Brief Communication Extinction partially reverts structural changes associated with remote fear memory

Structural synaptic changes occur in medial prefrontal cortex circuits during remote memory formation. Whether extinc-tion reverts or further reshapes these circuits is, however, unknown. Here we show that the number and the size of spines were enhanced in anterior cingulate (aCC) and infralimbic (ILC) cortices 36 d following contextual fear conditioning. Upon extinction, aCC spine density returned to baseline, but the enhanced proportion of large spines did not. Differently, ILC spine density remained elevated, but the size of spines decreased dramatically. Thus, extinction partially erases the remote memory network, suggesting that the preserved network properties might sustain reactivation of extinguished con-ditioned fear. The observation that dendritic spine growth develops in the anterior cingulate cortex during the formation of enduring fear memory (Restivo et al. 2009) has put forward the idea that in addition to changes in the strength and efficacy of existing syn-apses, large-scale modifications in prefrontal cortical connec-tivity, including the anterior cingulate cortex (aCC) and the infralimbic cortex (ILC), also underlie systemsmemory consolida-tion (Frankland et al. 2004). Nonetheless, how long the content o