Brain Networks and Addiction Susceptibility

Objective. Identification of genetic risk factors predisposing to nicotine abuse and dependence, and elucidation of their neurobiological mechanisms of action, is critical to individualized treatments and prevention of nicotine addiction. Nicotine exerts its effects on the brain through nicotinic acetylcholine receptors (nAChRs). The non-synonymous single nucleotide polymorphism (SNP) rs16969968 in the CHRNA5 gene encoding the 5 subunit of nAChRs has been unequivocally associated with smoking behavior and nicotine dependence in genome-wide association studies (GWAS). At the brain level, the salience network (SN) or the neural circuitry connecting the anterior insula (AI), dorsal anterior cingulate cortex (dACC), the ventral striatum (VS), and extended amygdala has been shown to be crucially involved in nicotine addiction. The SN detects salient events and initiates a rapid switch between large-scale brain networks, the default-mode network (DMN) and executive control network (ECN), in control of behavior. Genetic influences on the SN may therefore explain some of the individual differences in susceptibility to addiction. We have previously shown that resting-state connectivity of the SN is decreased in smokers and non-smokers with the rs16969968 risk allele. But the underlying neurobiological processes are still unknown. Given a well-established role of dopamine (DA) in addiction, and the presence of nAChRs on DA neurons, one plausible mechanism involves cholinergic modulation of DA transmission. Consequently, we will employ an integrative imaging pharmacogenetics approach to test for DA mediation of the rs16969968 effects on the SN in healthy non-smoking participants, with the goal of elucidating the neurobiological mechanism underlying the association between this SNP and susceptibility to nicotine dependence without the confound of chronic smoking. Study Population. Sixty pre-screened participants will be classified into two equal groups (n = 30 per group) based on their rs16969968 genotype: 1) rs16969968 risk allele homozygotes, or A/A genotype ( Risk Group ); and 2) rs16969968 non-risk allele homozygotes, or G/G genotype ( Non-Risk Group ). Participants will be healthy, right-handed males and females, aged 18-55, non-smokers and free of lifetime substance dependence. Design. A double-blind, placebo-controlled crossover design will be used. Each participant will complete a screening session (under the Screening Protocol 06-DA-N415); an orientation session, which will include a nicotine patch tolerance test; and 4 imaging visits, each with a different pharmacological pre-treatment prior to scanning: 1) placebo pill + placebo patch; 2) 20 mg oral methylphenidate + placebo patch; 3) 2 mg oral haloperidol + placebo patch; and 4) placebo pill + 7 mg nicotine patch. Outcome Measures. The study will use neuroimaging (fMRI) to assess the impact of rs16969968 genotype and drug condition (MPH vs. haloperidol vs. nicotine vs. placebo) on the SN, ECN, and DMN function at rest and during task performance. The primary outcome measures will be: 1) network coherence, as indexed by resting-state and task-related functional connectivity (FC); 2) dynamic task-related interactions; and 3) behavioral task performance. We will also test for genetic effects on 4) self-report measures of impulsivity and other traits associated with addiction susceptibility.