Touch domes are low threshold mechanosensory structures consisting of aggrgates of specialized Merkel cells and an overlying layer of epithelial keratinocytes. Merkel cells, located in the basal layer of the epidermis, appear to be highly differentiated epithelial cells which are innervated by large myelinated type I neurons. Synapse-like junctions join Merkel cell cytoplasm nearest to the nerve ending. Specialization is evident in the epithelium overlying touch domes because unlike nearby epithelium, it is strongly immunopositive for nerve growth factor (NGF). Although Merkel cells are ideally situated to act as primary mechanosensory transducer, their functional role in touch domes remains controversial. Our preliminary evidence obtained from a unique iin vitro skin-nerve preparation supports the notion that the neurotransmitter serotonin found both in Merkel cells and type I neurons first activates, and subsequently produces modes inhibition of type I afferent responses. The in vitro system will also be utilized in investigate other transmitter substances found in the Merkel cell/neurite complex (ATP, met-enkephalin and glutamate). Complementar immunocytochemical studies will localize neurotransmitter receptorss in touch domes. Careful histological counts of dense-cored vesicle profiles before and after mechanical depression of touch domes will also be undertaken to analyze whethr vesicle depletion accompanies natural stimulation in situ. Similar in vitro physiological and morphological techniques will be applied to preparations exposed to alpha-Latrotoxin, a spider venom which causes massive vesicle release frompresynaptic terminals. The functional roleof NGF and NGF-receptors located on Merkel cells and type I afferent neurons will be investigated by assessing mechanosensitivity of type I neurons in vivo in mice deficient in either the low- or high-affinity NGF receptor subtype. In situ hybridazation experiments will determine if NGF is sequesteredor produced by touch dome keratinocytes.
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