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:• 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)

Posztszinaptikusdenzitás (PSD)

Gray IISymmetric GABA

Gray IAsymmetric Glutamate

Clear vesicles

Dense core vesicles

Axodendritic

Axosomatic

Axoaxonal

Spine synapse

Spine

Shaftsnapse

Postsynaptic 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 vesicles2. Action potential at the presynaptic terminal3. Opening of voltage-gated calcium channels4. Influx of calcium5. Calcium induces vesicle fusion6. Transmitter released into the cleft7. Transmitter binds to postsynaptic receptors 8. Opening of postsynaptic ion channel/activation

of second messengers9. 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: TRIPARTITEsynapsis: pre-/postsynaptic + glia

1.

2.

3.

4.

5.

6.

7.

8.9.10.

11.

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-synaptotagminbinding

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

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

Excitatorytransmitter

Depolarization

Electrotonic spreading

Inhibitory transmitter

Hyperpolarization

Cl-/K+

channel

Electrotonic currents

Postsynapticneuron

Axon hillock

EPSP + IPSPsummation

Spatial summation: Simultaneous EPSPs of many dendrites(EPSP 1-3) spreading to the cell body and summed at theaxon hillock → reaching the threshold, axon actionpotential (APA)

Depolarizing currents

Summed EPSP

Action potential

Temporal summation: EPSPs following each other intime are summed → reaching the threshold, axon actionpotential (APA)

Depolarizing currents

Summed EPSP

Actionpotential

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

Spinalganglion

Cellbody

Axon

The primary sensory neuron

Extracellularspace

Intracellularspace

Ion channelsclosed

Membrane streched, channels

open

Mechanosensitive cation channelsat 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

Receptorpotential

Receptorpotential

Receptorpotential

Actionpotential

Sensory nerve ending

Sensory transduction, receptor potential, and 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 binding

2. Channelopening

3. Ion influx into the postsynaptic terminal

Postsynaptic

Synaptic cleft

1. Transmitterbinding

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

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

Transmitter Location of cell body Receptors Function

Norepinephrine • Locus coeruleus• Sympathetic postganglionic

• Metabotropic: Alpha 1-2Beta 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

DA

Thal/BG Limbic

Cortex

5HT – serotonin, NE – norepinephrine, DA – dopamineThal/BG – thalamus/basal ganglia

The functional organization of the brainstem monoaminergic systems

Three main targets:1. Thalamus/basal ganglia: vigilance,

movement control2. Limbic system (hippocampus,

amygdala): memory, emotions3. 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)

DOPAMINESubstantia nigraVentral tegmentalarea (VTA)

NOREPINEPHRINELocus coeruleus

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

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

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

Ionotropic receptorsCations• Nicotinic acetylcholine • Glutamate: NMDA, AMPA• Serotonin: 5-HT3Anion (chloride)• GABA-A/C • GlyR• HisCl

cAMP↑ (Gs)Norepinephrine: beta1-3Dopamine: D1,D5Histamine: H25-HT4-7

cAMP↓ (Gi)Acetylcholine: M2Norepinephrine: alfa2Dopamine: D2GABA-BmGLU5-HT1

IP3/DAG (Gq)M1Alfa1mGLUH15-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, volume

transmission

The intraneuronal (axonal) transport

Cell body Axon

Synapse

KINESIN: anterogradetransport• 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

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)• NMDA-modulation• Direct effect on DNA• Reactive free-radical

Endo-cannabinoid

NO NGF

Classic Retrograde

Non-synaptic neurotransmission: volume transmission

• Neurotransmitter A and B diffuse to distant targetsoutside 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)