Figure 208

Electron micrograph of rat posterior lobe. Neurosecretory granules and small vesicles are present in the terminal portions of the axonal processes of the hypothalamohypophyseal tract fibers. Capillaries with fenestrated endothelium are present in close proximity to the nerve endings. x20,000. (Courtesy of Drs. Sanford L. Palay and P. Orkland.)

also form accumulations that dilate portions of the axon near the terminals (Fig. 20.8). These dilations, called Herring bodies, are visible in the light microscope (Fig. 20.9).

• Nerve terminals also contain 30-nm vesicles that contain acetylcholine. These vesicles may play a specific role in the release of neurosecretory vesicles.

• Larger, 50- to 80-nm vesicles that resemble the dense core vesicles of the adrenal medulla and adrenergic nerve endings are present in the same terminal as the other membrane-bounded vesicles.

The membrane-bounded neurosecretory vesicles that aggregate to form Herring bodies contain either oxytocin or antidiuretic hoi-mone (ADH, vasopressin) (Table 20.4). Each hormone is a small peptide of nine amino acid residues. The two hormones differ in only two of these residues. Each vesicle also contains ATP and a neuro-physin, a protein that binds to the hormone by noncova-lent bonds. Oxytocin and ADH are synthesized as part of a large molecule that includes the hormone and its specific neurophysin. The large molecule is proteolytically cleaved into the hormone and neurophysin as it travels from the perikaryon to the axon terminal. Immunocytochemical staining demonstrates that oxytocin and ADH are secreted by different cells in the hypothalamic nuclei.

ADH controls blood pressure by altering the permeability of the collecting tubules in the kidney

ADH's original name, vasopressin, was derived from the observation that large nonphysiologic doses increase blood pressure by promoting the contraction of smooth muscle in small arteries and arterioles. The primary physiologic effect of ADH is to increase the permeability of the distal portions of the nephron, i.e., the distal convoluted tubule

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