01Begin with the measured direction of action

An agonist can produce receptor-mediated activation. An antagonist opposes an agonist's action; a receptor antagonist need not activate the same response itself. A partial agonist produces a smaller maximum response than a full agonist in the specified system. These labels concern function, rather than simply whether binding occurs.IUPHAR Pharmacology Education Project — Receptors (opens in a new tab)

An article that lists these words without the receptor or endpoint has left out essential context. Retrieve that context from the experiment before comparing it with another paper's label.

02Partial agonism is a comparison within a system

IUPHAR's quantitative terminology explicitly makes full and partial agonism system-dependent. A ligand described as partial in one tissue or measurement may behave as full in another. Receptor abundance and the way receptor activation is coupled to the measured response can matter.Neubig and colleagues — Terms and symbols in quantitative pharmacology (opens in a new tab)

Consider an illustrative response scale on which a reference agonist reaches 100 units. A test ligand plateaus at 35 units under the same conditions. That comparison may support partial agonism in that assay; it does not mean the ligand contains 35% active material, occupies 35% of receptors or will give 35% of a physiological effect elsewhere.

If a second assay uses a different readout and reports a different maximum, do not average the two percentages into a universal efficacy score. First ask whether the experiments address the same response and have compatible reference scales.

03A lower maximum is different from a shifted curve

Two illustrative curves can reach the same maximum while doing so at different concentrations. Alternatively, two curves can rise over a similar concentration range but reach different plateaus. Those visual differences answer different questions; a single point on either curve may not reveal them.

A response of 35 units at one tested concentration does not establish that 35 is the attainable maximum. The curve may simply not have reached a plateau. Before repeating a partial-agonist claim, check whether the study estimated a maximum and explained its comparison.

04Keep inverse agonism separate

An inverse agonist reduces receptor signalling below its constitutive, or ligand-independent, activity. That requires a system with measurable baseline receptor activity. Merely preventing another agonist's response is not the same observation.IUPAC Gold Book — Inverse agonist (opens in a new tab)

For a hypothetical baseline of 10 units, a fall from an agonist-stimulated 60 back to 10 demonstrates a different comparison from a fall below the unstimulated baseline. The numbers are explanatory, not data from a peptide study. Appropriate controls are still needed to connect either change to the receptor.

Avoid using opposite as a substitute for these definitions. It can blur the distinction between blocking activation, reducing constitutive activity and changing an unrelated downstream process.

05Carry the context into your notes

• ✓Name the receptor and biological preparation.
• ✓Record the endpoint and reference agonist where one is used.
• ✓Distinguish a tested response from an established response maximum.
• ✓Keep potency, maximum response and binding measurements in separate fields.

These terms can explain an experimental result without ranking reagents for human use. A larger assay response is not automatically a more useful research tool, and a functional label supplies no evidence about a product's analytical quality. Those are different questions with different supporting documents.