Lever-pressing Rates Research Articles

Differential Effect of Fixed Ratio Magnitude on the Rate of Lever-Pressing and Interinjection Intervals of Cocaine Self-Administration in Rats

Many features of self-administration behavior may be explained by reference to the properties of schedules of reinforcement. Schedules alter the probability of a behavior being reinforced and thereby increase, or decrease, the frequency of the behavior and fixed ratio (FR) magnitude reportedly alters the rate of responding to cocaine. A pharmacokinetic/pharmacodynamic interaction theory states that lever-pressing behavior is induced only when cocaine levels in the body are above the priming/remission threshold and below the satiety threshold-a range termed the compulsion zone. This theory successfully explains cocaine self-administration in rats on a progressive ratio and the FR1 schedule. To determine the effects of high FR magnitude on the rate of self-administration of cocaine and the rate of lever-pressing behavior when cocaine levels are within the compulsion zone. Rats acquired cocaine self-administration on an FR1 schedule and then were switched to sessions that started with FR1 and then FR 5, 10, 20, or 50. An only FR1 session was run each week between FR1/FR50 sessions and then only FR1 sessions were conducted for several weeks. Interinjection intervals at a unit dose of 3 µmol/kg were regular at both FR1 and FR50 but were longer by the time required to complete the 50 presses. When responding by rats was maintained under an FR50 schedule of cocaine presentations, compared to baseline FR1 sessions, dramatic increases in the number of lever-presses were observed after access to cocaine was terminated, a previously unreported finding. However, lever-pressing occurred only when cocaine levels were in the compulsion zone, and this duration was unchanged. The increase in lever-pressing persisted for weeks. Interinjection intervals at FR1 were not altered after exposure to FR50. Although previously considered key to understanding the regulation of cocaine self-administration behavior, FR magnitude simply increased interinjection intervals by the time required to complete 50 lever-presses. The dramatic increase in the rate of lever-pressing was caused by the high FR schedule rather than cocaine. The utility of the schedule-induced increase in the rate of lever-pressing is unclear. The compulsion zone theory provides a rational pharmacological basis for understanding cocaine self-administration behavior.

Chronic lipopolysaccharide impairs motivation when delivered to the ventricles, but not when delivered peripherally in male rats

Increased neuroinflammation relative to controls is observed in major depression. Moreover, depressive disorders are significantly elevated in conditions which increase neuroinflammation (e.g., brain injury, Parkinson's disease, Alzheimer's disease). To better understand the relationship between neuroinflammation and depression, additional research is needed. The current set of studies made use of the progressive ratio (PR) task in male rats, a stable measure of motivation which can be evaluated daily and thus is ideally suited for examining a potential role for chronic neuroinflammation in depressive-like behavior. Lipopolysaccharide (LPS) was used to induce an inflammatory response. Experiment 1 confirmed prior acute LPS administration experiments for sensitivity of the PR task, with a large effect at 2 mg/kg, a partial effect at 1 mg/kg, and no effect at 0.5 mg/kg. Experiment 2 evaluated a dose-response of continuous s.c. LPS infusion but found no significant elevation in brain cytokines after 14 days at any doses of 0.1, 0.5, 1, or 2 mg/kg/week. Experiment 3 assessed motivation during continuous s.c. infusion of a large 5 mg/kg/week LPS dose and found no significant impairments in motivation, but transient decreases in rates of lever pressing (i.e., only motoric deficits). Experiment 4 measured motivation during continuous ICV infusion of 10.5 μg/kg/week LPS and found significantly decreased motivation without changes to rates of lever pressing (i.e., only motivational deficits). Together these results suggest that the PR task is efficient for evaluating models of chronic inflammation, and that the adaptive response to chronic LPS exposure, even when delivered centrally, may necessitate alternative strategies for generating long-term neuroinflammation.

Dopamine D1 receptor and effort-based decision making in rats: The moderating effect of sex

Dopamine is a modulating factor in effort-based decision-making, and emerging evidence from pharmacological research suggests that the dopamine D1 receptor is the primary regulator. Given the limited selectivity of pharmacological tools, we further explored this hypothesis using dopamine D1 mutant (DAD1−/−) rats which have a specific genetic reduction in functional D1 receptors. Moreover, given the strong focus on males in neuroscience research in general and in the role of D1 receptors in effort-based learning, we compared both sexes in the present study.Adult male and female DAD1−/− mutant rats and wild type controls were trained to press a lever for a reinforcer. Once trained, subjects completed multiple fixed ratio, progressive ratio, and operant effort-choice (concurrent progressive ratio/chow feeding task [PROG/chow]) experiments. We predicted that DAD1−/− mutant rats would press the lever significantly less than controls across all experiments, have lower breakpoints, and consume more freely available food. As predicted, DAD1−/− mutant rats (regardless of sex) pressed the lever significantly less than controls and had lower breakpoints. Interestingly, there was a sex * genotype interaction for acquisition rates of lever pressing and change in breakpoints with free food available. Only 31% of DAD1−/− mutant males acquired lever pressing while 73% of DAD1−/− mutant females acquired lever pressing. Additionally, DAD1−/− mutant males had significantly larger decreases in breakpoints when free food was available. These findings extend the pharmacological research suggesting that the dopamine D1 receptor modulates decisions based on effort, which has implications for the development of treatment targeting amotivation in neuropsychiatric disorders. The sex * genotype interaction highlights the importance of including both sexes in future research, especially when there are sex differences in incidences and severity of neuropsychiatric disorders.

"You can be a behaviorist and still talk about the mind - as long as you don't put it into a person's head": An interview with Howard Rachlin1.

"You can be a behaviorist and still talk about the mind - as long as you don't put it into a person's head": An interview with Howard Rachlin1.

Discriminative stimulus effects of carfentanil in rats discriminating fentanyl: Differential antagonism by naltrexone

BackgroundA significant number of deaths caused by opioids involve fentanyl and/or one of its very potent analogs (e.g., carfentanil). Some clinical reports suggest larger doses of opioid receptor antagonists may be required to reverse the effects of carfentanil compared with other opioid receptor agonists, although this has not been examined extensively in vivo. The current study compared the discriminative stimulus effects of fentanyl, carfentanil, and heroin, and their antagonism by naltrexone. MethodsEight male Sprague-Dawley rats were trained to discriminate 10.0 μg/kg fentanyl from saline while responding under a fixed-ratio 10 schedule of food presentation. Dose-effect curves were determined for the opioid receptor agonists fentanyl (1.0–32.0 μg/kg), carfentanil (0.1–3.2 μg/kg), and heroin (10.0–320.0 μg/kg), then redetermined following a 15-minute pretreatment with 0.1 mg/kg naltrexone. ResultsFentanyl, carfentanil, and heroin dose-dependently increased responding on the fentanyl-associated lever and decreased the rate of lever pressing. Carfentanil and heroin were approximately 10-fold more and less potent, respectively, than fentanyl at eliciting >80 % responding on the fentanyl-associated lever. Pretreatment with 0.1 mg/kg naltrexone resulted in a significant rightward shift in the fentanyl and heroin but not carfentanil dose-effect curves. ConclusionsDifferences in the effectiveness of naltrexone to attenuate the discriminative stimulus effects of carfentanil, compared with fentanyl and heroin, suggest that there may be differences in how carfentanil exerts its discriminative stimulus effects compared with other opioids. Further evaluation is needed of potential pharmacological and behavioral differences between carfentanil and other opioids, particularly in the context of toxicity.

Discriminative Stimulus Effects of Mu Opioid Receptor Agonists in Rats Discriminating Fentanyl: Differential Antagonism by Naltrexone

The opioid crisis continues despite the availability of medications that are effective in some patients. A significant number of deaths caused by opioids involve fentanyl and/or one of its very potent analogues (e.g., carfentanil). Many fentanyl analogs have not been studied extensively in vivo. The current study compared the discriminative stimulus effects of fentanyl and carfentanil and their antagonism by naltrexone. Seven male Sprague‐Dawley rats were trained to discriminate a 0.01 mg/kg fentanyl from saline while responding under a fixed‐ratio 10 schedule of food presentation. Dose effect curves were determined for the opioid receptor agonists alone: fentanyl (0.001–0.1 mg/kg) and carfentanil (0.0001–0.0032 mg/kg). Dose effect curves for fentanyl and carfentanil were then redetermined following a 15‐minute pretreatment with 0.1 mg/kg naltrexone. All drugs were delivered intraperitoneally. Fentanyl and carfentanil dosedependently increased responding on the fentanyl‐associated lever and at larger doses decreased the rate of lever pressing. Carfentanil was approximately 10‐fold more potent than fentanyl at eliciting >90% responding on the fentanyl‐associated lever. Pretreatment with 0.1 mg/kg naltrexone shifted the fentanyl dose‐effect curve 10‐fold rightward whereas the same dose of naltrexone shifted the carfentanil curve only 3‐fold rightward. Differences in the effectiveness of naltrexone to attenuate the discriminative stimulus effects of fentanyl and carfentanil suggests that there may be differences in how fentanyl and carfentanil exert their discriminative stimulus effects. Further evaluation of potential pharmacological and behavioral differences between fentanyl and carfentanil is needed and might provide important insights regarding the apparently increased toxicity of carfentanil, compared with other opioids, in humans.Support or Funding InformationAQ‐0039 from the Welch Foundation

Unilateral accumbal dopamine depletion affects decision-making in a side-specific manner

Mechanisms underlying affective and cognitive deficits in Parkinson's disease (PD) remain less studied than motor symptoms. Nucleus accumbens (NAc) is affected in PD and due to its well-known involvement in motivation is an interesting target in this context. Furthermore, PD is frequently asymmetrical, with side-specific deficits aligning with evidences of accumbal laterality. We therefore used a 6-hydroxydopamine (6-OHDA) model to study the role of left and right NAc dopamine depletion in a battery of behavioral tasks.2 months old male rats were used in all experiments. Habitual-based and goal-directed decision-making, impulsivity, anxiety- and depressive-like behavior and motor performance were tested 3 weeks after left (6-OHDA L) or right (6-OHDA R) NAc lesion was induced. Upon contingency degradation, 6-OHDA R decrease their lever press rate less than Sham and 6-OHDA L, indicating an impairment in the shift from habit-based to goal-directed strategies. On the other hand, 6-OHDA L lesions lead to increased rates of premature responding when delays where increased in the variable delay-to-signal test. Importantly, in both paradigms task acquisition was similar between groups. In the same line we found no differences in the amount of sugared pellets eaten when freely available as well as in both general and fine motor behaviors.In conclusion, left and right NAc play distinct roles in the contingency degradation and impulsivity. More studies are needed to understand the mechanisms behind this functional lateralization and its implications for PD patients.

Local constraint of the contingent response controls rates of wheel-running and lever-pressing on an interval schedule of wheel-running reinforcement

Three experiments assessed whether changes in rates of wheel running and lever pressing on a schedule of wheel-running reinforcement were controlled by local or overall constraint of the contingent response. In Experiment 1 rats responded on a response-initiated variable interval 15-s schedule with wheel running as reinforcement. Overall constraint (total revolutions by session) was held constant at 600 revolutions while local constraint (revolutions per reinforcement) varied from 5 to 60 revolutions. Results showed that rates of wheel running and lever pressing decreased as local constraint decreased. In Experiment 2, rats responded on the same schedule, but local constraint (revolutions per reinforcement) was held constant at 10 revolutions. Overall constraint (total revolutions) was varied from 100 to 800 revolutions. Results showed no systematic changes in rates of wheel running and lever pressing with variation in overall constraint. In Experiment 3, rats responded on the same VI schedule for 30 revolutions as reinforcement. A 4.5-s interruption during the wheel-running reinforcement (30 revolutions) occurred after 2, 5, 10, and 15 revolutions as a manipulation of local constraint. Results showed that with overall constraint (total revolutions) and total reinforcement held constant, rates of wheel running before the interruption and overall lever-pressing rates varied in a bitonic relationship with local constraint. Comparison of the results across experiments demonstrates that local constraint of contingent wheel running controls the rate of wheel running and the rate of instrumental responding for wheel-running reinforcement.

Instrumental lever pressing for wheel running is a bitonic function of wheel revolutions per reinforcement: Effects of constraint and automatic reinforcement

The current study investigated the relationship between wheel-running reinforcement and operant lever pressing when an opportunity to run is defined by the number of wheel revolutions rather than duration of wheel access. Twelve female Long-Evans rats responded on response-initiated variable interval 15-s schedules for the opportunity to run for 1, 3, 5, 10, 20, 30, or 40 revolutions. Half the rats received an ascending order of revolutions/reinforcement; the other half, received a descending order. Results showed that wheel-running and lever-pressing rates were described by a bitonic relationship with revolutions/reinforcement, initially rising then falling, as revolutions/reinforcement increased. Long postreinforcement pause (PRP) durations occurred with few revolutions, short PRPs with an intermediate number, and long PRPs again with a high number of revolutions; thus, PRP showed an inverted U-shaped function. Rather than a reinforcement-magnitude interpretation, our findings suggest that number of revolutions/reinforcement (or duration of wheel access) be conceptualized as constraint on an automatically-reinforcing behavior (wheel running). Specifically, we propose that the automatic-reinforcement value of contingent wheel running varies with its rate of occurrence, which causes instrumental lever-pressing rates to vary with wheel-running rates. The current analysis of constraint and the automatic reinforcement of the contingent behavior is shown to further extend the response-deprivation hypothesis to wheel-running reinforcement.

Effects of Chronic, Intermittent Sucrose Consumption on the Discriminative Stimulus Effects of Dopamine or Opioid Antagonists

Chronic, intermittent sucrose access has been shown to induce neurochemical changes in the brain, including increasing the levels of dopamine and D3‐receptor mRNA, decreasing the levels of D2‐receptor mRNA, and increasing μ‐opioid receptor binding. We wondered if chronic, intermittent sucrose consumption would alter the ability of rats to discriminate drugs that affect endorphin and dopamine functioning. We successfully trained rats with chronic, intermittent sucrose access to discriminate various naltrexone training doses (ranging from 0.32 – 3.2 mg/kg) from saline. In the present series of studies, we sought to determine if chronic, intermittent sucrose consumption alters the ability of haloperidol to serve as a discriminative stimulus. Sprague‐Dawley rats were trained to discriminate between haloperidol [0.018–0.56 mg/kg, 30 min pretreatment (PT)] and vehicle in a two‐lever, operant choice procedure. Most subjects reached the discrimination criteria (80% or greater condition‐appropriate responding for 8 of 10 consecutive sessions). Among subjects that acquired the discrimination, there were no significant differences in acquisition between subjects with chronic, intermittent sucrose access and subjects with 24‐hr water access. Haloperidol (0.032 – 0.056 mg/kg) produced significant rate suppression. Increasing the PT to 60 minutes increased rates of lever pressing and reinforcers earned, but did not alter haloperidol generalization functions. Taken together, chronic sucrose consumption enhanced naltrexone's ability to function as a discriminative stimulus, but did not affect the discriminative stimulus effects of haloperidol.Support or Funding InformationUniversity of Wisconsin‐Eau Claire Office of Research and Sponsored ProgramsThis abstract is from the Experimental Biology 2018 Meeting. There is no full text article associated with this abstract published in The FASEB Journal.

Duration of wheel-running reinforcement: Effects on reinforcement value and motivation in free-feeding and food-deprived rats

Ten (pair housed) female Long-Evans rats were exposed to 5s, 30s, and 90s wheel-running reinforcement durations on a response-initiated variable interval 20s schedule as food deprivation was manipulated. On free feeding, never-deprived rats showed low wheel running and lever-pressing rates with long postreinforcement pauses (PRPs) for the 5-s reinforcement duration. Subsequently, when food deprived (Deprived 1), rats showed no effect of reinforcement duration on all measures. Under a second deprived condition (Deprived 2) with the rats maintained in single cages, there was no effect of housing (single vs. paired). When data from both deprivation assessments (Deprived 1 and Deprived 2) were combined, rats showed lower wheel running and overall lever-pressing rates with longer pauses on the 90-s duration compared to 30s and 5s bouts of wheel activity. The pattern of results challenges a reinforcement value interpretation, but is consistent with shifts in the motivational basis of wheel running. On free feeding, never-deprived rats were intrinsically motivated to run on wheels and operant lever-pressing was maintained at moderate rates by the automatic reinforcement of wheel running, except at the short reinforcement duration (5s). When food deprived, motivation became food-related and rats showed high rates of lever pressing even at the shortest duration. The weak effects under initial deprivation (Deprived 1) raise questions about equivalence between wheel-running reinforcement duration and reinforcement magnitude using food reinforcement.

Increased sign-tracking behavior in adolescent rats.

An autoshaping procedure was used to test the notion that conditioned stimuli (CSs) gain greater incentive salience during adolescence than young adulthood under conditions of social isolation rearing and food restriction. Rats were single-housed and placed on food restriction during 10 daily training sessions in which a lever (CS+ ) was presented then followed immediately by a food unconditioned stimulus (US). A second lever (CS- ) was presented on intermixed trials and was not reinforced. Despite the fact that food delivery was not contingent on the rats' behavior, all rats exhibited behaviors directed towards the lever (i.e., sign-tracking). In the adolescent group, the rate of lever pressing and the percentage of trials with a lever press were higher than in young adults. Initially, group differences were observed when rats were retrained when the adolescents had reached young adulthood. These findings support the hypothesis that cues that come to predict reward become imbued with excessive motivational value in adolescents, perhaps contributing to the hyper-responsiveness to reward-related stimuli typically observed during this period of development.

Reinforcement of a reinforcing behavior: Effect of sucrose concentration on wheel-running rate

Wheel running, unlike typical operant behavior, generates its own automatic reinforcement that alters the control exerted by extrinsic reinforcement on wheel running. The current study investigated the implications of the automatic reinforcement of wheel running by arranging different sucrose concentrations as extrinsic reinforcement for operant wheel running in ad-lib fed and food-deprived rats. Eleven female Long Evans rats ran on fixed revolution 30 schedules that delivered a drop of sucrose solution as reinforcement. Sucrose concentration varied across values of 0%, 2.5%, 5%, 10%, and 15% sucrose (w/v). Results showed that under ad-lib feeding, only the highest concentrations increased operant wheel-running rate. By contrast, under deprivation, all concentrations of sucrose increased the rate of wheel running. Despite the differences in sucrose-reinforced operant wheel-running rates by deprivation level (ad lib vs. deprived), wheel-running rates did not differ at the highest concentrations. Prior research on operant lever pressing, a response generating low (or no) automatic reinforcement, has shown considerably higher lever-pressing rates as a function of increasing amounts of sucrose reinforcement when rats are food deprived. Together, these previous observations and the current study suggest that automatic reinforcement generated by an operant decreases the control exerted by extrinsic reinforcement. Additionally, the regulation by extrinsic reinforcement on automatically reinforcing behavior depends on the organism’s motivation or deprivation level (ad lib vs. deprived).

Resistance to extinction of lever-pressing rates maintained by different wheel-running reinforcement durations

Previous research using resistance to extinction to assess response strength has shown slower attenuation of responding maintained by larger than smaller reinforcement magnitudes. The current study sought to generalize this reinforcement-magnitude effect to extinction of lever-pressing rates maintained by wheel-running reinforcement of different durations. Rats responded on a response-initiated variable interval 20-s schedule for the opportunity to run in a wheel for 5s, 30s, and 90s. Following training on each reinforcement duration, lever pressing for wheel running was placed on extinction. Results showed that responding maintained by 5s of wheel-running reinforcement declined slower in extinction than for the 90-s duration, while 30s of wheel-running reinforcement produced inconsistent effects on attenuation of response rates. These results suggest that shorter periods of wheel-running reinforcement have higher reinforcement value than longer durations—findings incompatible with the earlier research showing more resistance to extinction with large reinforcement magnitudes. We offer an interpretation based on response deprivation arising from restriction of the wheel-access interval and its motivational impact on wheel running as a reinforcing consequence and on responding for wheel-running reinforcement. An alternative analysis proposes that discrimination of the shift to extinction is more salient for the long wheel-running duration—accounting for the rapid attenuation of responding in extinction with the long periods of wheel-running reinforcement.

Evidence for positive, but not negative, behavioral contrast with wheel-running reinforcement on multiple variable-ratio schedules

Rats responded on a multiple variable-ratio (VR) 10 VR 10 schedule of reinforcement in which lever pressing was reinforced by the opportunity to run in a wheel for 30s in both the changed (manipulated) and unchanged components. To generate positive contrast, the schedule of reinforcement in the changed component was shifted to extinction; to generate negative contrast, the schedule was shifted to VR 3. With the shift to extinction in the changed component, wheel-running and local lever-pressing rates increased in the unchanged component, a result supporting positive contrast; however, the shift to a VR 3 schedule in the changed component showed no evidence of negative contrast in the unaltered setting, only wheel running decreased in the unchanged component. Changes in wheel-running rates across components were consistent in showing a compensation effect, depending on whether the schedule manipulation increased or decreased opportunities for wheel running in the changed component. These findings are the first to demonstrate positive behavioral contrast on a multiple schedule with wheel running as reinforcement in both components.

Motivational shift: Effects of unrestricted and restricted feeding on wheel running as an operant and reinforcing consequence on a multiple schedule of reinforcement

Rats were exposed to a multiple schedule on which they pressed a lever on a variable-ratio schedule for wheel-running reinforcement in one component and ran a fixed number of revolutions for 15% sucrose solution reinforcement in the other component. Feeding was varied from restricted to unrestricted (ad-libitum) and then returned to restricted. Across these changes in feeding, average body weight varied from 262g to 339g to 258g. Under ad-lib feeding, wheel-running and lever-pressing rates decreased while postreinforcement pause duration increased in the wheel-running reinforcement component. In the operant wheel-running component, wheel-running rates also decreased and postreinforcement pause duration increased. Notably, wheel-running rates in the operant running component were higher than in the wheel-running reinforcement component when rats were food restricted, but did not differ when rats were freely fed, indicating a reduced effectiveness of sucrose reinforcement. This reduction in response strengthening of operant wheel running by sucrose under ad-libitum feeding is consistent with a shift from food-related to intrinsic motivation of running. From a response deprivation perspective (Allison & Timberlake, 1974), this reduced effectiveness of sucrose reinforcement would occur if wheel running in freely fed rats became the more constricted behavior in the contingency between instrumental wheel running and contingent sucrose consumption.

Chemogenetic manipulation of ventral pallidal neurons impairs acquisition of sign‐tracking in rats

Cues associated with rewarding events acquire value themselves as a result of the incentive value of the reward being transferred to the cue. Consequently, presentation of a reward-paired cue can trigger reward-seeking behaviours towards the cue itself (i.e. sign-tracking). The ventral pallidum (VP) has been demonstrated to be involved in a number of motivated behaviours, both conditioned and unconditioned. However, its contribution to the acquisition of incentive value is unknown. Using a discriminative autoshaping procedure with levers, the effects of disrupting VP activity in rats on the emergence of sign-tracking was investigated using chemogenetics, i.e. Designer Receptors Exclusively Activated by Designer Drugs (DREADDs). Transient disruption of VP neurons [activation of the inhibitory hM4D(Gi) DREADD through systemic injections of clozapine N-oxide (CNO) prior to each autoshaping session] impaired acquisition of sign-tracking (lever press rate) without having any effect on approach to the site of reward delivery (i.e. goal-tracking) or on the expression of sign-tracking after it was acquired. In addition, electrophysiological recordings were conducted in freely behaving rats following VP DREADD activation. The majority of VP units that were responsive to CNO injections exhibited rapid inhibition relative to baseline, a subset of CNO-responsive units showed delayed excitation, and a smaller subset displayed a mixed response of inhibition and excitation following CNO injections. It is argued that disruption of VP during autoshaping specifically disrupted the transfer of incentive value that was attributed to the lever cue, suggesting a surprisingly fundamental role for the VP in acquiring, compared with expressing, Pavlovian incentive values.

Wheel-running reinforcement in free-feeding and food-deprived rats

Rats experiencing sessions of 30min free access to wheel running were assigned to ad-lib and food-deprived groups, and given additional sessions of free wheel activity. Subsequently, both ad-lib and deprived rats lever pressed for 60s of wheel running on fixed ratio (FR) 1, variable ratio (VR) 3, VR 5, and VR 10 schedules, and on a response-initiated variable interval (VI) 30s schedule. Finally, the ad-lib rats were switched to food deprivation and the food-deprived rats were switched to free food, as rats continued responding on the response-initiated VI 30-s schedule. Wheel running functioned as reinforcement for both ad-lib and food-deprived rats. Food-deprived rats, however, ran faster and had higher overall lever-pressing rates than free-feeding rats. On the VR schedules, wheel-running rates positively correlated with local and overall lever pressing rates for deprived, but not ad-lib rats. On the response-initiated VI 30s schedule, wheel-running rates and lever-pressing rates changed for ad-lib rats switched to food deprivation, but not for food-deprived rats switched to free-feeding. The overall pattern of results suggested different sources of control for wheel running: intrinsic motivation, contingencies of automatic reinforcement, and food-restricted wheel running. An implication is that generalizations about operant responding for wheel running in food-deprived rats may not extend to wheel running and operant responding of free-feeding animals.

Effects of extinction on wheel running and lever pressing as operant behaviors within a multiple schedule of reinforcement

Rats were exposed to a multiple schedule in which they pressed a lever on a variable-ratio schedule for wheel-running reinforcement in one component and pressed a lever or completed wheel revolutions on a fixed-ratio schedule for 15% sucrose solution reinforcement in the changed component. After 20 sessions, sucrose reinforcement was replaced with water (0% sucrose) and operant behavior (lever pressing or wheel running) placed on extinction in the changed component. Extinction markedly reduced lever-pressing rate and lengthened postreinforcement pause (PRP) duration in the changed component, but only modestly reduced wheel-running rate and actually shortened PRP duration. In the unchanged component, when either lever pressing or wheel running were placed on extinction in the changed component, wheel-running rate, lever-pressing rate, and wheel-running reinforcers decreased while PRP duration increased in the changed component. The effect of extinction differed in the changed component as a function of the type of operant, but not in the unchanged component. This ruled out the automatic reinforcement effect of wheel running as a bridge for transferring the extinction effect between components. Arousal, negative induction, and a shift from high to low valued reinforcement were considered as potential accounts for the effects in the unchanged component.

Effect of sucrose availability on wheel-running as an operant and as a reinforcing consequence on a multiple schedule: Additive effects of extrinsic and automatic reinforcement

As a follow up to Belke and Pierce’s (2014) study, we assessed the effects of repeated presentation and removal of sucrose solution on the behavior of rats responding on a two-component multiple schedule. Rats completed 15 wheel turns (FR 15) for either 15% or 0% sucrose solution in the manipulated component and lever pressed 10 times on average (VR 10) for an opportunity to complete 15 wheel turns (FR 15) in the other component. In contrast to our earlier study, the components advanced based on time (every 8min) rather than completed responses. Results showed that in the manipulated component wheel-running rates were higher and the latency to initiate running longer when sucrose was present (15%) compared to absent (0% or water); the number of obtained outcomes (sucrose/water), however, did not differ with the presentation and withdrawal of sucrose. For the wheel-running as reinforcement component, rates of wheel turns, overall lever-pressing rates, and obtained wheel-running reinforcements were higher, and postreinforcement pauses shorter, when sucrose was present (15%) than absent (0%) in manipulated component. Overall, our findings suggest that wheel-running rate regardless of its function (operant or reinforcement) is maintained by automatically generated consequences (automatic reinforcement) and is increased as an operant by adding experimentally arranged sucrose reinforcement (extrinsic reinforcement). This additive effect on operant wheel-running generalizes through induction or arousal to the wheel-running as reinforcement component, increasing the rate of responding for opportunities to run and the rate of wheel-running per opportunity.