Prolonged Restraint Stress Increases IL-6, Reduces IL-10, and Causes Persistent Depressive-Like Behavior That Is Reversed by Recombinant IL-10

<div><p>Altered inflammatory cytokine profiles are often observed in individuals suffering from major depression. Recent clinical work reports on elevated IL-6 and decreased IL-10 in depression. Elevated IL-6 has served as a consistent biomarker of depression and IL-10 is proposed to influence depressive behavior through its ability to counterbalance pro-inflammatory cytokine expression. Clinical and animal studies suggest a role for IL-10 in modifying depressive behavior. Murine restraint stress (RST) is regularly employed in the study of behavioral and biological symptoms associated with depressive disorders. While responses to acute RST exposure have been widely characterized, few studies have examined the ongoing and longitudinal effects of extended RST and fewer still have examined the lasting impact during the post-stress period. Consistent with clinical data, we report that a protocol of prolonged murine RST produced altered cytokine profiles similar to those observed in major depressive disorder. Parallel to these changes in circulating cytokines, IL-10 mRNA expression was diminished in the cortex and hippocampus throughout the stress period and following cessation of RST. Moreover, chronic RST promoted depressive-like behavior throughout the 28-day stress period and these depressive-like complications were maintained weeks after cessation of RST. Because of the correlation between IL-10 suppression and depressive behavior and because many successful antidepressant therapies yield increases in IL-10, we examined the effects of IL-10 treatment on RST-induced behavioral changes. Behavioral deficits induced by RST were reversed by exogenous administration of recombinant IL-10. This work provides one of the first reports describing the biological and behavioral impact following prolonged RST and, taken together, this study provides details on the correlation between responses to chronic RST and those seen in depressive disorders.</p> </div

Prolonged RST evoked sustained elevations in stress response.

<p>Bodyweight (A), splenic mass (B), adrenal mass (C), and thymic mass (D) were examined during (solid lines, Day 7–28) and following stress cessation (broken lines, Day 29–42). For each data point n = 10 individuals. Data shown is mean +SEM. *p<0.05; **p<.0001. Data were collected without repeated sampling of individuals.</p

IL-10 treatment exhibits a rescuing effect on restraint stress-induced behavioral deficits.

<p>As part of a 21-day RST protocol, mice were treated with vehicle (control and RST mice) or murine IL-10 (IL-10 mice) for seven days immediately preceding FST. For each data point n = 5–15 individuals. Data shown is mean +SEM. *p<0.05.</p

Mice exposed to prolonged RST demonstrated sustained depressive-like behavior.

<p>Mice were exposed to FST throughout the experimental period. Time to first immobility (A) as well as total immobility (B) were recorded. For each data point n = 10 individuals. Data shown is mean +SEM. *p<0.05. Data were collected without sampling of individuals.</p

RST decreased expression of IL-10 in the brain.

<p>mRNA expression of pro- and anti-inflammatory cytokines were measured in the cortex and hippocampus. For each data point n = 6–10 individuals. Data shown is mean +SEM. *p<0.05.</p

Prolonged RST elevated corticosterone levels throughout the stress period.

<p>Corticosterone levels were determined during the stress period (solid bars, Day 0–28) and following stress cessation (hashed bars, Day 29–42). For each data point, n = 5–15 individuals. Data shown is mean +SEM. *p<.05; **p<.0001. Data were collected without repeated sampling of individuals.</p

RST elevated circulating IL-6 and decreased IL4 and IL-10.

<p>Circulating levels of pro- and anti-inflammatory cytokines were measured throughout the experimental period. For each data point n = 3–9 individuals. Data shown is mean +SEM. *p<0.05. Data were collected without repeated sampling of individuals.</p