Parvalbumin-positive interneurons in the medial prefrontal cortex regulate stress-induced fear extinction impairments in male and female rats.

Stress has profound effects on fear extinction, a form of learning that is essential to behavioral therapies for trauma- and stressor-related disorders. Recent work reveals that acute footshock stress reduces medial prefrontal cortex (mPFC) activity that is critical for extinction learning. Reductions in mPFC activity may be mediated by parvalbumin (PV)-containing interneurons via feed-forward inhibition imposed by amygdala afferents. To test this hypothesis, footshock stress-induced Fos expression was characterized in PV+ and PV- neurons in the prelimbic (PL) and infralimbic (IL) cortices. Footshock stress increased the proportion of PV+ cells expressing Fos in both male and female rats; this effect was more pronounced in IL compared to PL. To determine if PV+ interneurons in the mPFC mediate stress-induced extinction impairments, we chemogenetically silenced these neurons prior to an immediate extinction procedure in PV-Cre rats. Clozapine- N -oxide (CNO) did not affect conditioned freezing during the extinction procedure. However, CNO exacerbated extinction retrieval in both male and female rats with relatively high PL DREADD expression. In contrast, in rats with relatively high IL DREADD expression, CNO produced a modest facilitation of extinction in the earliest retrieval trials, but in male rats only. Conversely, excitation of IL PV interneurons was sufficient to impair delayed extinction in both male and female rats. Finally, chemogenetic inhibition of IL-projecting amygdala neurons reduced the immediate extinction deficit in male, but not female rats. These results reveal that PV interneurons regulate extinction learning under stress in a sex-dependent manner, and this effect is mediated by amygdalo-prefrontal projections. Significance Statement: Stress significantly impairs the memory of fear extinction, a type of learning that is central to behavioral therapies for trauma- and anxiety-based disorders (e.g., post-traumatic stress disorder, PTSD). Here we show that acute footshock stress recruits parvalbumin (PV) interneurons in the medial prefrontal cortex (mPFC) of male and female rats. Silencing mPFC PV interneurons or mPFC-projecting amygdala neurons during immediate extinction influenced extinction retrieval in a sex-dependent manner. This work highlights the role for PV-containing mPFC interneurons in stress-induced impairments in extinction learning.