8. Drugs Acting on Central Nervous System: A Complete Guide for Pharmacy Students

DRUGS ACTING ON CENTRAL NERVOUS SYSTEM

Welcome, future pharmacists and healthcare professionals!

As a pharmacy educator with years of experience teaching pharmaceutical chemistry, I have always emphasized that drugs acting on the central nervous system (CNS) are among the most complex and clinically important classes of drugs. The central nervous system controls thinking, movement, emotions, memory, pain sensation, and consciousness. Drugs acting on the CNS either stimulate or depress brain activity and are used in conditions such as mental illness, seizures, pain, anxiety, sleep disorders, and anesthesia. Understanding these drugs is essential for pharmacy students to manage a wide range of neurological and psychiatric conditions.

In this comprehensive guide, I will take you through the classification, mechanisms of action, and therapeutic uses of drugs acting on the central nervous system. We will explore anaesthetics, sedatives and hypnotics, antipsychotics, anticonvulsants, and antidepressants. By the end of this article, you will have a solid understanding of how these drugs work and their role in clinical practice. Let us begin.

ANAESTHETICS

Anaesthetics are drugs that cause loss of sensation by depressing the central nervous system. They are used during medical procedures to prevent pain. Anaesthetics are broadly classified into two categories:

• Local Anaesthetics: Act on a specific area without affecting consciousness.
• General Anaesthetics: Cause complete unconsciousness and are used for major surgical procedures.

Stages of General Anaesthesia

General anaesthesia progresses through four stages:

• Stage I (Analgesia): Loss of pain sensation while consciousness is partially preserved.
• Stage II (Delirium): Excitement and involuntary movements; the patient may experience confusion.
• Stage III (Surgical Anaesthesia): Unconsciousness with adequate muscle relaxation for surgery.
• Stage IV (Medullary Paralysis): Dangerous stage with respiratory and cardiovascular depression.

Classification of General Anaesthetics

• Inhalational Agents: Halothane, Enflurane, Ether – administered through inhalation.
• Intravenous Agents: Thiopentone sodium, Ketamine, Propofol, Etomidate – administered intravenously.
• Gaseous Anaesthetics: Nitrous oxide – administered as a gas.

Important Anaesthetic Drugs

Thiopental Sodium

Mechanism of Action: Thiopental sodium is a short-acting barbiturate that enhances GABA-mediated inhibition in the CNS, producing rapid anaesthesia.

Therapeutic Uses: Used for rapid induction of anaesthesia in surgical procedures.

Side Effects: Respiratory depression, hypotension, and bradycardia.

Ketamine

Mechanism of Action: Ketamine is an NMDA receptor antagonist that produces dissociative anaesthesia. It provides analgesia and anaesthesia without significant respiratory depression.

Therapeutic Uses: Used in emergency procedures, paediatric anaesthesia, and as a sedative.

Side Effects: Hallucinations, increased intracranial pressure, and tachycardia.

Propofol

Mechanism of Action: Propofol enhances GABA-mediated inhibition in the CNS, producing rapid sedation and anaesthesia.

Therapeutic Uses: Used for induction and maintenance of anaesthesia, sedation in ICU, and procedural sedation.

Side Effects: Respiratory depression, hypotension, and pain at injection site.

SEDATIVES AND HYPNOTICS

Sedatives and hypnotics are drugs that depress the central nervous system. Low doses produce sedation (calming effect), while higher doses induce sleep. These drugs do not relieve pain but reduce anxiety and promote relaxation.

Benzodiazepines

Benzodiazepines are among the most commonly prescribed sedatives and hypnotics. They enhance the action of GABA, the major inhibitory neurotransmitter in the CNS.

Diazepam

Mechanism of Action: Diazepam enhances GABA-mediated inhibition in the CNS.

Therapeutic Uses: Used in anxiety, seizures, muscle relaxation, and alcohol withdrawal.

Side Effects: Drowsiness, dependence, and withdrawal symptoms.

Alprazolam

Mechanism of Action: Alprazolam enhances GABA-mediated inhibition.

Therapeutic Uses: Used in panic and anxiety disorders.

Side Effects: Drowsiness, dependence, and memory impairment.

Nitrazepam

Mechanism of Action: Nitrazepam enhances GABA-mediated inhibition.

Therapeutic Uses: Used in insomnia and seizures.

Side Effects: Drowsiness, dizziness, and dependence.

Barbiturates

Barbiturates are older sedative-hypnotic drugs that are less commonly used now due to their narrow therapeutic index and risk of dependence.

Phenobarbital

Mechanism of Action: Phenobarbital enhances GABA-mediated inhibition and prolongs the opening of chloride channels.

Therapeutic Uses: Used in epilepsy and sedation.

Side Effects: Drowsiness, dependence, and withdrawal symptoms.

ANTIPSYCHOTICS

Antipsychotics are used in schizophrenia, bipolar disorder, and severe behavioural conditions. They mainly act on dopamine and serotonin pathways to reduce psychotic symptoms.

Chlorpromazine

Mechanism of Action: Chlorpromazine is a dopamine D2 receptor antagonist that reduces psychotic symptoms.

Therapeutic Uses: Used in schizophrenia, nausea, and hyperactivity.

Side Effects: Extrapyramidal symptoms, sedation, and hypotension.

Haloperidol

Mechanism of Action: Haloperidol is a potent D2 receptor antagonist.

Therapeutic Uses: Used in psychosis and agitation.

Side Effects: Extrapyramidal symptoms and sedation.

Risperidone

Mechanism of Action: Risperidone is a D2 and 5-HT2A receptor antagonist.

Therapeutic Uses: Used in schizophrenia and bipolar disorder.

Side Effects: Weight gain, sedation, and extrapyramidal symptoms.

ANTICONVULSANTS

Anticonvulsants prevent abnormal neuronal activity and are used in epilepsy and seizures. They work by stabilising neuronal membranes, enhancing GABA-mediated inhibition, or blocking sodium channels.

Phenytoin

Mechanism of Action: Phenytoin blocks voltage-gated sodium channels, stabilising neuronal membranes and preventing seizure activity.

Therapeutic Uses: Used in seizures and neuralgia.

Side Effects: Gingival hyperplasia, ataxia, and nystagmus.

Carbamazepine

Mechanism of Action: Carbamazepine blocks voltage-gated sodium channels, reducing neuronal excitability.

Therapeutic Uses: Used in epilepsy and bipolar disorder.

Side Effects: Dizziness, drowsiness, and hyponatremia.

Valproic Acid

Mechanism of Action: Valproic acid enhances GABA-mediated inhibition and blocks sodium channels.

Therapeutic Uses: Used in seizures, mania, and migraine prevention.

Side Effects: Hepatotoxicity, weight gain, and teratogenicity.

ANTIDEPRESSANTS

Antidepressants improve mood by increasing neurotransmitters like serotonin and norepinephrine in the brain. They are used in depression, anxiety disorders, and neuropathic pain.

Amitriptyline

Mechanism of Action: Amitriptyline is a tricyclic antidepressant that inhibits the reuptake of serotonin and norepinephrine.

Therapeutic Uses: Used in depression and neuropathic pain.

Side Effects: Sedation, dry mouth, and cardiac arrhythmias.

Fluoxetine

Mechanism of Action: Fluoxetine is a selective serotonin reuptake inhibitor (SSRI) that increases serotonin levels in the brain.

Therapeutic Uses: Used in depression and OCD.

Side Effects: Nausea, insomnia, and sexual dysfunction.

Sertraline

Mechanism of Action: Sertraline is an SSRI that increases serotonin levels.

Therapeutic Uses: Used in depression, PTSD, and anxiety disorders.

Side Effects: Nausea, diarrhoea, and insomnia.

COMPARISON OF CNS DRUGS

Drug Class	Example	Mechanism	Primary Use
General Anaesthetic	Propofol	GABA agonist	Anaesthesia induction
Sedative-Hypnotic	Diazepam	GABA agonist	Anxiety, seizures
Antipsychotic	Chlorpromazine	D2 antagonist	Schizophrenia
Anticonvulsant	Phenytoin	Sodium channel blocker	Seizures
Antidepressant	Fluoxetine	SSRI	Depression

A TEACHER’S PRACTICAL INSIGHTS

Over my years of teaching, I have developed a few key insights about CNS drugs that I always share with my students:

• General anaesthetics have a narrow therapeutic index and require careful monitoring.
• Benzodiazepines are safer than barbiturates for sedation and anxiety.
• Antipsychotics can cause serious side effects like extrapyramidal symptoms and metabolic syndrome.
• Antidepressants may take several weeks to show full therapeutic effects.

FREQUENTLY ASKED QUESTIONS (FAQs)

1. What are the stages of general anaesthesia?

The four stages are analgesia, delirium, surgical anaesthesia, and medullary paralysis.

2. What is the mechanism of action of benzodiazepines?

Benzodiazepines enhance GABA-mediated inhibition in the CNS.

3. What is the mechanism of action of antipsychotics?

Antipsychotics mainly block dopamine D2 receptors in the brain.

4. What is the mechanism of action of phenytoin?

Phenytoin blocks voltage-gated sodium channels, stabilising neuronal membranes.

5. What is the mechanism of action of SSRIs?

SSRIs selectively inhibit the reuptake of serotonin, increasing its levels in the brain.

6. What is the difference between sedation and hypnosis?

Sedation is a calming effect with reduced anxiety, while hypnosis induces sleep.

7. What are the side effects of antipsychotics?

Antipsychotics can cause extrapyramidal symptoms, sedation, and metabolic syndrome.

SUMMARY

Drugs acting on the central nervous system are essential for managing a wide range of conditions, including anaesthesia, anxiety, seizures, psychosis, and depression. General anaesthetics like propofol and ketamine are used for surgical procedures. Sedatives and hypnotics like diazepam and phenobarbital reduce anxiety and induce sleep. Antipsychotics like chlorpromazine and risperidone are used in schizophrenia and bipolar disorder. Anticonvulsants like phenytoin and carbamazepine prevent seizures. Antidepressants like fluoxetine and sertraline improve mood by increasing neurotransmitter levels.

Understanding these drugs is essential for pharmacy students to ensure their safe and effective use in clinical practice.

As I always tell my students: “The central nervous system is the most complex system in the body. Understanding its pharmacology is the key to managing many neurological and psychiatric conditions.”

REFERENCES & FURTHER READING

• Government of India. (1948). The Pharmacy Act, 1948. Ministry of Health and Family Welfare.
• Indian Pharmacopoeia Commission (IPC). (2023). Indian Pharmacopoeia. Retrieved from IPC Official Website.
• World Health Organization (WHO). (2023). WHO Model List of Essential Medicines. Retrieved from WHO Official Website.
• National Institute of Mental Health (NIMH). (2023). Mental Health Medications. Retrieved from NIMH Official Website.
• International Pharmaceutical Federation (FIP). (2023). Pharmacy Practice and CNS Drugs. Retrieved from FIP Official Website.

Disclaimer: This article is for educational purposes only and does not constitute medical or legal advice. Always consult qualified healthcare professionals and regulatory authorities for professional and legal matters.