Stephanie L. Bowers

Stressor-Specific Alterations in Corticosterone and Immune Responses in Mice

Different stressors likely elicit different physiological and behavioral responses. Previously reported differences in the effects of stressors on immune function may reflect qualitatively different physiological responses to stressors; alternatively, both large and subtle differences in testing protocols and methods among laboratories may make direct comparisons among studies difficult. Here we examine the effects of chronic stressors on plasma corticosterone concentrations, leukocyte redistribution, and skin delayed-type hypersensitivity (DTH), and the effects of acute stressors on plasma corticosterone and leukocyte redistribution. The effects of several commonly used laboratory stressors including restraint, forced swim, isolation, and low ambient temperatures (4 degrees C) were examined. Exposure to each stressor elevated corticosterone concentrations, with restraint (a putative psychological stressor) evoking a significantly higher glucocorticoid response than other stressors. Chronic restraint and forced swim enhanced the DTH response compared to the handled, low temperature, or isolation conditions. Restraint, low temperature, and isolation significantly increased trafficking of lymphocytes and monocytes compared to forced swim or handling. Generally, acute restraint, low temperature, isolation, and handling increased trafficking of lymphocytes and monocytes. Considered together, our results suggest that the different stressors commonly used in psychoneuroimmunology research may not activate the physiological stress response to the same extent. The variation observed in the measured immune responses may reflect differential glucocorticoid activation, differential metabolic adjustments, or both processes in response to specific stressors.

Anxiety After Cardiac Arrest/Cardiopulmonary Resuscitation Exacerbated by Stress and Prevented by Minocycline

Background and Purpose Stress is an important risk factor for cardiovascular disease; however, most of the research on this topic has focused on incidence rather than outcome. The goal of this study was to determine the effects of prior exposure to chronic stress on ischemia-induced neuronal death, microglial activation, and anxiety-like behavior.Background and Purpose— Stress is an important risk factor for cardiovascular disease; however, most of the research on this topic has focused on incidence rather than outcome. The goal of this study was to determine the effects of prior exposure to chronic stress on ischemia-induced neuronal death, microglial activation, and anxiety-like behavior. Methods— In Experiment 1, mice were exposed to 3 weeks of daily restraint (3 hours) and then subjected to either 8 minutes of cardiac arrest/cardiopulmonary resuscitation (CA/CPR) or sham surgery. Anxiety-like behavior, microglial activation, and neuronal damage were assessed on postischemic Day 4. In Experiment 2, mice were infused intracerebroventricularly with minocycline (10 &mgr;g/day) to determine the effect of inhibiting post-CA/CPR microglial activation on the development of anxiety-like behavior and neuronal death. Results— CA/CPR precipitated anxiety-like behavior and increased microglial activation and neuronal damage within the hippocampus relative to sham surgery. Prior exposure to stress exacerbated these measures among CA/CPR mice, but had no significant effect on sham-operated mice. Treatment with minocycline reduced both neuronal damage and anxiety-like behavior among CA/CPR animals. Anxiety-like behavior was significantly correlated with measures of microglial activation but not neuronal damage. Conclusions— A history of stress exposure increases the pathophysiological response to ischemia and anxiety-like behavior, whereas inhibiting microglial activation reduces neuronal damage and mitigates the development of anxiety-like behavior after CA/CPR. Thus, modulating inflammatory signaling after cerebral ischemia may be beneficial in protecting the brain and preventing the development of affective disorders.

Enrichment and photoperiod interact to affect spatial learning and hippocampal dendritic morphology in white-footed mice (Peromyscus leucopus)

In seasonally changing environments, individuals must coordinate endogenous processes with ambient conditions. Winter is a challenging time to survive and reproduce. In order to anticipate decreased food availability and low temperatures in winter, many rodents use decreasing day lengths as a precise temporal cue. Short day lengths alter several adaptations, including reproduction, immune function, aggressive behavior and spatial learning in non‐tropical rodents. Specifically, short days impair spatial learning in white‐footed mice (Peromyscus leucopus) and alter dendritic complexity in the hippocampus. The goal of the current study was to determine whether short days constrain neural plasticity. If short days limit the capacity for plasticity, then environmental enrichment, a manipulation that induces morphological changes, should alter dendritic morphology in long, but not short, days. Male white‐footed mice were assigned to long (16 : 8 LD) or short (8 : 16 LD) photoperiod in either enriched or standard cages. Enrichment consisted of a large cage, cage mates, Habitrail® tubes, a nest box and a running wheel. Mice were tested in the Morris water maze. Reproductive tissues were collected and weighed; brains were processed for dendritic morphology. Short days impaired spatial learning. Short days also reduced spine density on apical dendrites within the CA3 region of the hippocampus. However, enrichment prevented short‐day‐induced deficits in learning and also increased hippocampal spine density in the CA1 and CA3 regions in short‐day mice. These results suggest that day length and other non‐photic environmental factors interact to regulate dendritic morphology, and that short photoperiods do not constrain the capacity for functional neural plasticity.

Cytoplasmic Prion Protein Induces Forebrain Neurotoxicity

The prion protein (PrP) is essential for the pathogenesis of prion disease. PrP has been detected in the cytosol of neurons and transgenic mice expressing PrP in the cytosol (cyPrP) under a pan-neuronal promoter developed rapid cerebellar granule neuron degeneration. Yet, it remains unclear whether cyPrP is capable to cause toxicity in other neuronal populations. Here, we report that transgenic mice expressing cyPrP in the forebrain neurons developed behavioral abnormalities including clasping and hyperactivity. These mice had reduced thickness in cortex and developed astrogliosis in hippocampal and cortical regions. Moreover, cyPrP in these mice was recognized by the A11 anti-oligomer antibody and was associated with the hydrophobic lipid core of membranes, indicating that cyPrP oligomer caused membrane perturbation contributes to cyPrP neurotoxicity. Together, our results clearly revealed that cyPrP is able to cause toxicity in different neuronal populations, supporting a role of cyPrP in PrP-mediated neurodegenerative disorders.

Sex-specific effects of glucose deprivation on cell-mediated immunity and reproduction in Siberian hamsters (Phodopus sungorus)

In most species, sexes differ in levels of parasitism. These differences have traditionally been believed to be static, but a capacity for adjusting anti-parasite investments would allow sexes to allocate resources adaptively contingent on environmental conditions. During stressful periods, such as a food shortage, allocation decisions would be mandated in males and females, but the biasing of resources may differ depending on the value of various physiological alternatives to the fitness of each sex. To determine whether sexes sacrifice immune or reproductive capacity when stressed, male and female Siberian hamsters (Phodopus sungorus) were pharmacologically deprived of glucose. Glucose deprivation was expected to compromise immune activity (delayed-type hypersensitivity) more than reproductive capacity in males because male fitness is limited by reproductive opportunities. The opposite was predicted for females because of the greater value of surviving to breed in favorable conditions. Contrary to expectations, glucoprivation compromised immune activity in female, but not male, hamsters. Conversely, glucoprivation reduced male, but not female, reproductive organ masses. These results may reflect the adjustments made by wild hamsters during food shortages, or they may be influenced by the study design; neither sex was permitted to incur other behavioral and physiological costs, such as lactation and parental care. Regardless, our results indicate that sex differences in parasitism are likely to be plastic in many circumstances, but further work in free-living animals is critical to ascertain whether results of the present study are naturally representative.

Maternal aggression persists following lipopolysaccharide-induced activation of the immune system

Lactating females direct aggressive behaviors towards intruders presumably to reduce the likelihood of infanticide of their pups. Infected animals display a constellation of responses that include lethargy, anorexia, and decreased social interactions. This suite of responses is referred to as sickness behavior, and is putatively part of an adaptive strategy to aid the organism in recovery from infection. Previous work has suggested that animals can suppress the behavioral symptoms of sickness in order to engage in adaptive behaviors. To test whether adaptive nest defense is affected by illness, dams received a peripheral injection of either saline or lipopolysaccharide (LPS [50, 400, or 1000 microg/kg]), a non-replicating component of bacterial cell walls that activates the immune system. Simulated infection with LPS reduced body mass and food intake in dams and interfered with litter growth in a dose-dependent manner. Generally, nest defense was unaffected by LPS; the proportion of dams displaying maternal aggression against a male intruder, as well as the latency and duration of aggressive encounters were only suppressed at the highest LPS dose tested. Further, LPS treatment also altered non-agonistic behavior during the aggression test as indicated by reduced social investigation of the intruder and an increased time spent immobile during the session. LPS administration also significantly increased serum corticosterone concentrations in lactating females. These findings suggest that maternal aggression is not suppressed by LPS-evoked immune activation at doses that attenuate other aspects of maternal and social behavior.

Photoperiod alters affective responses in collared lemmings

This study examined photoperiodic regulation of affective behaviours in collared lemmings (Dicrostonyx groenlandicus). Male and female lemmings were housed in either long (LD 22:2), intermediate (LD 16:8), or short days (LD 8:16) for 9 weeks. Exposure to short days induced moult to a winter pelage and gonadal regression in male, but not female, lemmings. Lemmings housed in long days reduced anxiety-like responses in the elevated plus maze. Depressive-like behaviours were decreased in the intermediate photoperiod relative to other photoperiod groups.

Photoperiod Alters the Time Course of Brain Cyclooxygenase-2 Expression in Siberian Hamsters

Fever is initiated by activation of the arachidonic acid cascade and the biosynthesis of prostaglandins within the brain. Inducible cyclooxygenase (COX‐2) is a rate‐limiting enzyme in prostaglandin synthesis, and the number of blood vessels expressing COX‐2 correlates with elevated body temperature following peripheral lipopolysaccharide (LPS). Despite its importance in host defense, fever is energetically expensive and we hypothesized that fever may be limited by available metabolic resources. During winter, when competing metabolic demands are constrained by low temperatures and food availability, it was predicted that fever duration would be reduced in seasonally breeding Siberian hamsters (Phodopus sungorus). We measured LPS‐induced COX‐2 expression in blood vessels of hamsters to test whether photoperiodic alterations in fever duration are centrally mediated, or whether they reflect changes in peripheral modulation of body temperature. Hamsters housed in long, ‘summer‐like’ or short, ‘winter‐like’ day lengths for 10 weeks were injected with LPS, and brains were collected 2, 4, or 8 h later. COX‐2 expression was comparably increased in long‐ and short‐day hamsters by 2 h and 4 h post‐LPS; however, short‐day hamsters exhibited significantly fewer COX‐2‐positive cells and blood vessels by 8 h post‐LPS compared to long‐day hamsters, corresponding with reduced fever duration in short‐day hamsters. Cortisol concentrations increased more than two‐fold in short‐day compared to long‐day hamsters by 4 h; this increase may have contributed to the decrease in COX‐2 expression observed by 8 h in short days. We conclude that short photoperiods significantly altered the time course of central COX‐2 protein expression in hamsters in a manner consistent with reduced fever duration.

Anxiety After Cardiac Arrest/Cardiopulmonary Resuscitation

Background and Purpose Stress is an important risk factor for cardiovascular disease; however, most of the research on this topic has focused on incidence rather than outcome. The goal of this study was to determine the effects of prior exposure to chronic stress on ischemia-induced neuronal death, microglial activation, and anxiety-like behavior.Background and Purpose— Stress is an important risk factor for cardiovascular disease; however, most of the research on this topic has focused on incidence rather than outcome. The goal of this study was to determine the effects of prior exposure to chronic stress on ischemia-induced neuronal death, microglial activation, and anxiety-like behavior. Methods— In Experiment 1, mice were exposed to 3 weeks of daily restraint (3 hours) and then subjected to either 8 minutes of cardiac arrest/cardiopulmonary resuscitation (CA/CPR) or sham surgery. Anxiety-like behavior, microglial activation, and neuronal damage were assessed on postischemic Day 4. In Experiment 2, mice were infused intracerebroventricularly with minocycline (10 &mgr;g/day) to determine the effect of inhibiting post-CA/CPR microglial activation on the development of anxiety-like behavior and neuronal death. Results— CA/CPR precipitated anxiety-like behavior and increased microglial activation and neuronal damage within the hippocampus relative to sham surgery. Prior exposure to stress exacerbated these measures among CA/CPR mice, but had no significant effect on sham-operated mice. Treatment with minocycline reduced both neuronal damage and anxiety-like behavior among CA/CPR animals. Anxiety-like behavior was significantly correlated with measures of microglial activation but not neuronal damage. Conclusions— A history of stress exposure increases the pathophysiological response to ischemia and anxiety-like behavior, whereas inhibiting microglial activation reduces neuronal damage and mitigates the development of anxiety-like behavior after CA/CPR. Thus, modulating inflammatory signaling after cerebral ischemia may be beneficial in protecting the brain and preventing the development of affective disorders.

ANXIETY FOLLOWING CARDIAC ARREST/CPR: EXACERBATED BY STRESS AND PREVENTED BY MINOCYCLINE

Background and Purpose Stress is an important risk factor for cardiovascular disease; however, most of the research on this topic has focused on incidence rather than outcome. The goal of this study was to determine the effects of prior exposure to chronic stress on ischemia-induced neuronal death, microglial activation, and anxiety-like behavior.Background and Purpose— Stress is an important risk factor for cardiovascular disease; however, most of the research on this topic has focused on incidence rather than outcome. The goal of this study was to determine the effects of prior exposure to chronic stress on ischemia-induced neuronal death, microglial activation, and anxiety-like behavior. Methods— In Experiment 1, mice were exposed to 3 weeks of daily restraint (3 hours) and then subjected to either 8 minutes of cardiac arrest/cardiopulmonary resuscitation (CA/CPR) or sham surgery. Anxiety-like behavior, microglial activation, and neuronal damage were assessed on postischemic Day 4. In Experiment 2, mice were infused intracerebroventricularly with minocycline (10 &mgr;g/day) to determine the effect of inhibiting post-CA/CPR microglial activation on the development of anxiety-like behavior and neuronal death. Results— CA/CPR precipitated anxiety-like behavior and increased microglial activation and neuronal damage within the hippocampus relative to sham surgery. Prior exposure to stress exacerbated these measures among CA/CPR mice, but had no significant effect on sham-operated mice. Treatment with minocycline reduced both neuronal damage and anxiety-like behavior among CA/CPR animals. Anxiety-like behavior was significantly correlated with measures of microglial activation but not neuronal damage. Conclusions— A history of stress exposure increases the pathophysiological response to ischemia and anxiety-like behavior, whereas inhibiting microglial activation reduces neuronal damage and mitigates the development of anxiety-like behavior after CA/CPR. Thus, modulating inflammatory signaling after cerebral ischemia may be beneficial in protecting the brain and preventing the development of affective disorders.