Multiphoton processes not only act to localize photoactivation to the focal spot but they allow access to high energy transitions not easily accesible by linear processes. We are using three-photon excitation to image the autofluorescence of serotonin-containing secretory granules in RBL-2H3 cells. Because we use red light (which is significantly less harmful than the 220nm wavelengths necessary to excite serotonin using one-photon processes), the cells remain viable and exhibit degranulation kinetics similar to those found using cuvette measurements. Studies at the single cell level suggest previously unknown mechanisms of release. A manuscript of these results is now in preparation.
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