CO2-saturated Saline Research Articles

The Insignificant Improvement of Corrosion and Corrosion Fatigue Behavior in Geothermal Environment Applying Boehmit Coatings on High Alloyed Steels

The efficacy of alumina-sol based coatings in a water-free atmosphere at high temperatures suggests a potential solution for enhancing the corrosion resistance of high-alloyed steels in Carbon Capture and Storage (CCS) environments. In this study, coupons of X20Cr13, designed for use as injection pipes with 13% Chromium and 0.20% Carbon (1.4021, AISI 420), were sol-gel coated with water and ethanol-based alumina. These coated coupons were then exposed to CO2-saturated saline aquifer water, simulating conditions in the Northern German Basin, for 1000 h at ambient pressure and 60 °C. Corrosion fatigue experiments were also conducted using specimens of X5CrNiMoCuNb16-4 (1.4542, AISI 630), a suitable candidate for geothermal applications, to assess the impact of the ethanol-based coating on the number of cycles to failure at different stress amplitudes. Unfortunately, the coating exhibited early spallation, resulting in corrosion kinetics and corrosion fatigue data identical to those of uncoated specimens. Consequently, the initially promising Boehmit coating is deemed unsuitable for CCS applications and further research therefore not advisable.

Guar Gum as an Eco-Friendly Corrosion Inhibitor for N80 Carbon Steel under Sweet Environment in Saline Solution: Electrochemical, Surface, and Spectroscopic Studies.

In this study, the corrosion inhibition performance of the natural polysaccharide guar gum (GG) for N80 carbon steel in CO2-saturated saline solution at different temperatures and immersion times was investigated by weight loss and electrochemical measurements. The results have revealed that GG showed good inhibition performance at lower and higher temperatures. The inhibition efficiency observed via weight loss measurements reached 76.16 and 63.19% with 0.4 g L-1 of GG, at 25 and 50 °C, respectively. The inhibition efficiency of GG increased as the inhibitor concentration and immersion time increased but decreased with increasing temperature. EIS measurements have shown that, even after prolonged exposure, GG was still able to protect the metal surface. Potentiodynamic measurements showed the mixed-type nature of GG inhibitive action. The Temkin and Dubinin-Radushkevich adsorption isotherm models give accurate fitting of the estimated data, and the calculated parameters indicated that the adsorption of GG occurred mainly via an electrostatic or physical adsorption process. The associated activation energy (Ea) and the heat of adsorption (Qa) supported the physical adsorption nature of GG. FTIR analysis was used to explain the adsorption interaction between the inhibitor and the N80 carbon steel surface. SEM-EDS and AFM confirmed the adsorption of GG and the formation of an adsorptive layer of GG on the metal surface.

Fault sealing and caprock integrity for CO<sub>2</sub> storage: an in situ injection experiment

Abstract. The success of geological carbon storage depends on the assurance of permanent containment for injected carbon dioxide (CO2) in the storage formation at depth. One of the critical elements of the safekeeping of CO2 is the sealing capacity of the caprock overlying the storage formation despite faults and/or fractures, which may occur in it. In this work, we present an ongoing injection experiment performed in a fault hosted in clay at the Mont Terri underground rock laboratory (NW Switzerland). The experiment aims to improve our understanding of the main physical and chemical mechanisms controlling (i) the migration of CO2 through a fault damage zone, (ii) the interaction of the CO2 with the neighboring intact rock, and (iii) the impact of the injection on the transmissivity in the fault. To this end, we inject CO2-saturated saline water in the top of a 3 m thick fault in the Opalinus Clay, a clay formation that is a good analog of common caprock for CO2 storage at depth. The mobility of the CO2 within the fault is studied at the decameter scale by using a comprehensive monitoring system. Our experiment aims to close the knowledge gap between laboratory and reservoir scales. Therefore, an important aspect of the experiment is the decameter scale and the prolonged duration of observations over many months. We collect observations and data from a wide range of monitoring systems, such as a seismic network, pressure temperature and electrical conductivity sensors, fiber optics, extensometers, and an in situ mass spectrometer for dissolved gas monitoring. The observations are complemented by laboratory data on collected fluids and rock samples. Here we show the details of the experimental concept and installed instrumentation, as well as the first results of the preliminary characterization. An analysis of borehole logging allows for identifying potential hydraulic transmissive structures within the fault zone. A preliminary analysis of the injection tests helped estimate the transmissivity of such structures within the fault zone and the pressure required to mechanically open such features. The preliminary tests did not record any induced microseismic events. Active seismic tomography enabled sharp imaging the fault zone.

Microbial Signatures in Deep CO2-Saturated Miocene Sediments of the Active Hartoušov Mofette System (NW Czech Republic).

The Hartoušov mofette system is a natural CO2 degassing site in the central Cheb Basin (Eger Rift, Central Europe). In early 2016 a 108 m deep core was obtained from this system to investigate the impact of ascending mantle-derived CO2 on indigenous deep microbial communities and their surrounding life habitat. During drilling, a CO2 blow out occurred at a depth of 78.5 meter below surface (mbs) suggesting a CO2 reservoir associated with a deep low-permeable CO2-saturated saline aquifer at the transition from Early Miocene terrestrial to lacustrine sediments. Past microbial communities were investigated by hopanoids and glycerol dialkyl glycerol tetraethers (GDGTs) reflecting the environmental conditions during the time of deposition rather than showing a signal of the current deep biosphere. The composition and distribution of the deep microbial community potentially stimulated by the upward migration of CO2 starting during Mid Pleistocene time was investigated by intact polar lipids (IPLs), quantitative polymerase chain reaction (qPCR), and deoxyribonucleic acid (DNA) analysis. The deep biosphere is characterized by microorganisms that are linked to the distribution and migration of the ascending CO2-saturated groundwater and the availability of organic matter instead of being linked to single lithological units of the investigated rock profile. Our findings revealed high relative abundances of common soil and water bacteria, in particular the facultative, anaerobic and potential iron-oxidizing Acidovorax and other members of the family Comamonadaceae across the whole recovered core. The results also highlighted the frequent detection of the putative sulfate-oxidizing and CO2-fixating genus Sulfuricurvum at certain depths. A set of new IPLs are suggested to be indicative for microorganisms associated to CO2 accumulation in the mofette system.

INFLUENCE OF SURFACE QUALITY ON THE CORROSION AND CORROSION FATIGUE BEHAVIOR OF HIGH ALLOYED STEELS EXPOSED TO DIFFERENT SALINE AQUIFER WATER ENVIRONMENTS

Coupons of X5CrNiCuNb16-4 with different surface roughness that may be utilized as injection pipe with 16% Chromium and 0.05% Carbon (1.4542, AISI 630) were exposed for 3000 h to CO2-saturated saline aquifer water simulating the conditions in the Northern German Basin at ambient pressure and 60 °C. Additionally, corrosion fatigue experiments (ambient pressure, technically clean CO2, saline aquifer water of Stuttgart Aquifer) were performed using specimen of X46Cr13 (1.4043, AISI 420C) with regard to the influence of the roughness of technical surfaces on the number of cycles to failure at different stress amplitudes. Specimen of duplex stainless steel X2CrNiMoN22-3-2 (1.4462) for corrosion fatigue experiments were provided with technical surfaces after machining as well as polished surfaces. Results were obtained at load amplitudes ranging from 175 MPa to 325 MPa in the geothermal brine of the Northern German Basin at 98 °C. The main precipitation phases on the surface as well as within pits reveal carbonates or hydroxides such as siderite (FeCO3) and ferrous hydroxide goethite (FeOOH) independent of the original surface roughness. Corrosion rates for polished and technical surfaces were below 0.005 mm/year compared to corrosion rates of 0.035 mm/year after shot peening. Specimen with technical surfaces tested at high stress amplitudes (>275 MPa) lasted longer (cycles to failure: P50% at Sa 300 MPa=5x105) than specimen with polished surfaces (cycles to failure: P50% at Sa 300 MPa=1.5x105). This behavior is emphasized by the slope coefficient (technical surfaces k = 19.006, polished surfaces k=8.78) meaning earlier failure for polished at high stress amplitude Sa. Although rather low scatter ranges (technical surface: TN=1:1.35, polished surface: TN=1.1.95) indicate no change in failure mechanism it may be assumed that at low stress pitting is the initiating crack growth process whereas at high stress amplitudes the formation of micro cracks is reason for crack propagation and failure. Article DOI: https://dx.doi.org/10.20319/mijst.2019.51.115137 This work is licensed under the Creative Commons Attribution-Non-commercial 4.0 International License. To view a copy of this license, visit http://creativecommons.org/licenses/by-nc/4.0/ or send a letter to Creative Commons, PO Box 1866, Mountain View, CA 94042, USA.

The Role of Surface Texture on the Corrosion Behaviour of High Alloyed Steels Exposed to Different Saline Aquifer Water Environments

Coupons of X5CrNiCuNb16-4 that may be used as injection pipe with 16% Chromium and 0.05% Carbon (1.4542, AISI 630) were exposed for 3000 h to CO2-saturated saline aquifer water similar to the conditions in the Northern German Basin at ambient pressure and 60 °C. Surface corrosion layers and pits reveal carbonate corrosion products on the surface such as FeCO3 and FeOOH as the main precipitation phases with no dependence on the original surface roughness. Corrosion rates for polished and technical surfaces were below 0.005 mm/year compared to corrosion rates of 0.035 mm/year after shot peening.

Corrosion Behavior of High-Strength Steel for Flexible Riser Exposed to CO2-Saturated Saline Solution and CO2-Saturated Vapor Environments

The corrosion behavior of high-strength steel used for flexible riser exposed to CO2-saturated saline solution and CO2-saturated vapor environments was studied through immersion experiment and electrochemical corrosion experiment. The corrosion behavior and mechanism of the tested steel were analyzed on the basis of corrosion kinetics, nature of corrosion products, corrosion product morphology, elemental distribution and polarization curves. The experimental results showed that the microstructure of the tested steel was bainitic microstructure. The corrosive activity of the tested steel exposed to CO2-saturated vapor environment was significantly lower than that exposed to CO2-saturated saline solution environment. On prolonging the exposure time, the corrosion rate gradually decreased, the corrosion heterogeneity increased, and the dimensions of FeCO3 crystals gradually became small. At later stages of corrosion, the corrosion current density decreased significantly and the anodic Tafel slope increased, indicating that the corrosion process was strongly inhibited. The corrosion mechanism of low-alloy steel with bainitic microstructure was proposed based on experimental results.

Degradation of AISI 630 exposed to CO2-saturated saline aquifer at ambient pressure and 100 bar

In general high alloyed steels are suitable as pipe steels for carbon capture and storage technology (CCS), because they provide sufficient resistance against the corrosive environment of CO2-saturated saline aquifer which serves as potential CCS-site in Germany. High alloyed martensitic steel AISI 630 has been proven to be sufficient resistant in corrosive environments, e.g. regarding heat, pressure, salinity of the aquifer, CO2-partial pressure), but reveals a distinct corrosion pattern in CCS environment. Therefore coupons of AISI 630 heat treated using usual protocols were kept at T=60 °C and ambient pressure as well as p=100 bar up to 8000 h in an a) water saturated supercritical CO2 and b) CO2-saturated synthetic aquifer environment similar to on-shore CCS-sites in the Northern German Basin. AISI 630 precipitates a discontinuous ellipsoidal corrosion layer after being exposed for more than 4000 hours. Best corrosion resistance in the CO2-saturated synthetic aquifer environment phase is achieved via normalizing prior to exposure. In water saturated supercritical CO2 tempering at medium temperatures after hardening gives lowest corrosion rates. Corrosion fatigue via push-pull tests with a series of 30 specimens was evaluated at stress amplitudes between 150 MPa and 500 MPa (sinusoidal dynamic test loads, R=-1; resonant frequency ~ 30 Hz). The endurance limit of AISI 630 is reduced by more than 50% when exposed to CCS environment (maximum number of cycles (10 x 106) at a stress amplitude of 150 MPa).

Comparative Study of the Performance of SAE 1010, API K55 and API N80 Steels Regarding to Corrosion Resistance in the Presence of High CO<sub>2</sub> Pressure

The performance of SAE 1010, API K55 and API N80 steels were evaluated in terms of resistance to corrosion with the use of two CO2-saturated saline solutions (NaCl and CaCl2, both at the concentration of 0.5 M) and wet supercritical CO2 at a temperature of 90°C under 15 MPa for 07 days. Mass loss tests were performed to determine corrosion rates and the films of corrosion product evaluated by scanning electron microscopy and potentiodynamic polarization technique. Lower corrosion rates were obtained in a wet supercritical CO2 medium. The degree of protection offered by corrosion products films depended not only on the steel but also on the corrosive medium and kind of salt present.

Persistent activity and interactions in the brainstem respiratory network during hyperventilatory apnea

Brainstem respiratory neuron interactions during hyperventilatory apnea (HA) and the dissolution and reemergence of the motor pattern for breathing remain poorly understood. We used multi‐array recording, cross‐correlation, and gravity clustering methods to evaluate spike train data from the ventral respiratory column (VRC), pontine respiratory group, and medullary raphé nuclei of decerebrate vagotomized neuromuscularly‐blocked cats. Phrenic activity, blood pressure, end‐tidal CO2, and arterial blood gas measures of O2, CO2 and pH were monitored. During HA: (i) tonic activities persisted in subsets of VRC inspiratory (I) and expiratory (E) neurons; (ii) short‐time scale correlations indicated interactions among neurons that expressed E and non‐respiratory modulated discharge patterns during control conditions, including neurons with functional inhibitory actions on I neurons; (iii) selective stimulation of carotid chemoreceptors by close injection of CO2‐saturated saline transiently evoked rhythmic respiratory network activity and suppressed tonic E neuron activity; (iv) respiratory and slower (0.04 – 0.1 Hz) network oscillations were disrupted. We conclude that VRC tonic E neurons contribute to I drive inhibition during HA (cf. Sears et al. 1982) and document the loss of multiple rhythms during HA. Support: NIH grant NS19814

Central chemoreceptor modulation of breathing via multipath tuning in medullary ventrolateral respiratory column circuits

Ventrolateral respiratory column (VRC) circuits that modulate breathing in response to changes in central chemoreceptor drive are incompletely understood. We employed multielectrode arrays and spike train correlation methods to test predictions of the hypothesis that pre-Bötzinger complex (pre-BötC) and retrotrapezoid nucleus/parafacial (RTN-pF) circuits cooperate in chemoreceptor-evoked tuning of ventral respiratory group (VRG) inspiratory neurons. Central chemoreceptors were selectively stimulated by injections of CO(2)-saturated saline into the vertebral artery in seven decerebrate, vagotomized, neuromuscularly blocked, and artificially ventilated cats. Among sampled neurons in the Bötzinger complex (BötC)-to-VRG region, 70% (161 of 231) had a significant change in firing rate after chemoreceptor stimulation, as did 70% (101 of 144) of the RTN-pF neurons. Other responsive neurons (24 BötC-VRG; 11 RTN-pF) had a change in the depth of respiratory modulation without a significant change in average firing rate. Seventy BötC-VRG chemoresponsive neurons triggered 189 offset-feature correlograms (96 peaks; 93 troughs) with at least one responsive BötC-VRG cell. Functional input from at least one RTN-pF cell could be inferred for 45 BötC-VRG neurons (19%). Eleven RTN-pF cells were correlated with more than one BötC-VRG target neuron, providing evidence for divergent connectivity. Thirty-seven RTN-pF neurons, 24 of which were chemoresponsive, were correlated with at least one chemoresponsive BötC-VRG neuron. Correlation linkage maps and spike-triggered averages of phrenic nerve signals suggest transmission of chemoreceptor drive via a multipath network architecture: RTN-pF modulation of pre-BötC-VRG rostral-to-caudal excitatory inspiratory neuron chains is tuned by feedforward and recurrent inhibition from other inspiratory neurons and from "tonic" expiratory neurons.

Raphe neuronal circuits and responses to central and peripheral chemoreceptor stimulation

Current models propose that raphe neurons are elements of both central and peripheral chemoreceptor reflex circuits. Focal acidification of the brainstem midline raphe can stimulate breathing and cultured raphe neurons have chemosensitive properties. However, little is known about the responses of individual raphe neurons in vivo and the network that integrates chemoreceptor signals. Using multi-electrode arrays and spike train analysis, we measured the activities of 340 raphe neurons together with the phrenic nerve during sequential transient central and/or peripheral chemoreceptor stimulation via CO2-saturated saline injections in vertebral and carotid arteries, respectively, in 11 decerebrate vagotomized cats. A subset of 267 neurons tested with both stimuli responded to either central (n=44) or peripheral (n=42) chemoreceptor challenges, but not both. Of 66 cells that did respond to both, 30 responded with firing rate changes in the same direction (14 INC, 16 DEC). Among 36 neurons with opposite changes in firing rate, 27 decreased during central chemoreceptor stimulation. Functional connections were detected between neurons in 320 of 4,021 pairs. Central cross-correlogram features include 136 peaks indicative of shared inputs of like sign and 31 troughs suggesting inputs with opposite actions. The presence of offset features, including 88 peaks and 33 troughs, suggest pauci-synaptic excitatory and inhibitory interactions, respectively. The results demonstrate functional convergence of central and peripheral chemoreceptor effects on raphe neuronal assemblies and suggest circuit chains and loops appropriate for gain control of both chemoreceptive inputs to the brainstem cardiorespiratory network. Support: NS19814.

Responses of simultaneously recorded respiratory-related medullary neurons to stimulation of multiple sensory modalities.

This study addresses the hypothesis that multiple afferent systems share elements of a distributed brain stem network that modulates the respiratory motor pattern. Data were collected from 18 decerebrate, bilaterally vagotomized, paralyzed, artificially ventilated cats. Up to 28 neurons distributed in the rostral and caudal ventral respiratory group, nucleus tractus solitarius, and raphe obscurus were recorded simultaneously with microelectrode arrays. Phases of the respiratory cycle and inspiratory drive were assessed from integrated efferent phrenic nerve activity. Carotid chemoreceptors were stimulated by injection of CO2-saturated saline solution via the external carotid artery. Baroreceptors were stimulated by increased blood pressure secondary to inflation of an embolectomy catheter in the descending aorta. Cutaneous nociceptors were stimulated by pinching a footpad. Four hundred seventy-four neurons were tested for respiratory modulated firing rates and responses; 403 neurons were tested with stimulation of all 3 modalities. Chemoreceptor stimulation and pinch, perturbations that tend to increase respiratory drive, caused similar responses in 52 neurons; 28 responded oppositely. Chemoreceptor and baroreceptor stimulation resulted in similar primary responses in 45 neurons; 48 responded oppositely. Similar responses to baroreceptor stimulation and pinch were recorded for 38 neurons; opposite effects were measured in 26 neurons. Among simultaneously recorded neurons, distinct combinations of firing rate changes were evoked in response to stimulation of the different modalities. The results show a functional convergence of information from carotid chemoreceptors, baroreceptors, and cutaneous nociceptors on respiratory-modulated neurons distributed in the medulla. The data are consistent with the hypothesis that brain stem neurons have overlapping memberships in multifunctional groups that influence the respiratory motor pattern.

Inspiratory drive and phase duration during carotid chemoreceptor stimulation in the cat: medullary neurone correlations.

1. This study addressed the hypothesis that there is a parallel processing of input from carotid chemoreceptors to brainstem neurones involved in inspiratory phase timing and control of inspiratory motor output amplitude. Data were from fifteen anaesthetized, bilaterally vagotomized, paralysed, artificially ventilated cats. Carotid chemoreceptors were stimulated by close arterial injection of 200 microliters of CO2-saturated saline solution. 2. Planar arrays of tungsten microelectrodes were used to monitor simultaneously up to twenty-two neurones in the nucleus tractus solitarii (NTS) and ventral respiratory group (VRG). Spike trains were analysed with two statistical tests of respiratory modulation, cycle-triggered histograms, peristimulus-time histograms, cumulative sum histograms and cross-correlograms. 3. In NTS, 16 of 26 neurones with respiratory and 12 of 27 without respiratory modulation changed firing rate during carotid chemoreceptor stimulation. In the VRG 72 of 112 respiratory and 14 of 48 non-respiratory neurones changed firing rate during stimulation. 4. The spike trains of 85 of 1276 pairs (6.7%) of cells exhibited short time scale correlations indicative of paucisynaptic interactions. Ten pairs of neurones were each composed of a rostral VRG phasic inspiratory neurone that responded to carotid chemoreceptor stimulation with a decline in firing rate and a caudal VRG phasic inspiratory neurone that increased its firing rate. Cross-correlograms from two of the pairs had features consistent with excitation of the caudal neurones by the rostral cells. A decrease in the duration of activity of the rostral VRG neurones was paralleled by the decrease in inspiratory time of phrenic nerve activity. Caudal VRG inspiratory neurones increased their activity as phrenic amplitude increased. Spike-triggered averages of all four neurones indicated post-spike facilitation of phrenic motoneurones. 5. The results support the hypothesis that unilateral stimulation of carotid chemoreceptors results in parallel actions. (a) Inhibition of rostral VRG I-Driver neurones decreases inspiratory duration. (b) Concurrent excitation of premotor VRG and dorsal respiratory group inspiratory neurones increases inspiratory drive to phrenic motoneurones. Other data suggest that responsive ipsilateral neurones act to regulate contralateral neurones.

Long-term facilitation of phrenic nerve activity in cats: responses and short time scale correlations of medullary neurones.

1. Stimulation of either peripheral chemoreceptors or nucleus raphe obscurus results in long-term facilitation of phrenic motoneurone activity. The first objective of this work was to measure the concurrent responses of neurones in the nucleus raphe obscurus, the nucleus tractus solitarii, and the regions of the retrofacial nucleus, nucleus ambiguus and nucleus retroambigualis during induction of long-term facilitation. A second goal was to assess functional relationships of the chemoresponsive raphe neurones with neurones in the other monitored locations and with phrenic motoneurones. 2. Up to thirty single medullary neurones and phrenic nerve efferent activity were recorded simultaneously in fifteen anaesthetized, paralysed, vagotomized, artificially ventilated adult cats. Carotid chemoreceptors were stimulated by close arterial injection of 200 microliters of CO2-saturated saline solution. Spike trains were analysed with cycle-triggered histograms and two statistical tests for respiratory modulation. Peristimulus-time histograms and cumulative sum histograms were used to assess responses to stimulation. Cross-correlation was used to test for non-random temporal relationships between spike trains. Spike-triggered average histograms provided evidence for functional associations with phrenic motoneurones. 3. One hundred and thirteen of 348 neurones were monitored in the nucleus raphe obscurus. The firing rates of twenty-nine raphe neurones increased during stimulation; eighteen decreased. In twenty-one pairs of concurrently monitored raphe neurones, the firing rate of one increased its activity during stimulation then decreased, while the other showed an increase that began as the rate of the former declined. Eighteen chemoresponsive raphe neurones had short time scale features in their phrenic spike-triggered averages. Short time scale features were found in cross-correlograms from 184 of 1407 neurone pairs. 4. The data suggest parallel routes by which carotid chemoreceptors influence medullary raphe neurones and support the hypotheses that mid-line respiratory-related neuronal assemblies transform information from those receptors and regulate the gain of respiratory motor output.

NMDA receptor-mediated transmission of carotid body chemoreceptor input to expiratory bulbospinal neurones in dogs.

1. This study tested the hypothesis that excitatory amino acid receptors mediate the excitatory response of expiratory bulbospinal neurones to carotid body chemoreceptor inputs. 2. Studies were carried out in thiopental sodium anaesthetized, paralysed, ventilated, vagotomized dogs. 3. Brisk, short-duration chemoreceptor activation was produced by bilateral bolus injections of CO2-saturated saline (PCO2 > 700 mmHg) into the autoperfused carotid arteries. A pressurized-reservoir-solenoid valve system was used to deliver the CO2 bolus injections just prior to the onset of the neural expiratory phase, as determined from the phrenic neurogram, about once per minute. 4. Multibarrelled micropipettes were used to record neuronal unit activity and deliver neurotransmitter agents. Net responses of expiratory bulbospinal neurones to peripheral chemoreceptor activation were determined by subtracting the mean discharge frequencies (Fn) during three control expiratory cycles from the Fn during administration of a CO2 test bolus. The role of excitatory amino acid receptors in mediating this response was determined by comparing the baseline and bolus expiratory neuronal Fn before, during and after the pressure microejection of the NMDA receptor antagonist 2-amino-5-phosphonovalerate (AP5) or the non-NMDA receptor antagonist 2,3-dihydroxy-6-nitro-7-sulphamoyl- benzo(f)quinoxaline (NBQX). Ejection rates of AP5 and NBQX were measured by monitoring the movement of the pipette meniscus. 5. AP5 reduced Fn during both the control and bolus cycles, as well as reducing the change in Fn between control and bolus cycles. NBQX had no effect on either baseline or bolus responses. 6. AP5 did not prevent excitation of expiratory bulbospinal neurones by AMPA. Coadministration of AMPA with AP5 prevented the AP5-mediated decrease in Fn but not the dose-dependent reduction in the CO2 bolus response. 7. Taken together, these data strongly suggest that the carotid chemoreceptor-mediated excitation of expiratory bulbospinal neurones is dependent on NMDA but not non-NMDA glutamate receptors.

Chemoreceptor-sensitive neurons in commissural subnucleus of nucleus tractus solitarius of the rat.

Experiments were carried out in pentobarbital-anesthetized, vagotomized, paralyzed, and artificially ventilated male Wistar rats with a pneumothorax. Blood pressure, heart rate, and phrenic nerve activity were recorded. Extracellular neuronal recordings were made by glass micropipettes in a midline area in the commissural subnucleus of the nucleus tractus solitarius. At the end of the experiment, the recording sites were marked. Chemoreceptors were stimulated by tracheal administration of N2 or injection of CO2-saturated saline (50 microliters) near the carotid body. The baroreceptors were stimulated by an intravenous injection of phenylephrine (1-3 micrograms). Neurons that were excited by chemoreceptor stimulation but not baroreceptor stimulation were found in the commissural subnucleus of nucleus tractus solitarius and were designated as chemoreceptor-sensitive neurons. They exhibited a continuous discharge (9.9 +/- 1.3 spikes/s) that showed no apparent relation to either phrenic nerve bursts or arterial pulses. Such neurons were not found in the rostral portions of the nucleus tractus solitarius. These results confirm and extend our previous reports in which a discrete chemoreceptor projection site was identified in the commissural subnucleus of the rat.

Adrenergic cell group in rostral ventrolateral medulla of cat: its correlation with central chemoreceptors

We have proposed a hypothesis that secondary neurons mediating central respiratory chemoreception are not restricted to the medullary superficial layer. This idea was further examined in the present physiological and morphological studies. We identified the ‘S’ area, i.e. the medullary surface area where cold blockade produced apnea, in anesthetized, spontaneously breathing cat. We then evaluated how the apnea was modified by injection of CO 2-saturated saline into the vertebral-basilar artery. The CO 2 injection caused immediate reappearance of respiratory rhythm, although intensity of inspiratory activity was smaller than in the control. This incomplete recovery suggests that the responsive structure extends deeply below the surface. The extent of the cell group underlying the ‘S’ area was evaluated by morphological study. Medullary transverse sections including the ‘S’ area were treated with three distinct antisera against phenylethanolamine N-methyltransferase (PNMT), dopamine-β-hydroxylase (DBH), and 5-hydroxytryptamine (5-HT). The PNMT-labelled cell group was found to exactly underlie the ‘S’ area, although the DBH- and the 5-HT-labelled cell groups did not show close topographic correlation with the ‘S’ area. PNMT cells were located in the region ventral to retrofacial nucleus within 0.5–1.5 mm depth beneath the surface. These results suggest that adrenergic cells in the rostral ventrolateral medulla are important candidates for secondary neurons mediating central respiratory chemoreception.

Interaction between chemoreceptor and stretch receptor inputs at medullary respiratory neurons

The interaction between afferent inputs from carotid body chemoreceptors (CCRs) and from slowly adapting pulmonary stretch receptors (PSRs) on the discharge patterns of medullary inspiratory (I) and expiratory (E) neurons was characterized in thiopental sodium-anesthetized, paralyzed, ventilated dogs. A cycle-triggered ventilator was used to produce control and test pulmonary afferent input patterns. The CCRs were stimulated by phase-synchronized bolus injections of CO2-saturated saline into the common carotid arteries. Only those neurons whose discharge time course was altered by both inflation and CCR activation were studied. The dorsal respiratory group (DRG) I inflation-insensitive neurons were also included. Cycle-triggered histograms of unit activity were obtained for the neuronal responses to inflation, CO2 bolus, and their combination, as well as for the spontaneous control condition. Linearity of the interaction was tested by comparing the sum of the net individual responses to the net response of the combined afferent inputs. The results suggest that a linear (additive) interaction between CCR and PSR inputs exists for the DRG I inflation-sensitive neurons, the ventral respiratory group (VRG) I decrementing, and caudal VRG E augmenting neurons, while a nonadditive interaction exists for caudal VRG E decrementing bulbospinal neurons. The implications of these findings are discussed.

Actions of carotid chemoreceptors on subretrofacial bulbospinal neurons in the cat

Thirty-one barosensitive bulbospinal neurons were recorded from the subretrofacial (SRF) nucleus in eight chloralose-anaesthetised, paralysed cats. Close arterial injections of CO 2-saturated saline were used to stimulate carotid body chemoreceptors. Seven neurons were abruptly excited and six neurons abruptly inhibited by chemoreceptor stimuli: these were primary responses, independent of changes in blood pressure or central respiratory drive (monitored from the phrenic nerve). A further eight neurons responded only indirectly to chemoreceptor stimuli, showing enhanced modulation of their activity coupled to the enhanced central respiratory drive. The distinction between primary and secondary responses was shown more clearly when central respiratory drive was inhibited by superior laryngeal nerve stimulation. The remaining ten neurons showed no clear response to chemoreceptor stimuli. SRF bulbospinal neurons thus show the same range of responses to chemoreceptor stimuli as the sympathetic neurons they are believed to drive.