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Nicotine has been used during pregnancy and lactation as a tobacco harm reduction strategy. However, it is unclear whether nicotine exposure during a critical development period negatively impacts stress responses in adulthood. This study investigated how nicotine, administered via breastfeeding, affects the brain‐derived neurotrophic factor (BDNF), synaptic proteins levels, and anxiety‐like behavior in adult female mice subjected to stress. Female Swiss mice were exposed to saline or nicotine (8 mg/kg/day) through breastfeeding between their fourth and 17th postnatal days (P) via implanted osmotic mini pumps. The unpredictable chronic mild stress (UCMS) protocol was performed during their adulthood (P65) for 10 consecutive days, followed by the elevated plus maze (EPM) test 1 day after the protocol. Animals were euthanized and their blood, collected for plasma corticosterone measurements and their brain structures, dissected for BDNF and synaptic proteins analyses. We found no significant differences in corticosterone levels between groups (Saline/Non‐stress, Nicotine/Non‐stress, Saline/Stress, and Nicotine/Stress). The UCMS protocol hindered weight gain. Mice exposed to nicotine through breastfeeding with or without the UCMS protocol in adulthood showed higher grooming and head dipping frequency; decreased BDNF levels in cerebellum and striatum; increased postsynaptic density protein 95 (PSD‐95), synapsin I, and synaptophysin levels in cerebellum; and decreased PSD‐95 and synapsin I levels in brainstem. Our results indicate that nicotine exposure through breastfeeding leads to long‐lasting behavioral effects and synaptic protein changes, most of which were independent of the UCMS protocol, even after a long nicotine‐free period, highlighting the importance of further studies on nicotine exposure during development.

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