Nitric oxide, cyclic guanosine monophosphate, and the anesthetic state

Sevoflurane postconditioning improves long-term learning and memory of neonatal hypoxia-ischemia brain damage rats via the PI3K/Akt-mPTP pathway

BACKGROUND

Volatile anesthetic postconditioning has been documented to provide neuroprotection in adult animals. Our aim was to investigate whether sevoflurane postconditioning improves long-term learning and memory of neonatal hypoxia-ischemia brain damage (HIBD) rats, and whether the PI3K/Akt pathway and mitochondrial permeability transition pore (mPTP) opening participate in the effect.

METHODS

Seven-day-old Sprague-Dawley rats were subjected to brain HI and randomly allocated to 10 groups (n=24 each group) and treated as follows: (1) Sham, without hypoxia-ischemia; (2) HI/Control, received cerebral hypoxia-ischemia; (3) HI+Atractyloside (Atr), (4) HI+Cyclosporin A (CsA), (5) HI+sevoflurane (Sev), (6) HI+Sev+ LY294002 (LY), (7) HI+Sev+ L-NAME (L-N), (8) HI+Sev+ SB216763 (SB), (9) HI+Sev+Atr, and (10) HI+Sev+CsA. Twelve rats in each group underwent behavioral testing and their brains were harvested for hippocampus neuron count and morphology study. Brains of the other 12 animals were harvested 24h after intervention to examine the expression of Akt, p-Akt, eNOS, p-eNOS, GSK-3β, p-GSK-3β by Western bolting and mPTP opening.

RESULTS

Sevoflurane postconditioning significantly improved the long-term cognitive performance of the rats, increased the number of surviving neurons in CA1 and CA3 hippocampal regions, and protected the histomorphology of the left hippocampus. These effects were abolished by inhibitors of PI3K/eNOS/GSK-3β. Although blocking mPTP opening simulated sevoflurane postconditioning-induced neuroprotection, it failed to enhance it.

CONCLUSIONS

Sevoflurane postconditioning exerts a neuroprotective effect against HIBD in neonatal rats via PI3K/Akt/eNOS and PI3K/Akt/GSK-3β pathways, and blockage of mPTP opening may be involved in attenuation of histomorphological injury.

Nitric oxide induced by ketamine/xylazine anesthesia maintains hepatic blood flow during hypothermia

Among the anesthetics influencing the nitric oxide (NO) pathway, ketamine is widely reported in the literature. We researched the variations in blood physiological parameters following ketamine/xylazine- or pentobarbital-induced anesthesia, with particular emphasis on plasmatic NO levels and oxidative stress-related factors. The effects of ketamine on hepatic blood flow during deep hypothermia were also examined. Adult male Sprague-Dawley rats were anesthetized intraperitoneally with ketamine/xylazine or with sodium pentobarbital. Animals underwent serial blood extraction to analyze acid-base balance and lactate levels in blood, as well as NO, MDA, SH groups, and AST levels in plasma samples. We demonstrated that ketamine leads to increased plasmatic NO levels, induces metabolic acidosis, and causes oxidative damage, though without reaching hepatic toxicity. When experimental hypothermia was induced, ketamine affected hepatic blood flow. Based on these results, we suggest that studies on physiological processes involving NO should exercise caution if anesthesia is induced by ketamine.

Selective phosphodiesterase 5 inhibition does not reduce propofol sedation requirements but affects speed of recovery and plasma cyclic guanosine 3',5'-monophosphate concentrations in healthy volunteers

Cyclic guanosine 3',5'-monophosphate (cyclic GMP) has been implicated in modulating the effects of anesthesia. We hypothesized that limiting the breakdown of cyclic GMP through selective phosphodiesterase inhibition would influence propofol sedation requirements and plasma cyclic GMP concentrations. Ten volunteers received 100 mg of sildenafil or placebo orally in this placebo-controlled, double-blind, randomized crossover pilot study. Propofol sedation was achieved using a target-controlled infusion system until loss of verbal contact (LVC). Plasma cyclic GMP concentrations were determined at baseline, LVC, and 30 min after LVC. There was no difference in the amount of propofol used, predicted plasma concentration, or duration of sedation in volunteers after sildenafil compared with placebo treatment. Return of spontaneous verbal contact was faster after sildenafil (4 [3-8] min versus 6 [3-5] min, median [range], P = 0.019). Cyclic GMP concentrations were reduced during propofol sedation in the placebo group compared with baseline (P < 0.004). The plasma cyclic GMP concentrations were larger (P = 0.004) at LVC in the sildenafil group compared with placebo. We have shown that selective phosphodiesterase 5 inhibition decreases recovery time from propofol sedation without affecting propofol requirements. The decrease of plasma cyclic GMP concentrations during propofol sedation in the placebo group indicates a potential role of cyclic GMP in propofol anesthesia in humans.

IMPLICATIONS

Plasma cyclic guanosine 3',5'-monophosphate (cyclic GMP) concentrations are reduced during propofol sedation. Selective phosphodiesterase 5 inhibition, however, does not reduce propofol sedation requirements or plasma cyclic GMP concentrations but affects speed of recovery in healthy volunteers.

Tumor necrosis factor-alpha reduces ketamine- and propofol-induced anesthesia time in rats

Tumor necrosis factor-alpha (TNFalpha) is a crucial neuromodulator in the brain. TNFalpha is involved in many physiological events including pain response and sleep. However, the interactions between TNFalpha and anesthetics have not been elucidated yet. In the present study, we investigated the effects of four intracerebroventricular (ICV) doses (1, 10, and 100 pg, and 1 ng) and two intraperitoneal (IP) doses (10 and 100 ng) of TNFalpha on anesthesia time of ketamine (100 mg/kg IP) and propofol (80 mg/kg IP) in rats. All ICV doses of TNFalpha reduced anesthesia time of ketamine and propofol compared with the saline ICV group (ketamine control group, 45.4 +/- 6.5 min; propofol control group, 43.5 +/- 11.0 min). The maximum effect was obtained after the ICV injection of 10 pg of TNFalpha (76% and 54% of ketamine and propofol control groups, respectively). Anesthesia time of ketamine or propofol was also decreased by IP injection of TNFalpha in a dose-dependent manner. Injection of 100 ng of TNFalpha IP reduced anesthesia time of ketamine and propofol by 67% and 64% of each control group, respectively. These data show that TNFalpha can modulate the anesthesia time of IV anesthetics, suggesting that anesthetic requirements might be altered in the presence of cerebral or systemic inflammation.

IMPLICATIONS

Tumor necrosis factor alpha (TNFalpha) regulates many physiological events in the brain. We investigated the effects of TNFalpha on anesthesia time in rats. Both central and peripheral administration of TNFalpha decreased anesthesia time induced by ketamine and propofol.

Thromboelastography: a reliable test?

The thromboelastograph (TEG), a measure of global haemostasis, is routinely used during cardiac and hepatic surgery to optimize blood product selection and usage. It has recently been suggested that it may also be a useful tool to screen patients with hypercoagulable states. Limited published data on performance characteristics has led to speculation regarding its consistency and, therefore, validity of the results. This study was designed to assess the effect of stability of blood samples prior to testing, repeated sampling, intra- and inter-assay variability using the native, celite, tissue factor (TF) and Reopro-modified TEG. Analysis of native and celite samples after storage over 90 min showed a period of instability up to 30 min. Thereafter, all parameters between 30 and 90 min were stable [P = not significant (NS)]. When the same sample was repeatedly assayed, both native and celite TEG parameters showed a significant change towards hypercoagulability (P < 0.01), whereas the TF and Reopro-modified TEG showed no change. Intra- and inter-assay variability on samples tested after 30 min showed excellent reproducibility for all parameters (P = NS). The data suggest that the TEG is a useful tool in haemostasis but requires a formal standard operating procedure to be adopted that takes into account the initial period of sample instability.

Neuroprotective effect of aminoguanidine on transient focal ischaemia in the rat brain

Using serial magnetic resonance imaging we have evaluated the effectiveness of aminoguanidine (AG) as a neuroprotective agent in a rat model of transient middle cerebral artery occlusion (MCAO). Because aminoguanidine's neuroprotective properties have primarily been ascribed to its action as iNOS inhibitor, we also performed a biochemical analysis of nitric oxide metabolites and NOS isoforms in our model of ischaemia. Daily injections of AG (100 mg/kg) or saline, were started at 6 h after the occlusion and the effects of this treatment on lesion progression monitored by T(2)-weighted MRI at 6 (pre-treatment scan), 24 and 72 h. Measurements of lesion volumes showed that between 6 and 72 h post-MCAO, lesion growth was slower in AG-treated rats than in control rats. This difference was most pronounced between 24 and 72 h post-MCAO when AG halted the lesion volume expansion observed in control rats. Measurements of plasma NOx (nitrite plus nitrate) at 0, 24, 48 and 72 h after MCAO, showed that NO levels did not differ significantly between the AG- and saline-treated groups at any time-point. Moreover, NOS activity assays revealed that no iNOS activity was present in any of the brains tested and that constitutive neuronal NOS activity was similar across the two hemispheres between both groups. The absence of iNOS protein in the ischaemic and contralateral hemispheres at 48 and 72 h after MCAO (control group only) was confirmed by Western blot analysis. These results suggest that AG treatment reduces the rate of growth of ischaemic lesions, perhaps preserving the functioning of perifocal neurons. Our observations contradict suggestions that high levels of NO generated by iNOS are partially responsible for exacerbating the neuronal damage in the postischaemic phase of MCAO. Although this does not rule out a role for AG as a neuroprotective agent via its ability to inhibit iNOS, these findings indicate that neuroprotective actions of AG may also be mediated via other cellular targets.

Differential nitric oxide synthase activity, cofactor availability and cGMP accumulation in the central nervous system during anaesthesia

We investigated the effects of anaesthesia on dynamic nitric oxide production, concentrations of tetrahydrobiopterin and the accumulation of cyclic GMP (cGMP) in the rat central nervous system (CNS). Rats were assigned to anaesthesia with halothane, isoflurane, pentobarbital, diazepam, ketamine or xenon (n=6 per group). After 30 min, [14C]L-arginine (i.v.) was given and, after a further 60 min of anaesthesia, rats were killed and exposed immediately to focused microwave radiation. After removal of the brain and spinal cord, nitric oxide production from radiolabelled arginine (and hence nitric oxide synthase activity during anaesthesia) was measured as [14C]L-citrulline by scintillation counting. cGMP was determined by enzyme immunoassay and tetrahydrobiopterin by fluorescence HPLC, in brain regions and the spinal cord. Nitric oxide synthase activity was similar in all brain regions but was lower in the spinal cord, and was unaffected by anaesthesia. cGMP was similar in all areas of the CNS and was significantly decreased in rats anaesthetized with halothane. Isoflurane produced similar effects. In contrast, ketamine and xenon anaesthesia increased cGMP in the spinal cord, brainstem and hippocampus. Diazepam and pentobarbital had no effect. Tetrahydrobiopterin concentrations were similar in all areas of the CNS and were increased in the cortex and hippocampus after anaesthesia. We have shown profound differential effects of anaesthesia on the nitric oxide pathway in the rat CNS.

Use of abciximab-modified thrombelastography in patients undergoing cardiac surgery

Thrombelastography (TEG) is a reliable coagulation monitoring system that can guide blood product transfusion in cardiac surgery. The maximum amplitude (MA) of TEG measures clot strength, which is dependent on both fibrinogen level and platelet function. Inhibition of platelet function with abciximab-fab is suggested to permit quantitative assessment of the contribution of fibrinogen to clot strength. We hypothesized that abciximab-modified TEG permits prediction of plasma fibrinogen levels and that the difference of standard MA and abciximab-modified MA (deltaMA) is a correlate for platelet function. We correlated abciximab-modified MA with plasma fibrinogen levels and deltaMA with platelet count in patients undergoing coronary revascularization. Correlation between plasma fibrinogen levels and abciximab-modified MA was significant (adjusted r2: 0.8; P < 0.0001). Correlation of deltaMA with platelet count was not significant when calculated in millimeters (adjusted r2: 0.04; P = 0.73). However, when deltaMA was calculated in dynes per square centimeter (deltaGMA), it correlated significantly with platelet count (adjusted r2: 0.51; P < 0.0001). We conclude that abciximab-modified TEG may therefore help to discriminate between hypofibrinogenemia and platelet dysfunction as a cause of decreased MA.

IMPLICATIONS

We examined the use of abciximab-modified thrombelastography in patients undergoing cardiac surgery. Modification of thrombelastography with abciximab-fab allows prediction of fibrinogen levels, despite coagulation altered by cardiac surgery. The difference of standard maximum amplitude and abciximab-modified maximum amplitude correlates with platelet function when expressed in dynes per square centimeter.