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Antioxidants 2020, 9, 425; doi:10.3390/antiox9050425 www.mdpi.com/journal/antioxidants
Review
Sobriety and Satiety: Is NAD+ the Answer?
Nady Braidy
1,
*, Maria D. Villalva
1
and Sam van Eeden
2
1
Centre for Healthy Brain Ageing, School of Psychiatry, University of New South Wales,
Sydney NSW 2052, Australia; m.villalva@unsw.edu.au
2
Centre for Cutaneous Research, Blizard Institute, Barts and The London School of Medicine and Dentistry,
Queen Mary University of London, London E1 4NS, UK; s.vaneden@qmul.ac.uk
* Correspondence: n.braidy@unsw.edu.au
Received: 30 March 2020; Accepted: 5 May 2020; Published: 14 May 2020
Abstract: Nicotinamide adenine dinucleotide (NAD+) is an essential pyridine nucleotide that has
garnered considerable interest in the last century due to its critical role in cellular processes
associated with energy production, cellular protection against stress and longevity. Research in
NAD+ has been reinvigorated by recent findings that components of NAD+ metabolism and NAD-
dependent enzymes can influence major signalling processes associated with the neurobiology of
addiction. These studies implicate raising intracellular NAD+ levels as a potential target for
managing and treating addictive behaviour and reducing cravings and withdrawal symptoms in
patients with food addiction and/or substance abuse. Since clinical studies showing the use of NAD+
for the treatment of addiction are limited, this review provides literature evidence that NAD+ can
influence the neurobiology of addiction and may have benefits as an anti-addiction intervention.
Keywords: NAD+, cocaine; addiction; alcohol; cellular energetics
1. Addiction: Today’s Most Common Modern Disease
Addiction represents the most common untreatable disorder in the 21st century. Addiction has
a profound effect on individuals, their family and carers, and represents a major socio-economic
burden on today’s healthcare system. Some epidemiological studies have suggested that addiction
may be endemic in some populations [1]. The most common forms of addiction are alcoholism and
smoking, both of which have been associated with increased risk of developing age-related disorders
which manifest in the cardiovascular and central nervous system [2–5]. Tobacco smoking is the most
preventable cause of morbidity and mortality worldwide. It has been estimated that of the 38 million
people in the United States of America alone who attempt to quit smoking, more than 85% relapse
within the first few weeks following withdrawal interventions. Abstinent smokers who successfully
withdraw in the first few months are susceptible to relapse after six months or even years later [6].
Alcoholism is the third leading cause of death after cardiovascular disease and cancer and is
associated with over 2.3 million deaths worldwide every year. Alcoholism is the ninth major
contributor to global disease burden as measured by the disability-adjusted life years (DALYs) [2–5].
As with other forms of addiction, relapse occurs in response to exposure to environmental stimuli
that are linked to the rewarding effects of the stimulant. Moreover, substance abuse (e.g., cocaine and
heroin), internet addiction and excessive gambling are growing problems among the younger
generation [7].
Food addiction has been associated with obesity, a global complex health problem that is
dependent on multidisciplinary treatment and various health practitioners including experts in
mental health, medicine and surgery [1]. Obesity has been associated with addictive behaviour that
has a profound effect on morbidity and mortality leading to a reduction in the overall quality of life.
The World Health Organization estimates that by the year 2030, over 57.8% of the world’s population
will be obese. Obesity represents the second cause of death in the United States alone, affecting nearly
Antioxidants 2020, 9, 425 2 of 25
35 million people alone. Obesity has a negative effect on metabolic and endocrine processes that can
lead to multiple organ disorders, malignant diseases such as cancers, mechanical impairments,
surgical complications, and psychosocial deficits [8–11]. Obesity is also associated with genetic
predisposition, impaired hormonal and metabolic processes, as well as confounding psychological
and lifestyle factors [12–15]. This suggests that obesity is not a single disease, but rather a
manifestation of several pathological, physiological and psychological processes leading to
endocrine-metabolic dysfunction.
Substantial research into the pathobiology and mechanisms of addiction, and addiction therapy
are yet to yield effective responses. The most successful addiction programs demonstrate a relapse-
response rate of 56% in subjects [16–20]. Given the importance of improving the prevention and
successful treatment of addiction, there is a growing need to investigate new factors that may be
associated with addictive behaviour. Various experimental approaches suggest that replenishment
or augmentation of cellular levels of the essential pyridine nucleotide, nicotinamide adenine
dinucleotide (NAD+) can provide ameliorative benefits and significantly lower relapse in addiction.
2. Neurobiology of Addiction
A wealth of evidence has identified several similarities between individuals who demonstrate
addictive food behaviour and those that are diagnosed as addicted to substances. DiLeone et al.
showed that individuals that are addicted to food describe similar processes (e.g., rewarding
properties, withdrawal symptoms and overeating) to those addicted to drug and alcohol addicts
[21,22]. Food in today’s contemporary society is rich in calories, saturated fats, sugars, synthetic
additives (e.g., colours) and preservatives which are of low nutritional value and widely accessible.
Compulsive over-consumption of foods rich in salt and additives or saturated with sugars and lots
of calories has the greatest potential for addiction [23]. These foods are termed ‘hyper palatable’, and
can induce hedonistic behaviours and inhibit negative thoughts similar to substance abuse [24,25].
This can induce excessive preoccupation of food and periodic eating of large amounts of food within
a very short time frame. These events may occur either once a day or once a week, leading to feelings
of culpability, ignominy and depression, triggering further over-consumption of foods due to
increased emotional stress [26,27]. Like substance abuse and sex addiction, ‘over-eating’ represents
an in-effective mechanism to overcome negative thoughts or mediate ‘self-control’ [1]. There are also
various similarities between the molecular basis of food and drug addiction which are detailed below.
2.1. Hyper-Activation of the Glutaminergic System
Overactivation of the glutaminergic system has been shown to play an important role in
substance abuse and obese conditions. In particular, it has been previously shown that treatment with
N-methyl-D-aspartate receptor (NMDAR) antagonists can attenuate drug cue associations, and
reduce cue-induced drug seeking and relapse-like behaviour [28]. Moreover, intracerebroventricular
and lateral hypothalamic injections of glutamate, or its excitatory amino acid agonists, kainic acid,
AMPA, and N-methyl-D-aspartate (NMDA), enhance food appetite in murine models, while the
mGluR5 receptor antagonist, (R,S)-2-chloro-5-hydroxyphenylglycine, suppresses appetite [29].
However, alternative therapies are warranted since NMDAR antagonism can induce psycho-mimetic
adverse effects (e.g., hallucinations, paranoia, cognitive deficits) due to NMDAR antagonism [30].
2.2. Impaired Mitochondria Function
There is also substantial evidence indicating that brain energy homeostasis is perturbed in
substance abuse and metabolic syndrome. This includes impairments in glucose metabolism,
increased oxidative stress and reduced cellular respiration [31–35]. However, very few studies are
available in the literature that investigate mitochondrial changes in selected brain regions associated
with the central reward pathway for substance abuse and food addiction. Recent studies have
reported evidence of mitochondrial dysfunction in impaired motivation states such as depression,
anxiety and stress [36,37]. For example, BCL-2, which regulates mitochondrial Ca
2+
homeostasis, has
Antioxidants 2020, 9, 425 3 of 25
been associated with impairments in mood, and increased BCL-2 has been reported to stabilise mood
[38]. In addition, mutations in several mitochondrial genes have been associated with increased
anxiety in several animal models [39]. Transcriptomic profiling of the nucleus accumbens (NAc)—an
important brain reward nucleus—identified enrichment in gene ontology of mitochondria-related
transcripts following repeated cocaine exposure [40]. Exposure to substances also impaired the
activity and function of mitochondrial complex I, oxidative phosphorylation and ATP production,
and mitochondrial membrane potential [31].
Furthermore, studies using functional magnetic resonance imaging (fMRI) reported increased
activity in regions of the brain associated with motivation and reward when a sample of 48 women
were drinking a chocolate milkshake or exposed to photographs of milkshake cups getting filled.
Higher activity was observed in women with greater addiction scores, and less activation was
reported in regions of the brain associated with inhibition of certain behaviours [25,26]. This suggests
that these women were less able to control their behaviours when exposed to milkshakes as stimuli.
Another study showed that food addiction was correlated with increased activation in the anterior
cingulate cortex, medial orbitofrontal cortex and amygdala in response to eating. Greater activation
was reported dorsolaterally in the prefrontal cortex and caudate nucleus, and less activation in the
lateral orbitofrontal cortex, similar to drug addicts [25,26].
2.3. Increased Neuroinflammation and Kynurenine Pathway Activation
Numerous studies have shown that neuroinflammation can induce depressive symptoms and
increase the likelihood of developing addiction [41–46]. Activation of the kynurenine pathway (KP)
of tryptophan (TRYP) degradation has been thought to play a major role in drug-related conditioned
behaviours [47]. Several products of the KP are neuroreactive. For example, quinolinic acid (QUIN)
is an NMDAR agonist and its accumulation can lead to excitotoxicity and impaired glutaminergic
transmission [48–50]. Kynurenic acid (KYNA) is an NMDAR antagonist that exerts neuroprotective
effects on the brain and can counteract the cytotoxic effects of QUIN [51]. We and others have
hypothesised that the QUIN/KYNA ratio represents a balance between neurodegeneration and
neuroprotection in the brain which may be associated with immune activation [49,52,53]. Inhibition
of kynurenine-3-monooxygenase (KMO) by Ro61-8048 and its prodrug JM6 has been previously
reported to induce a metabolic shift in the KP towards KYNA, thus reducing glutaminergic/NMDAR
activity [47]. KMO inhibition also abolished relapse-like alcohol drinking and alcohol and cocaine-
seeking behaviours [47]. However, given that nicotinamide adenine dinucleotide (NAD+) represents
the final end product of the KP, it is unclear whether NAD+ levels can be effectively maintained when
KMO is inhibited [54].
2.4. Alterations in the Mesolimbic-Fronto Cortical Dopamine Pathway
The mesolimbic-fronto cortical dopamine (DA) system (composed of the mesolimbic and
mesocortical DA systems) is an important pathway in brain reward [55]. The neurotransmitter, DA,
is a key component in the brain reward system. DA has been associated with both drug and food
addiction. Alcohol consumption has been reported to directly stimulate DA release and increase DA
levels [56]. In addition, the behavioural rewards of nicotine are associated with DA release in the
mesolimbic pathway [57], and lesions in the mesolimbic DA pathway have been reported to lead to
a reduction in self-administration of nicotine [55]. Cannabinoid receptors have also been identified
in regions of the brain associated with reward. The active component of cannabis, A’-
tetrahydrocannabinol (A9-THC) has been shown to increase DA levels [58].
The brain reward system also plays a crucial role in food addiction, promoting adaptive
behaviours (e.g., consuming palatable nutrients by linking them to pleasurable thoughts). Release of
gastrointestinal (GI) hormones in response to nutrients in the GI tract, limit food intake through
activation of the reward system by influencing motivational behaviour and its reward value [59]. For
instance, reduced GI production of the fat specific satiety factor oleoylethanolamide (OEA) can
induce high-fat-diet induced obesity in mice [60]. Negative feedback mechanisms also exist between
the brain and the gut for sugars, consumption of which is dependent on taste and post-ingestive
Antioxidants 2020, 9, 425 4 of 25
factors which have a direct effect on the brain reward system or mediating the production of
endocrine factors that influence sucrose satiety and reward [61].
2.5. Dysregulation of Endocrine Factors
The neuropeptide and hormone oxytocin (OXT) has been thought to play a role in reward, stress,
social and cognitive processes which are affected by addiction [62]. OXT is a nonapeptide that is
secreted by the brain from oxytocinergic neurons in the paraventricular and supraoptic nuclei of the
hypothalamus [63]. The association between OXT and addiction stems from similarities between
behaviours reported in human addiction and social behaviours [64]. Increased OXT levels following
exogenous OXT via intranasal administration has been shown to boost trust, generosity and social
recognition in humans without adverse effects [65]. However, these effects are context or situation
dependent. There is also evidence of a common neurobiology between social interaction and addition
as demonstrated in prairie voles and rodents [66]. Several preclinical studies have also shown that
OXT can reverse the neuroadaptations occurring with chronic drug and alcohol use [67].
The liver is thought to be involved in the production of endocrine factors that influence addiction
[68]. Fibroblast growth factor 21 (FGF21) is a liver-derived hormone that has various physiological
and pharmacological effects. FGF21 has been reported to normalise blood glucose levels in diabetic
animals, promote fatty acid oxidation, prevent β cell dysfunction and reduce weight gain in the high-
fat-diet induced mice models [69–75]. FGF21 has also been reported to reduce sweet consumption in
mice and humans whereas FGF21 knockout increases sugar consumption in murine models [76–78].
Additionally, overproduction of FGF21 signals to the brain to regulate food intake, energy
expenditure and fertility, and prevent age-related increases in weight gain and insulin resistance
[68,76–78]. FGF21 gene therapy has been demonstrated to be a potential therapy for obesity and type
2 diabetes [79]. Recently, FGF21 has also been reported to inhibit alcohol consumption in mice [80].
The levels of FGF21 are increased in human plasma following acute alcohol consumption and
sustained binge drinking [80]. Taken together, these studies suggest that FGF21 is an endocrine
inhibitor of alcohol and sugar consumption in humans.
2.6. Importance of Circadian Rhythms
The underlying pathobiology of addiction and DA release may be regulated diurnally by
interplay between circadian rhythms and metabolism. Circadian disruption has been reported to
influence behavioural responses to substance abuse [81–85]. For example, mutations in circadian
genes in mice mediate differential locomotor sensitisation and conditioned preference to cocaine [86].
It is thought that these circadian rhythms are controlled transcriptional-translational feedback loops
and selected protein coding genes governed by the molecular clock. In particular, the main circadian
transcription factors CLOCK and BMAL1 have been reported to couple with various intracellular
metabolic signalling pathways [87]. Circadian rhythms can regulate DA synthesis and release, which
are dependent on the availability and activity of tyrosine hydroxylase (TH), whose transcriptional
regulation is dependent on CLOCK/BMAL1 [88].
2.7. Role of Endogenous Opiates
Additionally, substance abuse and/or food have been shown to activate endogenous opiates,
which represent a group of peptides that are produced in several organs and the brain and pituitary
glands in particular [89]. The endogenous opioid system influences the mesolimbic DA and the
cortisol stress response, which are both implicated in addiction reward [90]. The opioid antagonist
naltrexone has been shown to inhibit the desire for sweet, salty and fatty foods, and alcohol and drugs
[91].
3. Historical Background of NAD+
Pellagra is a debilitating disorder caused by the deficiency of niacin and/or its precursor
tryptophan. Symptoms of pellagra were first documented in 18
th
century by the Spanish doctor
Antioxidants 2020, 9, 425 5 of 25
Gasper Casal, who described a disorder attributed to a diet deficient in meat [92]. Pellagra was
epidemic in malnourished regions of Europe and the southern states of the United States of America.
Pellagra has been characterised as a ‘disease with four D’s’—dermatitis, diarrhoea, dementia and
death [93]. However, these classic symptoms rarely occur together or follow a predictable pattern,
and are modified by environmental factors such as sun exposure, other concomitant vitamin
deficiencies and disease progression [94]. Excessive alcohol consumption represents a major risk
factor for pellagra and chronic alcohol use can induce niacin deficiency [95]. It has been reported that
more than 100,000 Americans died from pellagra between 1907 and 1940. In 1914, Sir Joseph
Goldberger linked pellagra to a nutrient deficiency due to a corn-rich diet. He suggested that dried
yeast could be a cheap and effective therapeutic strategy to prevent pellagra [96]. However, it was
not until 1937 that Dr. Conrad Elvehjem demonstrated that nicotinic acid (NA) and nicotinamide
(NAM) cured pellagra [97].
The metabolic significance of nicotinamide adenine dinucleotide (NAD+) was first identified by
Sir Arthur Harden in 1906, who demonstrated that boiling and filtering yeast extract enhanced
alcoholic fermentation in unboiled yeast extract. The active component was termed extract
conferment or cozymase [98]. In 1923, von Euler-Chelpin showed that cozymase was composed of a
nucleoside sugar phosphate [99]. Subsequently, the role of NAD+ as a hydrogen carrier in anaerobic
and aerobic oxidation was identified by Sir Otto Warburg in 1933 [100]. The pathways involved in
NAD+ anabolism were fully characterised by Sir Arthur Kornberg, and the work of Priess and
Handler in the 1940s and 1950s, respectively [101]. The redox roles of NAD+ in glycolysis, the
tricarboxylic acid cycle, oxidative metabolism and energy production were elucidated by several
scientists including Krebs. The non-redox roles of NAD+ as a substrate for ADP-ribosylation reactions
and histone deacetylase activities have been elucidated in the last few decades [101]. Remarkably,
maintaining NAD+ homeostasis is not only essential for the treatment of pellagra, but may also be
associated with addiction, cardiovascular and neurodegenerative diseases and metabolic syndrome
[54].
The significance of NAD+ in addictive disorders stems from the work of Dr. Paul O’ Hollaren
(1961) who claimed to have successfully utilised IV NAD+ for the prevention and treatment of over
104 cases of addiction to alcohol and other drugs of abuse, including heroin, opium extract, morphine,
dihydromorphine, meperidine, codeine, cocaine, amphetamines, barbiturates and tranquilisers [102].
In his retrospective case series, IV NAD+ was administered at a dose of 500–1000 mg added to 300 cc
normal saline daily for 4 days, twice per week for a month, followed by a maintenance dose twice
per month until addiction was ameliorated, with limited toxic effects [102]. NAD+ is likely to
represent a cheap and useful holistic approach for the estimated millions of addicts worldwide, and
may be an effective adjunct to psychotherapy, by ameliorating symptoms of physical addiction
through a variety of mechanisms.
4. Summary of NAD+ Dependent Processes
There is a growing consensus that NAD+ levels decline at the cellular, tissue and organismal
levels during ageing and progression of age-related degenerative diseases. NAD+ and its closely
related phosphate NADP are cofactors in several anabolic and catabolic processes, such as fatty acid
and cholesterol synthesis [103]. NAD+ and NADP, and their reduced forms, NADH and NADPH,
are important cofactors in more than 400 enzymatic reactions including dehydrogenases,
hydroxylases and reductases [104]. Recently, NAD+ has recently been reported to be an important
reductant and hydride donor in biological oxidation of carbohydrate pathways [105]. NAD+ is
essential for the dehydrogenation of acetylaldehyde, which is essential for alcohol metabolism.
Impaired oxidative phosphorylation in rat brain cortical slices following exposure to acetylaldehyde
was attenuated by the addition of NAD+ [106]. Although the reduced and phosphorylated forms can
interconvert, they do not alter the levels of NAD+. The activity of several NAD-dependent enzymes
or NAD+ ‘consumers’ are also affected by the decline in NAD+, influencing multiple processes and
age-associated pathophysiologies.
Antioxidants 2020, 9, 425 6 of 25
4.1. Poly(ADP-Ribose)Polymerases (PARPs)
Poly(ADP-ribose)polymerases (PARPs) are a family of DNA ‘nick’ sensors that detect DNA
strand breaks through its N-terminal zinc-finger domain [107]. Poly(ADP)ribosylation breaks down
NAD+ to NAM and an ADP-ribosyl product [108]. Oxidative damage and neuroinflammation have
been reported parallel to DNA damage. Therefore, NAD+ depletion due to hyperactivation of PARP1
and 2 in the nucleus may play an important role in the pathology of central nervous system (CNS)
disorders [109–113]. PARPs also regulate the tumour suppressor protein, p53. For instance, inhibition
of poly(ADP)ribosylation and inactivation of p53 following treatment with etoposide was reported
in PARP-deficient cell lines derived from Chinese hamster V79 cells [114]. PARP has also been
reported to activate DNA-dependent protein kinases which influence p53 activity via
phosphorylation [115]. Therefore, PARP activity is essential for the maintenance of DNA repair and
genomic stability.
4.2. CD38/NAD+ Glycohydrolase
CD38 is a major NADase in mammalian cells, and another NAD+-consuming enzyme [116].
CD38 and CD157 hydrolyse NAD+ to generate ADP ribose (ADPR) and NAM. CD38 also produces
the secondary messenger signalling molecule, cyclic-ADP-ribose (cADPR) which induces transient
intracellular calcium waves. It has been estimated that about 100 molecules of NAD+ are necessary
for hydrolysis to generate one molecular of cADPR. CD38 can also use the NAD+ precursors,
nicotinamide mononucleotide (NMN) and nicotinamide riboside (NR) to regulate intracellular
NAD+ levels [117]. It can also catalyse a base exchange between NADP and NA, generating nicotinic
acid adenine dinucleotide phosphate (NAADP). CD38 has various immunomodulatory roles and
increases in the levels of CD38 protein have been reported in several tissues over time, thus
contributing to age-related NAD+ decline. CD38/cADPR also regulate oxytocin release, which is
associated with social behaviours, and impaired CD38 function may induce several forms of
neurological deficits [118].
4.3. Sirtuins
Silent information regulators of gene transcription, or sirtuins, are a family of class III NAD+
dependent histone deacetylases enzymes that translate changes in NAD+ levels to the regulation of
several proteins associated with cellular metabolism, cellular stress responses, circadian rhythms and
endocrine functions. The deacetylation reaction involves acetyl group transfer to ADPR to form a
novel compound called 2’-O-acetyl-ADPR or acetylated ADP-ribose [119]. Cleavage of the glycosidic
bonds in NAD+, leads to production of NAM as a by-product [120]. Mammalian cells have seven
classes of sirtuins (SIRT1–7) located in various cellular organelles, and control a variety of important
biological processes [121]. SIRT1 is a nuclear protein that is found mainly in the nucleus but can
translocate to the cytoplasm, and is involved in promoting cellular longevity [122] by influencing the
acetylation/deacetylation status of several important transcription factors, including the metabolic
regulator, peroxisome proliferator-activated receptor-γ (PPARγ), tumour suppressor protein (p53)
and the cell growth linked FOXO forkhead family of transcription factors [123]. SIRT2 is found in the
cytoplasm but can also be found in the nucleus where it regulates gene expression and the
cytoskeletal structure [124]. SIRT3 [125], SIRT4 [126] and SIRT5 [127] are found in mitochondrial
compartments where they are involved in maintenance of mitochondrial redox status. SIRT6, another
nuclear sirtuin, is involved in mediating an age-resistant phenotype [128]. Finally, SIRT7 is found in
the nucleolus of mammalian cells where it mediates growth and metabolism [129]. Importantly, the
beneficial effects of sirtuin activity are dependent on optimal NAD+ levels.
4.4. Sterile Alpha and Toll/Interleukin-1 Receptor Motif-Containing 1 (SARM1)
Sterile alpha and Toll/interleukin-1 receptor motif-containing 1 (SARM1) is a recently discovered
NAD+ hydrolase. Axonal degeneration is associated with NAD+ depletion and inhibition of SARM1
Antioxidants 2020, 9, 425 7 of 25
function delays axonal degeneration. The Toll/interleukin-1 receptor (TIR) domain of SARM1 is
dependent on NAD+ for NAD+ hydrolase activity and enhances axonal degeneration [130–135].
4.5. Interactions between NAD+ Consumers
We and others have previously demonstrated that the activity of PARP and SIRT1 is regulated
by CD38, by potentially limiting the availability of NAD+ to its target enzymes [136]. NAM, which is
also produced by the catalytic activity of CD38, is also an endogenous inhibitor of SIRT1 and CD38.
Therefore, CD38 represents an important regulator of SIRT1 activity and SIRT1 functions, including
maintenance of cellular bioenergetics, obesity and senescence, mainly because it modulates the
availability of NAD+ to the SIRT1 enzyme [137]. There also exists a relationship between PARP1/2
and sirtuins, through their common substrate, NAD+. For example, PARP1/2 knockout enhances
SIRT1 activity leading to increased mitochondrial function, fatty acid oxidation and protection
against obesity [138,139]. However, while PARP1 knockout promotes NAD+ availability, PARP2
knockout promotes SIRT1 expression [140,141]. Therefore, chronic accumulation of oxidative stress
and increased PARP activity plays a causal role in the decline in NAD+ and SIRT1 activity.
5. Overview of NAD+ Biosynthetic Pathways
Given the importance of NAD+ in cellular bioenergetics and the need to maintain intracellular
NAD+ pools in cells, several pathways are involved in the synthesis of NAD+. These include but may
not be limited to: (1) de novo NAD+ synthesis from the amino acid TRYP via the KP; (2) NAD+
salvage pathway from vitamin B3 derivatives NA, NAM, NR and nicotinic acid riboside (NAR); and
(3) major recycling pathway through NAM (Figure 1) (reviewed in [54]). Evidence suggests that the
entire intracellular NAD+ pool may be consumed several times a day [142]. NAD+ anabolism is
dependent on the availability of potential precursors, and not all of these precursors are bioequivalent
[143].
Figure 1. NAD+ biosynthesis pathways.
The kynurenine pathway represents the de novo
pathway of NAD+ is synthesis from catabolism of the amino acid tryptophan. NAD+ can also be
synthesised via salvage of nicotinamide, nicotinic acid, nicotinic acid riboside and nicotinamide
riboside form of vitamin B3. Abbreviations: NR kinase, nicotinamide riboside kinase; NAMPT,
Antioxidants 2020, 9, 425 8 of 25
nicotinamide phosphoribosyltransferase; NMNAT, nicotinamide mononucleotide adenyltransferase;
NAPRT, nicotinic acid phosphoribosyltransferase; PNP, purine nucleoside phosphorylase.
5.1. NAD+ from Tryptophan
TRYP is converted to NAD+ via an eight-step process known as the KP. The conversion of TRYP
to NAD+ occurs when the substrate supply is greater than the enzymatic capacity of 2-amino-3-
carboxymuconate semialdehyde (ACMSD), an enzyme required for the synthesis of the NMDAR
antagonist and endogenous metal chelator, picolinic acid (PIC) [54]. The NAD+ equivalence of 60 mg
TRYP to 1 mg niacin may be partially explained by the fact that most of TRYP is used by the cell for
protein synthesis. The daily recommend TRYP intake is 4 mg/kg body weight for adults [101]. Studies
have shown that the levels of NAM have little effect on the de novo biosynthesis rate of NAD+ from
TRYP. Oral supplementation with TRYP at 15 g/day has been reported to induce some adverse effects
including drowsiness and headache [144]. Overconsumption of TRYP is capable of increasing the
levels of QUIN which has been associated with neurodegenerative disorders, anxiety and seizures
[48]. Therefore, TRYP is unlikely to represent an ideal pharmaceutical source to raise NAD+ levels.
5.2. NAD+ from Nicotinic Acid
NA is converted to nicotinic acid mononucleotide (NAMN) by the ATP-dependent enzyme
nicotinic acid phosphoribosyl transferase (NAPRT) (EC 6.3.4.21) using PRPP as a co-substrate. As
QUIN is converted to NAMN by the enzyme QPRT, the sequence of events leading to NAD+
production is identical to NAD+ from TRYP after NAMN formation [145]. NAPRT appears to be
expressed in several catabolic tissues including the colon, heart, kidney and liver [146]. Adenylation
of NAMN is dependent on the catalytic activity of NAD pyrophosphorylase or nicotinamide
mononucleotide adenylyltransferase (NMNAT) (EC 2.7.7.1) in the presence of ATP to produce
desamido NAD [147,148]. Three NMNAT isoforms are NMNAT-1 (nucleus), NMNAT-2 (Golgi
complex) and NMNAT-3 (mitochondria) [149]. This suggests an organelle-specific function for
enzymes, and specific nuclear, mitochondrial and Golgi-specific NAD+ anabolic pathways. NMNAT-
2 and -3 can also form NADH directly from reduced nicotinamide mononucleotide (NMN) [150].
NMNAT activity (and predominantly NMNAT-1) is high and non-rate-limiting in catabolic tissue,
but not in blood [151].
The process involved in the synthesis of NAD+ from NA is known as the Priess–Handler
process. NA has been shown to increase intracellular NAD+ levels in a kidney cell line [152]. Oral
supplementation of 1–3 g of NA daily lowers blood triglyceride levels and low-density lipoproteins
(LDLs), and increases the levels of high-density lipoproteins (HDLs) [153,154]. Exogenous NA also
increases intracellular NAD+ levels in brain cells [155]. However, NA therapy induces significant skin
flushing in most individuals, thus limiting its clinical use as an NAD+ precursor. A mild skin flush
has been reported in patients exposed to doses as low as 50 mg oral NA [156]. The lipid lowering
effects and adverse effects of NA are thought to be mediated by interaction of NA to the cell surface
of a G-protein coupled receptor known as HM74A or GPR109A, which enhances the conversion of
the omega-6 metabolite arachidonic acid (AA) into prostaglandin E2, stimulating vasodilation of skin
capillaries, causing unwanted skin flush [156].
5.3. NAD+ from Nicotinamide Recycling
Data in the literature suggest that half-lives of most enzymes involved in NAD+ consumption
are 4–10 h [142]. This suggests that 200–600 µmol/kg of NAM needs to be recycled back to NAD+ per
day per tissue in rats. This is equivalent to consumption of 3 g of NAM several times a day in a 75 kg
adult human [142]. These levels are considerably lower than the recommended doses of NAM from
diet (i.e., 1 lb tuna is required for 100 mg NAM, 1 lb beef generates 30 mg NAM while four cups of
broccoli retain 4 mg NAM). Therefore, effective NAM recycling pathways are necessary to protect
against NAD+ deficiency in humans. Recycling of NAM to NAD+ is dependent on the enzyme
nicotinamide phosphoribosyl transferase (NAMPT) (EC 2.4.2.12) using PRPP as a co-substrate which
converts NAM to NMN, and then to NAD+ by the action of NAD pyrophosphorylases in the presence
Antioxidants 2020, 9, 425 9 of 25
of ATP [157]. The rate of NAM recycling is highest in the liver and kidney, and lowest in blood [151].
The recommended dose of NAM is 14–16 mg per day and is suggestive of a net loss not exceeding
0.5% total NAD+ per day to maintain NAD+ homeostasis. Considering that the intracellular NAD+
pools may be replaced up to four times per day, this reflects a total loss of 0.1–0.2% NAM per cycle
[158]. NAM is methylated by the enzyme nicotinamide N-methyltransferase (NNMT) (EC 2.1.1.1) to
N-methylnicotinamide (MeNAM). MeNAM plays important roles in several metabolic and
epigenetic processes and can influence neurodegeneration and ageing. Therefore, while
supplementation with NAM can raise NAD+ and does not cause flushing, it is not considered an
ideal supplement due to its enzyme inhibiting (e.g., PARPs, sirtuins, CD38) and methyl depleting
potential.
5.4. NAD+ from Nicotinamide Riboside and Nicotinic Acid Riboside
NR or NAR represent newly identified precursors that can be used to synthesise NAD+ via the
NR kinase (NRK) (EC 2.7.1.173) pathway [159]. Two NRK enzymes have been identified, NRK1 and
NRK2, although, their exact physiological roles remain unclear. Dephosphorylation of NMN into NR,
which is required to produce NAD+ in yeast, represents a crucial step for increasing intracellular
NAD+ in mammalian cells [160]. NR can also be catabolised into a ribosyl product and NAM via an
NRK-independent pathway, which can then be further recycled to yield NAD+. Purine nucleoside
phosphorylase (PNP) (EC 2.4.2.1) can convert NAR to NA, which is then converted to NAMN by the
activity of NAPRT [161]. NR appears to be safe and orally bioavailable in mice and humans with very
few minor side effects reported [162]. NR is currently a lead candidate in several preclinical and
human clinical trials to evaluate whether it can be used for the treatment of age-related degenerative
disorders [163], given that NAD+ decline and/or increased NAD+ consumption may be a major risk
factor in these debilitating disorders.
6. NAD+ Metabolism: Cellular Energy, Secondary Messenger Signalling and Manipulation for
Addiction
Given the global effects of NAD+ and SIRT1 on human physiology and function, NAD+ levels
can influence anxiety, exploratory and depressive behaviour, and the brain reward system linked to
addiction. NAD+ is likely to represent a molecular link between metabolism and psychiatric
conditions [164]. It has been well established that substance abuse and food addiction can impair
metabolism, alter the cellular redox status, disrupt circadian rhythms and promote oxidative stress
and neuroinflammation [165]. NAD+ regulates intracellular calcium levels during DA release via
increased CD38 activity, therefore reducing NAD+ availability [166]. There is also evidence of a
functional relationship between NAD+ and DA levels in the brain which is likely to be critical to brain
function in physiological and pathological settings [101,167,168]. Mechanisms by which NAD+ and
its related processes can influence the neurobiology of addiction are described below.
6.1. SIRT1 Regulates Behavioural Responses Associated with Drug Addiction
The crucial role for SIRT1 and SIRT2 in modulating behavioural responses to cocaine and
morphine in the NAc has been previously studied. One study reported increases in SIRT1 and SIRT2
expression in the mouse NAc following chronic cocaine administration, while only SIRT1 expression
was increased following chronic morphine administration [169]. Interestingly, cocaine and morphine
had no effect on the expression of other sirtuin family members. Increased expression of SIRT1 and
SIRT2 following exposure to drugs of abuse is partly attributed to the drug-induced transcription
factor ΔFosB. In addition, the rewarding effects of cocaine and morphine were enhanced in mice
following viral-mediated overexpression of SIRT1 or SIRT2 in the NAc and localised knockdown of
SIRT1 in the NAc reduced drug reward [169]. Therefore, sirtuins and SIRT1 and SIRT2, play key roles
in mediating alterations in molecular and cellular plasticity induced by drugs of abuse in NAc, and
may regulate behavioural adaptations following exposure to substance abuse.
Antioxidants 2020, 9, 425 10 of 25
The exact mechanisms by which SIRT1 can mediate cocaine-induced plasticity in NAc remain
unclear. One study used chromatin immunoprecipitation after repeated cocaine (20 mg/kg) or saline
injections, to characterise the SIRT1 binding genome-wide in mice NAc [170]. The study reported
three major findings. Firstly, chronic cocaine causes depletion of SIRT1 from most affected gene
promoters and increases in H4K16ac (a deacetylation substrate of SIRT1). Secondly, cocaine-induced
SIRT1 induction in the NAc promotes deacetylation and activation of FOXO3a and upregulation of
various FOXO3a gene targets in other systems. Thirdly, overexpression of FOXO3a in NAc promotes
cocaine place conditioning [170]. Taken together, these findings suggest that SIRT1, an NAD-
dependent enzyme, can influence molecular adaptations associated with cocaine addiction.
6.2. NAD+ and SIRT1 Regulate Diurnal Rhythms Associated with Addiction Behaviour
The circadian transcription factors CLOCK and BMAL1 form heterodimers bind to enhancer E-
box promoter elements to directly regulate gene transcription (Figure 2) [171]. Owing to the location
of E-box promoter elements to the transcriptional start site and the cAMP response element (CRE),
direct transcriptional regulation of TH is mediated by CLOCK/BMAL1 [172]. TH is the enzyme
responsible for catalysing the synthesis of L-3,4-dihydroxyphenylalanine (L-DOPA) from the amino
acid L-tyrosine, using molecular oxygen (O
2
), iron (Fe
2+
) and tetrahydrobiopterin as cofactors [173].
L-DOPA is a precursor for DA, which is essential for the synthesis of the vital neurotransmitters
norepinephrine (noradrenaline) and epinephrine (adrenaline) [173]. TH enzyme represents the rate
limiting step in the synthesis of catecholamines and is encoded by the TH gene.
The TH gene is
expressed in the CNS, peripheral sympathetic neurons and the adrenal medulla [173].
Figure 2. Circadian rhythms are regulated by the availability of NAD+.
NAMPT is a
SIRT1/CLOCK/BMAL1-regulated circadian gene. SIRT1 and NAMPT and NMNAT form part of a
circadian regulatory feedback loop, regulating the availability of NAD+. NAD+ regulates SIRT1,
SIRT3 and SIRT6 activities. SIRT1 also regulates CLOCK/BMAL1 expression in the suprachiasmatic
nucleus. SIRT6 regulates chromatin recruitment of CLOCK/BMAL1. SIRT3 regulates oxidative
phosphorylation and fatty acid oxidation in the mitochondria via circadian deacetylation of
mitochondrial enzymes. Abbreviations: NAD+, nicotinamide adenine dinucleotide; NAMPT,
nicotinamide phosphoribosyltransferase; NMNAT, nicotinamide mononucleotide adenyltransferase;
SIRT, sirtuins.
Antioxidants 2020, 9, 425 11 of 25
CLOCK is disrupted in substance abuse and mice exhibiting mutations in CLOCK are
hyperdopaminergic and more susceptible to drug addiction [174]. TH expression is upregulated in
the mesolimbic DA system in mice harbouring the CLOCK19 mutation, providing evidence of the
regulatory role of CLOCK on the transcriptional activity of TH [175]. Recent evidence suggests that
the interaction between CLOCK and BMAL1 is regulated by the NAD-dependent histone deacetylase
enzyme SIRT1 to repress CLOCK-mediated transcription [176]. A recent study showed that CLOCK
and NAD-dependent SIRT1 antagonise CREB-mediated transcription during the day, which is
attenuated during the night, thus facilitating CREB-induced TH transcription at night. Impaired
CLOCK in CLOCK19 mice induces CREB-induced TH transcription during the day [176].
Nicotinamide phosphoribosyltransferase (NAMPT) is a rate-limiting enzyme in NAD+
biosynthesis pathway. NAMPT catalyses the conversion of NAM into NMN, which is subsequently
converted to NAD+. One study recently demonstrated that NAMPT expression was significantly
upregulated in the ventral tegmental area (VTA) of cocaine-conditioned mice [177]. Intraperitoneal
or intra-VTA injection of the NAMPT inhibitor FK866, attenuated cocaine reward, although the effect
was repressed by increasing intra-VTA expression of NAMPT or supplementation with NMN [177].
The levels of NAD+ and NMN, and SIRT1 expression were elevated in the VTA in cocaine-
conditioned mice. However, the inhibitory effect of FK866 on cocaine reward was markedly reduced
in SIRT1 midbrain conditional knockout mice [177]. These results suggest that NAMPT-mediated
NAD+ biosynthesis may influence cocaine behavioural effects via SIRT1.
NMN, an NAD+ precursor, also reduced CREB-mediated transcription, providing support for
the importance of maintaining NAD+ levels as a key regulator of SIRT1-mediated transcriptional
repression. Interestingly, the recycling of NAM to NMN is regulated by the CLOCK target gene,
NAMPT [176]. NAMPT and SIRT1 expression are highest during the diurnal phases and are opposite
to the levels of NAD+ and SIRT1 protein, suggestive of NAD-dependent SIRT1 mediated repression
of TH expression during the inactive phase. Daily changes in NAD+ levels regulate the
CLOCK/BMAL1/SIRT1 heterodimer complexes [178]. More specifically, CLOCK/SIRT1 preferentially
binds to the TH promoter during the day when the levels of NAD+ are highest and the expression of
TH is lowest.
Upregulation of SIRT1 and SIRT2 activities appears to be associated with increased excitability
of NAc neurons following repeated cocaine exposure
[179]. This raises the possibility for the
application of SIRT1 and SIRT2 inhibitors as potential agents to treat cocaine addiction.
6.3. NAD+ Increases Adenosine Levels Which Counteract the Effects of Dopamine
The modulatory effects of DA are mediated by postsynaptic D1 and D2 receptors. The
neuromodulator, adenosine, has been reported to attenuate DA signalling via modulation of
adenosine A1 and A2A receptors. Thus adenosine receptors serve as endogenous ‘brakes’ on
overactivated DA receptors and can attenuate drug-induced changes to neuronal function and
cognitive disorders [180]. Adenosine A1 receptors are found in the presynapsis where they compete
with A2A receptors by receptor–receptor mediated interactions to inhibit glutamate release.
Adenosine A1 and DA D1 receptors are expressed in the post synapsis on dynorphin-containing
neurons while adenosine A2A receptors are expressed with dopamine D2 receptors in encephalin
containing neurons [181]. These receptors mediate opposing effects including allosteric receptor-
receptor interactions and intracellular signalling cascades. Drugs of abuse have been reported to alter
the expression of adenosine and DA receptors in the mesolimbic DA system, this limiting adenosine
receptor activity in favour of increased DA stimulation and promoting addictive behaviour although
the exact mechanism(s) remain unclear [182].
Treatment with exogenous NAD+ has been shown to increase adenosine levels which are likely
to activate adenosine receptors to counteract the degenerative effects of DA on neuronal cells [183].
Endogenous adenosine levels have been reported to be in the nanomolar range [184], and the
potential beneficial effects of NAD+ in addiction may be partially mediated by increased adenosine
levels and activation of adenosine receptors. A recent study showed that treatment with exogenous
NAD+ increased intracellular adenosine levels in BV2 microglial cells which could enter cells through
Antioxidants 2020, 9, 425 12 of 25
equilibrative nucleoside transporters [185]. Increases in the intracellular adenylate pool can be
converted to AMP by the catalytic activity of adenosine kinase, which increases intracellular ATP
levels by enhancing AMP kinase (AMPK) activity and increasing ADP to promote mitochondrial ATP
synthase activity [186]. Therefore, extracellular NAD+ may have beneficial effects in addiction
therapy through its degradation into adenosine, attenuation of DA receptor activity and the activities
of adenosine kinase and AMPK.
6.4. NAD+ and SIRT1 Regulate Monoamine Oxidase A
It has been reported that SIRT1 regulates anxiety and addictive behaviour, although the exact
mechanism remains unclear [187–191]. It has been recently reported that the NAD-dependent SIRT1
deacetylates the brain-specific transcription factor NHLH2 on lysine 49, which leads to increases in
the activity of the monoamine oxidase A (MAO-A) promoter, thus activating MAO-transcription
[190]. Since MAO-A is responsible for the degradation of serotonin, increased MAO-A leads to
reduced serotonin levels, thus increasing anxiety and depression. MAO-A inhibitors have been
reported to normalise anxiety differences in murine models exhibiting altered brain SIRT1 levels.
Genetic analysis of unbiased human cohorts reported that the role of sirtuins in regulating anxiety
and behavioural disorders is conserved. These studies provide evidence for the role of sirtuins and
the essential substrate NAD+ in mediating psychological effects on stress-response pathways [190].
6.5. NAD+ Regulates FGF21 and Oxytocin Signalling via SIRT1 and CD38
NAD-dependent SIRT1 plays an important role in regulating cellular energy homeostasis. In
peripheral tissue, SIRT1 promotes the use of fat as a substrate [192–196]. On the other hand, brain
SIRT1 facilitates homeostatic feeding control enhancing hormone sensing. Recently, SIRT1 has been
reported to be a regulator of macronutrient preferences and regulates macronutrient preference via
FGF21-Nrf2-OXT signalling [197]. FGF21 enhances OXT-mediated neuronal activation via ERK
signalling and regulates OXT expression via AKT signalling. SIRT1 promotes FGF21 sensitivity in
OXT neurons and enhances negative feedback processes that regulate simple sugar preference [197].
Importantly, obesity is associated with FGF21 resistance in peripheral tissue and the CNS due
to reduced FGF21 receptor expression. FGF21 resistance in the CNS may play a critical role in obesity
and metabolic syndrome [198]. Hepatic SIRT1 is a key regulator of FGF21 transcription. Increased
systemic SIRT1 activity due to increased NAD+ levels can enhance FGF21 signalling by raising the
levels of FGF21 from the liver and increasing FGF21 signalling in OXT neurons [199]. Interestingly,
the circulating levels of OXT are reduced in obesity, and intranasal treatment with OXT led to a
metabolic shift from using carbohydrates to fat [197,200]. Given the brain FGF21 stimulates OXT
neurons and inhibits simple sugar preference, increasing FGF21 levels may also improve food
selection and improve health and promote stress resistance.
Apart from SIRT1, NAD+ can also regulate OXT through CD38 [201]. Effective social behaviour
is dependent of optimal CD38 activity to regulate OXT secretion from the hypothalamus and
pituitary [202] (Figure 3). Social amnesia may be induced by reducing OXT secretion from the
hypothalamus when CD38 activity is reduced due to gene manipulation or reduced availability of
NAD+ [203].
Antioxidants 2020, 9, 425 13 of 25
Figure 3. NAD+ regulates oxytocin activity via CD38 and cADPR. Social behaviour is regulated by
the amount of central OXT released due to NAD-dependent calcium release by CD38 activity and
cADPR generation. Abbreviations: NAD+, nicotinamide adenine dinucleotide; NADP, nicotinamide
adenine dinucleotide phosphate; CD38, NAD+ glycohydrolase; cADPR, cyclic adenosine
diphosphoribose; OXT, oxytocin; NAADP, nicotinic acid adenine dinucleotide phosphate; Ryr,
ryanodine receptor.
6.6. NAD+ and SIRT1 Regulate Drp1 and Mitochondrial Fission to Potentiate Drug Seeking
Impaired energy metabolism has been reported in the brains of subjects addicted to substances,
and cocaine, although the exact mechanism remains unclear. It has been recently demonstrated that
the dynamin-related protein-1 (Drp1), which mediated mitochondrial fission, is increased in the NAc
following chronic cocaine exposure [204]. A Drp1 inhibitor, Mdivi-1 inhibited cocaine seeking
Antioxidants 2020, 9, 425 14 of 25
behaviour, and antagonised c-Fos induction and excitation onto D1 DA receptors [204]. Since Drp1
expression and mitochondrial fission are regulated by SIRT1 [205] and NAD+ (Figure 4) by
deacetylating p53, reduced SIRT1 activity and NAD+ availability can alter brain energy homeostasis
affecting behavioural and cellular plasticity during addiction.
Figure 4. NAD+ regulates Drp1 associated in mitochondrial homeostasis. CLOCK/BMAL1 modulated
mitochondrial biogenesis and mitophagy via NAD-dependent SIRT1. Deacetylation of the
transcription factor PGC1A by SIRT1 regulates mitochondrial biogenesis. Drp1-mediated
mitochondrial fission precedes mitophagy leading to formation of fragmented mitochondria that are
taken up by autophagosomes. Abbreviations: : NAD+, nicotinamide adenine dinucleotide; NAMPT,
nicotinamide phosphoribosyltransferase; SIRT1, sirtuin-1; OPA1, mitochondrial dynamin like
GTPase; MFN1/2, mitofusin-1/2; FIS1, mitochondrial fission 1 protein; DRP1, dynamin-related
protein-1; PINK1, PTEN-induced kinase 1; BNIP1, BCL-2 interacting protein 1; PARKIN, E3 ubiquitin-
protein ligase parkin.
7. Is There Room for NAD+ in Addiction Therapy?
While NAD+ precursors (e.g., NAM, NMN and NR) have been shown to increase NAD+ levels
after treatment, IV NAD+ remains the most direct method of raising NAD+ levels, although it is yet
to gain U.S. Food and Drug Administration (FDA) approval. While data from experimental studies
are limited, significant clinical benefit of IV NAD+ infusion in alcohol and opioid withdrawal has
been previously reported [102]. These authors found significant improvements in the removal of
cravings and withdrawal symptoms. The benefits of NAD+ are likely to be extended to improving
emotional disorders in patients with psychological disorders. Whilst there has been growing
enthusiasm for the benefits of IV NAD+ therapy, the pharmacokinetics and pharmacodynamics of IV
NAD+ remain nascent in the current literature.
A recent study documented changes in levels of NAD+ and key metabolites in the NAD+
metabolome (henceforth the NADome) in both plasma and urine over 8 h using a typical clinical
dosing regimen of 750 mg NAD+ administered IV over a 6 h period [206]. The study was the first to
Antioxidants 2020, 9, 425 15 of 25
report that: (a) the level of NAD+ remained constant in the plasma for at least the first 2 h when
administered at a flow rate of 3 µmole/min; (b) increases in the levels of metabolic bi-products
analysed were consistent with the activity of NAD+ consumers (e.g., PARPs, CD38 and sirtuins); and
(c) by-products of NAD+ metabolism and unbound NAD+ itself can be excreted in urine [206].
However, further research is necessary to fully understand the complex metabolic fate of NAD+
following treatment.
Importantly, no adverse effect was reported following IV NAD+ infusion when administered at
an appropriate rate [102,206]. However, reduced plasma activities of enzymes associated with hepatic
stress such as the intrahepatic LD and AST and the post-hepatic (bile duct) enzyme GGT suggest that
NAD+ is essential for the maintenance of structural and functional integrity of both intra-hepatic and
post-hepatic tissue. Increases in bilirubin, a red cell degradation product, after 8 h may be indicative
of a minor increase in red cell turnover, possibly due to infusion-induced haemolysis, or reduced
heme metabolism. Since these increases are in a low magnitude of change, it was not considered to
be of clinical relevance [206]. IV NAD+ therapy may provide significant improvement in addictive
disorders likely due to increased NAD+ availability. Further longitudinal and follow-up studies are
necessary to cement the role of IV NAD+ in addiction therapy. In addition, the effect of oral
administration of the NAD+ precursors, NMN and NR, as potential therapeutic agents for raising
NAD+ and improving addictive symptoms warrants further clinical investigation.
8. Conclusions
The central role of NAD+ in the maintenance and regulation of cellular bioenergetics, and
modulation of numerous secondary messenger signalling pathways suggest that NAD+ is essential
for promoting cellular regeneration and repair in neuropsychiatric conditions such as addiction,
Alzheimer’s disease and other neurodegenerative dementias. A growing body of evidence suggests
that NAD+ levels decline with age and an imbalance in NAD+ anabolism and NAD+ consumption
can lead to a significant derailment of fundamental molecular processes leading to accelerated
degeneration and ageing [163,164,207]. This review attempts to provide renewed insight into current
knowledge of the NADome, and mechanisms by which NAD+ can interact with various processes in
cells and tissues to attenuate addictive behaviour and reduce the addictive phenotype in the clinic. If
NAD+ anabolism is impaired and/or if NAD+ consumption is increased during addiction, this is
likely to reduce intracellular NAD+ pools which contribute to functional decline. NAD+ consumers
such as PARPs, CD38, sirtuins and the more recently identified SARM1, may be affected not only in
the brain but in other cells and tissues leading to a loss in NAD+ homeostasis that plays a
contributory, if not causal, role in deficits in basic physiological processes associated with the
neurobiology of addiction.
For several decades, IV NAD+ has been used as a holistic ‘underground’ approach for the
treatment of various forms of addiction. IV NAD+ showed complete withdrawal of addictive drugs
without subjects experiencing ‘agony of withdrawal’ symptoms [102,206]. It also provides a cheap
and direct means for physicians to treat addiction without the necessity of using synthetic therapeutic
agents that are more costly, pose greater risk of adverse effects and induce addiction or regimens that
require ‘trafficking’ of addictive drugs for gradual withdrawal. However, it should be clarified that
the use of NAD+ does not provide allowance for continued use of addicted drugs. Rather, NAD+
therapy is aimed at improving ‘productive ageing’ by restoring health processes, removing cravings
and withdrawal symptoms and assisting in abstinence. Emerging studies using IV NAD+ in addiction
set the stage for deeper investigations into the mechanisms by which addiction is sensitive to NAD+
status, and how it can improve, if not attenuate, addiction in the clinic.
Apart from the historical benefits of niacin and IV NAD+ in addiction therapy, the identification
of NR, NAR and NMN as NAD+ precursors provide ‘newer’ alternatives for raising NAD+ levels and
to possibly alleviate biochemical processes associated with addiction. These NAD+ precursors have
already demonstrated protective effects in several animal models of disease including
neurodegenerative disorders, cardiovascular disease, metabolic syndrome, cancer and ageing
[54,163]. Given recent advances in quantifying the NADome in various biological specimens using
Antioxidants 2020, 9, 425 16 of 25
specific biosensors and mass spectrometry techniques [208], we are at an exciting time when we can
evaluate the importance of NAD+ metabolism not only for prevention and management of ageing
and age-related disorders, but in addictive behaviour as well.
Author Contributions: Conceptualization N.B. and S.v.E.—original draft preparation, N.B.; writing—review
and editing, N.B., M.D.V., and S.v.E.; supervision, N.B. All authors have read and agreed to the published
version of the manuscript.
Funding: This research received no external funding.
Conflicts of Interest: The authors declare no conflict of interest.
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