Immunohistochemical localization of substance P And cholecystokinin in the dorsal root ganglia and spinal cord of the bottlenose dolphin (Tursiops truncatus).

The presence of substance P (SP) and cholecystokinin (CCK) immunoreactive neurons was examined in the bottlenose dolphin dorsal root ganglia (DRGs) and spinal cord by immunohistochemical techniques. SP-positive and CCK-immunoreactive neurons were respectively approximately 50% and 1% of the total number of ganglion cells examined and especially belonged to small and medium-sized cell populations. Using double labeling techniques we observed that SP- and CCK-immunoreactivity coexisted in a very low number of primary afferent neurons (2.7%). Few SP-immunoreactive (IR) neurons (2.7%) were also CCK-positive. On the contrary, 65% of CCK-immunoreactive neurons contained SP. Interestingly, we observed CCK-immunoreactive satellite glial cells located around large cell class somata. Virtually no SP-IR and CCK-positive neurons were surrounded by peripheral CCK-immunoreactive satellite glial cells. The SP-IR and CCK-positive nerve fibers were particularly conspicuous in the superficial layers of the spinal cord. The present study indicates that SP and CCK only partially overlap in the thoracic, lumbar, and caudal DRGs of the bottlenose dolphin, suggesting that the majority of SP-IR ganglion neurons are lacking in CCK-immunoreactivity. The role of SP-containing DRG neurons is discussed also in relation to the huge vascular spinal retia mirabilia typical of cetaceans.