Autonomic Nervous system - Pharmacology of Cholinergic system

CHOLINERGIC SYSTEM

·         Acetylcholine is a major neurotransmitter at autonomic as well as somatic sites.

·         Neurons that release Ach are called cholinergic neurons and the neurotransmission is called the cholinergic transmission.

·         Ach acts as neurotransmitter at number of sites in body which include

1.      Post ganglionic parasympathetic nerve endings

2.      Post ganglionic sympathetic nerve endings to sweat glands and hair follicles

3.      Pre ganglionic fibers to autonomic ganglia in both sympathetic and parasympathetic nerves and fibers to adrenal medulla

4.      Somatic nerves to skeletal muscle

5.      Central nervous system.

Synthesis, storage, release and termination of neurotransmitter action

·         Acetylcholine is synthesized locally in the cholinergic nerve endings

   Acetic acid + Choline

  

                                                          Choline acetyl transferase (CAT)

                                                    

                                                    Acetyl choline (Ach)

·         In cholinergic nerve terminal acetylcholine is synthesized from choline and acetic acid and the reaction is catalyzed by the enzyme cholineacetyltransferase (CAT).

·         Transporter mediated uptake of choline into the nerve terminal presents rate limiting step of this synthetic pathway.

·         Ach is stored in the vesicles until action potential arriving at the nerve terminal.

·         The action potential induces calcium ion mediated fusion of secretory vesicles with the presynaptic membrane and neurotransmitter release.

·         Acetylcholinesterase a specific hydrolytic enzyme is responsible for the degradation of Ach in the synaptic cleft. Ach molecules after their action on receptors for about 2 milliseconds are dissociated and metabolized immediately to choline and acetate by acetylcholinesterase enzyme. This terminates the action of Ach and prevents its reoccupation of receptors.

·         After Ach hydrolysis, choline is taken up into the nerve terminal again where it is reutilized.

·         Butylcholinesterase / Pseuodocholinesterase – present in liver, plasma, intestine and whitematter and it slowly hydrolyses the Ach. But acetylcholinesterase / True cholinesterase enzyme is present in all cholinergic sites and it very fastly hydrolyses Ach.

Cholinergic receptors

Two types of receptors,

1.      Muscarinic receptors

2.      Nicotinic receptors

Muscarinic receptors

·         These are G- protein coupled receptors.

·         Five subtypes are M1 to M5. Among these M1, M2, M3 are present in autonomic system but M4 and M5 are present in CNS.

Nicotinic receptors

·         These are ligand gated ion channel receptors. Activation of receptors opens the cation channel predominantly sodium ions.

·         Two subtypes – N_M and N_N

·         N_M – Present in muscle of neuromuscular junction

·         N_N – Present in neurons of the CNS and autonomic ganglia.

Receptor	Location	Function	Nature	Transducer mechanism
M1	Autonomic ganglia	Depolarization (EPSP)	G- Protein coupled receptor	IP3 /DAG – increase cytosolic calcium
	Gastric glands	Acid secretion
	CNS	Not Known
M2	Heart	Negative ionotropic, choronotropic and Dromotropic effect	G- Protein coupled receptor	Potassium channel opening – decrease cAMP
M3	Visceral smooth muscles	Contraction	G- Protein coupled receptor	IP3 /DAG – increase cytosolic calcium
	Exocrine glands	Secretion
	Vascular endothelium	Release of NO results in vasodilation
	Eye	Miosis

Cholinergic agonist / Cholinomimetics

Drugs that produce actions similar to that of acetylcholine either by directly interacting with cholinergic receptors or indirectly by increasing the availability of Ach at its receptor site.

Classification

Depending on the mechanism of action cholinomimetics are classified into

1.      Direct acting cholinomimetics

a.       Cholinomimetic esters / Choline esters – Acetylcholine, Bethanechol, Carbachol and   

                                                                  Methacholine

b.      Cholinomemetic alkaloids – Pilocarpine, Arecolin, Muscarine and Nicotine

2.      Indirectly acting Cholinomimetics

a.       Reversible anticholinesterase agents

Neostigmine , Edrophonium, Pyridostigmine, Physostigmine, Tacrine

                        Carbamate insecticides (Carbaryl, Propoxur)

b.      Irreverible anticholinesterase agent

-          Organophosphates – Parathion and  Malathion, 

         Tabun, Sarin, Soman (Nerve gases for chemical warfare)

Directly acting Cholinomimetics

·         Activate cholinergic receptors and activate them to produce pharmacological effects.

Cholinomimetic ester

·         These are esters of choline that mimic the effects of neurotransmitter Ach at cholinergic receptor sites.

·         All these agents are poorly absorbed from the GI tract.

Acetylcholine

·         It acts both on muscarinic and nicotinic receptor and depending on the type of receptor, the actions may be classified into muscarinic or nicotinic.

Muscarinic actions

Heart

·         Decrease heart rate and force of contraction.

·         Decrease in rate of conduction in the specialized tissues of sinoartial and ariculoventricular nodes.

·         Cardiac output decreases due to decrease in force of contraction.

Blood vessels

·         Ach produces dilatation of all vascular beds. This is primarily due to the presence of M₃ subtype of muscarinic receptors located on the endothelial cells of vasculature.

·         Stimulation of these receptors causes release of Nitric oxide from endothelial cells, which diffuses to adjacent smooth muscle cells and causes relaxation.

Blood pressure

·         On intravenous administration of small dose of Ach produces a rapid fall in blood pressure because of generalized vasodilation accompanied by reflex tachycardia.

·         But large dose results in increased blood pressure by stimulation of nicotinic receptors located on adrenal medulla and autonomic ganglia which release adrenaline and noradrenaline.

Smooth muscles

·         Ach causes contraction of all non vascular smooth muscles including GI tract. It increases the tone and amplitude of contraction of stomach and intestine, but sphincters are relaxed.

·         Ach increases motility and contraction of uterus, urinary bladder, gall bladder and bronchiolar smooth muscles.

·         In urinary bladder detrusor muscle contracts leading to urination.

·         Contraction of bronchiolar smooth muscle.

Exocrine glands

·         Stimulates salivation, lacrimation, sweating, tracheobroncheolar secretion and gastric secretion.

Eye

·         Constriction of the pupil results in miosis. Also reduces intraoccular pressure especially in patients suffering from glaucoma.

Nicotinic actions

·         Skeletal muscle – Fasciculation noticed

·         Adrenal medulla and autonomic ganglia, release adrenaline and non adrenaline respectively.

Clinical use

·         No safe therapeutic use because of its multiplicity of actions and rapid inactivation by acetylcholinesterse.

Bethanechol

·         It is a synthetic choline ester.

·         It is structurally related to acetylcholine except the acetate is replaced by carbamate and choline is methylated.

·         These structural changes reduce the nicotinic property but not the muscarinic property.

·         The drug is resistant to the inactivation by acetylcholinesterase enzyme.

·         Have longer duration of action and its specific for muscarinic receptors only.

·         The effects of this drug are predominantly on GI and urinary tract.

·         It increases the tone and peristalsis of GI tract and increases the gastric secretions.

·         It stimulates detrusor muscle of bladder causing voiding of urine.

Clinical use

·         Useful to treat urinary retention and inadequate emptying of bladder especially after surgery or parturition or with spinal injury

·         It is used only in the absence of mechanical obstruction of the bowl or GI tract.

Carbachol

·         Potent synthetic choline ester

·         Like bethanechol it is poor substrate for acetylcholinesterase enzyme

·         It is active on both muscarinic and nicotinic receptors but muscarinic actions usually predominate.

Clinical use

·         Less used because of its high potency and relatively long duration of action.

·         Occasionally used for the treatment of ruminal stasis or impaction in cattle and for urinary retention.

·         Uses as miotic agent to decrease intraocular pressure when it is resistant to pilocarpine or physotigmine.

Methacholine

·         It is a methyl analogue of acetylcholine

·         It is a muscarinic agent with no or negligible nicotinic action.

·         Rarely used in veterinary medicine.

Cholinomimetic alkaloids

·         These are plant alkaloids found to exert parasympathomimetic action.

·         These agents were well absorbed from the site of administration.

Pilocarpine

·         It is a tertiary amine alkaloid

·         It has predominated muscarinic actions but also has minimal nicotinic actions.

·         It is one of the most potent stimulators of secretions like sweat, tears and saliva but it is not used for this purpose.

·         On eye it produces rapid miosis and contraction ciliary muscles when topically applied to cornea.

·         Miosis noticed within 15 minutes and lasts for 8 hrs. There is a transitory rise in intraocular pressure followed by a persistent fall.

·         Only on higher doses it shows ganglionic effects.

Clinical use

·         Drug of choice in rapid lowering of intraocular pressure of both narrow angle and wide angle glaucoma.

·         Used to control cycloplegia produced by atropine and ganglionic blockers.

·         Also used to improve tear and secretion in patients having problem in lachrymal gland function.

Arecoline

·         It is an alkaloid found in the betel nut the seed of betel plant Areca catechu.

·         It has both muscarinic and nicotinic actions.

Clinical use

·         Sometimes used as anticestodal agent and as purgative in veterinary medicine. But now it is infrequently used.

Muscarine

·         It is an alkaloid obtained from a poisonous mushroom named Amanita muscaria.

·         It is a potent poison and has no therapeutic use.

Nicotine

·         It is an alkaloid obtained from the tobacco plant Nicotiana tobaccum.

·         It is a potent poison and has no therapeutic use.

·         It is used as an aid in the cessation of smoking addiction (as patches)

Indirectly acting Cholinomimetics

·         These are the drugs that inactivate or inhibit the enzyme acetylcholinesterase (Ach esterase), thereby preventing Ach from hydrolysis.

·         The accumulated Ach in synaptic and neuroeffector junctional sites provoke response at all cholinergic receptors in the body, throughout peripheral and central nervous system.

·         So these are called anticholinesterase agent or cholinesterase enzyme inhibitors.

·         These are important as both therapeutic agent and potent toxicant.

Mechanism of Inhibition of enzyme

·         Ach esterase enzyme has two sites, namely anionic site (negatively charged) and esteratic site.

·         The cationic nitrogen and carboxyl portion of the acetyl ester of Ach bind to the anionic and esteratic site of the Ach esterase enzyme respectively and form covalent bond. The reaction is called acetylation

·         Normally, the acetylated enzyme formed after release of choline is regenerated very rapidly (within microseconds) after reaction with water (Hydrolysis).

Acetylcholine + Ach esterase enzyme

 

                                                     Acetylated enzyme    + Choline

                                                                                     H₂O

   Acetic acid + Regenerated enzyme

Reversible anticholinesterase agent

·         These are esters of carbamic acid (Carbamates) and have structural similarities with acetylcholine.

·         They form reversible complex with Ach esterase enzyme leading to temporary inhibition of enzyme and accumulation of Ach at cholinergic sites.

·         All reversible Ach esterase inhibitors bind with anionic and esteratic sites of Ach esterase and results in carbamylation. Then it is hydrolyzed in same manner but at very slow rate. Regeneration of carbamylated enzyme is slower (few minutes to few hours) than acetylated enzyme.

·         But edrophonium is a short acting drug that interacts primarily with anionic site of Ach esterase. It may also bind to esteratic site but without forming covalent bond. Reactivation of edrophonium does not involve hydrolysis of inhibitor. It acts only as a simple competitive inhibitor of Ach esterase and so its duration of action is shorter and it is less potent.

Neostigmine

·         It is a synthetic quaternary ammonium compound and poorly absorbed from the site of administration.

·         It mimics the pharmacological action of Ach.

Clinical use

·         Used to reverse the effect of competitive Nm blocking agents (Curare toxicity)

·         For the treatment of Myasthenia gravis (Autoimmune disease resulting in destruction of nicotinic receptors in the skeletal muscles)

Pyridostigmine

·         It is also a quaternary ammonium compound.

·         Used for the treatment of Myasthenia gravis

Edrophonium

·         It is also a quaternary ammonium compound.

·         Duration of action 10-20 minutes.

·         Occasionally used for reversal of competitive

·         Used for the differential diagnosis of Myasthenia gravis and cholinergic crisis

Physostigmine

·         It is a tertiary amine derivative

·         These agents are well absorbed from the site of administration.

·         Mimic the pharmacological actions of Ach. It can cross the blood brain barrier.

·         Mainly used to treat glaucoma alone or with pilocarpine.

·         Used to treat the atropine poisoning

·         Used to treat ruminal atony and impaction.

Carbamate insecticides (Carbaryl, Propoxur)

·         It is a cyclic / aliphatic acid derivative of carbamic acid

·         Highly lipid soluble and absorbed at all sites of administration

·         Used as insecticides - shampoos, flea and tick collars, sprays

·         It is a potent toxicant and results in excessive stimulation of both muscarinic and nicotinic receptors in peripheral and central nervous system.

Irreversible Ach esterase inhibitors

·         Organophosphates – Parathion and  Malathion, Tabun, Sarin, Soman

·         The esteratic site is permanently phosphorylated and restoration of Ach esterase requires synthesis of new enzyme molecule that may take several weeks.

·         It acts by covalently phosphorylating the hydroxyl group of serine on cholinesterase.

·         These compounds were developed initially as chemical warfare during World War II, but presently they are used as insecticides in Veterinary and Agriculture field.

·         These are extremely potent toxic agents.

·         Some organophosphates inhibit both Ach esterase and plasma cholinesterase and other esterases like neurotoxic esterase.

·         Inhibition of neurotoxic esterase cause demyelination of nerves called organophosphate induced delayed neuropathy (OPIDN).

·         Initial stages, enzyme complex can be dissociated with the use of chemical agent, pyridine – 2 – aldoxime (2- PAM) called cholinesterase enzyme reactivators like Pralidoxime.

·         However once covalent modification of enzyme loose one of its alkyl group (called ageing), it is impossible for the chemical reactivators to break the bond between the inhibitor and enzyme.

Mnemonic for symptoms of muscarinic excess

DUMBELS     -     Diarrhea

-          Urination

-          Miosis

-          Bronchoconstriction

-          Excitation

-          Lachrymation

-          Salivation and Sweating

  

Clinical Condition	Cholinomimetic agents used
Glaucoma	Pilocarpine, Physotigmine
Myasthenia Gravis , Reversal of neuromuscular blockade	Neostigmine, Pyridostigmine
Ruminal atony and impaction	Carbachol
Urinary retention	Bethanechol

Anticholinergic drugs (Parasympatholytics)

The anticholinergic drugs can be classified into

1.      Antinicotinic drugs – a. Ganglionic blockers b. Neuromuscular blockers

2.      Antimuscarinic drugs

Antimuscarinic drugs

·         These are muscarinic receptor antagonist.

Classification

A. Non selective muscarinic receptor antagonist

1.      Natural alkaloids – Atropine, Hyoscine (Scopalamine)

2.      Semisynthetic derivatives – Ipratropium bromide

3.      Synthetic compounds – Tropicamide, Glycopyrrolate, Dicylomine

B. Selective muscarinic receptor M₁ antagonist

1. M₁ selective antagonist – Pirenzipine

1.Natural alkaloids

Atropine

·         It is a tertiary amine alkaloid obtained from the plant Atropa belladonna (belladonna – beautiful lady in Italian language – because of dilatation of pupil)

·         The principle alkaloids of belladonna are atropine, hyoscyamine and hyoscine.

·         Highly lipid soluble agent that crosses blood brain barrier.

Mechanism of action

·         Atropine and related antimuscarinic agents are competitive antagonist of Ach and other muscarinic agonist at all muscarinic receptor sites.

·         The binding of atropine at muscarinic receptor is reversible as its action can be overcome by increasing the concentration of Ach.

Pharmacological effects

·         It inhibit all muscarinic functions

·         The pharmacological effects are generally dose related.

·         Small dose affect – salivary and sweat glands

·         Modest systemic dose – Pupil and heart

·         High dose - GI and urinary tract

·         Even higher dose – GI secretions.

CVS

·         Heart- Tachycardia due to blockade of M₂ receptor on SA node

·         Blood pressure- Atropine does not have any consistent or marked effect on blood pressure.

Smooth muscle

·         All parasympathetic nerve innervations are reversed by atropine

·         GI – Reduced tone and motility of the gut resulting in prolongation of gastric emptying time and closure of sphincters.

·         Bronchial smooth muscle – Cause relaxation of bronchial smooth muscle and increase diameter of the bronchi.

·         Urinary tract – It has relaxant action on urinary bladder and ureter which results in urinary retention.

Eye

·         Blocks all cholinergic activities in the eye and results in mydriasis, cycloplegia (paralysis of ciliary muscles), increase intraocular pressure and abolish light reflex.

Exocrine glands

·         Decreases secretion of glands activated by parasympathetic nervous system

Body temperature

·         It increase body temperature (Atropine fever) in some species (Eg: Human) due to inhibition of sweating as well as stimulation of temperature regulating centre in the hypothalamus.

Toxicity

·         Dose related and usually, excess of its pharmacological effects

·         Dry mouth, constipation, increase thirst, mydriasis, tachycardia, restlessness, urinary retention, ataxia, mania, delirium, hallucination, blurred vision, cycloplegia and photophobia are symptoms of the toxicity

Toxicity varies with species

·         Rabbit – resistant, due to atropine esterase (atropinase) enzyme

·         Dog, cat, human – highly susceptible

·         Horse, cattle, goat - relatively resistant.

Treatment –supportive and symptomatic. Extreme cases – physostigmine can be administered.

Contraindication and precaution

·         Glaucoma, CHF, Tachycardia, Myocardial Ischemia, Intestinal hypermotility

Clinical use

·         Preanaesthetic – to reduce secretions of salivary and respiratory tract

·         Rarely used as antispasmodic

·         Cardiac stimulant – to treat sinus Bradicardia, sinoartial arrest or incomplete AV block

·         As antidote to anticholinesterase poisoining (Carbamate and OPC poisoning)

·         Mushroom poisoning – Over exposure to Amanita muscaria which contains muscarine

·         As a mydriatic for the treatment of refractive error and other diagnostic procedures

Hyoscine / Scopalamine

·         Alkaloid found mainly in shrub Hyoscyamus niga, Scopolis carniolica and Datura fastuosa val alba.

·         Produces peripheral effects similar to atropine but greater action in CNS (depressant action but in higher dose it stimulate CNS)

Clinical use

·         Antispasmodic

·         Preanaesthetic

·         Antiemetic in motion sickness – one of the most effective anti-motion sickness drugs available. More effective prophylactically than treating motion sickness.

2. Semisynthetic drugs

Homatropine

·         Semisynthetic drug prepared from atropine, closely resembles atropine in most of its pharmacological actions but is about 10 times less potent than atropine.

·         Shorter duration of action and devoid of systemic effects when administered topically

Clinical use

·         Mydriatic agent

·         Paediatric preparations – used occasionally in small animals to give symptomatic relief from hypersecertion and bronchoconstriction of upper respiratory tract.

Ipratropium bromide

·         It acts selectively on bronchial smooth muscles without altering the volume and consistency or respiratory secretions.

Clinical use – Bronchodilator for treatment of chronic obstructive pulmonary disease or asthma

3.Synthetic drugs

Tropicamide

·         It is a non-cycloplegic mydriatic agent that has quickest (20-40 min) and briefest (3-6 hrs) action.

·         Mydriatic of choice for intraocular administration and fundoscopy.

Glycopyrrolate

·         It is a synthetic quaternary compound

·         Primarily used as an adjunct of general anaesthesia to decrease the secretions

Dicyclomine

·         It is a synthetic tertiary amine

·         It has direct smooth muscle relaxant effect in addition to competitive muscarinic receptor antagonistic action.

·         At therapeutic doses it decreases spasm in most smooth muscles without producing atropine like effects on heart, eye and exocrine glands.

Clinical use – Highly used as antispasmodic drug in human and dog/cat.

Antiparkinsonisam drug

·         Disease of old, characterized by dementia, facial rigidity, motor incordination caused by deficiency of dopamine in the basal ganglion

·         Drug of choice is L- Dopa

·         Cholineric neurons in basal ganglion have D2 receptors which inhibit Ach release when stimulated by dopamine

·         Loss of dopamine in parkinson’s disease increase Ach release

·         Anticholinesterase drugs Eg: Benztropine block the effects of increased Ach.

·         It is used as adjunct when patient becomes unresponsive to L-dopa.