The mitochondria in the cytoplasm of brown adipose tissue cells contain uncoupling protein (UCP-1), which uncouples the oxidation of fatty acids from the production of ATP. Therefore, protons are allowed to travel from the inter-membrane space back to the mitochondrial matrix along the gradient without passing through ATP synthase and thus without producing ATP. This can occur since an alternative pathway for the protons return is available through an UCP-1 that facilitates proton transport across the inner mitochondrial membrane. The movement of protons from the inner mitochondrial compartment dissipates the mitochondrial proton gradient, thus uncoupling respiration from ATP synthesis. The energy produced by the mitochondria is then dissipated as heat in process known as thermogenesis.
The metabolic activity of brown adipose tissue is regulated by the sympathetic nerve system and is related to ambient outdoor temperature.
The metabolic activity of brown adipose tissue is largely regulated by norepinephrine released from the sympathetic nerve terminals, which stimulates lipolysis and hydrolysis of triglycerydes as well as increases mitochondrial expression and activity of UCP-1 molecules. In experimental animals, chapter 9 Adipose Tissue B ROWN AD I POSE TISS U E TABLE Summary of Adipose Tissue Features 9.2 Features White Adipose Tissue Brown Adipose Tissue Location Subcutaneous layer, mammary gland, Large amounts in newborn greater omentum, mesenteries, Remnants in adults at the retroperitoneal retroperitoneal space, visceral space, deep cervical and supraclavicular pericardium, orbits (eye sockets), regions of the neck, interscapular, bone marrow cavity paravertebral regions of the back, mediastinum Function Metabolic energy storage, insulation, Heat production (thermogenesis) cushioning, hormone production, source of metabolic water Adipocyte morphology Unilocular, spherical, flatten nucleus, Multilocular, spherical, round rim of cytoplasm eccentric nucleus Large diameter (15–150 m) Smaller diameter (10–25 m) Transcription factors “master PPAR-/RXR PRDM16/PGC-1 switch” in differentiation UCP-1 genes expression No Yes (unique to brown fat) Mitochondria Few, poorly developed Many, well developed Innervation Few sympathetic nerve fibers High density of sympathetic nerve fibers Vascularization Few blood vessels Highly vascularized tissue Response to environmental Decreased lipogenesis Increased lipogenesis stress (cold exposure) Increased lipoprotein lipase activity Decreased lipoprotein lipase activity Growth and differentiation Throughout entire life from Only during fetal period stromal-vascular cells Decreases in adult life (exception: individuals with pheochromocytoma and hibernoma)  FOLDER 9.3 Clinical Correlation: PET Scanning and Brown Adipose Tissue Interference