NEURAL

COMMUNICATION

SYNAPTIC TRANSMISSION

Prof. Gábor JANCSÓ

17. Neurotransmission.

Characterize electric synapses including the description of the molecular structure of gap junction

operating in these synapses. Compare transmission between electric and chemical synapses

(direction of information, speed of transmission).

Describe the consecutive events of chemical neurotransmission (starting with the depolarization of

presynaptic membrane ending with the development of the graded electric response of the postsynaptic

membrane (postsynaptic potential, PSP). Describe the ion currents involved in the development of the

following local potentials: excitatory postsynaptic potential (EPSP), inhibitory postsynaptic potential

(IPSP), end plate potential (EPP).

Describe the temporal and spatial summation of postsynaptic potentials (EPSPs and IPSPs), and their

role to trigger an action potential

Describe the common features of classical neurotransmitters.

Group the classical and non-classical neurotransmitters based on their chemical structure: 1. acetyl-

choline, 2. amino acids (glutamate, glycine, GABA), 3. biogenic amines (dopamine, noradrenaline,

adrenaline, histamine, serotonin), 4. gases (NO, CO), 5. lipids (endocannabinoids, 6. peptides

(endophins, encephalins, dynorphins, substance P, CGRP, VIP).

Characterize the neurotransmitters in bold based on their synthesis, inactivation, receptors and signal

transduction mechanisms. Define the terms ionotropic and metabotropic neurotransmitter receptors.

Describe the role of intraneuronal (axonal) transport mechanisms in the maintenance of interneuronal

communication.

Nonsynaptic neurotransmission. Volume transmission.

Normal values: synaptic delay: 1-1,5 ms.

"in recognition of their

work on the structure

of the nervous

system"

"for their discoveries

regarding the functions

of neurons"

"for their discoveries

relating to chemical

transmission of nerve

impulses"

”their discoveries relating

to the highly differentiated

functions of single nerve

fibres"

"for their discoveries concerning

the ionic mechanisms involved in

excitation and inhibition in the

peripheral and central portions

of the nerve cell membrane"

"for their discoveries concerning the

humoral transmittors in the nerve

terminals and the mechanism for

their storage, release and

inactivation".

"for their discoveries concerning the

peptide hormone production of

the brain" and the other half to

Rosalyn Yalow "for the development of

radioimmunoassays of peptide

hormones".

"for their discoveries concerning

signal transduction in the

nervous system"

"for their discoveries of machinery

regulating vesicle traffic, a major

transport system in our cells”

(mechanisms of exocytosis of

synaptic vesicles)

Santiago RAMON y

CAJAL

Camillo GOLGI

Nobel prize 1906

‘reazione nera’ (black reaction) of Golgi

“neuron doctrine” of Cajal

Santiago RAMON Y CAJAL: Histologie Du Systeme Nerveux de

L’homme et Des Vertébrés. (1909, Maloine, Paris)

• Histodynamic polarity

• Specificity of

connections (wiring)

THE NEURON DOCTRINE

von Waldeyer-Hartz (1891) is generally credited

with formulating the neuron doctrine, although as

soon as one writes that one is obliged to cite Cajal

(see Cajal 1954, for the English translation) who

wrote: ‘ Professor Waldeyer, to whom poorly

informed persons attribute the neuron theory,

supported it with the prestige of his authority but did

not contribute a single personal observation. He

limited himself to a short, brilliant exposition (1891)

of the objective proofs adduced by His, Kölliker,

Retzius, Van Gehuchten and myself, and he invented

the fortunate term of neuron’.

CONDUCTION OF THE NERVE IMPULSE

Du Bois-Reymond (1877): ”Of known natural processes that

might pass on excitation, only two are, in my opinion, worth

talking about - either there exists at the boundary of the

contractile substance a stimulatory secretion…; or the

phenomenon is electrical in nature“

Chemical transmission of the nerve impulse

Dream, William Blake

Otto Loewi

Electric stimulation

Of the vagus nerve

Perfusion fluid

Electric stimulation

Perfusate

Otto LOEWI’S experiment proved the chemical nature of the

transmission of the nerve impulse

HR

HR

Skizze und Beschreibung der humoralen Übertragung

von Otto Loewi für seinen Sohn Guido, etwa 1950.

"If a nerve by a stimulus gets an impulse

this impulse is propagated within the nerve

and is transmitted to the respec tive effective

organs (heart, muscle, gland) innervated by

the nerve. The question arose by which means

the impulse coming from the nerve is transmitted

to the effector organ. I was able to solve this

question by proving that the impulse running

down within the nerve liberates from its endings

chemical substances (Acetylcholine or Adrenaline

respectively) which in their turn influence the

effector organ exactly like the stimulation of the

nerve. With other words: the influence of nervous

stimulation on an organ is not a direct one but an

indirect one mediated to the organ by chemical

substances released by the nerve stimulation in

its endings."

”the influence of nervous stimulation on an organ

is not a direct one but an indirect one mediated to

the organ by chemical substances released by the

nerve stimulation in its endings."

“So far as our present knowledge goes, we are led to

think that the tip of a twig of the arborescence is not

continuous with but merely in contact with the

substance of the dendrite or cell body on which it

impinges. Such a special connection of one nerve cell

with another might be called a synapse.”

(Sherrington, 1897)

The introduction of the functional term ”synapse”

{Synapse: ‘synaptein’ : Greek ‘syn-’ (together), ‘haptein’ (to clasp)}

Light microscopy (Cajal)

The morphological substrate of the ”synapse”

RASMUSSEN, 1957

Neuropil

pre

post

Elektronmikroszkópia spine synapse Electron microscopy

Gray Type 2

asymmetrical synapse

(GABA)

Gray Type 2

symmetrical synapse

(glutamate)

S and F types

synaptic vesicles

(spheroid/flattened

(excitatory/inhibitory)

Types of synapses:

Chemical synapses: Electrical synapses

(gap junctions)

excitatory inhibitory depolarization

(presynaptic, hyperpolarization

postsynaptic

inhibition)

Figure 1 Electrical postsynaptic potentials in neurons from the mammalian brain.

Each case illustrates simultaneous intracellular recordings from a pair of similar neurons,

with presynaptic action potentials above and electrical postsynaptic potentials

below. (A) Two types of inhibitory interneurons from the neocortex: fast-spiking (FS)

cells and low threshold–spiking (LTS) cells. Traces are averaged from ten and eight

neuron pairs, respectively, and the dashed lines are ±SD (J.R. Gibson, M. Beierlein,

B.W. Connors, unpublished report). (B) Recordings from a pair of thalamic reticular

neurons in tonically spiking mode (left) and bursting mode (right). Action potentials

are truncated (from Long et al. 2004). (C) Inferior olivary neurons (MA Long, unpublished

report). (D) AII amacrine neurons from the retina (Veruki & Hartveit 2002a).

(E) Cerebellar interneurons (Mann-Metzer & Yarom 1999).

Electrical synapses (gap junctions)

GAP JUNCTION (~2 nm [20 Å])

HEMICHANNELS (CONNEXONS)

6 CONNEXIN SUBUNITS

4 TRANSMEMBRANE PROTEIN MOLECULES

Lack of synaptic delay

Low resistance

Shorter latency

Connexons direct the nerve impulse

in both directions

Dye-coupling

Electrical postsynaptic potentials in neurons

from the mammalian brain. Each case

illustrates simultaneous intracellular

recordings from a pair of similar neurons,

with presynaptic action potentials above and

electrical postsynaptic potentials below.

Electrical postsynaptic potentials in neurons

from the mammalian brain. Each case

illustrates simultaneous intracellular

recordings from a pair of similar neurons,

with presynaptic action potentials above and

electrical postsynaptic potentials below.

POSTSYNAPTIC POTENTIAL

PRESYNAPTIC ACTION POTENTIAL

CHEMICAL SYNAPSE

ELECTRICAL SYNAPSE

DYE COUPLING

LUCIFER

YELLOW

GAP JUNCTION

DYE COUPLING

LUCIFER

YELLOW

GAP JUNCTION

Connexin mutations

and human diseases:

About 20 connexins identified in the

human genom. Their mutations cause

inherited diseases.

Con32: Charcot-Marie-Tooth-disease

motor+sensory neuropathy

(muscular dystrophy)

Con26 & 30: deafness, skin diseases

Con46 & 50: hereditery cataract

Chemical synapses

Postsynaptic electrical events localized changes in membrane potential

EPSP: excitatory postsynaptic potential

depolarization:

increased Na+ and/or Ca++ influx

summation of EPSPs (temporal, spatial summation)

IPSP: inhibitory postsynaptic potential

hyperpolarization:

opening of Cl– channels – increased Cl– influx

or opening of K+ channels – K+ efflux

or closure of Na+ or Ca++ channels)

summation of IPSPs (temporal, spatial summation)

Postsynaptic electrical events, EPSP

SPATIALIS (TÉRBELI) SUMMATIO

TEMPORALIS (IDŐBELI) SUMMATIO

spatial and

temporal

summations

Inhibition and facilitation at synapses

Postsynaptic or direct inhibition (IPSP, glycine)

Presynaptic inhibition

axo-axonal synapses

- mostly mediated by a decrease in the Ca++ entry and

the consequent reduction in the release of the excitatory

transmitter

- K+ channels may be also opened

- direct inhibitory effect on transmitter release

- mediated by GABA

- result: selective elimination of specific inputs

Presynaptic facilitation

- increase of Ca++ entry and the consequent increase in the

release of the excitatory transmitter

- closing of K+ channels

Presynaptic inhibition and facilitation

Chemical synapses:

mechanisms of neurotransmitter release

Neurexin

(3 gén:1000 típus)

200-500 μs

Synaptic cleft:

20-40 nm

Presynaptic nerve

ending

Postsynaptic

neuron

action

potential receptor

Synaptic delay: 200-500 μs

T

T

T T

T

Synaptic vesicles

Transmitter (T)

~ 2000 Synapses/Nerve cell

1011 Neurons in CNS

Synapses in CNS ~ 2 x 1014

Neurexine

(3 genes:1000 types)

T

T

T T

T

axon

terminal dendrite

The classical chemical

synapse

Bipartite synapse

LICHTMANN & SANES, 2008 RETZIUS, 1893

NEUROGLIA - ASTROCYTA

SILVER IMREGNATION COMBINATORIAL EXPRESSION OF

GREEN, BLUE & RED

FLUORESCENT PROTEINS

BRAINBOW

THE TRIPARTITE SYNAPSE

PRESYNAPTIC AXON TERMINAL

POSTSYNAPTIC ELEMENT (DENDRITE) + ASTROCYTE

Ax

Ax

Astro

D

D

Halassa 2007

„Tripartite” szinapszis

-Glutamát reciklizálás - K+ „spatial buffering”- Gliotranszmitter ürítés

FUNCTIONS OF ASTROCYTES: Maintaining the functional integrity of neurons which are in direct physical contact with the astrocytes. Spatial potassium buffering in the extracellular (perisynaptic) space. Glutamate/glutamine recycling between neurons and astroglia. Release of gliotransmitters, modulation of synaptic transmission (ATP, glutamate, D-serine).

MECHANISMS OF

TRANSMITTER RELEASE

Quantal release of

neurotransmitters

Transmitter is

packaged in

vesicles and each

vesicle contains

approximately

10000 transmitter

molecules. Release

of transmitter from

a single vesicle

results in a quantal

synaptic potential.

QUANTAL MECHANISM OF NEURAL

TRANSMITTER RELEASE

TRANSMITTERSTranszmitter

kvantális felszabadulása

Near motor

endplate region

Remote endplate

region

SYNAPTIC VESICLE = 10000

Ach molecule: quantum

Sir Bernard Katz

KATZ: On our present evidence, the sequence of events may

be described as follows: depolarization opens specific

<calcium gates> in the-terminal axon membrane-this leads to

an influx of calcium ions… Having reached the internal

surface of the axon membrane, calcium ions then initiate the

"quanta1 release reaction". … a plausible and, I think, very

strong hypothesis, namely that the quanta of transmitter

molecules are enclosed within synaptic vesicles which

undergo frequent transient collisions with the axon

membrane, that calcium brings about attachment and local

fusion between vesicular and axon membranes, and that this

is followed by all-or-none discharge of the vesicular content

into the synaptic cleft.

MEP = miniature

endplate potentials [minis] (0.5-1 mV)

Quantal release of the

transmitter under

conditions of low

extracellular[Ca++]

VESICULAR TRANSMITTER RELEASE

NATURE, 490:201(2012)

Transmitters are released through a calcium-dependent exocytosis

from synaptic vesicles after fusion with the presynaptic membrane

Synaptic vesicle cycle:

1. Docking in the active zone

2. Priming

3. Fusion, exocytosis

4. Endocytosis (clathrin)

5. Endosome fusion, recycling

6. Budding

7. Transmitter-uptake (proton pump)

8. Translocation into the active zone

vesicle

presynaptic axon terminal

Calcium imaging uses fluorescent dyes which change their spectral

characteristics after binding (ionic) calcium

[ ]

Synaptic vesicle fusion (omega profiles, electron microscopy

Synaptic vesicle docking proteins are

targets of neurotoxins (zinc endopeptidases)

Clostridium botulinum. Botulinum toxin A

(blepharospasm, strabismus, and

torticollis/cervical dystonia, wrinkles)

Tetanus: spastic paralysis by blocking

presynaptic transmitter release in CNS

Botulinismus: flaccid paralysis by

blocking Ach release at the NMJ

Proteins of the neuronal

fusion machine

RETROGRADE AXONAL

TRANSPORT OF TOXINS INTO

SPINAL MOTONEURONS

TRANSMITTER

INACTIVATION

enzymatic elimination

inactivation through diffusion

COMT

AChE

ATP-diphosphohydrolase

(AMP 5’nukleotidase

adenosine)

Receptor internalization (substance P receptor)

RETROGRADE SIGNALISATION:

NITRIC OXIDE

RETROGRADE (ENDOCANNABINOID) SIGNALISATION

(INHIBITION OF TRANSMITTER RELEASE)

GIRK: G protein-coupled inwardly-rectifying

potassium channel ANA=anandamide,

2-AG=2-arachidonoylglycerol

Release of glutamate

Increase in postsynaptic

Ca2+ level

synthesis and (non-vesicular)

release of anandamide and

2-arachidonylglycerol

Activation of presynaptic

CB1 receptors

Release of Gβγ dimer

modulation of voltage

dependent CaV2

channel function

activation of GIRK:

presynaptic

hyperpolarization

Inhibition of transmitter

release

Chemical synapses: neurotransmitters

Acetylcholine (Ach)

Monoamines,

Catecholamines:

serotonin [5-HT]

histamine

dopamine

noradrenaline

adrenaline

Amino acids:

glutamate

aspartate

GABA

glycine

purines: ATP

PEPTIDES:

Tachykinins: GI peptides:

substance P secretin

neurokinin A glucagon

Opioid VIP

peptides: CCK

enkephalins somatostatin

endorphins bombesin

dinorphins gastrin

Neurohypophyseal Substance P

peptides:

vasopressin

oxytocin

Lipids: Others:

endocannabinoids CGRP, galanin

endovanilloids neurotensin,

NPY

gas: NO, CO

Peptide

Meynert

NEUROTRANSMITTER SYSTEMS OF THE BRAIN [CHEMICAL NEUROANATOMY]

Temporal characteristics of synaptic events

produced by transmitter agents

Dale’s principle: the same transmitter

agent is released from all axon terminals of the

neuron („one neuron, one transmitter”)

Colocalization of chemically different

transmitter agents

in peripheral and central neurons

Co-release of neurotransmitters from

peripheral and central neurons

Co-localisation

Co-release

of neurotransmitters

e.g. Ach--VIP

NA--NPY

TRH--5HT--SP

SP--CGRP(--GLU)

Note the frequency-dependence

of the release of the different

transmitter agents.

Neurotransmitter receptors – ion channels

Multiple receptors for most transmitters (adrenergic, cholinergic, etc.)

Postsynaptic and presynaptic receptors for many transmitters

Receptor families

Receptors (in clusters) are bound to the membrane by specific binding

proteins (rapsyn-NMJ, GABAA – gephyrin)

Desensitization: loss of responsiveness of the receptor to the ligand

(phosphorylation, increased endocytosis-receptor internalization)

direct

ligand gated

channels

Indirect ligand gated channels

Direct via G-Proteine

Indirect via

second

messengers

ionotropic metabotropic

nikotinerger Ach Rezeptor

Funktionszustände

ligandgesteuerter Ionenkanäle

IONOTROP AND METABOTROP RECEPTORS OF TRANSMITTERS

NEUROTRANSMITTER IONOTROP RECEPTORS METABOTROP RECEPTORS

Ach nicotinic Ach receptor muscarinic Ach receptor

5-HT 5-HT3 other 5-HT receptors

Glycine glycine receptor -

GABA GABAA & GABAC GABAB

Glutamate NMDA, AMPA, metabotrop glutamate r. KAINATE

ATP P2X P2Y

Dopamine - D1 receptor

Noradrenaline - α & β receptors

Histamine - H1 receptor

Glutamate receptors

AMPA: 2-amino-3-(5-methyl-3-oxo-1,2- oxazol-4-yl)propanoic acid

NMDA: N-Methyl-D-aspartate

LTP

Long-term potentiation

LTD

Long-term depression

Mechanisms of LTP and LDP may involve

phosphorylation and dephosphorylation of

synaptic proteins, respectively, regulated by

changes in intracellular calcium levels.

MOLECULAR TARGETS OF ANXIETY

ELEVATED PLUS MAZE

Superfamily of ligand-gated pentameric channels

Ach, 5-HT, GABA, Glycine

INCREASED FREQUENCY

OF CHANNEL OPENING INCREASED DURATION

OF CHANNEL OPENING

GABAA receptor channel

Non-synaptic transmission

Volume transmission

GOLGI (1891): ‘‘The structural contact or fusion between two nerve

fibers is not a necessary condition to have functional relationship

between different neurons ….. since, studies on electricity show that

electrical currents can link two conductors not in direct contact’’

This specific hypothesis has been subsequently demonstrated since

intercellular communication via electrotonic currents takes place in

the CNS. Furthermore, Golgi’s position may have an even broader

meaning, since, nowadays, most neuroscientists, in a more or less

explicit way, believe that there exists in the brain some kind of non-

synaptic, hormone-like, modulatory transmission besides synaptic

transmission.

Fuxe, Agnati (1991): wiring transmission WT and

volume transmission VT.

Volume transmission

Intraneuronal communication:

axonal transport

Paul Alfred Weiss Wien, March 21, 1898 — September 8, 1989

„outgrowth of neurons and their maintenance

depended on axonal flow from the nucleated

cell bodies in nerve centers”

„A pressure block (by suture, by passage into

a fibrotic zone such as a scar, or by other

means) caused piling up of the material of an

intact axon on the nuclear side and reduction

in diameter on the distal side of the

constriction.”

leucine-3H

Figure 5-10 Early experiments on axonal

transport used radioactive labeling of proteins.

In the experiment illustrated here the distribution

of radioactive proteins along the sciatic nerve

of the cat was measured at various times after

injection of [3H]-leucine into dorsal root ganglia

in the lumbar region of the spinal cord. In order to

show transport curves from various times (2,

4, 6, 8, and 10 hours after the injection) in one

figure, several ordinate scales (in logarithmic

units) are used. Large amounts of labeled protein

stay in the ganglion cell bodies but with

time protein moves out along axons in the sciatic

nerve and the advancing front of the

labeled proteins is displayed progressively farther

from the cell body (arrows). The velocity

of transport can be calculated from the distances

displayed at the various times. From

experiments of this kind, Sidney Ochs found that

the rate of axonal transport is constant at 410

mm per day at body temperature. (Adapted from

Ochs 1972.)

Intraneuronal transport of neuronal proteins

following the incorporation of [3H]-leucine

transport-velocity

410 mm/day

KINESIN DYNEIN

Axoplasmatic transport (Paul Weiss)

anterograde (somatofugal): trophic factors (Merkel, taste buds)

retrograd (somatopetal): Viruses, Toxins (Rabies; Tetanus),

[viral neuronal ”tracing”, transneuronal labelling of neural pathways]

Trophic factors (NGF, CNTF, BDNF, GDNF, etc)

Regulation of gene expression

Transport block (colchicin, Vinca-alkaloids; Depolimerization of microtubules)

Fast transport (e.g., transmitters) cm/day

Slow transport (e.g., cell organelles) mm/day

Dendritic transport