Effects of seasonal variation on oxidative stress physiology in natural population of toad Bufo melanostictus; clues for analysis of environmental pollution.

Natural population of Bufo melanostictus in response to environmental cues shows several physiologic changes such as reproductive activity, hibernation, aestivation and metabolic depression in different seasons. We investigated the effects of seasonal fluctuations on oxidative stress (OS) physiology biomarkers, such as endogenous (ELPx) and induced (ILPx) lipid peroxidation, front-line redox regulatory enzymes (superoxide dismutase: SOD and catalase) and two non-enzyme antioxidant metabolites (ascorbic acid and reduced glutathione) in liver, gonad and cerebral hemisphere of toads collected from the Bhubaneswar area of India, where temperature fluctuates considerably rising to the highest in summer (∼46 °C) and being lowest in winter (<10 °C). Soil and air of the sampling site, although varying seasonally, were mostly found to be unpolluted, except for suspended particulate matter and respiratory particulate matter that were above recommended value. The magnitude of both ELPx and ILPx levels in most of the tissues, for example, ELPx in liver, cerebral hemisphere and ovary, and ILPx in liver of males and ovary, were found to be higher in rainy season in comparison to the other seasons. Nevertheless, levels of both ELPx and ILPx were low in testes in rainy season in comparison to the other two seasons. No correlation was observed between temperature and the studied OS parameters except a positive correlation with SOD and negative correlations with non-enzymatic small redox regulatory molecules in some selected tissues. Conversely, discriminant function analysis reveals a clear impact of the changing season on OS physiology of the toad. It implies that season considerably modulates OS physiology which be a reflection of the toads to abiotic pollutants alone and/or as results of metabolic changes under hibernation, aestivation and due to reproductive activities. Therefore, seasonal changes in OS physiological responses in poikilothermic models especially in toads must be cautiously used as indicators to assess environmental impact, mainly soil pollution. Results of the present study may be used as baseline data for any future analyses of the physiological impacts of environmental changes using toads as model organism.