Effect of Sex and Estrous Cycle on Nicotine withdrawal Syndrome in the Rat

Introduction: Severity of withdrawal syndrome in women during smoking cessation has reportedly been influenced by menstrual phase. There are few studies of female rats with their four-day estrous cycle. Methods: Histological examination determined the precise estrous phase at time of testing. The subjects were 14 qualifying female rats and 8 male rats, all five to six months old. Slides of vaginal fluid were examined for estrous phaseidentifying cell types. Nicotine withdrawal was evaluated at either the proestrus phase (n=7), a follicular portion of the cycle, and the metestrus phase (n=7) a luteal portion. Rats were continuously infused with 9 mg/kg/day s.c. nicotine bitartrate. On the seventh day, each subject was challenged with 1 mg/kg of the nicotinic antagonist mecamylamine, a dose that precipitates a vigorous withdrawal syndrome only in nicotine-dependent rats. Subjects were observed over 30 min. on a standard checklist of somatically expressed withdrawal behaviors. Results: Male rats displayed 26.00 ± 3.64 withdrawal signs (M ± SEM), while female rats in proestrus exhibited a similar 28.86 ± 3.39 signs. Female rats in metestrus displayed 41.57 ± 5.38 signs. One-way ANOVA revealed a significant difference among groups, p=0.039. Post hoc comparisons revealed significant differences between metestrus and proestrus females, p=0.049 and between metestrus females and males, p=0.016, but not between proestrus females and males. Conclusions: The results are consistent with reports of menstrual phase influence on withdrawal severity in smoking cessation, providing a laboratory model for studying this issue and its potential treatment. Implications: In the large literature on nicotine withdrawal in the rat, there are relatively few studies on female rats and hardly any on the effects of the estrous cycle on physical dependence and withdrawal. The results are consistent with reports on the menstrual cycle affecting withdrawal severity in women undergoing smoking cessation, supporting the translational relevance of the rat nicotine dependence model. The methods utilized here expand the ability of rat physical dependence models to compare the sexes and the estrous phases in nicotine physical dependence and withdrawal.