Neurobiology of Fear #3

Three types of optogenetic transgenes

• Channelrhodopsin is a sodium channel sensitive to blue light, which causes sodium to enter and depolarize the cell. (excites cell)
• Halorhodopsin is a chloride pump sensitive to yellow light, which causes chloride to be pumped in, hyperpolarizing the cell. (inhibit cell)
• Archaerhodopsin is a proton pump sensitive to green light, which causes hydrogen ions (protons) to be pumped out, hyperpolarizing the cell

 

Optogenetic stimulation of LA neurons substitutes for US

• What if the postsynaptic LA neuron was depolarized by light (optogenetic stimulation) instead of by the US?
• Johansen et al. (2010) transfected LA neurons in mice with either YFP-ChR2 (experimental group) or YFP alone (control group)
• They then paired an auditory CS with optical stimulation of the amygdala

++ optogenetic is also used to disect out the role of lateral amygdala

++ instead of using shock, used channelrhodopsin to direclty activate the LA. They optogenetically stimulat LA by blue light, no shock happening. Auditory tone is paired with blue light stimulation. You will get sound paired with artifically provoked action potential through optogenetics.

 

Optogenetic stimulation of LA neurons substitutes for US

• A) Pairing an auditory CS with optogenetic stimulation of LA neurons yields increased freezing across 4 CS-US pairing trials.
• B) Presenting the CS alone after training yields freezing in ChR2 infected rats (experimental group) that underwent CS-light pairing, but not in rats infected with YFP alone (control group #1) that received CS-light pairing, or in ChR2 infected rats that received unpaired CS-light presentations (control group #2).

++ optogentically stimulation instead of shock also increases freezing. (substitute)

 

Longer after pairing

• Ca2+ not only activates CAMKII, but also triggers adenylyl cyclase to convert ATP to cAMP (remember that olfactory g-protein receptors also did this in olfactory receptor neurons?)
• cAMP then activates cAMP-dependent protein kinase (better known as PKA), which in turn phosphorylates mitogen-activated protein kinase (MAPK)
• MAPK translocates to the soma and enters the nucleus of the LA neuron, where it activates a transcription factor called CREB
• CREB activates gene transcription in the nucleus of the cell, producing mRNA that is transcribed into new proteins
• These new proteins provide the raw "building materials" for constructing new dendritic spines on the LA neuron

++ when you get the calcium infux in the auditory synapse to the LA, will activate CAM KII protein and adenylyl cyclase which produce more cyclic AMP in turn will activate protein kinase A(PKA) which activate other MAPK that goes to nucleus and activate protein called CREB and then CREB is transcription factor that will activate the trascription of other genes and these new protein then used to contruct new spine that also receive information from these presynaptic that conveys auditory input

++ two different kind of mechanism strengthening input from the auditory pathway (one is inserting more AMPA receptors in the pre-existing dendrite) (other is making new dendrites by these protein controlled by CREB)

++ important concept is - these dendrites are not fixed structure. Depending on strengthening or weakening, you can get construction or destruction of dendrites

 

Protein synthesis is required for maintenance of amygdala LTP after fear conditioning

• Infusion of anisomycin (protein synthesis blocker) into amygdala during tone-shock pairing impairs long-term memory (LTM) but not short-term memory (STM) for the CS-US association, as assessed by tone-evoked freezing 1 hour versus 24 hours after training

++ block protein synthesis injecting a drug called amisomycin, then block large part of enhancing of freezing that happens in the fear conditioing, so you need this new protein to make new dendrites that participate in strengthening the sysnapse. If you do fear conditioing training with this protein blocked,/ then during the training when the shock is happening the animal will freeze to the tone and to the shock. But 24 hours later the test of memory, (playing just the tone and absence of shock) the animal that had blockage of protein synthesis will freeze less than animal where you didn't block. Since Block protein synthesis  the LA don't make new dendrite to strengthen pathway from the auditory input

++ if you block protein synthesis, there is still first mechanism which is insertion of more AMPA receptors, and it is difficult to block all the synthesis

 

Contextual Fear Conditioning

• Auditory Fear Conditioning Tone CS is presented in a novel context (different chamber from where US was delivered)
• Contextual Fear Conditioning Rat is returned to the training context (same chamber where US was delivered) without presenting the tone CS

++ there is no tone playing, but environment (blue box - shock) associate blue box to the shock

++ 만약에 tone이랑 context 동시에 하면 둘 다에 freeze 한다

 

Hippocampal Lesions Impair Context but not Tone Fear conditioning

• A: Rats with hippocampal lesions do not freeze to the training context
• B: Rats with hippocampal lesions still freeze to the auditory tone CS

++ it is kind of episodic memeory

++ contextual fear conditioing is impaired bc hippocampus is involved in space and context, but learning of auditory tone conditioing is not impaired bc hippocampus is not involved in auditory tone processing

++ 만약에 auditory pathway 중에 lesion이 있으면 tone fear conditioning learning은 impair 되지만, contextual learning은 impair 안됨

 

Synaptic plasticity in B nucleus during context fear conditioning

• Neurons in the basal (B) nucleus of the amygdala receive nociceptive inputs that are excited by the footshock US, as well as inputs from neurons in the ventral hippocampus that are thought to fire selectively in specific contexts. Prior to fear conditioning (baseline), the context input synapses onto B neurons are weak. Pairing the CS and context strengthens these synapses.

++ hippocampus which is more imporatant for contextual conditioning provide input to the basal nucleus of amygdala and then synaptic plasticity happens not in the LA rather in the input from the hippocampus. And then the same thing happens (Basal nucleus excites  Ce which causes disinhibition of freezing (more freezing to the context)