Forget It! The Role of Forget It! The Role of 22

Adrenergic Agonists in Fear Adrenergic Agonists in Fear ConditioningConditioning

Forget It! The Role of Forget It! The Role of 22

Adrenergic Agonists in Fear Adrenergic Agonists in Fear ConditioningConditioning

M. Frances Davies, Ph.D

Stanford University

Dept of Anesthesia

Common molecular and Common molecular and cellular substrates of cellular substrates of addiction and memoryaddiction and memory

. “Drugs of abuse cause long-lasting changes in the brain that underlie the behavioral abnormalities associated with drug addiction. Similarly, experience can induce memory formation by causing stable changes in the brain. Over the past decade, the molecular and cellular pathways of drug addiction, on the one hand, and of learning and memory, on the other, have converged. Learning and memory and drug addiction are modulated by the same neurotrophic factors, share certain intracellular signaling cascades, and depend on activation of the transcription factor CREB. They are associated with similar adaptations in neuronal morphology, and both are accompanied by alterations in synaptic plasticity (e.g., long-term potentiation, long-term depression) at particular glutamatergic synapses in the brain. “

Nestler EJ. Neurobiol Learn Mem. 2002

Nov;78(3):637-47

Can addiction be treated by Can addiction be treated by blocking learning or memory?blocking learning or memory?

Strategies to treat addiction:• inhibit neuroplastic changes • reverse established memories

The noradrenergic system in The noradrenergic system in fear learningfear learning

The noradrenergic system in The noradrenergic system in fear learningfear learning

• Intimately involved in vigilance and alertness

• NE activates 1, 2 and adrenergic receptors

• NE plays a role in the learning of fear

• 2 agonist dexmedetomidine reduces the activity of the central and peripheral noradrenergic system

• Reduction of fear is desirable for many anesthetic and ICU procedures

Supporting EvidenceSupporting Evidence Supporting EvidenceSupporting Evidence

Blockade of and 1 adrenoceptors reduces fear learning

Stimulation of the 2 adrenoceptors tends to:– reduce activity of the central noradrenergic

system– may also reduce the learning of fear

This hypothesis has not been rigorously tested

Fear Conditioning: TrainingFear Conditioning: Training

Fear Conditioning: The Fear Conditioning: The ConceptConcept

Fear Conditioning: The Fear Conditioning: The ConceptConcept

Day 1 Training

Day 2 Testing

Taken from Michael Davis in Neurobiology of Fear Responses: The Role of the Amygdala. Journal of Neuropsychiatry 1997 Summer 9(3):382-402.

Tone: Conditional stimulus-CS

Shock: Unconditional stimulus-US

Central Fear State

Freezing: Unconditional Response-UR

Tone: Conditional stimulus-CS

Central Fear State

Freezing: Unconditional Response-UR

Fear Conditioning: Discrete Fear Conditioning: Discrete Cue AssessmentCue Assessment

The Effect of The Effect of Dexmedetomidine on Dexmedetomidine on Discrete Cue MemoryDiscrete Cue Memory

Dexmedetomidine Injection Dexmedetomidine Injection and Testing Scheduleand Testing Schedule

Dexmedetomidine Injection Dexmedetomidine Injection and Testing Scheduleand Testing Schedule

Day 1 Training

Day 2 Testing

Reconsolidation

Dex Dex

Encoding Retrieval

Consolidation

Dexmedetomidine (10 µg/kg) given Dexmedetomidine (10 µg/kg) given before training reduced discrete before training reduced discrete

cue fear conditioningcue fear conditioning

SALINE DEX0

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ACTIVITY DISCRETE CUE

Dexmedetomidine (20 µg/kg) had Dexmedetomidine (20 µg/kg) had no effect on consolidation of no effect on consolidation of

discrete cue fear conditioningdiscrete cue fear conditioning

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ACTIVITY DISCRETE CUE

Does dexmedetomidine reduce Does dexmedetomidine reduce biochemical markers of learning?biochemical markers of learning?Does dexmedetomidine reduce Does dexmedetomidine reduce

biochemical markers of learning?biochemical markers of learning?

Amygdala is important in discrete cue memory

Discrete cue fear conditioning causes expression of c-Fos and P-CREB in the amygdala

Can dexmedetomidine affect this expression?

Dexmedetomidine reduced c-Dexmedetomidine reduced c-Fos and P-CREB in amygdalaFos and P-CREB in amygdala

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A B C

D E F

Lateral Nucleus

Basolateral Nucleus

Central Nucleus

What What 22 receptor subtypes are receptor subtypes are

involved in fear conditioning?involved in fear conditioning?What What 22 receptor subtypes are receptor subtypes are

involved in fear conditioning?involved in fear conditioning?

Adrenergic receptorsAdrenergic receptors

* Antihypertension* Sedation*Analgesia* Anesthetic sparing effect

ALPHA-2 A

ARTERIAL CONSTRICTION

ALPHA-2 B

*Analgesia*Mood

ALPHA-2 C

ALPHA-2 ADRENERGIC RECEPTORS

Dexmedetomidine (20µg/kg) did not affect encoding of discrete cue fear

conditioning in 2A AR KO mice

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ACTIVITY INITIAL EXPLORATION DISCRETE CUE

Dexmedetomidine did not Dexmedetomidine did not affect discrete cue fear affect discrete cue fear

conditioning in D79N miceconditioning in D79N mice

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ACTIVITY DISCRETE CUEINITIAL EXPLORATION

SummarySummary

• 2 receptor activation during training reduces discrete cue fear conditioning

• 2 receptor activation after training does not

• Dexmedetomidine reduced P-CREB and Fos production in amygdala

• The effect of 2 agonists on addiction is unknown

The role of the The role of the 2A2A AR in AR in

intrinsic fearfulness of miceintrinsic fearfulness of mice

Mice deficient in Mice deficient in 2A2A AR are more AR are more

fearful in the discrete cue testfearful in the discrete cue test

WT 2A AR KO D79N

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Is there any difference in the Is there any difference in the noradrenergic system in noradrenergic system in 2A2A AR AR

deficient mice?deficient mice?

LC and nucleus tractus solitarius (NTS) project to the amygdala

Are activated by the footshock (unconditioned stimulus)

2A2A AR KO mice have a longer AR KO mice have a longer

locus coeruleuslocus coeruleus

WT 2A KO

0.0

0.5

1.0

1.5

2.0 **

The LC cell bodies are bigger The LC cell bodies are bigger in in 2A2A AR KO mice AR KO mice

WT 2A KO

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There are more TH positive There are more TH positive neurons in the LC of neurons in the LC of 2A2A AR AR

KO miceKO mice

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There are more large neurons There are more large neurons in the LC of in the LC of 2A2A AR KO mice AR KO mice

0 1000 2000 3000 4000 5000 60000

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Soma Volume (µm3)

ConclusionsConclusions

2A adrenergic agonists block the creation of discrete cue fear conditioning memory

Block expression of transcription factors that have been linked to memory in critical area (amygdala)

2A adrenergic receptor knockout mice – are very sensitive to discrete cue fear conditioning– lose amnestic effect of dexmedetomidine– have hypertrophied central noradrenergic system

Relevance to Addiction Relevance to Addiction

Do individuals differ in their expression of 2A AR?– Yes known differences in promotor region– Linked to changes in memory, indirect hostility,

irritability, negativity, and verbal aggression Do individuals differ in their susceptibility to

learning to fear?? Do individuals differ in their susceptibility to

becoming addicted because of an altered noradrenergic system??

ContributorsContributors

Stanford University– Janet Tsui – Judy Flannery– Xiangqi Li– Brian Hoffman

Molecular Research Institute

–Tim DeLorey