Chao Yin Chen

Synaptic transmission in nucleus tractus solitarius is depressed by Group II and III but not Group I presynaptic metabotropic glutamate receptors in rats.

Presynaptic metabotropic glutamate receptors (mGluRs) serve as autoreceptors throughout the CNS to inhibit glutamate release and depress glutamatergic transmission. Both presynaptic and postsynaptic mGluRs have been implicated in shaping autonomic signal transmission in the nucleus tractus solitarius (NTS). We sought to test the hypothesis that activation of presynaptic mGluRs depresses neurotransmission between primary autonomic afferent fibres and second‐order NTS neurones. In second‐order NTS neurones, excitatory postsynaptic currents (EPSCs) synaptically evoked by stimulation of primary sensory afferent fibres in the tractus solitarius (ts) and currents postsynaptically evoked by α‐amino‐3‐hydroxy‐4‐isoxazoleproprionic acid (AMPA) were studied in the presence and absence of mGluR agonists and antagonists. Real‐time quantitative RT‐PCR (reverse transcription‐polymerase chain reaction) was used to determine whether the genes for the mGluR subtypes were expressed in the cell bodies of the primary autonomic afferent fibres. Agonist activation of Group II and III but not Group I mGluRs reduced the peak amplitude of synaptically (ts) evoked EPSCs in a concentration‐dependent manner while having no effect on postsynaptically (AMPA) evoked currents recorded in the same neurones. At the highest concentrations, the Group II agonist, (2S,3S,4S)‐CCG/(2S,1′S,2′S)‐2‐carboxycyclopropyl (l‐CCG‐I), decreased the amplitude of the ts‐evoked EPSCs by 39 % with an EC50 of 21 μm, and the Group III agonist, l(+)‐2‐amino‐4‐phosphonobutyric acid (l‐AP4), decreased the evoked EPSCs by 71 % with an EC50 of 1 mm. mRNA for all eight mGluR subtypes was detected in the autonomic afferent fibre cell bodies in the nodose and jugular ganglia. Group II and III antagonists ((2S,3S,4S)‐2‐methyl‐2‐(carboxycyclopropyl)glycine (MCCG) and (RS)‐α‐methylserine‐O‐phosphate (MSOP)), at concentrations that blocked the respective agonist‐induced synaptic depression, attenuated the frequency‐dependent synaptic depression associated with increasing frequencies of ts stimulation by 13–34 % and 13–19 %, respectively (P < 0.05, for each). We conclude that Group II and III mGluRs (synthesized in the cell bodies of the primary autonomic afferent fibres and transported to the central terminals in the NTS) contribute to the depression of autonomic signal transmission by attenuating presynaptic release of glutamate.

Postexercise Hypotension: Central Mechanisms

A single bout of exercise can lead to a postexercise decrease in blood pressure in hypertensive individuals, called postexercise hypotension. Compelling evidence suggests that the central baroreflex pathway plays a crucial role in the development of postexercise hypotension. This review focuses on the exercise-induced changes in brainstem nuclei involved in blood pressure regulation.

A presynaptic mechanism contributes to depression of autonomic signal transmission in NTS

With increasing frequencies of autonomic afferent input to the nucleus tractus solitarii (NTS), postsynaptic responses are depressed. To test the hypothesis that a presynaptic mechanism contributes to this frequency-dependent depression, we used whole cell, voltage-clamp recordings in an NTS slice. First, we determined whether solitary tract stimulation (0.4-24 Hz) resulted in frequency-dependent depression of excitatory postsynaptic currents (EPSCs) in second-order neurons. Second, because decreases in presynaptic glutamate release result in a parallel depression of α-amino-3-hydroxy-5-methylisoxazole-4-propionic acid (AMPA) and N-methyl-d-aspartic acid (NMDA) receptor-mediated components of EPSCs, we determined whether the magnitude, time course, and recovery from the depression were the same in both EPSC components. Third, to determine whether AMPA receptor desensitization contributed, we examined the depression during cyclothiazide. EPSCs decreased in a frequency-dependent manner by up to 76% in second- and 92% in higher-order neurons. AMPA and NMDA EPSC components were depressed with the same magnitude (by 83% and 83%) and time constant (113 and 103 ms). The time constant for the recovery was also not different (1.2 and 0.8 s). Cyclothiazide did not affect synaptic depression at ≥3 Hz. The data suggest that presynaptic mechanism(s) at the first NTS synapse mediate frequency-dependent synaptic depression.

Glutamate suppresses GABA release via presynaptic metabotropic glutamate receptors at baroreceptor neurones in rats

The nucleus tractus solitarii (NTS) is essential for coordinating arterial baroreflex control of blood pressure. The primary baroreceptor afferent fibres make their first excitatory synaptic contact at second‐order NTS neurones with glutamate as the major neurotransmitter. Glutamate regulates its own release by activating presynaptic metabotropic glutamate autoreceptors (mGluRs) on the baroreceptor central terminals to suppress its further release in frequency‐dependent manner. γ‐Aminobutyric acid (GABA) interneurones provide the major inhibitory synaptic input. It is the integration of excitatory and inhibitory inputs that shapes the NTS output of baroreceptor signals. We hypothesized that glutamate released from the primary central afferent terminals can spill over to presynaptic mGluRs on GABA interneurones to suppress GABA release at the second‐order baroreceptor neurones. We assessed GABA transmission in second‐order baroreceptor neurones identified by attached aortic depressor nerve (ADN) boutons. The medial NTS was stimulated to evoke GABA inhibitory postsynaptic currents (eIPSCs). Glutamate spillover, generated by brief 2 s, 25 Hz trains of stimuli applied to the tractus solitarius (TS), induced a small (10%) but significant reduction in the eIPSC amplitudes. The depression was enhanced to a 25% decrease by increasing glutamate in the cleft with a glutamate‐uptake inhibitor (M‐trans‐pyrrolidine‐2,4‐dicarboxylic acid, 1 μm), blocked by a Group II mGluR antagonist (LY341495, 200 nm) and mimicked by a Group II agonist ((2S,3S,4S)‐CCG/(2S,1′S,2′S)‐2‐carboxycyclopropyl; L‐CCG‐I). A presynaptic mGluR locus was established by the mGluR agonist‐mediated increase in the paired‐pulse ratio of two consecutive eIPSCs in conjunction with the decrease in the first eIPSC, and a decrease in the frequency (39–46% reduction at EC50 concentration), but not amplitude, of spontaneous and miniature GABA IPSCs. The data indicate that endogenous glutamate activation of Group II presynaptic mGluRs can decrease GABA release at the first central synapses, suggesting a heterosynaptic role for the Group II mGluRs in shaping baroreceptor signal transmission.

Plasticity of brainstem mechanisms of cough.

The cough reflex is a brainstem reflex, consisting of specific sensory afferent nerves which trigger the reflex, by transmitting the sensory input over vagal or laryngeal nerves to a brainstem circuitry which processes and ultimately transforms the sensory input into a complex motor output to generate cough. The first synaptic target for the primary cough-related sensory input is the second-order neurons in the nucleus tractus solitarius (NTS). This position in the reflex pathway and intricate local circuits within the nucleus make it a strategic site where the sensory information can be modified. Plasticity at this synapse will change the nature of the output--exaggerating it, suppressing it or transforming it into some other complex pattern. This review integrates evidence implicating the NTS in exaggerated cough with proof of the concept that NTS neurons undergo plasticity to contribute to an exaggeration of cough.

AKAP150 Contributes to Enhanced Vascular Tone by Facilitating Large-Conductance Ca2+-Activated K+ Channel Remodeling in Hyperglycemia and Diabetes Mellitus

Rationale: Increased contractility of arterial myocytes and enhanced vascular tone during hyperglycemia and diabetes mellitus may arise from impaired large-conductance Ca2+-activated K+ (BKCa) channel function. The scaffolding protein A-kinase anchoring protein 150 (AKAP150) is a key regulator of calcineurin (CaN), a phosphatase known to modulate the expression of the regulatory BKCa &bgr;1 subunit. Whether AKAP150 mediates BKCa channel suppression during hyperglycemia and diabetes mellitus is unknown. Objective: To test the hypothesis that AKAP150-dependent CaN signaling mediates BKCa &bgr;1 downregulation and impaired vascular BKCa channel function during hyperglycemia and diabetes mellitus. Methods and Results: We found that AKAP150 is an important determinant of BKCa channel remodeling, CaN/nuclear factor of activated T-cells c3 (NFATc3) activation, and resistance artery constriction in hyperglycemic animals on high-fat diet. Genetic ablation of AKAP150 protected against these alterations, including augmented vasoconstriction. D-glucose–dependent suppression of BKCa channel &bgr;1 subunits required Ca2+ influx via voltage-gated L-type Ca2+ channels and mobilization of a CaN/NFATc3 signaling pathway. Remarkably, high-fat diet mice expressing a mutant AKAP150 unable to anchor CaN resisted activation of NFATc3 and downregulation of BKCa &bgr;1 subunits and attenuated high-fat diet–induced elevation in arterial blood pressure. Conclusions: Our results support a model whereby subcellular anchoring of CaN by AKAP150 is a key molecular determinant of vascular BKCa channel remodeling, which contributes to vasoconstriction during diabetes mellitus.

Exercise Reduces GABA Synaptic Input onto Nucleus Tractus Solitarii Baroreceptor Second-Order Neurons via NK1 Receptor Internalization in Spontaneously Hypertensive Rats

A single bout of mild to moderate exercise can lead to a postexercise decrease in blood pressure in hypertensive subjects, namely postexercise hypotension (PEH). The full expression of PEH requires a functioning baroreflex, hypertension, and activation of muscle afferents (exercise), suggesting that interactions in the neural networks regulating exercise and blood pressure result in this fall in blood pressure. The nucleus tractus solitarii (NTS) is the first brain site that receives inputs from nerves carrying blood pressure and muscle activity information, making it an ideal site for integrating cardiovascular responses to exercise. During exercise, muscle afferents excite NTS GABA neurons via substance P and microinjection of a substance P–neurokinin 1 receptor (NK1-R) antagonist into the NTS attenuates PEH. The data suggest that an interaction between the substance P NK1-R and GABAergic transmission in the NTS may contribute to PEH. We performed voltage clamping on NTS baroreceptor second-order neurons in spontaneously hypertensive rats (SHRs). All animals were killed within 30 min and the patch-clamp recordings were performed 2–8 h after the sham/exercise protocol. The data showed that a single bout of exercise reduces (1) the frequency but not the amplitude of GABA spontaneous IPSCs (sIPSCs), (2) endogenous substance P influence on sIPSC frequency, and (3) sIPSC frequency response to exogenous application of substance P. Furthermore, immunofluorescence labeling in NTS show an increased substance P NK1-R internalization on GABA neurons. The data suggest that exercise-induced NK1-R internalization results in a reduced intrinsic inhibitory input to the neurons in the baroreflex pathway.

Short-term secondhand smoke exposure decreases heart rate variability and increases arrhythmia susceptibility in mice

Exposure to secondhand smoke (SHS), a major indoor air pollutant, is linked to increased cardiovascular morbidity and mortality, including cardiac arrhythmias. However, the mechanisms underlying the epidemiological findings are not well understood. Impaired cardiac autonomic function, indexed by reduced heart rate variability (HRV), may represent an underlying cause. The present study takes advantage of well-defined short-term SHS exposure (3 days, 6 h/day) on HRV and the susceptibility to arrhythmia in mice. With the use of electrocardiograph telemetry recordings in conscious mice, HRV parameters in the time domain were measured during the night after each day of exposure and 24 h after 3 days of exposure to either SHS or filtered air. The susceptibility to arrhythmia was determined after 3 days of exposure. Exposure to a low concentration of SHS [total suspended particle (TSP), 2.4 +/- 3.2; and nicotine, 0.3 +/- 0.1 mg/m(3)] had no significant effect on HRV parameters. In contrast, the exposure to a higher but still environmentally relevant concentration of SHS (TSP, 30 +/- 1; and nicotine, 5 +/- 1 mg/m(3)) significantly reduced HRV starting after the first day of exposure and continuing 24 h after the last day of exposure. Moreover, the exposed mice showed a significant increase in ventricular arrhythmia susceptibility and atrioventricular block. The data suggest that SHS exposure decreased HRV beyond the exposure period and was associated with an increase in arrhythmia susceptibility. The data provide insights into possible mechanisms underlying documented increases in cardiovascular morbidity and mortality in humans exposed to SHS.

Glutamatergic Neural Transmission In The Nucleus Tractus Solitarius: N-Methyl-D-Aspartate Receptors

1. The nucleus tractus solitarius (NTS) is the first central site where the reflex control of autonomic, including baroreceptor, reflex function is coordinated. Autonomic signals are transmitted from the first‐order visceral afferent fibres to second‐order NTS neurons by L‐glutamate. It is well established that activation of the α‐amino‐3‐hydroxy‐5‐methyl‐4‐isoxazole proprionic acid (AMPA) receptors, which mediate the fast component of L‐glutamate signalling, is required for generating changes in membrane potentials of the second‐order NTS neurons. The contribution of the slower‐developing, longer‐lasting N‐methyl‐D‐aspartate (NMDA) receptor‐mediated component of glutamate signalling to synaptic transmission at these synapses is less well understood.

Non-NMDA and NMDA receptors transmit area postrema input to aortic baroreceptor neurons in NTS

We sought to determine whether glutamate acting at both N-methyl-D-aspartate (NMDA) and non-NMDA receptors transmits area postrema (AP) excitatory inputs to nucleus tractus solitarii (NTS) neurons in the aortic baroreceptor or vagal afferent pathways in vivo. In alpha-chloralose-anesthetized rabbits, we recorded extracellular NTS neuronal responses to low-frequency aortic depressor nerve (ADN), vagus nerve, and AP stimulation and to iontophoresis of alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid and NMDA during control, iontophoresis of 2, 3-dihdroxy-6-nitro-7-sulfamoylbenzo(f)quinoxaline (NBQX), DL-2-amino-5-phosphonovaleric acid (AP5), or both, and recovery conditions. In neurons receiving AP and ADN inputs, NBQX attenuated AP- and ADN-evoked responses by 46 (P = 0.0206) and 49% (P = 0.0042). AP5 attenuated AP- and ADN-evoked responses by 39 (P = 0.0270) and 40% (P = 0.0157). NBQX + AP5 attenuated AP- and ADN-evoked responses by 74 (P = 0.0040) and 75% (P = 0.0028). In neurons receiving AP and vagal inputs, AP transmission was attenuated by 58, 60, and 98%; vagal transmission was attenuated by 62, 35, and 83% during NBQX, AP5, and both antagonists, respectively. These data suggest that both non-NMDA and NMDA receptors transmit AP input to NTS neurons in aortic baroreceptor or vagal afferent pathways.