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

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 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.

Cellular and molecular actions of Methylene Blue in the nervous system

“…Depolarizing effects of MB on neuronal structures have been demonstrated in several earlier studies (Burmistrov et al, 1967; Nozdrachev et al, 1984; Nemeth et al, 1985; Kress et al, 1997). Inhibition of Ca 2+ -activated K + channels (Nemeth et al, 1985), activation of Ca 2+ channels (Saitow and Nakaoka, 1997), and facilitation of Na + channel inactivation (Oxford, 1977; Kress et al, 1997) are suggested to be possible mechanisms for MB-induced depolarizations. It appears that the actions of MB, reported in the majority of these studies, are not mediated by the cGMP pathway.…”

“…In addition, the functions of voltage-sensitive Na + (Oxford, 1977; Starkus et al, 1984; 1993; Kress et al, 1997), Ca 2+ (Thuneberg, 1990) and Ca 2+ -activated K + channels (Nemeth et al, 1985; Stockand and Samson, 1996; Saitow and Nakaoka, 1997) are also modulated by MB. As a result of MB actions on ion channels, excitability of neurons is altered significantly.…”

“…It appears that the actions of MB, reported in the majority of these studies, are not mediated by the cGMP pathway. Molecular mechanisms mediating the effects of MB on these ion channels are complex and depend on multiple factors including the route of MB application (Oxford, 1977; Armstrong and Croop, 1982; Starkus et al, 1984; 1993), light exposure (Pooler, 1968; Starkus et al, 1993; Kress et al, 1997), membrane potential (Valenzeno et al, 1993) and the redox state of cells (Nanasi and Dely, 1995). Among these factors, light exposure deserves mention since it is particularly important during in vitro experiments in which various light sources are used routinely.…”

Sensory Neurons With Afferents From Hind Limbs

“…8 It seems unlikely that afferent innervation is lacking in the extremities or, if present, without influence on cardiovascular autonomic nerve function under the mentioned physiological conditions. It is known from other investigations that afferent nerve fibers from skin 9 and other organs can be electrophysiologi-cally characterized as C-fibers 10 that, in addition, were often observed to be peptidergic secreting substances like substance P (SP) and calcitonin gene-related peptide (CGRP). 11,12 Again, little is known about perivascular innervation of vessels relevant in hypertension.…”

A controlled trial of photodynamic therapy of actinic keratosis comparing different red light sources

“…This immediate pain is caused by stimulation of the epidermal receptors (polymodal nociceptors) which lie at the free ends of the fast A␦ and the slow C-fibers [12]. In detail, it is thought to be an activation of the TRPV1 (transient receptor potential vanilloid 1) receptor that controls heat sensation [13]. A second type of pain, a mediator-induced pain, is caused by the binding of prostaglandins and bradykinin to peripheral pain receptors (EP 2 and B 2 ).…”