Overview
This lecture covers hormone signaling in the body, focusing on how hormones are classified, transported, and interact with target cells, including their dual communication with the brain.
Cell Communication Mechanisms
- Cells communicate through four main mechanisms: cell-cell contact, paracrine signals (local), neuronal (fast, electrical), and endocrine (hormones via bloodstream).
- Endocrine signaling is slower but enables widespread, coordinated responses across the body.
Types and Properties of Hormones
- Two main hormone types: peptide (made from amino acids, hydrophilic) and steroid (from cholesterol, hydrophobic/lipophilic).
- Peptide hormones (e.g., insulin, vasopressin, oxytocin) travel freely in blood but act only via surface receptors on target cells.
- Steroid hormones (e.g., glucocorticoids, androgens, estrogen) require protein carriers in blood and pass directly through cell membranes to bind internal receptors.
Hormone Interaction with Target Cells
- Peptide hormones activate secondary messenger cascades, affecting protein activity quickly but for short durations.
- Steroid hormones enter cells, bind intracellular receptors, and influence gene transcription, leading to slower but longer-lasting effects.
- Hormone-receptor specificity ensures distinct cellular responses even for hormones with very similar structures.
Brain Control of Hormone Release
- The hypothalamus and pituitary gland are the brain’s major endocrine regulators, controlling many peripheral endocrine glands.
- The pituitary has two parts: anterior (regulated by hypothalamic hormone through blood) and posterior (direct neural connection, releases vasopressin and oxytocin).
Hormone Feedback and the HPA Axis
- The hypothalamic-pituitary-adrenal (HPA) axis exemplifies brain-to-gland signaling and negative feedback: hypothalamus releases CRH, stimulating ACTH from the pituitary, which triggers cortisol release from the adrenal cortex.
- Released cortisol feeds back to suppress further CRH and ACTH production, maintaining hormonal balance.
Hormone Action on the Brain
- Hormones reach the brain by crossing the blood-brain barrier (steroid hormones diffuse easily; peptides may use transporters).
- Brain areas differ in receptor type, location, and quantity, affecting regional sensitivity and behavioral outcomes.
- Hormone-receptor interactions in neurons can change membrane potential, gene transcription, protein activity, and synaptic function.
Key Terms & Definitions
- Peptide hormone — Water-soluble hormone made of amino acids, acts via cell surface receptors.
- Steroid hormone — Lipid-soluble hormone derived from cholesterol, acts via intracellular receptors.
- Hydrophilic — Water-loving; dissolves in water (peptides).
- Hydrophobic/Lipophilic — Water-fearing/lipid-loving; dissolves in fat (steroids).
- Blood-brain barrier — Selective barrier allowing certain substances, including steroid hormones, to access the brain.
- Negative feedback — Regulatory process where increased hormone levels suppress further hormone release.
- HPA axis — Hormonal pathway: hypothalamus → pituitary → adrenal gland, central to stress response.
Action Items / Next Steps
- Review hormone classifications and their mechanisms of action.
- Familiarize yourself with the HPA axis and negative feedback concepts.
- Complete assigned readings on hormone-behavior interactions.