lecture 6

lecture 6 - Synaptic transmission • ...

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Unformatted text preview: Synaptic transmission •  Synapses-functional contacts between neurons. •  Two general classes-electrical and chemical synapses. •  electrical- direct, passive flow of current between neurons. •  chemical- neurons talk to each other by release of neurotransmitters. electrical synapses •  Less common than chemical synapses. •  The cell membranes of two cells are linked together via gap junctions. •  Current actually flows from one neuron to another via gap junctions- form large pores between cells made up of connexin proteins. •  The signal is very fast-the only limit is diffusion. •  Signals can go in both directions. •  Are used to synchronize electrical activity among populations of neurons Structure and Function of Gap Junctions at Electrical Synapses •  PN05021.JPG Gap junctions allow current to flow from one cell to the next. Electrical and Chemical Synapses Differ in Their Transmission Mechanisms Chemical synapses •  The majority of connections use chemical synapses. •  They form at the synaptic cleft. •  Presynaptic cells have synaptic vesicles that have neurotransmitters in them. •  Post-synaptic cells have neurotransmitter receptors on the plasma membrane. Electrical and Chemical Synapses Differ in Their Transmission Mechanisms Synapses as seen by electron microscopy Synapse structure Synaptic cleft vesicles 10 stages of synaptic transmission •  •  •  •  •  •  •  •  •  •  Neurotransmitter is synthesized and packaged into vesicles. An action potential arrives at the presynaptic terminal. Depolarization causes opening of voltage gated Ca++ channels. There is an influx of Ca++ 10-4 outside 10-7 inside. Rushes in fast. Ca++ causes vesicles to fuse with membrane. Neurotransmitter is released into cleft. Transmitter binds to receptors on postsynaptic cell. This opens or closes postsynaptic channels. Postsynaptic current flows inside post-synaptic cell. Retrieval of membrane via endocytosis. Synaptic transmission 2 1 10 5 6 9 7 8 3 4 The discovery of acetylcholine(ACh) •  Otto Loewi- wanted to figure out how stimulation of vagus nerve caused the heart to slow down. Could transfer solution generated from one heart to slow down another even without stimulation. Showed a diffusible substance was released upon stimulation. Shared nobel in 1936, two years later the Nazi’s shut him down, made him give up all his possessions, but was allowed to leave the country. Eventually ended up at NYU Otto Loewi (austrian)- discovery of vagus nerve substance "In the night of Easter Saturday, 1921, I awoke, turned on the light, and jotted down a few notes on a tiny slip of paper. Then I fell asleep again. It occurred to me at six o'clock in the morning that during the night I had written down something most important, but I was unable to decipher the scrawl. That Sunday was the most desperate day in my whole scientific life. During the next night, however, I awoke again, at three o'clock, and I remembered what it was. This time I did not take any risk; I got up immediately, went to the laboratory, made the experiment on the frog's heart, described above, and at five o' clock the chemical transmission of nervous impulse was conclusively proved." Loewi's Experiment Demonstrating Chemical Neurotransmission Loewi's Experiment Demonstrating Chemical Neurotransmission Synaptic transmission is quantal •  The initial evidence obtained from studying the release of acetylcholine at neuromuscular junctions. •  The synapses between spinal motor neuron and skeletal muscle are simple, large, and peripherally located. •  The synapses form structures called end plates. Muscles have action potentials too, triggered by stimulus from Motor neurons - in the muscle, away from the neuromuscular junction -  the AP is again all-or-nothing Recordings in the junction reveal local potential changes before a regenerative action potential is produced. End plate potentials are generated at the end plate and only the end plate. These local membrane potentials are called end plate potentials (epps). End plate potential A presynaptic action potential releases a lot of ACh, opening channels in the muscle cell. The resulting depolarization is called an end plate potential (EPP). Miniature end plate potentials MEPP’s •  Spontaneous changes in potential even in the absence of an action potential. •  Same shape as EPPs but smaller (1mV vs 50mV). •  Sensitive to agents that block ACh receptors. •  Removing Ca2+ from media reduces EPPs to MEPPs. •  Thus EPPs are a bunch of MEPPs added up Comparison of MEPPs and subthreshold EPPs spontaneous Low Ca2+ Quantal Neurotransmission No release 1 2 3 •  By lowering Ca2+ one can reduce the amount of transmitter released by an AP. •  Here Ca2+ is so low that many presynaptic APs fail to release any Ach. •  Other APs release 1 to 6 quanta. Quantal Neurotransmission •  The MEPP is the quantal event of neurotransmission. It represents the postsynaptic response to the release of a single vesicle of neurotransmitter. •  The EPP is the result of the synchronized release of many vesicles. It is the sum of many MEPPs •  Bernard Katz nobel prize in 1970. 1 MEPP: 1synaptic vesicle •  Synaptic vesicles are full of neurotransmitter •  In motor neuron 1 vesicle contains approx 10,000 molecules of neurotransmitter. •  About the same amount needed to invoke an MEPP. Synaptic vesicles recycle •  All that vesicle fusion-why doesn’t the membrane keep growing and growing? •  Synaptic vesicle membranes get recycled quickly •  Are endocytosed in clathrin coated vesicles which fuse to endosome and bud off again. •  Used a pulse chase experiment to show this. Local Recycling of Synaptic Vesicles in Presynaptic Terminals Local Recycling of Synaptic Vesicles in Presynaptic Terminals Calcium is required for synaptic vesicle fusion •  Voltage clamping by Llinas showed that there is an inward Ca2+ flux in presynaptic cells that is voltage dependent. •  Ca2+ can be visualized entering cell after depolarization. •  Injection of Ca2+ into the presynaptic neuron can drive a post-synaptic potential. •  Chelating Ca2+ in the presynaptic cell can inhibit postsynaptic potential. The Role of If extracellular Ca2+ is removed or Ca2+ entry is blocked, there will be no release. 2+ Ca Voltage-gated Ca2+ channels in the presynaptic membrane provide Ca2+ to trigger the release of neurotransmitter. The role of Ca2+ •  Intracellular injection of Ca2+ into the presynaptic terminal will stimulate release. •  Intracellular injection of Ca2+ chelator will inhibit release. We know a lot about the proteins involved in vesicle fusion •  Yeast genetics and biochemistry have defined proteins involved in general vesicle fusion (SEC proteins). •  Homologues in synaptic vesicles •  Proteins have been found that are required in specific steps of fusion: budding, docking, priming, fusion. Presynaptic Proteins Implicated in Neurotransmitter Release Molecular Mechanisms •  SNAP-25 is a t-snare that regulates the assembly of two other SNAREs •  One SNARE (v-SNARE) in vesicle(synaptobrevin) •  tSNARE in plasma membrane (syntaxin) •  Together they tether the vesicle to the plasma membrane Molecular Mechanisms •  Vesicle docking with the major players in place. •  SNARES –  V-snare Synaptobrevin –  T-snares syntaxin and Snap-25 •  Synaptotagmin-Ca2+ sensor •  Ca2+ channel Molecular Mechanisms •  Primed and ready for exocytosis Molecular Mechanisms •  Depolarization opens the voltage-gated Ca2+ channels very close to the vesicle. •  Ca2+ enters and raises intracellular Ca2+ concentration very close to the vesicle. Molecular Mechanisms •  Exocytosis Vesicle proteins are the targets of many toxins •  Tetanus toxin cleaves synaptobrevin. •  Botulinum toxins cleave syntaxin and snap25. •  alpha-latrotoxin-black widow causes a massive exocytosis of vesicles. Somehow bypasses Ca2+ requirement, likely affecting synaptotagmin. Toxins •  The picture shows sites cleaved by tetanus toxin and various types of botulinum toxin. •  The action of toxins prevents exocytosis. Ⱥ  Dermatologists use botulinum toxin (or Botox) for cosmetic purposes. Ⱥ  When injected locally into a particular muscle or surrounding area, Botox causes a paralysis of that muscle due to a blockade of ACh release from the incoming motor nerve fibers. This leads to a reduction of wrinkle lines, although effective for only a few months. Neurotransmitters •  Lots of different kinds, over 100 or so. •  There are two main types- small molecule neurotransmitters and neuropeptides. •  Abnormalities of neurotransmitter function contributes to wide range of neurological diseases and psychiatric disorders. Formal criteria that define a neurotransmitter •  Must be present in the presynaptic neuron. •  Must be released in response to a depolarization and be Ca2+ dependent. •  Must have specific receptors localized on the post-synaptic cell. •  Note: It does not have to be uniquely a neurotransmitter, may have other functions. For example glutamate, glycine, ATP. Criteria That Define a Neurotransmitter present in presynaptic cell Calcium dependent release Criteria That Define a Neurotransmitter specific receptors on post-synaptic cell Localization of Neurotransmitter Action act locally can alter a few neurons at a time Localization of Neurotransmitter Action can act at long distances from the cell body Acetylcholine shown to be vagus factor •  Sir Henry Dale purified ACh and showed that it is vagus nerve substance. •  Can apply ACh to muscle and evoke an EPP. •  ACh action has same pharmacology as vagus nerve substance in that it is sensitive to curare. Competes with curare for receptor binding. Major categories of neurotransmitters •  Small molecule neurotransmitters- amino acids, purines, biogenic amines. •  Peptide neurotransmitters: 3-36 amino acid polypeptides, often derived from longer polypeptides. Examples of Small-Molecule Neurotransmitters Examples of Small-Molecule Neurotransmitters share hydroxylated benzene ring Examples of Small-Molecule and Peptide Neurotransmitters •  PN06033.JPG Neuropeptides Vary in Length, but Usually Contain Between 3 and 36 Amino Acids •  PN06042.JPG Neurotransmitter release can be regulated at many steps •  Synthesis- small molecules are generated from biosynthetic enzymes. Neuropeptides are generated by translation followed by protein processing. •  Packaging into vesicles- requires specific transporters on vesicle membrane, there are different types of vesicles, small clear-core (Ach, and amino acids) and large dense core (neuropeptides), biogenic amines do both. Location in synapses is different. •  Release-Small vesicles released fast, large-dense take more effort. The Synthesis, Packaging, Secretion, and Removal of Neurotransmitters Synthesis of Small Molecules Raw materials are collected by active transport. Neurotransmitter is synthesized and packaged right there. Synthesis of Neuropeptides Neuropeptides are synthesized in the nerve cell body, loaded into vesicles and transported down the axon via microtublules. Synthesis of Neuropeptides •  Neuropeptides are synthesized as prepropeptides in the nerve cell bodies. •  This includes a signal sequence that targets the peptides to the inside of the endoplasmic reticulum. •  The signal sequence is cleaved to form the propeptide. Synthesis of Neuropeptides ...
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