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- estradiol (E2) and 17, 20-dihydroxy-4-pregnen-3-one (17, 20 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 24h 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 annual cycle, melatonin titer was found to be maximum during the post-spawning phase (September-December). However, in each reproductive phase, a minimum daily value of melatonin was recorded in the mid-day. Serum E2, irrespective of reproductive phase, exhibited a daily peak in the mid-day and nadir in the early morning. However, in a seasonal cycle, E2 was found to be maximum in the spawning phase (July-August). The serum MIH, however, did not show any significant daily variations, and underwent significant changes in an annual cycle with a very low value in the preparatory phase followed by a gradual increase in the pre-spawning (April-June), a peak in the spawning phase, and was undetectable during the post-spawning phase. The simple correlation-coefficient analysis of the seasonal peak values of melatonin revealed a significant negative correlation with photoperiods, water temperature and serum E2, while a positive correlation was found between the seasonal values of serum E2, photoperiod and water temperature. In a seasonal cycle, serum MIH values exhibited a positive correlation with both water temperature and serum E2, and a negative correlation with melatonin. Collectively, the present study underlines the importance of environmental photo-thermal conditions, especially water temperature, as the functional correlate of seasonal changes in serum melatonin and ovarian steroids in carp.