Ultrastructure of the anuran pars intermedia following serverance of hypothalamic connection

Innervation of thyrotropin-releasing hormone-containing neurons in the bullfrog pars intermedia

We studied immunocytochemical localization of thyrotropin-releasing (TRH) in the bullfrog pats intermedia. Many TRH-like immunoreactive terminals containing immunoreactive granular vesicles 80-150 nm in diameter (116 nm mean diameter) were found in the pars intermedia. Some TRH-like immunoreactive terminals were in synaptic contact with the intermedia cells. These findings support the hypothesis that TRH directly activates the intermedia cells via synapses to secrete alpha-melanocyte-stimulating hormone.

Secretion of melanophore-stimulating hormone (MSH) in long-term cultures of pituitary neurointermediate lobes

Neurointermediate lobes from pituitaries of the frog, Rana berlandieri forreri (Rana pipiens, sensu lato), were maintained in organ culture in media with and without serum for up to six months. The cultured tissues were examined periodically by light microscopy and transmission electron microscopy and by bioassay of the melanophore-stimulating hormone (MSH) secreted and present in the culture media. Light-microscopic observations revealed a high degree of preservation of the pars intermedia at four weeks with isolated areas of some glands maintaining histological integrity for the entire six months. Similarly, at the ultrastructural level the cells appeared morphologically intact and to be actively synthesizing and secreting hormone. Bioassays showed the glands to be continuously secreting MSH; however, larger yields of hormone were obtained in media lacking serum. No significant ultrastructural differences between cells grown in the presence or absence of serum were detected. The difference in concentration of MSH between the two groups therefore apparently results from enzymatic degradation of the hormone by the serum. Organ culture of the vertebrate neurointermediate lobe may provide a unique method for the production of large quantities of MSH and for the study of other melanotropic and opiate peptides as they may be synthesized and secreted by the pars intermedia.

Effects of reserpine administration on the fine structure of the rat pars intermedia

Hormone release from the pars intermedia is under inhibitory control of the hypothalamus. Control may be mediated via dopaminergic fibers which directly contact secretory cells. Administration of reserpine in vivo to adult male rats for four consecutive days results in major alternations in pars intermedia cytology. Cells show expanded areas and whorls of rough endoplasmic reticulum, as well as extensive Golgi zones with numerous dense granules. Some nerve fibers exhibit alterations in vesicle content, while others retain a more normal appearance. Freeze-fracture of glands from reserpine-treated animals provides evidence for exocytosis of granules, although such phenomena are not observed in thin sections. The ultrastructural findings suggest that reserpine alters the content of local inhibitory neurotransmitters in the pars intermedia, leading to unrestrained hormone release, followed by renewed granule synthesis.