Receptor signaling mechanisms

Learning objective

Distinguish affinity from activation and trace ionotropic, GPCR and course-specific Trk signaling.

Binding is not the same as activation

Affinity describes how readily a ligand binds under specified conditions. Efficacy describes its capacity to produce a response in the measured system. An antagonist can bind strongly without producing an agonist response. Selectivity is a relative preference among targets and can depend on concentration; a drug is not necessarily limited to one receptor type. Isomers share a molecular formula but differ in arrangement, so their interactions with a target can differ. A racemic mixture contains equal proportions of a pair of enantiomers.

Trace the signal

An ionotropic receptor contains a ligand-gated channel; binding changes gating and ions carry the current. A GPCR activates a G protein and downstream targets. A route can involve an effector enzyme and second messenger, such as cAMP, or modulation of a separate channel. The extracellular ligand does not become the intracellular second messenger. GPCR pathways can amplify signals and alter longer-term cellular activity.

Keep the professor’s classification visible

The captured book section uses metabotropic largely as shorthand for GPCR. The professor also includes Trk receptors, including BDNF–TrkB, as a non-GPCR metabotropic category. For this course, identify TrkB as a receptor tyrosine kinase rather than inventing a G-protein step. Also keep location separate from mechanism: a receptor’s presynaptic location does not itself tell you whether it is ionotropic or metabotropic.

Different sites can influence the same receptor

A drug can bind a site distinct from the transmitter’s site and alter the receptor’s response. The book’s GABA-A figure illustrates separate transmitter and drug binding sites. Allosteric modulation therefore should not be described as every drug competing for the same pocket. Transporters are another target class and move substrates across membranes; they are not simply ligand-gated ion channels.

Check your understanding

Explain it without notes

Explain first messenger versus second messenger.

What is the course correction to “all metabotropic receptors are GPCRs”?

Revisit the connected course lessons and visuals

Receptor classification

Glutamate/NMDA and GABA comparisons

Julien’s Primer of Drug Action, supplied consolidated PDF pages 33, 34, 35 (Chapter 3). Locators refer to PDF captures, not printed pages. Original paraphrase and practice; source conformance reviewed, not independently clinically certified. Professor or supplementary clarifications are identified in the lesson and source list.