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Intragastric administration of glutamate increases REM sleep in rats
PHB-10156; No of Pages 4 Physiology & Behavior xxx (2013) xxx–xxx
Contents lists available at ScienceDirect
Physiology & Behavior journal homepage: www.elsevier.com/locate/phb
3Q1
Karuna Datta, Deependra Kumar 1, Hruda Nanda Mallick ⁎
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Department of Physiology, All India Institute of Medical Sciences, New Delhi 110029, India
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Intragastric administration of glutamate increases REM sleep in rats☆,☆☆
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• Effect of intragastric glutamate administration on sleep was studied in rats. • Sleep–wake parameters were studied for 6 h after intragastric administration of glutamate or saline. • Intragastric glutamate administration showed significant increase in REM sleep without much effect on slow wave sleep.
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a b s t r a c t
Study objective: To study the effect of intragastric infusion of glutamate on sleep–wake (S–W) parameters in rats. Design: Longitudinal study design. Setting: N/A. Patients/Participants: 10 adult male Wistar rats. Interventions: 0.12 M, 1.5 ml intragastric monosodium glutamate and saline. Measurements and results: S–W parameters were recorded by chronic implantation of EEG, EOG and EMG electrodes under thiopentone sodium anesthesia. After post-op recovery, sleep was recorded by a digital recording system (BIOPAC system Inc. BSL PRO 36, USA). 1.5 ml of 0.12 M MSG or saline on two different days, three days apart, was delivered intragastrically by a gavage tube between 0945 and 1000 h and S–W parameters were recorded for 6 h after the intragastric administration (10:00–16:00 h). Data was analyzed as wake, slow wave sleep (SWS) and rapid eye movement (REM) sleep using standard criteria. Total sleep time (TST) was also calculated. S–W records were then divided into 2 h bins (10:00–12:00 h, 12:00–14:00 h and 14:00– 16:00 h). Total duration of wake, SWS and REM sleep in 2 h bins after glutamate administration was compared with the same 2 h bins after saline. Episode frequency and maximum episode duration for REM sleep, SWS and wake after glutamate administration were also compared with saline. Intragastric application of glutamate, as compared to saline significantly increased REM sleep duration in all the three 2 h bins, 10:00–12:00 h (p = 0.037), 12:00–14:00 h (p = 0.037) and 14:00–16:00 h (p = 0.007). Episode frequency of REM sleep was also significantly increased. Conclusion: Intragastric glutamate administration increases REM sleep duration. © 2013 Published by Elsevier Inc.
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Article history: Received 13 August 2013 Accepted 7 September 2013 Available online xxxx
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Keywords: Glutamate REM sleep Intragastric Monosodium glutamate Rats
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☆ Institution at which work was conducted: All India Institute of Medical Sciences, New Delhi 110029. ☆☆ Disclosure of financial support: The study is a part of an ongoing project which is supported by Ajinomoto Co. Inc. Tokyo, Japan. The financial support had no role in study design; in the collection, analysis and interpretation of data; in the writing of the report; and in the decision to submit the article for publication. ⁎ Corresponding author at: Department of Physiology, All India Institute of Medical Sciences, Ansari Nagar, New Delhi 110029, India. Tel.: +91 11 26594881; fax: +91 11 26588663. E-mail address: [email protected] (H.N. Mallick). 1 Present Address: Max Planck Institute of Psychiatry, Kraepelinstr, 2-10, 80804 Munich, Germany.
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1. Introduction
51
Monosodium L glutamate (MSG), a sodium salt of L glutamate, is a commonly used flavor enhancer in food. L glutamate is a multifunctional amino acid. It is involved in umami taste sensation [1], acts as a neurotransmitter in central [2] and enteric nervous system [3], and also causes diet induced thermogenesis [4]. Studies show that MSG, taken orally has several postingestive effects [5–7]. Intragastric administration of glutamate activates gastric branch of vagus nerve [8]. Distinct gut brain axis has been known to mediate the effects of intragastric glutamate administration [9]. Kitamura and co-workers [10] in 2011 suggested that glutamate signaling plays an important role in the process of digestion, absorption and metabolism via activation of distinct brain regions. Considering that
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0031-9384/$ – see front matter © 2013 Published by Elsevier Inc. http://dx.doi.org/10.1016/j.physbeh.2013.09.007
Please cite this article as: Datta K, et al, Intragastric administration of glutamate increases REM sleep in rats, Physiol Behav (2013), http:// dx.doi.org/10.1016/j.physbeh.2013.09.007
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3. Results
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The S–W parameters on Wistar rats (n = 10) following intragastric administration of MSG and saline are shown in Fig. 1. Fig. 1(a), (b) and (c) is showing changes in duration (in minutes) of TST, SWS and REM sleep respectively during 10:00–12:00 h, 12:00–14:00 h and 14:00– 16:00 h after administration of MSG compared with saline. Wilcoxon Signed Ranks test was performed on the duration of each sleep wake parameter in the 2 h bins between intragastric administration of saline and glutamate.
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Fig. 1. (a): Total sleep time duration in 2 h bins after intragastric administration of saline and glutamate. (b): Slow wave sleep duration in 2 h bins after intragastric administration of saline and glutamate. (c): REM sleep duration in 2 h bins after intragastric administration of saline and glutamate (★—p b 0.05).
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Effect of intragastric administration of glutamate on S–W parameters was studied in ten adult male Wistar rats (BW, 225–275 g). They were housed individually in transparent polyethersulphone cage (floor, 445 × 295 × 185 mm and cage cover, 450 × 295 × 140 mm) with controlled temperature and ad libitum access to food and water (Millennium SPF Rack System R-series, Orient Co. Ltd., South Korea). The rats were maintained at 14 h lights (illumination above 200 lx) and 10 h dark (illumination below 5 lx) conditions with light on from 0600 h. Implantation of electrodes was conducted under thiopentone sodium (THIOSOL Sodium), anesthesia (40 mg/kg, body weight, i.p.). Electrodes for electroencephalogram (EEG), electromyogram (EMG) and electrooculogram (EOG) were chronically implanted and connected to an IC socket that was fixed to the skull with dental cement. All procedures were conducted in accordance with the rules of the Committee for the Purpose of Control and Supervision of Experiments on Animals (CPCSEA) and were approved by the Institutional Animal Ethics Committee. Flexible cables with connectors were plugged to the rat's head. Output from the plug was connected to BIOPAC system Inc. (BSL PRO 36, USA) for digital recordings of EOG, EEG, and EMG. After a ten-day recovery period, the rats were trained for two days to move freely in the recording cage with the attached cable. The rats were also acclimatized to a gavage tube by inserting it once per day and injecting 1.5 ml of sterile water intragastrically everyday at 09:45–10:00 h for 4 days prior to the saline administration (recording day I) and MSG ( Sigma-Aldrich, St Louis, Missouri, USA ) administration (recording day II). On recording day I, each rat was administered 1.5 ml of saline and recording of S–W parameters for 6 h after intragastric administration of saline was done. After an interval of three days, the same rat was administered 1.5 ml of MSG and 6 h recording of S–W parameters after intragastric administration of MSG was done. 1.5 ml of saline (0.12 M) or MSG (0.12 M) was intragastrically delivered between 0945 and 1000 h. The S–W parameters were recorded during 10:00–16:00 h after intragastric administration of MSG/saline. The 6 h data of S–W parameters obtained from all the ten rats were split into 15 s epochs, and visually classified as wake, slow wave sleep (SWS) and rapid eye movement (REM) sleep using standard criteria [11]. Six hour S–W data after intragastric administration of saline and MSG were both divided into 2 h bins. Duration of SWS, REM sleep, wake and total sleep time (TST) was calculated every 2 h. Episode frequency and maximum episode duration were also calculated every 2 h. S–W parameters of 2 h bins, post intragastric administration i.e. 10:00–12:00 h, 12:00– 14:00 h and 14:00–16:00 h of saline were compared with the same 2 h bins of MSG. Statistical analysis was done using Wilcoxon Signed Ranks test (IBM SPSS version 20).
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2. Methods
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distinct forebrain regions are activated following intragastric glutamate administration, we hypothesized that there may be effect of glutamate on sleep–wake (S–W) parameters. To test the above hypothesis, we studied the effect of intragastric glutamate administration on S–W parameters in rats. To rule out umami taste related changes, we studied this effect following intragastric administration of glutamate in rats.
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Episode frequency of SWS, REM sleep and wake in rats was noted. Mean ± SD of duration and episode frequency of the S–W parameters of ten rats are shown in Table 1. Maximum episode duration of SWS, REM sleep and wake was also calculated in the 2 h bins. There was no statistically significant difference in maximum episode duration of these S–W parameters after intragastric administration of saline or glutamate.
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4. Discussion
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In the present study, we found a significant increase in REM sleep duration and its episode frequency following intragastric application of MSG as compared to saline. Though there was a decrease in SWS duration following intragastric glutamate as compared to saline, but it was not found to be significant. No significant change was found in TST and wake duration. This effect on sleep could not be attributed to umami taste sensation per se, as MSG was administered intragastrically.
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Please cite this article as: Datta K, et al, Intragastric administration of glutamate increases REM sleep in rats, Physiol Behav (2013), http:// dx.doi.org/10.1016/j.physbeh.2013.09.007
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K. Datta et al. / Physiology & Behavior xxx (2013) xxx–xxx t1:1 t1:2
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Table 1 Duration and episode frequency in 2 h bins of various sleep–wake parameters after intragastric administration of saline and glutamate.
t1:3
Sleep–wake parameter
Time of the day (2 h bins)
Intragastric administration — substance used
Total duration in minutes Mean ± SD
Total duration in minutes (p values)
Episode frequency⁎ Mean ± SD
Episode frequency⁎ p values
t1:4 t1:5 Q2 t1:6 t1:7 t1:8 t1:9 t1:10 t1:11 t1:12 t1:13 t1:14 t1:15 t1:16 t1:17 t1:18 t1:19 t1:20 t1:21
SWS
10:00–12:00 h
Wake
10:00–12:00 h
SWS
12:00–14:00 h
REM sleep
12:00–14:00 h
Wake
12:00–14:00 h
SWS
14:00–16:00 h
29.8 32.3 14.9 21.8 17.6 14.2 34.2 34.9 23.5 18.3 17.1 13.4 39.5 42.1 19.7 29.4 21.3 14.8
p = 0.373
10:00–12:00 h
86.4 79.7 14.4 19.63 19.2 20.68 88.43 84.73 16.75 21.35 14.83 13.93 85.45 78.38 18.55 26.95 16 14.68
p = 0.059
REM sleep
Saline Glutamate Saline Glutamate Saline Glutamate Saline Glutamate Saline Glutamate Saline Glutamate Saline Glutamate Saline Glutamate Saline Glutamate
151 152 153 154 155 156 157 158 159 160 161 162 163 164 165 166 167 168 169 170 171 172 173 174 175 176 177 178 179 180 181
p = 0.037⁎⁎ p = 0.919 p = 0.059 p = 0.037⁎⁎
R O
p = 0.799
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p = 0.059 p = 0.007⁎⁎
8.83 5.42 5.55 6.16 5.70 6.27 10.64 9.34 10.84 6.04 4.25 5.76 13.02 6.47 7.36 8.00 12.00 5.96
F
p = 0.386
± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ±
p = 0.015⁎⁎ p = 0.239 p = 0.759 p = 0.109 p = 0.123 p = 0.333 p = 0.032⁎⁎ p = 0.083
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Acknowledgments
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studied by Ootsuka et al., BAT thermogenesis also occurs as a part of ultradian cycle. BAT thermogenesis heats up brain and body and during sleep phase increased brain temperature increases REM sleep [24]. Apart from BAT thermogenesis, leptin, white adipose tissue hormone is also found to affect sleep. In a study conducted on normal fed rats, leptin significantly reduced REM sleep by 30% and increased SWS duration by 13% [25]. A study conducted in rats, using chronic L glutamate ingestion ad libitum, produced reduced leptin levels [26]. With chronic glutamate ingestion, though, plasma levels of leptin were reduced but there was no difference in food intake. Same study also showed that intake of glutamate produced reduced weight gain and fat deposition. This may be due to increased thermogenesis or some other energy dissipating mechanism, is not clear. Leptin is also found to mediate BAT thermogenesis [27]. With glutamate administration leptin levels are reduced [26] and induction of BAT thermogenesis has been documented [4]. BAT thermogenesis can increase REM sleep [24] and leptin significantly reduces REM sleep, highlighting the possible link between BAT thermogenesis, leptin and REM sleep following intragastric glutamate administration. In our study we did not measure BAT thermogenesis and its correlation to increased REM sleep in the two groups. Increased BAT thermogenesis is found to be associated during food intake [28]. We also were unable to correlate the food intake to sleep wake parameters. In our study there was an increase in episode frequency but the episode maintenance was not significantly affected implying an effect on REM sleep generation mechanisms with MSG. The long lasting effect of glutamate administration on REM sleep can be explained in terms of synaptic plasticity. Vagus nerve stimulation in rats has shown to produce LTP in hippocampus [29]. It could be possible that intragastric administration of glutamate may produce stimulation of vagus nerve resulting in LTP in forebrain areas. The prolonged effect on REM sleep following glutamate administration, may be either due to direct activation of sleep promoting areas or a complex interplay of REM generating areas of brain and thermogenesis mechanisms.
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Ingestion of various food products is known to affect sleep. These effects may be either affecting sleep by changes in metabolism, changes in the plasma concentrations of melatonin [12] or CSF concentrations of serotonin [13] or tryptophan [14]. It can also affect either humorally or neurally by gut brain axis [15]. Many compounds, drugs and extracts affect a combination of these mechanisms to affect sleep. Hansen et al. reported that intact vagus was required for the effects of cafeteria diet, which is a high carbohydrate and high fat diet, on sleep. This diet as compared to normal chow caused increased SWS [16] and also produced diet induced thermogenesis [17]. This effect on sleep was found to be blocked by sub-diaphragmatic vagotomy in rats [16]. Vagal afferent stimulation is known to play an important role in induction of sleep [18]. MSG, unlike high carbohydrate and fat diet did not significantly affect total sleep duration or SWS duration, rather showed effects on REM sleep duration. Intragastric MSG is found to activate afferent vagal nerves [5,19,20] and a distinct gut-brain axis exists for glutamate signaling [9,19]. It is reported that abdominal vagotomy abolishes this response [21]. In our study, the effect of vagotomy on the sleep response has not been studied as vagotomy was not performed. Tsurugizawa et al. [7] in 2008, using fMRI studies showed that 60 mM intragastric administration of MSG caused activation of habenular nucleus, amygdala, medial preoptic area (mPOA) and DMH and the effect on these areas was not due to sodium ion in MSG but because of glutamate. Following intragastric administration of MSG, there was c-fos inductions in mPOA, lateral hypothalamic areas, DMH and arcuate nucleus [22]. Kondoh et al. [23] studied fMRI changes following intragastric administration of MSG and the activation of brain areas following MSG developed rapidly during infusion and reduced rapidly after cessation of infusion. Though the activation of brain areas following intragastric glutamate in the study done by Kondoh et al. [23] and by Tsurugizawa et al. [21] lasted only till the time the infusion occurred and did not have long lasting effects, we found increase in REM sleep for 6 h after administration of intragastric glutamate. The sustained increase in REM sleep may be due to activation of sleep promoting areas, the effect of which lasted even after cessation of infusion. Prolonged increase of REM sleep after intragastric glutamate may be probably as a result of long term potentiation (LTP) mechanisms in the neuronal circuitry inducing REM sleep. REM sleep increase may be linked to thermogenesis as 120 mM MSG used in the present study has been reported by Smriga et al. to cause diet induced BAT thermogenesis [4]. During active-wake states, as
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4.90 12.22 3.91 5.09 6.46 12.65 6.25 8.51 5.63 5.62 4.67 6.65 12.35 7.09 4.74 7.03 10.72 6.70
⁎ Episode frequency is the number of occurrences of episodes in two hours; p values are derived using Wilcoxon Signed Ranks test (**p b 0.05 considered as significant).
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REM sleep
± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ±
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The authors thank Mr. Ashish Datt Upadhyay, Department of Biosta- 218 tistics, All India Institute of Medical Sciences, New Delhi for the help pro- 219 vided in data analysis. 220
Please cite this article as: Datta K, et al, Intragastric administration of glutamate increases REM sleep in rats, Physiol Behav (2013), http:// dx.doi.org/10.1016/j.physbeh.2013.09.007
K. Datta et al. / Physiology & Behavior xxx (2013) xxx–xxx
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Please cite this article as: Datta K, et al, Intragastric administration of glutamate increases REM sleep in rats, Physiol Behav (2013), http:// dx.doi.org/10.1016/j.physbeh.2013.09.007
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Contents lists available at ScienceDirect
Physiology & Behavior journal homepage: www.elsevier.com/locate/phb
3Q1
Karuna Datta, Deependra Kumar 1, Hruda Nanda Mallick ⁎
4
Department of Physiology, All India Institute of Medical Sciences, New Delhi 110029, India
O
F
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Intragastric administration of glutamate increases REM sleep in rats☆,☆☆
1
P
• Effect of intragastric glutamate administration on sleep was studied in rats. • Sleep–wake parameters were studied for 6 h after intragastric administration of glutamate or saline. • Intragastric glutamate administration showed significant increase in REM sleep without much effect on slow wave sleep.
i n f o
a b s t r a c t
Study objective: To study the effect of intragastric infusion of glutamate on sleep–wake (S–W) parameters in rats. Design: Longitudinal study design. Setting: N/A. Patients/Participants: 10 adult male Wistar rats. Interventions: 0.12 M, 1.5 ml intragastric monosodium glutamate and saline. Measurements and results: S–W parameters were recorded by chronic implantation of EEG, EOG and EMG electrodes under thiopentone sodium anesthesia. After post-op recovery, sleep was recorded by a digital recording system (BIOPAC system Inc. BSL PRO 36, USA). 1.5 ml of 0.12 M MSG or saline on two different days, three days apart, was delivered intragastrically by a gavage tube between 0945 and 1000 h and S–W parameters were recorded for 6 h after the intragastric administration (10:00–16:00 h). Data was analyzed as wake, slow wave sleep (SWS) and rapid eye movement (REM) sleep using standard criteria. Total sleep time (TST) was also calculated. S–W records were then divided into 2 h bins (10:00–12:00 h, 12:00–14:00 h and 14:00– 16:00 h). Total duration of wake, SWS and REM sleep in 2 h bins after glutamate administration was compared with the same 2 h bins after saline. Episode frequency and maximum episode duration for REM sleep, SWS and wake after glutamate administration were also compared with saline. Intragastric application of glutamate, as compared to saline significantly increased REM sleep duration in all the three 2 h bins, 10:00–12:00 h (p = 0.037), 12:00–14:00 h (p = 0.037) and 14:00–16:00 h (p = 0.007). Episode frequency of REM sleep was also significantly increased. Conclusion: Intragastric glutamate administration increases REM sleep duration. © 2013 Published by Elsevier Inc.
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Article history: Received 13 August 2013 Accepted 7 September 2013 Available online xxxx
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Keywords: Glutamate REM sleep Intragastric Monosodium glutamate Rats
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☆ Institution at which work was conducted: All India Institute of Medical Sciences, New Delhi 110029. ☆☆ Disclosure of financial support: The study is a part of an ongoing project which is supported by Ajinomoto Co. Inc. Tokyo, Japan. The financial support had no role in study design; in the collection, analysis and interpretation of data; in the writing of the report; and in the decision to submit the article for publication. ⁎ Corresponding author at: Department of Physiology, All India Institute of Medical Sciences, Ansari Nagar, New Delhi 110029, India. Tel.: +91 11 26594881; fax: +91 11 26588663. E-mail address: [email protected] (H.N. Mallick). 1 Present Address: Max Planck Institute of Psychiatry, Kraepelinstr, 2-10, 80804 Munich, Germany.
27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 48 47
1. Introduction
51
Monosodium L glutamate (MSG), a sodium salt of L glutamate, is a commonly used flavor enhancer in food. L glutamate is a multifunctional amino acid. It is involved in umami taste sensation [1], acts as a neurotransmitter in central [2] and enteric nervous system [3], and also causes diet induced thermogenesis [4]. Studies show that MSG, taken orally has several postingestive effects [5–7]. Intragastric administration of glutamate activates gastric branch of vagus nerve [8]. Distinct gut brain axis has been known to mediate the effects of intragastric glutamate administration [9]. Kitamura and co-workers [10] in 2011 suggested that glutamate signaling plays an important role in the process of digestion, absorption and metabolism via activation of distinct brain regions. Considering that
52
0031-9384/$ – see front matter © 2013 Published by Elsevier Inc. http://dx.doi.org/10.1016/j.physbeh.2013.09.007
Please cite this article as: Datta K, et al, Intragastric administration of glutamate increases REM sleep in rats, Physiol Behav (2013), http:// dx.doi.org/10.1016/j.physbeh.2013.09.007
53 54 55 56 57 58 59 60 61 62 63
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3. Results
117
The S–W parameters on Wistar rats (n = 10) following intragastric administration of MSG and saline are shown in Fig. 1. Fig. 1(a), (b) and (c) is showing changes in duration (in minutes) of TST, SWS and REM sleep respectively during 10:00–12:00 h, 12:00–14:00 h and 14:00– 16:00 h after administration of MSG compared with saline. Wilcoxon Signed Ranks test was performed on the duration of each sleep wake parameter in the 2 h bins between intragastric administration of saline and glutamate.
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Fig. 1. (a): Total sleep time duration in 2 h bins after intragastric administration of saline and glutamate. (b): Slow wave sleep duration in 2 h bins after intragastric administration of saline and glutamate. (c): REM sleep duration in 2 h bins after intragastric administration of saline and glutamate (★—p b 0.05).
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Effect of intragastric administration of glutamate on S–W parameters was studied in ten adult male Wistar rats (BW, 225–275 g). They were housed individually in transparent polyethersulphone cage (floor, 445 × 295 × 185 mm and cage cover, 450 × 295 × 140 mm) with controlled temperature and ad libitum access to food and water (Millennium SPF Rack System R-series, Orient Co. Ltd., South Korea). The rats were maintained at 14 h lights (illumination above 200 lx) and 10 h dark (illumination below 5 lx) conditions with light on from 0600 h. Implantation of electrodes was conducted under thiopentone sodium (THIOSOL Sodium), anesthesia (40 mg/kg, body weight, i.p.). Electrodes for electroencephalogram (EEG), electromyogram (EMG) and electrooculogram (EOG) were chronically implanted and connected to an IC socket that was fixed to the skull with dental cement. All procedures were conducted in accordance with the rules of the Committee for the Purpose of Control and Supervision of Experiments on Animals (CPCSEA) and were approved by the Institutional Animal Ethics Committee. Flexible cables with connectors were plugged to the rat's head. Output from the plug was connected to BIOPAC system Inc. (BSL PRO 36, USA) for digital recordings of EOG, EEG, and EMG. After a ten-day recovery period, the rats were trained for two days to move freely in the recording cage with the attached cable. The rats were also acclimatized to a gavage tube by inserting it once per day and injecting 1.5 ml of sterile water intragastrically everyday at 09:45–10:00 h for 4 days prior to the saline administration (recording day I) and MSG ( Sigma-Aldrich, St Louis, Missouri, USA ) administration (recording day II). On recording day I, each rat was administered 1.5 ml of saline and recording of S–W parameters for 6 h after intragastric administration of saline was done. After an interval of three days, the same rat was administered 1.5 ml of MSG and 6 h recording of S–W parameters after intragastric administration of MSG was done. 1.5 ml of saline (0.12 M) or MSG (0.12 M) was intragastrically delivered between 0945 and 1000 h. The S–W parameters were recorded during 10:00–16:00 h after intragastric administration of MSG/saline. The 6 h data of S–W parameters obtained from all the ten rats were split into 15 s epochs, and visually classified as wake, slow wave sleep (SWS) and rapid eye movement (REM) sleep using standard criteria [11]. Six hour S–W data after intragastric administration of saline and MSG were both divided into 2 h bins. Duration of SWS, REM sleep, wake and total sleep time (TST) was calculated every 2 h. Episode frequency and maximum episode duration were also calculated every 2 h. S–W parameters of 2 h bins, post intragastric administration i.e. 10:00–12:00 h, 12:00– 14:00 h and 14:00–16:00 h of saline were compared with the same 2 h bins of MSG. Statistical analysis was done using Wilcoxon Signed Ranks test (IBM SPSS version 20).
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distinct forebrain regions are activated following intragastric glutamate administration, we hypothesized that there may be effect of glutamate on sleep–wake (S–W) parameters. To test the above hypothesis, we studied the effect of intragastric glutamate administration on S–W parameters in rats. To rule out umami taste related changes, we studied this effect following intragastric administration of glutamate in rats.
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Episode frequency of SWS, REM sleep and wake in rats was noted. Mean ± SD of duration and episode frequency of the S–W parameters of ten rats are shown in Table 1. Maximum episode duration of SWS, REM sleep and wake was also calculated in the 2 h bins. There was no statistically significant difference in maximum episode duration of these S–W parameters after intragastric administration of saline or glutamate.
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4. Discussion
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In the present study, we found a significant increase in REM sleep duration and its episode frequency following intragastric application of MSG as compared to saline. Though there was a decrease in SWS duration following intragastric glutamate as compared to saline, but it was not found to be significant. No significant change was found in TST and wake duration. This effect on sleep could not be attributed to umami taste sensation per se, as MSG was administered intragastrically.
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Please cite this article as: Datta K, et al, Intragastric administration of glutamate increases REM sleep in rats, Physiol Behav (2013), http:// dx.doi.org/10.1016/j.physbeh.2013.09.007
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K. Datta et al. / Physiology & Behavior xxx (2013) xxx–xxx t1:1 t1:2
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Table 1 Duration and episode frequency in 2 h bins of various sleep–wake parameters after intragastric administration of saline and glutamate.
t1:3
Sleep–wake parameter
Time of the day (2 h bins)
Intragastric administration — substance used
Total duration in minutes Mean ± SD
Total duration in minutes (p values)
Episode frequency⁎ Mean ± SD
Episode frequency⁎ p values
t1:4 t1:5 Q2 t1:6 t1:7 t1:8 t1:9 t1:10 t1:11 t1:12 t1:13 t1:14 t1:15 t1:16 t1:17 t1:18 t1:19 t1:20 t1:21
SWS
10:00–12:00 h
Wake
10:00–12:00 h
SWS
12:00–14:00 h
REM sleep
12:00–14:00 h
Wake
12:00–14:00 h
SWS
14:00–16:00 h
29.8 32.3 14.9 21.8 17.6 14.2 34.2 34.9 23.5 18.3 17.1 13.4 39.5 42.1 19.7 29.4 21.3 14.8
p = 0.373
10:00–12:00 h
86.4 79.7 14.4 19.63 19.2 20.68 88.43 84.73 16.75 21.35 14.83 13.93 85.45 78.38 18.55 26.95 16 14.68
p = 0.059
REM sleep
Saline Glutamate Saline Glutamate Saline Glutamate Saline Glutamate Saline Glutamate Saline Glutamate Saline Glutamate Saline Glutamate Saline Glutamate
151 152 153 154 155 156 157 158 159 160 161 162 163 164 165 166 167 168 169 170 171 172 173 174 175 176 177 178 179 180 181
p = 0.037⁎⁎ p = 0.919 p = 0.059 p = 0.037⁎⁎
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p = 0.799
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p = 0.059 p = 0.007⁎⁎
8.83 5.42 5.55 6.16 5.70 6.27 10.64 9.34 10.84 6.04 4.25 5.76 13.02 6.47 7.36 8.00 12.00 5.96
F
p = 0.386
± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ±
p = 0.015⁎⁎ p = 0.239 p = 0.759 p = 0.109 p = 0.123 p = 0.333 p = 0.032⁎⁎ p = 0.083
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Acknowledgments
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studied by Ootsuka et al., BAT thermogenesis also occurs as a part of ultradian cycle. BAT thermogenesis heats up brain and body and during sleep phase increased brain temperature increases REM sleep [24]. Apart from BAT thermogenesis, leptin, white adipose tissue hormone is also found to affect sleep. In a study conducted on normal fed rats, leptin significantly reduced REM sleep by 30% and increased SWS duration by 13% [25]. A study conducted in rats, using chronic L glutamate ingestion ad libitum, produced reduced leptin levels [26]. With chronic glutamate ingestion, though, plasma levels of leptin were reduced but there was no difference in food intake. Same study also showed that intake of glutamate produced reduced weight gain and fat deposition. This may be due to increased thermogenesis or some other energy dissipating mechanism, is not clear. Leptin is also found to mediate BAT thermogenesis [27]. With glutamate administration leptin levels are reduced [26] and induction of BAT thermogenesis has been documented [4]. BAT thermogenesis can increase REM sleep [24] and leptin significantly reduces REM sleep, highlighting the possible link between BAT thermogenesis, leptin and REM sleep following intragastric glutamate administration. In our study we did not measure BAT thermogenesis and its correlation to increased REM sleep in the two groups. Increased BAT thermogenesis is found to be associated during food intake [28]. We also were unable to correlate the food intake to sleep wake parameters. In our study there was an increase in episode frequency but the episode maintenance was not significantly affected implying an effect on REM sleep generation mechanisms with MSG. The long lasting effect of glutamate administration on REM sleep can be explained in terms of synaptic plasticity. Vagus nerve stimulation in rats has shown to produce LTP in hippocampus [29]. It could be possible that intragastric administration of glutamate may produce stimulation of vagus nerve resulting in LTP in forebrain areas. The prolonged effect on REM sleep following glutamate administration, may be either due to direct activation of sleep promoting areas or a complex interplay of REM generating areas of brain and thermogenesis mechanisms.
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Ingestion of various food products is known to affect sleep. These effects may be either affecting sleep by changes in metabolism, changes in the plasma concentrations of melatonin [12] or CSF concentrations of serotonin [13] or tryptophan [14]. It can also affect either humorally or neurally by gut brain axis [15]. Many compounds, drugs and extracts affect a combination of these mechanisms to affect sleep. Hansen et al. reported that intact vagus was required for the effects of cafeteria diet, which is a high carbohydrate and high fat diet, on sleep. This diet as compared to normal chow caused increased SWS [16] and also produced diet induced thermogenesis [17]. This effect on sleep was found to be blocked by sub-diaphragmatic vagotomy in rats [16]. Vagal afferent stimulation is known to play an important role in induction of sleep [18]. MSG, unlike high carbohydrate and fat diet did not significantly affect total sleep duration or SWS duration, rather showed effects on REM sleep duration. Intragastric MSG is found to activate afferent vagal nerves [5,19,20] and a distinct gut-brain axis exists for glutamate signaling [9,19]. It is reported that abdominal vagotomy abolishes this response [21]. In our study, the effect of vagotomy on the sleep response has not been studied as vagotomy was not performed. Tsurugizawa et al. [7] in 2008, using fMRI studies showed that 60 mM intragastric administration of MSG caused activation of habenular nucleus, amygdala, medial preoptic area (mPOA) and DMH and the effect on these areas was not due to sodium ion in MSG but because of glutamate. Following intragastric administration of MSG, there was c-fos inductions in mPOA, lateral hypothalamic areas, DMH and arcuate nucleus [22]. Kondoh et al. [23] studied fMRI changes following intragastric administration of MSG and the activation of brain areas following MSG developed rapidly during infusion and reduced rapidly after cessation of infusion. Though the activation of brain areas following intragastric glutamate in the study done by Kondoh et al. [23] and by Tsurugizawa et al. [21] lasted only till the time the infusion occurred and did not have long lasting effects, we found increase in REM sleep for 6 h after administration of intragastric glutamate. The sustained increase in REM sleep may be due to activation of sleep promoting areas, the effect of which lasted even after cessation of infusion. Prolonged increase of REM sleep after intragastric glutamate may be probably as a result of long term potentiation (LTP) mechanisms in the neuronal circuitry inducing REM sleep. REM sleep increase may be linked to thermogenesis as 120 mM MSG used in the present study has been reported by Smriga et al. to cause diet induced BAT thermogenesis [4]. During active-wake states, as
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4.90 12.22 3.91 5.09 6.46 12.65 6.25 8.51 5.63 5.62 4.67 6.65 12.35 7.09 4.74 7.03 10.72 6.70
⁎ Episode frequency is the number of occurrences of episodes in two hours; p values are derived using Wilcoxon Signed Ranks test (**p b 0.05 considered as significant).
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14:00–16:00 h
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Wake
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14:00–16:00 h
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t1:22
REM sleep
± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ±
183 184 185 186 187 188 189 190 191 192 193 194 195 196 197 198 199 200 201 202 203 204 205 206 207 208 209 210 211 212 213 214 215 216
The authors thank Mr. Ashish Datt Upadhyay, Department of Biosta- 218 tistics, All India Institute of Medical Sciences, New Delhi for the help pro- 219 vided in data analysis. 220
Please cite this article as: Datta K, et al, Intragastric administration of glutamate increases REM sleep in rats, Physiol Behav (2013), http:// dx.doi.org/10.1016/j.physbeh.2013.09.007
K. Datta et al. / Physiology & Behavior xxx (2013) xxx–xxx
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