HORMONES OF THE HYPOTHALAMUS: STIMULATORS AND INHIBITORS - NEUROPSYCHOLOGY - 2024

The hormones of the hypothalamus are very diverse and are responsible for carrying out actions such as the regulation of body temperature, the organization of eating, aggression and reproduction behaviors, as well as the structuring of visceral functions.

The hypothalamus is a nuclear region of the brain. It consists of a subcortical structure, is part of the diencephalon and is located just below the thalamus. This part of the brain is essential for the coordination of essential behaviors, which are linked to the maintenance of the species.

Hypothalamus marked in orange

One of the main functions of the hypothalamus is to release and inhibit hormones from the pituitary. The regulation of the functioning of these hormones makes it possible to carry out and modulate a large number of physical and biological processes.

Stimulating hypothalamic hormones

The hormones of the hypothalamus can be divided into two broad categories: stimulating hormones and inhibiting hormones.

The stimulating hormones are those that produce a direct stimulation on the hormonal liberation. These hormones work through the hypothalamic-pituitary axis. That is, by connecting these two structures of the body.

The hypothalamus receives information from the cerebral cortex and the autonomic nervous system. Likewise, it directly interprets a wide variety of environmental stimuli (such as temperature and lighting).

Upon receiving these stimuli, it sends signals to the pituitary gland to regulate the activity of the thyroid, adrenal and gonads, in order to satisfy the specific needs of the body. The main hypothalamic hormones are:

-Corticotropin-releasing hormone

Corticotropin-releasing hormone (CRF). Source: ProteinBoxBot.ProteinBoxBot at en.wikipedia

Corticotropin releasing factor or hormone is a 41 amino acid peptide. It is released by the ventromedial hypothalamus of the brain and is transported by the blood to the portal-pituitary system.

When the hormone reaches the pituitary, specifically the adenohypophysis, it is responsible for promoting the production and secretion of corticotropin (ACTH).

Corticotropin is a polypeptide hormone that stimulates the adrenal glands. It exerts its action on the adrenal cortex and stimulates steroidogenesis, the growth of the adrenal cortex and the secretion of corticosteroids.

The lack of this hormone in the blood causes a decrease in cortisol. Inducing the person a state of hypoglycemia and weakness. Likewise, it can also cause a decrease in adrenal androgens in the blood, producing a reduction in pubic axial hair and a reduction in libido.

Thus, corticotropin-releasing hormone is stimulated by states of positive energy balance and is reduced in a state of negative energy balance, such as lack of food.

On the other hand, the nutrients found in the blood also affect the levels of expression of the corticotropin-releasing hormone. In this sense, the hormone released by the hypothalamus allows regulating biological processes mainly related to hunger and sexual functioning.

-Hormone releasing growth hormone

Growth hormone structure. Source: Emw

Growth hormone-releasing hormone (GHRH) belongs to a family of molecules including secretin, glucagon, vasoactive intestinal peptide, and inhibitory gastric peptide.

The hormone is produced in the arcuate nucleus and in the ventromedial nucleus of the hypothalamus. When it occurs, it travels through the blood vessels to the pituitary.

There are two chemical forms of GHRH. The first has 40 amino acids and the second 44. Both types of hormones exert a large number of actions on somatotropic cells.

When GHRH binds to the membrane of pituitary cells, it produces a high stimulation of growth hormone (GH) secretion.

This stimulation is carried out by a calcium-dependent mechanism and activates adenyl cyclase through the accumulation of cyclic AMP. Likewise, it activates the phosphatidylinositsol cycle and exerts a direct action within the cell.

Growth hormone is a peptide hormone that stimulates cell growth and reproduction. Likewise, it allows the regeneration of muscles and tissues of the body.

The effects of GH can be broadly described as anabolic. And they carry out a great variety of actions on the organism. The main ones are:

1. Increases calcium retention and mineralization of bones.
2. Increase muscle mass.
3. Promotes lipolysis.
4. Increases protein biosynthesis.
5. Stimulates the growth of all internal organs except the brain.
6. Regulates the homeostasis of the body.
7. Reduces glucose consumption by the liver and promotes gluconeogenesis.
8. Contributes to the maintenance and function of the pancreatic islets.
9. Stimulates the immune system.

Thus, the hypothalamus plays an essential role in the development, growth and regeneration of the body through stimulation of the production of growth hormone.

-Gonadotropin-releasing hormone

Structure of gonadotropin-releasing hormone. Source: Author http://en.wikipedia.org/wiki/User:BorisTM -

Gonadotropin-releasing hormone (LHRH) acts directly on high-affinity pituitary receptors. When you stimulate these receptors, it causes an increase in the production of the hormone gonadotropin.

It is secreted mainly by neurons in the preoptic area and is made up of only 10 amino acids. The action of LHRH on the pituitary is initiated by binding to specific receptors on the cell surface.

The LHRH release process is activated through the mobilization of intracellular calcium. Adrenergic agonists facilitate the release of the hormone, while endogenous opioids inhibit it. Likewise, estrogens increase the amount of LHRH receptors and androgens reduce it.

The release of this hormone by the hypothalamus varies remarkably throughout the life of a human being. LHRH first appears during pregnancy. From the tenth week of gestation approximately.

During this time, LHRH triggers a surge in gonadotropins. Subsequently, the release of these hormones decreases markedly.

Gonadotropins are hormones that are involved in regulating the reproduction of vertebrates. Specifically, there are three different types (all of them released by the LRHR): luteinizing hormone, follicle-stimulating hormone, and chorionic gonadotropin.

Luteinizing hormone is responsible for initiating ovulation in women, and follicle-stimulating hormone stimulates the growth of the ovarian follicle that contains the ovum.

Finally, chorionic gonadotropin is responsible for managing nutritional factors and stimulating the production of the necessary amounts of other hormones for the embryo. For this reason, LHRH motivates a high gonadotropin stimulation during pregnancy.

Thyrotropin-releasing hormone

Structural formula of thyrotropin-releasing hormone (TSHRH). Source: Fvasconcellos

Thyrotropin-releasing hormone (TSHRH) is a tripeptide that is generated in the anterior hypothalamic area. Likewise, they can also be produced directly in the posterior pituitary and in other areas of the brain and spinal cord.

TSHRH circulates through the blood vessels until it reaches the pituitary, where it attaches to a series of specific receptors.

When it reaches the pituitary, TSHRH stimulates thyrotropin secretion through increased free cytoplasmic calcium. Phosphatidylinositol and membrane phospholipids are involved in thyrotropin secretion.

The action of TSHRH is carried out on the membrane and does not depend on internalization, although the latter takes place and causes an increase in thyrotropin secretion.

Thyrotropin, also called thyroid-stimulating hormone, is a hormone that regulates the production of thyroid hormones. Specifically, it is a glycoprotein substance that increases the secretion of thyroxine and triiodothyronine.

These hormones regulate cellular metabolism through the activation of metabolism, muscle tension, sensitivity to cold, increased heart rate and the execution of alert mental activities.

In this way, TSHRH is indirectly responsible for the regulation of basic processes in the body through the activation of the hormone that regulates the functioning of thyroid hormones.

-Prolactin releasing factors

Structure of prolactin. Source: BorisTM at English Wikipedia

Finally, prolactin-releasing factors (PRL) are a group of elements made up of neurotransmitters (serotonin and acetylcholine), opiate substances and estrogens.

These factors stimulate prolactin release through the collaboration of TSHRH, vasoactive intestinal peptide, substance P, cholecystokinin, neurotensin, GHRH, oxytocin, vasopressin, and galanin.

All these substances are responsible for increasing the secretion of prolactin in the pituitary. Prolactin is a peptide hormone that is responsible for producing milk in the mammary glands and synthesizing progesterone in the corpus luteum.

On the other hand, in the case of men, prolactin can affect adrenal function, electrolyte balance, breast development, and sometimes galactorrhea, decreased libido, and impotence.

Prolactin is mainly produced during pregnancy in women. The blood values ​​of this hormone range between 2 and 25 ng / mL in non-pregnant women and between 2 and 18 ng / mL in men. In pregnant women, the amount of prolactin in the blood increases between 10 and 209 ng / mL.

Thus, PRLs act specifically during pregnancy in women to increase milk production. When there is no pregnancy situation, the function of this hormone is greatly reduced.

Inhibitory hypothalamic hormones

The inhibitory hormones of the hypothalamus perform an opposite function to the stimulatory ones. That is, instead of stimulating the production of hormones in the body, they inhibit their secretion and generation.

These types of hypothalamic hormones also act on the pituitary. They are produced in the hypothalamus and travel to this region to perform certain functions.

There are two different types of inhibitory hypothalamic hormones: PRL inhibitory factors and GH inhibitory hormone.

-PRL inhibitory factors

Structure of dopamine. Source: NEUROtiker

PRL inhibitory factors are mainly made up of dopamine. This substance is generated in the arcuate and paraventricular nuclei of the hypothalamus.

Once produced, dopamine travels through the axons of neurons to nerve endings, where it is released into the blood. It is transported through blood vessels and reaches the anterior pituitary.

When it is coupled to the receptors of the pituitary, it performs a totally antagonistic action to the prolactin releasing factors. That is, instead of stimulating the secretion of this hormone, it inhibits its production.

Inhibition is carried out by interactions with D2 receptors (dopamine receptors linked to adenylate cyclase). Likewise, dopamine inhibits the formation of cyclic AMP and the synthesis of phosphonisitol, a highly relevant action in the regulation of PRL secretion.

In contrast to prolactin-stimulating factors, the action of dopamine in the pituitary is much more abundant.

This acts whenever prolactin production is not necessary, that is, when there is no pregnancy. In order to avoid the effects of this hormone, which are not necessary if there is no pregnancy.

Somatostatin hormone

Structure of somatostatin. Source: Ed (Edgar181)

Finally, somatostatin or hormone inhibitor of (GH) consists of a hormone of 14 amino acids that is distributed by multiple cells of the nervous system. It acts as a neurotransmitter in different regions of the spinal cord and brainstem.

Specific somatostatin-secreting cells are involved in the regulation of insulin and glucagon secretion, and are an example of paracrine hormonal control.

Somatostatin is a hormone that acts through five G protein-coupled receptors and uses various second messenger pathways. This hormone is responsible for inhibiting GH secretion and reducing the responses of this hormone to secretagogic stimuli.

The main effects of this hormone are:

1. Decreased rate of digestion and absorption of nutrients from the gastrointestinal tract.
2. Inhibition of glucagon and insulin secretion.
3. Inhibition of gastric, duodental and gallbladder motility.
4. Reduction of the secretion of hydrochloric acid, pepsin, gastrin, secretin, intestinal juice and pancreatic enzymes.
5. Inhibition of glucose and triglyceride absorption through the intestinal mucosa.

References

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