Neurochemical phenotype of sympathetic nervous system outflow from brain to white fat.

The sympathetic innervation of white adipose tissue (WAT) appears to be a dominant mechanism triggering lipolysis. The purpose of this study was to determine the neurochemical phenotype of neurons comprising the sympathetic outflow from brain to WAT. This was accomplished by injecting Siberian hamster WAT with a viral retrograde transneuronal tract tracer, the pseudorabies virus (PRV), in combination with immunocytochemical characterization of several neurotransmitters or their synthetic enzymes in the brain. Catecholaminergic (tyrosine hydroxylase [TH] and dopamine-beta-hydroxylase [DBH] immunoreactivity) and peptidergic (arginine vasopressin [AVP] and oxytocin [OXY] immunoreactivity) neurons were part of this outflow, but the percentage of double-labeled cells was small, consistent with previous studies. Brainstem PRV + TH- or PRV + DBH-labeled cells were in previously identified noradrenergic areas (A5, A6, and subcoeruleus, rostroventrolateral medulla [RVL], some reticular nuclei). Forebrain double labeling was greatest in the paraventricular (TH, AVP, OXY) and suprachiasmatic (AVP) nuclei, both implicated in the central control of lipolysis. Differences between the PRV double labeling reported here for WAT versus that of other sympathetic peripheral targets were PRV + DBH in A5 and RVL, and PRV + TH in RVL and in the lateral paragigantocellular and lateral reticular nuclei. Collectively, these results begin to identify the neurochemical identity of the sympathetic outflow from brain to WAT.