• Early life adversity is detrimental to psychological and somatic health across the lifetime.

• Individuals who have experienced early life stress (ELS) show heightened vulnerability for posttraumatic stress disorder (PTSD) following trauma exposure later in life. 

• Acute early life stress (aELS) can be modeled in rodents using footshock exposure on a precise day in early development.

• Incubation of fear is an increase in fear expression over time following fear learning. Since adult fear memory is typically stable over time, observations of incubation may be seen as maladaptive.

• We have previously demonstrated that mice who experience aELS demonstrate fear incubation in adulthood. Importantly, mice not exposed to aELS do not show fear incubation. 

• The present study aimed to replicate the aELS-induced adult fear memory incubation effect in male and female rats.

Hypothesis 

Acute ELS will yield fear memory incubation in adult male and female rats. 

Early Life Stress (ELS): Male and female rats were assigned to either no-aELS or aELS. The aELS exposure consisted of 15 footshocks (1 mA, 1 sec) across a 93 minute session in a novel conditioning chamber (Context A). No-aELS rats were placed into the conditioning chamber for 93 minutes without footshock delivery. All rats were individually isolated for 90 minutes following the aELS session to reduce potential for social buffering of stress (see Reichert et al., 2025).

Adult Fear Conditioning: In adulthood, rats were placed into a novel conditioning chamber (Context B) for 3.5 minutes where they received one mild footshock (0.4 mA, 1 sec). 

Adult Fear Test: Fear memory was assessed during an 8-minute footshock-free session either 1 day or 30 days following adult fear conditioning. Rats were placed into the same conditioning chamber they received fear conditioning in. Freezing during each minute and average freezing across the 8-minute session were analyzed.

Intracardial Perfusion and Brain Harvesting: Ninety minutes following the start of the adult fear test, rats were intracardially perfused with phosphate buffered saline followed by 4% paraformaldehyde. Brains were extracted and stored for subsequent immunohistochemical analyses.