ABSTRACT
T he incessant demand for energy by tissues, especially the central nervous system (CNS), is a challenge that must be met for normal physiological functioning and behavior to occur. This is an especially difficult task for small rodents because of their increased energy expenditure as a result of their high metabolic rates and because of their increased heat loss due to their high surface-to-volume ratio.1 When this demand for energy exceeds the calories available from recently ingested food, and from the relatively small carbohydrate stores (i.e., glycogen), the needed energy is liberated from white adipose tissue (WAT), the principal energy depot, through the process of lipolysis (i.e., breakdown of triglycerides). It now appears that the direct sympathetic nervous system (SNS) innervation ofWAT, rather than the indirect SNS control ofWAT via the secretion of adrenal medullary catecholamines into the circulation, may be the predominate trigger for lipolysis in adipocytes (for review see 2). Thus, the role of the innervation ofWAT by the SNS and sensory nerves is the major focus of this review. In addition, a more recently discovered role of the SNS innervation ofWAT, the control of adipocyte proliferation (i.e., fat cell number)5’4 also will be reviewed. Because we are unaware of any convincing data for the parasympathetic innervation ofWAT, this topic will not be discussed.5 6
Note that the neuroanatomy of the innervation ofWAT, as well as the SNS control of leptin secretion by WAT will be discussed here, but also will be discussed in greater detail in the chapters by S. Cinti and P. Trayhurn in this volume. In addition, there will be little discussion of adrenergic receptor and function in this review, but the interested reader should see reviews of this area by M. Lafontan (see e.g., 7-9). Finally, this review of the innervation ofWAT will not be exhaustively comprehensive, but such a review that spans the literature from 1898 to 1998 has been published recently.2
