Endocrine System Study Guide: FISH 4500

Nervous vs. Endocrine Communication

Nervous: electrical impulses, rapid, short-lived. Endocrine: chemical signals via blood, slower, longer-lasting.

Hormone

An internal chemical secretion from ductless glands, transported by blood to regulate target organs and tissues.

Endocrine Signaling

A hormone enters the bloodstream and acts on distant target organs.

Paracrine Signaling

A chemical secreted by a cell produces local effects on adjacent cells.

Pheromones

Chemical signals released into the external environment that affect other individuals.

Neurotransmitters vs. Neuroendocrine Compounds

Neurotransmitters cross synaptic clefts; neuroendocrine compounds are released by neurons into the bloodstream.

Hormone Receptors

Target organs recognize hormones through specific complementary receptors using a lock-and-key system.

Peptide Hormones

Short polypeptides that bind membrane receptors and activate intracellular second messengers such as cAMP.

Cyclic AMP (cAMP)

Cyclic adenosine monophosphate; a common intracellular second messenger activated by membrane receptors.

Steroid Hormones

Cholesterol-derived, lipid-soluble hormones that enter cells, bind nuclear receptors, and alter gene transcription.

Amino Acid-Derived Hormones

Hormones made from single amino acids, such as epinephrine and melatonin; they do not enter cells.

Organizational Effects

Permanent structural or behavioral changes during critical developmental periods.

Activational Effects

Reversible physiological or behavioral responses that occur without critical developmental periods.

Hypothalamus

Brain region that initiates release of many hormones by producing releasing or inhibiting paracrines.

Pituitary Gland

Located below the hypothalamus; releases tropic hormones into blood to control other endocrine glands.

Major Pituitary Hormones & Targets

FSH/LH → gonads; TSH → thyroid; ACTH → interrenal glands; prolactin → gills/kidneys; MSH → melanocytes; GH → muscle/bone/tissues.

Pineal Gland

Secretes melatonin, which regulates circadian rhythms and photoperiod responses.

Thyroid Gland

Produces thyroxin, which regulates growth, development, and metabolism.

Interrenal Gland

Fish equivalent of the mammalian adrenal cortex; located in the head kidney and secretes corticosteroids such as cortisol.

Chromaffin Tissue

Located in the head kidney; secretes catecholamines such as epinephrine and norepinephrine for stress responses.

Corpuscles of Stannius

Secrete stanniocalcin, which regulates calcium homeostasis.

Heart & Urophysis Hormones

Heart produces atrial natriuretic peptide; urophysis produces urotensin.

Gonadal Hormones

Gonads produce androgens such as testosterone/11-ketotestosterone and estrogens such as estradiol.

Endocrine Cascade

A sequence of hormonal activation reactions that amplifies an initial biological stimulus.

Negative Feedback System

Target hormone output signals control centers to inhibit further hormone production.

HPI Axis (Stress Axis)

Hypothalamus releases CRH → anterior pituitary releases ACTH → interrenal gland releases cortisol.

HPG Axis (Reproductive Axis)

Hypothalamus releases GRF/GnRH → anterior pituitary releases gonadotropins → gonads release estrogens/androgens → sexual maturation.

Practical Applications of Hormones

Hormones indicate maturity, reproduction, and health and can induce spawning or enhance growth.

Endocrine Disruptors

Environmental contaminants that mimic estrogens or androgens and disrupt reproduction, gonadal development, and sex determination.