Distinct sympathetic projections to brown fat regulate thermogenesis and glucose tolerance.

Brown adipose tissue (BAT) contributes to thermoregulation and glucose metabolism, but how these functions are coordinated remains unclear. While thermogenesis in the activated BAT typically coincides with increased blood flow and glucose uptake(1-5), several pathophysiological and nutritional states dissociate these processes(6,7), suggesting they are governed by distinct sympathetic circuits. Here we identify subpopulations of sympathetic neurons in the stellate ganglion that mediate distinct functions of intrascapular BAT (iBAT) in mice. Two main types of sympathetic neurons project to iBAT: those that innervate the organ parenchyma and those that innervate the large blood vessels feeding the depot(8-12). Here we develop a toolkit to parse the functions of these neuronal subclasses through targeted chemogenetic activation of projections to iBAT, while sparing other organs, and single-cell transcriptomics coupled to retrograde tracing from iBAT to the stellate ganglion. We find that stimulation of the parenchymal projections increases blood flow and thermogenesis in iBAT, without affecting circulating glucose levels. Conversely, stimulation of the vascular projections improves glucose tolerance but does not alter blood flow or thermogenesis in iBAT. These data provide a mechanistic explanation for the dissociation between the thermogenic and glycaemic effects of BAT activation(13-16).