A homologous salmonid leptin radioimmunoassay indicates elevated plasma leptin levels during fasting of rainbow trout.

The present study was conducted to establish a homologous radioimmunoassay (RIA) for quantifying plasma leptin (Lep) levels in salmonid species, and to study Lep levels in relation to nutritional status. A part of the Lep peptide, a 14 amino acid long sequence, identical between a Salmo and an Oncorhynchus species was synthesised. Polyclonal antibodies were raised in rabbit against this antigen and both were subsequently used in the development of a RIA protocol for assessing plasma Lep levels. The limit of detection of the assay was 0.3 nM, and intra- and interassay coefficient of variation (CV) were 8.4% and 13%, respectively. Apart from Atlantic salmon and rainbow trout, the assay exhibits measuring parallelism for a range of fish species, including arctic char, Atlantic cod and turbot, suggesting that the established RIA is useful for quantifying Lep levels in several fish species. The RIA indicates that Lep is found in salmonid plasma at levels of 0.5-5 nM, which is comparable with other peptide hormones, and well within the measuring range of the RIA. A study of fed and fasted rainbow trout showed elevated plasma Lep levels during fasting. In addition there was no correlation between Lep levels and condition factor. These data suggest that the relation between circulating Lep levels and energy status differs from that in mammals. While Lep is linked to energy balance, it may not act as an adiposity signal in salmonids, possibly pointing to functional divergence among ectothermic and endothermic vertebrates.