Carnitine is necessary to maintain the phenotype and function of brown adipose tissue.

The juvenile visceral steatosis (JVS) mouse is a mutant strain with an inherited systemic carnitine deficiency. Mice of this strain show clinical signs attributable to impaired heat production and disturbed energy production. Brown adipose tissue (BAT) is the primary site of non-shivering thermogenesis in the presence of uncoupling protein-1 (UCP-1) in rodents and humans, especially in infants. To investigate the possible cause of impaired heat production in BAT, we studied the morphological features, carnitine concentration, and UCP-1 production of BAT in JVS mice. The effect of carnitine administration on these parameters was also examined. JVS mice aged 5 or 10 days (60 each) and age-matched control mice were used in this study, along with 10-day-old JVS mice treated subcutaneously with L-carnitine once a day between postpartum days 5 and 10. JVS mice showed lower body temperatures and lower concentrations of carnitine in BAT. Morphologically, BAT cells in JVS mice contained large lipid vacuoles and small mitochondria, similar to those present in white adipose tissue cells. In addition, UCP-1 mRNA and protein expression levels were significantly reduced in JVS as compared with control mice. Carnitine treatment resulted in significant increases in body temperature and carnitine concentrations in BAT, together with the recovery of normal morphological features. UCP-1 mRNA and protein expression levels were also significantly increased. These findings strongly suggest that carnitine is essential for maintaining the function and morphology of BAT.