Hormones Lecture

Degradation of Hormonal Signaling

  • Definition and Overview

    • Degradation refers to the breakdown of hormones within the body, which ultimately affects cellular responses.

    • Hormones are transported through tissues and affect the metabolic activities of recipient cells.

  • Hormonal Types & Functions

    • Hormones are classified into various types such as ectohormones and exocrine hormones.

    • They can produce responses that are negative feedback mechanisms or simple signaling messages in organisms.

    • Ectohormones: hormones that exit glands and via the bloodstream convey messages to target organs or cells.

  • Hormone Signaling and Mechanisms

    • Systems & Processes

    • Hormonal signaling involves intricate biosynthesis leading to the release of molecules.

    • Primarily involves exocytosis, where hormones are released from cells into the bloodstream.

    • Feedback mechanisms play essential roles in regulating hormone secretion.

    • Homeostasis: Hormonal responses contribute to maintaining physiological balance within organisms.

  • Hierarchy of Hormonal Signaling

    • In multicellular organisms, hormonal signaling exhibits a hierarchical nature affecting various tissues and cells.

    • Hormonal signaling is specific and involves regulatory mechanisms such as down-regulation of receptor sites in response to prolonged hormone exposure.

Hypothalamus and Neuroendocrine Function

  • Structure of the Hypothalamus

    • Located at the anterior end of the neural tube; considered one of the most primitive parts of the brain.

    • Its anatomy includes various nuclei and components essential for neuroendocrine functions:

    • Pituitary Stalk (Infundibulum): Emerges from the ventral surface, connecting the hypothalamus to the pituitary gland.

    • Key structures include the supraoptic, paraventricular, and arcuate nucleus which have distinct neuroendocrine roles.

  • Neuroendocrine Cells

    • Magnocellular Output: Large neurons in the supraoptic and paraventricular nuclei secrete oxytocin and vasopressin.

    • Axons travel down the pituitary stalk to connect with the posterior pituitary gland.

    • Parvocellular Output: Smaller neurons situated at the floor of the 3rd ventricle which secrete releasing hormones into hypophysial portal vessels.

Gonadotropin-Releasing Hormone (GnRH)

  • GnRH Characteristics

    • A decapeptide that is conserved across all mammalian species.

    • Functions primarily as a pulse generator influencing the release of luteinizing hormone (LH).

  • Regulation of GnRH

    • Stimuli for Release:

    • Stimulatory signals include kisspeptin and glutamate.

    • Inhibition from substances includes dynorphin, beta-endorphin, and gamma-aminobutyric acid (GABA).

    • Receptors:

    • GnRH neurons do not express steroid hormone receptors, while kisspeptin neurons do expressing their sensitivity to hormonal regulation.

Circadian Rhythms and Hormonal Control

  • Circadian clock functions

    • Light-mediated signals activate photoreceptors and trigger electrical signals transmitted through the retinal hypothalamic tract.

    • These signals are chemical transformations occurring within the suprachiasmatic nucleus (SCN), influencing reproductive hormone patterns.

  • SCN Role in Reproduction

    • Regulates temporal patterns of LH concentrations with seasonal changes, particularly affecting species with breeding cycles linked to environmental changes.

GnRH Receptor and Anterior Pituitary Interaction

  • GnRH Receptor Characteristics

    • It's a G-protein coupled receptor that can undergo internalization for down-regulation in response to high GnRH concentrations.

  • Anterior Pituitary Hormones

    • Gonadotropes make up 3-15% of the anterior pituitary population, which increases post-gonadectomy leading to greater LH and FSH release.

Gonadotropin Subunit Regulation

  • Subunit Composition and Function

    • Common subunit exists for pituitary glycoprotein hormones such as LH and FSH as well as chorionic hormones.

    • Hormone expression is affected by GnRH pulse frequency;

    • High frequencies enhance LH levels; lower frequencies favor FSH biosynthesis.

    • Differences Between Male and Female Responses:

    • Male hormones respond similarly across varying frequencies while female hormones show more differentiation based on GnRH pulsing amounts.

Gonadal Hormones and Steroids

  • Sources and Target Tissues

    • Estradiol, progesterone, and testosterone are primary gonadal steroids regulating various reproductive functions.

    • Major Targets include the brain, pituitary gland, uterus, and mammary glands.

  • Steroidogenic Acute Regulatory Protein (StAR)

    • This protein is responsible for cholesterol transport within mitochondria and is crucial in the rate-limiting step of steroid hormone production.

Gonadotropin Interaction and Feedback Loops

  • Activin and Inhibin Functions

    • Activin enhances FSH gene expression, regulating folliculogenesis and gonadotrope population increases.

    • Inhibin is a feedback regulator reducing FSH synthesis and increasing responsiveness of LH in circulating blood.

  • Follistatin

    • It inhibits activin and is influenced by GnRH pulsing frequencies, contributing to overall FSH regulation by reducing its mRNA levels without influencing LH concentrations.

Conclusion: Critical Factors for Hormonal Action

  • Critical Developments

    • The development of ovaries versus testes is dependent directly on the Sry gene levels during early embryonic development.

    • Insufficient Sry levels may lead to ambiguous or mixed gonadal structures but does not completely rule out mature ovarian development under certain conditions.