Ontogenesis of tyrosine hydroxylase-immunopositive structures in the rat hypothalamus. Fiber pathways and terminal fields

The innervation of the hypothalamus and septal region by catecholaminergic fibers was studied in rats from the 12th fetal day until the 9th postnatal day. Catecholaminergic fibers were visualized with preembedding immunocytochemistry using antibodies to tyrosine hydroxylase. An intensification of diaminobenzidine product with silver and gold was additionally applied to increase the sensitivity and resolution power of the routine immunocytochemical technique. It has been demonstrated that, from the 13th fetal day, the hypothalamus and the septal region receive catecholaminergic fibers either belonging to the hypothalamic neurons or coming with the medial forebrain bundle from the outside of the hypothalamus. As the development of the hypothalamus proceeds, these fibers form the extensive networks within some neurosecretory centers either containing (the zona incerta, periventricular nucleus, etc.) or almost lacking (suprachiasmatic and paraventricular nuclei) the catecholaminergic neurons. In the former case, they terminate on the processes or perikarya of catecholaminergic neurons, while in the latter case their varicosities surround the immunonegative presumptive neurons in a basket-like manner. Moreover, from the 18th fetal day catecholaminergic fibers penetrate between the ependymal cells towards the 3rd ventricle and the primary capillary plexus of the hypophysial portal circulation, apparently providing the release of catecholamines to the cerebrospinal fluid and portal blood, respectively. The data obtained in this study are considered as the morphological basis for the involvement of the hypothalamic catecholamines in neuroendocrine regulations during ontogenesis.