Characterization of wheel running in rats as schedule-induced behavior

Resumo

Abstract Schedule-induced behaviors are distinct from operant behavior that arise excessively under conditions of intermittent reinforcement. They occur without any explicitly programmed contingency of reinforcement and develop a characteristic temporal distribution. Schedule-induced polydipsia (SIP) is generally considered a prototype par excellence. However, not all behaviors studied in these terms have so clearly been established as being schedule-induced behaviors. On example is schedule-induced wheel running (SIWR). Currently there are conflicting visions about its suitability as schedule-induced behavior. Based on this background, the aim of this thesis has been to study the development and maintenance of the SIWR, considering the characteristics of excessiveness, location and temporal distribution within the inter-food interval (IFI). This was achieved by manipulating the experimental conditions, to through two main studies, discussed in Chapters 2 and 3. A third study (Chapter 4) was based on the previous results obtained, and focused on studying another aspect of the schedule-induced behaviors. The reference to its excess as a characteristic feature in the diagnosis of psychopathological disorders related to impulse control. In Chapter 2, the development of the SIWR in rats under fixed-time (FT) schedules of different length (30, 60, 120, 240, and 480 s) was studied in a first experiment. The order of presentation was counterbalanced among the animals, (except for FT 480 s, which was presented at the end to complete the data set). Rats were also exposed to a massed-food control condition. The goal was to investigate the influence of intermittency reinforcement deliveries on maintenance of SIWR. The results showed that the SIWR was developed under the range of 30-240 s, but not the FT 480 s schedule, with a SIWR was a gradation as a function of inter-food interval (IFI) length. The data also shows that the wheel running adopts a post-reinforcing position within the IFI, presenting a temporary distribution in the form of an inverted Ushaped. Wheel running reduced in the control condition. Altogether, these data support the idea that wheel running is a schedule-induced behavior. In a second experiment, we proceeded to investigate more about the conditions under which the SIWR develops. In Experiment 2, the subjects were randomly divided into two groups. All animals were exposed to the same FT schedules as in Experiment 1, except for the FT 480-s. Half of the rats had a wheel in their home cages, while the other half had, a wheel and water bottle during the experimental session. The presence of the wheel in the home cage caused the reduction of the wheel running rate, as well as the shift of the temporal distribution curve to the right for the FT 30 and 60 s schedules. On the other hand, the availability of a water bottle in the conditioning chambers did not affect the wheel running rate, although the development of the SIP shifted the wheel running curve to a more central position within the IFI, resulting in temporary competition between behaviors. The results of this first study laid the groundwork for validating SIWR. The results were discussed by comparing them with SIP. In Chapter 3, the effect of operant contingencies over three behaviors, drinking, running and magazine entering was analyzed, according to Staddon’s classification (1977): interim, facultative and terminal behaviors, respectively. At first, the animals were exposed to an FT 60 s schedule for the establishment and acquisition of stable response rates. Rats were then randomly distributed in two groups. In both cases, the appearance of a protective contingency postponed the appearance of food when the animals performed any of the three responses during the last 1, 3, 5, 10, 20, 40 or 58 s of the IFI, under an FT 60- s schedule. For half of the animals the delay was signaled by the appearance of a tone and the extinction of the lights, while for the other group the delay was not signaled. In a final phase, the delays were eliminated. The average of the last three sessions of the acquisition condition was used as baseline. The results showed that the three behaviors are affected by protective delays based on their temporary location within the IFIs. Drinking was the behavior that most resisted the disruptive effects of delays, followed by wheel running and, finally, magazine entries. Magazine entries decreased, since the 1s delay, gradually due to the increase in the duration of the delay due to its close relationship with the reinforcer. The wheel running fell from the 1s delay but remained relatively stable with the increase in the duration of the delays. The effects of the delay on drinking were significant given the longer delays (40 and 58 s). The rates of all behaviors increased when delays were suppressed. However, no significant differences were found between signaled and unsignaled delays. These results indicate that the introduction of contingent delays to the response generates response gradients that reflect the effect of operating contingencies on the responses, showing different sensitivities depending on their temporal location within the IFI. In Chapter 4, impulsivity levels in sign-trackers (ST) and goal-trackers (GT) rats were studied through exposure to intermittent reinforcement schedules in the development of SIP (Experiment 1) and SIWR (Experiment 2) under different FTs, 5, 30, 60 and 120 s, and, 60 and 120 s, respectively; exposure to schedules was counterbalanced among animals. Subsequently, in Experiment 3, the levels of compulsivity of both groups were measured in an operant observing response task with uncertain reinforcement. Finally, in Experiment 4, cognitive impulsivity levels of both groups were recorded by a delay discounting procedure, the delay values for obtaining the greatest magnification were 5, 10, 20 and 40 s. The results showed the acquisition of SIP and SIWR in both groups, however, GT rats presented higher levels of drinking and wheel running than the ST group. While ST rats made more magazine entries than GT. Compulsivity and cognitive impulsivity levels did not differ between ST and GT. These results suggest that the schedule-induced behaviors are not related to the impulsivity traits as previously thought. Furthermore, SIP and SIWR behave as operant behavior, due to these behaviors being behaviors directed towards the goal. Together, these three studies offer a description of the wheel running as a schedule-induced behavior, presenting the same nature as the SIP, although with different sensitivity to delay to the reinforcer.