123I- or 125I-Metaiodobenzylguanidine Visualization of Brown Adipose Tissue

123I-Metaiodobenzylguanidine (MIBG) accumulations that do not correspond to any tumor are observed occasionally on the medial aspect of the upper back or shoulder of children. The true nature of such accumulations is unknown, and we hypoth-esized that they represent interscapular brown adipose tissue (IBAT) visualized by scintigraphy. Methods: Wistar rats (7 wk old) received MIBG labeled with 123I or 125I. Autoradiography was performed, and concentrations of the tracer in the interscapular subcutaneous tissue were identified histopathologically. The effects of 6-hydroxydopamine, reserpine, and 3–adrenergic receptor agonist (CL316,243) on the accumulation were inves-tigated to elucidate the mechanism of uptake into BAT. Results: Autoradiography showed well-defined distinct accumulation in the subcutaneous tissue on the upper back, and hematoxylin– eosin and anti-uncoupling protein 1 antibody staining confirmed that it was BAT. The percentage injected dose per gram in BAT was as high as that in the heart and was quite different from the concentration in white adipose tissue. Preadministration of 6-hydroxydopamine or reserpine resulted in lower MIBG con-centrations in BAT. Activation of the 3–adrenergic receptor accelerated the washout of MIBG in BAT and caused an in-crease in concentration in white adipose tissue. Conclusion: MIBG accumulates in the adrenergic nervous system in BAT, and IBAT is distinguished from the surrounding white adipose tissue. To our knowledge, BAT has not been visualized previ-ously. We showed that MIBG scintigraphy might be suitable for the investigation of BAT and treatment of human obesity. Key Words: brown adipose tissue; metaiodobenzylguanidine; sympathetic neuro