Sexually dimorphic effects of single prolonged stress on conditioned suppression behavior in Long Evans rats with no detection of altered mRNA expression of VGLUT1, GAD1, and Arc in the ventral hippocampus.

Introduction: Women are especially vulnerable to stress-related disorders such as posttraumatic stress disorder (PTSD), yet most preclinical studies have focused on males. Rodents often exhibit sexually-dimorphic coping strategies in fear behaviors, thus complicating neurobiological interpretations. To facilitate comparisons, an operant conditioned suppression paradigm was implemented where the measure of fear is not dependent on the coping strategy. The effect of single prolonged stress (SPS) on fear behavior was assessed. Since stress has been linked to excitatory phenotypes, mRNA expression of vesicular glutamate transporter 1 (VGLUT1) and glutamate decarboxylase (GAD1) was measured to assess excitation and inhibition-related markers in the ventral hippocampus (vHip), a stress-sensitive brain region. Activity-regulated cytoskeleton associated protein (Arc/Arg3.1) was used to identify neurons activated during fear extinction. The goal of this study was to determine whether SPS produces sex differences in conditioned-suppression fear behavior and to identify accompanying molecular changes in the vHip.

Methods: Thirty-two adult male and female Long Evans rats (n = 16/sex) were trained to lever press for sucrose on variable-interval (VI) schedules. After stable responding, a subset (8/sex) underwent single prolonged stress (SPS). All rats received fear conditioning with five presentations of a conditioned stimulus (CS)-shock pairing. Extinction sessions included baseline VI-60 responding followed by CS presentations. Fear was defined as the suppression of lever pressing during CS. Tissue containing the vHip was collected following the last session for RNA in situ hybridization (RNA-ISH) to examine mRNA expression of molecular markers of excitation/inhibition (VGLUT1, GAD1) and neuronal activation (Arc).

Results: SPS impaired extinction in males on day 1 but had no effect in females. On day 2, males showed greater fear than females regardless of SPS. No significant effects were observed on day 3 or the recall session. No significant effects of sex or stress were observed on vHip mRNA expression of VGLUT1, GAD1, or Arc under the experimental conditions.

Discussion: These findings demonstrate sex-specific effects of SPS on fear extinction measured by conditioned suppression. Future studies should explore earlier time points, additional regions (e.g., amygdala and medial prefrontal cortex) hormonal modulation, and interventions that may mitigate SPS effects in males and females.