Certain natural substances in the body, such as neurotransmitters and hormones, target specific receptors on the surface of cells. When these substances bind with the receptor on a cell, they stimulate that receptor to perform its function, which is to produce or to inhibit a specific action in the cell. Drugs can also target and bind with these receptors. Some drugs act as agonists, stimulating the receptor in the same way that the body’s natural substances do. Others act as antagonists, blocking the action of the natural substance on the receptor. Each type of receptor has many subtypes, and drugs may act on one or several subtypes of receptors. | |||
Type of Receptor | Body’s Natural Agonist | Resulting Action | Drugs That Target the Receptor |
Adrenergic | |||
Alpha 1 | Epinephrine and norepinephrine | “Fight-or-flight” reactions: Constriction of the blood vessels in the skin, digestive tract, and urinary tract Breakdown of glucose in the liver (releasing energy) Decrease in activity of the stomach and intestines Contraction of smooth muscle in the genital and urinary organs | Agonist: Methoxamine and phenylephrine Antagonist: Doxazosin, prazosin, tamsulosin, and terazosin |
Alpha 2 | Epinephrine and norepinephrine | A decrease in insulin secretion, in the clumping of platelets, in the constriction of blood vessels in the skin and intestines, and in the release of norepinephrine from nerves | Agonist: Clonidine Antagonist: Yohimbine |
Beta 1 | Epinephrine and norepinephrine | An increase in heart rate, in the force of heart contraction, and in secretion of renin (a hormone involved in controlling blood pressure) | Agonist: Dobutamine and isoproterenol Antagonist: Beta-blockers Beta-blockers The heart muscle needs a constant supply of oxygen-rich blood. The coronary arteries, which branch off the aorta just after it leaves the heart, deliver this blood. Coronary artery disease that... read more (used to treat hypertension and heart disease), such as atenolol and metoprolol |
Beta 2 | Epinephrine and norepinephrine | Dilation of smooth muscle in the blood vessels, airways, digestive tract, and urinary tract Breakdown of glycogen in skeletal muscles (releasing glucose for energy) | Agonist: Albuterol, isoetharine, and terbutaline Antagonist: Propranolol |
Cholinergic | |||
Muscarinic | Acetylcholine | A decrease in heart rate and the force of the heart’s contraction Constriction of airways Dilation of blood vessels throughout the body Increase in activity of the stomach, intestines, bladder, and salivary, lacrimal, and sweat glands | Agonist: Bethanechol and carbachol Antagonist: Atropine, ipratropium, and scopolamine |
Nicotinic | Acetylcholine | Contraction of skeletal muscles | Agonist: None commonly used Antagonist: Atracurium, pancuronium, and tubocurarine |
Histaminergic | |||
H1 | Histamine | Production of an allergic response Contraction of muscles in the airways and digestive tract Dilation of small blood vessels Drowsiness (sedation) | Agonist: None commonly used Antagonist: Cetirizine, chlorpheniramine, clemastine, diphenhydramine, fexofenadine, and loratadine |
H2 | Histamine | Stimulation of stomach secretions | Agonist: None commonly used Antagonist: Cimetidine, famotidine, and nizatidine |
Serotoninergic | |||
Serotonin | Constriction of blood vessels within the brain Stimulation of activity (motility) in the digestive tract Contraction of blood vessels Effects on sleep, memory, sensory perception, temperature regulation, mood, appetite, and hormone secretion | Partial agonist: Buspirone Agonist*: Sumitriptan and zolmitriptan Antagonist: Methysergide and ondansetron | |
Dopaminergic | |||
Dopamine | Involvement in movement, mood, thinking, learning, and reward-seeking Also increases blood flow to the kidneys, which allows for increased urine excretion | Agonist: Pramipexole and ropinirole Antagonist: Olanzapine and risperidone | |
* Antidepressants called selective serotonin reuptake inhibitors (SSRIs) act by enhancing the effects of serotonin but are not agonists (they do not act on the serotonin receptor). |