Analgesic Candidate Adenosine A 3 Receptors Are Expressed by Perineuronal Peripheral Macrophages in Human Dorsal Root Ganglion and Spinal Cord Microglia

Adenosine receptors are a family of purinergic G protein-coupled receptors that are widely distributed in bodily organs and in the peripheral and central nervous systems. Recently, antihyperalgesic actions have been suggested for the adenosine A 3 receptor, and its agonists have been proposed as new neuropathic pain treatments. We hypothesized that these receptors may be expressed in nociceptive primary afferent neurons. However, RNA sequencing across species, eg, rat, mouse, dog, and human, suggests that dorsal root ganglion (DRG) expression of ADORA3 is inconsistent. In rat and mouse, Adora3 shows very weak to no expression in DRG, whereas it is well expressed in human DRG. However, the cell types in human DRG that express ADORA3 have not been delineated. An examination of DRG cell types using in situ hybridization clearly detected ADORA3 transcripts in peripheral macrophages that are in close apposition to the neuronal perikarya but not in peripheral sensory neurons. By contrast, ADORA1 was found primarily in neurons, where it is broadly expressed at low levels. These results suggest that a more complex or indirect mechanism involving modulation of macrophage and/or microglial cells may underlie the potential analgesic action of adenosine A 3 receptor agonism.