The sympathetic nervous system in white adipose tissue regulation

Sympathetic stimulation has long been recognized to mobilise fatty acids fr om white adipose tissue. However, it is now apparent that adipose tissue is not only concerned with energy storage as fat, but is a major endocrine an d secretory organ. This change has resulted from the identification of lept in as a hormone of energy balance secreted by white adipose tissue. The sym pathetic system is a key regulator of leptin production in white fat. Sympa thomimetic amines, cold exposure or fasting (which lead to sympathetic stim ulation of white fat), decrease ob gone expression in the tissue and leptin production. On the other hand, sympathetic blockade often increases circul ating leptin and ob gene expression, and it is postulated that the sympathe tic system has a tonic inhibitory action on leptin synthesis. In rodents th is action is through stimulation of beta (3)-adrenoceptors. The adrenal med ulla (as opposed to the direct sympathetic innervation) has been thought to play only a minor role in the catecholaminergic regulation of white adipos e tissue. However, in rodents responses of the leptin system to adrenergic blockade vary with the method used. Changes in leptin and ob gene expressio n are considerably less using methods of blockade that only effect the term inal adrenergic innervation, rather than medullary secretions as well. Stim ulation of the leptin system increases sympathetic activity and hence metab olic activity in many tissues. As well as leptin, other (but not all) secre tions from white adipose tissue are subject to sympathetic regulation. In o besity the sympathetic sensitivity of adipose tissue is reduced and this fa ctor may underlie the dysregulation of leptin production and other adipose tissue secretions.