Viral labeling of the CNS origins of the sympathetic nervous system innervation of brown adipose tissue: Implications for torpor and hibernation responses

Brown adipose tissue (BAT) is intimately involved in cold exposure- and ove reating-induced thermogenesis. BAT receives a dense innervation by the symp athetic nervous system (SNS) and this innervation plays an important role i n BAT thermogenic responses. The BAT pad receiving the must attention is in terscapular BAT (IBAT) because of its size, accessibility;Ind clear SNS inn ervation. Although the first neuroanatomical demonstration of IBAT sympathe tic innervation was done using histofluorescence to visualize their catecho laminergic phenotype more than 35 years ago, the central nervous system (CN S) origins of this innervation were not identified neuroanatomatically unti l recently in our laboratory. This was accomplished using a highly specific , transneuronal viral tract tracer - the Bartha's K strain of the pseudorab ies virus (PRV), PRV was injected into TBAT of Siberian hamsters and the in fected neurons were visualized by immunocytochemistry. PRV-infected neurons were found in the spinal cord (intermediolateral cell group, central auton omic nucleus), brainstem (reticular area [lateral, intermediate, parvocellu lar, medullary and gigantocellular reticular nuclei], caudal raphe area [ra phe pallidus and raphe: obscurus nuclei], C1 adrenaline cells and rostroven trolateral medullary regions, nucleus of the solitary tract, lateral paragi gantocellular nucleus and the raphe magnus), midbrain (central gray, dorsal raphe) and forebrain (suprachiasmatic nucleus [SCN], retrochiasmatic area, paraventricular hypothalamic nucleus [PVN], medial preoptic area, lateral hypothalamus, dorsal hypothalamic area, zona incerta, accurate nucleus, lat eral septal region, bed nucleus of the stria terminalis - almost no infecte d neurons were seen in the ventromedial hypothalamic nucleus). Possible rol es of three of these virally labeled origins of SNS outflow fi om brain to BAT (SCN,PVN and raphe pallidus) were discussed. in terms of possible roles in thermogenesis in general, and in torpor/hibernation, specifically.