Role of the alpha 4-containing GABA A receptors in anesthetic and ethanol antagonist effects: Insights from a global knockout mouse model

Despite their widespread use, the precise molecular actions of anesthetics and alcohol are unknown. Although anesthetics have made surgical intervention palatable, anesthetics are not free of alarming side effects such as anesthetic awareness and post-operative cognitive deficits. While alcohol is consumed for positive effects such as anti-anxiety, euphoria, and relaxation, alcohol consumption also results in adverse behavioral effects such as sedation, motor incoordination, and cognitive impairment. Chronic alcohol consumption results in alcohol withdrawal syndrome, tolerance, and dependence. Understanding the mechanisms of action of anesthetics and alcohol are critical for preventing anesthetic side effects and for developing effective treatments for alcohol abuse and alcoholism. Many putative targets of anesthetics and alcohol have been identified in brain including sodium channels, potassium channels, glutamate receptors, glycine receptors, and especially GABA receptors. Phasic and tonic inhibitory currents mediated by GABA type A receptors are sensitive to modulation by anesthetics and alcohol. α4 subunit-containing GABA A receptors mediate tonic inhibitory currents, are highly sensitive to GABA, and are strongly implicated in the effects of volatile, intravenous, and neurosteroid anesthetics. Global α4 knockout mice showed reduced tonic current that was not potentiated by the volatile anesthetic isoflurane or the neurosteroid anesthetic, alphaxalone. Specific Aim 1 tested the hypothesis that volatile and intravenous anesthetic effects are mediated via a4-containing receptors by comparing behavioral responses to these drugs in wild type and α4 KO mice. Results obtained indicate that while α4-containing receptors are required for the amnestic effects of isoflurane, they are not required for mediating the effects of volatile and intravenous anesthetics on other behavioral endpoints. Interestingly, α4-containing receptors are required for low dose alphaxalone-induced locomotor stimulation, but not high dose effects. α4-containing receptors, when paired with the δ subunit, have been proposed to possess a common binding pocket for ethanol and pharmacologic antagonists of ethanol action. Previous studies in the Homanics lab indicated that the ethanol-reversing effects of Ro15-4513, an imidazobenzodiazepine ethanol-antagonist, were dramatically reduced in α4 KO mice both at the cellular and the behavioral level. Specific Aim 2 tested the hypothesis that a4-containing receptors are required for the ethanol antagonistic effects of RY023, a derivative of Ro15-4513. α4 KO mice showed differential sensitivity to the effects of RY023 in the presence and absence of ethanol on loss of righting reflex and locomotor behavior. We conclude that α4 containing receptors are involved in some intrinsic effects of RY023 but not in the ethanol-antagonistic effects of RY023. This study of α4-containing receptors has advanced our understanding of anesthetic action and eliminated the theory of a unitary target for ethanol antagonism