Background: Chronic cigarette smokers report withdrawal symptomology, including affective dysfunction and cognitive deficits. While there are studies demonstrating sex specific withdrawal symptomology in nicotine-dependent individuals, literature examining the underlying biological mediators of this is scant and not in complete agreement. Therefore, in this study, we evaluated the sex specific effects of nicotine and withdrawal on contextual fear memory, a hippocampally dependent aspect of cognition that is disrupted in nicotine withdrawal.
Methods: Male and female B6/129F1 mice (8-13 weeks old) were used in all experiments. For the acute nicotine experiment, mice received intraperitoneal saline or nicotine (0.5 mg/kg) prior to contextual fear conditioning and test. For the chronic nicotine experiment, mice received nicotine (18 mg/kg/day) or saline for 11 days, then underwent contextual fear conditioning and test. Following the test, mice underwent minipump removal to elicit withdrawal or sham surgery, followed by the fear extinction assay. Bulk cortical tissue was used to determine nicotinic acetylcholine receptor levels via single point [3H]Epibatidine binding assay. Gene expression levels in the dorsal and ventral hippocampus were quantified via RT-PCR.
Results: We found that female mice had a stronger expression of contextual fear memory than their male counterparts. Further, following acute nicotine treatment, male, but not female, subjects demonstrated augmented contextual fear memory expression. In contrast, no significant effects of chronic nicotine treatment on fear conditioning were observed in either sex. When examining extinction of fear learning, we observed that female mice withdrawn from nicotine displayed impaired extinction learning, but no effect was observed in males. Nicotine withdrawal caused similar suppression of fosb, cfos, and bdnf, our proxy for neuronal activation and plasticity changes, in the dorsal and ventral hippocampus of both sexes. Additionally, we found that ventral hippocampus erbb4 expression, a gene implicated in smoking cessation outcomes, was elevated in both sexes following nicotine withdrawal.
Conclusions: Despite the similar impacts of nicotine withdrawal on gene expression levels, fosb, cfos, bdnf and erbb4 levels in the ventral hippocampus were predictive of delays in female extinction learning alone. This suggests sex specific dysfunction in hippocampal circuitry may contribute to female specific nicotine withdrawal induced deficits in extinction learning.
Keywords: Extinction; Fear conditioning; Hippocampus; Nicotine; Withdrawal.
Smokers undergoing nicotine withdrawal report increased feelings of anxiety, depression, and cognitive deficits. However, there are sex differences in these symptoms, with women reporting higher feelings of anxiety compared to men and men having worse cognitive deficits than women. The mechanisms underlying these sex differences in nicotine withdrawal symptoms are not well understood. The hippocampus is a brain region highly implicated in both the cognitive and anxiety-like symptoms of nicotine withdrawal. Therefore, we evaluated the effects of nicotine and withdrawal on contextual fear memory, a hippocampally dependent learning and memory task, in male and female mice. We found that female mice had a stronger contextual fear memory expression than their male counterparts. However, following acute nicotine treatment male mice had enhanced contextual fear memory compared to non-nicotine treated males, while acute nicotine had no impact on female mice. When examining extinction of contextual fear, we found female mice withdrawn from nicotine displayed impaired extinction learning, but no effect was observed in males. The female specific deficits in extinction learning due to nicotine withdrawal were correlated to hippocampal gene expression related to neuronal activity. This suggests hippocampal dysfunction may be driving the female specific nicotine withdrawal induced deficits in extinction learning.
© 2024. The Author(s).