Although identification of significantly modulated genes provides clues about the expression pattern of each brain region’s response to nicotine, we were more interested in identifying the biological pathways potentially regulated by nicotine in these regions. By searching the annotated database with the GSEA algorithm for all genes included in our chip on the basis of GO information, we identified the overrepresented categories (Table 2). For the amygdala, nine enriched GO categories were identified, among which seven (i.e., electron transport, microtubule-based movement, mitochondrial function, protein folding, protein polymerization, DNA-dependent regulation of transcription, and zinc ion binding) were positively associated with nicotine exposure. Conversely, the GO categories related to cell proliferation and protein biosynthesis were negatively related to the treatment. For the hippocampus, eight GO categories were overrepresented, including three related to mitochondria (i.e., mitochondrial respiratory chain complex I, mitochondrion, and electron transport) and two others (namely, regulation of transcription from RNA polymerase II promoter and signal transduction), all of which were positively related to nicotine treatment. On the other hand, the GO categories related to protein biosynthesis and modification and nervous
