As shown above, both SAM and GSEA analyses indicated the mitochondrial electron transport system is regulated by chronic nicotine exposure in different brain regions. Figure 1 provides a summary of the expression changes of ten genes in these six regions. In the amygdala, two subunits of NADH dehydrogenase (complex I), Mt-nd4 and Mt-nd6, were significantly regulated, whereas two other subunits (Mt-nd1 and Mt-nd2) were slightly upregulated. Similarly, the expression of these genes was elevated in other brain regions, namely the hippocampus, PFC, and VTA. However, in the NA and striatum, their expression was decreased by nicotine. Mt-cyb of complex III was significantly induced by nicotine in the amygdala, PFC, and VTA, whereas it was suppressed in the NA. We also observed a slight elevation and inhibition of its expression in the hippocampus and striatum, respectively. The expression of three mitochondrial-DNA encoded subunits of complex IV (i.e., Mt-co1, Mt-co2, and Mt-co3) was differentially elevated in the amygdala, hippocampus, PFC, and VTA, whereas their expression was somewhat suppressed in the NA and striatum. We also identified two upregulated subunits of ATP synthase,
