Leptin-like immunoreactivity in the muscle of juvenile sea bass (Dicentrarchus labrax)

The mammalian hormone, leptin, is mainly synthesized in adipose tissue along with other tissues. Leptin plays a role in numerous processes such as in the control of food intake, homeostasis, immune function and reproduction. In this study, we detected and localized leptin im- munoreactivity to the muscle of early juvenile sea bass (Dicentrarchus labrax) by Western blot analysis and immunohistochemistry. A leptin immunopositive band with a molecular weight of $16 kDa, corresponding to mammalian leptin, was identified in trunk skeletal muscle homogenate. Furthermore, leptin immunopositive cells were detected in the endomysium of skeletal muscular fibers. These cells showed immunostained cytoplasmic granules and roundish and oval nuclei. The most intense immunostaining was observed in the endomysial space among the superficial red muscular fibers of the trunk. These findings suggest that in early juvenile sea bass, leptin is mostly produced by skeletal muscles. Therefore, during the developmental stage lacking adipose tissue, skeletal muscles can be considered an important source of leptin. As already suggested in mam- mals, we can hypothesize the potential roles of leptin not only in energy expenditure for muscle contraction but also during muscle differentiation and growth.