Action Of Barbiturates Research Articles

Modulation of the GABAA receptor by depressant barbiturates and pregnane steroids

1. The modulation of the gamma-aminobutyric acidA (GABAA) receptor by reduced metabolites of progesterone and deoxycorticosterone has been compared with that produced by depressant barbiturates in: (a) voltage-clamp recordings from bovine enzymatically isolated chromaffin cells in cell culture, and (b) an assay of the specific binding of [3H]-muscimol to a preparation of porcine brain membranes. 2. The progesterone metabolites 5 alpha- and 5 beta-pregnan-3 alpha-ol-20-one (greater than or equal to 30 nM) reversibly and dose-dependently enhanced the amplitude of membrane currents elicited by locally applied GABA (100 microM), and over the concentration range 30 nM-100 microM stimulated the binding of [3H]-muscimol. In contrast, 5 alpha- and 5 beta-pregnan-3 beta-ol-20-one (30 nM-100 microM) had little effect in either assay, indicating a marked stereoselectivity of steroid action. 3. Scatchard analysis of the ligand binding data suggested an apparent increase in the number, rather than the affinity, of detectable [3H]-muscimol binding sites as the principle action of the active steroid isomers. 4. GABA-evoked currents were also potentiated by androsterone (1 microM) and the deoxycorticosterone metabolite 5 alpha-pregnane-3 alpha,21-diol-20-one (100 nM). 5. Secobarbitone (10-100 microM), pentobarbitone (10-300 microM) and phenobarbitone (100-500 microM) reversibly and dose-dependently potentiated the amplitude of GABA-evoked currents in the absence of any change in their reversal potential. 6. At relatively high concentrations (greater than or equal to 30 microM) secobarbitone and pentobarbitone directly elicited a membrane current. It is concluded that such currents result from GABAA receptor-channel activation since they share a common reversal potential with GABA-evoked responses (approximately 0 mV), are reversibly antagonized by bicuculline (3 microM), and potentiated by either diazepam (1 microM) or 5 beta-pregnan-3 alpha-ol-20-one (500 nM). 7. Secobarbitone (1 microM-1 mM) dose-dependently enhanced the binding of [3H]-muscimol. In common with the active steroids, an increase in the apparent number of binding sites was responsible for this effect. 8. A saturating concentration (1 mM) of secobarbitone in the ligand binding assay did not suppress the degree of enhancement of control binding produced by 5 beta-pregnan-3 alpha-ol-20-one (30 nM-100 microM). Similarly the steroid, at a concentration of 100 microM, did not influence the enhancement of [3H]-muscimol binding by secobarbitone (1 microM-1 mM). In all combinations of concentrations tested, the effects of secobarbitone and 5#-pregnan-3a-ol-20-one on [3H]-muscimol binding were additive. 9. In conjunction with previously published observations, the present data indicate close similarities in the GABA-mimetic and potentiating actions of barbiturates and steroids. However, the results obtained with combinations of steroids and barbiturates in the ligand binding assay appear inconsistent with the two classes of compound interacting with a common site to modulate the GABAA receptor activity.

Differing actions of convulsant and nonconvulsant barbiturates: An electrophysiological study in the isolated spinal cord of the rat

The effects of various pairs of convulsant and nonconvulsant barbiturates on mono-and polysynaptic activity were studied in the isolated spinal cord of the immature rat, using extracellular recording. The convulsant barbiturates, 5-ethyl-5-(3-methylbut-2'-enyl) barbituric acid (3M2B), 5-ethyl-5-(1,3-dimethylbut-1'-enyl) barbituric acid (1,3M1B) and (+)-5-(1,3-dimethylbutyl)-5-ethyl barbituric acid [(+) DMBB] all increased the monosynaptic reflex at concentrations between 5 and 50 μM with no change in polysynaptic activity. When the concentration was raised to between 100 and 300 μM, however, the convulsants all reduced the monosynaptic reflex, thus producing a biphasic dose-response relationship. The nonconvulsant barbiturates phenobarbital, 5-ethyl-5-(3-methylbut-1'-enyl) barbituric acid (3M1B), amylobarbital (3MB) and (−)-5-(1,3-dimethylbutyl)-5-ethyl barbituric acid [(−)DMBB] produced only a decrease in mono- and polysynaptic reflexes. At concentrations which enhanced the monosynaptic reflex, the responses of motoneurones to glycine and eledoisin-related peptide (an analogue of substance P) were reduced by (+)DMBB, while 1,3M1B and 3M2B had no significant effects upon any of the neurotransmitters tested. At concentrations which depressed the monosynaptic reflex, the convulsants all reduced the response to glycine whereas the nonconvulsant barbiturates all increased the response to GABA. With the exception of phenobarbital, both convulsant and nonconvulsant barbiturates produced a direct depolarisation of the presynaptic terminal membrane, with only the convulsants producing a depolarisation of the membrane of the motoneurone. Using another convulsant barbiturate, 5-(2-cyclohexylideneethyl)-5-ethyl barbituric acid (CHEB), this direct depolarising action was found to be calcium-dependent.

Barbiturates and nifedipine have different and selective effects on calcium currents of mouse DRG neurons in culture

Barbiturates and dihydropyridines have been shown to block voltage-dependent calcium channels, but have differing sites of action and clinical effects. We have investigated the effects of the barbiturates, pentobarbital and phenobarbital, and the dihydropyridine, nifedipine, on isolated calcium currents in mouse dorsal root ganglion neurons in culture. Three different calcium channels exist in neurons, designated T, N, and L. None of the drugs affected the T calcium current, but all blocked the activation of the L calcium current. Nifedipine had no effect on the N calcium current. The activation of the N calcium current from holding potentials more negative than--80 mV was not affected by either barbiturate, though both caused a more rapid inactivation of the current at test potentials of--20 to 0 mV. To determine the possible functional significance of these different and selective effects on calcium currents, we studied synaptically driven spontaneous activity in spinal cord neurons in culture. Nifedipine had no effect on spontaneous activity, whereas pentobarbital completely suppressed it. This suggests that block of the L calcium channel alone is not sufficient to suppress spontaneous activity. The additional action of pentobarbital on N calcium current may explain in part the complete suppression of spontaneous activity; other possibilities are discussed. These data suggest a basis for the separate sites of action of barbiturates and dihydropyridines as well as a possible mechanism for the hypnotic and anesthetic effects of the barbiturates.

The effects of local intraparenchymal pentobarbital on intracranial hypertension following experimental subarachnoid hemorrhage.

Barbiturates are often utilized clinically in circumstances in which elevated intracranial pressure is expected. In this study, the mechanism of action of barbiturates was examined in dogs with intracranial hypertension induced by injecting autogenous incubated blood into the chiasmatic cistern. Intracranial pressure and systemic blood pressure were continuously monitored. A single bilateral administration of powdered pentobarbital (2 mg and 0.4 mg) in experimental animals and solid d-glucose (2 mg) in control animals was given into the posterior hypothalamus, pontine reticular formation, or medullary reticular formation when intracranial pressure reached 20-30 mmHg after the blood injection--usually in 3-6 h. The increased intracranial pressure following the experimental subarachnoid hemorrhage was always associated with either intracranial pressure irregularities or concomitant blood pressure variations, suggesting the presence of vasomotor instability. Administration of both 2 mg and 0.4 mg of pentobarbital into the medulla caused a significant (P less than 0.01) decrease of the intracranial pressure to 44 and 65% of control and stabilization of the intracranial pressure irregularities, whereas pentobarbital given at the other sites did not. The blood pressure was also decreased significantly (P less than 0.01) to 80 and 88% of control and the blood pressure variations were stabilized in animals after administration of pentobarbital into the medulla, whereas in those given pentobarbital at the other sites, it was not. The results suggest that, in the presence of elevated intracranial pressure following experimental subarachnoid hemorrhage, the mechanisms of action of barbiturates in reducing the intracranial pressure may result from alleviation of cerebral vasomotor instability by depression of the vasomotor center of the medulla.

Diethylstilbestrol counteracts barbiturate narcosis and hypothermia in male mice.

Treatment with DES one hour prior to pentobarbital injection resulted in diminution of the narcotic sleep and hypothermia usually found after pentobarbital (50 mg/kg 1) injection in male mice. The effect was biphasic: significantly countered relative to saline pretreated controls at low (.001-.10 mg/kg 1) and at high (10-50 mg/kg -1) DES doses only. Giving DES alone did not change core body temperature compared to saline injected controls, at either 25 degrees C or at 33 degrees C. At 33 degrees C, neither PeB narcosis nor body temperature loss was significantly inhibited by DES. Possible cytoplasmic, nonsex differentiated bases for these estrogen effects on barbiturate action in the brain is discussed.

Ethanol stimulates gamma-aminobutyric acid receptor-mediated chloride transport in rat brain synaptoneurosomes.

The effects of ethanol on Cl- uptake were studied using a cell-free subcellular preparation from brain that contains a gamma-aminobutyric acid (GABA)/barbiturate receptor-sensitive Cl- transport system. In isolated vesicles prepared from rat cerebral cortex, ethanol, at concentrations that are present during acute intoxication (20-50 mM), stimulated 36Cl- uptake in a concentration-dependent and biphasic manner. The ethanol-stimulated uptake of 36Cl- was markedly inhibited by the GABA antagonists picrotoxin and bicuculline but not by a variety of other neurotransmitter receptor antagonists. The effects of ethanol in stimulating 36Cl- uptake in isolated brain vesicles were qualitatively and quantitatively similar to that of pentobarbital. Ethanol also markedly potentiated both muscimol- and pentobarbital-stimulated 36Cl- uptake at concentrations below those that directly stimulate 36Cl- uptake. Under our incubation conditions, ethanol did not release GABA, suggesting that it interacts with the postsynaptic GABA/barbiturate receptor complex. The ability of ethanol to stimulate GABA/barbiturate receptor-mediated Cl- transport may explain many of its pharmacological properties and provides a mechanism for the common psychopharmacological actions of ethanol, barbiturates, and benzodiazepines.

Tumor-promoting barbiturates act on DNA repair of cultured hepatocytes

We have observed that two promoters of liver carcinogenesis, i.e. phenobarbital and barbital, markedly increase DNA-repair synthesis of cultured hepatocytes following treatment with the ultimate carcinogens methyl methanesulfonate, N-methyl- N′-nitro- N-nitrosoguanidine, N-acetoxy-2-acetylaminofluorene, and UV light of 254 nm. Phenobarbital also increased the incorporation rates of deoxynucleoside triphosphates into nuclear DNA of permeabilized hepatocytes following carcinogen treatment. The action of these barbiturates apparently correlates with their potential to promote hepatocarcinogenesis in vivo, since the non-promoting agent barbituric acid did not modify carcinogen-induced repair synthesis. Moreover, the mechanisms of action of tumor-promoting barbiturates is different from the known enhancing action on repair synthesis of inhibitors of nuclear poly(ADP-ribose) biosynthesis.

Significance of the enhancement of GABA receptor binding with low affinity in the assessment of central effects of benzodiazepine derivatives: Analysis using a 1H-1,2,4-triazolyl benzophenone derivative (450191-S)

The central actions of 1-(2-o-chlorobenzoyl-4-chlorophenyl)-5-glycylaminomethyl-3-dimethylcarbamoyl-1H-1,2,4-triazole hydrochloride dihydrate (450191-S), a potent sleep-inducing and anxiolytic drug, were re-evaluated in terms of the affinity for benzodiazepine (BZP) receptor and the activation of ?-aminobutyric acid (GABA) receptor binding. The 450191-S showed only very low capacity to displace the bindings of [(3)H]diazepam, [(3)H]?-carboline-3-carboxylate-ethylester, [(3)H]Rol5-1788, [(3)H]Ro5-4864 and [(3)H]naloxone to cerebral synaptic membranes. Similarly, this drug had a weak and undistinguishable affinity to both BZPtype 1 and 2 receptors determined under the presence of CL 218,872. On the other hand, 450191-S as well as its active metabolites (M-1, M-2, M-A, M-3 and M-4) showed a remarkable activating effect on the GABA receptor binding with low affinity in cerebral synaptic membranes. This enhancement of the low affinity GABA receptor binding was found to be due to the increase of affinity (K(d)) but not to the change in B(max). Furthermore, it has been found that the observed accentuation of low affinity GABA receptor binding is well-correlated with the potency of the central actions of 450191-S such as potentiation of the hypnotic action of barbiturates and muscle relaxation. These results suggest that the central actions of 450191-S may be due to, at least in part, the activation of central GABA receptor binding with low affinity. The present results also suggest that the activation of low affinity GABA receptor binding may be a better criterion than the affinity of BZP receptor for elucidating the central action of a certain type of BZP derivatives.

Are the toxicities of pentobarbital and ethanol mediated by the GABA-benzodiazepine receptor-chloride ionophore complex?

Both barbiturates and ethanol have been reported to interact with the GABA-benzodiazepine receptor-chloride ionophere ‘supramolecular complex’. These observations raise the possibility that some of the pharmacologic actions of barbiturates and ethanol may be mediated through this complex. In this study we have administered a series of drugs which bind to various components of the complex in an attempt to antagonize the lethality of sodium pentobarbital, and ethanol-induced loss of righting reflex in mice. It was found that isopropylbicyclophosphate (IPPO), a cage convulsant which binds at or near the chloride ionophore, greatly reduces the overall mortality (and increases latency to death) of animals pretreated with a lethal dose of pentobarbital. Picrotoxin also decreases pentobarbital lethality, but only at doses which were usually lethal when given alone. Picrotoxin shortened, rather than increased, latency to death. Strychnine did not prevent pentobarbital lethality, suggesting that the IPPO effect is not shared by convulsants in general. IPPO did not prevent ketamine-induced deaths, which supports the notion that the protective actions of IPPO are specific for depressant drugs which act at the chloride ionophore. IPPO also significantly reduced the duration of loss of righting reflex induced by ethanol. These observations suggest that the use of compounds which have a high affinity for the chloride ionophore in vitro might be fruitful in developing a clinical treatment for barbiturate or ethanol toxicity.

The actions of barbiturates on release of noradrenaline from rat hippocampal synaptosomes

Barbiturates are believed to work both by augmenting GABA action and independently perhaps by decreasing neurotransmitter release. By studying the effect of pentobarbitates on the release of 3H noradrenaline from rat hippocampal synaptosomes it was found that pentobarbitone (10 −6M and 10 −4M) augmented the GABA evoked release of 3H noradrenaline but 10 −4M depressed this GABA effect. This latter concentration of pentobarbitone also depressed the K +-evoked release of 3H noradrenaline by a Ca ++ dependent but picrotoxin insensitive mechanism.

Barbiturates, alcohols and the CNS excitatory neurotransmission: Specific effects on the kainate and quisqualate receptors

The effects of barbiturates and straight-chain aliphatic alcohols on the responses of rat striatal neurons to excitatory amino acids have been investigated. The responses to N-methyl- D-aspartate, quisqualate, kainate, L-glutamate and L-aspartate were measured by the increase in 22Na + efflux rate that they produce in brain slices. The responses to quisqualate and kainate, measured in the 22Na + efflux assay, were found to be partially blocked by barbiturates whereas the responses to N-methyl- D-aspartate, glutamate and aspartate were not. The kainate and quisqualate-induced increases in 22Na + efflux rate were much more readily blocked by the presence of aliphatic alcohols than were the responses to N-methyl- D-aspartate, glutamate and aspartate. These results strengthen the idea of the existence of 4 distinct receptors for excitatory amino acids in the rat striatum. They are consistent with the presence on the kainate and quisqualate receptors, but not on the N-methyl- D-aspartate and glutamate/aspartate receptors of a hydrophobic domain which would provide a site of interaction for barbiturates and alcohols. They suggest that receptors for excitatory amino acids can be targets for the actions of barbiturates and alcohols on the central nervous system, and may mediate some of the anesthetic and hypnotic effects of these drugs.

The pain threshold increasing effects by thiamine deficiency in rats.

It is well known that rats maintained on a thiamine deficient diet exhibit significant weight loss, decrease in heart frequency, behavioral disorders such as circling movements, muricide aggression as well as symptoms of polyneuritis. After a series of pharmacological studies on thiamine deficient rats, I reported that the actions of certain drugs affecting the central nervous system were modified by acute thiamine deficiency in animals. In brief, thiamine deficiency increased sensitivity to the hypnotic action of barbiturates and reduced the cataleptic action by tetrabenazine4). I am planning to investigate the effect of analgesics (morphine, pentazocine, aspirin……) on thiamine deficient animals, since little is known about changes in the pain threshold following severe thiamine deficiency. In this study, an attempt was made to investigate the changes in the pain threshold of rats as a consequence of thiamine deficiency.

Tracazolate: A novel nonsedative anxiolytic

AbstractTracazolate (ICI 136,753; 4‐butylamino‐1‐ethyl‐6‐methyl‐1H‐pyrazolo [3,4‐b] pyridine‐5‐carboxylic acid ethyl ester) exhibited dose‐related anticonflict activity in mice, rats, and squirrel monkeys. The potency of tracazolate appears to be one‐quarter to one‐half that of chlordiazepoxide, and their durations of activity are similar in rats. No tolerance to the anticonflict activity of either chlordiazepoxide or tracazolate was evident following 12 consecutive days of treatment. Tracazolate exhibits a much greater separation between sedative and therapeutic doses than does chlordiazepoxide; thus, it is predicted that tracazolate will not be sedative at anxiolytic doses in man. Furthermore, based upon studies in rodents, tracazolate should be much less likely than the benzodiazepines to potentiate the actions of barbiturates and ethanol in man. Although tracazolate is an anticonvulsant in rodents, it is considerably less potent in this respect than chlordiazepoxide. In contrast to benzodiazepine anxiolytics, tracazolate enhances the binding of 3H‐benzodiazepines to their binding sites in brain. Consistent with this finding, tracazolate potentiated both the anticonvulsant and anxiolytic effects of chlordiazepoxide in rodents. Although the benzodiazepine antagonists (RO 15‐1788 and CGS 8216) clearly antagonize the anticonflict activity of chlordiazepoxide, the activity of tracazolate was not significantly altered by either antagonist. Both chlordiazepoxide and tracazolate significantly enhanced 3H‐γ‐aminobutyric acid (GABA) binding. These results suggest that tracazolate is a novel psychoactive agent that should be a useful anxiolytic drug in man.

Chronopharmacology and Chronotoxicology of CNS Drugs: Interrelationships with Neuromodulators

It has been demonstrated that the effect of a drug, chemical, or poison is diurnal phase dependent. The circadian variation in drug response could be attributed to the diurnal fluctuations of several factors, including absorption, distribution, metabolism, and excretion. Another important factor in determining the response to CNS drugs is the circadian rhythm of brain sensitivity to drugs. The diurnal variability in the concentration of brain neuromodulators could in part explain the circadian rhythm of brain sensitivity. The present paper focuses on the interrelationships between the levels of neuromodulators and the chronopharmacology and chronotoxicology of CNS drugs. Evidence is presented to show that the circadian rhythm of specific brain neuromodulators plays an important role in the pharmacologic or toxicologic action of amphetamine, barbiturates, morphine, and ethanol.

Lung lipoprotein lipase: Inhibition by barbiturates

The action of barbiturates on the enzymatic activity of lung lipoprotein lipase has been studied in vitro and in vivo. Mouse lung preparations can be separated into two fractions containing lipoprotein lipase activity and designated as soluble and membrane-bound. In vitro, both enzyme fractions were inhibited in a graded manner by the barbiturates when present in concentrations of 1-4 mg/ml. Experiments to determine the mechanism for the inhibition of lipoprotein lipase suggest that it may be related to non-specific binding of the drug to the enzyme since other drugs e.g., phenylbutazone and a coumarin derivative which bind non-specifically to serum albumin showed similar inhibitory activity. When rat lungs were perfused in situ with heparin, 49% of the total lipoprotein lipase activity was released into the perfusate. However, when pentobarbital was perfused prior to heparin, no lipoprotein lipase activity was observed in the perfusate. The possible clinical implications resulting from inhibition of pulmonary lipoprotein lipase by barbiturates are discussed in this paper.

Potentiation of the antiepileptic activity of phenobarbital by nicotinamide.

Nicotinamide is a ligand of the benzodiazepine receptor and has been reported to have anticonvulsant activity. In addition, our previous clinical experience has raised the possibility that it may also potentiate the action of barbiturates. Therefore, we have examined the anticonvulsant activity and neurotoxicity of nicotinamide alone and in combination with phenobarbital in mice. Nicotinamide had its maximal anticonvulsant effect 15 min and its maximal sedative effect 45 min after intraperitoneal injection. At 15 min, the median effective dose was 586.5 mg/kg against bicuculline and 2,019 mg/kg against pentylenetetrazol. Nicotinamide was ineffective against maximal electroshock. It had a sedative effect, with a median toxic dose of 874.8 mg/kg by the Rotorod Toxicity Test at 45 min. At doses that were ineffective by themselves (0.01 effective dose) nicotinamide potentiated the anticonvulsant activity of phenobarbital against bicuculline and pentylenetetrazol, but the toxicity was not potentiated and therefore the therapeutic index of phenobarbital was improved by nicotinamide. These results suggest that nicotinamide may be useful as a therapeutic adjunct for the treatment of epilepsy with phenobarbital or primidone.

Quantification of the interaction between barbiturates and alcohol and interpretation of fatal blood concentrations.

Barbiturates and alcohol are frequently found in combination in cases of accidental or intentional fatal self-poisoning. Unfortunately, the lack of any precise knowledge concerning their interaction creates difficult problems when an interpretation of toxicological data is sought. 1 In the following work the joint action of single barbiturates and alcohol is examined and a means of quantifying it is presented. 2 The effects of different amounts of alcohol on fatal amylobarbitone, butobarbitone, pentobarbitone, phenobarbitone and quinalbarbitone blood concentrations are compared. 3 Combined alcohol-barbiturate blood concentration curves connecting those concentration pairs equally effective in causing death are used to quantify the increase in toxicity. The quantitative effects of alcohol appear to be greater with the shorter- and longer-acting barbiturates than with the intermediate-acting derivatives. The different modes of interaction are discussed in terms of the physico-chemical and pharmacokinetic properties of the drugs.

Effect of barbiturates on the GABA receptor of cat primary afferent neurones

1. The effects of the barbiturate anaesthetics, pentobarbitone and thiopentone, on the membrane properties and the gamma-aminobutyric acid (GABA)-induced responses of cat primary afferent neurones were studied with intracellular recording and voltageclamp techniques.2. At low concentrations (10(-7)-10(-5) M) both barbiturates slightly enhanced and prolonged GABA-induced depolarizations or currents without affecting the membrane properties. At these concentrations, barbiturates have no effect on the apparent dissociation constant of the GABA-GABA receptor interaction or the reversal potential for GABA-induced depolarizations or currents.3. At high concentrations (10(-4)-10(-3) M) barbiturates produced a few millivolts reduction in the resting membrane potential. Voltage-clamp analysis revealed that the depolarization was associated with one of the three types of conductance change, i.e., an initial increase followed by a decrease (40% of neurones examined), only an increase (40%) and only a decrease (20%).4. Analysis in different ionic media indicated that the depolarization with a reduced membrane resistance is associated with an increased chloride conductance and that the one with an increased membrane resistance is accompanied by a reduction in potassium conductance. Bath-application of GABA (10(-3) M) or picrotoxin (10(-5) M) inhibited the increase in chloride conductance but not the reduction in potassium conductance.5. Barbiturates at these high concentrations initially caused a marked augmentation and prolongation of GABA responses; this was followed by a depression. The depressant action did not appear to be voltage-dependent. These actions of barbiturates were not accompanied by changes in the apparent dissociation constant of the GABA-current dose-response curve or the reversal potential for GABA currents. In addition, the single exponential decay of GABA current was not changed despite a marked prolongation of its decay time.6. Picrotoxin (10(-5) M) antagonized the depressant effect of barbiturates at high concentrations on GABA currents, and barbiturates (5 x 10(-6) M) reduced the inhibitory action of picrotoxin (5 x 10(-6) M) on the GABA-currents.7. From all these results, it is suggested that the site of barbiturate actions on GABA-responses is mainly the allosteric site (the ionic conductance regulatory subunit) but not the agonist recognition site or the chloride channels linked with GABA receptors.

The action of barbiturates on contractile responses of canine and feline bronchial smooth muscle

Pentobarbitone, thiopentone and phenobarbitone in concentrations ranging from 10 −4 M to 10 −3 M reduced the contractile responses of isolated canine and feline trachealis muscle to electrical field stimulation without affecting contractile responses to acetylcholine. The order of potency was thiopentone, pentobarbitone and phenobarbitone. Experiments using carbachol and neostigmine or physostigmine suggested that the barbiturates act by preventing the release of acetylcholine from nerve endings. In vivo, pentobarbitone (2.5–20 mg/kg i.v.) produced dose-related reductions in the increase in total lung resistance induced by vagal stimulation in chloralose-anaesthetised cats.

Studies on the interaction of barbiturates with reactive oxygen radicals: implications regarding barbiturate protection against cerebral ischaemia.

Although the molecular basis of ischaemic damage of the brain is as yet unknown, it has been postulated that the uncontrolled production of reactive oxygenated species derived from molecular oxygen (including hydroxyl radicals, superoxide radicals and singlet oxygen) may play a major role in the production of such injury. The ability of various barbiturates to modify the nature and extent of membrane damage produced by various oxygen radicals generated under well-defined conditions in vitro has been directly examined using the human erythrocyte as model membrane system. Our results indicate that barbiturates are unlikely to exert their protective effects by directly scavenging singlet oxygen, superoxide or hydroxyl radicals. The highly lipophilic barbiturate thiopentone is capable of decreasing the susceptibility of membranes to oxidative degradation by a direct membrane action, a property shared by amphipathic membrane stabilizers such as propranolol. The barbiturates were found to stabilize the haeme moiety of haemoglobin preventing its conversion to methaemoglobin in the presence of hydrogen peroxide. It is postulated that a major aspect of barbiturate action in decreasing ischaemic injury to the brain may involve the stabilization of haeme-coordinated iron complexes, thereby preventing the participation of these ubiquitous substances in initiating and potentiating free radical-mediated processes which have been implicated in the production of such injury.