Methylene blue induces ongoing activity in rat cutaneous primary afferents and depolarization of DRG neurons via a photosensitive mechanism.

The dye methylene blue is known as a blocker of guanylyl cyclase and it has been widely used to deplete cells of internal cyclic GMP. The data presented demonstrate an activation of adult rat sensory neurons by methylene blue via a photosensitive mechanism. In single fiber recordings from primary afferents of the rat skin in vitro, methylene blue, applied to the receptive field, induced discharge activity: 2/2 A beta-, 2/4 A delta- and 5/7 C-fibers showed significantly enhanced firing upon 10 microM methylene blue in the presence of light, whereas the dye was ineffective when illumination was prevented. In whole cell current clamp experiments with dissociated dorsal root ganglion neurons, 100 microM methylene blue was ineffective in the dark but evoked a membrane depolarization of 15.3 +/- 3.5 mV (n = 5) accompanied by discharge activity upon illumination. In whole cell voltage clamp experiments, methylene blue (100 microM) caused a significant slowing of the inactivation of voltage-dependent sodium currents. In addition, an inhibition of fast and slow outward currents was observed with prolonged exposure. The impeded sodium inactivation together with the blockade of potassium currents may contribute to the depolarization and discharge activity observed in primary afferents in vitro as well as in dissociated sensory neurons in culture. We therefore suggest that methylene blue studies with excitable cells or tissues need to be interpreted with caution.