Animal Hormones and Chemical Signals

Chemical Signals in Animals

• Animal hormones are chemical signals secreted into the circulatory system.
• Hormones communicate regulatory messages within the body.
• Hormones reach all parts of the body, but only target cells have receptors for a specific hormone.
• Chemical signaling by hormones is the function of the endocrine system.
• The nervous system transmits signals along dedicated pathways via neurons.
• The nervous and endocrine systems often overlap in function.
• Animals use chemical signals to communicate in diverse ways.
• Intercellular communication via secreted signals is classified by the type of secreting cell and the signal's route to its target.
• In endocrine signaling, hormones secreted into extracellular fluids reach targets via the bloodstream.
• Endocrine signaling maintains homeostasis, mediates responses to stimuli, and regulates growth and development.
• Local regulators act over short distances, reaching target cells solely by diffusion.
• Local regulators act on their targets very fast, within seconds or milliseconds.
• In paracrine signaling, the target cells lie near the secreting cells.
• In autocrine signaling, the target cell is also the secreting cell.
• Paracrine and autocrine signaling play roles in blood pressure regulation, nervous system function, and reproduction.
• Prostaglandins are local regulators mediating such signaling that affect the immune system, inflammation, pain sensation, and blood clotting.
• In synaptic signaling, neurons form specialized junctions (synapses) with target cells.
• At synapses, neurotransmitters diffuse short distances and bind to receptors on target cells.
• In neuroendocrine signaling, neurosecretory cells secrete neurohormones.
• Neurohormones diffuse from nerve endings into the bloodstream.
• Examples of this are adrenal medulla (epinephrine) and posterior pituitary (ADH).
• Pheromones are chemicals released into the environment for communication among members of a species.
• Pheromones can mark trails to food, define territories, warn of predators, and attract potential mates.

Chemical Classes and Hormone Pathways

• Local regulators include modified fatty acids like prostaglandins, polypeptides, and gases.
• Nitric oxide (NO) is a gas that functions as a local regulator and neurotransmitter, and activates enzymes to increase blood flow to tissues when oxygen levels fall.
• Hormones are classified into three major classes: polypeptides, steroids, and amines.
• Polypeptides and most amines are water-soluble.
• Steroid hormones and other largely nonpolar hormones are lipid-soluble.
• Water-soluble hormones are secreted by exocytosis, travel freely in the bloodstream, and bind to cell-surface receptors.
• Lipid-soluble hormones diffuse across cell membranes, travel in the bloodstream bound to transport proteins, and diffuse through the membrane of target cells.
• Lipid-soluble hormones bind to receptors in the cytoplasm or nucleus of the target cells.
• Binding of a water-soluble hormone to its receptor initiates a signal transduction pathway.
• This leads to responses in the cytoskeleton, enzyme activation, or a change in gene expression.
• The hormone epinephrine (or adrenaline) regulates many organs in response to stressful situations.
• Epinephrine binds to receptors on the plasma membrane of liver cells.
• This binding triggers the release of messenger molecules that activate enzymes and result in the release of glucose into the bloodstream.
• The response to a lipid-soluble hormone is usually a change in gene expression.
• When a steroid hormone binds to its cytosolic receptor, a hormone-receptor complex forms that moves into the nucleus, acting as a transcriptional regulator.
• Steroid hormone receptors, such as those that bind to estrogens, are well-characterized.
• In female birds and frogs, estradiol binds to a cytoplasmic receptor in liver cells.
• The estradiol-bound receptor activates transcription of genes needed to produce egg yolk.
• Endocrine cells are grouped in ductless endocrine glands like the thyroid, parathyroid glands, testes, or ovaries.
• Exocrine glands, like salivary glands, have ducts to carry secreted substances onto body surfaces or into body cavities.

Hormone Regulation and Systems

• Hormones are assembled into regulatory pathways.
• In simple endocrine pathways, hormones are released from an endocrine cell, travel through the bloodstream, and interact with specific receptors within a target cell.
• This interaction then causes a physiological response.
• Acidic contents released from the stomach into the duodenum stimulate endocrine cells to secrete secretin, which causes target cells in the pancreas to raise the pH in the duodenum.
• In a simple neuroendocrine pathway, a sensory neuron receives a stimulus, which stimulates a neurosecretory cell to secrete a neurohormone that enters the bloodstream to travel to target cells.
• Suckling of an infant stimulates signals in the nervous system reach the hypothalamus.
• Nerve impulses from the hypothalamus trigger the release of oxytocin from the posterior pituitary which causes the mammary glands to secrete milk.
• A negative feedback loop inhibits a response by reducing the initial stimulus.
• This prevents excessive pathway activity.
• Positive feedback reinforces a stimulus to produce an even greater response.
• In mammals oxytocin causes the release of milk, causing greater suckling by offspring, which then stimulates the release of more oxytocin.
• The hypothalamus coordinates endocrine signaling and receives information from nerves throughout the body.
• It initiates appropriate neuroendocrine signals.
• The pituitary gland, composed of the posterior and anterior pituitary, is at the base of the hypothalamus.

Pituitary Hormones and their Effects

• Neurosecretory cells of the hypothalamus synthesize the two posterior pituitary hormones.
• Antidiuretic hormone (ADH) regulates physiology and behavior.
• Oxytocin regulates milk secretion by the mammary glands.
• The anterior pituitary controls diverse processes like metabolism, osmoregulation, and reproduction. Hormones secreted by the hypothalamus control the release of all anterior pituitary hormones. Releasing or inhibiting hormones are released for this process. Prolactin-releasing hormone from the hypothalamus stimulates the anterior pituitary.
• This causes it to secrete prolactin (PRL), which has a role in milk production.
• Sets of hormones from the hypothalamus, anterior pituitary, and a target endocrine gland are often organized into a hormone cascade.
• The anterior pituitary hormones in these pathways are called tropic hormones.
• Tropic hormones are hormones that have other endocrine glands as their target, and they are mostly produced and secreted by the anterior pituitary.
• In humans and other mammals thyroid hormone regulates many functions; helps maintain proper blood pressure, heart rate, muscle tone and regulate digestive and reproductive functions.
• If thyroid hormone level drops in the blood, the hypothalamus secretes thyrotropin-releasing hormone (TRH), which causes the anterior pituitary to secrete thyroid-stimulating hormone (TSH).
• TSH stimulates release of thyroid hormone by the thyroid gland.
• Disruption of thyroid hormone production and regulation can result in serious disorders.
• Thyroid hormone is the only iodine-containing molecule synthesized in the body.
• With low levels of thyroid hormone, due to insufficient iodine, the pituitary continues to secrete TSH that causes the thyroid to enlarge, resulting in a goiter.
• Growth hormone (GH) (somatotropin) is secreted by the anterior pituitary gland, and has tropic and nontropic effects.
• The liver is a major target and responds to GH by releasing insulin-like growth factors (IGFs).
• These stimulate bone and cartilage growth.
• An excess of GH can cause gigantism, while a lack of GH can cause dwarfism.

Calcium Regulation, Adrenal Hormones and Sex Hormones

• Homeostatic regulation of calcium (Ca2+) in the blood is vital.
• In mammals, parathyroid hormone (PTH) is released by the parathyroid glands when Ca2+ levels fall below a set point.
• PTH increases the level of blood Ca2+ and stimulates reabsorption of Ca2+ in the kidneys.
• It also indirectly affects Ca2+ by promoting production of vitamin D.
• Calcitonin decreases the level of blood Ca2+ and stimulates Ca2+ deposition in bones and secretion by kidneys.
• The adrenal glands are located atop the kidneys.
• Each adrenal gland consists of two glands: the adrenal medulla (inner portion) and adrenal cortex (outer portion).
• The adrenal cortex consists of true endocrine cells whereas adrenal medulla’s secretory cells develop from neural tissue.
• The adrenal medulla secretes epinephrine (adrenaline) and norepinephrine (noradrenaline).
• These hormones are members of a class of compounds called catecholamines.
• They coordinate a set of physiological responses that comprise the “fight-or-flight” response.
• Heart beats faster, breathing quickens, muscle tense and thoughts speed up
• Epinephrine and norepinephrine trigger the release of glucose and fatty acids into the blood.
• They increase oxygen delivery to body cells and direct blood towards the heart, brain, and skeletal muscles and away from skin, digestive system, and kidneys.
• The adrenal cortex becomes active under stressful conditions including low blood sugar, decreased blood volume and pressure, and shock.
• The hypothalamus secretes a releasing hormone that stimulates the anterior pituitary to release adrenocorticotropic hormone (ACTH).
• A series of hormonal signals leads to production and secretion of a family of steroids called corticosteroids.
• Humans produce two types of corticosteroids: glucocorticoids and mineralocorticoids.
• Glucocorticoids, such as cortisol, influence glucose metabolism and the immune system.
• They promote glucose synthesis and at high doses, act as immunosuppressant.
• Mineralocorticoids, such as aldosterone, affect salt and water balance.
• The gonads, testes and ovaries, produce most of the sex hormones: androgens, estrogens, and progesterone.
• All three sex hormones are found in both males and females, but in different proportions.
• The testes primarily synthesize androgens, mainly testosterone, which promote development of male reproductive structures.
• Testosterone is responsible for male secondary sex characteristics.
• Estrogens, most importantly estradiol, are responsible for maintenance of the female reproductive system.
• They are also responsible for development of female secondary sex characteristics.
• In mammals, progesterone is primarily involved in preparing and maintaining the uterus.
• Synthesis of the sex hormones is controlled by the gonadotropins.
• These are follicle-stimulating hormone and luteinizing hormone from the anterior pituitary.
• Gonadotropin secretion is controlled by gonadotropin-releasing (GnRH) hormone from the hypothalamus.

Biological Rhythms and Hormones

• The pineal gland, located in the brain, secretes melatonin, affecting skin pigmentation.
• Primary functions of melatonin relate to biological rhythms with reproduction and daily activity levels.
• Melatonin is secreted at night and the amount released depends on the length of the night.
• The release of melatonin by the pineal gland is controlled by the suprachiasmatic nucleus (SCN) in the hypothalamus.
• Melanocyte-stimulating hormone (MSH) regulates skin color in amphibians, fish, and reptiles by controlling pigment distribution in melanocytes.
• In mammals, MSH plays roles in hunger suppression, metabolism and coloration.