Central Nervous System Pharmacology — MCQs

Central Nervous System Pharmacology Indian Medical PG Practice Questions and MCQs

Question 531: Varenicline acts by?

A. Partial nicotine receptor agonist (Correct Answer)
B. Nicotine receptor antagonist
C. Both agonist and antagonist at nicotine receptor
D. None of the above

Explanation: **Explanation:** **Varenicline** is a high-yield drug in CNS pharmacology, specifically used for **smoking cessation**. Its mechanism of action is centered on its role as a **selective partial agonist** at the **α4β2 nicotinic acetylcholine receptors (nAChR)** located in the brain. 1. **Why Option A is correct:** As a **partial agonist**, varenicline provides a dual benefit. It stimulates the nicotinic receptors enough to maintain a low level of dopamine release, which helps **reduce withdrawal symptoms** and cravings. Simultaneously, because it has a high affinity for the receptor, it blocks nicotine from binding. 2. **Why Option B and C are incorrect:** While varenicline does have "antagonist-like" effects by blocking nicotine, it is pharmacologically classified as a **partial agonist**, not a pure antagonist. Option C is a distractor; although partial agonists exhibit both agonistic and antagonistic properties depending on the presence of a full agonist, the standard pharmacological classification for varenicline is a "Partial Agonist." **High-Yield Clinical Pearls for NEET-PG:** * **Indication:** First-line agent for smoking cessation (more effective than Bupropion). * **Side Effects:** The most common side effect is **nausea**. However, the most serious "red flag" side effects are **neuropsychiatric symptoms**, including suicidal ideation, depression, and vivid dreams/nightmares. * **Comparison:** Unlike **Bupropion** (which inhibits Dopamine and NE reuptake), Varenicline acts directly on the nicotinic receptor. * **Route:** Administered orally.

Question 532: What is the mode of action of depolarizing and non-depolarizing neuromuscular blockers?

A. Depolarizing - causes persistent depolarization; non-depolarizing - competitive inhibition (Correct Answer)
B. Both are competitive inhibitors
C. Non-depolarizing - no repolarization; depolarizing - no depolarization
D. Depolarizing - persistent depolarization; non-depolarizing - no repolarization

Explanation: **Explanation:** Neuromuscular blockers (NMBs) act at the nicotinic acetylcholine receptors (Nm) of the neuromuscular junction. They are classified into two distinct categories based on their mechanism of action: 1. **Depolarizing NMBs (e.g., Succinylcholine):** These act as acetylcholine (ACh) agonists. They bind to Nm receptors and cause initial depolarization (seen as fasciculations). However, unlike ACh, they are not degraded by acetylcholinesterase, leading to **persistent depolarization**. This keeps the sodium channels in an inactivated state, preventing further action potentials and causing "Phase I block." 2. **Non-depolarizing NMBs (e.g., Vecuronium, Atracurium):** These act as **competitive antagonists**. They bind to the Nm receptor but do not activate it, effectively blocking ACh from binding. This prevents depolarization entirely. **Analysis of Incorrect Options:** * **Option B:** Only non-depolarizing agents are competitive; depolarizing agents are agonists. * **Option C:** This reverses the logic; non-depolarizing agents prevent depolarization from occurring in the first place. * **Option D:** Non-depolarizing agents do not affect repolarization; they prevent the initial stimulus. **NEET-PG High-Yield Pearls:** * **Succinylcholine** is the only depolarizing agent used clinically; it is metabolized by **pseudocholinesterase** (plasma cholinesterase). * **Hofmann Elimination:** A spontaneous, non-enzymatic degradation seen with **Atracurium and Cisatracurium**, making them safe in renal or hepatic failure. * **Reversal:** Non-depolarizing blocks are reversed by Neostigmine (AChE inhibitor). Reversing a Phase I depolarizing block with Neostigmine will actually *prolong* the paralysis. * **Malignant Hyperthermia:** A dreaded complication of Succinylcholine, treated with **Dantrolene**.

Question 533: Which of the following drugs is NOT used in migraine prophylaxis?

A. Flunarizine
B. Propranolol
C. Cyproheptadine
D. Sumatriptan (Correct Answer)

Explanation: ### Explanation The management of migraine is divided into two categories: **Abortive (Acute) therapy** and **Prophylactic (Preventive) therapy**. **Why Sumatriptan is the correct answer:** Sumatriptan is a **5-HT$_{1B/1D}$ receptor agonist**. Its primary mechanism involves vasoconstriction of dilated cranial blood vessels and inhibition of pro-inflammatory neuropeptide release (Substance P, CGRP) from trigeminal nerve endings. Because of its rapid onset and short duration of action, it is used exclusively for the **acute abortive treatment** of migraine attacks, not for prophylaxis. **Analysis of other options (Prophylactic agents):** * **Propranolol (Option B):** A non-selective beta-blocker and the **drug of choice** for migraine prophylaxis. It works by stabilizing vascular tone and increasing the threshold for migraine triggers. * **Flunarizine (Option A):** A non-selective **Calcium Channel Blocker (CCB)** with additional H1-blocking activity. It is highly effective in reducing the frequency of attacks. * **Cyproheptadine (Option C):** A combined **5-HT$_2$ and H$_1$ receptor antagonist**. It is particularly used for migraine prophylaxis in **children**. **NEET-PG High-Yield Pearls:** 1. **Prophylaxis Criteria:** Indicated if attacks occur >2–3 times/month or are severe/disabling. 2. **First-line Prophylactic Drugs:** Beta-blockers (Propranolol), Anticonvulsants (Topiramate, Valproate), and TCAs (Amitriptyline). 3. **Newer Agents:** **CGRP Antagonists** (e.g., Erenumab) are used for prophylaxis, while **Gepants** (e.g., Rimegepant) and **Ditans** (e.g., Lasmiditan) are used for acute attacks. 4. **Contraindication:** Triptans are contraindicated in patients with Ischemic Heart Disease (IHD) or Prinzmetal angina due to their vasoconstrictive properties.

Question 534: Carbachol acts as

A. Agonist for M2 receptors
B. Agonist for M3 receptors (Correct Answer)
C. Agonist for M1 receptors
D. Antagonist for M3 receptors

Explanation: **Explanation:** **Carbachol** is a potent, synthetic **direct-acting cholinergic agonist**. It is a carbamate ester of choline that is structurally related to acetylcholine but is resistant to hydrolysis by both acetylcholinesterase and pseudocholinesterase, resulting in a prolonged duration of action. **Why Option B is Correct:** Carbachol is a **non-selective agonist** that stimulates both **Muscarinic (M1, M2, M3)** and **Nicotinic (Nn, Nm)** receptors. In clinical practice and exam contexts, its most significant therapeutic effect is mediated via **M3 receptors** located on the sphincter pupillae and ciliary muscle of the eye. Activation of M3 receptors causes miosis (pupillary constriction) and contraction of the ciliary muscle (accommodation), which facilitates the drainage of aqueous humor, thereby reducing intraocular pressure. **Why Other Options are Incorrect:** * **Options A & C:** While Carbachol does have agonist activity at M1 and M2 receptors, it is rarely used for these specific effects clinically. In the context of standard pharmacology questions, its primary "action" is associated with M3-mediated ocular effects. * **Option D:** Carbachol is a cholinomimetic (agonist), not an antagonist. An M3 antagonist (like Atropine) would cause mydriasis and cycloplegia, the opposite of Carbachol’s effect. **High-Yield Clinical Pearls for NEET-PG:** * **Clinical Use:** Primarily used topically in the eye to treat **glaucoma** or to produce miosis during ocular surgery. * **Nicotinic Activity:** Unlike Bethanechol (which is purely muscarinic), Carbachol has significant **nicotinic action**, which can cause the release of adrenaline from the adrenal medulla. * **Mnemonic:** Remember **"C"** for **C**arbachol and **"C"** for **C**omplete agonist (acts on both Muscarinic and Nicotinic receptors). * **Differentiating Choline Esters:** * *Acetylcholine:* Acts on M and N; rapidly hydrolyzed. * *Methacholine:* Acts mainly on M; used in bronchial challenge tests. * *Bethanechol:* Acts purely on M; used for urinary retention.

Question 535: Which of the following drugs inhibits the release of calcium from the sarcoplasmic reticulum?

A. Dantrolene (Correct Answer)
B. Caffeine
C. Rocuronium
D. Succinylcholine

Explanation: **Explanation:** **Dantrolene (Correct Answer):** Dantrolene is a peripherally acting muscle relaxant that directly interferes with excitation-contraction coupling in skeletal muscle. It acts by binding to the **Ryanodine Receptor 1 (RyR1)** located on the sarcoplasmic reticulum (SR). This binding inhibits the release of calcium ions from the SR into the cytosol, thereby preventing muscle contraction. This unique mechanism makes it the drug of choice for **Malignant Hyperthermia**, where there is an uncontrolled release of calcium. **Incorrect Options:** * **Caffeine:** Unlike Dantrolene, caffeine actually **stimulates** the Ryanodine receptors, promoting the release of calcium from the SR. In vitro, the "Caffeine-Halothane Contracture Test" is used to diagnose susceptibility to malignant hyperthermia. * **Rocuronium:** This is a **non-depolarizing neuromuscular blocker** that acts at the neuromuscular junction (NMJ) by competitively antagonizing nicotinic acetylcholine receptors ($N_m$). It does not act directly on the SR. * **Succinylcholine:** This is a **depolarizing neuromuscular blocker** that acts as an agonist at $N_m$ receptors, causing persistent depolarization. Notably, it is a known trigger for malignant hyperthermia in susceptible individuals. **NEET-PG High-Yield Pearls:** * **Drug of Choice:** Dantrolene is used for Malignant Hyperthermia, Neuroleptic Malignant Syndrome (NMS), and spasticity associated with upper motor neuron lesions. * **Side Effect:** The most significant chronic side effect of Dantrolene is **hepatotoxicity** (monitor LFTs). * **Malignant Hyperthermia Trigger:** Often triggered by volatile anesthetics (e.g., Halothane) and Succinylcholine due to mutations in the *RYR1* gene.

Question 536: All of the following drugs cause hyperprolactinemia EXCEPT:

A. Haloperidol
B. Chlorpromazine
C. Bromocriptine (Correct Answer)
D. Metoclopramide

Explanation: **Explanation:** The regulation of prolactin secretion is primarily under the inhibitory control of **Dopamine**, which acts on **D2 receptors** in the anterior pituitary (Tuberoinfundibular pathway). Any drug that increases dopaminergic activity will decrease prolactin, while any drug that blocks D2 receptors will cause **hyperprolactinemia**. **Why Bromocriptine is the Correct Answer:** Bromocriptine is a potent **D2 receptor agonist**. By stimulating the dopamine receptors in the pituitary, it mimics the natural inhibitory effect of dopamine, thereby **decreasing** prolactin levels. It is clinically used in the management of prolactinomas and galactorrhea. **Why the Other Options are Incorrect:** * **Haloperidol & Chlorpromazine:** These are typical antipsychotics that act by blocking D2 receptors in the brain. By blocking these receptors in the tuberoinfundibular pathway, they remove the "dopamine brake," leading to significantly elevated prolactin levels. * **Metoclopramide:** This is a prokinetic and antiemetic drug that also acts as a central D2 receptor antagonist. It is a well-known cause of drug-induced hyperprolactinemia and gynecomastia. **High-Yield NEET-PG Pearls:** 1. **Dopamine = Prolactin Inhibiting Hormone (PIH).** 2. **Tuberoinfundibular Pathway:** The specific dopaminergic pathway responsible for prolactin regulation. 3. **Other drugs causing hyperprolactinemia:** Reserpine (depletes dopamine), Methyldopa, Verapamil, and SSRIs. 4. **Clinical Presentation:** In females, hyperprolactinemia presents as galactorrhea, amenorrhea, and infertility; in males, it causes gynecomastia and erectile dysfunction.

Question 537: A patient with ischemic heart disease is diagnosed with migraine. Which drug is indicated in this case?

A. Propranolol
B. Triptans
C. Ergotamines
D. Butorphanol (Correct Answer)

Explanation: **Explanation:** The management of migraine in patients with **Ischemic Heart Disease (IHD)** requires caution because many standard migraine medications cause systemic vasoconstriction, which can exacerbate myocardial ischemia. **Why Butorphanol is correct:** Butorphanol is a mixed opioid agonist-antagonist (κ-agonist and μ-partial agonist/antagonist). Unlike specific anti-migraine drugs, it does not act on serotonin (5-HT) receptors that cause vasoconstriction. It provides potent analgesia for acute migraine attacks without compromising coronary blood flow, making it a safer alternative for patients with cardiovascular contraindications. **Why the other options are incorrect:** * **Triptans (e.g., Sumatriptan):** These are 5-HT$_{1B/1D}$ agonists. While they effectively constrict cranial vessels, they also cause **coronary vasospasm**. They are strictly contraindicated in patients with IHD, Prinzmetal angina, or uncontrolled hypertension. * **Ergotamines:** These act on 5-HT$_{1}$, 5-HT$_{2}$, and α-adrenergic receptors. They are potent, long-acting vasoconstrictors and are contraindicated in IHD and peripheral vascular disease. * **Propranolol:** While used for migraine **prophylaxis**, it is not indicated for the treatment of an acute attack. Furthermore, in certain cardiac conditions (like vasospastic angina), non-selective beta-blockers can worsen coronary vasoconstriction due to unopposed α-activity. **High-Yield Clinical Pearls for NEET-PG:** * **DOC for Acute Migraine:** Triptans (Sumatriptan). * **DOC for Migraine Prophylaxis:** Propranolol (others include Topiramate, Amitriptyline, and Valproate). * **Newer Drugs (Safe in IHD):** **Lasmiditan** (5-HT$_{1F}$ agonist) and **Gepants** (CGRP antagonists like Rimegepant) are preferred modern alternatives for patients with cardiovascular risks as they do not cause vasoconstriction.

Question 538: Which drug is primarily used for the treatment of absence seizures?

A. Lamotrigine (Correct Answer)
B. Carbamazepine
C. Phenytoin
D. Vigabatrine

Explanation: **Explanation:** The drug of choice for **Absence Seizures** (Petit Mal) is traditionally **Ethosuximide** (for pure absence) or **Sodium Valproate** (if associated with tonic-clonic seizures). However, among the given options, **Lamotrigine** is the correct answer as it is a broad-spectrum antiepileptic effective against absence seizures. **1. Why Lamotrigine is Correct:** Lamotrigine acts by blocking voltage-gated sodium channels and inhibiting the release of excitatory neurotransmitters (glutamate and aspartate) [1]. It also has some activity against T-type calcium channels, which are central to the pathophysiology of absence seizures. It is considered a first-line alternative, especially in women of childbearing age, due to its better safety profile compared to Valproate. **2. Why Other Options are Incorrect:** * **Carbamazepine & Phenytoin:** These are narrow-spectrum drugs that block sodium channels. They are strictly **contraindicated** in absence seizures as they can paradoxically worsen the condition or precipitate absence status. * **Vigabatrin:** This drug acts by irreversibly inhibiting GABA transaminase. It is primarily used for Infantile Spasms (West Syndrome) and focal seizures [3]. It is not effective for absence seizures. **Clinical Pearls for NEET-PG:** * **Drug of Choice (DOC):** Ethosuximide (Pure Absence); Valproate (Absence + GTCS). * **Mechanism of Ethosuximide:** Inhibition of **T-type Ca²⁺ channels** in thalamic neurons [2]. * **EEG Finding:** Classic **3 Hz spike-and-wave** discharges. * **Avoid in Absence:** Phenytoin, Carbamazepine, Oxcarbazepine, and Tiagabine (they aggravate the seizure) [2]. * **Lamotrigine Side Effect:** Watch for **Stevens-Johnson Syndrome (SJS)**; the dose must be titrated slowly.

Question 539: Which of the following is a preferred opioid for pain control in terminally ill cancer patients?

A. Pethidine
B. Fentanyl (Correct Answer)
C. Methadone
D. Remifentanyl

Explanation: **Explanation:** **Fentanyl (Option B)** is the preferred opioid for chronic pain management in terminally ill cancer patients, primarily due to its availability as a **transdermal patch**. This delivery system provides continuous, stable analgesia for up to 72 hours, ensuring better patient compliance and avoiding the "peak and valley" effect of oral medications. It is highly lipophilic, potent, and lacks active metabolites, making it safer in patients with renal impairment—a common complication in advanced malignancy. **Why other options are incorrect:** * **Pethidine (Option A):** It is contraindicated for chronic cancer pain. Its metabolite, **norpethidine**, has a long half-life and is neurotoxic, leading to tremors, myoclonus, and seizures, especially with repetitive dosing. * **Methadone (Option C):** While used for chronic pain, it has a very long and unpredictable half-life (15–60 hours), leading to a high risk of cumulative toxicity and respiratory depression. It requires complex titration. * **Remifentanil (Option D):** It has an ultra-short duration of action (metabolized by plasma esterases). It is used exclusively for induction/maintenance of anesthesia via continuous infusion and is unsuitable for long-term outpatient pain control. **High-Yield Clinical Pearls for NEET-PG:** * **Drug of Choice (DOC)** for terminal cancer pain (WHO Ladder Step 3): **Morphine** (Oral) is the gold standard; however, **Fentanyl** is the preferred long-term transdermal option. * **Renal Failure:** Fentanyl and Buprenorphine are the safest opioids as they do not have active metabolites excreted by the kidney (unlike Morphine and Hydromorphone). * **Miosis & Constipation:** These are two side effects of opioids to which **tolerance never develops**.

Question 540: What is the primary therapeutic strategy for dopamine deficiency in the substantia nigra in Parkinson's disease?

A. Feedback inhibition of dopamine oxidation
B. Competitive inhibition of dopamine biosynthesis from histidine
C. Provision of metabolites in the tyrosine pathway (Correct Answer)
D. Stimulation of monoamine oxidase

Explanation: ### Explanation **1. Why Option C is Correct:** The core pathology of Parkinson’s Disease (PD) is the progressive degeneration of dopaminergic neurons in the **substantia nigra pars compacta**, leading to a deficiency of dopamine in the striatum [1], [2]. Dopamine itself cannot cross the blood-brain barrier (BBB). Therefore, the primary therapeutic strategy is to provide **Levodopa (L-Dopa)**, which is a metabolic precursor in the **tyrosine pathway** (Tyrosine → L-Dopa → Dopamine) [3], [4]. L-Dopa can cross the BBB via large neutral amino acid transporters, where it is then decarboxylated to functional dopamine by the enzyme DOPA decarboxylase. **2. Why the Other Options are Incorrect:** * **Option A:** Inhibiting the feedback inhibition of dopamine oxidation is not a standard strategy. In fact, we aim to *prevent* dopamine oxidation (metabolism) using MAO-B inhibitors (e.g., Selegiline) to increase its synaptic availability. * **Option B:** Dopamine is synthesized from **Tyrosine**, not Histidine (which is the precursor for Histamine) [3]. Furthermore, we want to *promote* biosynthesis, not inhibit it competitively. * **Option C:** Stimulation of Monoamine Oxidase (MAO) would accelerate the breakdown of dopamine, further depleting levels and worsening the symptoms of Parkinsonism. **3. High-Yield Clinical Pearls for NEET-PG:** * **The "Gold Standard":** Levodopa remains the most effective drug for PD [4]. * **Peripheral Decarboxylase Inhibitors (Carbidopa/Benserazide):** These are always co-administered with L-Dopa to prevent its peripheral conversion to dopamine, thereby reducing systemic side effects (nausea, arrhythmias) and increasing CNS bioavailability. * **The "On-Off" Phenomenon:** A common long-term complication of L-Dopa therapy characterized by fluctuations in motor performance. * **Vitamin B6 (Pyridoxine) Warning:** High doses of B6 can increase the peripheral metabolism of L-Dopa (if taken without Carbidopa), reducing its therapeutic effect.

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Drugs of Abuse and Addiction

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