Cerebral gustatory activation in response to free fatty acids using gustatory evoked potentials in humans
2018; Elsevier BV; Volume: 60; Issue: 3 Linguagem: Inglês
10.1194/jlr.m086587
ISSN1539-7262
AutoresThomas Mouillot, Emilie Szleper, Gaspard Vagne, Sophie Barthet, Djihed Litime, Marie‐Claude Brindisi, Corinne Leloup, Luc Pénicaud, Sophie Nicklaus, Laurent Brondel, Agnès Jacquin-Piques,
Tópico(s)Advanced Chemical Sensor Technologies
ResumoThere is some evidence of specific oro-detection of FFAs in rodents and humans. The aim of this study was to record gustatory evoked potentials (GEPs) in response to FFA solutions and to compare GEPs in response to linoleic acid solution with GEPs obtained after stimulation with sweet and salty tastants. Eighteen healthy men were randomly stimulated with fatty (linoleic acid), sweet (sucrose), and salty (NaCl) solutions at two concentrations in the first experiment. Control recordings (n = 14) were obtained during stimulation by a paraffin oil mixture without FFA or by water. In the second experiment, 28 men were randomly stimulated with five FFA solutions and a paraffin emulsion. GEPs were recorded with electroencephalographic electrodes at Cz, Fz, and Pz. GEPs were observed in response to FFA in all participants. GEP characteristics did not differ according to the quality and the concentration of the solutions in the first experiment and according to the FFA in the second experiment. This study describes for the first time GEPs in response to FFA and demonstrates that the presence of FFA in the mouth triggers an activation of the gustatory cortex. These data reinforce the concept that fat taste could be the sixth primary taste. There is some evidence of specific oro-detection of FFAs in rodents and humans. The aim of this study was to record gustatory evoked potentials (GEPs) in response to FFA solutions and to compare GEPs in response to linoleic acid solution with GEPs obtained after stimulation with sweet and salty tastants. Eighteen healthy men were randomly stimulated with fatty (linoleic acid), sweet (sucrose), and salty (NaCl) solutions at two concentrations in the first experiment. Control recordings (n = 14) were obtained during stimulation by a paraffin oil mixture without FFA or by water. In the second experiment, 28 men were randomly stimulated with five FFA solutions and a paraffin emulsion. GEPs were recorded with electroencephalographic electrodes at Cz, Fz, and Pz. GEPs were observed in response to FFA in all participants. GEP characteristics did not differ according to the quality and the concentration of the solutions in the first experiment and according to the FFA in the second experiment. This study describes for the first time GEPs in response to FFA and demonstrates that the presence of FFA in the mouth triggers an activation of the gustatory cortex. These data reinforce the concept that fat taste could be the sixth primary taste. The gustatory processing system encodes the quality (sweet, salty, bitter, sour, or umami), intensity, and palatability of a sensory stimulus (1.Cabanac M. 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One previous study using fMRI reported lower activation of the gustatory and reward cerebral regions and higher activation in somatosensory cerebral regions following a fatty stimulus than that with sugar (10.Stice E. Burger K.S. Yokum S. Relative ability of fat and sugar tastes to activate reward, gustatory, and somatosensory regions.Am. J. Clin. Nutr. 2013; 98: 1377-1384Crossref PubMed Scopus (101) Google Scholar). However, other studies highlighted cerebral activation by fatty food in the orbito-frontal and cingulate cortices, suggesting that a fat stimulus play a role in the hedonic control of food intake (8.De Araujo I.E. Rolls E.T. The representation in the human brain of food texture and oral fat.J. Neurosci. 2004; 24: 3086-3093Crossref PubMed Scopus (293) Google Scholar, 9.Grabenhorst F. Rolls E.T. Parris B.A. D'Souza A.A. How the brain represents the reward value of fat in the mouth.Cereb. Cortex. 2010; 20: 1082-1091Crossref PubMed Scopus (147) Google Scholar). Other data demonstrated that long-chain fatty acids (LCFAs) could be detected through specific receptor located in the mouth of rodents (16.Laugerette F. Passilly-Degrace P. Patris B. Niot I. Febbraio M. Montmayeur J.P. Besnard P. CD36 involvement in orosensory detection of dietary lipids: impact of spontaneous fat preference and digestive secretions.J. Clin. Invest. 2005; 115: 3177-3184Crossref PubMed Scopus (501) Google Scholar, 17.Gaillard D. Laugerette F. Darcel N. El Yassimi A. Passily-Degrace P. Hichami A. Khan N. Montmayeur P. Besnard P. The gustatory pathway is involved in CD36-mediated oro-sensory perception of long-chain fatty acids in the mouse.FASEB J. 2008; 22: 1458-1468Crossref PubMed Scopus (181) Google Scholar, 18.Cartoni C. Yasumatsu K. Ohkuri T. Shigemura N. Yoshida R. Godinot N. Le Coutre J. Ninomiya Y. Damak S. Taste preference for fatty acids is mediated by GPR40 and GPR120.J. Neurosci. 2010; 30: 8376-8382Crossref PubMed Scopus (312) Google Scholar, 19.Martin C. 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Fraser N.J. et al.The orphan G protein-coupled receptors GPR41 and GPR43 are activated by propionate and other short chain carboxylic acids.J. Biol. Chem. 2003; 278: 11312-11319Abstract Full Text Full Text PDF PubMed Scopus (1579) Google Scholar) and medium-chain (23.Hirasawa A. Tsumaya K. Awaji T. Katsuma S. Adachi T. Yamada M. Sugimoto Y. Miyazaki S. Tsujimoto G. Free fatty acids regulate gut incretin glucagon-like peptide-1 secretion through GPR120.Nat. Med. 2005; 11: 90-94Crossref PubMed Scopus (1148) Google Scholar, 24.Itoh Y. Kawamata Y. Harada M. Kobayashi M. Fujii R. Fukusumi S. Ogl K. Hosoya M. Tanaka Y. Uejima H. et al.Free fatty acids regulate insulin secretion from pancreatic cells through GPR40.Nature. 2003; 422: 173-176Crossref PubMed Scopus (1243) Google Scholar) FAs. However, there is not sufficient evidence to define fat taste as a primary taste (25.Mattes R.D. Accumulating evidence supports a taste component for free fatty acids in humans.Physiol. 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We recently reported a noninvasive method with high time resolution to study the gustatory pathway: gustatory evoked potentials (GEPs) (26.Stewart J.E. Feinle-Bisset C. Keast R.S.J. Fatty acid detection during food consumption and digestion: associations with ingestive behavior and obesity.Prog. Lipid Res. 2011; 50: 225-233Crossref PubMed Scopus (74) Google Scholar, 27.Fushiki T. Why fat is so preferable: from oral fat detection to inducing reward in the brain.Biosci. Biotechnol. Biochem. 2014; 78: 363-369Crossref PubMed Scopus (14) Google Scholar). GEPs have already been obtained in response to primary taste (salty, sweet, sour, bitter, and umami) solutions (29.Jacquin-Piques A. Gaudillat S. Mouillot T. Gigot V. Meillon S. Leloup C. Pénicaud L. Brondel L. Prandial states modify the reactivity of the gustatory cortex using gustatory evoked potentials in Humans.Front. Neurosci. 2016; 9: 490Crossref PubMed Scopus (11) Google Scholar, 30.Jacquin-Piques A. Mouillot T. Gigot V. Meillon S. Leloup C. Pénicaud L. Brondel L. Preference for sucrose solutions modulates taste cortical activity in humans.Chem. Senses. 2016; 41: 591-599PubMed Google Scholar, 31.Ohla K. Busch N.A. Lundström J.N. Time for taste—a review of the early cerebral processing of gustatory perception.Chemosens. Percept. 2012; 5: 87-99Crossref PubMed Scopus (32) Google Scholar). In the present study, we hypothesized that GEPs could be observed in response to FFA solutions, with the same characteristics as GEPs in response to other known primary taste stimuli. Recording GEPs in response to FFA solutions could be an objective argument to demonstrate the activation of the gustatory pathway by FFAs. Therefore, the first aim was to record GEPs in response to unsaturated LCFA solutions, in two concentrations, applied in the oral cavity in healthy subjects, in comparison with control sessions (paraffin emulsion and water) (first experiment). We also aimed to compare the LCFA cortical responses with GEPs obtained in response to sweet and salty solutions in the same subjects (first experiment). The second aim was to compare GEPs in response to unsaturated LCFA solutions with other FFA solutions: saturated and unsaturated LCFAs and medium- and short-chain FAs (second experiment). The taste stimuli consisted of LCFA emulsions, sodium chloride (NaCl), and sucrose solutions. NaCl and sucrose (Cooper, Melun, France) were diluted in Evian water on the same day as GEP recordings. Evian water, which is almost deionized, was used as the control solution. The preparation of the LCFA emulsion was described elsewhere (32.Chevrot M. Passilly-Degrace P. Ancel D. Bernard A. Enderli G. Gomes M. Robin I. Issanchou S. Vergès B. Nicklaus S. et al.Obesity interferes with the orosensory detection of long-chain fatty acids in humans.Am. J. Clin. Nutr. 2014; 99: 975-983Crossref PubMed Scopus (55) Google Scholar). Linoleic acid (LA) were chosen because the Western diet is characterized by an overconsumption of LA and because they are known to bind and activate lipid receptors CD36 and G-protein-coupled receptor 120 (GPR120) in gustatory papillae (23.Hirasawa A. Tsumaya K. Awaji T. Katsuma S. Adachi T. Yamada M. Sugimoto Y. Miyazaki S. Tsujimoto G. Free fatty acids regulate gut incretin glucagon-like peptide-1 secretion through GPR120.Nat. Med. 2005; 11: 90-94Crossref PubMed Scopus (1148) Google Scholar, 33.Baillie A.G. Coburn C.T. Abumrad N.A. Reversible binding of long-chain fatty acids to purified FAT, the adipose CD36 homolog.J. Membr. Biol. 1996; 153: 75-81Crossref PubMed Scopus (185) Google Scholar). Briefly, LA (Sigma-Aldrich, Saint Quentin Fallavier, France) oil-in-water emulsions (LA emulsions) were prepared in a solution of 5% acacia gum (Cooper) and 5% paraffin oil (Cooper) diluted in Evian mineral water. A mixture of paraffin oil and acacia gum without LA was used as a control solution (paraffin emulsion), to limit differences in viscosity and lubricity between control and taste solutions, which could be detected by the subjects. LA and paraffin emulsions were prepared for less than 24 h before tasting. Healthy men were enrolled in this study. The mean age and BMI were 22 ± 2 years old (range: 19–34) and 23 ± 3 kg/m2 (range: 19–29), respectively. All of the subjects were nonsmokers or mild smokers who had not smoked during the day before GEP recordings. Heavy smokers (20 or more cigarettes per day) were not included. As shown in previous data (34.Vennemann M.M. Hummel T. Berger K. The association between smoking and smell and taste impairment in the general population.J. Neurol. 2008; 255: 1121-1126Crossref PubMed Scopus (361) Google Scholar), only heavy smoking alters gustatory pathways, and GEPs obtained in response to salty and sweet solutions were similar in mild and moderate smokers and nonsmokers (unpublished observations). None of the subjects had oral, dental, or neurological disorders or specific medical histories. Subjects who were currently undergoing medical treatment and obese subjects (BMI > 30 kg/m2) were excluded. Eighteen subjects were investigated in six sessions separated by an interval of at least 1 day. Each session corresponded to a specific stimulus quality and concentration, which was randomly assigned. Each stimulus was applied in two different concentrations: 0.25% and 1% LA solutions, which were higher than the LA threshold (32.Chevrot M. Passilly-Degrace P. Ancel D. Bernard A. Enderli G. Gomes M. Robin I. Issanchou S. Vergès B. Nicklaus S. et al.Obesity interferes with the orosensory detection of long-chain fatty acids in humans.Am. J. Clin. Nutr. 2014; 99: 975-983Crossref PubMed Scopus (55) Google Scholar), 5 and 20 g of sucrose per 100 ml of water, and 0.5 and 2 g of NaCl per 100 ml of water, also higher than the sweet and salty thresholds, respectively. Eighteen subjects were investigated in two control sessions of GEP recordings: one with the paraffin emulsion and another with water. The GEPs of only 14 subjects were assessable because of some artifacts that hindered the absence of GEPs in these recordings. The taste stimuli consisted of five FFA solutions. The FFAs were as follow: polyunsaturated long-chain FFA (solution containing 0.25% LA), monounsaturated long-chain FFA (solution containing 0.25% oleic acid), saturated long-chain FFA (solution containing 0.25% stearic acid), medium-chain FFA (solution containing 0.25% lauric acid), and short-chain FFA (solution containing 0.1% caproic acid). These FFAs were chosen because of their overconsumption in the Western diet. The concentrations that were tested were higher than the FFA threshold and were tested to be similarly intense in preliminary studies. The similar intensity of the prepared FFA emulsions was confirmed in the present study (see below). The preparation of the FFA solutions was similar to that described above. Each subject had also a sixth GEP recording session in response to a control paraffin solution. Each session of GEP recordings, which was randomly assigned, was separated by an interval of at least 1 day. Eighteen healthy men were enrolled in this second experiment. The mean age and BMI were 25 ± 6 years old (range: 19–37) and 22 ± 2 kg/m2 (range: 19–26), respectively. Age and BMI of the subjects were comparable in the two experiments (P > 0.05 for both). All of the subjects were nonsmokers or very mild smokers (<10 cigarettes per day) who had not smoked during the day before GEP recordings. The subjects were informed about the nature and aims of the experiments and provided informed consent. The study was approved by the regional Ethics Committee of Burgundy, France, in accordance with the latest revision of the Declaration of Helsinki and European Law (ISO EN 14155). The taste-delivery system has been described in detail in previous studies (29.Jacquin-Piques A. Gaudillat S. Mouillot T. Gigot V. Meillon S. Leloup C. Pénicaud L. Brondel L. Prandial states modify the reactivity of the gustatory cortex using gustatory evoked potentials in Humans.Front. Neurosci. 2016; 9: 490Crossref PubMed Scopus (11) Google Scholar, 30.Jacquin-Piques A. Mouillot T. Gigot V. Meillon S. Leloup C. Pénicaud L. Brondel L. Preference for sucrose solutions modulates taste cortical activity in humans.Chem. Senses. 2016; 41: 591-599PubMed Google Scholar). Briefly, control and taste solutions were driven through the system by compressed air. Two parallel silicone tubes were used: one for the control solution and the other for the taste solution. Switching between the control and taste solutions was performed by two electromagnetic valves controlled by an electronic device. This electronic device (stimulator) sent a signal to the computer software (SystemPLUS EVOLUTION, 2007 Micromed S.p.A) when the taste solution was administered (with 1 ms precision), resulting in a precise time recording of the GEPs. Participants put the two parallel tubes (silicone tubing, P/N 10025-02S, Bio-Chem valve) in their mouth, placed 1.5 ± 0.5 cm from the dental arch on the midline of the tongue. Air was purged from the taste delivery system to avoid delaying stimulus presentation. Solutions were delivered to the tongue through a hole at the end of each tube. A taste solution was intermittently delivered through the first tube. During the period without the taste solution, a control solution (water for salty and sweet solutions or paraffin emulsion for FFA emulsions) was continuously delivered through the second tube to minimize the likelihood that the subjects would feel different sensations from the injections from the two tubes. During the control sessions, the experimental protocol was similar to the one previously described, and the paraffin emulsion (or water) was present in both the tubes and was therefore used as the stimulus. All of the sessions were conducted at the same time of day, 2–4 h after lunch. The subjects were asked not to eat or drink anything except water during the time between lunch and the GEP recording. One session lasted approximately 40 min: 20 min to prepare for the GEP recording and 20 min for the GEP recording itself. In each session, the stimulus was presented for 1 s 20 times. Each stimulus was separated by a 1 min interval of water solution. During the GEP recordings, the subjects listened to quiet music through their headphones to mask the switching clicks of the electromagnetic valves. No evoked potentials were recorded in our experiment in response to quiet music (checked with control GEP recordings). The subjects also had to close their eyes to avoid light stimulation and to wear a nose clip for each of the taste stimulations in order to avoid retronasal olfaction, because FFA solutions can activate olfactory receptors (35.Ramirez I. Role of olfaction in starch and oil preference.Am. J. Physiol. 1993; 265: R1404-R1409PubMed Google Scholar). After GEP recordings were performed with the taste solutions, the subjects were asked to rate the hedonic value and the perceived intensity of each solution using a 10 cm visual analog scale (VAS) anchored by "not at all" and "extremely" at its extremities. They had to respond to the following questions: "How palatable was the taste solution?" and "How intense was the taste solution?" Electroencephalographic (EEG) measurements were recorded according to the international 10-20 system using a conventional EEG recording system. Five sites were recorded by surface electrodes defined by their scalp topography: centro-parietal electrode Pz, central electrode Cz, and frontal electrodes Fz, Fp1, Fp2. The electrodes were referenced against linked earlobes (ear clip electrodes enfolded by Ag, 10 mm diameter; SystemPLUS EVOLUTION). The ground electrode was placed on the forehead. Disposable cup electrodes enfolded by Ag-AgCl (6 mm diameter), with a long polyurethane cable (SystemPLUS EVOLUTION), were used. Electrodes were placed after using first a pumice paste and then a conductive and adhesive paste. The EEG measurements were amplified, filtered, and digitized using Micromed software (SystemPLUS EVOLUTION, 2007 Micromed S.p.A), as follows: time constant, 1 s; sampling frequency, 2,048 Hz; 200 Hz low-pass filter; 0.4 Hz high-pass filter; 50 Hz filter. GEPs were averaged after each recording session (average of 20 stimuli). No baseline correction was applied during averaging. GEP analysis was performed with the same software (SystemPLUS EVOLUTION). No contamination due to α waves was noted because baseline cortical activity in participants with closed eyes was mainly noted in occipital recordings. Moreover, averaging decreased α wave contamination. GEP was defined by three peaks, as described in previous studies (29.Jacquin-Piques A. Gaudillat S. Mouillot T. Gigot V. Meillon S. Leloup C. Pénicaud L. Brondel L. Prandial states modify the reactivity of the gustatory cortex using gustatory evoked potentials in Humans.Front. Neurosci. 2016; 9: 490Crossref PubMed Scopus (11) Google Scholar, 30.Jacquin-Piques A. Mouillot T. Gigot V. Meillon S. Leloup C. Pénicaud L. Brondel L. Preference for sucrose solutions modulates taste cortical activity in humans.Chem. Senses. 2016; 41: 591-599PubMed Google Scholar): P1 the first positive peak, N1 the higher negative peak, and P2 the second positive peak. P1 latency (in milliseconds), N1 latency (in milliseconds), and P1N1 amplitude (in microvolts) of the GEPs were registered for each recorded electrode. The P1 latency was defined as the time between stimulus delivery and the potential's positive peak P1. The N1 latency was defined as the time between stimulus delivery and the potential's negative peak. The amplitude of each response was calculated as the difference between the first positive and the negative peaks (P1N1 amplitude). The positive peak corresponded to the peak pointing down, whereas the negative peak corresponded to the peak pointing up. The software first averaged the GEPs (n = 20) and then detected the peaks. The GEP recordings were then analyzed by the same well-trained neurophysiologist and were processed with a standard and consistent method of EEG analysis, regardless of the quality and intensity of the taste solution and the hedonic value noted by the subject. The neurophysiologist was blinded to the taste solution applied. Because of constraints inherent to our software, prestimulus cerebral activity was not available. Many GEP recordings in Fp1 and Fp2 (in response to sweet, salty, and fatty stimuli), one GEP recording in Pz (in response to sweet stimulus), and 9 out of 108 GEP recordings in the second experiment were not analyzed because of artifacts. At the end of the recordings for each stimulus and for each patient, an average of the responses of all subjects was made: It was called the "grand average" (see Figs. 1, 2, and 5). P1N1 amplitudes are minimized in the "grand average" compared with the statistical mean: in fact, there is a smoothing of the amplitude in the graph because GEP peaks of each subject do not have the same latency.Fig. 2Grand averages (average of the responses of all subjects) of GEPs: recordings of GEPs in response to the three high concentrated taste solutions (LA, salty, and sweet solutions), on the Cz electrode, in all 18 participants. The start of the taste stimulation was at 0 ms. No difference of GEPs parameters was observed whatever the quality of the stimulus.View Large Image Figure ViewerDownload Hi-res image Download (PPT)Fig. 5Grand averages (average of the responses of all subjects) of recordings of GEPs, on the Cz electrode. The start of the stimulation was at 0 ms. P1N1 amplitude was higher in response to the five FFA solutions than in response to paraffin emulsion (P < 0.001 for all).View Large Image Figure ViewerDownload Hi-res image Download (PPT) P1 latency, N1 latency, and P1N1 amplitude of each GEP and the VAS results (for hedonic value and perceived intensity of the solutions) were expressed as means and standard deviations. In the first experiment, P1 latency, N1 latency, and P1N1 amplitude of GEPs obtained in Pz, Cz, and Fz after stimulation by LA, paraffin emulsions, and water were compared using two-way ANOVA for repeated measures, to test the subject effect and the solution effect. P1 latency, N1 latency, and P1N1 amplitude of GEPs obtained in Pz, Cz, and Fz after stimulation by sweet, salty, and fatty solutions, were then compared using three-way ANOVA for repeated measures to test the subject effect, the effect of the taste quality, and the effect of the concentration of taste. Hedonic value and perceived intensity of taste were also analyzed by three-way ANOVA to test the subject effect, the effect of the taste quality, and the effect of the concentration of taste. Post hoc analyses (Tukey's tests) were also performed when the result was found significant. In the second experiment, hedonic value, perceived intensity of taste, P1 latency, N1 latency, and P1N1 amplitude of GEPs obtained in Pz, Cz, and Fz after stimulation by FFA and paraffin emulsions were compared using two-way ANOVA for repeated measures, to test the subject effect and the solution effect. Post hoc analyses (Tukey's tests) were performed when the result was found to be significant. Then, GEP parameters in response to paraffin solutions on the one hand, and in response to LA solutions on the other hand, were compared between both experiments using one-way ANOVA. GEPs recorded in Fp1 and Fp2 electrodes were not analyzed because of the too large variability of the recording (artifacts due
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