These results suggest that astrocytes in the NTS region could act as a central gas sensor. This is the first study to reveal that the NTS includes hypercapnia and hypoxia dual-sensitive cells. In the presence of a glia preferential blocker, 5 mM fluoroacetate (plus 0.5 μM TTX), the percentage of hypoxia-sensitive cells was significantly reduced compared to those of all other conditions. Of note, 49% of the putative astrocytes identified by low K + stimulation were sensitive to hypercapnia, hypoxia or both. Some of these cells responded to low K +, and they were classified into astrocytes comprising 43% hypercapnia-sensitive cells, 56% hypoxia-sensitive cells and 54% of both stimulation-sensitive cells.
In the synaptic blockade solutions (cocktail or TTX solution), 7.6 and 8% of the NTS cells responded to hypercapnic and hypoxic stimulation, respectively, and approximately 2% of them responded to both stimulations. Excitation of cells was detected as changes of fluorescence intensity with a confocal calcium imaging system. At the end of the experiments, the superfusate potassium concentration was lowered to 0.2 from 3 mM to classify recorded cells into neurons and astrocytes. We tested these responses in standard solution and in two different synapse blockade solutions: (1) cocktail blockers solution including bicuculline, strychnine, NBQX and MK-801 or (2) TTX solution. We examined cellular responses initially to hypercapnic stimulation (to 8% CO 2 from 2% CO 2) and then to hypoxic stimulation (to 0% O 2 from 95% O 2 at 5% CO 2). To detect cellular responses, calcium indicator Oregon Green was pressure-injected into the NTS just beneath the cut surface of either the caudal or rostral block of the medulla, and the preparation was superfused with artificial cerebrospinal fluid (25–26☌). The brainstem and spinal cord were isolated from newborn rat (P0-P4) and were transversely cut at the level of the rostral area postrema. In the present study, we analyzed cellular responses to hypercapnia and hypoxia in the NTS region of newborn rat in vitro preparation. It is supposed that the nucleus of the solitary tract (NTS) in the dorsal medulla includes gas sensor cells responsive to hypercapnia or hypoxia in the central nervous system.
2Global Research Center for Innovative Life Science, Hoshi University School of Pharmacy and Pharmaceutical Sciences, Tokyo, Japan.1Department of Physiology, Showa University School of Medicine, Tokyo, Japan.Hiroshi Onimaru 1*, Itaru Yazawa 2, Kotaro Takeda 3, Isato Fukushi 4,5 and Yasumasa Okada 5
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