Forget It! The Role of a 2 Adrenergic Agonists in Fear Conditioning

1. Forget It! The Role of a2 Adrenergic Agonists in Fear Conditioning M. Frances Davies, Ph.D Stanford University Dept of Anesthesia

2. 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. 2002 Nov;78(3):637-47

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

4. The noradrenergic system in fear learning • Intimately involved in vigilance and alertness • NE activates a1,a2 and b adrenergic receptors • NE plays a role in the learning of fear • a2 agonist dexmedetomidine reduces the activity of the central and peripheral noradrenergic system • Reduction of fear is desirable for many anesthetic and ICU procedures

5. Supporting Evidence • Blockade of b and a1 adrenoceptors reduces fear learning • Stimulation of the a2 adrenoceptors tends to: • reduce activity of the central noradrenergic system • may also reduce the learning of fear • This hypothesis has not been rigorously tested

6. Fear Conditioning: Training

7. Fear Conditioning: The Concept

8. Fear Conditioning: Discrete Cue Assessment

9. The Effect of Dexmedetomidine on Discrete Cue Memory

10. Dex Dex Dexmedetomidine Injection and Testing Schedule Day 1 Training Day 2 Testing Encoding Retrieval Consolidation Reconsolidation

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

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

13. 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?

14. Dexmedetomidine reduced c-Fos and P-CREB in amygdala Lateral Nucleus Basolateral Nucleus Central Nucleus

15. What a2 receptor subtypes are involved in fear conditioning?

16. Adrenergic receptors

17. Dexmedetomidine (20µg/kg) did not affect encoding of discrete cue fear conditioning in a2A AR KO mice ACTIVITY INITIAL EXPLORATION DISCRETE CUE

18. Dexmedetomidine did not affect discrete cue fear conditioning in D79N mice

19. Summary • 2receptor activation during training reduces discrete cue fear conditioning • 2receptor activation after training does not • Dexmedetomidine reduced P-CREB and Fos production in amygdala • The effect of 2 agonists on addiction is unknown

20. The role of the a2A AR in intrinsic fearfulness of mice

21. Mice deficient in a2A AR are more fearful in the discrete cue test

22. Is there any difference in the noradrenergic system in a2A AR deficient mice? • LC and nucleus tractus solitarius (NTS) project to the amygdala • Are activated by the footshock (unconditioned stimulus)

23. a2A AR KO mice have a longer locus coeruleus

24. The LC cell bodies are bigger in a2A AR KO mice

25. There are more TH positive neurons in the LC of a2A AR KO mice

26. There are more large neurons in the LC of a2A AR KO mice

27. Conclusions • a2A 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) • a2A adrenergic receptor knockout mice • are very sensitive to discrete cue fear conditioning • lose amnestic effect of dexmedetomidine • have hypertrophied central noradrenergic system

28. Relevance to Addiction • Do individuals differ in their expression of a2A 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??

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