Magnocellular neurons of the hypothalamus synthesise ADH and oxytocin as prohormones. Which neurophysiological feature characterises their secretion into the systemic circulation?

• A Secretion by parvocellular neurons into the hypothalamo-hypophyseal portal system
• B Paracrine secretion within the hypothalamus acting on median eminence receptors
• C Axonal transport to posterior pituitary terminals and exocytosis directly into fenestrated sinusoids ✓
• D Release via gap junctions into cerebrospinal fluid before systemic absorption

Explanation

Magnocellular neurons in the supraoptic (SON) and paraventricular (PVN) nuclei synthesise ADH (vasopressin) and oxytocin, which are packaged with their respective neurophysins and transported along axons to nerve terminals in the posterior pituitary (neurohypophysis). Upon action potential-triggered Ca2+ influx, vesicles undergo exocytosis directly into the fenestrated capillaries of the posterior pituitary for systemic distribution. This is distinct from parvocellular PVN neurons that release CRH and other releasing hormones into the portal system.

Reference: Guyton & Hall, Textbook of Medical Physiology, 14th ed.

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