Neonatal isolation decreases cued fear conditioning and frontal cortical histone 3 lysine 9 methylation in adult female rats

Early life stress is thought to enhance adult susceptibility to stress and stress-related mood disorders. In this study, fear-potentiated startle was used to model the acquisition of a traumatic event-related memory in female rats experiencing early life stress. Daily 1-hr maternal and sibling separation throughout day 2-9 postpartum (D2-9 PP) caused a decrease in the fear-potentiated startle, but not acoustic startle baseline, in adult female rats. The separation procedure did not affect corticosterone secretion but produced an increase in serum estradiol concentration. Moreover, the separation procedure did not affect histone 3 lysine 9 (H3K9) acetylation but decreased H3K9 mono- and tri-methylation in frontal cortices. Treatment with 5-aza-2′-deoxycytidine (AZA) (5 mg/kg at alternative days from D2PP to D9PP or 10 mg/kg at D5PP and D9PP), a DNA methylation inhibitor, did not affect the separation-decreased fear-potentiated startle. Treatment with valproic acid (VPA), a histone deacetylase inhibitor, at 3 dosing regimens (300 mg/kg at D2-9PP; 100 mg/kg at D2-4PP, 200 mg/kg at D5-7PP, 300 mg/kg at D8-9PP; 100 mg/kg at D2-5PP, 200 mg/kg at D6-9PP) prior to daily separation reversed such a decrease in fear-potentiated startle. The lowest effective VPA dosing regimen used (100 mg/kg at D2-5PP, 200 mg/kg at D6-9PP) reversed the separation-decreased H3K9 mono- and tri-methylation in frontal cortices. Eight-day VPA (300 mg/kg/day) and AZA (5 mg/kg/day) administrations starting at D28PP were ineffective in altering the separation-decreased fear-potentiated startle. We, hereby, suggest that decreased frontal cortical H3K9 mono- and tri-methylation may be involved in early life separation-decreased fear memory of adult rats.