Substance P-containing sensory neurons in the rat dorsal root ganglia innervate the adrenal medulla

The adrenal medulla is innervated by both cholinergic and substance P (SP)-containing fibres via the splanchnic nerve. SP has been shown to modulate catecholamine (CA) secretion in isolated chromaffin cells and in the perfused rat adrenal gland, however, the origin of SP-containing fibres is not known. In the present study, we have combined the techniques of SP immunohistochemistry and retrograde tracing with Fast blue injected into the left adrenal medulla of the rat in order to study whether SP-containing sensory neurons in the dorsal root ganglia innervate the adrenal medulla. The results showed that there were on average 281 ± 31 SP-like immunoreactive cells in each left dorsal root ganglion, T3-T13 (range, 234 ± 19 in T4 to 372 ± 43 in T13, n = 8). The average total number of Fast blue-labelled cells (T3-T13) in 8 experiments was 172 ± 26, distributed normally about a peak at T8 (33.8 ± 6.3 cells) and T9 (33.3 ± 6.8 cells) with the least at T3 (1.5 ± 0.8) and T13 (5.2 ± 2.0). No Fast blue-labelled cells were found in the right DRG. In the left DRG, the average number of cells exhibiting both SP and Fast blue labelling in 8 experiments was 23 amounting to 14.0% of the total retrograde labelled cells. Most (67%) of the double labelled cells were distributed from T7 to T9. These results demonstrate that SP-containing sensory neurons in the dorsal root ganglia provide an ipsilateral innervation of the adrenal medulla in rat.