Synapses,
neurotransmitters and
neuromodulators
Lecture series
Model Systems in Neurobiology: From Molecules to Behaviour
Wintersemester 2007/2008
Outline
Synapses:
Electrical synapses
Chemical synapses
Neurotransmitters:
Underlying mechanisms of signal
transduction for electrial/ chemical synapses
Neuromodulators:
What is neuromodulation?
Levels of action
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 physiology
Sir 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
Types of contacts: Inferior olivary nucleus of the cat
Dendrite
Axon
Chemical synapse
Electrical synapse
Dendrite
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)
gap junctions = electrical synapses
Side view
Molecules of electrical synapses
Connexons, Connexins
(Innexins in invertebrate animals!)
current flow
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)
Electrical Synaptic Transmission at a Giant Synapse in the Crayfish CNS
B. Stimulation of presynaptic fiber
A. Experimental setup
Presynaptic lateral axon
Electrical synapse
Postsynaptic
m otor axon
Each cell reaches threshold and fires an action potential!
after Furspahn and Potter 1957, 1959
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 muscle
are 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 in
synchronizing neuron ensembles)
Types of contacts: Inferior olivary nucleus of the cat
Dendrite
Axon
Chemical synapse
Electrical synapse
Dendrite
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
Structure of
neuromuscular synapse
Longitudinal section through a portion of neuromuscular junction
PRESYNAPTIC
Source: From Neuron to Brain
Martin Nicholls Wallace,
Sinauer, Sunderland, Mass., USA
POSTSYNAPTIC
Frog
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)
Quantal nature of transmitter release
* mEPPs in unstimulated synapses (0,4 to 1 mV amplitude) can only be recorded in the
immediate vicinity of the end plate
* Estimates show a change in membrane potential of 0,3 µV as a consequence of a current
flow through one open ACh-channel. This means that for an endplate potential of 0,5 mV
about 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 remains
constant, 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 central
synapses between 1 and 10
Synapses of the giant axons in lamprey (Neunauge)
unstimulated
stimulated, 15 min at 20 Hz
Do depleted synaptic vesicles melt with the membrane or
do they pinch back after release into cytoplasm?
After vigorous stimulation synaptic membrane area is
increased!
60 min after stimulation
Life cycle of synaptic vesicles
The vesicles fuse by interactions between proteins of the vesicle
membrane and the cell membrane
SNARE Hypothesis
Interactions of vesicular membrane proteins and proteins of the
presynaptic cell membrane during the process of exocytosis
negative regulator
GTPase
SNARE = named after SNAP receptor, first identified recepetor protein involved in exocytosis process
SNARE = protein complex within active zone that is responsible for vesicle fusion with membrane and exocytosis
From Neuron to Brain, 4th edition, Nicholls,M artin, Wallace, Fuchs, Sinauer Associates, Sunderland, M ass., USA
Whether a transmitters is excitatory or inhibitory depends
solely on the properties of the postsynaptic receptor molecules.
* Two different receptor molecules:
* ionotropic receptors are ion channels with a binding site
for the respective transmitter, and cause fast changes in
the membrane potential of the postsynaptic neuron (in the
range of milliseconds)
* metabotropic receptors activate a signaling cascade in the
postsynaptic cell which leads to slow changes in the
electrical (and also biochemical) properties of the
postsynaptic cell (in the range of hundreds of
milliseconds, or seconds or even longer).
Formation of an (intracellular) second messenger
Ionotropic Receptor
α
Metabotropic Receptor
α
-pentamer of five subdomains (α,α,β, γ, δ) ACh binds to α subdomain
-all show a similar transmenbrane structure
e.g. nicotinic ACh-receptor
Seven transmembrane proteins
that activate other membrane associated proteins by
conformational change
e.g. muscarinic ACh-receptor
Neurotransmitters
acetylcholine (neuromuscular synapse of vertebrates, autonomic nervous system)
Biogenic amines
histamine
catecholamines: noradrenaline (norepinephrine), adrenaline (epinehrine), dopamine
octopamine (invertebrates)
serotonin (5-hydroxytryptamine, 5-HT)
Amino Acids
γ -aminobutyric acid (GABA), glycine, aspartate
glutamate (neuromuscular synapse of invertebrates, important transmitter of the vertebrate
brain)
Peptides
FMRF-amide, Proctoline,
Opioids, Enkephalins, Endorphins, Dynorphin (endogenous Opioids)
Peptides of Neurohypophysis: Vasopressin, Oxytocin,
Neurophysins, Neurotensin
Tachykinines: Substance P,
Insulins, Somatostatin, Polypeptides of pancreas, Gastrines: Gastrin, Cholecystokinin
Gaseous Transmitters
Nitric oxide (NO), Carbon monoxide (CO)
Synaptic plasticity:
Facilitation of transmitter release:
Increase of postsynaptic response due to increase of Ca2+
in presynapse and therefore increased vesicle release
Depression of transmitter release
Decrease of postsynaptic response due to decrease of Ca2+
in presynapse and therefore decreased vesicle release
e.g. Aplysia gill withdrawal reflex: homosynaptic depression leads to habituation and
heterosynaptic fascilitation leads to sensitisation
neurotransmitter
(„classical“ neurotransmitter)
* ionotropic postsynaptic receptors
fast action (milliseconds)
* metabotropic postsynaptic receptors
slow but lasting action (seconds to hours)
neuromodulator
* metabotropic postsynaptic
receptors
slow but lasting action
(minutes to hours to days to
weeks)
* specific targeted release
muscle
muscle, or
other targets (e.g. glands, neurons)
neurohormone
* released into hemolymph
global, systemic release
* metabotropic postsnaptic receptors
* long lasting effects: months to years to life
Neurotransmitter
* „classical“ transmitter, released at synapses
(type I terminals), with fast postsynaptic response
(milliseconds), ionotropic receptor, opens ion channel
* transmitter, released at synapses with slow synaptic
response that lasts for longer time, metabotropic receptor,
signalling cascade, often co-transmitter, phosphorylation of
ion channels, (seconds to minutes to hours)
Neuromodulator
Neurohormone
* 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, control
gene 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
allows independent state definition (independent controller)
* separate control of neuromodulator release
presynaptic neuron
compartment of release
neuromodulatory neuron
One form of intrinsic neuromodulation: Co-transmission
automatic state definition (automatic controller)
* frequency and time dependent neuromodulator release
postsynaptic neuron
synapse
Levels of Actions of Neuromodulators
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
Fin
Habituation
Kiemen-Rückziehreflex in
Aplysia californica
Kandel ER (2001) Science 294, 1030-1038
Habituation
Habituation
5 mV
10 mV
200 ms
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.
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
* If p is very small then the
number x of quanta should correspond to a Poisson-distribution:
nx = N (mx/x!) e –m
Neuropil:
Geflecht aus Dendriten
und Axonen
Astrozyt
Blutkapillare
Tsacopoulos and Magistretti, J. Neuroscience 16:877-885, 1996
Tsacopoulos and Magistretti, J. Neuroscience 16:877-885, 1996