Responses to sympathomimetics in rat sensory neurones after nerve transection.

Noradrenaline activation of sensory somata that project in damaged peripheral nerves has been postulated to underlie sympathetically-mediated pain. Intracellular recordings from some neurones with myelinated axons in acutely isolated rat dorsal root ganglia showed small prolonged depolarizations to brief applications of 0.1-5 mM noradrenaline whether or not the spinal nerve had been transected. Similar responses were evoked to noradrenaline when phentolamine was present, and also to 1-5 mM catechol, but not 1 mM clonidine, implying the responses were not adrenoceptor-mediated. In extracellular recordings from similar preparations after sciatic transection, many spontaneously active myelinated dorsal root axons were excited by noradrenaline and other sympathomimetics. Silent axons in injured or control ganglia did not respond. Thus, non-specific depolarizations may activate neurones that are hyperexcitable after a lesion but activation of neuronal alpha-adrenoceptors by sympathetically-released noradrenaline seems unlikely.