Triple Colocalization of Tyrosine Hydroxylase, Calretinin, and Calbindin D-28k in the Periventricu lar-Hypophyseal Dopaminergic Neuronal System

Abstract

Publisher Summary Studies have shown that the dopaminergic neurons terminating in the intermediate lobe of the pituitary (ILP) originate in the periventricular nucleus of the hypothalamus. A recent observation revealed that tyrosine hydroxylase (TH) colocalized with calretinin (CR), a calcium- binding protein, in the rat ILP. In addition, TH also colocalized with calbindin-D28k (CB), another calcium-binding protein (CaBP). The purpose of this chapter is to determine, using an immunohistochernical colocalization procedure, whether a triple colocalization of TH, CR, and CB existed in the DA fibers of the ILP. As it is already demonstrated that such a triple colocalization exists, the chapter sets out to map the dopaminergic cells in the rat hypothalamus in a search for the origin of these pituitary fibers. In immunocytochemical preparations of rat pituitary glands, varicose dopaminergic nerve fibers formed a plexus enveloping the endocrine cells of the intermediate lobe, and the lack of anti-DBH staining confirmed the DA nature of these fibers. Triple colocalization of TH, CR, and CB revealed that all three neurochemicals were present in these fibers in what appeared to be a 1:1 relationship between the CaBPs (CR, CB) and the TH-containing fibers. No immunoreactive cell bodies were observed in the ILP. Anatomical, neurochemical, and neuroendocrinological evidence indicates that dopamine terminals in the ILP originate from A14 cells in the hypothalamic periventricular nucleus. It has been observed that a triple colocalization of TH, CR, and CB was contained in about 3.3% of the A14 DA cell bodies, and it is suggested that these are the cells that innervate the ILP. Because of the triple colocalization within approximately 3.1% of the A11 cells, this system may also project to the ILP.