Astrocytic glutamate transporters reduce the neuronal and physiological influence of metabotropic glutamate receptors in nucleus tractus solitarii

• Published in: American journal of physiology. Regulatory, integrative and comparative physiology Vol. 318; no. 3; pp. R545 - R564
• Main Authors: Martinez, Diana, Rogers, Richard C, Hermann, Gerlinda E, Hasser, Eileen M, Kline, David D
• Format: Journal Article
• Language: English
• Published: United States American Physiological Society 01.03.2020
• Series: Neural Control
• Subjects: Amino Acid Transport System X-AG - drug effects; Amino Acid Transport System X-AG - metabolism; Animals; Aspartic Acid - analogs & derivatives; Aspartic Acid - pharmacology; Astrocytes - metabolism; Excitatory Amino Acid Antagonists - pharmacology; Excitatory Postsynaptic Potentials - drug effects; Excitatory Postsynaptic Potentials - physiology; Glutamic Acid - metabolism; Neurons - drug effects; Neurons - metabolism; Receptors, Metabotropic Glutamate - drug effects; Receptors, Metabotropic Glutamate - metabolism; Solitary Nucleus - metabolism; Synaptic Transmission - drug effects; Synaptic Transmission - physiology; glutamate signaling; astroglia; respiration; autonomic function
• ISSN: 0363-6119; 1522-1490
• DOI: 10.1152/ajpregu.00319.2019

Astrocytic excitatory amino acid transporters (EAATs) are critical to restraining synaptic and neuronal activity in the nucleus tractus solitarii (nTS). Relief of nTS EAAT restraint generates two opposing effects, an increase in neuronal excitability that reduces blood pressure and breathing and an attenuation in afferent [tractus solitarius (TS)]-driven excitatory postsynaptic current (EPSC) amplitude. Although the former is due, in part, to activation of ionotropic glutamate receptors, there remains a substantial contribution from another unidentified glutamate receptor. In addition, the mechanism(s) by which EAAT inhibition reduced TS-EPSC amplitude is unknown. Metabotropic glutamate receptors (mGluRs) differentially modulate nTS excitability. Activation of group I mGluRs on nTS neuron somas leads to depolarization, whereas group II/III mGluRs on sensory afferents decrease TS-EPSC amplitude. Thus we hypothesize that EAATs control postsynaptic excitability and TS-EPSC amplitude via restraint of mGluR activation. To test this hypothesis, we used in vivo recording, brain slice electrophysiology, and imaging of glutamate release and TS-afferent Ca . Results show that EAAT blockade in the nTS with (3 )-3-[[3-[[4-(trifluoromethyl)benzoyl]amino]phenyl]methoxy]-l-aspartic acid (TFB-TBOA) induced group I mGluR-mediated depressor, bradycardic, and apneic responses that were accompanied by neuronal depolarization, elevated discharge, and increased spontaneous synaptic activity. Conversely, upon TS stimulation TFB-TBOA elevated extracellular glutamate to decrease presynaptic Ca and TS-EPSC amplitude via activation of group II/III mGluRs. Together, these data suggest an important role of EAATs in restraining mGluR activation and overall cardiorespiratory function.