A quantitative analysis of the sensory and sympathetic innervation of the mouse pancreas

Pain from pancreatitis or pancreatic cancer can be both chronic and severe although little is known about the mechanisms that generate and maintain this pain. To define the peripheral sensory and sympathetic fibers involved in transmitting and modulating pancreatic pain, immunohistochemistry and confocal microscopy were used to examine the sensory and sympathetic innervation of the head, body and tail of the normal mouse pancreas. Myelinated sensory fibers were labeled with an antibody raised against 200 kD neurofilament H (clone RT97), thinly myelinated and unmyelinated peptidergic sensory fibers were labeled with antibodies raised against calcitonin gene-related peptide (CGRP) and post-ganglionic sympathetic fibers were labeled with an antibody raised against tyrosine hydroxylase (TH). RT97, CGRP, and TH immunoreactive fibers were present in parenchyma of the head, body and tail of the pancreas with the relative density of both RT97 and CGRP expressing fibers being head>body>tail, whereas for TH, a relatively even distribution was observed. In all three regions of the pancreas, RT97 fibers were associated mainly with large blood vessels, the CGRP fibers were associated with the large- and medium-sized blood vessels and the TH were associated with the large- and medium-sized blood vessels as well as capillaries. In addition to this extensive set of sensory and sympathetic nerve fibers that terminate in the pancreas, there were large bundles of en passant nerve fibers in the dorsal region of the pancreas that expressed RT97 or CGRP and were associated with the superior mesenteric plexus. These data suggest the pancreas receives a significant sensory and sympathetic innervation. Understanding the factors and disease states that sensitize and/or directly excite the nerve fibers that terminate in the pancreas as well as those that are en passant may aid in the development of therapies that more effectively modulate the pain that frequently accompanies diseases of the pancreas, such as pancreatitis and pancreatic cancer.