neurotransmission Flashcards
1
Q
chemical synapse
A
50 nm junction
Transmission speed 1-5 ms
Release of neurotransmitters
Excitatory or inhibitory
2
Q
electrical synapse
A
Close 3-5 nm
Joined by gap junction proteins
Fast response nearly no delay
3
Q
neuromodulators released from
A
neurosecretory terminals of modulatory neutrons
or conventional presynaptic terminals
4
Q
neuromodulators effect
A
alter the quality of information passing through a synapse
or the spontaneous activity of a population of post-synaptic neuron
5
Q
stats of neurons in brain
A
10^12 neurons
each with 1,000 synapses
–> 10^15 synapses in brain
10^15 glial cells ( capable of modulating aspects of neuronal functioning and synaptic transmission )
6
Q
tripartie synapses
A
refers to the functional integration and physical proximity of the
presynaptic membrane, postsynaptic membrane, and their intimate association with surrounding glia
as well as the combined contributions of these three synaptic components to the production of activity at the chemical synapse.
dynamic two way relationship between glial cells and neurons
7
Q
gliotransmitters
A
glutamate
adenosine
ATP
–> can modulate synaptic transmission by acting on neurons
8
Q
the synapse of Held
A
giant synapse surrounding post synaptic cell in auditory system
9
Q
ribbon synapses
A
spontaneous release
10
Q
synapses on different parts
A
affect functional role
11
Q
synapses on different parts
A
affect functional role eg dendrites, cell bodies, axons
12
Q
gap junctions
A
hemi channels of protein connexin in both pre and postsynaptic membrane –> aligned to form channels along which ions can flow from one cell to another
13
Q
connexin protein
A
tetra membrane spanning protein with cystine extracellular residues –> important for docking both halves of the hemi-channel
14
Q
three types of gap junctions
A
homomeric/homotypic: two identical hemichannels
heteromeric: more than one connexin isoform
heterotypic : two different types of hemichannels
each connexion: 6 connexion protein subunits
15
Q
Electrical synapses common features
A
- Direct coupling via gap junctions (connexins) (Invertebrates: Innexins and Pannexins)
- Bidirectional: Transmission in both directions (but examples of rectifying transmission in one direction only)
- Especially common among between rapidly firing interneurons in the neocortex
- Synchronize electrical activity between cells
not amplifable
no adaptation
16
Q
mixed synapses
A
chemical and electrical components of transmission
complex time dependent synaptic signalling, with the appearance of electrical excitation at a synapse when chemical inhibition for instance becomes fatigued. Neuromodulators can alter both chemical and electrical transmission
eg Mauthner neuron
17
Q
hetero-synaptic interactions
A
same postsynaptic cell
18
Q
Excitatory transmitters
A
increase time channel spends in open state
19
Q
chemical synapses , how it works basic
A
presynaptic action potential
calcium influx into presynaptic terminal
fusion of vesicles with synaptic membrane
transmitter release and diffusion across cleft
transmitter binds to receptors
postsynaptic response ( EPSP or IPSP)
20
Q
EPSP
A
excitatory postsynaptic potentials
–> if big enough opens voltage gated sensitive ion channels in postsynaptic membrane —> these excite postsynaptic cell
21
Q
IPSP
A
inhibitory postsynaptic potential
–> hyperpolarise postsynaptic membrane , less excitable
22
Q
Motor neurons
A
Katz at neuromuscular junction of frog
acetylcholine released and binds to receptors
ion channels open –> Na + influx
end plate potential ( like EPSP but at muscle )
TTX blocks opening of sodium channels –> no depolarisation that causes a.p –> graded response
23
Q
End Plate Potential
A
more than one ion involved
as current-voltage plots show that end plate potentials have a reversal potential of 0 mV
–> as no ion has a rp of 0 more than one have to be involved –> opening of non specific cation channels
–> end plate potential affected by extracellular sodium potassium calcium levels
24
Q
end plate potential decay
A
consistent with the rate of the time constant of the muscle fibre membrane +
muscle fibre cable properties predicted the amplitude with distance to plate
–> brief surge of inflowing current with passive propagation
–>? end plate potential declines in size as you move away from the junction
25
GPCRs
slower transmission
| indirectly modify the action of ion channels via G-proteins or second messenger pathways ( eg Calcium, Cyclic AMP )
26
mepps : miniature end plate potentials
in neuromuscular junction:
discrete spontaneous changes in membrane potential of around 0.5-0.8mV
--> evidence for vesicle hypothesis as when plotting the size of mepps against their frequency then they cluster at multiples of their initial size
27
what are synapses good for
enhances brain power through synaptic connection plasticity --> varying number of physical synapses between cells
varying strength of synaptic connections
functional networks formed through neuromodulation
28
neuromodulation
physiological process by which a given neuron uses one or more chemicals to regulate diverse populations of neurons
neuromodulators diffuse through neural tissue to affect slow-acting receptors of many neurons.
29
3 largest classes of transmitters
amino acids ( glutamate, Gabba, glycine )
biogenic amines ( acetylcholine, noradrenaline, dopamine, serotonine, histamine )
neuropeptides ( 80-100 and increasing )
30
purines
ATP, adenosine
31
acetylcholine synthesised from
( precursor ) AcetylCoA by the enzyme Choline Acetyltransferase ( CAT )
32
small neurotransmitters
are synthesised in terminals by precursors
are transported into vesicles in terminal by specific transporter molecules using energy of the proton gradient set up by the actions of the ATP driven proton pump
33
Neuropeptides synthesised
in cell bodies from larger protein precursor molecules ( DNA )
34
protein precursor molecules synthesised
on ribosomes attached to the endoplasmic reticulum
35
from the endoplasmic reticulum protein precursor molecules are
passed to the vesicular stacks of the Golgi apparatus --> mature there +
modified through sulphating and phosphorylation
packaged to vesicles from membranes of Golgi stacks + transported to release site by axonal transport
36
How are vesicles released ?
calcium sensitive mechanism at specific active zones on the nerve terminal
37
post synaptic densities
array of receptors and effector proteins held in place by scaffolding proteins ( eg gephryn)
38
in cell calcium channels localised
near the release sites in the active zones
39
vesicle release function RIM proteins
linking the calcium channels to the synaptic vesicles via Rabbles proteins
40
vesicle fusion
vesicular Synaptobrevin zips together with terminal SNAREs ( SNAP-25 + Syntaxin ) --> energy of the process brings together the vesicular and presynaptic membranes
calcium binding to synaptotagmin causes the complete zipping of the SNAREs and therefore the fusion of the membranes to form a fusion pore
41
terminal SNAREs
SNAP-25 + Syntaxin
42
calcium in terminal binds to
Synaptotagmin ( --> allows complete zipping of SNAREs)
43
fusion pore allows
neurotransmitter to diffuse into the synaptic cleft
44
kiss and run
partial release of vesicle content
vesicles pinch off after exocytosis without merging with the plasma membrane
45
vesicle fusion allows
complete release of vesicle content
46
what prevents the terminal from getting too big + recycling specific vesicular proteins
Cathrin-dependent mechanisms for membrane recycling
47
after exocytosis
vesicles flatten into the plasma membrane + components recycled via Cathrin -mediated endocytosis + formation of new vesicles
48
metabotropic: transmitter binding
and final effector different proteins
49
excitatory actions
ligand gated ion channels increasing conductance for:
sodium
calcium
decreasing conductance for:
potassium
50
inhibitory actions
ion channels :
increasing conductance for chloride
potassium
51
Goldman-Hodkin-Katz equation
lets calculate the reversal potential of the ion channel
52
reversal potential of ion channel
determines whether an EPSP or IPSP is induced
53
cys-loop family of receptors
five subunits pseudosymetrically arranged forming a rosette with a central ion-conducting pore
some cation selective some anion selective
54
cation selective ion channels
nACh and 5-HT3
55
anion selective
GABA a and Glycine
56
receptor containing
extracellular domain containing ligand binding sites
transmembrane domain that allows ions to pass across the membrane
intracellular domain that plays a role in channel conductance and receptor modulation
57
metabotropic receptors
7-transmembrane spanning g-protein coupled receptors
kinase-linked receptors
58
kinase linked receptors
act directly as enzyme effectors
59
GPCR activations
direct: g-protein activates ion channels
indirect: ion channels activated through second messenger pathways
60
cAMP signalling
Neurotransmitter docks on receptor
alpha g-protein activated
activity of enzyme adenylyl cyclase ( AC ) regulated
AC makes second messenger cAMP from ATP
cAMP activates protein kinase A ( PKA ) which can phosphorylate --> change activity of numerous proteins eg ion channels
61
modulation of AC by GPCRs
can be negative as well as positive
62
ca2+/ IP3 pathway
activated alpha g-protein activates phospholipase C which converts PIP2 into IP3 and DAG
IP3 acts on the IP3 gated receptors of the Endoplasmic reticulum . The opened channel allows for Calcium to flow into the cell
DAG activates Protein Kinase C ( PKC ) which phosphorylates targets eg membrane opine channels
63
intracellular Calcium
controls huge number of key cellular events
from exocytosis
to gene expression
64
3 main factors influencing transmitter life-time in cleft
Diffusion ( all synapses )
uptake ( most synapses eg GABA glutamate
enzymatic breakdown ( esp for cholinergic and peptidergic )
65
transmitter removal determines
response duration and frequency of postsynaptic response
66
ACh synapse neuromuscular junction
hard but brief postsynaptic hit
due to lots of transmitter vesicle released by each presynaptic AP
rapid breakdown by ACh esterase ( cholinesterase )
--> high fidelity transmission by a wide range of frequencies
67
Transmitter removal of most central synapses:
transmitter removed by diffusion and active uptake into presynaptic terminals or surrounding glial cell processes
fewer vesicles released
--> probabilistic transmission with less spill over
68
central synapses drug targets
uptake transporters as important drug targets
```
eg
function of cocaine blocking dopamine reuptake
```
SSRIs : specific serotonin reuptake inhibitors
69
synaptic cleft functionality
has structural organisation, Is not just an empty gap
nanocolumm organisation --> possibly by diffusible signals crossing synaptic cleft or interactions of pre and post synaptic proteins that project into cleft and interact
70
nanocolumn organisation
location of presynaptic active release sites may be coordinated by the RIM proteins to bring them into register with elements of the post-synaptic scaffold proteins ( eg PSD-95) --> organise location of various postsynaptic response elements eg neurotransmitter receptors
71
GPCRs
| anatomy
7 transmembrane spanning G-protein coupled receptors
variable sequence extracellular N-terminals ( amino )
3 extracellular loops influencing agonist binding
variable intracellular C-terminals ( carboxy )
3 intracellular loops which influence G-protein binding
+ scaffolding protein + other intracellular effectors
72
agonist binding influences
| ( GPCRs)
conformational change in shape of the receptor
--> including a change in relative position of TMs 3 and 6
--> change if the conformation of the intracellular portions of the receptor
--> can now interact and activate with g-protein
73
g-proteins
bind guanine nucleotides
trimers of three subunits : alpha beta gamma
74
metabotropic receptors (change)
indirect transmission:
not directly inhibiting or exciting
interact with other membrane proteins -->
changes in ion channel activity
changes in metabolic processes within the neuron
amplifiable at a number of different points in the signalling cascade
75
muscarinic receptors
binding of ACh activates βγ subunit which binds directly to and opens potassium channels
cell hyperpolarises with an outward potassium current
76
adrenergic receptors
GPCRs activated by norepinephrine
77
what mediates appropriate binding of g-protein
second and third cytoplasmic loops , amino terminal region of intracellular tail
78
G-proteins grouped into three classes
according to structure and target of their α-subunit
Gs
Gq
Gi
79
Gs ( effect)
stimulates adenylyl cyclase
--> more cyclic AMP
--> more protein kinase
80
Gi
inhibits adenylate cyclase
- -> less CAMP
- -> more Potassium channels open
- -> inhibition
includes
Gt ( transducin )
activates cGMP
Go : interacts with calcium ion channels
81
Gq
couples to enzyme phospholipase C ( PLC )
--> more of it --> ITP3
--> protein kinase C
82
activated g-protein
GDP displaced by GTP from α-subunit of the G-protein
---> dissociation of α-subunit from βγ-subunit complex
Free α-subunit and βγ-subunit diffuse and bind to target proteins: modulatory effects
--> GTP hydrolysed to GDP on α-subunit by endogenous GTPase activity
--> G-protein re-aggregates , activity terminated
83
resting state g-protein
GDP bound to the α-subunit and the three subunits associated as a trimer
84
modulation of ion channels by g-proteins
direct interaction: βγ-subunit
indirect interaction: α-subunit through activating one or more enzymes that activate second messengers which interact with ion channels
85
divergence transmitters (GPCRs )
when a transmitter interacts with a number of different receptor subtypes this results in divergening signals as each
interacts with a range of different effectors systems
86
convergence
each family of receptors contains several members however, C-terminus is indistinguishable
--> different transmitters thus act through the same effector systems although using different receptors
87
non-olfactory GPCRs 5 families
based on agonist and subtle structural differences
Rhodopsin family
metabotropic glutamate ( / GABA ) receptors
secretin/calcitonin-like receptors
smoothened/frizzzled-like receptors
adhesion receptors
-->different members of families characterised by different agonist binding sites and different length of N-terminals, C-terminals and third intracellular loops
88
β-2 adrenergic receptor
best understood GPCR
inactive crystal structure resolved :
fusion protein bound to its inverse agonist ( carazolol) + to T4 lysozyme to stabilise floppy intracellular loops
89
Ensemble theory
receptors are continuously oscillating between a number of different configurations each with different signalling properties
crystalline structures thus represent a snap shot of the receptor in a particular configuration
--> ligands determine the length of time a receptors spends in a configuration
90
GPCR crystalline structures--> why useful
diversity of agonist binding sites --> how ligands access those
understand the structural changes involved in receptor signalling : through accumulating info about inactive, agonist bound/ antagonist bound / agonist + g-protein bound structures for indv receptors
identifying dimer forming interfaces : for homo and heterodimeric GPCR forms
identifying action sites of allosteric modulators flexibility : important for internal water pathway
development of new drugs : by finding unknown molecular structures that can act as agonists or antagonists
91
negatives of GPCR crystalline structures
need to know influence of various proteins needed for crystallisation
92
Future technologies GPCR
• Single-particle negative-stain electron phase-plate cryo-microscopy – 3D structures at almost atomic level and protein-protein interactions – no modifications required
• Hydrogen-deuterium exchange mass spectrometry – dynamic changes
• NMR spectroscopy (Rotationally aligned solid-state NMR) of human chemokine receptor 1
(CXCR1) 2012 – real time dynamic changes
93
protein dimer
two monomer proteins ( single proteins ) that are non-covertly bound to form a complex
94
homodimer
A protein homodimer is formed by two identical proteins
95
heterodimer
a protein heterodimer is formed by two different proteins
96
allosteric
changing the activity of a protein by binding an effector ( conformational change )
97
GABA B heterodimers
needs to be formed between GABA B1 and GABA B2 subforms before signalling can take place
GABA B2 needed to get GABA B1 receptor to the cell surface --> dimers probably made in ER and transported to plasma membrane
98
GABA B activation
GABA B1: binds GABA to Venus fly trap region of N-terminus -->
conformational change --> passed allosterically to GABA B2
GABA B2: binds to g-proteins --> activates signal
99
Biased agonism
agonist specific coupling
different agonist induce different receptor conformation each with own distinct repertoire of signalling capabilities to second messenger systems
some compounds can be agonists for one second messenger pathway and antagonists at another through the same receptor
single G-protein coupled receptors --> different pharmacological profiles in different cell types in the same animal --> depending on their local G-protein environment.
100
biased agonism potential benefits
reduced side effects of drugs as agonist couple the receptor only to beneficial second messenger pathways and not to those responsible for the production of unwanted side effects
101
homologous desensitization
phosphorylation of g-protein coupled receptor specific kinase of agonist activated receptor --> β-arrestin binding
102
heterologous desensitization
PKA activated receptor phosphorylation
--> potentially leads to switching between different second messenger pathways
103
causes of receptor desensitisation 1
phosphorylation of sites on the third cytoplasm loop and the carboxy terminus by second-messenger related kinases ( eg cAMP dependent protein kinase )
104
Not Dale's principle of one neuron one transmitter but rather common that
one fast acting neurotransmitter is released with
- slower modulatory transmitter
- ATP
- Protons
eg NPY + GABA
105
3 criteria for transmiter system:
1. Substance present in specific neurons together with enzymes that make it, and stored in vesicles in nerve terminals.
2. Substance released in a Ca2+-dependent way upon depolarization of axon terminal.
3. Substance has receptors at the synapse and application of substance reproduces (at least partly) the effect of synaptic stimulation. (Note: Co-release means other substances can contribute to synaptic effects.)
106
invertebrate nervous systems transmitter system identification
easier
techniques:
large identifiable neurons accessed + dissected + biochemical experiments
107
mammalian brain transmitter system identification
harder to identify imdv neurons --> improved through laser techniques
techniques:
immunochemistry
in situ hybridisation --> identify neurotranmitter synthetic enzymes / neuropeptide precursors
induce neurotransmitter release through electrical stimulation / K+ depolarisation
( unsure about exact neuron of release, or is it glial cell? )
iontophoretically apply transmitters to cells in brain slices
- never sure how many different sites they are acting on and whether you are recording from identical neuronal types in different preparations
108
glutamate synthesis
from α-ketoglutarate and from glutamine
in presynaptic nerve terminals.
from glutamine made in astrocytic processes from glutamate taken up from synaptic clefts.
109
Glutamate Receptors
ionotopic: AMPA, Kainite, NMDA
metabotopic: mGluR ( 8 different )
110
AMPA receptors
fast transient excitatory transmissions
agonist binding ( glutamate) opens pore
permeable to Potassium and Sodium --> depolarisation
underlie synaptic plasticity
111
NMDA
act slower
coincidence detectors requiring depolarisation
magnesium blocks channel at resting potential due to transmembrane electrical gradient
once post-synaptic cell depolarised to 0, magnesium ion dissociates
--> NMDA receptor permeable to calcium potassium and sodium
glutamate increases Calcium permeability --> induces Long Term Potentiation ( LTP )
112
ionotropic glutamate receptors
tetramer of different subunits encoded by separate genes ( usu. dimers of dimers )
non-selective cation channels
differential distribution of subunits in the brain
eceptors with different pharmacology, ionic conductance, desensitization kinetics, calcium permeability and cell surface expression properties
113
Kainate receptors
non-NMDA LTP
114
Group II mGluR
mGluR 2, 3
Inhibition of adenylyl cylcase
Activation of K+ channels
presynaptically: Inhibition of Ca++ channels --> negative feedback of neurotransmitter release
115
mGluR1
predominantly postsynaptically
Phospholipase C stimulation
Stimulation of adenylyl cyclase
(some systems)
increase in Ca2+ concentration in cytoplasm
activate Gq --> produce inhibition of potassium channels ( M& Calcium activated)
116
Group I
mGluR1 mGluR5
117
Gaba A receptors
members of the Cys-Loop family of ionotropic receptors
pentameric arrangement of 7 different subunit ( 10^ 5 different versions )
gating central chloride channel
GABA binds to the β subunit
all forms contain the γ2 subunit which makes them sensitive to benzodiazepine allosteric modulators.
during neuronal maturation change from excitatory to inhibitory receptors
118
maturation change from excitatory to inhibitory receptors
early development:
NKCC transporter --> high intracellular chloride concentration --> m.p negative to equilibrium potential of chloride
receptor opened --> anions flow out of cell --> depolarisation
119
maturation change from excitatory to inhibitory receptors
early development:
NKCC transporter --> high intracellular chloride concentration --> m.p negative to equilibrium potential of chloride
receptor opened --> anions flow out of cell --> depolarisation
after maturation:
expression of K+/CL- co-transporter increases --> reduction of intracellular chloride concentration
receptor opened --> Cl- moves into cell ( as Cl- concentration lower than equilibrium potential )
--> outward current : hyper polarisation
120
maturation change from excitatory to inhibitory receptors
early development:
NKCC transporter --> high intracellular chloride concentration --> m.p negative to equilibrium potential of chloride
receptor opened --> anions flow out of cell --> depolarisation
after maturation:
expression of K+/CL- co-transporter increases --> reduction of intracellular chloride concentration
receptor opened --> Cl- moves into cell ( as Cl- concentration lower than equilibrium potential )
--> outward current : hyper polarisation
121
GABA inactivation
reuptake into presynaptic terminals or glial cells
metabolised to glutamine
transported back to neuron
122
GABA inactivation
reuptake into presynaptic terminals or glial cells
metabolised to glutamine
transported back to neuron
123
neuropeptides
most numerous transmitters
play specific and critical roles in regulation of brain state and behaviour
124
orexin
made by small group of hypothalamic neurons , project everywhere but cerebellum
essential for sustained wakefulness and consciousness --> loss narcolepsy
regulate reward and stress
125
noradrenaline
sonata in Locus Coeruleus
innervate: widespread regions of brain and spinal chord
regulates : attention, arousal, sleep/wake
126
Dopamine
somata in substantia nigra, ventral tegumental area and arcuate nucleus
regulates:
voluntary movement
reward, addiction
127
serotonin ( 5-HT )
somata in raphe nuclei
innervate:
caudal --> spinal chord
rostral: --> nearly all regions of brain
regulates: sleep-wake, mood, perception
128
Histamine
somata: tuberomammillary nucleus
regulates: arousal, energy metabolism , general effects
129
active energy currency transferred from glial cells to neurons
lactate
| not glucose
130
traditional roles of glial cells
take up glucose and pass to neuron ( however, pass lactate )
spatial buffering of potassium ions released from active neurons --> prevent building up of potassium ions , interfering with neuronal action potential generation
131
NG2+ glial precursor cells
can show Ca2+ dependent action potentials --> potentially to locally coordinate the actions of neurons
not clear if electric property is generalisable
release fibroblast growth factor 2 (FGF2) which supports glutamatrgic signalling
--> without: changes in neuronal activity that contribute to depression
132
examples of dynamic interaction of glial cells and neurons
astrocyte control of synaptic NMDA receptors can contribute to the progressive development of temporal lobe epilepsy
astrocyte activation can modulate the response selectivity of visual cortical neurones
133
glial and neuron interaction hippocampus during LTP
D-Serine --> co-agonist for NMDA
elease of both glutamate and D-Serine from small vesicles in glial cells
( co-localised in same vesicles )
buffering intracellular changes in Ca2+ in glial cells or blocking D-Serine synthesis
-->
reduce the release of D-Serine from glial cells surrounding hippocampal cell synapses
--> reduction of NMDA receptor induced LTP
glial cells are able to modulate synaptic properties by the release of “gliotransmitters”.
134
release of a wide variety of “gliotransmitters” from Schwann cells at the NMJ
Acetylcholine, ATP, Glutamate, Adenosine, Substance P and Calcitonin Gene Related Peptide
135
Denervated NMJs
Schwamm cells replace degenerating neuronal nerve terminal
--> memps due to spontaneously released acetylcholine
--> perhaps glial cells to keep neuromuscular architecture intact whilst axon regenerates , re-establishes NMJ
