Cold exposure restores the decrease in leptin receptors (OB-Rb) caused by neonatal leptin treatment in 30-day-old rats

We had previously shown that neonatal leptin treatment programs thyroid function in adulthood. As both thyroid hormones (TH) and leptin increased thermogenesis, it was interesting to evaluate the effect of cold exposure on the thyroid function of neonate rats treated with leptin. Pups were divided into two groups: Lep, injected with leptin (8 mg/100 g/BW, s.c.) for the first 10 days of lactation and control (C), injected with saline. When they were 30 days old, the groups were subdivided into two subgroups: LepC and CC, which were exposed to 8 8C for 12 h and compared with C and Lep groups, maintained at 25G1 8C. Serum leptin, TH, and TSH were measured by RIA. Type I liver deiodinase (D1) and mitochondrial a-glycerol-3-phosphate dehydrogenase (mGPD) activities were assayed by the release of 125 I from 125 I-reverse and colorimetric method respectively. Leptin receptor (OB-Rb) was evaluated by western blot. Lep group had hyperleptinemia (C22%) and lower free tri-iodothyronine (FT3; K33%). Cold exposure increased TH both in LepC and CC groups compared with respective controls free thyroxine (FT4:C63 and C39%; FT3:C75 and C40%). Liver D1 activity was lower in Lep (K22%) and increased with cold exposure (LepC C51% and CC C22%). The mGPD activity was lower in Lep (K34%) and increased (fourfold) when this group is cold exposed. Hypothalamic and thyroidal OB-Rb receptors were lower in Lep group (K47 and K36% respectively) and they were restored to normal levels after cold exposure. Leptin-programmed rats had higher TH response after cold exposure. OB-Rb had a fast response to cold exposure normalizing the lower levels observed in the leptin-programmed animals and may contribute to the higher TH cold responses.