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

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)