1. Neurotransmitter-4-BDS.pptx

A substance that is released at a synapse by a neuron and that effects another cell, either a neuron or an effectors organ, in a specialized manner , called neurotransmitter.

1. NEUROTRANSMITTERS Rajendra Dev Bhatt PhD Scholar Asst. Professor Clinical Biochemistry & Laboratory Medicine Fellow: Translational Research (2018-2022) in CVD in Nepal, NHLBI & NIH, USA

2. INTRODUCTION A substance that is released at a synapse by a neuron and that effects another cell, either a neuron or an effectors organ, in a specialized manner , called neurotransmitter. OR Neurotransmitters  are endogenous chemicals that transmit signals from a neuron to a target cell across a synapse Synapses are the junctions where neurons release a chemical neurotransmitter that acts on a postsynaptic target cell, which can be another neuron or a muscle or gland cell Some chemicals released by neurons have little or no direct effects on their own but can modify the effects of neurotransmitters. These chemicals are called neuromodulators.

3. Neurotransmitter Criteria Chemical messengers must meet following criteria to be considered transmitters: It is synthesized by a neuron. It is present in the presynaptic terminal and is released in amounts sufficient to exert a defined action on a postsynaptic neuron or effector organ. When given as a drug, it mimics the action of naturally occurring transmitter in the body exactly. A specific mechanism exists for removing it.

4. THE FATHER OF NEUROSCIENCE Otto L oewi (German pharmacologist) Discovered the chemical nature of neurotransmission (Acetylcholine) across synapse Loewi was awarded the Nobel Prize in Physiology (1936)

5. CLASSIFICATION OF NEUROTRANSMITTERS (Physiological)

6. CLASSIFICATION OF NEUROTRANSMITTERS (Biochemical)

7. Neurotransmitters can also be classified by another way dependent upon whether the receptor activated by the binding of transmitter is a metabotropic or an ionotropic receptor. Metabotropic ( muscarinic ) receptors activate signal transduction upon transmitter binding similar to many peptide hormone receptors which involves a second messenger. Many metabotropic receptors are members of the Gprotein coupled receptor (GPCR) family. Ionotropic (nicotinic) receptors constitute an ion channel, most often a ligand gated ion channel. Some neurotransmitters, for example glutamate and acetylcholine, bind to multiple receptors some of which are metabotropic and some of which are ionotropic . CLASSIFICATION OF NEUROTRANSMITTERS (Pharmacological)

8. This acquired knowledge about the neurotransmitters has led to the development of successful products for many brain disorders including epilepsy, schizophrenia, Parkinson’s disease, depression, anxiety disorders and migraine .

9. Neurotransmitter Receptors Once the molecules of neurotransmitter are released from a cell as the result of the firing of an action potential, they bind to specific receptors on the surface of the postsynaptic cell. The vast majority of neurotransmitter receptors belong to a class of proteins known as the G-protein coupled receptors, GPCRs.

10. Acetylcholine Acetylcholine ( ACh ) is a simple molecule synthesized from choline and acetylCoA through the action of choline acetyltransferase . Neurons that synthesize and release ACh are termed cholinergic neurons. When an action potential reaches the terminus of a presynaptic neuron a voltage gated calcium channel is opened. The influx of calcium ions, Ca2+, stimulates the exocytosis of presynaptic vesicles containing ACh , which is thereby released into the synaptic cleft.

11. Once released, ACh must be removed rapidly in order to allow repolarization to take place; this step, hydrolysis, is carried out by the enzyme, acetylcholinesterase ( AChE ). The released choline is then taken back up by the presynaptic neuron where it can once again serve as a substrate for ACh synthesis via choline acetyltransferase . Choline acetyl transferase

12. Two different mammalian AChE isoforms are produced from the single ACHE gene (chromosome 7q22.1) in humans via alternative splicing and posttranslational modification. These two AChE isoforms are termed T (tail) and H (hydrophobic). The T form ( AChET , also known as the hydrophilic form) is the predominant enzyme in the brain and at the neuromuscular junction. Two main classes of ACh receptors have been identified on the basis of their responsiveness to the toadstool (poisonous mushroom) alkaloid , muscarine and nicotine.

13. Acetylcholine Receptors

14. Muscarinic receptors Muscarine , the alkaloid responsible for the toxicity of toadstools, has little effect on the receptors in autonomic ganglia but mimics the stimulatory action of acetylcholine on smooth muscle and glands . These actions of acetylcholine are therefore called muscarinic actions, and the receptors involved are muscarinic cholinergic receptors. Five types , encoded by five separate genes, have been cloned (M1-M5). M 1 is abundant in the brain. The M 2 receptor is found in the heart. The M 4 receptor in pancreatic acinar and islet tissue. The M 3 and M 4 receptors are associated with smooth muscle .

15. Nicotinic receptors In Sympathetic Ganglia , the actions of Ach are unaffected by atropine but MIMICKED BY NICOTINE . Consequently, these actions of Ach are nicotinic actions and the receptors are nicotinic cholinergic receptors. Nicotinic receptors are subdivided into those at neuromuscular junctions and those found in autonomic ganglia and the central nervous system Both muscarinic and nicotinic acetylcholine receptors are found in large numbers in the brain . The nicotinic acetylcholine receptors are members of a super family of ligand-gated ion channels

16. Each nicotinic cholinergic receptor is made up of five subunits that form a central channel which, when the receptor is activated, permits the passage of Na + and other cations . A prominent feature of neuronal nicotinic cholinergic receptors is their high permeability to Ca2+. The 5 subunits come from a menu of 16 known subunits , α 1 – α 9 , β 1 – β 5 , γ , δ and ε , coded by 16 different genes .

17. CATECHOLAMINES : Dopamine, Epinephrine, Norepinephrine These three neurotransmitters are synthesized in a common pathway from the amino acid tyrosine. Tyrosine is supplied in the diet or is synthesized in the liver from the essential amino acid phenylalanine by phenylalanine hydroxylase. The tyrosine is then transported to catecholamine secreting neurons where a series of reactions convert it to dopamine, to norepinephrine and finally to epinephrine

18. The first and rate-limiting step in the synthesis of these neurotransmitters from tyrosine is the hydroxylation of the tyrosine ring by tyrosine hydroxylase, a tetrahydrobiopterin (BH4)-requiring enzyme. The product formed is dihydroxyphenylalanine (DOPA). The phenyl ring with two adjacent OH groups is a catechol , and hence dopamine, norepinephrine, and epinephrine are called catecholamines. The second step in catecholamine synthesis is the decarboxylation of DOPA to form dopamine. This reaction, like many decarboxylation reactions of amino acids, requires pyridoxal phosphate (PLP). SYNTHESIS OF CATECHOLAMINES

19. Neurons that secrete norepinephrine synthesize it from dopamine in a hydroxylation reaction catalyzed by dopamine -hydroxylase (DBH). This enzyme is present only within the storage vesicles of these cells. These neurons contain the above pathway for norepinephrine synthesis and in addition contain the enzyme that transfers a methyl group from SAM to norepinephrine to form epinephrine. Thus, epinephrine synthesis is dependent on the presence of adequate levels of B12 and folate .

20. Once synthesized, dopamine, norepinephrine and epinephrine are packaged in granulated vesicles for secretion in response to the appropriate nerve impulse. The catecholamines are transported into vesicles by the protein VMAT2 (vesicle membrane transporter) The primary effects of the catecholamines are exerted as neurotransmitters upon their stimulated release from presynaptic nerve terminals in the appropriate target organ. The catecholamines are also known as adrenergic neurotransmitters and the neurons that secrete them are referred to as adrenergic neurons.

21. Catecholamines exhibit peripheral nervous system excitatory and inhibitory effects as well as actions in the CNS such as respiratory stimulation and an increase in psychomotor activity. The excitatory effects are exerted upon smooth muscle cells of the vessels that supply blood to the skin and mucous membranes. Cardiac function is also subject to excitatory effects, which lead to an increase in heart rate and in the force of contraction.

22. Inhibitory effects, by contrast, are exerted upon smooth muscle cells in the wall of the gut, the bronchial tree of the lungs, and the vessels that supply blood to skeletal muscle. The actions of norepinephrine and epinephrine are exerted upon binding to and activating the adrenergic receptors of which are nine distinct forms.

23. Catecholamine Catabolism The action of catecholamines is terminated through reuptake into the presynaptic terminal and diffusion away from the synapse. Degradative enzymes are present in the presynaptic terminal, and in adjacent cells, including glial cells and endothelial cells. Epinephrine and norepinephrine are catabolized to inactive compounds through the sequential actions of catecholamine O methyltransferase (COMT) and monoamine oxidase (MAO).

24. Serotonin (5hydroxytryptamine, 5HT) Serotonin, an inhibitory neurotransmitter, is formed in the body by hydroxylation and decarboxylation of the essential amino acid TRYPTOPHAN Tryptophan hydroxylase in the human CNS is slightly different from the tryptophan hydroxylase in peripheral tissues, and is coded by a different gene .

25. The greatest concentration of 5HT (90%) is found in the enterochromaffin cells of the gastrointestinal tract. Most of the remainder of the body's 5HT is found in platelets and the CNS. The effects of 5HT are felt most prominently in the cardiovascular system, with additional effects in the respiratory system and the intestines. Vasoconstriction is a classic response to the administration of 5HT. Neurons that secrete 5HT are termed serotonergic . Following the release of 5HT, a portion is taken back up by the presynaptic serotonergic neuron in a manner similar to that of the reuptake of norepinephrine.

26. After release from serotonergic neurons, much of the released serotonin is recaptured by an active reuptake mechanism and inactivated by MONOAMINE OXIDASE (MAO) to form 5-hydroxyindoleacetic acid (5-HIAA). This substance is the principal urinary metabolite of serotonin, and its urinary output is used as an index of the rate of serotonin metabolism in the body

27. Serotonergic Receptors 5-HT 1 - 5-HT 7 receptors Most of these are G protein-coupled receptors 5-HT 3 receptors are ligand-gated ion channels present in the GIT & related to vomiting.

28. Histamine Histamine, an excitatory neurotransmitter, referred as a biogenic amine which is a potent neurotransmitter that binds to specific histamine receptors . Histamine is synthesized by the enzymatic decarboxylation of the amino acid histidine by the enzyme histidine decarboxylase (HDC). Within the gastrointestinal tract bacteria also produce histamine via a similar decarboxylation reaction.

29. Following its release, histamine is metabolized by two alternative pathways, one of which is an oxidation and the other a methylation reaction.

30. The oxidation of histamine is catalyzed by a diamine oxidase encoded by the AOC1 (amine oxidase , copper containing 1) gene. The product of diamine oxidase reaction is imidazole acetaldehyde which is itself further metabolized to imidazole acetate via the action of an aldehyde dehydrogenase . The methylation of histamine is catalyzed by the enzyme histamine Nmethyltransferase encoded by the HNMT gene.

31. Humans express four distinct histamine receptors identified as H1R, H2R, H3R, and H4R. All four histamine receptors are members of the G protein coupled receptor super family.

32. Glutamate & GABA Within the CNS glutamate is the main excitatory neurotransmitter. Neurons that respond to glutamate are referred to as glutamatergic neurons. Glutamate is primarily synthesized from the TCA cycle intermediate – ketoglutarate . This can occur via either of two routes. The first is via the enzyme glutamate dehydrogenase , which reduces - ketoglutarate to glutamate. The second route is through transamination reactions in which an amino group is transferred from other amino acids to - ketoglutarate to form glutamate.

33. GABA (Gama- aminobutyric acid) is the major inhibitory neurotransmitter in the central nervous system. GABA is synthesized by the decarboxylation of glutamate in a single step catalyzed by the enzyme glutamic acid decarboxylase (GAD). GABA is recycled in the central nervous system by a series of reactions called the GABA shunt, which conserves glutamate and GABA

34. GABA exerts its effects by binding to two distinct receptor subtypes. The GABA-A (GABAA) receptors are members of the ionotropic receptors, specifically the subfamily of ligandgated ion channels. The GABA-B (GABAB) receptors belong to the class C family of metabotropic G protein coupled receptors (GPCR).

35. Glycine Glycine is the major inhibitory neurotransmitter in the spinal cord. Most of the glycine in neurons is synthesized de novo within the nerve terminal from serine by the enzyme serine hydroxymethyltransferase , which requires folic acid. Serine, in turn, is synthesized from the intermediate 3-phosphoglycerate in the glycolytic pathway.

36. ASPARTATE Aspartate , like glutamate, is an excitatory neurotransmitter, but it functions in far fewer pathways. It is synthesized from the TCA cycle intermediate oxaloacetate via transamination reactions.

37. NITRIC OXIDE Nitric oxide (NO) is a biologic messenger in a variety of physiologic responses, including vasodilation , neurotransmission, and the ability of the immune system to kill tumor cells and parasites. First described in 1979 as a potent relaxant of peripheral vascular smooth muscle. Used by the body as a signaling molecule. Serves different functions depending on body system. i.e. neurotransmitter, vasodilator, bactericide. Environmental Pollutant First gas known to act as a biological messenger

38. Nitric oxide is a diatomic free radical consisting of one atom of nitrogen and one atom of oxygen Lipid soluble and very small for easy passage between cell membranes Short lived, usually degraded or reacted within a few seconds

39. Synthesis of Nitric Oxide NO is synthesized from arginine in a reaction catalyzed by NO synthase.

40. How does NO work? Diffuses to cells other than the one where it was synthesised cGMP acts to sequester intracellular calcium and close calcium channels Drop in intracellular calcium causes relaxation

41. Functions of NO in body Dilates blood vessels Displaces oxygen from oxyhaemoglobin ‘metabolic factor’ mediating increased blood flow to exercising muscle Appears to prevent hypoxia Released from organic nitrate and nitrite vasodilators Acts to relax bronchial smooth muscle NO is oxidised to nitrite and then nitrate excreted in urine

42. Thank You…