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

Electrophysiological recordings were made in vitro from primary afferent neurons with unmyelinated axons (C-neurons) in excised rat dorsal root ganglia. Spike activity triggered in neurons with myelinated axons (A- neurons) by stimulation of the peripheral nerve or the dorsal root produced a transient depolarization in passive neighboring C-neurons that share the same ganglion. About 90% of neurons sampled responded with this 'cross- depolarization'. Cross-depolarization was associated with functional excitation as indicated by an increase in firing probability in response to previously subthreshold intracellular test pulses. Furthermore, it yielded a net increase of the input resistance of the affected C-neurons. We suggest that functional coupling among DRG neurons could serve a metabolic role, providing a functionally relevant feedback signal useful for controlling the excitability of nociceptive sensory endings. In addition, the results provide a novel mechanism whereby afferent nociceptors could be stimulated by activity in low-threshold mechanoreceptors, particularly in the event of nerve injury. Hence, the coupling between afferent A- and C-neurons in dorsal root ganglia provides a novel candidate mechanism for neuropathic pain.