Lecture 25 Tripartite synapse

Glial Cells

There are 10-50 times as many glial cells than neurons

PNS:

• glial cells are called schwann cells

• make myelin for peripheral neurons

CNS:

• microglia

• oligodendrocytes

• astrocytes

Ion channel expression:

• potassium channels (so can maintain a resting membrane potential close to EK)

Astrocytes

• most common type of glial cell in the CNS

• found in gray and white matter

The tripartite (3-part) synapse

consists of:

• presynaptic neuron

• postsynaptic neuron

• astrocytes

Astrocytic processes affect synaptic transmission and shaoe

• astrocytes affect diffusion and uptake of neurotransmitter from the synaptic cleft

• Shape/proximity if astrocytes is not stable

• can change over the course of minutes

• change across the sleep-wake cycle

Astrocytes contributing to glutamate reuptake

• glutamate goes from synaptic cleft to surrounding astrocytes to be taken out of the cytoplasm sooner

Astrocytic glutamate transporter currents

• when you block glutamate transporters, the current going into the astrocyte decreases

• current also changes based on the resting membrane potential

GABA and glycine transporters on astrocytes

do same thing as glutamate

Ion channels in astrocytes

Potassium channels:

• maintain normal levels of K+

• drive Vm of astrocytes (80 mv)

Voltage-gated ion channels

• NA+

• Cl-

• Ca2+

Other cA2+ channels:

• intracellular

• mechano-sensitive

cannot produce AP’s but can respond to depolarization

Electrically-coupled groups of astrocytes

• astrocytes are connected to other astrocytes via gap junctions

• groups of 50-100 electrically coupled astrocytes can be regulated together

• These groups of electrically coupled astrocytes tend to be somewhat exclusive, forming “microdomains” of 200-200 micrometerss

Release of glutamate from astrocytes

• calcium dependent release from microvesicles

• glutmate transporters working in reverse

• glutamate conducting ion channels

There are mGLUR’s on the side of the presynaptic terminal

with mechanical stimulation and calcium entry, glutmate can be released by the astrocytes into the presynaptic terminal

• smaller epsp

Are astrocytes involved in “information processing”

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.