Variability in the characteristics of ultradian and circadian rhythms in plasma levels of growth hormone in the Indian walking catfish, Clarias batrachus

Abstract

The plasma profile of growth hormone (GH) was monitored in the freshwater catfish, Clarias batrachus over a single 24-h transverse timescale in the middle of each month starting from March to August. This period included different physiological status of the fish and also varying environmental photoperiod and ambient temperature. Statistically significant ultradian rhythm in GH was detected in all the months of study. Of the two ultradian peaks, one was located invariably in the early photophase with the other one in the early scotophase. During the period of intense reproductive activity (June and July), circadian rhythm in GH was also recorded in addition to ultradian rhythm. The 24-h average (Mesor) and amplitude of ultradian rhythm of GH were high in March and April coinciding with the increase in photoperiod, temperature and feeding activity. Mesor increased further in May, reaching the maximum in June with a further rise in the water temperature and intense gonadal activity, but the amplitude declined during these months. In the breeding month of July, Mesor declined moderately and amplitude declined further. In August, Mesor and amplitude both attained minimum levels. In July and August one of the ultradian peaks overlapped with the circadian peak of the GH rhythm and the circadian peaks appeared in the night, while in other months it appeared in the early morning or at midday. Synchronization of one of the ultradian peaks with that of the circadian peak in GH in the night of July and August might provide physiological advantage for the execution of breeding at the coolest part of the day. The rhythmic patterns of GH secretion might have a strong correlation with the changing photoperiod, temperature, and feeding and reproductive activity. The observed temporal organization in GH in C. batrachus may have physiological consequences that might provide suitable milieu for the regulation of its somatic and gonadal growth, and execution of reproductive activity.