• 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.
    • Paleobiology of ichthyosaurs: using osteohistology to test hypotheses of growth rates and metabolism in a clade of secondarily aquatic marine tetrapods

      Anderson, Katherine L.; Druckenmiller, Patrick; Erickson, Gregory; Horstmann, Lara; Fowell, Sarah (2019-08)
      Ichthyosaurians (Ichthyosauria) are one of the most prominent groups of secondarily aquatic Mesozoic marine reptiles. Over their 160 million years of evolution, the clade evolved a streamlined body plan with paddle-like limbs, convergent with modern cetaceans. Despite the fact that ichthyosaurians have been studied by paleontologists for over a century, very little is known about aspects of their biology, including quantification of their age structure and growth rates. Multiple lines of evidence, including oxygen isotope, swimming modality, and body shape analyses suggest that ichthyosaurians experienced elevated growth rates and likely maintained an elevated body temperature relative to ambient sea water. In this dissertation, I test these hypotheses using osteohistological methods. In the first manuscript, we describe new material of the small-bodied Upper Triassic ichthyosaurian Toretocnemus from the Nehenta Formation and the Hound Island Volcanics (both Norian, Upper Triassic) of Southeast Alaska. During the Upper Triassic, ichthyosaurians experienced their greatest size disparity, with large-bodied species rivaling the size of modern blue whales (Balaenoptera musculus; 20+ m body length) living alongside small-bodied species (1 m body length) like Toretocnemus. Prior to this study, Toretocnemus was known from Carnian deposits of California and possibly Sonora, Mexico. The referred material described here expands its geographic and temporal range. There are very few known ichthyosaurians from the Norian; thus, this material sheds light on the clade's diversity before the end Triassic extinction event. In the second and third manuscripts, we use osteohistological methods to describe the microstructure of various skeletal elements of two species of Stenopterygius from the Posidonia Shale (Lower Jurassic) of Germany. The Posidonia Shale is a Konservat-Lagerstätten that preserves over 3000 ichthyosaurian specimens, approximately 80 percent of which are referable to Stenopterygius. First, we sampled over 40 skeletal elements from one individual specimen referred to Stenopterygius quadriscissus to 1) describe the mineralized tissues across the skeleton, 2) infer relative growth rate, and 3) identify elements with growth marks. Almost all elements described demonstrate fibrolamellar primary bone, indicative of a rapid growth rate. We also identify growth marks in several elements, including the dentary and premaxilla, that will be used in future growth studies. In the third manuscript, we sample a scleral ossicle from Stenopterygius triscissus to describe its microstructure and investigate the use of ossicles for skeletochronology. The use of scleral ossicles for determining age structure has been documented in extant sea turtles as well as dinosaurs. We sectioned one ossicle in three planes and document conspicuous growth banding in the short axis section. Although this method requires further testing, we tentatively determine a minimum age of 7 years at the time of death for this individual. This dissertation lays critical groundwork for future studies of the paleobiology of ichthyosaurians. We are already in the preliminary stages of using these results to 1) quantify age structure and growth rates of an ichthyosaurian (Stenopterygius quadriscissus) for the first time, and 2) test the use of scleral ossicles for skeletochronology of ichthyosaurians. Through addressing these basic aspects of ichthyosaurian biology, we can begin to investigate how ichthyosaurian development and physiology changed over time and space and develop a greater understanding of this clade's 160 million years of evolution.