Central Nervous System Pharmacology — MCQs

Central Nervous System Pharmacology Indian Medical PG Practice Questions and MCQs

Question 821: What is the drug of choice (DOC) for smoking cessation?

A. Acamprosate
B. Varenicline (Correct Answer)
C. Thalidomide
D. Tryptophan

Explanation: ***Varenicline*** - **Varenicline** is a **partial nicotinic receptor agonist** that reduces both the cravings for nicotine and the pleasurable effects of smoking. - It is considered a **first-line agent** for smoking cessation due to its effectiveness in increasing abstinence rates. *Acamprosate* - **Acamprosate** is a medication used to maintain abstinence from **alcohol dependence** by modulating GABA and glutamate neurotransmission. - It is **not indicated** for smoking cessation and has no direct mechanism of action related to nicotine addiction. *Thalidomide* - **Thalidomide** is an **immunomodulatory drug** with anti-angiogenic and anti-inflammatory properties, primarily used in conditions like **multiple myeloma** and **leprosy**. - It has **severe teratogenic effects** and is not used for smoking cessation due to its complex mechanism and significant side effect profile. *Tryptophan* - **Tryptophan** is an **essential amino acid** that serves as a precursor to **serotonin**, a neurotransmitter involved in mood regulation and sleep. - While it can impact mood, it is **not an approved or effective treatment** for directly addressing nicotine dependence or cravings.

Question 822: What is the mechanism of action of Curare drugs as muscle relaxants?

A. Persistently depolarizing at Neuromuscular junction
B. Act competitively on Ach receptors blocking post-synaptically (Correct Answer)
C. Repetitive stimulation of Ach receptors on muscle end plate
D. Inhibiting the calcium channel on presynaptic membrane

Explanation: ***Act competitively on Ach receptors blocking post-synaptically*** - Curare-like drugs are **non-depolarizing neuromuscular blockers** that exhibit their effects by **competitively binding** to **nicotinic acetylcholine receptors** on the **postsynaptic membrane** of the neuromuscular junction. - This competitive binding prevents acetylcholine (ACh) from binding to its receptors, thereby inhibiting the generation of an **end-plate potential** and subsequent **muscle contraction**. *Persistently depolarizing at Neuromuscular junction* - This mechanism is characteristic of **depolarizing neuromuscular blockers** like **succinylcholine**, which first cause muscle fasciculations followed by paralysis due to persistent receptor activation and inactivation of voltage-gated sodium channels. - Curare-like drugs do not cause persistent depolarization; instead, they prevent depolarization by blocking ACh access to receptors. *Repetitive stimulation of Ach receptors on muscle end plate* - **Repetitive stimulation** of acetylcholine (ACh) receptors by ACh itself leads to muscle contraction, not relaxation. - Curare-like drugs *block* the ability of ACh to stimulate these receptors, thus preventing contraction. *Inhibiting the calcium channel on presynaptic membrane* - Inhibiting presynaptic calcium channels would reduce the **release of acetylcholine** from the presynaptic terminal. - While this would lead to muscle relaxation, it is not the primary mechanism of action for **curare-like drugs**, which act directly on the postsynaptic receptors.

Question 823: Suxamethonium primarily acts on which type of receptors?

A. Nicotinic acetylcholine receptors (Correct Answer)
B. Potassium channels
C. Calcium channels
D. Chloride channels

Explanation: ***Nicotinic acetylcholine receptors*** - **Suxamethonium** is a depolarizing muscle relaxant that acts as an **agonist at nicotinic acetylcholine receptors** at the neuromuscular junction. - This initial activation leads to muscle fasciculations followed by prolonged depolarization, causing **flaccid paralysis**. *Potassium channels* - While some drugs may affect potassium channels to alter neuronal excitability, suxamethonium's primary mechanism of action is not on these channels. - Blocking potassium channels is characteristic of drugs like **certain antiarrhythmics** or **sulfonylureas**. *Calcium channels* - **Calcium channels** play a role in muscle contraction, but they are not the primary target of suxamethonium. - Drugs like **dihydropyridines** (e.g., nifedipine) target calcium channels for their antihypertensive effects. *Chloride channels* - Chloride channels are involved in maintaining resting membrane potential and inhibitory neurotransmission. - Drugs such as **benzodiazepines** indirectly enhance GABA-mediated chloride influx, which is distinct from suxamethonium's action.

Question 824: Which of the following metal ions is associated with secondary Parkinsonisms?

A. Magnesium (Mg)
B. Selenium (Se)
C. Manganese (Mn) (Correct Answer)
D. Molybdenum (Mo)

Explanation: ***Manganese (Mn)*** - Chronic exposure to high levels of **manganese** can lead to **manganism**, a neurological disorder characterized by **Parkinsonian-like symptoms**, including bradykinesia, rigidity, and gait disturbances [1]. - This is due to manganese accumulation in the **basal ganglia**, particularly the **globus pallidus**, affecting dopaminergic pathways. *Magnesium (Mg)* - **Magnesium** is an essential mineral vital for numerous bodily functions, including nerve and muscle function. - While imbalances can cause neurological issues (e.g., tremors with hypomagnesemia), it is not directly associated with **secondary parkinsonism**. *Selenium (Se)* - **Selenium** is a trace element with antioxidant properties, important for thyroid hormone metabolism and immune function. - Both deficiency and toxicity can cause various health problems, but it is not known to cause **secondary parkinsonism**. *Molybdenum (Mo)* - **Molybdenum** is an essential trace element that functions as a cofactor for several enzymes. - No known association exists between molybdenum exposure, deficiency, or toxicity and the development of **secondary parkinsonism**.

Question 825: In the context of pharmacology, what is the term 'Mickey Finn' commonly associated with?

A. Chloroform
B. Methyl alcohol
C. Chloral hydrate (Correct Answer)
D. Ethylene glycol

Explanation: ***Chloral hydrate*** - A "Mickey Finn" is a slang term for a drink **laced with a psychoactive drug or incapacitating agent** given to an unsuspecting person. - Historically, **chloral hydrate** was a common substance used for this purpose due to its rapid sedative-hypnotic effects. *Chloroform* - While chloroform is a potent anesthetic and sedative, it is primarily used as an **inhalant** and is not typically administered orally in drinks. - Ingesting chloroform in large quantities can be **fatal due to severe hepatotoxicity and neurotoxicity**. *Methyl alcohol* - **Methyl alcohol (methanol)** is highly toxic and causes severe metabolic acidosis, blindness, and death, even in small amounts. - It does not induce the quick, incapacitating sedative effects associated with a "Mickey Finn" but rather a **delayed, severe poisoning syndrome**. *Ethylene glycol* - **Ethylene glycol** is an antifreeze agent that is also highly toxic, causing kidney failure and metabolic derangements. - Similar to methanol, its effects are **delayed and severe**, not the immediate incapacitating sedation implied by the term "Mickey Finn."

Question 826: Which anti-cholinesterase drug is known for its central nervous system activity?

A. Physostigmine (Correct Answer)
B. Edrophonium
C. Pyridostigmine
D. Neostigmine

Explanation: ***Physostigmine*** - Unlike most other anti-cholinesterases, **physostigmine** is a **tertiary amine** and is relatively lipid-soluble, allowing it to cross the **blood-brain barrier**. [2] - Its ability to increase acetylcholine in the CNS makes it useful in treating anticholinergic toxicity (e.g., atropine overdose). [2] *Pyridostigmine* - **Pyridostigmine** is a **quaternary amine** and does not readily cross the blood-brain barrier, primarily acting peripherally. - It is mainly used in the long-term management of **myasthenia gravis**. *Edrophonium* - **Edrophonium** is a very short-acting **quaternary amine** that does not cross the blood-brain barrier. [1] - It is primarily used for the **diagnosis of myasthenia gravis** (Tensilon test) due to its rapid onset and short duration of action. [1] *Neostigmine* - **Neostigmine** is also a **quaternary amine** with poor lipid solubility, meaning it has minimal central nervous system activity. [2] - It is commonly used to reverse the effects of **non-depolarizing neuromuscular blockers** and in the treatment of **myasthenia gravis**.

Question 827: What effect does morphine have on muscle tone?

A. Increased muscle tone (Correct Answer)
B. Respiratory stimulation
C. Decreased muscle tone
D. Mydriasis

Explanation: ***Increased muscle tone*** - Morphine **increases skeletal muscle tone** and can cause muscle rigidity, particularly with rapid IV administration (truncal rigidity). - It significantly increases **smooth muscle tone** in various organs including the sphincter of Oddi (causing biliary colic), bladder sphincter (causing urinary retention), and GI tract (causing constipation). - This increased tone in sphincters and smooth muscle is a well-documented effect mediated through **opioid receptor activation**. *Bradycardia (not increased heart rate)* - Morphine typically causes **bradycardia** (decreased heart rate) due to vagal stimulation and central effects, not tachycardia. - Increased heart rate would be atypical and not a primary pharmacological effect of morphine. *Miosis (not mydriasis)* - Morphine characteristically causes **miosis** (pinpoint pupils) due to stimulation of the Edinger-Westphal nucleus of the oculomotor nerve. - Mydriasis (dilated pupils) is seen with anticholinergics or sympathomimetics, not opioids. *Respiratory depression (not stimulation)* - Morphine causes **respiratory depression**, not stimulation, by reducing the responsiveness of brainstem respiratory centers to CO2. - This is one of the most dangerous adverse effects and the primary cause of death in opioid overdose.

Question 828: What is the primary mechanism of action of zonisamide?

A. GABA receptors
B. Cl- channels
C. Sodium channels (Correct Answer)
D. T-type calcium channels

Explanation: ***Sodium channels (Correct Answer)*** - Zonisamide's primary mechanism involves **blocking voltage-sensitive sodium channels**, which stabilizes neuronal membranes and inhibits repetitive neuronal firing. - This action helps to prevent the propagation of **seizure activity** in the brain. *GABA receptors* - While zonisamide has some weak effects on GABA, it is not its **primary mechanism of action** for antiepileptic efficacy. - Drugs like **benzodiazepines** and **barbiturates** primarily act by enhancing GABAergic transmission. *T-type calcium channels* - Zonisamide also blocks T-type calcium channels, contributing to its broad-spectrum antiepileptic activity, but this is a **secondary mechanism** compared to its sodium channel blockade. - **Ethosuximide** is a classic example of a drug primarily acting on T-type calcium channels, especially for absence seizures. *Cl- channels* - Zonisamide does not primarily act on **chloride channels**; these are often modulated by GABA receptors. - Drugs that act directly on chloride channels are not typically used as **antiepileptics** in the same way.

Question 829: Antidepressant drug used in nocturnal enuresis is:

A. Imipramine (Correct Answer)
B. Fluoxetine
C. Trazodone
D. Sertraline

Explanation: ***Imipramine*** - **Imipramine**, a **tricyclic antidepressant (TCA)**, is frequently used off-label for **nocturnal enuresis** in children [1]. - Its mechanism of action in enuresis is thought to involve a combination of anticholinergic effects (which relax the bladder detrusor muscle) and central nervous system effects (which may increase bladder capacity and arousal from sleep) [1]. *Fluoxetine* - **Fluoxetine** is a **selective serotonin reuptake inhibitor (SSRI)** and is primarily used for depression, anxiety disorders, and OCD [2]. - It is not indicated for the treatment of nocturnal enuresis and does not have the same bladder-relaxing or arousal-modulating properties as imipramine in this context. *Trazodone* - **Trazodone** is a **serotonin antagonist and reuptake inhibitor (SARI)**, commonly prescribed for depression and insomnia due to its prominent sedative effects. - It is not used for nocturnal enuresis and its mechanism of action does not confer benefits for bladder control. *Sertraline* - **Sertraline** is another **selective serotonin reuptake inhibitor (SSRI)** used for a wide range of psychiatric conditions, including depression, anxiety, and panic disorder [2]. - Like fluoxetine, it is not an appropriate treatment for nocturnal enuresis and lacks the specific known effects beneficial for this condition.

Question 830: Where is the benzodiazepine binding site located on GABA receptors?

A. β-subunit
B. δ-subunit
C. γ-subunit (Correct Answer)
D. α-subunit

Explanation: ***γ-subunit*** - The **benzodiazepine binding site** is located at the interface between the **α and γ subunits** of the GABA-A receptor, with the **γ-subunit (especially γ2) being essential** for benzodiazepine sensitivity. - The presence of the **γ2 subunit** is **mandatory** for benzodiazepine binding - receptors lacking this subunit are **insensitive to benzodiazepines**. - Benzodiazepines bind to this site and act as **positive allosteric modulators**, increasing the frequency of **chloride channel opening** in response to GABA. - This is the **standard answer** for NEET-PG and medical PG examinations in India. *α-subunit* - The **α-subunit** contributes to forming the benzodiazepine binding pocket at the α-γ interface. - Different **α-subunit isoforms** (α1, α2, α3, α5) determine the pharmacological profile and tissue distribution of benzodiazepine effects. - However, the **α-subunit alone** cannot bind benzodiazepines without the γ-subunit. *β-subunit* - The **β-subunit** contains the primary binding site for **GABA** itself. - It does not participate in benzodiazepine binding but is crucial for the receptor's overall function and GABAergic signaling. *δ-subunit* - The **δ-subunit** replaces the γ-subunit in certain GABA-A receptor subtypes that mediate **tonic inhibition**. - Receptors containing **δ-subunits** are **insensitive to benzodiazepines** but sensitive to neurosteroids and certain general anesthetics. - This is a key distinguishing feature between phasic (γ-containing) and tonic (δ-containing) GABA-A receptors.

Practice by Chapter

General Anesthetics

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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

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Drugs of Abuse and Addiction

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Psychostimulants

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Hallucinogens

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CNS Stimulants and Cognitive Enhancers

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