CHEMICALLY MEDIATED CROSS-EXCITATION IN RAT DORSAL-ROOT GANGLIA

Primary afferent neurons in mammalian dorsal root ganglia (DRGs) are a natomically isolated from one another and are not synaptically interco nnected, As such, they are classically thought to function as independ ent sensory communication elements. However, it has recently been show n that most DRG neurons are transiently depolarized when axons of neig hboring neurons of the same ganglion are stimulated repetitively. Here we further characterize this functional coupling. In electrophysiolog ical recordings made from excised rat DRGs, we found that DRG ''cross- depolarization'' is excitatory in that it is accompanied by an increas e in the probability of spiking in response to otherwise subthreshold test pulses delivered intracellularly. Cross-depolarization contribute s to this mutual cross-excitation. However, at least as important a co ntribution comes from a net increase in the neurons' input resistance (R(in)) triggered by the stimulation of neighboring neurons, This chan ge in R(in) occurs even when cross-depolarization is absent or is bala nced out. The amplitude of cross-depolarization was found to be voltag e-dependent, with a reversal potential at approximately -23 mV. Revers ibility and the change in R(in) both indicate that activity of neighbo ring neurons causes a membrane conductance change that is chemically m ediated. Thus, far from being isolated, most DRG neurons participate i n ongoing mutual interactions in which neuronal excitability is contin uously modulated by afferent spike activity. This intraganglionic dial og appears to be mediated, at least in part, by an activity-dependent diffusable substance(s) released from neuronal somata and/or adjacent axons, and detected by neighboring cell somata and/or axons.