ACOMMON PATHWAY FOR the reinforcing properties of most, if not all, addictive drugs is the mesolimbic brain dopamine (DA) system.1-4
In laboratory animal studies, the reinforcing property of nicotine has repeatedly been linked to DA release in the nucleus accumbens (NAc).5-14
Recent small studies of human smokers have used raclopride labeled with radioactive carbon (11
C) with positron emission tomography (PET) to assess change in DA concentration in response to smoking. These studies have generally supported the association between smoking and DA release, especially the relief from craving15
and hedonic responses16
that smokers experience when smoking. However, considerable unexplained interindividual variability in smoking-induced DA release was reported in these studies.
The high heritability of nicotine dependence17
and differences in individual sensitivity to the reinforcing properties of smoking implicate the influence of genetic differences on positive reinforcement from smoking.18
In the primary analysis of the present study, we sought to determine if known genetic variability in components of the brain DA pathway would help explain observed phenotypic variability in smoking-induced DA release found among human smokers. Although others have associated genetic variability with overall D2
dopamine receptor densities,19-22
we are aware of no prior studies that examined the link between DA genetic variability and DA release in humans. For this initial study, we restricted our examination to those well-characterized gene variants with reported functional effects while recognizing that many other genes could contribute to variation in smoking-induced DA release.
Within the mesolimbic DA pathway, nicotine stimulates nicotinic acetylcholine receptors on neurons of the ventral tegmental area that project to the NAc and release DA, resulting in positive reinforcement.9,23,24
Genotypic (and associated phenotypic) variation of any of the components of this pathway could affect the degree of smoking-induced DA release in individual smokers. From a large number of potential genes of interest, specific gene variants were chosen for analysis from well-studied parts of brain DA pathways, namely (1) the DA transporter (DAT),25
(2) the D2
dopamine receptor (DRD2) (a presynaptic and postsynaptic receptor26
that is activated indirectly by even low concentrations of nicotine27
), (3) the D4
dopamine receptor (DRD4) (a receptor that is expressed in low levels in the basal ganglia but may modulate excitatory neurotransmission28
), and (4) the enzyme catechol-O-methyltransferase (COMT).29
Four genetic variants were chosen from these DA system components based on substantial evidence of functional consequences of these variants, possible association with risk for nicotine dependence, and/or influence on brain DA system regulation. For DAT, the variable number of tandem repeat (VNTR) polymorphism of the 3′ untranslated region (SLC6A3)30
was selected because of reports linking the 10-repeat allele with risk for conditions associated with smoking behavior, namely attention-deficit/hyperactivity disorder,31
and reward dependence.33
Studies using single photon emission computed tomography have demonstrated associations between the 10-repeat allele and increased,34
DAT density (compared to alternate genotypes) with the larger (sample-size) studies demonstrating decreased or unaltered DAT density. If subjects with the 10-repeat allele indeed have decreased DAT density, we hypothesize that such subjects would have higher tonic levels of intrasynaptic DA and less phasic DA release in response to smoking.
For DRD2, A1
allelic variation of Taq1 was selected because the A1
allele is associated with low DRD2 density,19,20,40-42
increased likelihood of smoking behavior overall,43
progression to higher levels of smoking by adolescents,44
less positive responsiveness to bupropion hydrochloride45,46
(a medication with dopaminergic properties) or other treatments47
in smokers attempting to quit, and greater feelings of food reward following smoking cessation.48
Because the A1
allele is associated with decreased DRD2 density and function, and the DRD2 receptor is associated with inhibition of DA release,49,50
we hypothesized that subjects with the A1
allele would have greater DA release (due to less inhibition) than those with the alternate genotype.
For DRD4, variation in the 48–base pair (bp) VNTR in exon 3 was chosen based on the association of the 7-repeat allele with a reduced ability of DA to inhibit cAMP formation,51
reduced effectiveness of DA-releasing medications,52
and links with smoking behavior.53
Because DRD4 stimulation results in sensitization to a DA-releasing drug54
and blockade in the NAc results in diminished DA release,55
we hypothesized that subjects with the 7-repeat allele of DRD4 (having less DRD4 function) would have less smoking-induced DA release than subjects with the alternate genotype.
For COMT, we chose the 1947A>G (Val158Met) polymorphism because the Val substitution is associated with 3-fold to 4-fold higher enzyme activity56
as well as substance abuse57
and the personality traits novelty seeking and reward dependence.58
Additionally, a recent review of the functional effects of the COMT polymorphism59
concluded that the Met allele (low enzyme activity) results in increased levels of tonic DA and reciprocal reductions in phasic DA released in subcortical regions. Based on these reports, we hypothesized that subjects with the Met allele would have less smoking-induced DA release than subjects with the alternate genotype.
The 4 gene variants were studied with the recognition that research linking genetic variants with smoking and/or the brain DA system is an evolving field, and only modest links exist in the scientific literature between genotype and smoking.60,61
Other genotypes (such as α4
nicotinic acetylcholine receptor,62
dopamine β hydroxylase,63
functional polymorphisms) are certainly candidates for associations with smoking-12induced DA release; however, we felt that evidence for including these genotypes was not as strong as for the ones selected. We also recognize that some of the DA genotypes chosen here have had negative reports for associations with smoking,64-67
and some have had conflicting results regarding which allelic variants (if any) are associated with smoking.68-71
In addition to the primary analysis of this study (genotype/PET associations), we also tested for evidence of smoking-induced DA release as we have in the past15
with the present study having a larger sample of smokers and subjects being scanned on a newer PET scanner with improved sensitivity.