Sex differences in cholinergic systems in the interpeduncular nucleus following nicotine exposure and withdrawal

The medial habenula-interpeduncular nucleus (MHb-IPN) pathway modulates negative affective states produced by nicotine withdrawal. Sex differences in the contribution of acetylcholine (ACh) systems in this pathway have not been explored. Thus, this study assessed ACh levels and gene expression of alpha- and beta-containing nicotinic acetylcholine receptor (nAChR) subunits in the IPN of female and male rats following nicotine treatment and withdrawal. Rats were prepared with a pump that delivered nicotine for 14 days, and naive controls received a sham surgery. In Study 1, rats were prepared with a probe in the IPN, and ACh levels were measured following saline and then mecamylamine administration. In Study 2, separate groups of naive control or nicotine-treated rats received saline or mecamylamine and physical signs and anxiety-like behavior were assessed using elevated plus maze (EPM) procedures. The IPN was then dissected and mRNA levels were assessed using RT-qPCR methods. Nicotine treatment increased ACh levels to a larger extent in females than males. Nicotine withdrawal produced a similar increase in physical signs; however, females displayed greater anxiety-like behavior than males. In females, gene expression of alpha 5 increased following nicotine treatment and withdrawal. In males, alpha 7 increased following nicotine treatment and alpha 2 and alpha 3 increased during nicotine withdrawal. Both females and males displayed an increase in beta 3 and beta 4 during nicotine withdrawal. In females, anxiety-like behavior was correlated with alpha 4, alpha 5, and beta 2 gene expression in the IPN. These results suggest that sex differences in withdrawal are modulated via cholinergic systems in the IPN.