Increased GABAA Receptor ε-Subunit Expression on Ventral Respiratory Column Neurons Protects Breathing during Pregnancy

GABAergic signaling is essential for proper respiratory function. Potentiation of this signaling with allosteric modulators such as anesthetics, barbiturates, and neurosteroids can lead to respiratory arrest. Paradoxically, pregnant animals continue to breathe normally despite nearly 100-fold increases in circulating neurosteroids. epsilon subunit-containing GABA(A)Rs are insensitive to positive allosteric modulation, thus we hypothesized that pregnant rats increase e subunit-containing GABA(A)R expression on brainstem neurons of the ventral respiratory column (VRC). In vivo, pregnancy rendered respiratory motor output insensitive to otherwise lethal doses of pentobarbital, a barbiturate previously used to categorize the e subunit. Using electrode array recordings in vitro, we demonstrated that putative respiratory neurons of the preBotzinger Complex (preBotC) were also rendered insensitive to the effects of pentobarbital during pregnancy, but unit activity in the VRC was rapidly inhibited by the GABA(A)R agonist, muscimol. VRC unit activity from virgin and post-partum females was potently inhibited by both pentobarbital and muscimol. Brainstem e subunit mRNA and protein levels were increased in pregnant rats, and GABA(A)R epsilon subunit expression co-localized with a marker of rhythm generating neurons (neurokinin 1 receptors) in the preBotC. These data support the hypothesis that pregnancy renders respiratory motor output and respiratory neuron activity insensitive to barbiturates, most likely via increased epsilon subunit-containing GABA(A)R expression on respiratory rhythm-generating neurons. Increased epsilon subunit expression may be critical to preserve respiratory function (and life) despite increased neurosteroid levels during pregnancy.