Time-course studies on plasma glucose, insulin, and cortisol in sea bass (Dicentrarchus labrax) held under different photoperiodic regimes

Daily variations of insulin, cortisol, and glucose are studied in animals adapted to two different photoperiodic regimes, with the intervals between feeding times and photoperiodic events kept constant. Data support the existence of a daily rhythm of plasma glucose which seems to be photoperiodic. In contrast, the daily patterns of insulin in sea bass seem to be mainly influenced by feeding time; however, an effect of photoperiod can not be excluded. When the digestive tract is absent of food, insulin levels are generally minimal at feeding times and maximal during the inter-meals periods, suggesting the central control of insulin secretion during short-term food deprivation. Contrarily, the nadir values of plasma cortisol were reached at midday during the inter-meal period and peak plasma levels were evident at both light onset and offset. Disruption between metabolite and hormone patterns suggest that they are under different controls. Such results could be explained under the existence of a multioscillator system, including a food entrainable oscillator in addition to the master light entrainable oscillator.This research was supported by European Commission Project no: FAR Project AQ 2417. The authors wish to thank J. Gutiérrez and W. Hanke, who were also involved in EC FAR Project AQ. 2417, for kindly providing their facilities for the insulin and cortisol assays, respectively. We are also grateful to L. A. Sorbera for critical review of the manuscript and to J. Ramos, M. Blázquez, and X. Simó for their immense help in fish sampling