Effect of hippocampal feedback projections on the function of the early olfactory system - Project Summary/Abstract The neural circuits associated with encoding odors in patterns of activity throughout the brain are critical not only for chemosensory perception, but are intimately linked to emotion, learning and memory in humans and mice, the major animal model for studying disorders that impact chemosensory function. We propose to study the circuit between the ventral CA1 (vCA1) region of the hippocampus and the main olfactory bulb (MOB) to between understand how cognitive aspects of behavior, such as a sense of environment, or anxiety can influence the encoding of chemical signals. Improved understanding of these circuits could provide critical insight into the link between chemosensory function and neurological, neurodegenerative, and neuropsychiatric disorders. Mitral and tufted cells (M/T) are the principal neurons of the main olfactory bulb (MOB) in rodents, and relay chemosensory information they receive to a number of cortical regions including the anterior olfactory nucleus (AON) and the piriform cortex. Rather than active as passive relays of this information, mitral and tufted cell activity is reshaped in part by local inhibitory interneurons and by extensive feedback projections from a number of areas including olfactory cortex and the hippocampus. We have recently begun exploring the structure of one of these feedback circuits, projections from the pyramidal cells of the ventral CA1 (vCA1) region of the hippocampus. Although these connections were identified nearly a ½ century ago, little is known about their function. In this proposal, we will investigate the overall hypothesis that vCA1 projections relay behaviorally relevant cognitive information (representing where the animal is, the anxiety associated with that location in the environment, etc.) to the MOB. Using a combination of electrophysiology and behavioral measures, we will ask three critical questions about the vCA1->MOB circuit. (1) What behavioral information do the vCA1 neurons that project to the bulb encode? (2) What are the synaptic networks that link vCA1 to the M/T cells in the bulb? (3) What if any of these behavioral features are represented in the bulb and how does this affect chemosensory processing? Combining these studies will provide critical insight into the circuits that allow behavior to shape chemosensory perception.
