Endogenous Estrogen Influences Predator Odor-Induced Impairment of Cognitive and Social Behaviors in Aromatase Gene Deficiency Mice

Epidemiological studies have suggested that traumatic stress increases vulnerability to various mental disorders, such as dementia and psychiatric disorders. While women are more vulnerable than men to depression and anxiety, it is unclear whether endogenous estrogens are responsible for the underlying sex-specific mechanisms. In this study, the aromatase gene heterozygous (Ar+/-) mice were used as an endogenous estrogen deficiency model and age- and sex-matched wild type mice (WT) as controls to study the predator odor 2,3,5-trimethyl-3-thiazoline- (TMT-) induced short- and long-term cognitive and social behavior impairments. In addition, the changes in brain regional neurotransmitters and their associations with TMT-induced changes in behaviors were further investigated in these animals. Our results showed TMT induced immediate fear response in both Ar+/- and WT mice regardless of sexes. TMT induced an acute impairment of novel object recognition memory and long-term social behavior impairment in WT mice, particularly in females, while Ar+/- mice showed impaired novel object recognition in both sexes and TMT-elevated social behaviors, particularly in males. TMT failed to induce changes in the prepulse inhibition (PPI) test in both groups. TMT resulted in a slight increase of DOPAC/DA ratio in the cortex and a significant elevation of this ratio in the striatum of WT mice. In addition, the ratio of HIAA/5-HT was significantly elevated in the cortex of TMT-treated WT mice, which was not found in TMT-treated Ar+/- mice. Taken together, our results indicate that TMT exposure can cause cognitive and social behavior impairments as well as change catecholamine metabolism in WT mice, and endogenous estrogen deficiency might desensitize the behavioral and neurochemical responses to TMT in Ar+/- mice.

The Role of the Nucleus Reuniens in Regulating Contextual Conditioning with the Predator Odor TMT

Experiencing intrusive distressing memories of a traumatic event(s) is a prominent symptom profile for post-traumatic stress disorder (PTSD). Understanding the neurobiological mechanisms associated with this symptom profile can be invaluable for effective treatment for PTSD. Here, we investigated the functional role of the nucleus reuniens (RE), a midline thalamic in modulating stressor-related memory. Female Long Evans rats were implanted with a cannula aimed at the RE. The RE was pharmacologically inactivated via muscimol (0.5 mM) prior to exposure to the predator odor stressor trimethylthiazoline (TMT; synthetically derived fox feces component) or water (controls) in a distinct context with bedding material (Experiment 1) or no bedding (Experiment 2). To measure context reactivity, the index of the contextual memory, 2 weeks following exposure to TMT, rats were re-exposed to the TMT-paired context (in the absence of TMT). In Experiment 1, during context re-exposure (with bedding), inactivation of the RE had no effect on context reactivity. In Experiment 2, during context re-exposure (no bedding), rats previous exposed to TMT showed decreased immobility compared to controls, indicating reactivity to the context and likely related to increased exploration of the environment. Rats in the TMT group that received RE inactivation showed increased immobility relative to rats that received aCSF, suggesting that muscimol pre-treatment blunted context reactivity. In conclusion, recruitment of the RE in stressor-related contextual memory appears to be dependent on the contextual environment and whether the animal is able to engage in different stress coping strategies.

The effects of predator odor (TMT) exposure and mGlu3 NAM pretreatment on lasting behavioral and molecular adaptations in the insular cortex and BNST

A stressor can trigger adaptations that contribute to neuropsychiatric disorders. Predator odor (TMT) exposure is an innate stressor that produces lasting adaptations. TMT exposure may activate metabotropic glutamate receptor 3 (mGlu3), triggering excitatory corticolimbic adaptations that underlie behavioral changes. To evaluate functional involvement, the mGlu3 negative allosteric modulator (NAM, VU6010572; 3 mg/kg, i.p.) was administered before TMT exposure in male, Long Evans rats. Two weeks after stressor, rats underwent behavioral testing (context re-exposure, zero maze and acoustic startle response) followed by RT-PCR gene expression in the insular cortex and BNST. During the TMT exposure, rats displayed stress-reactive behaviors that were not affected by the VU6010572. During the context re-exposure, prior TMT exposure and VU6010572 pretreatment both produced a hyperactive response. TMT exposure did not affect zero maze or ASR measures, but VU6010572 increased time spent in the open arms and habituation to ASR, indicating anxiolytic-like effects. In the insular cortex, TMT exposure resulted in excitatory adaptations as shown by increased expression of mGlu (Grm3, Grm5), NMDA (GriN2A, GriN2B, GriN2C, GriN3A, GriN3B) and AMPA (GriA3) receptor transcripts. Interestingly, mGlu3 signaling during stressor mediated GriN3B upregulation. Stress reactivity during TMT exposure was associated with Grm5, GriN2A, GriN2C, and GriA3 upregulation in the insular cortex and context re-exposure reactivity in the TMT/vehicle, but not the TMT/mGlu3 NAM group. In the BNST, GriN2A, GriN2B and GriN3B were increased by VU6010572, but TMT prevented these effects. These data demonstrate that mGlu3 signaling contributes to the lasting behavioral and molecular adaptations of predator odor stressor.