Hypothalamic hormones

The hypothalamus is defined as a hypothalamus that occupies a part of the diencephalon located below the thalamus under the hypothalamic groove and is a cluster of nerve cells with numerous afferent and efferent connections. The hypothalamus is the highest vegetative center that coordinates the functions of various internal systems, adapting them to the overall activity of the body. It is essential in maintaining an optimal level of metabolism (protein, carbohydrate, fat, water and mineral) and energy, in regulating the body's temperature balance, the activity of the digestive, cardiovascular, excretory, respiratory and endocrine systems. The hypothalamus controls such endocrine glands as the pituitary gland, thyroid gland, sex glands, adrenal glands, and pancreas.

Regulation of the pituitary gland's tropic functions is carried out by secreting hypothalamic neurohormones that enter the gland through the portal vascular system. There is a feedback between the hypothalamus and the pituitary gland that regulates their secretory functions. This connection is usually called short, in contrast to the long one that connects the "target" glands and the hypothalamus or pituitary gland, and the ultrashort feedback that closes in the same structure where the hormone is secreted. The process of secretion of pituitary tropic hormones is controlled by both peripheral hormones and hypothalamic releasing hormones. Seven hypothalamic neurohormones that activate and three that inhibit the secretion of pituitary tropic hormones have been found in the hypothalamus. The classification of hypothalamic neurohormones is based on their ability to stimulate or inhibit the secretion of the corresponding pituitary hormone. The first group includes corticoliberin - releasing hormone ACTH, or corticotropic (CRH); thyrotropin-releasing hormone (TRH); luliberin - releasing hormone luteinizing hormone (LH-RH); folliberin - releasing hormone follicle-stimulating hormone (FSH-RH); somatoliberin - somatotropin-releasing hormone (SRH); prolactoliberin - prolactin-releasing hormone (PRH); melanoliberin - releasing hormone melanocyte-stimulating hormone (MSH); the second - prolactostatin - prolactinin-inhibiting hormone (PIF); melanostatin - inhibiting hormone melanocyte-stimulating hormone (MIF); somatostatin - somatotropin-inhibiting factor (SIF). Hypothalamic neurohormones also include vasopressin (VP) and oxytocin, produced by nerve cells of the large-cell nuclei of the hypothalamus, which are transported along their own axons to the posterior lobe of the pituitary gland. All hypothalamic neurohormones are substances of a peptide nature. Studies of the chemical structure of neurohormones, which began more than 25 years ago, have established the structure of only five hormones of this group of peptides: TRH, LH-RH, SIF, SRH and CRH. These compounds consist of 3, 10, 14, 44, 41 amino acids, respectively. The chemical nature of the remaining hypothalamic releasing hormones has not been fully established. The content of neurohormones in the hypothalamus is very insignificant and is expressed in nanograms. Synthesis of the five specified neuropeptides in large quantities allowed to develop radioimmunological methods of their determination and to specify their localization in the hypothalamic nuclei. Data of recent years indicate a wide distribution of neurohormones outside the hypothalamus, in other structures of the central nervous system, as well as in the gastrointestinal tract. There is every reason to believe that these hypothalamic neurohormones perform endocrine and neuromediator or neuromodulatory functions, being one of the components of physiologically active substances that determine a number of systemic reactions, such as sleep, memory, sexual behavior, etc.

Hypothalamic neurohormones are synthesized in the perikarya of neurons of the small-celled structures of the hypothalamus, from where they enter the nerve endings along the axons, where they accumulate in individual synaptic vesicles. It is assumed that the perikarya store a prohormone with a higher relative molecular weight than the true hormone released into the synaptic cleft. It should be noted that there is some discreteness in the localization of the sites of synthesis of luliberin in the hypothalamus (anterior hypothalamus) and the diffuseness of thyrotropin-releasing hormone and somatostatin. For example, the content of thyrotropin-releasing hormone in the hypothalamus is only 25% of its total content in the central nervous system. The discreteness of the localization of neurohormones determines the involvement of a particular area of the hypothalamus in the regulation of a certain tropic function of the pituitary gland. It is believed that the anterior region of the hypothalamus is directly involved in the regulation of the secretion of gonadotropins. Most researchers believe that the center of regulation of the thyroid function of the pituitary gland is the area located in the anterior basal part of the hypothalamus, below the perigastric nucleus, extending from the epioptic nuclei in front to the arcuate nuclei in back. The localization of the areas that selectively control the adrenocorticotropic function of the pituitary gland has not been studied sufficiently. A number of scientists associate the regulation of ACTH secretion with the posterior region of the hypothalamus. The localization of the hypothalamic areas involved in the regulation of the secretion of other tropic hormones of the pituitary gland remains unclear. It should be noted that the maximum concentration of all known hypothalamic neurohormones is found in the median eminence, i.e., at the final stage of their entry into the portal system. Functional isolation and delimitation of hypothalamic zones by their participation in controlling the tropic functions of the pituitary gland cannot be carried out clearly enough. Numerous studies have shown that the anterior region of the hypothalamus has a stimulating effect on sexual development, and the posterior region has an inhibitory effect. Patients with pathology of the hypothalamic region experience dysfunction of the reproductive system - sexual weakness, menstrual cycle disorders. There are many cases of accelerated puberty as a result of excessive irritation of the gray tubercle region by a tumor. Sexual dysfunction is also observed in adiposogenital syndrome associated with damage to the tuberal region of the hypothalamus. Reduced or even complete loss of smell in hypogenitalism is also associated with a decrease in the content of luliberin in the olfactory bulbs.

The hypothalamus is involved in the regulation of carbohydrate metabolism - damage to its posterior sections causes hyperglycemia. In some cases, changes in the hypothalamus are accompanied by obesity and cachexia. It usually develops with damage to the upper medial nucleus and the serous tuberculous region of the hypothalamus. The role of the supraoptic and periventricular nuclei in the mechanism of diabetes insipidus is shown.

Close connections of the hypothalamus with other structures of the central nervous system determine its participation in many other physiological processes of the organism's life - thermoregulation, digestion and regulation of blood pressure, alternation of sleep and wakefulness. It plays the main role in the formation of the main drives of the organism - motivations. This is based on the ability of hypothalamic neurons to specifically respond to changes in blood pH, carbon dioxide and oxygen tension, ion content, especially potassium and sodium. In other words, the cells of the hypothalamus perform the function of receptors that perceive changes in homeostasis and have the ability to transform humoral changes in the internal environment into a nervous process. The excitation arising in the cells of the hypothalamus spreads to neighboring structures of the brain. This leads to motivational excitation, accompanied by qualitative biological uniqueness of behavior.

Hypothalamic neurohormones are highly active physiological compounds that occupy a leading place in the feedback system between the hypothalamus, pituitary gland and target glands. The physiological effect of neurohormones is reduced to an increase or decrease in the concentration of the corresponding tropic hormones in the blood. It is necessary to pay attention to the lack of species specificity in hypothalamic neurohormones, which is very important for medical practice.

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