Localization and role of leptin in the thyroid gland of the lizard Podarcis sicula (Reptilia, Lacertidae)

Leptin, the product of the ob gene, is a hormone secreted by adipocytes thatregulates food intake and energy expenditure. The hypothalamus–pituitary–thyroid axis is markedlyinfluenced by the metabolism status, being suppressed during food deprivation. The present studywas designed to ascertain whether (1) lizard thyroid gland expresses the long form of leptin receptor(Ob-Rb) and (2) the leptin administration affects the thyroid gland activity in this species (and toverify whether leptin plays a similar role in reptiles as observed in the other vertebrates). Thepresence of leptin receptor in the thyroid gland of Podarcis sicula was demonstrated byimmunohistochemical technique (avidin–biotin–peroxidase complex—ABC method). The role ofleptin in the control of thyroid gland activity was studied in vivo using light microscopy (LM)technique coupled to a specific radioimmunoassay for thyroid-stimulating hormone (TSH) andthyroid hormones (T4 and T3). Leptin (0.1 mg/100 g body wt)/day increased T4 and T3 release for 3days but decreased the plasma concentration of TSH; using LM clear signs of stimulation in thethyroid gland were observed. These findings suggest that systemic administration of leptinstimulates the morphophysiology of the thyroid gland in the lizard through a direct mechanisminvolving Ob-Rb.