The Nucleus Accumbens: Involvement in the Modulation of Memory for Arousing Events

Some of the key factors that contribute to many memory disorders involve either a reduced or exaggerated capacity to experience arousal. These outcomes are related to impaired neurotransmitter release in brain areas that process memory such as the amygdala and hippocampus. Although there are multiple pathways that emotionally arousing information may be consolidated into memory, very little attention has been devoted to identifying how norepinephrine release in the nucleus accumbens contributes to this process. Of particular interest to this project is the shell region of the accumbens because it receives noradrenergic terminals exclusively from neurons in the nucleus of the solitary tract (NTS) that convey information regarding increased peripheral autonomic and neuroendocrine activity in response to emotionally arousing events. Noradrenergic input from the NTS may contribute to enhanced memory for emotional events by increasing responsiveness of accumbens neurons to the constellation of inputs transmitted to this area from the amygdala and the hippocampus. A major theme of the proposed studies is that the multitude of inputs that converge upon the accumbens during learning, places this structure in an ideal position to integrate and bind information regarding the individual features of learned events into memory storage. Thus, studies discussed in the dissertation were developed to examine specific elements of this central hypothesis.

The dissertation consists of five main chapters. The first presents background literature representing the foundation and rationale for conducting the proposed studies. Chapter 2 examines whether a functional relationship exists between noradrenergic A2 neurons and the nucleus accumbens shell in processing memory for emotional events. Findings from this chapter are the first to identify the specific noradrenergic receptor subtype in the nucleus accumbens shell that mediates the beneficial actions on memory produced by chemical stimulation of NTS neurons.

Experiments in Chapter 3 were integral in demonstrating that accumbens shell neurons play a fundamental role in consolidating converging information initially processed by the amygdala and hippocampus. These experiments also reveal that separate limbic structures provide a unique contribution in creating different representations in memory of an emotionally arousing event. Chapter 4 further investigates the integrative role of the accumbens in processing limbic information. These studies addressed a possible mechanism by which glutamate released from amygdala or hippocampal inputs may facilitate noradrenergic neurotransmission in the accumbens shell to modulate memory storage. Collectively, these findings indicate that, along with the amygdala and hippocampus, the shell division of the nucleus accumbens provides a critical contribution in integrating and consolidating information following learning. More importantly, the data presents a mechanism by which information emanating from these key limbic structures interacts with noradrenergic signals from the NTS. Implications and significance of these studies are discussed in Chapter 5.