Chapter 46: Chemical Signals in Animals

Chapter 46: Chemical Signals in Animals

Communication and Regulation

• The nervous system and the endocrine system are interconnected systems that facilitate communication and regulation within the body.
• Nervous System:
  • Conveys high-speed electrical and chemical signals.
• Endocrine System:
  • Secretes hormones.
  • Coordinates slower but longer-acting responses.

Hormones & Endocrine System

• Animal hormones are chemical signals:
  • Secreted into the circulatory system.
  • Communicate regulatory messages.
• Target cells:
  • Cells that can respond to a particular hormone.
• Hormone concentration:
  • Animal hormones are present in extremely small concentrations yet have large effects.

Chemical Signals

• Hormones and local regulators increase a cell’s response to a stimulus.
• Types of Chemical Signals in Animals:
  1. Hormones
  2. Local Regulators
  3. Neurotransmitters
  4. (Refer to transcript)
  5. (Refer to transcript)
• A chemical messenger may belong to more than one category.
• Some neurotransmitters are also considered hormones.

Pheromones

• Pheromones are produced by one individual and affect the behavior of another individual of the same species.
• Usually volatiles (evaporate easily).
• Pheromones are not hormones.

Hormone Signaling Pathways

• Endocrine Pathway
  • Stimulus triggers an endocrine cell to release an endocrine signal, which affects an effector cell, leading to a response. Feedback inhibition regulates this pathway.

Hormone Functions

• Single hormone: usually many effects.
• Several hormones may affect the same aspect of physiology.
• Hormones coordinate:
  • Development, reproduction, and growth.
  • Responses to environmental challenges.
  • Homeostasis.

Chemical Characteristics of Hormones

• Hormone categories:
  • Steroids: Similar to/synthesized from cholesterol. Lipid soluble; bind to receptors inside target cell.
  • Peptides and Polypeptides. Not lipid soluble; bind to receptors on surface of target cell.
  • Amino Acid Derivatives: Most not lipid soluble; bind to receptors on surface of target cell.
• Examples:
  • Steroid: Cortisol.
  • Peptide: Secretin.
  • Amino Acid Derivative: Epinephrine (aka adrenaline).

Steroid Hormone Action

• Steroid Hormone binds to a receptor in the cytoplasm.
• Hormone-receptor complex enters the nucleus.
• Complex binds to sites on chromatin, activating mRNA transcription.
• mRNA is produced which result in protein production and a cellular response.

Hormones Regulate Development and Growth

• Amphibian Metamorphosis: Thyroid hormone controls metamorphosis. T3 is a key hormone.
  • Juvenile frog (tadpole) characteristics:
    • Locomotion: Swims via movements of muscular tail.
    • Respiration: Gas exchange occurs in gills, across skin, and in lungs.
    • Nutrition: Most are herbivorous; have a long gut specialized for digesting algae and plants.
    • Excretion: Most nitrogenous wastes excreted as ammonia.
  • Adult frog characteristics:
    • Locomotion: Tailless; walk or hop.
    • Respiration: Gas exchange occurs across skin, and in lungs.
    • Nutrition: Most are carnivorous; have a large mouth or long tongue for catching prey; short gut specialized for digesting proteins.
    • Excretion: Most nitrogenous wastes excreted as urea.

Hormones Regulate Growth and Development in Insects

• Metamorphosis depends on interactions between two hormones:
  • Juvenile hormone (JH)
  • Ecdysone.
  • High JH + Ecdysone

Hormones Regulate Growth in Mammals

• Growth in mammals is stimulated by growth hormone (GH), which regulates the growth factors that control the cell cycle.
• GH declines with age.
• Some evidence suggests GH can be naturally increased in middle-aged adults with intense interval training (e.g., sprints).

Hormones Maintain Homeostasis

• Often controlled by pairs of antagonistic hormones.
  • Antidiuretic hormone and aldosterone control salt balance.
  • Insulin and glucagon control blood sugar.
• Erythropoietin (EPO) stimulates the production of red blood cells when blood oxygen levels fall (e.g., high altitudes).

Endocrine System Components

• Hypothalamus
  • Master gland.
  • Growth-hormone-releasing hormone: stimulates the release of GH from the pituitary gland.
  • Corticotropin-releasing hormone (CRH): stimulates the release of ACTH from the pituitary gland.
  • Thyrotropin-releasing hormone: stimulates the release of TSH from the pituitary gland.
  • Gonadotropin-releasing hormone (GnRH): stimulates the release of FSH and LH from the pituitary gland.
  • Antidiuretic hormone (ADH): promotes the reabsorption of H2O by kidneys.
  • Oxytocin: induces labor and milk release from mammary glands in females.
• Pineal Gland
  • Melatonin: regulates sleep-wake cycles and seasonal reproduction.
• Anterior Pituitary Gland
  • Growth hormone (GH): stimulates growth factors.
  • Adrenocorticotropic hormone (ACTH): stimulates adrenal glands to secrete glucocorticoids such as cortisol.
  • Thyroid-stimulating hormone (TSH): stimulates the thyroid gland to secrete thyroid hormones.
  • Follicle-stimulating hormone (FSH) and luteinizing hormone (LH): stimulate the production of gametes and sex steroid hormones.
  • Prolactin (PRL): stimulates mammary gland growth and milk production in females.
• Parathyroid Glands
  • Parathyroid hormone (PTH): increases blood Ca^{2+}.
• Thyroid Gland
  • Thyroid hormones, thyroxine (T4) and triiodothyronine (T3): increase metabolic rate and heart rate; promote growth.

*Cells in other tissues secrete hormones
*Exocrine glands deliver their secretions through ducts into a space other than the circulatory system (pancreas is both endocrine and exocrine)

Hypothalamus Role

• The hypothalamus receives info from the CNS and initiates a response.
• Key in coordinating nervous and endocrine systems.
• It makes several hormones that are stored and secreted by the posterior pituitary gland.

Anterior Pituitary Gland (Detailed)

• Growth hormone (GH): stimulates growth.
• Adrenocorticotropic hormone (ACTH): stimulates adrenal glands to secrete glucocorticoids such as cortisol.
• Thyroid-stimulating hormone (TSH): stimulates the thyroid gland to secrete thyroxine.
• Follicle-stimulating hormone (FSH) and luteinizing hormone (LH): involved in the production of sex hormones; regulate the menstrual cycle in females.
• Prolactin (PRL): stimulates mammary gland growth and milk production in females.
• The anterior pituitary makes and releases hormones under the control of the hypothalamus.

Hormone Examples

• Parathyroid Hormone (PTH)
  • Increases blood Ca^{2+}.
• Thyroxine
  • Increases metabolic rate and heart rate; promotes growth.
• Epinephrine (=adrenaline): short term stress--released by the “fight or flight” response of sympathetic nerves
• Cortisol: short and long term stress

Short-Term Responses to Stress

• Triggered by the sympathetic nervous system.
• The sympathetic nerves stimulate the adrenal gland to release epinephrine.
• Triggers a state of heightened alertness and increased energy use that prepares the body for rapid, intense action.

Epinephrine Action Example

• (a) Phosphorylase activation
  • Phosphorylase catalyzes the production of glucose from glycogen.
  • Inactive phosphorylase + P -> Active phosphorylase
  • Glycogen -> Glucose
• (b) Phosphorylase is activated in response to epinephrine
  • Experimental (epinephrine added) vs Control (no epinephrine)
  • Active phosphorylase increase with epinephrine stimulation.

Long-Term Responses to Stress

• In humans involves the hormone cortisol (produced in the adrenal gland).
• Ensures the continued availability of glucose for use by the brain.
• But comes at a cost:
  • Loss of muscle mass.
  • Suppression of the immune system and inflammatory response.
  • Insulin resistance.
• Inflammation (which leads to many other health problems) can cause chronic elevated cortisol.
• Chronic stress is bad for the body!

Sex Hormones

• The testes and ovaries produce most of the sex hormones: androgens, estrogens, and progestins.
• All three sex hormones are found in both males and females, but in different amounts.

Endocrine Disruptors

• Environmental pollutants.
• Some chemicals and pollutants are recognized by hormone receptors:
  • Pthalates
  • PCBs
  • Parabens
  • 2,4-D (common herbicide)
  • Other pesticides (some banned, some not)
  • Leachates from plastics
• Most of these are xenoestrogens - recognized by estrogen receptors.

Other Sources of Xenoestrogens

• Phytoestrogens in plants:
  • Unfermented soy
  • Lavender essential oil
  • Many other plant foods and extracts
• Excreted estrogens from birth control pills.

Xenoestrogens and Health

• Known and suspected to cause numerous health issues in humans and wildlife.
• Fish and other animals:
  • Reduced gonad size, disrupted gamete production.
  • Reduced reproductive rates.
  • Intersex individuals.
  • Altered sex-ratios.
• Humans:
  • Increases in estrogen-dependent cancers.
  • Feminizing affects in males.
  • Reduced fertility.
  • Altered sex-ratios.

Declining Sex Ratio Example

• Declining Sex Ratio in a First Nation Community C. A. Mackenzie, A. Lockridge, and M. Keith Environmental Health Perspectives 115, 2005

Learning Objectives

• Know the basic categories of chemical signals in animals and the pathways through which they act (table 46.1, Fig. 46.1).
• Know the major glands of the endocrine system and their functions. You don’t need to know every hormone, but should be familiar with those we discussed in more detail either in this chapter or in other chapters (e.g. T3, TSH, epinephrine, cortisol, insulin, glucagon, GnRH, FSH, estradiol, progesterone).
• Know the chemical classes of hormones.
• Know that hormones mediate major developmental stages in animals that undergo metamorphosis.
• Understand hormonal responses to short and long term stress.
• Be familiar with some examples of hormones that control homeostasis.
• Be familiar with some sources of endocrine disruptors and what their effects are on the health of humans and other animals.