Abstract
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.
Highlights
Chronic or recurring stress increases the risk of central nervous system (CNS) impairments and often leads to psychoneurological disorders through several synaptic plasticity mechanisms and changes the activity of stress-related systems, in particular, the hypothalamic–pituitary–adrenal (HPA)axis [1,2]
Females were more active than males when compared at 3, 5, and 8.5 months of age, while 1-month-old males and females did not differ (Figure 2A)
We believe one of the reasons for such inconsistency is the selection of indicators on which the authors base their conclusion on the efficiency of learning and memory As the present study shows, selecting as few as two or three indicators is not enough
Summary
Chronic or recurring stress increases the risk of central nervous system (CNS) impairments and often leads to psychoneurological disorders through several synaptic plasticity mechanisms and changes the activity of stress-related systems, in particular, the hypothalamic–pituitary–adrenal (HPA)axis [1,2]. A behavioral phenotype develops, which is characterized by a decline in cognitive abilities: atypical associative learning, spatial memory deficit, impairments of executive functions (inhibitory control, cognitive flexibility, and sustained attention) with a deficit in affective functioning (emotion regulation) [17,19,20]. Such changes increase the risk of social maladjustment and psychopathology. 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
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