Chemistry and Physiology of the Synapse PDF

Chemistry and Physiology of the Synapse The presentation is for personal use only and must not be copied or used outside of BSMS Dr Natasha Sigala [email protected] Module 202 Neuroscience & Behaviour Last lecture • neurotransmitters - synthesis, storage, release • termination - how these even...

Chemistry and Physiology of the Synapse The presentation is for personal use only and must not be copied or used outside of BSMS Dr Natasha Sigala [email protected] Module 202 Neuroscience & Behaviour Last lecture • neurotransmitters - synthesis, storage, release • termination - how these events can be regulated Four main types of transmitters, • amino acids, e.g. Glu, GABA, Gly • monoamines, e.g. DA and 5-HT • acetylcholine • neuropeptides, e.g. endorphins 2 But what do neurotransmitters do? Why is Glu excitatory and GABA inhibitory? How do the other neurotransmitters modulate neuronal activity? It comes down to the properties of the receptors that they activate Receptors are specific to neurotransmitters – BUT each neurotransmitter has multiple receptors. 3 Outline A) Ionotropic and B) Metabotropic Receptors (Rs) A) Ionotropic Rs, fast transmission o Synaptic Integration o Glu Rs (NMDA, non NMDA) B) Metabotropic Rs: shortcut pathway second messenger cascades 4 Learning outcomes By the end of this lecture you should be able to: Describe and compare structure, speed of action and function of ionotropic and metabotropic receptors. Describe different types of second messengers and the neurotransmitters and receptors they are associated with. 5 Two families of postsynaptic receptors PSPs with different time courses 6 Two families of postsynaptic receptors PSPs with different time courses e.g. ACh: heart, mAChR G-protein K+ channel hyperpolarisation skeletal muscle, nAChR Na+ ion channel depolarisation 7 A). Ionotropic receptors Ligand gated ion channels are responsible for fast transmission of information to the postsynaptic neuron Similar to the voltage gated Na+ and K+ channels that control the action potential but opened by ligand binding rather than voltage changes ligand = neurotransmitter It binds to the channel, changes its conformation, thus opening it and allowing ions to flux through central pore. Channels made of 4 or 5 subunits that fold together to form the central pore. (from Purves 2nd ed) Receptor variation: • pharmacology – what transmitter binds to the receptor and how o agonist - a drug that can combine with a receptor on a cell to produce a physiological reaction o antagonist – a drug that blocks the activity of the agonist or endogenous ligand (neurotransmitter) • kinetics - rate of transmitter binding and channel gating determine • selectivity – what ions are fluxed (Na+, Cl-, K+ and/or Ca2+) • conductance – the rate of flux helps determine effect magnitude drugs interact with them the duration of their effects 9 Fast synaptic transmission Glutamate ionotropic receptors in general flux Na+, which causes an EPSP (Excitatory Post Synaptic Potential) depolarizing the postsynaptic neuron. Enough depolarization, due to multiple receptors being activated or repeated activation, can cause the postsynaptic cell to fire an action potential. GABA ionotropic receptors flux Cl-, which causes an IPSP (Inhibitory Post Synaptic Potential) hyperpolarizing the postsynaptic neuron. This inhibits the neuron from firing unless there is sufficient glutamate stimulation to counteract the hyperpolarization. 10 Fast synaptic transmission Acetylcholine, serotonin and ATP also activate ionotropic receptors. Nicotinic receptors at the neuromuscular junction are the most well studied ionotropic receptors. Their activation by acetylcholine causes the excitation and contraction of muscle cells. An integration of all the changes in membrane potential will decide whether a postsynaptic neuron will fire an action potential or not. 11 Synaptic integration GABA molecules (from Bear, Connors & Paradiso) 12 Synaptic integration GABA molecules (from Bear, Connors & Paradiso) 13 Synaptic integration 14 Glutamate receptors (GluRs) Based on their pharmacology, three types of ionotropic receptor have been described that respond to glutamate: 1) NMDA 2) AMPA 3) Kainate names based on the agonists selective for them 15 Pharmacology of ionotropic GluRs 1) NMDA receptors Agonist NMDA (N-methyl D-aspartate) Antagonist APV (2-amino-5-phosphonovaleric acid) 2) AMPA receptors Agonist AMPA (a-amino-3-hydroxy-5-methylisoxazole-4-propionic acid) Antagonist CNQX (6-cyano-7-nitroquinoxaline-2,3-dione) 3) Kainate receptors Agonist Kainic acid Antagonist CNQX (6-cyano-7-nitroquinoxaline-2,3-dione) 16 Selectivity and conductance of GluRs Non-NMDA receptors (AMPA and Kainate) • • Fast opening channels permeable to Na+ and K+ Responsible for early phase EPSP NMDA receptor 1) Slow opening channel – permeable to Ca2+ as well as Na+ and K+ BUT also 2) requires an extracellular glycine as a cofactor to open the channel 3) it is also gated by membrane voltage – Mg2+ ion plugs pore at resting membrane potentials. When membrane depolarizes Mg2+ ejected from channel by electrostatic repulsion allowing conductance of the other cations, activity-dependent synaptic modification. • NMDA receptors responsible for a late phase EPSP • Activated only in an already depolarized membrane in the presence of glutamate 17 NMDA receptors - regulation of channel opening from higher In of EPSPs measured resting potential than Mg2+ blockade. presence or absence AMPA or NMDA antagonists. Slower (from Purves 2nd ed) kinetics of NMDA channel -late phase EPSP Influx of Ca2+ as well as Na+ leads to activation of a number of enzymes and other cellular events that cause widespread changes in the postsynaptic cell (neuroplasticity). This action of NMDA receptors and 18 the NMDA receptors - dysregulation • NMDA receptors and Schizophrenia? NMDA receptors also inhibited by phencyclidine (PCP, angel dust) and MK801; both bind in the open pore. Blockade of NMDA receptors in this way produces symptoms that resemble the hallucinations associated with Schizophrenia. Certain antipsychotic drugs enhance current flow through NMDA channels • Glutamate excitotoxicity Excessive Ca2+ influx into the cell, which activates calcium-dependent enzymes that degrade proteins, lipids, and nucleic acids. This kind of cell damage occurs after cardiac arrest, stroke, oxygen deficiency, and repeated intense seizures (status epilepticus). 19 Other ionotropic receptors (ligand-gated ion channels) Glutamate - excitatory GABA(A) - inhibitory (brain) Glycine - inhibitory (spinal cord and brain stem) Nicotine - excitatory at NMJ (neuromuscular junction) - excitatory or modulatory in the CNS Serotonin -excitatory or modulatory ATP - excitatory (GABA, serotonin and nicotinic receptors in lecture L11) 20 B). Metabotropic receptors They transduce signals into the cell not directly through an ion channel but through activation of a G-protein which in turn triggers a series of intracellular events (that can lead to ion channel opening) G-protein coupled receptors (GPCRs) seven transmembrane domain protein • • • • multiple receptors have been described for every known neurotransmitter transmitter binds to extracellular domain of receptor binding triggers uncoupling of a heteromeric G-protein on the intracellular surface transduces signal across the cell membrane (from Bear, Connors & Paradiso) 22 Synaptic second-messenger systems (from Purves et al.) G-proteins GTP–binding proteins composed of three subunits – a, b and g GDP 1) in resting state the heteromer is bound to GDP splits 2) on binding of a ligand to the receptor the is switched for a GTP and the heteromer in two and 3) the Ga subunit and Gbg complex divide diffuse separately through the membrane stimulate 4) these individual entities are able to activity of other effector proteins enzymatic transient: the (from Bear, Connors & Paradiso) switches off its 5) a subunits have intrinsic GTP-GDP activity allowing the signal to be break down from GTP to GDP activity 6) at this point the heteromer recomplexes and awaits activation by ligand binding to another receptor. 24 G-protein-coupled effector systems In comparison to the numbers of receptors there are relatively few G-proteins. cyclase phospholipase C g) directly channel). (from SIGMA-ALDRICH) action for receptors in the GABA receptor. a subunits (~20) Gs stimulates adenylyl Gi inhibits adenylyl cyclase Gq stimulates bg complexes (5 b and 12 Activate K+ channels (G-protein gated ion This is the mode of muscarinic ACh heart and the 25 Shortcut pathway receptor G-protein (from Bear, Connors & Paradiso) ion channel Second Messenger Cascades (from Bear, Connors & Paradiso) Second messenger cascades: cAMP Gs and Gi have opposite effects on adenylyl cyclase, thus stimulating or inhibiting the synthesis of cAMP and the subsequent activation of protein kinase A (PKA). (from Bear, Connors & Paradiso) 28 Second messenger cascades: PIP2 Gq activates phospholipase C (PLC) which converts PIP2 into IP3 and diacylglycerol (DAG). DAG activates protein kinase C (PKC) and IP3 releases Ca2+ from internal stores which activates Ca2+-dependent enzymes. PIP2 - phosphatidylinositol bisphosphate IP3 - inositol triphosphate (from Bear, Connors & Paradiso) 29 Kinases and phosphatases - activity of many proteins regulated by their phosphorylation state - maintenance of phosphorylation state an important level of control e.g. Phosphorylation gated channels Influences membrane potentials and affects excitation state (from Bear, Connors & Paradiso) 30 The same transmitter can have shortor long-term effects on an ion channel long term synaptic changes structural and biochemical recruitment of new receptors (from Kandel, Schwartz & Jessell) Amplification of G protein signals G-protein signalling provides a method of amplifying signals between neurons can every the one transmitter bound receptor uncouple multiple G-protein heteromers the signal can be amplified at stage. what begins as a weak signal at synapse can cause an amplified response in the postsynaptic cell (from Bear, Connors & Paradiso) 32 Modulation by receptor activation Presynaptic receptors - change amount of transmitter released autoreceptors regulate release of transmitter by modulating its synthesis, storage, release or reuptake e.g. phosphorylation of tyrosine hydroxylase heteroreceptors (axoaxonic synapses or extrasynaptic) regulate synthesis and/or release of transmitters other than their own ligand e.g. NE can influence the release of ACh by modulating αadrenergic receptors Postsynaptic receptors - change firing pattern or activity • • increase or decrease rate of cell firing (directly by action at ligand gated ion channels or indirectly G -protein or phosphorylationcoupled channels) long term synaptic changes 33 Metabotropic receptors metabotropic glutamate receptors Group I: mGluR1+5 Gq Group II: mGluR2+3 Gi Group III: mGluR4,6,7+8 Gi GABA(B) receptor muscarinic acetylcholine receptors dopamine receptors noradrenergic and adrenergic receptors serotonin receptors neuropeptide receptors 34 C). Other receptors found on or in neurons 1) Enzyme-linked receptors e.g. Receptor tyrosine kinases Transmembrane proteins with intrinsic tyrosine kinase activity activated by neurotrophin binding (e.g. NGF, BDNF) On activation autophosphorylate phosphorylate intracellular regulatory subunits signal transduction cascades 2) Membrane-permeant signalling molecules activate intracellular receptors. 35 Summary Synaptic receptors: 1. recognize specific transmitters Typical effectors: 2. activate effectors 1. ion channel (fast, brief, ms) 2. enzyme that produces second messenger (sec – min, or days/weeks if gene transcription involved) Second messengers: A. trigger biochemical cascades by: 1. 2. Activating specific protein kinases Mobilizing Ca2+ from intracellular stores Or B. act on an ion channel (shortcut pathway) 36 Summary communication through synapses is regulated at multiple levels both pre- and post-synaptically. activation of receptors can modulate both electrical and structural properties of the neurons and the synapse (neuroplasticity) (from Bear, Connors & Paradiso) 37 Reading materials (Lectures 1, 4, 7 and 10) Bear, Connors & Paradiso, Neuroscience: Exploring the Brain 2nd,3rd or 4th ed, Chapters 5 & 6 Carlson, Physiology of Behaviour 8th ed or 9th ed , Chapters 2 & 4 http://www.williams.edu/imput/synapse/index.html Purves et al,4th or 5th ed, Neuroscience Kandel, Schwartz & Jessell, 4th ed, Principles of neural science Chapters 10-16 38 Synaptic second-messenger systems (from Kandel, Schwartz & Jessell) Synaptic integration Glu GABA 40 Synaptic integration Glu GABA 41 Synaptic integration Glu GABA 42