Periodic arousal from hibernation is necessary for initiation of immune responses in ground squirrels

Abstract

Golden-mantled ground squirrels (Spermophilus lateralis) undergo seasonal hibernation during which core body temperature (Tb) values are maintained 1-2°C above ambient temperature. Hibernation is not continuous. Squirrels arouse at ϳ7-day intervals, during which Tb increases to 37°C for ϳ16 h; thereafter, they return to hibernation and sustain low Tbs until the next arousal. Over the course of the hibernation season, arousals consume 60-80% of a squirrel's winter energy budget, but their functional significance is unknown and disputed. Host-defense mechanisms appear to be downregulated during the hibernation season and preclude normal immune responses. These experiments assessed immune function during hibernation and subsequent periodic arousals. The acute-phase response to bacterial lipopolysaccharide (LPS) was arrested during hibernation and fully restored on arousal to normothermia. LPS injection (ip) resulted in a 1-1.5°C fever in normothermic animals that was sustained for Ͼ8 h. LPS was without effect in hibernating squirrels, neither inducing fever nor provoking arousal, but a fever did develop several days later, when squirrels next aroused from hibernation; the duration of this arousal was increased sixfold above baseline values. Intracerebroventricular infusions of prostaglandin E2 provoked arousal from hibernation and induced fever, suggesting that neural signaling pathways that mediate febrile responses are functional during hibernation. Periodic arousals may activate a dormant immune system, which can then combat pathogens that may have been introduced immediately before or during hibernation. circannual rhythms; Spermophilus lateralis; neural-immune interactions MAINTENANCE OF IMMUNE FUNCTION requires considerable energy expenditure (42, 62) and may limit or constrain the extent to which animals engage in other energetically demanding activities (39,40,58). Large seasonal increases in energy expenditure associated with reproduction or low ambient temperatures (5) may compete with the day-to-day demands of host defense. Sustained regulated hypothermia, in the form of hibernation or daily torpor, has been adopted by some mammals to contend with seasonal changes in temperature and energy availability. Golden-mantled ground squirrels (Spermophilus lateralis) spend 5-6 mo each year in deep hibernation, during which core body temperature (T b ) is maintained at 1-2°C above ambient temperature (T a ). Many physiological systems, including brain and renal metabolism, respiration, cardiac function, digestion, and mitosis, are arrested or substantially reduced during a hibernation bout (12,29,31,51,54,70,74).Although immune function during hibernation has been little studied, robust cell-mediated and humoral immune responses to pathogens are thought to be compromised. Impaired immune function in hibernators has been inferred from in vitro splenocyte proliferation or reduced antibody production over the course of an entire hibernation season (4,7,15,32,46,48,60). We are unaware of any studies that have assesse...

Skeletal muscle mass and composition during mammalian hibernation

“…Although evidence on skeletal muscle activity during arousal is largely anecdotal, one study conducted on hibernating bats demonstrated substantial recruitment of skeletal muscle with up to 3× increases in EMG spike amplitude over a period of several hours (Lee et al, 2010) and metabolic rates elevated by up to 10× basal levels (Geiser, 2004;Heldmaier and Ruf, 1992). While arousal bouts are likely necessary to pay off sleep debt (Strijkstra and Daan, 1997), clear metabolic waste products (Zancanaro et al, 1999) and modulate immune system function (Prendergast et al, 2002), the increased activity associated with them may also serve a secondary purpose to periodically increase muscle loading and myoplasmic calcium concentration, both of which may be important for maintaining skeletal muscle mass and avoiding alterations in fiber type associated with low activity during hibernation. While proximal muscles are most heavily involved in shivering thermogenesis (Bell et al, 1992;Meigal et al, 1998), it is unclear to what extent the more commonly studied distal muscles are involved in shivering thermogenesis during arousal, casting some doubt on the notion that shivering thermogenesis during arousal fully explains the maintenance of skeletal muscle during hibernation.…”

Mammalian hibernation: differential gene expression and novel application of epigenetic controls

“…Arousal from torpor is initially and primarily fueled by non-shivering thermogenesis in brown adipose tissue (BAT) but after Tb rises above about 15°C skeletal muscle shivering also contributes to rewarming. Multiple reasons for these periodic arousals have been postulated such as a need to eliminate metabolic wastes, readjust neural circuits, or recharge the immune system but no single regulatory factor has yet been identified (Prendergast et al, 2002;Heller and Ruby, 2004).…”

Body temperature and body mass of hibernating little brown bats Myotis lucifugus in hibernacula affected by white-nose syndrome

“…During hibernation, the immune system of most hibernators is partially or completely arrested (Carey et al, 2003). Following an immune challenge, the pattern of arousal from hibernation suggests a need to periodically raise body temperature (T b ) to mount an immune response against pathogens 4 (Prendergast et al, 2002;Luis and Hudson, 2006). Assuming the immune system of hibernating bats is suppressed as it is in other hibernating mammals, WNS may trigger a tradeoff in hibernation strategies.…”