Synaptic transmission Yang Zhou 10,13, 2023 Synapse Santiago Ramón y Cajal 1852 –1934 Charles Scott Sherrington 1857 – 1952 propagation of nervous action is made by contacts at the level of certain “Such a special connection of one nerve cell with another might be called synapse” apparatuses or dispositions of engagement ” The War of the Soups and the Sparks: The Discovery of Neurotransmitters and the Dispute Over How Nerves Communicate John Eccles Electrical? Chemical? 1903-1997 Henry Dale (1875-1968) Penton Elliott (Langley)1904 “Adrenaline might be the chemical stimulant liberated on each occasion when the impulse arrives at the periphery” Otto Loewi 1920 Otto Loewi 1873-1961 Stephen Kuffler: Convincing evidence of chemical transmission in neural-muscular junction (single nerve fiber) Synapses Are Predominantly Electrical or Chemical Electrical synaptic transmission was first described by Edwin Furshpan and David Potter in the giant motor synapse of the crayfish. 小龙虾 Electrical Synapses Provide Rapid Signal Transmission During excitatory synaptic transmission at an electrical synapse, voltage-gated ion channels in the presynaptic cell generate the current that depolarizes the postsynaptic cell, the presynaptic terminal must be big enough for its membrane to contain many ion channels , the postsynaptic cell must be relatively small. The synaptic delay— the time between the presynaptic spike and the postsynaptic potential—is remarkably short The change in potential of the postsynaptic cell is directly related to the size and shape of the change in potential of the presynaptic cell. Most electrical synapses can transmit both depolarizing and hyperpolarizing currents. Electrical transmission is graded It occurs even when the current in the presynaptic cell is below the threshold for an action potential. Cells at an Electrical Synapse Are Connected by Gap-Junction Channels At an electrical synapse, the pre- and postsynaptic components are apposed at the gap junction--specialized protein structures that conduct ionic current directly from the presynaptic to the postsynaptic cell. A gap-junction channel consists of a pair of hemichannels, or connexons (连接子), one in the presynaptic and the other in the postsynaptic cell membrane. Each connexon is composed of six identical subunits, called connexins (连接蛋白). The large pore of gap-junction channels does not discriminate among inorganic ions and is even wide enough to permit small organic molecules pass through. Many gap-junction channels in different cell types are formed by the products of different connexin genes and thus respond differently to modulatory factors that control their opening and closing pH, Ca2+ and neurotransmitters released from nearby chemical synapses can modulate the opening of gap-junction channels. A three-dimensional model of the gap-junction channel Electrical Transmission Allows Rapid and Synchronous Firing of Interconnected Cells Fast response, such as escape Orchestrating the actions of groups of neurons, which produce synchronous behaviors The fine structure of a presynaptic terminal The separation between the two cells at a chemical synapse---the synaptic cleft, is usually slightly wider (20–40 nm) than the nonsynaptic intercellular space (20 nm). Chemical synaptic transmission depends on a neurotransmitter, 神经递质 Transmitter is released from specialized swellings of the presynaptic axon—synaptic boutons—that typically contain 100 to 200 synaptic vesicles, each of which is filled with several thousand molecules of neurotransmitter The synaptic vesicles are clustered at specialized regions of the synaptic bouton called active zones. Synaptic transmission at chemical synapses involves several steps Influx of Ca2+ exocytosis 胞吐 binding of transmitter The Action of a Neurotransmitter Depends on the Properties of the Postsynaptic Receptor Chemical synaptic transmission can be divided into two steps: a transmitting step---the presynaptic cell releases a chemical messenger, and a receptive step---the transmitter binds to and activates the receptor molecules in the postsynaptic cell. The action of a transmitter depends on the properties of the postsynaptic receptors that recognize and bind the transmitter. In vertebrates, ACh acts on excitatory Ach receptors at all neuromuscular junctions to trigger contraction while also acting on inhibitory ACh receptors to slow the heart. All receptors for chemical transmitters have two biochemical features in common: 1. They are membrane-spanning proteins. 2. They carry out an effector function within the target cell. Neurotransmitters open postsynaptic ion channels either directly or indirectly Two classes of transmitter actions are mediated by receptor proteins : ionotropic receptors--the receptor directly controls ion flux. ligand-gated ion channels., binding of transmitter to an extracellular binding site triggers a conformational change that opens the channel pore, Fast Neurotransmitters open postsynaptic ion channels either directly or indirectly Metabotropic receptors--activate intracellular metabolic signaling pathways, often leading to the synthesis of second messengers, such as cAMP, that regulate levels of protein phosphorylation. Slower Electrical and Chemical Synapses Can Coexist and Interact Both forms of synaptic transmission can coexist in the same neuron and that electrical and chemical synapses can modify each other’s efficacy. Directly Gated Transmission: The Nerve-Muscle Synapse The Neuromuscular Junction Has Specialized Presynaptic and Postsynaptic Structures The neuromuscular junction Synaptic boutons The Postsynaptic Potential Results From a Local Change in Membrane Permeability Once ACh is released, the resulting excitatory postsynaptic potential (EPSP) is very large. At the nervemuscle synapse, the EPSP is also referred to as the end-plate potential . This change in membrane potential usually is large enough to rapidly activate the voltage-gated Na+ channels in the muscle membrane, converting the endplate potential into an action potential. The end-plate potential was first studied in detail in the 1950s by Paul Fatt and Bernard Katz using intracellular voltage recordings. 箭毒 The end-plate potential Fatt and Katz found that the EPSP is maximal at the end-plate and decreases progressively with distance In addition, the time course of the EPSP slows progressively with distance. From this, Fatt and Katz concluded that the endplate potential is generated by an inward ionic current that is confined to the end-plate and then spreads passively away. Electrophysiological evidence that the ACh receptors are localized to the end-plate was provided by Stephen Kuffler and his colleagues. Individual Acetylcholine Receptor-Channels Conduct All-or乙酰胆碱 None Currents The patch-clamp technique is used to record currents from single ACh receptor-channels. 1976, Erwin Neher and Bert Sakmann The time course of the total current at the endplate reflects the summation of contributions of many individual acetylcholine receptor-channels The end-plate potential resulting from the opening of acetylcholine receptor-channels opens voltage-gated sodium channels The nicotinic ACh receptor-channel is a pentameric macromolecule 五聚体 A high-resolution three-dimensional structural model of a neuronal nicotinic ACh receptor channel Synaptic Integration in the Central Nervous System synaptic transmission between central neurons is more complex 1. Although most muscle fibers are typically innervated by only one motor neuron, a central nerve cell receives connections from thousands of neurons. 2. Muscle fibers receive only excitatory inputs, whereas central neurons receive both excitatory and inhibitory inputs. 3. All synaptic actions on muscle fibers are mediated by one neurotransmitter ---ACh, which activates only one type of receptor; a single central neuron can respond to many different types of inputs, each mediated by a distinct transmitter that activates a specific type of receptor . 4. Every AP in the motor neuron produces an action potential in the muscle fiber; connections made by a presynaptic neuron onto a central neuron are only modestly effective—in many cases at least 50 to 100 excitatory neurons must fire together to trigger an action potential in postsynaptic neurons. stretch reflex The first insights into synaptic transmission in the central nervous system came from experiments by John Eccles and his colleagues in the 1950s on the synaptic inputs onto spinal motor neurons that control the stretch reflex. 膝跳反射 The combination of excitatory and inhibitory synaptic connections mediating the stretch reflex The two most common morphological types of synapses in the central nervous system Type I synapses are mostly glutamatergic and excitatory, have round synaptic vesicles, an electron-dense region (the active zone) on the presynaptic membrane, and an even larger electron dense region in the postsynaptic membrane (postsynaptic density), asymmetric appearance. Type II synapses are GABAergic and inhibitory. have oval or flattened synaptic vesicles , less obvious presynaptic membrane specializations and postsynaptic densities, resulting in a more symmetric appearance The proximity of a synapse to the axon initial segment is thought to determine its effectiveness. Excitatory Synaptic Transmission Is Mediated by Ionotropic Glutamate Receptor-Channels Permeable to Cations The EPSP in spinal motor cells results from the opening of ionotropic glutamate receptorchannels, which are permeable to both Na+ and K+. This ionic mechanism is similar to that produced by ACh at the neuromuscular junction. Glutamate receptors can be divided into two broad categories: ionotropic receptors and metabotropic Receptors. There are three major types of ionotropic glutamate receptors: AMPA , kainate, and NMDA. 卡英酸 The NMDA receptor channel is highly permeable to Ca2+, whereas most non-NMDA receptors are not. Different classes of glutamate receptors The action of all ionotropic glutamate receptors is excitatory or depolarizing , metabotropic receptors can produce either excitation or inhibition , depending on the reversal potential of the ionic currents that they regulate and whether they promote channel opening or channel closing. NMDA and AMPA Receptors Are Organized by a Network of Proteins at the Postsynaptic Density NMDA Receptors Have Unique Biophysical and Pharmacological Properties Opening of NMDA receptor depends on membrane voltage as well as transmitter binding. The contributions of the AMPA and NMDA receptorchannels to the excitatory postsynaptic current The Properties of the NMDA Receptor Underlie Long-Term Synaptic Plasticity In 1973, Tim Bliss and Terje Lomo found that a brief period of high-intensity and high-frequency synaptic stimulation leads to long-term potentiation (LTP) of excitatory synaptic transmission in the hippocampus. 海马体 LTP is completely blocked when the tetanus is delivered in the presence of the NMDA receptor antagonist APV. NMDA Receptors Contribute to Neuropsychiatric Disease A model for the mechanism of long-term potentiation Fast Inhibitory Synaptic Actions Are Mediated by Ionotropic GABA and Glycine Receptor- Channels Permeable to Chloride Cl− GABA acts on both ionotropic and metabotropic receptors: The GABA A receptor is an ionotropic receptor that directly opens a Cl− channel; The GABA B receptor is a metabotropic receptor that activates a second-messenger cascade, which often indirectly activates a K+ channel. Glycine, a less common inhibitory transmitter in the brain, also activates ionotropic receptors that directly open Cl− channels. Glycine is the major transmitter released in the spinal cord. 甘氨酸 Fast Inhibitory Synaptic Actions Are Mediated by Ionotropic GABA and Glycine Receptor- Channels Permeable to Chloride Inhibition can shape the firing pattern of a spontaneously active neuron Synaptic Inputs Are Integrated at the Axon Initial Segment Excitatory and Inhibitory Synaptic Actions Are Integrated by Neurons Into a Single Output. The net effect of the inputs at any individual excitatory or inhibitory synapse will therefore depend on several factors: the location, size, and shape of the synapse; the proximity and relative strength of other synergistic or antagonistic synapses; and the resting potential of the cell. In most neurons, AP output is initiated at the axon initial Segment, which has a lower threshold for AP generation than at the cell body or dendrites because of higher density of voltage-dependent Na+ channels。 Central neurons are able to integrate a variety of synaptic inputs through temporal and spatial summation of synaptic potentials Temporal summation , ---the process by which consecutive synaptic potentials are added together in the postsynaptic cell。 Spatial summation ---the inputs from many presynaptic neurons acting at different sites on the postsynaptic neuron are added together Dendritic spines compartmentalize calcium influx through NMDA receptors Transmitter Release Chemical neurotransmission is the primary mechanism by which neurons communicate, and process information; it occurs throughout the nervous system Transmitter release is triggered by changes in presynaptic membrane potential Transmitter release is not directly triggered by the opening of presynaptic voltage-gated Na+ or K+ channels Transmitter release is regulated by Ca2+ influx into the presynaptic terminals through voltage-gated Ca2+ channels Graded depolarizations activated a graded inward Ca2+ current, which in turn resulted in graded release of transmitter. The Ca2+ current is graded because the Ca2+ channels are voltage-dependent and stay open as long as the presynaptic depolarization lasts. Calcium channels are largely localized in presynaptic terminals at active zones Calcium flowing into the presynaptic site during synaptic transmission at the neuromuscular junction is concentrated at the active zone. These channels are concentrated at presynaptic “active zones,” very close to the sites at which release occurs The precise relation between presynaptic Ca2+ and transmitter release at a central synapse The calyx synapse includes almost a thousand active zones that function as independent release sites The synaptic delay is largely attributable to the time required to open Ca2+ channels. Several Classes of Calcium Channels Mediate Transmitter Release Neurons contain five broad classes of voltage-gated Ca2+ channels: the L-type, P/Q-type, N-type, R-type, and T-type, which are encoded by distinct but closely related genes. Transmitter Is Released in Quantal Units Katz and his colleagues showed that transmitter is released in discrete amounts they called quanta. Each quantum of transmitter produces a postsynaptic potential of fixed size, called the quantal synaptic potential. Transmitter Is Stored and Released by Synaptic Vesicles Del Castillo and Katz speculated that the vesicles (囊泡) were organelles for the storage of transmitter, each vesicle stored one quantum of transmitter. The efficacy of transmitter release from a single presynaptic cell onto a single postsynaptic cell varies widely in the nervous system and depends on several factors: (1) the number of individual synapses between a pair of presynaptic and postsynaptic cells; (2) the number of active zones in an individual synaptic terminal; (3) the probability that a presynaptic action potential will trigger release of one or more quanta of transmitter Synaptic Vesicles Discharge Transmitter by Exocytosis and Are Recycled by Endocytosis Vesicles release their contents by directly fusing with the presynaptic membrane, a process termed exocytosis. freeze-fracture electron microscopy, 冷冻蚀刻电子显微技术 Capacitance Measurements Provide Insight Into the Kinetics of Exocytosis and Endocytosis The Synaptic Vesicle Cycle Exocytosis of Synaptic Vesicles Relies on a Highly Conserved Protein Machinery SNARE Proteins Catalyze Fusion of Vesicles With the Plasma Membrane Calcium Binding to Synaptotagmin Triggers Transmitter Release The synaptotagmins, have been identified as the major Ca2+ sensors that trigger fusion of synaptic vesicles. Modulation of Transmitter Release Underlies Synaptic Plasticity The effectiveness of chemical synapses can be modulated dramatically and rapidly—by several-fold in a matter of seconds—and this change can be maintained for seconds, to hours, or even days or longer, a property called synaptic plasticity. Synaptic strength can be modified presynaptically, by altering the release of neurotransmitter, postsynaptically, by modulating the response to transmitter, or both. Synaptic depression synaptic facilitation Modulation of Transmitter Release Underlies Synaptic Plasticity Axo-axonic Synapses on Presynaptic Terminals Regulate Transmitter Release Neurotransmitters Four steps for chemical synaptic transmission • (1) synthesis and storage of a transmitter substance, • (2) release of the transmitter, • (3) interaction of the transmitter with receptors at the postsynaptic membrane, • (4)removal of the transmitter from the synapse. Four Criteria to Be Considered a Neurotransmitter 1. It is synthesized in the presynaptic neuron. 2. It is accumulated within vesicles present in presynaptic release sites and is released via exocytosis in amounts sufficient to exert a defined action on the postsynaptic neuron or effector organ. 3. When administered exogenously in reasonable concentrations, it mimics the action of the endogenous transmitter . 4. A specific mechanism usually exists for removing the substance from the extracellular environment. Two main classes of neurotransmitter Two main classes of neurotransmitter: Neuropeptides (神经肽) are short polymers of amino acids processed in the Golgi apparatus, where they are packaged in large dense-core vesicles. Small-molecule transmitters are packaged in small vesicles. 乙酰胆碱 生物胺 氨基酸 三磷酸腺苷 Acetylcholine Acetylcholine is the only low-molecular-weight aminergic transmitter substance that is not an amino acid or derived directly from one. Acetylcholine is released at all vertebrate neuromuscular junctions by spinal motor neurons . Cholinergic neurons form synapses throughout the brain; those in the nucleus basalis (基底核) have particularly widespread projections to the cerebral cortex. Acetylcholine is one of the principal neurotransmitter of the reticular activating system (网状激活 系统), which modulates arousal, sleep, wakefulness, and other critical aspects of human consciousness. Biogenic Amine Transmitters Includes the catecholamines (儿茶酚胺), serotonin (五羟色胺), and histamine (组胺). The catecholamine transmitters—dopamine (多巴胺), norepinephrine (去甲肾上腺素), and epinephrine (肾上腺素)—are all synthesized from the essential amino acid tyrosine (络氨酸) Biogenic Amine Transmitters Four major dopaminergic nerve tracts: Dopaminergic neurons in the substantia nigra (黑质) that project to the striatum are important for the control of movement and are affected in Parkinson disease and other disorders of movement, but projections to the associative striatum have also been implicated more recently in dopamine dysfunction in schizophrenia. The mesolimbic (中脑边缘系统通道) and mesocortical tracts (中脑皮层通路) are critical for affect, emotion, attention, and motivation and are implicated in drug addiction and schizophrenia. The tuberoinfundibular pathway, originates in the arcuate nucleus of the hypothalamus (下丘脑) and projects to the pituitary gland (脑垂体), where it regulates secretion of hormones. Biogenic Amine Transmitters Norepinephrinergic (adrenergic ) neurons --- cell bodies locate in the locus coeruleus (蓝斑), a nucleus of the brain stem with many complex modulatory functions; they project widely throughout the cortex, cerebellum, hippocampus, and spinal cord. Serotonergic neurons --- cell bodies locate in and around the midline raphe nuclei (中缝核) of the brain stem and are involved in regulating affect, attention, and other cognitive functions. They project widely throughout the brain and spinal cord. Serotonin and the catecholamines norepinephrine and dopamine are implicated in depression, Histamine is concentrated in the hypothalamus, one of the brain centers for regulating the secretion of hormones Amino Acid Transmitters The amino acids glutamate and glycine are not only neurotransmitters but also universal cellular constituents. Glutamate, the neurotransmitter most frequently used at excitatory synapses throughout the central nervous system. Glycine is the major transmitter used by inhibitory interneurons of the spinal cord. It is also a necessary cofactor for activation of NMDA glutamate receptors. GABA is present at high concentrations throughout the central nervous system.. It is used as a transmitter by an important class of inhibitory interneurons in the spinal cord. In the brain, GABA is the major transmitter of a wide array of inhibitory neurons and interneurons. ATP and Adenosine ATP and its degradation products (eg, adenosine (腺苷)) act as transmitters at some synapses by binding to several classes of G protein–coupled receptors (the P1 and P2Y receptors). ATP can also produce excitatory actions by binding to ionotropic P2X receptors. Caffeine’s stimulatory effects depend on its inhibition of adenosine binding to the P1 receptors. Purinergic transmission is particularly important for nerves mediating pain. 嘌呤 Small-Molecule Transmitters Are Actively Taken Up Into Vesicles Neurotransmitter substances are concentrated in vesicles by transporters that are specific to each type of neuron and energized by a vacuolar-type H+ATPase (V-ATPase) Small-molecule transmitters are transported from the cytosol into vesicles or from the synaptic cleft to the cytosol by transporters Neuroactive Peptides Neuroactive peptides are derived from secretory proteins that are formed in the cell body. 阿片 Many neuropeptides act as neurotransmitters 垂体神经肽 when released close to a target neuron, where they can cause inhibition, excitation, or both. 速激肽 Neuroactive peptides have been implicated in modulating sensory perception and affect. Some peptides, including substance P and the enkephalins, are preferentially located in regions of the CNS involved in the perception of pain. Other neuropeptides regulate complex responses to stress 分泌素 胰岛素 生长抑素 胃泌素 Peptides and Small-Molecule Transmitters Differ in Several Ways 1. Formation of the vesicle: Peptides---large dense-core vesicles are formed in the trans-Golgi network, then transported from the soma to presynaptic sites; whereas small-molecule transmitters --- small synaptic vesicles are not synthesized in the soma but produced by local processing. 2. Release of vesicle: The large dense-core vesicles release their contents in a way that does not require active zones; thus take place anywhere along the membrane of the axon; whereas small synaptic vesicles release the contents at the active zone of synapse. 3. Recycle of the membrane and transmitter: the membrane and peptides are not recycled to form new large dense core vesicles, and the vesicles must be replaced by transport from the soma; once small synaptic vesicles are released by exocytosis, synaptic vesicles can be rapidly recycled, and the transmitters are quickly uptake into the synapse. 4. Efficiency of exocytosis: exocytosis of large dense-core vesicles is slow and requires high stimulation frequencies to raise Ca2+ to levels sufficient to trigger release; whereas exocytosis of small synaptic vesicles is rapid following a single action potential. Removal of Transmitter From the Synaptic Cleft Terminates Synaptic Transmission Timely removal of transmitters from the synaptic cleft is critical to synaptic transmission. Transmitter substances are removed from the cleft by three mechanisms: diffusion, enzymatic degradation, and reuptake. Neuropeptides are removed relatively slowly from the synaptic cleft by slow diffusion and proteolysis by extracellular peptidases. Small molecule transmitters are removed more quickly from the synaptic cleft and extra synaptic space through the reuptake at the plasma membrane. High-affinity uptake is mediated by transporter molecules in the membranes of nerve terminals and glial cells. Electron-opaque gold particles linked to antibody are used to locate antigens in tissue at the ultrastructural level Fluorescent false neurotransmitter (FFN) labeling permits optical monitoring of neurotransmitter release Thank you ! Atomic structure of an ionotropic glutamate receptor Techniques for visualizing chemical messengers Gap Junctions Have a Role in Glial Function and Disease Gap junctions are formed between glial cells as well as between neurons. In the brain, individual astrocytes are connected to each other through gap junctions forming a glial cell network. Gap-junction channels also enhance communication within certain glial cells, such as the Schwann cells The net effect of the inputs at any individual excitatory or inhibitory synapse will therefore depend on several factors: the location, size, and shape of the synapse; the proximity and relative strength of other synergistic or antagonistic synapses; and the resting potential of the cell. And, in addition, all of this is exquisitely dependent on the timing of the excitatory and inhibitory input. Inputs are coordinated in the postsynaptic neuron by a process called neuronal integration. Exocytosis Involves the Formation of a Temporary Fusion Pore Exocytosis depends on the formation of a temporary fusion pore that spans the membranes of the vesicle and plasma membranes. The fusion pore can open and close rapidly and reversibly. The end-plate current increases and decays more rapidly than the end-plate potential.
