Alterations in hippocampal cholinergic receptors and hippocampal behaviors after early exposure to nicotine.

Mice were exposed to nicotine prenatally by injecting the mother with 1.5 mg/kg nicotine SC twice daily on gestation days 9-18 (PreN mice) or neonatally by daily SC injections of 1.5 mg/kg nicotine on postnatal days 2-21 (NeoN mice). At age 50 days, hippocampal muscarinic receptors Bmax of PreN and NeoN mice were 58% and 79% above control, respectively (p less than 0.01); Kd was unaffected by early nicotine exposure. Eight-arm maze performance of nicotine-exposed animals fell behind control level. Both PreN and NeoN made approximately 10% less correct responses in the first eight trials than controls throughout the test period (p less than 0.01). By the last day of testing, PreN needed 23% and NeoN 31% more trials than controls to enter all arms (p less than 0.001). In addition, PreN needed 35 and NeoN 42% more days than controls to reach criterion (p less than 0.05). Similarly, while 61% of controls reached criterion by day 6 only 17% of PreN and 25% of NeoN reached criterion (p less than 0.01). In the Morris maze, PreN needed from 43-119% more time to reach the platform (p less than 0.001). In the spatial probe test, PreN animals made 35% fewer crosses over the area of the missing platform (p less than 0.001). The study suggests that nicotine administered to the fetus or neonate alters septohippocampal chemistry and induces deficits in hippocampus-related behaviors. The possible reversal of the behavioral changes by manipulating the cholinergic innervations should be the subject of future investigations.