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

Vagotomy blunts cardiorespiratory responses to vagal afferent stimulation via pre- and postsynaptic effects in the nucleus tractus solitarii

Viscerosensory information travels to the brain via vagal afferents, where it is first integrated within the brainstem nucleus tractus solitarii (nTS), a critical contributor to cardiorespiratory function and site of neuroplasticity. We have shown that decreasing input to the nTS via unilateral vagus nerve transection (vagotomy) induces morphological changes in nTS glia and reduces sighs during hypoxia. The mechanisms behind post-vagotomy changes are not well understood. We hypothesized that chronic vagotomy alters cardiorespiratory responses to vagal afferent stimulation via blunted nTS neuronal activity. Male Sprague-Dawley rats (6 weeks old) underwent right cervical vagotomy caudal to the nodose ganglion, or sham surgery. After 1 week, rats were anaesthetized, ventilated and instrumented to measure mean arterial pressure (MAP), heart rate (HR), and splanchnic sympathetic and phrenic nerve activity (SSNA and PhrNA, respectively). Vagal afferent stimulation (2-50 Hz) decreased cardiorespiratory parameters and increased neuronal Ca2+ measured by in vivo photometry and in vitro slice imaging of nTS GCaMP8m. Vagotomy attenuated both these reflex and neuronal Ca2+ responses compared to shams. Vagotomy also reduced presynaptic Ca2+ responses to stimulation (Cal-520 imaging) in the nTS slice. The decrease in HR, SSNA and PhrNA due to nTS nanoinjection of exogenous glutamate also was tempered following vagotomy. This effect was not restored by blocking excitatory amino acid transporters. However, the blunted responses were mimicked by NMDA, not AMPA, nanoinjection and were associated with reduced NR1 subunits in the nTS. Altogether, these results demonstrate that vagotomy induces multiple changes within the nTS tripartite synapse that influence cardiorespiratory reflex responses to afferent stimulation. KEY POINTS: Multiple mechanisms within the nucleus tractus solitarii (nTS) contribute to functional changes following vagal nerve transection. Vagotomy results in reduced cardiorespiratory reflex responses to vagal afferent stimulation and nTS glutamate nanoinjection. Blunted responses occur via reduced presynaptic Ca2+ activation and attenuated NMDA receptor expression and function, leading to a reduction in nTS neuronal activation. These results provide insight into the control of autonomic and respiratory function, as well as the plasticity that can occur in response to nerve damage and cardiorespiratory disease.

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.

Glial Modulation of Energy Balance: The Dorsal Vagal Complex Is No Exception

The avoidance of being overweight or obese is a daily challenge for a growing number of people. The growing proportion of people suffering from a nutritional imbalance in many parts of the world exemplifies this challenge and emphasizes the need for a better understanding of the mechanisms that regulate nutritional balance. Until recently, research on the central regulation of food intake primarily focused on neuronal signaling, with little attention paid to the role of glial cells. Over the last few decades, our understanding of glial cells has changed dramatically. These cells are increasingly regarded as important neuronal partners, contributing not just to cerebral homeostasis, but also to cerebral signaling. Our understanding of the central regulation of energy balance is part of this (r)evolution. Evidence is accumulating that glial cells play a dynamic role in the modulation of energy balance. In the present review, we summarize recent data indicating that the multifaceted glial compartment of the brainstem dorsal vagal complex (DVC) should be considered in research aimed at identifying feeding-related processes operating at this level.

Loss of Group II Metabotropic Glutamate Receptor Signaling Exacerbates Hypertension in Spontaneously Hypertensive Rats

High blood pressure is a major risk factor of cerebro-cardiovascular outcomes. Blood pressure is partly regulated by the autonomic nervous system and its reflex functions; therefore, we hypothesized that pharmacological intervention in the brainstem that can regulate blood pressure could be a novel therapeutic strategy to control hypertension. We infused a group II metabotropic glutamate receptor (mGluR) antagonist (LY341495, 0.40 μg/day), using a mini-osmotic pump, into the dorsal medulla oblongata in young spontaneously hypertensive rats (SHRs), as this area is adjacent to the nucleus tractus solitarius (NTS), of which the neurons are involved in baroreflex pathways with glutamatergic transmission. Blood pressure was recorded for conscious rats with the tail cuff method. A 6-week antagonist treatment from 6 to 12 weeks of age slightly but significantly increased systolic blood pressure by >30 mmHg, compared to that in SHRs without treatment. Moreover, the effect continued even 3 weeks after the treatment ended, and concurred with an increase in blood catecholamine concentration. However, heart rate variability analysis revealed that LY341495 treatment had little effect on autonomic activity. Meanwhile, mRNA expression level of mGluR subtype 2, but not subtype 3 in the brainstem was significantly enhanced by the antagonist treatment in SHRs, possibly compensating the lack of mGluR signaling. In conclusion, mGluR2 signaling in the dorsal brainstem is crucial for preventing the worsening of hypertension over a relatively long period in SHRs, through a mechanism of catecholamine secretion. This may be a specific drug target for hypertension therapy.

The role of astrocytes in the nucleus tractus solitarii in maintaining central control of autonomic function

The nucleus tractus solitarii (nTS) is the first central site for the termination and integration of autonomic and respiratory sensory information. Sensory afferents terminating in the nTS as well as the embedded nTS neurocircuitry release and utilize glutamate that is critical for maintenance of baseline cardiorespiratory parameters and initiating cardiorespiratory reflexes, including those activated by bouts of hypoxia. nTS astrocytes contribute to synaptic and neuronal activity through a variety of mechanisms, including gliotransmission and regulation of glutamate in the extracellular space via membrane-bound transporters. Here, we aim to highlight recent evidence for the role of astrocytes within the nTS and their regulation of autonomic and cardiorespiratory processes under normal and hypoxic conditions.

Interplay Between Systemic Metabolic Cues and Autonomic Output: Connecting Cardiometabolic Function and Parasympathetic Circuits

There is consensus that the heart is innervated by both the parasympathetic and sympathetic nervous system. However, the role of the parasympathetic nervous system in controlling cardiac function has received significantly less attention than the sympathetic nervous system. New neuromodulatory strategies have renewed interest in the potential of parasympathetic (or vagal) motor output to treat cardiovascular disease and poor cardiac function. This renewed interest emphasizes a critical need to better understand how vagal motor output is generated and regulated. With clear clinical links between cardiovascular and metabolic diseases, addressing this gap in knowledge is undeniably critical to our understanding of the interaction between metabolic cues and vagal motor output, notwithstanding the classical role of the parasympathetic nervous system in regulating gastrointestinal function and energy homeostasis. For this reason, this review focuses on the central, vagal circuits involved in sensing metabolic state(s) and enacting vagal motor output to influence cardiac function. It will review our current understanding of brainstem vagal circuits and their unique position to integrate metabolic signaling into cardiac activity. This will include an overview of not only how metabolic cues alter vagal brainstem circuits, but also how vagal motor output might influence overall systemic concentrations of metabolic cues known to act on the cardiac tissue. Overall, this review proposes that the vagal brainstem circuits provide an integrative network capable of regulating and responding to metabolic cues to control cardiac function.

Mechanisms Underlying Neuroplasticity in the Nucleus Tractus Solitarii Following Hindlimb Unloading in Rats

Hindlimb unloading (HU) in rats induces cardiovascular deconditioning (CVD) analogous to that observed in individuals exposed to microgravity or bed rest. Among other physiological changes, HU rats exhibit autonomic imbalance and altered baroreflex function. Lack of change in visceral afferent activity that projects to the brainstem in HU rats suggests that neuronal plasticity within central nuclei processing cardiovascular afferents may be responsible for these changes in CVD and HU. The nucleus tractus solitarii (nTS) is a critical brainstem region for autonomic control and integration of cardiovascular reflexes. In this study, we used patch electrophysiology, live-cell calcium imaging and molecular methods to investigate the effects of HU on glutamatergic synaptic transmission and intrinsic properties of nTS neurons. HU increased the amplitude of monosynaptic excitatory postsynaptic currents and presynaptic calcium entry evoked by afferent tractus solitarii stimulus (TS-EPSC); spontaneous (s) EPSCs were unaffected. The addition of a NMDA receptor antagonist (AP5) reduced TS-EPSC amplitude and sEPSC frequency in HU but not control. Despite the increase in glutamatergic inputs, HU neurons were more hyperpolarized and exhibited intrinsic decreased excitability compared to controls. After block of ionotropic glutamatergic and GABAergic synaptic transmission (NBQX, AP5, Gabazine), HU neuronal membrane potential depolarized and neuronal excitability was comparable to controls. These data demonstrate that HU increases presynaptic release and TS-EPSC amplitude, which includes a NMDA receptor component. Furthermore, the decreased excitability and hyperpolarized membrane after HU are associated with enhanced GABAergic modulation. This functional neuroplasticity in the nTS may underly the CVD induced by HU.

Loss of excitatory amino acid transporter restraint following chronic intermittent hypoxia contributes to synaptic alterations in nucleus tractus solitarii

Peripheral viscerosensory afferent signals are transmitted to the nucleus tractus solitarii (nTS) via release of glutamate. Following release, glutamate is removed from the extrasynaptic and synaptic cleft via excitatory amino acid transporters (EAATs), thus limiting glutamate receptor activation or over activation, and maintaining its working range. We have shown that EAAT block with the antagonist threo-β-benzyloxyaspartic acid (TBOA) depolarized nTS neurons and increased spontaneous excitatory postsynaptic current (sEPSC) frequency yet reduced the amplitude of afferent (TS)-evoked EPSCs (TS-EPSCs). Interestingly, chronic intermittent hypoxia (CIH), a model of obstructive sleep apnea (OSA), produces similar synaptic responses as EAAT block. We hypothesized EAAT expression or function are downregulated after CIH, and this reduction in glutamate removal contributes to the observed neurophysiological responses. To test this hypothesis, we used brain slice electrophysiology and imaging of glutamate release and TS-afferent Ca²⁺ to compare nTS properties of rats exposed to 10 days of normoxia (Norm; 21%O₂) or CIH. Results show that EAAT blockade with (3S)-3-[[3-[[4-(trifluoromethyl)benzoyl]-amino]phenyl]methoxy]-l-aspartic acid (TFB-TBOA) in Norm caused neuronal depolarization, generation of an inward current, and increased spontaneous synaptic activity. The latter augmentation was eliminated by inclusion of tetrodotoxin in the perfusate. TS stimulation during TFB-TBOA also elevated extracellular glutamate and decreased presynaptic Ca²⁺ and TS-EPSC amplitude. In CIH, the effects of EAAT block are eliminated or attenuated. CIH reduced EAAT expression in nTS, which may contribute to the attenuated function seen in this condition. Therefore, CIH reduces EAAT influence on synaptic and neuronal activity, which may lead to the physiological consequences seen in OSA and CIH.NEW & NOTEWORTHY Removal of excitatory amino acid transporter (EAAT) restraint increases spontaneous synaptic activity yet decreases afferent [tractus solitarius (TS)]-driven excitatory postsynaptic current (EPSC) amplitude. In the chronic intermittent hypoxia model of obstructive sleep apnea, this restraint is lost due to reduction in EAAT expression and function. Thus EAATs are important in controlling elevated glutamatergic signaling, and loss of such control results in maladaptive synaptic signaling.