Cholinergic receptors in human prenetal brain : presence, distribution and influence of nicotine and ethanol

Abstract

Events early in life, including exposure to various compounds, may cause disturbances in brain development. Both maternal smoking and alcohol drinking during pregnancy can induce preand postnatal death, cognitive and neurobehavioral disturbances. Nicotine can, through stimulation of nicotinic receptors (nAChRs), interfere with proliferation, differentiation and synaptogenesis during brain development. Ethanol affects both proliferation and induction of apoptosis and is able to modulate the function of nAChRs.

This thesis focuses on the development of nAChRs, especially the alpha7 nAChR subtype, in the human prenatal brain and the effects of nicotine exposure. A significant positive correlation between gestational age (5-12 weeks of gestation) and the expression of both alpha7 nAChR protein and alpha7 mRNA was found. Both the number and the gene expression of alpha7 nAChRs were significantly higher in the medulla oblongata, pons, mesencephalon and spinal cord compared to cerebellum, cortical and subcortical forebrain at 911 weeks of gestation. Comparison of the presence of nAChRs in the prenatal (9-11 weeks of gestation), middleaged and aged brain, showed a significantly higher expression of alpha7 mRNA in prenatal pons, medulla oblongata, cortex and cerebellum than in corresponding regions in adult brain, except for aged cortex. Nonetheless, higher prenatal density of the alpha7 nAChRs themselves ([125I] -alpha-bungarotoxin binding) was observed only in brainstem, which is more mature at this stage.

The presence of muscarinic receptors (mAChRs) already at gestational week 5 was observed for the first time. Exposure to maternal smoking significantly changed the agerelated number of the alpha4 nAChRs ([3 H]-cytisine binding) but not the m2 mAChR ([3H]-AFDX 384 binding) in medulla oblongata, pons and cerebellum. In addition, the alpha4 and alpha7 mRNA expression was altered in all regions as was the mRNA levels for some mAChR subtypes.

A significantly dose-dependent decrease in cell survival was observed after co-exposure to nicotine and ethanol, at physiologically relevant concentrations, in primary cell cultures from human cortex (6-10.5 weeks of gestation). Neuronal cell cultures were more susceptible to exposure to low concentrations of ethanol than cultures with both neurons and astrocytes. High concentrations of nicotine and ethanol decreased the proliferation of cells in cultures with both neurons and astrocytes.

Conclusions: Subtypes of nAChRs and mAChRs are present early during development. Nicotine exposure during the first three months of gestation will affect the normal expression of these receptors. Maternal smoking provides excessive nAChR stimulation and discoordinate the numerous of events that are necessary for proper assembly of the human brain. Pregnant women should be advised to totally refrain from smoking and ethanol intake.