Pharmacology: Cholinoreceptor Activating and Cholinesterase Inhibiting Drugs

High-yield practice questions derived from lecture notes on cholinergic stimulants, acetylcholinesterase inhibitors, their mechanisms of action, clinical applications, and toxicological management.

Into what two major categories are cholinomimetics classified based on their mode of action?

Direct-acting agents (which bind and activate cholinoceptors) and indirect-acting agents (which inhibit the hydrolysis of endogenous acetylcholine).

Where are muscarinic receptors primarily located?

In the heart, nerves, smooth muscle, exocrine glands, and vascular endothelium.

Where are nicotinic receptors primarily located?

In the neuromuscular end plate, skeletal muscles, autonomic ganglia (including the adrenal medulla), and the central nervous system.

What is the primary chemical difference between tertiary and quaternary amines in terms of pharmacokinetics?

Tertiary amines (like pilocarpine) are lipid-soluble and can cross the blood-brain barrier ($\text{BBB}$), while quaternary amines (like muscarine) are lipid-insoluble, permanently charged, and poorly absorbed from the GIT.

Which choline ester is selective for muscarinic receptors and completely resistant to hydrolysis by acetylcholinesterase ($\text{AChE}$)?

Bethanechol.

What is the effect of the $\beta$-methyl group in methacholine and bethanechol?

It reduces the potency of these drugs at nicotinic receptors while maintaining activity at muscarinic receptors.

Which specific receptor activation leads to the breakdown of phosphoinositols and the formation of $IP_3$ and $DAG$?

Activation of $M_1$, $M_3$, and $M_5$ receptors.

How do indirect-acting agents produce their primary effect?

By inhibiting the enzyme acetylcholinesterase ($\text{AChE}$), thereby increasing the endogenous acetylcholine concentration in the synaptic cleft.

What is the mechanism for generalized vasodilation when $M_3$ receptors in the vascular epithelium are stimulated?

Activation of endothelium nitric oxide synthase ($\text{eNOS}$) produces nitric oxide, which diffuses to smooth muscle to increase $cGMP$, leading to relaxation.

What are the common effects of muscarinic agonists on the eye?

Miosis (contraction of the pupillary sphincter) and cyclospasm (contraction of the ciliary muscle for near vision).

Which drug is used as a selective $M_3$ agonist for treating xerostomia in patients with Sjogren's syndrome?

Cevimeline.

What are the components of the DUMBELS syndrome used to identify cholinomimetic overdose?

Diarrhea/Defecation, Urination, Miosis/Muscle weakness, Bronchoconstriction, Emesis, Lacrimation, Salivation/Seizures/Sweating.

What is the mechanism of toxicity for Amanita phalloides (death cap mushroom)?

Amatoxins inhibit RNA polymerase II, blocking $mRNA$ synthesis and causing cell death.

What is the primary diagnostic use of Edrophonium?

It is used as a rapid diagnostic test for Myasthenia Gravis and to differentiate myasthenic crisis from cholinergic crisis.

Which indirectly acting carbamate can cross the blood-brain barrier ($\text{BBB}$)?

Physostigmine (a tertiary amine).

What is the 'anti-curare action' of Neostigmine?

It reverses neuromuscular blockade caused by nondepolarizing competitive muscle relaxants at the neuromuscular junction ($\text{NMJ}$).

Why are organophosphates considered irreversible inhibitors of $AChE$?

They form a stable phosphorylated enzyme through covalent phosphorus-enzyme bonds that can undergo 'aging,' making the complex nearly impossible to break.

Which organophosphate is highly polar and used in the treatment of glaucoma?

Echothiophate.

What is the role of Pralidoxime in toxicology?

It acts as a cholinesterase reactivator for organophosphate poisoning by breaking the phosphorus-enzyme bond, but it must be administered before 'aging' occurs.

How does Edrophonium differentiate between a myasthenic crisis and a cholinergic crisis?

In a myasthenic crisis, muscle strength improves after administration; in a cholinergic crisis, muscle strength weakens further due to excess $ACh$.

Which drugs are intermediate to long-acting anticholinesterases used for Alzheimer's Disease?

Donepezil, Rivastigmine, and Galantamine.

What is a major contraindication for the use of cholinomimetic drugs involving the respiratory system?

Asthma, as these drugs can aggravate bronchoconstriction through $M_3$ and $M_2$ receptor stimulation.

Which acetylcholinesterase inhibitor used for Alzheimer's was stopped in development or used less frequently due to hepatotoxicity?

Tacrine.

In the absence of intact endothelium, how do muscarinic agonists affect vascular smooth muscle?

They cause vasoconstriction due to $IP_3$ production and the release of intracellular $Ca^{2+}$ through direct $M_3$ receptor activation on the smooth muscle cells.

What is the treatment for antimuscarinic drug intoxication (e.g., Atropine toxicity)?

Physostigmine is administered because it can traverse the CNS to reverse central anticholinergic symptoms.