RESUMEN
Introduction: Space is a unique environment characterized by isolation from community life and exposure to circadian misalignment, microgravity, and space radiation. These multiple differences from those experienced on the earth may cause systemic and local tissue stress. Autonomic nerves, including sympathetic and parasympathetic nerves, regulate functions in multiple organs. Saliva is secreted from the salivary gland, which is regulated by autonomic nerves, and plays several important roles in the oral cavity and digestive processes. The balance of the autonomic nervous system in the seromucous glands, such as the submandibular glands, precisely controls serous and mucous saliva. Psychological stress, radiation damage, and other triggers can cause an imbalance in salivary secretion systems. A previous study reported that amylase is a stress marker in behavioral medicine and space flight crews; however, the detailed mechanisms underlying amylase regulation in the space environment are still unknown. Methods: In this study, we aimed to elucidate how lunar gravity (1/6 g) changes mRNA expression patterns in the salivary gland. Using a multiple artificial gravity research system during space flight in the International Space Station, we studied the effects of two different gravitational levels, lunar and Earth gravity, on the submandibular glands of mice. All mice survived, returned to Earth from space, and their submandibular glands were collected 2 days after landing. Results: We found that lunar gravity induced the expression of the salivary amylase gene Amy1; however, no increase in Aqp5 and Ano1, which regulate water secretion, was observed. In addition, genes involved in the exocrine system, such as vesicle-associated membrane protein 8 (Vamp8) and small G proteins, including Rap1 and Rab families, were upregulated under lunar gravity. Conclusion: These results imply that lunar gravity upregulates salivary amylase secretion via Rap/Rab signaling and exocytosis via Vamp8. Our study highlights Amy1 as a potential candidate marker for stress regulation in salivary glands in the lunar gravity environment.
RESUMEN
According to the "hydrodynamic theory," dentinal pain or sensitivity is caused by dentinal fluid movement following the application of various stimuli to the dentin surface. Recent convergent evidence in Vitro has shown that plasma membrane deformation, mimicking dentinal fluid movement, activates mechanosensitive transient receptor potential (TRP)/Piezo channels in odontoblasts, with the Ca2+ signal eliciting the release of ATP from pannexin-1 (PANX-1). The released ATP activates the P2X3 receptor, which generates and propagates action potentials in the intradental Aδ afferent neurons. Thus, odontoblasts act as sensory receptor cells, and odontoblast-neuron signal communication established by the TRP/Piezo channel-PANX-1-P2X3 receptor complex may describe the mechanism of the sensory transduction sequence for dentinal sensitivity. To determine whether odontoblast-neuron communication and odontoblasts acting as sensory receptors are essential for generating dentinal pain, we evaluated nociceptive scores by analyzing behaviors evoked by dentinal sensitivity in conscious Wistar rats and Cre-mediated transgenic mouse models. In the dentin-exposed group, treatment with a bonding agent on the dentin surface, as well as systemic administration of A-317491 (P2X3 receptor antagonist), mefloquine and 10PANX (non-selective and selective PANX-1 antagonists), GsMTx-4 (selective Piezo1 channel antagonist), and HC-030031 (selective TRPA1 channel antagonist), but not HC-070 (selective TRPC5 channel antagonist), significantly reduced nociceptive scores following cold water (0.1 ml) stimulation of the exposed dentin surface of the incisors compared to the scores of rats without local or systemic treatment. When we applied cold water stimulation to the exposed dentin surface of the lower first molar, nociceptive scores in the rats with systemic administration of A-317491, 10PANX, and GsMTx-4 were significantly reduced compared to those in the rats without systemic treatment. Dentin-exposed mice, with somatic odontoblast-specific depletion, also showed significant reduction in the nociceptive scores compared to those of Cre-mediated transgenic mice, which did not show any type of cell deletion, including odontoblasts. In the odontoblast-eliminated mice, P2X3 receptor-positive A-neurons were morphologically intact. These results indicate that neurotransmission between odontoblasts and neurons mediated by the Piezo1/TRPA1-pannexin-1-P2X3 receptor axis is necessary for the development of dentinal pain. In addition, odontoblasts are necessary for sensory transduction to generate dentinal sensitivity as mechanosensory receptor cells.