Immunohistochemical identification of somatostatin-containing neurons projecting to the median eminence of the rat.

Immunohistochemical staining of somatostatin (SRIF) and a retrograde transport method with horseradish peroxidase (HRP) were simultaneously applied to the same section of the rat brain to identify the specific SRIF-containing neurons sending their fibers to the median eminence. After HRP injection into the median eminence, SRIF-positive neurons in the rostral parts of the periventricular nucleus were shown to be labeled with HRP, SRIF neurons in the other brain areas, such as the amygdala, the ventromedial nucleus, dorsomedial nucleus and arcuate nucleus, had no HRP-positive material. The present findings appear to demonstrate that SRIF neurons in the periventricular nucleus project directly to the median eminence.

Ultrastructural studies on the secretory cycle of the neurosecretory cells and the formation of Herring bodies in the paraventricular nucleus of the rat.

The ultrastructure of the neurosecretory cells in the paraventricular nucleus of the normal male rat was studied by electron microscopy during various functional states. Four morphologically distinct types of neurosecretory cells were observed. It appears that they do not represent different classes of cells but different phases of secretory activity of a single cell type. The perikarya of the neurosecretory cells show a definite cycle of formation and transportation of secretory granules. We have designated the phases of this cycle as: (1) phase of synthesis, (2) phase of granule production, (3) phase of granule storage and (4) phase of granule transport. The neurosecretory granules appear to be moved in bulk into the axons, forming a large axonal swelling filled with granules as a result of one cycle in the neurosecretory process. Thus it may be postulated that a secretory cycle in the perikaryon of the neurosecretory cell seems to result in the formation of a Herring body in its axon, and that its content is then conveyed to the posterior pituitary.

Morphological and morphometric studies on the terminal boutons on the neurosecretory cells of the rat paraventricular nucleus.

Ultrathin sections of the male rat hypothalamus, including the paraventricular nucleus prepared by perfusion fixation, were observed with a transmission electron microscope. The neurosecretory cells were classified into four types based on the difference in the structures of cell organelles and in the number and appearance of secretory granules present. This was reported elsewhere (Yukitake et al. Cell Tis. Res. 177: 1-8, 1977). Morphological and morphometric analyses on the terminal boutons abutting on the surface of the neursecretory cells were performed. The boutons were classified into three types: first type, having only small clear vesicles; second type, having large cored vesicles; and third type, having closely packed small vesicles. The incidence of the occurrence of each type of terminal bouton as well as the ratios of the surface of the perikarya of the neurosecretory cells covered by the terminal boutons were almost constant in all types of neurosecretory cells. As the stage of the secretory cycle proceeded, the numerical densities of the small synaptic vesicles in the terminal boutons clearly decreased. These results were consistent with the idea that each type of neurosecretory cell represented a different stage in the cell specific secretory cycle rather than a difference in neuron grouping, and that the terminal boutons probably transmitted some information regulating the secretory function of the cells.