Neurotransmission

Prof. Dr. Szabolcs Kéri

University of Szeged, Faculty of Medicine, Department of Physiology 2021 Why studying synapses?

Synaptopathy: diseases of the brain characterized by pathological synaptic structure and function Key points

1. Synapsis: definition and classification 2. Signal transduction in the synapsis 3. Neurotransmitters: definition and classification 4. Important transmitter systems and their functions 5. Non-conventional transmission: axon – glial connection, retrograde signals, and volume transmission 1. Definition and classification of synapses Definition and classification of synapses

Synapsis: Axons do not form a continuous network. They make contacts with dendrites or cell bodies. Synapse is a connection point to pass electrical or chemical signals to another neuron or to a target cell. A. CHEMICAL (neurotransmitter and receptor) B. ELECTRIC (gap junction)

I. Connection type: II. Transmitter type and function: • Axodendritic • Excitatory (Gray I: asymmetric, glutamate, spherical • Axosomatic vesicles) • Axoaxonal • Inhibitory (Gray II: symmetric, GABA, oval vesicles) • Axomyelinic • Modulatory (monoamines, small dense core vesicles) • Peptides (large dense core vesicles) Spine Clear vesicles

Spine synapse Dense core vesicles Shaft snapse

Gray I Gray II Asymmetric Symmetric Glutamate GABA

Axodendritic Axoaxonal

Posztszinaptikus Axosomatic Postsynapticdenzitás (PSD)density (PSD) Outlook: molecular diversity of the synapses 2. Signal transduction in the synapse Electric synapses: comparison with chemical synapses

ELECTRIC • Connexon pore (6 connexins) • Bidirectional diffusion of small molecules • Fast: minimal synaptic delay • Synchronization of neuronal groups • Glial networks • Passing second messengers (cAMP)

CHEMICAL • No pore in the membrane (transmitter and receptor needed) • Synaptic delay (1-1.5 ms) • One-way (pre → postsynaptic) Chemical neurotransmission

1. Transmitter stored in vesicles 2. 2. Action potential at the presynaptic terminal 1. 3. Opening of voltage-gated calcium channels 4. Influx of calcium 3. 5. Calcium induces vesicle fusion ASTROGLIA: 6. Transmitter released into the cleft TRIPARTITE synapsis: pre- 4. 7. Transmitter binds to postsynaptic receptors /postsynaptic + glia 8. Opening of postsynaptic ion channel/activation 5. of second messengers 11. 9. Generation of inhibitory or excitatory 6. postsynaptic potentials (IPSP/EPSP) 10. Transmitter elimination/inactivation (glial uptake, presynaptic reuptake, enzymatic degradation) 11. Vesicle retrieval from presynaptic membrane 7. (recirculation) 8. 10. 9. 1. Vesicle docking – The mechanism of synaptic vesicle fusion active zone • Proteins implicated in vesicle fusion: ▪ In the vesicle’s membrane: synaptobrevin, synaptotagmin 2. SNARE-complex ▪ In the presynaptic membrane: SNAP-25, syntaxin ▪ Botulinum toxin (BOTOX) and tetanus toxin: degradation of presynaptic proteins • N-type voltage-gated presynaptic calcium channels (inhibited by omega-conotoxin) 3. Calcium- • Quantal neurotransmitter release (neurotransmitter synaptotagmin content of 1 vesicle = 1 quantum) binding • Synaptic potentiation: higher postsynaptic response after high frequency presynaptic stimulation – calcium-calmodulin dependent protein kinase II → synapsin → docking of new vesicles 4. Membran fusion, pore formation SNARE = SNAP Receptor (Soluble NSF (N-ethymaleimide-sensitive factor) Attachment Protein Receptor) Reuptake: Monoamines Acetylcholine

GABA Glutamate Ionic mechanism of local potentials: postsynaptic potentials

Excitatory EPSP (excitatory postsynaptic transmitter potential) • Local and graded depolarization of the postsynaptic membrane • Influx of Na+ or Ca2+ into the postsynaptic terminal Inhibitory transmitter • Excitatory transmitters: glutamate, acetylcholine Depolarization Electrotonic spreading IPSP (inhibitory postsynaptic

- + potential) Cl /K EPSP + IPSP • Local and graded hyperpolarization channel summation of the postsynaptic membrane Hyperpolarization • Influx of Cl- (GABA-A receptor) or efflux of K+ Postsynaptic • Inhibitory transmitters: GABA, neuron Electrotonic currents glycine Axon hillock Spatial summation: Simultaneous EPSPs of many dendrites Temporal summation: EPSPs following each other in (EPSP 1-3) spreading to the cell body and summed at the time are summed → reaching the threshold, axon action axon hillock → reaching the threshold, axon action potential (APA) potential (APA)

Action potential Summed Action potential EPSP Depolarizing Depolarizing currents currents Summed EPSP The primary sensory neuron Dorsal horn

Receptor Synapse

Peripheral fiber (dendron) Receptor cells, nerve terminal: graded Cell body: ganglion spinale receptor potential (dorsal root ganglion cells) Cranial nerve ganglia (e.g. Gasserian ganglion) Spinal Cell ganglion body Central fiber

Axon Axon terminal Transmitter release: (dorsal horn) glutamate, aspartate, SP/CGRP, other peptides, NO Sensory transduction, receptor potential, and action potential

Sensory nerve ending Mechanosensitive cation channels at the sensory nerve endings

Extracellular space

Receptor potential:

Membrane Influenced by stimulus Intracellular Ion channels streched, channels strength, open space closed graded, Weak stimulus Moderate stimulus Strong stimulus local, spreading with

Receptor decrement, Receptor Receptor Action potential potential potential potential depolarization → Threshold threshold → action potential 3. The definition and classification of neurotransmitters The features of classic neurotransmitters

• Synthesized and present in the presynaptic terminal • Released following depolarization and calcium-influx • Specific receptors are present in the postsynaptic membrane • Action is terminated by specific mechanisms (reuptake transporter in the presynaptic membrane, enzyme, glial uptake) • Dale-principle: each axon terminal of a neuron releases the same transmitter • Co-transmitter: peptides released after high-frequency stimulation, inducing late and prolonged EPSP • acetylcholine - vasoactive intestinal polipeptid (VIP) • norepinephrine - neuropeptid Y (NPY) • glutamate - substance P (SP)/calcitonin-gene related peptide (CGRP) Classification of neurotransmitters

1. Acetylcholine 2. Amino acids (glutamate, glycine, GABA) 3. Biogenic amines (dopamine, noradrenalin, adrenalin, histamine, serotonin) 4. Peptides (opiates [endorphins, enkephalins, dynorphins], SP, CGRP, VIP)

5. Gases (NO, CO, H2S) 6. Lipids (endocannabinoids, prostaglandins) 7. Purines (adenosine, ADP, ATP) Classification of neurotransmitter receptors: ionotropic and metabotropic

Ionotropic: ligand-gated ion channel Metabotropic: G-protein coupled receptors

1. Transmitter Synaptic cleft binding 1. Transmitter binding

2. Channel opening 5. Ion influx

Postsynaptic 4. Ion channel opening

3. Ion influx into 2. G-protein 3. G-protein subunit or the postsynaptic activation second messenger terminal modulates the ion channel 4. Organization and function of important transmitter systems Acetylcholine and amino acid transmitters

Transmitter Location of cell body Receptors Function Acetylcholine • N. basalis Meynerti • Ionotropic: nicotinic • Attention, memory • Autonomic neurons • Metabotropic: • Sympathetic • Motor endplate muscarinic (M1-M4) preganglionic • Parasympathetic pre- /postganglionic Glutamate • Neocortex pyramidal cells • Ionotropic: NMDA, • General excitatory (most abundant AMPA, kainate transmitter neurotransmitter) • Metabotropic: • Learning, plasticity mGluR1-R8 • Neurodegeneration GABA (gamma- • Neocortex interneurons • Ionotropic: GABA-A/C • General inhibitory amino-butiric- • Purkinje-cells • Metabotropic: GABA-B transmitter acid) (cerebellum) • Cortical oscillation • Striatum • Anxiety, vigilance

Glycine • Spinal cord • Ionotropic: GlyR • Inhibitory transmitter • Brainstem Biogenic amines

Transmitter Location of cell body Receptors Function Norepinephrine • Locus coeruleus • Metabotropic: • Attention, vigilance, • Sympathetic postganglionic Alpha 1-2 anxiety (alarm Beta 1-3 reaction) • Sympathetic effect Dopamine • Substantia nigra (pars • Metabotropic: D1-D5 • Reward, motivation compacta) • Movement control • Ventral tegmental area • Higher cognitive functions Serotonin • Raphe nuclei • Metabotropic: 5-HT1- • Emotional functions 2, 4-7 • Sleep, appetite, sex • Ionotropic: 5-HT3 • Neuroendocrine regulation Histamine • N. tuberomammalis • Metabotropic: H1-4 • Sleep-wakefulness (posterior hypothalamus) • Ionotropic: HisCl cycle, vigilance (histamine-gated chloride • Appetite channel) The functional organization of the brainstem monoaminergic systems

Function: improving signal-noise ratio in glutamate/GABA synapses Three main targets: 1. Thalamus/basal ganglia: vigilance, movement control Cortex 2. Limbic system (hippocampus, amygdala): memory, emotions Thal/BG Limbic 3. Prefrontal cortex: higher cognition

DA Dopaminergic neurons: histology and PET (positron emission tomography)

5HT – serotonin, NE – norepinephrine, DA – dopamine Thal/BG – thalamus/basal ganglia Imaging brainstem monoaminergic nuclei in humans (neuromelanin-sensitive MRI)

DOPAMINE Substantia nigra Ventral tegmental area (VTA)

NOREPINEPHRINE Locus coeruleus Production, inactivation, and receptors of some key transmitters

1. The glutamate – GABA system

Glia Glucose → glutamine ↔ glutamate ↔ GABA

Glutamine Glutamate decarboxylase Glutamine (GAD) + vitamin B6

Glutamate GABA → succinate, gamma-hydroxybutirate

Glutamate The universal mechanism of re-uptake elimination of conventional transmitters: • Presynaptic: Na+-associated secondary active symport • Uptake into the vesicles: H+-associated secondary active antiport The most important receptors of the glutamate-GABA system

GABA GABA Glycine Benzodiazepin Glutamate

Volatile anesthetics

Ethanol

Inhibitory chloride-channel Excitatory non-selective cation-channel NMDA – N-methyl-D-aspartate 2. Acetylcholine and catecholamines (norepinephrine, epinephrine, dopamine)

3. Serotonin

• Production: tryptophan → 5-hydroxy-tryptophane → 5-hydroxy-tryptamine • Elimination: ▪ Presynaptic reuptake (SERT = serotonin transporter) ▪ Enzymatic degradation: Monoamine Oxidase-A (MAO-A) (main metabolite: 5- hydroxy-indolacetate) 4. Hisztamin

• Production: histidine → histamine • Elimination: rapid inactivation by Synaptic Histamine-N-Methyltransferase Signal transduction of neurotransmitter receptors

Ionotropic receptors Metabotropic receptors Cations cAMP↑ (Gs) IP3/DAG (Gq) • Nicotinic acetylcholine Norepinephrine: beta1-3 M1 • Glutamate: NMDA, AMPA Dopamine: D1,D5 Alfa1 • Serotonin: 5-HT3 Histamine: H2 mGLU Anion (chloride) 5-HT4-7 H1 • GABA-A/C 5-HT2 • GlyR cAMP↓ (Gi) • HisCl Acetylcholine: M2 cGMP ↑ Norepinephrine: alfa2 NO Dopamine: D2 GABA-B mGLU 5-HT1

AMPA: α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptor 5. Non-conventional neurotransmission: axon- glia connection, retrograde signals, volume transmission The intraneuronal (axonal) transport

Synapse

Cell body Axon

KINESIN: anterograde DYNEIN: retrograde transport transport • Degradation products • Synaptic elements (e.g. • Neurotrophic signals vesicles) • Neuroinvasive viruses (e.g. • Peptide transmitters herpes simplex) • Cytoskeleton Microtubule-associated proteins (e.g. tau) – neurodegeneration (e.g. Alzheimer’s) The axomyelitic synapse

Oligodendroglia

AMPA NMDA Axon Classic and retrograde neurotransmission

1. CB1 receptor: endocannabinoid (EC) signal (anandamide, 2-arachidonoylglycerol)

2. NGF (nerve growth factor): retrograde trophic signal

3. NO (nitrogen monoxide) • Arginine → citrulline (neuronal NO-synthase, Endo- NOS1) cannabinoid • cGMP – protein kinase G • S-nitrosylation (posttranslational NGF NO modification, e.g. cysteine) • NMDA-modulation • Direct effect on DNA • Reactive free-radical Classic Retrograde Non-synaptic neurotransmission: volume transmission

• Neurotransmitter A and B diffuse to distant targets outside the synapse (1), and act on their receptors (2) • Extrasynaptic receptors, medication effects • Example: dopamine (DA) in the prefrontal cortex (link between higher cognition and motivation/attention)