CUTANEOUS MERKEL CELLS OF THE RAT CONTAIN BOTH DYNORPHIN-A AND VESICULAR MONOAMINE TRANSPORTER TYPE-1 (VMAT1) IMMUNOREACTIVITY

To delineate fully opioid peptide function in cutaneous inflammatory a nd nociceptive responses, it is necessary to know first which opioid p eptides are present in the skin and which cellular elements in the ski n store and secrete them. Merkel cells are cutaneous neuroendocrine ce lls, which may derive from the neural crest or from undifferentiated k eratinocytes with stem cell character. The neuroendocrine character of Merkel cells is supported by their immunoreactivity for chromogranin A (CGA) and a variety of neuropeptides, among them the opioid peptide [Met]enkephalin as shown in guinea-pig and mouse. This study investiga tes in the rat whether the preprodynorphin derived opioid peptide dyno rphin A is expressed in cutaneous Merkel cells and possibly related to an aminergic phenotype. Light microscopic immunohistochemistry reveal ed dynorphin A immunoreactivity in Merkel cells to be codistributed wi th immunoreactivity for calcitonin gene-related peptide (CGRP) and CGA ,two well-established marker peptides of mammalian Merkel cells. Vibri ssal Merkel cells stained for the neuroendocrine vesicular monoamine t ransporter isoform 1 (VMAT1) but not for the predominantly neuronal is oform 2 (VMAT2). Merkel cell staining for dynorphin A, VMAT1, CGA, and CGRP was unaffected by experimental denervation. Dynorphin A and a st ill unidentified monoamine, possibly serotonin, may cofunction as auto crine or paracrine mediators in the mechanosensory Merkel cell - axon complex and are potentially involved in peripheral analgesia.