Pharmacology of the Parasympathetic Nervous System

Cholinergic transmission

The terminals of cholinergic neurons contain many small vesicles near the synaptic membrane full with ACh, and fewer large dense-core vesicles that are located farther from the synapse and contain peptides that may act as co-transmitters.

The vesicles are synthesized in the cell body and transported to the terminal.

ACh is composed of acetyl-CoA & choline by choline acetyltransferase. Choline enters the cell via Na⁺ dependent symporter, which can be blocked by hemicholinium.
After synthesis, ACh enters the vesicle through an ACh/H⁺ antiport, which can be blocked by vesamicol. Each vesicle stores 1000-50,000 ACh molecules.

AP will cause a Ca²⁺ influx. The Ca²⁺ interacts with proteins on the vesicle, which then fuses with the synaptic terminal → exocytosis of ACh.

Autonomic postggl. terminals release less ACh than released in motor end-plates.

The vesicular release can be blocked by botulinum toxin.

After exocytosis, ACh can bind and activate ACh receptors and it can break to choline & acetate by ACh-esterase. The t½ of ACh in the synapse is very short. ACh is found in:

CNS both nicotinic & muscarinic receptors can be found. ACh is found in the striatum, as part of the extrapyramidal motor pathway. Since DA inhibits ACh release by acting on D₂-receptors, when there is no DA → excess ACh production → Parkinsonism.
ACh is also found in the hypothalamus, related to short term memory.

Autonomic ggl. acting on neuronal type N-receptors, both in Psy & Sym ggl.

Neuromuscular junction as part of the somatomotor system. It activates N-receptors on mm.

Psy postggl. nerve terminals all visceral autonomic organs that receive Psy innervation have M-receptors.

Sweat glands receive sympathetic cholinergic innervation that acts on M-receptors.

Cholinergic receptors:

o       N (nicotinic) receptors mostly ligand-gated Na⁺ channels, causing cellular stimulation. Long binding may block the receptor → spastic paralysis. These receptors are found in: (1) autonomic ggl. of both divisions; (2) between preggl. cells & adrenal medulla; (3) neuromuscular junctions.

o       M (muscarinic) receptors there are 5 different genes (M₄ & M₅ are rather unknown yet). All are G-protein coupled. As long as the agonist is bound they remain active:

M₁: it acts through IP₃ → Ca²⁺ → inhibit adenylyl cyclase → positive feedback. It is located mainly on CNS neurons, presynaptic cholinergic neurons and the stomach ( secretion).

M₂: ACh binding will open mainly K⁺ channels → hyperpolarization. M₂-receptors are found in the supraventricular heart → reduce HR by (-) chronotropic, dromotropic & inotropic effects → bradycardia. It is also found on inhibitory presynaptic neurons, which following ACh stimulation will inhibit transmission.

M₃: ACh will ↑IP₃ & Ca²⁺. Located postsynaptically on smooth mm and autonomic organs.

Actions of ACh:

CNS related to memory & extrapyramidal motor system.

Muscles smooth mm contraction (by M₃); skeletal mm contraction (by N); cardiac mm relaxation (by M₂).

Autonomic organs in preggl. of both divisions & in Psy. postggl.

a.       Eyes pupil constriction (mediated through nerve III) & ↑lacrimation (through VII).

b.      Saliva ↑saliva secretion, containing amylase (by secretomotor fibers in VII & IX).

c.       Lungs bronchoconstriction (through vagus).

d.      Heart bradycardia (through X).

e.       Stomach increased gastric acid secretion, both directly and by increasing histamine (X).

f.        Gut increased motility and sphincter relaxation (through X & pelvic splanchnic nerves)

g.       Urinary tract urination by constricting the detrusor mm and relaxing the sphincter.

h.       Penis ↑erection.

i.         Sweat glands ↑sweating (Sym cholinergic nn.).

Presynaptic stimulation of cholinergic transmission

↑synthesis by stimulating choline acetyltransferase (in theory, no such drug yet).

↑release by changing ion concentration in the terminal (↑↑ Mg²⁺ or Ca²⁺).
Release can be stimulated by acting on presynaptic receptors, causing a positive feedback →
↑ACh, e.g. by using PG, 5-HT, and NE (in the skin, acting on α1-receptors → ↑ACh release).
Ephedrine will cause NE release and NE will act on α1 receptors → ↑ACh in neuromuscular junctions → used as treatment for myasthenia gravis.
4-aminopyridine can block K⁺ channels → depolarization → Ca²⁺ influx → ACh release.

Postsynaptic stimulation of cholinergic transmission

Direct cholinomimetics by agonists on N, M or both receptors → ↑ACh release.

Choline esters similar to ACh. None of them can cross the BBB due to positive charge.

a.       ACh not used as a drug except in ophthalmology during eye operations. It can be used as eye drops for short duration.

b.      Methacholine (β-methyl-ACh) short acting M-agonist. The methyl group completely eliminates the nicotinic effect. Has less susceptibility to cholinesterase.

c.       Carbachol similar to ACh, only with NH₂ instead of CH₃. It is long acting highly resistant to cholinesterase and can act both on N & M receptors. Indicated topically as eye drops for glaucoma when the aqueous humor cannot be drained normally. Drainage can be improved by pupil constriction.

d.      Bethanechol (β-methyl-carbachol) the same action as carbachol, but resistant to cholinesterase and acts only on M-receptors. It is used in paralytic ileus, stomach atony, and to induce bladder constriction.

Alkaloids derived from different plants:

a.       Nicotine at first it stimulates N-receptors, but after prolonged depolarization it closes the pore, becoming an antagonist.

b.      Muscarine derived from the Amanita muscaria mushroom. It stimulates M-receptors. Eating the mushroom will cause extreme Psy symptoms → bradycardia, salivation, diarrhea & bronchoconstriction. Toxic amounts may cause seizures. Easily treated with atropine.

c.       Pilocarpine a short acting M-agonist used locally as eye drops for glaucoma.

d.      Arecoline M-agonist that derives from the betel nut. It is centrally acting.

Indirect cholinomimetics by blocking ACh-esterase, either reversibly or irreversibly.

Reversible cholinesterase inhibitors can be either competitive inhibitors or used as an enzyme substrate that is metabolized much slower. These drugs are contraindicated for muscle operations and for mechanical ileus (↑the obstruction).

a.       Competitive inhibitors used in myasthenia gravis, by increasing the ACh acting on mm.
E.g. edrophonium, was used also for SVPT. It slows the conduction and disturbs the macro-reentry, thus blocking the arrhythmia.

b.      Enzyme substrates cholinesterase usually recovers quickly, but by using carbamides the enzyme remains inactive longer. E.g.:

A)    Physostigmine a tertiary amine, lipid soluble drug that can act peripherally & centrally. It has 3 systemic indications: (1) atropine poisoning; (2) poisoning by an atropine-like containing mushroom; (3) Alzheimers disease.
Another local indication is for glaucoma (by 0.25-1% solution).

B)     Neostigmine a quaternary amine with only peripheral action, since it is water soluble with a positive charge. The intestinal absorption is minimal so it must be given IV.
Mostly indicated for paralytic ileus and it can antagonize skeletal mm relaxants.

C)    Distigmine very similar to neostigmine and with the same indications.

D)    Pyridostigmine & ambenonium long acting oral drugs (2hrs), for myasthenia gravis.
Pyridostigmine is used against nerve gases, which are irreversible inhibitors of cholinesterase. Since it is metabolized by the enzyme and inactivates it, pyridostigmine protects it from the irreversible inhibition.

E)     Demecarium used as eye drops for glaucoma.

Irreversible cholinesterase inhibitors all are phosphate esters → organophosphates that phosphorylate the enzyme. Since the phosphate never hydrolyses spontaneously, the inactivation is irreversible. Within min-hrs the phosphate will lose an oxygen (aging process) → enzyme death. There is a time frame to remove the phosphate group by drugs (cholinesterase
re-activators), but this will only work before the aging process. Such dephosphorylating drugs contain NOH group, e.g. pralidoxime & obidoxime.
Irreversible inhibitors are used in agriculture for killing insects. Out of the medically important drugs → echothiophate, a quaternary phosphate compound, water soluble, and is used for glaucoma. It has a long local action, but it may cause cataract.
Other irreversible inhibitors include the nerve gases.

Presynaptic inhibition of cholinergic transmission

Inhibition of synthesis by hemicholiniums, a group of drugs that block the Na⁺ dependent transport of choline into the terminal. The block is never complete.

Prevent ACh storage by vesamicol, an experimental drug that blocks the ACh/H⁺ antiport, preventing vesicular uptake of ACh into storage vesicles.

Inhibition of vesicular release by botulinum toxin. It can be used during blepharospasm (mm spasms of neck or face, mainly eye ticks), causing mm. paralysis.

Negative feedback on transmitter release through presynaptic receptors:

a.       Presynaptic α₂ receptors NE binds α₂ receptors, blocking cholinergic transmission → uses as autonomic transmission regulator.

b.      Presynaptic D₂-receptors DA inhibits ACh release by acting on D₂. In parkinsonism there is ↓DA → ↓inhibition → ↑ACh.

c.       μ, κ & δ opioid receptors bind opioids, e.g. morphine. Its peripheral action will inhibit ACh release → constipation.

Positive feedback prevention by inhibiting PG, M₁ & M₂ → inhibition of stimulation.

Block AP preventing release (e.g. procaine). In neuromuscular junctions, some antibiotics may inhibit transmission (e.g. tetracycline & aminoglycosides). There is no effect in healthy people but in myasthenia gravis patients → mm. weakness (side effect of the ATB).

Postsynaptic inhibitors of cholinergic transmission (parasympatholytics)

M-antagonists

Tropeines including natural & semi-synthetic drugs, derived from plants ( alkaloids). The prototype is atropine (was called atropa belladonna, since high class prostitutes in Italy used it for pupil dilation, and since it accommodation, the girls didnt have to see their clients very well). Atropine is a lipid soluble tertiary amine that is absorbed by the GI and penetrates the BBB → CNS & PNS actions. Its systemic actions last for several hours and since its not metabolized, it is excreted unchanged in the urine (rabbits can metabolize it).
The dose is 0.5mg. Increasing dosage will cause same effects, only stronger. Peripheral actions:

a.       Eyes pupil dilation. It is topically active (drops) for 8 days. The maximal dilatation lasts for 3-4 days and is eliminated by day 8. It causes loss of accommodation double (diplopia) and blurred vision that last for ~5 days.

b.      Salivary glands atropine inhibits saliva production → xerostomia.

c.       Lungs causes bronchodilation. Was used in the past for asthma.

d.      Heart causes tachycardia, depending on the original Psy tone in the heart → mainly effective during bradycardia & AV blocks but basically not very effective in normal or fast rates. The best treatment for bradycardia is a pacemaker.

e.       Stomach it inhibits gastric acid secretion and was used in the past for peptic ulcers.

f.        Intestines it relaxes the peristalsis and closes the sphincters → constipation. It can treat diarrhea, but opioids are better for it.

g.       Billiary tract given together with smooth mm relaxants to treat billiary cramps.

h.       Urinary bladder dilates the detrusor m. and closes the sphincter → dysuria & retention.

i.         Sweating & perspiration atropine inhibits sweating by inhibiting Sym cholinergic nerves. This is important in children, which have a bigger body surface and lose heat by sweating. Atropine will prevent heat loss atropine fever → dont use it on children.

The CNS effects are more dose-dependent, demonstrated here by increasing dosage:

a.       Antiemetic action 0.5mg can treat kinetosis problems (air, sea & motion sickness). The action is better for prevention, not during an attack. It acts on the area postrema.

b.      Anxiety & nervousness, restlessness when increasing the dose to 1-2mg the effects of atropine become excitatory on the CNS.

c.       Extrapyramidal dyskinesia & tremor when the dose exceeds 10mg.

d.      Rage & hallucinations with even higher dose (>>10mg). Atropine-like drugs were used by medicine men in Asian tribes for seeing the future hallucinations.

e.       Convulsions when increasing the dose.

f.        Coma & death above 100mg its actions become inhibitory. 100-150mg is lethal.

Additional drugs have the same actions as atropine, with some differences:

Scopolamine also a tertiary amine, with better penetration → can penetrate even the skin. The peripheral actions are the same, but centrally the antikinetosis action is better than atropine. It is used as a patch placed behind the ear. People might forget and leave the patch → blurred vision for a while.
Other CNS effects are inhibitory → the drug causes depression, respiratory depression, coma and death (with increasing dosage).
In the 19^th century it was used together with morphine as a narcotic drug.

Homatropine a tertiary amine, that causes mydriasis for 4 days only. The rest is the same. It can be used for skeletal mm relaxation in abdominal cramps.

Methylatropine, methlyscopolamine & methylhomatropine quaternary amines, with poor GI absorption 5mg are needed to get the same action as 0.5mg IV atropine.
The methylation cancels the CNS effects no BBB penetration.
They have a negative result, since they also antagonize N-receptors (though weakly), which may block Sym ganglia → hypotension.

Ipratropium poorly absorbed so cant be taken orally. Applied by inhalation for treating bronchial asthma. Since it not absorbed from the lung as well, there are no peripheral effects, only bronchodilation.

Non-tropeines semi-synthetic drugs, aimed for acting on certain organs only:

a.       Pirenzepin strong action on gastric acid secretion and peptic ulcer disease. There is no true selectivity, only weaker parasympatholytic effect than atropine.

b.      Tropicamide & cyclopentolate are active for 1day & 6hrs, respectively, causing short term pupil dilation. They have a risk of glaucoma, since the drainage of Schlemms canal is worse during pupil dilation → glaucoma attack (may be without symptoms) → eventual blindness.

c.       Procyclidine, biperiden & orphanedrine they are supposed to have only central action, for treating Parkinsons disease. Practically, they treat only the resting tremor.
They are the only usable drugs for drug-induced Parkinson syndrome, since its caused by DA drugs, so we must use M-antagonists instead. In the past benzatropine was used.

N-antagonists

Ggl. blockers including both competitive & competitive-like drugs that bind different sites but behave like competitive. E.g. hexamethonium (enters into the pore), trimethaphane (quaternary amine) & mecamylamine (tertiary amine). They have a very short action.
They were used by infusions to treat hypertensive surges occurring during surgery, but not anymore, since they also block Sym ggl.
The sympatholytic effect will cause orthostatic hypotension and rebound HT when the drug effect is over once it leaves the receptor. It can also cause sexual dysfunction. The parasympatholytic effect is the same as atropine constipation, dry mouth, blurred vision, etc.
IV trimethaphane is given in hypertensive crises, acute aortic dissection and in order to induce hypotension in neurosurgery. The antiHT effect is mainly by blocking the baroreflex & by pooling of blood in the capacitance vessels.
Depolarizing ggl. blockers e.g. nicotine are not used. Every N-agonist will eventually block the receptor, becoming an antagonistic, since nicotine is not metabolized. It is only important in nicotine poisoning.

Skeletal mm. relaxants:

b.      Competitive bind and antagonize N-receptors, causing mm paralysis. The prototype is
D-tubocurarine, the active compound of curare, which is not used since it also blocks ggl. & releases histamine → vasodilation and severe hypotension. Its derivatives pancuronium, pipecuronium, vecuronium, rocuronium & atracurium are used during surgery for complete mm. relaxation. This requires also artificial ventilation.
They are used IV with no CNS penetration. Only atracurium is metabolized (both spontaneously & enzymatically). All others are excreted through the kidney.
Theres a loading dose followed by a maintenance dose. In order to antagonize these drugs → give choline esterase inhibitor, such as neostigmine.

c.       Depolarizing these are N-receptor agonists, but with slower elimination than ACh, e.g. succinylcholine & decamethonium. When given IV they cause mm fibrillation (not a synchronized contraction, since it comes through the blood, reaching everywhere).
The action is prolonged since they are not metabolized by ACh-esterase → flaccid paralysis for ~7min. It is eventually metabolized by pseudocholinesterase. In patients lacking this enzyme, the action will last for hours, but eventually will be removed by the kidney.
After surgery, as an after-effect, it may cause hyperkalemia due to some myocyte destruction. It might also cause malignant hyperthermia, which is treated by dantrolene.