This study aims to investigate the process of “memory replay”, a phenomenon in which a particular neuronal firing pattern seen during learning is reinstated during sleep. Rats will learn to run specific spatial sequences within an associated context. At various points in the training paradigm, any time between a task being completely novel versus quite familiar, the hippocampus will be inactivated via muscimol, a GABA receptor agonist, or infused with saline by way of chronically implanted cannulae. To assess the degree of reactivation during subsequent sleep, electrical activity of the retrosplenial cortex will be measured by way of a chronically implanted microelectrode array or “hyperdrive” with independently movable tetrodes. In addition to cortical reactivation, the rat’s performance on the task the following day will be analyzed. By inactivating the hippocampus at various time points within the training schedule, we should be able to deduce at what time cortical reactivation and/or maze performance becomes hippocampal independent.
For this project I am assisting in surgeries, cannulae and stimulating electrode fabrication, experimental design, behavior training and testing, and data analysis. During the rat’s initial surgery, guide cannulae are implanted bilaterally into the hippocampus and stimulating electrodes are implanted bilaterally into the medial forebrain bundle, a region that upon stimulation releases dopamine and gives the rat a rewarding sensation. After the first surgery the rats are trained with this positively reinforcing brain stimulation to run to a blinking light to assess the effectiveness of the stimulation and the rat’s ability to learn. Once the rat has been thoroughly trained, another surgery is performed to install the “hyperdrive”. During the experimental tasks the rat will sleep, run the assigned sequence, then sleep again, all while neural activity is being recorded. Cortical activity will be compared between pre and post-task sleep as well as across subsequent days of learning the same sequence to determine the effects of hippocampal inactivation at various time points in the trials.