Metabolic depression and non-specific immune response during hibernation of common Asian toad, Duttaphrynus melanostictus.

To assess metabolic depression and non-specific immune response during hibernation in male common Asian toads, Duttaphrynus melanostictus, we measured activities of different enzymes of both aerobic (oxygen-dependent) and anaerobic (oxygen-independent) metabolic pathways in liver tissue and some non-specific immune responses in blood and liver tissue by obtaining hibernating toads directly from their hibernaculum in nature. Though decreased activities of enzymes and suppressions of non-specific immune responses were hypothesised, some contrasting results were found. Activities of citrate synthase (CS) and isocitrate dehydrogenase (ICDH) enzymes of aerobic metabolic pathways showed a significant decrease in their activities during hibernation up to 29% and 61% respectively of their active period value. Contrary to our hypothesis enzymes of oxygen-independent metabolic pathways i.e. pyruvate kinase (PK) and lactate dehydrogenase (LDH) showed no significant changes in their activities during hibernation compared to the active period. This shows aerobic metabolic depression during normoxic hibernation in common Asian toads and maintenance of vital activities at a minimum level with use of energy (ATP) generated from the oxygen-independent metabolic pathway. Likewise, the non-specific immune response comprising total leucocyte count, individual leucocytes like neutrophil, eosinophil, basophil, lymphocyte and monocytes showed a significant decrease in their count during hibernation along with a reduction in complement proteins indicated by serum bacteria-killing ability, compared to active period. In contrast, the levels of reactive oxygen species (ROS) in liver tissue resulting in oxidative stress in terms of TBARS formed and GSSG/GSH ratio were significantly higher during hibernation, suggesting some components of non-specific immunity remain elevated. We conclude that, though there is suppression of non-specific immune response during hibernation to a maximum extent to conserve energy, some components of it in terms of oxidative stress are still in an active state to provide the signal to adaptive immunity for a quick response that is expected during post hibernation phase. Further, it indicates that non-specific immune response during hibernation is variable and tissue-specific.