Condiciones para la evaluación

Understanding the role of the central control of body fat is important given that obesity is at epidemic proportions with serious secondary health consequences such as Type II diabetes, cardiovascular disease, some cancers and dementias, as well as being a significant economic burden (28; 43; 44; 66). White adipose tissue (WAT) stores energy in the form of triacylglycerol

for mobilization during times of increased energetic demands. By contrast, when there is a surfeit of calories relative to energy expenditure, lipid is stored but depending upon the storage location of the WAT depot, the excess lipid stores can be detrimental. The culprit most often pointed to as being detrimental to health is visceral WAT, which is thought to contribute to comorbidities previously mentioned above [for review see: (10; 24; 32)]. One hypothesis suggests that visceral WAT secretes pro-inflammatory cytokines, such as interleukin-6, that contribute to insulin resistance (33; 40), whereas another hypothesis, the ‘hepatic-portal theory’, proposes that the visceral WAT has a high lipolytic rate resulting in large amounts of free fatty acids (FFAs) being transported to the liver through the portal vein and ultimately resulting in liver insulin resistance, although visceral WAT as the source of FFAs for this negative response has been questioned [e.g., (32)].

Larger subcutaneous thigh WAT is seen as a potentially beneficial WAT depot because it is associated with the more favorable glucose and lipid profile in humans (51) and provides a potentially enormous location for lipid storage without the more direct organ-associated aspects of visceral WAT (29) or ectopic deposition of lipid in key organs involved in energy homeostasis (e.g., liver, muscle). Subcutaneous WAT also is associated with reductions in plasma insulin concentrations and hepatic steatosis as seen in improved muscle insulin sensitivity of

lipodystrophic animal models, which are characterized by the small amount or the lack of body fat (46). Despite a deficit of body fat, some of the same metabolic characteristics associated with obesity are seen in lipodystrophic models because lipids are ectopically deposited in sites where they are typically not stored thus being the hypothesized problem in the lipodystrophic disorders (46).

The central nervous system is now clearly recognized as a participator in the control of peripheral lipid storage and as an activator of the sympathetic nervous system (SNS) innervation of WAT. The SNS initiates lipolysis in humans and other mammals via norepinephrine (NE) release from its postganglionic nerve terminals [for review see: (6; 7)]. Using traditional

monosynaptic neuronal and transneuronal viral tract tracers injected into WAT, the sympathetic postganglionic innervation of WAT from the sympathetic chain (64) as well as the origins of the central circuits ultimately terminating in WAT depots have been described in Siberian hamsters, domestic swine and laboratory rats (1; 4; 19; 20; 47; 52; 54; 56). More importantly in terms of function, destruction of the sympathetic nerves innervating WAT by either chemical or surgical denervation significantly blocks lipid mobilization in response to several lipolytic stimuli including food deprivation, short photoperiods, and estradiol replacement in ovariectomized obese rats (9; 14; 21; 62; 65).

Of the WAT depots in rodents, the two that represent visceral and subcutaneous WAT that have analogous counterparts in humans are mesenteric WAT (MWAT) and inguinal WAT (IWAT), respectively (60). MWAT is the only true visceral WAT in rodents, if the definition of visceral WAT is that it drains into the hepatic portal vein because although epididymal WAT (EWAT) and retroperitoneal WAT (RWAT) are intra-abdominal WAT depots, they drain systemically through the inferior vena cava (16). In addition, the often cited EWAT as visceral WAT does not exist in humans [(60); J. G. Kral, personal communication]. The rodent literature, however, is rife with supposed manipulations involving ‘visceral fat’, which was actually intra- abdominal WAT (e.g., RWAT, EWAT).

In our previous studies testing for changes in the sympathetic drive associated with energy challenges, as measured by a direct neurochemical assessment – norepinephrine turnover

(NETO), we found different sympathetic drives across various WAT depots for each stimulus we tested (i.e., food deprivation, cold exposure, glucoprivation, central melanocortin receptor

agonism, short photoperiod exposure) in Siberian hamsters (12; 13). To date, the central circuits defining the SNS outflow to MWAT have not been examined, nor has there been a functional in vivo test of sympathetic drive (i.e., NETO) to this WAT depot. In an attempt to understand human WAT depots functioning using non-human rodent animal models, it is important to test whether MWAT and IWAT have similar or separate central SNS outflow circuitries such that, potentially, a therapy could be designed to mobilize lipids from visceral WAT depots while relatively sparing that of subcutaneous WAT depots given that decreases in visceral WAT of as little as 5% has health benefits (16; 41) and the perhaps beneficial aspects of subcutaneous WAT.

Therefore, the purpose of the present investigation was to define the central SNS outflow circuits to MWAT and IWAT and to functionally test the sympathetic drive to these two depots in response to a lipolytic challenge (i.e., food deprivation). We used Siberian hamsters, our model of naturally-occurring obesity that we have employed previously to define the sympathetic outflow to various WAT depots (4; 64) and sensory inflow from WAT depots to the brain (38; 55), in addition to assessing SNS drive using NETO with various energy challenges (12; 13). To define the separate/shared central SNS outflow circuitry, we used isogenic strains of

pseudorabies virus (PRV) that have distinct reporters – PRV 152 [green fluorescent protein (GFP) (50)] and PRV 614 [monomeric red fluorescent protein (mRFP) (5)], each injected into one of the two WAT depots. Single- and double-labeled (infected) neurons were quantified across the neuroaxis as well as in the spinal cord and the sympathetic chain. The functional test of differential SNS drive was accomplished by food depriving hamsters for 16 h with ad libitum- fed hamsters as a control and measuring NETO.

2.3 Materials and Methods