Introduction
This page the second part of a study of the article
Engineering a memory with LTD and LTP
by researchers from the university of California at San Diego and the Howard Hughes Medical Institute
Experiment number 1
The base conditions
The rats were first trained to press a lever in response to a cue. Then they were conditioned to associate a tone with an electric shock to the foot.
This led to a CR, less pressing of the lever after the tone.
Manipulation of the brain
This tone was then replaced with a light flashed directly to a region of the brain ( the axon terminals of the lateral amygdala ).
The researchers injected viruses in the rats brain. These viruses were adeno associated viruses holding a variation of a Cholinergic receptor called ChR2.
This variant is sensitive to light and its channel are activated by a light instead of being fully dependent on certain kinds of molecules.
The viruses reached the geniculate nucleus and the auditory cortex and spread to the axon terminals of the lateral amygdala where the neurons received the receptors.
The results
As a control experiment, they first gave, only the light to the brain ( optical CS ).
This as expected did nothing to the rats and they continued to press the lever as before.
Then, they gave an electric shock with the optical CS.
This cued fear conditioning was able to cause the rats to stop pressing the lever during the optical CS as expected.
However, this association of electric shock and light was susceptible to extinction
Extinction
= the loss of the CR after repeated exposure of the CS without the US
and was blocked by a NMDA receptor inhibitor.
Inhibitor
= a molecule that blocks the receptor and it’s functions
The blocking of cued fear conditioning, a type of memory, due to the receptors inhibitor shows the need of the NMDA receptor and maybe LTP/LTD for memory.
NMDA receptor
= an ionotropic glutamate receptor involved in most kinds of synaptic plasticity.
It is thought to be the main receptor for LTP and LTD
The activity of the synapses can come from AMPA receptors or NMDA receptors. The ratio of the response from AMPARs to NMDARs can be used to see the occurrence of LTP.
After conditioning of the optical CS and the shock, rats with paired conditioning had a higher ratio of AMPAR to NMDAR response in the amygdala than unconditioned rats or rats with unpaired conditioning.
Paired conditioning
= conditioning when a stimulus is paired to another
Ex : do A and be rewarded by B
Unpaired conditioning
= conditioning when a stimulus is paired with the absence of another
Ex : do A to not get B
Showing that LTP had happened where the light was flashed at in the group of rats with paired conditioning.
The next part of the results is where the engineering begins.
Next Part : The brain and memory, simple but complex engineering
Part 1 : What is fear conditioning and how is it used ?
Part 4 : Memories forever in our brains
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