Cholinergic Transmission & Receptors....

The cholinergic system plays a crucial role in maintaining homeostasis, coordinating various physiological processes, and facilitating communication within the body. Disruptions in this system can lead to significant health implications.

1. Cholinergic Transmission And Cholinergic Receptors Submitted To :- Dr. Amit Kumar Verma

2. ‹#› Medicinal Chemistry S. No. Topic Name 1 Cholinergic Neurotransmitter 2 Cholinergic Neurochemistry 3 Cholinergic Transmission 4 Cholinergic Receptors 5 Biochemical Effects 6 Reference Contents :-

3. ‹#› Dale described the actions of esters and ethers of choline on isolated organs and their relationship to Muscarinic Receptors, this helped in understanding action of cholinergic nerve and its NT. Drugs can mimic the action of ACh by either acting on ACh receptors or inhibiting AChE, the enzyme that inactivates ACh. Cholinergic Nerves are found in PNS and CNS, presence in CNS is under extensive research. Synaptic terminals in the cerebral cortex, hippocampus and corpus striatum are rich in ACh, however its function in CNS is not clear, implicated in memory and behavioral activity. Cholinergic Neurotransmitter

4. ‹#› Cholinergic neurons synthesize ACh, AChE, ChAT. ChAT stands for choline acetyltransferase and catalyze transfer of acetyl group(by conversion of glucose to pyruvate in cytosol of neuron, formation of acetyl CoA) from CoA to choline, this results in production of ACh. ACh is present in cystosol & nerve endings, presynaptic nerve uptake the choline part of ACh via Na+ since choline( by hydrolysis of ACh ) is a limiting substrate for production of ACh. QAC can behave as a competitive inhibitor of choline uptake, e.g. Hemicholinium (HC3), 2-hydroxyethyltriethylammonium. Cholinergic Neurochemistry

5. ‹#› Serine gets metabolized by seine decarboxylase which gives ethanolamine and S-adenosylmethionine act over it to produce choline. Depolarisation of the nerve terminal causes release of ACh & it functions as a NT. Further this ACh is degraded by the action of AChE (acetylcholinesterase) forming choline & acetate. Cholinergic Neurochemistry

6. Cholinergic Transmission ‹#›

7. Cholinergic Receptors They are of two types namely Nicotinic and Muscarinic, based on their ability to bind with naturally occurring alkaloids nicotine and muscarine respectively. ‹#› Cholinergic Receptors Nicotinic Muscarinic

8. ‹#› Action of ACh on muscarinic receptors results in secretion from salivary & sweat glands, contraction & secretion in gut and constriction of respiratory tract. It also relaxes the smooth muscles and slows down contraction of heart muscle. Guanosine triphosphate binding protein (G-protein) is responsible for actions of muscarinic receptors. Muscarinic receptors consist of 7 transmembrane protein helices that create four intracellular and four extracellular domains with the extracellular domain having ACh binding site and intracellular domain attached to the G-protein. Muscarinic receptors have subtypes namely M 1 , M 2 , M 3 , M 4 and M 5 . Muscarinic Receptors

9. Muscarinic Receptors ‹#› M-1 Receptors These receptors have a high affinity for pirenzepine & low affinity for AF-DX116. Also termed as neural as present in the brain. They tend to stimulate the gastric secretions. Agonist :- McN-A343. Antagonist :- Pirenzepine Hydrochloride. Location :- CNS, Exocrine Glands, Autonomic Ganglia, Parietal Cells(GIT), PNS & Submucosal Glands. Functions :- Rapid Eye Movement, Arousal Attention, Sleep, Emotional Responses, Affective Disorders (stress, depression), Memory & Learning.

10. Muscarinic Receptors ‹#› M-2 Receptors They show high affinity for methoctramine (polyamine) & low affinity for pirenzepine. Since they are present in atria, also called cardiac muscarinic receptors. Agonist :- Bethanechol Antagonist :- Atropine Location :- Lungs, Heart, Presynaptic terminals of postganglionic cholinergic nerves & Visceral Smooth Muscles. Function:- Decrease strength & rate of heart rate, hyperpolarization of of cardiac cells (bradycardia), lowers cAMP levels (Ca 2+ also decrease) in cardiac cells via G i protein, Act as autoreceptor on presynaptic terminals of postganglionic cholinergic nerves to inhibit ACh release.

11. Muscarinic Receptors ‹#› M-3 Receptors Also known as glandular muscarinic receptors & have stimulatory effect on organs. Functions:- Glandular Secretion from Pancreatic, Bronchial, Lacrimal, Salivary and Mucosal Cells, Contraction of Visceral Smooth Muscle, Mediated via G Protein activation of PLC that give DAG & IP 3 (second messenger). Endothelium control tone of Vascular Smooth Muscles by potent relaxant (EDRF) nitric oxide & a vasoconstrictor Endothelium Derived Contracting Factor. Location:- Exocrine Glands, Smooth Muscles, Iris, & Vascular Endothelium.

12. Muscarinic Receptors ‹#› M-4 & M-5 Receptors M 4 Receptors :- Regulate K + and Ca 2+ ion channels like M 2 , present in tracheal smooth muscle & inhibit ACH synthesis like M 2 receptors. Antagonist:- Tropicamide M 5 Receptors :- M 5 receptor mRNA found in substantia nigra & may regulate dopamine release at terminals within the striatum.

13. Nicotinic Receptors They are coupled to ion channels, mediate very rapid response (0.1_10 milliseconds) and provide aqueous pathway to ions through plasma membrane. Charge and size of ions affect selectivity of ion pore. water around the ion determines the energy required to remove the hydration shell. water surrounds ion as a tightly bound, highly ordered layer immediately surrounding ion and a second less structured layer. First NT isolated & purified in active form was nicotinic ACh receptor, a glycoprotein embedded into polysynaptic membrane. It is a cylindrical protein of 250000 Da with five subunit polypeptide chains (α 2 βɣδ). These peptide chains are arranged in such a way that have a central opening and each ɑ chain contains a negatively charged binding site for QAG of ACh. Receptor exist as a dimer of two five subunit polypeptide chain monomers linked via disulfide bond b/w 𝛿 chains. ‹#›

14. Nicotinic Receptors When ACh binds to the receptor it causes a change in permeability of the membrane, allowing passage of cations such as Ca 2+ , Na + , K + . So depolarisation occurs and signal transfer to adjacent entities. Nicotinic receptors in CNS and NMJ differ in ligand specificity, at NMJ (N 1 ) blocked by succinylcholine, d-tubocurarine and decamethonium while stimulated by phenyltrimethylammonium. Nicotinic receptors (N 2 ) are found in autonomic ganglia, blocked by hexamethonium, trimethaphan while stimulated by tetramethylammonium and dimethyl 1,-4-phenyl piperazinium (DMPP). ‹#›

15. Nicotinic Receptor ‹#›

16. Biochemical Effects If transmission occurs via second messenger it is generally slow (100ms) as compared to ion channel transmission at synapse. Agonist activate receptor, involves activation of G protein (interact with GTP & GDP) part bound to the intracellular domain of M receptor. In G protein ɑ subunit (may cause activation G s or inactivation of enzymes or channels G i ) with enzymatic activity catalyze GTP to GDP. M 1 , M 3 & M 5 activate PLC hence DAG & IP3 forms that cause release of intracellular Ca 2+ & activation of protein kinase. While M 2 & M 4 inhibit Adenylate Cyclase. ‹#›

17. Conversion of PIP 2 to IP 3 & DAG, IP3 metabolizes Ca2+ in E.R. this increases cytosolic free Ca2+ & activates Ca2+ dependent kinases (troponin C in muscle) or binds to calmodulin-dependent kinase, which phosphorylase cell specific protein & causes muscle contraction. DAG, lipid-like, also activates protein kinaseC & results in muscle contraction. Phosphoinositol System :-

18. Another target for M receptors it metabolizes formation of cAMP from ATP, this cAMP activates various protein kinases which catalyzes phosphorylation of enzymes & ion channels which alter ion(Ca2+) concentration hence regulating muscle contraction. GTP protein (Gi) may cause smooth muscle relaxation by reducing activity of Adenyl Cyclase. M receptor stimulation causes cAMP level to fall, cAMP protein dependent kinase activity decreases & relaxation of muscle contraction. Adenylate Cyclase

19. G proteins are present on the ion channel, Ca2+ channel & activate them without need of a second messenger. E.g. G protein in heart tissue activate K+ channel & causes hyperpolarization ( heart rate decreases ). Ion Channel

20. ‹#›

21. Reference Wilson, C. O., Beale, J. M., & Block, J. H. (2011). Wilson and Gisvold's textbook of organic medicinal and pharmaceutical chemistry (12th ed.). Baltimore, MD: Lippincott Williams & Wilkins. THANKYOU ‹#›