Anesth Analg 2004;99:49-55
© 2004 International Anesthesia Research Society
doi: 10.1213/01.ANE.0000117226.45704.65
PEDIATRIC ANESTHESIA
A Neuronal Mechanism of Propofol-Induced Central Respiratory Depression in Newborn Rats
Masanori Kashiwagi, MD*,
Yasumasa Okada, MD
,
Shun-ichi Kuwana, PhD
,
Shigeki Sakuraba, MD*,
Ryoichi Ochiai, MD*, and
Junzo Takeda, MD*
*Department of Anesthesiology, School of Medicine, Keio University;
Department of Physiology, Teikyo University School of Medicine, Tokyo; and
Department of Medicine, Keio University Tsukigase Rehabilitation Center, Shizuoka-ken, Japan
Address correspondence and reprint requests to Masanori Kashiwagi, MD, Department of Anesthesiology, School of Medicine, Keio University, 35 Shinanomachi, Shinjuku-ku, Tokyo, 160-8582, Japan. Address e-mail to mkashiwagi{at}1990.jukuin.keio.ac.jp
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Abstract
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The neural mechanisms of propofol-induced central respiratory depression remain poorly understood. In the present study, we studied these mechanisms and the involvement of 
-aminobutyric acid (GABA)A receptors in propofol-induced central respiratory depression. The brainstem and the cervical spinal cord of 1- to 4-day-old rats were isolated, and preparations were maintained in vitro with oxygenated artificial cerebrospinal fluid. Rhythmic inspiratory burst activity was recorded from the C4 spinal ventral root. The activity of respiratory neurons in the ventrolateral medulla was recorded using a perforated patch-clamp technique. We found that bath-applied propofol decreased C4 inspiratory burst rate, which could be reversed by the administration of a GABAA antagonist, bicuculline. Propofol caused resting membrane potentials to hyperpolarize and suppressed the firing of action potentials in preinspiratory and expiratory neurons. In contrast, propofol had little effect on resting membrane potentials and action potential firing in inspiratory neurons. Our findings suggest that the depressive effects of propofol are, at least in part, mediated by the agonistic action of propofol on GABAA receptors. It is likely that the GABAA receptor-mediated hyperpolarization of preinspiratory neurons serves as the neuronal basis of propofol-induced respiratory depression in the newborn rat.
IMPLICATIONS: We analyzed the effects of propofol on medullary respiratory neurons in brainstem-spinal cord preparations from newborn rats in vitro using a perforated patch-clamp technique. Our findings suggest that the -aminobutyric acid (GABA)A receptor-mediated hyperpolarization of preinspiratory neurons serves as the neuronal basis of propofol-induced respiratory depression.
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Introduction
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There have been several experimental studies on the patterns and mechanisms of propofol-induced respiratory depression in humans (1–3) and in animals in vivo (4,5). However, most of these studies have focused only on the dose-response relationship. Based on these reports, it has been suggested that propofol induces respiratory depression by inhibiting the central inspiratory drive. However, the precise neural mechanisms of propofol-induced respiratory depression remain poorly understood. There are no reported studies on brainstem neuronal responses to propofol, indicating the need for experimental measurements of respiratory neuron activity in vitro to elucidate the central respiratory effects of propofol at the cellular level.
Brainstem-spinal cord preparations from the newborn rat provide an established model for the investigation of the mammalian respiratory neuronal network (6,7). This model maintains spontaneous respiratory activity that can be recorded from the C4 ventral root of the spinal cord, enabling the study of the central mechanisms of respiratory control free from the influence of supramedullary and peripheral afferent inputs (8). The rat brainstem model is also advantageous for pharmacological studies (9–12) because it is possible to superfuse preparations with drugs at controlled concentrations and to observe the pharmacological effects in an anesthetic- and muscle relaxant-free condition.
The present study was conducted to investigate (a) the response of medullary respiratory neurons to propofol, including its effects on membrane properties, and (b) the role of -aminobutyric acid (GABA)A receptors in propofol-induced respiratory depression. It is hoped that results of this study will provide a new base of knowledge regarding this phenomenon and lead to a better understanding of the mechanisms underlying propofol-induced respiratory depression.
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Methods
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This study was approved by the Keio University Laboratory Animal Care and Use Committee. All experiments in the study (Experiments 1, 2, and 3) were conducted using isolated brainstem-spinal cord preparations. The surgical procedure used to make these preparations has been described in detail elsewhere (13). Briefly, brainstems (including spinal cords) of 1- to 4-day-old Wistar rats of either sex were isolated under deep ether anesthesia. The brainstem was transected at the level of the roots of cranial nerve VI. The preparation consisted of the isolated brainstem and spinal cord extending from the rostral medulla oblongata to the spinal cord segment C8. Each preparation was placed ventral side up in a recording chamber (volume, 2 mL) and superfused continuously at a flow rate of 7 mL/min with the following modified Krebs solution (in mM): NaCl 124, KCl 5.0, KH2PO4 1.2, CaCl2 2.4, MgSO4 1.3, NaHCO3 26, glucose 30, equilibrated with 95% O2–5% CO2. The temperature was maintained at 25°C–26°C, and the superfusate pH value was 7.4. In Experiment 2, we used a chamber, partitioned at the spinomedullary junction to permit the selective application of drugs. The partition was made using two thin polyvinyl chloride plates placed parallel to each other with woven nylon packed between them. Each partitioned section had a capacity of 1 mL and was superfused continuously at a flow rate of 4 mL/min.
Propofol was obtained from Tokyo Kasei Kogyo (Tokyo, Japan). Stock solutions of 1 M of propofol were prepared in dimethylsulfoxide (Wako Pure Chemical, Osaka, Japan) and diluted with the control superfusate to give final propofol concentrations of 5–100 µM. The final concentration of dimethylsulfoxide was <0.01% (vol/vol). Bicuculline was obtained from Sigma (St. Louis, MO). After saturation at 95% O2–5% CO2, the pH value of the solutions was adjusted to 7.4 using HCl or NaHCO3.
Rhythmic inspiratory burst activity was continuously recorded from the C4 ventral root (which contains the phrenic nerve fibers) using a glass suction electrode (9,13,14). The C4 electrical signals were bandpass-filtered (50–10,000 Hz), amplified, halfwave-rectified, and integrated with a time constant of 100 ms.
In Experiment 3, neuronal activities in the rostral ventrolateral medulla were recorded intracellularly using a perforated patch-clamp configuration. The protocol was as detailed in previous reports (13,15). Briefly, a glass micropipette (GC100-TF-10; Clark; Reading, United Kingdom) was pulled with a horizontal puller to a tip size of approximately 2 µm. Electrode resistance ranged from 10 to 14 M
when filled with a solution containing (mM) K-gluconate 140, KCl 3, EGTA 10, HEPES 10, CaCl2 1, MgCl2 1, and nystatin (100 µg/mL); pH value was adjusted to 7.2–7.3 by using KOH. The micropipette was inserted into the rostral ventrolateral medulla using a manual hydraulic micromanipulator. Membrane potentials were recorded with a whole-cell patch amplifier (CEZ 3100, Nihon Kohden, Tokyo, Japan). While neurons were searched, positive pressure (10–20 cm H2O) was applied inside the pipette. After a giga-ohm seal was obtained, the recorded membrane potential gradually became negative and stabilized in approximately 10 min. Membrane potentials have been presented without correction for liquid junction potential. Perforated patch recording was stably maintained for more than 60 min. When a patch formation could not be obtained for a given respiratory neuron, we conducted extracellular neuronal recording experiments in which membrane potentials were monitored using an oscilloscope and stored in a DAT tape recorder for offline analysis.
In Experiments 1 and 2, the preparations were superfused with the control solution in the recording chamber for at least 30 min before recording to stabilize C4 activity; in Experiment 3, the preparations were superfused with the control solution for approximately 10 min before recording. Preparations in which respiratory output from C4 roots was unstable were excluded.
Experiment 1
Forty preparations were randomly allocated to one of four groups: a control group and three propofol groups at concentrations 5, 10, or 20 µM; each group contained 10 preparations. In the propofol groups, after obtaining a baseline recording, the superfusate was replaced by solution containing propofol at the specified concentration, and recording was resumed. In the control group, a recording was made at 120 min with the control superfusate.
Experiment 2
Experiments were performed in 10 preparations. After C4 activity stabilized, the superfusate bathing the medulla was replaced as follows: control solution for 15 min, 20 µM of propofol for 15 min, 20 µM of propofol with 4 µM of bicuculline for 15 min, and 20 µM of propofol again for 60 min. The spinal cord was continuously bathed in the control solution throughout the experiment.
Experiment 3
After obtaining a baseline recording with the control solution, the preparation was superfused with a solution containing 20–100 µM (mostly 50–100 µM) of propofol for 4–10 min and then by the control superfusate again.
Medullary neurons were categorized into four groups according to their firing patterns: preinspiratory, inspiratory, expiratory, and nonrespiratory neurons (13,14,16). Neurons that showed burst activity during preinspiratory and postinspiratory phases were defined as preinspiratory. Inspiratory neurons were identified as those discharging bursts during the C4 burst activity phase. Expiratory neurons are characterized by hyperpolarization or absence of firing synchronous with C4 burst firing. Neurons that exhibited tonic patterns and showed no respiratory modulation were categorized as nonrespiratory.
Data are presented as mean and SD for all preparations in the study groups. C4 inspiratory burst rates were calculated from the total number of bursts within a 5-min period before switching the superfusate in Experiments 1 and 2. Intraburst firing frequencies of preinspiratory and expiratory neurons are represented by the mean firing frequency during the expiratory phase, and intraburst firing frequencies of inspiratory neurons are represented by the mean firing frequency during the inspiratory phase. The mean and SD of the intraburst firing frequencies, resting membrane potentials, and C4 inspiratory burst rates were calculated based on recordings from 5 respiratory cycles in Experiment 3. Apnea was defined as the absence of C4 inspiratory burst activity for 60 s or longer. Changes in C4 inspiratory burst rates were compared by using one-way analysis of variance, followed by either Dunnett test (in Experiment 1) or the Student-Newman-Keuls test (in Experiment 2). In Experiment 3, a paired t-test was used to compare resting membrane potentials, intraburst firing frequencies, and C4 inspiratory burst rates before and during drug administration. P < 0.05 was considered statistically significant.
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Results
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Experiment 1
Representative tracings of integrated C4 activity (
C4) before and during superfusion with propofol-containing solutions are shown in Figure 1. There were no significant differences among the four groups in the baseline values of C4 inspiratory burst rates (Fig. 2A). Preparations in the control group maintained the baseline C4 inspiratory burst rate throughout the entire 120-min recording period. C4 inspiratory burst rates decreased over time in the propofol groups. Larger concentrations of propofol tended to produce greater and steeper decreases in C4 inspiratory burst rate, although this effect was not linearly dose-dependent. C4 inspiratory burst rates after 30 min of superfusion with 5, 10, or 20 µM of propofol were significantly less than in the control group (P < 0.05). The incidence of apnea increased in a concentration- and time-dependent manner. After 90 min of superfusion with propofol, the percentages of preparations in which apnea had occurred were as follows: 5 µM, 0%; 10 µM, 40%; and 20 µM, 80%.
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Figure 1. Representative sample tracing of integrated C4 activity before and during superfusion with control, 5, 10, and 20 µM of propofol solutions. The horizontal bar indicates the duration of propofol superfusion. Propofol slowed C4 inspiratory burst rates, which represent respiratory frequency. At the largest concentration of propofol (20 µM), a decrease of the C4 inspiratory burst rate appeared quickly and markedly. C4 = integrated C4.
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Experiment 2
Bicuculline, a GABAA antagonist, induced the complete recovery of C4 inspiratory burst rates that had been decreased by propofol (Fig. 2B). In the same preparations, C4 inspiratory burst rates decreased once again after bicuculline washout.
Experiment 3
To examine the effects of propofol on the function of medullary neurons, we recorded 25 neurons in the rostral ventrolateral medulla: 8 preinspiratory neurons (2 intracellular and 6 extracellular recordings), 4 expiratory neurons (2 intracellular and 2 extracellular recordings), and 13 inspiratory neurons (6 intracellular and 7 extracellular recordings). These neurons could be divided into two categories according to the patterns of their responses to propofol. The first category comprises preinspiratory and expiratory neurons, in which propofol produced inhibitory effects. In the second category, comprising inspiratory neurons, propofol had little effect. The firing of action potentials ceased during propofol superfusion in 4 of 12 preinspiratory/expiratory neurons; this effect was not observed in any of the 13 inspiratory neurons. Propofol also induced the hyperpolarization of resting membrane potentials (P < 0.01 versus baseline) and a decrease in intraburst firing frequencies (P < 0.05 versus baseline) in preinspiratory/expiratory neurons, whereas these variables were not significantly affected in inspiratory neurons (Table 1). The propofol-induced reduction in C4 inspiratory burst rate was –39% ± 30% in preinspiratory and expiratory neurons (P < 0.01 versus baseline) and –52% ± 25% in inspiratory neurons (P < 0.01 versus baseline). Representative recordings of membrane potentials in a preinspiratory neuron and an inspiratory neuron are shown in Figure 3.
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Table 1. Effects of Propofol on Intraburst Firing Frequency and Resting Membrane Potential of Respiratory Neurons
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Figure 3. (A) Representative recording of membrane potentials of a medullary preinspiratory neuron before, during, and after superfusion with 50 µM of propofol. Propofol reversibly hyperpolarized the cell and suppressed action potential firing. (B) Representative recording of membrane potential in a medullary inspiratory neuron before, during, and after superfusion with 100 µM of propofol. Although propofol-induced decrease in burst rates of the inspiratory neuron synchronized with those of C4, propofol did not affect the resting membrane potential. Pre-I = preinspiratory neuron, Insp = inspiratory neuron, and C4 = integrated C4.
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Discussion
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Our major findings are that propofol exerts a depressive effect on neuronal output in the medullary respiratory center and that this depression can be reversed by a GABAA receptor antagonist. Propofol inhibits spontaneous burst activity in medullary preinspiratory and expiratory neurons, but not in inspiratory neurons.
In the present study, we used the in vitro brainstem-spinal cord preparation. This preparation enables analyzing effects of various drugs on central respiratory control. It is also especially suitable for perforated patch recording of identified respiratory neurons in an anesthetic- and muscle relaxant-free condition (7). Because this preparation lacks blood circulation, it is also possible to exclude the influences of the changes of cardiovascular function when analyzing drug effects on the respiratory neuronal network. However, to maintain the viability of the preparation that has limited oxygen supply and to obtain stable respiratory rhythm, a lowered temperature (approximately 25°C–26°C) that reduces the oxygen demand is the most suitable for the preparation (17). Also, only small newborn animals (e.g., rat and mouse, age 0–5 days old) can be successfully used for this preparation because newborn animals can more easily survive than adult animals, even when oxygen supply is reduced and because oxygen is more readily supplied to respiratory neurons by diffusion through a shorter diffusion distance in a smaller preparation (7,17). Although it is conceivable that substantial changes in respiratory network organization or membrane properties occur within the first few weeks after birth, there are already the basic synaptic connections among respiratory neurons, even in newborn rats (18,19). The baseline value of inspiratory burst rates observed in the in vitro preparation is less than that seen in intact newborn rats. This reduction is caused by the experimental conditions, i.e., deafferentation and the low temperature of the preparation (9,20). Nonetheless, this type of preparation has been extensively used to study the effects of neurotransmitters, neuromodulators, and other neuroactive substances on the respiratory rhythm-generating network with good results (6,7,9–12,16).
In the present study, we used propofol at concentrations of 5–100 µM. Blood or plasma concentrations of propofol in clinical situations are also often in the range of 10–30 µM (2–6 µg/mL) (21) and up to 110 µM (20 µg/mL) (22). However, we consider that propofol concentrations of the superfusate of in vitro experiments and those of blood or plasma in humans need not precisely coincide because various factors, such as species (rats and humans) and delivery routes for propofol to target neurons, are quite different between an in vitro experimental condition and a clinical situation. There has been a study to measure the propofol concentration in cerebrospinal fluid in humans (approximately 0.2 µM) (23), and it is much smaller than that in human plasma and in the superfusate in our experiments. We consider that this is because only a part of IV propofol moved to cerebrospinal fluid and was diluted in the in vivo situation. However, propofol concentration in the brain tissue of the in vivo rat is approximately 80–200 µM when an anesthetic dose of propofol was given IV (24). Indeed, in pharmacological experiments using in vitro brain-slice preparations, propofol has been typically applied at concentrations up to several hundred micromoles per liter (25). Therefore, the propofol concentration in the superfusate in our study is considered to be appropriate. The concentration of propofol in Experiment 3 was larger than that in Experiments 1 and 2. This was because it is advantageous to complete each analysis in a short time to obtain high-quality intracellular neuronal recordings and because when the concentration of propofol is larger, the effect appears more quickly. In Experiment 3, the responses of C4 inspiratory burst activity to propofol were essentially similar to those in Experiments 1 and 2 and characterized by decreases in inspiratory burst rates with constant integrated C4 amplitudes (Figs. 1–3). Nonetheless, extreme caution is required to extrapolate the results obtained in in vitro experiments to an in vivo situation, and our results must be confirmed in in vivo animal experiments.
In the present study, propofol, when selectively applied to the medulla, induced a decrease in C4 inspiratory burst rate. The inspiratory burst rate in brainstem-spinal cord preparations from newborn rat is widely considered to provide a reliable physiological index of central respiratory output (9,11,12). IV propofol decreases ventilation mainly by reducing tidal volume (1–3). Similarly, in an in vivo rabbit experiment (5), the amplitude of integrated phrenic nerve activity, which represents tidal volume (26), was reduced after the administration of propofol. However, this reduction in the amplitude of C4 activity in the in vitro brainstem-spinal cord preparation is less informative because it can be affected by C4 inspiratory burst rates (6,7). Indeed, more frequent inspiratory burst rates can diminish C4 burst amplitude because of the refractory periods of some of medullary neurons (6,7). In summary, the propofol-induced decrease in C4 inspiratory burst rate indicates that propofol reduces the output of the medullary respiratory neuronal network irrespective of C4 amplitude.
Respiratory neurons in the ventrolateral medulla of the isolated brainstem-spinal cord preparation form a neuronal network that is responsible for central respiratory control. It has been suggested that the excitatory network of preinspiratory neurons in the rostral ventrolateral medulla constitutes the respiratory rhythm generator and that inspiratory neurons in the rostral and caudal ventral medulla constitute the inspiratory pattern generator (6,7). In this model, the respiratory rhythm generator periodically triggers the inspiratory pattern generator to produce inspiratory burst activity. The output of the inspiratory pattern generator drives motor neurons, such as the phrenic or cranial nerves (6,7). Therefore, the burst rates of inspiratory neurons and C4 are thought to be dependent on those of preinspiratory neurons (14,16). The propofol-induced decrease in burst rates of inspiratory neurons does not seem to be the result of direct inhibition because we observed neither a significant change in resting membrane potentials nor obvious arrest of burst activity in these neurons, even at a large concentration of propofol. Our results suggest that propofol primarily suppresses preinspiratory neurons, resulting in the indirect suppression of inspiratory neurons and the C4. However, the number of intracellularly recorded respiratory neurons was small, and further study will be required to confirm the effect of propofol on respiratory neurons.
Propofol facilitates inhibitory synaptic transmission by promoting the function of the ß1 subunit of the GABAA receptor through the activation of chloride channels (27,28). Propofol anesthesia in adult mice is potentiated by a GABAA agonist muscimol and reversed by bicuculline (29). Bath-applied GABA (16,30) and muscimol (30) also reduce the burst rate of preinspiratory neurons, and bicuculline blocks the muscimol-induced respiratory arrest (30) in isolated brainstem-spinal cord preparations from the newborn rat. Bicuculline also blocks the hyperpolarization evoked by GABA and muscimol in preinspiratory neurons (30). In the present study, we found that propofol reduced C4 inspiratory burst rate, and this effect was blocked by bicuculline. This prompts us to suggest that the propofol-induced decrease in C4 inspiratory burst rate is, at least partly, caused by the activation of GABAA receptors. Preinspiratory neurons (16,30) and expiratory neurons (31) have GABAA receptor-mediated inhibitory postsynaptic potentials. In the present study, propofol induced hyperpolarization of the resting membrane potentials and arrest of burst activity in preinspiratory and expiratory neurons. It is probable that propofol-induced hyperpolarization modulates the excitability of preinspiratory and expiratory neurons through the activation of GABAA receptors. Further study is required to identify the type of receptor that is involved in propofol-induced hyperpolarization of respiratory neurons.
In summary, we suggest that propofol suppresses the activity of preinspiratory and expiratory neurons, but not that of inspiratory neurons. The propofol-induced decrease of respiratory output is considered to be mediated through the inhibition of preinspiratory neurons. Our findings also suggest that GABAA receptor activation is involved in propofol-induced respiratory depression.
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Acknowledgments
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Supported, in part, by Keio University Grant-in-Aid, Tokyo, Japan; Keio University Medical Science Fund, Tokyo, Japan; Keio Gijuku Academic Development Funds, Tokyo, Japan; Grant-in-Aid from the Japanese Ministry of Education, Culture, Sports, Science and Technology, Tokyo, Japan; and Research Grant for Specific Diseases from the Japanese Ministry of Health, Labour and Welfare.
We express our appreciation to Dr. A. Kawai for his advice on experimental techniques.
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Footnotes
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Presented, in part, as an abstract at the 2001 annual meeting of the American Society of Anesthesiologists, New Orleans, LA, October 2001, and at the 49th annual meeting of the Japanese Society of Anesthesiologists, Fukuoka, Japan, April 2002.
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References
|
|---|
- Grounds RM, Maxwell DL, Taylor MB, et al. Acute ventilatory changes during i.v. induction of anaesthesia with thiopentone or propofol in man: studies using inductance plethysmography. Br J Anaesth 1987; 59: 1098–102.[Abstract/Free Full Text]
- Goodman NW, Black AM, Carter JA. Some ventilatory effects of propofol as sole anaesthetic agent. Br J Anaesth 1987; 59: 1497–503.[Abstract/Free Full Text]
- Kulkarni P, Brown KA. Ventilatory parameters in children during propofol anaesthesia: a comparison with halothane. Can J Anaesth 1996; 43: 653–9.[Abstract]
- Muir WW, Gadawski JE. Respiratory depression and apnea induced by propofol in dogs. Am J Vet Res 1998; 59: 157–61.[ISI][Medline]
- Ma D, Sapsed-Byrne SM, Chakrabarti MK, Whitwam JG. Synergistic interaction between the effects of propofol and midazolam with fentanyl on phrenic nerve activity in rabbits. Acta Anaesthesiol Scand 1998; 42: 670–7.[ISI][Medline]
- Onimaru H, Arata A, Homma I. Neuronal mechanisms of respiratory rhythm generation: an approach using in vitro preparation. Jpn J Physiol 1997; 47: 385–403.[ISI][Medline]
- Ballanyi K, Onimaru H, Homma I. Respiratory network function in the isolated brainstem-spinal cord of newborn rats. Prog Neurobiol 1999; 59: 583–634.[ISI][Medline]
- Feldman JL. Neurophysiology of breathing in mammals. In: Handbook of physiology: Section 1: The nervous system, Volume IV: Intrinsic regulatory systems in the brain. Washington, DC: American Physiology Society, 1986: 463–524.
- Murakoshi T, Suzue T, Tamai S. A pharmacological study on respiratory rhythm in the isolated brainstem-spinal cord preparation of the newborn rat. Br J Pharmacol 1985; 86: 95–104.[ISI][Medline]
- Otsuka H. Effects of volatile anesthetics on respiratory activity and chemosensitivity in the isolated brainstem-spinal cord of the newborn rat. Hokkaido Igaku Zasshi 1998; 73: 117–36.[Medline]
- Takeda S, Eriksson LI, Yamamoto Y, et al. Opioid action on respiratory neuron activity of the isolated respiratory network in newborn rats. Anesthesiology 2001; 95: 740–9.[ISI][Medline]
- Akada S, Takeda S, Ogawa R. Salicylate action on medullary inspiratory neuron activity in a brainstem-spinal cord preparation from newborn rats. Anesth Analg 2003; 96: 407–11.[Abstract/Free Full Text]
- Kuwana S, Okada Y, Natsui T. Effects of extracellular calcium and magnesium on central respiratory control in the brainstem-spinal cord of neonatal rat. Brain Res 1998; 786: 194–204.[ISI][Medline]
- Onimaru H, Homma I. Respiratory rhythm generator neurons in medulla of brainstem-spinal cord preparation from newborn rat. Brain Res 1987; 403: 380–4.[ISI][Medline]
- Okada Y, Chen Z, Jiang W, et al. Anatomical arrangement of hypercapnia-activated cells in the superficial ventral medulla of rats. J Appl Physiol 2002; 93: 427–39.[Abstract/Free Full Text]
- Onimaru H, Arata A, Homma I. Inhibitory synaptic inputs to the respiratory rhythm generator in the medulla isolated from newborn rats. Pflügers Arch 1990; 417: 425–32.[ISI][Medline]
- Suzue T. Respiratory rhythm generation in the in vitro brain stem-spinal cord preparation of the neonatal rat. J Physiol (London) 1984; 354: 173–83.[Abstract/Free Full Text]
- Richter DW, Ballanyi K, Schwarzacher S. Mechanisms of respiratory rhythm generation. Curr Opin Neurobiol 1992; 2: 788–93.[Medline]
- Funk GD, Feldman JL. Generation of respiratory rhythm and pattern in mammals: insights from developmental studies. Curr Opin Neurobiol 1995; 5: 778–85.[ISI][Medline]
- Peever J, Tian GF, Duffin J. Temperature and pH affect respiratory rhythm of in vitro preparations from neonatal rats. Respir Physiol 1999; 117: 97–107.[ISI][Medline]
- Adachi YU, Watanabe K, Higuchi H, Satoh T. The determinants of propofol induction of anesthesia dose. Anesth Analg 2001; 92: 656–61.[Abstract/Free Full Text]
- Kazama T, Ikeda K, Morita K, et al. Propofol concentration required for endotracheal intubation with a laryngoscope or fiberscope and its interaction with fentanyl. Anesth Analg 1998; 86: 872–9.[Abstract]
- Engldahl O, Abrahams M, Bjornsson A, et al. Cerebrospinal fluid concentrations of propofol during anaesthesia in humans. Br J Anaesth 1998; 81: 957–9.[Abstract/Free Full Text]
- Shyr MH, Tsai TH, Tan PP, et al. Concentration and regional distribution of propofol in brain and spinal cord during propofol anesthesia in the rat. Neurosci Lett 1995; 184: 212–5.[ISI][Medline]
- Wakasugi M, Hirota K, Roth SH, Ito Y. The effects of general anesthetics on excitatory and inhibitory synaptic transmission in area CA1 of the rat hippocampus in vitro. Anesth Analg 1999; 88: 676–80.[Abstract/Free Full Text]
- Eldridge FL. Relationship between phrenic nerve activity and ventilation. Am J Physiol 1971; 221: 535–43.[Free Full Text]
- Collins GG. Effects of the anaesthetic 2,6-diisopropylphenol on synaptic transmission in the rat olfactory cortex slice. Br J Pharmacol 1988; 95: 939–49.[ISI][Medline]
- Sanna E, Garau F, Harris RA. Novel properties of homomeric beta 1 gamma-aminobutyric acid type A receptors: actions of the anesthetics propofol and pentobarbital. Mol Pharmacol 1995; 47: 213–7.[Abstract]
- Irifune M, Sugimura M, Takarada T, et al. Propofol anaesthesia in mice is potentiated by muscimol and reversed by bicuculline. Br J Anaesth 1999; 83: 665–7.[Abstract/Free Full Text]
- Brockhaus J, Ballanyi K. Synaptic inhibition in the isolated respiratory network of neonatal rats. Eur J Neurosci 1998; 10: 3823–39.[ISI][Medline]
- Arata A, Onimaru H, Homma I. Possible synaptic connections of expiratory neurons in the medulla of newborn rat in vitro. Neuroreport 1998; 9: 743–6.[ISI][Medline]
Accepted for publication December 26, 2003.
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Abstract
Introduction
