Abstract

Deficits in fear extinction are exacerbated by exposure to stress and are seen in many psychopathologies, including posttraumatic stress disorder.  Connections from the medial prefrontal cortex to the basolateral amygdala are critical to fear extinction and are sensitive to both chronic and acute stress. Chronic stress impairs fear extinction recall, leads to retraction of apical dendrites and a decrease in spine density in medial prefrontal cortex, but induces dendritic growth in basolateral amygdala. Acute stress impairs fear extinction and causes dendritic retraction in both medial prefrontal cortex and basolateral amygdala.  We examined the effects of a single elevated platform stressor on fear extinction and spine density in rat infralimbic cortex. Spines were counted on 12 apical and 12 basilar terminal branches per rat, evenly distributed across hemispheres and cortical layers.  Fear extinction deficits were seen following acute stress, but there was no effect of stress on spine density. Fear conditioning and extinction, however, resulted in an overall decrease in spine density in apical dendrites, as seen across all spine types. Although acute stress did not alter spine density in medial prefrontal cortex, that conditioning and extinction was sufficient in decreasing spine density reveals the sensitivity of medial prefrontal cortex to behavioral manipulations.  In sum, medial prefrontal cortex is an important part of the fear conditioning and extinction pathway but stress may be acting differentially throughout the pathway to impair extinction.