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Receptors can be activated by either endogenous agonists (such as hormones and neurotransmitters) or exogenous agonists (such as drugs), resulting in a biological response.A physiological agonist is a substance that creates the same bodily responses but does not bind to the same receptor.
Agonist vs. antagonist. In pharmacology the term agonist-antagonist or mixed agonist/antagonist is used to refer to a drug which under some conditions behaves as an agonist (a substance that fully activates the receptor that it binds to) while under other conditions, behaves as an antagonist (a substance that binds to a receptor but does not activate and can block the activity of other agonists).
The mechanisms of sympathomimetic drugs can be direct-acting (direct interaction between drug and receptor), such as α-adrenergic agonists, β-adrenergic agonists, and dopaminergic agonists; or indirect-acting (interaction not between drug and receptor), such as MAOIs, COMT inhibitors, release stimulants, and reuptake inhibitors that increase the levels of endogenous catecholamines.
A molecule that binds to a receptor is called a ligand and can be a protein, peptide (short protein), or another small molecule, such as a neurotransmitter, hormone, pharmaceutical drug, toxin, calcium ion or parts of the outside of a virus or microbe. An endogenously produced substance that binds to a particular receptor is referred to as its ...
The definition is context-dependent, and can refer to the biological target of a pharmacologically active drug compound, the receptor target of a hormone (like insulin), or some other target of an external stimulus. Biological targets are most commonly proteins such as enzymes, ion channels, and receptors.
An adrenergic agonist is a drug that stimulates a response from the adrenergic receptors.The five main categories of adrenergic receptors are: α 1, α 2, β 1, β 2, and β 3, although there are more subtypes, and agonists vary in specificity between these receptors, and may be classified respectively.
The M 3 receptors are also located in many glands, which help to stimulate secretion in, for example, the salivary glands, as well as other glands of the body. Like the M 1 muscarinic receptor, M 3 receptors are G proteins of class G q that upregulate phospholipase C and, therefore, inositol trisphosphate and intracellular calcium as a ...
In the example shown to the right, two different ligands bind to the same receptor binding site. Only one of the agonists shown can maximally stimulate the receptor and, thus, can be defined as a full agonist. An agonist that can only partially activate the physiological response is called a partial agonist.