Neural Activity in the Mediodorsal Thalamus around Hippocampal ripples

Objectives: Highly synchronized population bursts, or ripples (~150Hz), in hippocampus (HPC) promote replay of recently acquired memories and facilitate information transfer to cortex for long term storage. Aside hippocampal-cortical interactions, the activity within a larger scale neural network supporting declarative memory consolidation is essentially unknown. Mapping of the whole brain activity around the times of ripple occurrence revealed a very characteristic pattern of positive BOLD responses in many cortical and limbic structures and negative BOLD responses in a number of subcortical regions, including sensory and association thalamus (Logothetis, 2012). Mediodorsal thalamus (MD) has been long implicated to play a role in mnemonic functions. The MD receives input from the entorhinal cortex and has dense reciprocal connection with the medial prefrontal cortex (mPFC). We would like to characterize how thalamic neural activity contributes to hippocampal-cortical dialogue underlying memory consolidation. Purpose: In the present study, we sought to characterize in detail the ripple-associated neural activity in the MD. Methods: We recorded broad-band (0.1Hz – 8kHz) extracellular activity in the MD, HPC, and mPFC in behaving rats. Results: The MD firing rate was suppressed ± 2.34 sec around ripple peaks (63 compared to the baseline). The modulation profiles differed for the ripples occurring during awake or sleep. The temporal window of modulation was much narrower during ‘awake’-ripples (± 1.74 sec compared to ± 2.43 sec during sleep). We further subdivided ripples depending on the presence or absence of co-occurring sleep spindles in the mPFC. Notably, the MD activity was not suppressed when ripples were coupled with sleep spindles. Conclusions: Our results support the hypothesis that thalamic suppression during ripples may reduce interference for hippocampal-cortical interaction, yet suggest that there may be temporal windows when thalamic activity plays a functional role for cross-regional communication underlying memory