Animal Communication: Pheromones and Hormones

Signaling by Pheromones

• Pheromones are chemicals released into the environment by some animal species to communicate with each other.
• Pheromones serve various functions, including:
  • Marking trails leading to food
  • Defining territories
  • Warning of predators
  • Attracting potential mates

Hormone Chemical Classes

• Hormones can be classified into three major chemical classes:
  • Polypeptides
  • Steroids
  • Amines
• Polypeptides and most amines are water-soluble, while steroid hormones and other non-polar hormones are lipid-soluble.

Cellular Hormone Response Pathways

• 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.
• They bind to receptors in the cytoplasm or nucleus of the target cells.

Response Pathway for Water-Soluble Hormones

• Binding of a hormone to its receptor initiates a cellular response.
• The response may be:
  • Activation of an enzyme
  • Change in uptake or secretion of certain molecules
  • Rearrangement of the cytoskeleton
  • Initiation of changes in transcription of certain genes
• Signal transduction is the process by which the chemical signal is converted to an intracellular response.

Multiple Responses to a Single Hormone

• The same hormone can have different effects on target cells that have:
  • Different receptors for the hormone
  • Different signal transduction pathways
• For example, the hormone epinephrine has multiple effects that form the basis of the “fight-or-flight” response.

Endocrine Tissues and Organs

• Endocrine cells are often grouped in ductless organs called endocrine glands, such as:
  • Thyroid and parathyroid glands
  • Testes or ovaries
• In contrast, exocrine glands, such as salivary glands, have ducts to carry secreted substances onto body surfaces or into body cavities.

Feedback Regulation and Coordination with the Nervous System

• Hormones are assembled into regulatory pathways that involve feedback regulation and coordination with the nervous system.
• In a simple endocrine pathway, endocrine cells respond directly to a stimulus by secreting a particular hormone.
• The hormone travels in the bloodstream to target cells, where it interacts with its specific receptors, and signal transduction within target cells brings about a physiological response.

Simple Neuroendocrine Pathways

• In a simple neuroendocrine pathway, the stimulus is received by a sensory neuron, which stimulates a neurosecretory cell.
• The neurosecretory cell secretes a neurohormone, which enters the bloodstream and travels to target cells.

Feedback Regulation

• In a negative feedback loop, the response reduces the initial stimulus.
• For example, the increase in pH in the intestine caused by secretin release shuts off further secretin release.
• Positive feedback reinforces a stimulus to produce an even greater response.
• For example, in mammals, oxytocin causes the release of milk, causing greater suckling by offspring, which stimulates the release of more oxytocin.

Coordination of Endocrine and Nervous Systems

• In a wide range of animals, endocrine organs in the brain integrate the function of the endocrine system with that of the nervous system.
• The hypothalamus coordinates endocrine signaling, receives information from nerves throughout the body, and initiates appropriate neuroendocrine signals.
• Signals from the hypothalamus travel to the pituitary gland, composed of the posterior pituitary and anterior pituitary.

Posterior Pituitary Hormones

• 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

Anterior Pituitary Hormones

• The anterior pituitary controls diverse processes, such as metabolism, osmoregulation, and reproduction.
• Hormones secreted by the hypothalamus control the release of all anterior pituitary hormones.
• For example, prolactin-releasing hormone from the hypothalamus stimulates the anterior pituitary to secrete prolactin (PRL), which has a role in milk production.

Hormone Cascades

• 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.
• Thyroid hormone regulates many functions, and low levels of thyroid hormone in the blood stimulate the hypothalamus to secrete thyrotropin-releasing hormone (TRH), causing the anterior pituitary to secrete thyroid-stimulating hormone (TSH).

Disorders of Thyroid Function and Regulation

• 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, causing the thyroid to enlarge, resulting in a goiter, a marked swelling of the neck.

Hormonal Regulation of Growth

• Growth hormone (GH) is secreted by the anterior pituitary gland, and has tropic and nontropic effects.
• The liver, a major target, responds to GH by releasing insulin-like growth factors (IGFs), which stimulate bone and cartilage growth.
• An excess of GH can cause gigantism, while a lack of GH can cause dwarfism.

Parathyroid Hormone and Vitamin D: Control of Blood Calcium

• 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.

Learn about the role of pheromones in animal communication, including marking trails, defining territories, and attracting mates. Also, explore the chemical classes of hormones.