Cigarette smoking persists as the premier cause of preventable death in the United States and is the direct cause of most lung cancer, chronic obstructive pulmonary disease, emphysema, and numerous other devastating and fatal diseases (Centers for Disease Control and Prevention, 1993
; Cancer Facts and Figures, 2007
). In total, 40% of the approximately 36 million smokers in the United States attempt to quit each year however less than 7% succeed, even when assisted with nicotine replacement (Baillie et al, 1995
; Hughes et al, 2003
). During abstinence, smokers are often obsessively fixated on smoking to alleviate intolerable cravings, induced by nicotine withdrawal and/or reminders to smoke, both of which contribute to relapse (Baker et al, 1986
; Shiffman et al, 1996
). Nicotine withdrawal symptoms abate within 1 month of quitting (Hughes, 2007
). However, smokers report that reminders to smoke such as the smell of a burning match, another person smoking, and even internal mood states repeatedly associated with smoking can trigger relapse months or even years after quitting.
A wealth of preclinical literature examining the neuro-biology underlying drug dependence and the cues that predict their availability point to a final common brain substrate, the mesolimbic dopaminergic reward system (Di Chiara and Imperato, 1988
; Di Ciano et al, 1998
; Duvauchelle et al, 2000
; Gratton and Wise, 1994
). Current neuroimaging techniques provide the opportunity to examine dopaminergic systems in humans. Volkow et al (2006)
, followed by Wong et al (2006)
were the first to provide direct evidence that cocaine cues on their own elicit craving and increased dopamine (DA) release in the striatum of cocaine-addicted individuals. This work was followed by the study of Boileau et al (2007)
who showed that only three pairings with amphetamine produced conditioned DA release in the ventral striatum (VS) of normal volunteers accompanied by amphetamine-like behavioral responses.
We showed robust and consistent brain perfusion, independent of withdrawal, to smoking cues (SCs) in reward-related mesocorticolimbic circuitry (amygdala, VS, orbitofrontal cortex, parahippocampus, medial thalamus, and insula) that correlated with craving in posterior cingulate and dorsolateral prefrontal cortex (Franklin et al, 2007
) supporting hypotheses implicating this circuitry in stimulus-evoked craving and relapse (Corrigall et al, 1992
; Di Chiara, 2000
; Franklin and Druhan, 2000a
). Despite a robust overall cue effect, there was considerable interindividual variability in the brain response. As heritability estimates for various smoking behaviors range from 46 to 84% (Batra et al, 2003
; Heath and Madden, 1995
), and as DA is a critical neurotransmitter for drug reward and is released by its signals, we hypothesized that genetically driven variation in DA transmission contributes to differences in the strength of the attribution of incentive salience to cues associated with smoking cigarettes and may be reflected in different neural activation patterns and/or intensity of activations.
In the present studies, we linked functional neuroimaging and behavioral measurements of subjective craving with a candidate gene approach to evaluate the impact of genetic variation in the dopamine transporter (DAT) gene (SLC6A3
) on brain perfusion to SCs. The DAT is abundantly present in the VS and rapidly clears DA from the synapse after phasic release that occurs in response to primary rewards and predictive stimuli (Jaber et al, 1997
; Mash et al, 2002
; Zahniser and Sorkin, 2004
). Evidence suggests that a 40 bp variable number of tandem repeat (VNTR) polymorphism in the 3′-untranslated region of the SLC6A3
effects DAT expression (Vandenbergh et al, 1992
). The 9-repeat allele (9-repeats) is associated with lower DAT expression (Fuke et al, 2001
; Mill et al, 2002
) (although see van Dyck et al, 2005
), which may result in slow DA clearance potentially reflecting prolongation of the DA-related ‘reward message’. In support of this view, Erblich et al (2005)
found that smokers carrying a 9-repeat reported stronger SC-induced cravings. Further support is provided by a study in which 11
C raclopride competition in striatal regions was greater in 9-repeats immediately after smoking (Brody et al, 2006
), which might indicate either compromised DAT function or reduced availability in the 9-repeat probands.
These and other studies provide the foundation for our hypothesis that synaptic DA levels are sustained in smokers with the less ‘active’ or less available form of the DAT and may be reflected in a heightened neural response to SCs. Particularly, as in our previous study, we expected carriers of a 9-repeat to exhibit increased activity in the VS, orbitofrontal cortex (OFC), amygdala, parahippocampal gyrus, thalamus and insula, and would report greater craving to the SCs compared to probands homozygous for the 10-repeat allele (10/10-repeats). To test this hypothesis, we utilized the technique of continuous arterial spin-labeled (CASL) perfusion fMRI, which is quantitative and stable over time and thus well suited for the study of low frequency (sustained) brain changes (Detre et al, 1992
), such as drug craving, which once triggered, can persist for several minutes (Childress et al, 1999
; Franklin et al, 2007
; Volkow et al, 2006
The study, approved by the University of Pennsylvania Institutional Review Board, adhered to the Declaration of Helsinki. Smokers were compensated $100 for completion of both MRI scans. Subjects (~75%) were recruited from those presenting for treatment for nicotine dependence at the University of Pennsylvania Treatment Research Center and through word of mouth (~25%). Subjects were screened, tested on study knowledge, and consented prior to psychological and physical evaluations. The Minnesota International Neuropsychiatric Interview (Sheehan et al, 1998
) is a short accurate structured diagnostic psychiatric interview, which was used to determine current DSM-IV diagnosis of psychoactive substance dependence other than nicotine and to diagnose current severe psychiatric symptoms. Individuals with other current psychoactive substance dependence or current DSM-IV psychiatric diagnoses were excluded. Severity of nicotine dependence was determined from a laboratory-developed Smoking History Questionnaire that included a modified Fagerstrom Test for Nicotine Dependence (FTND; Fagerstrom and Schneider, 1989
Individuals with an abnormal structural MRI, a history of head trauma or injury causing loss of consciousness lasting greater than 3 min or associated with skull fracture or intercranial bleeding, or had magnetically active objects on or within their body were excluded.
DNA samples were acquired and functional scanning was performed on 22 subjects, 3 of whom were excluded due to incomplete image acquisition. A subgroup of these subject's imaging data has been published previously (Franklin et al, 2007
). Thus, the final sample consisted of 19 (N
= men, 32% African American, 43% European American, 21% multiple ethnicity) physically healthy and mentally stable smokers between the ages of 18 and 60 (35.8±2.38) who met DSM-IV criteria for nicotine dependence (FTND: 4.77 (±0.52); indicating moderate dependence). Subjects smoked from 12 to 35 cigarettes per day (21.4±1.61) and averaged 13.3 (±0.47) years of education.