Expression of the alpha 4 neuronal nicotinic acetylcholine receptor subunit in the developing mouse hippocampus

Neurotransmitters such as acetylcholine can control neuritogenesis of hippocampal cells. The timing of its receptors expression consequently may influence synaptogenesis and neuronal activity in the developing hippocampus. We investigated the mRNA expression of the nicotinic acetylcholine-gated ion channel receptor (nAChR) alpha 4 subunit in the embryonic and postnatal hippocampal formation. Although its expression level is low in the adult hippocampus, this protein constitutes the major nAChR subunit in the central nervous system. We carried out in-situ hybridization experiments to determine whether or not the alpha 4 AChR subunit mRNA distributions show evidence of regional and developmental regulation during hippocampal maturation. Our studies reveal that alpha 4 AChR mRNA expression was low at the embryonic stage, but increased transiently during postnatal development reaching a maximum during the second week of life and decreasing thereafter, to a minimum at adulthood. In hippocampal regions, the peak values of alpha 4 AChR expression were between 400 and 800% of adult alpha 4 messenger levels. In the postnatal hippocampus, most of the cells from the pyramidal layer of the CA3 and CA2 areas displayed a strong hybridization signal for the alpha 4 AChR subunit. In the hilus and the CA1 regions, the localization of the alpha 4 transcripts seemed to be restricted to some interneurons and pyramidal cells, respectively. Moderate and uniform in-situ hybridization signals were observed in granular cells from the dentate gyrus. The transient profile of alpha 4 expression suggests that nAChRs may participate in the early postnatal maturation of hippocampal circuity.