Fisiología Endocrina - Eje Hipotálamo Hipófisis (hipotálamo-hipofisario) (IG:@doctor.paiva)

Introduction to the Hypothalamic-Pituitary Axis

Overview of the Class

• The class is introduced by Eduardo Paiva, focusing on the hypothalamic-pituitary axis.

• Topics covered include physiological anatomy of the hypothalamus and pituitary gland, hypophysis hormones, and neuroendocrine axes.

Anatomical Structure

• A lateral view of a human brain is presented, highlighting key structures: cerebrum, cerebellum, brainstem, and eye.

• The hypothalamus (in blue) and pituitary gland are identified; anterior (adenohypophysis) and posterior (neurohypophysis) parts are discussed.

Understanding the Portal System

Importance of the Portal System

• The portal system's unique structure is explained: it consists of veins connecting capillary beds directly to another set of capillaries.

• This system allows for efficient hormone secretion from the hypothalamus to the pituitary gland.

Capillary Networks

• The arterial supply from superior hypophyseal artery forms a primary capillary plexus in the pituitary region.

• Secondary capillary plexuses are formed through long portal vessels that connect back to venous circulation.

Hormonal Communication Between Hypothalamus and Pituitary

Hormone Secretion Mechanism

• Neuronal groups in the hypothalamus secrete hormones into primary plexuses; these hormones stimulate adenohypophysis functions.

• Neurohypophysis acts as an extension of neurons from the hypothalamus where specific hormones like ADH and oxytocin are synthesized.

Cellular Functions in Adenohypophysis

• Important secretory cells in adenohypophysis release trophic hormones into systemic circulation via veins.

Communication Pathways

Types of Communication

• Hormonal communication occurs between hypothalamus and adenohypophysis through blood vessels; this is crucial for hormone delivery.

• In contrast, communication with neurohypophysis involves direct neuronal connections due to its structural continuity with hypothalamic neurons.

Understanding the Hypophysis: Structure and Function

Overview of the Hypophysis

• The hypophysis, also known as the pituitary gland, consists of two main parts: the anterior (adenohypophysis) and posterior (neurohypophysis). The anterior is primarily epithelial, while the posterior is neuronal.

• The adenohypophysis synthesizes hormones due to specialized cells that both synthesize and secrete them. In contrast, the neurohypophysis serves mainly as a storage site for hormones produced in the hypothalamus.

Hormonal Functions

• Specialized endocrine cells in the adenohypophysis respond to hormonal stimuli from the hypothalamus via portal vessels, leading to hormone secretion into circulation.

• An example includes TRH (Thyrotropin-Releasing Hormone), which stimulates TSH (Thyroid-Stimulating Hormone) production in response to signals from the hypothalamus.

Role of the Hypothalamus

• The hypothalamus is a complex structure with various functions; its endocrine role involves several nuclei grouped into four areas: pre-optic, supraoptic, median eminence, and posterior regions.

• Key areas relevant to endocrinology include supraoptic and median eminence regions where specific nuclei are responsible for hormone release.

Nuclei of Interest

• Important nuclei include:

• Supraoptic nucleus

• Suprachiasmatic nucleus

• Anterior hypothalamic nucleus

• Paraventricular nucleus

• Dorsomedial nucleus

• Ventromedial nucleus

• Arcuate nucleus

• These nuclei are crucial for releasing but not synthesizing hypothalamic hormones.

Adenohypophyseal Cells and Their Hormones

• The adenohypophysis contains specialized secretory cells called trophic hormone-secreting cells that synthesize various hormones.

• There are five main types of these cells:

• Somatotropes (growth hormone)

• Lactotropes (prolactin)

• Corticotropes (ACTH)

• Thyrotropes (TSH)

• Gonadotropes (LH & FSH)

Cell Distribution and Functionality

• Somatotropes make up about 30% to 40% of all adenohypophyseal cells; corticotropes account for around 20%, while other cell types comprise only a small percentage.

• Tumors arising from somatotropes are referred to as acidophilic tumors due to their staining properties with acidic dyes.

Hormonal Regulation Mechanisms

• Hormones released by the hypothalamus regulate these adenohypophyseal cells. For instance:

• GH-RH stimulates somatotropes,

• Somatostatin inhibits them,

• Dopamine inhibits lactotropes,

Hormonal Regulation and Hypothalamic Function

Overview of Hormones Released by the Hypothalamus

• The hypothalamus releases hormones that stimulate various pituitary cells, leading to the production of growth hormone (GH) and other key hormones.

• Prolactin is unique as it is the only hormone under chronic inhibition; its regulation differs from other stimulating hormones.

• Various hormones such as adrenocorticotropic hormone (ACTH), thyroid-stimulating hormone (TSH), luteinizing hormone (LH), and follicle-stimulating hormone (FSH) are also produced in response to hypothalamic signals.

Mechanism of Hormone Release

• The release mechanism involves hypothalamic hormones traveling through portal vessels to stimulate specific pituitary cells, which then produce their respective hormones.

• Unlike anterior pituitary hormones, posterior pituitary hormones like antidiuretic hormone (ADH) and oxytocin are synthesized in the hypothalamus but stored in the neurohypophysis for later release.

Types of Hypothalamic Hormones

• Hypothalamic releasing and inhibiting hormones primarily regulate anterior pituitary functions. Most are peptide-based, except dopamine, which is derived from tyrosine.

• Key releasing factors include corticotropin-releasing hormone (CRH), thyrotropin-releasing hormone (TRH), gonadotropin-releasing hormone (GnRH), and growth hormone-releasing hormone (GHRH).

Specific Functions of Releasing Hormones

• TRH stimulates TSH secretion; GnRH promotes FSH and LH secretion; CRH triggers ACTH release; GHRH stimulates GH production while somatostatin inhibits it.

• Dopamine acts as an inhibitor for prolactin secretion, highlighting its role in regulating lactation-related processes.

Localization of Hormone Synthesis

• The synthesis location for these hormones varies within different hypothalamic nuclei, with some being produced in multiple nuclei.

• Notably, ADH and oxytocin do not act directly on the hypophysis but have significant physiological roles elsewhere.

Hormonal Regulation and Feedback Mechanisms in the Endocrine System

Corticotropin and Thyroid Stimulating Hormones

• Corticotropin-releasing hormone (CRH) stimulates the adrenal cortex to produce glucocorticoids and androgens, maintaining the size of fasciculata and reticular zones.

• Thyroid-stimulating hormone (TSH) promotes thyroid hormone production by follicular cells, also increasing their size; excess TSH can lead to goiter.

Gonadotropins: Follicle-Stimulating Hormone (FSH) and Luteinizing Hormone (LH)

• LH stimulates ovarian follicle development in women and regulates testicular function in men; FSH induces ovulation and corpus luteum formation while promoting estrogen and progesterone production.

• Prolactin, released by lactotropic cells, stimulates milk secretion while oxytocin facilitates milk ejection during breastfeeding.

Growth Hormone Effects

• Growth hormone has widespread effects on body tissues, primarily stimulating growth processes; its specific actions will be discussed later.

Neurohypophysis Functions

• The neurohypophysis is an extension of the hypothalamus composed of neuronal processes that transport hormones synthesized in hypothalamic nuclei.

• Key hormones include antidiuretic hormone (ADH), which regulates osmolarity and volume, and oxytocin, which aids in childbirth by inducing uterine contractions.

Feedback Mechanisms in Endocrine Regulation

• The hypothalamic-pituitary axis regulates hormone synthesis through trophic factors; most hormones exhibit excitatory feedback except prolactin, which is chronically inhibited.

• Negative feedback loops are crucial for maintaining hormonal balance. For example, TRH from the hypothalamus stimulates TSH release from the pituitary gland.

Types of Feedback Circuits

• Peripheral hormones inhibit both the pituitary gland and hypothalamus to prevent overproduction of hormones—a key aspect of negative feedback regulation.

• An increase in thyroid hormones like T3/T4 leads to inhibition signals sent back to both the pituitary gland and hypothalamus to reduce further stimulation.

Short-Circuit Feedback Mechanisms

Hormonal Regulation and Feedback Mechanisms

Hypothalamic-Pituitary Axis Overview

• The hypothalamus stimulates the pituitary gland to release hormones that affect peripheral hormone release, which can have ultra-short, short, long, and very long feedback loops.

• The hypothalamus releases Thyrotropin-Releasing Hormone (TRH), stimulating the pituitary to produce Thyroid-Stimulating Hormone (TSH), which in turn prompts the thyroid to produce thyroid hormones T3 and T4.

• These thyroid hormones exert negative feedback on both the hypothalamus and pituitary to regulate their own production.

Gonadal Hormones and Feedback

• Gonadotropin-Releasing Hormone (GnRH) from the hypothalamus stimulates the pituitary to release Luteinizing Hormone (LH) and Follicle-Stimulating Hormone (FSH), affecting ovaries and testes.

• Testosterone from testes inhibits both the hypothalamus and pituitary; estradiol has a similar inhibitory effect on these structures.

Cortisol Production Regulation

• The hypothalamus releases Corticotropin-Releasing Hormone (CRH), stimulating ACTH release from the pituitary, which then acts on the adrenal cortex to produce cortisol.

• Cortisol exerts negative feedback on both the hypothalamus and pituitary, inhibiting further production of CRH and ACTH.

Growth Hormone Dynamics

• Growth hormone-releasing hormone (GHRH) stimulates GH release from the pituitary while somatostatin inhibits it. This creates a complex regulatory mechanism for growth hormone levels.

• Insulin-like Growth Factor 1 (IGF-1), produced in response to GH in liver tissues, also provides negative feedback on GH secretion.

Prolactin Regulation

• Prolactin is uniquely subject to chronic inhibition by dopamine from the hypothalamus; its secretion increases when communication between these two structures is disrupted.

Antagonism of Dopamine and Its Effects

Impact of Antidepressants and Antipsychotics on Prolactin Levels

• The discussion highlights that certain antidepressants and antipsychotics antagonize dopamine, which can lead to increased prolactin levels (hyperprolactinemia).

• It is noted that hyperprolactinemia can also be stimulated by other factors such as breast suction and estrogen levels.

• The physiological implications of these effects will be explored further in the context of lactation and pregnancy.