Central Nervous System Pharmacology Practice Questions

Q: Which neurotransmitter is primarily responsible for the action of Barbiturates?

GABA. **Explanation:** **1. Why GABA is correct:** Barbiturates act as positive allosteric modulators of the **GABA-A receptor** complex. They bind to a specific site on the chloride channel, distinct from the benzodiazepine binding site. Their primary mechanism involves **increasing the duration** of chloride channel opening (mnemonic: *Barbi-DUR-ate* for duration). At higher concentrations, barbiturates can also act as **GABA-mimetics**, directly activating the receptor even in the absence of GABA, which contributes to their lower therapeutic index and higher risk of fatal overdose compared to benzodiazepines. **2. Why the other options are incorrect:** * **Glutamate & Aspartate:** These are the primary **excitatory** neurotransmitters in the CNS. While barbiturates do inhibit glutamate receptors (specifically AMPA receptors) at high doses, this is a secondary effect. Their sedative-hypnotic action is fundamentally mediated through GABA. * **Glycine:** This is the major inhibitory neurotransmitter in the **spinal cord** and brainstem. Barbiturates do not primarily target glycine receptors to exert their central sedative effects. **3. High-Yield Clinical Pearls for NEET-PG:** * **Mechanism Difference:** Benzodiazepines increase the **frequency** of channel opening, whereas Barbiturates increase the **duration**. * **Enzyme Induction:** Barbiturates are potent **CYP450 enzyme inducers**, leading to numerous drug interactions (e.g., decreasing the efficacy of Warfarin or Oral Contraceptives). * **Contraindication:** They are strictly contraindicated in **Acute Intermittent Porphyria** because they induce ALA synthase, the rate-limiting enzyme in porphyrin synthesis. * **Antidote:** Unlike benzodiazepines (Flumazenil), there is **no specific antidote** for barbiturate poisoning; management is purely supportive (alkalinization of urine for Phenobarbital).

Q: All of the following act through GABA except?

Carbamazepine. **Explanation:** The question tests the understanding of the mechanisms of action of various CNS drugs, specifically focusing on the GABAergic system. **Why Carbamazepine is the correct answer:** Carbamazepine is an anticonvulsant that primarily acts by **blocking voltage-gated sodium channels** in their inactivated state. This stabilizes neuronal membranes and inhibits repetitive firing. It does **not** have a direct or indirect effect on the GABA receptor complex. **Analysis of incorrect options:** * **Phenobarbitone:** This is a barbiturate that acts as a positive allosteric modulator of the **GABA-A receptor**. It increases the **duration** of chloride channel opening and, at higher doses, can directly mimic GABA (GABA-mimetic). * **Zopiclone:** This is a "Z-drug" (non-benzodiazepine hypnotic). It acts selectively on the **α1 subunit of the GABA-A receptor** to enhance the inhibitory effects of GABA, similar to benzodiazepines but with a different chemical structure. * **Muscimol:** This is a potent, selective **GABA-A receptor agonist** derived from the mushroom *Amanita muscaria*. It binds directly to the same site as the endogenous neurotransmitter GABA. **High-Yield Clinical Pearls for NEET-PG:** * **Barbiturates vs. Benzodiazepines:** Barbiturates increase the *duration* of Cl- channel opening, while Benzodiazepines increase the *frequency* (Mnemonic: **Ben**zodiazepines = **Fr**equency; **Bar**biturates = **Dur**ation). * **Carbamazepine** is the drug of choice for **Trigeminal Neuralgia**. * **Zopiclone/Zolpidem** are preferred for insomnia because they cause less disruption of sleep architecture and have a lower potential for dependence compared to benzodiazepines. * **GABA-B Agonist:** Remember that **Baclofen** acts on GABA-B receptors (G-protein coupled) to treat spasticity.

Q: Which of the following is an antagonist of benzodiazepines?

Flumazenil. ### Explanation **Correct Option: D. Flumazenil** Flumazenil is a competitive antagonist at the **GABA-A receptor** benzodiazepine (BZD) binding site. It specifically reverses the sedative, psychomotor, and anticonvulsant effects of benzodiazepines and "Z-drugs" (Zolpidem, Zopiclone). It is the drug of choice for managing BZD overdose and reversing anesthesia induced by midazolam. **Analysis of Incorrect Options:** * **A. Nalorphine:** This is a mixed opioid agonist-antagonist. While it can antagonize opioid effects, it is rarely used today due to its psychotomimetic side effects. * **B. Carbamazepine:** This is an anticonvulsant and mood stabilizer that works primarily by blocking **voltage-gated sodium channels**. It has no antagonistic activity at the BZD receptor. * **C. Naloxone:** This is a pure **opioid receptor antagonist**. It is the specific antidote for opioid overdose (e.g., morphine, heroin) but has no effect on BZD-induced respiratory depression. **High-Yield Clinical Pearls for NEET-PG:** * **Mechanism:** Flumazenil has a high affinity for the BZD site but lacks intrinsic activity (neutral antagonist). * **Duration of Action:** Flumazenil has a very **short half-life** (~1 hour). Since most BZDs (like Diazepam) last longer, "re-sedation" can occur, necessitating repeated doses or an infusion. * **Contraindication:** Avoid flumazenil in patients with long-term BZD dependence or those who have co-ingested TCAs, as it can precipitate **acute withdrawal seizures**. * **Z-drugs:** Remember that Flumazenil also reverses non-benzodiazepine hypnotics like **Zolpidem and Zaleplon**.

Q: Which antiepileptic drug can cause folate deficiency anemia?

Phenytoin. **Explanation:** **Phenytoin** is the correct answer because it is a well-known inducer of **folate deficiency**, which can lead to **megaloblastic anemia**. The underlying mechanism involves Phenytoin’s ability to interfere with the intestinal absorption of dietary folates and increase the hepatic metabolism of folate by inducing microsomal enzymes. Chronic use leads to depleted folate stores, resulting in impaired DNA synthesis in red blood cell precursors. **Analysis of Options:** * **Valproate (A):** While Valproate is associated with bone marrow suppression (thrombocytopenia) and is highly teratogenic (neural tube defects due to folate interference in pregnancy), it is not the classic cause of megaloblastic anemia in routine clinical practice compared to Phenytoin. * **Phenobarbitone (C) & Carbamazepine (D):** Both are enzyme inducers and can occasionally lower folate levels, but they are significantly less likely to cause clinically symptomatic megaloblastic anemia than Phenytoin. **High-Yield Clinical Pearls for NEET-PG:** * **Mnemonic for Phenytoin Side Effects (PHENYTOIN):** **P**-P450 induction, **H**-Hirsutism, **E**-Enlarged gums (Gingival hyperplasia), **N**-Nystagmus, **Y**-Yellow-brown skin (pigmentation), **T**-Teratogenicity (Fetal Hydantoin Syndrome), **O**-Osteomalacia (Vitamin D interference), **I**-Interference with B9 (Megaloblastic Anemia), **N**-Neuropathy. * **Management:** Patients on long-term Phenytoin therapy should be monitored for macrocytosis and often require supplemental **Folic Acid**. * **Gum Hyperplasia:** This occurs due to increased expression of Platelet-Derived Growth Factor (PDGF).

Q: Which of the following agents enhances the bioavailability of levodopa in patients with Parkinson's disease?

Entacapone. **Explanation:** The correct answer is **Entacapone**. **Why Entacapone is correct:** Levodopa is the precursor to dopamine. When administered orally, it is rapidly metabolized in the periphery by two enzymes: **DOPA decarboxylase (DDC)** and **Catechol-O-methyltransferase (COMT)**. While Carbidopa inhibits DDC, it leads to a compensatory increase in COMT activity, which converts levodopa into 3-O-methyldopa. **Entacapone** is a selective, reversible **peripheral COMT inhibitor**. By blocking this pathway, it prevents the peripheral degradation of levodopa, thereby increasing its plasma half-life and **bioavailability** to the brain. This helps reduce "off" periods in patients with motor fluctuations. **Why the other options are incorrect:** * **Amantadine (A):** An antiviral drug that acts by increasing dopamine release and blocking NMDA receptors. It does not affect levodopa metabolism. * **Ropinirole (B):** A non-ergot **Dopamine Agonist** that acts directly on postsynaptic D2/D3 receptors. It mimics dopamine but does not alter levodopa levels. * **Selegiline (D):** A selective **MAO-B inhibitor**. It acts primarily within the **Central Nervous System** to prevent the breakdown of dopamine already formed in the brain. It does not significantly enhance the peripheral bioavailability of levodopa. **High-Yield Clinical Pearls for NEET-PG:** * **Tolcapone vs. Entacapone:** Entacapone acts only peripherally, whereas Tolcapone acts both peripherally and centrally. However, Tolcapone is rarely used due to **hepatotoxicity** (requires LFT monitoring). * **Orange Discoloration:** Entacapone can cause harmless orange discoloration of urine. * **Stalevo:** A fixed-dose combination of Levodopa + Carbidopa + Entacapone.

Q: Which of the following drugs is known to cause aplastic anemia as a side effect?

Carbamazepine. **Explanation:** **Carbamazepine** is a first-line antiepileptic drug for focal seizures and trigeminal neuralgia [1]. Its association with **aplastic anemia** and agranulocytosis is a classic, high-yield side effect. While rare (occurring in approximately 2–5 per million patients per year), it is a serious idiosyncratic reaction involving bone marrow suppression. This necessitates baseline and periodic complete blood counts (CBC) during treatment. **Analysis of Incorrect Options:** * **A. Valproate:** Most commonly associated with **hepatotoxicity** (especially in children <2 years), weight gain, alopecia, and **thrombocytopenia** (low platelets), rather than global aplastic anemia. It is also highly teratogenic (neural tube defects). * **C. Lamotrigine:** The most significant life-threatening side effect is **Stevens-Johnson Syndrome (SJS)** or Toxic Epidermal Necrolysis (TEN). It does not typically cause bone marrow failure. * **D. Lithium:** Known for causing **leukocytosis** (an increase in white blood cell count), which is sometimes used therapeutically in Felty’s syndrome. Other side effects include nephrogenic diabetes insipidus, tremors, and hypothyroidism. **Clinical Pearls for NEET-PG:** * **HLA-B*1502 Link:** In patients of Asian descent, this allele significantly increases the risk of SJS/TEN when taking Carbamazepine. * **Enzyme Induction:** Carbamazepine is a potent **CYP450 inducer**, leading to many drug-drug interactions (e.g., decreasing the efficacy of oral contraceptives). * **Other drugs causing Aplastic Anemia:** Chloramphenicol, Phenylbutazone, Gold salts, and Felbamate [2].

Q: Which drug is used for smoking cessation?

Varenicline. **Explanation:** **Varenicline** is the correct answer. It is a **selective partial agonist at the α4β2 nicotinic acetylcholine receptors (nAChR)** in the brain. Its mechanism of action is twofold: 1. **Agonist Effect:** It provides a low-level release of dopamine, which helps mitigate the symptoms of nicotine withdrawal. 2. **Antagonist Effect:** By binding to the receptor with high affinity, it prevents inhaled nicotine from binding. This reduces the "reward" or reinforcement associated with smoking, making a relapse less satisfying. **Analysis of Incorrect Options:** * **Theophylline (Option A):** A methylxanthine used as a bronchodilator in asthma and COPD. It acts primarily by inhibiting phosphodiesterase (PDE) and antagonizing adenosine receptors. * **Bicalutamide (Option B):** A non-steroidal anti-androgen used in the management of prostate cancer. * **Salmeterol (Option C):** A Long-Acting Beta-2 Agonist (LABA) used for the maintenance treatment of asthma and COPD; it has no role in addiction medicine. **High-Yield Clinical Pearls for NEET-PG:** * **First-line agents** for smoking cessation include Varenicline, Bupropion (an atypical antidepressant), and Nicotine Replacement Therapy (NRT). * **Varenicline Side Effects:** The most common side effect is **nausea**. It is also associated with **neuropsychiatric symptoms** (vivid dreams, depression, suicidal ideation) and should be used cautiously in patients with pre-existing psychiatric illness. * **Bupropion** is specifically preferred in smokers who are concerned about weight gain or have comorbid depression, but it is contraindicated in patients with a history of seizures.

Q: Tolerance is seen in all, except?

Cocaine. The correct answer is **Cocaine**. In pharmacology, tolerance refers to the requirement of higher doses of a drug to achieve the same effect previously produced by a lower dose. **Why Cocaine is the correct answer:** Cocaine is a unique CNS stimulant because it primarily exhibits **reverse tolerance (sensitization)** rather than traditional tolerance [1]. With repeated use, the user may become *more* sensitive to its effects, particularly its psychomotor and convulsant actions. While some tolerance can develop to its euphoric effects, the lack of significant physiological tolerance to its dangerous side effects (like seizures and cardiac arrhythmias) distinguishes it from the other options [3]. **Analysis of Incorrect Options:** * **Morphine (Opioids):** Exhibits profound tolerance to most effects (analgesia, euphoria, respiratory depression) except for **miosis and constipation**, which are high-yield "no-tolerance" exceptions [2]. * **Amphetamine:** Rapid tolerance (tachyphylaxis) develops to its anorectic and pressor effects due to the depletion of neuronal norepinephrine stores. * **Barbiturates:** These induce hepatic microsomal enzymes (pharmacokinetic tolerance) and cause CNS adaptation (pharmacodynamic tolerance). **High-Yield NEET-PG Pearls:** 1. **Miosis and Constipation:** Remember these two effects of Morphine never show tolerance. 2. **Tachyphylaxis:** This is a form of rapid-onset tolerance seen with Ephedrine, Tyramine, and Amphetamines. 3. **Cross-tolerance:** Often occurs between drugs of the same class (e.g., between different Benzodiazepines or between Alcohol and Barbiturates). 4. **Cocaine Mechanism:** It acts by blocking the reuptake of Dopamine, Norepinephrine, and Serotonin (NET, DAT, and SERT inhibitors) [3].

Q: Which antidepressant is relatively contraindicated in young men secondary to its side effect of priapism?

Trazodone. **Explanation:** **Trazodone** is a SARI (Serotonin Antagonist and Reuptake Inhibitor) primarily used for insomnia and depression. The correct answer is Trazodone because it is notorious for causing **priapism** (a painful, persistent erection lasting >4 hours), which is considered a urological emergency. * **Mechanism:** This side effect is attributed to its potent **$\alpha_1$-adrenergic receptor antagonism**. Blockade of these receptors in the corpora cavernosa prevents the sympathetic-mediated vasoconstriction required for detumescence, leading to "trapped" blood flow. Due to this association, it is often humorously referred to as **"Trazobone."** **Analysis of Incorrect Options:** * **Sertraline (SSRI):** The most common sexual side effect of SSRIs is **delayed ejaculation** or erectile dysfunction, not priapism. In fact, SSRIs are sometimes used therapeutically to treat premature ejaculation. * **Imipramine & Amitriptyline (TCAs):** These Tricyclic Antidepressants are more commonly associated with **anticholinergic side effects** (dry mouth, blurred vision, urinary retention) and cardiac arrhythmias. While they can cause sexual dysfunction, priapism is extremely rare compared to Trazodone. **High-Yield Clinical Pearls for NEET-PG:** * **Management of Priapism:** If Trazodone-induced priapism occurs, the immediate treatment involves intracavernosal injection of an $\alpha$-agonist like **Phenylephrine**. * **Other Side Effects:** Trazodone is highly sedating (due to $H_1$ blockade) and can cause postural hypotension (due to $\alpha_1$ blockade). * **Drug of Choice:** Trazodone is frequently used "off-label" as a hypnotic for patients with depression-associated insomnia because it lacks the addictive potential of benzodiazepines.

Q: True about phenytoin?

Causes nystagmus and ataxia. **Explanation:** Phenytoin is a high-yield hydantoin derivative used primarily for focal and generalized tonic-clonic seizures. Its mechanism involves the blockade of voltage-gated sodium channels in their inactivated state. **Why Option D is Correct:** Nystagmus and ataxia are the hallmark signs of **dose-dependent cerebellar-vestibular toxicity**. At therapeutic levels (10–20 µg/mL), mild nystagmus may appear. As levels exceed 20–30 µg/mL, ataxia, slurred speech, and incoordination develop. This occurs because phenytoin follows **zero-order kinetics** (saturation kinetics) at higher doses; a small increase in dose leads to a disproportionately large increase in plasma concentration, rapidly reaching toxic levels. **Analysis of Incorrect Options:** * **Option A:** Phenytoin has high oral bioavailability (~90%) and does **not** undergo significant first-pass metabolism. It is metabolized by hepatic CYP2C9/2C19 enzymes. * **Option B:** Phenytoin is highly **teratogenic**. It causes **Fetal Hydantoin Syndrome**, characterized by craniofacial anomalies (cleft lip/palate), microcephaly, and hypoplastic phalanges/nails. * **Option C:** Phenytoin is **ineffective** in absence seizures and may even exacerbate them. The drug of choice for absence seizures is **Ethosuximide** (or Valproate). **High-Yield Clinical Pearls for NEET-PG:** * **Non-dose dependent side effects (Mnemonic: P-H-E-N-Y-T-O-I-N):** **P**-450 induction, **H**irsutism, **E**nlarged gums (Gingival hyperplasia due to PDGF), **N**ystagmus, **Y**ellow-brown skin (pigmentation), **T**eratogenicity, **O**steomalacia (Vitamin D deficiency), **I**nterference with B12/Folate (Megaloblastic anemia), **N**europathy. * **Fosphenytoin:** A water-soluble prodrug of phenytoin used IV to avoid the local irritation and "Purple Glove Syndrome" associated with phenytoin.

Question 1

Which neurotransmitter is primarily responsible for the action of Barbiturates?

Accepted Answer

GABA

Answer

Glutamate

Answer

Glycine

Answer

Aspartate

Question 2

All of the following act through GABA except?

Accepted Answer

Carbamazepine

Answer

Phenobarbitone

Answer

Zopiclone

Answer

Muscimol

Question 3

Which of the following is an antagonist of benzodiazepines?

Accepted Answer

Flumazenil

Answer

Nalorphine

Answer

Carbamazepine

Answer

Naloxone

Question 4

Which antiepileptic drug can cause folate deficiency anemia?

Accepted Answer

Phenytoin

Answer

Valproate

Answer

Phenobarbitone

Answer

Carbamazepine

Question 5

Which of the following agents enhances the bioavailability of levodopa in patients with Parkinson's disease?

Accepted Answer

Entacapone

Answer

Amantadine

Answer

Ropinirole

Answer

Selegiline

Question 6

Which of the following drugs is known to cause aplastic anemia as a side effect?

Accepted Answer

Carbamazepine

Answer

Valproate

Answer

Lamotrigine

Answer

Lithium

Question 7

Which drug is used for smoking cessation?

Accepted Answer

Varenicline

Answer

Theophylline

Answer

Biclutamide

Answer

Salmeterol

Question 8

Tolerance is seen in all, except?

Accepted Answer

Cocaine

Answer

Morphine

Answer

Amphetamine

Answer

Barbiturates

Question 9

Which antidepressant is relatively contraindicated in young men secondary to its side effect of priapism?

Accepted Answer

Trazodone

Answer

Sertraline

Answer

Imipramine

Answer

Amitriptyline

Question 10

True about phenytoin?

Accepted Answer

Causes nystagmus and ataxia

Answer

Undergoes first-pass metabolism

Answer

Is not teratogenic

Answer

Is the drug of choice for absence seizures

Central Nervous System Pharmacology — MCQs

Central Nervous System Pharmacology — MCQs

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