Thiopental and methohexital depress Ca2+ entry into and glutamate release from cultured neurons

Hippocampal recording via the RNS system reveals marked ipsilateral activation of epileptiform activity during Wada testing

Wada testing remains an important component of pre-surgical testing to assess the feasibility of temporal lobectomy for patients with intractable epilepsy. In this procedure, an anesthetic is injected into either internal carotid artery while memory and language testing is performed, simulating the effect of temporal lobe resection. The mechanism remains poorly understood because the hippocampal vasculature is predominantly via the posterior circulation. We recorded hippocampal EEG during bilateral methohexital Wada testing in three patients who had previously been implanted with a responsive neurostimulation system (RNS) to determine the effect of the injections on hippocampal activity. In all six injections from three patients, methohexital caused immediate, transient increases in hippocampal spikes. With at least two of these injections, the electrographic changes were consistent with electrographic seizures. In all cases, the epileptiform activity was not apparent on scalp EEG and was without obvious clinical correlate other than the negative findings expected from the anesthetic. The results demonstrate the utility of intracranial EEG during Wada testing and suggest that the elicitation of seizures or continuous spiking might contribute to dysfunction of the hippocampus during the Wada test. We hypothesize that this effect is due to disconnection and disinhibition of medial temporal structures.

Severe hypokalemia and rebound hyperkalemia during barbiturate coma in patients with severe traumatic brain injury

OBJECTIVES

To evaluate the incidence of severe potassium disturbances during barbiturate coma therapy in patients with severe traumatic brain injury (TBI), and the characteristics of these patients.

METHODS

The study comprised 37 patients with severe TBI who were treated for barbiturate coma between 2015 and 2017 in level 3 intensive care units of two hospitals.

RESULTS

No potassium disturbance occurred in 14 patients. Seventeen patients developed mild-moderate hypokalemia (2.6-3.5mEq/L), and 6 patients developed severe hypokalemia (<2.5mEq/L) following the induction of barbiturate therapy. The incidence of mild-to-severe barbiturate-induced hypokalemia was 62.2% and the rate of severe hypokalemia was 16.2%. The mean potassium supply per day during thiopentone therapy was statistically significantly different between patients with mild-to-moderate hypokalemic and those with severe hypokalemic (p<0.001). Four of 6 patients with severe hypokalemia developed rebound hyperkalemia exceeding 6mEq/L following the cessation of barbiturate infusion. The nadir potassium concentration was 1.5mEq/L and the highest value was 6.8mEq/L. The mean time to reach nadir potassium concentrations was 2.8 days. The mortality rate of the 6 patients was 66.7%. Of the 2 survivors of severe hypokalemia, the Glasgow Outcome Scale (GOS) on discharge and the extended GOS one year after the trauma were 5 and 8 respectively.

CONCLUSIONS

Severe hypokalemia refractory to medical treatment and rebound hyperkalemia is a serious adverse effect of thiopentone coma therapy in patients with severe TBI. Excessive and aggressive potassium replacement during the barbiturate-induced hypokalemia period must be avoided. Weaning barbiturate treatment over time may be advantageous in the management of severe serum potassium disturbances.

(+)-Dehydrofukinone modulates membrane potential and delays seizure onset by GABAa receptor-mediated mechanism in mice

(+)-Dehydrofukinone (DHF), isolated from Nectandra grandiflora (Lauraceae) essential oil, induces sedation and anesthesia by modulation of GABAa receptors. However, no study has addressed whether DHF modulates other cellular events involved in the control of cellular excitability, such as seizure behavior. Therefore, the aim of the present study was to investigate the effect of DHF on cellular excitability and seizure behavior in mice. For this purpose, we used isolated nerve terminals (synaptosomes) to examine the effect of DHF on the plasma membrane potential, the involvement of GABAa receptors and the downstream activation of Ca²⁺ mobilization. Finally, we performed an in vivo assay in order to verify whether DHF could impact on seizures induced by pentylenetetrazole (PTZ) in mice. The results showed that DHF induced a GABA-dependent sustained hyperpolarization, sensitive to flumazenil and absent in low-[Cl^-] medium. Additionally, (1-100μM) DHF decreased KCl-evoked calcium mobilization over time in a concentration-dependent manner and this effect was prevented by flumazenil. DHF increased the latency to myoclonic jerks (10mg/kg), delayed the onset of generalized tonic-clonic seizures (10, 30 and 100mg/kg), and these effects were also blocked by the pretreatment with flumazenil. Our data indicate that DHF has anticonvulsant properties and the molecular target underlying this effect is likely to be the facilitation of GABAergic neuronal inhibition. The present study highlights the therapeutic potential of the natural compound DHF as a suppressor of neuronal excitability.

Taurine and glutathione levels in plasma before and after ECT treatment

Taurine has been shown to be elevated in plasma and lymphocytes of depressed patients, but the level normalises after successful drug therapy. During depression, levels of glutathione (GSH) are decreased in the plasma and blood. This study was performed to examine taurine and GSH levels in depressed patients before and after electroconvulsive therapy (ECT). Fasting blood samples were collected from 23 patients before the first and after the third ECT treatment. The severity of depression was estimated with the Montgomery-Åsberg Depression Rating Scale (MADRS). We analysed GSH in blood and the levels of taurine and total GSH in plasma. After three ECTs, a significant decrease in MADRS scores was found for the entire group. Simultaneously, the decrease in the plasma taurine levels was significant for the seven responders but not for the sixteen non-responders. We observed no differences in blood or plasma GSH levels after three ECT treatments when compared to values before the therapy. Plasma taurine levels decrease significantly after three ECT treatments in patients who respond to treatment. GSH levels were not affected by ECT treatment. The results indicate that taurine may play a role in the pathophysiology of depression.

Quantitative Evaluation of the Neuroprotective Effects of Thiopental Sodium, Propofol, and Halothane on Brain Ischemia in the Gerbil: Effects of the Anesthetics on Ischemic Depolarization and Extracellular Glutamate Concentration

Although propofol and thiopental are commonly used as neuroprotective agents, it has not been determined which is more neuroprotective. This study was designed to quantitatively evaluate the neuroprotective effects of thiopental, propofol, and halothane on brain ischemia by determining P50, ischemic time necessary for causing 50% neuronal damage. Gerbils were anesthetized with thiopental, propofol, or halothane and underwent 2-vessel occlusion (0, 3, 5 or 10 min). Direct current potentials were measured in bilateral CA1 regions, in which histologic evaluation was performed 5 days later. In some animals, extracellular glutamate concentrations (microdialysis) were measured during 7.5 minutes of ischemia. P50 in the thiopental, propofol, and halothane groups were estimated to be 8.4, 6.5 (P<0.05, vs. thiopental), and 5.1 (P<0.05) minutes, respectively. Durations of ischemic depolarization were equally reduced in the thiopental and propofol groups compared with that in the halothane group. Severity of neuronal damage with identical duration of ischemic depolarization was attenuated by thiopental compared with the effect of propofol. Maximum glutamate concentrations in the thiopental and propofol group were significantly reduced compared with that in the halothane groups but were comparable. By using P50, we found that the neuroprotective effect of thiopental was greater than that of propofol. Although duration of ischemic depolarization was equally reduced in thiopental and propofol groups, thiopental has a greater suppressive effect on neuronal injury during identical duration of ischemic depolarization than propofol does. Glutamate concentration during brain ischemia tended to be attenuated more by thiopental than by propofol, but it was not statistically significant.

Changes in plasma amino acids after electroconvulsive therapy of depressed patients

There are indications that mood disorders may be related to perturbations in the amino acid transmitters. The amino acids may thus be targets of treatment of depression. The purpose of this pilot study was to measure the acute effects of a single administration of electroconvulsive therapy (ECT) on the plasma levels of amino acids in depressed patients. ECT was administered to 10 patients with major depressive disorder. Altogether 23 plasma amino acids were analyzed before and at 2, 6, 24 and 48 h after ECT. The levels of glutamate and aspartate increased at 6 h after ECT compared with the baseline. Also the levels of total tryptophan increased 2-24 h after ECT. There were also elevations in other amino acids at 6 and 24 h. The levels of gamma-aminobutyric acid (GABA) decreased at 2 h. In this study the acute effects of single ECT were associated with changes in the levels of glutamate, aspartate, GABA, tryptophan and some other amino acids. The preliminary data suggest that the therapeutic effects of ECT in depression may be due to mechanisms involving these amino acid transmitters.

Thiopental sodium reduces glutamate extracellular levels in rat intact prefrontal cortex

To investigate the effect of thiopental sodium on glutamate extracellular levels in the prefrontal cortex (PFC) of rats, a microdialysis probe was inserted into the PFC, the perfusate was collected every 10 min throughout the experiment with thiopental sodium ip or perfused into the PFC locally. The concentrations of glutamate in the perfusate were determined by reversed-phase high performance liquid chromatography. Thiopental sodium 30 mg kg(-1) ip significantly decreased glutamate levels in the perfusate after 10, 20, 30, and 40 min; glutamate levels in the perfusate were also decreased from 10 to 90 min after thiopental sodium 50 mg kg(-1) ip. Thiopental sodium with concentrations of 30, 100, or 300 microM perfused into the PFC also decreased glutamate levels in the perfusate significantly. The results suggest that thiopental sodium decreases glutamate extracellular levels in rat intact PFC.

Effect of thiopental sodium on the release of glutamate and gamma-aminobutyric acid from rats prefrontal cortical synaptosomes

To investigate the effect of thiopental sodium on the release of glutamate and gamma-aminobutyric acid (GABA) from synaptosomes in the prefrontal cortex, synaptosomes were made, the spontaneous release and the evoked release by 30 mmol/L KCl or 20 micromol/L veratridine of glutamate and GABA were performed under various concentrations of thiopental sodium (10-300 micromol/L), glutamate and GABA concentrations were determined by reversed-phase high-performance liquid chromatography. Our results showed that spontaneous release and evoked release of glutamate were significantly inhibited by 30 micromol/L, 100 micromol/L and 300 micromol/L thiopental sodium, IC50 of thiopental sodium was 25.8 +/- 2.3 micromol/L for the spontaneous release, 23.4 +/- 2.4 micromol/L for KCl-evoked release, and 24.3 +/- 1.8 micromol/L for veratridine-evoked release. But GABA spontaneous release and evoked release were unaffected. The study showed that thiopental sodium with clinically related concentrations could inhibit the release of glutamate, but had no effect on the release of GABA from rats prefrontal cortical synaptosomes.

Effects of methohexital on bioelectrical reactions in guinea pig hippocampal slices during hypoxia

The barbiturate methohexital (42 and 140 micromol/l) was tested for an acute effect on anoxic depolarization (AD) and evoked potentials (EP) in hippocampal slices of guinea pigs exposed to repeated hypoxic conditions (n = 78). The dosages of methohexital resemble the range of plasma levels measured in patients with an intraoperative burst suppression electroencephalogram. Direct current potential and EP were recorded in the CA1 region. Hypoxia was terminated either when AD had reached its peak, or 2 min after maximum AD. Excluding the actions of methohexital on cerebral blood flow, reperfusion phase and the delayed mechanisms of cellular protection, a dose-dependent direct effect on EP even after repeated hypoxic conditions could be observed.

Amobarbital inhibits K(+)-stimulated glucose oxidation in cerebellar granule neurons by two mechanisms

The study aimed at determining mechanism(s) by which amobarbital (amytal) suppresses glucose oxidation in cerebellar granule neurons in primary cultures, a glutamatergic preparation. When challenged with a depolarizing K(+) concentration (55 mM), the cells doubled their rate of glucose oxidation (production of 14CO(2) from U-[14C]glucose) and glycolysis (lactate accumulation). At normal K(+) concentration, amobarbital reduced 14CO(2) production with half-maximum effect at 0.5-1 mM; at 55 mM K(+), the inhibition was more potent, with more than half of the K(+)-induced stimulation abolished at 50 microM. Dixon plot analysis showed a single inhibitory mechanism at 5.4 mM K(+), but at 55 mM K(+), two kinetically different mechanisms could be distinguished. A more pronounced compensatory amobarbital-induced increase in glycolysis at 5.4 than at 55 mM K(+) suggested that amobarbital in addition to its inhibition of mitochondrial respiration inhibited K(+)-induced increase in energy demand, probably by its known suppression of stimulated glutamate release.

Free radicals, antioxidants, and neurologic injury: possible relationship to cerebral protection by anesthetics

Oxygen-centered free radicals cause brain injury associated with trauma and stroke. These reactive oxygen species may be detoxified by endogenous antioxidants, but cell death occurs after antioxidants become depleted. General anesthetics penetrate into brain parenchyma, where they may abrogate oxidative injury to neurons by several mechanisms that prevent the initiation of free radical chain reactions or terminate the propagation of highly reactive radicals. First, general anesthetics may inhibit free radical generation because these drugs slow cerebral utilization of oxygen and glucose, inhibit oxidative metabolism in neutrophils, and prevent redox changes in hemoglobin. Second, antioxidant anesthetics, such as thiopental and propofol, directly scavenge reactive oxygen species and inhibit lipid peroxidation. Finally, anesthetics may prevent the elevation of extracellular glutamate concentration and inhibit the activation of excitatory glutamatergic receptors that augment oxidative stress after ischemia.

Thiopentone and methohexital, but not pentobarbitone, reduce early focal cerebral ischemic injury in rats

PURPOSE

Although barbiturates are considered to be cerebral protectants, little is known regarding the relative efficacy of different barbiturates to reduce ischemic brain injury. In a model of middle cerebral artery occlusion (MCAo), we compared the relative effects of 1.0 and 0.4 burst-suppression doses of thiopentone, methohexital, and pentobarbitone on cerebral infarct.

METHODS

During isoflurane anesthesia, MCAo was achieved via a temporal craniotomy. Thirty minutes before MCAo the rats were randomized to receive one of the following which was maintained throughout the study. Halothane (n=20)-1.2 MAC halothane, thiopentone (n=20), methohexital (n=20), or pentobarbitone (n=20). The first ten animals in each barbiturate group received the respective barbiturate in a dose sufficient to maintain burst-suppression of the electroencephalogram (3-5 bursts x min(-1)). The subsequent ten animals in each barbiturate group received 40% of the burst-suppression dose. After 180 min of MCAo and 120 min of reperfusion, cerebral injury was assessed.

RESULTS

For the burst-suppression animals, injury volume (mm3, mean +/- SD) was less in the thiopentone group (88 +/- 14) than the halothane (133 +/- 17), methohexital (126 +/- 19), or pentobarbitone (130 +/- 17) groups (P <0.05). For 0.4 burst-suppression animals, injury volume was less for the methohexital group (70 +/- 22) than the halothane (124 +/- 24), thiopentone (118 +/- 15), or pentobarbitone (121 +/- 20) groups (P <0.05).

CONCLUSIONS

These data are inconsistent with the longstanding assumption that electrophysiologically comparable doses of the various classes of barbiturates have equivalent protective efficacy. They in turn suggest that mechanisms other than, or at least in addition to, metabolic suppression may contribute to the protective effect of barbiturates.

Prolonged thiopentone infusion for neurosurgical emergencies: usefulness of therapeutic drug monitoring

Serial serum thiopentone concentrations were measured during and following completion of an intravenous infusion of thiopentone in 20 patients with neurosurgical emergencies. The concentration data from a further 55 patients who had had some such measurements were reviewed retrospectively. The patients received an infusion for longer than 24 hours at a rate adjusted to maintain EEG burst suppression. The data were interpreted in terms of thiopentone pharmacokinetics and used to produce statistical models relating to clinical outcomes. In these patients, the one-month mortality rate following commencement of thiopentone treatment was 20%; the mean durations of pupillary and motor unresponsiveness following cessation of an infusion were 22 and 91 hours, respectively. Predictors of a prolonged duration of motor unresponsiveness included a prolonged duration of pupillary unresponsiveness, a low thiopentone clearance and a high maximum serum concentration of thiopentone. From pooled logistic regression, median effective serum thiopentone concentrations (EC50) were found to be 50 mg x l(-1) for recovery of pupillary responsiveness and 12 mg x l(-1) for the recovery of motor responsiveness. Because prolonged high-dose thiopentone leads to prolonged residual serum concentrations, it is difficult to distinguish the residual pharmacological effects of thiopentone from the clinical condition. This study suggests that, based on EC50 values for responses, monitoring of post-infusion serum thiopentone concentrations may help determine whether a patient's clinical state is due to residual thiopentone pharmacological effects.

Barbiturates inhibit K(+)-evoked noradrenaline and dopamine release from rat striatal slices--involvement of voltage sensitive Ca(2+) channels

The cellular target site(s) for anaesthetic action remain unclear. In rat striatal slices we have previously demonstrated that K(+)-evoked noradrenaline (NA) and dopamine (DA) release is mediated predominantly via P/Q-type voltage sensitive Ca(2+) channels (VSCC). Using this model of Ca(2+) dependent transmitter release we have evaluated the effects of anaesthetic and non-anaesthetic barbiturates. Rat brain striatal slices were incubated in the absence and presence of barbiturate for 10 min at 37 degrees C. The slices were then incubated for 6 min with 40 mM KCl. All anaesthetic barbiturates produced a concentration-dependent inhibition of K(+)-evoked NA and DA release. Non-anaesthetic barbiturate, barbituric acid was ineffective. The pIC(50) for NA and DA release (thiopental: 4.90+/-0.13 and 5.00+/-0.10, pentobarbital: 4.39+/-0.07 and 4.43+/-0.14, phenobarbital: 3.85+/-0.08 and 3.59+/-0.10, respectively) correlated with lipid solubility (NA: r(2)=0.999, DA: r(2)=0.987). We therefore suggest that barbiturates inhibit catecholamine release via an interaction with P/Q VSCC further implicating this channel in anaesthetic action.

Effects of temperature on calcium-sensitive fluorescent probes

The effect of temperature on the binding equilibria of calcium-sensing dyes has been extensively studied, but there are also important temperature-related changes in the photophysics of the dyes that have been largely ignored. We conducted a systematic study of thermal effects on five calcium-sensing dyes under calcium-saturated and calcium-free conditions. Quin-2, chlortetracycline, calcium green dextran, Indo-1, and Fura-2 all show temperature-dependent effects on fluorescence in all or part of the range tested (5-40 degrees C). Specifically, the intensity of the single-wavelength dyes increased at low temperature. The ratiometric dyes, because of variable effects at the two wavelengths, showed, in general, a reduction in the fluorescence ratio as temperature decreased. Changes in viscosity, pH, oxygen quenching, or fluorescence maxima could not fully explain the effects of temperature on fluorescence. The excited-state lifetimes of the dyes were determined, in both the presence and absence of calcium, using multifrequency phase-modulation fluorimetry. In most cases, low temperature led to prolonged fluorescence lifetimes. The increase in lifetimes at reduced temperature is probably largely responsible for the effects of temperature on the physical properties of the calcium-sensing dyes. Clearly, these temperature effects can influence reported calcium concentrations and must therefore be taken into consideration during any investigation involving variable temperatures.

The influence of the timing of administration of thiopentone sodium on nitric oxide-mediated neurotoxicity in vitro

Thiopentone sodium is a highly useful pharmacological agent that provides a neuroprotection against cerebral ischaemia. Since not all patients can receive thiopentone sodium before cerebral ischaemia occurs, we investigated the influence of timing of thiopentone sodium administration on the neurotoxicity induced by nitric oxide (NO) using Shibuta's established model of primary brain cultures. Cortical neurones prepared from 16-day gestational rat foetuses were used after 13-14 days in culture. The cells were exposed to an NO-donor, NOC-5 at 30 microM. Thiopentone sodium administered at 30 and 10 min before or 5, 10 and 15 min after exposure to NOC-5, but not thereafter, significantly attenuated NO-induced neurotoxicity compared with controls. The survival rate of the neurones in which thiopentone sodium was administered at 15 min after exposure to NOC-5 was 55.7+/-2.4%, compared to a 10.0+/-1.6% survival rate in neurones when thiopentone sodium was administered at 30 min after exposure to NOC-5. These findings demonstrate that thiopentone sodium, which protects cerebral cortical neurones against NO-mediated cytotoxicity, should be given as soon as possible in case ischaemic or hypoxic neuronal damage is predicted.

[Neuroprotective effects of anesthetics]

During cerebral ischaemia, energetic failure of injured cells together with excessive release of glutamate the most common excitatory amino acid in the brain, lead to excitotoxicity and immediate or delayed neuronal death. There is strong experimental evidence to support the neuroprotective role played by anaesthetic agents. Hence, barbiturates, volatile anesthetics or ketamine exhibit significant protective effects against ischaemic injury in numerous experimental models of ischaemia in vitro or in vivo. The neurobiological substrate of this action is probably a reduction of the activity of glutamate receptors (N-methyl-D-aspartate and kainate), and/or downstream biochemical events. Reduction of cerebral metabolism by these agents seems not to be their primary neuroprotective mechanism. However, no data are available at the present time to support any clinical benefit of these actions in neurosurgical patients, head trauma in contrast to mild hypothermia or cerebrovascular disease. Future research should develop models as close as possible to the clinical situation to examine further pathophysiological hypotheses and clinical implications.