Unit 2: Drugs Acting on Autonomic Nervous System

March 1, 2026

Semester 4
BP402T

Introduction to Drugs Acting on Autonomic Nervous System

The Autonomic Nervous System (ANS) regulates involuntary body functions. This unit focuses on the Adrenergic system (Sympathetic). It explores how drugs mimic (sympathomimetics) or block (sympatholytics) the actions of naturally occurring neurotransmitters like Adrenaline and Noradrenaline. The core focus is on the Structure-Activity Relationships (SAR) explaining how adding or modifying functional groups changes the drug’s specificity from an alpha-receptor target (blood vessels) to a beta-receptor target (heart, lungs). Vital syntheses include Salbutamol, Phenylephrine, and Propranolol.

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Syllabus & Topics

  • 1Sympathomimetic agents: Direct acting (Noradrenaline, Adrenaline, Phenylephrine, Salbutamol).
  • 2Sympathomimetic agents: Indirect acting (Ephedrine, Amphetamine).
  • 3SAR of Sympathomimetic agents.
  • 4Adrenergic Antagonists: Alpha adrenergic blockers (Tolazoline, Phentolamine, Prazosin).
  • 5Adrenergic Antagonists: Beta adrenergic blockers (Propranolol, Atenolol, Metoprolol).
  • 6SAR of beta blockers.
  • 7Synthesis of Salbutamol, Phenylephrine, Dopamine, Tolazoline, and Propranolol.

Learning Objectives

Memorize the detailed Structure-Activity Relationships of Adrenergic agonists and antagonists.
Classify sympathomimetics into direct, indirect, and mixed acting agents.
Design the chemical syntheses of marked compounds like Salbutamol and Propranolol.
Correlate the chemical structure of beta-blockers with their pharmacological selectivity.

Frequently Asked Questions (FAQs)

Q1. What is the Structural Basis for Adrenergic Agonist Activity?

The structure–activity relationship (SAR) of sympathomimetic agents is based on the phenylethylamine skeleton. Optimal activity requires a primary or secondary amine separated by two carbon atoms from a benzene ring. A catechol ring (3,4-dihydroxybenzene) confers maximum alpha and beta agonist activity. Substitution of a bulky alkyl group on the amine nitrogen, such as the isopropyl group in Isoprenaline, increases beta-receptor selectivity.

Q2. Differentiate Between Direct and Indirect Acting Sympathomimetics.

Direct-acting sympathomimetics, such as Adrenaline and Salbutamol, bind directly to and activate alpha or beta adrenergic receptors. Indirect-acting agents, such as Ephedrine and Amphetamine, do not directly activate receptors but instead promote the release of stored endogenous norepinephrine or inhibit its reuptake.

Q3. What Makes Salbutamol Selective for the Lungs (Asthma)?

Salbutamol (also known as albuterol) contains a bulky tert-butyl group on the nitrogen atom and a hydroxymethyl substitution at the meta position of the aromatic ring. These structural features enhance its selectivity for beta-2 adrenergic receptors in bronchial smooth muscle, thereby reducing beta-1 mediated cardiac side effects.

Q4. What is the SAR of Beta-Blockers (Aryloxypropanolamines)?

Beta-blockers such as Propranolol possess an aryloxypropanolamine structure. An ether linkage (–O–) connects the aromatic ring to the ethylamine side chain, and the presence of a hydroxyl group at the beta carbon of the side chain is essential for beta-adrenergic antagonist activity.