Animal Hormones

1. Animal Hormones

2. Animal Hormones • Introduction • Hormones and Their Actions • Hormonal Control of Molting and Development in Insects • Vertebrate Endocrine Systems • Hormone Actions: The Role of Signal Transduction Pathways

3. Introduction • Chemical messages, or hormones, produce and coordinate anatomical, physiological, and behavioral changes in an animal. • An example are the cichlid fish that live in shallow pools around Lake Tanganyika, Africa. • Dominant male cichlids are brightly colored and aggressive. • Nondominant male cichlids look like females. • Russell Fernald showed that hormones control the production of characteristics of a dominant male.

4. Hormones and Their Actions • Control and regulation require information. • In multicellular animals, nerve impulses provide electric signals; hormones provide chemical signals. • Hormones are secreted by cells, diffuse into the extracellular fluid, and often are distributed by the circulatory system. • Hormones work much more slowly than nerve impulse transmission and are not useful for controlling rapid actions. • Hormones coordinate longer-term developmental processes such as reproductive cycles.

5. Hormones and Their Actions • Hormone-secreting cells are called endocrine cells. • Cells receiving the hormonal message are called target cells and must have appropriate receptors. • The binding of the receptor activates a response. • The distance over which the signal operates distinguishes hormone groups; some act close to the release site, others at distant body locations.

6. Hormones and Their Actions • Hormones can be classified into three main groups: • Peptides or proteins. They are water soluble and transported by vesicles out of the cell that made them. • Steroid hormones are lipid-soluble and can diffuse out of the cell that made them but in the blood they must be bound to carrier proteins. • Amine hormones are derivatives of the amino acid tyrosine. Some are water-soluble and some are lipid-soluble.

7. Hormones and Their Actions • The receptors for lipid-soluble hormones are inside cells, either in the cytoplasm or in the nucleus. • The action of lipid-soluble hormones is mediated by intracellular hormone receptors that usually alter gene expression.

8. Hormones and Their Actions • The receptors for water-soluble proteins are large glycoproteins on the cell surface with three domains: • A binding domain projecting outside the plasma membrane • A transmembrane domain that anchors the receptor in the membrane • A cytoplasmic domain that extends into the cytoplasm of the cell • The cytoplasmic domain initiates the target cell’s response by activating protein kinases or protein phosphatases.

9. Hormones and Their Actions • Some hormones act locally. • Autocrine hormones act on the secreting cell itself. • Paracrine hormones act on cells near the site of release. • Paracrine hormones are released in tiny amounts, or are inactivated rapidly by enzymes, or are taken up efficiently by local cells. They never get into the circulatory system.

10. Figure 42.1 Chemical Signaling Systems

11. Hormones and Their Actions • Growth factors, which stimulate growth and differentiation of cells, are a major class of paracrine hormones. • Growth factors also act as autocrine hormones: Some of the hormone influences the cell that secreted it, preventing the cell from secreting too much hormone. • Neurons may also be considered to be paracrine cells because they use chemicals called neurotransmitters to send messages to another cell.

12. Hormones and Their Actions • Most hormones diffuse into the blood, which distributes them throughout the body. • When the hormone message encounters a cell with the proper receptor, it binds and triggers a response. • The same hormone can cause different responses in different types of cells. • An example is epinephrine. The nervous system reacts to an emergency very quickly and stimulates adrenal cells to secrete epinephrine. The result is the fight-or-flight response.

13. Hormones and Their Actions • The epinephrine acts on different cells in the body: • In the heart, it stimulates faster and stronger heartbeat. • Blood vessels in some areas constrict to send more blood to muscles. • In the liver, glycogen is broken down to glucose to provide quick energy. • In fat tissue, fats are mobilized as another energy source.

14. Hormones and Their Actions • Endocrine refers to cells or glands that do not have ducts leading to the outside of the body; they secrete their products directly into the extracellular fluid. • Some endocrine cells are single cells within a tissue. • Digestive hormones, for example, are secreted by isolated endocrine cells in the wall of the stomach and small intestine. • Some endocrine cells aggregate into secretory organs called endocrine glands. • In vertebrates, nine major endocrine glands make up the endocrine system.

15. Figure 42.2 The Endocrine System of Humans

16. Hormonal Control of Molting and Development in Insects • Because insects have rigid exoskeletons, they have episodic growth patterns and must molt periodically. • The growth stage between each molt is called an instar. • Experiments by British physiologist Wigglesworth showed how molting is triggered by a hormone from the brain.

17. Figure 42.3 A Diffusible Substance Triggers Molting (Part 1)

18. Figure 42.3 A Diffusible Substance Triggers Molting (Part 2)

19. Hormonal Control of Molting and Development in Insects • Two hormones work together in insects to regulate molting: brain hormone and ecdysone. • Cells in the brain produce brain hormone which is stored in a pair of structures attached to the brain called the corpora cardiaca. • After appropriate stimulation (e.g., a blood meal for Rhodnius), the corpora release brain hormone, which diffuses to an endocrine gland called the prothoracic gland. • The prothoracic gland releases ecdysone which stimulates molting.

20. Hormonal Control of Molting and Development in Insects • Wigglesworth also demonstrated that another hormone is responsible for determining when an insect molts into an adult. • By removing only the front part of the head, it was shown that the rear part, containing the corpora allata, produces a substance preventing a molt to the adult stage. • The substance is called juvenile hormone. If it is present, Rhodnius molts to another juvenile instar. • Normally, during the fifth instar, the corpora allata stop making this hormone and the insect molts to the adult stage.

21. Hormonal Control of Molting and Development in Insects • Hormonal control is more complex in insects having complete metamorphosis. • An example is the silkworm. The egg hatches into a larva that has a high amount of juvenile hormone in its body. • As long as the level of juvenile hormone stays high, larvae molt into larvae; when the juvenile hormone level wanes, pupae are formed. • No juvenile hormone is found in the pupae, so they molt into adults.

22. Figure 42.4 Complete Metamorphosis

23. Vertebrate Endocrine Systems • The pituitary gland of mammals is a link between the nervous system and many endocrine glands and plays a crucial role in the endocrine system. • The pituitary gland sits in a depression at the bottom of the skull and is attached to the hypothalamus. • The pituitary is made of two parts: anterior and posterior.

24. Vertebrate Endocrine Systems • The posterior pituitary releases two hormones: antidiuretic hormone and oxytocin. • They are made by neurons in the hypothalamus, are called neurohormones, and are packaged in vesicles. • The vesicles are transported down the axons of the neurons that made them and are stored in the posterior pituitary. • This movement of the vesicles is achieved by kinesin proteins, powered by ATP, that “walk” down the microtubules of the axon.

25. Figure 42.5 The Posterior Pituitary Releases Neurohormones

26. Vertebrate Endocrine Systems • The function of antidiuretic hormone (ADH) is to increase water conservation by the kidney. • If there is a high level of ADH secretion, the kidneys resorb water. • If there is a low level of ADH secretion, the kidneys release water in dilute urine. • ADH release by the posterior pituitary increases if blood pressure falls or blood becomes too salty. • ADH causes peripheral blood vessel constriction to help elevate blood pressure and is also called vasopressin.

27. Vertebrate Endocrine Systems • The function of oxytocin is to stimulate uterine muscle contraction for the birth process. • It also stimulates milk flow in the mother’s breasts. Suckling by the baby, or even the sight or sound of the baby, can cause the mother to secrete oxytocin and release milk.

28. Vertebrate Endocrine Systems • The anterior pituitary releases four tropic hormones, which control activities of other endocrine glands. • They are peptide and protein hormones; each is produced by a different type of pituitary cell. • The four tropic hormones are: thyrotropin, adrenocorticotropin, luteinizing hormone, and follicle-stimulating hormone.

29. Figure 42.7 Hormones from the Hypothalamus Control the Anterior Pituitary

30. Vertebrate Endocrine Systems • Other peptide and protein anterior pituitary hormones influence tissues that are not endocrine glands. • These include: growth hormone, prolactin, melanocyte-stimulating hormone, endorphins, and enkephalins.

31. Vertebrate Endocrine Systems • Growth hormone (GH) acts on many tissues to promote growth. • GH stimulates cells to take up amino acids. • GH also stimulates the liver to produce chemical messages (insulin-like growth factors) that stimulate bone and cartilage growth. • Gigantism is the result of overproduction of GH in children. • Underproduction of GH causes pituitary dwarfism. GH is now produced by genetically engineered bacteria.

32. Figure 42.6 Effects of Excess Growth Hormone

33. Vertebrate Endocrine Systems • Prolactin stimulates the production and secretion of milk in female mammals. • It is also important in pregnancy and, in males, has a role in controlling the endocrine functions of the testes.

34. Vertebrate Endocrine Systems • Endorphins and enkephalins are the body’s natural opiates. In the brain, these molecules act as neurotransmitters in pain pathways. • The production of these hormones, as well as some other anterior pituitary hormones, is governed by one gene. • The gene encodes for a protein called pro-opiomelanocortin. • This large molecule is later cleaved into several peptides including adrenocorticotropin, melanocyte-stimulating hormone, endorphins, and enkephalins.

35. Vertebrate Endocrine Systems • The anterior pituitary is controlled by neurohormones from the hypothalamus. • The hypothalamus obtains data about body conditions and the external environment through both neuronal and hormonal signals. • The hypothalamus and the anterior pituitary are connected by portal blood vessels. • Secretions from hypothalamic nerves are transported by these blood vessels to the anterior pituitary.

36. Figure 42.7 Hormones from the Hypothalamus Control the Anterior Pituitary

37. Vertebrate Endocrine Systems • Thyrotropin-releasing hormone (TRH) was the first releasing hormone extracted from the hypothalamus. • It causes anterior pituitary cells to release thyrotropin, which in turn stimulates the thyroid gland. • Gonadotropin-releasing hormone (GnRH) causes the anterior pituitary to release tropic hormones that control gonad activity. • Now many more hypothalamic neurohormones are known.

38. Table 42.2 Releasing and Release-Inhibiting Neurohormones of the Hypothalamus

39. Vertebrate Endocrine Systems • The anterior pituitary cells are also under negative feedback control by the hormones of the glands that they stimulate. • For example, cortisol is produced by the adrenal gland in response to adrenocorticotropin. It returns to the pituitary in the blood, and inhibits further release of adrenocorticotropin. • Cortisol also exerts negative feedback control on the hypothalamus, inhibiting release of adrenocorticotropin-releasing hormone.

40. Figure 42.8 Multiple Feedback Loops Control Hormone Secretion

41. Vertebrate Endocrine Systems • The thyroid gland, located near the trachea, is an example of an endocrine gland that is controlled by negative feedback. • The thyroid gland produces the hormone thyroxine in specialized structures called follicles. • Two forms of thyroxine, T3 and T4, are made from tyrosine. T3 (triiodothyronine) has three iodine atoms. T4 has four iodine atoms. • More T4 is produced, but it can be converted to T3 by an enzyme in the blood. T3 is the more active form of the hormone.

42. In-Text Art p. 809(1)

43. In-Text Art p. 809(2)

44. Vertebrate Endocrine Systems • Thyroxine has many roles in regulating metabolism. • It stimulates the transcription of many genes in nearly all cells in the body. These include genes for enzymes of energy pathways, transport proteins, and structural proteins. • It elevates metabolic rates in most cells and tissues. • It promotes the use of carbohydrates over fats for fuel. • It promotes amino acid uptake and protein synthesis and so is critical for growth and development. Insufficient thyroxine may result in cretinism.

45. Vertebrate Endocrine Systems • Thyrotropin (or thyroid-stimulating hormone, TSH) from the anterior pituitary activates thyroid gland cells to produce thyroxine. • Thyrotropin-releasing hormone (TRH) from the hypothalamus activates TSH-producing cells in the anterior pituitary. • In a negative feedback loop, thyroxine inhibits the response of pituitary cells to TRH. • Therefore, less TSH is released when thyroxine levels are high, and more is released when levels are low.

46. Vertebrate Endocrine Systems • A goiter is an enlarged thyroid gland associated with either very low (hypothyroidism) or very high (hyperthyroidism) levels of thyroxine. • A thyroid follicle is a layer of epithelial cells surrounding a mass of glycoprotein called thyroglobulin. • Thyroglobulin consists of iodinated tyrosines and is digested by the epithelial cells to make thyroxine. • If there is no iodine present when thyroglobulin is made, the released molecules will be neither T3 nor T4 and will not bind to appropriate receptors.

47. Vertebrate Endocrine Systems • Goiter occurs when thyroglobulin production is above normal and the follicles are enlarged. • Hyperthyroid goiter results when the negative feedback mechanism fails even though blood levels of thyroxine are high. • A common cause is an autoimmune disease in which an antibody to the TSH receptor is produced. This antibody binds the TSH receptor, causing the thyroid cells to release excess thyroxine. • The thyroid remains maximally active and grows larger, causing symptoms associated with high metabolic rates.

48. Vertebrate Endocrine Systems • Hypothyroid goiter results when there is insufficient thyroxine to turn off TSH production. • The most common cause is a deficiency of dietary iodine. • With high TSH levels, the thyroid gland continues to produce nonfunctional thyroxine and becomes very large. • The body symptoms of this condition are low metabolism, cold intolerance, and physical and mental sluggishness.

49. Vertebrate Endocrine Systems • Calcium levels in the blood must be regulated within a narrow range. Small changes in blood calcium levels have serious effects. • Most calcium in the body is in the bones (99%). About 1% is in the cells, and only 0.1% is in the extracellular fluids. • Blood calcium levels are regulated by: • Deposition and absorption of bone • Excretion of calcium by the kidneys • Absorption of calcium from the digestive tract

50. Vertebrate Endocrine Systems • Calcitonin, released by the thyroid gland, acts to lower calcium levels in the blood. • Bone is constantly remodeled by absorption of old bone and production of new bone. • Osteoclasts break down bone and release calcium. • Osteoblasts use circulating calcium to build new bone. • Calcitonin decreases osteoclast activity and stimulates the osteoblasts to take up calcium for bone growth.