Central role for fast nociceptors in mechanical nocifensive behavior and sensitization

Nociceptors, primary afferent nerve fibers that signal noxious stimuli, are broadly divided into slowly conducting unmyelinated C fibers and fast-conducting myelinated A fibers. Whereas C-nociceptors have been extensively studied, considerably less is known about the function of A-nociceptors. To address this gap, we developed an intersectional genetic approach for robust and selective interrogation and manipulation of mechanically responsive A-nociceptors (A-MNs) in mice. Optogenetic A-MN stimulation induced rapid and precise withdrawal reflexes as well as place aversion and facial expression changes consistent with pain affect, while inhibition strongly impaired mechanical nociceptive withdrawal reflexes, demonstrating that A-MNs are necessary and sufficient for rapid avoidance of noxious mechanical stimuli. Prolonged A-MN activation induced mechanical allodynia and central sensitization. In a rare individual lacking thickly myelinated A{beta} fibers, mechanical withdrawal reflexes were completely absent, and pain perception reduced. Together, these findings identify fast-conducting mechano-nociceptors as essential drivers of nocifensive behaviors in mice and humans.