Neuroendocrinology

Neuroendocrinology is the study of the interactions between the nervous systemand the endocrine system. The concept arose from the recognition that the secretion of hormonesfrom the pituitary glandwas closely controlled by the brain, and especially by the hypothalamus.

Neuroendocrine systems of the hypothalamus

Oxytocinand vasopressin, the two hormones of the posterior pituitary gland(the neurohypophysis), are secreted from the nerve endings of neurosecretory neuronsinto the systemic circulation. The cell bodies of these oxytocin and vasopressin neurons are in the supraoptic nucleusand the paraventricular nucleus, and the electrical activity of these neurons is regulated by afferent synaptic inputs from other brain regions. By contrast, the hormones of the anterior pituitary gland(the adenohypophysis) are secreted from endocrine cells that, in mammals, are not directly innervated, yet the secretion of these hormones (adrenocorticotrophic hormone(ACTH), luteinizing hormone(LH), follicle stimulating hormone(FSH), thyroid stimulating hormone(TSH), prolactinand growth hormone) remains under the control of the brain. The brain controls the anterior pituitary gland by ?releasing factors? and ?release-inhibiting factors?; these are blood-borne substances released by hypothalamic neurons into blood vessels at the base of the brain, at the median eminence. These vessels, the hypothalamo-hypophysial portal vessels, carry the hypothalamic factors to the adenohypophysis where they bind to specific receptors on the surface of the hormone-producing cells.

For example, the secretion of growth hormone is controlled by two neuroendocrine systems: the growth-hormone releasing hormone(GHRH) neurons and the somatostatinneurons, which stimulate and inhibit GH secretion respectively. The GHRH neurones are located in the arcuate nucleusof the hypothalamus, while the somatostatin cells involved in growth hormone regulation are in the periventricular nucleus. These two neuronal systems project axons to the median eminence where they release their peptidesinto portal blood vessels for transport to the anterior pituitary. Growth hormone is secreted in pulses, which arise from alternating episodes of GHRH release and somatostatin release, which may reflect neuronal interactions between the GHRH and somatostatin cells, and negative feedback from growth hormone.

So why are these systems of interest to physiologists and neuroscientists? Firstly, neuroendocrine systems regulate things that matter to most of us. They control reproduction in all its aspects, from bonding to sexual behaviour, they control spermatogenesisand the ovarian cycle, parturition, lactationand maternal behaviour. They control the way we respond to stressand infection. They regulate our metabolism? they influence our eating and drinking behaviour, and influence how the energy intake is utilised ? i.e. how fat we get. They influence our mood. They regulate body fluid and electrolyte homeostasis, and blood pressure. In other words, these are systems of central importance to many problems that are major health concerns, as well of sometimes of intimate personal interest.

Secondly, these neurons are large; they are mini ? factories? for producing secretory products; their nerve terminal are large and organised in coherent terminal fields; their output can often be measured easily in the blood; and what these neurons do and what stimuli they respond to are readily open to hypothesis and experiment. For all these reasons and more, neuroendocrine neurons are good "model systems" for studying general questions, like ?how does a neurone regulate the synthesis, packaging and secretion of its product?? and ?how is information encoded in electrical activity??

Today, neuroendocrinology embraces a wide range of topics that arose directly or indirectly from the core conception of neuroendocrine neurons. Neuroendocrine neurones control the gonads ? and gonadal steroidsin turn influence the brain; and so do corticosteroidssecreted from the adrenal glandunder the influence of ACTH. The study of these feedbacks became naturally the province of neuroendocrinologists. The peptides secreted by hypothalamic neuroendocrine neurons into the blood proved to be released also into the brain, and the central actions often appeared to complement the peripheral actions, so understanding these central actions also became the province of neuroendocrinologists, sometimes even when these peptides cropped up in quite different parts of the brain apparently serving functions unrelated to endocrine regulation. Neuroendocrine neurons were discovered in the peripheral nervous system, regulating for instance digestion. The cells in the adrenal medullathat release adrenalineand noradrenalineproved to have properties between endocrine cells and neurons, and proved to be outstanding model systems for instance for the study of the molecular mechanisms of exocytosis, and these too have become, by extension, ?neuroendocrine? systems.

Neuroendocrine systems have been important to our understanding of many basic principles in neuroscience and physiology ? for instance our understanding of stimulus-secretion coupling. One of the dominant themes in neuroendocrinology today is the origins and significance of patterning in neuroendocrine secretion.

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It uses material from the http://en.wikipedia.org/wiki/Neuroendocrinology Wikipedia article Neuroendocrinology.