Tyrosine hydroxylase-immunoreactive neurons of the hypothalamus: A light and electron microscopic study

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The localization and morphology of neurons, processes, and neuronal groups in the rat hypothalamus containing tyrosine hydroxylase-like immunoreactivity were studied using an antiserum to bovine tyrosine hydroxylase. This antiserum was thoroughly characterized by precipitation of enzyme activity, immunoblotting, and precipitation of cell-free translation products; a single molecular weight band was recognized by the antiserum. Absorption of the antiserum with purified tyrosine hydroxylase abolished immunocytochemical staining, while addition of bovine dopamine β-hydroxylase had no effect on immunostaining. Immunoreactive cells were found throughout the hypothalamus. Significant numbers of cells were found in the arcuate, periventricular, dorsomedial hypothalamus/zona incerta, posterior hypothalamic regions (A11–A14), and paraventricular nucleus, as previously described, and in addition, in the preoptic area, adjacent to the anterior commissure, medial and lateral to the suprachiasmatic nucleus, dorsal to and in the supraoptic nucleus, at the lateral borders of the ventromedial nucleus, and in the dorsal and ventral lateral hypothalamus. None of the immunoreactive cell groups are totally separated from adjacent cell groups. Dendritic overlap occurs between any two adjacent groups. From cell counts of 30 μm coronal sections, we estimate the hypothalamus has about 12,000 cells based on raw counts, or 8000 immunoreactive cells after correction for possible split cells. Mean soma size varied considerably from one immunoreactive group to another. Cells in the caudal part of the dorsomedial hypothalamus/zona incerta region were the largest, with a mean diameter of 25 μm, while cells in the anterior commissural and posterior hypothalamic group were among the smallest, with mean diameters of 10 μm. The largest immunoreactive cells in the hypothalamus had volumes in excess of ten times greater than the smallest immunoreactive cells. Tyrosine hydroxylase immunoreactivity was found in dendrites in every region of the hypothalamus, sometimes extending hundreds of micrometers from the perikaryon of origin. Although adjacent cell groups were not distinctly separated, the dendritic arbors of the different cell groups differed greatly. Dendritic and somatic appendages were found on some cells, particularly in the paraventricular nucleus. Immunoreactive dendritic arbors were particularly large in cells seen on horizontal sections through the caudal dorsomedial hypothalamic group and through the anterior hypothalamus. Only slight dendritic trees were observed in the rostral dorsomedial hypothalamus/zona incerta region, and in the pericommissural group. Immunoreactive axons, containing dopamine, norepinephrine or epinephrine, were found in every area of the hypothalamus. Axon morphology, size, density and terminals varied from region to region. Axons of hypothalamic origin arose from both immunoreactive somata and primary dendrites. A large number of non-synapsing immunoreactive axons were found in the median eminence, and many were also found throughout the hypothalamus. Retrograde transport of substances injected in the neurohypophysis labeled cells in the most rostral part of the arcuate nucleus and periventricular area; some labeled cells also showed tyrosine hydroxylase immunoreactivity. Many of the immunoreactive boutons contained primarily clear vesicles, while others contained both clear and large dense-core vesicles. Immunoreactive axons made synaptic contact with unlabeled dendrites.