Forget It! The Role of  2 Adrenergic Agonists in Fear Conditioning M. Frances Davies, Ph.D Stanford University Dept of Anesthesia

Common molecular and cellular substrates of addiction 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 Nov;78(3):637-47

Can addiction be treated by blocking learning or memory? Strategies to treat addiction: inhibit neuroplastic changes reverse established memories

The noradrenergic system in fear 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 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: Training

Fear Conditioning: The Concept

Fear Conditioning: Discrete Cue Assessment

The Effect of Dexmedetomidine on Discrete Cue Memory

Dexmedetomidine Injection and Testing Schedule Day 1 Training Day 2 Testing Reconsolidation Dex EncodingRetrieval Consolidation

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

Dexmedetomidine (20 µg/kg) had no effect on consolidation of discrete cue fear conditioning

Does dexmedetomidine reduce 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- Fos and P-CREB in amygdala Lateral Nucleus Basolateral Nucleus Central Nucleus

What  2 receptor subtypes are involved in fear conditioning?

Adrenergic receptors

Dexmedetomidine (20µg/kg) did not affect encoding of discrete cue fear conditioning in  2A AR KO mice ACTIVITYINITIAL EXPLORATIONDISCRETE CUE

Dexmedetomidine did not affect discrete cue fear conditioning in D79N mice

Summary  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  2A AR in intrinsic fearfulness of mice

Mice deficient in  2A AR are more fearful in the discrete cue test

Is there any difference in the noradrenergic system in  2A AR deficient mice? LC and nucleus tractus solitarius (NTS) project to the amygdala Are activated by the footshock (unconditioned stimulus)

 2A AR KO mice have a longer locus coeruleus

The LC cell bodies are bigger in  2A AR KO mice

There are more TH positive neurons in the LC of  2A AR KO mice

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

Conclusions  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 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??

Contributors Stanford University – Janet Tsui – Judy Flannery – Xiangqi Li – Brian Hoffman Molecular Research Institute – Tim DeLorey