l-Glutamate stimulates cholecystokinin secretion via the T1R1/T1R3 mediated PLC/TRPM5 transduction pathway

Central amygdala is related to the reduction of aggressive behavior by monosodium glutamate ingestion during the period of development in an ADHD model rat

Introduction

Monosodium glutamate (MSG), an umami substance, stimulates the gut-brain axis communication via gut umami receptors and the subsequent vagus nerves. However, the brain mechanism underlying the effect of MSG ingestion during the developmental period on aggression has not yet been clarified. We first tried to establish new experimental conditions to be more appropriate for detailed analysis of the brain, and then investigated the effects of MSG ingestion on aggressive behavior during the developmental stage of an ADHD rat model.

Methods

Long-Evans, WKY/Izm, SHR/Izm, and SHR-SP/Ezo were individually housed from postnatal day 25 for 5 weeks. Post-weaning social isolation (PWSI) was given to escalate aggressive behavior. The resident-intruder test, that is conducted during the subjective night, was used for a detailed analysis of aggression, including the frequency, duration, and latency of anogenital sniffing, aggressive grooming, and attack behavior. Immunohistochemistry of c-Fos expression was conducted in all strains to predict potential aggression-related brain areas. Finally, the most aggressive strain, SHR/Izm, a known model of attention-deficit hyperactivity disorder (ADHD), was used to investigate the effect of MSG ingestion (60 mM solution) on aggression, followed by c-Fos immunostaining in aggression-related areas. Bilateral subdiaphragmatic vagotomy was performed to verify the importance of gut-brain interactions in the effect of MSG.

Results

The resident intruder test revealed that SHR/Izm rats were the most aggressive among the four strains for all aggression parameters tested. SHR/Izm rats also showed the highest number of c-Fos + cells in aggression-related brain areas, including the central amygdala (CeA). MSG ingestion significantly decreased the frequency and duration of aggressive grooming and attack behavior and increased the latency of attack behavior. Furthermore, MSG administration successfully increased c-Fos positive cell number in the intermediate nucleus of the solitary tract (iNTS), a terminal of the gastrointestinal sensory afferent fiber of the vagus nerve, and modulated c-Fos positive cells in the CeA. Interestingly, vagotomy diminished the MSG effects on aggression and c-Fos expression in the iNTS and CeA.

Conclusion

MSG ingestion decreased PWSI-induced aggression in SHR/Izm, which was mediated by the vagus nerve related to the stimulation of iNTS and modulation of CeA activity.

Expression of the umami taste receptor T1R1/T1R3 in porcine testis of: Function in regulating testosterone synthesis and autophagy in Leydig cells

Testosterone is a vital male hormone responsible for male sexual characteristics. The taste receptor family 1 subunit 3 (T1R3) regulates testosterone synthesis and autophagy in non-taste cells, and the links with the taste receptor family 1 subunit 1 (T1R1) for umami perception. However, little is known about these mechanisms. Thus, we aimed to determine the relationship between the umami taste receptor (T1R1/T1R3) and testosterone synthesis or autophagy in testicular Leydig cells of the Xiang pig. There was a certain proportion of spermatogenic tubular dysplasia in the Xiang pig at puberty, in which autophagy was enhanced, and the testosterone level was increased with a weak expression of T1R3. Silenced T1R3 decreased testosterone level and intracellular cyclic adenosine monophosphate (cAMP) content and inhibited the messenger RNA (mRNA) expression levels of testosterone synthesis enzyme genes [steroidogenic acute regulatory protein (StAR), hydroxy-delta-5-steroid dehydrogenase, 3 beta- and steroid delta-isomerase 1 (3β-HSD1), cytochrome P450 family 17 subfamily A member 1 (CYP17A1) and hydroxysteroid 17-beta dehydrogenase 3 (17β-HSD3)]. In addition, T1R3 increased the number of acidic autophagy bubbles and upregulated the expression levels of autophagy markers [Microtubule-associated protein 1 A/1B-light chain 3 (LC3) and Beclin-1] in testicular Leydig cells of the Xiang pig. Using an umami tasting agonist (10 mM L-glutamate for 6 h), the activation of T1R1/T1R3 enhanced the testosterone synthesis ability by increasing the intracellular cAMP level and upregulated the expression levels of StAR, 3β-HSD1, CYP17A1 and 17β-HSD3 in Leydig cells. Furthermore, the number of acidic autophagy bubbles decreased in the T1R1/T1R3-activated group with the downregulation of the expression levels of the autophagy markers, including LC3 and Beclin-1. These data suggest that the function of T1R1/T1R3 expressed in testicular Leydig cells of the Xiang pig is related to testosterone synthesis and autophagy.

The co-expression of steroidogenic enzymes with T1R3 during testicular development in the Congjiang Xiang pig

Testosterone is a key crucial hormone synthesized by steroidogenic enzymes that initiate and maintain spermatogenesis and secondary sexual characteristics in adult males. The taste receptor family 1 subunit 3 (T1R3) is reported to be associated with male reproduction. T1R3 can regulate the expressions of steroidogenic enzymes and affect testosterone synthesis. In this study, we addressed the question of whether the expression of steroid synthase was associated with T1R3 and its downstream-tasting molecules during testicular development. The results showed an overall upward trend in testosterone and morphological development in testes from Congjiang Xiang pigs from pre-puberty to sexual maturity. Gene expression levels of testicular steroidogenic acute regulatory protein (StAR), 3β-hydroxysteroid dehydrogenase (3β-HSD), cytochrome P450c17 (CYP17A1) and 17β-hydroxysteroid dehydrogenase (17β-HSD) were increased from pre-puberty to sexual maturity. Protein expression changes of CYP17A1 and 3β-HSD were consistent with mRNA. The relative abundance of tasting molecules (TAS1R3, phospholipase Cβ2, PLCβ2) was increased from pre-puberty to puberty (P < 0.05), with no further significant changes in expression from puberty to sexual maturity. Steroidogenic enzymes (3β-HSD and CYP17A1) were strongly detected in Leydig cells from pre-puberty to sexual maturity, while tasting molecules were localized in Leydig cells and spermatogenic cells. Correlation analysis showed that the genes mentioned above (except for PLCβ2) were positively correlated with testosterone levels and morphological characteristics of the testes at different developmental stages of Congjiang Xiang pigs. These results suggest that steroidogenic enzymes regulate testosterone synthesis and testicular development, and that taste receptor T1R3, but not PLCβ2, may associate with this process.

Synergistic Effect of Kokumi-Active γ-Glutamyl Peptides and l-Glutamate on Enhancing Umami Sensation and Stimulating Cholecystokinin Secretion via T1R1/T1R3 Activation in STC-1 Cells

This study aimed to investigate the synergistic effect of γ-glutamyl peptides (γEL, γEV, and γEγEV) and l-glutamate (MSG) on the activation of the umami receptor (T1R1/T1R3) in relation to enhanced umami taste and promoted cholecystokinin (CCK) secretion. The synergy of γ-glutamyl peptides and MSG (1-15 mM, 1:1) caused a significant increase in both the umami taste score by 0.218 ± 0.015-1.216 ± 0.031 times and the CCK secretion by 41.41 ± 6.46-201.16 ± 12.91% when compared to the group treated with individual MSG. The increase in CCK secretion promoted by γ-glutamyl peptides was only reduced by 11.54 ± 0.01-45.65 ± 3.58% after adding yjr CaSR inhibitor (NPS 2143), implying that there were other receptors besides CaSR involved in the stimulation of CCK secretion. The mixture of γEγEV and MSG synergistically increased the intracellular calcium release by 111.26 ± 11.94-135.28 ± 16.60% in STC-1 and 108.47 ± 7.89-152.33 ± 26.26% in HEK 293 compared to MSG. The protein expression for T1R1/T1R3 was increased, indicating that the mixture can activate T1R1/T1R3. The amino acids V277, S147, and D190 of T1R3 can be critical for the binding of γEγEV to T1R3. This is the first report on the synergistic effect of taste-active substances on taste sensation and hormone release via taste receptor activation.