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Aims. Animal studies have shown that nicotine releases dopamine, a neurotransmitter implicated in drug reinforcement. We hypothesized that bromocriptine would decrease smoking behavior in humans. Design. The study was conducted double blind and subjects' order of dose exposure was randomized. Participants. The smoking behavior of 20 heavy smokers was recorded for 5 hours after ingesting placebo or one of two doses of bromocriptine (2.50 mg, 3.75 mg) over three sessions (one dose per session). Findings. There was a significant negative linear trend by dosage indicating shorter total puffing time with increasing bromocriptine dosages ( p < 0.02). Other significant negative linear trends by increasing dosage include fewer number of puffs, fewer number of cigarettes smoked and mean latency to smoke after 3 hours (expected C MAX on the drug (all p s < 0.05). There was a negative significant linear trend showing decreased plasma nicotine ( p < 0.02) and cotinine ( p < 0.005) with increasing dosages of bromocriptine. Shiffman/Jarvik Withdrawal Scale (SJWS) cigarette craving subscale scores decreased significantly across increasing dosages (linear trend p < 0.02). There was a significant negative linear trend ( p < 0.05) on the Profile of Mood States (POMS) Vigor and Depression subscales, with subjects reporting decreased vigor and depression with increasing bromocriptine doses. No other mood effects were observed. Conclusion. These results support the hypothesis that dopaminergic mechanisms mediate cigarette smoking reinforcement.