• Mechanisms regulating the circannual rhythm of hibernation

      Frare, Carla; Drew, Kelly L.; Bult-Ito, Abel; Green, Thomas K.; Kuhn, Thomas B. (2019-08)
      Hibernation is a unique adaptation to conserve energy entering a hypometabolic (low metabolic rate) and hypothermic (low body temperature) state called torpor. Torpor is characterized by a drop in metabolism to 1-2% of basal metabolic rate and a decrease in body temperature to one to two degrees above ambient temperature. Metabolic rate is restored to basal metabolic rate and body temperature increases from 2-3⁰C to 36⁰C during the regularly timed arousal. The adenosine A1 receptor agonists promote the onset of hibernation and torpor in different species, through a yet undefined neuronal circuit. In the Arctic ground squirrel, CHA, an adenosine A1 receptor agonist, induces hibernation during the winter- hibernation season but not in summer even when the environmental conditions are kept constant (ambient temperature of 2⁰C and a light cycle of 4L:20D). Thus, the phenomenon of CHA-induced hibernation is entrained to an endogenous circannual rhythm. In this work, I aim to identify the changes in neuronal activation that reflect the circannual rhythm regulating the seasonal difference in response to CHA. Arctic ground squirrels, housed at constant ambient temperature (2°C) and light cycle (4L:20D), were implanted with body temperature transmitters. I collected tissue during Summer, Fall, Winter and Torpor conditions for seasonal analysis. For treatment analysis, I collected tissue form animals treated with CHA or vehicle in Summer and Winter. Primarily, I used immunohistochemistry to identify cell groups affected by season and treatment. I used cFos to identify neuronal activity and other immunohistochemical markers to identify neuronal phenotypes, based on specific cytoplasmic proteins. An overall seasonal decrease in thermogenesis, measured as reduced neuronal activity in the thermoregulatory pathways, and increase in vasoconstriction reflected the higher order processing necessary for CHA-induced hibernation. CHA inhibited the histaminergic neurons in the hypothalamus suppressing wakefulness and dis-inhibited the nucleus tractus solitarius, further suppressing thermogenesis. Preliminary data also suggested a seasonal change in the adenosine metabolic pathway, which may have increased adenosine receptor sensitivity during the hibernation season. Our results suggest that histaminergic neurons in the hypothalamus and the nucleus tractus solitarius are likely targets to manipulate metabolic demand in the clinical setting inducing therapeutic hypothermia or increasing metabolic rate.
    • The molecular basis of aerobic metabolic remodeling in threespine stickleback in response to cold acclimation

      Orczewska, Julieanna Inez (2011-05)
      Increases in mitochondrial density during cold acclimation have been documented in many fish species, however the mechanism regulating this process is not understood. The present study sought to characterize metabolic changes in response to cold acclimation and identify how these changes are regulated in oxidative muscle, glycolytic muscle and liver tissue of threespine stickleback, Gasterosteus aculeatus. Fish were warm (20°C) or cold (8°C) acclimated for 9 weeks and harvested during acclimation. Mitochondrial volume density was quantified using transmission electron microscopy and aerobic metabolic capacity assessed by measuring the maximal activity of citrate synthase and cytochrome c oxidase. The molecular mechanism mediating changes in aerobic metabolic capacity were assessed by quantifying transcript levels of aerobic metabolic genes and known regulators of mammalian mitochondrial biogenesis using quantitative real-time PCR. Our results indicate that while the maximal activity of aerobic metabolic enzymes increased in all tissues, mitochondrial biogenesis only occurred in oxidative muscle. Our results also suggest that the time course of metabolic remodeling is tissue specific. Lastly, we identified differences in the magnitude and timing of transcriptional and co-transcriptional activators driving metabolic remodeling between each tissue. These results suggest aerobic metabolic remodeling may be triggered by different stimuli in different tissues.