Stress-induced dynamic changes in mouse brain polyamines. Role in behavioral reactivity

Recent findings indicate that rapid and transient changes in polyamine metabolism, termed the polyamine-stress-response, may occur repeatedly in the brain after chronic intermittent stress. Here, we sought to examine the effects of chronic intermittent restraint stress, or of daily intraperitoneal dexamethasone injections on polyamine concentrations in the hippocampus of adult male C57BL/6 mice. Additionally, we studied the effects of alpha-difluoromethylornithine, an irreversible ornithine decarboxylase inhibitor, on stress-induced changes in polyamines and on behavioral reactivity to novelty stress measured in an open-field arena. As previously observed, following a single stress episode putrescine concentration increased transiently, but the polyamines spermidine and spermine remained unchanged. Following chronic restraint stress, putrescine concentration was increased after each daily stress episode with the largest increase observed after the 4th episode, while spermidine was increased just after the 2nd and 4th episodes and spermine only after the 4th daily episode. In contrast, all polyamine concentrations were increased after 10 injections of either dexamethasone or vehicle (0.9% NaCl). A 14-day course of alpha-difluoromethylornithine treatment resulted in a complete putrescine depletion and over 50% reduction in polyamines, and led to changes in open field activity indicative of altered emotional behavior. CONCLUSIONS: (a) while putrescine concentration increases in the hippocampus after each restraint stress episode, spermidine and spermine undergo a delayed but transient increase; (b) in contrast, chronic dexamethasone treatment may lead to a permanent increase in the concentrations of all polyamines and; (c) chronic alpha-difluoromethylornithine treatment reduces brain polyamine concentrations and modulates emotional reactivity to novelty stress. The study indicates that the brain polyamine-stress-response is a dynamic process that varies with the type, intensity and length of stressful stimuli, and implicates this response as an adaptive mechanism in the reaction to stressors.