Central Nervous System Pharmacology Practice Questions

Q: Which drug has inverse agonist activity at benzodiazepine receptors?

Beta carboline. **Explanation:** The benzodiazepine (BZD) receptor is a modulatory site on the **GABA-A receptor-chloride channel complex**. Drugs acting at this site are classified based on their intrinsic activity: 1. **Agonists (e.g., Diazepam):** Increase the frequency of chloride channel opening, leading to CNS depression (anxiolysis, sedation). 2. **Antagonists (e.g., Flumazenil):** Occupy the receptor with zero intrinsic activity, blocking the effects of both agonists and inverse agonists. 3. **Inverse Agonists (e.g., Beta-carbolines):** These produce effects **opposite** to those of BZD agonists. By reducing the frequency of chloride channel opening, they cause CNS stimulation, manifesting as intense anxiety, restlessness, and convulsions (pro-convulsant). **Analysis of Options:** * **B. Beta-carboline (Correct):** It is the classic example of a BZD inverse agonist. It decreases GABA-mediated chloride conductance. * **A. Flumazenil:** It is a **competitive antagonist** at BZD receptors. It is used to reverse BZD overdose but does not possess inverse agonist activity. * **C. Naltrexone:** It is an **opioid receptor antagonist** used in the management of opioid addiction and alcohol dependence; it has no action at BZD receptors. * **D. Zopiclone:** It is a **non-benzodiazepine hypnotic** ("Z-drug"). Despite a different chemical structure, it acts as a full **agonist** at the BZD-1 (alpha-1 subunit) receptor. **High-Yield Pearls for NEET-PG:** * **Flumazenil** has a shorter half-life than most BZDs; watch for "re-sedation" in clinical scenarios. * **Z-drugs** (Zolpidem, Zopiclone, Zaleplon) are preferred for insomnia because they preserve sleep architecture better than BZDs. * **Inverse agonists** are primarily used in research to study the GABA-A receptor complex and are not used clinically due to their convulsant properties.

Q: Which of the following is a known side effect of ketamine?

Hallucination. **Explanation:** **Ketamine** is a unique intravenous anesthetic agent that acts as a non-competitive antagonist at **NMDA receptors**. It induces a state known as **"Dissociative Anesthesia,"** where the patient appears awake (eyes open) but is unconscious and feels no pain. **Why Hallucinations are the Correct Answer:** The most characteristic side effect of ketamine is **Emergence Delirium**. During recovery, patients often experience vivid unpleasant dreams, sensory distortions, and **hallucinations**. This occurs due to the stimulation of the limbic system while cortical sensory perception is depressed. These effects are more common in adults than children and can be minimized by co-administering benzodiazepines like midazolam. **Why the Other Options are Incorrect:** * **B & C (Hypotension and Myocardial Depression):** Unlike most anesthetics (like propofol or thiopentone) which depress the heart, ketamine is a **sympathomimetic**. It inhibits the reuptake of catecholamines, leading to **increased heart rate, cardiac output, and blood pressure**. This makes it the induction agent of choice in patients with hypovolemic shock. * **D (Decrease in ICP):** Ketamine is a potent cerebral vasodilator. It **increases cerebral blood flow and intracranial pressure (ICP)**. Therefore, it is traditionally contraindicated in patients with head injuries or intracranial space-occupying lesions. **High-Yield Clinical Pearls for NEET-PG:** * **Drug of Choice:** For induction in asthmatics (due to its potent **bronchodilatory** effect) and in patients with compensated shock. * **Reflexes:** Pharyngeal and laryngeal reflexes are usually maintained. * **Secretions:** It increases salivation (sialagogue effect); pretreatment with atropine or glycopyrrolate is often required. * **Analgesia:** Provides profound analgesia even at sub-anesthetic doses.

Q: Which of the following is a long-acting barbiturate?

Phenobarbitone. **Explanation:** Barbiturates are classified based on their **duration of action**, which is primarily determined by their lipid solubility and metabolic rate [2]. **1. Why Phenobarbitone is Correct:** **Phenobarbitone** is a classic **long-acting barbiturate** with a duration of action typically exceeding 10–12 hours (half-life of 50–120 hours). It has low lipid solubility, meaning it enters the brain slowly and remains in the systemic circulation for a longer period [2]. It is primarily used as an antiepileptic (GTCS and status epilepticus) and for daytime sedation [1]. **2. Analysis of Incorrect Options:** * **Thiopentone (Option A):** This is an **ultra-short-acting** barbiturate. Due to its extremely high lipid solubility, it crosses the blood-brain barrier almost instantly, making it ideal for the induction of anesthesia. Its action is terminated rapidly by **redistribution** from the brain to skeletal muscle and fat [2]. * **Secobarbitone (Option B) & Pentobarbitone (Option D):** These are classified as **short-to-intermediate-acting** barbiturates (duration 3–8 hours). They are more lipid-soluble than phenobarbitone and are historically used as hypnotics or for preoperative sedation. **3. High-Yield Clinical Pearls for NEET-PG:** * **Mechanism:** Barbiturates increase the **duration** of GABA-A chloride channel opening (unlike benzodiazepines, which increase frequency) [1]. * **Metabolism:** They are potent **inducers of Cytochrome P450 enzymes**, leading to numerous drug interactions (e.g., decreasing the efficacy of Warfarin or Oral Contraceptives) [3]. * **Contraindication:** Absolutely contraindicated in **Acute Intermittent Porphyria** as they induce ALA synthase. * **Excretion:** Phenobarbitone is a weak acid; its excretion can be enhanced by **alkalinization of urine** with Sodium Bicarbonate in cases of toxicity [3].

Q: Which CB1 antagonist is used in smoking cessation?

Rimonabant. **Explanation:** **Correct Answer: B. Rimonabant** Rimonabant is a selective **CB1 receptor antagonist** (inverse agonist) located in the endocannabinoid system. The endocannabinoid system plays a crucial role in regulating energy balance and the reward pathways associated with nicotine and food. By blocking CB1 receptors in the brain, Rimonabant reduces the dopamine release in the nucleus accumbens triggered by nicotine, thereby decreasing the craving for smoking. It was also used for weight loss due to its effect on appetite suppression. **Analysis of Incorrect Options:** * **A. Naloxone:** This is a competitive **opioid receptor antagonist** used primarily for the emergency reversal of opioid overdose. It has no significant role in smoking cessation. * **C. Varenicline:** This is a **partial agonist at α4β2 nicotinic acetylcholine receptors**. While it is a first-line drug for smoking cessation, its mechanism involves nicotinic receptors, not CB1 receptors. * **D. Bupropion:** This is an atypical antidepressant that acts as a **Norepinephrine-Dopamine Reuptake Inhibitor (NDRI)**. It is used in smoking cessation to reduce withdrawal symptoms but does not act on cannabinoid receptors. **High-Yield Clinical Pearls for NEET-PG:** * **The "Rimonabant Withdrawal":** Although effective for smoking cessation and obesity, Rimonabant was withdrawn from the global market due to serious **psychiatric side effects**, including severe depression, anxiety, and increased suicidal ideation. * **CB1 vs. CB2:** CB1 receptors are primarily central (CNS), while CB2 receptors are primarily peripheral (immune cells). * **First-line Smoking Cessation:** Currently, Varenicline, Bupropion, and Nicotine Replacement Therapy (NRT) are the preferred agents.

Q: Which of the following local anesthetics has the lowest intrinsic potency?

Procaine. **Explanation:** The **intrinsic potency** of a local anesthetic (LA) is primarily determined by its **lipid solubility**. Since the neuronal membrane consists of a phospholipid bilayer, drugs with higher lipid solubility can penetrate the membrane more easily to reach the sodium channel binding sites, thus requiring a lower concentration to achieve a block. **Why Procaine is correct:** Procaine is an ester-linked local anesthetic with very low lipid solubility. It is considered the **standard reference (potency = 1)** against which other LAs are measured. Due to its low lipid solubility and rapid hydrolysis by plasma pseudocholinesterase, it has the lowest intrinsic potency and the shortest duration of action among the options provided. **Analysis of Incorrect Options:** * **Lidocaine:** An amide-linked LA with intermediate lipid solubility. It is significantly more potent than procaine (Potency ≈ 2-3). * **Prilocaine:** Also an amide with intermediate potency, similar to lidocaine. It is notably associated with methemoglobinemia at high doses. * **Tetracaine:** An ester-linked LA but with very high lipid solubility. It is one of the most potent local anesthetics (Potency ≈ 8-10), often used for spinal anesthesia. **NEET-PG High-Yield Pearls:** 1. **Potency ∝ Lipid Solubility:** Determined by the aromatic ring. 2. **Duration of Action ∝ Protein Binding:** Determined by the affinity for plasma proteins (e.g., Bupivacaine has high protein binding and long duration). 3. **Onset of Action ∝ pKa:** The closer the pKa is to physiological pH (7.4), the faster the onset (Exception: Chloroprocaine). 4. **Order of Potency (Low to High):** Procaine < Lidocaine < Bupivacaine < Tetracaine.

Q: Which of the following agents does not bind to GABA receptor chloride channels?

Buspirone. ### Explanation The **GABA-A receptor** is a ligand-gated chloride channel. Several classes of drugs act as positive allosteric modulators by binding to specific sites on this receptor complex to increase chloride conductance, leading to hyperpolarization and CNS depression. **Why Buspirone is the Correct Answer:** **Buspirone** is a non-benzodiazepine anxiolytic that does **not** act on the GABA system. Its mechanism of action involves acting as a **selective partial agonist at the 5-HT1A (Serotonin) receptor**. It lacks sedative, hypnotic, anticonvulsant, and muscle relaxant properties, making it unique among anxiolytics. **Analysis of Incorrect Options:** * **Ethanol (A):** Alcohol facilitates GABA-A receptor-mediated chloride currents by binding to a specific transmembrane site, contributing to its sedative-hypnotic effects. * **Alphaxolone (B):** This is a synthetic neurosteroid used in anesthesia. Neurosteroids bind to a distinct site on the GABA-A receptor (different from the benzodiazepine site) to enhance chloride channel opening. * **Zolpidem (C):** A "Z-drug" (non-benzodiazepine sedative), it binds selectively to the **α1 subunit** of the GABA-A receptor (BZ1 site). **High-Yield Clinical Pearls for NEET-PG:** 1. **Buspirone Lag Time:** Unlike benzodiazepines, Buspirone takes **2–4 weeks** to show therapeutic effects; it is used for Generalized Anxiety Disorder (GAD) but not for acute anxiety or panic attacks. 2. **No Abuse Potential:** Buspirone does not cause physical dependence or withdrawal symptoms and does not potentiate the effects of alcohol. 3. **GABA-A vs. GABA-B:** Remember that GABA-A is ionotropic (Chloride channel), while GABA-B is metabotropic (G-protein coupled, linked to Potassium channels). **Baclofen** is the prototype GABA-B agonist.

Q: Which local anesthetic agent has antimuscarinic action on heart muscle receptors?

Cocaine. **Explanation:** **Cocaine** is unique among local anesthetics because of its complex sympathomimetic and parasympatholytic properties. While most local anesthetics are vasodilators and can cause bradycardia at high doses, cocaine is a potent vasoconstrictor and causes tachycardia. It exerts an **antimuscarinic (atropine-like) action** on the heart muscle receptors, which blocks the inhibitory effects of the vagus nerve. Additionally, it inhibits the reuptake of norepinephrine (Uptake-1) at sympathetic nerve endings. The combination of these two mechanisms leads to significant tachycardia, hypertension, and potential arrhythmias. **Analysis of Incorrect Options:** * **Procaine:** An ester-linked local anesthetic with a short duration of action. It lacks antimuscarinic properties and typically causes mild vasodilation. * **Chloroprocaine:** A halogenated derivative of procaine known for its rapid onset and low systemic toxicity due to fast metabolism by plasma cholinesterase. It does not possess antimuscarinic activity. **High-Yield Clinical Pearls for NEET-PG:** * **Vasoconstriction:** Cocaine is the **only** local anesthetic that naturally causes vasoconstriction (all others, except perhaps ropivacaine/levobupivacaine at low doses, are vasodilators). * **Metabolism:** Like other esters, cocaine is metabolized by **plasma pseudocholinesterase**. * **Toxicity:** Cocaine overdose is managed with benzodiazepines; **pure beta-blockers are contraindicated** due to the risk of "unopposed alpha-stimulation," which can lead to severe hypertension and coronary vasospasm. * **Surface Anesthesia:** Cocaine is primarily used in ENT surgeries for its excellent topical anesthesia and shrinking of mucous membranes.

Q: What drug is given for a metoclopramide-induced dystonic reaction?

Promethazine. **Explanation:** **Mechanism of Action:** Metoclopramide is a potent **D2 receptor antagonist**. By blocking dopamine in the nigrostriatal pathway, it creates a chemical imbalance where cholinergic (acetylcholine) activity becomes relatively overactive. This results in **Extrapyramidal Side Effects (EPS)**, most commonly **acute dystonia** (e.g., torticollis, facial grimacing, or oculogyric crisis). To treat this, a drug with strong **central anticholinergic properties** is required to restore the dopamine-acetylcholine balance. **Why Promethazine is Correct:** **Promethazine** is a first-generation antihistamine that possesses potent **antimuscarinic (anticholinergic)** activity. It effectively crosses the blood-brain barrier to antagonize the excess cholinergic activity, providing rapid relief from dystonic spasms. **Analysis of Incorrect Options:** * **Pheniramine:** While it is an antihistamine, its central anticholinergic potency is significantly lower than promethazine or diphenhydramine, making it less effective for acute EPS. * **Chlorpropamide:** This is a first-generation sulfonylurea used in Diabetes Mellitus; it has no role in treating movement disorders. * **Prochlorperazine:** This is a dopamine antagonist (antipsychotic/antiemetic) itself. Giving it would **worsen** the dystonia by further blocking D2 receptors. **NEET-PG High-Yield Pearls:** 1. **Drug of Choice (DOC):** For drug-induced acute dystonia, the parenteral DOC is usually **Benztropine** or **Diphenhydramine**. In the Indian context, **Promethazine** is frequently used and tested. 2. **Akathisia:** The most common EPS of metoclopramide; managed by Propropanol. 3. **Tardive Dyskinesia:** A late-onset EPS; managed by Valbenazine or Deutetrabenazine (VMAT2 inhibitors). 4. **Avoid in Children:** Metoclopramide is prone to causing dystonia in children and young adults; use with caution.

Q: What is the drug of choice for insomnia in night shift workers?

Modafinil. The question addresses the management of **Shift Work Sleep Disorder (SWSD)**, a circadian rhythm sleep disorder characterized by excessive sleepiness during night shifts and insomnia during the day [1, 2]. **Why Modafinil is the Correct Answer:** Modafinil is a non-amphetamine **eugeroic (wakefulness-promoting agent)**. It is the FDA-approved drug of choice for excessive daytime sleepiness associated with narcolepsy, obstructive sleep apnea, and SWSD. Its mechanism involves increasing synaptic concentrations of dopamine by inhibiting reuptake and modulating the hypothalamic **orexin/hypocretin** system, which regulates the sleep-wake cycle. Unlike traditional stimulants, it has a lower potential for abuse and fewer sympathomimetic side effects. **Analysis of Incorrect Options:** * **A. Methylphenidate:** A potent CNS stimulant used primarily for ADHD and narcolepsy. It carries a high risk of addiction and cardiovascular side effects, making it a second-line choice compared to Modafinil. * **C. Amitriptyline:** A Tricyclic Antidepressant (TCA) with sedative properties due to H1-receptor blockade. While it helps with sleep onset, it is not used to manage the primary issue of shift-work alertness and has significant anticholinergic side effects. * **D. Adrenaline:** A catecholamine used in anaphylaxis and cardiac arrest; it has no role in the chronic management of sleep disorders. **High-Yield Clinical Pearls for NEET-PG:** * **Armodafinil** is the R-enantiomer of modafinil with a longer half-life, also used for SWSD. * **Side Effect:** A rare but serious side effect of Modafinil is **Stevens-Johnson Syndrome (SJS)**. * **Neuroanatomy Link:** The **Suprachiasmatic Nucleus (SCN)** in the hypothalamus is the "master clock" that is disrupted in shift workers. * **Melatonin** is often used as an adjunct to help shift workers sleep during the day, but Modafinil is the gold standard for maintaining alertness during the shift.

Question 1

Which drug has inverse agonist activity at benzodiazepine receptors?

Accepted Answer

Beta carboline

Answer

Flumazenil

Answer

Naltrexone

Answer

Zopiclone

Question 2

Which of the following is a known side effect of ketamine?

Accepted Answer

Hallucination

Answer

Hypotension

Answer

Myocardial depression

Answer

Decrease in intracranial pressure

Question 3

Which of the following is a long-acting barbiturate?

Accepted Answer

Phenobarbitone

Answer

Thiopentone

Answer

Secobarbitone

Answer

Pentobarbitone

Question 4

Which CB1 antagonist is used in smoking cessation?

Accepted Answer

Rimonabant

Answer

Naloxona

Answer

Varenicline

Answer

Bupropion

Question 5

Which of the following local anesthetics has the lowest intrinsic potency?

Accepted Answer

Procaine

Answer

Lidocaine

Answer

Prilocaine

Answer

Tetracaine

Question 6

Which of the following agents does not bind to GABA receptor chloride channels?

Accepted Answer

Buspirone

Answer

Ethanol

Answer

Alphaxolone

Answer

Zolpidem

Question 7

Which local anesthetic agent has antimuscarinic action on heart muscle receptors?

Accepted Answer

Cocaine

Answer

Procaine

Answer

Chloroprocaine

Answer

None of the above

Question 8

What drug is given for a metoclopramide-induced dystonic reaction?

Accepted Answer

Promethazine

Answer

Pheniramine

Answer

Chlorpropamide

Answer

Prochlorperazine

Question 9

Which of the following is used with local anesthetics to potentiate their action?

Accepted Answer

Sodium bicarbonate

Answer

Adrenaline

Answer

Both adrenaline and sodium bicarbonate

Answer

Dantrolene

Question 10

What is the drug of choice for insomnia in night shift workers?

Accepted Answer

Modafinil

Answer

Methylphenidate

Answer

Amitriptyline

Answer

Adrenaline

Central Nervous System Pharmacology — MCQs

Central Nervous System Pharmacology — MCQs

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