Role of the ventral subiculum-to-nucleus accumbens circuit in reinforcement and choice of delayed outcomes.

摘要

In order for organisms to survive in changing environments with limited resources, it is essential that they form and maintain associations between their actions, environmental stimuli, and biologically salient outcomes. Moreover, these associations motivate organisms to continue beneficial activities, and are used to guide decision making between actions for different potential outcomes. Half a century of research has identified a distributed brain reward network centered around the nucleus accumbens (NAc) that is thought to mediate these behavioral mechanisms, however with classic techniques such as electrical stimulation and pharmacology it was impossible to delineate the function of specific inputs to the NAc. The experiments described here take advantage of recently developed optogenetic techniques to selectively stimulate the strong glutamatergic projection from the ventral subiculum (vSUB) to the shell subregion of the NAc (NAcSh) in order to characterize the functionality of this pathway in motivated behavior. First, this procedure was used to determine the general reinforcing properties of vSUB-NAcSh pathway stimulation as compared to broad stimulation of vSUB cell bodies. The results of these experiments revealed a disconnection between the role of the vSUB-NAcSh pathway and vSUB cell bodies in reinforcement. Rats readily self-stimulated the vSUB-NAcSh pathway, and would regularly escape prolonged pathway stimulation, but these effects were not produced by vSUB cell body stimulation. In a second set of experiments, this procedure was applied to evaluate the precise role of vSUB-NAcSh connectivity in delay-based decision making. Here, vSUB-NAcSh pathway or vSUB stimulation was delivered during cues or following responses while animals performed a simple delay choice task for immediate and delayed rewards of the same objective value. However, despite previous reports that both the vSUB and NAc are involved in delay-based decision making, no stimulation parameter was able to influence behavioral performance or outcome preference on this task. These results suggest animals do not need information transmitted between the vSUB and NAcSh during periods associated with the use and maintenance of behavior-outcome representations to make simple delay-based decisions. Instead, it is proposed that the vSUB-NAcSh pathway is only necessary for more complex decision making.