Gamma-Aminobutyric Acid Transporters in the Nucleus Tractus Solitarii Regulate Inhibitory and Excitatory Synaptic Currents That Influence Cardiorespiratory Function

The brainstem nucleus tractus solitarii (nTS) processes and modulates the afferent arc of critical peripheral cardiorespiratory reflexes. Sensory afferents release glutamate to initiate the central component of these reflexes, and glutamate concentration is critically controlled by its removal via astrocytic neurotransmitter transporters. Gamma-aminobutyric acid (GABA) is the primary inhibitory neurotransmitter in the nTS providing tonic and phasic modulation of neuronal activity. GABA is removed from the extracellular space through GABA transporters (GATs), however, the role of GATs in nTS synaptic transmission and their influence on cardiorespiratory function is unknown. We hypothesized that GATs tonically restrain nTS inhibitory signaling and given the considerable nTS GABA-glutamate cross-talk, modify excitatory signaling and thus cardiorespiratory function. Reverse transcription real-time polymerase chain reaction (RT-PCR), immunoblot and immunohistochemistry showed expression of GAT-1 and GAT-3 mRNA and protein within the rat nTS, with GAT-3 greater than GAT-1, and GAT-3 colocalizing with astrocyte S100B. Recordings in rat nTS slices demonstrated GAT-3 block decreased spontaneous inhibitory postsynaptic current (IPSC) frequency and reduced IPSC amplitude evoked from electrical stimulation of the medial nTS. Block of GAT-3 also increased spontaneous excitatory postsynaptic current (EPSC) frequency yet did not alter sensory afferent-evoked EPSC amplitude. Block of GAT-3 in the nTS of anesthetized rats increased mean arterial pressure, heart rate, sympathetic nerve activity, and minute phrenic nerve activity. These results demonstrate inhibitory and excitatory neurotransmission in the nTS is significantly modulated by endogenous GAT-3 to influence basal cardiorespiratory function.

Sustained Hypoxia Alters nTS Glutamatergic Signaling and Expression and Function of Excitatory Amino Acid Transporters

Glutamate is the major excitatory neurotransmitter in the nucleus tractus solitarii (nTS) and mediates chemoreflex function during periods of low oxygen (i.e. hypoxia). We have previously shown that nTS excitatory amino acid transporters (EAATs), specifically EAAT-2, located on glia modulate neuronal activity, cardiorespiratory and chemoreflex function under normal conditions via its tonic uptake of extracellular glutamate. Chronic sustained hypoxia (SH) elevates nTS synaptic transmission and chemoreflex function. The goal of this study was to determine the extent to which glial EAAT-2 contributes to SH-induced nTS synaptic alterations. To do so, male Sprague-Dawley rats (4-7 weeks) were exposed to either 1, 3, or 7 days of SH (10% O2, 24 h/day) and compared to normoxic controls (21% O2, 24 h/day, i.e., 0 days SH). After which, the nTS was harvested for patch clamp electrophysiology, quantitative real-time PCR, immunohistochemistry and immunoblots. SH induced time- and parameter-dependent increases in excitatory postsynaptic currents (EPSCs). TS-evoked EPSC amplitude increased after 1D SH which returned at 3D and 7D SH. Spontaneous EPSC frequency increased only after 3D SH, which returned to normoxic levels at 7D SH. EPSC enhancement occurred primarily by presynaptic mechanisms. Inhibition of EAAT-2 with dihydrokainate (DHK, 300 µM) did not alter EPSCs following 1D SH but induced depolarizing inward currents (Ihold). After 3D SH, DHK decreased TS-EPSC amplitude yet its resulting Ihold was eliminated. EAAT-2 mRNA and protein increased after 3D and 7D SH, respectively. These data suggest that SH alters the expression and function of EAAT-2 which may have a neuroprotective effect.

Hydrogen peroxide inhibits neurons in the paraventricular nucleus of the hypothalamus via potassium channel activation

The paraventricular nucleus (PVN) of the hypothalamus is an important homeostatic and reflex center for neuroendocrine, respiratory, and autonomic regulation, including during hypoxic stressor challenges. Such challenges increase reactive oxygen species (ROS) to modulate synaptic, neuronal, and ion channel activity. Previously, in the nucleus tractus solitarius, another cardiorespiratory nucleus, we showed that the ROS H₂O₂ induced membrane hyperpolarization and reduced action potential discharge via increased K⁺ conductance at the resting potential. Here, we sought to determine the homogeneity of influence and mechanism of action of H₂O₂ on PVN neurons. We recorded PVN neurons in isolation and in an acute slice preparation, which leaves neurons in their semi-intact network. Regardless of preparation, H₂O₂ hyperpolarized PVN neurons and decreased action potential discharge. In the slice preparation, H₂O₂ also decreased spontaneous excitatory postsynaptic current frequency, but not amplitude. To examine potential mechanisms, we investigated the influence of the K⁺ channel blockers Ba²⁺, Cs⁺, and glibenclamide on membrane potential, as well as the ionic currents active at resting potential and outward K⁺ currents (I_K) upon depolarization. The H₂O₂ hyperpolarization was blocked by K⁺ channel blockers. H₂O₂ did not alter currents between -50 and -110 mV. However, H₂O₂ induced an outward I_K at -50 mV yet, at potentials more positive to 0 mV H₂O₂, decreased I_K. Elevated intracellular antioxidant catalase eliminated H₂O₂ effects. These data indicate that H₂O₂ alters synaptic and neuronal properties of PVN neurons likely via membrane hyperpolarization and alteration of I_K, which may ultimately alter cardiorespiratory reflexes.

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

KEY POINTS

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.

ABSTRACT

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.

Excitatory amino acid transporters tonically restrain nTS synaptic and neuronal activity to modulate cardiorespiratory function

The nucleus tractus solitarii (nTS) is the initial central termination site for visceral afferents and is important for modulation and integration of multiple reflexes including cardiorespiratory reflexes. Glutamate is the primary excitatory neurotransmitter in the nTS and is removed from the extracellular milieu by excitatory amino acid transporters (EAATs). The goal of this study was to elucidate the role of EAATs in the nTS on basal synaptic and neuronal function and cardiorespiratory regulation. The majority of glutamate clearance in the central nervous system is believed to be mediated by astrocytic EAAT 1 and 2. We confirmed the presence of EAAT 1 and 2 within the nTS and their colocalization with astrocytic markers. EAAT blockade withdl-threo-β-benzyloxyaspartic acid (TBOA) produced a concentration-related depolarization, increased spontaneous excitatory postsynaptic current (EPSC) frequency, and enhanced action potential discharge in nTS neurons. Solitary tract-evoked EPSCs were significantly reduced by EAAT blockade. Microinjection of TBOA into the nTS of anesthetized rats induced apneic, sympathoinhibitory, depressor, and bradycardic responses. These effects mimicked the response to microinjection of exogenous glutamate, and glutamate responses were enhanced by EAAT blockade. Together these data indicate that EAATs tonically restrain nTS excitability to modulate cardiorespiratory function.