Sub-population of capsaicin sensitive primary afferent neurons in thoracic, lumbar and sacral dorsal root ganglion in young rats revealed by stimulated cobalt uptake.

The number of dorsal root ganglion (DRG) neurons, the relative number of capsaicin sensitive DRG cells and the diameter distribution of these neurons were investigated in the thoracic 11, lumbar 5 and sacral 1 DRG ganglia in young rats. The capsaicin sensitivity of DRG cells was shown by the stimulated cobalt uptake technique in in vitro conditions. Cobalt labelled and non-labelled neurons were counted using the dissector method. Our results show that the total number of DRG cells in the Th11, L5 and S1 segments (4200-6500 per ganglion) were not significantly different from each other and about 8% of these cells were capsaicin sensitive in the segments studied. These findings also show that the capsaicin sensitive DRG cells belong to the small diameter DRG cell population. Control experiments on neonatally capsaicin injected animals indicate that the capsaicin stimulated cobalt uptake provides selective and specific staining of capsaicin sensitive DRG neurons.