Methylene blue induces ongoing activity in rat cutaneous primary afferents and depolarization of DRG neurons via a photosensitive mechanism |
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Authors: | M Kress M Petersen P W Reeh |
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Institution: | Institut für Physiologie und Experimentelle Pathophysiologie, Universit?tsstrasse 17, D-91054 Erlangen, Germany, DE Physiologisches Institut der Universit?t Würzburg, R?ntgenring 9, D-97070 Würzburg, Germany, DE
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Abstract: | 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 Aβ-, 2/4 Aδ- and 5/7 C-fibers showed significantly enhanced firing upon 10 μM 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 μM 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 μM)
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.
Received: 2 April 1997 / Accepted: 29 July 1997 |
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Keywords: | Sensory neuron Excitation Ongoing discharge Ionic current Second messenger Cyclic nucleotide Phototoxicity |
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