Repeated stress exposure in mid-adolescence attenuates behavioral, noradrenergic, and epigenetic effects of trauma-like stress in early adult male rats

Abstract

Stress in adolescence can regulate vulnerability to traumatic stress in adulthood through region-specific epigenetic activity and catecholamine levels. We hypothesized that stress in adolescence would increase adult trauma vulnerability by impairing extinction-retention, a deficit in PTSD, by (1) altering class IIa histone deacetylases (HDACs), which integrate effects of stress on gene expression, and (2) enhancing norepinephrine in brain regions regulating cognitive effects of trauma. We investigated the effects of adolescent-stress on adult vulnerability to severe stress using the single-prolonged stress (SPS) model in male rats. Rats were exposed to either (1) adolescent-stress (33-35 postnatal days) then SPS (58-60 postnatal days; n = 14), or (2) no adolescent-stress and SPS (58-60 postnatal days; n = 14), or (3) unstressed conditions (n = 8). We then measured extinction-retention, norepinephrine, HDAC4, and HDAC5. As expected, SPS exposure induced an extinction-retention deficit. Adolescent-stress prior to SPS eliminated this deficit, suggesting adolescent-stress conferred resiliency to adult severe stress. Adolescent-stress also conferred region-specific resilience to norepinephrine changes. HDAC4 and HDAC5 were down-regulated following SPS, and these changes were also modulated by adolescent-stress. Regulation of HDAC levels was consistent with the pattern of cognitive effects of SPS; only animals exposed to SPS without adolescent-stress exhibited reduced HDAC4 and HDAC5 in the prelimbic cortex, hippocampus, and striatum. Thus, HDAC regulation caused by severe stress in adulthood interacts with stress history such that seemingly conflicting reports describing effects of adolescent stress on adult PTSD vulnerability may stem in part from dynamic HDAC changes following trauma that are shaped by adolescent stress history.

Figure 1

Timeline of procedures. Repeated variable stress consisted of repeated, unpredictable exposure to visual, olfactory, and auditory predation cues (a swooping hawk model, fox urine, large cat vocalizations). The single prolonged stress (SPS) model is defined by three stressors in succession (restraint, forced swim, ether exposure). Following SPS, rats were socially isolated for seven days. During fear learning testing, rats were first trained to associate a tone with a shock, then the tone was repeatedly presented in a novel context to facilitate extinction learning. Rats were then returned to the second context to test the retention of extinction learning. The day following extinction retention testing, brains were collected for region-specific measurement of HDAC4, HDAC5, NE, and NPAS4. All groups were age-matched for SPS, fear learning, and time in the laboratory; groups 2 and 3 were reared in the laboratory under control conditions.

Figure 2

Effects of repeated variable stress during mid-adolescence on fear cognition after traumatic-stress in adulthood. (A) All rats showed equivalent fear learning regardless of stress history. (B) Fear behavior during fear extinction learning was heightened in rats exposed to SPS stress in adulthood compared with control rats (^#P < 0.05). Exposure to the combination of adolescent-stress and adult SPS increased freezing in the early phase of fear extinction learning, compared with control rats (⁺P < 0.05), but this effect abated over time and was not present during the second half of extinction learning. (C) Exposure to adult SPS without prior adolescent-stress enhanced fear behavior during both extinction retention test phases (^#P < 0.05). Conversely, the combination of repeated variable stress in adolescence and adult SPS enhanced the retention of safety information during the late phase of extinction retention compared with traumatic stress alone (^{^}P < 0.05).

Figure 3

Norepinephrine levels in brain regions mediating fear cognition in adult male rats that were exposed to either repeated variable stress in adolescence followed by SPS, only age-matched SPS exposure, or unstressed control conditions. (a) indicates an effect of adolescent-stress exposure; (b) indicates an effect of SPS exposure in adulthood; (*P < 0.05, ⁺P = 0.11). In the prelimbic cortex of young adult rats, following extinction retention testing, norepinephrine levels were elevated by the combination of repeated variable stress and adult SPS but were not affected by adult SPS alone. In the striatum, adult SPS elevated norepinephrine, but this effect was reversed by prior exposure to repeated variable stress in mid-adolescence. In the hippocampus, exposure to adult SPS decreased levels of norepinephrine; there was a trend towards a norepinephrine increase from adolescent-stress exposure.

Figure 4

Histone deacetylase (HDAC) 4 and 5 levels in brain regions mediating fear cognition in adult male rats that were exposed to either repeated variable stress in adolescence followed by SPS in adulthood, only age-matched SPS exposure, or unstressed control conditions. (a) indicates an effect of adolescent-stress exposure; (b) indicates an effect of adult SPS exposure; (*P < 0.05, ⁺P = 0.10). In the prelimbic cortex, SPS exposure decreased levels of HDAC4 and HDAC5, while prior exposure to repeated adolescent-stress mitigated these effects (HDAC4 levels were shaped by exposure to adolescent-stress; HDAC5 levels were decreased by SPS). In the hippocampus, HDAC4 and HDAC5 levels were decreased by SPS, but this effect was reversed by prior adolescent-stress. In the striatum, exposure to repeated variable stress in mid-adolescence followed by SPS increased HDAC4 and HDAC5, whereas SPS alone decreased HDAC4 and HDAC5; however, only adolescent-stress had a statistically detectable effect. In the amygdala, neither mid-adolescent stress nor SPS had detectable effects on HDAC levels (P > 0.15).