Effects Of Barbiturates Research Articles

Effects of Barbiturates on the Expression of the Inducible Nitric Oxide Synthase in Vascular Smooth Muscle

Certain cytokines stimulate the expression of the inducible nitric oxide synthase (iNOS) in vascular smooth muscle cells (VSMCs) and in many other cell types. The large amounts of nitric oxide (NO) generated by iNOS in the vascular wall contribute to the unrelenting hypotension in septic shock. Because septic patients are often treated with barbiturates, we examined the effect of these anesthetic agents on the expression of iNOS in VSMCs. The induction of iNOS was elicited either in cultured rat aortic SMCs [interleukin-1beta (IL-1beta), 60 U/ml for 24 h] or in endothelium-denuded segments of the rabbit carotid artery [IL-1beta (100 U/ml) for 7 h]. The activity of iNOS was assessed by the accumulation of nitrite in the conditioned medium of cultured VSMCs and by the hyporeactivity of carotid arteries to phenylephrine. Moreover, iNOS protein abundance was determined by Western blot analysis, iNOS messenger RNA (mRNA) by reverse transcription followed by the polymerase chain reaction (PCR), and activation of the transcription factor NF-kappaB by gel electrophoretic mobility-shift analysis of nuclear extracts from VSMCs. The IL-1beta-stimulated increase in nitrite formation, iNOS protein, and mRNA abundance in VSMCs was significantly augmented in the presence of thiopental (100 microM), whereas methohexital, hexobarbital, pentobarbital, and phenobarbital were without effect. The potentiating effect of thiopental was observed only when the barbiturate was administered during the first 2 h of the 24-h incubation period of cultured VSMCs with IL-1beta. Thiopental did not affect the IL-1beta-stimulated activation of NF-kappaB in VSMCs. This barbiturate also significantly augmented the hyporeactivity to phenylephrine in carotid arteries exposed to IL-1beta, an effect that was abolished by N(G)-nitro-L-arginine. Exposure of either cultured or native VSMCs to thiopental alone did not stimulate iNOS expression. These findings demonstrate that the thiobarbiturate thiopental, but not oxybarbiturates, augments the IL-1beta-stimulated synthesis of NO in both cultured and native VSMCs. This effect of thiopental is the result of an increased expression of iNOS, involving most likely mechanisms distinct from NF-kappaB activation. The use of thiopental for long-term treatment of septic patients might possibly potentiate the biosynthesis of NO in the vascular wall and thus cause a further deterioration of the hemodynamic state.

Nocifensive reflex thresholds in rats: measures of central nervous system effects of barbiturates.

To characterize the pharmacodynamic relationships between plasma pentobarbitone and thiopentone concentrations and nocifensive reflexes during emergence from anaesthesia. Forty-nine rats were studied. Plasma barbiturate concentrations were measured with high performance liquid chromatography. Nocifensive reflexes were assessed with the hindlimb withdrawal latency (WL) to heat and the somatic motor response threshold (SMRT) to tail pressure. In Protocol I, SMRT, WL, sedation, and the presence of paw-licking and the righting reflex were assessed in unrestrained rats before and every 10 min for two hours after an intraperitoneal injection of pentobarbitone (30 mg.kg-1). Plasma pentobarbitone kinetics were determined in a separate group of rats. In Protocol II, SMRT and drug concentrations were measured concurrently in partially restrained animals before and for 35 min after a computer-controlled i.v. bolus of thiopentone. In Protocol III the SMRT-plasma thiopentone relationship was determined during increasing and decreasing plasma thiopentone concentrations. Enhancement of both nocifensive reflexes was observed in the unrestrained animals. Enhancement of SMRT was maximal [175% (153-197) of control values] at a mean plasma thiopentone concentration of 11 (9-13) micrograms.ml-1. The SMRT-plasma thiopentone curve showed a mean efflux-influx difference in plasma thiopentone concentration of 4(2.3-5.7) micrograms.ml-1. Barbiturate-associated nocifensive reflex enhancement occurs in unrestrained animals with both thermal and pressure stimuli. The SMRT-plasma thiopentone concentration relationship during emergence from anaesthesia was similar to that observed previously during induction. The thiopentone plasma concentration-SMRT plot showed an equilibrium delay similar to that previously described by others for thiopentone at an electroencephalographic effect site.

Anticonvulsant treatment in old age--principles and differential indications

When administering anticonvulsive drugs to the elderly, a number of peculiarities should be taken into consideration. Age-related changes in pharmacokinetics and drug interactions can make such treatment a complicated issue. Some of the side effects which hardly play a role among younger patients can lead to fatal consequences among the elderly. Both phenytoin (PHT), if submitted intravenously (but not in oral form), and carbamazepine (CBZ) may cause life-threatening cardiac arrhythmias. Valproate (VPA), otherwise well tolerated, seems to be less effective than CBZ and PHT in partial seizures. Cognitive dysfunction is a known side effect of barbiturates, but also seems to occur among the other drugs of first choice. In contrast to a widely held opinion, VPA, CBZ and PHT hardly differ in their effect on cognitive function if administered correctly.

Thiobarbiturates suppress depolarization-induced contraction of vascular smooth muscle without suppression of calcium influx

We have studied the effects of barbiturates on vascular smooth muscle tension and cytosolic calcium concentrations ([Ca2+]i) in endothelium-denuded rat aortic rings, preloaded with fluo-3. Changes in [Ca2+]i were estimated by the fluorescence intensity of the calcium-bound form of fluo-3. In aortic rings under basal conditions, thiobarbiturates (thiopentone and thiamylal 100-300 mumol litre-1) increased [Ca2+]i, concomitantly with an increase in tension, although oxybarbiturates (pentobarbitone and secobarbitone up to 300 mumol litre-1) had no effect. Thiopentone (300 mumol litre-1)-induced increases in tension and fluorescence intensity were mean 25.1 (SD 3.2)% and 55.0 (6.0)%, respectively, of those induced by KCl 30 mmol litre-1 (n = 8, each). In KCl (30 mmol litre-1)-precontracted aortic rings, thiopentone decreased tension without reduction of [Ca2+]i, whereas pentobarbitone decreased tension and [Ca2+]i, KCl (30 mmol litre-1)-induced contraction was suppressed by pretreatment with all barbiturates (100-300 mumol litre-1); thiopentone 300 mumol litre-1 suppressed contraction to 64.8 (2.5)% (n = 6) and pentobarbitone 300 mumol litre-1 to 57.5 (2.2)% (n = 9). However, the increase in [Ca2+]i was suppressed by oxybarbiturates (pentobarbitone 300 mumol litre-1 to 77.9 (5.2) %; n = 9), but not altered by thiobarbiturates. These results suggest that thiobarbiturates and oxybarbiturates affect vascular smooth muscle differently and that the affected site in thiobarbiturate-induced vasodilatation is distal to regulation of [Ca2+]i.

Pentobarbital enhances cyclic adenosine monophosphate production in the brain by effects on neurons but not glia.

Cyclic adenosine monophosphate (cAMP) is an important regulator of neuronal excitability. The effects of barbiturates on cAMP production in intact neurons are not known. This study used cultures of cortical neurons, cultures of glia, and slices of cerebral cortex from the rat to study the effects of barbiturates on cAMP regulation in the brain. Primary cultures of cortical neurons or glia were prepared from 17-day gestational Sprague-Dawley rat fetuses and were used after 12-16 days in culture. Cross-cut slices (300 microns) were prepared from cerebral cortex of adult rats. Cyclic AMP accumulation was determined by measuring the conversion of [3H]adenosine triphosphate (ATP) to [3H]cAMP in cells preloaded with [3H]adenine. Pentobarbital enhanced isoproterenol- and forskolin-stimulated, but not basal, cAMP accumulation in cultures of cerebral neurons. Cyclic AMP production was enhanced by pentobarbital in a dose-dependent fashion up to a concentration of 250 microM; This concentration of pentobarbital increased cAMP production by 40-50% relative to that in controls without pentobarbital. At 500 microM pentobarbital, the magnitude of the enhancement was less. Pentobarbital had no effect on isoproterenol-stimulated cAMP production in cultures containing only glia. Pentobarbital also enhanced isoproterenol-stimulated, but not basal, cAMP production in slices of cerebral cortex by approximately 30% at concentrations of 62.5-250 microM and by almost 100% at 500 microM. Pentobarbital enhances stimulated cAMP accumulation in cultured preparations from brain and fresh cortical slices. Neurons are required for this effect. Because cAMP modulates neuronal excitability, this effect of pentobarbital may be an important mechanism by which this anesthetic influences brain function.

Effect of dexamethasone, barbiturate and hypothermy on edema induced by CO2 laser brain lesion in the dog: light and electron microscopic study.

Experiments were carried out to compare the effectiveness of dexamethasone, a barbiturate, and hypothermy on experimental cerebral edema caused by CO2 laser in dogs. Experimental brain lesions were created over the right frontal cortex of the dogs through the intact dura mater with CO2 laser energy (40 W impact, 0.5 second duration, for a total time of 4 seconds on a 12.5 mm surface). Animals were divided into four groups and treated with dexamethasone, a barbiturate, hypothermy, and a crystalloid (control group). The brains were examined 48 hours after injury. Histologically all brain lesions showed three distinct layers with a vaporized center bordered by a zone of coagulation necrosis surrounded by edema. The main finding in the surrounding coagulation and edematous layers was dilatation of the vessels. Hemorrhage was sometimes observed mainly in the edematous layer. The effect of these therapies on the laser lesion and the effectiveness of these therapies on surrounding cerebral edema were evaluated by both light and electron microscopy. The control group showed significantly greater edema than the dexamethasone group. There was only a minimal difference between the control group and the barbiturate group, and there was no significant difference in amount of edema between control group and the hypothermy group. There was less edema in the dexamethasone group than in the other ones. These data suggest that dexamethasone inhibits edema in CO2 laser lesions with the same efficacy as shown in the treatment of vasogenic edema.

Effects of barbiturates on hypoxic cultures of brain derived microvascular endothelial cells

An in vitro model of the blood-brain barrier (BBB) consisting of porcine brain derived microvascular endothelial cells (BMEC) seeded onto collagen-coated polycarbonate membranes was used to investigate the effects of the barbiturates, methohexital and thiopental, on permeability properties of the endothelial cell monolayer under hypoxia. The permeability of cultured BMEC to ions and sucrose increased significantly during 6 h of hypoxia in a reversible manner. Cells were resistant to hypoxia for up to 24 h, but 48 h resulted in marked damage as assessed by the release of lactate dehydrogenase activity into the culture medium. The hypoxia-induced increase of the permeability was unchanged in the presence of superoxide dismutase (SOD) and catalase. Methohexital and thiopental decreased the hypoxia-induced permeability increase in a concentration-dependent manner and permeability changes were abolished completely at the barbiturate concentration of 50, μg/ml. The barbiturates had no effect on the intracellular cAMP content which started to decline after 3 h of hypoxia. Results suggest that barbiturates at high concentrations might be able to prevent permeability changes of the BBB during cerebral ischemia.

Inhibition of gap junctional intercellular communication by barbiturates in long-term primary cultured rat hepatocytes is correlated with liver tumour promoting activity.

Rodent liver tumor formation can be promoted by certain barbiturates and this may involve their ability to inhibit hepatocyte gap junctional intercellular communication (GJIC). In order to address the mechanisms and specificity of action of barbiturates on hepatocyte gap junctions, we have compared the effects of liver tumor-promoting barbiturates (phenobarbital, sodium barbital and amobarbital: PB, SB and AB, respectively) and a non-liver tumor-promoting barbiturate (barbituric acid: BA) on primary cultured rat hepatocyte GJIC and connexin32 (Cx32) expression after short (1-24 h) and long (2-14 days) treatment. GJIC was evaluated by fluorescent dye microinjection (dye-coupling); Cx32 expression was monitored by Northern blot, Western blot and immunohistochemistry. Both parameters were maintained at high levels over 14 days by coculture of the cells with WB-F344 rat liver epithelial cells in the presence of dexamethasone. Treatment with PB (2 mM) for 1 h sharply reduced dye-coupling from approximately 90-30%, but the cells fully recovered by 24 h. No inhibition was seen with the other barbiturates over this 1-day treatment period. Longer treatments (2-14 days) with the promoters PB, SB and AB, however, gradually reduced hepatocyte dye-coupling to approximately 30-50%. The non-promoter, BA, did not affect hepatocyte GJIC. These decreases in hepatocyte dye-coupling occurred without changes in Cx32 or gap junction expression. Dye-coupling of WB-F344 cells and expression of their predominant gap junction protein, connexin43 (Cx43), were also not affected. Thus, the inhibition of GJIC was specific to liver tumor promoting barbiturates in hepatocytes, was time-dependent and was not due to altered Cx32 expression.

Mechanism of inhibition of the plasma membrane Ca(2+)-ATPase by barbiturates.

We have demonstrated that sodium pentobarbital inhibited the activation of the human red blood cell plasma membrane Ca(2+)-ATPase produced by dimerization of enzyme monomers or by calmodulin binding to enzyme monomers. The effects of the barbiturate were dose-dependent. Both Vmax and Ca2+ affinity were reduced. The Ca(2+)-ATPase activity of the dimeric enzyme was distinctly less sensitive with respect to the effective inhibitory concentrations of pentobarbital and to the rate of onset of inhibition than was the calmodulin-dependent activation of enzyme monomers. Temperature dependence of the inhibition was in agreement with direct, nonpolar interactions of pentobarbital with a water-exposed nonpolar patch on the surface of this transmembrane protein. The barbiturate prevented the increase of intrinsic tryptophan fluorescence associated with substrate Ca2+ binding to the enzyme dimer. On the basis of the barbiturate effects we propose a model for the action of detergent-like compounds on the enzyme. They inhibit Ca(2+)-ATPase activity by binding to a nonpolar patch on the water-exposed dimerization surface of the enzyme monomer, part of which is also the binding site for calmodulin. The model assumes that their binding to the nonpolar patch on the monomer interferes with dimerization and weakens but does not prohibit calmodulin binding, whose activation of the enzyme is then submaximal. The model should be applicable to other proteins as the two activation pathways studied have been demonstrated for various enzymes.

Chimeric GABAA/Glycine Receptors: Expression and Barbiturate Pharmacology

GABAA and glycine receptors are close relatives in the ‘gene superfamily’ of ligand-gated ion channels, but have distinctly different pharmacology. For example, barbiturates have two effects on GABAA receptors (GABAA-R): at low micromolar concentrations (2–5 μM), the anesthetic barbiturate methohexital potentiates submaximal chloride current responses to GABA; at higher concentrations (20–50 μM), the barbiturate causes direct gating of the channel in the absence of agonist. Neither of these barbiturate effects is seen on the glycine receptor (Gly-R). In order to study the structural parts of the GABAA-R involved in this barbiturate pharmacology, two unique restriction sites were introduced into the cDNAs encoding the α2 and β1 subunits of the human GABAA-R and the α1 subunit of the human gly-R. The first site (‘X’) corresponded to the C-terminal end of the third transmembrane domain (M3) in each subunit and enabled exchange of a C-terminal fragment of ∼ 100 amino acids (which includes the large ‘cytoplasmic loop’ and M4 segment) between GABAA-R and Gly-R subunits. The second site (‘S’) was ∼ 30 amino acids 3′- from the N-terminal end of each subunit and enabled exchange of a small N-terminal fragment between GABAA-R and Gly-R subunits. Several chimeric receptor subunit cDNAs were constructed and the resulting receptors tested for their ability to respond to GABA and glycine and for sensitivity to the barbiturate methohexital (MTX). The results show that neither the large C-terminal fragment nor the smaller N-terminal fragment is associated with the enhancement or direct activation of the GABAA-R by MTX. These results demonstrate the viability of chimeric GABAA/Gly-R and suggest that the method will be suitable for further investigation of the molecular basis of the barbiturate pharmacology of the GABAA-R. Copyright © 1996 Elsevier Science Ltd.

Anesthetic barbiturates enhance Gs alpha-dependent cyclic AMP production in S49 mouse lymphoma cells.

Cyclic AMP (cAMP) regulates many important physiological processes. Barbiturates influence cAMP regulation, possibly through effects on G proteins. This study used intact S49 mouse lymphoma cells to characterize the role of G proteins in the effect of barbiturates on cAMP regulation. cAMP accumulation was determined in intact S49 WT (wild-type) and S49 cyc- cells (the Gs alpha-deficient mutant) by measuring the conversion of [3H]-ATP to [3H]cAMP in cells preloaded with [3H]adenine. Pentobarbital enhanced cAMP accumulation in WT cells in the absence (basal) or presence of isoproterenol but had no effect on the EC50 for isoproterenol. This effect was dose dependent with a 50-60% enhancement at 2 mM pentobarbital. Pentobarbital did not affect forskolin-stimulated cAMP accumulation in WT cells. In cyc- cells, basal and forskolin-stimulated cAMP accumulation were stimulated only at the highest concentration of pentobarbital used (2 mM). Pentobarbital did not affect the inhibition of cAMP accumulation by somatostatin in WT cells, and pertussis toxin treatment of WT cells did not affect the action of pentobarbital on cAMP accumulation. Pentobarbital did not affect isoproterenol-stimulated adenylyl cyclase activity in whole-cell homogenates or membranes prepared from WT cells. The S-(-)-isomer of pentobarbital enhanced isoproterenol-stimulated cAMP accumulation more than the R-(-)-isomer. Phenobarbital and barbituric acid did not enhance isoproterenol-stimulated cAMP accumulation, whereas the anesthetic barbiturates hexobarbital, pentobarbital, and thiopental all enhanced activity. These results suggest that pentobarbital enhances cAMP accumulation in intact WT cells by a mechanism that is dependent on Gs alpha but independent of Gi.(ABSTRACT TRUNCATED AT 250 WORDS)

Hyperalgesia during sedation: effects of barbiturates and propofol in the rat

Subhypnotic doses of thiopentone are considered to possess antianalgesic or hyperalgesic properties. In this study, we have tested the hypothesis that the coincidence of sedation and hyperalgesia is a property of both barbiturate and non-barbiturate anaesthetic agents. In a randomized, prospective, blinded study, the effects of slow (20 min) iv infusions of thiopentone, pentobarbitone, methohexitone or propofol on nociceptive threshold were measured in rats by tail pressure analgesimetry and compared with saline-infused control animals. Nociceptive thresholds were correlated with measurements of plasma drug concentrations and behavioural assessments. Comparison of pre-infusion nociceptive threshold with the lowest threshold obtained during drug infusion revealed decreases in all four treatment groups. As a percentage of the pre-infusion values, the decreases were: thiopentone: 42.5% (P < 0.001), pentobarbitone: 27.8% (P = 0.014), methohexitone: 24.9% (P = 0.013), propofol: 21.6% (P = 0.006). There were no changes in nociceptive threshold in the control groups. The relationship between nociceptive threshold and plasma drug concentration was usually characterized by an initial decline followed by a rise in nociceptive threshold as the plasma concentration and degree of sedation increased. The results support the hypothesis that hyperalgesia is a property of different anaesthetic agents when administered at sub-hynotic concentrations.

Electrophysiological and metabolic effects of a convulsant barbiturate on dissociated mouse primary sensory neurons.

1. The convulsant barbiturate 5-(2-cyclohexylidene-ethyl)-5-ethyl barbituric acid (CHEB) depolarizes dorsal root ganglion (DRG) neurons. We have applied microfluorimetric and whole-cell patch clamp techniques to investigate the mechanisms underlying this response in freshly dissociated mouse DRG cells. 2. Application of CHEB (2-200 microM) raised cytosolic calcium concentration ([Ca2+]i) rapidly and reversibly in 55% of eighty-three neurons tested. This population did not correlate with other classifications of sensory neurons based on either cell size or the expression of membrane currents. 3. The response was dependent on external calcium and was reduced by 81 +/- 22% by Ruthenium Red. A rise in [Ca2+]i was still seen with the membrane potential clamped at -70 mV, excluding membrane depolarization and activation of voltage-dependent Ca2+ channels as the principal mechanism for the response. 4. The rise in [Ca2+]i was associated with an increase in membrane conductance and a current, ICHEB, which was inward at -70 mV. Both the rise in [Ca2+]i and the current showed 'run-down' under whole-cell recording conditions. When K+ conductances were blocked, the reversal potential of ICHEB was close to 0 mV. This was independent of the Cl- reversal potential, suggesting that ICHEB is carried as a non-specific cation current. 5. In contrast to the change in [Ca2+]i, ICHEB was not dependent on external Ca2+ and the current was still seen when [Ca2+]i as strongly buffered by the pipette filling solution. These data suggest that CHEB opens a non-selective cation channel permeant to Ca2+, raising [Ca2+]i and further depolarizing the cell membrane potential. The exact nature of this conductance remains unknown. These actions could readily account for the convulsant actions of the drug, depolarizing neurons and increasing transmitter release. 6. It was also noted that CHEB increases autofluorescence derived from mitochondrial NAD(P)H. Further examination of this phenomenon using the dye rhodamine 123 to follow changes in mitochondrial potential (psi m) suggested that CHEB is a potent inhibitor of mitochondrial respiration, probably acting at complex I. These effects appeared to be quite distinct from the action of CHEB at the level of the plasma membrane.

Barbiturates inhibit hexose transport in cultured mammalian cells and human erythrocytes and interact directly with purified GLUT-1.

Barbiturates reduce cerebral blood flow, metabolism, and Glc transfer across the blood-brain barrier. The effect of barbiturates on hexose transport in cultured mammalian cell lines and human erythrocytes was studied. Pentobarbital inhibits [3H]-2-dGlc uptake in 3T3-C2 murine fibroblasts by approximately 95% and approximately 50% at 10 and 0.5 mM, respectively. Uptake of [3H]-2-dGlc is linear with time in the presence or absence of pentobarbital, and the percent inhibition is constant. This suggests that hexose transport, not phosphorylation, is inhibited by barbiturates. Inhibition by pentobarbital of hexose transport in 3T3-C2 cells is rapid (< 1 min), is not readily reversible, is not altered by the presence of albumin [1% (w/v)], and is independent of temperature (4-37 degrees C) and the level of cell surface GLUT-1. The IC50's for inhibition of hexose transport in 3T3-C2 cells by pentobarbital, thiobutabarbital, and barbital are 0.8, 1.0, and 4 mM, respectively. This is consistent with both the Meyer-Overton rule and the pharmacology of barbiturates. Neither halothane (< or = 10 mM) nor ethanol [< or = 0.4% (v/v)] significantly inhibits hexose transport. Inhibition by pentobarbital (0.5 mM) of [3H]-2-dGlc uptake by 3T3-C2 cells decreases the apparent Vmax (approximately 50%) but does not alter the apparent Km (approximately 0.5 mM). Inhibition of hexose transport by barbiturates, but not ethanol [< or = 0.4% (v/v)], is also observed in human erythrocytes and four other cultured mammalian cell lines. Pentobarbital quenches (Qmax approximately 75%) the intrinsic fluorescence of purified and reconstituted GLUT-1 (Kd approximately 3 mM). Quenching is independent of Glc occupancy, is unchanged by mild proteolytic inactivation, and does not appear to directly involve perturbations of the lipid bilayer. We propose that barbiturates can interact directly with GLUT-1 and inhibit the intrinsic activity of the carrier. Glc crosses the blood-brain barrier primarily via the GLUT-1 of the endothelial cells of cerebral capillaries. Partial inhibition of this process by barbiturates may be of significance to cerebral protection.

Differences of central mechanisms of the discriminative effects of benzodiazepines and barbiturates

Our previous studies have shown that central-type benzodiazepine (BZD) receptors (CBR) and neurbsteroids capable of modulating GABAA receptor function are involved in the decrease of pentobarbital (PB)-induced sleep caused by social isolation stress in mice. In this study, to further clarify the mechanism underlying this decrease, we investigated the possible involvement of peripheraltype BZD receptors (PBR) which play an important role in neurosteroidogenesis in PB sleep in socially isolated mice. Socially isolated mice showed significantly shorter duration of PB-induced sleep than group-housed animals. When injected intracerebroventricularly (i.c.v.), FGIN-1–27 (FGIN, 25–100 nmol), a selective PBR agonist, and PK11195 (PK, 14–28 nmol), a PBR antagonist, and pregnenolone (PREG, 15–30 nmol), a neurosteroid precursor, dose-dependently normalized the PB sleep in isolated mice without having an effect on the group-housed animals. In contrast, pregnenolone sulfate (PS, 24 nmol), an endogenous neurosteroidal negative allosteric modulator of the GABAA receptor, reduced PB sleep in group-housed but not isolated mice. PS, at the same dose, significantly attenuated the effects of FGIN (100 nmol), PK (28 nmol) and PREG (30 nmol) in isolated mice, while FGIN (100 nmol), PK (28 nmol) and pregnenolone (30 nmol) significantly blocked the effect of PS (24 nmol) in group-housed mice. These results suggest that the PBR-mediated decrease in the genesis of neurosteroid(s) possessing a GABAA receptor agonistic profile is also partly involved in the down regulation of the GABAA receptor following long-term social isolation and contributes to the decrease of PB-induced sleep in isolation stressed mice.

Protection cérébrale : apport des agents anesthésiques intraveineux

The administration of an intravenous anaesthetic agent before experimental cerebral ischaemia in animals improves the functional and histological outcome. Cerebral ischaemia may be global or focal, complete or incomplete. Intravenous anaesthetic agents reduce the cerebral metabolic demand for oxygen (CMRO 2) and abolish electrophysiological activity. This reflects a discontinuation of the functionnal neuronal activity with maintenance of its basic metabolic activity. The oxygen spared by the decrease in consumption, while reducing the functional activity, might be used by the neurons to sustain longer periods of ischaemia. This protective effect is also observed after pretreatment with either lidocaine or volatile agents, but their potentially deleterious vasodilating effect must be considered. Ketamine has recently been shown to antagonize NMDA receptors. The protective effect of barbiturates was experimentally demonstrated more than 30 years ago. They are still used as a reference. They reduce CMRO 2, optimise the ratio between oxygen consumption and oxygen delivery and thus reduce cerebral blood flow and cerebral blood volume, as a result of the decrease of the metabolic demand. This might explain why a protective effect is seen in case of global or focal hypoxia with increased intracranial pressure, while no protection is documented in case of global cerebral ischaemia, such as after cardiac arrest, where EEG is immediately flat and ICP low. However, at doses required to obtain a protective effect, barbiturates induce deleterious side effects such as severe arterial hypotension, which limits their use. Cerebrovascular and cardiac surgery or surgery of the carotids are characterised by potentially ischaemic episodes which can be predicted. This allows proper application of protective measures, such as administration of intravenous anaesthetic agents. Not surprisingly, the only clinical documentation of a protective effect of barbiturates in man was achieved in patients undergoing cardiac surgery under extracorporeal circulation. The concept of pharmacological cerebral protection in case of cerebral ischaemia was investigated with other agents than barbiturates, such as etomidate, gamma-OH and propofol. Anaesthetic agents with their properties of reducing CMRO 2 might be of benefit if applied early, by « buying timein the very first phase of the ischaemic insult.

Barbiturates inhibit endothelium-dependent and independent relaxations mediated by cyclic GMP.

The inhibitory effects of volatile and local, but not intravenous, anesthetics on endothelium-dependent relaxations of blood vessels have been demonstrated in vitro by several investigators. The aim of this study was to determine the effects of barbiturates on endothelium-dependent arterial relaxation and elucidate the mechanism(s) responsible. Canine mesenteric arteries and rat aortae were isolated, and tension changes in helical strips were recorded. Endothelium-dependent relaxations elicited by acetylcholine and bradykinin in canine mesenteric arteries, and those by acetylcholine in rat aortae, were significantly attenuated by thiopental (3 x 10(-4) M) pretreatment. Sodium nitroprusside (SNP)-induced, endothelium-independent relaxations were significantly attenuated by thiopental (10(-4)-3 x 10(-4) M). The effects of pentobarbital were less marked than those of thiopental. Acetylcholine (10(-5) M)-stimulated levels of 3',5'-cyclic guanosine monophosphate (cGMP) in rat aortae were reduced significantly by thiopental and pentobarbital (both 10(-3) M), and SNP (3 x 10(-7) M)-stimulated levels were reduced by thiopental (3 x 10(-4)-10(-3) M) and pentobarbital (10(-3) M). We conclude that barbiturates inhibit cGMP-mediated endothelium-dependent and independent arterial relaxations. Inhibition of endothelium-dependent relaxation by barbiturates may be mediated by their effects on vascular smooth muscle itself and not on endothelium.

Acute, chronic and differential effects of several anesthetic barbiturates on glutamate receptor activation in neuronal culture.

Acute, chronic and differential effects of several anesthetic barbiturates on glutamate receptor activation in neuronal culture.

Pentobarbital sodium inhibits the release of noradrenaline in the medial preoptic area in the rat

The effect of pentobarbital sodium (PB) on noradrenaline (NA) release in the medial preoptic area was studied in ovariectomized rats by a microdialysis method coupled to HPLC. Dialyzates were collected at 6-min intervals before and after the injection in freely moving rats. PB injected i.p. at a dose of 35 mg/kg caused a marked, ∼40–50% decrease in NA release, which became evident 50 min after the injection and lasted for another 60 min during which the observation was performed. The results suggest that the anesthetic effect of barbiturates is related to the reduction in NA release in the brain.

Effect of anesthetic and convulsant barbiturates on N-methyl-D-aspartate receptor-mediated calcium flux in brain membrane vesicles.

The effects of anesthetic and convulsant barbiturates on brain N-methyl-D-aspartate (NMDA) receptor function were examined in a cell-free membrane vesicle preparation from mouse hippocampus. Increases in intracellular free calcium concentrations (Cai) were determined using a fluorescent dye, Indo-1, after stimulation with the NMDA receptor agonist, L-glutamate. Anesthetic barbiturates inhibited NMDA responses in a concentration-dependent manner with a rank order of potency of secobarbital > amobarbital > pentobarbital > mephobarbital = phenobarbital >> barbital. However, the IC50 values for these barbiturates were larger than probable blood anesthetic concentrations. Barbiturates with both anesthetic and convulsant effects in mice [optical isomers of pentobarbital and secobarbital, 5-(2-cyclohexylideneethyl)-5-ethylbarbituric acid and (+/-)-dimethylbutylbarbituric acid] also reduced NMDA responses. Inhibition of NMDA responses by racemic pentobarbital or isomers of pentobarbital was noncompetitive. Resting Cai was altered by all barbiturates tested except secobarbital and barbital, but not in a consistent manner, suggesting that the effect of barbiturates on resting Cai is not related either to their effects on NMDA receptor responses or to their behavioral effects. These results show that anesthetic and convulsant barbiturates inhibit NMDA responses, but their anesthetic and convulsant activities may be primarily due to their effects on other brain targets.