Photoperiodic regulation of nuclear melatonin receptor RORα in lymphoid organs of a tropical rodent Funambulus pennanti: role in seasonal oxidative stress.

The orphan nuclear receptor RORα of RZR/ROR family has been suggested to mediate the genomic actions of melatonin on the expression of antioxidant enzymes. However, no direct evidences exist. In the present study we explored the role of photoperiod (natural and artificial) in regulation of RORα and its association with the photoperiod induced antioxidant defense system in the lymphoid organs (spleen and thymus) of seasonally breeding, tropical squirrels, Funambulus pennanti. The photoperiod mediated regulation of antioxidant status was checked along with the RORα expression and circulatory melatonin level in the squirrels. The enhancement of the antioxidant capacity of serum and lymphoid organ was concomitant with the short photoperiod (10L:14D) induced high levels of plasma melatonin. Further, peripheral melatonin level enhanced the AANAT activity as well as the melatonin synthesis in the lymphoid tissues. RORα expression presented an inverse correlation with the plasma level of melatonin as well as the short day induced antioxidant enzyme activity in the lymphoid organs. The results suggest that for reduction of seasonal oxidative stress melatonin might not be utilizing the nuclear receptor RORα pathway; rather the rise in circulatory melatonin collectively with tissue specific melatonin might be protecting the splenic and thymic lymphocytes from the seasonal oxidative stress.