Download Elyse Coolidge

Elyse Coolidge
BIOL 303, 501
November 8th, 2007
The Genetics of Opioid Addiction
Heroin, a member of the opioid family, is considered to be the most addictive of
commonly abused drugs such as nicotine, cocaine and marijuana, and the most devastating.
Heroin addicts are three times more likely to have a history of homicide than other substance
abusers, and 94% of heroin addicts are reported to have contracted Hepatitis C (Comings et al.,
1999). As a result, heroin, its metabolite morphine, the opioid system, and the cannabinoid
system, which has a demonstrated relationship to opioid addiction, have been studied extensively
in the past 20 years through population genotyping and knockout mice.
The endogenous opioid system consists of μ-, δ-, and κ-opioid receptors (MOR, DOR,
and KOR respectively) and their associated peptidic ligands: enkephalins, dynorphins, and βendorphin. Early in the 1980s, the precursor opioid ligand genes were identified;
preproenkephalin (Penk) and preprodynorphin (Pdyn) genes encode copies of enkephalins and
dynorphins respectively, and the proopiomelanocorin (POMC) gene encodes β-endorphin. In the
early 1990s, three separate genes encoding the three opioid receptors MOR, DOR, and KOR
were identified and are referred to as Oprm, Oprd1, and Oprk1 respectively (Keiffer, 2001).
Of the three opioid receptors, MOR has been identified as the primary target for
morphine through gene knockout experiments. In at least five different cases in the late 1990s,
deficient mice were generated and treated with morphine to examine behavioral and analgesic
effects (Gavériaux-Ruff, 2002). In models of thermal, mechanical, and inflammatory pain, the
analgesic effects of morphine were abolished (Gavériaux-Ruff, 2002). In a study by Horace
Loh, et al. (1997), the analgesic effects of morphine and two major morphine metabolites, M-6-G
and M-6-S on wild-type, heterozygote, and homozygote MOR-deficient mice were studied via
tail flick and hot plate tests. M-6-G and M-6-S in doses 40 times the dose required to achieve an
analgesic effect in wild-type and heterozygote mice showed no analgesic effect on homozygous
MOR-deficient mice. The dosage of M-6-G and M-6-S administered was limited to 120 mg/kg
and 125 mg/kg respectively due to availability. Morphine demonstrated an analgesic effect on
homozygous MOR-deficient mice at a dosage of 413.9 mg/kg for the tail flick test, and 491.59
mg/kg for the hot plate test, more than 100 times the analgesic dosage for wild type mice in both
studies (Loh et al., 1997).
As a result of the connection between MOR and morphine, it has been hypothesized that
the MOR and its endogenous peptidic ligands, enkephalins and dynorphins, play an important
role in morphine dependence. Ensuing studies focused specifically on the Penk and Oprm genes
which both demonstrated a connection to dependence on morphine as well as Δ9tetrahydrocannabinol (THC) and alcohol (Gavériaux-Ruff, 2002). A population study by
Katarina Drakenberg, et al.(2006) which focused on the A118G polymorphism in the MOR
examined the frequency of the A/A, A/G, and G/G genotypes in heroin abusers as compared to
control subjects. Of the initial sample of 65 subjects, 91% of those with the A/G genotype were
heroin abusers. However, the heroin abusers composed 60% of the individuals included in the
study, and 54% of those with the A/A genotype were heroin abusers as well. This study was
repeated with an additional 53 subjects consisting of 14 controls and 39 heroin abusers, and it
was found that 88% of all heterozygous individuals were heroin abusers.
Polymorphisms in the Penk gene in relation to opioid dependence yielded similar results.
In an earlier study, the Penk alleles were divided into two groups: alleles smaller than 79bp (< =
79), and alleles larger than 81bp (> = 81). The range of size of Penk alleles was 75bp to 83bp.
Thirty-one subjects with opioid dependence, 89 subjects with substance dependence other than
opioid, and 132 controls were genotyped using DNA extracted from whole blood and PCR.
While the controls exhibited equal frequencies of the < = 79 allele and the > = 81 allele, the
opioid dependent individuals exhibited an allelic frequency of 66.1% for the > = 81 allele. The
control subjects had a roughly equal frequency of homozygote < = 79 and > = 81 genotypes, and
43.2% were heterozygote. In contrast, only 16.1% of opioid dependent individuals were < = 79
homozygous, whereas 48.4% of opioid dependent individuals were > = 81 homozygous
(Comings et al., 1999).
Despite the fact that both studies indicate that the > = 81 allele of the Penk gene and the
118G SNP of the Oprm gene are risk factors for opioid dependence, both focused on small
populations of between 100 and 200 people. Larger population studies, although difficult due to
the infrequency of heroin addiction, are necessary to confirm this trend. A recent family-based
association study by Xiang et al. (2007) surveyed a population of 1923 individuals and focused
on 18 Oprm SNPs, 18 Oprd1 SNPs, 7 Penk SNPs, and 7 POMC SNPs and the relationship
between these genes and alcohol dependence, illicit drug dependence, and opioid dependence.
Of the 18 Oprm SNPs examined, the A118G SNP was included. Neither this SNP, nor any of
the other 35 SNPs in Oprm and Oprd1 were found to be associated with alcohol, illicit drug, or
opioid dependence. However, three SNPs on the Penk gene and two SNPs on the POMC gene
were found to be associated with opioid dependence specifically.
In addition to the opioid pathway, the cannabinoid pathway has also been examined as a
possible player in opioid addiction due to the previously mentioned relationship between the two
pathways. Much like opioids, the administration of cannabinoids results in analgesia and can
develop a physical dependence, and it has been demonstrated that Penk KO mice exhibit a
reduction in the severity of cannabinoid withdrawal (Valverde et al., 2000). The cannabinoid
pathway consists of two receptors, CB1 and CB2, which are bound by the exogenous
cannabinoid, THC, commonly found in marijuana. The CB1 receptor is thought to mediate the
majority of the effects of exogenous cannabinoids on the central nervous system much like the
MOR mediates the effects of morphine; therefore studies of the effect of the cannabinoid
pathway on morphine addiction focus on the CB1 receptor.
Multiple studies have been conducted on CB1 receptor KO mice confirming the role of
the cannabinoid system in morphine addiction. In a study by Cossu et al. (2000), CB1 receptor
KO drug-naïve mice homozygous for the presence (CB1+/+) or absence (CB1-/-) of CB1 receptors
were placed in cages and administered illicit drugs intravenously based on a nose-poke response.
These drugs consisted of morphine, cocaine, amphetamine, and nicotine. As shown in Figures 14 below, mice with the genotype CB1-/- exhibited an increased number of nose-pokes equivalent
to the mice with the genotype CB1+/+ for all drugs except morphine. Those mice homozygous for
the absence of CB1 receptors self-administered morphine in quantities equivalent to the amount
of vehicle administered by the controls. In contrast mice with the CB1+/+ genotype selfadministered twice as much morphine as controls did vehicle indicating that the CB1 could be
essential in morphine reward. A second study duplicated these results using the same methods
confirming the role of the CB1 receptor of the cannabinoid system in reinforcing effects of
morphine (Ledent et al., 2006).
Fig. 1. Intravenous morphine (0.002 mg/kg per inj)
self-administration in drug-naive CB1 receptor
knockout (CB1−/−) and wild type (CB1+/+) mice. Each
bar represents the mean (±S.E.M.) of the cumulative
number of nose-poke responses (NPRs) of the active
(solid bars) and passive (open bars) mice in a 30-min
session. ** P<0.01 vs. vehicle and corresponding
passive group (n=4–8 pairs of animals).
Fig. 2. Intravenous cocaine (0.1 mg/kg per inj) selfadministration in drug-naive CB1 receptor knockout
(CB1−/−) and wild type (CB1+/+) mice. Each bar
represents the mean (±S.E.M.) of the cumulative
number of nose-poke responses (NPRs) of the active
(solid bars) and passive (open bars) mice in a 30-min
session. * P<0.05 and ** P<0.01 vs. vehicle and
corresponding passive group (n=6–8 pairs of animals).
Fig. 3. Intravenous amphetamine (0.1 mg/kg per inj)
self-administration in drug-naive CB1 receptor
knockout (CB1−/−) and wild type (CB1+/+) mice. Each
bar represents the mean (±S.E.M.) of the cumulative
number of nose-poke responses (NPRs) of the active
(solid bars) and passive (open bars) mice in a 30-min
session. ** P<0.01 vs. vehicle and corresponding
passive group (n=10–11 pairs of animals).
Fig. 4. Intravenous nicotine (0.075 mg/kg per inj) selfadministration in drug-naive CB1 receptor knockout
(CB1−/−) and wild type (CB1+/+) mice. Each bar
represents the mean (±S.E.M.) of the cumulative
number of nose-poke responses (NPRs) of the active
(solid bars) and passive (open bars) mice in a 30-min
session. ** P<0.01 vs. vehicle and corresponding
passive group (n=4–12 pairs of animals).
Through the use of KO mice and population studies, multiple theories for the
development of opioid abuse have been developed. While multiple conflicting population
studies have been conducted concerning the Penk and Oprm genes, both implicated in opioid
addiction, the results are inconclusive for Oprm SNPs, and incomplete for Penk alleles. Mice
KO studies of Oprm and CB1 have demonstrated the importance of MOR in the metabolism and
reward of morphine, and the key role the CB1 receptor in morphine addiction. To the knowledge
of the author, no Penk KO mice studies have been conducted in regard to morphine and opioid
addiction. Most likely, heroin addiction is controlled by the interaction between these three
genes and the cannabinoid and opioid systems. Further population and KO mice studies that
focus on these three genes will be necessary to fully understand the relationship between these
genes and heroin addiction.
Works Cited
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Cossu, Gregorio, et al. “Cannabinoid CB1 receptor knockout mice fail to self-administer
morphine but not other drugs of abuse.” Behavioural Brain Research 118 (2001):6165. Elsevier. 6 Nov 2007 <http://www.elsevier.com/locate/bbr>.
Drakenberg, Katarina, et al. “μ Opioid receptor A118G polymorphism in association with
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