Evidence for nodose ganglion as the source of innervation to the anterior lobe of the pituitary gland.

Recent studies have provided convincing evidence for the presence of peptidergic nerve fibers in the pituitary anterior lobe in several animal species. This study was aimed at elucidating the origin of this innervation by neuroanatomical tracing, denervation experiments, and immunohistochemistry. Immunohistochemistry against substance P and growth-associated protein 43 revealed a dense fiber plexus within the anterior lobe, and these markers were mostly colocalized. Retrograde tracing with Fluorogold from the pituitary gland stained neurons in the hypothalamus, superior cervical ganglia and the nodose ganglia. None of the Fluorogold-labelled neurons in the hypothalamus or superior cervical ganglion were substance P-immunoreactive, while many of the neuronal cell bodies in the nodose ganglion exhibited substance P immunoreactivity. There were no Fluorogold-labelled neurons in the trigeminal, otic or cervical dorsal root ganglia. Surgical transection of the pituitary stalk or bilateral removal of the superior cervical ganglion did not abolish the anterior lobe nerve fibers, and anterograde tracing with 1,1'-dioctadecyl-3,3,3',3'-tetramethylindo-carbocyanine perchlorate from the pituitary stalk failed to stain any nerve fibers within the anterior lobe. Our findings suggest that the nodose ganglion neurons likely innervate the pituitary anterior lobe, while neither hypothalamus nor sympathetic superior cervical ganglion may be a source of this innervation. By showing a distinct neuronal system in the pituitary anterior lobe our findings (i) support the previous functional studies demonstrating a distinct regulation of the morphology of the anterior lobe innervation by hormonal changes, and (ii) suggest that the innervation of the pituitary anterior lobe is a part of the visceral innervation by the vagus nerve rather than a part of the other intracranial innervation. These findings provide a neuroanatomic basis for the reported observations about the neural regulation of the pituitary anterior lobe.