Early life social experience affects adulthood fear extinction deficit and associated dopamine profile abnormalities in a rat model of PTSD

Abstract

Individuals may develop fear extinction deficits after life-threatening traumatic events; such deficits indicate posttraumatic stress disorder (PTSD). Because the occurrence of this disorder differs among people who have experienced trauma, hidden underlying factors should be determined. Increasing evidence suggests the involvement of neuronal dysregulation of information processes or cognitive function during development. This neuronal dysregulation is caused by disturbances in dopamine (DA) transmission within the fear circuit, which comprises the medial prefrontal cortex (mPFC), amygdala, and hippocampus. Single prolonged stress (SPS) combined with an isolation rearing (IR) paradigm was used to randomly assign rats to four groups [social rearing-no SPS (SR-NS), SR-SPS, IR-NS, and IR-SPS], and their performance in prepulse inhibition (PPI) and on Pavlovian fear conditioning tests was assessed. Tissue DA levels and the expression of DA receptors (D1R and D2R) in the fear circuit were measured at the end of the experiment. Our results indicated that PPI deficits and fear extinction problems were specific to rats subjected to IR and SPS, respectively. Furthermore, IR-induced PPI deficits were not influenced by SPS, but SPS-induced fear extinction retrieval impairment could be adjusted according to previous IR experiences. Neurochemically, tissue DA levels and D1R expression in the mPFC and amygdala were nonspecifically reduced by IR and SPS, whereas D2R expression in the mPFC and amygdala was higher in IR-SPS than in SR-SPS rats. These findings suggest that early life experiences may influence fear responses in adulthood through a change in DA profiles within the fear circuit.

Keywords: Dopamine; Fear memory; Isolation rearing; Posttraumatic stress disorder; Prepulse inhibition.