Study protocols. (A) Effect of prior exposure to cold. Rats were placed in cold (4 °C) environment for 4 h before tracer administration. Rats were replaced in cold for additional 60 min before sacrifice. Control rats underwent similar treatment, but without exposure to cold. (B) Effect of propranolol. Propranolol (5 mg/kg) was administered intraperitoneally to rats. After 30 min at room temperature (22 °C), rats were placed in cold environment for 4 h. Radioactive tracers were then administered intravenously, and rats were put back in cold for 60 min before sacrifice. Control rats underwent same cold treatment, but without propranolol administration. (C) Effect of cold on 18F-FBnTP retention in BAT. 18F-FBnTP was administered at room temperature, and after 60 min, rats were placed in cold environment for either 120 or 240 min before sacrifice. Control rats were kept at room temperature and sacrificed at 60, 180, and 240 min after tracer administration. IP = intraperitoneal; MIBI = sestamibi; PROP = propranolol; Sac = sacrifice.