Neurobiology of Fear #2

Anatomy of the Amygdala

• The "striatal amygdala" has inhibitory projection neurons, and consists of the central (Ce) and medial (M) nuclei. The central and medial nucleus together are sometimes referred to as the "extended amygdala"
• The "cortical amygdala" has excitatory projection neurons, and consists of the Lateral (LA), Basal (B) and accessory basal (AB) nuclei. The lateral and basal nucleus together are often referred to as the basolateral (BLA) amygdala(lateral + basal nucleus). The amygdala derives its name from the almond-like shape of the BLA ("amygdala" means almond in Latin)
• Central amygdala cells are the main fear-inducing output of the amygdala

++ disected that pathway to find out how the sound detected by the ear eventually influence some brain regions that causes the mouse to freeze

++ Amygdala can be divided into this 'pink' cortex (excitatory) and 'yellow' stiatum (inhibitory-GABAergic)

 

Amygdala activation correlated with fear cues

• Human subjects were placed into an fMRI scanner where they viewed fearful versus happy faces (three examples of fearful faces are shown at left)
• BOLD signal measuring regional cerebral blood flow (rCBF) was greater in the amygdala (yellow region) when subjects viewed fearful faces

++ Amygdala is also important to detection of fear in human. If you show human scared faces, then you see activation of amygdala of human subject

++ also patients that have lesion on amygdala, they don't get scared at things that ordinary scary

 

 

Necessity: auditory fear conditioning requires LA & Ce

• For the specific case of auditory fear conditioning (fear conditioning to an auditory CS), the lateral (LA) and central (Ce) nuclei of the amygdala are necessary for learning that an auditory tone predicts a footshock. Pre- training lesions of either LA or Ce, but not other amygdala nuclei, impair the rat's conditioned freezing response(CR) to the C. B:basal amygdala, M:medial amygdala, Ce: central amygdala, LA: lateral amygdala
• Accessory basal (AB) is also called the basomedial amygdala

++ researchers have carefully lesioned various part of the amygdala (only Ce, only LA, only M, only B, only AB). Lesion of central amygdala or lateral amygdala before the fear conditioning learning impairs very strongly freezing to the tone. (lesion of these region interferes the learning fear conditioning)

 

 

Synaptic plasticity in LA during auditory fear conditioning

• Neurons in the amygdala’s LA nucleus excite neurons the Ce nucleus, which in turn sends inhibitory projections to the ventral periaqueductal gray (vPAG). Projections from Ce are thought to disinhibit neurons in vPAG to drive defensive freezing behavior.
• Neurons in the LA nucleus of the amygdala receive nociceptive inputs that are excited by the footshock US, as well as thalamic and cortical inputs that are excited by the auditory CS. Prior to fear conditioning (baseline), the CS inputs synapses onto LA neurons are weak. Pairing the CS and US (acquisition training) strengthens these synapses, so after being paired with the US, the CS acquires the ability to activate LA.

++ auditory tone goes from the ear -> cochlear nucleus -> superior olive -> inferior colliculus -> auditory thalamus -> auditory cortex (these region send input into the lateral amygdala region and this excites the central nucleus output neuron and those output neurons (which are inhibitory in the ventral periaqueductal gray)

++ if you get more freezing, this ouput is that ventral periaqueductal gray is activated, but usually this neuron is being inhibited by this local inhibitory cell. During high fear situation, this central amygdala inhibits this inhibitory neuron, so then you don't get inhibition on to these freezing provoking neurons to get more freezing because you take off this inhibition

 

 

Presentation of CS alone during Pre-training (Baseline) phase

• When the CS alone is presented before training, thalamic or cortical input(presynaptic) neurons fire and release glutamate onto LA neurons
• The glutamate binds to both AMPA and NMDA receptors, but only the AMPA receptors pass current, because the NMDA receptors are blocked by Mg2+
• Sodium entry through AMPA receptors generates an EPSP, but this EPSP is too small to trigger an action potential in the LA neuron, because there are very few AMPA receptors at the CS input synapse, and therefore it is a WEAK synapse

++ just auditory tone (not freeze to it), synapse from the auditory (cortical thalamus) that in amygdala release some amount of glutamate onto the lateral amygdala cell. But this synapse is very weak. Small EPSP is not enough to depolarize. (not depolarization Mg++ blocks)

 

Presentation of US alone during Pre-training (Baseline) phase

• When the US alone is presented before training, nociceptive inputs to the amygdala fire and release glutamate onto LA neurons(postsynaptic)
• The glutamate binds to AMPA receptors, which generate an EPSP, and this EPSP is large enough to trigger an action potential in the LA neuorn, because there are lots of AMPA receptors in this synapse
• Depolarization of the LA pops the Mg2+ cork off of the NMDA receptor at the mossy fiber synapse, but the receptor does not open because it is no binding glutamate

++ just a shock, no tone.

 

Pairing of CS and US during acquisition phase

• When the CS and US are paired duirng training, the CS causes auditory thalamus/cortex neurons to fire and release glutamate onto WEAK input synapses, and at the same time, the US causes nociceptive neurons to fire and release glutamate onto STRONG LA synapses
• Strong US-evoked depolarization of the LA neuron by the US-input synapse kicks the magnesium block off of the NMDA receptor at CS input synapses, at the same time that the NMDA receptor is binding CS-evoked glutamate
• The NMDA receptor can now pass current, and it allows calcium (Ca2+) to enter the LA neuron

++ depolarizatino from the shock will pop off the Mg from the NMDA in the synapse between auditory tone and LA, Mg++ comes out and glutamate can flow both NMDA and AMPA.

++ Pain conduction pathway (strong pathway naturally - enough to generate Action potential)

 

 

Long-term potentiation (LTP) at CS inputs to LA neurons

• Calcium entry through NMDARs activates calmodulin (CAM) and CAM kinase II (CAMKII)
• CAMKII activation leads to the insertion of more AMPA receptors into the postsynaptic membrane of the thalamic/cortical auditory CS input synapse
• These additional AMPA receptors make the synapse stronger, so that if the CS is presented alone after training, the LA neuron will fire and generate a CR

== input from the auditory pathway is very weak, and the input from pain pathway is strong on to the same cell -> pair these two at the same time, you will get enough depolarization from the shock -> Mg out -> caclium influx -> activate calmodulin and CAM kinase II -> insertion of more AMPA receptors (strengthend synapse)

 

 

Correlation: LA neurons exhibit conditioned enhancement of CS-evoked responses

• In this study, amygdala neurons were recorded in awake, behaving rats during fear conditioning
• Before conditioning (black), amygdala neurons exhibited a small response to the onset of an auditory CS (vertical black line)
• After conditioning with CS-US pairings (blue), the CS elicited a significantly larger response from amygdala neurons, suggesting (but not proving) that synapses relaying the CS to amygdala neurons had gotten stronger

++ tone by itself is enough strong to elicit action potential in the LA because it has extra AMPA receptors.

++ record the activity of LA and exposed rats to a tone. (before learning, some amount of activation) (after learning, much bigger response action potential in LA -> enough to provoke the animal to freeze without the shock)

 

NMDARs in LA are required for acquisition but not expression of fear conditioning

• A) Pretraining blockade of NMDA receptors dose-dependently impairs acquisition of fear conditioning when animals are tested drug-free after 24h
• B) Pretesting blockade of NMDA receptors (NR2B) has no effect on expression of fear conditioning in animals that were previously trained drug free

++ if you block NMDA receptor in LA by infusing locally antagonist for the NMDA receptor, then the mouse doesn't learn to associate the tone with the shock

++ learning without block anything -> after learning then you block NMDA receptors, no effects on expression of fear conditioning

== thus, the NMDA receptors are important to learn behavior not expression after the association have already been learned.

++ Ce is inhibitory structure, and LA send excitatory output to Ce, activating inhibotry Ce, -> Ce send inhibitory projection to another inhibitory cell in the ventral periaqueductual gray. / ventral periaqueductal gray itself inhibiting freezing provoking neuron

== so inhibit that inhibition.