U-50, 488H and [D-Pen2,D-Pen5] enkephalin on the concentration increases intracellular ... minimum alveolar concentration for isoflurane, with no of brain nitric ...
British Journal of Anaesthesia 84 (2): 141–3 (2000)
Editorial II Anaesthesia and the nitric oxide–cyclic GMP pathway in the central nervous system Twenty years ago, the concept that a simple gas, known to anaesthetists as a toxic contaminant of nitrous oxide cylinders, was a biological mediator, was unthinkable. However, nitric oxide has roles in blood pressure regulation, neurotransmission, immunity and cardiovascular function. Nitric oxide is synthesized from L-arginine by the action of the enzyme nitric oxide synthase (NOS). There are three distinct isoforms: type I (also known as ‘neuronal’) NOS; type II (‘inducible’) NOS; and type III (‘endothelial constitutive’) NOS. It is now clear that many cells can generate nitric oxide, and many cells express more than one isoform of the enzyme. NOS enzymes are cytochrome P450-like haem proteins with binding sites for calmodulin, NADPH, flavine adenine dinucleotide (FAD) and flavine mononucleotide (FMN), and a requirement for tetrahydrobiopterin for catalytic activity. Tetrahydrobiopterin is a member of the pterin family and is synthesized in many cells. GTP cyclohydrolase I is the first enzyme in its synthesis and is upregulated in response to the same stimuli which regulate NOS synthesis. Each NOS enzyme catalyses the five electron oxidation of L-arginine to generate nitric oxide, with citrulline as a byproduct. Type I NOS is the predominant isoform in the central nervous system. It is constitutively expressed but requires stimulation of the calcium–calmodulin pathway for activation, through increased intracellular calcium. The synthesis and release of nitric oxide by type I NOS is rapid and independent of de novo protein synthesis. The primary way in which nitric oxide mediates cellular and intercellular communication is through activation of soluble guanylyl cyclase resulting in cGMP formation and activation of downstream signalling pathways. Both excitatory and inhibitory neurotransmitter pathways are implicated in the state of anaesthesia and the nitric oxide pathway has also been shown to be a component.1 Depending on location, NOS activation in the central nervous system is coupled to one of two types of physiological stimuli: postsynaptic neurotransmitter receptor stimulation leading to calcium influx, or action potentials in presynaptic nerves resulting in calcium influx through voltage-sensitive calcium channels.2 Postsynaptically, activation of glutamate receptors has been shown to result in increases in cGMP, and is attenuated by NOS inhibitors and NMDA receptor antagonists.3 Type I NOS in the brain is apparently associated almost exclusively with neurones4 and co-localized with argininosuccinate synthase, the enzyme involved in the metabolic conversion of citrulline to arginine. The distribution of nitric oxide producing cells and those that accumulate
cGMP are usually distinct, supporting the concept of a role for nitric oxide as an inter-neuronal messenger.5 cGMP and cGMP-dependent kinases are also detectable in the spinal cord.6 7 Nitric oxide release in response to anaesthesia has been assessed in vitro in terms of nitric oxide itself, measured as its breakdown products, nitrite and nitrate; as 14C-citrulline from 14C-arginine; by conversion of oxy- to met-haemoglobin; or by chemiluminescence or electrochemical detection. In addition, cGMP release has been used as an index of nitric oxide production. In vivo studies are limited, but the use of microdialysis probes directly in the brains of rats to measure either nitrite–nitrate8 9 or cGMP10 has been described. The study by Wu and colleagues in this issue11 used such a technique, coupled with measurement of nitrite after reduction of nitrate using a cadmium-coated column and reaction with an azo dye to measure the effect of ketamine anaesthesia on nitric oxide release. A direct inhibitory effect of halothane and isoflurane12 on NOS activity in rat brain homogenates has been described. However, another study found no effect of halothane, isoflurane or enflurane on the activity of partially purified rat brain NOS.13 Ketamine and pentobarbital had no effect on rat brain NOS activity in one study,12 whereas ketamine, thiopental and midazolam inhibited NOS activity in a rat brain preparation.14 Different techniques were used in these in vitro studies and it is unclear if measurement of nitric oxide production from brain homogenates in an artificial in vitro situation provides a clear insight into the effects in vivo. There have also been conflicting results from studies investigating the effect of anaesthetics on cGMP in the central nervous system. In vitro, halothane attenuated NMDA-mediated accumulation of cGMP, but not cGMP from exogenous nitric oxide donors, in studies using rat cerebellar slices.15 Isoflurane also suppressed NMDA-stimulated cGMP but not that from nitroprusside stimulation. In contrast, thiopental suppressed cGMP release regardless of the stimulant. These authors suggested that potent inhalation agents inactivate either NOS or its co-factors, and that thiopental suppresses guanylate cyclase activity.15 However, in cultured fetal rat cortical neurones, it was found that isoflurane, in contrast, potentiated cGMP release in response to glutamatergic agonists but enflurane and halothane had no effect.16 There is an inverse correlation between cerebellar cGMP and PCO2,17 illustrating the need to maintain arterial bloodgas values during in vivo studies. It is not clear if this is a
© The Board of Management and Trustees of the British Journal of Anaesthesia 2000
Editorial II
direct effect on cGMP, although prolonged hypoxia increases cerebellar nitric oxide formation through upregulation of GTP cyclohydrolase I expression, which is the first enzyme in the pathway of tetrahydrobiopterin synthesis.18 In the study by Wu and colleagues, animals did not undergo ventilation and it is not known if normocapnia was maintained.11 There are several older reports which describe the effects of various drugs on cGMP in the brains of rats and mice. In pentobarbital and halothane anaesthetized animals, cGMP decreased in most brain regions studied.19 Benzodiazepines, morphine, sodium barbital and haloperidol also decreased cGMP.20–23 However, these changes in cGMP were attributed to altered motor function as a result of anaesthesia or sedation, as paralysis for even short periods of time also resulted in decreased cGMP.24 In addition, increased locomotor activity induced by agents which caused tremors and seizures25 or forced running26 resulted in an increase in cGMP. In rat cerebellar slices, ketamine depressed NMDA or glutamate stimulated cGMP but not non-receptor mediated cGMP.27 These in vitro studies are in contrast with the findings of increased in vivo nitric oxide production in response to the NMDA receptor antagonist ketamine, reported by Wu and colleagues.11 It might be expected that ketamine would decrease cGMP by blocking NMDA receptor-mediated NOS activation. However, acetylcholine does not activate NOS via NMDA receptors, yet acetylcholine stimulated-cGMP was reduced by ketamine in a study using rat thoracic aorta.28 Although other in vitro studies have shown decreased cGMP in response to ketamine after NDMA, quisqualate or glutamate treatment of cultured cerebral neurones, closer examination of the data reveal that ketamine did not decrease endogenous, unstimulated basal cGMP production.29 It is also worth noting that stimulation of neurones with kainate, which is a selective ionotropic agonist similar to NMDA, and which also increases intracellular calcium, was unaffected by ketamine.29 Other glutamate receptor antagonists had neither basal nor stimulated effects on cGMP. The study by Wu and colleagues11 investigated the effect of ketamine on unstimulated nitric oxide release. Alterations in calcium mobilization or inositol triphosphate are unlikely as these have been shown to be unaffected by ketamine in studies using cultured vascular smooth muscle cells. Glutamate uptake into cultured rat synaptosomes is also unaltered by ketamine. The concentration of ketamine used in in vitro studies is also relevant. Many studies have used concentrations above those encountered clinically in the circulation during ketamine anaesthesia, and local concentrations in the brain are not known. In addition, it should be appreciated that changes in nitric oxide release may not necessarily be reflected by changes in cGMP. Type I NOS knockout mice show no difference in minimum alveolar concentration for isoflurane, with no effect of NOS inhibitors, although inhibitors in normal mice
reduce MAC, thus revealing a compensatory mechanism for the absence of type I NOS.30 As another example, the increase in cerebral blood flow which occurs in response to hypercapnia is nitric oxide-dependent and is blocked by NOS inhibitors. However, hypercapnic cerebral blood flow responses remain intact in type I knockout mice, and NOS inhibitors do not block the response.31 This shows that the hypercapnic blood flow response in these animals is not caused by upregulation of other NOS isoforms but is mediated by an alternative, nitric oxide-independent mechanism. Compensation by alternative pathways appears to be a common physiological reaction to deficits in nitric oxide biosynthesis and has been observed in several systems. These findings and the interesting results presented by Wu and colleagues,11 coupled with the discrepancies in previous studies, dictate that further work is required to elucidate the complex relationship between anaesthesia and the nitric oxide pathway. H. F. Galley Academic Unit of Anaesthesia and Intensive Care University of Aberdeen Aberdeen AB25 2ZD, UK
References 1 Johns RA. Nitric oxide, cyclic GMP and the anaesthetic state. Anesthesiology 1996; 85: 457–9 2 Garthwaite J, Boulton CL. Nitric oxide signalling in the central nervous system. Annu Rev Physiol 1995; 57: 683–706 3 East SJ, Garthwaite J. NMDA receptor activation in rat hippocampus induces cyclic GMP formation through the L-arginine–nitric oxide pathway. Neurosci Lett 1991; 123: 17–9 4 Bredt DS, Hwang PM, Snyder SH. Localization of nitric oxide synthase indicating a neural role for nitric oxide. Nature 1990; 347: 768–70 5 Southam E, Garthwaite J. Nitric oxide–cyclic GMP pathway in rat brain. Neuropharmacology 1993; 32: 1267–77 6 Bhargava HN, Cao YJ. Effect of chronic administration of morphine, U-50, 488H and [D-Pen2,D-Pen5] enkephalin on the concentration of cGMP in brain regions and spinal cord of the mouse. Peptides 1997; 18: 1629–34 7 Qian Y, Chao DS, Santillano DR, et al. cGMP-dependent protein kinase in dorsal root ganglion: relationship with nitric oxide synthase and nociceptive neurons. J Neurosci 1996; 16: 3130–8 8 Luo D, Knezevich S, Vincent SR. N-methyl-D-aspartate-induced nitric oxide release: an in vivo microdialysis study. Neuroscience 1993; 57: 897–900 9 Luo D, Vincent SR. NMDA-dependent nitric oxide release in the hippocampus in vivo: interactions with noradrenaline. Neuropharmacology 1994; 33: 1345–50 10 Luo D, Leung E, Vincent R. Nitric oxide-dependent efflux of cGMP in rat cerebellar cortex: an in vivo microdialysis study. J Neurosci 1994; 14: 263–71 11 Wu J, Kikuchi T, Wang Y, Sato K, Okumura F. NOX– concentrations in the rat hippocampus and striatum have no direct relationship to anaesthesia induced by ketamine. Br J Anaesth 2000; 84: 183–9 12 Tobin JR, Martin LD, Breslow MJ, Traystman RJ. Selective inhibition of brain nitric oxide synthase. Anesthesiology 1994; 81: 1264–9 13 Rengasamy A, Pajeweski TN, Johns RA. Inhalational anaesthetic
142
Editorial II
14
15
16
17
18
19
20
21
22
effects on rat cerebellar nitric oxide and cyclic guanosine monophosphate production. Anesthesiology 1997; 86: 689–98 Galley HF, Webster NR. Brain nitric oxide synthase activity is decreased by intravenous anaesthetics. Anesth Analg 1996; 83: 591–4 Terasako K, Nakamura K, Miyawaki I, Toda H, Kakuyama M, Mori K. Inhibitory effects of anaesthetics on cyclic GMP accumulation in rat cerebellar slices. Anesth Analg 1994; 79: 921–6 Loeb AL, Gonzales JM, Reichard PS. Isoflurane enhances glutamatergic agonist stimulated nitric oxide synthesis in cultured neurons. Brain Res 1996; 734: 295–300 Mueller RA, Lundberg D, Frye GD, Mailman RB, Bresse GR. Cerebellar cGMP varies with motor function and respiratory exchange. Neurosci Lett 1978; 10: 89–92 Guo Y, Ward ME, Beasjours S, Mori M, Hussain SN. Regulation of cerebellar nitric oxide production in response to prolonged in vivo hypoxia. J Neurosci Res 1997; 49: 89–97 Kant GJ, Muller TW, Lenox RH, Meyerhoff JL. In vivo effects of pentobarbital and halothane anaesthesia on levels of adenosine 3⬘,5⬘-monophosphate and guanosine 3⬘,5⬘-monophosphate in rat brain regions and pituitary. Biochem Pharmacol 1980; 29: 1891–6 Mao CC, Guidotti A, Costa E. Inhibition by diazepam of the tremor and increase in cGMP elicited by harmaline. Brain Res 1975; 83: 516–19 Biggio G, Guidotti A, Costa E. On the mechanism of the decrease in cerebellar cyclic GMP content elicited by opiate receptor agonists. Arch Pharmacol 1977; 296: 117–21 Biggio G, Corda MG, Casu M, Gessa GL. Effect of chronic treatment with neuroleptics on the content of 3⬘,5⬘-cyclic guanosine monophosphate in cerebellar cortex of rats. Life Sci 1978; 23: 649–52
23 Lenox RH, Wray HL, Kant GJ, Hawkins TD, Meyerhoff JL. Changes in brain levels of cyclic nucleotides and γ-aminobutyric acid in barbiturate dependence and withdrawal. Eur J Pharmacol 1979; 55: 159–69 24 Lundberg DBA, Breese GR, Mailman RB, Frye GD, Mueller RA. Depression of some drug-induced in vivo changes of cerebellar guanosine 3⬘,5⬘-monophosphate by control of motor and respiratory responses. Mol Pharmacol 1979; 15: 246–56 25 Vulliemoz Y, Verosky M, Alpert M, Triner L. Effect of enflurane on cerebellar cGMP and on motor activity in the mouse. Br J Anaesth 1983; 55: 79–84 26 Kant GJ, Meyerhoff JL, Bunnell BN, Lenox RH. Cyclic AMP and cyclic GMP response to stress in brain and pituitary: stress elevates pituitary cyclic AMP. Pharmacol Biochem Behav 1982; 17: 1067–72 27 Miyawaki I, Nakamura K, Yokubol B, Kitamura R, Mori K. Suppression of cGMP formation in rat cerebellar slices by propofol, ketamine and midazolam. Can J Anaesth 1997; 44: 1301–7 28 Miyawaki I, Nakamura K, Terasako K, Toda H, Kakuyama M, Mori K. Modification of endothelium dependent relaxation by propofol, ketamine and midazolam. Anesth Analg 1995; 81: 474–9 29 Gonzales JM, Loeb AL, Reichard PS, Irvine S. Ketamine inhibits glutamate-, N-methyl-D-aspartate-, and quisqualate-stimulated cGMP production in cultured cerebral neurons. Anesthesiology 1995; 82: 205–13 30 Ichinose F, Huang PL, Zapol WM. Effects of targeted neuronal nitric oxide synthase gene disruption and nitroG-L-arginine methylester on the threshold for isoflurane anesthesia. Anesthesiology 1995; 83: 101–8 31 Irikura K, Huang PL, Ma WS, et al. Cerebrovascular alterations in mice lacking neuronal nitric oxide synthase gene expression. Proc Natl Acad Sci USA 1995; 92: 6823–7
143
