Autonomic Nervous System Drugs (Adrenergics & Cholinergics)

Sympathetic catecholamines (norepinephrine and epinephrine) activate five canonical G-protein-coupled adrenergic receptors with distinct vascular, cardiac, and metabolic effects. α₁-receptors (G_q → PLC → IP₃/DAG → ↑ Ca²⁺) constrict vascular smooth muscle in skin, splanchnic, and renal arterioles (↑ SVR and diastolic BP), contract the radial dilator pupillae muscle (producing mydriasis without cycloplegia), and contract the internal urethral sphincter and prostatic capsule. α₂-receptors (G_i → ↓ cAMP) reside on presynaptic sympathetic nerve terminals as autoinhibitory feedback brakes suppressing further norepinephrine exocytosis, on pancreatic β-islet cells (inhibiting insulin release), and on platelets (promoting aggregation). β₁-receptors (G_s → ↑ cAMP → PKA) drive SA nodal chronotropy, AV nodal dromotropy, ventricular inotropy/lusitropy (via phospholamban and L-type Ca²⁺ channel phosphorylation), and juxtaglomerular apparatus renin secretion. β₂-receptors (G_s → ↑ cAMP) activate myosin light-chain phosphatase to relax bronchial, uterine, and skeletal muscle arteriolar smooth muscle while stimulating hepatic glycogenolysis/gluconeogenesis and Na⁺/K⁺-ATPase-mediated intracellular potassium uptake. β₃-receptors (G_s) stimulate lipolysis in brown adipose tissue and relax bladder detrusor smooth muscle during urine storage (targeted by mirabegron and vibegron).

Parasympathetic postganglionic fibers release acetylcholine (ACh) onto metabotropic muscarinic receptors, plus sympathetic postganglionic fibers innervating eccrine sweat glands uniquely utilize ACh on M₃ receptors (sympathetic cholinergic exception). M₁ receptors (G_q) mediate cortical/hippocampal memory consolidation and enteric plexus excitation; M₂ receptors (G_i) hyperpolarize SA nodal pacemaker cells (opening I_KACh channels to slow phase 4 depolarization) and prolong AV nodal conduction velocity; and M₃ receptors (G_q) contract bronchial smooth muscle, intestinal peristaltic smooth muscle, bladder detrusor muscle, and the pupillary sphincter/ciliary muscles (miosis and accommodation for near vision). On intact vascular endothelium, M₃ activation stimulates endothelial nitric oxide synthase (eNOS) to generate nitric oxide (NO), which diffuses into adjacent vascular smooth muscle to activate soluble guanylyl cyclase (↑ cGMP) and cause vasodilation. Meanwhile, D₁ receptors (G_s) dilate renal, mesenteric, and coronary vascular beds (exploited by fenoldopam in hypertensive emergencies with renal insufficiency).

Organophosphate compounds—including agricultural insecticides (parathion, malathion, chlorpyrifos), ophthalmic echothiophate, and chemical warfare nerve agents (sarin, soman, tabun, VX)—phosphorylate the catalytic serine hydroxyl group within the active gorge of acetylcholinesterase (AChE). Unlike reversible carbamates (neostigmine, physostigmine, pyridostigmine, carbaryl), which carbamylate AChE for only 2 to 8 hours before spontaneous hydrolysis restores enzyme function, organophosphate-AChE conjugates undergo a time-dependent dealkylation reaction termed "aging". Once an alkyl group is cleaved from the phosphorylated enzyme, the phosphorus-oxygen bond becomes permanently resistant to nucleophilic attack. Clinically, patients present with fulminant DUMBBELLS (Diarrhea, Urination, Miosis, Bronchospasm/Bronchorrhea, Bradycardia, Emesis, Lacrimation, Lethargy, Salivation/Sweating) alongside nicotinic N_M muscle fasciculations that rapidly fatigue into depolarizing flaccid paralysis of the diaphragm and intercostals—the primary cause of death alongside drowning in copious bronchial secretions.

Resuscitation of organophosphate poisoning follows a strict pharmacological sequence: following personal protective decontamination and dermal washing, intravenous atropine must be administered immediately and doubled every 3 to 5 minutes until tracheobronchial secretions dry and ventilation normalizes (endpoint is pulmonary drying, not pupil dilation). Because atropine competitively blocks only central and peripheral muscarinic receptors, it has zero effect on nicotinic N_M diaphragmatic paralysis. Therefore, pralidoxime (2-PAM)—a quaternary oxime nucleophile that regenerates phosphorylated AChE at both muscarinic and nicotinic neuromuscular junctions—must be co-administered with atropine prior to enzyme aging. In patients with myasthenia gravis on chronic pyridostigmine who present with acute respiratory weakness, clinicians must distinguish underdosing (myasthenic crisis, accompanied by ptosis/diplopia and normal secretions) from overdosing (cholinergic crisis, marked by miosis, sialorrhea, diarrhea, and muscle fasciculations); airway stabilization and withholding anticholinesterases take precedence over legacy edrophonium testing.

Antimuscarinic agents are stratified by their charge state at physiological pH. Tertiary amines (atropine, scopolamine, benztropine, trihexyphenidyl, tropicamide, oxybutynin) are uncharged, lipid-soluble molecules that readily cross the blood-brain barrier and corneal epithelium. Scopolamine targets vestibular M₁ nuclei to prevent motion sickness; benztropine and trihexyphenidyl restore striatal dopamine-acetylcholine balance in acute antipsychotic-induced dystonia and pseudoparkinsonism; and short-acting tropicamide (4–6 hours) or long-acting atropine (7–14 days) produces mydriasis and cycloplegia for funduscopic exam. Conversely, quaternary ammonium compounds (glycopyrrolate, ipratropium, tiotropium, trospium) carry a permanent positive charge that prevents CNS penetration. Inhaled ipratropium (short-acting SAMA) and tiotropium/umeclidinium (long-acting LAMAs) block bronchial M₃ receptors in COPD and asthma without systemic dry mouth or delirium, while parenteral glycopyrrolate reduces perioperative sialorrhea and vagal bradycardia during neostigmine neuromuscular reversal.

Overdosage of antimuscarinic medications—including diphenhydramine, tricyclic antidepressants (amitriptyline), low-potency first-generation antipsychotics (chlorpromazine), jimsonweed (Datura stramonium containing atropine/scopolamine), or deadly nightshade (Atropa belladonna)—produces the classic anticholinergic toxidrome: " Blind as a bat" (mydriasis and cycloplegia causing blurry near vision), "Mad as a hatter" (central M₁ blockade causing agitated delirium and visual hallucinations), "Red as a beet" (cutaneous vasodilation replacing blocked thermoregulatory sweating), "Hot as a hare" (anhidrotic hyperthermia), "Dry as a bone" (xerostomia, dry axillae), and "Full as a flask" (detrusor atony with α₁-unopposed sphincter tone causing acute urinary retention, especially in older men with BPH). Because of Beers Criteria delirium, falls, and acute angle-closure glaucoma precipitation, antimuscarinics are avoided in older adults. Pure central and peripheral anticholinergic toxicity is reversed by physostigmine (a tertiary carbamate AChE inhibitor that crosses the blood-brain barrier), provided the ECG shows a normal QRS duration (physostigmine is contraindicated in tricyclic antidepressant overdose due to refractory bradyasystole).

When high-dose epinephrine is infused alone, potent α₁ vasoconstriction overrides β₂ vasodilation, producing a net rise in mean arterial pressure (MAP). However, if a patient is pretreated with an α₁-antagonist (e.g., phenoxybenzamine, phentolamine, or a low-potency antipsychotic like chlorpromazine), subsequent epinephrine administration leaves vascular β₂-receptors completely unopposed—converting a pressor response into profound hypotension (Dale's Vasomotor Reversal). Consequently, hypotension from chlorpromazine or phenoxybenzamine overdose must be treated with pure α₁-agonists (phenylephrine or norepinephrine), never epinephrine. The inverse phenomenon—unopposed α₁ vasoconstriction—occurs when a patient with acute cocaine intoxication, amphetamine toxicity, or pheochromocytoma receives a pure β-blocker (e.g., metoprolol or propranolol): blocking vasodilatory β₂ receptors leaves coronary and systemic α₁ receptors unopposed, precipitating fulminant hypertensive crisis and coronary vasospasm. Cocaine-associated chest pain and hypertension are instead managed with benzodiazepines, nitrates, and phentolamine.

Interpreting arterial blood pressure and heart rate tracings requires integrating direct receptor affinities with the carotid sinus (CN IX, glossopharyngeal) and aortic arch (CN X, vagus) baroreceptor reflex. Pure phenylephrine (α₁ > α₂) causes intense arteriolar vasoconstriction (↑↑ SVR, ↑ systolic and diastolic BP), which stretches arterial baroreceptors and triggers a powerful vagal efferent discharge that markedly slows heart rate (↓↓ reflex bradycardia). Norepinephrine (α₁ > α₂ > β₁, negligible β₂) elevates both systolic and diastolic blood pressure (↑↑ MAP) while baroreceptor-mediated vagal tone overrides its direct β₁ positive chronotropy, resulting in a net decrease in heart rate (↓ HR) despite increased contractility. Isoproterenol (β₁ = β₂, zero α) causes massive β₂ arteriolar vasodilation (↓↓ SVR and diastolic BP, ↓ MAP), so both direct β₁ stimulation and baroreceptor reflex unloading synergize to produce dramatic tachycardia (↑↑↑ HR) with widened pulse pressure. Low-dose epinephrine preferentially activates higher-affinity β₂ and β₁ receptors (↑ SBP, ↓ DBP, ↑ HR), whereas high-dose epinephrine recruits α₁ vasoconstriction.

Primary open-angle glaucoma (POAG) is painless, progressive optic neuropathy treated first-line with topical prostaglandin F_2α analogs (latanoprost, bimatoprost, travoprost), which remodel extracellular matrix metalloproteinases in the ciliary body to increase uveoscleral outflow, or topical β-blockers (timolol, betaxolol), which inhibit G_s-cAMP signaling in the non-pigmented ciliary epithelium to decrease aqueous humor secretion. In contrast, acute angle-closure glaucoma is an ophthalmic emergency presenting with a rock-hard, painful red eye, corneal epithelial edema (halos around lights), frontal headache, nausea/vomiting, and a fixed mid-dilated pupil (4–6 mm) that crowds the iridocorneal trabecular meshwork. Emergency medical therapy Prior to definitive laser peripheral iridotomy combines topical pilocarpine (a direct muscarinic M₃ agonist that contracts the pupillary sphincter and ciliary muscle to pull the peripheral iris away from the trabecular meshwork), topical timolol and apraclonidine/brimonidine (α₂ agonists), and intravenous acetazolamide or hyperosmotic mannitol. Mydriatic α₁ agonists (phenylephrine) and antimuscarinics (atropine, tropicamide) are strictly contraindicated in narrow angles because pupillary dilation bunches the iris into the iridocorneal angle.

Direct muscarinic agonists and reversible cholinesterase inhibitors restore cholinergic tone in targeted organ systems. Bethanechol—a carbamic acid ester engineered with a β-methyl group that confers complete resistance to acetylcholinesterase hydrolysis and pure muscarinic (M₃) selectivity without nicotinic activity—stimulates detrusor contraction and GI peristalsis in postoperative non-obstructive urinary retention and neurogenic ileus (after mechanical obstruction has been excluded). Cevimeline and oral pilocarpine stimulate residual salivary and lacrimal gland M₃ receptors to relieve xerostomia and keratoconjunctivitis sicca in Sjögren syndrome and post-head-and-neck radiation. In myasthenia gravis, autoantibodies against postsynaptic skeletal muscle nicotinic N_M receptors cause fatigable ptosis, diplopia, bulbar dysarthria, and proximal limb weakness; oral pyridostigmine (a long-acting quaternary carbamate AChE inhibitor that cannot cross the blood-brain barrier) prolongs synaptic ACh dwell time at the neuromuscular junction, often paired with glycopyrrolate if muscarinic abdominal cramping or diarrhea becomes dose-limiting.

Alpha-adrenergic blockers are divided by receptor subtype selectivity and covalent vs. competitive binding kinetics. Phenoxybenzamine forms an irreversible covalent carbonium ion bond with both α₁ and α₂ receptors (duration 14–48 hours, requiring de novo receptor synthesis), making it the gold-standard oral agent initiated 10 to 14 days prior to adrenalectomy for pheochromocytoma to prevent intraoperative hypertensive crisis when the tumor is manipulated. Phentolamine is a short-acting, reversible competitive α₁/α₂ antagonist used intravenously for tyramine-MAOI hypertensive crises, cocaine toxicity, and local infiltration into tissues after accidental extravasation of norepinephrine or dopamine (preventing ischemic dermal necrosis). Selective α₁-antagonists (prazosin, terazosin, doxazosin) relax vascular smooth muscle and internal urethral sphincter tone in patients with concurrent hypertension and benign prostatic hyperplasia (BPH), but carry a high risk of first-dose orthostatic syncope (managed by bedtime initiation); tamsulosin and silodosin selectively block prostatic α_1A/α_1D receptors over vascular α_1B receptors, relieving BPH symptoms and facilitating distal ureteral stone expulsion without orthostasis.

Beta-blockers are stratified into three clinical generations. First-generation non-selective β₁/β₂ antagonists (propranolol, nadolol, timolol) treat essential tremor, migraine prophylaxis, portal hypertension variceal bleeding prophylaxis (blocking splanchnic β₂ vasodilation to lower portal venous inflow), and thyroid storm (propranolol additionally inhibits peripheral 5'-deiodinase conversion of T₄ to T₃), but are contraindicated in asthma and severe COPD due to β₂-blockade-induced bronchospasm. Second-generation β₁-cardioselective antagonists ("BEAM": Bisoprolol, Esmolol, Atenolol, Metoprolol) preferentially block cardiac β₁ receptors at standard doses and are safer in patients with mild bronchospastic disease or diabetes (where β₂ blockade masks adrenergic symptoms of hypoglycemia except diaphoresis, which is cholinergic M₃). Third-generation vasodilatory beta-blockers include carvedilol and labetalol (combined α₁ + β₁/β₂ antagonists) and nebivolol (β₁-selective blocker that stimulates endothelial nitric oxide release), while pindolol and acebutolol possess intrinsic sympathomimetic activity (partial agonists).

Prostaglandin F_2α analogs (latanoprost, bimatoprost, travoprost, tafluprost) are the first-line pharmacotherapy for chronic open-angle glaucoma. They activate FP receptors to upregulate matrix metalloproteinases (MMPs) in the ciliary muscle, remodeling the extracellular matrix and markedly increasing uveoscleral outflow. Administered once nightly, they reduce IOP by 25–35%. Classic board adverse effects include irreversible iris hyperpigmentation (increased melanosome number within stromal melanocytes), hypertrichosis (eyelash lengthening/thickening), periorbital fat atrophy, and exacerbation of cystoid macular edema or herpetic keratitis/uveitis.

Rho-kinase (ROCK) inhibitors (netarsudil) directly relax trabecular meshwork myofibroblasts, decrease episcleral venous pressure, and increase conventional outflow through the Canal of Schlemm. Characteristic adverse effects include conjunctival hyperemia, corneal verticillata (whorl-like lipid deposits), and subconjunctival hemorrhages.

Topical beta-blockers (non-selective: timolol, carteolol, levobunolol; beta₁-selective: betaxolol) block beta₂-adrenergic receptors on the ciliary epithelium, decreasing cAMP-mediated aqueous humor secretion. Betaxolol carries lower pulmonary risk than timolol in patients with mild reactive airway disease, though all ophthalmic beta-blockers can cause bradycardia and mask hypoglycemia.

Alpha₂-adrenergic agonists (brimonidine, apraclonidine) exhibit a dual mechanism: both agents acutely decrease aqueous humor synthesis via G_i-coupled inhibition of adenylyl cyclase, while brimonidine additionally increases uveoscleral outflow during chronic therapy. Apraclonidine is primarily used short-term to blunt post-laser trabeculoplasty IOP spikes due to high rates of allergic follicular conjunctivitis; brimonidine is contraindicated in children under 2 years (central apnea/CNS depression).

Carbonic anhydrase inhibitors (topical: dorzolamide, brinzolamide; systemic: acetazolamide, methazolamide) inhibit carbonic anhydrase II in the ciliary processes, reducing bicarbonate (HCO₃⁻) formation and subsequent sodium and fluid transport into the posterior chamber. Systemic acetazolamide is critical in acute angle-closure crisis but causes proximal renal tubular acidosis (Type 2 RTA), paresthesias, hypokalemia, calcium phosphate nephrolithiasis, and sulfonamide cross-reactivity.

Direct/indirect parasympathomimetic miotics (pilocarpine, carbachol, echothiophate) stimulate M₃ receptors on the ciliary muscle and iris sphincter. Ciliary muscle contraction pulls on the scleral spur, opening the trabecular meshwork spaces to enhance conventional outflow. In acute angle-closure glaucoma, pilocarpine pulls the peripheral iris away from the trabecular meshwork to break pupillary block once IOP is partially lowered with acetazolamide, timolol, and hyperosmotic mannitol.

Succinylcholine consists of two acetylcholine molecules linked together. It acts as a full nicotinic N_M agonist, producing rapid onset (30–60 seconds) and ultrashort duration (5–10 minutes) ideal for rapid-sequence endotracheal intubation. Because it is resistant to synaptic acetylcholinesterase, clearance depends entirely on plasma butyrylcholinesterase (pseudocholinesterase).

During Phase I (depolarizing) blockade, persistent endplate depolarization produces transient generalized muscle fasciculations followed by flaccid paralysis because voltage-gated perijunctional Na⁺ channels become inactivated and cannot reopen until the endplate repolarizes. Train-of-Four (TOF) peripheral nerve stimulation shows equal attenuation of all 4 twitches (TOF ratio 1.0, no fade) and no post-tetanic potentiation. Crucially, acetylcholinesterase inhibitors (neostigmine) potentiate and worsen Phase I blockade by increasing synaptic ACh and inhibiting plasma pseudocholinesterase. With prolonged infusion or high cumulative doses, the receptor undergoes conformational desensitization into Phase II (desensitizing) blockade, where the membrane repolarizes but remains unresponsive to ACh; TOF now demonstrates classic tetanic fade and can be cautiously reversed with anticholinesterases once succinylcholine has cleared.

Non-depolarizing NMBAs act as competitive nicotinic N_M antagonists without causing fasciculations. On Train-of-Four monitoring, they exhibit progressive twitch fade (T4 < T3 < T2 < T1) and post-tetanic potentiation. They are divided chemically into aminosteroids (rocuronium, vecuronium, pancuronium—cleared hepatically/renally; pancuronium causes vagolytic tachycardia) and benzylisoquinoliniums (cisatracurium, atracurium, mivacurium). Cisatracurium and atracurium undergo spontaneous, non-enzymatic Hofmann elimination (temperature- and pH-dependent degradation in plasma), making cisatracurium the agent of choice in combined severe hepatic and renal failure (unlike atracurium, cisatracurium avoids histamine release and produces minimal epileptogenic laudanosine metabolite).

Reversal of non-depolarizing blockade is achieved via two distinct mechanisms: (1) Neostigmine + Glycopyrrolate (or edrophonium + atropine), which floods the synapse with ACh to outcompete the blocker, or (2) Sugammadex, a modified gamma-cyclodextrin ring that directly chelates and encapsulates steroidal NMBAs (rocuronium > vecuronium) in a 1:1 complex in plasma for renal excretion, enabling immediate rescue from "cannot intubate, cannot ventilate" deep blockade without muscarinic side effects (note: sugammadex also binds progesterone and reduces hormonal contraceptive efficacy for 7 days).

Malignant Hyperthermia (MH) is a life-threatening pharmacogenetic hypermetabolic crisis triggered by succinylcholine or halogenated volatile anesthetics (sevoflurane, desflurane, isoflurane, halothane) in patients with autosomal dominant mutations in the ryanodine receptor (RYR1) on the skeletal muscle sarcoplasmic reticulum. Uncontrolled Ca²⁺ efflux causes ATP-depleting sustained actin-myosin cross-bridging, presenting first with masseter muscle spasm and an abrupt, steep rise in end-tidal CO₂ (ETCO₂), followed by generalized rigidity, tachycardia, hyperthermia, severe lactic/respiratory acidosis, rhabdomyolysis, and hyperkalemia. Immediate treatment requires stopping the volatile agent, hyperventilating with 100% O₂, and administering IV dantrolene (RYR1 antagonist that blocks sarcoplasmic Ca²⁺ release). By contrast, centrally acting spasmolytics include baclofen (GABA_B agonist in spinal cord), tizanidine (central alpha₂ agonist), and cyclobenzaprine (structurally related to TCAs with strong antimuscarinic effects).