Synaptic Transmission and Neurotransmitter System PDF

20th January 2025 XY3291 - Neuroscience Constantin-Iulian Chiță Where opportunity creates success Synaptic transmission Where opportunity creates success  Contents ❖ Action Potential ❖ Neurotransmitter system ❖ Types of synapses Res...

20th January 2025 XY3291 - Neuroscience Constantin-Iulian Chiță Where opportunity creates success Synaptic transmission Where opportunity creates success  Contents ❖ Action Potential ❖ Neurotransmitter system ❖ Types of synapses Research methods to study Electrical synapses neurotransmitters Dale’s principle Chemical synapses Categories of neurotransmitters ❖ Chemical synaptic transmission Neurotransmitter receptor Neurotransmitter ❖ Amino acids Vesicle cycling ❖ Biogenic amines Neurotransmitter receptor ❖ Purines ❖ Peptides Neurotransmitter recovery and degradation ❖ Unconventional neurotransmitters ❖ Principles of synaptic integration Action Potential Action Potential Represents the temporary change of the neuronal membrane potential to facilitate the transmission of electric impulses and the release of neurotransmitters A ‘resting state’ neuronal membrane potential is around -70 mV This negative potential is attributed to an inequal distribution of ions between the inside and outside of the neuronal membrane Action Potential The movement of these ions is possible due to active and passive transport mechanisms The active mechanisms require energy (ATP – adenosine triphosphate) and they are called Na+/K+ pumps. The passive mechanisms do not require energy and they are represented by ionic Na and K channels. During the resting state of the neuron, the Na channels are always closed, whereas only some K channels are open. This is the reason why Na cannot enter the membrane by itself. Action Potential The action potential is an ‘all or nothing’ response. 3 main stages: 1. Depolarization – caused by an increased permeability for Na+ ions These ions will pass through the membrane via the specialised channels that open after the firing threshold has been achieved 2. Repolarization - caused by an efflux of K+ ions outside of the membrane During this stage, the membrane potential returns to negative values Action Potential 3. Hyperpolarisation - stage where the neuronal membrane returns to negative values - caused by an efflux of K+ through K+ channels * Refractory Period – represents a time interval where the neuron is unable to fire again - Can be absolute (impossible to produce a second action potential) or relative (the neuron exhibits resistance to being triggered but can still fire under certain conditions)  Types of synapses Information transfer at a synapse Plays role in all the operations of the nervous system Direction of information flow generally in one direction: neuron to target cell - First neuron: presynaptic neuron - Target cell: postsynaptic neuron  Electrical synapses Gap junction Channel : Connexon—formed by six connexins Cells said to be “electrically coupled” Flow of ions from cytoplasm of one cell to cytoplasm of another cell Unlike most chemical synapses, bidirectional Very fast transmission – Postsynaptic potentials (PSPs) Synaptic integration: several PSPs occurring simultaneously to excite a neuron (causes AP) Examples: myocardium, smooth muscle, some brain regions  Chemical synapses I. Presynaptic membrane II. Synaptic cleft III. Postsynaptic membrane Synaptic vesicles Neurotransmitter Transmitter receptor Examples: Almost all CNS synapses; Autonomic PNS synapses I. Presynaptic membrane – In most cases, it is represented by the terminal regions of the axons, also called terminal buttons This is where the neurotransmitters are located II. Synaptic cleft – space between the two membranes III. Postsynaptic membrane – can be one of the following: dendrite, soma, axon. Based on which part of neuron is postsynaptic Axodendritic: axon to dendrite Axosomatic: axon to cell body Axoaxonic: axon to axon Axospinous: axon to dendritic spine Dendrodendritic: dendrite to dendrite By Membrane Differentiations Gray’s type I: – Asymmetrical→ membrane differentiation on the postsynaptic side is thicker than that on the presynaptic side – usually excitatory Gray’s type II: – Symmetrical→ membrane differentiations are of similar thickness – usually inhibitory  Chemical synaptic transmission Neurotransmitter - Neurotransmitter synthesis and storage Vesicle cycling - Vesicle release - Vesicle recycling Neurotransmitter receptor Neurotransmitter recovery and degradation  Neurotransmitter Amino acids: small organic molecules—vesicles – Examples: glutamate, glycine, GABA Amines: small organic molecules—vesicles – Examples: dopamine, acetylcholine, histamine Peptides: short amino acid chains (proteins)— secretory granules – Examples: dynorphin, enkephalins Neurotransmitter synthesis and storage Synthesis peptides: rER—Golgi —Secretory vesicles — stored in the axon terminal Synthesis amine and amino acids: Enzymes in cytosol — transporter proteins — stored in synaptic vesicles at the axon terminal  Vesicle cycling Vesicle release 1) Vesicle pools: ❖ Classical model 3 different pools of vesicles 200 vesicles → 170 for the reserve pool/ 20 for the recycling pool and around 10 for the vesicles that are ready to go ❖ Modern theory – No clear distinction between location of reserve and recycled vesicles – Reserve vesicles “fixed” – Recycling vesicles mobile 2) Vesicle docking: Vesicle is transported to an active site close to the cell membrane - Free SNARES on vesicle plasma membrane - SNARE complexes form as vesicle docks 3) Vesicle priming: Vesicle is bound to active site by the SNARE complex ready for release Synaptotagmin binds to SNARE complex Entering Ca2+ binds to synaptotagmin, leading to curvature of plasma membrane with brings membranes together 4) Fusion: Ca2+ signal initiates merging of the vesicular and cell membranes Vesicle membrane merges with cell membrane; shift of SNARE from trans to cis (i.e. on same membrane) Energy dependent process; supplied by SNARE complex Merging of cell and vesicle membranes expels lumen contents to EC space  “Kiss and Run” vs full exocytosis Incomplete merging of vesicle and membrane Partial release of neurotransmitter Vesicle remains able for release Burgoyne, R. D., Fisher, R. J., & Graham, M. E. (2001). Regulation of kiss-and-run exocytosis. Trends in cell biology, 11(10), 404-405. Vesicle recycling NSF and SNAP proteins mediate disassembly of SNARE complex Clathrin mediated endocytosis SNARE proteins recycling also via endocytosis Occurs in zone lateral to exocytosis  Neurotransmitter receptor Ionotropic receptor - Neurotransmitter binds to the channel - Channel opens - Ions flow across the membrane Metabotropic receptor - Neurotransmitter binds - G-protein subunits or intracellular messengers modulate ion channels - Ion channels open and ions flow across the membrane  Neurotransmitter recovery and degradation Recovery → simple diffusion aided by NT transporter proteins which locate the NT in cytosol and reload into vesicles or degraded - Presynaptic membrane - Glia membrane Degradation - Cytosol: enzymatically degraded and recycled - Synaptic cleft (AChE) Principles of synaptic integration  Quanta Vesicles are packed with transmitter Assume each synaptic vesicle has the same amount of transmitter (NMJ, estimated ~6000 ACh molecules; CNS, ~3600 Glu) Therefore 1 vesicle = 1 quantum At the same synapse, each quanta should generate the same size response Reasons for variability in measured quanta?  Quantal release Response = N x Pr x Q N = number vesicles can be released Pr = probability of release ( Ca+ dependent) Q = quantal amplitude (postsynaptic response to 1 quantum) Traditionally assumed only one vesicle released per synapse; some evidential support, but also contrary evidence. Pr tends to vary by type of synapse (~0.1 – 0.9) Q determined by postsynaptic receptor population  Vesicles Receptors Number of release (n) sites dependent on? Vesicles ready for release Vesicle release (Pr) dependent on? Calcium channels Presynaptic [Ca2+]i Postsynaptic response dependent on? Number & sensitivity of Pre Post postsynaptic receptors  mEPSCs Synapse Presynaptic cell Recording electrode Postsynaptic cell Vesicle docking at the synapse is not totally stable Vesicles occasionally may have their release triggered by random chance or events without an AP trigger: “mEPSC” or “mini. Each mEPSC should be about the size of one quanta  mEPSCs: individual quantal release Ca2+ increases probability of release (N) but not postsynaptic response to one quanta (Q) AP-dependent quantal events Presynaptic cell Synapse Recording APs likely to cause release of electrode multiple quanta Postsynaptic cell Stimulus In theory, should see quantal increases in PSC amplitude In practice, often difficult to clearly observe  Excitatory and inhibitory postsynaptic potential Neurotransmitter binding to postsynaptic receptors → postsynaptic current (PSC)→ postsynaptic potential (PSP) PSP – Excitatory→ increase probability of firing PSP – Inhibitory→ decrease probability of firing PSP Reversal potential→ membrane potential at which the direction of ionic current reverses – reversal potential for a PSP (0 mV) is more positive than the action potential threshold (–40 mV) → excitatory – reversal potential for a PSP is more negative than the action potential threshold → inhibitory EPSP summation Spatial→ adding different EPSPs from different synapses Temporal→ adding together a succession of EPSP in the same synapse Inhibition Temporal Integration Two stimuli, same strength and 1 location with short delay 2 Two stimuli, same strength and location with longer delay Inputs within a short timeframe will summate; less summation with greater latency of second pulse Spatial Integration Two simultaneous stimuli at different distances 1 2 1+2= Why do they summate so effectively despite the distance? Inhibition Simultaneous stimulations 1 2 3 1+2+3= Inhibitory inputs opposite from excitatory. In this example, excitatory inputs cancelled out by inhibitory “Veto principle” GABAergic synapses are capable of exerting a “veto principle” – preventing excitation via excitatory synapses Works most effectively via: a) inhibitory synapses that are close to the excitatory inputs b) inhibitory synapses that are somatic or proximal dendritic GABAergic synapses frequently involved in controlling synchrony and oscillations  Intrinsic activity and modulation Pacemakers: currents that generate APs without the need for excitatory inputs (see oscillations lecture) Ion channels: e.g. K+ channels, T-type Ca2+ channels Control of membrane potential & resistance Modulation by metabotropic signalling; altering activity of ion channels & receptors  Dendritic properties Dendrites play a critical role in integrating these synaptic inputs Distal synapses (further from the soma) are weaker than proximal ones Synapses on smaller (distal) dendrites generate larger signals in the local area - Smaller cables with higher resistance (V=IR) Neurotransmitter system  Research methods to study neurotransmitters Localization of neurotransmitters 1. Immunocytochemistry→ neurotransmitter antibody is created and then tagged with a marker, applied to the brain tissue and the antibody just label those cells which contain the candidate 2. In situ hybridization→ a synthetic probe is constructed containing a sequence of complementary nucleotides that will allow it to stick to the mRNA. If the probe is labeled, the location of cells containing the mRNA will be revealed Studying receptors 1. Neuropharmacological analysis→ agonist and antagonist of the receptor 2. Ligand-binding methods→ radioactively labeled compounds and applied them in small quantities to neuronal membranes → If appropriate receptors existed in the membrane, it should bind tightly to them 3. Molecular analysis→ studying protein molecules (it has enabled us to divide the neurotransmitter receptor proteins into two groups: transmitter-gated ion channels and G- protein-coupled (metabotropic) receptors)  Categories of neurotransmitter Small-molecules neurotransmitter Peptide neurotransmitter  Neurotransmitter receptor Membrane receptor protein Activated by neurotransmitter located on the surface of neuronal and glia cells There are two major types: - Ionotropic: Ligand-gated ion channels - Metabotropic: G protein-coupled receptors Ionotropic receptors Metabotropic receptors  Acetylcholine Neuromuscular junctions and ganglionic synapses Excitatory postsynaptic effect Precursor: Choline + Acetyl CoA Removal mechanism: AChE Receptors - Nicotinic Ach receptors (muscle and CNS) - Muscarinic Ach receptors Amino acids  Glutamate Most important transmitter for normal brain function Excitatory postsynaptic effect Precursor: Glutamine Removal mechanism: EAATs (glia and presynaptic terminals) Receptors - AMPA - NMDA - Kainate receptors - mGluRs  GABA Brain and spinal cord Inhibitory postsynaptic effect Precursor: Glutamate Removal mechanism: GAT Receptors - GABAa - GABAb  Glycine Brain and spinal cord Inhibitory postsynaptic effect Precursor: Serine Removal mechanism: GlyT Receptors - Glycine receptor Biogenic amines  Dopamine Essential role in coordination of movements, motivation and reinforcement Excitatory postsynaptic effect Precursor: Tyrosine Removal mechanism: DAT/MAO/ COMT Receptors - Dopamine receptors  Norepinephrine (noradrenaline) Essential role in sleep and wakefulness, arousal, attention and feeding behaviour Excitatory postsynaptic effect Precursor: Tyrosine→ Da + Da β hydroxylase Removal mechanism: NET/MAO/ COMT Receptors - Adrenergic receptors  Epinephrine Essential role in respiration and cardiac function Excitatory postsynaptic effect Precursor: Tyrosine→ Da → Ne + phenylenthanolamine-N- methyltranferase Removal mechanism: NET/MAO/ COMT Receptors - Adrenergic receptors  Serotonin Essential role in sleep and wakefulness Excitatory postsynaptic effect Precursor: Tryptophan Removal mechanism: SERT/ MAO Receptors - 5-HT₃ - 5-HT₁ ₂ ₄ ₅ ₆ ₇  Histamine Essential role in arousal and attention and reactivity vestibular system Influence brain blood flow Excitatory postsynaptic effect Precursor: histidine decarboxylase Removal mechanism: No transporters/ MAO Receptors - Histamine receptors Purines  ATP Neurotransmitter in motor neurons spinal cord, sensory and autonomic ganglia and hippocampal neurons Acts as co-transmitter (no consider a classical neurotransmitter) Excitatory postsynaptic effect Precursor: ADP Removal mechanism: Nucleoside transporters Receptors - P2X receptors - Adenosine receptors  Peptide neurotransmitters  Many peptides are hormones and act as neurotransmitters Peptide neurotransmitter: - Biological activity depend on their amino acid sequence - opioids family - 3 classes: Endorphins, Enkephalins and Dynorphins - Co-localized with GABA and 5-HT  Essential role in respiration and cardiac function Excitatory and inhibitory postsynaptic effect Precursor: Amino acids Removal mechanism: Proteases Unconventional neurotransmitters  Endocannabinoids Postsynaptic effect: Inhibits inhibition Precursor: Membrane lipids Removal mechanism: Hydrolysis by FAAH Receptors - CB₁: brain - CB₂: periphery system  Nitric oxide Can mediate forms of synaptic plasticity Consider a second messenger Excitatory and inhibitory postsynaptic effects Precursor: Arginine Removal mechanism: Spontaneous oxidation