Neurotransmitter Termination in Adrenergic and Dopaminergic Neurons
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Questions and Answers

What is the primary mechanism by which norepinephrine (NE) is removed from the synaptic cleft?

  • Uptake by the postsynaptic neuron
  • Degradation by catechol-O-methyltransferase (COMT)
  • Metabolism by monoamine oxidase (MAO)
  • Uptake by the presynaptic neuron through a carrier protein (correct)

    • What is the result of the uptake 2 process in the removal of norepinephrine (NE)?

  • NE is stored in the presynaptic neuron
  • NE is removed from the synaptic cleft and degraded by COMT
  • NE is taken up by the postsynaptic neuron (correct)
  • NE is metabolized by MAO

    • What is the function of catechol-O-methyltransferase (COMT) in the metabolism of norepinephrine (NE)?

  • It breaks down NE into amino acids
  • It methylates NE to form 3-methoxy-4-hydroxymandelic acid (correct)
  • It oxidizes NE to form an aldehyde
  • It converts NE to an inactive metabolite

    • What is the classification of adrenergic receptors that respond more strongly to norepinephrine (NE) than to isoproterenol?

    <p>Alpha-adrenergic receptors</p> Signup and view all the answers

    What is the secondary messenger involved in the signaling pathway of adrenergic receptors?

    <p>cAMP</p> Signup and view all the answers

    What is the characteristic of alpha-adrenergic receptor agonists?

    <p>They have small N-substituents and at least one OH group</p> Signup and view all the answers

    What is the result of the metabolism of norepinephrine (NE) by monoamine oxidase (MAO)?

    <p>NE is converted to an aldehyde</p> Signup and view all the answers

    What is the mechanism by which norepinephrine (NE) is reabsorbed into neurons?

    <p>Active transport through a carrier protein</p> Signup and view all the answers

    What is the effect of stimulating α1 receptors in blood vessels?

    <p>Vasoconstriction</p> Signup and view all the answers

    Which neurotransmitter is NOT a member of the catecholamine family?

    <p>Serotonin</p> Signup and view all the answers

    What is the function of β2 receptors in the cardiac muscles?

    <p>Increase in heart rate</p> Signup and view all the answers

    What is the effect of NE administration on the uterus?

    <p>Stimulation of uterine contractions</p> Signup and view all the answers

    What is the primary function of α2 receptors?

    <p>Vasoconstriction</p> Signup and view all the answers

    Which of the following is NOT a physiological function regulated by adrenergic neurotransmitters?

    <p>Muscle contraction</p> Signup and view all the answers

    What is the effect of adrenergic drugs on carbohydrate metabolism?

    <p>Increase in carbohydrate metabolism</p> Signup and view all the answers

    What is the primary function of β2 receptors in skeletal muscles?

    <p>Contraction of skeletal muscles</p> Signup and view all the answers

    What is the essential feature of direct acting sympathomimetics?

    <p>Presence of β-OH group</p> Signup and view all the answers

    What is the effect of replacing H with an alkyl group in direct acting sympathomimetics?

    <p>Increases lipophilicity and duration</p> Signup and view all the answers

    Why do direct acting sympathomimetics exhibit stereoselectivity?

    <p>Due to the arrangement of phenyl group, amino group, and β-OH group in space</p> Signup and view all the answers

    Which of the following is NOT inactivated orally?

    <p>Isoproterenol</p> Signup and view all the answers

    What is the effect of β-OH group on receptor binding?

    <p>Increases receptor binding</p> Signup and view all the answers

    What is the characteristic of R(-) enantiomer of direct acting sympathomimetics?

    <p>Has greater affinity for the receptor</p> Signup and view all the answers

    What is the consequence of metabolic inactivation of direct acting sympathomimetics?

    <p>Inactivation of the drug orally</p> Signup and view all the answers

    What is the pharmacological effect of direct acting sympathomimetics?

    <p>Vasoconstriction</p> Signup and view all the answers

    Study Notes

    Termination of Action

    • Norepinephrine (NE) may be removed from the action site through two processes: uptake and metabolism
    • Uptake process: the greatest amount of NE is rapidly reabsorbed into neurons and then into its storage sites through an active transport mechanism (carrier protein)
    • Two types of uptake: uptake 1 (highly stereospecific, occurs at nerve terminal on presynaptic cell membrane) and uptake 2 (less stereospecific, occurs at the receptor site on the postsynaptic cell membrane)

    Metabolism

    • NE is metabolized to inactive metabolites excreted in the urine
    • Two enzymes involved in metabolism: Monoamine Oxidase (MAO) and Catechol-O-Methyltransferase (COMT)
    • MAO converts NE into an acid or aldehyde
    • COMT converts NE into 3-methoxy-4-hydroxymandelic acid

    Adrenergic Receptors

    • Adrenoreceptors are G protein-coupled receptors that are targets of catecholamines (NE and E)
    • Classified into two types: α and β receptors
    • α-adrenergic receptor primary binding involves a small N-substituent and at least one OH
    • β-adrenergic receptor primary binding involves a bulky N-substituent and two catecholic OHs
    • α1 or α2 response is stronger to NE or E than isoproterenol
    • β1, β2, or β3 response is stronger to isoproterenol than E or NE

    Sympathomimetic Drugs

    • Direct-acting sympathomimetics are phenylethylamine derivatives
    • Structure-activity relationship (SAR) requirements:
      • β-OH is essential
      • N atom should bear at least one H (1ry or 2ry)
      • N atom should be separated from the ring by 2 C
      • 3-OH and 4-OH are important for activity
    • Vasoconstrictor activity is increased by replacing H with alkyl groups (increases lipophilicity)

    Adrenergic Drugs

    • Adrenergic drugs stimulate or inhibit adrenergic receptors (α and β receptors)
    • Adrenergic neurotransmitters regulate many physiological functions, including:
      • Blood pressure
      • Heart rate
      • GIT motility
      • Bronchial tone
      • Carbohydrate metabolism
      • Transport of lipid from fat depots to the liver
    • NE is a neurotransmitter in the autonomic nervous system and in the CNS

    Adrenergic Receptors and Physiological Functions

    • α1 receptors: stimulation causes vasoconstriction
    • α2 receptors: mostly postsynaptic
    • β2 receptors: stimulation causes:
      • Increased heart rate and various arrhythmias
      • Contraction of skeletal muscles
      • Increased GIT motility and pancreatic secretion

    Catecholamines

    • Include NE, E, and dopamine
    • Chemical structures:
      • NE: OH, NH2, HO
      • E: OH, N, HO, NH2
      • Dopamine: OH, NH2, HO

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    Description

    This quiz covers the termination of action of neurotransmitters, including uptake process and metabolism, in adrenergic and dopaminergic neurons in the CNS and ANS.

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