White and brown adipocytes are usually located in distinct depots; however, in response to cold, brown adipocytes appear in white fat. This response is mediated by b-adrenoceptors but there is a controversy about the subtype(s) involved. In the present study, we exposed to cold b 3 -adrenoceptor knockout mice (b 3 KO) on a C57BL/6J genetic background and measured in white adipose tissue the density of multilocular cells and the expression of the brown adipocyte marker uncoupling protein-1 (UCP1). In brown fat of b 3 KO mice, UCP1 expression levels were normal at 24°C as well as after a 10-day cold exposure. Strikingly, under both conditions, in the white fat of b 3 KO mice the levels of UCP1 mRNA and protein as well as the density of multilocular cells were decreased. These results indicate that b 3 -adrenoceptors play a major role in the appearance of brown adipocytes in white fat and suggest that the brown adipocytes present in white fat differ from those in brown fat.Keywords: adipose tissue; cold exposure; differentiation; knockout; uncoupling protein 1.Brown adipose tissue (BAT) and white adipose tissue (WAT) both play an important role in the control of energy balance in mammals. Indeed, BAT is involved in the control of body temperature and body weight via nonshivering and diet-induced thermogenesis [1,2] whereas the primary function of WAT is to store and release energy. BAT function is particularly important for thermoregulation in small mammals in which the surface to volume ratio is unfavorable [1]. Moreover, activation of BAT prevents obesity induced by overfeeding in rodents [2].Cold exposure, via activation of the sympathetic nervous system, induces heat production in BAT [3,4] and lipolysis in WAT [5]. The effects of norepinephrine released at the nerve endings are mediated in brown and white adipocytes mainly by the b 3 -adrenoceptor subtype [6].Brown and white adipocytes are usually located in distinct depots [4] and can be distinguished morphologically [7]: brown adipocytes contain multiple (multilocular) lipid droplets and numerous mitochondria whereas white adipocytes display a main (unilocular) lipid droplet and few mitochondria. Nevertheless, the morphological aspect alone is not sufficient to distinguish between these two cell types [8]. In fact, the only criterion is the specific expression in mitochondria of brown adipocytes, of the uncoupling protein-1 (UCP1), which uncouples the oxidative phosphorylations from electron transport, thus dissipating energy as heat [9].An observation made initially by Young et al.[10] was the presence of multilocular mitochondria-rich brown adipocytes in depots previously defined as typical WAT. The emergence of these ectopic cells was found to be induced by cold acclimation in WAT of rats [11,12] and of mice [13,14]. The new cells were found to be sympathetically innervated [15] and to remain present as long as a sympathetic nervous system stimulation persisted [16]. Several reports also showed that the administration of selective b 3 -adrenoceptor agonis...