Reinforcement omission effects in rats with bilateral lesions in the substantia nigra pars compacta and ventral tegmental area

Reinforcement omission effects (ROEs) are characterized by higher response rates after reinforcement omission than after reinforcement delivery. This pattern of behavior is interpreted in terms of motivational and attentional processes. Recent studies from our laboratory have shown that the amygdala, nucleus accumbens, and medial prefrontal cortex are involved in ROE modulation. Also, the literature has demonstrated a role of other areas such as substantia nigra pars compacta (SNc) and the ventral tegmental area (VTA) in processes related to surprising events, such as prediction error and presentation or omission of an event (exteroceptive stimulus and reinforcement). Since these structures send projections to areas related to ROE modulation such as the amygdala, nucleus accumbens, and prefrontal cortex, the objective of the present study was to determine whether the SNc and VTA also integrate the circuit involved in ROE modulation. Rats were trained on a fixed-interval 12 s with limited-hold 6 s signaled schedule of reinforcement (Pre-lesion training). After acquisition of stable performance, the rats received bilateral neurotoxic lesions of the SNc (Experiment 1) and VTA (Experiment 2). Following postoperative recovery, the rats were submitted to two refresher sessions (Post-lesion training). Subsequently, the training was changed from a 100 to a 50% schedule of reinforcement (Post-lesion testing). In both experiments, the results showed that there was no difference in performance between sham rats and rats with bilateral lesions of the SNc or the VTA.

Lesions of the orbitofrontal cortex do not affect the reinforcement omission effect in rats

The reinforcement omission effect (ROE), reflected by response rates that are higher after reinforcement omission than after reinforcement delivery, has been attributed to both motivational and attentional consequences of the surprising reinforcement omission. These processes depend on the operation of separate amygdala areas and their connections with other brain systems. The interaction between the amygdala and orbitofrontal cortex has been suggested to be important in the modulation of motivational processes. The present study sought to verify whether the mechanisms involved in the ROE depend on the integrity of the orbitofrontal cortex. Prior to acquisition training, rats received bilateral excitotoxic lesions of the orbitofrontal cortex or sham lesions. Following postoperative recovery, the rats were trained on a fixed-interval 12 s limited-hold 6 s signaled schedule of reinforcement. After the acquisition of stable performance, the training was changed from a 100% to 50% schedule of reinforcement. The results showed that rats in both groups exhibited the ROE, with no differences in performance between groups following nonreinforcement. These data do not support the hypothesis that the orbitofrontal cortex is included in the neural substrates related to ROE modulation. The results also showed no difference in response rates between groups in the periods that preceded and followed nonreinforcement. These findings confirm previous studies that showed that the ROE is not related to the facilitation of behavior induced by nonreinforcement...

Operant-Behavior Tests in Rats Prenatally Exposed to Low Level Methylmercury / 环境与健康杂志

Objective To establish the computer controlled operant behavior test system for rats,and to explore the effects of prenatal exposure to low level methylmercury on the ability of learning and memory in rats Methods The Wistar rats were perorally perfused by 0,0.01,0.05,2.00 mg/(kg?d) methylmercury chloride(MMC) during the period from the 6th day to the 9th day after conception respectively.In every dose group,8 offspring aged 10 weeks were randomly selected to be tested by operant behavior test system.The operant performance was measured in automated nocturnal 15 hour session(16∶00~7∶00).The trial pragram Differential Reinforcement of High (DRH) rate and of low (DRL) rate was carried out by a double blind method.The DRH tests required observed animals to press levers frequently,while the DRL test required the subjects to respect a blocking interval after a reinforcement for pressing lever again. Results The performances scores of 3 exposure groups at doses of 0.01,0.05 and 2.00 mg/(kg?d) methylmercury were significantly lower than that of the control group( P