Role of physiological patterns in hippocampal-prefrontal interactions

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Project Summary/ Abstract The hippocampus and medial prefrontal cortex (PFC) are both critical for learning and memory-guided behavior. Coordination of neural activity between these regions is necessary for memory and cognitive processes, however, the nature of these interactions and their roles are still unclear. We have established multiple timescale neural representations in these regions for spatial learning and memory-guided behavior, with inter-regional coordination during spatial firing and theta oscillations in exploratory behavior, and during sharp-wave ripple (SWR) associated replay in sleep and awake immobility periods. Here, we will investigate the roles of these physiological network patterns in learning and memory-guided navigation by combining behavioral methods in rats, high-density recordings, and causal manipulation methods. (Aim 1) The influence of reward on mnemonic representations is key to understanding the role of hippocampal-prefrontal activity patterns in memory-guided behavior. In particular, it is known that reward changes influence hippocampal replay, and replay is thought to play a role in linking actions to reward for reinforcement learning. To address this question directly, we have developed methods in TH-Cre rats for optogenetic tagging and identification of dopaminergic ventral tegmental area (VTA) neurons which encode reward prediction error, simultaneously with recordings from hippocampal and prefrontal ensembles. We will record and manipulate VTA dopamine neuron firing to examine the influence on replay and task representations in the hippocampal-prefrontal network during learning of new spatial rules. (Aim 2) The hippocampus and prefrontal cortex are both known to be important for contextual encoding and generalizing experiences across contexts, but the mechanisms are unclear. Our preliminary data show rule- selective encoding by prefrontal neurons that is maintained across spatial contexts, whereas hippocampal CA1 neurons remap across contexts. Further, we have also found coordinated hippocampal-prefrontal ripples vs independent cortical ripples with distinct reactivation patterns. We will examine whether hippocampal-prefrontal ensembles underlie rule representations and contextual encoding, whether theta sequences maintain encoding of current context, and test the hypothesis that replay during coordinated ripples enables associations across contexts. (Aim 3) Goal representations are considered central to navigation, but the mechanisms for goal coding and hippocampal-prefrontal representations for memory-guided navigation are still unclear. Our preliminary data using a complex 2-d maze with flexible goal locations and barriers supports goal representations and replay by hippocampal and prefrontal ensembles during navigation. We will test the hypotheses that hippocampal and prefrontal replay events during immobility support planning of upcoming trajectories, and goal representations impact theta sequences for online maintenance during trajectory execution. Overall, our results will provide novel insight into fundamental learning and memory mechanisms that are affected in aging and memory disorders such as Alzheimer’s disease.