BIO 3350 - Lec 6 Neurotransmitters.pdf

REVERSAL OF POLARITY OF PSPS NEUROPATHIC PAIN 1 REVERSAL OF POLARITY OF PSPS Spasticity after injury to the spinal cord may result from a similar depolarizing change in Erev of GABAergic synapses 2 LECTURE 6: SYSTEMS OF NEUROTRANSMITTERS • • • • • • • Localisation of neurotransmitters and syn...

REVERSAL OF POLARITY OF PSPS NEUROPATHIC PAIN 1 REVERSAL OF POLARITY OF PSPS Spasticity after injury to the spinal cord may result from a similar depolarizing change in Erev of GABAergic synapses 2 LECTURE 6: SYSTEMS OF NEUROTRANSMITTERS • • • • • • • Localisation of neurotransmitters and synthesizing enzymes Cholinergic neurons Monoaminergic neurons Other systems Ionotropic receptors Metabotropic receptors: Coupled to G-proteins Intracellular effectors of metabotropic receptors Readings • Bear, chapter 6 3 INTRODUCTION • 3 classes of neurotransmitters • Amino acids, amines and peptide • Each system of neurotransmitter has specific … • Molecules, machinerie for synthesis, filling, recapture, degradation, etc., that are specific and sometimes unique 4 MAJOR NEUROTRANSMITTERS Amino acids Amines Peptide G-aminobutyric acid (GABA) Acetylcholine (ACh) Cholecystokinin (CCK) Glutamate (Glu) Dopamine (DA) Dynorphin Glycine (Gly) Adrenaline Enkephalin (Enk) Serotonin (5-HT) Neuropeptide Y Histamine Somatostatin Noradrenaline (NA) Substance P Vaspactive intestinal polypeptide (VIP) 5 NEUROTRANSMITTERS: 3 CRITERIA 1. Synthetized and packed in presynaptic neurons 2. Released by axon terminals 3. Produces a post-synaptic response 6 STUDYING NEUROTRANSMITTER SYSTEMS For each neurotransmitter, we can study: 1. Which neurons use a particular neurotransmitter? 2. Types of receptors? 3. Structures of receptors 4. Consequence of the activation of receptors? 7 STUDYING NEUROTRANSMITTER SYSTEMS • Localisation of neurotransmitters • 2 methods: I. II. I. In situ hybridization Immunocytochemistry In situ hybridization • Probes binding to mRNA of machinerie associated with a neurotransmitter • Labels cell bodies where mRNA is located • Considerations: Only labels cell bodies, binding effectiveness of probe can be variable, time-intensive process, probes are cheap 8 STUDYING NEUROTRANSMITTER SYSTEMS Motoneuron labelled by antibody against acetylcholine transferase II. Immunocytochemistry • Antibody recognising machinerie associated with a neurotransmitter Neurotransmitters (in only some cases), enzymes of biosynthesis or of transport • Considerations: Can label different parts of the cell (soma, axon terminals), shorter process than in situ hybridization, antibodies can be costly, if the axon terminals are labelled but not the cell body, than the origin of the axon terminals is not known 9 STUDYING NEUROTRANSMITTER SYSTEMS • Mimic the action of neurotransmitters using agonists • molecules evoking similar response to the unknown neurotransmitter released by the presynaptic neuron • Combination of: I. Micro-ionophoresis • Injection of candidate molecule • Can reproduce effects of synaptic transmission II. Microelectrode • Measure the effect of candidate molecule on postsynatpic neuron (e.g. EPSP/IPSP) 10 STUDY OF RECEPTOR SUBTYPES • Neuropharmacology • agonists and antagonists • Each neurotransmitter has many subtypes of receptors that respond to specific pharmacological agonists 3 subtypes of receptors to neurotransmitter X but each subtype responds to a specific agonist EXTRACELLULAR SPACE CYTOSOL 11 11 STUDY OF RECEPTOR SUBTYPES • 2 subtypes of cholinergic receptors • Nicotinergic • Muscarinic • Each subtype has a different antagonists (curare and atropine) 12 STUDY OF RECEPTOR SUBTYPES • 3 subtypes of receptors glutamatergic • AMPA • Kainate • NMDA • Each subtype has different antagonists and modes of operation 13 SUBTYPES OF RECEPTORS AND PHARMACOLOGY 14 MOLECULAR STUDY OF RECEPTORS • Molecular analysis of classes of receptors • Ionotropic receptors • subunits, structure, etc. • Metabotropic receptors • Structure, G protein, enzymatic cascade 15 ACETYLCHOLINE (ACH) 16 SYNTHESIS OF ACH • Acetylcholine (ACh) is made from acetyl-CoA and choline by choline acetyltransferase • ACh is degraded into choline and acetate by acetylcholinesterase (in the synaptic cleft) • Recapture of choline 17 ACETYLCHOLINE • In the CNS, ACh is involved in attention, memory, and learning. In the PNS, it is involved in muscular contractions and vegetative function By OpenStax College - Anatomy & Physiology, Connexions Web site. http://cnx.org/content/col11496/1.6/, Jun 19, 2013., CC BY 18 3.0, https://commons.wikimedia.org/w/index.php?curid=30148020 AMINO ACIDS 19 GLUTAMATERGIC AND GABAERGIC NEURONS 1. Glutamic acid (Glutamate) • 80% of excitatory synapses of CNS 2. GABA • Principal NT of inhibitory synapses • Synthetized from glu via Glutamate decarboxylase (GAD) 3. Glycine • Two modes of action I. II. NT co-activated by glutamate Inhibitory NT 20 GLUTAMATERGIC RECEPTORS • 3 types of receptors • AMPA • Kainate • NMDA • The 3 types can be found at the same synapse 21 GLUTAMATERGIC RECEPTORS • EGluR=0mV • AMPAR and KainateR are rapid • Initial phase of glu EPSP 22 GLUTAMATERGIC NMDA RECEPTORS  NMDAR are blocked at rest by Mg++  Depolarization removes Mg++ block (-30mV)  Normally, the Mg++ block is removed by depolarization caused by the opening of AMPAR  NMDAR are slower  Long phase of glutamatergic EPSP  Ca++ permeable  Associated with synaptic plasticity EXTRACELLULAR SPACE Mg2 + Mg2 + CYTOSOL Vm = -65 mV Vm = -30 mV 23 GABAERGIC AND GLYCINERGIC RECEPTORS • GABA is responsable for most inhibitory transmission • Glycine is responsable for non-GABAergic inhibitory transmission • GABARs bind ethanol, benzodiazepine, barbituric 24 OTHER NEUROTRANSMITTERS AND INTRACELLULAR MESSENGERS • ATP • Stimulates neurons • Binds to purinergic receptors • Involved in nociception • Endocannabinoids • Retrograde messenger • Synthetises and secreted by postsynaptic neuron • CB1R receptors on presynaptic neuron • Reduction of synaptic transmission • Involved in memory, mood, and movements 25 IONOTROPIC RECEPTORS • Rapid synaptic transmission • Sensitive to molecules and sometimes, membrane potential • Mediates significant membrane currents • Selective for specific ions 26 IONOTROPIC RECEPTORS: STRUCTURE • Basic structure of ionotropic receptors • • • • Pentamer: 5 subunits Subunits with 4 transmembrane domains Pore at the centre of subunits Pairs of subunit can bind two ligand molecules • E.g. Cholinergic receptor • Pentamer • 4 transmembrane domains • 2 α-subunits bind 2 ACh 27 METABOTROPIC RECEPTORS • G-protein coupled receptor • Structure of metabotropic receptors • A single polypeptide with 7 transmembrane alpha helix domains • Neurotransmitters that bind to metabotropic receptors • Amines (e.g. dopamine, serotonin, noradrenalin) • Peptides • Amino acids have a few metabotropic receptors 28 METABOTROPIC RECEPTOR ACTIVAITON OF G-PROTEINS • G-protein structure • α, β, and γ subunits • Inactive: GDP-bound; • Active: GTP-bound • 5 steps: 1. “Floating” G-protein 2. Binding of NT to metabotropic receptor activates Gα, which then binds to GTP while shedding the GDP. This leads to the separation of two complexes: Gα-GTP and Gβγ 29 METABOTROPIC RECEPTOR ACTIVAITON OF G-PROTEINS 5 steps (continuation): 3. Activation of effector proteins by Gα-GTP and Gβγ complexes 4. Gα inactives itself by hydrolyzing GTP into GDP 5. Re-assembly into inactive Gprotein (Gαβγ-GDP) 30 G-PROTEIN: CASCADE OF SECONDS MESSAGERS • G-protein links the neurotransmitter with the activation of an enzymatic cascade (series of reactions) downstream 31 CASCADE OF G-PROTEINS • Push-pull principle • Many types of G-proteins • Gs: stimulates the activity of adenylyl cyclase, which synthesizes cAMP • Gi: inhibits the activity of of adenylyl cyclase • … many others 32 CASCADE OF G-PROTEINS  Some cascades split into multiple branches  E.g. G-protein activates PLC (Phospholipase C)  PLC genates DAG and IP3 from PIP2  DAG (diacylglycerol) activates PKC  IP3 (inositol-3P) stimulates influx of Ca++ of internal storage 33 CASCADE OF G-PROTEINS • Divergence • 1 NT can activate many effectors in a cell • Convergence • Multiple NTs can act on the same effector (e.g. push-pull on adenylate cyclase) 34 CONCLUDING REMARKS • Neurotransmitters • • Transmit information between neurons Essential link between neurons and effector cells • Signaling pathways • • • • Signaling network within a neuron somewhat resembles brain’s neural network Inputs vary temporally and spatially to increase and/or decrease drive Delicately balanced Signals regulate signals - drugs can shift the balance of signaling power 35