Glutamate Receptor-Like Immunoreactivity in Rat Vibrissal Merkel Cells

Recent functional data has raised the possibility that an excitatory amino acid, such as glutamate, may act as neurotransmitter in vibrissal type I slowly adapting (St I) mechanoreceptors. It is supposed that Merkel cells respond to mechanical stimulation by releasing a neurotransmitter which acts on the nerve terminals in contact with the Merkel cells. Therefore, we sought to establish whether or not the appropriate signalling molecules for glutamate neurotransmission are to be found in the vibrissa. Using frozen sections from vibrissa isolated from adult Wistar rats, we stained for NMDA and AMPA glutamate receptors and the glutamate transporter GLT-1. Distinct patterns of expression of NMDA-type receptors were found in the vibrissal sections. There was intense immunoreactivity to the NR1 antibody in the layer of cells in the outer root sheath, adjacent to the glassy membrane. However, NR2A/B staining was restricted to a subpopulation of cells adjacent to the glassy membrane, but only above the level of the ring wulst and the innermost cells of the rete ridge region – suggesting that the NR2A/B stained cells are Merkel cells. GLT-1 immunoreactivity was largely restricted to the epithelial cells lining the thick sinus body capsule enclosing the blood sinuses. There was no remarkable staining with the AMPA-type GluR1–3 antibodies. The data suggest the presence of glutamate signaling in the outer root sheath cells, including Merkel cells. However, it remains to be seen if this is related to mechanosensory function in Merkel cells or whether it is part of the already described glutamate signaling in keratinocytes. 164 Senok et al.