Sex-specific transcriptomic profiles of nicotine sensitization in rats inform the genetic basis of human smoking behavior

Tobacco cigarette smoking, with nicotine (NIC) as the addictive component, is a large risk factor for human mortality. In animals, repeated NIC exposure leads to sensitization (SST) and enhances self-administration (SA) of NIC. However, the molecular basis of SST and SA and their genetic relevance to smoking behavior are poorly understood. Using F1 progeny of inbred Envigo rats (F344/BN), we carried out a transcriptional profiling of NIC SST and SA in ventral tegmental area (VTA), nucleus accumbens core (Nac) and shell (Nash). We observed male-specific NIC SST and a parental effect of NIC with SA only present in paternal F344 crosses. Gene differential expression (DE) analysis revealed sex and brain region-specific transcriptomic signatures of SST and SA, with genes downregulated in male VTA associated with both SST and SA, while genes upregulated in male Nac were associated with SA. DE genes associated with SST and SA are enriched for those related to synaptic processes, myelin sheath, and tobacco use disorder or chemdependency. Interestingly, we found that for SST the downregulated genes in the male VTA tended to be upregulated in female VTA, and the overlapping genes were strongly enriched for smoking genome-wide association study (GWAS) risk variants, which may thus explain male-specific SST. To gain mechanistic insight on the observed parental effect of SA, we analyzed the allelic imbalance of expression (AIE) in reciprocally crossed F1 rats that exhibited differential tendency of SA and found widespread region-specific AIE. For the SA-inclined rats, the genes showing AIE bias towards paternal F344 alleles in Nac were strongly enriched for genes upregulated in Nac DE—the gene set most relevant to NIC SA—and also for GWAS risk variants of smoking initiation. These findings show a prominent role for sex and region-specific gene expression and imprinting in NIC SST and SA, and suggest a mechanistic link between genes underlying these processes and human NIC addiction. This study provides a resource for understanding the biology underlying the genetic findings on human smoking phenotypes. F1 progeny were generated by crossing two inbred Envigo strains (Fischer-344 [F344] and Brown Norway [BN]). Initial cross (F1i) was performed by crossing male F344 and female BN. Reciprocal cross (F1r) was performed by crossing female F344 and male BN. The proven breeding pairs (F344/NHsd and BN/RijHsd) were purchased from Envigo. The breeding was carried out at the University of Chicago animal facility. For sensitization, F1 males (n=5-6/group) and females (n=7/group) were randomly assigned the to three groups and tested in early adulthood (~250 g). The first group received 0.1 mg/kg of NIC (base), the second group received 0.4 mg/kg of NIC (base) and the third group received SAL (1.0 ml/kg). F1 rats were daily administered either NIC or SAL for 4 days. On days 1 and 4, rats were placed in an open field immediately after the injection and their locomotion measured for 2 hours. Rats were then left undisturbed in their home cages for 2 weeks. On day 19, all F1 rats were administered one injection of NIC (0.4 mg/kg, IP, base), immediately placed in the open fields and their locomotion measured for two hours. For the SST transcriptomic analysis F1 males (n=16) and females (n=16) were divided in 2 groups. In the first group (n=8) rats were exposed to 0.4 mg/kg of NIC (base) every day for 4 days. In the second group (n=8) rats were exposed to SAL (1.0 ml/kg) every day for 4 days. Brain tissues (~20mg) from Nac, Nash and VTA regions were harvested on day 17.