Receptoragonists

A receptor agonist is a molecule that binds to a receptor and activates it, producing a physiological response. Agonists imitate endogenous ligands such as hormones or neurotransmitters, stabilizing the receptor in an active conformation. They can be classified by efficacy into full agonists, which produce the maximal possible response, and partial agonists, which generate a submaximal response even at full receptor occupancy. Inverse agonists suppress constitutive receptor activity, but these are not agonists.

When a receptor is activated, signaling pathways are engaged according to receptor type: G protein–coupled receptors, tyrosine kinase receptors, or ligand-gated ion channels, among others. Pharmacodynamics describes an agonist's potency, the amount needed for an effect, and efficacy, the size of the maximal response. Selectivity for receptor subtypes influences therapeutic effects and safety.

Applications: agonists are used to compensate for underactive signaling or to elicit specific physiological responses. Examples include epinephrine, which activates adrenergic receptors to raise blood pressure and open airways; albuterol, a selective beta-2 agonist for asthma; dopamine or pramipexole and ropinirole, which stimulate dopamine receptors in Parkinson's disease; mu-opioid receptor agonists such as morphine for analgesia; and serotonin receptor agonists like sumatriptan for migraine.

Some agonists are partial or biased, producing desirable effects with fewer side effects in some cases. Adverse effects can arise from excessive receptor activation, and chronic exposure can lead to desensitization or downregulation of receptors.