Brain and adipose tissue global gene network responses to photoperiod in pre-pubertal European sea bass

Abstract

Puberty is a developmental process in which the hypothalamus-pituitarygonadal axis is activated and animals become reproductively active. In European sea bass, Dicentrarchus labrax, precocious puberty among males results in smaller size at the time of marketing. Photoperiod manipulations are able to inhibit (continuous light - CC) or advance (shift from long to short days - AP) this process, although the underlying mechanisms are largely unknown. In this study, we explored the effects of CC and AP compared to natural photoperiod (NP) in the brain and adipose/pancreatic transcriptomes of pre-pubertal sea bass applied before the summer solstice (from April). We hypothesized that larger fish, who become reproductive during the first year, would have a different response to AP compared to non-reproductive smaller fish and other photoperiods. We constructed SuperSAGE libraries from brains and adipose/pancreatic tissue of immature fish of two sizes (Llarge fish and S-small fish) reared in NP, CC or AP, and sampled at different times after photoperiod change. Comparison of the brain transcriptome under NP showed the larger fish with higher levels of circadian clock, glycolysis-related and fatty acid sensing genes, as well as differential expression of growth regulating neuropeptides - higher PACAP/PAC1 and lower somatostatin (SST). In the adipose/pancreatic tissue, downregulation of pancreatic hormones ¿ insulin (INS), STT, and glucagon (GCG) - and upregulation of lipolysis-related genes in L compared to S. When L were exposed to AP, the opposite expression was seen, as well as the activation of SST in the brain and SST and INS/GCG genes in the pancreatic tissue, indicating inhibition of lipolysis and growth stimulating signals. Exposure to CC downregulated growth promoting signals in L but had no effect on SST in the brain nor on SST or INS/GCG levels in pancreatic cells. This may explain the stimulation of lipolysis related genes and previously reported reduced growth/weight seen in fish under CC. Overall these results suggest that precocious puberty is associated to growth/metabolic networks which respond differently according to fish size and photoperiod ¿ activation of puberty seems to be associated with lipid storage and activation of SST and INS/GCG axis.