Responses to GABA(A) receptor activation are altered in NTS neurons isolated from renal-wrap hypertensive rats

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 GABA_A 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.

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.

Targeting angiotensin type 2 receptors located on pressor neurons in the nucleus of the solitary tract to relieve hypertension in mice

AIMS

These studies evaluate whether angiotensin type-2 receptors (AT2Rs) that are expressed on γ-aminobutyric acid (GABA) neurons in the nucleus of the solitary tract (NTS) represent a novel endogenous blood pressure lowering mechanism.

METHODS AND RESULTS

Experiments combined advanced genetic and neuroanatomical techniques, pharmacology, electrophysiology and optogenetics in mice to define the structure and cardiovascular-related function of NTS neurons that contain AT2R. Using mice with Cre-recombinase directed to the AT2R gene, we discovered that optogenetic stimulation of AT2R-expressing neurons in the NTS increases GABA release and blood pressure. To evaluate the role of the receptor, per se, in cardiovascular regulation, we chronically delivered C21, a selective AT2R agonist, into the brains of normotensive mice and found that central AT2R activation reduces GABA-related gene expression and blunts the pressor responses induced by optogenetic excitation of NTS AT2R neurons. Next, using in situ hybridization, we found that the levels of Agtr2 mRNAs in GABAergic NTS neurons rise during experimentally-induced hypertension, and we hypothesized that this increased expression may be exploited to ameliorate the disease. Consistent with this, final experiments revealed that central administration of C21 attenuates hypertension, an effect that is abolished in mice lacking AT2R in GABAergic NTS neurons.

CONCLUSIONS

These studies unveil novel hindbrain circuits that maintain arterial blood pressure, and reveal a specific population of AT2R that can be engaged to alleviate hypertension. The implication is that these discrete receptors may serve as an access point for activating an endogenous depressor circuit.

TRANSLATIONAL PERSPECTIVE

Hypertension is a widespread health problem and risk factor for cardiovascular disease and stroke. Although treatment options exist, many patients suffer from resistant hypertension, which is associated with enhanced sympathetic drive. Thus, many available therapeutics focus on dampening pressor mechanisms. The present studies take the alternative approach of treating hypertension by exploiting an endogenous depressor mechanism.

H2O2 augments cytosolic calcium in nucleus tractus solitarii neurons via multiple voltage-gated calcium channels

Reactive oxygen species (ROS) play a profound role in cardiorespiratory function under normal physiological conditions and disease states. ROS can influence neuronal activity by altering various ion channels and transporters. Within the nucleus tractus solitarii (nTS), a vital brainstem area for cardiorespiratory control, hydrogen peroxide (H₂O₂) induces sustained hyperexcitability following an initial depression of neuronal activity. The mechanism(s) associated with the delayed hyperexcitability are unknown. Here we evaluate the effect(s) of H₂O₂ on cytosolic Ca²⁺ (via fura-2 imaging) and voltage-dependent calcium currents in dissociated rat nTS neurons. H₂O₂ perfusion (200 µM; 1 min) induced a delayed, slow, and moderate increase (~27%) in intracellular Ca²⁺ concentration ([Ca²⁺]_i). The H₂O₂-mediated increase in [Ca²⁺]_i prevailed during thapsigargin, excluding the endoplasmic reticulum as a Ca²⁺ source. The effect, however, was abolished by removal of extracellular Ca²⁺ or the addition of cadmium to the bath solution, suggesting voltage-gated Ca²⁺ channels (VGCCs) as targets for H₂O₂ modulation. Recording of the total voltage-dependent Ca²⁺ current confirmed H₂O₂ enhanced Ca²⁺ entry. Blocking VGCC L, N, and P/Q subtypes decreased the number of cells and their calcium currents that respond to H₂O₂ The number of responder cells to H₂O₂ also decreased in the presence of dithiothreitol, suggesting the actions of H₂O₂ were dependent on sulfhydryl oxidation. In summary, here, we have shown that H₂O₂ increases [Ca²⁺]_i and its Ca²⁺ currents, which is dependent on multiple VGCCs likely by oxidation of sulfhydryl groups. These processes presumably contribute to the previously observed delayed hyperexcitability of nTS neurons in in vitro brainstem slices.

Chronic hypertension enhances presynaptic inhibition by baclofen in the nucleus of the solitary tract

The selective gamma-aminobutyric acid B-subtype receptor agonist baclofen activates both presynaptic and postsynaptic receptors in the brain. Microinjection of baclofen into the nucleus of the solitary tract increases arterial pressure, heart rate, and sympathetic nerve discharge consistent with inhibition of the arterial baroreflex. The magnitude of these responses is enhanced in hypertension and is associated with increased postsynaptic GABA(B) receptor function. We tested whether a presynaptic mechanism contributes to the enhanced baclofen inhibition in hypertension. Whole-cell recordings of second-order baroreceptor neurons, identified by 4-(4-(dihexadecylamino)styryl)-N-methylpyridinium iodide labeling of aortic nerve, were obtained in brainstem slices from normotensive control and renal-wrap hypertensive rats. After 4 weeks, arterial blood pressure was 162+/-9 mm Hg in hypertensive (n=6) and 107+/-3 mm Hg in control rats (n=6/11; P<0.001). Baclofen reduced the amplitude of excitatory postsynaptic currents evoked by solitary tract stimulation and the EC(50) of this inhibition was greater in control (1.5+/-0.5 micromol/L; n=6) than in hypertensive cells (0.6+/-0.1 micromol/L; n=9; P<0.05). Baclofen (1 micromol/L) elicited greater inhibition on evoked response in hypertensive (58+/-6%; n=9) than in control cells (40+/-6%; n=8; P<0.05). Another index of presynaptic inhibition, the paired-pulse ratio (ratio of second to first evoked response amplitudes at stimulus intervals of 40 ms), was greater in hypertensive (0.60+/-0.08; n=8) than in control cells (0.48+/-0.06; n=5; P<0.05). The results suggest that in renal-wrap hypertensive rats, baclofen causes an enhanced presynaptic inhibition of glutamate release from baroreceptor afferent terminals to second-order neurons in the nucleus of the solitary tract. This enhanced presynaptic inhibition could contribute to altered baroreflex function in hypertension.

New insights into the electrophysiology of brainstem circuits controlling blood pressure

The brainstem contains the necessary circuitry for the maintenance and regulation of arterial blood pressure. It has become increasingly clear in the past few years that the characteristics of the neurons that constitute these circuits are not static, but can be altered in the face of chronic changes in physiological state. Alterations in voltage-gated and ligand-gated ion channels have been reported in neurons located within the nucleus of the solitary tract and the nucleus ambiguus in response to hypertension and exposures to hypoxia and environmental pollutants (eg, ozone and cigarette smoke). A discussion of these neuronal adaptations, the mechanisms that might initiate and sustain the adaptations, and their potential significance is the focus of this brief review.

Voltage-dependent calcium currents are enhanced in nucleus of the solitary tract neurons isolated from renal wrap hypertensive rats

The nucleus of the solitary tract (NTS) is the central site of termination of baroreceptor afferents. We hypothesize that changes occur in voltage-gated calcium channels (VGCCs) within NTS neurons as a consequence of hypertension. Whole-cell patch-clamp recordings were obtained from adult normotensive (109+/-2 mm Hg; n=6 from 6 sham-operated and 31 nonsurgically treated) and hypertensive (158+/-6 mm Hg; n=24) rats. In some experiments, 4-(4-[dihexadecylamino]styryl)-N-methylpyridinium iodide was applied to the aortic nerve to visualize NTS neurons receiving baroreceptor synaptic contacts. Ba(2+) currents (500 ms; -80 mV prepotential; 500 ms voltage steps in 5-mV increments to +15mV) peaked between -20 and -10 mV and were blocked by 100 mum of Cd(2+). Peak VGCCs were not different comparing non-4-(4-[dihexadecylamino]styryl)-N-methylpyridinium iodide-labeled and 4-(4- [dihexadecylamino]styryl)-N-methylpyridinium iodide-labeled NTS neurons in hypertensive and normotensive rats. The peak VGCC was significantly greater in cells from hypertensive compared with normotensive rats for both non-DiA-labeled (P=0.02) and DiA-labeled (P=0.04) neurons. To separate high-voltage activated (HVA) and low-voltage activated (LVA) components of VGCCs, voltage ramps (-110 mV to +30 mV over 50 ms) were applied from a holding potential of -60 mV (LVA channels inactivated) and a holding potential of -100 mV (both LVA and HVA currents activated). HVA currents were subtracted from HVA+LVA currents to yield the LVA current. Peak LVA currents were not different between hypertensive (8.9+/-0.8 pA/pF) and normotensive (7.8+/-0.6 pA/pF) groups of NTS neurons (P=0.27). These results demonstrate that 4 weeks of renal wrap hypertension induce an increase in Ca(2+) influx through HVA VGCCs in NTS neurons receiving arterial baroreceptor inputs.

Chronic intermittent hypoxia alters NMDA and AMPA-evoked currents in NTS neurons receiving carotid body chemoreceptor inputs

Chronic exposure to intermittent hypoxia (CIH) has been used in animals to mimic the arterial hypoxemia that accompanies sleep apnea. Humans with sleep apnea and animals exposed to CIH have elevated blood pressures and augmented sympathetic nervous system responses to acute exposures to hypoxia. To test the hypothesis that exposure to CIH alters neurons within the nucleus of the solitary tract (NTS) that integrate arterial chemoreceptor afferent inputs, we measured whole cell currents induced by activation of alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionate (AMPA) and N-methyl-D-aspartate (NMDA) receptors in enzymatically dispersed NTS neurons from normoxic (NORM) and CIH-exposed rats (alternating cycles of 3 min at 10% O2 followed by 3 min at 21% O2 between 8 AM and 4 PM for 7 days). To identify NTS neurons receiving carotid body afferent inputs the anterograde tracer 4- (4-(dihexadecylamino)styryl-N-methylpyridinum iodide (DiA) was placed onto the carotid body 1 wk before exposure to CIH. AMPA dose-response curves had similar EC50 but maximal responses increased in neurons isolated from DiA-labeled CIH (20.1 +/- 0.8 microM, n = 9) compared with NORM (6.0 +/- 0.3 microM, n = 8) rats. NMDA dose-response curves also had similar EC50 but maximal responses decreased in CIH (8.4 +/- 0.4 microM, n = 8) compared with NORM (19.4 +/- 0.6 microM, n = 9) rats. These results suggest reciprocal changes in the number and/or conductance characteristics of AMPA and NMDA receptors. Enhanced responses to AMPA receptor activation could contribute to enhanced chemoreflex responses observed in animals exposed to CIH and humans with sleep apnea.

Chronic hypertension enhances the postsynaptic effect of baclofen in the nucleus tractus solitarius

Microinjection of the inhibitory neurotransmitter gamma-aminobutyric acid B-subtype receptor agonist baclofen into the nucleus tractus solitarius increases arterial blood pressure and sympathetic nerve discharge. The baclofen-induced pressor response is enhanced in chronic hypertension. We hypothesized that a postsynaptic mechanism contributes to the enhanced responses to baclofen in hypertension. We investigated the postsynaptic effect of baclofen on second-order baroreceptor neurons, identified by 1,1'-dilinoleyl-3,3,3',3'-tetra-methylindocarbocyanine, 4-chlorobenzenesulphonate labeling of the aortic nerve, in nucleus tractus solitarius slices from sham-operated normotensive and unilateral nephrectomized, renal-wrap hypertensive rats. After 4 weeks, arterial blood pressure was 153+/-7 mm Hg in hypertensive rats (n=9) and 93+/-3 mm Hg in normotensive rats (n=8; P<0.05). There was no difference in resting membrane potential (54.5+/-0.7 versus 53.3+/-0.6 mV) or input resistance (1.07+/-0.11 versus 1.03+/-0.11 GOmega) between hypertensive and normotensive neurons (both n=18). Baclofen induced a net outward current in nucleus tractus solitarius neurons in the presence of 1 micromol/L tetrodotoxin. The EC50 of the baclofen effect was greater in normotensive cells (9.1+/-3.2 micromol/L; n=5) than hypertensive cells (3.0+/-0.5 micromol/L; n=7; P<0.05), and baclofen (10 micromol/L) induced a greater decrease in input resistance in hypertensive cells (61+/-2%; n=6) than in normotensive cells (45+/-4%; n=9; P<0.05). Both potassium and calcium channels were involved in the baclofen-evoked whole-cell current. The results suggest an enhanced postsynaptic response to activation of inhibitory neurotransmitter gamma-aminobutyric acid B-subtype receptors in second-order baroreceptor neurons in the nucleus tractus solitarius in renal-wrap hypertensive rats. This enhanced inhibition could alter baroreflex function in chronic hypertension.

Vesicular glutamate transporters and neuronal nitric oxide synthase colocalize in aortic depressor afferent neurons

The aortic depressor nerve (ADN) primarily transmits baroreceptor signals from the aortic arch to the nucleus tractus solitarii. Cell bodies of neurons that send peripheral fibers to form the ADN are located in the nodose ganglion (NG). Studies have implicated glutamate and nitric oxide in transmission of baroreflex signals; therefore, we tested the hypothesis that ADN neurons contain either vesicular glutamate transporters (VGLUTs) or neuronal nitric oxide synthase (nNOS) or both. We applied a fluorescent tracer, tetramethyl rhodamine dextran (TRD), to rat ADN to identify ADN neurons and then performed immunofluorescent labeling for nNOS and VGLUTs 1, 2, and 3 in NG sections. We found that VGLUT2-immunoreactivity (IR) and VGLUT3-IR was present in a significantly higher proportion of TRD positive neurons than in TRD negative neurons. In contrast, the percentage of TRD positive neurons containing VGLUT1-IR or nNOS-IR did not differ from that of TRD negative neurons. We also observed that the percentage of TRD positive neurons containing both VGLUT2-IR and nNOS-IR and the percentage of TRD positive neurons containing both VGLUT3-IR and nNOS-IR were significantly higher than that of TRD negative neurons. On the other hand, colocalization of VGLUT1-IR and nNOS-IR in TRD positive neurons did not differ from that of TRD negative neurons. These results support our hypothesis and suggest prominent roles of VGLUT2-IR containing neurons and VGLUT3-IR containing neurons in transmitting cardiovascular signals via the ADN to the brain stem.

Transient Voltage-Dependent Potassium Currents Are Reduced in NTS Neurons Isolated From Renal Wrap Hypertensive Rats

Whole cell patch-clamp measurements were made in neurons enzymatically dispersed from the nucleus of the solitary tract (NTS) to determine if alterations occur in voltage-dependent potassium channels from rats made hypertensive (HT) by unilateral nephrectomy/renal wrap for 4 wk. Some rats had the fluorescent tracer DiA applied to the aortic nerve before the experiment to identify NTS neurons receiving monosynaptic baroreceptor afferent inputs. Mean arterial pressure (MAP) was greater in 4-wk HT (165 ± 5 mmHg, n = 26, P < 0.001) rats compared with normotensive (NT) rats (109 ± 3 mmHg measured in 10 of 69 rats). Transient outward currents (TOCs) were observed in 67–82% of NTS neurons from NT and HT rats. At activation voltages from −10 to +10 mV, TOCs were significantly less in HT neurons compared with those observed in NT neurons ( P < 0.001). There were no differences in the voltage-dependent activation kinetics, the voltage dependence of steady-state inactivation, and the rise and decay time constants of the TOCs comparing neurons isolated from NT and HT rats. The 4-aminopyridine–sensitive component of the TOC was significantly less in neurons from HT compared with NT rats ( P < 0.001), whereas steady-state outward currents, whether or not sensitive to 4-aminopyridine or tetraethylammonium, were not different. Delayed excitation, studied under current clamp, was observed in 60–80% of NTS neurons from NT and HT rats and was not different comparing neurons from NT and HT rats. However, examination of the subset of NTS neurons exhibiting somatic DiA fluorescence revealed that DiA-labeled neurons from HT rats had a significantly shorter duration delayed excitation ( n = 8 cells, P = 0.022) than DiA-labeled neurons from NT rats ( n = 7 cells). Neurons with delayed excitation from HT rats had a significantly broader first action potential (AP) and a slower maximal downstroke velocity of repolarization compared with NT neurons with delayed excitation ( P = 0.016 and P = 0.014, respectively). The number of APs in the first 200 ms of a sustained depolarization was greater in HT than NT neurons ( P = 0.012). These results suggest that HT of 4-wk duration reduces TOCs in NTS neurons, and this contributes to reduced delayed excitation and increased AP responses to depolarizing inputs. Such changes could alter baroreflex function in hypertension.

Rapid substrate-induced charge movements of the GABA transporter GAT1

The GABA transporter GAT1 removes the neurotransmitter GABA from the synaptic cleft by coupling of GABA uptake to the co-transport of two sodium ions and one chloride ion. The aim of this work was to investigate the individual reaction steps of GAT1 after a GABA concentration jump. GAT1 was transiently expressed in HEK293 cells and its pre-steady-state kinetics were studied by combining the patch-clamp technique with the laser-pulse photolysis of caged GABA, which allowed us to generate GABA concentration jumps within <100 micros. Recordings of transport currents generated by GAT1, both in forward and exchange transport modes, showed multiple charge movements that can be separated along the time axis. The individual reactions associated with these charge movements differ from the well-characterized electrogenic "sodium-occlusion" reaction by GAT1. One of the observed electrogenic reactions is shown to be associated with the GABA-translocating half-cycle of the transporter, in contradiction to previous studies that showed no charge movements associated with these reactions. Interestingly, reactions of the GABA-bound transporter were not affected by the absence of extracellular chloride, suggesting that Cl- may not be co-translocated with GABA. Based on the results, a new alternating access sequential-binding model is proposed for GAT1's transport cycle that describes the results presented here and those by others.

Responses to GABA(A) receptor activation are altered in NTS neurons isolated from chronic hypoxic rats

The inhibitory amino acid GABA is released within the nucleus of the solitary tract (NTS) during hypoxia and modulates the respiratory response to hypoxia. To determine if responses of NTS neurons to activation of GABA(A) receptors are altered following exposure to chronic hypoxia, GABA(A) receptor-evoked whole cell currents were measured in enzymatically dispersed NTS neurons from normoxic and chronic hypoxic rats. Chronic hypoxic rats were exposed to 10% O(2) for 9-12 days. Membrane capacitance was the same in neurons from normoxic (6.9+/-0.5 pF, n=16) and hypoxic (6.3+/-0.5 pF, n=15) rats. The EC(50) for peak GABA-evoked current density was significantly greater in neurons from hypoxic (21.7+/-2.2 microM) compared to normoxic rats (12.2+/-0.9 microM) (p<0.001). Peak and 5-s adapted GABA currents evoked by 1, 3 and 10 microM were greater in neurons from normoxic compared to hypoxic rats (p<0.05) whereas peak and 5-s adapted responses to 30 and 100 microM GABA were not different comparing normoxic to hypoxic rats. Desensitization of GABA(A)-evoked currents was observed at concentrations greater than 3 microM and, measured as the ratio of the current 5 s after the onset of 100 microM GABA application to the peak GABA current, was the same in neurons from normoxic (0.37+/-0.03) and hypoxic rats (0.33+/-0.04). Reduced sensitivity to GABA(A) receptor-evoked inhibition in chronic hypoxia could influence chemoreceptor afferent integration by NTS neurons.