Central Nervous System Pharmacology — MCQs

Central Nervous System Pharmacology Indian Medical PG Practice Questions and MCQs

Question 311: Beta-carboline at benzodiazepine receptors acts as?

A. Agonist
B. Antagonist
C. Inverse agonist (Correct Answer)
D. Physiological antagonist

Explanation: **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. **Inverse Agonists (Beta-carbolines):** These bind to the BZD receptor but produce effects **opposite** to those of benzodiazepines. While BZDs increase chloride channel opening frequency (facilitating GABA), beta-carbolines decrease it. This results in **anxiogenic** (anxiety-inducing) and **pro-convulsant** effects. 2. **Agonists (Benzodiazepines like Diazepam):** These facilitate GABA action, leading to sedation, anxiolysis, and anticonvulsant effects. 3. **Antagonists (Flumazenil):** These have no intrinsic activity but competitively block both agonists and inverse agonists. Flumazenil is used clinically to reverse BZD overdose. 4. **Physiological Antagonists:** This term refers to two drugs acting on different receptors to produce opposite physiological effects (e.g., Histamine and Adrenaline on bronchial smooth muscle). Beta-carbolines act on the *same* receptor as BZDs, making this option incorrect. **High-Yield Clinical Pearls for NEET-PG:** * **Flumazenil** is the specific antidote for BZD poisoning but can precipitate seizures in chronic BZD users or TCA overdose. * **GABA-A** is an ionotropic receptor (chloride channel), while **GABA-B** is a metabotropic G-protein coupled receptor (baclofen acts here). * **Z-drugs** (Zolpidem, Zopiclone) are selective agonists at the **alpha-1 subunit** of the GABA-A receptor, primarily used for hypnosis without significant anxiolysis.

Question 312: Which of the following antiepileptic drugs can be used to treat neonatal jaundice?

A. Phenytoin
B. Phenobarbital (Correct Answer)
C. Sodium valproate
D. Vigabatrin

Explanation: **Explanation:** **Phenobarbital** is the correct answer because of its potent ability to act as a **microsomal enzyme inducer**. In the context of neonatal jaundice, it induces the hepatic enzyme **UDP-glucuronosyltransferase (UGT1A1)**. This enzyme is responsible for the conjugation of free (unconjugated) bilirubin with glucuronic acid, making it water-soluble and excretable. By increasing the synthesis of this enzyme and Y-binding proteins (ligandin), phenobarbital enhances the clearance of bilirubin, particularly in conditions like **Crigler-Najjar Syndrome Type II** and occasionally in severe physiological jaundice. **Incorrect Options:** * **Phenytoin:** While it is a hepatic enzyme inducer, it does not significantly target the UGT1A1 pathway required for bilirubin conjugation and is associated with potential toxicity in neonates. * **Sodium Valproate:** This drug is an **enzyme inhibitor**. It would theoretically worsen jaundice by inhibiting metabolic pathways and carries a high risk of hepatotoxicity. * **Vigabatrin:** This drug acts by irreversibly inhibiting GABA transaminase. It has no significant effect on hepatic microsomal enzymes or bilirubin metabolism. **High-Yield Clinical Pearls for NEET-PG:** * **Drug of Choice:** Phenobarbital remains the first-line drug for **Neonatal Seizures**. * **Crigler-Najjar Syndrome:** Phenobarbital is effective in **Type II** (partial enzyme deficiency) but ineffective in **Type I** (total enzyme deficiency). * **Adverse Effect:** A key side effect in children is **hyperactivity/behavioral changes**, whereas in adults, it causes sedation. * **Metabolism:** It follows first-order kinetics (unlike Phenytoin, which follows zero-order at high doses).

Question 313: All of the following are therapeutic effects for which cannabinoids are commonly used, except?

A. Decreased intraocular pressure
B. Reduced muscle spasticity
C. Relief of nausea and vomiting
D. Weight loss (Correct Answer)

Explanation: **Explanation:** The correct answer is **D. Weight loss**. Cannabinoids, specifically those acting on **CB1 receptors** in the hypothalamus and mesolimbic system, are potent **appetite stimulants** (orexigenic effect). This leads to increased food intake and weight gain, often referred to as "the munchies." In clinical practice, synthetic cannabinoids like **Dronabinol** are FDA-approved to treat anorexia and weight loss associated with AIDS. Conversely, CB1 receptor *antagonists* (e.g., Rimonabant) were once studied for weight loss but were withdrawn due to psychiatric side effects. **Analysis of Incorrect Options:** * **A. Decreased intraocular pressure:** Cannabinoids effectively lower intraocular pressure (IOP) by increasing aqueous humor outflow and potentially decreasing production. While not first-line due to systemic side effects, this is a recognized pharmacological effect. * **B. Reduced muscle spasticity:** Cannabinoids (e.g., **Nabiximols**) are used as adjunctive therapy for symptomatic relief of spasticity in patients with **Multiple Sclerosis** who have not responded to other anti-spasticity medications. * **C. Relief of nausea and vomiting:** Cannabinoids possess significant antiemetic properties. Dronabinol and Nabilone are used specifically for **Chemotherapy-Induced Nausea and Vomiting (CINV)** in patients who fail to respond to conventional antiemetics. **High-Yield Clinical Pearls for NEET-PG:** * **Receptors:** CB1 (mainly CNS/Presynaptic inhibition) and CB2 (mainly Peripheral/Immune cells). * **Dronabinol:** Synthetic Δ9-THC used for CINV and AIDS-related cachexia. * **Nabiximols (Sativex):** 1:1 ratio of THC:CBD, used for MS spasticity. * **Epidiolex:** Purified CBD used for refractory pediatric seizures (Lennox-Gastaut and Dravet syndromes).

Question 314: A patient on antipsychotic drugs develops a temperature of 104°F, a BP of approximately 150/100 mmHg, and abnormal behavior. What is the likely diagnosis?

A. Aggravation of psychosis
B. Parkinsonism
C. Dystonia
D. Neuroleptic malignant syndrome (Correct Answer)

Explanation: ### Explanation The patient is presenting with the classic triad of **Neuroleptic Malignant Syndrome (NMS)**: hyperpyrexia (104°F), autonomic instability (hypertension), and altered mental status (abnormal behavior). **1. Why Neuroleptic Malignant Syndrome is correct:** NMS is a life-threatening idiosyncratic reaction to dopamine antagonists (antipsychotics). It is characterized by the **"FEVER"** mnemonic: **F**ever, **E**ncephalopathy (abnormal behavior), **V**itals unstable, **E**levated enzymes (CPK), and **R**igidity ("lead-pipe" type). The underlying pathophysiology involves massive dopamine blockade in the hypothalamus (causing hyperthermia) and the basal ganglia (causing rigidity). **2. Why the other options are incorrect:** * **Aggravation of psychosis:** While it explains abnormal behavior, it does not account for high-grade fever or autonomic instability. * **Parkinsonism:** This is an Extrapyramidal Side Effect (EPS) characterized by tremors, bradykinesia, and rigidity, but it lacks the systemic features of high fever and hemodynamic instability. * **Dystonia:** This involves acute muscle spasms (e.g., torticollis, oculogyric crisis) occurring shortly after starting antipsychotics. It is localized and does not cause systemic collapse. **3. NEET-PG High-Yield Pearls:** * **Drug of Choice:** **Dantrolene** (a muscle relaxant that acts on Ryanodine receptors) or **Bromocriptine** (a dopamine agonist). * **Key Lab Finding:** Significantly elevated **Creatine Phosphokinase (CPK)** due to muscle rigidity. * **Differential Diagnosis:** **Serotonin Syndrome** (presents with hyperreflexia and myoclonus, whereas NMS has "lead-pipe" rigidity and bradyreflexia). * **Most common causative agent:** Haloperidol (High-potency typical antipsychotics).

Question 315: Which antipsychotic drug has the least extrapyramidal side effect?

A. Triflupromazine
B. Thioridazine (Correct Answer)
C. Pimozide
D. Trifluoperazine

Explanation: **Explanation:** The occurrence of **Extrapyramidal Side Effects (EPS)** in antipsychotic therapy is determined by the balance between **Dopaminergic (D2) blockade** and **Antimuscarinic (M1) activity** in the nigrostriatal pathway. **Why Thioridazine is correct:** Thioridazine is a low-potency typical antipsychotic. It possesses **inherent high central anticholinergic activity**. Since EPS results from a relative deficiency of dopamine and an excess of acetylcholine in the basal ganglia, the strong anticholinergic property of Thioridazine "self-neutralizes" the dopaminergic blockade, leading to the **lowest incidence of EPS** among typical antipsychotics. **Analysis of Incorrect Options:** * **Triflupromazine & Trifluoperazine:** These are high-potency piperazine phenothiazines. They have strong D2 receptor affinity but very weak anticholinergic properties, making them highly likely to cause severe EPS (dystonias, parkinsonism). * **Pimozide:** This is a potent diphenylbutylpiperidine used primarily for Tourette’s syndrome. Like other high-potency agents, it has a high propensity for inducing EPS. **High-Yield Clinical Pearls for NEET-PG:** * **The "Thio" Rule:** Thioridazine has the *least* EPS but is associated with unique side effects: **Retinal pigmentation** (at doses >800mg/day) and significant **QTc prolongation** (cardiotoxicity). * **Most EPS:** Haloperidol and Fluphenazine (High potency). * **Atypical Antipsychotics:** Among all antipsychotics (including atypicals), **Clozapine** has the absolute lowest risk of EPS. * **Mechanism:** EPS occurs when D2 receptor occupancy in the striatum exceeds **80%**.

Question 316: All of the following act on the GABA receptor except?

A. Thiopentone
B. Propofol
C. Zolpidem
D. Ketamine (Correct Answer)

Explanation: **Explanation:** The correct answer is **Ketamine**. The primary mechanism of action for Ketamine is the non-competitive antagonism of **NMDA (N-methyl-D-aspartate) receptors**, which are excitatory glutamate receptors. Unlike most other intravenous anesthetics, Ketamine does not act through the GABAergic system; instead, it produces "dissociative anesthesia" by interrupting transmission between the thalamocortical and limbic systems. **Analysis of Options:** * **Thiopentone (Option A):** A barbiturate that acts by increasing the **duration** of GABA-A receptor channel opening. At higher doses, it can directly activate the receptor (GABA-mimetic). * **Propofol (Option B):** Acts primarily by facilitating inhibitory neurotransmission through **GABA-A receptors**. It increases the binding affinity of GABA and, at high concentrations, directly activates the chloride channel. * **Zolpidem (Option C):** A non-benzodiazepine hypnotic ("Z-drug") that selectively binds to the **α1 subunit of the GABA-A receptor** (BZ1 site), mediating its sedative-hypnotic effects. **High-Yield Clinical Pearls for NEET-PG:** * **Ketamine "Triple High":** It is unique because it increases **BP (sympathomimetic)**, **Heart Rate**, and **Intracranial Pressure (ICP)**. It is the induction agent of choice in hypovolemic shock but contraindicated in head injuries. * **Dissociative Anesthesia:** Characterized by catalepsy, amnesia, and profound analgesia while the patient appears awake (eyes open). * **Emergence Delirium:** A common side effect of Ketamine, which can be prevented by pre-medication with Benzodiazepines (e.g., Midazolam).

Question 317: Which drug acts through the alpha subunit of the GABA receptor?

A. Benzodiazepine (Correct Answer)
B. Barbiturate
C. Haloperidol
D. TCA

Explanation: **Explanation:** The GABA-A receptor is a ligand-gated chloride channel composed of five subunits (typically $2\alpha$, $2\beta$, and $1\gamma$). The correct answer is **Benzodiazepines (BZDs)** because they bind specifically to the interface of the **$\alpha$ and $\gamma$ subunits**. This binding increases the **frequency** of chloride channel opening in the presence of GABA, leading to hyperpolarization and CNS depression. **Analysis of Options:** * **Benzodiazepines (A):** Bind to the $\alpha/\gamma$ subunit interface. They are positive allosteric modulators that require GABA to be present to exert their effect. * **Barbiturates (B):** Bind to the **$\beta$ subunit** of the GABA-A receptor. Unlike BZDs, they increase the **duration** of chloride channel opening and can act as GABA-mimetics at high doses, making them more toxic in overdose. * **Haloperidol (C):** An antipsychotic that primarily acts as a **D2 receptor antagonist** in the mesolimbic pathway; it has no direct action on GABA subunits. * **TCA (D):** Tricyclic antidepressants primarily inhibit the reuptake of **Norepinephrine and Serotonin (5-HT)**; they do not act through GABA receptors. **High-Yield Clinical Pearls for NEET-PG:** * **BZD Antagonist:** Flumazenil (competitive antagonist at the BZD binding site). * **Z-drugs (Zolpidem, Zaleplon):** Bind selectively to the **$\alpha_1$ subunit** of the GABA-A receptor, which mediates sedation (hence used as hypnotics). * **$\alpha_2$ and $\alpha_3$ subunits:** Mediate the anxiolytic and muscle relaxant effects of BZDs. * **Barbiturate vs. BZD:** Remember "Barbi-**dur**-ate" (increases **dur**ation) and "Ben-**fre**-diazepine" (increases **fre**quency).

Question 318: What are the clinical uses of Amantadine?

A. Prophylaxis and treatment of influenza A
B. Treatment of Parkinson's symptoms
C. Treatment of drug-induced extrapyramidal symptoms
D. All of the above (Correct Answer)

Explanation: **Explanation:** Amantadine is a unique pharmacological agent with a dual role as both an antiviral and a dopaminergic drug. Its multifaceted mechanism of action makes it effective for all the conditions listed in the options. 1. **Antiviral Action (Option A):** Amantadine inhibits the **M2 ion channel protein** of the Influenza A virus. This prevents the "uncoating" of the viral RNA within the host cell, thereby inhibiting viral replication. While its use has declined due to widespread resistance, it remains a classic indication for the prophylaxis and treatment of **Influenza A** (not B). 2. **Antiparkinsonian Action (Option B & C):** Amantadine increases dopamine release from presynaptic terminals, inhibits dopamine reuptake, and possesses mild **anticholinergic** properties. Most importantly, it acts as a **low-affinity non-competitive NMDA receptor antagonist**. This helps in: * Improving bradykinesia and rigidity in **Parkinson’s disease**. * Managing **Drug-Induced Extrapyramidal Symptoms (EPS)**, such as pseudoparkinsonism caused by antipsychotics. * Reducing **Levodopa-induced dyskinesias** (a high-yield clinical use). **Why "All of the above" is correct:** Since Amantadine targets both viral replication (M2 protein) and the dopaminergic/glutamatergic systems (NMDA antagonism), it is clinically utilized for influenza, Parkinson’s disease, and EPS. **High-Yield Clinical Pearls for NEET-PG:** * **Livedo Reticularis:** A characteristic side effect of Amantadine presenting as a reddish-blue net-like mottling of the skin. * **Excretion:** It is excreted unchanged by the kidneys; dose adjustment is mandatory in renal failure. * **Drug of Choice:** It is specifically preferred for managing **Levodopa-induced dyskinesias** in advanced Parkinson's.

Question 319: Carbamazepine is NOT used in which of the following conditions?

A. Mania
B. Partial seizure
C. Trigeminal neuralgia
D. Migraine (Correct Answer)

Explanation: Explanation: **Carbamazepine** is a sodium channel blocker that stabilizes hyperexcitable neuronal membranes [1], [4]. While it is a versatile drug in neurology and psychiatry, it has no established role in the acute treatment or prophylaxis of **Migraine** [2]. **Why Migraine is the Correct Answer:** The first-line drugs for migraine prophylaxis include Beta-blockers (Propranolol), Anticonvulsants (Topiramate, Valproate), Tricyclic Antidepressants (Amitriptyline), and Calcium Channel Blockers (Flunarizine) [2]. Carbamazepine is not effective for migraine and may even exacerbate headaches in some patients. **Analysis of Incorrect Options:** * **Mania:** Carbamazepine is a recognized **mood stabilizer** [5]. It is used as a second-line agent in Acute Mania and Prophylaxis of Bipolar Disorder, especially in patients who do not respond to Lithium [3]. * **Partial Seizure:** It is a **drug of choice (DOC)** for Focal (Partial) seizures and Generalized Tonic-Clonic Seizures (GTCS) [3]. It works by prolonging the inactivated state of voltage-gated sodium channels [5]. * **Trigeminal Neuralgia:** Carbamazepine is the **DOC** for Trigeminal Neuralgia [1]. It effectively reduces the paroxysmal neuropathic pain associated with this condition. **High-Yield Clinical Pearls for NEET-PG:** * **Mechanism:** Blocks voltage-gated $Na^+$ channels [4]. * **Pharmacokinetics:** It is a potent **enzyme inducer** and exhibits **auto-induction** (induces its own metabolism). * **Side Effects:** Most common is diplopia/ataxia. Most serious include **Aplastic Anemia** and **Stevens-Johnson Syndrome** (associated with HLA-B*1502 allele). * **Other Uses:** Glossopharyngeal neuralgia and Diabetic Neuropathy. * **Contraindication:** It can **worsen Absence and Myoclonic seizures**.

Question 320: Which of the following agents binds to the GABA receptor chloride channel complex?

A. Ethanol
B. Alphaxolone
C. Zolpidem (Correct Answer)
D. Buspirone

Explanation: **Explanation:** The **GABA-A receptor** is a ligand-gated chloride channel complex. It contains specific binding sites for various drugs that modulate inhibitory neurotransmission [1], [2]. **1. Why Zolpidem is Correct:** **Zolpidem** is a non-benzodiazepine hypnotic (Z-drug). It binds selectively to the **α1 subunit** of the GABA-A receptor chloride channel complex (specifically at the benzodiazepine-1 or BZ1 site) [1]. By binding here, it increases the frequency of chloride channel opening, leading to hyperpolarization and a sedative-hypnotic effect [2]. **2. Analysis of Incorrect Options:** * **Ethanol (A):** While ethanol facilitates GABAergic transmission, its primary mechanism involves non-specific fluidization of membranes and modulation of multiple targets (NMDA, 5-HT3). It does not have a specific, high-affinity "binding site" on the GABA complex in the same way as Z-drugs or benzodiazepines. * **Alphaxolone (B):** This is a neurosteroid anesthetic. While neurosteroids do modulate GABA-A receptors, Alphaxolone is primarily used in veterinary medicine and is not the standard answer for human pharmacology questions regarding the GABA-chloride complex unless specified as a neurosteroid. * **Buspirone (D):** This is an anxiolytic that acts as a **selective 5-HT1A partial agonist**. It has no action on the GABA receptor complex, which is why it lacks sedative, anticonvulsant, or muscle relaxant properties [3]. **High-Yield Clinical Pearls for NEET-PG:** * **Z-drugs (Zolpidem, Zaleplon, Eszopiclone):** Preferred for insomnia due to minimal "hangover" effect and low potential for dependence [1]. * **Antidote:** **Flumazenil** reverses the effects of both Benzodiazepines and Zolpidem because they share overlapping binding domains [1]. * **Barbiturates vs. Benzodiazepines:** Barbiturates increase the **duration** of chloride channel opening, while Benzodiazepines/Zolpidem increase the **frequency** [2].

Practice by Chapter

General Anesthetics

Practice Questions

Local Anesthetics

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Sedative-Hypnotics

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Antiepileptic Drugs

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Antiparkinsonian Drugs

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Opioid Analgesics

Practice Questions

Drugs of Abuse and Addiction

Practice Questions

Psychostimulants

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Hallucinogens

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CNS Stimulants and Cognitive Enhancers

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