Evidence for a noradrenergic projection to the subcommissural organ.

Several physiological studies have shown that the subcommissural organ (SCO) is influenced by catecholamines. This study provides immunohistochemical evidence for a noradrenergic input to the SCO of rats. A light plexus of tyrosine hydroxylase (TH)-and dopamine-beta-hydroxylase (D beta H)-positive axons present in the SCO of both Long-Evans and Sprague-Dawley rats. The innervation density was greatest in the hypendymal wing of the rostral aspect of the SCO and it declined both caudally in the hypendymal wing and medially in the hypendymal layer. Some TH- and D beta D beta H-immunoreactive fibers entered the lateral margin of the ependymal layer along the basal surface of ependymal cells; others coursed medially in the transverse plane to ramify along the base of the ependymal cells. These fibers are presumed to be noradrenergic because phenylethanolamine N-methyltransferase immunoreactivity was absent in adjacent sections through the SCO. Considering the potential role of the SCO region in sodium homeostasis, these data suggest that central noradrenergic input to the SCO may parallel peripheral catecholaminergic mechanisms that regulate sodium balance.