Endocrine Signaling: Hormones, Receptors, and Hypothalamic-Pituitary Axis Lecture 2

Fifty vocabulary flashcards covering key terms from the lecture notes on hormones, receptors, signaling mechanisms, feedback, and the hypothalamic-pituitary axis.

Ligand

A chemical messenger (hormone, neurotransmitter, or drug) that binds to a receptor to initiate a cellular response.

Receptor

A binding site—often a protein—that recognizes and binds a specific ligand to trigger signal transduction.

Ligand-receptor complex

The bound form of ligand and receptor that activates downstream cellular responses.

Second messenger

Intracellular chemical that relays signals from membrane receptors to intracellular targets (e.g., cAMP, DAG, IP3, Ca2+).

cAMP

Cyclic adenosine monophosphate; a common second messenger produced by many receptor pathways.

DAG

Diacylglycerol; second messenger produced from PIP2 that activates protein kinase C.

IP3

Inositol triphosphate; second messenger that triggers Ca2+ release from intracellular stores.

Ca2+

Calcium ion; versatile second messenger in many signaling pathways.

PIP2

Phosphatidylinositol 4,5-bisphosphate; plasma membrane lipid cleaved to yield DAG and IP3.

Lipid-soluble hormone

Hormone that can diffuse through the plasma membrane and binds intracellular receptors.

Intracellular receptor

Receptor located inside the cell; binds lipid-soluble hormones to regulate gene activity.

Receptor enzyme

An enzyme-linked receptor; hormone binding activates the receptor’s enzymatic activity (e.g., tyrosine kinase).

Tyrosine kinase pathway

Signaling cascade activated by receptor tyrosine kinases that phosphorylate substrates to propagate the signal.

Phosphorylation

Addition of a phosphate group to a protein, often activating signaling proteins.

G protein-coupled receptor (GPCR)

Membrane receptor that activates G proteins to modulate second messenger systems.

Promoter region

DNA region upstream of a gene where transcription factors bind to regulate transcription.

Hormone response element (HRE)

DNA sequence in a gene promoter recognized by hormone-receptor complexes to control transcription.

Transcription

Process of synthesizing messenger RNA from DNA.

Translation

Process of protein synthesis from mRNA at ribosomes.

Upregulation

Increase in receptor density on a cell in response to low hormone levels, increasing sensitivity.

Downregulation

Decrease in receptor density in response to high hormone levels, reducing sensitivity.

Affinity

Strength of binding between receptor and its ligand; high affinity means greater receptor occupancy.

Half-life

Time required for a hormone’s blood concentration to decrease by half.

Onset

Time from hormone release to the start of its effects.

Duration

Length of time the hormone’s effects persist.

Humoral stimuli

Hormone release triggered by changes in blood levels of ions or nutrients.

Neural stimulus

Nervous system input triggering hormone release (often via the sympathetic system).

Hormonal stimuli

Release of hormones in response to other hormones (hypothalamic-pituitary axis).

Hypothalamus

Brain region that links nervous and endocrine systems; releases releasing/inhibiting hormones and controls the pituitary.

Pituitary gland

Master endocrine gland with anterior (adenohypophysis) and posterior (neurohypophysis) lobes.

Adenohypophysis

Anterior pituitary; glandular tissue that secretes hormones in response to hypothalamic releasing hormones.

Neurohypophysis

Posterior pituitary; neural tissue that stores and releases hypothalamic hormones (oxytocin, ADH).

Hypophyseal portal system

Vascular connection between hypothalamus and anterior pituitary via two capillary plexuses.

Primary capillary plexus

Capillary network in the hypothalamus that collects releasing/inhibiting hormones.

Secondary capillary plexus

Capillary network in the anterior pituitary that receives hypothalamic hormones.

Hypophyseal portal veins

Veins connecting the primary and secondary capillary plexuses in the portal system.

Oxytocin

Posterior pituitary hormone; targets uterus (contractions) and breast (milk ejection); positive feedback in childbirth.

Vasopressin (ADH)

Antidiuretic hormone; posterior pituitary; regulates water balance and can cause vasoconstriction.

Negative feedback

Regulatory mechanism that reduces the original stimulus as hormone levels rise.

Positive feedback

Regulatory mechanism that amplifies a process to completion (e.g., oxytocin during childbirth).

Permissiveness

One hormone enables another to act by increasing receptor expression or signaling readiness.

Synergism

Two or more hormones produce a greater-than-additive effect.

Antagonism

Hormones with opposing actions that fine-tune blood levels.

Target cell

Cell that bears the appropriate receptor and responds to a hormone.

Receptor density

Number of receptors on/within a cell; can be up- or down-regulated in response to signals.

Lipid-soluble receptor location

Intracellular or nuclear receptor location for lipid-soluble hormones.

Releasing hormone (RH)

Hypothalamic hormone that stimulates the anterior pituitary to release a specific hormone.

Anterior pituitary hormone (examples)

Glandular hormones secreted by the adenohypophysis in response to hypothalamic RHs (e.g., TSH, ACTH, GH, FSH/LH, prolactin).

Posterior pituitary hormone functions

Neural storage and release of hypothalamic hormones (oxytocin, ADH).

Thyroid-stimulating hormone (TSH)

Anterior pituitary hormone that stimulates the thyroid to release thyroid hormones.

Hypothalamic-pituitary axis

Hormone production and regulation pathway linking hypothalamus to pituitary and target organs.

Promoter region vs HRE

Promoter is DNA region for general transcription; HRE is hormone-receptor-specific DNA element for regulation.

Thyroid hormone receptor localization

Receptors for lipid-soluble thyroid hormones can be located in the nucleus or cytoplasm, influencing gene activation.

Ghrelin/CRH/TRH (hypothalamic RH examples)

Specific releasing hormones exampled in notes (e.g., gonadotropin-releasing hormone, thyrotropin-releasing hormone) that regulate pituitary secretion.