In pharmacology, an agonist binds to a receptor , typically a protein on or within a cell , and triggers a conformational change that initiates a downstream signalling cascade. The magnitude of that response is described by two key properties: affinity (how strongly the molecule binds to the receptor) and efficacy (its capacity to activate the receptor once bound). A molecule can bind with high affinity yet still produce only a partial response, which is why the two properties are considered separately.
Agonists are classified by the degree of response they produce relative to the maximum possible for that receptor system. A full agonist produces the maximal response the receptor can generate. A partial agonist binds to the same site but produces a submaximal response regardless of concentration. An inverse agonist binds to the receptor and produces the opposite effect to the endogenous agonist , reducing baseline receptor activity rather than increasing it. This last category is distinct from an antagonist, which blocks the receptor without itself producing a functional response.
Endogenous agonists; molecules produced naturally within the body, include neurotransmitters, hormones, and other signalling compounds such as endocannabinoids, which act at cannabinoid receptors. Exogenous agonists include plant-derived compounds, synthetic pharmaceuticals, and research chemicals. The receptor selectivity of an agonist (whether it acts preferentially at one receptor subtype over another) is a central consideration in understanding its pharmacological profile and any downstream effects.
Within Australian regulatory frameworks, substances that act as agonists at specific receptor classes, such as opioid receptors or cannabinoid receptors; may be subject to scheduling under the Poisons Standard (the current version of which is maintained by the Therapeutic Goods Administration). Schedule classification is based on factors including potency, risk profile, and therapeutic utility, and determines whether a compound is available over the counter, by prescription only, or prohibited from supply.
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