Immunohistochemical features of substance P-immunoreactive chromaffin cells and nerve fibers in the rat adrenal gland.

The distribution of substance P (SP) immunoreactivity and the colocalization of SP with other bioactive substances in chromaffin cells and nerve fibers were investigated in the rat adrenal gland at the light microscopic level. In the capsule and cortex, SP immunoreactivity was seen in some nerve fibers around blood vessels and in thick nerve bundles passing through the cortex directly into the medulla. In the medulla, the SP immunoreactivity was observed in a small number of chromaffin cells; these SP-immunoreactive chromaffin cells were either phenylethanolamine N-methyltransferase (PNMT) immunoreactive or immunonegative, indicating that they were either adrenaline cells or noradrenaline (NA) cells. SP-immunoreactive varicose nerve fibers were also found in the medulla and were in contact with a cluster of the NA cells showing catecholamine fluorescence, which suggests that SP from medullary nerve fibers may regulate the secretory activity of the NA cells. Because no SP-immunoreactive ganglion cell was present in the rat adrenal gland, the intra-adrenal nerve fibers were considered to be extrinsic in origin. The double-immunostaining method further revealed that the SP-immunoreactive chromaffin cells also exhibit immunoreactivities for calcitonin gene-related peptide (CGRP), and neuropeptide tyrosine (NPY), suggesting that these peptides can also be released from the chromaffin cells by certain stimuli. The intra-adrenal nerve fibers in the medulla were composed of SP-single immunoreactive, and SP/CGRP-, SP/choline acetyltransferase (ChAT)-, SP/nitric oxide synthase (NOS)-, SP/pituitary adenylate cyclase activating polypeptide (PACAP)-, ChAT/NOS-, and ChAT/PACAP-immunoreactive nerve fibers, which may affect the secretory activity of the NA cells. In the adrenal capsule, the nerve fibers were present around blood vessels and showed immunoreactivities for SP/ CGRP, SP/NPY, SP/NOS, and SP/vasoactive intestinal polypeptide, suggesting that the origin of nerve fibers in the capsule may differ from those in the medulla.