Both acute and chronic effects of nicotine/cigarette exposure have been elucidated with functional brain imaging. Replicated responses to acute administration of nicotine/smoking include: a reduction in global brain activity (perhaps most prominently in smokers with high levels of hostility as a personality trait); activation of the prefrontal cortex, thalamus, and visual system; activation of the thalamus and visual cortex (and possibly ACC) during visual cognitive tasks; and increased DA concentration in the ventral striatum/NAc. Replicated responses to chronic nicotine/cigarette exposure include decreased MAO A and B activity and a substantial reduction in α4β2 nAChR availability in the thalamus and putamen (accompanied by an overall up-regulation of these receptors).
This group of findings demonstrates a number of ways in which smoking might enhance neurotransmission through cortico-basal ganglia-thalamic circuits (
Alexander et al., 1990) (in addition to demonstrating direct effects of chronic nicotine exposure on nAChR availability) (). Given that the thalamus (
Groenewegen et al., 1999;
Herrero et al., 2002;
Sommer, 2003) and ventral striatum/Nac (
Groenewegen et al., 1999;
Herrero et al., 2002) function as relay centers for information and for paralimbic and motor processing in the brain, the net effect of smoking may be to enhance neurotransmission along cortico-basal ganglia-thalamic loops originating in prefrontal and paralimbic cortices. Neurotransmission through these circuits may be stimulated directly by the interconnected (
Sherman, 2001;
Sillito and Jones, 2002) nAChR-rich thalamus and visual systems, and/or indirectly through effects on MAO inhibition and DA release in the ventral striatum/NAc (as well as through nicotine stimulation of excitatory glutamatergic input to the dopamine system (
Mansvelder et al., 2002)). In the thalamus, for example, nicotine has direct agonist action on excitatory thalamocortical projection neurons and local circuit neurons, although nicotine also stimulates GABAergic interneurons, so that the relationship between nicotine stimulation and thalamocortical stimulation may be complex (
Clarke, 2004). There is mixed evidence as to whether or not nicotine stimulates corticothalamic neurons (
Clarke, 2004).
Enhancement of neurotransmission through prefrontal and paralimbic cortico-basal ganglia-thalamic circuits may account for the most commonly reported cognitive effect of cigarette smoking, namely improved attentional performance (
Newhouse et al., 2004), and also related effects, such as improvements in reaction times (
Hatsukami et al., 1989;
Pritchard et al., 1992;
Shiffman et al., 1995), arousal (
Parrott and Kaye, 1999), motivation (
Powell et al., 2002), and sustained attention (
Rusted et al., 2000). Prefrontal (including both dorsolateral and ventrolateral) (
Duncan and Owen, 2000;
Rees and Lavie, 2001;
Smith and Jonides, 1999) and ACC (
Carter et al., 1999;
Duncan and Owen, 2000;
Peterson et al., 1999;
Smith and Jonides, 1999) cortices are reported to activate during attentional control tasks (especially visuospatial tasks) (
Pessoa et al., 2003). Cigarette smoking may enhance attentional control through direct stimulation of nAChRs within these structures or perhaps through subcortical stimulation of nAChRs in the thalamus and via DA release and/or MAO inhibition in the basal ganglia.
In addition to improvement in attention, smoking improves withdrawal symptoms, such as depressed mood, anxiety, and irritability in tobacco dependent smokers (
Cohen et al., 1991;
Parrott, 2003), and all of these effects depend (at least in part) on the expectations of the smoker (
Perkins et al., 2003). Though nicotine administration generally results in increased activity along prefrontal and paralimbic brain circuits, it is interesting that both increased and decreased ACC activation during cognitive task performance has been reported (see Section 2.2). ACC activity has been associated with anxiety and mood, with increased activity being associated with greater anxiety (
Chua et al., 1999;
Kimbrell et al., 1999) and decreased activity being associated with depressed mood (
Drevets et al., 1997). This combination of findings suggests a potential interaction between expectation of the effects of smoking (e.g. mood improvement, anxiety reduction, or decreased irritability) and direction of ACC activity change during cognitively demanding tasks. Perhaps smokers who expect to and do have anxiety alleviation from smoking have deactivation or decreased activation of the ACC while performing cognitive tasks, while those who expect to and do experience mood improvement from smoking have increased activation of the ACC.
In addition to these primary effects of nicotine and smoking, other functional imaging studies reviewed here focus on smoking-related states, such as cue-induced cigarette craving. Such studies are part of a large body of literature examining cue-induced craving for addictive drugs. Studies specific for cigarette cues/craving reveal that exposure to visual cigarette cues immediately activates mesolimbic (ventral tegmental area, amygdala, and hippocampus) and visuospatial cortical attention areas of the brain, and acutely (over a 30 min time period) activate paralimbic regions (ACC and OFC), and that this cue-induced activation may be diminished by a course of bupropion treatment. These results are similar to those of functional imaging studies for drugs other than tobacco (
Goldstein and Volkow, 2002;
Miller and Goldsmith, 2001). and it has been posited that at least some of the activations seen with cigarette-related cues (cortical attention areas and OFC) are associated with an expectation of smoking in the non-treatment seeking subjects who participated in these studies (
Wilson et al., 2004).
In summary, functional brain imaging studies of nicotine/cigarette smoking have demonstrated a link between nicotine/cigarette administration and brain circuitry that mediates visuospatial attentional processing and withdrawal symptoms. Future studies utilizing newer PET tracers and enhanced MRI techniques will undoubtedly further elucidate the brain mediation of tobacco dependence, and may accelerate the development of targeted smoking cessation therapies.