Lecture 25: Tripartite Synapse

1. Called Schwann cells
2. Make myelin for peripheral axons

What is the role of glial cells within the peripheral nervous system (PNS)?

Microglia- Brain macrophages, “shape shifters”

Oligodendrocytes- myelin providers

Astrocytes- participate in signaling

* structural and environmental support

What are the different types of glial cells within the central nervous system (CNS) and what are their role

Potassium channels

* Around 80 Vm

If astrocytes assist with ion channel expression, what ion do astrocytes work on what is the resting membrane potential?

Participates in signling in PNS

The Perisynaptic Schwann cell expresses:

* Ion channels
* Neurotransmitter receptors coupled to G proteins

\
Overall, the Persynaptic Schwann cell:

* releases calcium
* responds to neurotransmitter
* influences signaling between motor neuron and postsynaptic muscle fiber

Explain the role for Schwann cells on the frog neuromuscular junction nerve terminal

low frequency:

* Not a high amount of neurotransmitter being released
* Not a high concentration of diffused to perisynaptic sites to influence Schwann cell

If there is a low frequency of neurotransmitter being released, how does this relate to presynaptic action potentials within Schwann Cells?

high frequency:

* Burst of action potentials
* Increased release of neurotransmitter
* Works on Schwann cells to increase calcium concentration

If there is a high frequency of neurotransmitter being released, how does this relate to presynaptic action potentials within Schwann Cells?

Schwann Cell

Transmitters released from motor nerve terminals act on __________________ receptors to tigger signaling cascades

1. L-Type voltage-gated calcium channels
2. Muscarinic ACh receptors
3. Substance P receptors
4. ATP and adenosine purine receptors

What types of channels/receptors to Schwann cells express? (4)

reduced

After GTP-y-S injection into Schwann cells, neurotransmitter release is ____________

synaptic depression

After GDP-B-S injection into Schwann cells, a train of action potentials that normally leads to synaptic __________,__ now has less __________

prevents activation of G-Proteins

What is the role of GDP-B-S?

the pattern of activity in presynaptic neuron

The calcium signal within a Schwann cell is dependent on what?

1. Single response


1. Regular action potential
2. Robust increase in intracellular calcium
2. Oscillatory


1. Burst of APs, quiet, burst of APs
2. increases and decreases back and forth over time

What are the two types of activity patterns for action potentials for determination of calcium signals in a Schwann cell?

astrocytes

\
found in both gray and white matter

What is the most common type of glial cell found in the CNS?

1. Presynaptic neuron
2. Postsynaptic neuron
3. Astrocyte

What makes up the tripartite synapse?

Post-Synaptic Density

* Dense
* Increased concentration of proteins
* Directly across from neurotransmitters

What is PSD?

1. Astrocytes affect diffusion and uptake of neurotransmitter from the synaptic cleft


1. Neurotransmitter does not have ability to diffuse on synaptic cleft
2. Shape/proximity of astrocytes is not stable
3. the shape can change over the course of minutes
4. the shape can change across sleep-wake cycle

What are the four ways that astrocytic processes affect synaptic transmission?

Net positive charge

* 1 Glutamate (negative)
* 1 Hydrogren (positive)
* 3 Sodium (positive)

What is the net charge for entry in astrocyte based on transporter? What are the molecules involved?

Excitatory Amino Acid Transmitters

What is EAAT?

* Current is largest at resting negative potential
* Current NEVER reverses
* Not ionic
* Transporter will NOT reverse direction

Explain the IV plot for astrocytic glutamate transporter currents

20-25%

GABA and gylcine transporter on astrocytes are responsible for what percentage for clearance inhibitory amino acids?

* Maintain normal levels of Potassium out
* Drive Vm of astrocytes (around -80 mV)

Explain the topics of potassium channels within astrocytes

1. Voltage-gated Sodium Channels- depolarize
2. Voltage-gated Chloride Channes- hyperpolarize
3. Voltage-gated Calcium Channels- depolarize

What are the three types of voltage gated ion channels in astrocytes? Do they hyperpolarize or depolarize?

* Intacellular
* Mechano-sensitive

What are two other types of calcium channels for astrocytes?

* Astrocytes are not electrically excitable
* Astrocytes will NEVER spike an action potential

Regarding stabilization and action potentials, what do astrocytes exhibit?

gap junctions

\
microdomains

With electrically-coupled groups of astrocytes, astrocytes are connected to other astrocytes via ____________________.

\
Groups of electrically-coupled astrocytes tend to be somewhat exclusive, forming ___________ of 200-300 um

* pipette, poke astrocyte, activates mechanoreceptors
* First poked: small calcium response, little to no microdomain (1s)
* 8s: entire microdomain is stimulated, increased elevation in calcium

Explain calcium waves experiment and electron micrograph

1. Calcium-dependent release from microvesicles
2. Glutamate transporters working in reverse
3. Glutamate conducting ion channels

Explain broad ideas of how astrocytes can release glutamate

1. Astrocytes respond to external signals from other elements of the nervous system, including neurons
2. Responses to external signals can change cytosolic calcium levels inside astrocytes rapidly and at high temporal precision
3. Astrocytes release neurotransmitters in a calcium-dependent manner
4. Release of neurotransmitter by astrocytes can affect neuronal responses

What are the four ideas in which astrocytes are involved in information processing?