Sex differences in the effects of adult short-term isolation rearing on contextual fear memory and extinction

Importance of home cage condition for contextual fear memory, fear extinction and spontaneous recovery: Cage size and bedding material

Home cage condition influences the central nervous system of experimental animals. However, little is known about the effect of home cage size and bedding material on fear-related behaviors. Thus, in this study, the effects of home cage size (large or small) and/or bedding material (paper or wood) on acquisition, retrieval, extinction, and spontaneous recovery of contextual fear memory were investigated in both male and female mice. The present study demonstrated that males housed in small cages with wood bedding showed a low fear response during fear extinction when compared to males housed in small or large cages with paper bedding. In females, mice housed in small cages with wood bedding showed low fear response during fear conditioning and extinction when compared to mice housed in large cages with paper bedding. Moreover, small cages with wood bedding, but not small or large cages with paper bedding, prevented the spontaneous recovery of fear memory in females. Thus, home cage conditions, and particularly bedding material, influence contextual fear extinction and spontaneous recovery. This finding may help to obtain reproducibility of results by researchers and explain discrepancies of results among research groups.

EFFECTS OF LITTER SIZE AND CAGING ON PHYSICAL AND MENTAL DEVELOPMENT IN RATS

As a multidisciplinary field, laboratory animal science promotes or accelerates the emergence of innovative ideas and products. As research has increased, so has the demand for laboratory animals with reliable, standardized characteristics. Thus, the breeding, reproduction, and welfare of laboratory animals are now animals reliable and more. The aim of this study to investigate whether different litter sizes of mothers and different husbandry methods affect the physical and mental development of pups. 30 adults female Wistar Hanover albino rats weighing 200-250 g were used for the study. The weight of the pups was measured once a week from birth until the end of the study, and their physical development was observed. After the pups were weaned, they were randomly divided into cages by sex. The 45 male and 45 female pups were housed in groups of three, five, and seven per cage. When the pups were 12 weeks old, open field test, elevated plus-maze test and Morris water maze behavioral tests were performed every other day, and then plasma corticosterone levels were measured. When the male and female pups in the groups were 14 weeks old, six females were taken from each housing group and mated, and the conception rates and maternal behavior of the pups were observed. During lactation, physical developmental parameters and the body weight of the rats were affected by litter size. Among the post-weaning housing groups, cage density was found to affect weight gain and body weight between groups. It was found that only the sex factor caused significant differences in the behavior of the animals. Females housed with seven rats per cage had higher corticosteroid levels than other females. As a result, it was observed that cages with seven female rats were more physically and psychologically affected than those with three and five rats.

Behavioral and histochemical characterization of sexually dimorphic responses to acute social isolation and reunion in mice

In many mammalian species, females exhibit higher sociability and gregariousness than males, presumably due to the benefit of group living for maternal care. We have previously reported that adult female mice exhibit contact-seeking behaviors upon acute social isolation via amylin-calcitonin receptor (Calcr) signaling in the medial preoptic area (MPOA). In this study, we examined the sex differences in the behavioral responses to acute social isolation and reunion, and the levels of amylin and Calcr expression in the MPOA. We found that male mice exhibited significantly less contact-seeking upon social isolation. Upon reunion, male mice contacted each other to a similar extent as females, but their interactions were more aggressive and less affiliative compared with females. While Calcr-expressing neurons were activated during social contacts in males as in females, the amylin and Calcr expression were significantly lower in males than in females. Together with our previous findings, these findings suggested that the lower expression of both amylin and Calcr may explain the lower contact-seeking and social affiliation of male mice.

Apolipoprotein E Isoform-specific changes related to stress and trauma exposure

Post-Traumatic Stress Disorder (PTSD) is a highly prevalent mental health disorder. Due to the high level of variability in susceptibility and severity, PTSD therapies are still insufficient. In addition to environmental exposures, genetic risks play a prominent role and one such factor is apolipoprotein E. The protein (apoE) is functionally involved in cholesterol transport and metabolism and exists as 3 major isoforms in humans: E2, E3, and E4. To model the role of apolipoprotein E isoform in stress-related changes in behavior and cognition, female and male mice (3-5 months of age) expressing E2, E3, or E4 were used. Mice were either placed into control groups or exposed to chronic variable stress (CVS), which has been shown to induce PTSD-like behavioral and neuroendocrine changes. E2 mice showed a unique response to CVS compared to E3 and E4 mice that included impaired spatial learning and memory, increased adrenal gland weight, and no increase in glucocorticoid receptor protein levels (normalized to apoE levels). In addition, the cholesterol metabolite 7-ketocholesterol was elevated in the cortex after CVS in E3 and E4, but not E2 female mice. E2 confers unique changes in behavioral, cognitive, and biomarker profiles after stress exposure and identify 7-ketocholesterol as a possible novel biomarker of the traumatic stress response. We further explored the relationship between E2 and PTSD in an understudied population by genotyping 102 patients of Cambodian and Vietnamese ethnicity. E2 carriers demonstrated a higher odds ratio of having a PTSD diagnosis compared to E3/E3 carriers, supporting that the E2 genotype is associated with PTSD diagnosis after trauma exposure in this population.

Amylin-Calcitonin receptor signaling in the medial preoptic area mediates affiliative social behaviors in female mice

Social animals actively engage in contact with conspecifics and experience stress upon isolation. However, the neural mechanisms coordinating the sensing and seeking of social contacts are unclear. Here we report that amylin-calcitonin receptor (Calcr) signaling in the medial preoptic area (MPOA) mediates affiliative social contacts among adult female mice. Isolation of females from free social interactions first induces active contact-seeking, then depressive-like behavior, concurrent with a loss of Amylin mRNA expression in the MPOA. Reunion with peers induces physical contacts, activates both amylin- and Calcr-expressing neurons, and leads to a recovery of Amylin mRNA expression. Chemogenetic activation of amylin neurons increases and molecular knockdown of either amylin or Calcr attenuates contact-seeking behavior, respectively. Our data provide evidence in support of a previously postulated origin of social affiliation in mammals.

Overexpression of catalase in mitochondria mitigates changes in hippocampal cytokine expression following simulated microgravity and isolation

Isolation on Earth can alter physiology and signaling of organs systems, including the central nervous system. Although not in complete solitude, astronauts operate in an isolated environment during spaceflight. In this study, we determined the effects of isolation and simulated microgravity solely or combined, on the inflammatory cytokine milieu of the hippocampus. Adult female wild-type mice underwent simulated microgravity by hindlimb unloading for 30 days in single or social (paired) housing. In hippocampus, simulated microgravity and isolation each regulate a discrete repertoire of cytokines associated with inflammation. Their combined effects are not additive. A model for mitochondrial reactive oxygen species (ROS) quenching via targeted overexpression of the human catalase gene to the mitochondria (MCAT mice), are protected from isolation- and/or simulated microgravity-induced changes in cytokine expression. These findings suggest a key role for mitochondrial ROS signaling in neuroinflammatory responses to spaceflight and prolonged bedrest, isolation, and confinement on Earth.

Prolonged Social Isolation, Started Early in Life, Impairs Cognitive Abilities in Rats Depending on Sex

Background: The chronic stress of social isolation is a valid predictor of cognitive pathology. This study aimed to compare the effects of long-term social isolation on female versus male Wistar rats’ learning and memory. We hypothesized that prolonged social isolation stress, which starts early in life, would affect learning in a sex-dependent manner. Methods: Social isolation started at the edge of early to mid-adolescence and lasted 9 months. The rat’s cognitive abilities were assessed by habituation and reactivity to novelty in the open field (OF) test, spatial memory in the Morris water maze (MWM), and the conditioned passive avoidance (PA) reflex. Basal serum corticosterone levels were assessed using an enzyme-linked immunosorbent assay. Results: Regardless of the housing conditions, females habituated to the OF under low illumination slower than males. Under bright light, the single-housed rats showed hyporeactivity to novelty. In the MWM, all the rats learned to locate the platform; however, on the first training day, the single-housed females’ speed was lower relative to other groups. Four months later, in the post-reminder probe trial, the single-housed rats reached the area around the platform site later, and only males, regardless of housing conditions, preferred the target quadrant. Single-housed rats, irrespective of sex, showed a PA deficit. There was a more pronounced conditioned fear in the single-housed males than in females. In both male and female rats, basal corticosterone levels in rat blood serum after 9 months of social isolation did not differ from that in the group-housed rats of the corresponding sex. Meanwhile, females’ basal corticosterone level was higher than in males, regardless of the housing conditions. The relative weight of the adrenal glands was increased only in single-housed females. Conclusions: Under long-term social isolation, started early in life, single-housed females compared with males showed more pronounced cognitive impairments in the MWM and PA paradigm, findings that specify their greater vulnerability to the stress of prolonged social isolation.

Sex-dependent opposite effects of a tropomyosin-related kinase B receptor (TrkB) agonist 7,8-dihydroxyflavone on cued fear extinction in mice

Tropomyosin-related kinase B receptor (TrkB) is one of the new candidate receptors for drugs targeting psychiatric and neurodegenerative disorders. Recently, 7,8-dihydroxyflavone (7,8-DHF) has been identified as a selective TrkB agonist that crosses the blood-brain barrier after oral or intraperitoneal administration, and it enhances cued fear extinction in male rodents. However, its effects on females remain unclear. Preclinical research including both sexes is important for the development of treatment, particularly, for stress-related disorders such as post-traumatic stress disorder because such disorders are more prevalent in women. Therefore, we investigated the effects of 7,8-DHF on cued and contextual fear extinction in both male and female mice. Here we demonstrated that the administration of 7,8-DHF before each extinction session attenuated cued fear extinction in females; conversely, it enhanced cued fear extinction in males. However, administration of 7,8-DHF immediately after each extinction session did not affect cued fear extinction in either sex. Moreover, in contextual fear extinction, administration of 7,8-DHF before each extinction session did not affect fear extinction in either sex. Thus, 7,8-DHF showed sex-dependent opposite effects on cued fear extinction in mice when administered before but not immediately after each extinction session. Our results could contribute to the development of pharmacotherapy involving 7,8-DHF, particularly for stress-related disorders.

Influence of Social Isolation During Prolonged Simulated Weightlessness by Hindlimb Unloading

The hindlimb unloading (HU) model has been used extensively to simulate the cephalad fluid shift and musculoskeletal disuse observed in spaceflight with its application expanding to study immune, cardiovascular and central nervous system responses, among others. Most HU studies are performed with singly housed animals, although social isolation also can substantially impact behavior and physiology, and therefore may confound HU experimental results. Other HU variants that allow for paired housing have been developed although no systematic assessment has been made to understand the effects of social isolation on HU outcomes. Hence, we aimed to determine the contribution of social isolation to tissue responses to HU. To accomplish this, we developed a refinement to the traditional NASA Ames single housing HU system to accommodate social housing in pairs, retaining desirable features of the original design. We conducted a 30-day HU experiment with adult, female mice that were either singly or socially housed. HU animals in both single and social housing displayed expected musculoskeletal deficits versus housing matched, normally loaded (NL) controls. However, select immune and hypothalamic-pituitary-adrenal (HPA) axis responses were differentially impacted by the HU social environment relative to matched NL controls. HU led to a reduction in % CD4⁺ T cells in singly housed, but not in socially housed mice. Unexpectedly, HU increased adrenal gland mass in socially housed but not singly housed mice, while social isolation increased adrenal gland mass in NL controls. HU also led to elevated plasma corticosterone levels at day 30 in both singly and socially housed mice. Thus, musculoskeletal responses to simulated weightlessness are similar regardless of social environment with a few differences in adrenal and immune responses. Our findings show that combined stressors can mask, not only exacerbate, select responses to HU. These findings further expand the utility of the HU model for studying possible combined effects of spaceflight stressors.