Long-range synchrony between medial prefrontal cortex, thalamus and hippocampus underlies working memory behavior in mice

Long-range synchrony between medial prefrontal cortex, thalamus and hippocampus underlies working memory behavior in mice. Pia-Kelsey O’Neill Presently, there are no antipsychotic drugs capable of treating the cognitive dysfunctions of schizophrenia. In order to inform the development of better therapies, it is essential to understand the mechanism behind dysfunctional cognition, which requires an understanding of functional cognition. Spatial working memory, a measure of cognitive function, can be assessed in the mouse using a task of delayed alternation: the T-maze. In this thesis, I focus on spatial working memory behavior in the mouse and three brain regions that are implicated in this behavior: the medial prefrontal cortex (mPFC), the hippocampus (HPC) and the medial dorsal thalamus (MD). Lesion and electrophysiological studies in each structure have demonstrated their importance during working memory behavior. Disconnection studies also show that the coordination between the mPFC and either the HPC or MD is important for the behavior, but little is known about the mechanism by which they coordinate. The MD and the ventral region of the hippocampus (vHPC) have robust projections into the mPFC. They are therefore in a good position to influence mPFC activity. Previous reports show that the mPFC and the dorsal region of the hippocampus (dHPC) synchronize activity in the theta range (4-12 Hz) with working memory demand. However, the dHPC does not directly connect with the mPFC so it is unclear how this coordination occurs. We hypothesized that the vHPC may also be involved in spatial working memory behavior and that it may mediate the dHPC-mPFC theta synchrony observed. To test these hypotheses, we recorded neural activity simultaneously from the mPFC, dHPC and vHPC in mice performing the T-maze task. Local field potential oscillations (LFPs), thought to be a measure of synchronized synaptic activity, were obtained from each area. We observed an increase in theta synchrony between the mPFC and both the dHPC and vHPC. Removing the influence of vHPC both analytically and experimentally, we found a decrease in synchrony of the dHPC-mPFC. Aside from the disconnection studies, little is known about the MD-mPFC pathway in rodents. However, due to evidence from schizophrenia patients of altered correlation specifically between the MD and PFC, we hypothesized that an electrophysiological correlate of working memory exists in the MD-mPFC pathway as well and that a decrease in MD activity may lead to prefrontal dysfunction. To test these hypotheses, we recorded LFPs from the mPFC and both single unit activity and LFPs from the MD in mice performing the T-maze task. We observed an increase in phase locking of MD cells to mPFC LFPs in beta (13-30Hz) range during the choice phase of the task. We then utilized a pharmacogenetic technique to decrease firing rate in a small portion of MD cells, which resulted in a deficit in both task acquisition and performance. The increase in MD-mPFC beta phase locking we had observed was not present in MDinactivated animals. Interestingly, beta coherence between the two structures across learning was highly correlated with choice accuracy on the task. This suggests that MD-PFC coordination is predictive of working memory performance. These findings illustrate how long-range synchrony of the mPFC with HPC in the theta frequency range and with the MD in the beta frequency range may be important markers for normal working memory behavior and if disrupted in humans, could contribute to the cognitive symptoms of schizophrenia.