台大心理系

The Nucleus Reuniens (RE) of the midline thalamus interconnects the hippocampus (HPC) and the medial prefrontal cortex (mPFC). Several studies have suggested that the RE participates in various cognitive tasks. However, little is known about the contribution of the RE in Pavlovian trace fear conditioning, a procedure with a temporal gap between the conditioned stimulus (CS) and the unconditioned stimulus (US), and therefore making it harder for the animals to acquire. Because the HPC and mPFC are involved in trace, but not delay, fear conditioning and given the role of the RE in mediating this neurocircuitry, we hypothesized that RE inactivation leads to a learning deficit only in trace conditioning. In a series of experiments, we first examined the c-Fos expression in male Long-Evans rats and established that the RE was generally recruited in the encoding, but not the retrieval phase, of fear memory. Next, we performed behavioral pharmacology experiments and in line with the c-Fos results, RE inactivation impaired only the acquisition, but not the consolidation or retrieval, of trace fear. However, although the RE was recruited during the encoding of delay fear demonstrated by c-Fos results, RE inactivation in any phases did not interfere with delay conditioning. Finally, we found that trace fear acquired under RE inactivation reprised when the RE was brought off-line during retrieval. Together, our data revealed the essential role of the RE in a learning task with temporally discontinuous stimuli.