Introduction: The mechanism underlying post-stroke cognitive impairment is not well understood. The hippocampus is involved in memory function yet remains structurally intact after cortical stroke. During the slow-wave state, the hippocampus exhibits Sharp-wave associated ripples (SPW-Rs), a high-frequency activity in the pyramidal layer of the hippocampal CA1 that plays a critical role in memory consolidation. However, it is unclear whether cortical stroke chronically affects SPW-Rs or any other hippocampal oscillations.
Hypothesis: Stroke affects network oscillations that bind neurons into transient assemblies with coherent activity.
Methods: Adult male rats (n=28) were subjected to distal middle cerebral artery occlusion (dMCAO), compared to controls (n=19) by sham-operation. After time periods of either two weeks or one month, we recorded the neural activity from different depths starting at deep cortical layers down to hippocampus using 16-site linear electrode arrays under urethane anesthesia. We defined two states of high theta (HT) and low theta (LT) by calculating the ratio of Hilbert amplitudes of theta (4-7 Hz) to delta (0.1-3 Hz) filtered local field potential (LFP) recorded from the CA1 stratum lacunosum moleculare. A threshold was manually set to capture sustained periods in which the theta:delta ratio was elevated. Periods of SWRs were also detected from the filtered LFP (150-250Hz) recordings of CA1 pyramidal layer.
Results: Chronic stroke increased the frequency of alternation between HT and LT states by shortening the duration of HT states. Stroke also increased the rate of occurrence of SWRs during LT state. Furthermore, chronic stroke at one month reduced theta and delta power at the hippocampal pyramidal layer and the cortex ipsilaterally, while it reduced gamma power bilaterally in the pyramidal layer during the HT states.
Conclusions: Ischemic stroke affects brain states and several major oscillations not only in the cortex but also in the hippocampus. The aberrant brain oscillation may disrupt neural network activity that gives rise to complex cognitive behaviors, underlying post stroke cognitive impairment.