McCormick CM, Smith C, Mathews IZ
Behav. Brain Res. 2008 Mar;187(2):228-38
Using a rat model of adolescent social stress (SS, daily 1 h isolation and change of cage partner, 30-45 days of age), we have reported sex-specific effects on neuroendocrine function over the course of SS, and enduring effects of SS in females, and not males, on drug-related behaviour. Here, we investigated both the immediate and enduring impact of SS in adolescence on anxiety-like behaviour in the elevated plus maze (EPM) and determined the temporal pattern of corticosterone release after confinement to the open arm of the EPM. When tested as adolescents, SS decreased anxiety-like behaviour in females and had no effect in males. When tested as adults several weeks after the chronic stress, overall, SS tended to increase anxiety-like behaviour in both sexes. However, estrous cycle moderated the effect in females, in that reduced anxiety-like behaviour was observed for SS females in the estrous group. Confinement to the open arm of the EPM increased plasma corticosterone concentrations, which declined markedly upon return to home cage for all except adolescent control males for which corticosterone concentrations at 45 and 90 min were elevated compared other groups. Among controls, anxiety-like behaviour decreased in females and increased in males with age, and confinement to the open arm of the EPM led to a greater increase in corticosterone concentrations in adult males compared to adolescent males. In sum, modest effects of adolescent social stress were observable several weeks after the stress exposure, indicating that sex-specific developmental trajectories and vulnerability to anxiety may be shaped by experiences in adolescence.
McCormick CM, Mathews IZ
Prog. Neuropsychopharmacol. Biol. Psychiatry 2010 Jun;34(5):756-65
Chronic exposure to stress is known to affect learning and memory in adults through the release of glucocorticoid hormones by the hypothalamic-pituitary-adrenal (HPA) axis. In adults, glucocorticoids alter synaptic structure and function in brain regions that express high levels of glucocorticoid receptors and that mediate goal-directed behaviour and learning and memory. In contrast to relatively transient effects of stress on cognitive function in adulthood, exposure to high levels of glucocorticoids in early life can produce enduring changes through substantial remodeling of the developing nervous system. Adolescence is another time of significant brain development and maturation of the HPA axis, thereby providing another opportunity for glucocorticoids to exert programming effects on neurocircuitry involved in learning and memory. These topics are reviewed, as is the emerging research evidence in rodent models highlighting that adolescence may be a period of increased vulnerability compared to adulthood in which exposure to high levels of glucocorticoids results in enduring changes in adult cognitive function.
McCormick CM, Thomas CM, Sheridan CS, Nixon F, Flynn JA, Mathews IZ
Hippocampus 2012 Jun;22(6):1300-12
The ongoing development of the hippocampus in adolescence may be vulnerable to stressors. The effects of social instability stress (SS) in adolescence (daily 1 h isolation and change of cage partner postnatal days 30-45) on cell proliferation in the dentate gyrus (DG) in adolescence (on days 33 and 46, experiment 1) and in adulthood (experiment 2) was examined in Long Evans male rats and compared to nonstressed controls (CTL). Additionally, in experiment 2, a separate group of SS and CTL rats was tested on either a spatial (hippocampal-dependent) or nonspatial (nonhippocampal dependent) version of an object memory test and also were used to investigate hippocampal expression of markers of synaptic plasticity. No memory impairment was evident until the SS rats were adults, and the impairment was only on the spatial test. SS rats initially (postnatal day 33) had increased cell proliferation based on counts of Ki67 immunoreactive (ir) cells and greater survival of immature neurons based on counts of doublecortin ir cells on day 46 and in adulthood, irrespective of behavioral testing. Counts of microglia in the DG did not differ by stress group, but behavioral testing was associated with reduced microglia counts compared to nontested rats. As adults, SS and CTL rats did not differ in hippocampal expression of synaptophysin, but compared to CTL rats, SS rats had higher expression of basal calcium/calmodulin-dependent kinase II (CamKII), and lower expression of the phosphorylated CamKII subunit threonine 286, signaling molecules related to synaptic plasticity. The results are contrasted with those from previous reports of chronic stress in adult rats, and we conclude that adolescent stress alters the ongoing development of the hippocampus leading to impaired spatial memory in adulthood, highlighting the heightened vulnerability to stressors in adolescence.