BIO 3350 Lecture 5: Synaptic Transmission PDF

LECTURE 5: SYNAPTIC TRANSMISSION • Types of synapses and neurotransmitters • How neurotransmitters are released at synaptic terminals • The forces that drive ion flow at the postsynaptic membrane Readings Bear, chapiter 5 1 FLOW OF INFORMATION AT A SYNAPSE • Information flows in one direction •...

LECTURE 5: SYNAPTIC TRANSMISSION • Types of synapses and neurotransmitters • How neurotransmitters are released at synaptic terminals • The forces that drive ion flow at the postsynaptic membrane Readings Bear, chapiter 5 1 FLOW OF INFORMATION AT A SYNAPSE • Information flows in one direction • Neuron -> Target cell or organ • First neuron • Presynaptic neuron Presynaptic neuron • Target cell • Postsynatic cell • Neuron • Muscle cell • Gland Postsynaptic neuron 3 TYPES OF SYNAPSE 1. Electrical synapse • Very rapid transmission • Bidirectional transmission 2. Chemical synapse • Slower • Generates postsynaptic potentials (PSPs) • Synaptic integration • Many PSPs can summate to reach the threshold to generate action potentials 4 TYPES OF SYNAPSES 1. Electrical synapse • Very rapid transmission • Bidirectional transmission 2. Chemical synapse • Slower • Generates postsynaptic potentials (PSPs) • Synaptic integration • Many PSPs can summate to reach the threshold to generate action potentials 5 ELECTRICAL SYNAPSE • Gap junction • Channel : connexon • Formed by six connexins • Cells are electrically coupled • Ion flows from one cytoplasm to the other 6 POTENTIAL PRODUCED BY ELECTRICAL SYNAPSES • Postsynaptic potential has the same shape as an action potential • The amplitude depends on the coupling efficiency, a product of the number of gap-junctions linking the two cells 7 SYNAPTIC TRANSMISSION CHEMICAL SYNAPSE a) b) c) d) Axo-dendritic synapse: axon to dendrite Axo-somatic: axon to soma Axo-axonic: axon to another axon Dendro-dendritic synapse : One dendrite to another 9 TRANSMISSION AT CHEMICAL SYNAPSES 1. 2. 3. 4. 5. 6. 7. Neurotransmitter synthesis Loading of neurotransmitter into vesicles Fusion of vesicle to membrane at presynaptic nerve terminal Release of neurotransmitters into synaptic cleft Binding of neurotransmitter to post-synaptic receptors Postsynaptic response 1. Electrical (PSPs) 2. Biochemical (intracellular pathways) Removal of neurotransmitters from synaptic cleft 10 TYPES OF NEUROTRANSMITTERS a) Amino acids • Small organic molecules b) Amines • Small organic molecules c) Peptides • Small chains of amino acids 11 TYPES OF NEUROTRANSMITTERS Amino acids Amines Peptides G-aminobutyric acid (GABA) Acetylcholine (ACh) Cholecystokinin (CCK) Glutamate (Glu) Dopamine (DA) Dynorphin Glycine (Gly) Adrenaline Enkephalin (Enk) Serotonin (5-HT) Neuropeptide Y Histamine Somatostatin Noradrenaline (NA) Substance P Vaspactive intestinal polypeptide (VIP) 12 NEUROTRANSMITTER SYNTHESIS AND STORAGE - PEPTIDES • Peptides • Synthetized in the endoplasmic reticulum • Activated and packed in Golgi apparatus before being transported to nerve terminals 13 NEUROTRANSMITTER SYNTHESIS AND STORAGE – AMINO ACIDS AND AMINES • Amino acids and amines • Synthetized by enzymes at nerve terminals • Transported to empty vesicles at nerve terminals 14 LOADING OF NEUROTRANSMITTERS INTO VESICLES Each vesicle: • 50 different transmembrane proteins including V-snares that are the most abundant • H+ ATPase-V pumps H+ into vesicle • H+/glutamate transporter uses the proton gradient to transport glutamate inside vesicles Figure 13-74 Molecular Biology of the Cell (© Garland Science 2008) RELEASE OF NEUROTRANSMITTERS • Exocytosis • Process by which vesicles release their content 1. Vesicles are loaded with neurostransmitters (NT) 2. The arrival of an action potential activates voltagedependent Ca2+-channels 3. Ca2+ stimulates movement of vesicles to the membrane, fusion, and release of NTs into synaptic cleft. 4. Recapture of vesicle for reuse 16 NEUROMUSCULAR JUNCTION (NMJ) • Study of NMJ has enabled the establishment of fundamental principles of synaptic transmission • Large • Ease of access • Ease of verifying activity of NMJ 17 PRESYNAPTIC RELEASE AND CALCIUM AT NMJ • Experiments by Katz and Miledi at the frog NMJ (1967) • Extracellular medium was modified so that there was no Ca2+ and tetrodotoxin was applied to block Na+ channels • Recordings of end-plate potentials (EPPs), which are homologs of PSPs in neurons • B. Stimulus pulse alone -> no EPP • Ca++ before pulse -> EPP • Pulse before Ca++ -> no EPP Conclusion Ca++ is necessary for synaptic transmission synaptique P: Electric pulse Ca++: injection of calcium 18 PRESYNAPTIC RELEASE AND CALCIUM AT THE SQUID NMJ • Experiments of Llinas and Heuser, 1977: • Lateral giant fiber (pre) • Giant motor fiber (post) • The greater the Ca++ presynaptic influx, the greater the PSPs are in the postsynaptic membrane 19 NEUROTRANSMITTER RELEASE • Mechanisms • Exocytosis activated by [Ca++]intra • Activated proteins change their conformation • SNARES: VAMP (vesicle), SNAP-25 (nerve terminal membrane), syntaxin (nerve terminal membrane) • Vesicle membrane fuses with presynaptic membrane • Release of neurotransmitters into synaptic cleft 20 HOW ARE NEUROTRANSMITTERS RELEASED? ? ? ? 21 NEUROTRANSMITTER RELEASE: STOCHASTIC NATURE • Left: Recordings of miniature postsynaptic potentials (“mini”). Minis are events of spontaneous neurotransmitter release • Analysis of mini-EPSPs shows: • Amplitudes of “minis” have discrete values • Each EPSP is a multiple of other mini-EPSPs • Synaptic vesicle = unit of synaptic transmission • Therefore, Quantum: indivisible unit Fatt and Katz, 1952 Squid 22 NEUROTRANSMITTER RELEASE : QUANTAL HYPOTHESIS • Quantal analysis used to determine the number of vesicles released during transmission • A histograms of the minis recorded (panel B) and curve-fitting of this histogram suggest that the amplitude of each mini is a multiple of some indivisible unit (i.e. a quantum) Del Castillo and Katz, J.Physiol 23 1954 NEUROTRANSMITTER RELEASE : QUANTAL HYPOTHESIS • Amplitude of minis are multiples of a quantum • E.g: At the neuromuscular junction • Approx. 200 vesicles -> EPSP of 40 mV. A quantum = 40 mV/200= 0.5 mV • In the CNS: a quantum has been estimated to be about 0.1-1 mV Del Castillo and Katz, J.Physiol 24 1954 QUESTIONS 1. You are studying a particular synapse. You notice that the values ​of 10 spontaneous mini EPSPs recorded are 5 mV, 10 mV, 2.5 mV, 5 mV, 2.5 mV, 2.5 mV, 2.5 mV, 7.5 mV, 5 mV, 2.5 mV. At this synapse, what is the size of a quantum? 2. Is the answer to the first question the size of a quantum for all synapses? 25 RECAPTURE AND DEGRADATION OF NTS 2 1 Recapture (e.g. DAT recaptures dopamine, implicated in the treatment of depression) Diffusion (e.g. dopamine) Acetylcholin e Choline Acetate 3 Enzymatic breakdown Acetylcholinestarase 26 EXCITATORY POSTSYNAPTIC POTENTIALS • • EPSP -> Excitatory postsynaptic potentials Short-lasting depolarization following presynaptic release of NTs • Net inward flow of + charges • Results from an equilibrium potential, a.k.a. reversal potential (Erev), that is more depolarized that the Vm at the time that ligand gated channels are open. I.e. EPSPs occur when Vm < Erev 27 EXCITATORY POSTSYNAPTIC POTENTIALS Some ligand-gated channels allow Na+ and K+ flow A depolarization occurs (despite the presence of an outward flow of K+) because of net inward flow of + charges • Occurs when Vm < Erev • According to Ohm’s law: Isyn = gsyn (Vm - Erev) • • Na K + + + Na EXTRACELLULAR SPACE CYTOSOL + Na K+ + K 28 INHIBITORY POSTSYNAPTIC POTENTIALS IPSP -> Inhibitory postsynaptic potentials Short-lasting hyperpolarization following presynaptic release of NTs • Net inward flow of - charges • Occurs when Vm > Erev • • Cl- Cl- EXTRACELLULAR SPACE CYTOSOL Cl- 29 IONOTROPIC RECEPTORS: REVERSAL POTENTIAL • The amount of synaptic current (Isyn) is determined by Ohm’s law, which says that it is a product of two factors: 1. The permeability of the ionotropic receptor channel (gsyn) 2. The electrical motive force (Vm-Erev) Isyn= gsyn (Vm - ERev) • Erev is the reversal potential of the synapse (think Eion). • Erev is equal to a weighted sum of the Eions of the ions that can flow through that receptor channel • For glutamatergic synapses: ERev= PNa x ENa + PK x EK • For GABAergic synapses: Erev = PCl x Ecl = ECl = -65 mV 30 IONOTROPIC RECEPTORS: REVERSAL POTENTIAL • Glutamatergic synapses • ERev= PNa x ENa + PK x EK; • ENa = 60 mV; EK = - 90 mV; PNa = 0.6; PK = 0.4. Thus, ERev = 0 mV Na+ K + + Na EXTRACELLULAR SPACE ENa = +60 mV EK = -90 mV CYTOSOL + Na + K K+ 31 IONOTROPIC RECEPTORS: REVERSAL POTENTIAL Postsynaptic currents are the sum of the ionic currents that flow through the receptor channel (e.g. AMPA receptors IK and INa). IK INa Each ionic current is determined by (as seen in Lecture 3): Iion = gion (Vm – Eion) Note in the recording on the right how IK and INa of the post-synaptic current (PSC) vary according to the membrane potential (Vm), which then varies the PSC (which is a sum of IK and INa) 32 IONOTROPIC RECEPTORS: REVERSAL POTENTIAL A EPSP can change polarity depending on the membrane potential Vm at the time that EPSP is produced • i.e. a glutamatergic synapse could produce a IPSP (if Vm > Erev = 0 mV) and a GABAergic synapse could produce an EPSP (if Vm < Erev = - 65 mV) IK INa For any synapse: • If Vm is below Erev (for that synapse) • I >0 ; EPSP • If Vm is above Erev (for that synapse) • I<0 ; IPSP 33 QUESTIONS • You discover a synapse where the postsynaptic receptor produces Ca2+ and Clflow. The reversal potential (E_rev) of this receptor is 30 mV. If E_Ca = 120 mV and E_Cl = -60 mV, can you determine the relative permeability at this synapse to Ca2+ and Cl-? • You also discover a version of this receptor which only passes Ca2+? What is the E_rev of this receiver? 34 SYNAPTIC INTEGRATION • Process by which multiple postsynaptic potentials (EPSP and IPSP) combine to initiate action potentials or to modulate the firing of action potentials Σ 35 SYNAPTIC INTEGRATION • Summation of PSPs • Enables neurons to make complex “calculations” • Integration: PSPs sum to make a large postsynaptic depolarization or hyperpolarization • Spatial integration: PSPs generated simultaneously at nearby regions sum together • Temporal: PSPs generated successively at the same synapse 36 DENDRITIC PROPERTIES AFFECT SYNAPTIC INTEGRATION • Most dendrites act like leaky electric cables – Passive transmission • Depolarization of the membrane diminishes exponentially with distance from the site of the EPSP Vx = Vx/ex/λ + + 37 37 + + SYNAPTIC INTEGRATION: EXCITABLE DENDRITES • SOME dendrites have voltage-dependent ion channels that make dendrites more excitable • Sodium (INa) • Calcium (ICa) • Dendritic sodium channels • Can amplify EPSPs as they travel towards the cell body • Can facilitate the propagation of action potentials in the opposite direction… from soma to the dendrites N a 38 SYNAPTIC INTEGRATION: INHIBITED DENDRITES • SOME dendrites have potassium voltage-dependent ion channels that make dendrites • These K+ channels can dampen EPSPs to prevent the firing of action potentials K 39 39 CONCLUSION • Chemical synaptic transmission • Rich diversity enabling complex patterns of neural activity • Provide explanation for effects of many drugs • Transmission dysfunction is at the basis of many neurological and psychiatric disorders • Understanding chemical synaptic integration in neurons can lead to insights into learning, memory, perception, action, etc… 40