Developmental changes and day-night expression of the gonadotropin-inhibitory hormone system in the European sea bass: Effects of rearing temperature

The role of rearing temperature on fish development, sex differentiation and puberty has been largely addressed, but the impact of water temperature on the ontogeny of the main neuroendocrine systems controlling reproduction has received little attention. Gonadotropin-inhibitory hormone (GnIH) has been shown to act on gonadotropin-releasing hormone (GnRH) neurons and on the pituitary to inhibit gonadotropin release and synthesis in vertebrates, including sea bass, Dicentrarchus labrax. In the present study we investigated the effects of rearing temperature during the thermosensitive period (5–60 days post-fertilization, dpf) on the expression of the GnIH gene (gnih) and its receptor (gnihr). Animals were maintained under two different conditions, low temperature (LT, 15 °C) or high temperature (HT, 21 °C), throughout the thermosensitive period and sampled from 5 to 360 dpf at mid-light (ML) and mid-dark (MD). Our results showed significant effects of temperature on gnih and gnihr expression during the thermosensitive period, with higher transcript levels under LT condition. Some differences were also evident after the completion of the sex differentiation process. Moreover, we revealed daily variations in the developmental expression of gnih and gnihr, with higher diurnal mRNA levels at early stages (until 25 dpf), and a shift to higher nocturnal expression levels at 300–360 dpf, which corresponded with the beginning of the winter (reproductive season). To the best of our knowledge, this work represents the first study reporting the effects of rearing temperature on the transcription of gnih system genes, as well as its daily variations during the development of a fish species.