Immunohistochemical expressions of mGluR5, P2Y2 receptor, PLC-beta1, and IP3R-I and -II in Merkel cells in rat sinus hair follicles.

We previously found that Merkel cells (MCs) of the rat and monkey show a strong immunoreaction of the alpha-subunit of Gq protein. The Galphaq-subunit isoform activates isozymes of phospholipase C-beta (PLC-beta), which produces inositol-(1,4,5)-triphosphate (IP3) which mobilizes intracellular Ca(++) from calcium stores via IP3 receptors. Glutamate and adenosine triphosphate (ATP), which are candidates for neurotransmitters in Merkel endings, are known to couple to Galphaq. Although MCs showed positive immunoreactions of metabotropic glutamate receptor 5 (mGluR5) in our preliminary study, these cells were not reactive to all antibodies to PLC-beta isozymes. We, therefore, reinvestigated immunohistochemical affinities to MCs of antibodies to PLC-beta isozymes and mGluRs using frozen sections of rat sinus hair follicles that were briefly postfixed in formaldehyde. We also studied the immunohistochemical expressions of P2Y receptors for ATP and IP3 receptor subtypes using similar sections. Merkel cells showed positive immunoreactions of PLC-beta1 and mGluR5. It was also found that MCs show positive immunoreactions of P2Y2, IP3R-I, and IP3R-II receptors. These results suggest that the Galphaq isoform in MCs couples to both the P2Y2 receptor and mGluR5 and regulates the intracellular Ca(++) concentration via the PLC-beta-IP3 cascade.