Central Nervous System Pharmacology — MCQs

Central Nervous System Pharmacology Indian Medical PG Practice Questions and MCQs

Question 541: On which receptor do barbiturates act?

A. alpha-adrenergic receptors
B. beta-adrenergic receptors
C. cholinergic receptors
D. GABA receptor (Correct Answer)

Explanation: **Explanation:** **Mechanism of Action (Correct Answer: D)** Barbiturates are sedative-hypnotic drugs that 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 action is to **increase the duration** of chloride channel opening (mnemonic: "Barbi-Dura-tion"). At higher concentrations, barbiturates can also act as **GABA-mimetic** agents, directly opening the chloride channels even in the absence of GABA. This leads to hyperpolarization of the postsynaptic neuron and CNS depression. **Analysis of Incorrect Options:** * **A & B (Adrenergic Receptors):** Alpha and beta receptors are part of the sympathetic nervous system and respond to catecholamines like epinephrine and norepinephrine. Barbiturates do not have a primary affinity for these receptors. * **C (Cholinergic Receptors):** These receptors (Nicotinic and Muscarinic) respond to Acetylcholine. While barbiturates may inhibit some excitatory neurotransmitters (like glutamate) at very high doses, their therapeutic effect is not mediated through cholinergic pathways. **NEET-PG High-Yield Pearls:** * **Site of Action:** Barbiturates also inhibit **AMPA receptors** (glutamate subtype), contributing to their potent CNS depressant effects. * **Enzyme Induction:** Barbiturates are potent **inducers of Cytochrome P450 enzymes** (specifically CYP1A2, 2C9, 2C19, and 3A4), 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 heme synthesis. * **Safety Profile:** Unlike benzodiazepines, barbiturates lack a specific antagonist (like Flumazenil), making overdose management purely supportive.

Question 542: Vigabatrin acts by:

A. GABA receptor inhibitor
B. Glutamate receptor inhibitor
C. GABA transaminase inhibitor (Correct Answer)
D. GABA transport inhibitor

Explanation: **Explanation:** **Mechanism of Action (The Correct Answer):** Vigabatrin is an antiepileptic drug that acts as an **irreversible inhibitor of GABA transaminase (GABA-T)**. GABA-T is the enzyme responsible for the metabolic degradation of Gamma-Aminobutyric Acid (GABA), the primary inhibitory neurotransmitter in the brain. By inhibiting this enzyme, Vigabatrin significantly increases the concentration of GABA in the synaptic cleft, leading to enhanced inhibitory neurotransmission and suppression of seizure activity. **Analysis of Incorrect Options:** * **Option A (GABA receptor inhibitor):** This would be pro-convulsant (e.g., Bicuculline). Vigabatrin enhances GABAergic tone rather than inhibiting the receptor. * **Option B (Glutamate receptor inhibitor):** While drugs like Perampanel (AMPA antagonist) or Felbamate (NMDA antagonist) work here, Vigabatrin does not affect glutamate receptors directly. * **Option C (GABA transport inhibitor):** This describes the mechanism of **Tiagabine**, which inhibits the GAT-1 transporter to prevent GABA reuptake. **High-Yield Clinical Pearls for NEET-PG:** * **Drug of Choice (DOC):** Vigabatrin is the first-line treatment for **Infantile Spasms (West Syndrome)** associated with **Tuberous Sclerosis**. * **Adverse Effect:** The most characteristic side effect is **permanent bilateral concentric visual field contraction** (visual field defects), necessitating regular perimetry monitoring. * **Mnemonic:** **Vi**gabatrin **G**ABA **T**ransaminase inhibitor (**Vi-GA-T**). * **Excretion:** It is primarily excreted unchanged by the kidneys; dose adjustment is required in renal impairment.

Question 543: Morphine can be used in all the following conditions except?

A. Head injury (Correct Answer)
B. Asthma
C. Hypothyroidism
D. Diabetes

Explanation: **Explanation:** **1. Why Head Injury is the Correct Answer (Contraindication):** Morphine is strictly contraindicated in head injuries due to two primary reasons: * **Respiratory Depression & Increased ICP:** Morphine causes respiratory depression, leading to CO₂ retention (hypercapnia). CO₂ is a potent cerebral vasodilator, which increases cerebral blood flow and further elevates **Intracranial Pressure (ICP)**, potentially causing brain herniation. * **Masking of Clinical Signs:** Morphine causes **miosis** (pinpoint pupils) and sedation. This masks critical neurological signs such as pupillary changes and level of consciousness (GCS score), which are essential for monitoring the progression of a head injury. **2. Analysis of Other Options:** * **Asthma:** While Morphine can cause histamine release and worsen bronchospasm, it is a *relative* contraindication compared to the *absolute* danger in head injuries. * **Hypothyroidism:** Patients with myxedema are highly sensitive to the respiratory depressant effects of opioids due to a low metabolic rate, but it is not the primary contraindication in this specific question context. * **Diabetes:** There is no direct contraindication for using Morphine in diabetic patients, although caution is always advised in chronic metabolic conditions. **3. High-Yield Clinical Pearls for NEET-PG:** * **Drug of Choice (DOC):** Morphine is the DOC for pain in **Myocardial Infarction (MI)** and **Acute Left Ventricular Failure (Cardiac Asthma)** because it reduces preload and anxiety. * **Biliary Colic:** Morphine is generally avoided in biliary colic because it causes constriction of the **Sphincter of Oddi** (Pethidine is preferred). * **Triad of Morphine Poisoning:** Coma, Pinpoint pupil, and Depressed respiration. * **Specific Antidote:** **Naloxone** (pure opioid antagonist) is used for acute toxicity.

Question 544: Which of the following is a potassium channel opener?

A. Stiripentol
B. Retigabine (Correct Answer)
C. Lacosamide
D. Modafinil

Explanation: **Explanation:** The correct answer is **Retigabine** (also known as Ezogabine). **1. Why Retigabine is correct:** Retigabine is a unique anti-seizure medication that acts as a **positive allosteric modulator of KCNQ2-5 (Kv7.2-7.5) potassium channels**. By opening these voltage-gated potassium channels, it facilitates an outward potassium current (the "M-current"). This leads to hyperpolarization of the neuronal membrane, effectively stabilizing the resting membrane potential and reducing neuronal excitability. It is primarily used as an adjunctive treatment for focal-onset seizures. **2. Why the other options are incorrect:** * **Stiripentol (A):** This is an orphan drug used for Dravet syndrome. Its primary mechanism is increasing GABAergic transmission by acting as a positive allosteric modulator of the **GABA-A receptor** and inhibiting CYP450 enzymes. * **Lacosamide (C):** This drug works by enhancing the **slow inactivation of voltage-gated sodium channels**, unlike traditional sodium channel blockers (like Phenytoin) which affect fast inactivation. * **Modafinil (D):** This is a CNS stimulant used for narcolepsy. Its exact mechanism is complex but involves inhibiting the reuptake of **dopamine** and increasing levels of orexin and histamine in the hypothalamus. **High-Yield Clinical Pearls for NEET-PG:** * **Retigabine Side Effects:** Be aware of **Blue-grey skin discoloration** and **retinal pigmentation** (macular abnormalities), which led to its restricted use. * **Mnemonic:** "Retiga-**B**ine opens the **B**ank (channel) for Potassium." * **Lacosamide Key Point:** Always associate Lacosamide with "**Slow Inactivation**" of Sodium channels—a frequent MCQ favorite. * **Stiripentol Key Point:** Specifically indicated for **Dravet Syndrome** (Severe Myoclonic Epilepsy of Infancy).

Question 545: Which drug is not currently used in the management of Alzheimer's disease?

A. Tacrine (Correct Answer)
B. Galantamine
C. Donepezil
D. Rivastigmine

Explanation: ### Explanation **1. Why Tacrine is the Correct Answer:** Tacrine was the first centrally acting acetylcholinesterase (AChE) inhibitor approved for Alzheimer’s disease. However, it is **no longer used** in clinical practice due to its significant **hepatotoxicity** (elevation of serum transaminases) and the requirement for frequent dosing (four times daily). Modern clinical guidelines have replaced it with safer, more potent alternatives with better side-effect profiles. **2. Analysis of Incorrect Options:** * **Donepezil (Option C):** A reversible, non-competitive AChE inhibitor. It is the most commonly used drug for Alzheimer’s because of its long half-life (once-daily dosing) and lack of hepatotoxicity. * **Galantamine (Option B):** A competitive AChE inhibitor that also acts as a **nicotinic receptor modulator**, enhancing the action of acetylcholine. It is used for mild-to-moderate Alzheimer's. * **Rivastigmine (Option D):** A "pseudo-irreversible" inhibitor of both AChE and **butyrylcholinesterase**. It is unique because it is available as a **transdermal patch**, which reduces gastrointestinal side effects. **3. High-Yield Clinical Pearls for NEET-PG:** * **Mechanism of Action:** Alzheimer’s is characterized by a cholinergic deficit. These drugs increase acetylcholine levels in the synaptic cleft. * **Memantine:** A NMDA receptor antagonist used for moderate-to-severe Alzheimer’s; it is often combined with Donepezil. * **Side Effects:** Common side effects of AChE inhibitors include "SLUDGE" symptoms (Salivation, Lacrimation, Urination, Diarrhea, GI distress, Emesis) and bradycardia. * **Newer Monoclonal Antibodies:** Keep an eye on **Aducanumab** and **Lecanemab**, which target amyloid-beta plaques.

Question 546: Which of the following is the least narcotic opioid?

A. Morphine
B. Codeine
C. Heroin
D. Papaverine (Correct Answer)

Explanation: **Explanation:** The correct answer is **Papaverine**. To understand why, we must look at the chemical composition of Opium, which contains two distinct classes of alkaloids: 1. **Phenanthrene derivatives:** These include Morphine, Codeine, and Thebaine. They act on opioid receptors and possess significant analgesic and narcotic (sleep-inducing/addictive) properties. 2. **Benzylisoquinoline derivatives:** These include **Papaverine** and Noscapine. Unlike the phenanthrene group, these do **not** act on opioid receptors. **Why Papaverine is the correct answer:** Papaverine is a non-narcotic alkaloid. Its primary mechanism of action is the inhibition of phosphodiesterase (PDE), leading to an increase in cAMP and direct relaxation of smooth muscles. It lacks analgesic, euphoric, or addictive properties, making it the "least narcotic" (effectively non-narcotic) among the choices. **Analysis of Incorrect Options:** * **Morphine:** The prototypical opioid agonist. It is a potent narcotic used for severe pain but has high abuse potential. * **Codeine:** A natural phenanthrene alkaloid. While less potent than morphine, it is still a narcotic used as an antitussive and mild analgesic. * **Heroin (Diacetylmorphine):** A semi-synthetic phenanthrene. It is highly lipid-soluble, crosses the blood-brain barrier rapidly, and is more narcotic and addictive than morphine. **NEET-PG High-Yield Pearls:** * **Clinical use of Papaverine:** Historically used for visceral spasms and erectile dysfunction (intracavernosal injection), though largely replaced by PDE-5 inhibitors like Sildenafil. * **Noscapine:** Another benzylisoquinoline (non-narcotic) often used as a non-addictive antitussive; it does not cause constipation or respiratory depression. * **Opium source:** Derived from the air-dried milk (latex) of the unripe seed capsule of *Papaver somniferum*.

Question 547: Respiratory center depression can be caused by all of the following except?

A. Opium
B. Strychnine (Correct Answer)
C. Barbiturates
D. Gelsemium

Explanation: **Explanation:** The correct answer is **Strychnine**. The respiratory center in the medulla is highly sensitive to drugs that modulate inhibitory neurotransmission or opioid receptors. **1. Why Strychnine is the correct answer:** Strychnine is a potent **CNS stimulant**, not a depressant. It acts as a competitive antagonist at **glycine receptors** (primarily in the spinal cord). By blocking glycine—an inhibitory neurotransmitter—strychnine causes disinhibition, leading to generalized excitatory motor activity and "spinal convulsions." Death occurs due to asphyxia caused by sustained spasms of the diaphragm and thoracic muscles, rather than depression of the respiratory center itself. **2. Why the other options are incorrect:** * **Opium (Morphine):** Opioids are classic respiratory depressants. They act on $\mu$-receptors in the brainstem to decrease the responsiveness of the respiratory center to carbon dioxide ($CO_2$). * **Barbiturates:** These drugs enhance GABAergic transmission. In high doses, they directly depress the medullary respiratory center and the mechanisms responsible for rhythmic breathing. * **Gelsemium:** Derived from the "yellow jasmine," it contains alkaloids (like gelsemine) that act as potent CNS depressants, leading to respiratory failure and paralysis. **Clinical Pearls for NEET-PG:** * **Strychnine Poisoning:** Characterized by *Risus sardonicus* (facial grimace) and *Opisthotonus* (archback), mimicking Tetanus. However, unlike Tetanus, muscles relax between convulsions in Strychnine poisoning. * **Antidote for Opioids:** Naloxone (Competitive antagonist). * **Management of Strychnine:** Diazepam (to control convulsions) and avoiding external stimuli.

Question 548: Which drug, with chronic use in seizure states, has adverse effects including coarsening of facial features, hirsutism, gingival hyperplasia, and osteomalacia?

A. Ethosuximide
B. Carbamazepine
C. Gabapentin
D. Phenytoin (Correct Answer)

Explanation: **Explanation:** **Phenytoin** is a classic hydantoin anticonvulsant that acts by blocking voltage-gated sodium channels in their inactive state. Its chronic use is associated with a distinct profile of adverse effects due to its impact on connective tissue and vitamin metabolism. **Why Phenytoin is correct:** The symptoms described—**gingival hyperplasia** (due to increased PDGF and collagen production), **hirsutism**, and **coarsening of facial features**—are hallmark side effects of long-term Phenytoin therapy. Furthermore, Phenytoin induces hepatic microsomal enzymes (CYP450), which accelerates the metabolism of Vitamin D, leading to hypocalcemia and **osteomalacia**. **Why other options are incorrect:** * **Ethosuximide:** The drug of choice for absence seizures; its primary side effects are GI distress, fatigue, and Stevens-Johnson Syndrome, but it does not cause connective tissue changes. * **Carbamazepine:** While it also induces CYP450 and can cause osteomalacia, it is more typically associated with diplopia, ataxia, SIADH (hyponatremia), and blood dyscrasias (agranulocytosis). * **Gabapentin:** Primarily used for neuropathic pain; its side effects are generally mild, such as sedation and peripheral edema. **High-Yield Clinical Pearls for NEET-PG:** * **Mnemonic for Phenytoin side effects (PHENYTOIN):** **P**-450 induction, **H**irsutism, **E**nlarged gums, **N**ystagmus (earliest sign of toxicity), **Y**ellow-brown skin, **T**eratogenicity (Fetal Hydantoin Syndrome), **O**steomalacia, **I**nterference with B12/Folate (Megaloblastic anemia), **N**europathy. * **Kinetics:** Phenytoin follows **Zero-order kinetics** (capacity-limited metabolism) at therapeutic or high doses, making its plasma levels highly unpredictable. * **Fetal Hydantoin Syndrome:** Characterized by cleft lip/palate and digital hypoplasia.

Question 549: A patient with Parkinsonism being treated with one of the following drugs developed the skin condition shown in the image. Which drug is most likely responsible for this adverse effect?

A. Levo-dopa
B. Amantadine (Correct Answer)
C. Selegiline
D. Pramipexole

Explanation: ***Amantadine*** - The skin condition shown is **livedo reticularis**, a characteristic adverse effect of amantadine causing a **reticular (net-like) mottling pattern** on the skin. - This occurs due to amantadine's effect on **dopamine and catecholamine-mediated local vasoconstriction**, leading to the distinctive lace-like skin appearance. *Levo-dopa* - Common adverse effects include **dyskinesias**, **nausea**, and **wearing-off phenomena**, but does not cause livedo reticularis. - Skin-related effects are typically **hyperpigmentation** or **melanoma risk**, not the reticular mottling pattern seen here. *Selegiline* - As a **MAO-B inhibitor**, its main adverse effects include **insomnia**, **dizziness**, and potential **hypertensive crisis** with tyramine-rich foods. - Does not cause **livedo reticularis** or any characteristic skin mottling patterns. *Pramipexole* - This **dopamine agonist** commonly causes **impulse control disorders**, **somnolence**, and **peripheral edema**. - Skin manifestations are not typical adverse effects of pramipexole therapy.

Question 550: Rivastigmine is used in the management of which condition?

A. Dementia (Correct Answer)
B. Dissociation
C. Depression
D. Delusions

Explanation: **Explanation:** **Rivastigmine** is a **pseudo-irreversible cholinesterase inhibitor** that inhibits both acetylcholinesterase (AChE) and butyrylcholinesterase (BChE). In Alzheimer’s disease and other forms of dementia, there is a significant deficiency of acetylcholine in the cortical and subcortical regions. By inhibiting the breakdown of acetylcholine, Rivastigmine increases cholinergic neurotransmission, which helps improve cognitive function and memory. **Analysis of Options:** * **A. Dementia (Correct):** Rivastigmine is FDA-approved for the treatment of mild-to-moderate **Alzheimer’s disease** and **Parkinson’s disease dementia**. Its unique dual inhibition (AChE and BChE) may offer additional benefits in certain neurodegenerative processes. * **B. Dissociation:** This is a psychological defense mechanism or symptom of dissociative disorders (e.g., DID). It is not treated with pro-cholinergic drugs. * **C. Depression:** Depression is primarily managed with drugs affecting serotonin, norepinephrine, or dopamine (e.g., SSRIs, SNRIs). * **D. Delusions:** These are fixed false beliefs treated with antipsychotics (dopamine antagonists), not cholinesterase inhibitors. **High-Yield Clinical Pearls for NEET-PG:** * **Route of Administration:** Rivastigmine is available as a **transdermal patch**, which is preferred in dementia patients to reduce gastrointestinal side effects (nausea/vomiting) and improve compliance. * **Selectivity:** It is "brain-selective," meaning it has a higher affinity for AChE in the CNS than in the periphery. * **Other Drugs in Class:** Donepezil (long-acting) and Galantamine (also acts on nicotinic receptors) are other common treatments for Alzheimer’s. * **Side Effects:** Common side effects are cholinergic in nature—nausea, vomiting, diarrhea, and bradycardia.

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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