Functional enrichment analysis revealed a more specific function pattern of these genes. Among the GO terms significantly enriched in the candidate genes (Table 1), including those associated with neurodevelopment or synaptic transmission. In the biological process, terms directly related to neurodevelopment, e.g., synaptic transmission (PBH = 1.21×10–36), transmission of nerve impulse (PBH = 1.69×10–36) and neurological system process (PBH = 3.23×10–32) were identified (Table 1). Consistently, for the molecular function category, terms related to the activity of neurotransmitter receptor or channel were enriched, such as neurotransmitter receptor activity (PBH = 1.36×10–26), excitatory extracellular ligand-gated ion channel activity (PBH = 2.62×10–22) and acetylcholine receptor activity (PBH = 5.12×10–22). In the cellular component category, the significantly enriched terms included synaptic membrane (PBH = 5.18×10–27), neuron projection (PBH = 4.51×10–24), axon (PBH = 3.55×10–21), dendritic spine (PBH = 9.87×10–10). Similarly, GO terms related to drug response (e.g., response to alkaloid, response to nicotine, and response to alcohol) and metabolism (e.g., monooxygenase activity, and oxidoreductase activity), were also enriched in NAGenes. These results were consistent with the pathophysiological background of nicotine addiction, which also indicated the candidate genes are relatively reliable for the following up bioinformatics analysis.
