Developmental changes in the expression of nicotinic acetylcholine receptor alpha-subunits in the rat heart

Neuronal nicotinic acetylcholine receptors (nAChR) are ligand-gated ion channels that consist of various subunits. During ontogeny, muscular and neuronal nAChR undergo changes in the distribution and subunit composition in skeletal muscle and brain, respectively. Here, we have investigated the occurrence of the ligand-binding alpha-subunits of neuronal nAChR by means of reverse transcription/polymerase chain reaction and immunohistochemistry in the rat heart during prenatal and postnatal development and after capsaicin-induced sensory denervation. mRNAs coding for the alpha4, alpha5, alpha7 and alpha10 subunits were detected throughout all developmental stages. Messenger coding for the alpha2 subunit was first detectable at developmental stage E20; alpha3 subunit mRNA was expressed throughout all prenatal developmental stages, whereas it was restricted postnatally to the atria. mRNA for alpha6 was observed at E14-P8 but was absent thereafter. At no developmental stage could an unequivocal signal for alpha9 nAChR subunit mRNA be obtained. The expression pattern was unchanged by capsaicin treatment. Immunohistochemistry demonstrated alpha7 subunits on cardiac neurons, fibroblasts and cardiomyocytes and alpha2/4 subunits on cardiomyocytes with a postnatal redistribution to intercalated discs, as shown by cryo-immunoelectron microscopy. Our results indicate an additional non-neuronal expression of nAChR subunits in the rat heart that, as in skeletal muscle, precedes functional innervation and then undergoes changes in its distribution on the surface of cells.