The subject population consisted of adults (mean age ± S.E.M., 35.7 ± 3.1 yr), was mostly male (nine males, six females), and smoked moderately (18.7 ± 0.5 cigarettes/d) for many years (13.5 ± 2.6 yr). Subjects were moderately nicotine-dependent (mean total FTND scores: 5.7 ± 0.3), had low levels of depression and anxiety (HAMD and HAMA scores: 2.2 ± 0.4 and 2.3 ± 0.4, respectively), and experienced moderate withdrawal during the initial visit (mean per item SJWS score=3.9 ± 0.1). All subjects reported compliance with the abstinence protocol, and exhaled CO levels at the time of scanning (2.8 ± 0.4 ppm) were consistent with these subject reports. The groups that underwent the three scanning conditions did not significantly differ in demographic or rating scale variables ().
From before to after the smoke break, subjects who smoked a cigarette had significant reductions in craving (UTS scores: 4.8 ± 0.6 to 2.4 ± 0.4 and 5.0 ± 0.5 to 3.6 ± 0.6 for the low-nicotine and denicotinized cigarette conditions, respectively, paired Student’s t tests, p = 0.03 and p = 0.04, respectively). Although the reduction in mean UTS score was more pronounced in the low-nicotine cigarette group than the denicotinized cigarette group (UTS score change: −2.4 vs. −1.4, respectively), this difference did not reach statistical significance. The no-smoking group had no change in mean UTS score from before to after the break in scanning (UTS scores: 2.5 ± 0.5 to 2.5 ± 0.5, paired Student’s t test, n.s). For the last break in scanning (which corresponded to the middle of the post-smoking PET scanning frames used for data analysis, 3.1 h after smoking), mean UTS scores returned towards baseline for both the low-nicotine and the denicotinized cigarette groups (3.8 ± 0.7 and 4.2 ± 0.7, respectively), and were no longer significantly different from baseline (paired Student’s t tests, n.s.).
Nervousness and mood scores did not change significantly from the beginning to end of the smoke break for the three conditions (nervousness: 4.4 ± 0.8 to 3.7 ± 0.7, 3.0 ± 0.6 to 3.4 ± 0.5, and 1.7 ± 0.3 to 1.4 ± 0.3 for the low-nicotine cigarette, denicotinized cigarette, and no-smoking conditions, respectively; paired Student’s t tests, n.s.; mood: 3.9 ± 0.7 to 3.9 ± 0.4, 4.7 ± 0.4 to 3.7 ± 0.5, and 4.3 ± 0.7 to 4.6 ± 0.6 for the three groups, respectively; paired Student’s t tests, n.s.). [As an aside, subjects undergoing the no-smoking condition had significantly lower UTS and nervousness scores prior to the break in scanning than those undergoing the two smoking conditions (Student’s t tests, p = 0.002 for both comparisons for UTS scores and p < 0.05 for both comparisons for nervousness scores).]
Determination of nAChR occupancy from smoking a denicotinized and a low-nicotine cigarette
Time–activity curves for the three ROIs () demonstrated that displacement of 2-FA was consistent with the amount of nicotine in the cigarette smoked. Compared to the no-smoking control condition (which had a mean 2.5% per h increase in radioactivity), smoking a denicotinized cigarette resulted in 17 ± 1%, 16 ± 2%, and 16 ± 2% displacement of total radioactivity in the thalamus, brainstem, and cerebellum, respectively. Smoking a low-nicotine cigarette resulted in 52 ± 2%, 53 ± 2%, and 43 ± 2% displacement of total radioactivity in these three brain regions, respectively.
Receptor occupancy from smoking a denicotinized cigarette (displacement of specifically bound radiotracer) was 25 ± 2%, 25 ± 3%, and 28 ± 4% (mean 26%) for the thalamus, brainstem, and cerebellum, respectively, while receptor occupancy from smoking a low-nicotine cigarette was 78 ± 3%, 80 ± 3%, and 79 ± 3% (mean 79%) for the three brain regions, respectively. These receptor occupancies were similar to the predicted values of 22–24% and 77–79%, respectively, and indicate that nicotine inhalation is primarily responsible for α4β2* nAChR occupancy from smoking. Plasma nicotine levels were also consistent with the amount of nicotine in the cigarette smoked ().
Comparison of α4β2* nAChR occupancy between the no-smoking, denicotinized cigarette, and low-nicotine cigarette conditions
For the comparison of α4β2* nAChR occupancy between the no-smoking and low-nicotine cigarette conditions, all three brain regions had significantly different occupancies (unpaired Student’s t test, two-tailed, p < 0.0001, for the three regions, following Bonferroni correction) ( and ). These results indicate that smoking a low-nicotine cigarette results in significant receptor occupancy compared to no smoking.
Figure 2 (a) The mean (±S.E.M.) percent displacements of total radioactivity for the three smoking conditions [no smoking, Quest 3 (denicotinized cigarette), Quest 1 (low-nicotine cigarette)] for the three regions of interest (Th, thalamus, BS, brainstem, (more ...)
Figure 3 Radioactivity levels on mean positron emission tomography (PET) images 3.1 h after the smoke break in scanning. Column 1 shows a magnetic resonance imaging (MRI) scan with representative slices from three levels of interest for this study (row 1 at the (more ...)
For the comparison of α4β2* nAChR occupancy between the no-smoking and denicotinized cigarette conditions, all three brain regions were significantly different with both statistical methods applied (for the first analysis, unpaired Student’s t tests, two-tailed, p < 0.01 for the three regions; for the second analysis using sequential tests, p = 0.01, 0.03, and 0.04 for the thalamus, brainstem, and cerebellum, respectively; all results are Bonferroni-corrected) ( and ). These results indicate that smoking a denicotinized cigarette results in significant receptor occupancy compared to no smoking.
For the comparison of α4β2* nAChR occupancy between the low-nicotine and denicotinized cigarette conditions, all three brain regions were significantly different with both statistical analysis methods (for the first analysis, unpaired Student’s t tests, two-tailed, p < 0.0001, for the three regions; for the second analysis using sequential tests, p < 0.0001 for the three regions; all results are Bonferroni-corrected) ( and ). These results indicate that smoking a low-nicotine cigarette results in significantly greater receptor occupancy than smoking a denicotinized cigarette.
Determination of EC50
The effective venous plasma concentration of nicotine needed to occupy 50% of α4β2* nAChRs (EC50) was 0.79 ± 0.08, 0.77 ± 0.10, and 0.68 ± 0.10 ng/ml for the thalamus, brainstem, and cerebellum, respectively (), with the mean EC50 being 0.75 ± 0.08 ng/ml ().
Correlations between α4β2* nAChR occupancy and cigarette withdrawal symptoms
There were no significant correlations between receptor occupancy and change in withdrawal symptoms (correlation values −0.27 to −0.13, d.f. = 13, p values > 0.05), indicating that factors other than nicotine binding to α4β2* nAChRs are at least partly responsible for the improvement in withdrawal symptoms seen with smoking either low-nicotine or denicotinized cigarettes, or (alternatively) that the sample size here was too small to detect such correlations.