Temporal relationship between the circulating profiles of melatonin and ovarian steroids under natural photo-thermal conditions in an annual reproductive cycle in carp Catla catla

A temporal relationship between the serum profiles of pineal-hormone melatonin (N-acetyl-5-methoxytryptamine) and two major ovarian steroids, i.e., 17-β estradiol (E2) and 17α, 20β-dihydroxy-4-pregnen-3-one (17α, 20β DHP) or maturation inducing hormone (MIH), has been sought for the first time in any free-living teleost. The study was carried under natural conditions in a carp Catla catla following collection of blood samples at different time-points in a diurnal cycle during each of the four distinct reproductive phases in an annual cycle. The levels of melatonin and the steroids were measured following specific RIA and ELISA techniques, respectively. A time-bound change in the serum melatonin was noted in a 24 h cycle, though the pattern of fluctuations was not identical throughout the reproductive cycle. Melatonin reached its peak during the mid-night in each reproductive phase, other than the preparatory phase (January–March) when the diurnal peak value was recorded in the late dark phase. In an annua...