2019.09.18.

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Neurotransmission

Prof. Dr. Szabolcs Kéri

University of Szeged, Faculty of Medicine, Department of Physiology

2019

Why studying synapses?

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

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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

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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:• Axodendritic

• Axosomatic

• Axoaxonal

• Axomyelinic

II. Transmitter type and function:• Excitatory (Gray I: asymmetric, glutamate, spherical

vesicles)

• Inhibitory (Gray II: symmetric, GABA, oval vesicles)

• Modulatory (monoamines, small dense core vesicles)

• Peptides (large dense core vesicles)

Posztszinaptikus

denzitás (PSD)

Gray II

Symmetric

GABA

Gray I

Asymmetric

Glutamate

Clear vesicles

Dense core vesicles

Axodendritic

Axosomatic

Axoaxonal

Spine

synapse

Spine

Shaft

snapse

Postsynaptic density

(PSD)

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Outlook: molecular diversity of the synapses

2. Signal transduction in the synapse

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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. Action potential at the presynaptic terminal

3. Opening of voltage-gated calcium channels

4. Influx of calcium

5. Calcium induces vesicle fusion

6. Transmitter released into the cleft

7. Transmitter binds to postsynaptic receptors

8. Opening of postsynaptic ion channel/activation

of second messengers

9. Generation of inhibitory or excitatory

postsynaptic potentials (IPSP/EPSP)

10. Transmitter elimination/inactivation (glial

uptake, presynaptic reuptake, enzymatic

degradation)

11. Vesicle retrieval from presynaptic membrane

(recirculation)

ASTROGLIA:

TRIPARTITE

synapsis: pre-

/postsynaptic + glia

1.

2.

3.

4.

5.

6.

7.

8.9.10.

11.

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The mechanism of synaptic vesicle fusion

• Proteins implicated in vesicle fusion:

� In the vesicle’s membrane: synaptobrevin,

synaptotagmin

� 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)

• Quantal neurotransmitter release (neurotransmitter

content of 1 vesicle = 1 quantum)

• Synaptic potentiation: higher postsynaptic response

after high frequency presynaptic stimulation –

calcium-calmodulin dependent protein kinase II →

synapsin → docking of new vesicles

1. Vesicle docking –

active zone

2. SNARE-complex

3. Calcium-

synaptotagmin

binding

4. Membran fusion,

pore formation SNARE = SNAP Receptor

(Soluble NSF (N-ethymaleimide-sensitive factor) Attachment Protein Receptor)

Ionic mechanism of local potentials: postsynaptic potentials

EPSP (excitatory postsynaptic

potential)• Local and graded depolarization of

the postsynaptic membrane

• Influx of Na+ or Ca2+ into the

postsynaptic terminal

• Excitatory transmitters: glutamate,

acetylcholine

IPSP (inhibitory postsynaptic

potential)• Local and graded hyperpolarization

of the postsynaptic membrane

• Influx of Cl- (GABA-A receptor) or

efflux of K+

• Inhibitory transmitters: GABA,

glycine

Excitatory

transmitter

Depolarization

Electrotonic spreading

Inhibitory transmitter

Hyperpolarization

Cl-/K+

channel

Electrotonic

currents

Postsynaptic

neuron

Axon hillock

EPSP + IPSP

summation

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Spatial summation: Simultaneous EPSPs of many dendrites

(EPSP 1-3) spreading to the cell body and summed at the

axon hillock → reaching the threshold, axon action

potential (APA)

Depolarizing

currentsSummed EPSP

Action potential

Temporal summation: EPSPs following each other in

time are summed → reaching the threshold, axon action

potential (APA)

Depolarizing

currents

Summed

EPSP

Action

potential

Cell body: ganglion spinale

(dorsal root ganglion cells)

Cranial nerve ganglia (e.g. Gasserian ganglion)

Receptor cells,

nerve terminal: graded

receptor potential

Peripheral fiber

(dendron)

Central fiber

Axon terminal

(dorsal horn)

Transmitter release:

glutamate, aspartate, SP/CGRP,

other peptides, NO

Dorsal horn

Synapse

Receptor

Spinal

ganglionCell

body

Axon

The primary sensory neuron

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Extracellular

space

Intracellular

space

Ion channels

closed

Membrane

streched, channels

open

Mechanosensitive cation channels

at the sensory nerve endings

Receptor potential:

Influenced by stimulus

strength,

graded,

local,

spreading with

decrement,

depolarization →

threshold →

action potential

Threshold

Weak stimulus Moderate stimulus Strong stimulus

Receptor

potential

Receptor

potential

Receptor

potentialAction

potential

Sensory nerve ending

Sensory transduction, receptor potential, and action potential

3. The definition and classification of

neurotransmitters

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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).

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Classification of neurotransmitter receptors: ionotropic and metabotropic

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

1. Transmitter

binding

2. Channel

opening

3. Ion influx into

the postsynaptic

terminal

Postsynaptic

Synaptic cleft

1. Transmitter

binding

2. G-protein

activation

3. G-protein subunit or

second messenger

modulates the ion channel

4. Ion

channel

opening

5. Ion

influx

4. Organization and function of important transmitter systems

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Transmitter Location of cell body Receptors Function

Acetylcholine • N. basalis Meynerti

• Autonomic neurons

• Motor endplate

• Ionotropic: nicotinic

• Metabotropic:

muscarinic (M1-M4)

• Attention, memory

• Sympathetic

preganglionic

• Parasympathetic pre-

/postganglionic

Glutamate • Neocortex pyramidal cells

(most abundant

neurotransmitter)

• Ionotropic: NMDA,

AMPA, kainate

• Metabotropic:

mGluR1-R8

• General excitatory

transmitter

• Learning, plasticity

• Neurodegeneration

GABA (gamma-

amino-butiric-

acid)

• Neocortex interneurons

• Purkinje-cells

(cerebellum)

• Striatum

• Ionotropic: GABA-A/C

• Metabotropic: GABA-B

• General inhibitory

transmitter

• Cortical oscillation

• Anxiety, vigilance

Glycine • Spinal cord

• Brainstem

• Ionotropic: GlyR • Inhibitory transmitter

Acetylcholine and amino acid transmitters

Transzmitter Sejttest helye Receptorok Funkció

Norepinephrine • Locus coeruleus

• Sympathetic postganglionic

• Metabotropic:

Alpha 1-2

Beta 1-3

• Attention, vigilance,

anxiety (alarm

reaction)

• Sympathetic effect

Dopamine • Substantia nigra (pars

compacta)

• Ventral tegmental area

• Metabotropic: D1-D5 • Reward, motivation

• Movement control

• Higher cognitive

functions

Serotonin • Raphe nuclei • Metabotropic: 5-HT1-

2, 4-7

• Ionotropic: 5-HT3

• Emotional functions

• Sleep, appetite, sex

• Neuroendocrine

regulation

Histamine • N. tuberomammalis

(posterior hypothalamus)

• Metabotropic: H1-4

• Ionotropic: HisCl(histamine-gated chloride

channel)

• Sleep-wakefulness

cycle, vigilance

• Appetite

Biogenic amines

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DA

Thal/BG Limbic

Cortex

5HT – serotonin, NE – norepinephrine, DA – dopamine

Thal/BG – thalamus/basal ganglia

The functional organization of the brainstem monoaminergic systems

Three main targets:

1. Thalamus/basal ganglia: vigilance,

movement control

2. Limbic system (hippocampus,

amygdala): memory, emotions

3. Prefrontal cortex: higher cognition

Dopaminergic neurons: histology and PET

(positron emission tomography)

Function: improving signal-noise ratio in

glutamate/GABA synapses

Imaging brainstem monoaminergic nuclei in humans

(neuromelanin-sensitive MRI)

DOPAMINE

Substantia nigra

Ventral tegmental

area (VTA)

NOREPINEPHRINE

Locus coeruleus

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Production, inactivation, and receptors of some key transmitters

Glucose → glutamine ↔ glutamate ↔ GABA

Glutamate decarboxylase

(GAD) + vitamin B6

GABA → succinate, gamma-hydroxybutirate

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

1. The glutamate – GABA system

Glia

Glutamine

Glutamate

Glutamate

Glutamine

Reuptake: Monoamines Acetylcholine

GABA Glutamate

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The most important receptors of the glutamate-GABA system

Inhibitory chloride-channel Excitatory non-selective cation-channel

GABAGABA

Benzodiazepin

Volatile

anesthetics

Ethanol

Glutamate

Glycine

NMDA – N-methyl-D-aspartate, PCP – phencyclidine (angel dust)

GluN1 (NR1),

GluN2 (NR2),

GluN3 (NR3),

subunits,

heterotetramer

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

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Ionotropic receptorsCations

• Nicotinic acetylcholine

• Glutamate: NMDA, AMPA

• Serotonin: 5-HT3

Anion (chloride)

• GABA-A/C

• GlyR

• HisCl

cAMP↑ (Gs)

Norepinephrine: beta1-3

Dopamine: D1,D5

Histamine: H2

5-HT4-7

cAMP↓ (Gi)

Acetylcholine: M2

Norepinephrine: alfa2

Dopamine: D2

GABA-B

mGLU

5-HT1

IP3/DAG (Gq)

M1

Alfa1

mGLU

H1

5-HT2

cGMP ↑

NO

Signal transduction of neurotransmitter receptors

Metabotropic receptors

AMPA: α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptor

5. Non-conventional neurotransmission: axon-glia

connection, retrograde signals, volumetransmission

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The intraneuronal (axonal) transport

Cell body Axon

Synapse

KINESIN: anterograde

transport

• Synaptic elements (e.g.

vesicles)

• Peptide transmitters

• Cytoskeleton

DYNEIN: retrograde transport

• Degradation products

• Neurotrophic signals

• Neuroinvasive viruses (e.g.

herpes simplex)

Microtubule-associated proteins (e.g. tau) –

neurodegeneration (e.g. Alzheimer’s)

Oligodendroglia

Axon

AMPA NMDA

The axomyelitic synapse

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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,

NOS1)

• cGMP – protein kinase G

• S-nitrosylation (posttranslational

modification, e.g. cysteine)

• Direct effect on DNA

• NMDA-modulation

• Reactive free-radical

Endo-

cannabinoid

NO NGF

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)