The Effects of Chronic Hypercarbia on Morphological and Ventilatory Development in Crayfish

Instances of abnormally high CO2 levels are becoming increasingly common in freshwater ecosystems undergoing eutrophication. Chronic hypercarbia (long-term elevation of the partial pressure of carbon dioxide (Pco2)) is pervasive in these eutrophic ecosystems. Elevated Pco2 increases the ventilation and metabolism of some tadpoles and aquatic frogs and hinders their morphological development23. We expect that chronic hypercarbia will affect most aquatic organisms in the same way, but it remains to be verified that chronic hypercarbic conditions also alter aquatic organisms similarly to intermittent and acute hypercarbic challenges

Long-Term Exposure to an Invasive Fungal Pathogen Decreases Eptesicus fuscus Body Mass With Increasing Latitude

Abstract Invasive pathogens threaten wildlife health and biodiversity. Physiological responses of species highly susceptible to pathogen infections following invasion are well described. However, the responses of less susceptible species (relative to highly susceptible species) are not well known. Latitudinal gradients, which can influence body condition via Bergmann\u27s rule and/or reflect the time it takes for an introduced pathogen to spread geographically, add an additional layer for how mammalian species respond to pathogen exposure. Our goal was to understand how hosts less susceptible to pathogen infections respond to long‐term pathogen exposure across a broad latitudinal gradient. We examined changes in body mass throughout pathogen exposure time across the eastern United States (latitude ranging 30.5° N–44.8° N) in Eptesicus fuscus, a bat species classified as less susceptible to infection (relative to highly susceptible species) by the invasive fungal pathogen that causes white‐nose syndrome, Pseudogymnoascus destructans (Pd). Using 30 years of spring through fall adult capture records, we created linear mixed‐effects models for female and male bats to determine how mass or mass variation changed across the eastern United States from pre‐Pd invasion years through Pd invasion (0–1 years with Pd), epidemic (2–4 years with Pd), and established years (5+ years with Pd). By Pd establishment, all female and male bats decreased body mass with increasing latitude across a spatial threshold at 39.6° N. Differences in bat mass north and south of the spatial threshold progressively increased over Pd exposure time‐steps such that body mass was lower in northern latitudes compared to southern latitudes by Pd establishment. Results indicated that the progressive differences in E. fuscus body mass with latitude across the eastern United States are due to long‐term pathogen exposure; however, other environmental and ecological pressures may contribute to decreases in E. fuscus body mass with latitude and long‐term pathogen exposure. As pathogen introductions and emerging infectious diseases become more prevalent on the landscape, it is imperative that we understand how less susceptible species directly and indirectly respond to long‐term pathogen exposure in order to maintain population health in surviving species

Central Chemosensitivity in Mammals

More than a century has passed since the beginning of direct experimentation on control of ventilation, and the ensuing years have brought considerable insight into the mechanisms of this control. Much of what we know about cellular chemosensitivity in mammals comes from a limited number of species; yet, given the diversity of circumstances in which mammals exist, their potential has been greatly underused. Here we review some of the environmental situations for plasticity of mammalian central chemosensitivity and function of chemosensors. “Normal” breathing patterns change during sleep, hibernation, and exercise, and central chemosensitivity must be altered during acclimation or adaptation to altitude, burrowing, or disease states. Where central chemosensitive cells are located, and what qualifies a cell as chemosensitive, is currently debated. The chemosensitivity of these cells changes over development, and the signaling mechanisms of these cells vary between chemosensitive regions, probably accounting for plasticity in response to environmental perturbations

Activation of Respiratory Muscles Does Not Occur During Cold-Submergence in Bullfrogs, Lithobates Catesbeianus

Semiaquatic frogs may not breathe air for several months because they overwinter in ice-covered ponds. In contrast to many vertebrates that experience decreased motor performance after inactivity, bullfrogs, Lithobates catesbeianus, retain functional respiratory motor processes following cold-submergence. Unlike mammalian hibernators with unloaded limb muscles and inactive locomotor systems, respiratory mechanics of frogs counterintuitively allow for ventilatory maneuvers when submerged. Thus, we hypothesized that bullfrogs generate respiratory motor patterns during cold-submergence to avoid disuse and preserve motor performance. Accordingly, we measured activity of respiratory muscles (buccal floor compressor and glottal dilator) via electromyography in freely behaving bullfrogs at 20 and 2°C. Although we confirm that ventilation cycles occur underwater at 20°C, bullfrogs did not activate either respiratory muscle when submerged acutely or chronically at 2°C. We conclude that cold-submerged bullfrogs endure respiratory motor inactivity, implying that other mechanisms, excluding underwater muscle activation, maintain a functional respiratory motor system throughout overwintering

Activation of Respiratory Muscles Does Not Occur During Cold-Submergence in Bullfrogs, Lithobates Catesbeianus

Semiaquatic frogs may not breathe air for several months because they overwinter in ice-covered ponds. In contrast to many vertebrates that experience decreased motor performance after inactivity, bullfrogs, Lithobates catesbeianus, retain functional respiratory motor processes following cold-submergence. Unlike mammalian hibernators with unloaded limb muscles and inactive locomotor systems, respiratory mechanics of frogs counterintuitively allow for ventilatory maneuvers when submerged. Thus, we hypothesized that bullfrogs generate respiratory motor patterns during cold-submergence to avoid disuse and preserve motor performance. Accordingly, we measured activity of respiratory muscles (buccal floor compressor and glottal dilator) via electromyography in freely behaving bullfrogs at 20 and 2°C. Although we confirm that ventilation cycles occur underwater at 20°C, bullfrogs did not activate either respiratory muscle when submerged acutely or chronically at 2°C. We conclude that cold-submerged bullfrogs endure respiratory motor inactivity, implying that other mechanisms, excluding underwater muscle activation, maintain a functional respiratory motor system throughout overwintering

Activation State of the Hyperpolarization-Activated Current Modulates Temperature-Sensitivity of Firing in Locus Coeruleus Neurons from Bullfrogs

Locus coeruleus neurons of anuran amphibians contribute to breathing control and have spontaneous firing frequencies that, paradoxically, increase with cooling. We previously showed that cooling inhibits a depolarizing membrane current, the hyperpolarization-activated current (I h) in locus coeruleus neurons from bullfrogs, Lithobates catesbeianus (Santin JM, Watters KC, Putnam RW, Hartzler LK. Am J Physiol Regul Integr Comp Physiol 305: R1451–R1464, 2013). This suggests an unlikely role for I h in generating cold activation, but led us to hypothesize that inhibition of I h by cooling functions as a physiological brake to limit the cold-activated response. Using whole cell electrophysiology in brain slices, we employed 2 mM Cs+ (an I h antagonist) to isolate the role of I h in spontaneous firing and cold activation in neurons recorded with either control or I h agonist (cyclic AMP)-containing artificial intracellular fluid. I h did not contribute to the membrane potential (V m) and spontaneous firing at 20°C. Although voltage-clamp analysis confirmed that cooling inhibits I h, its lack of involvement in setting baseline firing and Vm precluded its ability to regulate cold activation as hypothesized. In contrast, neurons dialyzed with cAMP exhibited greater baseline firing frequencies at 20°C due to I h activation. Our hypothesis was supported when the starting level of I h was enhanced by elevating cAMP because cold activation was converted to more ordinary cold inhibition. These findings indicate that situations leading to enhancement of I h facilitate firing at 20°C, yet the hyperpolarization associated with inhibiting a depolarizing cation current by cooling blunts the net V m response to cooling to oppose normal cold-depolarizing factors. This suggests that the influence of I h activation state on neuronal firing varies in the poikilothermic neuronal environment

Impact of a CO2 Gradient on the Behavior of the Red Crayfish, Procambarus clarkii

With carbon dioxide levels on the rise, studies to investigate the possible detriment that climate change will have on our ecosystems and the organisms that live within them are essential. The field lacks an abundance of studies focusing on the effects of rising CO2 levels on freshwater organisms. This study looks at the effects of a CO2 gradient on the freshwater crayfish Procambarus clarkii. The gradient allows the crayfish to choose to avoid or prefer the higher carbon dioxide levels. Previous studies have looked at the effect of high CO2 levels, decreased pH, on a variety of crustaceans, but did not use the gradient. Crayfish were introduced into a control environment and observed for normal behavior then introduced into a CO2 gradient environment. The crayfish did not prefer a particular section of the tank in the control environment, making the CO2 gradient experiment possible. When in the CO2 gradient, the crayfish significantly preferred Sections 1, 2, and 3 over Section 4 (where the highest CO2 levels were present). In the CO2 gradient, the crayfish exhibited less hiding behavior and did not acclimatize to the CO2 levels over time. The crayfish left themselves to be more vulnerable to their surroundings. However, exploratory and feeding behavior were surprisingly not affected by the CO2 gradient environment. Rising carbon dioxide levels have the potential to negatively affect freshwater organisms such as the crayfish, but crayfish also may have the potential to adapt to these alterations brought about by climate change, especially if the change takes place over a significantly longer period of time