Day and night variation of plasma melatonin and glucocorticoids in a subterranean rodent endemic to north-west Argentina in two seasons.

Tuco-tucos (Ctenomys coludo) are Neotropical rodents that inhabit the extreme photic environment of the subterranean, thus being intriguing models for chronobiological studies of circadian and seasonal rhythms. Here we characterized day/night profiles of plasma melatonin and glucocorticoids along with their activity/rest rhythms from freshly caught animals during two seasons. Animals were live-trapped in their native Monte Desert (Argentina) during summer (n = 27) and winter (n = 18), housed under constant dark conditions with wheel-running activity recording prior to sample collection. Sampling occurred at six timepoints across day and night to obtain cross-sectional data. Unlike typical lab-based seasonal studies using artificial photoperiods, our approach captures the complexity of environmental cycles by studying wild animals exposed to the full array of environmental variations across seasons. Melatonin, cortisol and corticosterone were analyzed via ELISA. Melatonin levels showed circadian rhythmicity with higher levels during subjective night. Results revealed higher levels of cortisol secretion during subjective night, consistent with the profile of a nocturnal-active animal. Cortisol levels were 25 to 50 times higher than corticosterone. These findings provide the first circadian melatonin and glucocorticoid baseline for C. coludo in two seasons paving the way for future work on their endocrine rhythms and interplay with activity/rest rhythms and other circadian patterns.