Synapses,

neurotransmitters and neuromodulators

Lecture series

Model Systems in Neurobiology: From Molecules to Behaviour

Wintersemester 2007/2008

Synapses:Electrical synapses

Chemical synapses

Neurotransmitters:

Underlying mechanisms of signal

transduction for electrial/ chemical synapses

Neuromodulators:

What is neuromodulation?

Levels of action

Outline

Synapses

Contacts between neurons, or between neuron and muscle (neuromuscular

synapse, neuromuscular junction, in vertebrates: motor endplate) are called

synapses.

The term „synapse“ was introduced by the Oxford professor of physiologySir Charles Scott Sherrington (1857– 1952)

Synapses are distinguished depending on the nature of

transmission: electrical or chemical synapse.

A synapse consists of a presynaptic part and a postsynaptic part

Neuromuscular synapse: the axon terminal of the motoneuron

Neuromuscular synpase: the muscle is the postsynaptic part

Axon

Dendrite

Dendrite

Electrical synapse

Types of contacts: Inferior olivary nucleus of the cat

Chemical synapse

gap junctions = electrical synapses

gap junctions = electrical synapses

Freeze-fracture through the electrical synapse

Face view through the presynaptic membrane

(each particle in the cluster represents a single connexon)

Side view

Molecules of electrical synapses

current flow

Connexons, Connexins

(Innexins in invertebrate animals!)

Electrical synapses

* first identified by E. Furshpan and D. Potter 1957 in the nervous system

of crayfish

* very small gap (3,5 nm), gap junctions,

Vertebrates: Connexins form pores (diameter 2 nm) between pre- and

postsynaptic cell, current can flow in both directions without a noticable

time delay

Invertebrates: Innexins, a different family of channel proteins

In principle, therefore, electrical synapses can conduct in both directions,

but rectifying (gleichrichtende) electrical synapses exist, and electrical

synapses can also be influenced by neuromodulators!

(for example, size difference between pre- and postsynaptic neuron

iinduces a rectifying property)

* Exchange of low molecular weight material through gap junctions (ions,

small dye molecules such as Lucifer yellow)

after Furspahn and Potter 1957, 1959

Electrical Synaptic Transmission at a Giant Synapse in the Crayfish CNS

Electrical synapse

Presynaptic lateral axon

Postsynaptic motor axon

B. Stimulation of presynaptic fiber

Each cell reaches threshold and fires an action potential!

A. Experimental setup

Where can electrical synapses be found?

* during development (all neuroblasts are electrically coupled)

* whenever speed is required (giant fibre systems in Crustaceans and Annelids,

(escape behaviour), or in vertebrates in the Ciliar ganglion,

eye muscles (rapid contractions).

* heart muscle fibres and muscle fibres of smooth muscleare connected via gap junctions

* most likely, electrical synapses exist in greater numbers in

the CNS than anticipated

(for example in the mammalian brain they may be involved insynchronizing neuron ensembles)

Axon

Dendrite

Dendrite

Electrical synapse

Types of contacts: Inferior olivary nucleus of the cat

Chemical synapse

Chemical synapses

* synaptic cleft, about 20 - 40 nm wide

* presynaptiv neuron releases transmitter via vesicles which diffuses

through the synaptic cleft to the postsynaptic cell where it binds to

specific receptor molecules and changes the state of (ion) conductivity

* amount of transmitter released is dependent on the membrane potential

of the presynaptic neuron

* chemical snapses are rectifying (gleichrichtend), and conduct only in

one direction with a time delay of about 1 ms

StructureStructure of of

neuromuscularneuromuscular synapsesynapseFrogFrog

Source: From Neuron to Brain

Martin Nicholls Wallace,

Sinauer, Sunderland, Mass., USA

Longitudinal section through a portion of neuromuscular junctionLongitudinal section through a portion of neuromuscular junction

PRESYNAPTICPRESYNAPTIC

POSTSYNAPTICPOSTSYNAPTIC

Quantal release shown first

from Katz and Miledi,1952 on

frog neuromuscular junction

Miniature endplate potentials of the frog neuromuscular synapse

Quantal nature of transmitter release(Katz und Miledi 1952)

miniature endplate potentials

(„miniatures“, mEPPs)

* mEPPs in unstimulated synapses (0,4 to 1 mV amplitude) can only be recorded in theimmediate vicinity of the end plate

* Estimates show a change in membrane potential of 0,3 µV as a consequence of a currentflow through one open ACh-channel. This means that for an endplate potential of 0,5 mVabout 5000 AChR have to be activated

* All EPSPs/IPSPs are manyfolds of a single mEPP (quantum)

* If the Calcium concentration of the presynapse changes the size of the quantum remainsconstant, however the probability of its release changes(if Ca-concentration is increased: failures decrease, and the probability of the simultaneous

release of two quanta increases)

* At the neuromuscular synapse one AP releases approx. 150 transmitter quanta, at centralsynapses between 1 and 10

Quantal nature of transmitter release

Synapses of the giant axons in lamprey (Neunauge)

unstimulated

60 min after stimulation

stimulated, 15 min at 20 Hz

Do depleted synaptic vesicles melt with the membrane ordo they pinch back after release into cytoplasm?

After vigorous stimulation synaptic membrane area is

increased!

Life cycle of synaptic vesicles

The vesicles fuse by interactions between proteins of the vesicle

membrane and the cell membrane

Interactions of vesicular membrane proteins and proteins of thepresynaptic cell membrane during the process of exocytosis

From Neuron to Brain, 4th edition, Nicholls,Martin, Wallace, Fuchs, Sinauer Associates, Sunderland, Mass., USA

SNARE Hypothesis

SNARE = named after SNAP receptor, first identified recepetor protein involved in exocytosis processSNARE = protein complex within active zone that is responsible for vesicle fusion with membrane and exocytosis

negative regulator

GTPase

* Two different receptor molecules:

* ionotropic receptors are ion channels with a binding sitefor the respective transmitter, and cause fast changes inthe membrane potential of the postsynaptic neuron (in therange of milliseconds)

* metabotropic receptors activate a signaling cascade in thepostsynaptic cell which leads to slow changes in theelectrical (and also biochemical) properties of thepostsynaptic cell (in the range of hundreds of milliseconds, or seconds or even longer). Formation of an (intracellular) second messenger

Whether a transmitters is excitatory or inhibitory dependssolely on the properties of the postsynaptic receptor molecules.

Ionotropic Receptor Metabotropic Receptor

e.g. nicotinic ACh-receptor e.g. muscarinic ACh-receptor

-pentamer of five subdomains (α,α,β, γ, δ) ACh binds to α subdomain

-all show a similar transmenbrane structure

αααα ααααSeven transmembrane proteinsthat activate other membrane associated proteins by

conformational change

Neurotransmitters

acetylcholine (neuromuscular synapse of vertebrates, autonomic nervous system)

Biogenic amineshistaminecatecholamines: noradrenaline (norepinephrine), adrenaline (epinehrine), dopamineoctopamine (invertebrates)serotonin (5-hydroxytryptamine, 5-HT)

Amino Acidsγγγγ-aminobutyric acid (GABA), glycine, aspartateglutamate (neuromuscular synapse of invertebrates, important transmitter of the vertebratebrain)

PeptidesFMRF-amide, Proctoline, Opioids, Enkephalins, Endorphins, Dynorphin (endogenous Opioids)

Peptides of Neurohypophysis: Vasopressin, Oxytocin,Neurophysins, NeurotensinTachykinines: Substance P, Insulins, Somatostatin, Polypeptides of pancreas, Gastrines: Gastrin, Cholecystokinin

Gaseous TransmittersNitric oxide (NO), Carbon monoxide (CO)

Synaptic plasticity:

Facilitation of transmitter release: Depression of transmitter release

e.g. Aplysia gill withdrawal reflex: homosynaptic depression leads to habituation and heterosynaptic fascilitation leads to sensitisation

Increase of postsynaptic response due to increase of Ca2+

in presynapse and therefore increased vesicle releaseDecrease of postsynaptic response due to decrease of Ca2+

in presynapse and therefore decreased vesicle release

muscle muscle, or

other targets (e.g. glands, neurons)

neurotransmitter(„classical“ neurotransmitter)

* ionotropic postsynaptic receptors

fast action (milliseconds)* metabotropic postsynaptic receptors

slow but lasting action (seconds to hours)

neuromodulator

* metabotropic postsynaptic

receptorsslow but lasting action

(minutes to hours to days to weeks)

* specific targeted release

neurohormone

* released into hemolymph

global, systemic release* metabotropic postsnaptic receptors

* long lasting effects: months to years to life

Neurotransmitter

Neuromodulator

Neurohormone

* „classical“ transmitter, released at synapses

(type I terminals), with fast postsynaptic response

(milliseconds), ionotropic receptor, opens ion channel

* transmitter, released at synapses with slow synapticresponse that lasts for longer time, metabotropic receptor,

signalling cascade, often co-transmitter, phosphorylation of

ion channels, (seconds to minutes to hours)

* modulator released from type II terminals (varicosities),

targeted release by special neurones, changing either

transmitter relase of other neurones or properties of postsynaptic neurones,

both pre- and postsynaptic metabotropic receptors,effects last a long time (minutes to hours to days to weeks)

phosphorylation of ion channels, other proteins, affecting

metabolic pathways, gene expression (learning, memory)

* transmitter released into circulatory system

(haemolymph, blood) by special neurosecretory cells,long lasting responses (months to years to life long)

metabotropic receptors or cytoplasmic receptors, controlgene expression and protein synthesis

One of the most important excitatory (ionotropic/ metabotropic)

receptor: NMDA – receptor in vertebrate brain

Recepors are named after their agonist(e.g. NMDA: N-Methyl-D-Aspartate)

Neuromodulation

“Modulator substance” is used for any compound of cellular or nonsynaptic origin that affects the excitability of nerve cells and

represents a normal link in the regulatory mechanisms that govern

the performance of the nervous system. Such modulator substances can affect the responsiveness of nerve cells to transsynaptic actions

of presynaptic neurones and they can alter the tendency to spontaneous activity“.

* an early definition by E. Florey (1967) Federation Proc. 26: 1164 – 1178

“Neuromodulation” occurs when a substance released from one neuron

alters the cellular or synaptic properties of another neuron“.

* Kupferman I (1979) Annu Rev Neurosci 2: 447-465 , Kacmarek LK and Levitan IB (1987) Neuromodulation..., Oxford University Press

“Any communication between neurons, caused by release of a chemical,

that is either not fast, or not point-to-point, or not simply excitation or inhibition

will be classified as neuromodulatory.“

* Katz P (1999) Beyond Neurotransmission...., Oxford University Press

Neuromodulation

“Modulator substance” is used for any compound of cellular or nonsynaptic origin that affects the excitability of nerve cells and

represents a normal link in the regulatory mechanisms that govern

the performance of the nervous system. Such modulator substances can affect the responsiveness of nerve cells to transsynaptic actions

of presynaptic neurones and they can alter the tendency to spontaneous activity“.

* an early definition by E. Florey (1967) Federation Proc. 26: 1164 – 1178

A good working definition(of a 2004 Dahlem conference on microcircuits)

Neuromodulation is the targeted release of a substance

from a neuron (or glial cell ?) that either alters the

efficacy of synaptic transmission, or the

cellular properties of a pre- and/or postsynaptic neuron(or glial cell) via metabotropic receptors.

Extrinsic neuromodulation

One form of intrinsic neuromodulation: Co-transmission

presynaptic neuron

postsynaptic neuron

neuromodulatory neuron

allows independent state definition (independent controller)

* separate control of neuromodulator release

automatic state definition (automatic controller)

* frequency and time dependent neuromodulator release

compartment of release

synapse

BEHAVIOR: selection and induction of behavior (neuron ensembles, systems of networks)

Neuromodulators, Neurohormones

BIOMECHANICS/MUSCULATURE: execution of behavior (effector organs)Neuromodulators, Neurohormones

NEURONAL NETWORKS: rhythm, pattern generation, reconfiguration of networks

(affects timing, amplitude, phase), Neuromodulators (change synaptic gain,

electrical properties)

SINGLE NEURONS (elecrical properties)Neuromodulators (ionic currents)

SIGNALLING CASCADES (regulation of electrical

and biochemical properties incl. energy metabolism)

Neuromodulators

GENES AND PROTEIN BIOSYNTHESIS

(long term changes), Neuromodulators,

Neurohormones

Levels of Actions of Neuromodulators

Fin

Kandel ER (2001) Science 294, 1030-1038

Habituation

KiemenKiemen--RRüückziehreflex in ckziehreflex in

AplysiaAplysia californicacalifornica

HabituationHabituation

Habituation

200 ms

5 mV

10 mV

Habituation / Adaptation

Habituation

ist die Abnahme der Reaktionsstärke auf einen

wiederholt einwirkenden Reiz.

Adaptation

ist die Abnahme der Reaktionsstärke auf einen

lang andauernden Reiz

Quantal hypothesis of transmitter release (Fatt und Katz 1952)

* Each endplate potential, EPP, consists of a certain number of quanta

(a normal EPP has approximately 200 quanta), quantal content of an EPSP* reduction of quantal content by using solutions reduced in Ca2+ (less vesicles fuse)

* Del Castillo and Katz (1954): statistical analysis

* each motor terminal contains n quantal packages ACh, each of which is released

by the probability p.* If many eyperiments are performed, the mean quantal value released in each trial

is m and equals n p, and the number of events with 0,1,2,3,...x quanta would

correspond to a binomial distribution.

* If p is very small then the

number x of quanta should correspond to a Poisson-distribution:

nx = N (mx/x!) e –m

Problem: n and p are unknown and are not measurable, so the idea was to reduce the quantal release

by changing the extrcellular Ca2+ concentration

Tsacopoulos and Magistretti, J. Neuroscience 16:877-885, 1996

Blutkapillare

Astrozyt

Neuropil:

Geflecht aus Dendriten und Axonen

Tsacopoulos and Magistretti, J. Neuroscience 16:877-885, 1996