Effects of chronic neonatal nicotine exposure on nicotinic acetylcholine receptor binding, cell death and morphology in hippocampus and cerebellum

Abstract

Nicotine, the major psychoactive ingredient in tobacco interacting with nicotinic acetylcholine receptors (nAChR), is believed to have neuroprotective and neurotoxic effects on the developing brain. Neurotoxicity has been attributed to activation of homomeric α7 nAChRs, neuroprotection to heteromeric α4β2 nAChRs. Thus, developmental nicotine could have opposite effects in different brain regions, depending on nAChR subtype expression. Here, we determined if chronic neonatal nicotine exposure (CNN), during a period of brain growth corresponding to the third human trimester, differentially regulates nAChR expression, cell death, and morphological properties in hippocampus and cerebellum, two structures maturing postnatally. Rat pups were orally treated with 6 mg/kg/day nicotine from postnatal day (P)1 to P7. On P8, expression for α4, α7 and β2 mRNA was determined by in situ hybridization; nAChR binding sites by receptor autoradiography, dying neurons by TUNEL and Fluoro-Jade staining and morphological properties by analysis of Cresyl Violet–stained sections. In control cerebellum, strong expression of α4, β2 mRNA and heteromeric nAChRs labeled with [125I]-epibatidine was found in granule cells, and α7 mRNA and homomeric nAChRs labeled with [125I]-α-bungarotoxin were in the external germinal layer. In control hippocampus, low expression of α4 mRNA and heteromeric nAChRs and high expression of α7 mRNA and homomeric nAChRs were detected. CNN increased heteromeric nAChR binding in hippocampus but not cerebellum and significantly decreased neuronal soma size and increased packing density in hippocampal principal cells but not in cerebellum. CNN did not increase the number of dying cells in any area, but significantly fewer TUNEL-labeled cells were found in CA3 strata oriens and radiatum and cerebellar granule layer. Thus, the hippocampus seems to be more sensitive than the cerebellum to CNN which could result from different nAChR subtype expression and might explain long-lasting altered cognitive functions correlated with gestational nicotine exposure due to changes in hippocampal cell morphology.