Photoperiodic modulation of circadian clock and reproduction related genes in the European sea bass brain at the early onset of puberty

Trabajo presentado en el 17th International Congress of Comparative Endocrinology (ICCE 2013), celebrado en Barcelona (España) del 15 al 19 de julio de 2013
Fish puberty is a developmental period that is rhythmically programmed and covers the transition from a juvenile to a mature adult. In the European sea bass long term exposure to continuous light (LL) alters the rhythm of reproductive hormones, delays spermatogenesis and reduces the incidence of precocious males, while a shift from long to short photoperiod (CP) accelerates spermatogenesis. However, how photoperiod affects key genes in the brain to trigger the onset of puberty is still largely unknown. One possibility is that the integration of the light stimulus by clock proteins is sufficient to activate key genes that trigger the brain- pituitary-gonad (BPG) axis. In order to shed light on these regulatory mechanisms, the promoter sequences of circadian clock genes (CLOCK, NPAS2, ARTLN) were compared with those of GnRH, KISS and KISSR and found that they share between themselves and other 173 gene promoters conserved transcription factor (TF) frameworks (same sets of equally spaced TF binding sites). Cluster analysis according to functional role showed an enrichment of genes linked to regulation of hormones and reproduction in general. Gene expression analysis in the brain of male European sea bass exposed to LL, CP and natural photoperiod (NP) provided evidence for co-regulation of the predicted genes by the different light regimes. LL fish, compared to other photoperiods, had low expression of sbGnRH, KISS1, GALR1b and ER¿, suggesting a suppressed BPG axis. Conversely, up-regulation of NPAS2, sbGnRH, cGnRH, KISS1, KISSr1b and GALR1b was detected within the first 48 hours in CP fish. These results show that clock and reproductive axis related genes are strongly modulated by light, prior to the first detectable signs of endocrine brain-pituitary-gonadal axis activation.