Unit 4: Drugs Acting on Central Nervous System (Part II)

Semester 4

BP404T

Introduction to Drugs Acting on Central Nervous System (Part I)

The Central Nervous System (CNS) is complexly regulated by a delicate balance of excitatory (Glutamate) and inhibitory (GABA) neurotransmitters. This unit explains how CNS depressants shift this balance toward inhibition. It covers the stages of General Anesthesia and the drugs used to achieve it. It details the pharmacology of Sedatives and Hypnotics (focusing on the massive safety difference between Benzodiazepines and Barbiturates), life-saving Anti-epileptic medications used to suppress seizures, and the pharmacological impact and treatment of Alcohol addiction.

Syllabus & Topics

1Neurohumoral transmission in CNS: Role of GABA, Glutamate, Dopamine, and Serotonin.
2General anesthetics: Inhalation and Intravenous agents, Preanesthetic medication.
3Sedatives and hypnotics: Benzodiazepines, Barbiturates, and non-benzodiazepines.
4Centrally acting muscle relaxants (Baclofen, Tizanidine).
5Anti-epileptics: Phenytoin, Carbamazepine, Valproate, Ethosuximide.
6Alcohol and related drugs: Pharmacology of Ethanol, Methanol toxicity, Disulfiram.

Learning Objectives

Describe the opposing roles of GABA and Glutamate in brain physiology.

Compare the safety profiles and modes of action of various sleep-inducing drugs.

Understand the specific ion-channel mechanisms of antiepileptic therapy.

Explain the biochemical basis of aversion therapy in chronic alcoholism.

Frequently Asked Questions (FAQs)

Q1. What is the Role of GABA in the Central Nervous System?

Gamma-aminobutyric acid (GABA) is the primary inhibitory neurotransmitter in the brain. When GABA binds to the GABAA receptor, it opens chloride (Cl⁻) channels, allowing influx of chloride ions that hyperpolarize the neuron and reduce its excitability. Many CNS depressants, such as Benzodiazepine drugs and Barbiturate drugs, enhance the effects of GABA.

Q2. How Do Barbiturates and Benzodiazepines Differ Pharmacologically?

Both drug classes enhance GABA-mediated inhibition. Benzodiazepines increase the frequency of chloride channel opening and exhibit a safety ceiling effect. Barbiturates increase the duration of chloride channel opening and, at high doses, can directly activate the GABA receptor, eliminating the safety ceiling and increasing the risk of severe respiratory depression.

Q3. How Does Phenytoin Control Epilepsy Without Severe Sedation?

Phenytoin blocks voltage-gated sodium (Na⁺) channels and prolongs their inactivated state. This selectively suppresses high-frequency repetitive neuronal firing characteristic of seizures while allowing normal neuronal activity to continue, thereby controlling seizures without causing marked sedation.

Q4. What is the Mechanism of Disulfiram in Treating Alcohol Addiction?

Disulfiram irreversibly inhibits the enzyme Aldehyde dehydrogenase. Normally, ethanol is metabolized to acetaldehyde and then to acetic acid. Inhibition of aldehyde dehydrogenase causes accumulation of toxic acetaldehyde when alcohol is consumed, leading to nausea, flushing, and discomfort, thereby creating an aversion to alcohol.