Comparison of progressive hold and progressive response schedules of reinforcement.

Progressive ratio (PR) schedules have been widely used to study motivation to work for a reinforcer. After a post-reinforcer pause, subjects engage in pressing a lever until a reinforcer is obtained. However, the discrete nature of lever presses allows alternative behaviors during inter-response time and has led to several behavioral explanations of pauses and work time. A progressive hold-down (PH) is incompatible with alternative responses and may allow a precise estimation of work time. Performance of rats trained in both PR and PH that received sucrose or intracranial self-stimulation (ICSS) as reinforcer were compared. We observed that rats mastered the PR and PH schedules. Post-reinforcer pauses (PSRP), work time and inter-reinforcer time increased as a function of the response or hold requirement. However, rats' performance suggested that the PH progression may be experienced by the rats as easier that the PR progression. Elimination of consummatory behavior with ICSS reduced post-reinforcer pause in accordance with predictions of explanatory models of fixed and variable schedules of reinforcement. In the case of PH performance, pauses showed little variation across intermediate requirements but increased rapidly on later requirements; since rats controlled their pause length and work time was close to the hold requirement, time allocation between PR and PH schedules diverged. Finally, the Mathematical Principles of Reinforcement model of Bradshaw and Killeen (Psychopharmacology 2012, 222: 549) rendered a good description of the performance in both PR and PH schedules.

Exposure to a hypercaloric diet produces long lasting changes in motivation.

Changes in motivation have been observed following induction of diet-induced obesity. However, to date, results have been contradictory, some authors reporting an increase in motivation to obtain palatable food, but others observing a decrease. Observed differences might be associated with the length of both the evaluation period and exposure to the diet. Therefore, the aim of this study was to evaluate changes in motivation during 20 weeks of exposure to a hypercaloric diet. Performance of the subjects in a progressive ratio schedule was evaluated before and during the exposure to a high-fat, high-sugar choice diet (HFHSc). A decrease in motivation was observed after 2 weeks of diet exposure, low levels of motivation remained throughout 20 weeks. A comparable decrease in motivation took longer (3 weeks) to develop using chow diet in the control group. Overall, our results suggest that, when changes in motivation are being evaluated, long periods of diet exposure made no further contribution, once motivation decreased, it remained low up to 18 weeks. Exposure to a HFHSc diet is a useful animal model of obesity, since it replicates some pathophysiological and psychological features of human obesity such as an increase in fasting glucose levels, body weight and the weight of adipose tissue.

Binge eating behavior and incentive motivation with a cafeteria diet.

Binge-like eating behavior (BLE) has been characterized as an eating disorder in which subjects have an enhanced intake of food, mainly fats. However, intake of fats and carbohydrates may have differential effects on motivation. Previously it was shown that BLE produces an increase in operant responding for vegetable shortening, but others were unable to replicate the finding using sucrose as the reinforcer. Our aim was to determine if BLE behavior induced with a cafeteria-like diet (CaLD) with several options with fat content would produce an increment in performance. Male Wistar rats were trained under an exponential progressive ratio schedule of sucrose reinforcement; thereafter, the limited access model was used to induce BLE using CaLD options. Finally, subjects were tested for increments in break points (BPs) in the progressive ratio schedule. Rats with intermittent access to CaLD options showed a clear BLE with an escalation in their intake; however they showed compensatory decrements of chow intake that rendered a similar body weight gain to a continuous access group. Although we were unable to observe an increase in BPs after BLE we were able to observe a protection against the decrements of BP previously observed with sugar. Different mechanisms for processing high fat and high carbohydrate reinforcers are variables worth exploring to gain a better understanding of BLE behavior in rodent models.

Dissociation between binge eating behavior and incentive motivation.

Binge-like eating behavior (BLE) has been characterized as an eating disorder in which subjects have an enhanced intake of food, mainly fats. However, intake of fats and carbohydrates may have differential effects on motivation. Previously it was shown that BLE produces an increase in operant responding for vegetable shortening. Our aim was to determine if BLE behavior induced with a sucrose solution would produce an increment in performance for sucrose reinforcers. Male Wistar rats were trained under an exponential progressive ratio schedule of sucrose reinforcement; thereafter, the limited access model was used to induce BLE. Finally, subjects were tested for increments in break points (BPs) in the progressive ratio schedule. We were unable to observe an increase in BPs after BLE. No increments in BPs were observed when a distinctive flavor (vanilla-flavored sucrose) was correlated with BLE induction and reinforcement, or when different types of ratio progression in the operant schedules were employed. However, rats adjusted their BPs according to reinforcer concentration after BLE induction, demonstrating that valuation (cost/benefit decision) of reinforcers was intact. Extent of training, alterations of reward processing after extended exposure to sucrose, and different mechanisms for processing high fat and high carbohydrate reinforcers are variables worth exploring to gain a better understanding of BLE behavior in rodent models.

Efecto de la ingesta tipo atracón sobre la preferencia hacia diferentes concentraciones de aceite de maíz en ratas / Effects of binge eating behavior upon the preference of rats for different corn oil solutions

Resumen La ingesta tipo atracón (binge eating behavior), es un desorden alimentario que se caracteriza por un sobreconsumo de alimentos sabrosos en períodos breves sin que haya un déficit energético. Previamente se ha reportado que es posible inducir ingesta tipo atracón en ratas empleando grasa vegetal; sin embargo, se ha descrito que el consumo de grasas depende de su forma de presentación. Los sujetos que presentan atracones incremental el consumo del alimento palatable con el que fueron entrenados, pero se desconoce su preferencia cuando se ofrecen otras alternativas con diferente contenido calórico. Los objetivos del presente trabajo fueron evaluar el establecimiento de la ingesta tipo atracón empleando una solución de aceite de maíz y, si hay cambios en la preferencia de los sujetos por diferentes concentraciones de aceite de maíz después de la inducción de ingesta tipo atracón. Para ello, se realizó una línea base de la preferencia de las ratas hacia diferentes concentraciones de aceite de maíz en condiciones de acceso libre al alimento, empleando la prueba de dos botellas. Posteriormente los sujetos fueron divididos en grupo control y experimental con base en su consumo de aceite y se indujo ingesta tipo atracón en el grupo experimental; finalmente, se reevalúo de nuevo la preferencia de los sujetos en condiciones de acceso libre y, posteriormente, de privación de alimento. Los resultados obtenidos, permiten concluir que es posible inducir ingesta tipo atracón con aceite de maíz; además, la inducción de la conducta atracón aumenta la preferencia por las concentraciones altas de aceite de maíz y la privación de alimento modula dicha preferencia. Estos hallazgos implican que la regulación energética se mantiene intacta a pesar de la inducción de la conducta de atracón.

Abstract Binge eating behavior is a food intake disorder characterized by an overconsumption of palatable food without an energy deficit. It has been reported the induction of binge eating with vegetable shortening, but also it has been reported that differences may exist on consumption depending upon the fat presentation (as vegetable shortening or corn oil). In addition, there is limited information on whether binge eating may modulate preference for options that differ on their caloric content. Therefore, main objectives were to determine whether is possible to induce binge eating using corn oil as the palatable food and whether preference for different oil concentrations is modified after binge induction. First, we determined the preference of the subjects to different oil concentrations; thereafter subjects were assigned to control (no binge induction) or experimental (binge eating induction protocol) on the basis of their oil consumption. Binge induction lasted one month and then a second two-bottle preference test was carried. Results showed that corn oil may be used as a palatable substance to induce binge eating behavior; also, it was observed an increase in preference for higher oil concentrations induced by binge, but preference was also modulated by deprivation conditions, suggesting normal regulation of caloric intake despite binge induction.

Persistencia Temporal de la Conducta de Atracón en un Modelo con Ratas / Temporal Persistency of Binge Eating Behavior in Rats

Resumen La conducta de ingesta tipo atracón, se define como un consumo considerable de alimento sabroso en un periodo reducido y discreto. Los modelos experimentales con roedores que se han enfocado en este patrón de conducta asumen implícita o explícitamente que se genera un cambio permanente en el consumo de alimento sabroso, no obstante, aún no se describe la persistencia del patrón de ingesta tipo atracón, lo cual resulta de interés ya que para evaluar cualquier tratamiento es necesario conocer por cuánto tiempo los sujetos continúan presentando atracones, por lo tanto los objetivos del presente trabajo fueron: a) determinar si la conducta de ingesta tipo atracón permanece estable después de su inducción con el acceso durante 2 horas una solución de sacarosa concurrente con el acceso al alimento estándar durante 24 horas y, b) evaluar si la presencia o ausencia de alimento estándar afecta el mantenimiento de la conducta de atracón. Con este fin se dio acceso diario durante 2 horas a la solución sacarosa al 10%, mientras el acceso al alimento estándar y agua fue libre. Se observó que en 25 días los sujetos desarrollaron el patrón de ingesta tipo atracón. La conducta de atracón se mantuvo estable a lo largo de ocho semanas; ni el acceso ni la privación de alimento estándar afectaron el mantenimiento de la conducta de ingesta de atracón y el patrón de ingesta del alimento estándar se mantuvo similar al del inicio de la inducción. La estabilidad de la conducta de atracón reproduce observaciones con humanos y posibilita el estudio a largo plazo de los cambios neuronales generados durante la inducción de la conducta de atracón.

Abstract Binge eating behavior has been defined as an increased intake of palatable food during a short time period. The experimental models with rodents that had studied this eating behavior had implicitly or explicitly assumed that the induction protocol produced a permanent change in palatable eating, although there is no description of the persistence of the behavioral pattern despite this pice of information may be needed to evaluate any therapeutic strategy. Therefore, present objectives were: a) determine whether binge eating behavior persist after its induction with a 2h access to sucrose solution concurrent to free access to rat chow food for 24 h and, b) determine whether the deprivation level of chow food modulates maintenance of binge eating behavior. To this aim, rats had a 2h daily access to 10% sucrose with concurrent access to ad lib food and water. It was observed that after 25 days subjects develop binge eating behavior. It was also observed that along 8 weeks at least, binge eating behavior was stable and neither ad lib access or food deprivation modulated binge eating behavior, that remained similar to the last induction day. Stability of binge eating behavior reproduced observations with human patients and may aloud the study of long term neural changes induced after binge induction.