The medial prefrontal cortex is both necessary and sufficient for the acquisition of conditioned defeat

We have previously demonstrated that the basolateral amygdala (BLA) is a key component of a neural circuit mediating memory formation for emotionally relevant stimuli in an ethologically-based model of conditioned fear, termed conditioned defeat (CD). In this model, subjects are socially defeated by a larger, more aggressive hamster. Upon subsequent exposure to a smaller, non-aggressive intruder, the defeated animal will show high levels of submissive behaviors and fail to defend its territory. Here we examined whether the medial prefrontal cortex (mPFC), an area with extensive connections with the amygdala, is also a component of this circuit. Temporary inactivation of the mPFC using muscimol, a GABA(A) receptor agonist, significantly enhanced the acquisition but not expression of CD, while blockade of GABA(A) receptors in the mPFC using bicuculline, a GABA(A) antagonist, impaired acquisition of CD. Given these findings, we next sought to test whether plasticity related to the defeat experience occurs in the mPFC. We infused anisomycin, a protein synthesis inhibitor, in the mPFC but this treatment did not alter the acquisition of CD. In our final experiment, we demonstrated that bicuculline failed to alter the acquisition of CD. Together, these results demonstrate for the first time that while the mPFC is both necessary and sufficient for the acquisition of CD, it does not appear to mediate plasticity related to the defeat experience. In contrast, while plasticity underlying CD does appear to occur in the BLA, GABAergic receptor inhibition in the BLA is not sufficient to enhance CD. This article is part of a Special Issue entitled 'Post-Traumatic Stress Disorder'.