Functional cross-excitation between afferent A- and C-neurons in dorsal root ganglia

Electrophysiological recordings were made in vitro from primary afferent ne urons with unmyelinated axone (C-neurons) in excised rat dorsal root gangli a. Spike activity triggered in neurons with myelinated axons (A-neurons) by stimulation of the peripheral nerve or the dorsal root produced a transien t depolarization in passive neighboring C-neurons that share the same gangl ion. About 90% of neurons sampled responded with this "cross-depolarization ". Cross-depolarization was associated with functional excitation as indica ted by an increase in firing probability in response to previously subthres hold intracellular test pulses. Furthermore, it yielded a net increase of t he input resistance of the affected C-neurons. We suggest that functional coupling among DRG neurons could serve a metabol ic role, providing a functionally relevant feedback signal useful for contr olling the excitability of nociceptive sensory endings. In addition, the re sults provide a novel mechanism whereby afferent nociceptors could be stimu lated by activity in low-threshold mechanoreceptors, particularly in the ev ent of nerve injury. Hence, the coupling between afferent A- and C-neurons in dorsal root ganglia provides a novel candidate mechanism for neuropathic pain. (C) 1999 IBRO. Published by Elsevier Science Ltd.