Brown adipocytes (BAs) are specialized for adaptive thermogenesis and, upon 
sympathetic stimulation, activate mitochondrial uncoupling protein (UCP)-1 and 
oxidize fatty acids to generate heat. The capacity for brown adipose tissue 
(BAT) to protect against obesity and metabolic disease is recognized, yet 
information about which signals activate BA, besides β3-adrenergic receptor 
stimulation, is limited. Using single-cell transcriptomics, we confirmed the 
presence of mRNAs encoding traditional BAT markers (i.e., UCP1, expressed in 
100% of BAs Adrb3, expressed in <50% of BAs) in mouse and have shown single-cell 
variability (>1000-fold) in their expression at both the mRNA and protein 
levels. We further identified mRNAs encoding novel markers, orphan GPCRs, and 
many receptors that bind the classic neurotransmitters, neuropeptides, 
chemokines, cytokines, and hormones. The transcriptome variability between BAs 
suggests a much larger range of responsiveness of BAT than previously recognized 
and that not all BAs function identically. We examined the in vivo functional 
expression of 12 selected receptors by microinjecting agonists into live mouse 
BAT and analyzing the metabolic response. In this manner, we expanded the number 
of known receptors on BAs at least 25-fold, while showing that the expression of 
classic BA markers is more complex and variable than previously thought.