Brown adipose tissue (BAT) is a specialized type of “fat” that is responsible for the dissipation of energy in response to either lowered ambient temperature or elevated caloric intake. The realization that BAT is present in adult humans in inverse proportion to BMI and fat mass as well as the discovery that white adipose tissue (WAT) can be transformed to “brown-like” (Brite or Beige) fat has necessitated a more complete understanding of the central neural control of BAT or “brown-like” fat cell function.
We used neurotropic viruses injected into fat depots in rats to trace multisynaptic central neural pathways directed to WAT, BAT and Brite / Beige fat. Specifically, pseudorabies virus (PRV Bartha) was injected into inguinal WAT (iWAT), interscapular BAT (iBAT) and iWAT transformed to include Brite/Beige fat cells by exposure of rats to 8°C for 7 days. After injection of PRV with different fluorescent reporters (PRV-red / PRV-green) into the various fat pads, rats were allowed to survive for 5 days to allow transport through the autonomic neuraxis before sacrifice and subsequent histolological analysis to assess the distribution of transynaptically viral-infected neurons. After injection of PRV-red or PRV-green into each of the fat depots in the same animal, distinct labeling patterns were observed in 1st, 2nd, 3rd and 4th order neurons in paravertebral ganglia, spinal cord, brain stem, midbrain and hypothalamus. In addition to these “private lines” of communication to various fat pads, populations of “command neurons” were identified which had collateral axonal projections to different fat pads including those to both brown and white fat. Moreover, the relative percentage of these “command” neurons projecting to iBAT and beiged iWAT increased under conditions of cold exposure.
These data including the unique identification of “command” controllers of fat involved with both storage and burning of energy provide a neuroanatomical basis for differentiating the central neural control of white, brown and white fat transferred into brite/beige fat.