Metabolic depression is a ubiquitous characteristic of dormancy, allowing animals to survive off limited endogenous fuels supplies for extended periods of time. Metabolic depression is characterised by a complex set of physiological and biochemical changes (respiratory depression, bradycardia, hypophagia and cessation of renal function), in addition to a remarkable ability to prevent or minimise damage usually associated with such depressive changes (e.g. gut and muscle atrophy, renal failure and ischemic injuries). A coordinated shut-down of systems is essential to surviving long periods of dormancy, however, the mechanisms involved in regulating metabolic depression remain poorly understood. Aestivation, a period of dormancy that occurs in response to desiccating environments, provides an excellent model system for investigating the regulation of metabolic depression, because unlike other forms of dormancy such as hibernation, it is not confounded by changes in temperature or PO2 and thus represents a true intrinsic metabolic depression. The green striped burrowing frog, Cyclorana alboguttata, is an abundant species of burrowing frog inhabiting arid to semi-arid regions of Queensland and Northern New South Wales. During the dry season, C. alboguttata aestivates in an underground clay chamber; the aestivating period lasts on average for nine months a year, but in drought conditions may last for as long as several years. In recent years, C. alboguttata has become a well studied organism for investigating the physiological processes involved in aestivation, especially regarding muscle disuse atrophy and gut maintenance. Despite this, our understanding of this frog’s capacity to metabolically depress remains limited. The first aim of this study was to extend our current knowledge of metabolic depression during aestivation in C. alboguttata. C. alboguttata reduced whole animal metabolism by 82% within five weeks of aestivation. The effects of aestivation on mass specific in vitro tissue metabolic rate (VO2) varied among individual organs, with muscle and liver slices showing significant reductions in metabolism; kidney VO2 was elevated and there was no change in the VO2 of small intestine tissue slices. Organ size was also affected by aestivation, with significant reductions in the mass of all tissues, except the gastrocnemius. These reductions in organ size, combined with changes in mass specific VO2 of tissue slices, resulted in further energy savings to aestivating animals. Mitochondrial VO2 was significantly reduced during aestivation, by an average of 83%; in addition respiratory control ratios (RCRs), a measurement of mitochondrial coupling efficiency, significantly increased during aestivation, suggesting increased mitochondrial coupling efficiency. The second aim of this study was to investigate the potential role of the endogenous opioid system (EOS) and thyroid hormones in the regulation of metabolic depression in C. alboguttata. When incubated in the presence of opioid agonists, liver and muscle tissue slices from Cyclorana alboguttata underwent tissue- and agonist- specific metabolic depression. In most cases, the effect of opioid agonists on metabolism was more pronounced in tissue slices from aestivating animals. The delta (δ) opioid agonist DADLE was the most consistent in producing a metabolic depression, however the effect of DADLE on the metabolism of tissue slices was not reversible by the general opioid antagonist naloxone. Gene transcript levels for the δ-opioid receptor remained constant in the brain and in the muscle of aestivating C. alboguttata compared to controls. Opioid receptor XOR1 transcript levels also remained constant in the brain during aestivation compared to controls. There was a significant reduction in total thyroid hormone concentration in the plasma of C. alboguttata during aestivation, with thyroxine (T4) decreasing by 75% and an 84% reduction in triiodothyronine (T3). We also examined changes in gene expression of deiodinase 2 (D2) and deiodinase 3 (D3), the major activating and deactivating enzymes of thyroid hormones, respectively. There were no significant changes in gene expression of D2 in the brain or muscle, or of D3 in the brain; however there was a significant down-regulation of D3 transcripts in muscle tissue. In the liver, D2 was significantly down-regulated and D3 showed an increase in transcript levels that was approaching significance. During dormancy energy conservation is a key priority and as such dormant animals undergo a major metabolic depression to conserve their limited endogenous fuel supplies. At the same time, dormant animals are faced with the challenge of maintaining functional organs for immediate use upon arousal. In this study C. alboguttata appeared to selectively down- or up-regulating individual tissues, using both changes in metabolic rate and morphology. This strategy would allow maximal energy savings during aestivation without compromising organ functionality and survival at arousal. In addition, the frogs appeared to decrease rates of mitochondrial proton leak to a greater extent than ATP synthesis, consistent with an increase in mitochondrial coupling efficiency. Again, this ability to selectively down-regulate one process to a greater extent than another allows energy savings to be maximised without compromising processes essential for survival. C. alboguttata can survive in a dormant state for several years and it is hypothesised that these frogs become more energy efficient during aestivation. The regulation of metabolic depression still remains poorly understood. In this study the potential role of the EOS and thyroid hormones in regulating metabolic depression was investigated. The results of this study suggest the δ-opioid system may be responsible for the initiation of metabolic depression, while the mu (μ) and kappa (κ) opioid systems may be important during long term maintenance of aestivation. In addition, the EOS may exert its control over metabolism via both receptor and non-receptor mediated pathways. The significant down-regulation of total thyroid hormones, as well as the changes in gene expression of the thyroid deiodinases indicates thyroid hormones may also play a significant role in regulating metabolic depression. This suggests that the regulation of metabolic depression may involve several interacting systems, or perhaps a ‘systems cascade’.
| Identifer | oai:union.ndltd.org:ADTP/283956 |
| Creators | Sara Kayes |
| Source Sets | Australiasian Digital Theses Program |
| Detected Language | English |
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