Brian J. Wiltgen, Ph.D. - US grants

DESCRIPTION (provided by applicant): These studies are designed to determine the role of alpha-CaMKII in memory formation. Specifically, the effects of alpha-CaMKII deletion in distinct hippocampus subregions (CA1, dentate gyrus) will be examined. Deletion of alpha-CAMKII will be accomplished using contemporary knockout technology and targeted viral-vector infusions. Electrophysiological analyses will determine the effects of alpha-CaMKII deletion on long-term potentiation (LTP), a cellular model of learning and memory. Behavioral analyses will examine the effects of alpha-CaMKII loss on specific mnemonic functions mediated by each hippocampus subregion. The ultimate goal of these studies is to further our understanding of the molecular mechanisms that underlie memory formation in humans. Hippocampal dysfunction caused by disease (e.g., Alzheimer's, Korsakoff's), aging or damage produces severe memory loss. Understanding the molecular mechanisms that underlie hippocampus-dependent memory formation should provide insight into disorders caused by the dysfunction of this brain region.

DESCRIPTION (provided by applicant): The goal of this project is to examine the motivational effects of reward cues on habitual responding. Habits are behaviors that have become unconscious and automatic as a result of repeated pairings with reinforcement (A. Dickinson, 1985;H. H. Yin and B. J. Knowlton, 2006). They are elicited reflexively by stimuli in the environment and are not responsive to feedback. Not surprisingly then, habits play a major role in addiction where maladaptive behavior persists despite the fact that drugs lose their rewarding effects over time and produce many unwanted, aversive consequences (P. W. Kalivas and N. D. Volkow, 2005;T. W. Robbins et al., 2008). Although reward cues are known to influence goal-directed actions, the extent to which they motivate habitual responding has not been well characterized (P. C. Holland, 2004). This process is critical to our understanding of addiction, as environmental cues associated with drugs are major contributors to relapse (B. J. Everitt and T. W. Robbins, 2005;P. W. Kalivas and N. D. Volkow, 2005). The current experiments will examine this motivational process using operant conditioning techniques in mice. The purpose is to determine if habits are particularly responsive to reward cues compared to goal-directed actions. Animals will be trained to press levers in order to obtain food rewards. For some animals these rewards will be delivered on variable interval schedules (where reward is contingent on responding after a specific amount of time has passed) that have been shown to promote habitual responding. Other animals will be trained on ratio schedules (where reward delivery is contingent on the number of responses made), which have been shown to promote the acquisition of goal-directed actions (Dickinson, Nicholas, &Adams, 1983;Yin, Knowlton, &Balleine, 2004). The motivational effects of reward cues will be modeled with two procedures: Pavlovian-instrumental transfer (PIT) and reinstatement (Wiltgen, Law, Ostlund, Mayford, &Balleine, 2007). In the PIT procedure, environmental cues acquire significance as predictors of reward through Pavlovian conditioning and are then able to exert motivational control over instrumental actions (Colwill, 1988). Reinstatement is an assay of outcome-mediated initiation of instrumental behavior in which delivery of the reward itself primes and motivates behavior after a period of extinction (Delamater, 1997;Ostlund &Balline, 2007). Our prediction is that habitual responding will be more susceptible to the effects of reward cues (i.e. increased rate of responding) than goal-directed actions. PUBLIC HEALTH RELEVANCE: This project will determine how automatic, unconscious behaviors called habits are motivated by reward cues in the environment. Habits are normally adaptive because they allow reliably reinforced behaviors to become efficient and automatic. However, they can also be maladaptive when they produce behaviors like those that maintain drug addiction. Therefore, understanding the mechanisms by which reward cues motivate habitual responding will lead to a better understanding of drug relapse and its underlying neural mechanisms.

? DESCRIPTION (provided by applicant): It is well established that new episodic and contextual memories are stored in the hippocampus. Over time, these memories are transferred to the cortex through a process called systems consolidation. This process is assumed to occur during periods of inactivity and sleep when the hippocampus replays newly acquired information. Replay is thought to drive the formation of intra-cortical connections that eventually allow memory to be retrieved without input from the hippocampus. Although these assumptions are widely accepted in the field, there is little direct evidence to support them. To address this significant gap in our knowledge, we will use recently developed genetic tools to: 1) identify and control hippocampal neurons that are active during learning and 2) determine if reactivation of these cells is necessary and sufficient for memory retrieval and long-term storage in the cortex. We will accomplish these goals by using newly generated transgenic mice to permanently label neurons that are active during learning. Tagging these cells will allow us to identify networks in the hippocampus and cortex that encode memory and follow their activity during the consolidation period. Next, we will use optogenetic and pharmacogenetic tools to control the activity of labeled hippocampal neurons and determine the effects on long-term memory storage in the cortex. Standard models of consolidation predict that hippocampal stimulation will reactivate cortical neurons that were tagged during learning and induce long-term storage. In contrast, silencing hippocampal ensembles after learning should prevent consolidation and induce amnesia. Our experiments will either: a) substantiate these long-held assumptions and provide mechanistic insight or b) refute these assumptions and provide a new framework for understanding the contributions of the hippocampus to memory consolidation.

Project Summary Human and animal data indicate that the hippocampus encodes events and the spatial context in which they occur. To understand how this is accomplished, we will use a systems-level approach that compares and contrasts the function of specialized microcircuits within the mouse hippocampus. We will focus on the proximal and distal segments of CA1, which are thought to encode an animal?s spatial location and the location of objects in the environment. To control the activity of these regions during behavior, we will develop new genetic tools that can be used to express the inhibitory opsin ArchT in specified segments of CA1. Targeted laser stimulation will then be used to silence these segments during newly developed place and object learning tasks. We predict that visuospatial learning will require activity in proximal CA1 while object learning will require activity in distal CA1. The information gained from these analyses will improve our models of hippocampal organization and allow us to better understand how this structure stores and retrieves distinct types of memory. In addition, these tools can be used in mouse models of human disease to modify activity or control gene expression in distinct segments of the hippocampus.