Nucleus Tractus Solitarius Neurons Research Articles

Acid-sensing ion channel 1 in nucleus tractus solitarii neurons contributes to the enhanced CO2-stimulated cardiorespiratory effect in spontaneously hypertensive rats

AimsActivation of central respiratory chemoreceptors provides excitatory drive to both respiratory and sympathetic outputs. The enhanced respiratory-sympathetic coupling contributes to the onset and development of hypertension. However, the specific central targets and molecular mechanisms involved in this process remain elusive. This study aimed to investigate the role of acid-sensing ion channel 1 (ASIC1) in nucleus tractus solitarii (NTS) neurons in CO2-stimulated cardiorespiratory effects in spontaneously hypertensive rats (SHRs). Main methodsRespiration and blood pressure of conscious rats were recorded by whole-body plethysmography and telemetry, respectively. Western blot was used to detect the expression difference of ASIC1 protein in NTS region between Wistar-Kyoto (WKY) rats and SHRs. Excitability of NTS neurons were assessed by extracellular recordings. Key findingsCompared to WKY rats, the enhanced CO2-stimulated cardiopulmonary effect and up-regulation of ASIC1 in the NTS were already observed in 4-week-old prehypertensive SHRs. Furthermore, specific blockade of ASIC1 effectively attenuated the CO2-stimulated increase in firing rate of NTS neurons in anesthetized adult SHRs. Intracerebroventricular injections of the ASIC1a blocker PcTx1 or knockdown Asic1 in NTS neurons significantly reduced the heightened CO2-stimulated ventilatory response, and diminished the CO2-stimulated increase in arterial pressure and heart rate in adult SHRs. SignificanceThese findings showed that dysregulated ASIC1 signaling in the NTS contribute to the exaggerated CO2-stimulated cardiorespiratory effects observed in SHRs.

Impaired excitability of nucleus tractus solitarius neurons in a neonatal rat model of hyperbilirubinemia

Hyperbilirubinemia is associated with multiple morbidities including apnea of prematurity in preterm infants. Bilirubin, a byproduct of hemoglobin breakdown, is not excreted normally and results in high circulating levels giving rise to a state of hyperbilirubinemia. Free bilirubin (not bound to albumin) can cross the blood-brain barrier and be neurotoxic. Prior studies have shown that a systemic injection of bilirubin accumulates in the brainstem and is associated with an impaired ventilatory response to hypercapnia and an augmented hypoxic ventilatory depression. Here, using the Gunn rat model of endogenous hyperbilirubinemia, we investigated its effects on the excitability of neurons of the nucleus tractus solitarius (NTS). The homozygous Gunn rat (jj) model of hyperbilirubinemia lacks uridine diphosphate-glucuronosyltransferase-1A1, an enzyme which is needed to catabolize bilirubin. Heterozygous Nj Gunn rat pups were used for control. On postnatal (P) day 5, Nj and jj rat pups received either a 200mg/kg intraperitoneal injection of sulfadimethoxine (SDMX), which dissociates bilirubin from albumin resulting in increased levels of circulating free bilirubin, or an equivalent amount of saline as a vehicle control. Whole cell patch clamp was performed on NTS neurons two days later (P7) to assess characteristics of evoked excitatory postsynaptic currents (eEPSC).The amplitude of the evoked stimulus response to a 20Hz stimulus train was significantly increased in NTS neurons from jj+saline pups when compared to cells from Nj+saline and Nj+SDMX animals (p<0.05). This increased amplitude was no longer present after SDMX treatment of jj pups. Additionally, jj+saline NTS neurons did not exhibit any failure to solitary tract stimulation, whereas SDMX treatment significantly increased the failure rate during a 20Hz stimulus train (jj+saline =0.0%±0.0, jj+SDMX= 18.63%±8.82, p<0.05). Comparison of failure rate of jj+saline to Nj+saline (6.63%±3.12, p<0.05) was also statistically significant. The hyper-responsive NTS neurons of jj+saline rats may be an adaptive response to the deleterious effects of circulating (bound and unbound) bilirubin. However, the impaired response of neurons in jj+SDMX rats may reflect neurotoxic effects of increased free bilirubin leading to impaired neurotransmission. The latter is consistent with the high failure rate of NTS neurons in jj+SDMX rats. These data may have implications for impaired respiratory control mechanisms involved in mediating essential vagally mediated ventilatory defense responses to hypoxemia and may contribute to the apnea of prematurity associated with hyperbilirubinemia in preterm infants. William and Lois Briggs Endowment. This is the full abstract presented at the American Physiology Summit 2024 meeting and is only available in HTML format. There are no additional versions or additional content available for this abstract. Physiology was not involved in the peer review process.

Control of breathing by orexinergic signaling in the nucleus tractus solitarii

Orexin signaling plays a facilitatory role in respiration. Abnormalities in orexin levels correlate with disordered breathing patterns and impaired central respiratory chemoreception. Nucleus tractus solitarii (NTS) neurons expressing the transcription factor Phox2b contribute to the chemoreceptive regulation of respiration. However, the extent to which orexinergic signaling modulates respiratory activity in these Phox2b-expressing NTS neurons remains unclear. In the present study, the injection of orexin A into the NTS significantly increased the firing rate of the phrenic nerve. Further analysis using fluorescence in situ hybridization and immunohistochemistry revealed that orexin 1 receptors (OX1Rs) were primarily located in the ventrolateral subdivision of the NTS and expressed in 25% of Phox2b-expressing neurons. Additionally, electrophysiological recordings showed that exposure to orexin A increased the spontaneous firing rate of Phox2b-expressing neurons. Immunostaining experiments with cFos revealed that the OX1R-residing Phox2b-expressing neurons were activated by an 8% CO2 stimulus. Crucially, OX1R knockdown in these NTS neurons notably blunted the ventilatory response to 8% CO2, alongside an increase in sigh-related apneas. In conclusion, orexinergic signaling in the NTS facilitates breathing through the activation of OX1Rs, which induces the depolarization of Phox2b-expressing neurons. OX1Rs are essential for the involvement of Phox2b-expressing NTS neurons in the hypercapnic ventilatory response.

Synaptic mechanisms underlying the elevated sympathetic outflow in fructose-induced hypertension.

Metabolic syndrome is associated with cardiovascular dysfunction, including elevated sympathetic outflow. However, the underlying brain mechanisms are unclear. The nucleus tractus solitarius (NTS) critically regulates autonomic reflexes related to cardiovascular function and contains neurons projecting to the caudal ventrolateral medulla (CVLM). Nitric oxide (NO) is a diffusible free-radical messenger in the vascular, immune, and nervous systems. In this study, we determine if NO in the NTS is involved in the synaptic plasticity underlying the elevated sympathetic outflow in fructose-induced hypertension. We retrogradely labeled CVLM-projecting NTS neurons through the injection of FluoSpheres into the CVLM in a fructose-fed rat model to determine the cellular mechanism involved in increased sympathetic outflow. Fructose feeding increased the blood pressure and glucose levels, which represent metabolic syndrome. We found that fructose feeding reduces the NO precursor L-arginine-induced increase in the firing activity of CVLM-projecting NTS neurons. Furthermore, fructose feeding reduces the L-arginine-induced increase in presynaptic spontaneous glutamatergic synaptic inputs to NTS neurons, while NO donor DEA/NO produces an increase in glutamatergic synaptic inputs in fructose-fed rats similar to that in vehicle-treated rats. In addition, fructose feeding reduces the NO-induced depressor response and sympathoinhibition. These data suggested that fructose feeding reduced NO production and, thus, the subsequent NO-induced glutamate releases in the NTS and depressor response. The findings of this study provide new insights into the central mechanisms involved in the neural control of cardiovascular and autonomic functions in the NTS in metabolic syndrome.

Vagus nerve stimulation inhibits cortical spreading depression via glutamate-dependent TrkB activation mechanism in the nucleus tractus solitarius.

Vagus nerve stimulation (VNS) was recently found to inhibit cortical spreading depression (CSD), the underlying mechanism of migraine aura, through activation of the nucleus tractus solitarius (NTS), locus coeruleus (LC) and dorsal raphe nucleus (DRN). The molecular mechanisms underlying the effect of VNS on CSD in these nuclei remain to be explored. We hypothesized that VNS may activate glutamate receptor-mediated tropomyosin kinase B (TrkB) signaling in the NTS, thereby facilitating the noradrenergic and serotonergic neurotransmission to inhibit CSD. To investigate the role of TrkB and glutamate receptors in non-invasive VNS efficacy on CSD, a validated KCl-evoked CSD rat model coupled with intra-NTS microinjection of selective antagonists, immunoblot and immunohistochemistry was employed. VNS increased TrkB phosphorylation in the NTS. Inhibition of intra-NTS TrkB abrogated the suppressive effect of VNS on CSD and CSD-induced cortical neuroinflammation. TrkB was found colocalized with glutamate receptors in NTS neurons. Inhibition of glutamate receptors in the NTS abrogated VNS-induced TrkB activation. Moreover, the blockade of TrkB in the NTS attenuated VNS-induced activation of the LC and DRN. VNS induces the activation of glutamate receptor-mediated TrkB signaling in the NTS, which might modulate serotonergic and norepinephrinergic innervation to the cerebral cortex to inhibit CSD and cortical inflammation.

Systemic Glucose Regulation by a Hindbrain Inhibitory Circuit in a Mouse Model of Type 1 Diabetes

Introduction: Previous work showed that increasing the electrical activity of inhibitory neurons in the dorsal vagal complex (DVC) is sufficient to increase whole-body glucose concentration in normoglycemic mice. Here we tested the hypothesis that deactivating GABAergic neurons in the dorsal hindbrain of hyperglycemic mice decreases synaptic inhibition of parasympathetic motor neurons in the dorsal motor nucleus of the vagus (DMV) and reduces systemic glucose levels. Methods: Chemogenetic activation or inactivation of GABAergic neurons in the nucleus tractus solitarius (NTS) was used to assess effects of modulating parasympathetic output on blood glucose concentration in normoglycemic and hyperglycemic mice. Patch-clamp electrophysiology in vitro was used to assess cellular effects of chemogenetic manipulation of NTS GABA neurons. Results: Chemogenetic activation of GABAergic NTS neurons in normoglycemic mice increased their action potential firing, resulting in increased inhibitory synaptic input to DMV motor neurons and elevated blood glucose concentration. Deactivation of GABAergic DVC neurons in normoglycemic mice altered their electrical activity but did not alter systemic glucose levels. Conversely, stimulation of GABAergic DVC neurons in mice that were hyperglycemic subsequent to treatment with streptozotocin changed their electrical activity but did not alter whole-body glucose concentration, while deactivation of this inhibitory circuit significantly decreased circulating glucose concentration. Peripheral administration of a brain impermeant muscarinic acetylcholine receptor antagonist abolished these effects. Conclusion: Disinhibiting vagal motor neurons decreases hyperglycemia in a mouse model of type 1 diabetes. This inhibitory brainstem circuit emerges as a key parasympathetic regulator of whole-body glucose homeostasis that undergoes functional plasticity in hyperglycemic conditions.

Enhancement of the Evoked Excitatory Transmission in the Nucleus Tractus Solitarius Neurons after Sustained Hypoxia in Mice Depends on A2A Receptors

The first synapses of the afferents of peripheral chemoreceptors are located in the Nucleus Tractus Solitarius (NTS) and there is evidence that short-term sustained hypoxia (SH – 24 h, FiO2 0.1) facilitates glutamatergic transmission in NTS neurons of rats. Adenosine is an important neuromodulator of synaptic transmission and hypoxia contributes to increase its extracellular concentration. The A2A receptors mediate the excitatory actions of adenosine and are active players in the modulation of neuronal networks in the NTS. Herein, we used knockout mice for A2A receptors (A2AKO) and electrophysiological recordings of NTS neurons were performed to evaluate the contribution of these receptors in the changes in synaptic transmission in NTS neurons of mice submitted to SH. The membrane passive properties and excitability of NTS neurons were not affected by SH and were similar between A2AKO and wild-type mice. The overall amplitude of spontaneous glutamatergic currents in NTS neurons of A2AKO mice was lower than in Balb/c WT mice. SH increased the amplitude of evoked glutamatergic currents of NTS neurons from WT mice by a non-presynaptic mechanism, but this enhancement was not observed in NTS neurons of A2AKO mice. Under normoxia, the amplitude of evoked glutamatergic currents was similar between WT and A2AKO mice. The data indicate that A2A receptors (a) modulate spontaneous glutamatergic currents, (b) do not modulate the evoked glutamatergic transmission in the NTS neurons under control conditions, and (c) are required for the enhancement of glutamatergic transmission observed in the NTS neurons of mice submitted to SH.

Inhibition of SOCS3 signaling in the nucleus tractus solitarii and retrotrapezoid nucleus alleviates hypoventilation in diet-induced obese male mice

The central leptin signaling system has been found to facilitate breathing and is linked to obesity-related hypoventilation. Activation of leptin signaling in the nucleus tractus solitarii (NTS) and retrotrapezoid nucleus (RTN) enhances respiratory drive. In this study, we investigated how medullary leptin signaling contributes to hypoventilation and whether respective deletion of SOCS3 in the NTS and RTN could mitigate hypoventilation in diet-induced obesity (DIO) male mice. Our findings revealed a decrease in the number of CO2-activated NTS neurons and downregulation of acid-sensing ion channels in DIO mice compared to lean control mice. Moreover, NTS leptin signaling was disrupted, as evidenced by the downregulation of phosphorylated STAT3 and the upregulation of SOCS3 in DIO mice. Importantly, deleting SOCS3 in the NTS and RTN significantly improved the diminished hypercapnic ventilatory response in DIO mice. In conclusion, our study suggests that disrupted medullary leptin signaling contributes to obesity-related hypoventilation, and inhibiting the upregulated SOCS3 in the NTS and RTN can alleviate this condition.

Inhibitory Neurons in Nucleus Tractus Solitarius Are Involved in Decrease of Heart Rate Variability and Development of Depression-Like Behaviors in Temporal Lobe Epilepsy.

Diminished heart rate variability (HRV) has been observed in epilepsy, especially in epilepsy with depressive disorders. However, the underlying mechanism remains elusive. We studied HRV, spontaneous recurrent seizures, and depression-like behaviors in different phases of pilocarpine-induced temporal lobe epilepsy (TLE) in mice. Single-cell RNA sequencing analysis was used to identify various nerve cell subsets in TLE mice with and without depression. Differentially expressed gene (DEG) analysis was performed in epilepsy, depression, and HRV central control-related brain areas. We found decreased HRV parameters in TLE mice, and alterations were positively correlated with the severity of depression-like behaviors. The severity of depression-like behaviors was correlated with the frequency of spontaneous recurrent seizure. Characteristic expression of mitochondria-related genes was significantly elevated in mice with depression in glial cells, and the enrichment analysis of those DEGs showed an enriched GABAergic synapse pathway in the HRV central control-related brain area. Furthermore, inhibitory neurons in the nucleus tractus solitarius, which is an HRV central control-related brain area, were specifically expressed in TLE mice combined with depression compared with those in mice without depression. A significantly enriched long-term depression pathway in DEGs from inhibitory neurons was found. Our study reported correlations between HRV and epilepsy-depression comorbidity in different phases of TLE. More importantly, we found that HRV central control-related inhibitory neurons are involved in the development of depression in TLE, providing new insights into epilepsy comorbid with depression.

Sex difference in the activation of central autonomic control regions in chronic intermittent hypoxia

Obstructive sleep apnea (OSA) results in sustained daytime hypertension. Chronic intermittent hypoxia (CIH) mimics the repetitive bouts of arterial hypoxemia associated with OSA. Male Sprague Dawley rats treated with CIH develop sustained hypertension and increased activation of central autonomic regions that regulate mean arterial pressure (MAP). However, gonadally intact female rats exposed to modest CIH treatment are not hypertensive. In male rats, lesions of the median preoptic nucleus (MnPO) prevent CIH-induced increase of MAP. We hypothesize that activation of MnPO and other autonomic control regions may contribute to the sex differences in the MAP response to CIH. To test this hypothesis, adult gonadally intact male and female rats (250-300 g bw) were exposed to either continuously normoxic (CON) or treated with CIH (10% O2 every 3 mins alternating with 21% O2 every 3 mins, 8 h/day) for 7 days. One week before the experiment started, some rats were instrumented with radiotelemetry transmitters to measure MAP and heart rate (HR). After one week of baseline recording, the rats were exposed to either normoxia or CIH and were euthanized (inactin 100 mg/kg ip) on the 8th day for immunohistochemistry. Forebrain and brainstem sections were stained for FosB/ΔFosB. Forebrain sections were also stained for neuronal nitric oxide synthase (NOS1) while brainstem sections were processed for dopamine-β-hydroxylase (DBH). The numbers of cells positive for NOS1 and FosB neurons in MnPO were counted and DBH and FosB positive neurons were counted in the hindbrain autonomic regions. CIH was associated with increased FosB staining in males but not females. Male exhibited an increase in the average number of FosB positive neurons (CON male 20 ± 2 cells/section, CIH male 35 ± 3; CON female 11 ± 1, CIH female 12 ± 2,) and colocalization of FosB and NOS1 (CON male 10 ± 1 cells/section, CIH male 18 ± 4; CON female 5 ± 1, CIH female 6 ± 1) in the MnPO. CIH females (n = 3) did not demonstrate increases in the numbers of FosB positive cells or DBH positive neurons in the commissural nucleus tractus solitarius (CON 7 ± 2, CIH 8 ± 2), rostral ventrolateral medulla (CON 2 ± 1, CIH 3 ± 1), caudal ventrolateral medulla (CON 5 ± 1, CIH 6 ± 2), and area postrema (CON 2 ± 1, CIH 2 ± 1) compared to CON females (n = 2). These preliminary results suggests that CIH is associated with increased FosB staining in the autonomic regions of male rats as opposed to female rats which is consistent with our working hypothesis. In addition, CIH was associated with increased FosB staining in NOS1 positive MnPO neurons suggesting that they may play a role in the sustained hypertension reported in male rats. The research is funded by NIH grant RO1 HL155977 This is the full abstract presented at the American Physiology Summit 2023 meeting and is only available in HTML format. There are no additional versions or additional content available for this abstract. Physiology was not involved in the peer review process.

High Fat Diet Suppresses Energy Expenditure Via Neurons in the Brainstem

Oxidation of fat by brown adipose tissue (BAT) contributes to energy balance and heat production. During cold exposure, BAT thermogenesis produces heat to warm the body. Obese subjects and rodents, however, show impaired BAT thermogenesis to the cold. Our previous studies suggest that vagal afferents synapsing in the nucleus tractus solitarius (NTS), tonically inhibit BAT thermogenesis to the cold in obese rats. NTS neurons send projections to the dorsal aspect of the lateral parabrachial nucleus (LPBd), which is a major integrative center that receives warm afferent inputs from the periphery and promotes inhibition of BAT thermogenesis. This study investigated the contribution of LPBd neurons in the impairment of BAT thermogenesis in rats fed a high-fat diet (HFD). By using a targeted dual viral vector approach, we found that chemogenetic activation of an NTS-LPB pathway inhibited BAT thermogenesis to the cold. We also found that the number of Fos-labelled neurons in the LPBd was higher in rats fed a HFD than in chow diet-fed rats after exposure to a cold ambient temperature. Nanoinjections of a GABAA receptor agonist into the LPBd area rescued BAT thermogenesis to the cold in HFD rats. These data reveal the LPBd as a critical brain area that tonically suppresses energy expenditure in obesity during skin cooling. These findings reveal novel effects of high-fat diets in the brain and in the control of metabolism and can contribute to the development of therapeutic approaches to regulate fat metabolism.

Circuit-Specific Control of Blood Pressure by PNMT-Expressing Nucleus Tractus Solitarii Neurons

The nucleus tractus solitarii (NTS) is one of the morphologically and functionally defined centers that engage in the autonomic regulation of cardiovascular activity. Phenotypically-characterized NTS neurons have been implicated in the differential regulation of blood pressure (BP). Here, we investigated whether phenylethanolamine N-methyltransferase (PNMT)-expressing NTS (NTSPNMT) neurons contribute to the control of BP. We demonstrate that photostimulation of NTSPNMT neurons has variable effects on BP. A depressor response was produced during optogenetic stimulation of NTSPNMT neurons projecting to the paraventricular nucleus of the hypothalamus, lateral parabrachial nucleus, and caudal ventrolateral medulla. Conversely, photostimulation of NTSPNMT neurons projecting to the rostral ventrolateral medulla produced a robust pressor response and bradycardia. In addition, genetic ablation of both NTSPNMT neurons and those projecting to the rostral ventrolateral medulla impaired the arterial baroreflex. Overall, we revealed the neuronal phenotype- and circuit-specific mechanisms underlying the contribution of NTSPNMT neurons to the regulation of BP.

Brainstem activation of GABAB receptors in the nucleus tractus solitarius increases gastric motility.

Background and aimLocal GABAergic signaling in the dorsal vagal complex (DVC) is essential to control gastric function. While the inhibitory GABAA receptor action on motility in the DVC is well-documented, the role of the GABAB receptor on gastric function is less well-established. Microinjection of baclofen, a selective GABAB receptor agonist, in the dorsal motor nucleus of the vagus (DMV) increases gastric tone and motility, while the effect on motility in the nucleus tractus solitarius (NTS) needs to be investigated. Previous in vitro studies showed that GABAB receptors exert a local inhibitory effect in unidentified NTS neurons. Since the NTS and DMV nuclei have differential control of gastric motility, we compared GABAB receptor activation in the NTS to that reported in the DMV. We microinjected baclofen unilaterally in the NTS while monitoring intragastric pressure and compared its action to optogenetic activation of somatostatin (SST) neurons in transgenic sst-Cre::channelrhodopsin-2 (ChR2) mice. We also performed patch-clamp recordings from SST and DMV neurons in brainstem slices from these mice.MethodsIn vivo drug injections and optogenetic stimulation were performed in fasted urethane/α-chloralose anesthetized male mice. Gastric tone and motility were monitored by an intragastric balloon inserted in the antrum and inflated with warm water to provide a baseline intragastric pressure (IGP). Coronal brainstem slices were obtained from the sst-Cre::ChR2 mice for interrogation with optogenetics and pharmacology using electrophysiology.ResultsThe unilateral microinjection of baclofen into the NTS caused a robust increase in gastric tone and motility that was not affected by ipsilateral vagotomy. Optogenetic activation of SST neurons that followed baclofen effectively suppresses the gastric motility in vivo. In brain slices, baclofen suppressed spontaneous and light-activated inhibitory postsynaptic currents in SST and gastrointestinal-projection DMV neurons and produced outward currents.ConclusionOur results show that GABAB receptors in the NTS strongly increase gastric tone and motility. Optogenetic stimulation in vivo and in vitro suggests that these receptors activated by baclofen suppress the glutamatergic sensory vagal afferents in the NTS and also inhibit the interneurons and the inhibitory neurons that project to the DMV, which, in turn, increase motility via a cholinergic excitatory pathway to the stomach.

Frequency-dependent depression of the NTS synapse affects the temporal response of the antihypertensive effect of auricular vagus nerve stimulation (aVNS)

Objective. Auricular vagus nerve stimulation (aVNS) has recently emerged as a promising neuromodulation modality for blood pressure (BP) reduction due to its ease of use although its efficacy is still limited compared to direct baroreflex stimulation. Previous studies have also indicated that synaptic depression of nucleus tractus solitarius (NTS) in the baroreflex pathway depends on stimulus frequency. However, the nature of this frequency dependence phenomenon on antihypertensive effect has been unknown for aVNS. We aimed to investigate the antihypertensive effect of aVNS considering frequency-dependent depression characteristic in the NTS synapse. We explored NTS activation and BP reduction induced by aVNS and by direct secondary neuron stimulation (DS). Approach. Both protocols were performed with recording of NTS activation and BP response with stimulation for each frequency parameter (2, 4, 20, 50, and 80 Hz). Main results. The BP recovery time constant was significantly dependent on the frequency of DS and aVNS (DS—2 Hz: 8.17 ± 4.98; 4 Hz: 9.73 ± 6.3; 20 Hz: 6.61 ± 3.28; 50 Hz: 4.93 ± 1.65; 80 Hz: 4.00 ± 1.43, p < 0.001, Kruskal–Wallis (KW) H-test/aVNS—2 Hz: 4.02 ± 2.55; 4 Hz: 8.13 ± 4.05; 20 Hz: 6.40 ± 3.16; 50 Hz: 5.18 ± 2.37; 80 Hz: 3.13 ± 1.29, p < 0.05, KW H-test) despite no significant BP reduction at 2 Hz compared to sham groups (p > 0.05, Mann–Whitney U-test). Significance. Our observations suggest that the antihypertensive effect of aVNS is influenced by the characteristics of frequency-dependent synaptic depression in the NTS neuron in terms of the BP recovery time. These findings suggest that the antihypertensive effect of aVNS can be improved with further understanding of the neurological properties of the baroreflex associated with aVNS, which is critical to push this new modality for clinical interpretation.

Nucleus Tractus Solitarius Neurons Activated by Hypercapnia and Hypoxia Lack Mu Opioid Receptor Expression.

Impaired chemoreflex responses are a central feature of opioid-induced respiratory depression, however, the mechanism through which mu opioid receptor agonists lead to diminished chemoreflexes is not fully understood. One brainstem structure involved in opioid-induced impairment of chemoreflexes is the nucleus of the solitary tract (NTS), which contains a population of neurons that express mu opioid receptors. Here, we tested whether caudal NTS neurons activated during the chemoreflex challenge express mu opioid receptors and overlap with neurons activated by opioids. Using genetic labeling of mu opioid receptor-expressing neurons and cFos immunohistochemistry as a proxy for neuronal activation, we examined the distribution of activated NTS neurons following hypercapnia, hypoxia, and morphine administration. The main finding was that hypoxia and hypercapnia primarily activated NTS neurons that did not express mu opioid receptors. Furthermore, concurrent administration of morphine with hypercapnia induced cFos expression in non-overlapping populations of neurons. Together these results suggest an indirect effect of opioids within the NTS, which could be mediated through mu opioid receptors on afferents and/or inhibitory interneurons.

Exercise Improves Spatial Learning and Memory Performance through the Central GLP-1 Receptors.

The glucagon-like peptide 1 (GLP-1) is a hormone which is produced in the enteroendocrine L-cells in the ileum and the neurons of nucleus tractus solitarius (NTS) in the brain which has numerous metabolic effects. The central GLP-1R's role in cognitive functioning is well known. On the contrary, it has been shown that exercise has positive effects on brain function. So, we decided to elucidate whether the central GLP-1 has a role in memory and learning. Thirty-two rats were used in this experiment in 4 groups. After anesthetizing the rats, the right lateral ventricle was detected, and a cannula was directed to the ventricle. Ten micrograms of exendin-3 or sterile saline, according to the group, was injected via ICV once daily for seven days. The rats in the exercise group considered an exercise period of one hour each day (17 meters per minute) for seven consecutive days. To evaluate the performance of memory and learning, a standard Morris water maze (MWM) tank was utilized. According to the results, the TE-exendin group showed a statistically significant difference from the TE-SAL group in both parameters of latency and time in the zone. In summary, memory and learning were improved by GLP-1R in the exercise group, but not in the sedentary group, which we can hypothesize that exercise can affect memory and learning through this pathway.

Cardiovascular deconditioning increases GABA signaling in the nucleus tractus solitarii.

The nucleus tractus solitarii (nTS) is the major integrative brainstem region for autonomic modulation and processing of cardiovascular reflexes. GABA and glutamate are the main inhibitory and excitatory neurotransmitters, respectively, within this nucleus. Alterations in the GABA-glutamate regulation in the nTS are related to numerous cardiovascular comorbidities. Bedridden individuals and people exposed to microgravity exhibit dysautonomia and cardiovascular deconditioning that are mimicked in the hindlimb unloading (HU) rat model. We have previously shown in the nTS that HU increases glutamatergic neurotransmission yet decreases neuronal excitability. In this study, we investigated the effects of HU on nTS GABAergic neurotransmission. We hypothesized that HU potentiates GABA signaling via increased GABAergic release and postsynaptic GABA receptor expression. Following HU or control postural exposure, GABAergic neurotransmission was assessed using whole cell patch clamp whereas the magnitude of GABA release was evaluated via an intensity-based GABA sensing fluorescence reporter (iGABASnFR). In response to GABA interneuron stimulation, the evoked inhibitory postsynaptic current (nTS-IPSC) amplitude and area, as well as iGABASnFR fluorescence, were greater in HU than in control. HU also elevated the frequency but not the amplitude of spontaneous miniature IPSCs. Picoapplication of GABA produced similar postsynaptic current responses in nTS neurons of HU and control. Moreover, HU did not alter GABAA receptor α1 subunit expression, indicating minimal alterations in postsynaptic membrane receptor expression. These results indicate that HU increases GABAergic signaling in the nTS likely via augmented release of GABA from presynaptic terminals. Altogether, our data indicate GABA plasticity contributes to the autonomic and cardiovascular alterations following cardiovascular deconditioning (CVD).NEW & NOTEWORTHY Gravity influences distribution of blood volume and autonomic function. Microgravity and prolonged bed rest induce cardiovascular deconditioning (CVD). We used hindlimb unloading (HU), a rat analog for bed rest, to investigate CVD-induced neuroplasticity in the brainstem. Our data demonstrate that HU increases GABA modulation of nucleus tractus solitarii (nTS) neurons via presynaptic plasticity. Given the importance of nTS in integrating cardiovascular reflexes, this study provides new evidence on the central mechanisms behind CVD following HU.

The carotid body detects circulating tumor necrosis factor-alpha to activate a sympathetic anti-inflammatory reflex

Recent evidence has suggested that the carotid bodies might act as immunological sensors, detecting pro-inflammatory mediators and signalling to the central nervous system, which, in turn, orchestrates autonomic responses. Here, we confirmed that the TNF-α receptor type I is expressed in the carotid bodies of rats. The systemic administration of TNF-α increased carotid body afferent discharge and activated glutamatergic neurons in the nucleus tractus solitarius (NTS) that project to the rostral ventrolateral medulla (RVLM), where many pre-sympathetic neurons reside. The activation of these neurons was accompanied by an increase in splanchnic sympathetic nerve activity. Carotid body ablation blunted the TNF-α-induced activation of RVLM-projecting NTS neurons and the increase in splanchnic sympathetic nerve activity. Finally, plasma and spleen levels of cytokines after TNF-α administration were higher in rats subjected to either carotid body ablation or splanchnic sympathetic denervation. Collectively, our findings indicate that the carotid body detects circulating TNF-α to activate a counteracting sympathetic anti-inflammatory mechanism.

Minocycline Activates the Nucleus of the Solitary Tract-Associated Network to Alleviate Lipopolysaccharide-Induced Neuroinflammation

ObjectiveTo examine the neuroanatomical substrates underlying the effects of minocycline in alleviating lipopolysaccharide (LPS)-induced neuroinflammation. MethodsForty C57BL/6 male mice were randomly and equally divided into eight groups. Over three consecutive days, saline was administered to four groups of mice and minocycline to the other four groups. Immediately after the administration of saline or minocycline on the third day, two groups of mice were additionally injected with saline and the other two groups were injected with LPS. Six or 24 hours after the last injection, mice were sacrificed and the brains were removed. Immunohistochemical staining across the whole brain was performed to detect microglia activation via Iba1 and neuronal activation via c-Fos. Morphology of microglia and the number of c-Fo-positive neurons were analyzed by Image-Pro Premier 3D. One-way ANOVA and Fisher's least-significant differences were employed for statistical analyses. ResultsMinocycline alleviated LPS-induced neuroinflammation as evidenced by reduced activation of microglia in multiple brain regions, including the shell part of the nucleus accumbens (Acbs), paraventricular nucleus (PVN) of the hypothalamus, central nucleus of the amygdala (CeA), locus coeruleus (LC), and nucleus tractus solitarius (NTS). Minocycline significantly increased the number of c-Fo-positive neurons in NTS and area postrema (AP) after LPS treatment. Furthermore, in NTS-associated brain areas, including LC, lateral parabrachial nucleus (LPB), periaqueductal gray (PAG), dorsal raphe nucleus (DR), amygdala, PVN, and bed nucleus of the stria terminali (BNST), minocycline also significantly increased the number of c-Fo-positive neurons after LPS administration. ConclusionMinocycline alleviates LPS-induced neuroinflammation in multiple brain regions, possibly due to increased activation of neurons in the NTS-associated network.

Obesity Combined with Chronic Restraint Stress-Induced Hypertension in Mice Is Associated with the Damage of Noradrenergic Neurons in Nucleus Tractus Solitarii

To investigate whether obesity combined with chronic restraint stress (CRS) can increase blood pressure in mice and its relationship with the damage of the intermediate part of the nucleus tractus solitarius (iNTS). The CRS mouse model was constructed, and 51 mice were assigned to four groups, low-fat diet non-restraint group (LF group), low-fat diet restraint group (LS group), high-fat diet non-restraint group (HF group), and high-fat diet restraint group (HS group). Interventions were carried out in four cycles (over the course of 40 consecutive days), with each cycle consisting of 7 days of restraint and 3 days of free movement. The body weight and the arterial systolic blood pressure of the mice were measured on the day 9 of every cycle. The mice were sacrificed on day 40 and the brain tissues of the mice were collected afterwards in order to perform immunohistochemical staining and Western blot to examine the expression of glial fibrillary acidic protein (GFAP) and tyrosine hydroxylase (TH). The protein expression of vascular endothelial growth factor A (VEGFA) was examined with Western blot on epididymal fat pad to assess the vascular density of lipid tissue. On day 40, the arterial systolic pressure of mice in HS group was significantly higher than that of mice in the three other groups. Body mass of high-fat diet group (HF group and HS group) increased significantly. Mice in the four groups did not present significant difference in VEGFA protein expression. INTS astrocytes were activated in the brain of mice in the restraint groups (LS group and HS group), and iNTS TH expression was decreased in HS group. Mice in HF group and LS group did not show abnormal changes in their blood pressure. Blood pressure of mice in the HS group generally rose, and hypertension (arterial systolic blood pressure ≥140 mmHg, 1 mmHg=0.133 kPa) was observed in 37.5% of the mice in this group. Obesity combined with CRS may cause an increase in arterial blood pressure in mice, the mechanism of which may be related to the damage of noradrenergic neurons in the nucleus tractus solitarius.