1 bi 1 lecture 9 thursday, april 13, 2006 synapses: biophysical machines reminder: henry lester’s...
Post on 20-Jan-2016
215 Views
Preview:
TRANSCRIPT
1
Bi 1 Lecture 9
Thursday, April 13, 2006
Synapses: Biophysical Machines
Reminder: Henry Lester’s “office” hoursMon, Thurs 1-1:30 PM, outside the Red Door
2
Video on synaptic transmission
3
Narrated by
Leonard Nemoy
4
Cerebral cortexAxonsDendrites SynapsesSynaptic vesicles (volleyballs), mitochondria (watermelons)
1. Movements do not occur in 1 ms; vesicles are already “docked”.
2. Neurotransmitter diffuses across the synaptic cleft in a few s
3. There is little space between neurons
4. “Slight” errors cause mental illness? Unproven!(But single-codon mutations do cause some neuroscience diseases)
5Nestler Figure 2-2(rotated)
Parts of two neurons
synaptic cleft
direction of information flow
dendrites
Excitatoryterminal
cell body
nucleus
axon
presynaptic terminal postsynaptic
dendrite
Inhibitoryterminal presynaptic
terminal
neuronPresynaptic
neuronPostsynaptic
Greek, “tree”
Greek, “axis”
from Lecture 4
6
presynaptic neuron postsynaptic neuron
The synapse is a point of information processing
An adult human brain contains ~ 1011 neurons,
and each of these might receive 103 synapses apiece,
for a total of 1014 synapses.
Most of these synapses form during the first 2 yr of life.
Thus 1014synapses/108 s = 106 synapses/s form in a fetus and infant!
Nestler Box 2-3 Figure A
Greek, “connection, junction”
from Lecture 4
7
cytosol
receptor
cytosolsynaptic cleft
receptor
receptor
Chemistry is a language of the nervous system, for instance at synapses
presynaptic terminal
postsynaptic dendrite
direction of information flow
from Lecture 4
transmitter moleculesin synaptic vesicles
8
9
Electron microscope image (“micrograph”) of a portion of a synapse
synaptic vesicles
presynaptic terminal
postsynaptic dendrite
It takes 3 different pump / transporter proteins to position the transmitter within the vesicles
10
Na+-coupled cell membrane neurotransmitter transporters:
Antidepressants (“SSRIs” = serotonin-selectivereuptake inhibitors):Prozac, Zoloft, Paxil, Celexa, Luvox
Drugs of abuse: MDMA
Attention-deficit disorder medications:
Ritalin, Dexedrine, Adderall,Strattera (?)
Drugs of abuse: cocaine amphetamine
Na+-coupledcell membrane serotonintransporter
Na+-coupledcell membrane dopamine transporter
NH
HO NH3+
HO
HO
H2C
CH2
NH3+
cytosol
outside
major targets for drugs of therapy and abuse
Presynapticterminals
From Lecture 5
Trademarks:
11
from Lecture 5:
Could cells utilize plasma membrane H+ fluxes?
Probably not.
There are not enough protons to make a bulk flow, required for robustly
maintaining the ion concentration gradients.
(but some very small organelles (~ 0.1 m) do indeed store energy as H+ gradients).
12
NeurotransmitterandATP
(1,000 to 10,000 molecules of each)
3. Proton-coupled neurotransmitter transporter
cytosol
2. ATP-driven proton pump
H+
cytosol
cytosol
H +
transmitter
~ isotonic!
How synaptic vesicles fill from the cytosolvesicle interior
vesicle interior
13
Neurotransmitter
and
ATP
kinesin
cell body presynapticterminal
~ 20 distinct proteins
vesicle transport;pumping protons;pumping neurotransmitter; docking;fusion;recycling.
cytosol
50 nm
Lecture 10
Synaptic vesicles have many proteins
14
NeurotransmitterandATP
(1,000 to 10,000 molecules of each)
proton-coupled vesicularneurotransmitter transporter
cytosol
ATP-driven proton pump
pH effects also account for some drug actions on synaptic vesicles
HN
CH3O
H2CO CH3
3,4-methylenedioxymethamphetamine(MDMA, “ecstasy”, XTC) pKa ~ 8.5
15
proton-coupledvesicular serotonin transporter cytosol
ATP-driven proton pump
synaptic cleftNa+-coupledcell membrane
serotonin transporterMDMA
serotonin vesicle
MDMA MDMA-H+
H+
MDMA (“ecstasy”) dissipates the vesicle’s H+ store, preventing the vesicle from pumping serotonin
serotonin
depletedserotonin vesicle
MDMA-H+
Weak acids and weak bases short-circuit many vesicles!
16
These proteins have evolved in a natural—perhaps necessary--way to provide that
• The resting potential arises via selective permeability to K+
This selective permeability also leads to the Nernst potential. Transient breakdowns in membrane potential are used as nerve signals.
• Neuronal and non-neuronal cells also signal via transient influxes of Na+ and Ca2+.
3 classes of proteins that transport ions across membranes:
Little Alberts 12-4© Garland
Ion channels that flux many ions per event
Ion-coupled transporters
“Active” pumps that split ATP
from Lecture 5
17
How does the electric field across a biological membrane compare with other electric fields in the modern world?
1. A “high-voltage” transmission line1 megavolt = 106 V.The ceramic insulators have a length of ~ 1 m.The field is ~ 106 V/m.
2. A biological membraneThe “resting potential” ~ the Nernst potential for K+, 60 mV.The membrane thickness is ~ 3 nm = 30 A.The field is (6 x 10-2 V) / (3 x 10-9 m) = 2 x 107 V/m!
Dielectric breakdown voltages (V/m)
Ceramic 8 x 107
Silicone Rubber 3 x 107
Polyvinyl chloride 7 x 106
from Lecture 6: Introduction to Voltage-Gated Channels
18
K+ ions lose their waters of hydration and
are co-ordinated by backbone carbonyl groups
when they travel through a channel.
H2O K+ ion
carbonyl
From Lecture 5
19
H2O K+ ion
carbonyl
From Lecture 5
As of 13 April 2006, there are no crystal structures of voltage-gated Na+ and Ca2+ channels.
But the similarities in sequence allow us to assume that the secondary and tertiary structures resemble those of K+ channels.
A voltage-gated Na+ channel can be changed to a voltage-gated Ca2+ channel by mutating . . .just 2 out of 1800 amino acids
20
docked vesicle
nerve impulse
voltage-gated Ca2+ channel
neurotransmitter
Electricity, then chemistry triggers synaptic vesicle fusion
21
voltage-gated Ca2+ channel
Electricity, then chemistry triggers synaptic vesicle fusion
Ca2+
docked vesicle
neurotransmitternerve impulse
22
fused vesicle
Ca2+
neurotransmitter
Electricity, then chemistry triggers synaptic vesicle fusion
23
A conotoxin:25 amino acidsheld together by disulfide bondsindividual conotoxins specifically block individual ion channels
This conotoxin blocks Ca2+ channels.Slightly modified, it is now the drug, ziconotide.It suppresses transmission at pain synapses in the spinal cord.
(Swiss-prot viewer must be installed on your computer)
http://www.its.caltech.edu/~lester/Bi-1/conotoxin-annotated.pdb
from Lecture 8:
24
V
measured postsynaptic response
1 ms 5
mV
-60
+60
large“synaptic potential” leads to postsynaptic action potential
subthreshold synaptic events(revealed in low Ca2+)
stimulus to presynaptic neuron, producing action potential
Electrophysiological analysis of
quantal synaptic transmission(slide 1)
Nestler Box 2-3 Figure A
25
V
we stimulate the presynaptic neuron, producing an action potential
we measure thepostsynaptic responses,“postsynaptic potentials”
repeated identical stimuli to the presynaptic neuron . . .
. . . yield variable postsynaptic responses!
5 mV
5 ms
Electrophysiological analysis of
quantal synaptic transmission(slide 2)
26
no stimulus; spontaneous “miniature” postsynaptic potentials
repeated stimuli to presynaptic neuron
5 mV
50 - 1000 channels (differs among types of synapse).
This is the contents of a single vesicle.
Electrophysiological analysis of quantal synaptic transmission
(slide 3)
Analysis of Quantal Synaptic Transmission
00.10.20.30.40.50.60.70.80.9
1
1 2 3 4 5 6Amplitude of Postsynaptic Response (mV)
Fra
ctio
n o
f O
bse
rvat
ion
s
Stimulated
Spontaneous
0 1 2 3 4 5
27
nNpnpn
NnP
1)(
N vesicles per terminal (3 in this example)
p probability of release per vesicle
what is the probability P of releasing n vesicles?
(n = 2 for this action potential)
N and p sometimes change during memory, learning, and drug addiction
Electrophysiological analysis of quantal synaptic transmission(slide 4):
Binomial statistics of vesicle release
28
1. Stimulated postsynaptic potentials have variable amplitudes
2. Spontaneous “miniature” postsynaptic potentials occur
• The amplitudes of the stimulated psp’s are integral multiples
of the spontaneous “miniature” psp’s
Electrophysiological analysis of quantal synaptic transmission (slide 5):
Summary of the classical evidence:
29
fused vesicle adds capacitance
C
E
G
Na+ K+ Cl-inside
outside
C
inside
outside
A more direct electrical measurement of quantal release:Measuring the presynaptic capacitance increase due to vesicle fusion
30
To measure the conductances, we set IC = CdV/dt = 0, but G/dt 0.
To measure capacitance, we set IC = CdV/dt 0, but G/dt = 0.
C
E
G
Na+ K+ Cl-
C
Measuring the presynaptic capacitance increase due to vesicle fusion
C ~ 1 femtofarad
= 1 fF = 10-15 F
like Lecture 6
http://www.srl.caltech.edu/sal/ph1bc/phys1c.html
http://www.theory.caltech.edu/people/politzer/web2.html
Phys1c reminders. Anal:
Practical:
31
The light chain of botulinum toxin is an enzyme that cleaves synaptic vesicle fusion proteins
32
Popular press:http://search.news.yahoo.com/search/news/?p=botox
What’s the latest new use for Botox ?
Scientific journals:http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?CMD=Details&DB=PubMed
((((("botulinum toxins"[MeSH Terms] OR botulinum toxin[Text Word]) AND hasabstract[text]) AND Clinical Trial[ptyp]) AND English[Lang]) AND (("human"[MeSH Terms] OR "hominidae"[MeSH Terms]) OR "Human"[MeSH Terms]))
33
Membrane trafficking, fusion and sorting: crucial to many aspects of cell function.
© Garland publishing
34
Membrane trafficking, fusion and sorting: crucial to many aspects of cell function.
© Garland publishing
35
Date: Thursday, 13 April 2006 11:38:53 -700To: Henry Lester lester@caltech.eduFrom “Mary B. Kennedy” kennedym@its.caltech.edu
Subject: I wish to interrupt Bi 1
Hi Henry,
I wish to tell the Bi 1 students about the biochemistry of postsynaptic signalling and regulation.
Mary
Mary B. Kennedy, 4/14/05, I wish to interrupt Bi 1
36
Email from Professor Mary Kennedy
http://www.search.caltech.edu/CIT_People/action.lasso?-database=CIT_People&-response=Detail_Person.html&-layout=all_fields&person_id=11610&-search
37
How do synapses Learn?
The answer is mostly Biochemistry
38
The NMDA receptor conducts only when 1. The membrane potential is more positive than -30 mV2. Glutamate is present
outside
inside
Action potential plus glutamate functioning channel
Na+, Ca2+
A molecular coincidence detector leading to Na+ and Ca2+ influx,with many intracellular effects (lectures 12, 14)
-30 mV
(intracellular concentrations of glutamate and Mg2+ are nearly irrelevant)
39
Calcium ion is a biochemical trigger
(Often called a “Second Messenger”)
40
41
42
43
Ca2+-dependent protein kinase (12 subunits)
44
Protein kinases are enzymes that catalyze transfer of phosphate to hydroxyl groups on proteins.
The phosphate moiety is transferred from ATP to serine, threonine, or tyrosine side chains
45Phosphorylation of synaptic proteins can change the behavior of synapses
46
End of Lecture 9
Reminder: Henry Lester’s “office” hoursMon, Thurs 1-1:30 PM, outside the Red Door
47
Electrical studies on synapses
Nestler Box 2-3 Figure A
presynaptic neuron postynaptic neuron
48
no stimulus; spontaneous events
repeated stimuli to presynaptic neuron
5 mV
50 - 1000 channels (differs among synapses).This responds to the contents of a single vesicle.
Analysis of Quantal Synaptic Transmission
0
0.1
0.2
0.3
0.4
1 2 3 4 5 6
Amplitude of Postsynaptic Response (mV)
Fra
ctio
n o
f O
bse
rvat
ion
s
Stimulated
Spontaneous
1.0
0 1 2 3 4 5
Electrophysiological analysis of
synaptic transmission(slide 3)
top related