the effects of intra-cea parp-1 inhibition on cocaine

Running Head: PARP-1 INHIBITION AND COCAINE SEEKING

The effects of intra-CeA PARP-1 inhibition on cocaine reinstatement

By: Megan Brickner

Honors Thesis

Advisor: Dr. Kathryn Reissner

Department of Psychology & Neuroscience

University of North Carolina at Chapel Hill

PARP-1 INHIBITION AND COCAINE SEEKING

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Acknowledgements

I would first like to thank my advisor and mentor, Dr. Kathryn Reissner, for supporting me as

both a professor and a research mentor. Her class and passion for the material allowed me to

translate my interests into opportunities. Thank you for seeing potential in me by investing time

and energy into my development – as a student, researcher, and person.

I am also grateful to Emily Witt who has been a fantastic graduate student mentor to me for the

past two years. Thank you for your role in shaping this thesis, as well as for teaching me so many

valuable skills that I will take with me for the rest of my research training and career. I would not

have been able to do this without you.

To the rest of the Reissner lab –Eden, Janay, Anze, and Ron – thank you for creating a

welcoming environment. I have enjoyed getting to know you all, appreciate your help, and

cherish your support.

Dr. Rachel Penton and Dr. Regina Carelli, thank you for serving on my Honors Thesis

Committee. I appreciate the time and consideration you put into my thesis and my presentation.

Finally, thank you to my family, friends, and loved ones who have supported me throughout my

time at UNC. Their love and encouragement gave me the foundation I needed to pursue research

and complete an Honors Thesis. I am so fortunate to have found such wonderful people through

the Women’s Rugby Team, Tri Sigma Sorority, and my first year dorm; a special nod to my

friends who have been in my corner since high school. To mom, dad, and Rebecca: I am so lucky

to have you by my side, thank you for providing me with so many opportunities.

This project was supported by the Gump Family Undergraduate Research Fund administered

by Honors Carolina. Additional funding was provided by the Lindquist Undergraduate Research

Award.


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Abstract

Currently, there are no FDA-approved treatments for cocaine or psychostimulant use disorders,

compounding the need for the development of effective interventions. The DNA repair enzyme

and transcription regulator, Poly(ADP-ribose)polymerase (PARP-1), is activated by cocaine and

contributes to mechanisms of cocaine action in the brain (Dash et al., 2017; Lax et al., 2017;

Scobie et al., 2014). While it is known that inhibition of PARP-1 with the compound PJ34 within

the central nucleus of the amygdala (CeA) reduces cocaine conditioned place preference (Lax et

al., 2017), the effects of PARP-1 inhibition on cocaine relapse after cessation of self-

administration have not been investigated. Accordingly, the goal of this project was to test the

hypothesis that pharmacological inhibition of PARP-1 in the CeA would inhibit cocaine seeking

in the rat reinstatement model of cocaine addiction. Results indicate that while vehicle-treated

rats significantly reinstated over extinction baseline, PJ34-treated rats did not, suggesting an

inhibitory effect of CeA PARP-1 inhibition against reinstatement. Future studies will better

inform the involvement of CeA PARP-1 in cocaine seeking, and guide future research toward

PARP-1 as a candidate pharmacotherapeutic intervention for cocaine use disorders.


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Introduction

In 2016, 1.9 million Americans were current users of cocaine, while approximately

867,000 people met the clinical criteria for a cocaine use disorder (CUD; NSDUH, 2017).

However, no FDA-approved pharmacological treatments currently exist specifically for CUD.

Accordingly, investigations into candidate pharmacotherapies are critical, and elucidation of

cellular mechanisms that drive drug seeking and relapse are essential to this endeavor. One such

mechanism may be the link between drug-paired cues and relapse. Existing research indicates

that drug use is reinforced by the addictive drugs themselves as well as the drug related cues, and

that intervention into both of these reinforcers (the drug and drug-related cues) can reduce drug

seeking in animal models (Namba, Tomek, Olive, Beckmann, & Gipson, 2018; Perry, Zbukvic,

Kim, & Lawrence, 2014).

Specifically, the rat self-administration and reinstatement model is an approach used to

model drug addiction based on the principles of operant conditioning. It provides a system in

which candidate treatments can be evaluated, and has face and construct validity as a model of

human drug use and relapse (Panlilio & Goldberg, 2007). The model we utilized has three phases:

self-administration, extinction, and reinstatement (Panlilio & Goldberg, 2007). In this model,

reinstatement to cocaine seeking following a period of extinction training can be precipitated in

numerous ways, including reintroduction of drug-paired cues (Farrell, Schoch, & Mahler, 2018).

This cue-primed reinstatement can be used to evaluate mechanisms and interventions for relapse

vulnerability.

Research in the Reissner lab has shown that chronic systemic administration of

nicotinamide (NAM), a form of vitamin B3, reduces cue-primed reinstatement to cocaine,

without effects on locomotor activity or food seeking behavior (Witt & Reissner, 2019). These


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results were only found in males, and not in female rats. This suggests that NAM may play a role

in modifying the salience of cocaine-associated cues in a sex-specific manner (Witt & Reissner,

2019). While the mechanism by which NAM may reduce cue-primed seeking is currently still

unknown, several known effects of NAM indicate potential mechanisms by which it may be

beneficial in combating cellular adaptations induced by cocaine self-administration.

First, NAM is a known inhibitor of poly(ADP-ribose) polymerase (PARP) (Javle &

Curtin, 2011; Riklis, Kol, & Marko, 1990). PARP is a family of enzymes composed of at least 17

members, each with a specific function and specialization (Amé, Spenlehauer, & de Murcia,

2004). Of the 17 PARP family members, PARP-1 is the most active and readily studied enzyme

(Morales et al., 2014). In general, PARPs function by attaching ADP-ribose units onto substrate

proteins in order to regulate the cell’s response to oxidative stress conditions such as DNA

damage to aid in DNA repair (Malanga & Althaus, 2005; Vyas, Chesarone-Cataldo, Todorova,

Huang, & Chang, 2013). This process is known as PARylation. DNA in the brain can be

damaged after cocaine exposure (Alvarenga et al., 2010), indicating an important role for PARP-

mediated repair. Secondly, PARPs also regulate transcription (Kraus & Hottiger, 2013) by

binding to gene promoter regions to regulate gene expression (Gupte, Liu, & Kraus, 2017), and

cocaine has been found to modulate this PARP-mediated transcription (Lax et al., 2017; Scobie

et al., 2014). Additionally, PARP-1 activation can trigger the mitochondria to release apoptosis-

inducing factor, which can lead to apoptosis (Yu et al., 2002; Du et al., 2003). This suggests that

cocaine use may be harmful to the brain, as PARP-1 expression is increased in the nucleus

accumbens (NAc) after cocaine exposure (Dash et al., 2017).

This cocaine exposure-induced increased PARP-1 expression within the NAc (Dash et al.,

2017) may also promote behaviors associated with drug seeking (Scobie et al., 2014). For


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example, experimenter-induced overexpression of PARP-1 in mice resulted in increased cocaine

self-administration, increased motor responses to cocaine, and increased conditioned place

preference (CPP) – a behavioral paradigm used to assess of cocaine-associated memories and

seeking (Scobie et al., 2014). PARP-1 is also activated during retrieval of cocaine-associated

contextual memory in the central nucleus of the amygdala (CeA), indicating a role of PARP-1 in

the retrieval of cocaine-associated contextual memory (Lax et al., 2017). Further, inhibition of

PARP-1 in the CeA with PJ34 reduced CPP (Lax et al., 2017). However, the effects of intra-CeA

PARP-1 inhibition on cocaine reinstatement following self-administration and extinction have

not been investigated.

The CeA is one of a number of sub-nuclei within the amygdala (Rasia-Filho, Londero, &

Achaval, 2000). The amygdala is a prominent and key structure in the brain’s motivation-related

circuitry and helps create and strengthen relationships between stimuli, significance, and

motivation (for review, see Kim, 2013). In relation to addiction, the amygdala is activated by

drug cues that can promote drug craving and seeking in humans (Tang, Fellows, Small, &

Dagher, 2012). In animal models, the CeA in particular has been found to be involved in cue-

related drug craving (Lu et al., 2005; Lu, Uejima, Gray, Bossert, & Shaham, 2007). For example,

optogenetic stimulation of the CeA augmented the motivation for and intake of cocaine, while

optogenetic inhibition of the CeA attenuated both the acquisition of cocaine self-administration

and cocaine intake in female rats (Warlow, Robinson, & Berridge, 2017). This CeA stimulation

also increased the attractiveness of the cocaine-associated cues themselves as the rats displayed

biting behavior towards the nose port that pairs the CeA stimulating laser with intravenous

cocaine, indicating amplified incentive salience towards the cue (Warlaw et al., 2017). As

mentioned, inhibition via intra-CeA PJ34 administration reduced cocaine CPP (Lax et al., 2017).


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This suggests that PARP-1 activation may play a role in cocaine-associated memory retrieval

(Lax et al., 2017) and as such, PARP-1 inhibition within the CeA may lead to similar reductions

in cocaine seeking behaviors after a self-administration and extinction paradigm.

PARP inhibition has not been examined in a self-administration and extinction model of

reinstatement; however, I hypothesized that the reported effects of NAM against reinstatement

may occur through an effect of PARP inhibition. If this is true, direct inhibition of PARP should

inhibit reinstatement. Moreover, because PARP activation in the CeA plays a particularly salient

role in responsiveness to drug-paired cues, I hypothesized that NAM might inhibit cue-primed

reinstatement through inhibition of PARP in the CeA. Accordingly, the goal of this project was

to determine if pharmacological inhibition of PARP via intra-CeA administration of PJ34 would

inhibit cocaine seeking in the rodent reinstatement model of cocaine seeking. .


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Methods

Subjects

This study utilized thirty-two adult male Sprague-Dawley rats purchased from Envigo.

They were housed individually in temperature and humidity controlled cages. Rats were exposed

to a 12-hour light-dark cycle, where the dark period lasted from 7 AM to 7 PM. All experiments

were performed during the dark cycle at the same time daily. All experiments were approved by

the Institutional Animal Care and Use Committee (IACUC) at the University of North Carolina

at Chapel Hill, and were in accordance with the guidelines of the American Association for the

Accreditation of Laboratory Animal Care (AAALAC) and the National Research Council’s

Guide for Care and Use of Laboratory Animals.

Surgery

Rats were anesthetized with ketamine (100 mg/kg, i.m.) and xylazine (7 mg/kg, i.m.).

Catheters were then implanted into the right jugular vein with an exit on the back to allow for

intravenous (i.v.) administration of cocaine. Intracranial cannulas were then implanted via

stereotaxic surgery into the CeA according to the coordinates of Paxinos and Watson: AP -2.56

mm, ML +4 mm, DV -7 mm (Lax et al., 2017). Upon completion of the surgeries, rats received

meloxicam analgesia (4 mg/ml) for 3 days during post-surgical recovery and care. Catheters were

flushed daily with gentamicin antibiotic (0.1 mL; 5 mg/mL), and an anticoagulant, heparinized

saline (0.1 mL; 100U/mL), throughout surgery recovery as well as cocaine self-administration in

order to support catheter patency. Twenty-four hrs before self-administration began, catheter

patency was validated with propofol (1 mg/0.1 mL, i.v.).


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Behavioral Training

All operant conditioning was performed in standard rat operant boxes (Med Associates).

Before undergoing surgical procedures, each rat was trained in operant responding for food.

During this session, presses on the active lever in operant box were paired with a release of a 45

mg food pellet, while presses on the inactive had no consequence. Food training sessions ended

after at least 100 pellets were received or after 6 hours had elapsed. Surgery was performed as

described above before beginning the cocaine self-administration paradigm.

At the start of each daily 2 hr session, the house light was illuminated and two levers

were extended: an active and inactive lever. Throughout the self-administration sessions, under a

fixed-ratio 1 (FR1) schedule, active lever presses resulted in an infusion of cocaine (0.75

mg/kg/infusion), as well as presentation of a light and tone cue for 5 s. Reward and cue delivery

were followed by a 20 s time out, during which time presses were recorded but had no effect.

Presses on the inactive lever had no programmed consequences throughout the session. On the

first two self-administration days, all rats were restricted to a maximum of 40 cocaine infusions;

all following sessions were unrestricted. To qualify as a successful self-administration session,

the animal must have received 10 or more infusions of cocaine per session.

Following 12 successful self-administration sessions of 10 or more infusions, the

extinction period began and continued for 15 days. Extinction sessions began similarly to self-

administration sessions with house light illumination and extension of levers; however, cocaine,

tone, and light stimuli were no longer presented following active lever presses.

Half of the rats were randomly assigned to receive the PARP-1 inhibitor, PJ34 in

artificial cerebrospinal fluid (aCSF; n=16), while the other half received a vehicle solution, aCSF

(n=16), as a control. PJ34 (50μM) or vehicle was administered bilaterally via microinjection


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(0.5L per hemisphere; 1L/min) directly into the CeA as described by Lax et al. (2017).

Microinjections occurred 1mm below the base of the cannula. After the infusion, the

microinjectors were kept in the guide cannulas for 60s, to allow for diffusion. Microinjections

were administered 30 min prior to reinstatement testing, based on the methods employed by Lax

et al. (2017).

Thirty min after the microinjection, animals were placed into the operant boxes for the 2

hour cue-primed reinstatement test. During reinstatement, the active lever was once again paired

with the light and tone cues, while the inactive lever continued to have no programmed

consequences. Cocaine was not reintroduced.

Histology

Following the cue-primed reinstatement session, animals were anesthetized with

pentobarbital and transcardially perfused with 4% paraformaldehyde in phosphate buffer. The

brains were extracted and cryoprotected in a 30% sucrose solution prior to cryostat sectioning.

100 m brain slices of the CeA were collected for Cresyl Violet staining for histological

confirmation of cannula placement.

Data Analysis

Behavioral data were collected on lever presses during each phase of training and testing,

and were analyzed using GraphPad Prism. A repeated-measures two-way analysis of variance

(ANOVA) test was used to compare the number of cocaine infusions between groups during

self-administration, as well as active lever responses during cocaine self-administration and

extinction. Reinstatement responding was also analyzed using an two-way repeated measures


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ANOVA between groups (PJ34 vs. vehicle) comparing operant behavior on the last day of

extinction with reinstatement behavior via mean levels of active lever presses. Sidak’s test of

multiple comparisons was used to examine the direction of mean difference in lever pressing by

condition. Results are reported as mean ± variance (SEM), where p < 0.05 was considered

significant.


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Results

No significant differences were found between future treatment groups for active lever

presses or infusions during cocaine self-administration (lever presses: Fig. 1A; F(1, 30) = 0.68, p

= .42; infusions: Fig. 1B; F(1, 30) = 0.29, p = .59). The number of active lever presses

throughout extinction was also not significantly different between future treatment groups (Fig.

1A; F(1, 30) = 0.10, p = .76). However, there was a significant main effect of session across self-

administration (Fig. 1A; F(11, 330) = 6.99, p < 0.0001) and extinction (Fig. 1A; F(15, 540) =

73.71, p < 0.0001 ), indicating that the number of lever presses was significantly different based

on the day of the paradigm. There was also a significant effect of session across self-

administration when comparing cocaine infusions (Fig. 1B; F(11, 330) = 10.59, p < 0.0001),

indicating that the number of cocaine rewards significantly differed based on the session number

of the self-administration phase.

Two-way repeated measures ANOVAs were also utilized to analyze cue-primed

reinstatement responding, to determine whether active lever presses during the final extinction

session and the reinstatement test differed as a function of treatment group. A significant main

effect was found for session, indicating a significant difference in active lever presses during the

last day of extinction and reinstatement (Fig. 2; F(1, 30) = 35.84, p < .0001). Sidak’s test of

multiple comparisons indicated that the active lever presses were significantly higher during the

cue-primed reinstatement than during the last day of extinction for both the PJ34 (Fig. 2; p

= .002) and vehicle group (Fig. 2; p < .0001). No main effect of treatment group observed for

lever pressing between the PJ34 and vehicle groups during reinstatement (Fig. 2; F(1, 30)= 0.88,

p= .36).

Histology of the brains was performed to examine cannula placement and PJ34 delivery


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to the CeA. Analysis indicated that 20 of the 32 rats had cannula tracts in the proper positions, 5

bilateral hits and 15 unilateral hits. The data from the other 12 rats were removed, and analysis

was repeated to include only the hits. Note that while histological analysis has been completed,

plans to generate placement maps were disrupted due to SARS-CoV-2 interruption of research

activities.

Behavioral analysis was repeated, using only data from rats with either bilateral or

unilateral hits of the CeA. The updated analyses revealed no significant differences between the

PJ34 (n=9) and the vehicle (n=11) future treatment groups for the number of active lever presses

throughout self-administration or extinction (self-administration: Fig. 3A; F(1, 18) = 1.98, p

= .18; extinction: Fig. 3A; F(1, 18) = 0.16, p = .70), nor infusions during cocaine self-

administration (Fig. 3B; F(1, 18) = 0.55, p = .47).There was a significant main effect of session

across self-administration (Fig. 1A; F(11, 198) = 4.93, p < 0.0001) and extinction (Fig. 1A; F(15,

270) = 48.88, p < 0.0001 ). There was also significant effect of session across self-administration

when comparing cocaine infusions (Fig. 1B; F(11, 198) = 8.38, p < 0.0001).

Additionally, when comparing active lever presses during the last day of extinction and

the cue-primed reinstatement test, a significant main effect was found for session (Fig. 4; F(1, 18)

= 18.46, p = .0004), such that the active lever presses were significantly higher during the cue-

primed reinstatement than the last day of extinction for the vehicle group (Fig. 4; p < .0026).

However, there were no significant differences between the active lever presses during the last

extinction session and the reinstatement test for the PJ34 group (Fig. 4; p = .059). Finally, there

was no main effect of treatment group observed for lever pressing during reinstatement between

the PJ34 and vehicle groups (Fig. 4; F(1, 18)= 0.53, p= .48).


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Figure 1. (A) Comparison of active lever presses throughout the self-administration and

extinction phases for all rats, separated by future treatment groups. Analyses showed no

significant difference between the active lever presses in either phase between the treatment

groups. (B) Cocaine infusions during self-administration sessions for vehicle (open circles) and

PJ34 (filled circles) future groupings. No significant differences in cocaine infusions were found

between the experimental and control groups.

A)

B)


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Figure 2. Active lever pressing during extinction and cue-primed reinstatement test for

vehicle (open bars) and PJ34-treated rats (filled bars) for all rats used in the study. Post-hoc

analyses indicated significant reinstatement in both the vehicle-treated and PJ34-treated group.

However, there was no main effect of treatment group. *=p<0.05, indicating a main effect of

session.


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Figure 3. (A) Comparison of active lever presses throughout the self-administration and

extinction phases of the paradigm for the rats with bilateral or unilateral cannula hits, separated

by future treatment groups (Vehicle n=11; PJ34 n=9). Analyses showed no significant difference

between the active lever presses in either phase between the treatment groups. (B) Cocaine

infusions during self-administration sessions for vehicle (open circles) and PJ34 (filled circles)

future groupings. No significant differences in cocaine infusions were found between the

experimental and control groups.

A)

B)


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Figure 4. Active lever pressing during extinction and cue-primed reinstatement test for

vehicle (open bars) and PJ34-treated rats (filled bars). Post-hoc analyses indicated significant

reinstatement in the vehicle-treated, but not PJ34-treated group. * indicates main effect of

session.

11 9


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Discussion

This study investigated the role of intra-CeA PARP-1 in cue-primed reinstatement via

intra-CeA administration of PJ34 in a rat reinstatement model of cocaine seeking. While CeA

PARP-1 has been shown to mediate cocaine conditioned place preference (Lax et al., 2017), the

involvement of PARP-1 in reinstatement to cocaine seeking is unknown. I hypothesized that

administration of a 50 μM (total volume 0.5μL/hemisphere) dose of PJ34 delivered 30 min

directly into the CeA before cue-primed reinstatement would reduce reinstatement responding.

When analyzing only unilateral and bilateral hits confirmed by histology, only the vehicle group

significantly reinstated as compared to the final day of extinction, while the PJ34 group exhibited

no significant difference in active lever presses when comparing the last day of extinction and

the cue-primed reinstatement test. However, there was no significant difference between the

vehicle and control groups in active lever presses during reinstatement. This suggests that while

the treatment of PJ34 was not effective in significantly reducing seeking behaviors when

compared to the vehicle group, PJ34 has the potential to reduce the salience of light and tone

cues to inhibit seeking behaviors. More studies will be required to confirm this result and more

effectively power the study.

Strengths & Weaknesses of Current Study

A strength of the study was the ability to assess cue-induced reinstatement. In humans,

drug-associated cues can cause cravings, induce seeking behaviors, and initiate relapse

(Childress et al., 1993), suggesting that targeting cue-primed reinstatement is clinically relevant.

Past studies have shown drug seeking behaviors can be profoundly influenced by drug cues for a

long period following cessation of drug use in both animal models (Lu, Grimm, Hope, &


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Shaham, 2004; Madangopal et al., 2019) and humans (Parvaz, Moeller, & Goldstein, 2016).

Secondly, the PARP-1 inhibitor used in this study, PJ34, is a highly specific and potent inhibitor.

Use of PJ34 results in 10,000 times more PARP inhibition than other PARP inhibitors such as 3-

AB (Abdelkarim et al., 2001). PJ34 also exhibits more specific inhibition, via IC50 values, of

PARP-1 than inhibition of TNKS1 and PARP-2 (Wahlberg et al., 2012).

However, a limitation of this study was intra-CeA cannula placements. Histological

analysis confirmed that only 20 cannulas were positioned correctly above the CeA, indicating

that those other 12 animals did not have PJ34 or vehicle delivery to the CeA during

microinjections. Elimination of rats with inaccurate cannula placement changed our results to

suggest an inhibitory effect of CeA PARP-1 inhibition against reinstatement. However, while the

rats in the PJ34 group did not significantly reinstate, the vehicle-treated rats did. With proper

cannula placement and PJ34 administration to the CeA, it is possible the results would have

confirmed our hypothesis: intra-CeA PARP-1 inhibition would result in a significant reduction of

reinstatement behaviors in comparison to a control group, as previously observed for cocaine

CPP (Lax et al., 2017).

Before repeating this study with adjusted stereotaxic cannula placements, it may be

beneficial to conduct an experiment to see the effects of a systemic PARP-1 inhibitor. Past

research on the use of systemic NAM, a known PARP inhibitor, found that cue-primed

reinstatement can be reduced with chronic administration of NAM (Witt & Reissner, 2019). The

current experiment was designed as a follow-up study to investigate the CeA based on previous

findings involving PARP-1 inhibition and the CeA (Lax et al., 2017) as well as the experimental

overexpression of PARP-1 (Scobie et al., 2014). An experiment utilizing a systemic PARP-1

inhibitor could provide evidence that, when applied across the whole brain, PARP-1 inhibition


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has the desired effect on cocaine seeking during a reinstatement paradigm without limiting PJ34

to one, specific brain area.

The Relationship of PARP and NAD+

As well as a PARP inhibitor, NAM is also a precursor for oxidized nicotinamide adenine

dinucleotide (NAD+), which may be beneficial in reversing the neurodegeneration associated

with drug use, as it is a coenzyme necessary for many cell reactions – including metabolism.

Further, NAD+ and PARP-1 affect each other. PARP-1 consumes NAD+ in order to respond to

DNA breakage and complete cellular repair tasks within the nucleus (Morales et al., 2014).

PARPs aid in the repair mechanism for damaged DNA by synthesizing poly-ADP-ribose chains

on proteins, which in turn, signal for DNA repair enzymes to recruit to and mend the breakage

point (Yelamos, Farres, Llacuna, Ampurdanes, & Martin-Caballero, 2011).

While it is involved in DNA repair, too much PARP activation can result in diminished

levels of NAD+ (Zhang et al., 2019) and ATP (Ha & Snyder, 1999). This depletion of NAD+ by

PARP-1 during routine and drug-induced repair can lead to lack of cellular repair and

neurodegeneration (Liu et al., 2009). For this study, these findings were interpreted to suggest

that inhibiting PARP-1, and therefore helping maintain NAD+ levels after drug exposure, may

aid in DNA repair, as DNA in the brain and other organs can be damaged after just a single

exposure to cocaine (Alvarenga et al., 2010). However, the results of systemic administration of

NAM being associated with reduced cue-induced reinstatement (Witt & Reissner, 2019) may be

due to NAM’s role as an NAD+ precursor. It may be advantageous to investigate the effects of

increasing NAD+ levels in the CeA or NAc as the protective nature of NAD+ by NAM


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suppresses DNA fragmentation throughout both healthy and necrotic cell death (Klaidman,

Mukherjee, Hutchin, & Adams, 1996).

Brain Regions Involved in Cocaine Seeking

While the CeA’s involvement in motivation and stimuli makes this brain region a

promising target for pharmacotherapies to reduce cocaine seeking, the NAc may also be a

promising target for PARP-1 inhibition for reducing seeking behaviors, especially if systemic

PARP inhibition proves successful. The NAc is a central nucleus in the brain’s mesolimbic

pathway (Adinoff, 2004). Past research has found an increase of PARP-1 levels in the CeA

during CPP (Lax et al., 2017), but similar findings have been found following cocaine exposure

in the NAc (Dash et al., 2017; Scobie et al., 2014). After cocaine exposure in rats, the NAc

undergoes other cocaine-induced adaptions thought to contribute to long-lasting cocaine seeking

(Kalivas, 2009; Scofield et al., 2016). Neuropathological adaptions include changes in

methylation of DNA that alters gene expression (for review: Vaillancourt, Ernst, Mash, &

Turecki, 2017), while subsequent behavioral adaptions include cocaine-induced behavioral

sensitization (Anier, Malinovskaja, Aonurm-Helm, Zharkovsky, & Kalda, 2010), incubation of

cocaine craving (Massart et al., 2015), and addiction (Sadri-Vakili, 2015). Within the self-

administration paradigm, the presentation of cocaine-paired cues trigger rapid synaptic plasticity

in the NAc, which makes the neurons hypersensitive to the incoming excitatory signals (Gipson

et al., 2013). In turn, this sensitization promotes the physical seeking behaviors due to the

outputs from the NAc to the brain’s motor circuits (Gipson et al., 2013). Accordingly,

pharmacologically targeting PARP within the NAc to attenuate cue-induced relapse may be a

worthwhile next step in this study.


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Role of Gene Transcription

As addressed above, PARP-1 activity is important for regulation of gene transcription.

PARP-1 can promote the activation of transcription through engagement with gene promoter and

enhancer complexes (Kraus & Lis, 2003). In the NAc, chronic cocaine increased the association

between PARP-1 and NF-kB, a transcription factor, suggesting that cocaine exposure may

enhance PARP-1’s involvement in activating transcriptional complexes (Scobie et al., 2014).

PARP-1 can also repress transcription in certain genes (Kraus, 2008). For example, within the

CeA, PARP-1 may play a role in the transcription regulation involved in cocaine CPP (Lax et al.,

2017). A new gene, D3ZLJ1, was discovered and while it is unknown how D3ZLJ1 is exactly

involved in drug addiction, PARP-1 binds to its promoter to repress gene expression, which in

turn, augments cocaine CPP memory retrieval (Lax et al., 2017). Administration of PJ34 inhibits

this PARP-1 binding to D3ZLJ1, which is associated with increased D3ZLJ1 expression. The

subsequent upregulation of D3ZLJ1 and/or downregulation of PARP-1 reduced cocaine-CPP

memory retrieval (Lax et al., 2017).

Additionally, PARP-1 can affect transcription by preventing Histone H1 from binding to

PARP-1 regulated promotors (Krishnakumar et al., 2008). H1 modulates the chromatin

arrangement (Woodcock, Skoultchi, & Fan, 2006) and is a transcription regulator that can

repress or enhance the transcription of genes (Hergeth & Schneider, 2015). Blocking H1 from

binding has implications for proper regulation of gene expression and chromatin structure

(Krishnakumar et al., 2008). Following chronic cocaine administration, increased expression of

PARP-1 in the mouse NAc is associated with increased levels of PARylated H1, which changes

the chromatin structure to be ready for transcription (Scobie et al., 2014).

Perhaps PARP-1’s modulatory role in transcription contributes to cocaine addiction by


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activating transcription on genes that promote drug seeking or repressing transcription on genes

within the brain that help suppress those behaviors. The expression of the PARP-1 gene itself

may be regulated by the binding of PARP-1 to hairpin loops in the DNA molecules (Soldatenkov

et al., 2001), suggesting that cocaine mediated PARP-1 increases may also serve as a feedback

loop that modulates PARP-1 levels within the brain. However, future research should investigate

and build upon these claims.

Conclusion

There is no current evidence to suggest that PARP-1 activity increases in the human brain

after cocaine use. Past research on rodent models, however, provide promising indications that

PARP-1 plays a role in cocaine addiction, which can presumably be translated into the human

model. In support of this translational utility, PARP inhibitors are currently used as treatments

for cancers that are caused by mistakes in the DNA repair genes (for a review paper on clinical

trials see Chen, 2011). PARP inhibition stops the cancer cells from being repaired, leading to cell

death and tumor size reduction (Chen, 2011). In the example of breast cancer, the BRCA DNA

gene is faulty. If the cancerous cells only have PARP to repair the DNA because the BRCA is

compromised, the DNA repair will be incomplete but the compromised cell can still replicate

(Chen, 2011). This can lead to tumor growth as usually cells with broken DNA will trigger their

own death. Inhibiting PARP would cause the cancer cells with broken DNA to undergo

apoptosis and stunt tumor growth (Chen, 2011). An increase of PARP activity has also been

found in inflammation-related diseases, such as neurodegenerative diseases (Pazzaglia & Pioli,

2019).


23

Presently, there are no FDA-approved pharmacological treatments for CUD or other

psychostimulant use disorders. The most common treatment plans revolved psychosocial

therapies such as contingency management and cognitive behavioral therapy, but none of the

latest therapeutic approaches are particularly effective for most patients (Kampman, 2019). It is

important to find better, more effective treatments for cocaine use. This can be accomplished by

discovering the complex neural mechanism that underlies cocaine addiction and relapse. Strides

have been made to further develop pharmacological treatments, as the effects of cocaine on the

brain reward circuitry are being discovered. Accordingly, the dissemination and conclusions

drawn from this study have the potential to be of importance in pharmacological research,

specifically uncovering the role of PARP-1 in drug addiction.


24

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the effects of intra-cea parp-1 inhibition on cocaine

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