MN: Molecular and Cellular neuroscience
Lecture 1
Timing of development of the brain in a mouse: firstly, in E8 to E16, neurons will grow in large
amounts. In E14 the astrocytes begin to grow until P13. In E14 also the oligodendrocytes begin to
grow but become more abundant in a later time scale. In the picture you can see this development.
Neurons are the output cells and are supported by the astrocytes and oligodendrocytes. This means
that every cell type has a different time of abundancy in development, this is the same for humans.
Non-neuronal cells are typically called glial cells. Rudolf Virchow searched for connective tissue in the
brain. He named this material nervenkitt or neuroglia. There are different types:
- Astrocytes: tripartite synapse gliotransmission (BBB and absorption of neural transmitters)
- Microglia: synaptic maturation (development), learning and memory. Remove toxic agents.
- Oligodendrocytes: axon myelination
- Ependymal cells: ependymal wall of the ventricles and choroid plexus
A presynaptic neuron fires an action potential which goes to the synapse. The pre synapse and post
synapse are very distinct of each other. The vesicles are in the pre-synaptic side, the post-synaptic
density belongs to the post-synapse. This density is so dark because there are a lot of receptors and
proteins that keep the receptors in place. The vesicles in the pre-synapse are aligned with the
receptors of the post-synapse. The action potential will cause the vesicles to go to the membrane,
fuse and release their neurotransmitters in the synaptic cleft to travel to the receptors. The process
of synaptic transmission goes very fast (milliseconds). The synapse has three parts: pre-synaptic,
post-synaptic side and the synaptic cleft.
Dendritic spines are protrusions of the dendrite where excitatory synapses are formed. Excitatory
synapses (Glutamatergic) are asymmetric. The inhibitory synapses (GABAergic) are very symmetric
and are formed on the dendritic shaft. There is no structure from the dendrite that makes it clear
that there is a GABAergic synapse.
One post-synapse can have two PSDs, which means that you have two synapses there.
An excitatory synapse depolarizes the cell with the help of sodium. An excitatory synapse is
a synapse in which an action potential in a presynaptic neuron increases the probability of an action
potential occurring in a postsynaptic cell. Positive ions flow into the cell and the membrane potential
will go to -40/-30 mV which will depolarize the cell. Inhibitory synapses will have an action potential
that will make it more difficult to get an action potential occurring in a postsynaptic cell.
Lecture 2
With an action potential, Ca2+ channels will open in the pre-synapse, calcium will flow in which leads
to the transportation of vesicles to the membrane. The vesicles will fuse and let the neurotransmitter
go to the post-synapse. The neurotransmitters will open the receptors here and Na+ will flow in the
post-synapse to cause a new action potential. neurotransmitters can open the ion channels directly
or indirectly. The indirect activation will activate a receptor that will cause a whole cascade that will
eventually open the channels.
AMPA receptors are receptors that allow sodium to flow in. NMDA receptors allow sodium and
calcium to flow in. NMDA have very specific properties: they have a magnesium block at resting
potential (-60/-70 mV in the cells) which prevent ions to flow in.
Again: the excitatory (glutamatergic) synapses are situated on dendritic spines. 80% of synapses in
central nervous system are excitatory. The dendritic spine with the strongest synapse is the spine
,with the largest surface. This is because more receptors can be placed on here, more sodium can go
into the post-synapse, a larger action potential.
There are different distinct phases of synapse development: before and after birth there is a huge
phase of synapse formation. This period lasts until a few years into your childhood. After that people
will begin to lose synapses (synapse elimination) in adolescent years. The system will eliminate the
synapses that will not be used. This is an active phase that is done by microglia. In adulthood you will
eventually have synapse maintenance, you now have the tuned synapse system that will be used in
your lifetime. AD will dramatically decrease the amount of synapses. Developmental disorders will
decrease the amount of synapse formation in childhood.
The more AMPA receptors you have, the more depolarization you will have.
The factors that play a role in mediating the action potential in the pre-synapse:
- The release probability (the chance that vesicles will fuse and release neurotransmitters
when Ca2+ flows in),
- The number/size of vesicles
- The transmitter content
Dendritic spines can be colored by the actin filaments. Actin is only present in the dendritic spines
and not in the dendritic shaft. In the dendrite there is a lot of microtubule, which is not present in the
spines. The pathways that control actin are enriched in the spines. Rho GTPases can transfer
extracellular signals to changes in actin, which will come back in a second.
Strong and weak synapses are the basis for learning and memory. Particular synapses are
strengthened and weakened, the chance in synapse strength is called synaptic plasticity (a cellular
correlate of learning and memory).
Long-term potentiation (LTP) and long-term depression (LTD) will change the strength of the synapse.
Both are important for memory and learning. LTP will strengthen the connection, LTD will weaken it.
‘Cells that fire together wire together’.
Cell A receives input from cell B and C. When Cell A and B are engaged in the same memory trace,
that means that A and B are active at the same time (B fires at the same time as A). So when they fire
together, there will be LTP where the synapses of both neurons will be strengthened. A and C never
fire at the same time, they can still be connected but they will not function for the same memory.
The synapses between these two are weakened via LTD.
LTP: more AMPAR on the surface and a bigger surface of the post-synapse (dendritic spine needs to
grow, thus actin cytoskeleton needs to grow). How this happens: Ca2+ will enter the cell and
activates CAMKII. This will activate Rho-GTPases so that the structure will increase. It will also chance
the function (AMPAR will go to the surface).
Example: dentate gyrus has neurons that go to the CA3,
which has neurons that go to CA1. When you stimulate the
CA3 cell, that leads to a response in the CA1 cell. you can
mimic LTP by giving a lot of action potentials in CA3 (stimuli)
for 30 seconds. If you then give another stimulus after that,
the response is much larger. The structure and function has
changed so more receptors were available at the grown
post-synapse. Once the action of LTP happened, it can
happen for your entire life. The same goes for LTD.
The role of AMPA and NMDA receptors: to get LTP, the
post-synapse needs to grow and the amount of AMPAR
needs to increase. Glutamate from the pre-synapse can
bind to the AMPAR which will open and Na+ flows in, but
this isn’t enough to depolarize the post-synapse. Glutamate
, can also bind to the NMDAR but there is still a
magnesium block so nothing happens with the
NMDAR. What happens during post-synaptic
depolarization, the magnesium block from the
NMDAR will be released, glutamate can bind to
NMDAR and Ca2+ will flow into the cell. When
both cells A and B fire together, the glutamate
from cell A will go towards the post-synapse of
cell B and at the same time the post-synaptic
cell is depolarized. Only in this way (the firing
together) the NMDAR will open and a massive
amount of Ca2+ will flow into the cell in a short
time. Ca2+ will activate Calmodulin kinase II
(CAMKII) and protein kinase C which will ensure
substrate phosphorylation via protein kinases.
This will ensure that the AMPAR will be inserted on the surface of the post-synapse. The protein
kinases will go to the cell nucleus and activate transcriptional regulators like CREB. This will regulate
gene transcription that will make sure that the amount of AMPAR will stay at the surface.
The CAMKII can activate Rho-GTPases so that actin will be reorganized, Cdc42 (reorganization of actin
skeleton) and HRas which will transport AMPAR and insert them into the surface.
Lecture 3
Astrocytes: there are two types of astrocytes, which are the fibrous astrocytes which make contact
with the blood vessels and the nodes of Ranvier. The other one is the protoplasmic astrocyte which
forms close connections to the synapses. These cell types are the most abundant glial cells in the
CNS. GFAP is a protein that is specifically expressed in the astrocytes.
One astrocyte has contact with 100.000 of synapses. It is an important cell for controlling those
synapses. Astrocytes do many things but the most important role is to modulate synaptic and neural
activity.
Astrocytes are important during development and adulthood. They secrete molecules that promote
synapse formation and function (synapse development). Neurons are born first but before the
astrocytes are made, the synapses are not well developed. Without astrocytes, there is a tenfold
decrease of synapses.
Tripartite synapse: pre- and post-synapse, as well as the astrocyte. When glutamate is being
released, not only the receptors on the post-synapse are active. The astrocytes have channels that
take up the glutamate from the synaptic cleft and prevents glutamate to spill over and become toxic.
It reuses glutamate to signal back to the pre-synapse.
Astrocytes can be visualized by staining for astrocyte specific markers: GFAP, CX43, GLAST, GS,
S100beta, etc.
Neurons can be visualized by staining for Tuj1 or MAP2
Two lines of evidence that show that astrocytes are part of the synapse formation: stimulate the CA3
cells and look at the CA1 cells. when you look at CA1, you can stain the calcium production and see
that the astrocytes will activate. There must be some active interaction between neurons and
astrocytes. When you touch a whisker of a mouse, you will see an increase of calcium in the
astrocytes of the specific brain area for that whisker. These two experiments show that the
astrocytes will be activated when you activate the neurons of these brain areas.
