Nicotinic acetylcholine receptors in rat and human placenta

Smoking during pregnancy causes low birth weight, premature delivery, neonatal morbidity, and mortality. Nicotine is a main pathogenic compound of cigarette smoke, and depresses active amino-acid uptake by human placental villi. It binds to the acetylcholine binding site of the α-subunits of nicotinic acetylcholine receptors (nAChR). Eight different neuronal nAChR α-subunits have been identified in mammals. Here, we investigated their localisation and distribution in the human and rat placenta by RT-PCR and immunofluorescence. The mRNAs of all α-subunits are expressed in the human and rat placenta. Immunohistochemically, subunits α2–5, α7, α9 and α10 are localised in different combinations in rat cytotrophoblast, human and rat syncytiotrophoblast, vascular smooth muscle cells, endothelial cells, Hofbauer cells, human amnion epithelium and rat visceral yolk sac epithelium. Thus, all human and rat placental cell types exhibit receptor subunits with binding sites for the endogenous ligand ACh and nicotine. ACh is suggested to be an important placental signalling molecule that, through stimulation of nAChR, controls the uptake of nutrients, blood flow and fluid volume in placental vessels, and the vascularisation during placental development. Chronic stimulation of nAChR by nicotine might result in unbalanced receptor activation or functional desensitisation followed by the known pathological effects of smoking.

Introduction

Smoking during pregnancy causes low birth weight, premature delivery, neonatal morbidity, mortality [1], [2], and increases the risk of behavioral problems, attention deficits, hyperactivity and learning disabilities, sudden infant death syndrome and nicotine dependence later in life ([3], [4], for review). Nicotine is the component of tobacco smoke that causes addiction [5]. It is considered to be a main pathogenic compound of cigarette smoke [6], and depresses active amino-acid uptake by human placental villi [7]. Part of this depression (10–16%) is irreversible [7].

Nicotine stimulates nicotinic acetylcholine receptors (nAChR) by binding at the acetylcholine binding site of the nAChR α-subunits [8]. The nAChR restricted to the motor endplate of skeletal muscle are distinguished from receptors of the neuronal type. The nAChR of the motor endplate are Na⁺-permeable heteropentamers, composed of two α-, one β-, γ- or ɛ- and δ-subunit each. Receptors of the neuronal type are permeable to mono- and divalent cations. They are heteromers composed of two α-subunits and three β-subunits, homopentamers of five α-subunits (e.g. α7, α9) or α-heteropentamers (e.g. α9/10) [9], [10], [11], [12]. Until recently, 10 different α-subunits and four different β-subunits have been identified. The α1 and β1 subunits are found exclusively at the motor endplate. Receptors of the neuronal type consist of α2–10 and β2–4 subunits [10], [13], [14], [15], [16]. Among these, the α8 subunit has only been found in chick [17]. The composition of subunits in the receptor determines ligand specificity, ligand affinity, cation permeability and channel kinetics [18], e.g. α-homomers and α-heteromers have an increased Ca²⁺-permeability [10], [12], [19], [20]. Data on the detailed expression and distribution of the individual nAChR α-subunits in the placenta are required as a basis for a better understanding of both the endogenous placental cholinergic signalling pathways and their perturbation by nicotine as it occurs during maternal smoking. In this study, we investigated the presence, localisation and distribution of the nAChR α-subunits in placenta utilizing RT-PCR and immunofluorescence. In addition to the human placenta, we also studied rat material since this species often serves as experimental model, and similarities or dissimilarities to the situation found in human are of interest when experimental data are to be interpreted.