Excitatory Amino Acid Transporters (EAATs) in nTS Limit Metabotropic Glutamate Receptor (mGluRs) Modulation of Synaptic and Neuronal Activity in Chronic Intermittent Hypoxia

Abstract

Glutamate (Glu) is the primary excitatory neurotransmitter in the nucleus Tractus Solitarii (nTS), the first central site for integration and modulation of the carotid body chemoafferent reflex. We have shown that nTS extracellular Glu concentration, ionotropic Glu receptor activation, synaptic and neuronal activity, and ultimately cardiorespiratory reflexes are tonically restrained by EAATs. Specifically, EAAT block (EAAT‐X) increased spontaneous excitatory postsynaptic current (sEPSC) frequency, depolarized neurons, enhanced discharge, but reduced the amplitude of afferent (TS)‐evoked EPSCs (TS‐EPSCs). The contribution of mGluRs in EAAT‐X, which decrease TS‐EPSCs via Group II/III mGluRs, and depolarizes nTS neurons via Group I mGluRs, is unknown. Interestingly, chronic intermittent hypoxia (CIH), a model for obstructive sleep apnea, produces similar synaptic and neuronal responses as EAAT‐X in nTS. This study sought to determine the contribution of a) mGluRs on synaptic alterations after EAAT‐X, b) EAATs on CIH‐induced alterations, and 3) mGluRs and EAATs in CIH responses. Male Sprague‐Dawley rats (3–6 weeks) were exposed to 10d normoxia (Norm, 21% O2) or 10d CIH (alternating 21% O2 and 6% O2, 8h/day). Horizontal brainstem nTS slices were generated and sEPSCs, TS‐EPSCs, holding currents (Ihold), and membrane potential (Vm) were recorded from patch‐clamped monosynaptic nTS neurons. Electrophysiological properties were examined during aCSF control, EAAT‐X (TFB‐TBOA, 500 nM) alone, and subsequently during EAAT‐X and block of either mGluR (i.e, EAAT+mGluR‐X) Group II/III (EGLU and MSOP, 200 mM each) or Group I (LY367385, 160 mM). As a time control, a separate group of neurons were exposed to a second EAAT‐X (EAAT‐X2) for comparison with EAAT+mGluR‐X. In Norm, EAAT‐X increased (depolarized) Ihold and depolarized Vm, and EAAT‐X2 enhanced this depolarization. EAAT+mGluR‐II/III‐X eliminated the enhancement of Ihold and Vm in comparison to EAAT‐X2. After CIH, EAAT‐X did not alter Ihold or Vm, and EAAT+mGluR‐II/III‐X did not further alter these parameters. sEPSC frequency in Norm increased in EAAT‐X, indicating an increase in network activity. EAAT‐X2 enhanced the increase in sEPSC frequency, which was blocked by EAAT+mGluR II/III‐X but not EAAT+mGluR I‐X. After CIH, sEPSC frequency was enhanced vs. Norm, and EAAT‐X had no additional effect on sEPSC frequency. In CIH, EAAT+mGluR‐II/III‐X did not enhance sEPSC frequency, which did not differ from EAAT‐X alone. TS‐EPSCs (0.5 Hz) in Norm were significantly decreased following EAAT‐X, and EAAT+mGluR‐II/III‐X attenuated the amplitude reduction compared to EAAT‐X alone. After CIH, the amplitude of TS‐EPSCs was decreased vs. Norm; EAAT‐X further decreased TS‐EPSC amplitude which was not blocked by mGluR‐II/III‐X. These data suggest CIH reduces EAAT influence on synaptic and neuronal activity, and mGluRs contribute. Additionally, these data further suggest a role for EAATs and mGluRs on cardiorespiratory reflexes.Support or Funding InformationSupported by RO1 HL128454This abstract is from the Experimental Biology 2018 Meeting. There is no full text article associated with this abstract published in The FASEB Journal.