Tolerance to and residual effects of cocaine in squirrel monkeys depend on reinforcement-schedule parameter

Effects of ethanol on negative phototaxis and motility in brown planarians (Dugesia tigrina)

The behavioral effects of ethanol in brown planarians were studied in four experiments. In the first two experiments, acute administrations of ethanol increased travel time in a dose-dependent fashion in individual planarians moving away from a light source. Orderly results were obtained using both within-subject and between-group designs. In a third experiment, ethanol dose was arranged by time rather than concentration. Ethanol increased travel time overall, but variability between subjects was considerable. In a final experiment, ethanol administration reduced motility and altered movement patterns in planarians in an open-field test. These experiments demonstrated that negative phototaxis by planarians may exhibit sufficient stability to allow for experimental determinations of dose-response curves utilizing both within-subject and between-subject designs.

Tolerance to cocaine's effects following chronic administration of a dose without detected effects on response rate or pause

To observe tolerance to drug effects on operant behavior, the dose that researchers have often selected for chronic administration is one that disrupts, but does not abolish, responding. Some evidence suggests that tolerance may develop after chronic administration of relatively smaller doses. The purpose of the present experiment was to assess systematically effects of chronic administration of a dose without detected effect on responding. Specifically, response rates and post-reinforcement pauses of five pigeons key pecking under a three-component multiple fixed-ratio schedule of food reinforcement were observed under chronic cocaine administration. We evaluated the effects of a range of doses (1.0 mg/kg to 17.0  mg/kg) during acute administration. The largest dose that failed to alter responding acutely then was administered chronically (1.0  mg/kg for 1 pigeon, 3.0  mg/kg for 3 pigeons, and 5.6  mg/kg for 1 pigeon). After 30 consecutive sessions of chronic administration, smaller and larger doses occasionally were substituted for the chronic dose. Pigeons then received pre-session saline administration for 30 consecutive sessions, and the post-chronic effects of the series of doses on responding were determined. All subjects developed tolerance to doses of cocaine that initially had caused large decreases in rate, with the magnitude of the effects varying across components of the multiple schedule and subjects. Specifically, tolerance generally was greatest in the components with smaller ratios. Following post-chronic saline administration, tolerance was usually diminished. Overall, the results demonstrate that under these conditions, repeated experience with disruptive effects of cocaine on food-maintained responding is not a necessary factor in the development of tolerance.

Tolerance to cocaine's effects on schedule-controlled behavior: role of delay between pause-ending responses and reinforcement

The schedule of reinforcement under which behavior is maintained is an important contributor to whether tolerance to the behavioral effects of cocaine develops. Schedule parameter value (for example, fixed-ratio size) has been shown to affect the development of tolerance under some schedule types but not others, but the specific procedural variables causing this effect remain to be identified. To date, schedule-parameter-related tolerance has developed when a longer pause after reinforcement does not lead to a shorter delay between the response that ends the pause and reinforcement. The current study investigated the importance of this variable in pigeons using a multiple chained Fixed-Ratio 1, Fixed-Time x schedule, in which the first key peck in a trial produced a stimulus change and initiated a delay at the end of which food was presented regardless of whether or not additional pecks were made during the delay. Dose-response curves were assessed before, during and after chronic (daily) administration of cocaine. Tolerance to the pause-increasing effects of cocaine occurred to a similar degree regardless of the scheduled time between the end of the pause and reinforcement. Therefore, the relationship between pause length and delay to reinforcement does not provide an explanation for schedule-parameter-related tolerance.

Influences on cocaine tolerance assessed under a multiple conjunctive schedule of reinforcement

Under multiple schedules of reinforcement, previous research has generally observed tolerance to the rate-decreasing effects of cocaine that has been dependent on schedule-parameter size in the context of fixed-ratio (FR) schedules, but not under the context of fixed-interval (FI) schedules of reinforcement. The current experiment examined the effects of cocaine on key-pecking responses of White Carneau pigeons maintained under a three-component multiple conjunctive FI (10 s, 30 s, & 120 s) FR (5 responses) schedule of food presentation. Dose-effect curves representing the effects of presession cocaine on responding were assessed in the context of (1) acute administration of cocaine (2) chronic administration of cocaine and (3) daily administration of saline. Chronic administration of cocaine generally resulted in tolerance to the response-rate decreasing effects of cocaine, and that tolerance was generally independent of relative FI value, as measured by changes in ED50 values. Daily administration of saline decreased ED50 values to those observed when cocaine was administered acutely. The results show that adding a FR requirement to FI schedules is not sufficient to produce schedule-parameter-specific tolerance. Tolerance to cocaine was generally independent of FI-parameter under the present conjunctive schedules, indicating that a ratio requirement, per se, is not sufficient for tolerance to be dependent on FI parameter.

Cocaine and automaintained responding in pigeons: rate-reducing effects and tolerance thereto with different durations of food delivery

Pigeons were exposed to an automaintenance procedure in which 6-s key illuminations in one color (red or white) were immediately followed by 3-s food deliveries and key illuminations in the other color were followed by 9-s food deliveries. Both conditions engendered consistent responding. With both durations of food delivery, acute and chronic cocaine administrations (1.0-17.8 mg/kg) produced dose-dependent decreases in mean percent trials (key illuminations) with a response and mean total response per session. Tolerance developed to the disruptive effects of cocaine on both response measures. Food duration did not significantly affect either response measure or significantly interact with cocaine dose or drug regimen. The orderliness of the present findings, like those of a related study examining whether probability of food delivery modulated the effects of cocaine on automaintained responding [Porritt, M., Arnold, M., Poling, A., Cocaine and automaintained responding in pigeons: rate-reducing effects and tolerance thereto with different CS-US pairing probabilities. Pharmacol Biochem Behav 2007; 87:405-411.], suggests that the automaintenance procedure is a useful assay for examining tolerance to drug effects on classically-conditioned responding. Unlike the results of that study, however, the present findings are inconsistent with a behavioral momentum analysis of drug effects on such responding.

Tolerance to effects of cocaine on behavior under a response-initiated fixed-interval schedule

Tolerance to effects of cocaine can be modulated by schedules of reinforcement. With multiple ratio schedules, research has shown an inverse relationship between ratio requirement and amount of tolerance that resulted from daily administration of the drug. In contrast, tolerance to the effects of cocaine on behavior under multiple interval schedules generally has developed regardless of interval value. Under interval schedules reinforcement depends on the animal making one response following a time interval. Thus, as time to respond increases, the time to reinforcement decreases. On the other hand, fixed ratio schedules require a specified number of responses to be made prior to reinforcement. Therefore, delaying the initiation of responding does not coincide with a significant decrease in the time to reinforcement. In the current experiment, 6 pigeons were trained to respond under a three-component multiple schedule, with a different tandem fixed-ratio 1 fixed-interval schedule in each component. The multiple schedule required one response, which was followed by one of three fixed-interval values (5, 15, or 60 s). Thus, the multiple schedule was interval-like because after the fixed-ratio 1, only one more response was required for reinforcement, but it was also ratio-like because the length of the pause at the beginning of each interreinforcer interval affected the time until the next reinforcer. Acute administration of cocaine generally resulted in dose-dependent decreases in responding. Chronic (i.e., daily) administration of a rate-decreasing dose resulted in tolerance patterns similar to those usually obtained with multiple ratio schedules. That is, the magnitude of tolerance was related inversely to schedule size. These results suggest that delay to reinforcement from the initial response may play a role in the development of schedule-parameter-related tolerance.

Cocaine and automaintained responding in pigeons: rate-reducing effects and tolerance thereto with different CS-US pairing probabilities

To examine whether CS-US pairing probability, hence rate (or probability) of food delivery, modulated the effects of cocaine, pigeons were exposed to an autoshaping procedure in which 6-s red, green, and white illuminations of a response key were immediately followed by response-independent food delivery with a probability of 1.0, 0.5, and 0.25, respectively. Substantial responding occurred at all probabilities. Pre- and post-chronic administrations of cocaine (1.0-17.8 mg/kg) produced dose-dependent decreases in mean percent trials (key illuminations) with a response and total responses per session at all probabilities. In general, sensitivity to the drug was lowest at the highest probability, suggesting that rate (or probability) of food delivery influenced the behavioral effects of cocaine.

Morphine tolerance as a function of ratio schedule: response requirement or unit price?

Key pecking by 3 pigeons was maintained by a multiple fixed-ratio 10, fixed-ratio 30, fixed-ratio 90 schedule of food presentation. Components differed with respect to amount of reinforcement, such that the unit price was 10 responses per 1-s access to food. Acute administration of morphine, l-methadone, and cocaine dose-dependently decreased overall response rates in each of the components. When a rate decreasing dose of morphine was administered daily, tolerance, as measured by an increase in the dose that reduced response rates to 50% of control (i.e., the ED50 value), developed in each of the components; however, the degree of tolerance was smallest in the fixed-ratio 90 component (i.e., the ED50 value increased the least). When the l-methadone dose-effect curve was redetermined during the chronic morphine phase, the degree of cross-tolerance conferred to l-methadone was similar across components, suggesting that behavioral variables may not influence the degree of cross-tolerance between opioids. During the chronic phase, the cocaine dose-effect curve shifted to the right for 2 pigeons and to the left for 1 pigeon, which is consistent with predictions based on the lack of pharmacological similarity between morphine and cocaine. When the morphine, l-methadone, and cocaine dose-effect curves were redetermined after chronic morphine administration ended, the morphine and l-methadone ED50s replicated those obtained prior to chronic morphine administration. The morphine data suggest that the fixed-ratio value (i.e., the absolute output) determines the degree of tolerance and not the unit price.

Effects of cocaine on performance under fixed-interval schedules with a small tandem ratio requirement

Daily administration of cocaine often results in the development of tolerance to its effects on responding maintained by fixed-ratio schedules. Such effects have been observed to be greater when the ratio value is small, whereas less or no tolerance has been observed at large ratio values. Similar schedule-parameter-dependent tolerance, however, has not been observed with fixed-interval schedules arranging comparable interreinforcement intervals. This experiment examined the possibility that differences in rate and temporal patterning between the two types of schedule are responsible for the differences in observed patterns of tolerance. Five pigeons were trained to key peck on a three-component multiple (tandem fixed-interval fixed-ratio) schedule. The interval values were 10, 30, and 120 s; the tandem ratio was held constant at five responses. Performance appeared more like that observed under fixed-ratio schedules than fixed-interval schedules. Effects of various doses of cocaine given weekly were then determined for each pigeon. A dose that reduced responding was administered prior to each session for 50 days. A reassessment of effects of the range of doses revealed tolerance. The degree of tolerance was similar across components of the multiple schedule. Next, the saline vehicle was administered prior to each session for 50 days to assess the persistence of tolerance. Tolerance diminished in all subjects. Overall, the results suggested that schedule-parameter-dependent tolerance does not depend on the temporal pattern of responding engendered by fixed-ratio schedules.

Interactions among unit-price, fixed-ratio value, and dosing regimen in determining effects of repeated cocaine administration

Previous research has shown tolerance to cocaine that was dependent on fixed-ratio (FR) parameter size in the context of a multiple FR schedule of food reinforcement. Completion of the FR requirement in these studies resulted in the same magnitude of reinforcement, regardless of ratio size. The cost-to-benefits ratio (unit-price) was therefore not equated across the different FR components. The current study examined the effects of repeated administration of cocaine to pigeons when unit-price under FR schedules was either the same or different. Additionally, the role of a chronic variable-dosing versus chronic fixed-dosing procedure was examined when unit-price was equated across different ratio values. Pigeons were trained to key peck in daily sessions under a three-component multiple FR schedule of food presentation, according to which either 10, 30, or 100 pecks were required for each delivery of food. In Experiment 1, completion of the FR 10, FR 30, and FR 100 resulted in 1.5, 4.5, and 15.0 s access to food, respectively. That is, the response requirement was correlated with access to food time so that unit-price (pecks per second of access to food) was equated across components. After assessing acute effects of a range of doses of cocaine, drug administration occurred daily before each session, with the dose varying from day to day. Tolerance, the magnitude of which was unrelated to the peck requirement, developed under the repeated-dosing regimen. In order to assess whether having equal unit-price was responsible for producing similar levels of tolerance across components, daily drug administration continued in Experiment 2 using the variable-dose regimen, but the amount of food presented each time was fixed at 4.5 s access to food, yielding different unit-prices under the three pecking requirements. Subsequently, the conditions of Experiment 1 were reinstated, i.e., unit-price was equated. Making unit-price different or the same had little influence on effects of cocaine. In Experiment 3, a fixed dose of cocaine was administered before each session while programmed unit-price remained the same across components. Under these conditions, tolerance became peck-requirement related. Specifically, tolerance was most prevalent under the smaller requirements and less robust or absent when the largest requirement was in effect. Differences in unit-price, therefore, were not related to degree of tolerance, but work requirement was. Differences in effects of cocaine across responses requirements, however, were observed only when each session was preceded by the same dose, not when dose varied from session to session.

Repeated post- or presession cocaine administration: roles of dose and fixed-ratio schedule

Effects of repeated administration of cocaine to animals behaving under operant contingencies have depended on when the drug is given. Moderate doses given presession have generally led to a decrease in the drug's effect, an outcome usually referred to as tolerance. When these same doses have been given after sessions, the usual result has been no change or an increase in the drug's effects, with the latter usually referred to as sensitization. In the present study, repeated postsession administration of a relatively small dose of cocaine (3.0 or 5.6 mg/kg) to pigeons responding under a multiple fixed-ratio 5, fixed-ratio 100 schedule of food presentation generally resulted in tolerance to the rate-decreasing effects of the drug. When the same dose was given before sessions, little additional tolerance was observed, although some subjects showed further tolerance in the small-ratio component. A regimen of repeated postsession injection of larger (10.0-23.0 mg/kg) doses suppressed key pecking during the session; responding resumed following discontinuation of postsession administrations. Effects of postsession administration of cocaine, therefore, depended on the dose, with smaller doses leading to tolerance and larger ones to suppression of behavior during the session. Effects of postsession drug administration of either small or large doses were not related to whether effects of postsession drug were experienced mainly in the operant test chamber or in the pigeon's home cage. The results with large postsession doses are compatible with a view that the drug acted as a Pavlovian unconditional stimulus, with the session-related stimuli acting as a long-duration Pavlovian conditional stimulus. Tolerance following postsession administration of the smaller doses challenges the view that it depended on experiencing the drug's effects while the arranged reinforcement contingencies were in effect.

Effects of chronic d-amphetamine treatment on cocaine- and food-maintained responding under a second-order schedule in rhesus monkeys

Effective treatment of opioid dependence with methadone and of tobacco dependence with nicotine illustrates the potential usefulness of agonist medications for drug abuse treatment. The monoamine-releaser d-amphetamine is one drug under consideration as an agonist pharmacotherapy for cocaine dependence. To assess the concordance between preclinical evaluations and ongoing clinical trials, the present study examined the effects of chronic treatment with saline or d-amphetamine on cocaine- and food-maintained responding in rhesus monkeys. Cocaine injections and food pellets were available under a second-order schedule during alternating daily sessions of cocaine and food availability. d-Amphetamine (0.01-0.1 mg/kg per h i.v. for 7 consecutive days) dose-dependently decreased self-administration of a unit dose of cocaine (0.01 mg/kg per injection) at the peak of the cocaine self-administration dose-effect curve. d-Amphetamine (0.032-0.1 mg/kg per h for 7 days) also decreased self-administration of a broad range of cocaine doses (0.0032-0.1 mg/kg per injection) and produced rightward and downward shifts in the cocaine dose-effect curve. Food-maintained responding was usually decreased less than cocaine self-administration, and few signs of toxicity were noted. To evaluate the effects of a longer treatment regimen, d-amphetamine (0.1 mg/kg per h) was administered for 28 consecutive days. d-Amphetamine nearly eliminated self-administration of cocaine (0.01 mg/kg per injection) throughout this treatment, whereas food-maintained responding returned to baseline levels after approximately 9 days. These preclinical findings are concordant with recent clinical studies and suggest that chronic d-amphetamine may selectively decrease cocaine-taking behavior in rhesus monkeys, possibly by producing a selective decrease in the reinforcing effects of cocaine.

Tolerance to the effects of cocaine on performance under behavior-correlated reinforcement magnitude

Four pigeons responded under a fixed-interval 8-min schedule of food delivery in which the amount of food delivered at the end of each interval depended on performance during the interval (i.e., a correlated schedule). Specifically, duration of access to grain was contingent upon the number of responses made during the first 4 min of the interval. This differential outcome did not affect response rates or patterning relative to performance under a simple fixed-interval 8-min schedule. Behavior under the correlated schedule was then investigated under doses of cocaine ranging from 0.3 to 10.0 mg/kg. A bitonic dose-response function was obtained for response rates and the time with head in the food hopper, whereas dose-dependent decreases were observed in the mathematical index of curvature (Fry, Kelleher, & Cook, 1960). The dose that produced the greatest increase in the head-in-hopper time was then administered prior to each session. Following repeated administration of cocaine, disruptions in response patterning were attenuated for all 4 pigeons; tolerance was also observed to the rate-increasing effects and increased head-in-hopper time for 2 pigeons after chronic cocaine administration. Tolerance therefore developed despite the fact that the initial effect of cocaine was to increase the amount of food obtained.

Effects of cocaine and morphine under mixed-ratio schedules of food delivery: support for a behavioral momentum analysis

Previous studies have shown that ratio size influenced the development of tolerance under simple and multiple schedules, but not under progressive-ratio (PR) schedules. PR schedules share certain features with mixed-ratio (MR) schedules, and pilot data suggested that ratio size fails to modulate tolerance to cocaine or morphine under MR schedules. The present study examined more comprehensively the pre- and postchronic effects of cocaine and (in separate birds) morphine under MR schedules with fixed-ratio (FR) 5 and FR 95, FR 25 and FR 75, and FR 50 and FR 50 components. Acute doses of cocaine and morphine initially were given in an ascending series (beginning with 0.56 mg/kg) until responding was reduced to near-zero levels. Chronic (daily) dosing with a dose that reduced, but did not eliminate, responding then occurred until response rates stabilized. Finally, postchronic dose-response determinations were conducted. Both cocaine and morphine reduced response rates at all FR values. Tolerance was consistently observed to the effects of morphine, but not to those of cocaine. With both drugs the degree of tolerance observed did not vary as a function of FR value. These findings, like those obtained under PR schedules, indicate that ratio size does not always modulate drug tolerance. A behavioral momentum analysis of drug action appears to account for whether or not ratio size modulates tolerance, and such an analysis is provided.

Acute and chronic effects of morphine under a progressive-ratio 25 schedule of food delivery

The present study examined the effects of morphine in pigeons responding under a progressive-ratio 25 schedule of food delivery. Morphine initially reduced response rates and breaking points. With chronic exposure, tolerance developed to these effects. The magnitude of the observed tolerance was not obviously different from that previously reported under a PR 5 schedule of food delivery. In addition, when drug effects were compared under the fixed-ratio 25 and fixed-ratio 100 components comprised by the progressive-ratio schedule, comparable tolerance was observed. Although prior studies using other procedures have shown that ratio size modulates the development of tolerance to morphine and other drugs, the present data suggest that this relation is constrained, and is not easily observed under progressive-ratio schedules.

In pursuit of general behavioral relations

Efforts to develop behavioral technologies from advances in basic research assume that results from studies with nonhuman subjects can, in some instances, be applied to human behavior. The behavioral principles likely to be most useful for application are those that represent robust general behavioral relations. Basic and applied research on behavioral momentum suggests that there is a general behavioral relation between the persistence of behavior and the rate of reinforcement obtained in a given situation. Understanding the factors that affect behavioral persistence may have important implications for applied behavior analysts that justify studies aimed at establishing the generality and limits of the functional relation between reinforcement rate and behavioral persistence. Strategies for establishing the generality of behavioral relations are reviewed, followed by a brief summary of the evidence for the generality of behavioral momentum.

A review of the effects of dopaminergic agents on humans, animals, and drug-seeking behavior, and its implications for medication development. Focus on GBR 12909

Medication development for cocaine abuse has focused on potential mechanisms of action related to the abuse of cocaine. The hypothesis that mesolimbic dopamine (DA) is the key neurochemical mediator of cocaine's addictive and reinforcing effects is well supported by a wide variety of data from animal studies. On the other hand, medications that increase DA or block its action in humans can produce effects that appear incompatible with this hypothesis. This article reviews these incompatibilities between animal and human data with a focus on the DAergic actions of drugs, including DA reuptake inhibitors, direct DA agonists, DA increasers, and DA antagonists. Possible reasons for these discrepancies are discussed, and the potential role of high-affinity DA uptake inhibitors, such as GBR12909, for pharmacotherapies for treating cocaine addiction in humans is likely to come from understanding its mechanisms of action, it is clear that further research on the effects of cocaine in humans and animals will be critical to the medication development effort.

Cocaine's effects on food-reinforced pecking in pigeons depend on food-deprivation level

Four pigeons deprived to 80% of their laboratory free-feeding weights pecked keys under a multiple fixed-ratio 30 fixed-interval 5-min schedule of food presentation. Components alternated strictly with 15-s timeouts separating them; each was presented six times. When rates of pecking were stable, 2 pigeons' weights were reduced to 70%, and the other 2 pigeons' weights were increased to 82.5% to 85% of free-feeding levels. Cocaine (1.0, 3.0, 5.6, and 10.0 mg/kg and saline) was administered 5 min prior to sessions. When each dose had been tested twice, pigeons' weights were adjusted to the level that they had not yet experienced, and cocaine was tested again. Cocaine reduced response rates in a dose-dependent manner under the fixed-ratio schedule and under the fixed-interval schedule at high doses, and increased rates under the fixed-interval schedule at low low doses. Reductions in pecking rates occurred at lower doses under both schedules in 3 of 4 pigeons when they were less food deprived compared to when they were more food deprived. Low doses of cocaine increased low baseline rates of pecking in the initial portions of the fixed-interval schedules by a greater magnitude when pigeons were more food deprived. Thus, food-deprivation levels altered both the rate-decreasing and rate-increasing effects of cocaine. The implications of these results for the mechanisms by which food deprivation increases cocaine self-administration and for the dependence of cocaine's effects on the baseline strength of operant behavior are discussed.

The effects of cocaine on behavior maintained by timeout from avoidance

Rats were trained on concurrent schedules under which responses on one lever postponed shock (avoidance) and responses on the other lever produced brief (2-min) periods of signaled timeout from avoidance. For 6 rats, timeout from avoidance was programmed on a variable-interval 45-s schedule that generally resulted in rates that were lower than those on the avoidance lever. For another 6 rats, timeout was arranged on a variable-ratio 15 schedule that produced higher baseline rates. Cocaine (3 to 40 mg/kg) produced large, dose-dependent increases in behavior maintained by timeout in both groups of rats. Avoidance responding was also generally increased by cocaine, but the increases were of lesser magnitude. Increases in response rates were seen across a broad range of doses on behavior maintained by either interval or ratio schedules, an outcome that was unexpected on the basis of most studies of cocaine on food-maintained behavior. These results were similar to those of previous studies of the effects of amphetamine on behavior maintained by timeout from avoidance and suggest that stimulant drugs affect behavior maintained under a shock-postponement schedule differently than they affect behavior maintained by timeout from avoidance.

Behavioral effects of enantiomers of dizocilpine under two "counting" procedures in rats

Stereoisomers of the N-methyl-D-aspartate antagonist dizocilpine (MK-801) were studied to determine whether behavioral effects on complex operants depend on reinforcement loss accompanying behavioral disruption. Rats earned food pellets if the run of consecutive left-lever presses preceding a trial-terminating right-lever press approximated a target of 12. A percentile schedule reinforced any run closer to the target than two-thirds of the runs on the most recent 24 trials. Once the sequence was learned, half the subjects were shifted to a procedure that yoked reinforcement for each length run to the probability that length generated pellets during asymptototic percentile performance. Although these two procedures generate similar control run and reinforcement distributions, disrupting behavior reduced reinforcement probability far more under the yoked than the percentile procedure. Despite this difference in drug-induced reinforcement loss, both enantiomers produced similar dose-related decreases in run length and response rate under both procedures, with the (-) isomer approximately one log unit less potent than the (+) isomer. The absence of differential effects under these procedures diminishes the likelihood that reinforcement loss contributes to dizocilpine's effects, indirectly bolstering claims that dizocilpine directly affects learning.

Effects of cocaine on fixed-ratio responding of rats: modulation by required response force

The effects of acute cocaine administrations (5.6-32 mg/kg) were determined in rats responding under a multiple fixed-ratio 15 (FR 15) FR 15 schedule of food delivery. The minimum response effort required in one schedule component was 25 g, whereas in the other component it was 200 g. Cocaine produced generally dose-dependent decreases in rate of responding and increases in preratio pause time under each component. There was, however, a significant interaction between force and drug dose, and drug effects were larger in the component requiring 200 g for lever operation. Although a number of other parameters have been shown previously to modulate the effects of cocaine on schedule-controlled responding, the present data constitute the first demonstration that minimum response effort does so.

The effects of acute and chronic cocaine administration on paced responding in intact and gonadectomized male and female Wistar rats

Intact and gonadectomized male and female Wistar rats pressed a lever on a multiple (tandem Random-Interval 30-s, Differential Reinforcement of High Rate 0.5-s) (tandem Random-Interval 30-s, Differential Reinforcement of Low-Rate 5.0-s) schedule of reinforcement. The pacing requirements maintained high and low response rates under equal reinforcement frequencies. Low doses of cocaine (1 and 3 mg/kg) either did not affect or slightly increased high and low response rates of intact and gonadectomized female rats, while they did not affect or decrease high and low response rates of intact and castrated male rats. Higher doses of cocaine (up to 30 mg/kg) dose-dependently decreased both high and low response rates for all subjects. Intact male rats were less sensitive to the rate-decreasing effects of these doses of cocaine than castrated male rats or intact and ovariectomized female rats. Chronic cocaine administration consistently resulted in behavioral sensitization only in intact male subjects. The results of this experiment provide further support for the notion that the behavioral effects of psychomotor stimulant drugs are not necessarily rate dependent but may depend upon the extent to which schedule contingencies allow for behavioral variability without negatively affecting behavioral outcomes.

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.