Lecture 11 - Excitatory Amino Acids

Question 1

What are the 4 Excitatory Amino Acids?

Answer 1

-Glutamate (Glu) [80-90% of synapses involve Glu; most research on this]
-Aspertate (Asp)
-Cysteate
-Homocysteate

Question 2

What is the role of Glu and GABA?

Answer 2

-Glu and GABA make up the vast majority of synapses in the brain
-Glu synapses are the excitatory drive of the central nervous system
-GABA synpases are the main brake of the CNS (inhibitory)

Question 3

What is the importance of Glu synapses?

Answer 3

-forming new synapses as we learn (LTP)

Question 4

How was Glu identified as a NT?

Answer 4

-took a while for Glu to be considered a NT
-early requirements to be considered a NT was to be released via vesicles into synapse by nerve cell (not easy to discover without advanced tech)
-also since Glu is an amino acid, it was hard to identify that it plays a 2nd role as a NT

Question 5

What is glutamatergic?

Answer 5

-The injectable form of glutamate is called glutamatergic.

Question 6

What are the 2 sources of Glu?

Answer 6

-glucose (by-product of the Kerbs cycle) [external source]
-glutamine [internal; from within cell]

Question 7

How do glucose and glutamine make Glu?

Answer 7

-glucose [from outside the cell] is turned into Glu by an enzyme called Glutaminase
-glutamine [from inside the cell] is turned into Glu by Glutaminase

Question 8

What happens once Glu is synthesized?

Answer 8

-once Glu is synthesized, it’s brought into the vesicle via the Vesicular Glu Transporter (VGlut).
-like ACh, it is brought into vesicle via proton (H+) antiporter

Question 9

How does the proton antiporter work?

Answer 9

-proton antiporter brings H+ into the vesicle, making it very acidic
-the VGlut works against the concentration gradient of these H+ –> as they exit the cell they bring Glu into the cell (same as ACh)
[only difference from ACh is that this transporter is selective for Glu and the other one was selective for ACh]

Question 10

What happens when the NT is inside the vesicle?

Answer 10

-once the NT is inside the vesicle: Ca2+ comes in, mobilizes the vesicles, and they release their content into the synaptic cleft
-then it binds on the receptors on the post-synaptic neuron

Question 11

What happens when Glu is released in the synaptic cleft?

Answer 11

-[transmission is terminated by a reuptake]
-Glu is recycled back either into the pre-synaptic terminal by Excitatory Amino Acid Transporter (EAAT)
-EAAT vacuums Glu out of the synapse and back into the cell to recycle it

Question 12

What surrounds the glutamatergic synapse?

Answer 12

-astrocytes make an insulation around the synaptic cleft (same as myelin on axon)
-they help monitor and regulate synaptic transmission

Question 13

What do the astrocytes do?

Answer 13

-these astrocytes have EAAT, and they can vacuum Glu from the synapse and into the astrocyte
-and it turns Glu into glutamine via the enzyme glutamine synthase

Question 14

What is excitotoxicity?

Answer 14

-if too much glutamate is released in the space outside the neurons (neuropil), we can get a massive activation or excitation of surrounding neurons
-overactivation of the neurons that eventually leads to the neurons death
-we don’t want this to happen

Question 15

Why does Glu get turned into glutamine in astrocytes?

Answer 15

-glutamine is less reactive and not so prone to excitotoxicity
-so it is a safe format in which to store excess Glu

Question 16

What happens when our neuron is firing and it needs to synthesize more Glu?

Answer 16

-the astrocyte shifts the glutamine from the astrocyte into the neuron via 2 transporters: system N transporter (brings Glu out of astrocyte); and system A transporter (brings glu into neuron)

Question 17

What kind of energy do EAAT use?

Answer 17

-it uses a Na+ concentration gradient
-a lot more Na+ outside the cell, so both the fusional and electrostatic forces are trying to drive Na+ in
-EAAT has a channel that allows Na+ to come through, and as it brings 1 Na+ in the cell, it also grabs a Glu

Question 18

What are the EAA receptors?

Answer 18

-LGICs: NMDA; AMPA; Kainate
-GPCRs: mGluR –> group I (mGluR1 & 5; Gq); group II (mGluR2 & 3; Gi/o); group III (mGluR4, 6, 7, & 8; Gi/o)

Question 19

What are the characteristics of NMDA receptors?

Answer 19

-have 5 subunits
-highest density in hippocampus and cortex
-has various binding sites: Glu; Mg2+; Polyamine; Glycine; PCP (Phencyclidine)
-when NMDA channel is blocked, it has a psychedelic/hallucinogenic effect
-Zn+ is another channel blocker

Question 20

What are the characteristics of Mg2+ binding site?

Answer 20

-it is inside the channel
-important for learning and LTP
-membrane at rest, when the channel opens Mg2+ is drawn to the negative charge and binds to the Mg2+ binding site and blocks the channel (always blocked at rest)
-when it is activated and unblocked, it will allow Ca2+ to enter the cell
-thus, we say that the NMDA receptor is silenced.

Question 21

What are the characteristics of Polyamine binding site?

Answer 21

-this is a site where allosteric modulators can dictate how wide or long the channel is open

Question 22

What are the characteristics of Glycine binding site?

Answer 22

-glycine is required to be bound to it for the channel to function
-so we require a certain degree of extracellular glycine for this to work

Question 23

What are the characteristics of PCP binding site?

Answer 23

-hallucinogenic/psychedelic
-2 compounds bind to PCP site: ketamine and MK-801;
-these drugs bind to PCP and block the channel of the NMDA receptor –> disable the receptor

Question 24

What is NMDA permeable to when the channel is open (excitatory)?

Answer 24

-permeable to Na+ and K+ —> more likely to have an action potential
-also permeable to Ca2+ (which can act as second messenger)

Question 25

What are the similarities and differences between NMDA and AMPA receptors?

Answer 25

-similarities: LGICs, bind to Glu to open channel, permeable to Na+ & K+
-differences: NMDA has Mg2+ channel blocker [silent] (AMPA doesn’t), and NMDA is also permeable to Ca2+ (not AMPA)