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

Molecular targets and mechanisms for ethanol action in glycine receptors

Glycine receptors (GlyRs) are recognized as the primary mediators of neuronal inhibition in the spinal cord, brain stem and higher brain regions known to be sensitive to ethanol. Building evidence supports the notion that ethanol acting on GlyRs causes at least a subset of its behavioral effects and may be involved in modulating ethanol intake. For over two decades, GlyRs have been studied at the molecular level as targets for ethanol action. Despite the advances in understanding the effects of ethanol in vivo and in vitro, the precise molecular sites and mechanisms of action for ethanol in ligand-gated ion channels in general, and in GlyRs specifically, are just now starting to become understood. The present review focuses on advances in our knowledge produced by using molecular biology, pressure antagonism, electrophysiology and molecular modeling strategies over the last two decades to probe, identify and model the initial molecular sites and mechanisms of ethanol action in GlyRs. The molecular targets on the GlyR are covered on a global perspective, which includes the intracellular, transmembrane and extracellular domains. The latter has received increasing attention in recent years. Recent molecular models of the sites of ethanol action in GlyRs and their implications to our understanding of possible mechanism of ethanol action and novel targets for drug development in GlyRs are discussed.

Targets for ethanol action and antagonism in loop 2 of the extracellular domain of glycine receptors

The present studies used increased atmospheric pressure in place of a traditional pharmacological antagonist to probe the molecular sites and mechanisms of ethanol action in glycine receptors (GlyRs). Based on previous studies, we tested the hypothesis that physical-chemical properties at position 52 in extracellular domain Loop 2 of alpha1GlyRs, or the homologous alpha2GlyR position 59, determine sensitivity to ethanol and pressure antagonism of ethanol. Pressure antagonized ethanol in alpha1GlyRs that contain a non-polar residue at position 52, but did not antagonize ethanol in receptors with a polar residue at this position. Ethanol sensitivity in receptors with polar substitutions at position 52 was significantly lower than GlyRs with non-polar residues at this position. The alpha2T59A mutation switched sensitivity to ethanol and pressure antagonism in the WTalpha2GlyR, thereby making it alpha1-like. Collectively, these findings indicate that (i) polarity at position 52 plays a key role in determining sensitivity to ethanol and pressure antagonism of ethanol; (ii) the extracellular domain in alpha1- and alpha2GlyRs is a target for ethanol action and antagonism and (iii) there is structural-functional homology across subunits in Loop 2 of GlyRs with respect to their roles in determining sensitivity to ethanol and pressure antagonism of ethanol. These findings should help in the development of pharmacological agents that antagonize ethanol.

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.

Low-level hyperbaric exposure antagonizes locomotor effects of ethanol and n-propanol but not morphine in C57BL mice

Low-level hyperbaric exposure antagonizes a broad range of behavioral effects of ethanol in a direct, reversible, and competitive manner. This study investigates the selectivity of the antagonism across other drugs. C57BL/6 mice were injected with saline, ethanol, n-propanol, or morphine sulfate, and then were exposed to 1 atmosphere absolute (ATA) air, 1 ATA helium-oxygen gas mixture (heliox), or 12 ATA heliox. Locomotor activity was measured from 10 to 40 min following injection. N-propanol produced a dose-dependent depression of locomotor activity from 1.0 g/kg. Morphine produced a dose-dependent stimulation of locomotor activity at doses of 3.75-12.0 mg/kg. Exposure to 12 ATA heliox significantly antagonized the locomotor depressant effects of 1.0 g/kg n-propanol and 2.5 g/kg ethanol, without significantly affecting blood concentrations of these drugs measured at 40 min postinjection. Exposure to 12 ATA heliox did not significantly antagonize the locomotor-stimulating effects of the two morphine doses tested (3.75 and 7.5 mg/kg). These findings suggest that exposure to 12 ATA heliox antagonizes the behavioral effects of intoxicant-anesthetic drugs like ethanol and n-propanol, which are believed to act via perturbation or allosteric modulation of functional proteins, but does not antagonize the effects of drugs like morphine, which act via more direct mechanisms. This demonstration of selective antagonism adds important support for the hypothesis that low-level hyperbaric exposure is a direct mechanistic ethanol antagonist, with characteristics similar to a competitive pharmacological antagonist.

Low-level hyperbaric antagonism of ethanol's anticonvulsant property in C57BL/6J mice

This study investigated the ability of hyperbaric exposure to antagonize ethanol's anticonvulsant effect on isoniazid (INH)-induced seizures. Drug-naive, male C57BL/6 mice were injected intraperitoneally with saline, 1.5, 2.0, or 2.5 g/kg ethanol followed immediately by an intramuscular injection of 300 mg/kg of INH. The mice were then exposed to either 1 atmosphere absolute (1 ATA) air, 1 ATA helium-oxygen gas mixture (heliox), or 12 ATA heliox at temperatures that offset the hypothermic effects of helium. Ethanol increased the latency to onset of myoclonus in a dose-dependent manner. Exposure to 12 ATA heliox antagonized ethanol's anticonvulsant effect at 2.0 and 2.5 g/kg, but not at 1.5 g/kg. Ethanol also increased the latency to onset of clonus in a dose-dependent manner beginning at 2.0 g/kg. Exposure to 12 ATA heliox antagonized this anticonvulsant effect. When exposed to 12 ATA heliox, the blood ethanol concentrations at time to onset of myoclonus were significantly higher in mice treated with 2.5 g/kg of ethanol as compared with blood ethanol concentrations of mice exposed to 1 ATA air. These findings extend the acute behavioral effects of ethanol known to be antagonized by hyperbaric exposure and support the hypothesis that low-level hyperbaric exposure blocks or reverses the initial action(s) of ethanol leading to its acute behavioral effects.

Low-level hyperbaric antagonism of ethanol-induced locomotor depression in C57BL/6J mice: dose response

This study characterized the antagonistic effects of hyperbaric exposure on the dose-response curve for ethanol-induced depression of locomotor activity. Drug-naive, male C57BL/6 mice were injected intraperitoneally with saline, 1.5, 2.0, 2.5, or 3.0 g/kg ethanol, and were exposed to 1 atmosphere absolute (ATA) air or 12 ATA helium-oxygen gas mixtures (heliox) at temperatures that offset the hypothermic effects of ethanol and helium. Locomotor activity was measured 10-30 min after injection. In addition, the effects of exposure to 12 ATA heliox on blood ethanol concentrations were tested in a separate group of mice injected with 2.5 g/kg ethanol. Ethanol produced a dose-dependent depression of locomotor activity beginning at 2.0 g/kg. Exposure to 12 ATA heliox completely antagonized the locomotor depressant effects of 2.0 and 2.5 g/kg ethanol and partially blocked the effects of 3.0 g/kg. Activity in mice given 1.5 g/kg ethanol was not significantly affected at 1 ATA air, but was significantly increased at 12 ATA heliox. Low-level hyperbaric exposure shifted the ethanol dose-response curve to the right with a resultant increase in the ED50 of ethanol for locomotor depression from 2.6 to 3.3 g/kg. Exposure to 12 ATA heliox did not alter blood ethanol concentrations in mice injected with 2.5 g/kg ethanol. These findings with 12 ATA heliox present key new evidence for the hypothesis that low-level hyperbaric exposure acts directly, with a pattern analogous to a competitive, mechanistic antagonist of ethanol.

Genetically determined differences in the antagonistic effect of pressure on ethanol-induced loss of righting reflex in mice

Hyperbaric exposure antagonizes ethanol's behavioral effects in a wide variety of species. Recent studies indicating that there are genetically determined differences in the effects of body temperature manipulation on ethanol sensitivity suggested that genotype might also influence the effects of hyperbaric exposure on ethanol intoxication. To investigate this possibility, ethanol injected long sleep (LS)/Ibg (2.7 g/kg), short sleep (SS)/Ibg (4.8 g/kg), 129/J (2.9 g/kg), and C57BL/6J (3.6 g/kg) mice were exposed to one atmosphere absolute (ATA) air or to one or 12 ATA helium-oxygen (heliox) at ambient temperatures selected to offset ethanol and helium-induced hypothermia. Hyperbaric exposure significantly reduced loss of righting reflex (LORR) duration in LS, 129, and C57 mice, but not in SS mice. A second experiment found that hyperbaric exposure significantly reduced LORR duration and increased the blood ethanol concentration (BEC) at return of righting reflex (RORR) in LS mice, but did not significantly affect either measure in SS mice. These results indicate that exposure to 12 ATA heliox antagonizes ethanol-induced LORR in LS, 129 and C57 mice, but not in SS mice. Taken with previous results, the present findings suggest that the antagonism in LS, 129, and C57 mice reflects a pressure-induced decrease in brain sensitivity to ethanol and that the lack of antagonism in SS mice cannot be explained by pressure-induced or genotypic differences in ethanol pharmacokinetics.(ABSTRACT TRUNCATED AT 250 WORDS)

Ethanol-induced depression of aggression in mice antagonized by hyperbaric exposure

The present study investigated the effect of hyperbaric exposure on ethanol-induced depression of aggressive behavior measured by resident-intruder confrontations. Adult male CFW mice (residents) were paired with females and housed together for 26 days. Then, resident mice were intubated with either ethanol (2 g/kg) or water (20 ml/kg) and were exposed to 1 atmosphere absolute (ATA) air, 1 ATA helium oxygen (heliox) or 12 ATA heliox using a within-subjects counterbalanced design. Thirty minutes after intubation an intruder was introduced. Ethanol significantly decreased aggressive behaviors (attack latency, attack bites, sideways threats, tail rattles and pursuit) in 1 ATA-treated animals. Pressure completely antagonized the depression of aggression induced by ethanol. Ethanol alone and pressure alone did not significantly affect nonaggressive behaviors. There were no statistically significant differences between groups in blood ethanol concentrations 50 minutes after intubation. These results suggest that ethanol's effects on aggressive behavior result from the same membrane actions leading to loss of righting reflex, depression of locomotor activity, tolerance and dependence.

Interaction of high pressure and a narcotic dose of ethanol on spontaneous behavior in rats

This study analyses the spontaneous motor activity of rats that had received a narcotic dose of ethanol (3.5 g/kg) and were then exposed to 1 atmosphere absolute pressure (ATA) air or to 1 or 72 ATA of helium-oxygen (heliox). The ambient temperature was adjusted to offset ethanol-and helium-induced hypothermia. Ethanol administration prevented the occurrence of convulsions but did not alter the total number of myoclonic jerks at stable pressure. The ethanol-intoxicated animals exposed to high pressure did not exhibit normal locomotion but showed a trend towards increased activity during the last observation period. Similar blood and brain concentrations of ethanol were found in the 1 and 72 ATA groups. These results show that exposure to 72 ATA for 40 min started to exert some antagonistic effects, and they suggest that exposure to higher pressures or for a longer period of time may be sufficient to significantly offset the depressant effects of a narcotic dose of ethanol on spontaneous behavior in rats. At the same time, ethanol seems to protect against some aversive effects of high pressure.

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.

Antagonism of ethanol-induced depression of mouse locomotor activity by hyperbaric exposure

Previous studies have shown that exposure to hyperbaric helium + oxygen (HEOX) antagonizes the acute depressant effect of hypnotic doses of ethanol on rodent behavior, precipitates and exacerbates withdrawal in ethanol-dependent mice, and attenuates the development of chronic functional ethanol tolerance. The present study extends these investigations to the sub-hypnotic dose range by determining the effect of hyperbaric exposure on ethanol-induced depression of locomotor activity. Male C57BL/6J mice were given two treatments, 2.5 g/kg ethanol and saline, spaced one week apart according to a within subjects, balanced crossover design. Following injection, animals were exposed individually to 1 atmosphere absolute (ATA) air or to 1 ATA or 12 ATA HEOX inside a 15 liter hyperbaric chamber. Chamber temperatures were adjusted to offset ethanol hypothermia and the cooling effect of helium. Locomotor activity was measured continuously, beginning 10 min after injection, and recorded at prescribed intervals for 60 min. Multivariate analysis of variance of the measured activity revealed statistically significant differences between groups based on atmospheric condition, treatment, and time after injection. Within group comparisons indicated that ethanol treatment induced a significant reduction in locomotor activity in mice exposed to either 1 ATA air or 1 ATA HEOX. In contrast, ethanol-injected mice exposed to 12 ATA HEOX did not show a significant ethanol-induced decrease in locomotor activity, indicating antagonism of ethanol's effect. Hyperbaric exposure did not significantly alter blood ethanol concentrations measured 70 min after ethanol injection, thus making a pharmacokinetic explanation for these results unlikely. These findings are consistent with, and extend, previous evidence suggesting that hyperbaric exposure antagonizes molecular actions of ethanol leading to intoxication.

Ethanol withdrawal in mice precipitated and exacerbated by hyperbaric exposure

Mice were fed an ethanol-containing liquid diet for 9 days. On removal of the diet, exposure to 12 atmospheres absolute of a mixture of helium and oxygen precipitated earlier withdrawal, increased withdrawal scores for the first 6 hours, and increased the peak withdrawal intensity compared to dependent animals exposed to control conditions. The enhanced withdrawal did not appear to reflect alterations in ethanol elimination, oxygen or helium partial pressures, body temperature, or general excitability. These results extend to chronically treated animals the evidence that hyperbaric exposure antagonizes the membrane actions of ethanol.

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.

The importance of experience in the development of tolerance to ethanol hypothermia

Studies were conducted to determine if an animal has to experience a reduction in body temperature during the acquisition period in order to develop tolerance to the hypothermic effect of ethanol. Adult, drug-naive C57BL/6J mice were injected with 2.6 or 3.6 g/kg ethanol or normal saline once daily for 6 days. During the tolerance acquisition period, days 1-5, mice were placed into warmed chambers (36 +/- 2(0)C) which offset ethanol hypothermia or into chambers at room temperature (24 +/- 1(0)C). On day 6, all mice were injected with ethanol and placed into chambers at room temperature. Tolerance to ethanol's hypothermic effect did not develop in the ethanol-warm acquisition group. These mice had a significantly greater degree of hypothermia on test day than the ethanol-room temperature acquisition group, which showed tolerance, and their degree of hypothermic response was similar to that of mice injected with saline during acquisition. The differences between groups cannot be attributed to pharmacokinetic alterations or to conditioned responses since there were no differences between groups in blood or brain ethanol concentrations on test day and all groups were exposed to the same acquisition and test situations. These results extend previous work to suggest that the development of tolerance to the physiological, as well as behavioral, aspects of ethanol intoxication requires more than simple exposure to ethanol.