Neurohormone

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A chemical messenger molecule secreted (or released) by endocrine cells in endocrine glands. By definition, a hormone molecule is released into the bloodstream as it travels throughout the body to find its target cells. Although target cells can be found within the intravascular compartment (i.e., within blood vessels), the majority of hormones have target cells in tissues other than blood vessels.

About Neurohormone

In the majority of cases, neurohormones are neuropeptides (short, functional peptides) acting in the CNS. They can be produced locally in neuroendocrine cells or in the CNS parenchyma (including brain and spinal cord), and can be transported by exocytosis from neuroendocrine cells to target cells.

Many neurohormones act on the target cell via receptor proteins found on the surface of the cell membrane (G protein-coupled receptor, GPCR). Another possibility is that neurohormones may act on the target cell via a soluble receptor protein or on the surface of adjacent cells, which will be considered in more detail below.

All neurohormones are secreted by neural cells. Neurons of the CNS and peripheral nervous system are involved in all neurohormone synthesis and release. Cells containing neurohormones, i.e. the "neuroendocrine cell" may be found in the CNS parenchyma, brainstem, as well as the hypothalamus, thalamus and amygdala.

It is produced by neurons and involved in the control of behavior. Neurohormones act in the brain and the central nervous system to control appetite, reproduction, emotions and memory. Some neurohormones are also involved in many aspects of the immune system.

The most researched neurohormones are serotonin, norepinephrine and dopamine.

History 

Neurohormones were first described by the Belgian researcher, Paul de Kruif in the 1920s and were named after his discovery, the neurohypophyseal hormones. Neurohormones were named after the hypophysis, which means pituitary gland in Latin. Later on, neurohormones were found to have other functions in addition to the pituitary gland. They are produced in other areas of the brain, mainly in the hypothalamus, cerebral cortex and cerebellum. Most of the effects of neurohormones are controlled via the adrenal medulla, though this is not the only area of production. These hormones are also found in the periphery and are called peripheral neurohormones.

Brain 

There are two branches of the axis; the anterior pituitary branch and the posterior pituitary branch. The anterior pituitary branch regulates the hypothalamus. The hypothalamus is a portion of the brain that is responsible for regulating homeostasis. It is composed of the arcuate, ventromedial and dorsomedial nuclei. The dorsomedial nuclei are the most important nuclei for neurohormone release. There is also a link between neurohormones in the brain and the cerebral cortex.Neurohormones are made and released to the bloodstream and transported to the brain.

Endocrine 

The endocrine system is the system that deals with hormone release, and so it is important to understand the role neurohormones play in this system. Endocrine organs include the thyroid, pancreas, parathyroid, adrenal glands, gonads, and placenta. Neurohormones regulate the release of endocrine hormones from these organs. If these organs are disrupted, neurohormones cannot release hormones, and so endocrine hormones cannot be released, which causes malfunction.

Neurohormones Definition

A neurohormone is any hormone that is produced and released into the bloodstream by neuroendocrine cells. Hormones are secreted into the circulation for systemic effect, but they can also function as neurotransmitters or in other roles such as autocrine or also as paracrine messengers.

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Neurohormones in Brief

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A neurohormone is any of a class of substances produced by specialized cells (neurosecretory cells) that are structurally similar to those found in the nervous system rather than the endocrine system. Neurohormones are hormones that travel along nerve-cell extensions (axons) before entering the bloodstream at neurohemal organs. As a result, neurohormones serve as an intermediary between sensory stimuli (events or conditions perceived by the nervous system) and chemical reactions (endocrine secretions that act on other tissues of the endocrine system or on tissues of other systems, such as those involved with excretion or reproduction).

In most mammals, neurohormones such as oxytocin and vasopressin are produced in the hypothalamic region of the brain and secreted into the bloodstream by the neurohypophysis (part of the pituitary gland). The hypothalamus also produces a second class of neurohormones known as releasing hormones (the first of which was chemically identified in 1969). Members of this group, on the other hand, are transmitted within neural cells to a second location in the brain, from which they travel in the bloodstream to the adenohypophysis, which is also a part of the pituitary gland. They either stimulate or inhibit adenohypophyseal hormone release.

Enkephalins and other endorphins are a third class of neurohormones that were discovered in 1975 during research into the mechanism of action of morphine and other analgesics. Endorphins are effective pain relievers, which appears to be related to their function as neurotransmitters, which transmit nerve impulses from one neuron to another. Their neurohormonal activity is manifested by an indirect process involving a site other than the secretory neuron in the central nervous system that stimulates somatotropin and vasopressin secretion.

Function of Neurohormones

Neurohormones are chemical messenger molecules released by neurons that travel to distant target sites throughout the body via the bloodstream. As a result, neurohormones have properties that are similar to those of neurotransmitters and hormones.

The Seven “small Molecule” Neurotransmitters Listed Below Do the Majority of the Work -

1. Acetylcholine

2. Dopamine 

3. Gamma-aminobutyric acid (GABA)

4. Glutamate

5. Histamine

6. Norepinephrine

7. Serotonin

Where are Neurohormones Produced?

The hypothalamus produces neurohormones that regulate pituitary gland hormone biosynthesis and secretion, as well as mediating interactions between the external and internal environments and producing hormones that regulate metamorphosis.

Types of Neurohormone

• Hormone.

• Oxytocin.

• Vasopressin.

• Neurotransmitter.

• Norepinephrine.

• Gonadotropin-releasing hormone.

• Corticotropin-releasing hormone.

• Thyrotropin-releasing hormone.

What is Oxytocin?

Oxytocin is a neurohormone in mammals whose main functions are to stimulate uterine contractions during labour, to stimulate milk ejection (letdown) during lactation, and to promote maternal nurturing behaviour. Oxytocin is thought to influence a variety of other physiological and behavioural processes, including sexual and social behaviour in both men and women. The hypothalamus produces oxytocin in both sexes, which is then stored and secreted into the bloodstream by the posterior pituitary gland. Other tissues that synthesise and secrete it includes the brain, uterus, placenta, ovaries, and testes.

What is Vasopressin?

Vasopressin, also known as antidiuretic hormone (AVP), is a hormone that is important in maintaining osmolality (the concentration of dissolved particles in the serum, such as salts and glucose) and thus the volume of water in the extracellular fluid (the fluid space that surrounds cells). This is necessary to protect cells from sudden changes in water content, which can interfere with proper cell function. In healthy people, normal serum osmolality ranges between 285 and 300 milliosmoles per kilogramme (mOsm/kg). Vasopressin as well as the hormone oxytocin evolved from a single primordial neurohypophyseal hormone called vasotocin, which is found in trace amounts in humans. 

Vasopressin's Primary Function

Although it is also a vasoconstrictor and pressor agent, the primary function of AVP in the body is to regulate extracellular fluid volume by regulating renal water handling (hence, the name "vasopressin"). AVP increases water permeability in the renal collecting ducts via V2 receptors (a cAMP-dependent mechanism), resulting in decreased urine formation (hence, the antidiuretic action of "antidiuretic hormone"). Blood volume, cardiac output, and arterial pressure are all increased as a result.

Heart failure is associated with a seemingly paradoxical increase in AVP. AVP secretion should be reduced by increased blood volume and atrial pressure associated with heart failure, but it is not. It is possible that sympathetic and renin-angiotensin system activation in heart failure overrides the volume and low pressure cardiovascular receptors (as well as the hypothalamic control of AVP release) and results in an increase in AVP secretion. Nonetheless, an increase in AVP during heart failure may contribute to an increase in systemic vascular resistance as well as increased renal fluid retention that occurs with heart failure.

What are Neurotransmitters?

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Neurotransmitters are chemical messengers that send signals from one neuron to another across the synapse to a target cell, which can be another neuron, muscle cell, or gland cell. Neurotransmitters are chemical substances produced by neurons specifically to transmit a message. 

Neurotransmitter Signaling

Neurotransmitters are produced by neurons and stored in vesicles, which are typically found at the terminal end of an axon, also known as the presynaptic terminal.

Nociception 

Neurotransmitters produced by the peripheral nervous system (PNS) may play a role in pain processing. The PNS is responsible for transmitting pain signals to the central nervous system (CNS).  These pain signals can be transmitted as fast impulses in the form of electrical impulses (electrical nociception) or as slower signals in the form of chemical mediators released by sensory nerve endings (chemical nociception).   Pain signals from the PNS travel along specialized nerves, the nociceptors, which are in close physical contact with other sensory neurons and thus can influence them (autonomic reflex). This influence can lead to sensory information that the body doesn't require. For example, a feeling of pain in response to a small injury.  However, in people who suffer from neuropathic pain, this process doesn't normally occur.

A study performed on mice found that the nociceptor cells were activated by the neurotransmitter glutamate and in turn released glutamate into the space between the nerve and the nociceptor, thus activating the nociceptor.

Pain perception 

Pain perception is the process by which the brain and body react to noxious stimuli. It is generally believed that noxious stimuli initiate activation of specific nociceptors. These are distributed throughout the body and function as specialized peripheral chemoreceptors. When a nociceptor is stimulated, a nerve signal is sent to the central nervous system (CNS). The CNS then processes this signal.  A study performed on anesthetized rats found that the neurotransmitter acetylcholine influences pain perception in the rat brain. Acetylcholine was found to increase the size of the neurons in the brain, increasing excitability of the neurons. The neurotransmitter glutamate, on the other hand, decreased the size of the neurons in the brain, thus decreasing excitability.

An electrical pain stimulus is transmitted to the CNS by way of peripheral neurons, and then processed by the spinal cord, brainstem and thalamus before the information is sent back to the brain.  It is believed that the way in which a nerve responds to a noxious stimulus can change over time.  For example, a person who suffers from chronic pain may learn to interpret some stimuli as painful (i.e. the person could learn to interpret stimuli as painful even when they are not), and thus the brain and body may adapt to these changes in pain perception.

What is Norepinephrine?

Norepinephrine is a chemical that occurs naturally in the body and functions as both a stress hormone and a neurotransmitter (a substance that sends signals between nerve cells). When the brain perceives a stressful event, it releases it into the blood as a stress hormone.