**2.2 Visible light**

The effects of ultraviolet or infrared radiation on skin are well known, but only a few reports describe the effect of visible radiation. We have shown that visible radiation influences the epidermal barrier recovery rate after barrier disruption (Denda & Fuziwara 2008). The effects of visible radiation on epidermal permeability barrier recovery were evaluated by using light-emitting diodes as light sources. The flank skin of hairless mice was tapestripped, and immediately exposed to blue (430-510 nm), green (490-560 nm), red (550-670 nm) or white (400-670 nm) light (20 W each) for 1 hour, followed by measurement of transepidermal water loss. Control mice were kept in a dark box during the experiments. During the irradiation, the skin surface temperature was kept constant at 37oC in all mice. Irradiation with red light significantly accelerated barrier recovery, while irradiation with blue light delayed it, compared with the control. White or green light did not affect the barrier recovery rate. We next carried out a study using hairless mouse skin organ culture. The permeability barrier was disrupted by means of acetone treatment, then each section was incubated afloat on the medium (37oC) and irradiated with blue, red or white light (20 w) for one hour. Immediately after the end of irradiation, we evaluated the barrier

Fig. 2. Temperature ranges within which various TRP receptors are activated, and effect of

The effects of ultraviolet or infrared radiation on skin are well known, but only a few reports describe the effect of visible radiation. We have shown that visible radiation influences the epidermal barrier recovery rate after barrier disruption (Denda & Fuziwara 2008). The effects of visible radiation on epidermal permeability barrier recovery were evaluated by using light-emitting diodes as light sources. The flank skin of hairless mice was tapestripped, and immediately exposed to blue (430-510 nm), green (490-560 nm), red (550-670 nm) or white (400-670 nm) light (20 W each) for 1 hour, followed by measurement of transepidermal water loss. Control mice were kept in a dark box during the experiments. During the irradiation, the skin surface temperature was kept constant at 37oC in all mice. Irradiation with red light significantly accelerated barrier recovery, while irradiation with blue light delayed it, compared with the control. White or green light did not affect the barrier recovery rate. We next carried out a study using hairless mouse skin organ culture. The permeability barrier was disrupted by means of acetone treatment, then each section was incubated afloat on the medium (37oC) and irradiated with blue, red or white light (20 w) for one hour. Immediately after the end of irradiation, we evaluated the barrier

their activation on epidermal permeability barrier homeostasis.

**2.2 Visible light** 

function. Again, red light accelerated barrier recovery, while blue light delayed it. An electronmicroscopic study suggested that red light accelerated lamellar body secretion, while blue light blocked it. These results indicate that visible radiation affects skin barrier homeostasis. That is, epidermal keratinocytes might have a sensory system for visible radiation.

Rhodopsin is a well-known photosensitive protein found in rod cells of the retina and detects light/dark contrast. Cone opsins are also photosensitive receptors in the cone cells of the retina and detect color. We have reported immunochemical studies using antirhodopsin and anti-opsin antibodies on human skin (Tsutsumi 2009). Both mouse retina and human epidermis showed clear immunoreactivity with each antibody. Interestingly, immunoreactivity against longer-wavelength opsin antibody was observed in the basal layer of the epidermis, while immunoreactivity against rhodopsin and shorterwavelength opsin was observed in the upper layer. PCR analysis confirmed the expression of rhodopsin-like and opsin-like genes in human retina and skin. These results suggest that a series of proteins, which play a crucial role in visual perception, are also expressed in human epidermis.

In retina, transducin and phosphodiesterase 6 play key roles in signal transmission. Thus, we hypothesized that these proteins might exist in epidermal keratinocytes and be associated with barrier homeostasis (Goto 2011). Immunohistochemical study and reverse transcription-PCR assays confirmed the expression of both transducin and phosphodiesterase 6 in epidermal keratinocytes. Topical application of 3-isobutyl-1 methylxanthine, a non-specific phosphodiesterase inhibitor, blocked the acceleration of barrier recovery by red light. Topical application of zaprinast, a specific inhibitor of phosphodiesterases 5 and 6, also blocked the acceleration, while T0156, a specific inhibitor of phosphodiesterase 5, had no effect. Red-light exposure reduced the epidermal hyperplasia induced by barrier disruption under low humidity, and the effect was blocked by pretreatment with zaprinast. Our results indicate phosphodiesterase 6 is involved in the recovery-accelerating effect of red light on the disrupted epidermal permeability barrier. Also, epidermal keratinocytes have a similar energy conversion system to that of the retina.

Fig. 3. Effects of visible radiation on epidermal permeability barrier homeostasis.

Physical and Chemical Factors that Improve Epidermal Permeability Barrier Homeostasis 203

It has been demonstrated that cultured human keratinocytes migrate to the negative pole in direct current electrical fields (Nishimura 1996). This result suggested that keratinocytes might have a sensory system for the external electrical field. Thus, we hypothesized that external electrical potential would influence epidermal barrier homeostasis. We applied negative and positive direct electric potential (0.5 V) to hairless mouse flank skin immediately after barrier disruption for one hour, and then we evaluated barrier recovery by the measurement of transepidermal water loss. At the area of applied negative potential, the barrier recovery rate was significantly accelerated, while the recovery was delayed at the

We subsequently found that several interfacial electrical conditions also affect barrier homeostasis. For example, topical application of barium sulfate or aqueous solution of ionic polymers formed an electrical double layer on the skin surface and affected the barrier recovery rate (Fuziwara 2004)(Denda 2005). Moreover, just placing metals on the skin surface after barrier disruption accelerated the barrier recovery, presumably because free electrons were supplied from metal to the skin surface (Denda & Kumazawa 2010). When chemically different materials are in contact, electro-chemical phenomena, such as formation of an electrical double layer, are induced. We previously demonstrated that a voltage-gated calcium channel is expressed at the upper layer of the epidermis (Denda 2006). Thus, when

Lipid metabolism is regulated by a series of enzymes in the epidermis (Feingold & Elias 2000) and each of them has optimal conditions of pH (Mauro 1998), concentrations of other ions (Denda 1999), etc. for activity. For example, the pH value of the healthy stratum corneum is kept acidic because the lipid-processing enzymes have an acidic optimal pH. Mauro et al. demonstrated that topical application of a basic buffer after barrier disruption delayed the

It has also been shown that topical application of calcium or potassium reduced barrier repair [Lee 1992], while magnesium and a mixture of calcium and magnesium salts accelerated the repair process [Denda 1999]. Topical application of an aqueous solution containing 10 mM magnesium chloride, magnesium sulfate, and magnesium lactate accelerated barrier repair. Application of magnesium bis(dihydrogen phosphate) or magnesium chloride in PBS solution did not affect the barrier recovery rate. Application of 10 mM calcium chloride aqueous solution delayed barrier repair, but a mixture of calcium chloride and magnesium chloride accelerated it when the calcium-to-magnesium molar ratio was lower than 1. Application of the mixture also improved the condition of dry, scaly skin induced by SDS treatment. These

Hexose is known to influence the stability of phospholipid bilayers. Therefore, the effects of topical application of all 12 stereoisomers of dextro-hexose on the epidermal barrier recovery rate after barrier disruption were evaluated (Denda 2011). Immediately after tape stripping, a 0.1 M aqueous solution of each hexose was applied on hairless mouse skin. Among the 8 dextro-aldohexoses, topical application of altose, idose, mannose and talose

the skin touches other materials, physiological phenomena might be induced.

repair process because the basic condition perturbed lipid processing (Mauro 1998).

area of positive applied potential (Denda & Kumazawa 2002).

**3. Chemical factors that influence barrier function** 

results suggest that ions are important in barrier homeostasis.

**2.4 Electrical potential** 

**3.1 Ions** 

**3.2 Hexose** 
