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

Abstract

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 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.