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Growth hormone (GH), a key regulator of somatic and reproductive growth in vertebrates, has been extensively studied, although primarily in female fish. Despite numerous reports about sex- and species-specific growth patterns in fish, to our knowledge, there is no report about the 24 h rhythm of plasma GH in male fish. Thus, we aimed to investigate temporal variations in plasma GH levels and the existence of any rhythms therein during the reproductively active months of March to August in the male walking catfish, Clarias batrachus. We also aimed to compare the secretory temporal patterns of GH in male–female specimens of C. batrachus to decipher sexual dimorphism in GH secretions in fish. After 14 days of acclimation to the natural environment, male catfish (N = 240 in total) were sorted and randomly divided into eight groups for study at ZT0 (sunrise ~06:00 h), 3, 6, 9, 12, 15, 18, and 21. During each month, physical parameters like duration of photoperiod and water temperature were measured. Male catfish (n = 40/month) in all eight groups were sampled (n = 5/group) at each time point under the natural time-of-year 24 h light-dark (LD) cycle. Male catfish were anesthetized and blood was collected through a caudal puncture, centrifuged, and plasma isolated. Plasma GH was measured using a competitive homologous enzyme-linked immunosorbent assay. Further, testes were removed, weighed, and the gonadosomatic index (GSI) was calculated. A significant effect of time and season (p ˂ 0.05, two-way ANOVA) on plasma GH level was detected. Cosinor analyses verified the existence of statistically significant (p ˂ 0.05) ultradian (12 h) and 24 h rhythms of plasma GH in male C. batrachus, with the higher values of Mesor (time series mean) and amplitude (one-half peak-to-trough difference) of the periodicities from March to July. Mapping of the acrophases (peak times) showed two ultradian and one 24 h acrophase of GH during the early photophase and early scotophase from March to August. Distinct sexual-dimorphism in plasma GH Mesors and acrophases was noticed between male and female catfish. GSI values of male catfish indicate males mature a little earlier than females in terms of size and reproductive activity. The findings that plasma GH show 24 h and seasonal fluctuations in a sex-specific manner collectively demonstrate the importance of considering the effect of biological 24 h and seasonal time and sex on the GH level in regulating the physiology of somatic growth and reproduction in catfish. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]