Sobrinho Cleyton R, Gonçalves Christopher M, Takakura Ana C, Mulkey Daniel K, Moreira Thiago S
Department of Physiology and Biophysics, University of São Paulo, São Paulo, Brazil.
Department of Physiology and Neurobiology, University of Connecticut, Storrs, Connecticut; and.
J Neurophysiol. 2017 Sep 1;118(3):1690-1697. doi: 10.1152/jn.00032.2017. Epub 2017 Jul 5.
Evidence indicates that CO/H-evoked ATP released from retrotrapezoid nucleus (RTN) astrocytes modulates the activity of CO-sensitive neurons. RTN astrocytes also sense H by inhibition of Kir4.1 channels; however, the relevance of this pH-sensitive current remains unclear since ATP release appears to involve CO-dependent gating of connexin 26 hemichannels. Considering that depolarization mediated by H inhibition of Kir4.1 channels is expected to increase sodium bicarbonate cotransporter (NBC) conductance and favor Ca influx via the sodium calcium exchanger (NCX), we hypothesize that depolarization in the presence of CO is sufficient to facilitate ATP release and enhance respiratory output. Here, we confirmed that acute exposure to fluorocitrate (FCt) reversibly depolarizes RTN astrocytes and increased activity of RTN neurons by a purinergic-dependent mechanism. We then made unilateral injections of FCt into the RTN or two other putative chemoreceptor regions (NTS and medullary raphe) to depolarize astrocytes under control conditions and during P2-recepetor blockade while measuring cardiorespiratory activities in urethane-anesthetized, vagotomized, artificially ventilated male Wistar rats. Unilateral injection of FCt into the RTN increased phrenic (PNA) amplitude and frequency without changes in arterial pressure. Unilateral injection of pyridoxal-phosphate-6-azophenyl-2',4'-disulfonate (PPADS, a P2-receptor antagonist) into the RTN dampened both PNA amplitude and frequency responses to FCt. Injection of MRS2179 (P2Y1-receptor antagonist) into the RTN did not affect the FCt-induced respiratory responses. Fluorocitrate had no effect on breathing when injected into the NTS or raphe. These results suggest that depolarization can facilitate purinergic enhancement of respiratory drive from the RTN. Astrocytes in the retrotrapezoid nucleus (RTN) are known to function as respiratory chemoreceptors; however, it is not clear whether changes in voltage contribute to astrocyte chemoreception. We showed that depolarization of RTN astrocytes at constant CO levels is sufficient to modulate RTN chemoreception by a purinergic-dependent mechanism. These results support the possibility that astrocyte depolarization can facilitate purinergic enhancement of respiratory drive from the RTN.
有证据表明，延髓头端腹外侧区（RTN）星形胶质细胞释放的一氧化碳/氢离子诱发的三磷酸腺苷（ATP）可调节对一氧化碳敏感的神经元的活性。RTN星形胶质细胞也通过抑制内向整流钾通道4.1（Kir4.1）来感知氢离子；然而，这种对pH敏感的电流的相关性仍不清楚，因为ATP的释放似乎涉及连接蛋白26半通道的一氧化碳依赖性门控。鉴于氢离子抑制Kir4.1通道介导的去极化预计会增加碳酸氢钠协同转运体（NBC）的电导，并有利于通过钠钙交换体（NCX）的钙离子内流，我们推测在有一氧化碳存在的情况下的去极化足以促进ATP的释放并增强呼吸输出。在此，我们证实急性暴露于氟柠檬酸（FCt）会使RTN星形胶质细胞可逆性去极化，并通过嘌呤能依赖性机制增加RTN神经元的活性。然后，我们在乌拉坦麻醉、迷走神经切断、人工通气的雄性Wistar大鼠中测量心肺活动时，将FCt单侧注射到RTN或其他两个假定的化学感受器区域（孤束核和延髓中缝），以在对照条件下和P2受体阻断期间使星形胶质细胞去极化。将FCt单侧注射到RTN会增加膈神经（PNA）的幅度和频率，而动脉血压没有变化。将磷酸吡哆醛 - 6 - 偶氮苯 - 2',4' - 二磺酸盐（PPADS，一种P2受体拮抗剂）单侧注射到RTN会减弱对FCt的PNA幅度和频率反应。将MRS2179（P2Y1受体拮抗剂）注射到RTN中不影响FCt诱导的呼吸反应。当注射到孤束核或中缝时，氟柠檬酸对呼吸没有影响。这些结果表明去极化可促进嘌呤能增强来自RTN的呼吸驱动。已知延髓头端腹外侧区（RTN）的星形胶质细胞作为呼吸化学感受器发挥作用；然而，尚不清楚电压变化是否有助于星形胶质细胞的化学感受。我们表明，在恒定的一氧化碳水平下，RTN星形胶质细胞的去极化足以通过嘌呤能依赖性机制调节RTN的化学感受。这些结果支持星形胶质细胞去极化可促进嘌呤能增强来自RTN的呼吸驱动的可能性。
