Neurosecretion (Regulated Exocytosis)

The Basic Concept of Neurosecretion

In 1928, Ernst Scharrer accidentally discovered and hypothesized that there were large “glandular cells” in the preoptic region of the diencephalon of the fish, having secretory activity similar to that of endocrine gland cells. Lately confirmed by histochemical staining of neurons by Wolfgang Bargmann, neurosecretion was established.

Currently, neurosecretion is referred to as a physiological process of the storage, synthesis, and release of neuronal hormones. Since the neurosecretions from the neurons axonal terminals are transported and released to the outside of the neurons via dense core neurosecretory vesicles, this process is also known as regulated exocytosis. These secretory products, or neurohormones, synthesized and released from neurosecretory neurons circulate in the bloodstream and reach the target site of action, having a regulatory effect on both endocrine and nonendocrine cells. Therefore, neurosecretion has an important influence on brain structure, functions, and many other physiological responses.

Neurosecretory Vesicles for Exocytosis

Neurosecretory vesicles are dense-cored vesicles or electron-lucent synaptic vesicles produced in the cell body of neurosecretory neurons. Like other vesicles presenting in all cells, neurosecretory vesicles are synthesized in the rough endoplasmic reticulum, and transported to the Golgi for modification, condensing into immature vesicles. These precursor vesicles mature into neurosecretory vesicles of different sizes and shapes when they pass through axons or dendrites of neurons. Mature neurosecretory vesicles containing a major neurosecretory product usually bind to microtubules, which are responsible for transportation neurosecretory vesicles to the plasma membrane. The actin filaments on the plasma membrane of neuroendocrine neurons control the docking and exocytosis of neurosecretory vesicles.

Fig.1 Neurosecretory vesicle production, transport, and release in the neurosecretory neurons. (Morris, 2020)

Current Neurosecretory Model System

Currently, there are well-established neurosecretory systems that have been elucidated, one is the hypothalamic-neurohypophysial system, and the other is the hypothalamic-adenohypophyseal system, both of which play an important role in the regulation of brain function and other physiological activities.

The hypothalamic-neurohypophysial system, also known as the magnocellular neurosecretory system, consists of magnocellular neurosecretory neurons whose somata are in the supraoptic nucleus and paraventricular nucleus while nerve terminals located in the posterior pituitary. The magnocellular neurons synthesize and release neuropeptides (oxytocin and arginine vasopressin) into the circulation, both of which are critical factors for body fluid homeostasis and reproduction.

The hypothalamic-adenohypophyseal neurosecretory system, or the parvocellular neurosecretory system, synthesizes and releases several neurohormones, including 4 releasing hormones (growth hormone-releasing hormone, gonadotrophin-releasing hormone, thyrotrophin-releasing hormone, and corticotrophin-releasing hormone), Dopamine, and somatostatin. These neurohormones are widely implanted in a variety of physiological activities, such as stress response, growth, development, and metabolism.

Fig.2 Schematic illustration neurosecretory model systems. (Miyata, 2017)

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References

1. Morris, J.F. Neurosecretory Vesicles: Structure, Distribution, Release and Breakdown. In: Neurosecretion: Secretory Mechanisms. Masterclass in Neuroendocrinology. 2020, 8: pp 81-102.
2. Miyata, S.Advances in Understanding of Structural Reorganization in the Hypothalamic Neurosecretory System. Frontiers in Endocrinology (Lausanne). 2017, 8: 275.