Chronic neonatal nicotine exposure increases mRNA expression of neurotrophic factors in the postnatal rat hippocampus.

Nicotine, the psychoactive ingredient in tobacco, can be neuroprotective but the mechanism is unknown. In the adult hippocampus, chronic nicotine can increase expression of growth factors which could contribute to nicotine's neuroprotective effects. During development, nicotine could also increase expression of neurotrophic factors. Therefore, we determined whether chronic neonatal nicotine (CNN) exposure increased mRNA expression levels of brain-derived neurotrophic factor (BDNF), nerve-growth factor (NGF), neurotrophin-3 (NT-3), fibroblast growth factor-2 (FGF-2), and insulin-like growth factor-1 (IGF-1). Nicotine (6 mg/kg/day in milk formula) or milk formula (controls) were delivered in three daily doses via oral gastric intubation to rat pups from postnatal day (P)1 to P8, and then sacrificed. Brains were processed for in situ hybridization using specific (35)S-labeled cRNA probes. At P8, CNN had a significant stimulant treatment effect on the expression of BDNF, FGF-2, NT-3 and IGF-1 [p<0.01], but not NGF. Specifically, BDNF mRNA expression, detected in CA1, CA3 stratum (s.) pyramidal and granule cell layer of the dentate gyrus (DG), was increased by 27.4%, 23.26% and 27.3%, respectively. FGF-2 mRNA expression, detected in neurons and astrocytes in CA1 s. radiatum, CA2 and CA3 s. pyramidale, and molecular layer of the DG, was increased by 34.0%, 8.9%, 31.0% and 23.1%, respectively. NT-3 mRNA expression in CA2 s. pyramidale was increased by 80.0%, and CNN increased the number of IGF-1-expressing cells in CA1 (18.0%), CA3 (20.9%) and DG (17.7%). Thus, nicotine exposure during early postnatal development differentially up-regulated expression of neurotrophic factor mRNAs in the hippocampus, which could increase neurotrophic tone and alter developmental processes.