Antagonism of ethanol narcosis in mice by hyperbaric pressures of 4-8 atmospheres

Benzodiazepine agonist and inverse agonist coupling in GABAA receptors antagonized by increased atmospheric pressure

Past work found that exposure to 12 times normal atmospheric pressure (ATA) of helium-oxygen gas (heliox) selectively antagonizes (uncouples) and differentiates allosteric coupling in GABA(A) receptors initiated by benzodiazepines versus neurosteroids. The present study tested the hypothesis that pressure can differentiate coupling initiated by a spectrum of benzodiazepine receptor ligands by measuring the effects of pressure on benzodiazepine ligand modulation of GABA-activated 36Cl(-) uptake in mouse brain membranes. 12 ATA completely antagonized allosteric modulation by: benzodiazepine receptor agonists diazepam and flunitrazepam; Type-1 selective benzodiazepine receptor agonist zolpidem and the benzodiazepine receptor partial inverse agonist ethyl-8-azido-5,6-dihydro-5-methyl-6-oxo-4H-imidazo[1,5-alpha][1,4]benzodiazepine-3-carboxylate (Ro15-4513). The similar, non-competitive-like characteristics of pressure antagonism of these ligands suggest common structural/functional elements underlying their coupling. Pressure also antagonized allosteric modulation by the benzodiazepine receptor inverse agonist methyl 6,7-dimethoxy-4-ethyl-beta-carboline-3-carboxylate (DMCM), but the antagonism was not complete and appeared to be surmountable (competitive-like) suggesting unexpected differences in coupling for DMCM versus Ro15-4513. These studies represent the first attempt to use pressure as a tool to dissect benzodiazepine receptor coupling. The results suggest that there is a common, pressure antagonism sensitive structural/functional element underlying coupling for benzodiazepine receptor ligands and that coupling for the full inverse benzodiazepine receptor agonist DMCM differs from coupling for benzodiazepine receptor agonists and benzodiazepine receptor partial inverse agonists.

Ethanol potentiation of glycine receptors expressed in Xenopus oocytes antagonized by increased atmospheric pressure

BACKGROUND

Behavioral and biochemical studies indicate that exposure to 12 times normal atmospheric pressure (12 ATA) of helium-oxygen gas (heliox) is a direct, selective ethanol antagonist. The current study begins to test the hypothesis that ethanol acts by a common mechanism on ligand-gated ion channels by expanding previous hyperbaric investigations on gamma-aminobutyric acid type A (GABA(A)) receptors (GABA(A)Rs) at the biochemical level to alpha(1)glycine (GlyRs) expressed in Xenopus oocytes.

METHODS

Oocytes expressing wild-type alpha(1) homomeric GlyRs were voltage-clamped (-70 mV) and tested in the presence of glycine (EC(2)) +/- ethanol (50-200 mM) under 1 ATA control and 3 to 12 ATA heliox conditions. Glycine concentration response curves, strychnine/glycine interactions, and zinc (Zn2+) modulation of GlyR function was also tested.

RESULTS

Pressure reversibly antagonized the action of ethanol. The degree of antagonism increased as pressure increased. Pressure did not significantly alter the effects of glycine, strychnine, or Zn2+, indicating that ethanol antagonism by pressure cannot be attributed to alterations by pressure of normal GlyR function. The antagonism did not reflect tolerance to ethanol, receptor desensitization, or receptor rundown.

CONCLUSION

This is the first use of hyperbarics to investigate the mechanism of action of ethanol in recombinant receptors. The findings indicate that pressure directly and selectively antagonizes ethanol potentiation of alpha(1)GlyR function in a reversible and concentration- and pressure-dependent manner. The sensitivity of ethanol potentiation of GlyR function to pressure antagonism indicates that ethanol acts by a common, pressure-antagonism-sensitive mechanism in GlyRs and GABA(A)Rs. The findings also support the hypothesis that ethanol potentiation of GlyR function plays a role in mediating the sedative-hypnotic effects of ethanol.

Effects of ethanol on body temperature of rats at high ambient pressure

Separately, ethanol and high ambient pressure cause hypothermia in laboratory animals. However, ethanol and high pressure have mutually antagonistic effects on several biological functions and the present experiments investigate their combined action on body temperature. Rats given saline, 1.5 g/kg ethanol or 3.5 g/kg ethanol were exposed to 1 bar air at 25-26 degrees C, 1 bar helium-oxygen at 30-31 degrees C, or 48 bar helium-oxygen at 33.5-34.5 degrees C. Ambient, colonic and tail-skin temperatures were monitored for 60 min. There were no significant differences in mean ambient or tail-skin temperatures between groups belonging to the same ambient condition. Colonic temperatures under the 1 bar conditions were 1.5-2 degrees C lower in the 3.5 g/kg ethanol group than in the saline and 1.5 g/kg ethanol groups, while no significant differences were observed between the groups at 48 bar. Comparisons of the colonic temperatures at the end of the observation period, i.e., 60 min after administration of ethanol, demonstrated that their values at 48 bar were significantly lower than at 1 bar after saline, significantly higher after 3.5 g/kg ethanol and identical across conditions in the 1.5 g/kg groups. The results suggest that high ambient pressure may counteract rather than potentiate the hypothermic effect of ethanol.

Mechanisms of alcohol intoxication in a rodent model: blunted alcohol-opposing reaction in "alcohol-sensitive" rats

In this paper we describe new data and review some studies on the mechanisms of alcohol-induced motor impairment in rats. Habituation to handling did not affect the naive behavioural differences between the alcohol sensitive and alcohol insensitive rat lines. Nor was there any effect on the differential sensitivities of the lines to the motor impairing and hypnotic effects of alcohol. Peripheral mechanisms may be involved in the differential behaviours of these lines, as the plasma corticosterone response was much weaker in the alcohol sensitive animals, suggesting a limited capacity to react to stress and alcohol. A similar blunted response to acute ethanol exposure was found in the uptake of the benzodiazepine antagonist [3H]Ro 15-1788 in vivo by the cerebellum of alcohol sensitive rats. The finding that these rat lines do not have any general differences in their brain inhibitory GABAergic receptors was extended to the spinal cord inhibitory glycinergic receptors, which showed only a modest line difference in their dissociation constant. The apparent localisation of the two main receptor differences (high-affinity [3H]muscimol binding and diazepam sensitivity of [3H]Ro 15-4513 binding) to the cerebellar granule layer suggests a genetic modification in the granule cells of alcohol-sensitive rats. In conclusion, our studies on acute intoxication by moderate alcohol doses show that several central nervous and peripheral factors may be involved in this behaviour. As many of these factors mitigate the effects of alcohol, alcohol antagonistic treatments should be aimed at activating and supporting multiple adaptive phenomena.

Pressure reversal of the depressant effect of ethanol on spontaneous behavior in rats

This study deals with the interaction between high pressure and a sub-hypnotic dose of ethanol in rats. Male Sprague-Dawley rats were given either ethanol 1.5 g/kg or saline IP and subsequently exposed to 1 atmosphere absolute pressure (ATA) air or to 1, 12, 24 or 48 ATA of helium-oxygen (heliox). The gas temperature was adjusted to offset ethanol and helium-induced hypothermia. Ethanol induced a characteristic unsteady pattern of locomotion which was completely reversed at 48 ATA, partially reversed at 24 ATA, but not affected at 12 ATA. Other behavioral effects of ethanol such as depression of total motor activity and rearing were similarly affected. Blood and brain concentrations of ethanol in the pressure groups did not differ significantly from concentrations measured in the 1 ATA groups. A similar pattern of reversal was observed whether the compression was initiated 4, 10 or 16 min after injection. These results show that hyperbaric exposure antagonizes the depressant effect of ethanol on spontaneous behavior in rats. This antagonism does not appear to be due to changes in ethanol distribution or elimination.

Reduced lethality from ethanol or ethanol plus pentobarbital in mice exposed to 1 or 12 atmospheres absolute helium-oxygen

The present experiments investigated the effects of 1 and 12 atmospheres absolute (ATA) helium-oxygen on potentially lethal doses of ethanol given alone or in combination with pentobarbital. Drug-naive, male C57BL/6J mice were injected IP with 5.4-6.5 g/kg ethanol, 4.5-6.9 g/kg ethanol plus 20 mg/kg pentobarbital, or 50-110 mg/kg pentobarbital plus 2.5 g/kg ethanol. Following injection, the mice were placed into chambers and exposed to environments of 1 ATA air, 1 ATA helium-oxygen, or 12 ATA helium-oxygen. Exposure to 1 or 12 ATA helium-oxygen significantly reduced the lethal effect (percent mortality at given doses and LD50) of ethanol given alone or with 20 mg/kg pentobarbital when compared to animals exposed to 1 ATA air. The pattern and degree of reduction in lethality for the 1 and 12 ATA helium-oxygen treatments were similar, suggesting that the antagonism resulted from increased helium or decreased nitrogen and not from increased atmospheric pressure. Exposure to these environments did not reduce lethality in mice given 2.5 g/kg ethanol in combination with relatively high doses (50-110 mg/kg) of pentobarbital. These findings suggest that helium-oxygen breathing mixtures may be useful in the treatment of some overdose patients.

Alleviation of drug withdrawal symptoms by treatment with a potent mixture of natural lipids

Acute treatment with morphine, as well as with other addictives, causes an initial decrease in brain membrane lipid microviscosity. Upon chronic exposure to the addictive drug, the original fluidity of brain membranes is restored, mostly due to an increase in the cholesterol content. Withdrawal of the drug leaves the membranes hyperviscous and the central nervous system supersensitive, as manifested in the abstinence syndrome. Based on this scheme of events, we have designed treatments of addicted mice with a special lipid mixture extracted from hen egg-yolk (Active-lipid, AL), which is a highly potent membrane fluidizer. In vivo treatment of morphine-addicted mice with AL, either by injection or as a diet supplement, reversed the brain membrane hyperviscosity and, concomitantly, markedly reduced, or even completely abolished the withdrawal symptoms precipitated by naloxone. Treatments with the fully saturated, rigidifying phospholipid dipalmitoyl-lecithin, exacerbated the withdrawal symptoms. These results could be the basis for a novel innocuous treatment which may facilitate the rehabilitation of drug and alcohol addicts.

Interaction of apomorphine and amantadine with ethanol in men

Male moderate drinkers (n = 12) drank ethanol (0.8 g/kg) and then ingested one of the following: apomorphine (5 mg), amantadine (200 mg), or placebo. Subjects were tested on a battery of physiological and behavioral measures using a double-blind, within-subjects, crossover design. Postethanol ingestion of apomorphine significantly increased ethanol's effect on 3 out of the 8 measures employed (divided attention, objective and subjective inebriation ratings) without significantly altering blood ethanol concentrations or the rate of blood ethanol decline. There was no indication that apomorphine antagonized ethanol's effects. In contrast to reports indicating that amantadine antagonized ethanol depression in rodents, amantadine did not significantly alter the degree of ethanol intoxication in humans. The increase in intoxication induced by apomorphine supports suggestions that dopaminergic systems may be involved in mediating ethanol intoxication and that the sobering effect of catecholamine-augmenting drugs results from noradrenergic, rather than combined noradrenergic and dopaminergic, stimulation. Further studies are necessary to elucidate the role of pre- and postsynaptic dopaminergic receptors in mediating these effects.

Hyperbaric ethanol antagonism: role of temperature, blood and brain ethanol concentrations

Three studies investigated the mechanism of hyperbaric ethanol antagonism. C57 Bl/6j mice were injected intraperitoneally with 3.6 g/kg ethanol and were exposed to 1-12 atmospheres absolute (ATA) helium-oxygen or to 1 ATA air at temperatures that offset the cooling effects of helium or helium and ethanol. Hyperbaric treatment significantly reduced sleep-time and increased wake-up brain ethanol concentrations compared to temperature matched controls. Treatment with 12 ATA helium-oxygen increased the brain ethanol concentration and the brain-blood ethanol ratio at 150 minutes after ethanol administration. These results exclude helium or pressure-induced changes in body temperature, rate of blood ethanol decline or distribution to the brain as mechanisms mediating the antagonism. The data support a membrane site of antagonism.

Hyperbaric ethanol antagonism in mice: studies on oxygen, nitrogen, strain and sex

Male and female C57BL/6J and male BALBc/J mice were injected with 3.6 g/kg ethanol or saline and exposed to 1-10 atmospheres absolute (ATA) air, to 1 ATA 80% helium-20% oxygen, or to 12 ATA helium-oxygen having oxygen partial pressures between 0.5 and 18 times normal. Hyperbaric helium-oxygen significantly reduced sleep-time and increased wake-up brain ethanol concentrations in all mice tested. The degree of antagonism was not enhanced by increasing the oxygen partial pressure. Hyperbaric air increased sleep-time and decreased wake-up brain ethanol concentration in C57 mice. Hyperbaric air induced a pressure-related lethal effect beginning at 6 ATA in intoxicated BALBs. These findings demonstrate that hyperbaric ethanol antagonism extends across strains and sexes, that the degree of antagonism cannot be enhanced by increasing the oxygen partial pressure, and that air is not suitable as an antagonistic hyperbaric gas. The findings are consistent with membrane theories of anesthesia.