Changes in ventral respiratory column GABA_aR ε- and δ-subunits during hibernation mediate resistance to depression by EtOH and pentobarbital

During hibernation in the 13-lined ground squirrel, Ictidomys tridecemlineatus, the cerebral cortex is electrically silent, yet the brainstem continues to regulate cardiorespiratory function. Previous work showed that neurons in slices through the medullary ventral respiratory column (VRC) but not the cortex are insensitive to high doses of pentobarbital during hibernation, leading to the hypothesis that GABA_A receptors (GABA_AR) in the VRC undergo a seasonal modification in subunit composition. To test whether alteration of GABA_AR subunits are responsible for hibernation-associated pentobarbital insensitivity, we examined an array of subunits using RT-PCR and Western blots and identified changes in ε- and δ-subunits in the medulla but not the cortex. Using immunohistochemistry, we confirmed that during hibernation, the expression of ε-subunit- containing GABA_ARs nearly doubles in the VRC. We also identified a population of δ-subunit-containing GABA_ARs adjacent to the VRC that were differentially expressed during hibernation. As δ-subunit- containing GABA_ARs are particularly sensitive to ethanol (EtOH), multichannel electrodes were inserted in slices of medulla and cortex from hibernating squirrels and EtOH was applied. EtOH, which normally inhibits neuronal activity, excited VRC but not cortical neurons during hibernation. This excitation was prevented by bicuculline pretreatment, indicating the involvement of GABA_ARs. We propose that neuronal activity in the VRC during hibernation is unaffected by pentobarbital due to upregulation of ε-subunit-containing GABA_ARs on VRC neurons. Synaptic input from adjacent inhibitory interneurons that express δ-subunit-containing GABA_ARs is responsible for the excitatory effects of EtOH on VRC neurons during hibernation.