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Salivary Secretion under theInfluence of Bright/Dim LightExposure in the Morning andEvening in HumansKi Ja Hyun , Hiromi Tokura , Koichi Gotoh , MasayukiKondo , Takako Akimoto , Dominika Kanikowska , YukiYamabayashi & Keiko TsuchidaPublished online: 09 Aug 2010.

To cite this article: Ki Ja Hyun , Hiromi Tokura , Koichi Gotoh , Masayuki Kondo , TakakoAkimoto , Dominika Kanikowska , Yuki Yamabayashi & Keiko Tsuchida (2002) SalivarySecretion under the Influence of Bright/Dim Light Exposure in the Morning and Evening inHumans, Biological Rhythm Research, 33:2, 129-140

To link to this article: http://dx.doi.org/10.1076/brhm.33.2.129.1316

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Salivary Secretion under the Influence of Bright/DimLight Exposure in the Morning and Evening in Humans

Dominika Kanikowska, Yuki Yamabayashi, Keiko Tsuchida, Ki Ja Hyun, Hiromi Tokura, Koichi Gotoh2, Masayuki Kondo2 and Takako Akimoto2

1Department of Environmental Health, Nara Women’s University, Nara, Japan;2Sekisui House R&D Institute, Kyoto, Japan

Abstract

Recent studies show that bright and dim light intensities during the daytime haveimportant regulatory functions. Our present study was performed to evaluate theeffect of exposure to different light intensities during the morning and evening onsalivary secretion and its sodium concentration. The study involved 6 healthy, femalevolunteers who were exposed to dim light (100 lx) from 7:00 to 17:00 and to brightlight (3000 lx) from 17:00 to 23:00 one day, and to bright light (3000 lx) from 7:00to 17:00 and dim light (100 lx) from 17:00 to 23:00 on the next day. We collectedsalivary samples every 10 minutes during 2 hours in the morning and in the eveningby means of a Lashley cup. Saliva secretion was stimulated by sweet candy. Theamount of saliva secreted was significantly greater in the morning under bright lightexposure, while it was significantly greater in the evening under dim light exposure.We discuss these findings in terms of changes in activity of the parasympatheticnervous system (PNS) and sympathetic (SNS) nervous system produced by exposureto different light intensities at different times of the day.

Keywords: Saliva secretion, morning and evening bright light, autonomic nervoussystem.

Introduction

Our studies have been directed toward identifying changes in the amount of salivarysecretion and its sodium concentration in response to different light intensities duringmorning and evening times. Tokura and his group have shown that changed lightintensities during the daytime have a profound influence on the nocturnal levels ofcore temperature and urinary melatonin, and they have attributed this to effects on

Address correspondence to: Hiromi Tokura, Department of Environmental Health, Nara Women’s University, Nara 630–8506, Japan. Tel.: 81/742/20/3469; Fax: 81/742/20/3499; E-mail: tokura@cc.nara-wu.ac.jp

Biological Rhythm Research 0929-1016/02/3302-129$16.002002, Vol. 33 No. 2. pp. 129–140 © Swets & Zeitlinger

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the autonomic nervous system (Kim & Tokura, 1995; Park & Tokura, 1999; Aizawa& Tokura, 1998). Nishmura et al. (2000) measured heart rate variability in order toanalyze the effects of diurnal exposure to bright light during the daytime on the auto-nomic nervous system; they have found that it could enhance the parasympatheticnervous system around midnight. Furthermore, Lee et al. (unpublished) have foundthat urinary VMA (vanillylmandelic acid) and plasma noradrenalne were significantlysmaller both during the daytime and nighttime under the influence of bright lightexposure during the daytime, suggesting that the human body was less excited underthe bright light. On the contrary, bright light exposure enhanced the activity of sym-pathetic nervous system in rat (Niijima et al. 1992) and man (Saito et al., 1996). It isimportant to collect more physiological parameters under the influence of bright anddim light conditions. With these in mind, it is tempting to know whether such changesin light intensity might influence the amount of salivary secretion because this alsois controlled by the autonomic nervous system (Shannon & Suddick, 1973). One ofthe effects of biological rhythms is that organisms respond differently to light stimu-lation as a function of the time of day (Pittendrigh, 1981). Therefore, our presentexperiments investigated if different amounts of exposure to light in the morning andin the evening could influence salivary secretion.

Methods

Six healthy, young, females (mean age 21 ± 1.78 S.D. years, range 19–24 years, meanbody mass index 19.43 ± 0.659 S.D.) gave written informed consent to take part inthe investigation. They were all non-smokers (Maier et al., 1988; Maier et al., 1991).The Ethics Committee of Nara Women’s University had approved the experimentalprotocol. All subjects were tested in the follicular phase of the menstrual cycles(Dawes, 1974; Maier et al., 1988). The experiment was performed between July andAugust 2000. All volunteers refrained from heavy exercise (Chicharro et al., 1988)and from drinking alcohol (Maier et al., 1988; Martin et al., 1971) or caffeine (Melaet al., 1992) for at least 24h before the start of experiment. They led a regular routinefor a week prior to the measurements. During this, meals were served at 7:30, 12:30,15:00 and 17:30 and consisted of a general diet totaling 1800 calories; 255g carbo-hydrate, 80g proteins, 93g fat (Dawes, 1970; Legott et al., 1987; Wisniewski et al.,1992). Each experimental session was preceded by a day of adaptation to the exper-imental chamber. Participants arrived at the chamber (temperature 26°C, humidity60%) at 10:00 and stayed in the chamber for 4 days (4 nights) with a short (7 hours)beak during third day (from 10:00 to 17:00). Volunteers were exposed to dim light(200 lx) from 10:00 to 23:00 first adaptation day and from 17:00 to 23:00 third adap-tation day.

Participants were exposed to dim light (100 lx) from 7:00 to 17:00 and to brightlight (3000 lx) from 17:00 to 23:00 on first experimental day. On the second experi-mental day, subjects staying in the bright room (3000 lx) from 7:00 to 17:00 and inthe dim light (100 lx) from 17:00 to 23:00. Complete darkness was kept during thesleep period (23:00–7:00) (Fig. 1). We collected 10min salivary samples for 6 times

130 D. Kanikowska et al.

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per hour in the morning (9:00–10:00 and 11:00–12:00) and also 6 times per hour inthe evening (19:00–20:00 and 21:00–22:00). Saliva was collected by siting a Lashleycup over the orifice of the parotid duct, on one side of the mouth. The subject chewedsweet candy (one candy each 10min), which resulted in the reflex secretion of parotidsaliva (Scott et al., 1999; Bonnans & Noble, 1995). Sodium, potassium, chloride andamylase concentrations of the sample were measured by routine laboratory analysis,and the salivary flow rate was calculated (mg/min) in the normal way.

Statistical methods

The statistical analysis employed for these data was 2-way ANOVA with repeatedmeasure, which was applied for a comparison of amounts of saliva in the morningbetween bright and dim light conditions and these in the evening between bright anddim light conditions separately. Student paired t-test was applied for a comparison ofNa concentration in the morning and evening.

Results

Figure 2 compares the profile of salivary secretion rates in bright and dim light in themorning (top) and in the evening (bottom) as typical example in a participant (S–5).

Salivary Secretion Influence by Bright/Dim Light Exposure 131

Protocol

6:00 12:00 18:00 24:00

First experimental day

7:00 100lux 17:00 3000lux 23:00 0lux

wake up retire

salivary sampling salivary sampling

Second experimental day

7:00 3000lux 17:00 100lux 23:00 0lux

wake up retire

salivary sampling salivary sampling

Figure 1. Experimental protocol.

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As seen in the figure salivary secretion rates were mostly higher in the morning underbright light, while in the evening was higher under dim light.

Figure 3a (top) compares temporal changes in the mean parotid flow rate from allsubjects in the morning between bright and dim light conditions, which was expressedrelatively Figure 3b. As seen in the figure, the actual and relative values were sig-nificantly higher in bright than in dim light conditions (F (1,11) = 4.63; p < 0.05 foractual; F (1,11) = 17.35; p < 0.01 for relative). On the contrary relative values in theevening were significantly higher in dim than in bright light (F (1,11) = 5.22; p <0.05) (Figure 4b), although raw values were not significantly different.

Figure 5 compares sodium concentration between bright and dim light exposureboth in the morning and evening. As seen in the figure the values were 20.7 ±2.96mmol/min in the morning under bright light and 15.1 ± 3.19mmol/l under dim

132 D. Kanikowska et al.

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Figure 2. Typical examples of temporal changes of salivary flow rate in a participant underthe influence of morning (top) and evening (bottom) bright/dim light exposure.

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Salivary Secretion Influence by Bright/Dim Light Exposure 133

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Figure 3. Average temporal changes of salivary flow rate under the influence of bright anddim light conditions in the morning A: actual values; B: elative values. Open circles: Bright.Closed circles: Dim. Mean ± SE (n = 6).

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134 D. Kanikowska et al.

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Figure 4. Average temporal changes of salivary flow rate under the influence of bright anddim light conditions in the evening A: actual values; B: elative values. Open circles: Bright.Closed circles: Dim. Mean ± SE (n = 6).

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light, which was significantly different, (p < 0.05). On the contrary, the values werenot different in the evening between the two light conditions.

Figure 6 showed a correlation between salivary flow rate (mg/min) and sodiumconcentration (mmol/l). A positive correlation existed only under morning bright, butnot under morning dim light.

The correlation seemed to be more closely linked between the two values underbright light than under dim light. A correlation between the lights and the values ofpotassium, chloride and amylase concentration did not exist in the morning and inthe evening.

Discussion

Circadian variations have been reported in a wide range of physiological parameters,including salivary flow rate and composition (Dawes, 1974). The time of day whensaliva samples are collected can influence the composition and flow of saliva, there-fore (Dawes, 1974, 1970). Both salivary flow and composition are dependent on theactivity of the autonomic nervous system (Ekstrom, 1989; Wong, 1989; Garrett, 1987;Malfertheiner, 1987). Our present results suggest that diurnal exposure to bright and dim light during the morning and evening altered salivary secretion and that the effect depended on time of day. Thus, it is possible that bright light exposure forseveral hours in the morning could have increased PNS and inhibited SNS activity,resulting in the increased amount of salivary secretion, while evening bright light

Salivary Secretion Influence by Bright/Dim Light Exposure 135

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Figure 5. A comparison of sodium concentration between morning (left) and evening (right)bright/dim light exposure. Mean ± SE (n = 6); p < 0.05.

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136 D. Kanikowska et al.

Morning bright lighty = 0.0269x + 9.8639 (r = 0.654)

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Figure 6. A correlation between flow rate and sodium concentration under the influence of morning bright (top) and dim (bottom) light exposure. Data were collected from six participants.

exposure for several hours could have produced the opposite changes in ANS activ-ity, resulting in the decreased amount of salivary secretion. This position could havearisen because morning bright light exposure increased diurnal melatonin secretion(Aizawa & Tokura, 1999; Park & Tokura, 1999; Teramoto et al., 1986) and decreases

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diurnal noradrenaline secretion (Lee et al., unpublished), while the evening brightlight exposure decreases the evening melatonin rise (Lewy et al., 1980) and mayincrease noradrenaline secretion at this time, because melatonin and noradrenalineare inversely linked (Markey et al., 1985; Bartsch et al., 1992). It should be noticeddifferences of salivary secretion in the evening between bright and dim light condi-tion were observed only in the relative values, not in the actual ones. The reason forthis was that the light conditions during the daytime were different; 100 lux duringfirst experimental day and 3000 lux during the second experimental day, which mighthave make the differences value. The studies on effects of previous different lightintensities on salivary secretion remain to be studied systematically. On the contrary,several reports are available, indicating that bright light exposure for 10–20 min couldincrease sympathetic nerve outflow innervating kidney in the rat (Niijima et al., 1992)and skeletal muscle in humans (Saito et al., 1996). These discrepancies between ourpresent data and those reported in other laboratories might be due to different periodsof bright light exposure (2–4 hours bright light exposure before the saliva collectionin the morning and the evening in our present case and 10–20min in other laborato-ries). There is also a report that bright light acts as a stimulant, preventing body tem-perature from evening fall (Dijk et al., 1991). However, in this case bright light wasexposed for humans in the evening. In this case, evening bright light might have stimu-lated sympathetic nervous system, having results in inhibition of salivary secretionrate. Therefore, there is no inconsistency between our case and Dijk et al. (1991).

It should also be noted that both the pineal and the parotid glands receive sympa-thetic postganglionic connection from the superior cervical ganglion, suggesting thatlight could influence salivary secretion (Shannon et al., 1973; Gallara et al., 1995).Morning bright light exposure increased also the sodium concentration of saliva (Fig. 4) and regulated in a stronger correlation between salivary flow rate and sodiumconcentration (Fig. 5). It is well known that salivary secretion rate and sodium con-centrations in saliva are positively correlated (Dawes, 1974; Schneyer et al., 1972).Our results suggest that such correlation may become stronger under the influence of morning bright light exposure.

It is concluded that morning bright light can increase the amount of saliva secre-tion, while evening bright light can suppress it.

Acknowledgements

We thank Dr J. Waterhouse in Liverpool, U.K. for his critical reading.

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