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Questions and Answers
What is the primary mediator of the sympathetic nervous system?
Where are adrenergic synapses primarily located?
What is the role of adrenaline in the brain?
Which of the following processes describes neuronal uptake of catecholamines?
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What is the primary function of dopamine within the nervous system?
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What are catecholamines primarily synthesized from?
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Which mechanism is NOT involved in the inactivation of noradrenaline?
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Which adrenergic mediator is secreted into the blood from the adrenal medulla?
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What is the primary effect of stimulating β1-adrenoreceptors on the myocardium?
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Which of the following tissues contains β2-adrenoceptors that facilitate bronchodilation?
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Which of the following is NOT a function of β2-adrenoceptors?
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In which location are β3-adrenoreceptors primarily found?
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What is the role of β2-adrenoceptors in the renal system?
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What mechanism do adrenergic receptors employ to affect cellular activity?
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Which action is associated with the stimulation of β3-adrenoreceptors?
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What is one of the effects of presynaptic β2-adrenoceptors?
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What is the primary action of β2-adrenergic agonists on smooth muscles?
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Which β2-adrenergic agonist is specifically used as a tocolytic agent?
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What distinguishes β2-adrenergic agonists from non-selective adrenergic agonists?
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Which of the following β3-adrenergic agonists is used to treat overactive bladder?
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What forms of administration are commonly used for β2-adrenergic agonists?
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What is the primary effect of α2-receptors activated via Gi-protein?
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Which of the following correctly describes the action of β1 receptors?
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What type of agonists are Naphazoline and Oxymethazoline classified as?
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The primary function of β3 receptors is to increase what?
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Which of the following receptors is NOT coupled via G-protein to adenylyl cyclase?
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What is the effect of β2 receptors on smooth muscle?
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What type of adrenergic agonists directly stimulate all types of adrenoceptors?
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Which of the following β-adrenergic agonists is specifically cardioselective?
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What is the effect of α1 receptor activation on the prostate?
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How do adrenergic receptors affect the human uterus during pregnancy?
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What is the result of activating β2 receptors in skeletal muscle?
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What are the central nervous system effects of adrenergic agonists in clinically used doses?
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Which of the following processes is stimulated by adrenaline via β2 receptors?
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What is the effect of α2 receptor activation in the brainstem?
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Which adrenergic effect contributes to the tremors induced by β2 agonists?
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Which of the following is an effect of α2 receptors on insulin secretion?
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Study Notes
Adrenergic Synapses
- The sympathetic nervous system's primary mediator is norepinephrine
- Sympathetic innervation is also known as noradrenergic
- Adrenergic agents act on adrenergic synapses
- Adrenergic synapses can be found both peripherally and centrally
Adrenergic Mediators
- Norepinephrine is the main mediator at postganglionic sympathetic terminals (excluding sweat glands, hair follicles, and some vasodilating fibers) and in certain brain regions
- Adrenaline plays a role as a transmitter in the brain
- Dopamine serves as a major transmitter in basal ganglia, limbic system, chemoreceptor trigger zone (CTZ), anterior pituitary, and the periphery (to a lesser extent).
- Catecholamines are produced from tyrosine
Uptake of Catecholamines
- After the "norepinephrine-adrenoceptor complex" dissociates, the mediator is inactivated through several mechanisms
- Neuronal uptake occurs in two steps:
- Axonal uptake (uptake-1): Active sodium-coupled transport by an amine pump across the presynaptic membrane
- Vesicular uptake: Another amine pump exchanges with hydrogen ions
Beta Adrenoceptors
-
Beta-1 adrenoceptors:
- Located on the postsynaptic membrane of myocardial cells
- Stimulation increases all cardiac functions:
- Automatism
- AV conduction
- Excitability
- Heart rate
- Contractility
- Increases myocardial oxygen demand
- Tachycardia occurs with excess catecholamines in the blood
- Located in the juxtaglomerular apparatus, where they increase renin secretion
-
Beta-2 adrenoceptors:
- Can be located presynaptically, postsynaptically, and extrasynaptically
- Extrasynaptic:
- Located in smooth muscles and glands of bronchi, trachea, and platelets
- Promote bronchodilation, decrease bronchial secretions, and inhibit platelet aggregation
- In the pancreas, they stimulate glucagon secretion
- In the liver, they stimulate glycogenolysis and gluconeogenesis, increasing blood glucose levels
-
Beta-2 adrenoceptors (postsynaptic):
- Located in the uterus, urinary bladder, gall bladder, and gastrointestinal tract (GIT)
- Decrease tone of myometrium, urinary bladder detrusor, GIT, and biliary tract
- Located in blood vessels of skeletal muscle, coronary, pulmonary, cerebral, and hepatic blood vessels
- Responsible for dilation of vessels in skeletal muscles, coronary arteries, pulmonary arteries, hepatic blood vessels, and cerebral blood vessels
-
Beta-2 adrenoceptors (presynaptic):
- Function according to positive feedback, stimulating norepinephrine release when adrenoceptor activation is insufficient
-
Beta-3 adrenoceptors:
- Found on the membranes of adipocytes
- High concentrations of catecholamines stimulate them, leading to lipolysis in adipose tissue and thermogenesis in skeletal muscles
- Participate in regulating carbohydrate, lipid, and energy metabolism
- Excitation by catecholamines stimulates metabolism and increases oxygen demand
- Thought to cause relaxation of the bladder and prevent urination
Transducer Mechanisms of Adrenoceptors
- Adrenergic receptors are membrane-bound G-protein coupled receptors
- They primarily function by increasing or decreasing intracellular production of second messengers (cAMP or IP3/DAG)
- In some cases, the activated G-protein itself operates K+ or Ca2+ channels or increases prostaglandin production
Transducer Mechanisms of Adrenoceptors
-
Alpha-1 receptors:
- Coupled to phospholipase C via Gq-protein
- Activation of membrane phospholipases leads to increased Ca2+ influx across the membrane and liberation of deposited Ca2+ from intracellular stores
-
Alpha-2 receptors:
- Located presynaptically and centrally postsynaptically
- Inhibit adenylyl cyclase via Gi-protein and decrease cAMP formation
- Increase membrane permeability to K+, leading to hyperpolarization and blockage of Ca2+ channels
Transducer Mechanisms of Adrenoceptors
-
Beta-1 receptors:
- Stimulate adenylyl cyclase via Gs-proteins, leading to phosphorylation of calcium channels and their opening
- Ca2+ flows into the sarcoplasm and is mobilized from the sarcoplasmic reticulum of the myocardium
-
Beta-2 receptors:
- Activate adenylyl cyclase and increase cyclic AMP content in smooth muscle
- Cyclic AMP binds free Ca2+, leading to membrane hyperpolarization
-
Beta-3 receptors:
- Increase cAMP-dependent lipolysis
Classification of Adrenergic Agonists
-
Increase transmission of nerve impulses in adrenergic synapses
-
Direct-acting adrenergic agonists:
- Alpha, beta-adrenergic agonists (non-selective): Stimulate all types of adrenoceptors:
- Norepinephrine hydrotartrate
- Adrenaline hydrochloride
- Dopamine
- Alpha, beta-adrenergic agonists (non-selective): Stimulate all types of adrenoceptors:
-
Alpha-adrenergic agonists:
- Alpha-1, alpha-2 agonists: Naphazoline, xylametazoline, oxymetazoline, tetrizoline
- Alpha-1 agonists: Phenylephrine, etilofrine, midodrine, methoxamine
- Alpha-2 agonists: Clonidine, alpha-methyldopa, apraclonidine, brimonidine
-
Beta-adrenergic agonists:
- Beta-1, beta-2-adrenergic agonists: Isoprenaline, orciprenaline
- Beta-1-adrenergic agonists (cardioselective): Dobutamine
- Beta-2-adrenergic agonists: Salbutamol, fenoterol, terbutaline, salmeterol, pirbuterol, bambuterol
- Clinically insignificant effects: Beta-4 agonists and other drugs are not clinically relevant
Pharmacological Effects of Alpha, Beta-Adrenergic Agonists
- Bladder, prostate, vas deferens:
- Detrusor relaxation via beta-2 and beta-3 receptors
- Trigone constriction via alpha-1 receptors, opposing bladder voiding
- Increased prostatic muscle tone via alpha-1 receptor activation
- Rhythmic contractions of the vas deferens and seminal vesicles, involved in ejaculation, are mediated by alpha-1 receptors
Pharmacological Effects of Alpha, Beta-Adrenergic Agonists
- Uterus:
- Adrenaline can both contract and relax uterine muscle, depending on receptor types
- Overall effect varies with species, hormonal status, and gestational stage
- Human uterus is relaxed by adrenaline at term, but its effects are enhanced at other times
Pharmacological Effects of Alpha, Beta-Adrenergic Agonists
- Skeletal muscle neuromuscular transmission:
- Facilitated by adrenaline
- Alpha receptor activation on motor nerve endings augments acetylcholine (ACh) release, likely due to the alpha-1 subtype
- Direct effect on muscle fibers is exerted through beta-2 receptors
- Red fibers (slow contracting): Active state is abbreviated, less tension developed, and incomplete fusion of individual responses
- White fibers (rapid contracting): Active state is prolonged, increased tension develops
Pharmacological Effects of Alpha, Beta-Adrenergic Agonists
- Central Nervous System (CNS):
- Adrenaline does not produce marked CNS effects in clinical doses due to poor penetration
- Restlessness, apprehension, and tremor may occur
- Activation of alpha-2 receptors in the brainstem (by selective alpha-2 agonists like clonidine) results in decreased sympathetic outflow, lowered blood pressure (BP), and bradycardia
Pharmacological Effects of Alpha, Beta-Adrenergic Agonists
- Metabolism:
- Adrenaline stimulates glycogenolysis (due to beta-2 receptor stimulation of muscle cells and the liver)
- Alpha-2 receptors inhibit insulin secretion
- Beta-2 receptors stimulate glucagon release, leading to hyperglycemia
Beta-2 Adrenergic Agonists
- Representatives: Salbutamol, fenoterol, terbutaline, salmeterol, pirbuterol, bambuterol
- Selective stimulants of beta-2 adrenoceptors
- Marked action on smooth muscles of the bronchi, causing dilation
- Fewer adverse effects than non-selective adrenergic agonists
- Stimulate beta-2 adrenoceptors of the uterus, causing myometrium relaxation
- Available as sprays for inhalation, oral formulations, and powder inhalations
- Used as bronchodilators to relieve bronchial obstruction
- Used to prevent preterm delivery in threatened abortion, with fenoterol frequently used as "Partusisten"
- "Salbupart," ritodrine, and isoxsuprine are also tocolytics (uterine relaxants)
Beta-3 Adrenergic Agonists
- Mirabegron is used for overactive bladder with urinary incontinence, frequency, and urgency.
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Description
Test your knowledge on adrenergic synapses and mediators within the sympathetic nervous system. This quiz covers the roles of norepinephrine, adrenaline, and dopamine, as well as the mechanisms of catecholamine uptake. Perfect for students studying neurobiology or pharmacology.
