Transient Inactivation of the Thalamic Nucleus Reuniens Produces Deficits of a Delayed Spatial Alternation Task

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University of Delaware
Electrophysiological evidence has shown that the hippocampus and prefrontal cortex functionally synchronize during working memory tasks in rodents, indicating that the two brain structures form a neural circuit that is important for working memory performance (Hallock et al., 2013; Gordon, 2011). Current hypotheses propose a possible time-dependent functional relationship between HPC and PFC, with results of functional inactivation studies suggesting that HPC and PFC act together during performance of tasks that require working memory over long delays, and operate in parallel or have dissociable functions over short delays (Churchwell & Kesner 2011). The nucleus reuniens of the thalamus (RE) is anatomically well positioned to gate the flow of information between HPC and PFC, and has been shown to be necessary for the performance of spatial working memory tasks (Hallock et al., in preparation; Hembrook & Mair 2010); however, the extent to which RE is necessary for the maintenance of information over both long and short delays remains unclear. Our lab first collected behavioral data from rats on a delayed-alternation Tmaze task (DA30) that requires spatial working memory over a 30-second delay (Hallock et al., 2013). DA30 requires rats to alternate between a left and a right goal arm to obtain a reward, relying on their previous choice of direction to determine the location of the next reward. We found that RE inactivation impaired DA30 performance (Hallock et al., 2013). To better understand the importance of RE in working memory during a delay, we inactivated RE prior to the performance of the same DA task without a delay – continuous alternation (CA). We found that performance on DA30 was significantly more impaired when compared to the performance on CA. This suggests that the dependence on RE for working memory tasks increases as the delay period for the tasks increases, providing evidence for a time-dependent component of RE activity during working memory tasks.