2024 TARIK Lab Lecture - Synthesis and Degradation of Neurotransmitters PDF

Autonomic Imbalance Synthesis and Degradation of Neurotransmitter at Cholinergic and Adrenergic Synapses Lab Lecture YEAR 2 - UNIT 1 10/5/2017 Academic year 1 2024/2025...

Autonomic Imbalance Synthesis and Degradation of Neurotransmitter at Cholinergic and Adrenergic Synapses Lab Lecture YEAR 2 - UNIT 1 10/5/2017 Academic year 1 2024/2025  Synthesis of Acetylcholine (ACh) ACh is synthesized in the cytoplasm of the nerve terminal from choline and acetylcoenzyme-A in the presence of an enzyme, choline acetyltransferase. Acetyl-CoA + Choline Choline acetyltransferase Acetylcholine ACh is then released into the synaptic cleft by exocytosis. 10/5/2017 2  Termination of Action of Acetylcholine ACh is hydrolyzed into an Acetate ion and Choline, This mechanism takes place at cholinergic synapses catalyzed by the enzyme Acetylcholine esterase that is and at the neuromuscular junctions of skeletal nerve bound with collagen and glycosaminoglycans in the fibers. local connective tissue. Pralidoxime chloride ( 2-PAM) is a cholinesterase regenerator. sused to treat organophosphate poisoning Choline is then transported back into the terminal nerve ending, where it is used again for synthesis of new ACh. Choline is taken back into the presynaptic cell by a high affinity choline uptake (HACU) system. Acetate quickly diffuses into the surrounding medium. effect 2 Parathion (as in the given problem in tutorial) inhibits the same as Parasympathetic action of the enzyme acetylcholinesterase leading to bluz more Ach accumulation of ACh in the synaptic cleft. Once acetylcholine (ACh) is secreted into tissue by cholinergic nerve ending, it persists in the tissue for a few seconds while it performs its nerve signal transmitter function. 26/09/2024 3 - Ach iS hydrolized to Acetyl COA and choline bound with collagen by acetylcholinesterase and gam. - choline is transported back for later use Its taken back to Pre-synaptic cell by HACU - Acetyl diffuses Parathion - Inhibits leading to accumulatylcholinesterase - This takes place at 1 cholinergic synapses. Neuromuscular junction at skeletal 2 nerve fibre  Cholinergic Blockers (Antagonists) Divided into 2 groups, depending on their receptor affinities. 1. Muscarinic antagonists (Parasympatholytic effect): Selectively block the muscarinic postganglionic synapses of the parasympathetic nerves. Muscarine, a poison derived from a toadstool (frogs) or certain types of mushrooms. Atropine-like drugs 2. Nicotinic antagonists: Blocks autonomic ganglia, and the NMJ in skeletal muscle. Curare-like drugs: Curare is the first paralytic used in anaesthesia (It produces flaccidity in skeletal muscle). Was first used in poison arrows for hunting. Derived from Tobacco plants Inhibits ACH in Skeletal muscle Parasympathetic has Muscurinic muscurinic : Post ganglion - Nicotinic - Sympathetic has Nicotinic: Ganglia - nicotinic adrenergic -  Cholinergic blockers( antagonists) and acetylcholinesterase regenerator Atropine sulphate : a cholinergic blocker sgives sympathetic effects Pralidoxime chloride ( 2-PAM) : a cholinesterase regenerator & in Problem (2-Pam) is. given 2-pam regenerate cholinesterase to break down Ach and Prevent It from accumulating 10/5/2017 5 Muscarine, or muscarin: ACTS ON MUSCARINIC RECEPTORS. is a natural product found in certain wild mushrooms or toad's stool. Muscarinic antagonists are often called parasympatholytic because they have the same effect as agents that block postganglionic parasympathetic nerves. Curare Poisons: ACT ON NICOTINIC RECEPTORS. These poisons cause weakness of the skeletal muscles and, when administered in a sufficient dose, eventual death by asphyxiation due to paralysis of the diaphragm. Curare is prepared by boiling the bark of one of the dozens of plant sources, leaving a dark, heavy paste that can be applied to arrow or dart heads. In medicine, curare has been used as a treatment for tetanus (exotoxin= tetanospasmin) and strychnine poisoning and as a paralyzing agent for surgical procedures. Curare acts as a neuromuscular blocking agent by binding to the acetylcholine receptors at the neuromuscular junction and preventing nerve impulses from activating skeletal muscles leading to muscle paralysis, (especially paralysis of diaphragm= asphyxiation).  KNOW THE STEPS Synthesis of Norepinephrine and Epinephrine Epinephrine is synthesized from NE It begins in the axoplasm of the terminal nerve endings of adrenergic within the adrenal medulla, by an nerve fibers, but it is completed inside the secretory vesicles. enzyme that adds a methyl group to NE to form epinephrine. The basic steps are the following: 1. Tyrosine Hydroxylation DOPA 2. DOPA Decarboxylation Dopamine In the vesicles: 3. Dopamine Hydroxylation NE Norepinephrine adding methyl group Tyrosine y Epinephrine hydroxylation I DOPA decarboxylation 7 ~ NE Dopamine When methyl group added to , we get Epinephrine hydroxylation I NE Process happen in adrenal medulla  Termination of Action of Catecholamines The norepinephrine (NE) secreted directly into a synapse remains active for only a few seconds. After secretion of NE by the terminal nerve endings, it is removed from the secretory site in three ways: 1. Reuptake of NE back into the nerve terminals. 2. Diffusion of NE into capillaries (carried out of the tissue by the circulation). 3. Destruction by tissue enzymes: in Pre-synaptic a) Monoamine Oxidase “MAO” (found in the nerve endings) and b) Catechol-O- Methyl Transferase “COMT” (found diffused in the tissue) Normetanephrine: a metabolite (a product) created by The NE and E secreted into the blood by the adrenal medulla action of COMT on remain active until they diffuse into some tissue, where they NE. can be destroyed by COMT, mainly in the liver. both NE & E remain active for 10 to 30 seconds, but their activity declines to extinction for over 1 to several minutes. -NE remains only for few seconds NE and E are active for 10-30 See After three ways its removed by - but declined in one to several minutes 1. reuptake 2. diffusion 3. destruction by +issue enzyme A MOA. (n nerve endings) B. COMT com+ destroyed mainly in liver  24 Adrenergic Agonists same as adregenc · she consumed ephedrine Ephedrine acts like an adrenergic drug ( as in the problem given in the tutorial). Phenylephrine: α1 agonist Clonidine: α2 agonist Isoproterenol: β1 agonist heart Salbutamol: β2 agonist p2 , 10/5/2017 9 Receptors and Signal Transduction in the ANS  Signal Transduction Signal transduction is the process by which a chemical or physical signal is transmitted through a cell as a series of molecular events (most commonly, protein phosphorylation catalyzed by protein kinases). Signal transduction pathway is the diverse sequence of events between receptor activation and cellular response. Each component of a signalling pathway is classified according to the role it plays with respect to the initial stimulus. 1. Ligands are termed first messengers. Activation of receptors. 2. Receptors are the signal transducers, which then activate primary effectors (typically proteins). 3. Second messengers: linked to primary effectors and can activate secondary effectors. ↑ binds to lig and Signal transduction is how cells receive and respond to signals from outside the cell. Here’s a simpler breakdown: 1. First Messengers: These are the signaling molecules, like hormones, that come from outside the cell and bind to receptors on the cell surface. 2. Receptors: These are proteins on the cell’s surface (or inside) that detect the first messenger. When the receptor binds to the first messenger, it gets activated and starts passing the signal inside the cell. 3. Primary Effectors: After the receptor is activated, it passes the signal to proteins inside the cell called primary effectors. These proteins help carry the signal further into the cell. 4. Second Messengers: Some primary effectors create small molecules called second messengers, which spread the signal inside the cell and make it stronger. 5. Secondary Effectors: These are proteins that receive the signal from the second messengers and carry out the cell’s response, like activating certain genes or changing how the cell behaves. In short, the signal from outside the cell moves through a series of steps, each with different players, to create a specific response inside the cell. In essence, the receptor functions as a molecular switch that elicits the cell’s response when “switched on” by the messenger binding to it. Just as identical types of switches can be used to turn on a light or a radio, a single type of receptor can be used to produce quite different responses in different cell types.  Action of Water-Soluble Messengers (First Messengers or Ligands) – Receptor Activation Receptors are proteins or glycoproteins located either in the cell’s plasma membrane (transmembrane proteins) or inside the cell, in the cytosol or the nucleus. either in cell Or inside the cell membrane The majority of signal transduction pathways involve the binding of ligands to receptors that trigger events inside the cell. causes a change in the conformation of the receptor, known as receptor activation. The resulting cellular responses from receptor activation and signal transduction can take the form of changes in: 1. permeability, transport properties, or electrical state of the plasma membrane 2. metabolism Specificity of 3. secretory activity of the cell Receptors to 4. rate of proliferation and differentiation ligands 5.. contractile activities 13 Most ligands are lipid-insoluble molecules from the extracellular medium which bind to cell surface receptors. These include growth factors, cytokines and neurotransmitters. Components of the extracellular matrix such as fibronectin and hyaluronan can also bind to such receptors (integrins and CD44, respectively). In addition, some molecules such as steroid hormones are lipid-soluble and thus cross the plasma membrane to reach cytoplasmic or nuclear receptors  Types of Signal Transduction Different ways of signal transduction mechanisms in which the receptor complex: a) A Acts as an ion channel. b) B Functions as an enzyme, usually a tyrosine kinase. c) C Activates a JAK kinase in the cytoplasm. d) E Is coupled to a G protein. 10/5/2017 15 Here’s a simpler way to understand it: 1. Signal Binding: A signal molecule (first messenger) attaches to a receptor on the cell’s surface. 2. G-Protein Activation: This changes the shape of the receptor, activating a helper protein inside the cell called a G-protein. The G-protein’s α subunit grabs onto a GTP molecule, which turns it on. 3. Subunit Split: The activated α subunit separates from the other parts of the G-protein (β and γ). 4. Action: The α subunit moves along the cell membrane and either: Opens ion channels, changing ion levels like sodium (Na+) or calcium (Ca++) inside the cell. Activates an enzyme, which produces second messengers that continue the signal inside the cell. These actions eventually lead to the cell’s response to the original signal.  Ligand-Gated Ion Channels – Direct Gating Ion Channel is part of the receptor. Example: Nicotinic cholinoreceptors 28/09/2024 17 G-protein Coupled Receptors – Indirect Gating of Ion Channel Binding of a first messenger to the receptor changes the conformation of the receptor increases the affinity of the α subunit of the G protein for GTP (guanosine triphosphate) When bound to GTP, alpha subunit dissociates from the remaining two (β and γ). This dissociation allows the activated α subunit to link up with an ion channel May cause the ion channel to open or close, with a resulting change in the cytosolic ion concentration 2 18 G-Protein Coupled Receptors- Generation of Intracellular Second Messengers The G protein may activate or inhibit membrane enzymes with which it interacts. Such enzymes, when activated, cause the generation of second messengers inside the cell. Mechanism of Action of α1 Mechanism of Action of β Adrenoreceptors Adrenoreceptors 10/5/2017 20 Types of Cholinergic Receptors GQ Cholinergic Muscarinic Receptors Muscarinic receptors are G-coupled protein receptors involved in the parasympathetic nervous system. o The only exception to these receptors is the sweat glands, which possess muscarinic receptors but are part of the sympathetic nervous system. Same mechanism as the activation of Same messengers as the activation of β (but opposite α1 adrenoreceptors via G proteins effect) or α2 adrenoreceptors (Same effect) via G 13, 5 proteins 2, 4 Abbreviations to Know in Signal Transduction Phosphotidylinositol Biphospate = PIP2 Diacylglycerol = DAG Inositol Triphosphate = IP3 10/5/2017 24  for memorization nerves muscle fire fast muscles , mucus , membr M1 , m3 , m5 = A1 10/5/2017 m2 , m3 = A2 25