BSCI 353 Exam 2 Lecture Notes, Lecture notes of Neuroscience

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Synaptic Transmission (Chapter 5) Week 6: Synaptic Transmission (Chapter 5) Signaling Between Two Neurons  The Concept of Synapses  Santiago Ramón y Cajal (1852-1934) o 1906 Nobel Prize  Charles Sherrington (1857-1952) o 1932 Nobel Prize  The information transfer from one neuron to another occurs at specialized sites of contact: the synapses o 1897  Early ideas of synaptic transmission: Sparkers vs. Soupers o Sparkers: Electrical current flows from one neuron to the next o Soupers: Chemical substances transfer information from one neuron to the next at the synapse  Synaptic transmission: Both types exist o Electrical synapse (1957 Furshpan & Potter)  3 nm wide  Bidirectional  Invertebrate escape circuit, pacemaker, and liver cells o Chemical synapse (1921 Loewi)  20-50 nm wide  Unidirectional  Electrical and chemical synapses differ fundamentally in their transmission mechanisms o o Electrical synapses Synaptic Transmission (Chapter 5)  Ion current flows through connexon channels o Chemical synapses  Neurotransmitter released from pre-synaptic cell  Ions flow through postsynaptic channels Structure of Electrical Synapses  Gap junctions o Hemi-channel (connexon) made up of 6 connexins on each cell o Two hemi-channels are aligned o Channel pore is large, permitting ions and small proteins to go through o  Current directly flows through the connexon pores  Fast (<0.1 ms delay) & reliable  Bidirectional  Synchronize electrical activity (e.g., pacemakers, hormone secretion, etc.) Structure of Chemical Synapses  1) Transmitter in synthesized in presynaptic neuron and then stored in vesicles  2) An action potential invades the presynaptic terminal  3) Depolarization of the presynaptic terminal causes opening of voltage-gated Ca2+ channels  4) Influx of Ca2+ through channels  5) Ca2+ causes vesicles to fuse with presynaptic membrane  6) Neurotransmitter is released into synaptic cleft via exocytosis  7) Transmitter binds to receptor molecules in postsynaptic membrane  8) Opening or closing of postsynaptic channels  9) Postsynaptic current causes excitatory or inhibitory postsynaptic potential that changes the excitability of the postsynaptic cell  10) Removal of neurotransmitter by glial uptake (endocytosis) or enzymatic degradation  11) Retrieval of vesicular membrane from plasma membrane o Synaptic Transmission (Chapter 5) o Role of Calcium  Ca2+-dependent synaptic transmission at the neuromuscular junction o In a low-Ca2+ solution, stimulating the motor axon only leads to subthreshold EPP o Neurotransmitter release is dependent on Ca2+  Calcium concentrations o Squid neuron  Intracellular: 0.0001 mM  Extracellular: 10 mM o Mammalian neuron  Intracellular: 0.0001 mM  Extracellular: 1-2 mM  The role of calcium o Ca2+ enters presynaptic terminal  C2+ influx causes NT release o How do we test this?  1) Does Ca2+ enter?  Calcium imaging  2) Does blocking Ca2+ entry prevent NT release?  Apply Ca2+ blockers  3) Does Ca2+ alone cause NT release?  Inject Ca2+ directly  Evidence that a rise in presynaptic Ca2+ concentration triggers transmitter release from presynaptic terminals (1) o Using Ca2+-sensitive dyes under a fluorescent microscope, we can directly observe a rise in Ca2+ concentration (Ca2+ influx) at the presynaptic terminal following a chain of action potentials in the presynaptic neuron  Entry of Ca2+ through presynaptic voltage-gated calcium channels causes neurotransmitter release (2) o Blocking Ca2+ influx with Cd2+ prevents the ride of the postsynaptic membrane potential (EPSP), demonstrating that Ca2+ influx is necessary for synaptic transmission Synaptic Transmission (Chapter 5)  Evidence that a rise in presynaptic Ca2+ concentration triggers neurotransmitter release from presynaptic terminals (3) o Injecting Ca2+ into the presynaptic terminal alone triggers a rise of the postsynaptic membrane potential, demonstrating that the presence of Ca2+ alone is sufficient for synaptic transmission Endocytosis and Exocytosis of Vesicles  Synaptic vesicles fuse with the presynaptic neuronal membrane via exocytosis and get recycled via endocytosis  The synaptic vesicle membrane is densely covered by proteins o Synaptobrevin o Synaptotagmin o Synapsin  Presynaptic proteins play a large role in synaptic vesicle cycling  Mobilization  Synapsins  Docking  SNAREs  Priming  SNAREs  Fusion  Synaptotagmins  SNAREs  Coating  Clathrin  Budding  Dynamin  Clathrin  Uncoating  Clathrin o  Molecular mechanisms of exocytosis during neurotransmitter release o SNARE complex Synaptic Transmission (Chapter 5)  o 1) Free SNARE proteins on vesicle and plasma membranes  Synaptobrevin and synaptotagmin on vesicle  Syntaxin and SNAP-25 on plasma membrane o 2) SNARE complexes form as vesicle docks o 3) Synaptotagmin binds to SNARE complex o 4) Entering Ca2+ binds to synaptotagmin, leading to curvature of plasma membrane, which brings membranes together  Synaptotagmin is the calcium sensor (mutation of its gene is lethal) o 5) Fusion of membranes leads to exocytic release of neurotransmitter o  Molecular mechanisms of endocytosis following neurotransmitter release o Clathrin attaches to membrane to be retrieved o Dynamin pinches off membrane o 1) Adaptor proteins connect clathrin to vesicular membrane o 2) Clathrin triskelia assemble into coat, curving membrane to form coated pit o 3) Assembled clathrin cage constricts lipid stalk connecting two membranes o 4) Dynamin ring forms and pinches off lipid stalk o 5) Coated vesicle is translocated by actin filaments o 6) Hsc70 and auxilin uncoat vesicle Neurotransmitter & Receptor Diversity (Chapter 6) Week 7: Neurotransmitters & Receptors (Chapter 6) Two Types of Vesicles  Small (40-60 nm in diameter) o Acetylcholine (ACh) o Glutamate o GABA o Others  Dense core (90-250 nm) o Neuropeptides Neurotransmitters  Small-molecule neurotransmitters o Acetylcholine o Amino Acids  Glutamate  Aspartate  GABA  Glycine o Purines  ATP o Biogenic Amines  Catecholamines  Dopamine  Norepinephrine  Epinephrine  Indoleamine  Serotonin (5-HT)  Imidazoleamine  Histamine  Peptide neurotransmitters o More than 100 peptides, usually 3-36 amino acids long Neurotransmitter Synthesis and Storage  Small-molecule neurotransmitters are synthesized locally o 1) Enzymes convert precursor molecules into neurotransmitter molecules in the cytosol in the terminals o 2) Transporter proteins load the neurotransmitter into synaptic vesicles in the terminal, where they are stored  Peptide neurotransmitters are synthesized in the cell body and transported to the terminals o 1) A precursor peptide is synthesized in the rough ER o 2) The precursor peptide is split in the Golgi apparatus to yield the active neurotransmitter o 3) Secretory vesicles containing the peptide bud off from the Golgi apparatus o 4) The secretory granules are transported down the axon to the terminal where the peptide is stored Neurotransmitter Degradation and Recycling  Reuptake by transporters into presynaptic axon terminal Neurotransmitter & Receptor Diversity (Chapter 6)  Degrade by enzymes and metabolites reuptake  Glutamate & acetylcholine synthesis and recycling o Acetylcholine Pathways  First neurotransmitter to be identified  Synthesis of acetylcholine o Acetylcholine is synthesized in the presynaptic terminal from acetyl CoA and choline by the enzyme choline acetyltransferase o Neurons that use acetylcholine are called cholinergic o After release, acetylcholine is rapidly metabolized by the enzyme acetylcholinesterase, which breaks it down into acetate and choline  The choline is transported back into the presynaptic terminal by an Na+/choline transporter where it used to produce acetylcholine o  Distribution of acetylcholine o In the brain, several distinct clusters of cholinergic neurons are present  Project to hippocampus, reticular formation, and thalamus  Involved in arousal and sleep/wake cycle  Cholinergic systems in the cortex are crucial for learning and memory  Loss of these cells is characteristic of Alzheimer’s Neurotransmitter & Receptor Diversity (Chapter 6) o In the PNS, acetylcholine is present at the neuromuscular junction and in the visceral motor system  Pharmacology of acetylcholine o Two broad categories of acetylcholine receptors:  Nicotinic  Named for nicotine, which activates the receptor  Most nicotinic receptors are fast and ionotropic, usually having an excitatory effect o 1) Neurotransmitter binds directly to the channel o 2) Channel opens immediately o 3) Ions flow across membrane for a brief time  Curare blocks nicotinic ACh receptors o Because the synapses between nerves and muscle cells are nicotinic, curare paralyzes all muscles, including those used in breathing  Muscarinic  Named for muscarine, which also activates the receptor  Most muscarinic receptors are slow and metabotropic (G protein- coupled), can be excitatory or inhibitory o 1) Neurotransmitter binds G protein-coupled receptor o 2) G protein activated o 3) Activated G protein subunit moves to adjacent ion channel, which imposes a brief delay o 4) Channel opens, ions flow across membrane for a longer period of time  Atropine and scopolamine blocks muscarinic receptors o DDT and nerve gases inhibit acetylcholinesterase, leading acetylcholine to build up in the synapse and continuously depolarize the postsynaptic cell (neuromuscular paralysis) o Agonists that act at cholinergic synapses  Nicotine  Muscarine  Organophosphates  Black widow spider venom o Antagonists that act at cholinergic synapses  Atropine  Scopolamine  Botulinum toxin  Tetanus toxin Glutamate Pathways  Glutamate is an amino acid neurotransmitter o Main excitatory neurotransmitter in the brain o Over half of all brain synapses release glutamate o Does not cross the blood-brain barrier  Must be synthesized in neurons from local precursors Neurotransmitter & Receptor Diversity (Chapter 6)  L-amino acid decarboxylase catalyzes the removal of a carboxyl group from L-DOPA to produce dopamine o Once synthesized, catecholamines are packaged into vesicles for subsequent release  Distribution of dopamine o About 1 million nerve cells in the brain contain dopamine o Dopaminergic neurons are found in several areas of the brain, including the mesostriatal pathway and the mesolimbocortical pathway  Mesostriatal pathway: Substantia nigra to striatum (caudate and putamen)  Originates in midbrain, projects to forebrain  Plays crucial role in motor control o Damage to area causes tremors  Mesolimbocortical pathway: Ventral tegmental area to nucleus accumbens, cortex, and hippocampus  Originates in midbrain, projects to limbic system  Plays crucial role in motivation, reward, and reinforcement  Pharmacology of dopamine o Catecholamines are subject to multiple rounds of inactivation o Following release, both dopamine and norepinephrine are taken back into the nerve terminal by membrane transporters  Although this is an effective method for terminating the synaptic actions of catecholamines, degradative processes must also exist to prevent excess accumulation of catecholamines  Several important enzymes participate in the breakdown of catecholamines:  Monoamine oxidase (MAO) o MAO inhibitors act as dopaminergic agonists and have been used to treat depression  Cytosolic O-methyl transferase (COMT)  o Cocaine and amphetamines act as dopamine agonists by inhibiting reuptake of dopamine at the nerve terminal o Several dopamine receptors have been discovered and labeled D1, D2, D3, D4, and D5 Neurotransmitter & Receptor Diversity (Chapter 6)  The effects of some drugs are specific to one or a few of these subtypes o Dopaminergic antagonists include reserpine, which prevents the storage of monoamines in synaptic vesicles, and AMPT, which inhibits tyrosine hydroxylase Serotonin Pathways  Serotonin (5-HT) is an indolamine  Synthesis of serotonin o Synthesized from the amino acid tryptophan o The enzyme tryptophan hydroxylase adds a hydroxyl group and L-amino acid decarboxylase removes a carboxyl group from tryptophan to yield 5- hydroxytryptamine (serotonin)  Distribution of serotonin o Serotonergic cell bodies are relatively few and concentrated in the midbrain and brainstem along the midline o Large areas of the brain are innervated by serotonergic fibers  Serotonin has been implicated in the control of sleep/wake cycle, mood, anxiety, and many other functions  Pharmacology of serotonin o Termination of the effects of synaptic-released serotonin occurs largely by reuptake processes o Cytoplasmic serotonin is also metabolized by MAO o A large number of serotonin receptors have been identified (5HT-1A, 5HT-1B, 5HT- 2, 5HT-3, 5HT-4  All but one are metabotropic o o A large number of drugs have been found to stimulate 5-HT release  Fenfluramine  Amphetamines  Drugs that inhibit the reuptake of serotonin (SSRIs) Postsynaptic Neurotransmitter Receptors  Ionotropic receptors (fast) o Ligand-gated o Steps Neurotransmitter & Receptor Diversity (Chapter 6)  1) Neurotransmitter binds  2) Channel opens  3) Ions flow across membrane  Metabotropic receptors (slow) o G protein-coupled receptors o Amine & peptide receptors o Steps  1) Neurotransmitter binds  2) G protein is activated  3) G protein subunits or intracellular messengers modulate ion channels  4) Ion channel opens  5) Ions flow across membrane  Acetylcholine (cholinergic) receptors o Ionotropic  nAChR  K+  Na+ o Metabotropic  mAChR  Glutamate (glutamatergic) receptors o Ionotropic  AMPA  NMDA  Both ligand & voltage dependent ion channel  Blocked by Mg2+   Kainate o Metabotropic  mGluR  GABA (GABAergic) receptors o Ionotropic  GABAA  Cl- o Metabotropic Neurotransmitter & Receptor Diversity (Chapter 6) o In the muscle cell, ENa = +70 mV; EK = -100 mV   Activation of ACh receptors at neuromuscular synapses o Patch-clamp measurement of single ACh receptor current   Na+ and K+ movements during EPCs and EPPs o Ions flowing through nAChRs depend entirely on equilibrium and reversal potentials, not selectivity filter! o ENa = +70, EK = -100 o Erev = 0 Neurotransmitter & Receptor Diversity (Chapter 6) o  Summation of postsynaptic potentials (PSPs) o Excitatory postsynaptic potential (EPSP) o Inhibitory postsynaptic potential (IPSP) o Spatial summation  o Temporal summation Neurotransmitter & Receptor Diversity (Chapter 6)  Summation of Postsynaptic Potentials  The postsynaptic potentials that are caused in cells are either excitatory (EPSPs) inhibitory (IPSPs)  Postsynaptic potentials (PSPs) generally move passively along the membrane, gradually becoming smaller as they spread o The postsynaptic potentials from more distant sources will decay more than PSPs from sources close to the integration zone at the axon hillock o The PSPs produced in most synapses are well below the threshold for producing a post-synaptic action potential  Temporal and spatial summation of inputs leads to action potential  When inhibitory synapses are active, the membrane potential tends to be stabilized below threshold due to hyperpolarization Steps of Neurotransmission  11 basic steps of neurotransmission in chemical synapses o Pre  1) NT synthesized and stored in vesicles  2) Action potential invades terminal  3) Depolarization causes opening of Ca2+ channels  4) Influx of Ca2+  5) Ca2+ causes vesicles to fuse  6) NT released by exocytosis into synaptic cleft  7) Removal of NT by reuptake or enzyme degradation  8) Retrieval of vesicular membrane and recycling o Post  9) NT binds to postsynaptic receptors  10) Opening or closing postsynaptic channels  11) Postsynaptic current makes EPSP (or IPSP)  12) Summation determines whether or not an AP occurs  The four functional components of a neuron: A bigger picture o Dendrites  Input (receptive) component o Axon hillock  Integrative (summing) component o Axon Molecular Signaling (Chapter 7) o  G protein-coupled second messenger cascade o Neurotransmitter  G-protein  Enzyme  Second messengers o o Three common effector pathways associated with G protein coupled receptors  Amplification in signal transduction pathways o  Chemical signaling mechanisms and amplification o Chemical signaling requires several steps  Signaling cell  Signal  Receptor  Target molecule  Cellular response Molecular Signaling (Chapter 7) o Intracellular signaling  Receptor activation  G-protein activation (1st amplification step)  Adenylyl cyclase activation  cAMP production (2nd amplification step)  Protein kinase activation  Phosphorylation of proteins (3rd amplification step)   2nd messenger-mediated long-term effect o Nuclear signaling in transcriptional regulation by CREB  cAMP response element binding protein (CREB)   Long-term potentiation in memory Summary  Intracellular signal transduction pathways provide a wide range of temporal and spatial controls over the functions of individual neurons Molecular Signaling (Chapter 7) Optogenetics  A modern method of manipulating neuronal activity  Four major classes of ion channels o 1) Voltage-gated ion channels o 2) Ligand-gated ion channels o 3) Stretch and heat activated ion channels o 4) Light-gated ion channels  Braun & Hedgemann, 1999 o “The ultra-fast appearance of the rhodopsin current suggested that the rhodopsin and ion channel were intimately linked, forming a single protein complex”  Rhodopsins o Animal-type rhodopsins  G protein-coupled receptors indirectly modulating ion channel activity via signaling molecules o Microbrial-type rhodopsins  Light-gated ion channels  Channelrhodopsin 2  Light-activated excitation  Halorhodopsin  Light-activated inhibition  Experimental steps o 1) Viruses are used to ferry genes encoding light-sensitive receptors (opsins) into specific neurons o 2) Animals are fitted with an “optrode” (a fiber-optic cable with an electrode) o 3) Light beamed down the optrode will open or close ion channels while the electrode records neuronal firing and researchers recorded behaviors Vision (Chapter 11)  Do photoreceptors fire action potentials? o Input: Light absorption  Membrane potential change  Output: Neurotransmitter release o Hyperpolarization of a photoreceptor   Graded response!  Cyclic GMP-gated channels and light-induced changes in the electrical activity of photoreceptors o Vm at about -40 mV (depolarized) o Inward current of Na+ & Ca2+ through cGMP-gated channels (open in the dark) o Outward current of K+ through potassium (leak) channels o Continuous neurotransmitter release o Light-Sensing Mechanisms  Microorganisms o Light-activated ion channels (channelrhodopsin, halorhodopsin)  Other animals Vision (Chapter 11) o Light-activated metabotropic receptors (G-protein coupled receptors) o  Phototransduction in rod receptors via retinal o Photopigment: Opsin protein + chromophore (retinal) o 1) Light absorption leads to configuration change of retinal (cis retinal  trans retinal)  o 2) Activated rhodopsin facilitates GTP binding to transducin  Rhodopsin is the G protein coupled receptor  Transducin is the G protein that binds to rhodopsin  Vision (Chapter 11) o 3) GTP-bound  subunit dissociates from  and  subunit, and activates phosphodiesterase (PDE)  o 4) Activated phosphodiesterase (PDE) hydrolyzes cGMP, changing it into GMP  cGMP concentration therefore is reduced  o 5) cGMP-gated cation channels in the photoreceptor membrane close; Na+ and Ca2+ can no longer come into the cell  o Does light depolarize or hyperpolarize membrane potential?  Light  Hyperpolarize  Dark  Depolarize  Summary of phototransduction o 1) Light absorption leads to configuration change of retinal in rhodopsin (GPCR)  Amplification 1:800  Rhodopsin kinase + arrestin Vision (Chapter 11)  o Differential responses of primate rods and cones  Sensitivity: 15-30 rods converging onto 1 rod bipolar cell  Patch clamp recording from cGMP-gated channel shows rods stay activated after light stimulus  Resolution (acuity): 1 cone converging onto 1 cone bipolar cell  Patch clamp recording from cGMP-gated channel shows cone show initial activation and then decrease  Both patch clamp recordings show a constant inward current in the dark  Distribution of photoreceptors in the human retina o o Cone specialization: High resolution vision in the fovea due to high number of cones  Absorption spectra and distribution of cone opsins o Cone specialization: Color vision o Blue opsin (short wavelength pigment)  Rhodopsin  Short o Green opsin (medium wavelength pigment)  Short  Medium o Red opsin (large wavelength pigment)  Medium  Large Vision (Chapter 11) o Rods vs. Cones  Rods o Black & white o Very sensitive (1 photon) o Slow o Many rods converge onto one bipolar cell o Very low spatial resolution o Saturates at high light levels o Dense at edges of retina  Cones o Color o Relatively insensitive (many photons) o Fast o One cone to one bipolar cell o High spatial resolution o Adapts at high light levels o Dense in fovea