Circadian and Ultradian Variations in the Plasma Level of MaturationalGonadotropin (GTH II) in the Indian Catfish, Clarias batrachus.

Analysis of changes in the plasma levels of GTH II over a 24-h day/nightcycle revealed statistically significant circadian or ultradian variationor both in its circulating level during the gonadally active months of thereproductive cycle. The 24-h average and amplitude of circadian or ultradianrhythm in GTH II increased with the advancement of ovarian development andmaturation. In April, a single peak in GTH II level was noticed in the nightat 20.6 h. In contrast, in May, June and July a biphasic pattern (ultradianrhythm) was noticed with two characteristic peaks, one in the photophase andthe other in the scotophase. In addition, in May a statistically significantcircadian rhythm in plasma GTH II was validated with a peak located at 20.1h. These rhythms seem to have physiological significance. The ultradian rhythmwith two peaks during the reproductively active phase appears to provide asuitable physiological milieu for the temporally different yet synchronouspopulation of oocytes for the secretion of steroids. Thus, the observed temporalorganization in GTH II may have physiological consequences leading to accomplishmentof reproductive process at appropriate time of the year. [ABSTRACT FROM AUTHOR]