Glial EAAT2 regulation of extracellular nTS glutamate critically controls neuronal activity and cardiorespiratory reflexes.

Abstract

Excitatory amino acid transporter 2 (EAAT2) is present on astrocytes in the nucleus tractus solitarii (nTS), an important nucleus in cardiorespiratory control. Its specific role in influencing nTS neuronal activity and thereby basal and reflex cardiorespiratory function is unknown. The specific role of nTS EAAT2 was determined via whole animal and brainstem slice patch clamp experiments. Astrocytic EAAT2 buffers basal glutamate activation of AMPA-type glutamate receptors and therefore decreases baseline excitability of nTS neurons. EAAT2 modulates cardiorespiratory control and tempers excitatory cardiorespiratory responses to activation of the peripheral chemoreflex. This study supports the concept that nTS astrocyte transporters influence sympathetic nervous system activity and cardiorespiratory reflex function in health and disease. Glutamatergic signalling is critical in the nucleus tractus solitarii (nTS) for cardiorespiratory homeostasis and initiation of sensory reflexes, including the chemoreflex activated during hypoxia. Maintenance of nTS glutamate concentration occurs in part through astrocytic excitatory amino acid transporters (EAATs). We previously established the importance of EAATs in the nTS by demonstrating their inhibition produced neuronal excitation to alter basal cardiorespiratory function. Since EAAT2 is the most expressed EAAT in the nTS, this study specifically determined EAAT2's role in nTS astrocytes, its influence on neuronal and synaptic properties, and ultimately on basal and reflex cardiorespiratory function. The EAAT2-specific antagonist dihydrokainate (DHK) was microinjected into the anaesthetized rat nTS or applied to rat nTS slices. DHK produced depressor, bradycardic and sympathoinhibitory responses and reduced neural respiration in the intact rat, mimicking responses to glutamate excitation. DHK also enhanced responses to glutamate microinjection. DHK elevated extracellular nTS glutamate concentration, depolarized neurons and enhanced spontaneous EPSCs. EAAT2 block also augmented action potential discharge in chemosensitive nTS neurons. Glial recordings confirmed EAAT2 is functional on nTS astrocytes. Neuronal excitation and cardiorespiratory effects following EAAT2 inhibition were due to activation of putative extrasynaptic AMPA receptors as their antagonism blocked DHK responses in the intact rat nTS and the slice. The DHK-induced elevation of extracellular glutamate and neuronal excitation augmented chemoreflex-mediated pressor, sympathoexcitatory and minute neural ventilation responses in the rat. These data shed new light on the important role astrocytic EAAT2 plays on buffering nTS excitation and overall cardiorespiratory function.