Guinea pig pancreatic ganglia: projections, transmitter content, and the type-specific localization of monoamine oxidase.

The ganglionated plexus of the guinea pig pancreas was investigated by using histochemical, immunocytochemical, and tract-tracing methods in order to determine whether pancreatic ganglia are analogous to the ganglia of the enteric nervous system (ENS). Three lines of evidence suggest that the ganglia of the pancreas appear to be interconnected with one another, as are enteric ganglia. First, microinjections of the retrograde tracer Fluoro-Gold into individual pancreatic ganglia labeled the perikarya of neurons in distant pancreatic ganglia, whereas no labeling of neurons was observed if injections were placed in the connective tissue adjacent to pancreatic ganglia. Second, when the intercalating dye DiI was microinjected into single pancreatic ganglia in fixed tissues, DiI-labeled terminals were found in additional pancreatic ganglia. Finally, microinjections of the beta subunit of cholera toxin into individual pancreatic ganglia yielded similar results. The ganglionated plexus of the pancreas also expresses a diversity of transmitter content and cell type-specific localization of monoamine oxidase (MAO) that is analogous to the ENS. In common with guinea pig enteric ganglia, pancreatic ganglia contain highly varicose 5-hydroxytryptamine (5-HT)-immunoreactive axons and intrinsic neuropeptide Y (NPY)- and substance P (SP)-immunoreactive neurons. The vast majority, but not all, of SP-immunoreactive fibers in the pancreatic parenchyma also contain calcitonin gene-related peptide (CGRP) immunoreactivity. MAO-B was the primary type of MAO found in the intrinsic elements of the pancreas where it was located in neurons and fibers in the pancreatic parenchyma. In common with serotoninergic enteric neurons, MAO-B immunoreactivity was not found at the LM level in pancreatic serotoninergic neurites. In contrast, NPY- and tyrosine hydroxylase (TH)-immunoreactive perivascular axons were found to contain abundant MAO-A, but no MAO-B immunoreactivity. It is concluded that MAO-B immunoreactivity is characteristic of a portion of the intrinsic innervation of the pancreas, whereas MAO-A immunoreactivity is a marker for the extrinsic sympathetic innervation of the pancreas. Because of its receipt of a direct neural innervation from myenteric ganglia of the bowel (Kirchgessner and Gershon, '90: J. Neurosci 10:1626-1642), similar connections, transmitter content and localization of type-specific MAO, the ganglionated plexus of the pancreas should be regarded as an extension or subset of the ENS.