CATECHOLAMINERGIC INNERVATION OF WHITE ADIPOSE-TISSUE IN SIBERIAN HAMSTERS

When Siberian hamsters are transferred from long summerlike days (LDs) to short winterlike days (SDs) they decrease their body weight, prima rily as body fat. These SD-induced decreases in lipid stores are not u niform. Internally located white adipose tissue (WAT) pads are deplete d preferentially of lipid, whereas the more externally located subcuta neous WAT pads are relatively spared. These data suggest a possible di fferential sympathetic neural control over catecholamine-induced lipol ysis and that lipolytic rates are greater for internal vs. external WA T pads. Moreover, if these differential rates of lipolysis are due to differential sympathetic nervous system (SNS) drives on the pads, then fat pad-specific catecholaminergic innervation may exist. Therefore, we tested whether inguinal WAT (IWAT; an external pad) and epididymal WAT (EWAT; an internal pad) were innervated differentially. In additio n, we tested whether norepinephrine (NE) turnover (TO) reflected the p resumed greater SNS drive on EWAT vs. IWAT after SD exposure. Injectio ns of fluorescent tract tracers [Fluoro-Gold or indocarbocyanine perch lorate (DiI)] demonstrated projections from the SNS ganglia T-13-L(3) to both fat pads. Retrograde labeling revealed a relatively separate p attern of distribution of labeled neurons in the ganglia projecting to each pad. In vivo anterograde transport of DiI resulted in labeling i n both IWAT and EWAT that included staining around individual adipocyt es and occasionally retrogradely labeled cells. The proportionately gr eater decrease in EWAT compared with IWAT mass after 5 wk of SD exposu re was reflected in greater EWAT NE TO than found in their LD counterp arts for this pad. After 10 wk of SD exposure, NE TO in both EWAT and TWAT is elevated significantly vs. that of their LD conspecifics. Coll ectively, these results suggest a neural mechanism mediating the diffe rential changes in fat pad mass seen in Siberian hamsters exposed to S Ds. In conclusion, the present study provides the first direct anatomi c evidence of innervation of TWAT and EWAT by the SNS in rodents. More over, these data support an important role for the SNS innervation of WAT that has been neglected traditionally.