L1: General Principles of Hormones, HPA, Pineal Gland

Endocrine system

The collection of glands that produce hormones to regulate various body functions, including metabolism, growth, reproduction, and behavior

Hormones

Chemical messengers secreted into the bloodstream by endocrine glands that regulate physiological processes

Peptide hormones are composed of

amino acids (insulin, glucagon)

Steroid hormones are derived from

cholesterol (cortisol, testosterone)

Amine hormones are derived from

a single amino acid (thyroid hormones, epinephrine)

Hypothalamus

main structure that integrates the nervous system with the endocrine system

Hormonal axis

A sequence of hormone interactions, typically involving the hypothalamus, pituitary gland, and target endocrine glands (e.g., HPA axis: hypothalamic-pituitary-adrenal)

Anabolic hormones

promote tissue building (insulin, growth hormones)

Catabolic hormones

promote the breakdown of molecules for energy (glucagon, cortisol)

Peptide hormones are

Water-soluble hormones

Steroid hormones are

Lipid-soluble hormones

Hypothalamic-Pituitary Portal System (HPPS)

network of blood vessels that links the hypothalamus & pituitary gland

Target cells

cells that have receptors for specific hormones

Where are the receptors of water-soluble hormones located in the target cells?

plasma membrane

What are the chemical messengers of the endocrine system?

hormones

How do hormones reach their target cells?

through the bloodstream

Hormones influence

metabolism, growth, reproduction, homeostasis

Endocrine glands

made of ductless glandular epithelial cells & secrete hormones directly into bloodstream

Endocrine hromone

chemical messenger produced by endocrine glands & use the bloodstream to reach target cells

Autocrine signaling

a cell secretes a hormone or signaling molecule that binds to receptors on its own surface

Paracrine signaling

a cell releases hormones or signaling molecules that affect nearby cells

Water-soluble hormones are transported in blood

freely

Lipid-soluble hormones are transported in blood

bound to a carrier protein

How does the way a hormone is transported in the blood affect its half-life?

hormones bound to carrier proteins have a longer-half life because they are protected from degradation and filtration by the kidneys

Cell-surface receptors are located

on the plasma membrane & utilize second messengers (cAMP or Ca ions)

Intracellular receptors are located

in the cytoplasm or nucleus & regulate gene expression

Cellular responses to hormones

1. metabolic changes

2. changes in gene expression

3. cell division (replication) or differentiation

How long does it take to see the effect of hormones?

minutes, hours, days

Termination of hormone signaling is done by

1. negative feedback mechanisms

2. hormone degradation (clearance) by enzymes

3. reduction in the number of receptors available (down-regulation)

Steroid hormones biochemical structure

- formed from cholesterol

- lipophilic

- intracellular receptor

- bound to carrier proteins in blood

- quickly released upon stimuli

Peptide hormones biochemical structure

- majority of hormones

- water-soluble

- unbound in blood

- cell-surface receptors

- stored in vesicles, fast secretory response

Amine hormones biochemical structure

- smallest hormone

- quickly exerts effects on target cells

- receptors on plasma membrane (except for thyroid hormones which are intracellular)

Eicosanoids are derived from

arachidonic acid

Eicosanoids biochemical structure

- produced by enzymatic action of COX & LOX

- cell-surface receptors

- water-soluble

Synthesis of peptide hormones

1. endocrine gland stimulus

2. hormone gene transcription

3. enzymatic processing to become the active hormone

4. active hormone stored in vesicles

5. exocytosis via Ca++ or cAMP

What is the first step in steroid hormone synthesis?

the conversion of cholesterol to pregnenolone in the mitochondria by P450

Pregnenolone

precursor to all steroid hormones

What converts pregnenolone to progesterone?

3 beta hydroxysteroid dehydrogenase (3B-HSD)

What is the primary reason why steroid hormone synthesis relies so heavily on mitochondria?

mitochondria house the enzymes that catalyze the conversion of cholesterol into pregnenolone

Peptide hormone MOA

hormone activates second messenger system:

1. activation/inhibition of enzymes

2. second messenger will increase concentration in cytosol

3. enzymes stimulated or inhibited

Steroid hormone MOA

1. diffuse freely through cell membrane & bind to intracellular receptors

2. active hormone-receptor complex will bind to nuclear DNA (change rate of synthesis of proteins)

3. stimulation or inhibition of metabolic pathways

Hypothalamic-pituitary-endocrine axis feedback

hierarchic control system

1. hypothalamus

2. pituitary

3. endocrine gland

Response-driven configuration

secretion of a hormone is stimulated or inhibited by a change in the level of a specific extracellular parameter (does not depend on HPA)

Tropic hormones

stimulate the secretion of peripheral endocrine glands (thyroid hormones, cortisol, estrogen, progesterone, testosterone)

Hypothalamus largely controls

homeostasis & endocrine system

Hypothalamic hormones examples

- thyrotropin-releasing hormone (TRH)

- corticotropin-releasing hormone (CRH)

- gonadotropin-releasing hormone (GnRH)

- growth hormone-releasing hormone (GHRH)

- growth hormone-inhibiting hormone (GHIH)

- prolactin releasing hormone (PRH)

- dopamine

Hypothalamic hormones

either stimulate or inhibit the release of hormones from the anterior pituitary gland

Anterior pituitary hormones

tropic hormones stimulate by hypothalamic hormones that enter the systemic circulation & reach target peripheral endocrine glands

Anterior pituitary hormones examples

- growth hormone (GH)

- thyroid-stimulating hormone (TSH)

- adrenocorticotropic hormone (ACTH)

- prolactin (PRL)

- follicle-stimulating hormone (FSH)

- leutinizing hormone (LH)

What is the correct pair of hypothalamic factor and the anterior pituitary hormone it stimulates?

gonadotropin-releasing hormone (GnRH) → follicle-stimulating hormone (FSH)

Posterior pituitary

does not produce hormones, stores & releases them into the bloodstream as needed

Posterior pituitary hormones

- antidiuretic hormone (ADH)

- oxytocin

Pineal gland is located

at the caudal end of the roof of the third ventricle

Pinealocytes

pineal gland cells that produce melatonin

Melatonin

a monoamine neurotransmitter derived from serotonin

Melatonin is important for

circadian rhythms, reproduction in seasonal breeders (horse, sheep), coordinating endocrine responses

The timing of molt in penguins is influenced by

daylength and seasonal changes in light exposure, which are detected by the pineal gland through melatonin