Evidence for leptin expression in fishes.

Tissues from bony fish were screened with anti-mouse leptin antibodies to detect the presence of the fat-regulating hormone in fishes. Low molecular-weight (16 kDa) immunoreactive bands were detected in blood, brain, heart and liver of green sunfish (Lepomis cyanellus), bluegill sunfish (Lepomis macrochirus), largemouth bass (Micropterus salmoides), white crappie (Pomonix annularis), channel catfish (Ictalurus punctatus), and rainbow trout (Oncorhynchus mykiss). To further verify that we had identified leptin, the response of fish "leptin" was measured in fed and fasted green sunfish. Fed sunfish had approximately threefold higher concentration of leptin in blood than did fasted sunfish (fed vs. fasted; 0.599 +/- 0.03 microg/microl vs. 0.196 +/- 0.04 microg/microl; P > F = 0.0001), which is consistent with mammalian models of leptin function. Brain leptin concentration is also positively correlated with percent body fat in white crappie and bluegill. Based upon electrophoretic mobility, immunoreactivity, response to fasting, and correlation with adiposity, we believe we have the first evidence for leptin expression in an ectotherm.