Getting to the Core of the issue between the Nucleus Accumbens and Impulsivity

1. Getting to the Core of the issue between the Nucleus Accumbens and Impulsivity Justin Achua

2. Impulsive Choice Induced in Rats by Lesions of the Nucleus Accumbens Core Rudolf N. Cardinal, David R. Pennicott, C. Lakmali Sugathapala, Trevor W. Robbins, and Barry J. Everitt

3. Why? • Impulsive choice – “Choosing a small or poor reward that is available immediately, in preference to a larger but delayed reward” • Yet the neural mechanisms underlying impulsivity and delayed reinforcement is not understood • Impulsive choice contributes to drug addiction, attention-deficit/hyperactivity disorder (ADHD), mania, and personality disorders

4. Why the Nucleus Accumbens? • Several studies suggest the Nucleus Accumbens (NAc) and afferents are involved in regulating choice between alternative reinforcers • Anterior cingulate cortex (ACC) and medial prefontal cortex (mPFC)

5. Why the Nucleus Accumbens? • A key site for reinforced learning and motivational impact of impending reinforcers • Regulated by Dopamine (DA) and Serotonin (5-HT) • Manipulation of these systems affect impulsive choice • Abnormalities of limbic systems have been observed in impulsive individuals • Animal models of ADHD have shown abnormal DA release in the NAc and mPFC • Humans have shown abnormalities in the mPFC and ACC associated with ADHD

6. How to test this? • Lesions to the nucleus accumbens core (NAcC), ACC, or mPFC in rats testing impulsivity • Rats would select between a smaller immediate appetitive reinforcer and delayed larger reinforcer • The delay to reinforcement would be increase

7. Materials and Methods • Lister-hooded rats were trained on the task • Ranked into pairs according to sensitivity to delay • Randomly assigned one rat from each pair to recieveexcitotoxic lesions and one to recieve sham surgeries • Lesion rats were separated into NAcC, ACC, and mPFClesioned groups

8. Delayed Reinforcement Choice Task

9. Results • Prior to surgery rats shifted preference from the large to small reinforcer as the delay increased • Lesions to NAcC caused a deficit in rats’ ability to choose the delayed reinforcer • Rats became more impulsive • Not due to pre-testing bias • NAcC rats chose larger reinforcer at zero second delay • Lesioned rats were hypersensitive to delays when reintroduced

10. Results • NAaC lesioned rats were hyperactive, ~10% lighter than controls, and took longer to habituate to novel testing apparatus • Ate slower than control rats, but did not differ in total amount consumed • Unlikely that differences in motivation affected impulsive choice

11. Results • NAaC lesioned rats displayed two signs of ADHD • Locomotor hyperactivity • Impulsive choice • Attention deficts can not be seen in rats • NAaC lesions can represent hyperactive/impulsive subtype of ADHD

12. Results • Lesions to the ACC did not affect impulsivity • No change in rats’ ability to choose a delayed reinforcer • Lesions to the mPFC showed an insignificant shift from large to small reinforcer • Lesions to the NAaC induced impulsive choice • Basolateral amygdala and orbitofrontal cortex may promote delayed reinforcers in the NAaC

13. Effect of NAaC, ACC, and mPFC lesions

14. Closing Notes • Found that NAc is involved in impulsive choice • NAc could contribute to ADHD, addiction, and other impulse control disorders

15. Double dissociation of the effects of selective nucleus accumbens core and shell lesions on impulsive-choice behaviour and salience learning in rats Helen H. J. Pothuizen, Ana L. Jongen-Relo, Joram Feldon, and Benjamin K. Yee

16. The Nucleus Accumbens • Consists of two subregions • Dorsolateral Core • Ventromedial Shell • The regions are distinguished by: • Locomotion • Explorative behavior • Latent inhibition • Spatial working memory • Prepulse inhibition of the acoustic startle reflex

17. The Nucleus Accumbens • Also involved in the control of choice behavior • Excitotoxic lesions of the Core result in impulsive behavior • Eg. Choosing a small immediate reward over a larger delayed reward (Cardinal et al., 2001)

18. Introduction • Compared core and shell lesions of the NAc using: • An initial evaluation of latent inhibition • Similar delayed reward choice paradigm • Differential reinforcement for low rates of responding (DRL) operant task

19. Materials and Methods • Male Winstar rats were used for all experiments • Animals were separated into core lesion, shell lesion, sham operation, and no operation groups • All test conducted during dark phase • Stereotaxic bilateral lesions were made by injecting N-methyl-D-aspartate (NMDA) • All animals were tested for latent inhibition (LI) then separated into two groups • Delayed reward choice experiment • DRL experiment

20. Figure 1. Extent of NAcC and NAcS in a coronal plane

21. Figure 2. Selective NAcC and NAcS lesions

22. Experiment 1 : Latent inhibition • Rats from the four surgical groups were subdivided into two groups • Pre-exposure (PE) • Nonpre-exposure (NPE) • During pre-exposure PE rats were placed into the testing chamber with the tone stimulus playing • NPE rats were placed into the chamber without tone • The number of crossings was measured as basal locomotor activity

23. Experiment 1 : Latent inhibition • During conditioning rats were placed into testing chamber for 100 trials • Trials consist of: • 10s tone • Followed by 2s foot shock • If subject crosses barrier during first 10s of tone, no foot shock, avoidance response is recorded • If subject crosses barrier during foot shock, foot shock and tone are terminated, escape response is recorded • If subject fails to cross barrier after 2s foot shock, escape failure is recorded

24. Experiment 1 : Latent inhibition • The number of avoidance response over successive 10-trial blocks were recorded • Measurement of conditioned avoidance learning • LI effect present – lower avoidance response in PE when compared to NPE • LI was reduced in the shell lesioned group when compared to core and sham groups • Selective core lesion did not affect integrity of the shell lesion

25. Experiment 1 : Latent inhibition

26. Experiment 2 : Delayed reward choice paradigm • Testing was conducted in phases • 3-9 days of forced-trial training • 2-10 days of choice-trial testing • No-delay conditioning occurred before every choice-trial • Inside the test chamber there is a CRF lever and PRF lever • Continuously reinforced (CRF) lever dispense a food pellet after a set delay • 0, 20, 0, 10, 0, 15, 0, 20s delay respective to each phase • Partially reinforced (PRF) lever has a probability of 25% of dispensing a food pellet

27. Experiment 2 : Delayed reward choice paradigm • Forced-trial training consisted of giving the subject 12 forced CRF trials followed by 12 forced PRF trials • Only one lever was given in testing chamber • Choice-trial training consisted of the presence of both levers • The nonselected lever was immediately removed • The selected lever as removed after 5 presses • If CRF lever selected, light switched on, nose-poke initiates deliver of food pellet • If PRF lever selected, an immediate food pellet or nothing is delivered

28. Experiment 2 : Delayed reward choice paradigm • 20s (1) delayed CRF lever dropped to near chance levels • Shifting of CRF lever towards PRF lever was observed over 5 days of testing for shell lesion and sham groups • Core lesion group remained at chance levels

29. Experiment 2 : Delayed reward choice paradigm • 10s delayed CRF lever shifted away from CRF, but CRF bias remained over 5 days of testing

30. Experiment 2 : Delayed reward choice paradigm • 15s delayed CRF lever comparable to 10s delayed CRF lever with lower CRF bias

31. Experiment 2 : Delayed reward choice paradigm • 20s (2) delayed CRF lever showed a shift of preference away from CRF • Initial shift maintained above chance levels for shell lesion and sham groups • Most pronounced shift in core lesion group

32. Experiment 3 : Differential reinforcement for low rates of response task • After the first lever press only responses made after a specific delay are reinforced with a food pellet • Premature responses are not rewarded and reset the time to zero • The ration of mean lever-presses per reward is used to analyze DRL performance

33. Experiment 3 : Differential reinforcement for low rates of response task • DRL-4s – performance improved over the three 2-day blocks

34. Experiment 3 : Differential reinforcement for low rates of response task • DRL-8s – increase in delay resulted in decreased performance • Performance improved over the three 2-day blocks • Improvement to a lesser degree for the core lesion group

35. Experiment 3 : Differential reinforcement for low rates of response task • DRL-12s – initial reduction in performance followed by improvement • Continued trend of poor improvement in core lesion group

36. Experiment 3 : Differential reinforcement for low rates of response task • DRL-18s – initial reduction in performance followed by improvement • Shell lesion and sham groups performance at similar levels • Continued trend of inferior performance in core lesion group

37. Discussion • Expands upon finding of Cardinal et al., 2001 using a similar delayed reward choice test • Utilizes contrast between reward probability and reward size • Impulsive-like behavior was associated with NAc core damage • DRL performance was impaired by core lesion • Core lesion lead to impulsive-like behavior in delayed reward choice test • NAc shell lesions did not show similar results

38. Conclusion • LI is not associated with enhanced impulsive behavior • Lesions of the NAc shell abolished LI • Dysfunction of the NAc core, not shell, may be associated with the appearance of impulsive-like behaviors • Evidence by a pronounced shift from choosing the CRF lever to PRF lever • Impaired performance on DRL task

39. Gamma Aminobutyric Acidergic and Neuronal Structural Markers in the Nucleus Accumbens Core Underlie Trait-like Impulsive Behavior Daniele Caprioli, Stephen J. Sawiak, Emiliano Merlo, David E.H. Theobald, Marcia Spoelder, Bianca Jupp, Valerie Voon, T. Adrian Carpenter, Barry J. Everitt, Trevor W. Robbins, and Jeffrey W. Dalley

40. Introduction • Impulsivity – defined as a wide variety of behaviors, including, “A failure of motor inhibition to individual predisposition to choose small, immediate rewards as opposed to large but delayed rewards” • Motor impulsivity – motor response inhibition • Decisional impulsivity – delay discounting and reflection impulsivity • High levels of impulsivity have been associated with: • ADHD • Conduct disorders • Antisocial behaviors • Substance use disorders

41. Introduction • Mechanisms of impulsivity are not well understood • Deficiencies in norepinephrine (NE) and dopamine (DA) • Abnormalities in PFC and striatum’ • Previous research points towards nucleus accumbens • Behavior output in the NAc is governed by GABA-ergic medium-spiny neurons (MSN) • May play a critical role in synaptic transmission in the NAc and impulsivity

42. Materials and Methods • Lister-hooded rats • Assessed impulsivity using a five-choice serial reaction time task (5-CSRTT) • Subjects were trained on the task and separated into high impulsivity (HI), medium impulsivity (MI), and low impulsivity (LI) groups • Conversion of LI group to HI behavior using IC injection of glutamate decarboxylase (GAD)

43. Materials and Methods • 5-CSRTT – rats were trained to locate a brief visual stimuli in one of 5 apertures • Correct response was rewarded with a food pellet • Incorrect responses, no responses, and premature responses resulted in a 5s time out and no food pellet • HI rats – over 50% of trials with premature responses • LI rats – lowest premature responses • MI rats – intermediate levels of premature responses • MRI was given after the 5-CSRTT

44. Materials and Methods • LI rats’ NAcC was intracerebrally cannulated • Received bilateral injections of GAD antisense scramble sequence (ASO) or scramble pairs (Scr) • Unilateral injections of ASO and Scr • Cannulated rats were then ran on the 5-CSRTT • Western blot analysis was performed on LI and HI rats

45. Results • When delay to 5-CSRTT visual stimuli was increased from 5s to 7s • Greatest increase in impulsivity observed in HI rats • MRI revealed a significant reduction of gray matter in the left NAcC of HI rats • Correlated inversely with impulsivity on 5-CSRTT • Western blot analysis of the left NAcC in HI rats showed significantly lower levels of: • GAD • Marker microtubule associated protein (MAP2) • Dendritic spine marker spinophilin

46. Figure 1

47. Figure 2

48. Results • GAD ASO caused a significant increase in impulsivity in LI rats • No effect on locomotor activity, speed, or accuracy on 5-CSRTT • Bilateral injections of GAD ASO resulted in observed increase in impulsivity • Unilateral injections to left or right NAcC showed no significant effect • Confirmed localization of GAD ASO to NAcC core • No presence in NAcS

49. Figure 3

50. Discussion • Relationship between nucleus accumbens core and impulsivity on the 5-CSRTT • Reduction of grey matter density in NAcC of HI rats corresponds with impulsivity • Reduction in GAD • Reduction in dendritic spine and microtubule markers