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Joad JP, Sekizawa SI, Chen CY, Bonham AC. Air pollutants and cough. Pulm Pharmacol Ther 2007; 20:347-54. [PMID: 17174132 DOI: 10.1016/j.pupt.2006.10.013] [Citation(s) in RCA: 33] [Impact Index Per Article: 1.9] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Received: 10/05/2006] [Accepted: 10/20/2006] [Indexed: 11/22/2022]
Abstract
Epidemiological studies have shown that exposure to air pollution is associated with respiratory symptoms and decreases in lung function. This paper reviews recent literature showing that exposure to particulate matter, irritant gases, environmental tobacco smoke (ETS), mixed pollutants, and molds is associated with an increase in cough and wheeze. Some pollutants, like particulate matter and mixed pollutants, appear to increase cough at least as much as wheeze. Others, like irritant gases, appear to increase wheeze more than cough. For ETS, exposure during childhood is associated with cough and wheeze in adulthood, suggesting that the pollutant permanently alters some important aspect of the lungs, immune system or nervous system. We have shown in animal studies that pollutants change the neural control of airways and cough. Second hand smoke (SHS) exposure lengthened stimulated apnoea, increased the number of stimulated coughs, and augmented the degree of stimulated bronchoconstriction. The mechanisms included enhanced reactivity of the peripheral sensory neurones and second-order neurones in the nucleus tractus solitarius (NTS). NTS effects were due to a substance P mechanism at least in part. Ozone and allergen increased the intrinsic excitability of second-order neurones in the NTS. The animal studies suggest that the cough and wheeze experienced by humans exposed to pollutants may involve plasticity in the nervous system.
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Chen C, Chow D, Pinkerton KE, Bonham AC. Biphasic heart rate variability response to short‐term second‐hand smoke exposure in mice. FASEB J 2007. [DOI: 10.1096/fasebj.21.5.a474] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/11/2022]
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Chen C, Pinkerton KE, Bonham AC. Time‐dependent decrease in heart rate variability to particulate matter exposure in mice. FASEB J 2007. [DOI: 10.1096/fasebj.21.5.a473-d] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/11/2022]
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Chen C, Bechtold AG, Bonham AC. Postexercise hypotension is associated with a reduced NK1 receptor mechanism on GABAergic inter neurons in nucleus tractus solitarii (NTS) in spontaneously hypertensive rats (SHRs). FASEB J 2007. [DOI: 10.1096/fasebj.21.5.a463-d] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/11/2022]
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Bechtold AG, Zhang Y, Accorsi‐Mendonca D, Chen C, Bonham AC. Exercise decreases the spiking response to excitation of a subpopulation of 2nd‐order aortic baroreceptor neurons in the nucleus tractus solitarii (NTS) of spontaneously hypertensive rats (SHRs). FASEB J 2007. [DOI: 10.1096/fasebj.21.5.a463-c] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/11/2022]
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Sekizawa S, Bonham AC, Pinkerton KE, Joad JP. Developmental changes in two types of neurons in the nucleus tractus solitarius (NTS) of non‐human primates. FASEB J 2007. [DOI: 10.1096/fasebj.21.6.a1442-d] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/11/2022]
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Bonham AC, Sekizawa SI, Chen CY, Joad JP. Plasticity of brainstem mechanisms of cough. Respir Physiol Neurobiol 2006; 152:312-9. [PMID: 16554189 DOI: 10.1016/j.resp.2006.02.010] [Citation(s) in RCA: 47] [Impact Index Per Article: 2.6] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 12/11/2005] [Revised: 02/07/2006] [Accepted: 02/09/2006] [Indexed: 11/27/2022]
Abstract
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.
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Chen C, Bonham AC. NK1 receptor mechanism mediates exercise‐induced reduction in inhibitory inputs to baroreceptor second‐order neurons in nucleus tractus solitarii (NTS) in spontaneously hypertensive rats (SHRs). FASEB J 2006. [DOI: 10.1096/fasebj.20.4.a361-b] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/11/2022]
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Bonham AC, Chen CY, Sekizawa SI, Joad JP. Plasticity in the nucleus tractus solitarius and its influence on lung and airway reflexes. J Appl Physiol (1985) 2006; 101:322-7. [PMID: 16484366 DOI: 10.1152/japplphysiol.00143.2006] [Citation(s) in RCA: 60] [Impact Index Per Article: 3.3] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/22/2022] Open
Abstract
The nucleus tractus solitarius (NTS) is the first central nervous system (CNS) site for synaptic contact of the primary afferent fibers from the lungs and airways. The signal processing at these synapses will determine the output of the sensory information from the lungs and airways to all downstream synapses in the reflex pathways. The second-order NTS neurons bring to bear their own intrinsic and synaptic properties to temporally and spatially integrate the sensory information with inputs from local networks, higher brain regions, and circulating mediators, to orchestrate a coherent reflex output. There is growing evidence that NTS neurons share the rich repertoire of forms of plasticity demonstrated throughout the CNS. This review focuses on existing evidence for plasticity in the NTS, potential targets for plasticity in the NTS, and the impact of this plasticity on lung and airway reflexes.
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Sekizawa SI, Bonham AC. Group I Metabotropic Glutamate Receptors on Second-Order Baroreceptor Neurons Are Tonically Activated and Induce a Na+–Ca2+Exchange Current. J Neurophysiol 2006; 95:882-92. [PMID: 16192328 DOI: 10.1152/jn.00772.2005] [Citation(s) in RCA: 25] [Impact Index Per Article: 1.4] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/17/2023] Open
Abstract
The nucleus tractus solitarius (NTS) is essential for coordinating baroreflex control of blood pressure. The baroreceptor sensory fibers make glutamatergic synapses onto second-order NTS neurons. Glutamate spillover activates Group II and III presynaptic metabotropic glutamate receptors (mGluRs) on the baroreceptor central terminals to inhibit synaptic transmission, but the role of postsynaptic mGluRs is less understood. We used whole cell patch-clamping in anatomically identified second-order baroreceptor neurons in a brain stem slice to test whether Group I, II, and III mGluRs had postsynaptic effects at this first central synapse in the baroreceptor afferent pathway. The Group I agonist DHPG induced a depolarization and spiking that was mimicked by endogenous glutamate. Group I mGluR blockade prevented the depolarization and slightly hyperpolarized the neurons, suggesting a small tonic Group I mGluR activation. The DHPG-induced inward current consisted of voltage-dependent and -independent components; the former was blocked by TEA and the latter was blocked by replacing extracellular NaCl with LiCl or Tris-HCl. The DHPG current was potentiated in a Ca2+-free external solution and was diminished by intracellular dialysis with BAPTA and by perfusion with Na+–Ca2+exchanger blockers, KB-R7943 or 3′,4′-dichlorobenzamil. Intracellular dialysis with GDPβS or heparin and perfusion with the PLC inhibitor U-73122 or the Ca2+-calmodulin inhibitor W-7 significantly decreased the DHPG current. The data suggest that Group I mGluRs on baroreceptor neurons are functional; are activated by endogenous glutamate; and activate a Na+–Ca2+exchanger through G-protein, PLC, IP3, and Ca2+-calmodulin mechanisms to excite the cell, thus providing postsynaptic mechanisms to enhance or prolong baroreceptor signal transmission.
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Bonham AC, Sekizawa SI, Joad JP. Plasticity of central mechanisms for cough. Pulm Pharmacol Ther 2005; 17:453-7; discussion 469-70. [PMID: 15564091 DOI: 10.1016/j.pupt.2004.09.008] [Citation(s) in RCA: 43] [Impact Index Per Article: 2.3] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Received: 09/08/2004] [Accepted: 09/13/2004] [Indexed: 11/19/2022]
Abstract
Cough is associated with plasticity of putative cough afferent fibres, but whether plasticity in the brainstem network contributes is less well understood. A key site in the CNS network is the nucleus tractus solitarius (NTS), the first synaptic contact of the primary afferent fibres. We sought to develop a conscious guinea pig model to detect enhanced cough, to focus on the NTS as a potential site for plasticity, and to test a role for substance P in the NTS since the neuropeptide has been implicated in plasticity of the vagal afferent fibres. Guinea pigs were exposed to second-hand tobacco smoke (SHS) or filtered air (FA) from 1-6 weeks of age. At 5 weeks, cannulae were implanted in the NTS. At 6 weeks, either vehicle or a neurokinin 1 (NK-1) receptor antagonist was injected into the NTS of the conscious guinea pigs who were then exposed to citric acid aerosol. SHS exposure significantly enhanced citric acid-induced cough (56%, P<0.05), an effect attenuated by NTS NK-1 receptor blockade (P<0.05). The findings suggest that one possible mechanism for plasticity in cough is related to substance P effects in the NTS. Future studies will be required to investigate the possible mechanisms underlying the role of substance P as well as other mechanisms in generating SHS-induced cough.
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Chen CY, Bonham AC. Glutamate suppresses GABA release via presynaptic metabotropic glutamate receptors at baroreceptor neurones in rats. J Physiol 2004; 562:535-51. [PMID: 15539399 PMCID: PMC1665512 DOI: 10.1113/jphysiol.2004.076885] [Citation(s) in RCA: 56] [Impact Index Per Article: 2.8] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/24/2023] Open
Abstract
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. Gamma-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 mum), 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 EC(50) 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.
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Sekizawa SI, Joad JP, Bonham AC. Substance P presynaptically depresses the transmission of sensory input to bronchopulmonary neurons in the guinea pig nucleus tractus solitarii. J Physiol 2004; 552:547-59. [PMID: 14561836 PMCID: PMC2343393 DOI: 10.1113/jphysiol.2003.051326] [Citation(s) in RCA: 55] [Impact Index Per Article: 2.8] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/08/2022] Open
Abstract
Substance P modulates the reflex regulation of respiratory function by its actions both peripherally and in the CNS, particularly in the nucleus tractus solitarii (NTS), the first central site for synaptic contact of the lung and airway afferent fibres. There is considerable evidence that the actions of substance P in the NTS augment respiratory reflex output, but the precise effects on synaptic transmission have not yet been determined. Therefore, we determined the effects of substance P on synaptic transmission at the first central synapses by using whole-cell voltage clamping in an NTS slice preparation. Studies were performed on second-order neurons in the slice anatomically identified as receiving monosynaptic input from sensory nerves in the lungs and airways. This was done by the fluorescent labelling of terminal boutons after 1,1'-dioctadecyl-3,3,3',3'-tetra-methylindocarbo-cyanine perchlorate (DiI) was applied via tracheal instillation. Substance P (1.0, 0.3 and 0.1 microM) significantly decreased the amplitude of excitatory postsynaptic currents (eEPSCs) evoked by stimulation of the tractus solitarius, in a concentration-dependent manner. The decrease was accompanied by an increase in the paired-pulse ratio of two consecutive eEPSCs, and a decrease in the frequency, but not the amplitude, of spontaneous EPSCs and miniature EPSCs, findings consistent with a presynaptic site of action. The effects were consistently and significantly attenuated by a neurokinin-1 (NK1) receptor antagonist (SR140333, 3 muM). The data suggest a new site of action for substance P in the NTS (NK1 receptors on the central terminals of sensory fibres) and a new mechanism (depression of synaptic transmission) for regulating respiratory reflex function.
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Joad JP, Munch PA, Bric JM, Evans SJ, Pinkerton KE, Chen CY, Bonham AC. Passive smoke effects on cough and airways in young guinea pigs: role of brainstem substance P. Am J Respir Crit Care Med 2003; 169:499-504. [PMID: 14644932 DOI: 10.1164/rccm.200308-1139oc] [Citation(s) in RCA: 73] [Impact Index Per Article: 3.5] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/16/2022] Open
Abstract
Children raised with extended exposure to environmental tobacco smoke (ETS) experience increased cough and wheeze. This study was designed to determine whether extended ETS exposure enhances citric acid-induced cough and bronchoconstriction in young guinea pigs via a neurokinin-1 (NK-1) receptor mechanism at the first central synapse of lung afferent neurons, the nucleus tractus solitarius. Guinea pigs were exposed to ETS from 1 to 6 weeks of age. At 5 weeks of age, guide cannulae were implanted bilaterally in the medial nucleus tractus solitarius at a site that produced apnea in response to the glutamate agonist D,L-homocysteic acid. At 6 weeks of age, either vehicle or a NK-1 receptor antagonist, SR 140333, was injected into the nucleus tractus solitarius of the conscious guinea pigs who were then exposed to citric acid aerosol. ETS exposure significantly enhanced citric acid-induced cough by 56% and maximal Penh (a measure of airway obstruction) by 43%, effects that were attenuated by the NK-1 receptor antagonist in the nucleus tractus solitarius. We conclude that in young guinea pigs extended exposure to ETS increases citric acid-induced cough and bronchoconstriction in part by an NK-1 receptor mechanism in the nucleus tractus solitarius.
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Chen CY, Bonham AC, Plopper CG, Joad JP. Neuroplasticity in nucleus tractus solitarius neurons after episodic ozone exposure in infant primates. J Appl Physiol (1985) 2003; 94:819-27. [PMID: 12433861 DOI: 10.1152/japplphysiol.00552.2002] [Citation(s) in RCA: 49] [Impact Index Per Article: 2.3] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/22/2022] Open
Abstract
Acute ozone exposure evokes adverse respiratory responses, particularly in children. With repeated ozone exposures, however, despite the persistent lung inflammation and increased sensory nerve excitability, the central nervous system reflex responses, i.e., rapid shallow breathing and decreased lung function, adapt, suggesting changes in central nervous system signaling. We determined whether repeated ozone exposures altered the behavior of nucleus tractus solitarius (NTS) neurons where reflex respiratory motor outputs are first coordinated. Whole cell recordings were performed on NTS neurons in brain stem slices from infant monkeys exposed to filtered air or ozone (0.5 ppm, 8 h/day for 5 days every 14 days for 11 episodes). Although episodic ozone exposure depolarized the membrane potential, increased the membrane resistance, and increased neuronal spiking responses to depolarizing current injections (P < 0.05), it decreased the excitability to vagal sensory fiber activation (P < 0.05), suggesting a diminished responsiveness to sensory transmission, despite overall increases in excitability. Substance P, implicated in lung and NTS signaling, contributed to the increased responsiveness to current injections but not to the diminished sensory transmission. The finding that NTS neurons undergo plasticity with repeated ozone exposures may help to explain the adaptation of the respiratory motor responses.
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Chen CY, Munch PA, Quail AW, Bonham AC. Postexercise hypotension in conscious SHR is attenuated by blockade of substance P receptors in NTS. Am J Physiol Heart Circ Physiol 2002; 283:H1856-62. [PMID: 12384463 DOI: 10.1152/ajpheart.00827.2001] [Citation(s) in RCA: 23] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Indexed: 11/22/2022]
Abstract
In hypertensive subjects, a single bout of dynamic exercise results in an immediate lowering of blood pressure back toward normal. This postexercise hypotension (PEH) also occurs in the spontaneously hypertensive rat (SHR). In both humans and SHRs, PEH features a decrease in sympathetic nerve discharge, suggesting the involvement of central nervous system pathways. Given that substance P is released in the nucleus tractus solitarius (NTS) by activation of baroreceptor and skeletal muscle afferent fibers during muscle contraction, we hypothesized that substance P acting at neurokinin-1 (NK-1) receptors in the NTS might contribute to PEH. We tested the hypothesis by determining, in conscious SHRs, whether NTS microinjections of the NK-1 receptor antagonist SR-140333 before exercise attenuated PEH. The antagonist, in a dose (60 pmol) that blocked substance P- and spared D,L-homocysteic acid-induced depressor responses, significantly attenuated the PEH by 37%, whereas it had no effect on blood pressure during exercise. Vehicle microinjection had no effect. The antagonist also had no effect on heart rate responses during both exercise and the PEH period. The data suggest that a substance P (NK-1) receptor mechanism in the NTS contributes to PEH.
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Kajekar R, Chen CY, Mutoh T, Bonham AC. GABA(A) receptor activation at medullary sympathetic neurons contributes to postexercise hypotension. Am J Physiol Heart Circ Physiol 2002; 282:H1615-24. [PMID: 11959623 DOI: 10.1152/ajpheart.00725.2001] [Citation(s) in RCA: 32] [Impact Index Per Article: 1.5] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Indexed: 11/22/2022]
Abstract
A single bout of exercise results in a postexercise hypotension (PEH) that is accompanied by a reduced baroreflex function. Based on the role of rostral ventrolateral medulla (RVLM) neurons in controlling sympathetic nerve activity (SNA) and blood pressure, the role of gamma-aminobutyric acid (GABA) in controlling RVLM neuronal activity, and the reduced baroreflex-SNA relationship during PEH, we determined whether: 1) RVLM neuronal activity is decreased during PEH, 2) GABA(A)-receptor mechanisms mediate the decrease, and 3) baroreflex control of RVLM activity is reduced. Spontaneously hypertensive rats (SHR) were subjected to 40 min of treadmill or sham exercise (Sham PEH). PEH lasted 10 h in conscious and anesthetized SHR, indicating that the anesthetics did not affect the expression of PEH. Extracellular RVLM neuronal activity having a cardiac and sympathetic rhythm, lumbar SNA, and blood pressure were recorded at rest and during baroreflex function curves. Resting RVLM neuronal activity was lower and was increased to a greater extent by GABA(A)-receptor antagonism in PEH versus Sham PEH (P < 0.05). Baroreflex control of RVLM neuronal activity operated with a reduced gain (P < 0.05). Thus increased GABA signaling at RVLM neurons may contribute to PEH.
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Bonham AC, Chen CY. Glutamatergic neural transmission in the nucleus tractus solitarius: N-methyl-D-aspartate receptors. Clin Exp Pharmacol Physiol 2002; 29:497-502. [PMID: 12010198 DOI: 10.1046/j.1440-1681.2002.03662.x] [Citation(s) in RCA: 28] [Impact Index Per Article: 1.3] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/20/2022]
Abstract
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 alpha-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. 2. The aim of this work is to highlight evidence that functional NMDA receptors exist on second-order NTS neurons in autonomic, including baroreceptor, afferent pathways by determining whether NMDA receptors can be activated by: (i) exogenous application of NMDA; and (ii) endogenous release of L-glutamate from autonomic afferent fibres. Studies were performed on second-order neurons in transverse and horizontal brainstem slices containing the intermediate NTS and the tractus solitarius. Second-order NTS neurons were identified by electrophysiological criteria or by attached fluorescent-labelled aortic depressor nerve (ADN) boutons. 3. N-Methyl-D-aspartate (50 nmol(-2) micromol) dose-dependently evoked excitatory post-synaptic currents in second-order NTS neurons (P = 0.004; n = 4). The NMDA receptor-mediated currents were also synaptically evoked by low-frequency stimulation of the autonomic afferent fibres in the tractus solitarius. The NMDA receptor-mediated currents were blocked by the NMDA receptor antagonist AP5 (n = 7; P = 0.027). 4. The findings suggest that functional NMDA receptors exist on second-order NTS neurons. While the NMDA receptor- mediated currents may not be required for signal transmission when the second-order neurons are at resting membrane potential, their activation may help to modulate autonomic signal transmission in the NTS under conditions in which the membrane is depolarized by high frequency or convergent inputs.
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Chen CY, Ling Eh EH, Horowitz JM, Bonham AC. Synaptic transmission in nucleus tractus solitarius is depressed by Group II and III but not Group I presynaptic metabotropic glutamate receptors in rats. J Physiol 2002; 538:773-86. [PMID: 11826164 PMCID: PMC2290096 DOI: 10.1113/jphysiol.2001.012948] [Citation(s) in RCA: 81] [Impact Index Per Article: 3.7] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/08/2022] Open
Abstract
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 alpha-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 microM, 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)-alpha-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.
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Chen CY, Bonham AC, Schelegle ES, Gershwin LJ, Plopper CG, Joad JP. Extended allergen exposure in asthmatic monkeys induces neuroplasticity in nucleus tractus solitarius. J Allergy Clin Immunol 2001; 108:557-62. [PMID: 11590381 DOI: 10.1067/mai.2001.118132] [Citation(s) in RCA: 67] [Impact Index Per Article: 2.9] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/18/2023]
Abstract
BACKGROUND Extended exposure to allergen exacerbates asthma symptoms, in part via complex interactions between inflammatory cells and mediators. One consequence of these interactions is the triggering of local and central nervous system (CNS) neuronal activity that might further exacerbate the asthma-like symptoms by causing bronchoconstriction, mucous secretion, increased microvascular leak, and cough. One CNS region that might be particularly important is the caudomedial nucleus tractus solitarius (NTS). NTS neurons not only integrate primary afferent inputs from lung sensory nerve fibers but also have direct exposure to inhaled allergens and allergen-induced blood-borne inflammatory mediators via a deficient blood-brain barrier. Given the capacity of CNS neurons to undergo plasticity, allergen-induced changes in NTS neuronal properties could contribute to the exaggerated respiratory responses to extended allergen exposure. OBJECTIVE In a recently developed rhesus monkey model of allergic asthma, we tested the hypothesis that extended exposure to allergen increases the intrinsic excitability of NTS neurons. METHODS Three adult monkeys were sensitized and then repeatedly exposed to aerosols of house dust mite allergen; 4 monkeys served as controls. Whole-cell current-clamp recordings were made to measure 3 indices of excitability: resting membrane potential, input resistance, and number of action potentials evoked by current injections. RESULTS Extended allergen exposure depolarized the resting membrane potential by 14% and increased the number of action potentials evoked by current injections (5-fold). CONCLUSION The finding that NTS neurons in a primate model of allergic asthma undergo intrinsic increases in excitability suggests that CNS mechanisms might contribute to the exaggerated symptoms in asthmatic individuals exposed to allergen.
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Bonham AC, Chen CY, Mutoh T, Joad JP. Lung C-fiber CNS reflex: role in the respiratory consequences of extended environmental tobacco smoke exposure in young guinea pigs. ENVIRONMENTAL HEALTH PERSPECTIVES 2001; 109 Suppl 4:573-578. [PMID: 11544166 PMCID: PMC1240584 DOI: 10.1289/ehp.01109s4573] [Citation(s) in RCA: 15] [Impact Index Per Article: 0.7] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 05/23/2023]
Abstract
Environmental tobacco smoke (ETS) exposure harms the respiratory health of children and is associated with an increased risk of asthma and sudden infant death syndrome (SIDS). The mechanisms by which ETS causes these effects are not understood. We hypothesized that one mechanism is an upregulation of the lung C-fiber central nervous system (CNS) reflex responses, which would result in exaggerated reflex responses of apnea, bronchoconstriction, and mucous hypersecretion. The purpose of this work is to highlight evidence obtained in an animal model of postnatal ETS exposure supporting the hypothesis and present data suggesting that actions of the neuropeptide substance P in the nucleus tractus solitarius (NTS) may contribute. Exposing young guinea pigs to sidestream smoke, the surrogate for ETS, for 5 weeks during the equivalent of human childhood, increased the excitability of afferent lung C fibers and NTS neurons in the CNS reflex pathway and prolonged the expiratory apnea. The findings suggest that an increased excitability of NTS neurons that can augment reflex output may contribute to respiratory symptoms in children exposed to ETS. Besides ETS exposure, substance P can also excite NTS neurons and augment lung C-fiber CNS reflex responses. Others have shown that substance P synthesis in lung C fibers is upregulated by another environmental stimulant, allergen. Thus, an upregulation of the substance P system at NTS synapses could contribute to the increased NTS excitability and enhanced reflex responses to lung C-fiber stimulation, providing a potential mechanism to help explain the association of ETS exposure with respiratory symptoms and SIDS.
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Mutoh T, Bonham AC, Joad JP. Substance P in the nucleus of the solitary tract augments bronchopulmonary C fiber reflex output. Am J Physiol Regul Integr Comp Physiol 2000; 279:R1215-23. [PMID: 11003986 DOI: 10.1152/ajpregu.2000.279.4.r1215] [Citation(s) in RCA: 67] [Impact Index Per Article: 2.8] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/22/2022]
Abstract
Bronchopulmonary C fibers defend the lungs against injury from inhaled agents by a central nervous system reflex consisting of apnea, cough, bronchoconstriction, hypotension, and bradycardia. Glutamate is the putative neurotransmitter at the first central synapses in the nucleus of the solitary tract (NTS), but substance P, also released in the NTS, may modulate the transmission. To test the hypothesis that substance P in the NTS augments bronchopulmonary C fiber input and hence reflex output, we stimulated the C fibers with left atrial capsaicin (LA CAP) injections and compared the changes in phrenic nerve discharge, tracheal pressure (TP), arterial blood pressure (ABP), and heart rate (HR) in guinea pigs before and after substance P injections (200 microM, 25 nl) in the NTS. Substance P significantly augmented LA CAP-evoked increases in expiratory time by 10-fold and increases in TP and decreases in ABP and HR by threefold, effects prevented by neurokinin-1 (NK1) receptor antagonism. Thus substance P acting at NTS NK1 receptors can exaggerate bronchopulmonary C fiber reflex output. Because substance P synthesis in vagal airway C fibers may be enhanced in pathological conditions such as allergic asthma, the findings may help explain some of the associated respiratory symptoms including cough and bronchoconstriction.
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Mutoh T, Joad JP, Bonham AC. Chronic passive cigarette smoke exposure augments bronchopulmonary C-fibre inputs to nucleus tractus solitarii neurones and reflex output in young guinea-pigs. J Physiol 2000; 523 Pt 1:223-33. [PMID: 10673557 PMCID: PMC2269785 DOI: 10.1111/j.1469-7793.2000.00223.x] [Citation(s) in RCA: 51] [Impact Index Per Article: 2.1] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/29/2022] Open
Abstract
1. Children chronically exposed to environmental tobacco smoke (passive cigarette smoke) have more wheeze, cough, bronchoconstriction, airway hyper-reactivity and mucous secretion, which may result, in part, from stimulation of the vagal bronchopulmonary C-fibre reflex. 2. Environmental tobacco smoke increases the sensitivity of bronchopulmonary C-fibre endings, but the physiological relevance of this sensitization is unknown. If this exposure augments the reflex responses via a central mechanism, then the responses of higher-order neurones in the reflex pathway and some components of the reflex output should also be augmented. 3. Guinea-pigs were chronically exposed to sidestream tobacco smoke (surrogate for environmental tobacco smoke) or filtered air for 5 days week-1 from age 1 to 6 weeks (age equivalent of human childhood) and were then anaesthetized, paralysed, ventilated and prepared with pneumothoraces. Baseline and left atrial capsaicin (0.5 and 2.0 microg kg-1)- evoked changes in the impulse activity of vagal C-fibre-activated neurones in nucleus tractus solitarii (NTS), phrenic nerve activity, tracheal pressure, arterial blood pressure and heart rate were compared in the two groups. 4. Sidestream smoke exposure significantly augmented the peak (P = 0.02) and duration (P = 0.01) of the NTS neuronal responses and the prolongation of expiratory time (P = 0.003) at the higher capsaicin dose. 5. Thus, the sensitization of the bronchopulmonary C-fibre endings by chronic exposure to sidestream tobacco smoke is transmitted to the NTS and is associated with a prolonged reflexively evoked expiratory apnoea. The findings may help to explain some related respiratory symptoms in children and be a factor in sudden infant death syndrome.
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Abstract
The frequency of baroreceptor volleys to the central nervous system can influence the fidelity of baroreceptor signal transmission and thus may affect baroreflex regulation of blood pressure. We examined 1) the extent to which frequency-dependent depression of aortic baroreceptor signals was initiated at the first central synapse between primary baroreceptor fibers and second-order nucleus tractus solitarii (NTS) neurons; 2) whether the pattern of baroreceptor input influenced the depression; and 3) the potential relevance to baroreflex sympathoinhibition. In urethan-anesthetized rats, NTS action potential responses of neurons classified as second or higher order and averaged lumbar sympathetic nerve activity responses were simultaneously measured during 100 aortic depressor nerve stimuli delivered in constant or phasic patterns (0.8-48 Hz). Frequency-dependent depression was initiated at second-order neurons, with NTS responses decreasing to a 72% response rate at 48 Hz; the depression was greater at higher-order neurons; responses decreased to a 30% response rate. The depression was slightly but significantly greater with phasic inputs. Curve fitting suggested that synaptic depression may limit baroreflex sympathoinhibition. Thus frequency limits on baroreceptor inputs at NTS synapses may affect baroreflex function.
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Chen CY, Horowitz JM, Bonham AC. A presynaptic mechanism contributes to depression of autonomic signal transmission in NTS. THE AMERICAN JOURNAL OF PHYSIOLOGY 1999; 277:H1350-60. [PMID: 10516169 DOI: 10.1152/ajpheart.1999.277.4.h1350] [Citation(s) in RCA: 40] [Impact Index Per Article: 1.6] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 11/22/2022]
Abstract
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 alpha-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.
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