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In a previous work we showed that only unilocular brown adipocytes express leptin. In order to investigate the relationship between leptin gene expression, brown adipocyte activity (UCP1) and morphology, we studied brown adipose tissues of mice (C57BL, female, 7 weeks old) acclimated at different temperatures (19°C and 28°C). Northern blot analysis revealed higher leptin and lower UCP1 mRNA levels in mice exposed to 28°C than in the group acclimated at 19°C. Also protein expression (immunohistochemistry) differed in the two groups: at 28°C brown adipocytes were positive for leptin and only weakly positive for UCP1, while at 19°C they were leptin-negative and UCP1-positive. In the former group the morphology was mainly unilocular. Our data suggest that in brown adipocytes of warm- acclimated mice leptin expression is closely related to their hypoactive functional stage, as evidenced by their low …