| Summary: | The nucleus accumbens is a heterogeneous brain structure involved in the integration of limbic and cortical input and the coordination of motor output during behavior. Made up primarily of two major subregions, the nucleus accumbens core (NAcC) and shell (NAcS), this region has been suggested to contribute to dissociable aspects of appetitive behavior on the basis of differential functions localized within these subregions. Briefly, the NAcC may promote states of behavioral action during reward-seeking, while the NAcS may refine such behavior by actively inhibiting inappropriate or irrelevant actions. In Chapter 1, we discuss relevant research related to the dissociability of the NAcC and NAcS at the circuit and behavioral levels. In Chapters 2, 3, and 4, we examine the contribution of these two NAc subregions, as well as associated cortical and limbic structures, to Pavlovian and instrumental suppression. Results suggested that the NAcC acted to promote behavioral indices of reward-seeking vigor, while the NAcS was necessary for the appropriate instantiation and expression of conditioned suppression. In Chapter 5, we probed the relevance of these NAc subregions to the performance of a novel active/passive avoidance behavior. On this task, rats had to dynamically promote or inhibit their responding, guided by discrete cues, to avoid a painful stimulus. While both NAc subregions were necessary for promoting behavior during active avoidance trials, only the NAcS was required for inhibiting responding during presentations of the passive avoidance stimulus. A control study suggested that neither NAc subregion was necessary for unconditioned responding to foot-shock, indicating that the previous results could not be explained by changes in pain sensitivity. We also probed the role of monoaminergic transmission to motivational conflict and active/passive avoidance by systemically administering d-amphetamine (AMPH) to a subset of animals in Chapter 3 and 4. These results suggested that AMPH promoted punishment induced inhibition of behavior during motivational conflict, but had the opposite effect during passive avoidance trials, inducing pressing despite punishment. Chapter 5 discusses these results in the framework of a dichotomy between response-promotion and response-inhibition, relating these findings to extant literature in the appetitive and aversive domains. === Arts, Faculty of === Psychology, Department of === Graduate
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