Chemical Messengers and the Endocrine System

4 Amino Acid Messengers

Glutamine, aspartate, glycine, and gamma-aminobutyric acid (GABA).

Amino acid messenger solubility

Hydrophilic.

Where amino acid messengers are synthesized

In the cytoplasm of the secreting neuron.

Why amino acid messengers are called neurotransmitters

Because they are synthesized within neurons.

Amine messengers chemical group

Amino group (NH₂).

Catecholamines origin

Derived from the amino acid tyrosine.

3 Catecholamine examples

Dopamine, norepinephrine, and epinephrine.

Serotonin functions

Neurotransmission and blood clotting.

Histamine functions

Neurotransmission and paracrine signaling.

Thyroid hormones T3/T4 solubility

Hydrophobic (unlike other amine messengers which are hydrophilic).

T3/T4 synthesis and release

Synthesized upon demand and exit via simple diffusion.

Peptide vs. Protein size

Peptides have ≤50 amino acids; proteins have ≥51 amino acids.

Peptide/Protein messenger solubility

Large and hydrophilic (e.g., insulin).

Steroid messenger solubility

Hydrophobic.

Steroid chemical structure

Three 6-carbon rings and one 5-carbon ring.

Steroid precursor and synthesis site

Derived from cholesterol; synthesized in the smooth ER.

Why steroid hormones cannot be stored

They are hydrophobic and diffuse straight through cell membranes.

5 Steroid examples

Testosterone, estrogen, cortisol, aldosterone, and Vitamin D.

Eicosanoid messenger solubility and function

Hydrophobic lipids; mostly act as paracrine signals.

Eicosanoid precursor

Arachidonic acid (a 20-carbon fatty acid from phospholipids/DAG).

Two pathways of eicosanoid synthesis

Cyclooxygenase (COX) pathway and Lipoxygenase pathway.

4 Eicosanoid examples

Prostacyclins, thromboxanes, prostaglandins, and leukotrienes.

Aspirin mechanism of action

Inhibits cyclooxygenase (COX) to reduce prostaglandin synthesis.

Hormone definition

A chemical messenger traveling from source to target through blood.

Hydrophilic hormone transport & half-life

Dissolve in blood; half-life of only minutes.

Hydrophobic hormone transport & half-life

Bind to carrier proteins; half-life of hours or more.

Nervous vs. Endocrine communication speed

Nervous is fast and direct; Endocrine is slower and diffuse.

Command center of the endocrine system

Hypothalamus (made of neural tissue).

Neurohormone definition

Produced by neural tissue but travels through blood.

2 Hormones secreted by hypothalamus

Oxytocin and antidiuretic hormone (ADH / vasopressin).

Oxytocin & ADH transport pathway

Somas in hypothalamus down axons in infundibulum to posterior pituitary.

Anterior Pituitary tissue type

Epithelial tissue (making it a true gland).

Posterior Pituitary tissue type

Neural tissue (axon terminals of hypothalamic neurons).

Hypophyseal portal system

Specialized blood vessels connecting hypothalamus to anterior pituitary.

3 Hypothalamic releasing hormones

TRH, CRH, and GnRH.

6 Anterior Pituitary hormones

TSH, LH, FSH, GH, ACTH, and prolactin.

Hypothalamus-Pituitary-Thyroid pathway

TRH stimulates TSH release; TSH stimulates T3/T4 release.

Anterior Pituitary nickname

The "master gland".

Negative feedback in endocrine system

A hormone feeds back to suppress its own releasing organ.

Positive feedback definition

The end product further stimulates the releasing organ.

Main functions of thyroid hormones (T3 and T4)

To increase basal metabolic rate (BMR) and increase heat production.

Effect of large doses of cortisol on immune organs

Causes them to shrink due to its immunosuppressive properties.

Target cells of LH in males

Leydig cells of the testes, which then release testosterone.

Hypertrophy vs. Atrophy

Hypertrophy is organ/gland enlargement from activation; atrophy is shrinkage from inhibition.

Graves' disease

The most common cause of hyperthyroidism, characterized by an enlarged thyroid (goiter).

Symptoms of hypothyroidism

Low BMR, feeling cold, tiredness, and weight gain.

Cushing's syndrome

Hypercortisolism causing hypertension, osteoporosis, and muscle wasting.

Addison's disease

Hyposecretion of cortisol characterized by defective metabolism and decreased stress adaptation.

Which type of feedback inhibits further hormone secretion?

negative feedback

Name of hormone that increases BMR.

thyroid

what hormone can act as immunosuppressive and anti-inflammatory agent.

cortisol

What is atrophy?

decrease in size

The excessive production of thyroid hormones is called

Hyperthyroidism