Yoshitaka Shimizu

Propofol-induced Anesthesia in Mice Is Mediated by γ-aminobutyric Acid-a and Excitatory Amino Acid Receptors

To elucidate the role of &ggr;-aminobutyric acid (GABA)A receptor complex and excitatory amino acid receptors (N-methyl-d-aspartate [NMDA] and non-NMDA receptors) in propofol-induced anesthesia, we examined behaviorally the effects of GABAergic and glutamatergic drugs on propofol anesthesia in mice. All drugs were administered intraperitoneally. General anesthetic potencies were evaluated using a righting reflex assay. The GABAA receptor agonist muscimol potentiated propofol (140 mg/kg; 50% effective dose for loss of righting reflex) induced anesthesia. Similarly, the benzodiazepine receptor agonist diazepam and the NMDA receptor antagonist MK-801 augmented propofol anesthesia, but the non-NMDA receptor antagonist CNQX did not. In contrast, the GABAA receptor antagonist bicuculline antagonized propofol (200 mg/kg; 95% effective dose for loss of righting reflex) induced anesthesia. However, neither the benzodiazepine receptor antagonist flumazenil, the GABA synthesis inhibitor l-allylglycine, nor the NMDA receptor agonist NMDA reversed propofol anesthesia. Conversely, the non-NMDA receptor agonist kainate enhanced propofol anesthesia. These results suggest that propofol-induced anesthesia is mediated, at least in part, by both GABAA and excitatory amino acid receptors.

Riluzole, a glutamate release inhibitor, induces loss of righting reflex, antinociception, and immobility in response to noxious stimulation in mice

BACKGROUND:The general anesthetic state comprises behavioral and perceptual components, including amnesia, unconsciousness, analgesia, and immobility. In vitro, glutamatergic excitatory neurons are important targets for anesthetic action at the cellular and microcircuits levels. Riluzole (2-amino-6-[trifluoromethoxy]benzothiazole) is a neuroprotective drug that inhibits glutamate release from nerve terminals in the central nervous system. Here, we examined in vivo the ability of riluzole to produce components of the general anesthetic state through a selective blockade of glutamatergic neurotransmission. METHODS:Riluzole was administered intraperitoneally in adult male ddY mice. To assess the general anesthetic components, three end-points were used: 1) loss of righting reflex (LORR; as a measure of unconsciousness), 2) loss of movement in response to noxious stimulation (as a measure of immobility), and 3) loss of nociceptive response (as a measure of analgesia). RESULTS:The intraperitoneal administration of riluzole induced LORR in a dose-dependent fashion with a 50% effective dose value of 27.4 (23.3–32.2; 95% confidence limits) mg/kg. The behavioral and microdialysis studies revealed that time-course changes in impairment and LORR induced by riluzole corresponded with decreased glutamate levels in the mouse brain. This suggests that riluzole-induced LORR (unconsciousness) could result, at least in part, from its ability to decrease brain glutamate concentrations. Riluzole dose-dependently produced not only LORR, but also loss of movement in response to painful stimulation (immobility), and loss of nociceptive response (analgesia) with 50% effective dose values of 43.0 (37.1–49.9), and 10.0 (7.4–13.5) mg/kg, respectively. These three dose–response curves were parallel, suggesting that the behavioral effects of riluzole may be mediated through a common site of action. CONCLUSIONS:These findings suggest that riluzole-induced LORR, immobility, and antinociception appear to be associated with its ability to inhibit glutamatergic neurotransmission in the central nervous system.

MK-801 enhances gabaculine-induced loss of the righting reflex in mice, but not immobility.

Purpose: γ-Aminobutyric acid (GABA) and N-methyl-D-aspartate (NMDA) receptors are important targets for anesthetic action at thein vitro cellular level. Gabaculine is a GABA-transaminase inhibitor that increases endogenous GABA in the brain, and enhances GABA activity. We have recently shown that unconsciousness is associated with the enhanced GABA activity due to gabaculine, but that immobility is not. MK-801 is a selective NMDA channel blocker. In this study, we examined behaviourally whether gabaculine in combination with MK-801 could produce these components of the general anesthetic state. We further compared the effect of MK-801 with ketamine, another NMDA channel blocker.Methods: All drugs were administered intraperitoneally to adult male ddY mice. To assess the general anesthetic components, two endpoints were used. One was loss of the righting reflex (LORR; as a measure of unconsciousness) and the other was loss of movement in response to tail-clamp stimulation (as a measure of immobility).Results: Large doses of MK-801 alone (10–50 mg·kg−1) induced neither LORR nor immobility in response to noxious stimulation. However, even a small dose (0.2 mg·kg−1) significantly enhanced gabaculine-induced LORR (P<0.05), although gabaculine in combination with MK-801 (0.2–10 mg·kg−1) produced no immobility. However, gabaculine plus a subanesthetic dose of ketamine (30 mg·kg−1), which acts on NMDA, opioid and nicotinic acetylcholine receptors and neuronal Na+ channels, suppressed the pain response, but did not achieve a full effect. Ketamine alone dose-dependently produced both LORR and immobility.Conclusion: These findings suggest that gabaculine-induced LORR is modulated by blocking NMDA receptors, but that immobility is not mediated through GABA or NMDA receptors.RésuméObjectif: Les récepteurs GABA (acide gamma-aminobutyrique) et NMDA (N-méthyl-D-aspartate) constituent d’importantes cibles pour l’action des anesthésiques au niveau cellulaire in vitro. La ga-baculine est un inhibiteur des GABA-transaminases qui augmente le GABA endogène dans le cerveau, et stimule l’activité GABA. Nous avons récemment démontré que la perte de conscience est associée à l’activité GABA stimulée par la gabaculine, mais que l’immobilité ne l’est pas. Le MK-801 est un bloqueur sélectif du canal NMDA. Dans cette étude, nous avons examiné si la gabaculine combinée à du MK-801 pouvait produire ces composantes de l’état d’anesthésie générale au niveau comportemental. Nous avons également comparé l’effet du MK-801 à celui de la kétamine, un autre bloqueur du canal NMDA.Méthode: Tous les médicaments ont été administrés à des souris mâles adultes ddY par voie intrapéritonéale. Deux paramètres ont été utilisés afin d’évaluer les composantes de l’anesthésie générale. L’un était la perte du réflexe de redressement (LORR — loss of righting reflex ; pour mesurer la perte de conscience), et l’autre l’absence de mouvement en réaction à la stimulation d’une pince à la queue (pour mesurer l’immobilité).Résultats: D’importantes doses de MK-801 seul (10–50 mg·kg−1) n’ont provoqué ni LORR ni l’immobilité en réaction à une stimulation nociceptive. Toutefois, une dose même faible (0.2 mg·kg−1) a significativement accentué le LORR provoqué par la gabaculine (P<0,05), bien que la gabaculine associée à du MK-801 (0,2–10 mg·kg−1)n’ait pas provoqué d’immobilité. Cependant, la gabaculine additionnée d’une dose sous-anesthésique de kétamine (30 mg·kg−1),laquelle agit sur les récepteurs NMDA, opiacés et cholinergiques nicotiniques ainsi que sur les canaux Na+, a supprimé la réaction douloureuse, mais n’a pas eu un effet complet. La kétamine seule a provoqué LORR et immobilité, de façon dose-dépendante.Conclusion: Ces résultats suggèrent que le LORR provoqué par la gabaculine est modulé en bloquant les récepteurs NMDA, mais que l’immobilité n’est pas médiée par les récepteurs GABA ou NMDA.

Increased γ-aminobutyric acid levels in mouse brain induce loss of righting reflex, but not immobility, in response to noxious stimulation

BACKGROUND:The general anesthetic state comprises behavioral and perceptual components, including amnesia, unconsciousness, and immobility. &ggr;-Aminobutyric acidergic (GABAergic) inhibitory neurotransmission is an important target for anesthetic action at the in vitro cellular level. In vivo, however, the functional relevance of enhancing GABAergic neurotransmission in mediating essential components of the general anesthetic state is unknown. Gabaculine is a GABA-transaminase inhibitor that inhibits degradation of released GABA, and consequently increases endogenous GABA in the central nervous system. Here, we examined, behaviorally, the ability of increased GABA levels to produce components of the general anesthetic state. METHODS:All drugs were administered systemically in adult male ddY mice. To assess the general anesthetic components, two end-points were used. One was loss of righting reflex (LORR; as a measure of unconsciousness); the other was loss of movement in response to tail-clamp stimulation (as a measure of immobility). RESULTS:Gabaculine induced LORR in a dose-dependent fashion with a 50% effective dose of 100 (75–134; 95% confidence limits) mg/kg. The behavioral and microdialysis studies revealed that the endogenous GABA-induced LORR occurred in a brain concentration-dependent manner. However, even larger doses of gabaculine (285–400 mg/kg) produced no loss of tail-clamp response. In contrast, all the tested volatile anesthetics concentration-dependently abolished both righting and tail-clamp response, supporting the evidence that volatile anesthetics act on a variety of molecular targets. CONCLUSIONS:These findings indicate that LORR is associated with enhanced GABAergic neurotransmission, but that immobility in response to noxious stimulation is not, suggesting that LORR and immobility are mediated through different neuronal pathways and/or regions in the central nervous system.

An Alternative Approach to the Monitoring of Respiration by Dynamic Air-Pressure Sensor

Monitoring and assessing of patient respiratory function during conscious sedation are important because many drugs used for conscious sedation produce respiratory depression and subsequent hypoventilation. The purpose of this study is to assess the value of a dynamic air-pressure sensor for respiratory monitoring of clothed patients. Eight clothed adult volunteers were reclined on a dental chair positioned horizontally. The air bag for measuring air-pressure signals corresponding to respiration was placed on the seat back of the dental chair in the central lumbar area of the subject. The subject breathed through a face mask with a respirometer attached for measuring expiratory tidal volume. The air-pressure signals corresponding to respiration were obtained and the time integration values for air pressure during each expiration (integral P(exp)) were calculated. The expiratory tidal volume (TV(exp)) was measured simultaneously by respirometer. The relationship between TV(exp) and integral P(exp) for each subject was assessed by a Pearson correlation coefficient. A strong correlation between TV(exp) and integral P(exp) was observed in all subjects. Measuring integral P(exp) by dynamic air-pressure sensor makes it possible to estimate respiratory volume breath by breath, and the respiratory pressure-time integral waveform was useful in visually monitoring the respiration pattern. We believe that in the future this device will be used to monitor respiratory physiology in clothed patients, contributing to safer sedative procedures.