Selective phototoxic destruction of quinacrine-loaded Merkel cells is neither selective nor complete

Experiments were performed on slowly adapting type I mechanoreceptors in an isolated rat skin-nerve preparation (SA I receptors) and in an isolated rat sinus hair preparation (St I receptors). Merkel cells were stained in vitro with the fluorescent dye quinacrine and irradiated with ultraviolet (UV) light (2 mW for up to 1 h) while recording receptor responses to standard mechanical stimuli every 30 s. In addition, thresholds for electrically evoked action potentials were tested by applying electrical stimuli to the skin through the same stylus used for mechanical stimulation. UV irradiation resulted in abrupt failure to respond to mechanical stimuli in 73% of the SA I receptors examined (n = 37) within less than 1 h. This confirms previous reports of phototoxic destruction of Merkel cells. However, several minutes after the receptors failed to respond to mechanical stimulation, thresholds for electrical stimuli applied to the receptive field increased sharply. About 40% of the St I receptors (n = 13) irradiated with UV light following quinacrine staining stopped responding to bending of the hair within 1 h. In contrast, none of the seven St II receptors treated in the same way showed significant changes in the responses. Electron microscopic examination of sinus hairs after quinacrine staining alone showed slight changes in the appearance of Merkel cells, and in particular enlargement of the perinuclear space. These changes did not affect receptor responses. Electron microscopic studies of sinus hairs with receptors that had maintained normal responses to mechanical stimuli after quinacrine staining and 1 h of UV irradiation revealed that a substantial number of Merkel cells still had a normal ultrastructure while adjacent nerve terminals were severely swollen and partially compressing the Merkel cells. No changes were observed in lanceolate nerve terminals forming the morphological substrate of St II receptors. These results demonstrate that sensitivity to phototoxic destruction following quinacrine staining varies greatly among Merkel cells, with some maintaining normal function and ultrastructural appearance even after 1 h of UV irradiation. On the other hand there is clear evidence that the phototoxic damage affects the nerve terminals as well. Such experiments can therefore not provide conclusive proof about the role of Merkel cells in these mechanoreceptors.