Circadian rhythms of gonadal and extra-gonadal hormonal and glycemic profiles during the breeding phase of the ovarian cycle of Indian estuarine grey mullets, Mugil cephalus L.

Abstract

Circadian rhythms of ovarian activity during the breeding phase of the circannual ovarian cycle were poorly known in estuarine teleosts. This topic was resolved in an Indian estuarine teleost, the grey mullet, Mugil cephalus. Several hormones and blood glucose profiles were investigated at four different time intervals (06.00, 12.00, 18.00 and 24.00 h) of a 24-h period. Sex hormones [estradiol-17β, testosterone and 17α,20β-dihydroxy-4-pregnen-3-one (DHP)] were quantitated by RIA, FSH, LH, prolactin, T3, T4 and TSH and insulin by ELISA and cortisol, catecholamines and indoleamines by fluorometric analysis. Estradiol-17β, testosterone and DHP profiles were higher in the photophase (06.00–18.00 h) than the scotophase (18.00–06.00 h), with a peak at 12.00 h, whereas, FSH, LH and prolactin concentrations were higher in the scotophase than the photophase with a peak at 24.00 h. But cortisol, epinephrine, norepinephrine, and insulin levels were higher in the photophase than the scotophase, with peaks at 06.00 h for T3 and T4, at 12.00 h for adrenal hormones and at 06.00–18.00 h for insulin. Blood glucose profile was lower in the photophase than the scotophase with a peak at 12.00 h unlike insulin. Pineal hormonal levels (NAS and melatonin) were higher in the scotophase than the photophase with a peak at 24.00 h. Serotonin level was at peak at 12.00 h. Environmental factor, such as photoperiod is likely to be responsible for the induction of circadian rhythms of hormones and consequently on blood glucose rhythms via hypothalamic SCN, pineal and retino-hypophysial tract in the mullet. A probable involvement of clock genes in the induction of circadian rhythms of hormones and blood glucose profiles is indicated in the mullet ovarian function.