Agonist and antagonist are terms used in pharmacology and physiology to describe two different types of molecules or substances that can affect the function of a receptor or target cell in the body.
An agonist is a molecule or substance that binds to a receptor and activates it, producing a physiological response. Agonists can be natural ligands (endogenous) such as hormones or neurotransmitters, or synthetic (exogenous) such as drugs. Examples of agonists include adrenaline, which binds to adrenergic receptors to increase heart rate and blood pressure, and morphine, which binds to opioid receptors to produce pain relief.
On the other hand, an antagonist is a molecule or substance that binds to a receptor but does not activate it, preventing an agonist from binding and producing a response. Antagonists can be competitive or non-competitive. Competitive antagonists bind to the same site on a receptor as an agonist, and thus compete with the agonist for binding. Non-competitive antagonists bind to a different site on the receptor and change the shape of the receptor, preventing agonists from binding. Examples of antagonists include naloxone, which binds to opioid receptors to reverse the effects of morphine, and propranolol, which binds to adrenergic receptors to block the effects of adrenaline.
In summary, an agonist activates a receptor and produces a response, while an antagonist binds to a receptor but does not activate it, preventing an agonist from binding and producing a response.