agonistes

Agonistes, or agonists, are substances that activate receptors to produce a biological response. They bind to receptor sites with a specific affinity and a degree of intrinsic activity that determines their efficacy. Agonists can be endogenous ligands such as neurotransmitters and hormones, or synthetic compounds used as medicines. By binding to a receptor, an agonist stabilizes an active receptor conformation and initiates intracellular signaling appropriate to the receptor type, which may involve G proteins, ion channels, enzymes, or gene regulation.

Agonists are classified by efficacy. Full agonists produce the maximal response possible at a receptor; partial agonists yield a submaximal response even at full occupancy; inverse agonists reduce constitutive receptor activity, though they act opposite to agonists rather than as true agonists.

Key pharmacodynamic concepts include potency (often reflected by the EC50) and efficacy (the maximal effect, Emax). Affinity describes how tightly an agonist binds to its receptor.

Common clinical examples include morphine and other μ-opioid receptor agonists for analgesia; salbutamol, a β2-adrenergic receptor agonist used in asthma; epinephrine acting on multiple receptors; nicotine at nicotinic acetylcholine receptors; and dopamine or pramipexole as dopaminergic agonists in Parkinson’s disease. Serotonin receptor agonists such as sumatriptan illustrate selective targeting in headaches.

Therapeutic use depends on receptor selectivity to minimize adverse effects. Receptor desensitization and tolerance can develop with sustained exposure, and excessive agonism can cause toxicity in off-target tissues.