Interactions of leptin and leptin receptors play crucial roles during animal development and regulation of appetite and energy balance. In this study we analyzed expression pattern of a zebrafish leptin receptor gene in both developing and adult zebrafish using in situ hybridization and Q-PCR methods. Zebrafish leptin receptor message (lepr) was detected in all embryonic and larval stages examined, and in adult zebrafish. In embryonic zebrafish, lepr was mainly expressed in the notochord. As development proceeded, lepr expression in the notochord decreased, while its expression in several other tissues, including the trunk muscles and gut, became evident. In both larval and adult brains, large lepr expressing cells were detected in similar regions of the hindbrain. In adult zebrafish, lepr expression was also observed in several other brain regions including the hypothalamic lateral tuberal nucleus, the fish homolog of the arcuate nucleus. Q-PCR experiments confirmed lepr expression in the adult fish brain, and also showed lepr expression in several adult tissues including liver, muscle and gonads. Our results showed that lepr expression was both spatially and temporally regulated.

All organisms face tradeoffs with regard to how limited energy resources should be invested. When is it most favorable to grow, to reproduce, how much lipid should be allocated to storage in preparation for a period of limited resources (e.g., winter), instead of being used for growth or maturation? These are a few of the high consequence fitness “decisions” that represent the balance between energy acquisition and allocation. Indeed, for animals to make favorable decisions about when to grow, eat, or reproduce, they must integrate signals among the systems responsible for energy acquisition, storage, and demand. We make the argument that leptin signaling is a likely candidate for an integrating system. Great progress has been made understanding the leptin system in mammals, however our understanding in fishes has been hampered by difficulty in cloning fish orthologs of mammalian proteins and (we assert), underutilization of the comparative approach.

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