Peptide immunoreactivities in the ganglionated plexuses and nerve fibers innervating the human gallbladder

The mammalian gallbladder is innervated by a well-developed intrinsic neural network. However, little is known about the neurochemistry and organization of the innervation of this organ in humans. The aim of this study was to analyze the distribution of immunoreactivity (IR) for the neuropeptides, vasoactive intestinal polypeptide (VIP), neuropeptide Y (NPY), tachykinins (TK) and calcitonin gene-related peptide (CGRP) in the human gallbladder by means of immunohistochemistry. Neuropeptide-IRs are found in neurons and processes of the two ganglionated plexuses, i.e., the innermost plexus located in the lamina propria at the base of the mucosal folds, and the outermost plexus situated within the fibro-muscular layer. In these two plexuses, VIP-, NPY- and TK-IRs are present in ganglion cells and varicose fibers, whereas CGRP-IR is confined to nerve processes. VIP-IR is present in most, if not all, neurons. NPY-and TK-IRs are also found in many neurons. The densities of the peptide-IR nerves in the mucosa are NPY and VIP > TK ≫ CGRP, and in the fibro-muscular layer are NPY > VIP and TK > CGRP. The vasculature is richly innervated by NPY-IR nerves, which are mostly perivascular. CGRP-, VIP- and TK-IR processes are found only occasionally around blood vessels and in a paravascular position. Double-label studies demonstrated that a large number of VIP-containing neurons expresses NPY- or TK-IR. On the other hand, all neurons positive for either NPY- or TK-IR are immunostained for VIP. In agreement with these findings, most of the NPY-IR fibers in the lamina propria and fibro-muscular layer contain VIP-IR, and numerous TK-IR fibers are positive for VIP. However, the perivascular NPY-IR processes do not contain VIP-IR, suggesting an extrinsic origin. In addition, a population of TK-IR processes contains CGRP-IR and presumably originates from extrinsic sources, since CGRP/TK-IR intrinsic neurons could not be detected in the gallbladder. Peptide-IRs have a similar distribution in the neck, body and fundus of the gallbladder. No peptide-containing endocrine/paracrine cells are observed in the epithelium. The presence of peptide-IRs in the ganglionated plexuses and the abundance of peptidergic innervation suggest that peptides exert their effects on gallbladder function by acting directly on tissue targets and influencing intrinsic ganglion cells. Furthermore, the co-localization of more than one peptide in the same neuron raises the possibility that peptides are co-released upon stimulation and might interact at the same target.