Synaptic Transmission

Transcription

1 Synaptic Transmission Jianhong Luo, Ph.D. Department of Neurobiology Zhejiang University School of Medicine Main Reference: Neuroscience Exploring the Brain, 3 rd Ed. By M.F. Bear, B.W. Connors, and M.A. Paradiso

2 Introduction Types of synapses Electrical synapses Chemical synapses Principles of chemical synaptic transmission - Neurotransmitters (NT) - Synthesis and storage - Release - Receptors and effectors - Recovery and degradation - Neuropharmacology Principles of synaptic integration The integration of EPSPs The contribution of dendritic properties Inhibition Modulation

3 Introduction By the end of 19th century, it was recognized that this transfer of information from one neuron to another occurs at specialized sites of contact. ( 例 : 疼痛的反应 )

4 Introduction Synapse (1897 Charles Sherrington) Synaptic transmission two hypothesis Argued for a century on its physical nature. Electrical synapse (Proven in 1959 by E. Furshpan and D. Potter in crayfish) Chemical synapse 1. Solid evidence given in 1921 by Otto Loewi; 2. B. Katz et al. demonstrated fast transmission at NMJ chemically mediated. 3. By 1951, John Eccles studied the synaptic transmission in the mammalian CNS using the glass microelectrode 4. During the last decade, new methods of studying the molecules revealed that synapses are far more complex. A large and fascinating topic

5 Introduction Otto Loewi ( ), a German born pharmacologist, discoverer of acetylcholine, nobel prize laureate in Physiology or Medicine in His most famous experiment came from his dream in the night of Easter Sunday, 1921 and found vagusstoff, turned out to be acetylcholine, showing that synaptic signaling used chemical messengers.

6 Types of synapses A synapse is the specialized junction where one part of a neuron contacts and communicates with another neuron or cell type (such as a muscle or glandular cell). Information tends to flow in one direction, from a neuron to its target cell. The first is said to be presynaptic and the target is said to be postsynaptic. Electrical synapses ( 电突触 ) Six connexins form a channel (connexon), and two connexons (one from each cell) form a gap junction channel. The pore of channels is relatively large with diameter 1 2 nm, enough for all the major cellular ions, and many small organic molecules, to pass through directly from the cytoplasm of one cell to the other s.

7 Types of synapses Neurites of two cells connected by a gap junction. Six connexin subunits form one connexon, two connexons form one gap junction channel, and many channels comprise one gap junction.

8 Types of synapses Functional properties of electrical synapses: Equally pass in both direction Electrically coupled Very fast, and if the synapse is large, fail-safe. Thus, an AP in the presynaptic neuron can produce, almost instantaneously, an AP in the postsynaptic neuron. In invertebrate species, such as the crayfish, electrical synapses are sometimes found between sensory and motor neurons in neural pathways mediating escape reflexes.

9 Types of synapses Electrical synapses also occur in the vertebrate brain. Are common in every part of the mammalian CNS Among electrically coupled neurons, AP in the presynaptic neuron can cause a small amount of ionic current to flow across the gap junction channels into the other neuron, producing postsynaptic potential (PSP). The PSP generated by a single electrical synapse in the mammalian brain is usually small about 1 mv or less at its peak and may not, by itself, be large enough to trigger an AP in the postsynaptic cell. The precise roles of electrical synapses vary from one brain region to another (synchronize; developmental coordination; in non-neuron cells). Box 5-2 by Michael V. L. Bennett

10 Types of synapses Electrical synapses. (a) A gap junction coupling the dendrites of two neurons constitutes an electrical synapse. (b) An AP generated in one neuron causes a small amount of ionic current to flow through gap junction channels into a second neuron, inducing an PSP.

11 Types of synapses Chemical synapses General description: synaptic cleft (20 50 nm ), filled with a matrix of fibrous extracellular protein. One function of this matrix is to make the preand postsynaptic membranes adhere to each other. presynaptic element, is usually an axon terminal. synaptic vesicles (50 nm in diameter), store neurotransmitter used to communicate with the postsynaptic neuron. secretory granules (larger vesicles, about 100 nm diameter) contain soluble protein (dark in EM, large dense-core vesicles) Membrane differentiations on either side of the synaptic cleft Active zone looks like pyramid, the sites of NT release postsynaptic density contains receptors converting signal from intercellular to intracellular

12 Types of synapses The components of a chemical synapse.

13 Types of synapses Presynaptic axon Postsynaptic spine Postsynaptic density Active zone Astrocyte Coated vesicle Dense-core vesicle Double-walled vesicle Endo. Reticulum Mitochondrion Punctum adhaerens Synaptic cleft Synaptic vesicle

14 Types of synapses Different types in of synapse in the CNS (CNS synapses) The sizes and shapes of CNS synapses also vary widely Axodendritic, Axosomatic, axoaxonic, dendrodendritic synapses. Mt V DCV Presynaptic Postsynaptic AZ Chemical synapses as seen with EM (left) A fast excitatorysynapse in the CNS (right) A synapse in the PNS, with numerous dense-core vesicles

15 Types of synapses Synaptic arrangements in the CNS. (a) An axodendritic synapse. (b) An axosomatic synapse. (c) An axoaxonic synapse.

16 Types of synapses Various sizes of CNS synapses. Notice that larger synapses have more active zones.

17 Types of synapses Two categories of CNS synaptic membrane differentiations. (a) A Gray s type I synapse is asymmetrical and usually excitatory. (b) A Gray s type II synapse is symmetrical and usually inhibitory.

18 Types of synapses Synaptic junctions also exist outside the central nervous system. Axons of the autonomic nervous system innervate glands, smooth muscle, and the heart. Neuromuscular junctions occur between the axons of motor neurons of the spinal cord and skeletal muscle. NMJ has many of the structural features of chemical synapses in the CNS. Neuromuscular synaptic transmission is fast and reliable. An AP in the motor axon always causes an AP in the muscle cell it innervates (What structural features for this reliability?) Most knowledge from the research on NMJ transmission.

19 Types of synapses The neuromuscular junction. The postsynaptic membrane, known as the motor endplate, contains junctional folds with numerous neurotransmitter receptors.

20 Principles of chemical synaptic transmission There basic requirements for chemical synaptic transmission: Synthesis and package into vesicles of neurotransmitter (NT); Release of vesicle NT to cleft in response to a presynaptic AP; Induction of an electrical or biochemical response to NT in the postsynaptic neuron Clearance of NT from the synaptic cleft And, occur very rapidly to be useful for sensation, perception, and the control of movement.

21 Principles of chemical synaptic transmission Neurotransmitters (three chemical categories) g- 氨基丁酸 乙酰胆碱 胆囊收缩素 谷氨酸 多巴胺 强腓肽 甘氨酸 肾上腺素 脑啡肽 组胺 N- 乙酰门冬氨酰谷氨酸 去甲肾上腺素 神经肽 Y 5- 羟色胺 生长抑素 P 物质促甲状腺素释放激素 血管活性肠肽

22 Principles of chemical synaptic transmission Neurotransmitters Three chemical categories Amine, amino acid, peptide Secretory granules and synaptic vesicles Often co-exist in the same axon terminals amine + peptide amino acid + peptide Different neurons release different neurotransmitters Fast transmission; NMJ; Slow transmission

23 Principles of chemical synaptic transmission Representative neurotransmitters (a) glutamate, GABA, and glycine. (b) acetylcholine and norepinephrine. (c) substance P.

24 Principles of chemical synaptic transmission Neurotransmitter Synthesis and Storage Amine and amino acid neurotransmitters: ➀ Enzymes are transported to the axon terminal and convert precursor molecules into neurotransmitter molecules in the cytosol. ➁ Transporter proteins load the neurotransmitter into synaptic vesicles in the terminal, where they are stored. Glu, Gly vs GABA, the amines Peptides: ➀ A precursor peptide (a long peptide) synthesis in the rough ER in cell body. ➁ Then split in the Golgi apparatus to yield the active one. ➂ Secretory vesicles with the peptide bud off from the Golgi apparatus. ➃ The secretory granules are transported (axoplasmic) down the axon to the terminal where the peptide is stored.

25 Principles of chemical synaptic transmission Transporters, proteins in the vesicle membrane, take up and concentrate the amino acid and amaine neurotransmitters inside the vesicle.

26 Principles of chemical synaptic transmission Neurotransmitter Release An action potential in the axon terminal depolarization of the terminal membrane voltage-gated calcium channels in the active zones to open ([Ca 2+ ]i mm 0.1 mm) vesicles release(exocytosis) the contents to spill out into the synaptic cleft The exocytosis occurs very rapidly within 0.2 msec of the Ca 2+ influx into the terminal. Why? The mechanism by which [Ca 2+ ] i stimulates exocytosis: Reserve pool of vesicles bound to the cytoskeleton Docking of vesicles to active zone SNARE protein complex, conformation altered by [Ca 2+ ]i Endocytosis Recycled vesicle refilled with neurotransmitter

27 Principles of chemical synaptic transmission The Release of Neurotransmitter by Exocytosis

28 Principles of chemical synaptic transmission A receptor s eye view of neurotransmitter release (a) This is a view of the extracellular surface of the active zone of a neuromuscular junction in a frog. The particles are believed to be calcium channels. (b) In this view, the presynaptic terminal had been stimulated to release neurotransmitter. The exocytotic fusion pores are where synaptic vesicles have fused with the presynaptic membrane and released their contents.

29 Principles of chemical synaptic transmission SNAREs and vesicle fusion (Box 5.3) SNARE: SNAP Receptor SNAP: Soluble NSF Attach Protein NSF: N-ethylmaleimide-sensitive factor (N- 乙基马来酰亚胺敏感的融合因子 )

30 Principles of chemical synaptic transmission Secretory granules also release peptide neurotransmitters by exocytosis: in a calcium-dependent fashion typically not at the active zones requires high-frequency trains of AP and more calcium influx. a leisurely process to taking 50 msec or more.

31 Principles of chemical synaptic transmission Neurotransmitter Receptors and Effectors binding to specific receptor proteins in the postsynaptic density. key in a lock, induce conformational changes in the receptor and lead to different functions. More than 100 different receptors can be classified into two types: transmitter-gated ion channels and G-proteincoupled receptors.

32 Principles of chemical synaptic transmission Receptors Ion channels G-protein Coupled Receptors Enzyme linked receptors Nuclear receptors Receptor channels Voltage-gated Mechanically-gated Non-gated or Ionotropic receptors, or Ligand-gated ion channels Transmitter-gated ion channels

33 Principles of chemical synaptic transmission Transmitter-Gated Ion Channels Membrane-spanning proteins consisting of four or five subunits to form a pore. Closed to open, neurotransmitter, binds to specific sites, induces a conformational change The functional consequence depends on which ions. The structure of an ACh-gated ion channel

34 Principles of chemical synaptic transmission Ion selectivity of transmitter-gated channels and postsynaptic potential Channels permeable to Na +, Depolarization, to be excitatory Excitatory postsynaptic potential (EPSP) Ach- or Glutamate-gated channels

35 Principles of chemical synaptic transmission Channels permeable to Cl -, Hyperpolarization, to be inhibitory Inhibitory postsynaptic potential (IPSP) BABA- or Glycine-gated channels

36 Principles of chemical synaptic transmission G-Protein-Coupled Receptors (GPCR) Fast chemical synaptic transmission is mediated by amino acid and amine neurotransmitters acting on transmitter-gated ion channels. However, all three types of neurotransmitter, acting on GPCR, can also have slower, longer-lasting, and much more diverse postsynaptic actions. This type of transmitter action involves three steps: 1 Transmitters bind to receptors in the postsynaptic membrane. 2 The receptors activate G-proteins, free to move along the intracellular face of the postsynaptic membrane. 3 The activated G-proteins activate effector proteins.

37 Principles of chemical synaptic transmission Effector proteins G-protein-gated ion channels in the membrane (left) Enzymes that synthesize second messengers (right) Second messengers can activate additional enzymes in the cytosol that can regulate ion channel function and alter cellular metabolism. GPCR often referred to as metabotropic receptors.

38 Principles of chemical synaptic transmission The same neurotransmitter can have different postsynaptic actions, depending on what receptors it binds to. In the heart, a metabotropic ACh receptor is coupled by a G-protein to a potassium channel. It slows the rhythmic contractions of the heart by causing a slow hyperpolarization of the cardiac muscle cells. In skeletal muscle, the receptor is an ACh-gated ion channel, permeable to Na +. ACh induces contraction by causing a rapid depolarization of the muscle fibers.

39 Principles of chemical synaptic transmission The shortcut pathway. (a) G-proteins in heart muscle are activated by ACh binding to muscarinic receptors. (b) The activated G subunit directly gates a potassium channel.

40 Principles of chemical synaptic transmission Autoreceptors Neurotransmitter receptors are also commonly found in the membrane of the presynaptic axon terminal. Sensitive to the neurotransmitter, called autoreceptors. Typically, autoreceptors are GPCR The common consequences of activating autoreceptors is inhibition of neurotransmitter release. This allows a presynaptic terminal to regulate itself

41 Principles of chemical synaptic transmission Neurotransmitter Recovery and Degradation Neurotransmitter in the synaptic cleft must be cleared to allow another round of synaptic transmission. Simple diffusion (For most of the amino acid and amine neurotransmitters) Reuptake occurs by the action of specific transporter proteins located in the presynaptic membrane (once inside the cytosol, enzymatically destroyed, or reloaded into synaptic vesicles) Neurotransmitter transporters also exist in the membranes of glia surrounding the synapse, which assist in such removal. Enzymatic destruction in the cleft. Ach is removed at the NMJ by enzyme acetylcholinesterase, deposited in the cleft. Importance of removal: desensitization ( 脱敏 ); nerve gases

42 Principles of chemical synaptic transmission Neuropharmacology Each of the steps of synaptic transmission is chemical, and therefore can be affected by specific drugs and toxins. Inhibitors: e.g. Nerve gases inhibite the enzyme AChE. Inhibitors of neurotransmitter receptors, called receptor antagonists (e.g. Curare, an arrow-tip poison, binds tightly to the ACh receptors) Receptor agonists. e.g. nicotine, binds to, and activates, the ACh receptors in skeletal muscle and CNS. nicotinic ACh receptors (nachr). Wrong neurotransmission is the root cause of many neurological and psychiatric disorders. Knowledge of neuropharmacology of synaptic transmission will be helpful for development of new and effective therapeutic drugs.

43 Principles Of Synaptic Integration Principles of synaptic integration The integration of EPSPs The contribution of dendritic properties Inhibition Modulation

44 Principles Of Synaptic Integration The postsynaptic neuron integrates thousands of synaptic inputs (complex ionic and chemical signals) and gives rise to a simple form of output: AP The transformation constitutes a neural computation. The brain performs billions of neural computations every second. Synaptic integration is the process by which multiple synaptic potentials combine within one postsynaptic neuron.

45 Principles Of Synaptic Integration The Integration of EPSPs The opening of a single transmitter-gated channel A patch-clamp recording from a transmitter-gated ion channel. Ionic current passes through the channels when the channels are open. In the presence of neurotransmitter, they rapidly alternate between open and closed states.

46 Principles Of Synaptic Integration Patch Clamp ( 膜片钳 ) Patch Clamps Permit Measurement of Ion Movements through Single Channel (not only in a whole cell) Different configurations

47 Principles Of Synaptic Integration Quantal Analysis ( 量子分析 ) of EPSPs: a method of comparing the amplitudes of miniature and evoked postsynaptic potentials. The neurotransmitter content in a single synaptic vesicle. Spontaneous release w/o AP, one vesicle miniature EPSP (miniepsp, mepsp) Multiple vesicle release w AP (evoked) EPSP (multiples of mepsp) i.e. postsynaptic EPSPs at a given synapse are quantized; they are multiples of an indivisible unit, the quantum, that reflects the number of transmitter molecules in a single synaptic vesicle and the number of postsynaptic receptors available at the synapse.

48 Principles Of Synaptic Integration There is a big difference between excitatory transmission at NMJ and CNS synapses. Most neurons in CNS perform more sophisticated computations, requiring that many EPSPs add together to produce a significant postsynaptic depolarization. This is what is meant by integration of EPSPs. EPSP summation is the simplest form of synaptic integration. Spatial summation is the adding together of EPSPs generated simultaneously at many different synapses on a dendrite. Temporal summation is the adding together of EPSPs generated at the same synapse if they occur in rapid succession, within about 1 15 msec of one another.

49 Principles Of Synaptic Integration (a) An AP triggers a small EPSP in a postsynaptic neuron. (b) Spatial summation: When two or more presynaptic inputs are active at the same time, their individual EPSPs add together. (c) Temporal summation: When the same presynaptic fiber fires APs in quick succession, the individual EPSPs add together.

50 Principles Of Synaptic Integration The Contribution of Dendritic Properties to Synaptic Integration The current of synaptic contact must spread down the dendrite and the soma, and cause the membrane of the spike-initiation zone to be depolarized beyond threshold, before an AP can be generated. The effectiveness of an excitatory synapse in triggering an AP, therefore, depends on how far the synapse is from the spikeinitiation zone and on the properties of the dendritic membrane.

51 Principles Of Synaptic Integration Dendritic Cable Properties: To simplify, let s assume that dendrites function as cylindrical cables that are electrically passive; that is, lacking voltage-gated ion channels (in contrast, of course, with axons). Imagine that the current at a synapse is like turning on the water that will flow down a leaky garden hose (the dendrite). Similarly, two paths that synaptic current can take: One is down the inside of the dendrite; the other is across the dendritic membrane. At some distance from the site of current influx, the EPSP amplitude may approach zero because of the dissipation of the current across the membrane.

52 Principles Of Synaptic Integration To simplify the mathematics, we assume the dendrite is infinitely long, unbranched, and uniform in diameter. The amount of depolarization falls off exponentially with increasing distance: V x =V 0 /e x/λ when x=λ, then V x =V 0 /e. Put another way, V λ =0.37 (V 0 ). This distance λ, where the depolarization is 37% of that at the origin, is called the dendritic length constant. (Remember that this analysis is an oversimplification).

53 Principles Of Synaptic Integration Decreasing depolarization as a function of distance along a long dendritic cable (a) The depolarization measured at a distance from the site of current injection is smaller than that measured right under it. (b) A plot of membrane depolarization as a function of distance along the dendrite.

54 Principles Of Synaptic Integration The length constant is an index of how far depolarization can spread down a dendrite or axon. The longer the length constant, the more likely it is that EPSPs generated at distant synapses will depolarize the membrane at the axon hillock( 轴丘 ). λ depends on two factors: (1) the internal resistance (r i ); and (2) the membrane resistance (r m ). r i depends only on the diameter of the dendrite and the electrical properties of the cytoplasm (relatively constant in a mature neuron) r m, depends on the number of open ion channels, which changes from moment to moment depending on what other synapses are active. The dendritic length constant, therefore, is not constant at all!

55 Principles Of Synaptic Integration Excitable Dendrites. Assumption: The dendrite s membrane is electrically passive. The dendrites of spinal motor neurons are very close to passive. However, many other neuronal dendrites are decidedly not passive. The voltage-gated channels in dendrites can act as important amplifiers of small EPSPs generated far out on dendrites. Paradoxically, in some cells dendritic sodium channels may also serve to carry electrical signals in the other direction from the soma outward along dendrites.

56 Principles Of Synaptic Integration A cortical pyramidal neuron with a long apical dendrite that has voltage-gated ion channels

57 Principles Of Synaptic Integration Inhibition EPSP AP output depends on: the number of coactive excitatory synapses the distance the synapse is from the spike-initiation zone the properties of the dendritic membrane Plus: inhibitory synapses that take the membrane potential away from action potential threshold, and exert a powerful control over a neuron s output.

58 Principles Of Synaptic Integration IPSPs and Shunting Inhibition ( 分流抑制 ) The postsynaptic inhibitory receptors are GABA or glycinegated ion channels that they only allow Cl - to pass through their channels. Opening of the chloride channel brings the membrane potential toward the chloride equilibrium potential, ECl -, about - 65 mv. So, whether its activation causes a hyperpolarizing IPSP or not depend on the resting membrane potential. If there is no visible IPSP, is the neuron really inhibited? The answer is yes. Shunting inhibition ( 分流抑制 ). The actual physical basis of shunting inhibition is the inward movement of negatively charged chloride ions, which is formally equivalent to outward positive current flow. Thus, inhibitory synapses also contribute to synaptic integration

59 Principles Of Synaptic Integration Shunting inhibition. (a) Stimulation of the excitatory input causes inward postsynaptic current that spreads to the soma, where it can be recorded as an EPSP. (b) When the inhibitory and excitatory inputs are stimulated together, the depolarizing current leaks out before it reaches the soma.

60 Principles Of Synaptic Integration The Geometry of Excitatory and Inhibitory Synapses Inhibitory synapses (GABA or glycine), Gray s type II. Excitatory synapses (glutamate), Gray s type I Inhibitory synapses on many neurons are found clustered on the soma and near the axon hillock.

61 Principles Of Synaptic Integration Modulation ( 调制 ) In addition to synaptic transmitter-gated channels, there are many synapses with G-protein-coupled neurotransmitter receptors that do not directly evoke EPSPs and IPSPs, but instead modifies the effectiveness of EPSPs generated by other synapses. This is called modulation. e.g. norepinephrine β receptor. The binding of norepinephrine (NE) to the receptor triggers a cascade of biochemical events within the cell to produce the second messager camp

62 Principles Of Synaptic Integration Modulation by the NE receptor. ➀ The binding of NE to the receptor activates a G-protein in the membrane. ➁ The G-protein activates the enzyme adenylyl cyclase. ➂ Adenylyl cyclase converts ATP into the second messenger camp. ➃ camp activates a protein kinase. ➄ The protein kinase causes a potassium channel to close by attaching a phosphate group to it.

63 Principles Of Synaptic Integration decreasing the K + conductance increases the dendritic membrane resistance and therefore increases the length constant λ. Distant or weak excitatory synapses will become more effective in depolarizing the spike-initiation zone beyond threshold. i.e. the cell becomes more excitable. It is why excitability of a neuron is increased when NE is released presynaptically. END

64 谢谢!