Selective phototoxic destruction of rat Merkel cells abolishes responses of slowly adapting type I mechanoreceptor units.

1. The fluorescent dye quinacrine which accumulates in Merkel cells in touch domes was administered to rats and the effects of excitation light irradiation on the mechanical responses of slowly adapting (SA) type I units innervating the touch domes were investigated. 2. Histological examination showed that after 10 min of irradiation degeneration was specifically localized to Merkel cells loaded with quinacrine. Nerve terminals associated with Merkel cells remained intact, even after treatment. 3. In SA type I units, responses to standard stimulation (a 100 ms ramp followed by a 2.9 s plateau of 400 microns constant displacement) decreased significantly after irradiation of the domes with quinacrine-excitation light through a 'B' filter ('B' light). With 5 min irradiation, the response decreased to 52 +/- 7% (n = 10, mean +/- S.E.M.) of the pretreated value, to 17 +/- 4% with a 10 min treatment and practically disappeared within 20 min. 4. In SA type I units with non-loaded Merkel cells, the response increased to 119 +/- 8% (n = 13) with 5 min irradiation and was 99 +/- 9% with the 10 min treatment. At around 15 min after the onset of irradiation there was a gradual decrease and within 60 min the response disappeared. 5. When responses were divided into phasic (0-120 ms after the onset of stimulation) and tonic (120-3000 ms) components, 'tonic' responses were more affected than 'phasic' ones in quinacrine-loaded SA type I units. 6. Stimulus-response curves shifted to the right and downwards in SA type I units with quinacrine-loaded Merkel cells after irradiation, but no significant change was seen in SA type I units without quinacrine. 7. Our observations are consistent with the hypothesis that Merkel cells are responsible for mechanoelectric transduction in SA type I units.