug dependence

Drug abuse and dependence are among the most important problems facing society today. Understanding the determinants of drug abuse has been advanced by a considerable quantity of research on environmental and pharmacological factors that control drug taking in a variety of settings and species. Behavioral economics, which is the application of economic principles to the behavior of the individual, may have the potential to integrate a number of these empirical observations in a novel quantitative framework. In this paper the utility of behavioral economics for the study of drug dependence is reviewed. Specifically, we reviewed (i) the parsimony behavioral economics affords via the integration of variables, (ii) a ubiquitous behavioral process it has identified, (iii) the precise quantification of that behavioral process and its predictive utility, (iv) a novel independent variable suggested by behavioral economics, and (v) the utility of behavioral economic notions for the process of medication development. We conclude that behavioral economics provides a novel conceptual framework that has utility for the study of drug dependence.

Concurrent pentobarbital- and saccharin-maintained responding: effects of saccharin concentration and schedule conditions

Responses of rhesus monkeys were reinforced by delivery of either a pentobarbital (4.0 mg/ml) solution or a vehicle (water) or saccharin solution under a concurrent signaled differential reinforcement of low rates 30-s schedule. After 30 s of no responding, the first response on the pentobarbital or saccharin spout resulted in the delivery of the appropriate solution and reset the timing on both spouts (i.e. a mutually exclusive choice). In the first experiment, the concentration of saccharin was gradually increased across sessions. As saccharin concentration increased, pentobarbital deliveries decreased and saccharin as well as total session deliveries increased. In a second experiment, pentobarbital and 0.24 (mg/ml) saccharin were made available under concurrent signaled differential reinforcement of low rates 30-s schedules which operated independently. Under these conditions responding on one spout had no consequences with respect to the other spout. The reduction of pentobarbital deliveries was substantially attenuated when the choice was not mutually exclusive.

Oral self-administration of pentobarbital by rhesus monkeys: relative reinforcing effects under concurrent signalled differential-reinforcement-of-low-rates schedules

During daily 3-h sessions two separate pentobarbital solutions were concurrently available to rhesus monkeys under signalled differential-reinforcement-of-low-rates (signalled DRL) schedules of mouth contacts with spouts. The schedules were synchronized so that each time the 30-s DRL interval expired, lights above both spouts were illuminated and a liquid delivery could be obtained either from the left or right spout, but not both. First water and then each of four 'comparison-concentration' pentobarbital solutions (0.0625, 0.25, 1 and 4 mg/ml) were successively available under one schedule for a block of sessions. Concurrently, deliveries of a 'standard concentration' solution were available from the second spout under an identical DRL schedule; the concentration of this standard solution remained constant throughout the testing of the series of comparison solutions. Three pentobarbital concentrations (4, 1 and 0.25 mg/ml) in turn served as the standard concentration. Relative reinforcing effects were directly related to pentobarbital concentration: in general, within pairs of concurrently available pentobarbital solutions more behavior was maintained by the higher of the two drug concentrations. These findings are discussed in the context of previous studies using ratio and interval schedules which have found relative reinforcing effects to be directly related to reinforcer magnitude.

Etonitazene delivered orally serves as a reinforcer for Lewis but not Fischer 344 rats

Oral etonitazene self-administration was systematically investigated in two inbred strains of rats, Lewis (LEW) and Fischer 344 (F344). For LEW rats, etonitazene maintained higher rates of lever pressing and was consumed in larger volumes than the water vehicle when the reinforcement schedule was fixed ratio (FR) 8. In contrast, with F344 rats responding did not systematically exceed water values at any etonitazene concentration. LEW rats also drank substantially more etonitazene than F344 rats, and at FR 8 only LEW rats showed the typical inverted U-shaped function between etonitazene concentration and number of responses. For the LEW strain, response rate increased as FR size increased from FR 1 to FR 2 and FR 4, but decreased at FR 8. For the F344 strain, as FR size increased response rate showed small increases, but the response rates were far lower than those of the LEW strain. The results support the conclusion that etonitazene was an effective reinforcer for LEW but not F344 rats. These findings demonstrate genetic differences in opioid reinforcement of operant behavior and indicate that genotype can be an important determinant of whether etonitazene serves as a reinforcer.

An integrative model for the study of behavioral momentum

Behavioral momentum is the product of response rate and resistance to change. The data on relative resistance to change are summarized for pigeons responding on single-key two-component multiple schedules, in the initial links of two-key multiple chained schedules, and in equivalent components of two-key serial schedules. For single-key procedures, the ratio of resistance to change in two schedule components is shown to depend on the ratio of reinforcer rates obtained in the presence of the component stimuli. For two-key procedures, the ratio of resistance to change in equivalent components is shown to depend on the ratio of reinforcer rates correlated with key locations. A model based on stimulus-reinforcer contingencies that combines the reinforcer rates in schedule components summed over key locations and reinforcer rates correlated with key locations summed over components, each expressed relative to the session average reinforcer rate, gives a good account of the data. An extension of the relative law of effect for multiple schedules fails to provide a complete account of resistance to change, but both approaches are needed for a comprehensive understanding of behavioral momentum.

Effects of increasing response requirement on choice between cocaine and food in rhesus monkeys

Rhesus monkeys were trained in a discrete-trials choice procedure and allowed to choose between intravenous injections of cocaine (0.01-1.0 mg/kg/injection) and food presentation (1 or 4 pellets; 1 g/pellet) during daily 7-h experimental sessions. When each reinforcer was available under a fixed-ratio (FR) 30 schedule, the frequency of cocaine choice and the total drug intake increased in a dose-related manner for all monkeys. When the FR for cocaine was differentially increased, the frequency of cocaine choice decreased, shifting the cocaine dose-response function to the right and/or downward. When the FR for cocaine was at least 480, cocaine preference could not be recovered up to doses of 1.0 mg/kg/injection. In a second experiment, when the response requirement for food was differentially increased, the frequency of cocaine choice increased. These results demonstrate that altering the response requirement for cocaine or for alternative reinforcers that are available can substantially affect cocaine self-administration.

Analysis of fixed-ratio behavior maintained by drug reinforcers

Behavior maintained by intravenously delivered alfentanil, cocaine, or ketamine was assessed using a fixed-ratio schedule of reinforcement. As the dose of each drug was increased, rate of responding also increased up to a maximum. Further increases in dose resulted in decreased response rates (inverted U-shaped curve). An analysis of postreinforcement-pause-time and run-time measures for the ascending limb of the inverted U-shaped functions revealed that behavior was characterized by systematic decreases in both pause time and run time as dose and rate increased. An examination of the descending limb of the dose-response functions revealed that lowered response rates for cocaine and ketamine were correlated with increases in run time and small and inconsistent effects on postreinforcement pause time. Behavior maintained by rate-reducing doses of alfentanil was characterized by lengthened postreinforcement pauses with small increases in run time. These data suggest that at larger doses, drug reinforcers may have unconditioned or direct effects on the behavior that the drug is maintaining, and more important, that the nature of these unconditioned effects depends on the drug that is maintaining behavior.

Modifying drug-reinforced behavior by altering the economic conditions of the drug and a nondrug reinforcer

Six rhesus monkeys were trained to self-administer orally delivered phencyclidine (0.25 mg/mL) and saccharin (0.03% wt/vol) under concurrent fixed-ratio 16 schedules. In Condition 1 the fixed-ratio requirement for phencyclidine was changed from 16 to 4, 8, 16, 32, 64, 128 and 16 while the fixed-ratio requirement for saccharin deliveries remained constant at 16. In Condition 2 the fixed-ratio value for saccharin was systematically altered while the fixed-ratio requirement for phencyclidine remained at 16, and in Condition 3 the fixed-ratio requirements for both phencyclidine and saccharin were altered simultaneously. Water was then substituted for saccharin, and the series of fixed-ratio manipulations was replicated. The phencyclidine concentration was reduced to 0.125 mg/mL and Conditions 1 and 3 were repeated. When the fixed-ratio requirement for phencyclidine was increased and the fixed-ratio requirement for saccharin or water remained fixed at 16, phencyclidine deliveries decreased when saccharin (vs. water) was concurrently available. The magnitude of the decrease ranged from 20% to 90% (of the concurrent water condition) as the fixed-ratio requirement for phencyclidine increased from 4 to 128. When the fixed-ratio requirement for phencyclidine remained at 16 and the fixed-ratio requirements for concurrent saccharin or water varied between 4 and 128, phencyclidine deliveries decreased by 30% to 40% due to the concurrent availability of saccharin (vs. water). This decrease occurred only at the three lowest fixed-ratio values when saccharin intake was relatively high. When the fixed-ratio requirements for both phencyclidine and concurrent saccharin or water were varied simultaneously, phencyclidine deliveries were reduced from 20% to 45% when saccharin (vs. water) was concurrently present. There was little effect of reducing the phencyclidine concentration when the data were analyzed in terms of unit price (responses per milligram). Thus, changes in the fixed-ratio requirement or drug concentration were functionally similar, and unit price of phencyclidine was the variable that was influenced by the presence of concurrent saccharin. These data indicate that drug-reinforced behavior is substantially reduced when the environment is enriched with an alternative nondrug reinforcer. The economic context in which these substances are presented is an important determinant of drug-reinforced behavior.

Reinforcing effects of a pentobarbital-ethanol combination relative to each drug alone

The reinforcing effects of an orally delivered combination of 1 mg/ml pentobarbital plus 1% ethanol were evaluated in four rhesus monkeys. The drug combination and another liquid (either water, 1 mg/ml pentobarbital, or 1% ethanol) were concurrently available under identical fixed-ratio (FR) schedules. Substantially higher response rates were maintained by the drug combination than by any of the three other liquids. Thus, the reinforcing effects of the pentobarbital-ethanol combination were greater than those of either component drug. In a second experiment, water (the vehicle) was concurrently available with one other liquid (1 mg/ml pentobarbital, 1% ethanol, or the pentobarbital-ethanol combination). All three drug solutions functioned as reinforcers since they maintained much higher response rates than water. These results demonstrate that the greater relative reinforcing effects of the drug combination in the first experiment were not due to a lack of reinforcing effects of the 1 mg/ml pentobarbital or 1% ethanol solutions. In a final experiment, the drug combination was scheduled concurrently with 1 mg/ml pentobarbital, and FR size was systematically varied. The drug combination was then scheduled concurrently with 1% ethanol, and FR size was again varied. As FR size increased, the relative amount of responding maintained by the drug combination increased. Thus, differences in relative reinforcing effects that were evident in the first experiment were again evident in the final experiment when appropriate schedule-parameter values were used.

Orally delivered cocaine as a reinforcer for rhesus monkeys

Orally delivered cocaine was established as a reinforcer for six rhesus monkeys. Cocaine and its vehicle, water, were available from separate spouts under independent concurrent fixed-ratio schedules. The positions of cocaine and water were reversed between spouts from session to session. Cocaine consistently maintained higher response rates than water. Cocaine concentration was systematically varied for three of the six monkeys tested, and cocaine intake (mg of drug/kg of body wt.) increased with increases in drug concentration.

Behavioral economics of drug self-administration. I. Functional equivalence of response requirement and drug dose

Response requirement and dose of drug per administration are two separate factors that have been demonstrated to control drug self-administration. Recent developments in behavioral economics have shown that these two factors are in fact functionally equivalent for nondrug reinforcers, as indicated by a unit-price analysis. In this review, the unit-price notion was tested for drugs as reinforcers via a re-analysis of ten drug self-administration studies. The results of the re-analysis indicated that response requirement and reinforcer magnitude, the constituents of unit price, have functionally equivalent effects on drug consumption and that a positively decelerating demand curve is produced as unit price increases. This suggests that the behavioral-economic notion of unit price is a more parsimonious explanation of the effects of response requirement and dose in drug self-administration studies, in that it integrates and describes what was previously considered to be two distinct operations.

Oral self-administration of pentobarbital by rhesus monkeys: maintenance of behavior by different concurrently available volumes of drug solution

For 4 rhesus monkeys, mouth-contact responses with either of two brass spouts were reinforced according to fixed-ratio schedules by 0.65-mL liquid deliveries during daily 3-hr sessions. Three experiments were conducted. In each experiment, independent fixed-ratio schedules were concurrently in effect at the two spouts. Following completion of each fixed ratio on a spout, a specified number of liquid deliveries were available from that spout under a continuous-reinforcement schedule. The number of such deliveries available at each spout was manipulated independently. In Experiment 1, a 1-mg/mL pentobarbital solution was simultaneously available with water (the drug vehicle) under concurrent fixed-ratio schedules of 32 responses for 3 subjects and 64 responses for the remaining subject. The number (N) of liquid deliveries that were available after completion of each fixed ratio was varied in the following order: 8, 4, 2, 1, and 8 (retest). For each subject at each condition, drug maintained more responding than water. The number of drug deliveries obtained per session was directly related to the amount of drug available per fixed ratio (i.e., to N), whereas the number of fixed ratios completed per session generally was inversely related to the value of N. In Experiment 2, fixed-ratio size was the same for each subject as in Experiment 1, but deliveries of a 1-mg/mL pentobarbital solution were available at both spouts. The number of drug deliveries available under one fixed-ratio schedule (Ns, the "standard" reinforcer amount) was held at eight, and the number of drug deliveries available under the second schedule (Nc, the "comparison" reinforcer amount) was changed across blocks of six sessions of stable responding in the following order: 1, 2, 4, 8, 4, 2, and 1. The identical series of comparison reinforcer amounts (Nc) was then tested twice more, but with the standard reinforcer (Ns) held first at four and then at two deliveries. Across the three choice series, reinforcing effects were directly related to reinforcer magnitude. In Experiment 3, deliveries of a 1-mg/mL pentobarbital solution again were available at both spouts. However, the two reinforcer amounts were held constant at N = 8 deliveries under one schedule and N = 4 deliveries under the second schedule, and fixed-ratio size was systematically varied. Across the range of fixed-ratio sizes from low to high, the degree to which behavior was better maintained by the larger of the two drug quantities was an inverted U-shaped function of fixed-ratio size.(ABSTRACT TRUNCATED AT 400 WORDS)

Effects of fixed ratio size and dose on phencyclidine self-administration by rats

Female Sprague-Dawley rats were trained to self-administer phencyclidine (PCP; 0.125, 0.25, or 0.5 mg/kg/injection) on a fixed ratio (FR) schedule of reinforcement under limited access conditions (3 h). Initial training began with cocaine, which was later replaced with ketamine and then one of the three unit doses of PCP. Baseline rates of injection were determined at FR 10. The size of the ratio was then incremented geometrically every fifth daily session. Increasing the ratio resulted in a decrease in the number of injections per session. Furthermore, this decrease was greater for the 0.25 mg/kg dose than for the 0.5 mg/kg unit dose. The self-administration of the 0.125 mg/kg dose was variable and rapidly extinguished upon the increase in fixed ratio. The results indicate that PCP is self-administered by rats under the conditions imposed in this study. Furthermore, the relative reinforcing efficacy of the different unit doses of PCP could be discriminated using this type of response cost procedure.

Oral pentobarbital intake in rhesus monkeys: effects of drug concentration under conditions of food deprivation and satiation

Pentobarbital-reinforced behavior was studied in four rhesus monkeys. A pentobarbital solution and water were concurrently available during 3-hr sessions; water was freely available between sessions. Both pentobarbital concentration and feeding conditions (deprivation versus satiation) were varied. In two food-restricted monkeys subsequent food satiation eliminated pentobarbital-maintained responding. In two other food-restricted monkeys the effects of food satiation varied with the drug concentration. At the highest concentration, 4 mg/ml, food satiation did not alter responding, whereas at 2 mg/ml a moderate decrease occurred and at 1 mg/ml responding was greatly reduced. During the food satiation phase, when the concentration was 4 mg/ml, responding was well maintained under several fixed-ratio sizes. Large quantities of pentobarbital were consumed, and intoxication was observed. Water-maintained responding occurred at low rates and did not vary across feeding conditions or drug concentration. The results support an interpretation in terms of a behavioral mechanism of action. Specifically, the effects of food deprivation on drug self-administration are to increase the magnitude of the reinforcing effects of the drug.

Oral self-administration of pentobarbital by rhesus monkeys: relative reinforcing effects under concurrent fixed-ratio schedules

During daily 3-hr sessions, orally delivered pentobarbital solutions and water, or two separate pentobarbital solutions, were concurrently available to rhesus monkeys according to fixed-ratio schedules of mouth contacts with a spout. First water, and then each of four "comparison-concentration" pentobarbital solutions (0.0625, 0.25, 1, and 4 mg/mL), was successively available from one spout for a block of sessions under a fixed-ratio-64 (three monkeys) or fixed-ratio-16 (one monkey) schedule. Under an identically sized fixed-ratio schedule, deliveries of a "standard-concentration" pentobarbital solution were concurrently available from a second spout. The concentration of the standard solution remained unchanged throughout testing of the series of comparison solutions. Each of three pentobarbital concentrations (4, 1, and 0.25 mg/mL) in turn served as the standard concentration. Within each pair of concurrently available solutions, the higher drug concentration maintained more behavior than the lower concentration. Thus when monkeys were provided with concurrent access to different pentobarbital concentrations, relative reinforcing effects were directly related to drug concentration. Further, the amount of behavior maintained by a particular drug concentration was dependent on the concentration of the concurrently available drug solution. Thus, the relative effectiveness of a reinforcer in maintaining behavior is a function of both the reinforcer's magnitude and the availability of alternative reinforcers in the environment.

Fixed-ratio schedules of oral ethanol self-administration in inbred mouse strains

Previous studies of ethanol reinforcement in BALB/cJ and C57BL/6J mice have shown that over a range of concentrations oral ethanol appeared to serve as a reinforcer only for the C57BL/6J mice. In the previous studies BALB/cJ mice maintained rates of responding for ethanol that only slightly exceeded the rates maintained by the vehicle, water. However, the quantity of ethanol consumed with the continuous reinforcement schedule (fixed ratio one) may have led to pharmacologically significant effects, given the high sensitivity to ethanol of this genotype. The present study tested whether and to what extent ethanol would maintain responding under increasing fixed ratio size in these two strains of mice at ethanol concentrations of 0%, 8%, and 16% (w/v). For the C57BL/6J mice, as fixed-ratio size increased from 1 to 2, 4, and 8, there were almost directly proportional increases in response rate at ethanol concentrations of 8% and 16% (w/v), but not at 0%. Post-session blood ethanol levels confirmed intake of pharmacologically significant quantities. The volume consumed per unit of body weight decreased as fixed-ratio size increased. For the BALB/cJ mice, at no condition did ethanol maintain responding at levels that significantly exceeded vehicle maintained responding. BALB/cJ mice did not differ from C57BL/6J mice as fixed-ratio size was increased during vehicle conditions. These results, along with earlier findings, demonstrate that ethanol can serve as a reinforcer for C57BL/6J mice but not in BALB/cJ mice over a range of schedule conditions. They further support the conclusion that genotype is an important determinant of ethanol reinforced behavior.

Factors controlling drug reinforced behavior

An overview is provided of factors controlling drug reinforced behavior. Drug reinforced behavior is defined, and control procedures for rigorously identifying such behavior are discussed. Factors affecting drug reinforced behavior include the drug itself, animal species, route of administration, current circumstance variables, subject's experimental history, and response consequences. Current circumstance variables concern conditions present during opportunities for drug self-administration and include such factors as stimulus control, food deprivation, drug-access conditions, and brain lesions. Response consequence variables include reinforcement schedule, punishment, drug dose, and competing reinforcers. Drug reinforced behavior is a member of a more general class of behavior, namely operant behavior. Over the last 25 years there has been a rapid increase in understanding drug reinforcement, and this pattern of expanding knowledge suggests that the high rate of progress will continue.

Self-administration of barbiturates and benzodiazepines: a review

Studies of barbiturate and benzodiazepine self-administration are categorized by species and route of administration. Reinforcement, defined as self-administration of drug greater than of a non-drug control, has been demonstrated most often in studies employing the IV route, and there has been greater reliability in this result for a given drug among barbiturates rather than among benzodiazepines. Most studies of PO self-administration in rodents have not demonstrated reinforcement, despite a number of behavioral manipulations to induce drug intake. Studies of PO barbiturate self-administration in monkeys have demonstrated reinforcement but recent studies of PO benzodiazepine self-administration in baboons have not, although physical dependence was demonstrated. Reinforcement via the IG route has not been reliably demonstrated. Behavioral variables, including interreinforcement interval and drug self-administration history, appear to be important determinants of whether or not reinforcement will be demonstrated, particularly among the benzodiazepines; but the range of conditions under which behavioral and pharmacological variables interact to promote or lessen the likelihood of self-administration of these drugs remains to be determined experimentally.

Concurrent access to two concentrations of orally delivered phencyclidine: effects of feeding conditions

Two experiments addressed the effects of food satiation and deprivation on oral self-administration of two concurrently available phencyclidine concentrations. In the first experiment, 8 rhesus monkeys self-administered either of two concentrations of phencyclidine ("PCP, angel dust") and water under concurrent fixed-ratio 16 schedules. One concentration was always held constant (0.25 mg/mL) while a series of other phencyclidine concentrations, ranging from 0 (water) to 1.0 mg/mL, was presented in a nonsystematic order. Initially the monkeys were tested while food satiated, and the procedure was then repeated during food deprivation. The monkeys usually selected the higher concentration within the first few minutes of the session, indicating that taste and/or other immediate postingestional effects were important factors. Contrary to a number of previous reports, there were no consistent differences across subjects in the mean number of liquid deliveries or mean drug intake (mg/kg) during food satiation and deprivation. However, for all monkeys the within-session time course of responding during food satiation consistently differed from that during deprivation. A second experiment assessed whether the failure to find consistent differences in drug intake during food satiation and deprivation had been due to the history of concurrent access to different phencyclidine concentrations or to the extended experience with phencyclidine under food-satiation conditions. Six additional monkeys (Group 2) were exposed to the phencyclidine self-administration procedure (during food satiation and deprivation) for the same length of time as the monkeys in Experiment 1 (Group 1), except they received only concurrent access to phencyclidine (0.25 mg/mL) and water. Both groups then received concurrent access to phencyclidine and water during five repeated cycles of food deprivation and satiation. There were also marked individual differences in Group 2: During food satiation, 2 of the monkeys' responding increased, 1 showed no change, and 3 decreased. Examination of a number of historical variables indicated that the greater the percentage of total sessions spent during food satiation with phencyclidine available (before these experiments began), the greater the amounts of phencyclidine consumed during food satiation and the smaller the differences in phencyclidine intake when the two feeding conditions were compared.

Self-administration of orally-delivered phencyclidine and ethanol under concurrent fixed-ratio schedules in rhesus monkeys

Five monkeys were trained to self-administer orally-delivered phencyclidine (0.25 mg/ml) and water under concurrent fixed-ratio 16 (FR 16) schedules during daily sessions that lasted 3 h. An 8% (w/v) ethanol solution was substituted for water and then the following series of phencyclidine concentrations was presented: 0.25, 0.5, 1, 0.03, 0.06, 0.12, 0 (water), and 0.25 mg/ml (retest). Next, the phencyclidine concentration was held constant (0.25 mg/ml), and the ethanol concentration was varied as follows: 8, 16, 32, 1, 2, 4, 0 (water), and 8% w/v (retest). Each drug concentration series was tested again with water concurrently available. At low phencyclidine concentrations the ethanol solution was preferred, but phencyclidine deliveries exceeded ethanol deliveries at higher phencyclidine concentrations. Ethanol-maintained responding was increased by the lower phencyclidine concentrations (0.03, 0.06 and 0.12 mg/ml), and it was slightly suppressed by the highest concentration (1 mg/ml). Phencyclidine was preferred to all concentrations of ethanol except 4%. Ethanol (8%) suppressed phencyclidine-maintained behavior at lower concentrations, but it had no effect at higher phencyclidine concentrations. The ethanol concentration-response functions were nearly identical whether phencyclidine (0.25 mg/ml) or water was concurrently present. Drug preferences were usually evident within the first 10 min of the session, suggesting they were based on olfaction, taste, or other immediate postingestional effects. These results show that two orally-delivered drugs may be concurrently self-administered under independently-operating FR schedules. Behavior maintained by one drug depends on the magnitude of the concurrently available, alternative reinforcing drug.

Oral drug self-administration in rhesus monkeys: interactions between drug amount and fixed-ratio size

During daily 3-hr sessions, 5 rhesus monkeys drank drug solutions and water that were concurrently available. The drug solutions were: 1 milligram per milliliter (mg/mL) pentobarbital (2 monkeys), 1 mg/mL pentobarbital plus 0.5% ethanol (1 monkey), 1 mg/mL pentobarbital plus 1% ethanol (1 monkey), and 8% ethanol (1 monkey). The drug solution and water were available under identical two-component tandem fixed-ratio continuous-reinforcement N schedules. Two variables were manipulated: the size of the fixed-ratio component and the number of liquid deliveries (N) in the second component. Deliveries of the drug solution maintained higher rates of responding than did deliveries of the drug vehicle, water. The number of drug deliveries per session increased with increases in the number of deliveries per fixed ratio and decreased with increases in fixed-ratio size. Analysis of the results in terms of the proportion of deliveries to responses showed that the number of drug deliveries per session was directly related to the size of this quotient. Finally, when fixed-ratio size was repeatedly doubled, the following orderly relationship emerged: The greater the number of available drug deliveries per fixed ratio, the less was the percent decrease in the number of fixed ratios completed per session. It was concluded that increases in the number of liquid deliveries per fixed ratio resulted in increases in reinforcing efficacy.

Concurrent phencyclidine and saccharin access: presentation of an alternative reinforcer reduces drug intake

Six monkeys self-administered orally delivered phencyclidine ("angel dust") and saccharin under concurrent fixed-ratio 16 schedules during daily three-hour sessions. Liquid deliveries were contingent upon lip-contact responses on solenoid-operated drinking spouts. Three saccharin concentrations (0.003%, 0.03% and 0.3%, wt/vol) were tested in a nonsystematic order. For each saccharin concentration, the following series of phencyclidine concentrations (mg/ml) was presented: 0.25, 0.5, 1, 0.25 (retest), 0.125, 0.0625, 0.0312, 0.25 (retest) and 0 (water with stimuli signaling phencyclidine). As the saccharin concentration increased, the number of drug deliveries decreased, and the peaks of the concentration-response functions were shifted to the right. The lowest saccharin concentration (0.003%, wt/vol) maintained responding in excess of phencyclidine levels in only one monkey. The two higher saccharin concentrations maintained behavior far in excess of phencyclidine, but saccharin deliveries decreased in some monkeys as phencyclidine concentration and intake (mg/kg) increased. The time course and patterns of phencyclidine-reinforced responding were also altered when saccharin was concurrently available. The results are discussed in terms of strategies to reduce drug-reinforced behavior, preference between different reinforcers, and measures of reinforcing efficacy.