Autonomic Nervous System Drugs Practice Questions

Q: Tyramine acts by:

Releasing norepinephrine. **Explanation:** **Tyramine** is a classic example of an **indirectly acting sympathomimetic**. It does not act directly on adrenergic receptors. Instead, it is taken up into the presynaptic nerve terminal via the Norepinephrine Transporter (NET). Once inside, it enters the storage vesicles through the Vesicular Monoamine Transporter (VMAT), displacing **Norepinephrine (NE)** into the cytoplasm. This high concentration of cytoplasmic NE causes the NET to work in reverse, pumping NE into the synaptic cleft, where it activates post-synaptic receptors. **Analysis of Options:** * **A. Inhibiting acetylcholinesterase:** This is the mechanism of action for parasympathomimetics like Neostigmine or Organophosphates, which increase acetylcholine levels. * **B. Stimulating co-receptors:** Tyramine specifically targets the displacement of primary neurotransmitters (NE) rather than modulating co-receptors (like NPY or ATP). * **D. Stimulating beta-3 receptors:** Direct stimulation of $\beta_3$ receptors is seen with drugs like Mirabegron (used for overactive bladder). **High-Yield Clinical Pearls for NEET-PG:** * **The Cheese Reaction:** Tyramine is found in fermented foods (aged cheese, red wine, pickled fish). It is normally metabolized by **MAO-A** in the gut. In patients taking **MAO inhibitors**, tyramine reaches the systemic circulation in high amounts, leading to a massive release of NE and a life-threatening **Hypertensive Crisis**. * **Tachyphylaxis:** Tyramine shows the phenomenon of tachyphylaxis (rapidly diminishing response to repeated doses) because it depletes the available stores of NE. * **Drug Interaction:** The effects of Tyramine are blocked by **Reserpine** (which inhibits VMAT) and **Cocaine/TCAs** (which inhibit NET).

Q: All of the following drugs act on dopaminergic receptors except?

Dobutamine. **Explanation:** The correct answer is **Dobutamine** because its primary mechanism of action involves adrenergic receptors, not dopaminergic receptors. **1. Why Dobutamine is the correct answer:** Dobutamine is a synthetic catecholamine that acts as a **selective $\beta_1$-agonist**. While it has minor effects on $\beta_2$ and $\alpha_1$ receptors, it has **no significant activity at dopamine (D) receptors**. It is primarily used as an inotropic agent in cardiogenic shock and acute heart failure to increase cardiac output without significantly increasing heart rate. **2. Analysis of other options:** * **Domperidone:** This is a peripheral **$D_2$ receptor antagonist**. It is used as an antiemetic and prokinetic. Unlike metoclopramide, it does not cross the blood-brain barrier easily, resulting in fewer extrapyramidal side effects. * **Fenoldopam:** This is a selective **$D_1$ receptor agonist**. It causes systemic vasodilation, particularly in the renal and mesenteric beds. It is used clinically in the management of hypertensive emergencies. * **Metoclopramide:** This is a central and peripheral **$D_2$ receptor antagonist**. It also possesses $5-HT_4$ agonist properties. It is a potent antiemetic and prokinetic agent but can cause extrapyramidal symptoms (EPS) due to central dopamine blockade. **Clinical Pearls for NEET-PG:** * **Dopamine (The Drug):** Unlike Dobutamine, Dopamine acts on $D_1$ receptors (low dose), $\beta_1$ receptors (medium dose), and $\alpha_1$ receptors (high dose). * **Renal Dose Myth:** Low-dose dopamine was historically thought to protect the kidneys via $D_1$ vasodilation, but clinical trials (SOAP II) have shown no mortality benefit in acute renal failure. * **Drug of Choice:** Fenoldopam is unique as it is the only parenteral dopamine agonist used for hypertensive crisis, especially when renal perfusion is a concern.

Q: Which of the following drugs is not given alone in a patient of pheochromocytoma?

Atenolol. In **Pheochromocytoma**, there is a massive release of catecholamines (Epinephrine and Norepinephrine). These act on both **$\alpha$-receptors** (causing vasoconstriction and hypertension) and **$\beta$-receptors** (causing tachycardia and vasodilation in skeletal muscle). ### **Why Atenolol (Option A) is the Correct Answer** Atenolol is a selective $\beta_1$-blocker. If a $\beta$-blocker is given alone, it inhibits $\beta_2$-mediated vasodilation while leaving $\alpha_1$-mediated vasoconstriction unopposed. This leads to a **paradoxical rise in blood pressure** (hypertensive crisis). * **Rule:** In pheochromocytoma, **always give an $\alpha$-blocker first**, followed by a $\beta$-blocker. ### **Why Other Options are Incorrect** * **B. Prazosin:** This is a selective $\alpha_1$-blocker. It is safe and often used to control hypertension in these patients by reducing peripheral vascular resistance. * **C. Nitroprusside:** This is a potent vasodilator used intravenously to manage **hypertensive emergencies** during surgery for pheochromocytoma. * **D. Metyrosine:** This drug inhibits **Tyrosine Hydroxylase** (the rate-limiting enzyme in catecholamine synthesis). It is used to decrease the total catecholamine pool before surgery. ### **Clinical Pearls for NEET-PG** 1. **Drug of Choice (Pre-op):** Phenoxybenzamine (a non-selective, irreversible $\alpha$-blocker). 2. **The "7-10 Day Rule":** Start $\alpha$-blockade at least 7–10 days before surgery; add $\beta$-blockers only after adequate $\alpha$-blockade is achieved. 3. **Diagnosis:** Best initial screening test is **Urinary/Plasma Metanephrines**. 4. **Rule of 10s:** 10% are bilateral, 10% are malignant, 10% are extra-adrenal (Paraganglioma), and 10% are familial.

Q: What effect does a lower dose of dopamine have on a patient in cardiogenic shock?

Urine output. ### Explanation Dopamine is a unique catecholamine because its effects are strictly **dose-dependent**. Understanding these dose ranges is a high-yield concept for NEET-PG. **1. Why "Urine Output" is correct:** At **low doses (0.5–2 µg/kg/min)**, dopamine primarily acts on **D1 receptors** located in the renal, mesenteric, and coronary vascular beds. Activation of D1 receptors causes vasodilation, specifically increasing renal blood flow and the glomerular filtration rate (GFR). This results in increased **urine output** (diuresis) and sodium excretion (natriuresis). This is often referred to as the "renal dose" of dopamine. **2. Analysis of Incorrect Options:** * **Cardiac Output (Option A):** This is the primary effect of **medium doses (2–10 µg/kg/min)**. At this range, dopamine stimulates **β1-adrenergic receptors** in the heart, increasing myocardial contractility (inotropy) and stroke volume. * **Heart Rate (Option C):** While β1 stimulation at medium doses can increase heart rate (chronotropy), it is not the specific effect of a *low* dose. Tachycardia is often a limiting side effect at higher doses. * **Blood Pressure (Option D):** Significant increases in blood pressure occur at **high doses (>10 µg/kg/min)**. At this level, dopamine stimulates **α1-adrenergic receptors**, leading to systemic vasoconstriction and increased peripheral resistance. **3. Clinical Pearls for NEET-PG:** * **The "D-B-A" Rule:** Remember the sequence of receptor activation as dose increases: **D**opaminergic (Low) → **B**eta (Medium) → **A**lpha (High). * **Fenoldopam:** A selective D1 agonist used in hypertensive emergencies to maintain renal perfusion. * **Current Status:** While "renal dose" dopamine increases urine output, clinical trials (like the SOAP II trial) have shown it does not necessarily prevent acute renal failure or improve mortality, leading to a preference for Norepinephrine in many shock scenarios.

Q: Which of the following medications can cause hyperkalemia?

Succinylcholine. **Explanation:** **Succinylcholine** is a depolarizing neuromuscular blocker that acts as an agonist at the nicotinic acetylcholine receptors (nAChR) of the motor endplate. Upon binding, it causes prolonged depolarization, leading to an efflux of intracellular **potassium (K+)** into the extracellular space. In healthy individuals, this typically results in a minor rise in serum potassium (0.5 mEq/L). However, in patients with "upregulation" of extrajunctional receptors (e.g., burns, massive trauma, or prolonged immobilization), this efflux can be massive and life-threatening. **Analysis of Incorrect Options:** * **Amphotericin B:** This antifungal is notorious for causing **hypokalemia** and hypomagnesemia due to its toxic effects on the distal renal tubules (Type 1 RTA), leading to increased potassium excretion. * **Beta agonists (e.g., Salbutamol):** These drugs stimulate the Na+/K+-ATPase pump, shifting potassium from the extracellular fluid into the cells. Consequently, they cause **hypokalemia** and are actually used in the emergency management of hyperkalemia. * **Gentamicin:** As an aminoglycoside, it can cause nephrotoxicity and electrolyte disturbances, most commonly **hypokalemia** and hypomagnesemia (Bartter-like syndrome). **NEET-PG High-Yield Pearls:** * **Succinylcholine Contraindications:** Avoid in patients with major burns (>24 hours old), crush injuries, spinal cord injuries, and muscular dystrophies (risk of rhabdomyolysis). * **Drug-Induced Hyperkalemia Mnemonic (K-BANK):** **K**-sparing diuretics, **B**eta-blockers, **A**CE inhibitors/ARBs, **N**SAIDs, and **K** (Potassium) supplements/Succinylcholine. * **Antidote:** There is no direct pharmacological reversal agent for Succinylcholine; it is metabolized by plasma pseudocholinesterase.

Q: Which one of the following drugs does not induce mydriasis?

Phentolamine. To understand mydriasis (pupillary dilation), one must remember that it is mediated by the **Alpha-1 ($\alpha_1$) receptors** located on the radial (dilator) muscle of the iris. ### **1. Why Phentolamine is the Correct Answer** **Phentolamine** is a non-selective **alpha-adrenergic blocker** (blocks both $\alpha_1$ and $\alpha_2$). By blocking $\alpha_1$ receptors on the iris dilator muscle, it prevents contraction, thereby inhibiting mydriasis. In fact, alpha-blockers can lead to miosis (pupillary constriction) due to unopposed parasympathetic action. ### **2. Why the Other Options are Incorrect** * **Phenylephrine (Option C):** A selective **$\alpha_1$ agonist**. It directly stimulates the radial muscle, causing "active mydriasis" without affecting accommodation (no cycloplegia). * **Ephedrine (Option B):** A **mixed-acting sympathomimetic**. It acts directly on $\alpha$ and $\beta$ receptors and indirectly by releasing stored norepinephrine, leading to pupillary dilation. * **Cocaine (Option D):** An **indirect sympathomimetic**. It blocks the reuptake of norepinephrine at the synaptic cleft. The increased concentration of norepinephrine in the neuro-effector junction of the iris results in mydriasis. ### **3. High-Yield Clinical Pearls for NEET-PG** * **Active vs. Passive Mydriasis:** Sympathomimetics cause *active* mydriasis (contraction of dilator). Anticholinergics (like Atropine) cause *passive* mydriasis (relaxation of sphincter) accompanied by **cycloplegia** (loss of accommodation). * **Diagnostic Use:** Phenylephrine is preferred for routine fundus examination when cycloplegia is not required. * **Cocaine Test:** Historically used to diagnose **Horner’s Syndrome**. A normal eye dilates with cocaine, but a Horner's eye (sympathetic denervation) does not. * **Phentolamine Use:** Clinically used in the management of pheochromocytoma and extravasation of catecholamines.

Q: Which of the following drugs increases gastrointestinal motility?

Neostigmine. **Explanation:** The gastrointestinal (GI) tract is primarily regulated by the parasympathetic nervous system via **muscarinic (M3) receptors**. Activation of these receptors increases intestinal tone and peristalsis while relaxing sphincters, thereby promoting GI motility. **Why Neostigmine is Correct:** Neostigmine is an **indirect-acting cholinergic agonist** (Anticholinesterase). It inhibits the enzyme acetylcholinesterase, leading to an accumulation of endogenous acetylcholine at the neuromuscular junction and neuroeffector sites. By increasing acetylcholine levels at M3 receptors, it stimulates intestinal smooth muscle contraction. Clinically, it is used to treat paralytic ileus and Ogilvie’s syndrome (acute colonic pseudo-obstruction). **Why Other Options are Incorrect:** * **Glycopyrrolate & Atropine:** These are **muscarinic antagonists** (Anticholinergics). They block M3 receptors in the gut, leading to decreased secretions and reduced motility (antispasmodic effect). They are often used pre-operatively to reduce secretions or to treat bradycardia. * **Fentanyl:** This is a potent **opioid analgesic**. Opioids act on $\mu$-receptors in the myenteric plexus to inhibit acetylcholine release, significantly *decreasing* GI motility and causing constipation. **High-Yield Clinical Pearls for NEET-PG:** * **Neostigmine Side Effects:** Being a quaternary ammonium compound, it does not cross the Blood-Brain Barrier (BBB), but it can cause systemic "DUMBELS" effects (Diarrhea, Urination, Miosis, Bradycardia, etc.). * **Prokinetic of Choice:** While Neostigmine increases motility, **Metoclopramide** (D2 antagonist) is more commonly used as a prokinetic for GERD and gastroparesis. * **Atropine vs. Glycopyrrolate:** Glycopyrrolate is preferred in anesthesia because it is polar, does not cross the BBB, and causes less tachycardia than atropine.

Q: Which of the following is not an endogenous catecholamine?

Dobutamine. **Explanation:** The core concept here is the distinction between **endogenous catecholamines** (naturally produced in the body) and **synthetic catecholamines** (man-made pharmacological agents). **Why Dobutamine is the correct answer:** **Dobutamine** is a **synthetic** catecholamine. It is a derivative of dopamine but is not produced naturally within the human body. Clinically, it acts primarily as a selective **$\beta_1$-agonist** with mild $\beta_2$ and $\alpha_1$ activity, making it a "positive inotrope" used in the management of acute heart failure and cardiogenic shock. **Why the other options are incorrect:** * **Dopamine (A):** An endogenous catecholamine and a precursor to Noradrenaline. It acts on D1, D2, $\beta_1$, and $\alpha_1$ receptors depending on the dose. * **Adrenaline/Epinephrine (C):** The primary hormone secreted by the **adrenal medulla**. It acts on all $\alpha$ and $\beta$ receptors. * **Noradrenaline/Norepinephrine (D):** The primary neurotransmitter of the **postganglionic sympathetic nerves**. It acts mainly on $\alpha_1$, $\alpha_2$, and $\beta_1$ receptors (with negligible effect on $\beta_2$). **High-Yield NEET-PG Pearls:** 1. **Chemical Structure:** Catecholamines consist of a catechol nucleus (benzene ring with two adjacent hydroxyl groups) and an amine side chain. 2. **Biosynthesis Pathway:** Tyrosine $\rightarrow$ L-Dopa $\rightarrow$ **Dopamine** $\rightarrow$ **Noradrenaline** $\rightarrow$ **Adrenaline**. 3. **Metabolism:** Endogenous catecholamines are rapidly metabolized by **MAO** (Monoamine oxidase) and **COMT** (Catechol-O-methyltransferase), which is why they have a very short half-life and are ineffective orally. 4. **Other Synthetic Catecholamines:** Isoprenaline (Isoproterenol) is another common synthetic catecholamine frequently tested alongside Dobutamine.

Q: Which of the following is a parasympatholytic agent?

Atropine. ### Explanation **Correct Answer: A. Atropine** **Mechanism and Concept:** A **parasympatholytic** agent (also known as an anticholinergic or muscarinic antagonist) is a drug that reduces the activity of the parasympathetic nervous system. **Atropine** is the prototype competitive antagonist at muscarinic receptors. It blocks the action of acetylcholine (ACh) at effector organs, leading to effects such as tachycardia, mydriasis, decreased secretions, and bronchodilation. **Analysis of Incorrect Options:** * **B. Neostigmine & C. Pyridostigmine:** These are **parasympathomimetics**. Specifically, they are reversible acetylcholinesterase (AChE) inhibitors. By preventing the breakdown of endogenous ACh, they increase cholinergic activity. They are used in treating Myasthenia Gravis and reversing neuromuscular blockade. * **D. Acetylcholine:** This is the primary neurotransmitter of the parasympathetic nervous system. It is a direct-acting **parasympathomimetic** that stimulates both muscarinic and nicotinic receptors. **High-Yield Clinical Pearls for NEET-PG:** * **Atropine Flush:** High doses of atropine can cause cutaneous vasodilation (redness), especially in children. * **Drug of Choice (DOC):** Atropine is the DOC for **symptomatic sinus bradycardia** and **organophosphate poisoning** (where it antagonizes muscarinic effects). * **Contraindication:** Parasympatholytics like atropine are strictly contraindicated in patients with **narrow-angle glaucoma** (as they can precipitate an acute attack) and **Benign Prostatic Hyperplasia (BPH)** (due to risk of urinary retention). * **Mnemonic for Atropine Toxicity:** "Hot as a hare, red as a beet, dry as a bone, blind as a bat, and mad as a hatter."

Q: All of the following effects are seen with cholinergic muscarinic receptor stimulation except?

Rise in blood pressure. **Explanation:** The correct answer is **B. Rise in blood pressure**. Muscarinic receptor (M1-M5) stimulation primarily activates the parasympathetic nervous system. Stimulation of **M3 receptors** on vascular endothelial cells triggers the release of **Nitric Oxide (NO)**, which leads to vasodilation and a **fall in blood pressure**, not a rise. While high doses of acetylcholine can cause a rise in BP via nicotinic receptors in sympathetic ganglia (the "Nicotinic effect of ACh"), pure muscarinic stimulation results in hypotension. **Analysis of other options:** * **A. Sweating:** Although sweat glands are part of the sympathetic nervous system anatomically, they are unique because they are **cholinergic** (mediated by **M3 receptors**). Thus, muscarinic agonists increase sweating (diaphoresis). * **C. Bradycardia:** Stimulation of **M2 receptors** in the SA node of the heart leads to hyperpolarization and a decrease in heart rate (negative chronotropic effect). * **D. Urination:** Muscarinic stimulation (**M3 receptors**) causes contraction of the detrusor muscle and relaxation of the trigone/sphincter, facilitating voiding of urine. **NEET-PG High-Yield Pearls:** * **Mnemonic for Cholinergic effects:** **DUMBELS** (Diarrhea, Urination, Miosis, Bradycardia/Bronchospasm, Emesis, Lacrimation, Salivation/Sweating). * **M1:** Gastric glands (CNS); **M2:** Heart; **M3:** Smooth muscles, Exocrine glands, and Eye (Ciliary muscle/Sphincter pupillae). * **Vascular Paradox:** Blood vessels have M3 receptors but no parasympathetic innervation. They respond only to exogenous muscarinic agonists or circulating ACh.

Question 1

Tyramine acts by:

Accepted Answer

Releasing norepinephrine

Answer

Inhibiting acetylcholinesterase

Answer

Stimulating co-receptors

Answer

Stimulating beta-3 receptors

Question 2

All of the following drugs act on dopaminergic receptors except?

Accepted Answer

Dobutamine

Answer

Domperidone

Answer

Fenoldopam

Answer

Metoclopramide

Question 3

Which of the following drugs is not given alone in a patient of pheochromocytoma?

Accepted Answer

Atenolol

Answer

Prazosin

Answer

Nitroprusside

Answer

Metyrosine

Question 4

What effect does a lower dose of dopamine have on a patient in cardiogenic shock?

Accepted Answer

Urine output

Answer

Cardiac output

Answer

Heart rate

Answer

Blood pressure

Question 5

Which of the following medications can cause hyperkalemia?

Accepted Answer

Succinylcholine

Answer

Amphotericin B

Answer

Beta agonists

Answer

Gentamicin

Question 6

Which one of the following drugs does not induce mydriasis?

Accepted Answer

Phentolamine

Answer

Ephedrine

Answer

Phenylephrine

Answer

Cocaine

Question 7

Which of the following drugs increases gastrointestinal motility?

Accepted Answer

Neostigmine

Answer

Glycopyrrolate

Answer

Atropine

Answer

Fentanyl

Question 8

Which of the following is not an endogenous catecholamine?

Accepted Answer

Dobutamine

Answer

Dopamine

Answer

Adrenaline

Answer

Noradrenaline

Question 9

Which of the following is a parasympatholytic agent?

Accepted Answer

Atropine

Answer

Neostigmine

Answer

Pyridostigmine

Answer

Acetylcholine

Question 10

All of the following effects are seen with cholinergic muscarinic receptor stimulation except?

Accepted Answer

Rise in blood pressure

Answer

Sweating

Answer

Bradycardia

Answer

Urination

Autonomic Nervous System Drugs — MCQs

Autonomic Nervous System Drugs — MCQs

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