Chapter 46: Animal Hormones

What is an autocrine signal?

• Act on the same cell that secretes them, often amplifying the cell’s response to a stimulus

• Ex: cytokines released by immune cells can stimulate their own activity

What is a paracrine signal?

• Diffuse locally and act on nearby cells, allowing coordinated responses in small groups of cells

• Typically short-lived and rapidly degraded.

What is an endocrine signal?

• Hormones released into the bloodstream and transported to distant target cells

• Produce slower but longer-lasting effects compared to local signaling.

What is a neural signal?

• Involve neurotransmitters released across synapses between neurons

• Enables rapid and highly specific communication.

• Act over very short distances.

What is a neuroendocrine signal?

• Hormones released by neurons into the bloodstream, combining features of neural and endocrine signaling

• Can affect distant target tissues

What is the endocrine system?

A collection of glands and tissues that produce hormones and release them into the bloodstream to regulate physiological processes. It works closely with the nervous system to maintain homeostasis. Organs include:

• Pineal gland

• Hypothalamus

• Pituitary gland

• Thyroid

• Parathyroid

• Adrenal glands

• Pancreas

• Ovaries

• Testes

Why are hormones effective at low concentrations?

• Signal transduction pathways amplify their effects within target cells.

  • One hormone-receptor interaction activates many downstream molecules in a cascade increasing magnitude of response

• A single hormone molecule can trigger large intracellular responses

How do target cells respond specifically to hormones?

• Only cells with the appropriate receptors for a hormone can respond to it, ensuring specificity of signaling

• Different cell types can produce different responses to the same hormone based on receptor type and signaling pathways

Overview of peptide hormones

• Structure: chains of amino acids (anywhere from 3 to 200 AA’s linked together)

• Solubility: water soluble

• Synthesis and storage: made in rough ER, stored in vesicles

• Secretion: exocytosis

• Transport: dissolved in plasma

• Half-life: decay in minutes

• Receptors: on the cell-surface

  • Water-soluble so they can’t cross the plasma membrane

• Actions: stimulate/activate an internal secondary messenger

• Result: modifies enzymatic activity

Step-by-step: how do peptide hormones affect target cells?

1. A peptide hormone binds to a membrane receptor

2. That activates a G protein that stimulates enzymes such as adenylyl cyclase to produce second messengers like cAMP

3. Uses ATP to activate cAMP-dependent protein kinase A, then more ATP to active phosphorylase kinases. These phosphorylation cascades amplify the signal

4. Enzymes are rapidly modified to then go on to cause changes in cellular activity

How are steroid hormones made?

Steroid hormones are all derived from cholesterol. To make a specific steroid hormone, cholesterol is modified in a variety of ways via enzymes. The main enzymes that cause big changes between the steroid hormone classes are:

• Cholesterol side-chain cleavage enzyme

• 2,1-hydroxylase

• Aromatase

Overview of steroid hormones

• Structure: derived from cholesterol

• Solubility: lipid-soluble

• Synthesis and storage: made on-demand, are not stored

• Secretion: diffusion

• Transport: carrier protein

• Half-life: lasts for hours or more

• Receptors: located intracellularly (can diffuse through plasma membrane)

• Actions: transcription modification by acting on DNA

• Result: change type of enzymes

Step-by-step: how do steroid hormones affect target cells?

1. Steroid hormones diffuse through the membrane and bind intracellular receptors

2. Binding forms a hormone-receptor complex that can enter the nucleus

3. This complex binds DNA at hormone-response elements, altering gene transcription and creating mRNA which amplifies the signal

4. Each transcript is translated multiple times, further amplifying the signal and producing long-lasting changes in protein expression

5. These proteins (like enzymes, structural proteins, receptors, etc.) go on to alter cellular function, growth, and metabolism

What are biogenic amines?

• Derivatives of individual amino acids that include tyrosine and tryptophan

• Tyrosine is a precursor for two different chains.

  • (1) L-DOPA, dopamine, norepinephrine, and epinephrine.

  • (2) Thyroid hormones: T3 and T4

• Tryptophan is a precursor for serotonin and melatonin

• Only certain endocrine cells can produce these hormones due to enzymatic differences

Overview of catecholamines

• Structure: derived from tyrosine

• Solubility: water soluble

• Synthesis and storage: made in cytoplasm, stored in vesicles

• Secretion: exocytosis

• Transport: dissolved in plasma

• Half-life: decay seconds to minutes

• Receptors: on the cell-surface (plasma membrane)

• Actions: stimulate/activate an internal secondary messenger

• Result: modifies enzymatic activity

Overview of thyroid hormones

• Structure: derived from tyrosine and iodine

• Solubility: lipid-soluble

• Synthesis and storage: stored in the thyroid

• Secretion: diffusion

• Transport: carrier protein

• Half-life: lasts for days

• Receptors: located intracellularly (can diffuse through plasma membrane)

• Actions: transcription modification by acting on DNA

• Result: change type of enzymes

What is the hypothalamus and its role in the endocrine system?

• Region of the brain that links the nervous and endocrine systems by producing hormones that regulate the pituitary gland

• It controls many physiological processes through this connection (“master regulator”)

How is the hypothalamus connected to the posterior pituitary?

• Posterior pituitary is an extension of the hypothalamus (made of nervous tissue)

• Neurosecretory cells release hypothalamic hormones such as ADH and oxytocin directly into the posterior pituitary where they are stored and can be released

• Release is controlled by an actional potential from the hypothalamus

How is the hypothalamus connected to the anterior pituitary?

• Hypothalamus communicates with the anterior pituitary (make of endocrine tissue) through a portal blood system

• Releasing hormones travel through blood vessels to regulate pituitary hormone secretion

• Allows for extremely localized delivery

Step-by-step: what is the hypothalamic-pituitary cascade?

Process is regulated by negative feedback

1. The hypothalamus increases the release of a tropic hormone (the releasing hormone)

2. Increases the concentration of that tropic hormone in portal circulation

3. The anterior pituitary increases secretion of the pituitary hormone (this is the target cells for hypothalamic hormone)

4. Increases concentration of the pituitary hormone in general circulation

5. Endocrine gland increases secretion of the 3rd hormone (this is the target cells for the pituitary hormone)

6. Increases plasma levels of the 3rd hormone

7. Causes/increases hormone binding at target cells

Step-by-step: how does the HPA axis regulate stress?

1. Stress stimulates the hypothalamus to release corticotropin-releasing hormone (CRH)

2. This triggers the anterior pituitary to release adrenocorticotropic hormone (ACTH)

3. This stimulates the adrenal cortex to release cortisol

4. Cortisol then inhibits CRH and ACTH release through negative feedback, regulating the stress response.

What hormones are produced by the hypothalamus?

• Dopamine

• Neurohormones

  • ADH

  • Oxytocin

• Releasing hormones (regulate anterior pituitary)

  • Corticotropin-releasing hormone (CRH)

  • Gonadotropin-releasing hormone (GnRH)

  • Growth hormone releasing hormone (GHRH)

  • Thyroid releasing hormone (TSH)

What hormones are produced by the anterior pituitary?

Hormones that regulate growth, metabolism, stress, and reproduction and act on other adrenal glands

• Growth hormone (GH)

• Adrenocorticotropic hormone (ACTH)

• Thyroid stimulating hormone (TSH)

• Follicle-stimulating hormone (FSH)

• Luteinizing hormone (LH)

• Prolactin (PRL),

What hormones are produced by the thyroid gland?

• Thyroid produces T3 and T4

• Increase metabolic rate and energy use in cells/regulate overall metabolism.

What hormones are produced by the adrenal glands?

• Produce epinephrine for rapid stress responses

• Steroid hormones such as cortisol and aldosterone

• Regulate stress, metabolism, and ion balance

What hormones are produced by the pancreas?

• Produces insulin and glucagon

• Regulate blood glucose levels

  • Insulin lowers glucose

  • Glucagon raises it

What is endocrine disruption?

Occurs when external chemicals interfere with hormone signaling by mimicking or blocking hormones or altering their levels, potentially leading to developmental and physiological abnormalities.

What are examples of endocrine disruptors?

BPA and atrazine, which can mimic or interfere with hormone signaling pathways, ultimately affecting reproduction, development, and behavior.

How was the concept of hormones first demonstrated experimentally?

• Berthold’s experiment showed that transplanted testes restored male characteristics in castrated chickens

• Demonstrated that a chemical signal in the blood controlled development, providing early evidence for hormones.

What are the four types of endocrine disorders?

• Hyposecretion: not enough hormone is produced

  • Damage/disease to cells that make the hormone or a genetic mutation

  • Ex: Type I diabetes

• Hypersecretion: too much hormone produced

  • Hormone producing tumor

• Hyporesponsiveness: cells do not respond as much as they should

  • Downregulation of receptors

  • Ex: Type II diabetes

• Hyperresponsiveness: cells overreact to the hormone

  • Upregulation of receptors