Influence of Two‐Hit Lipopolysaccharide (LPS) Administration on Acute Hypoxic Ventilatory Response (HVR) in Urethane‐Anesthetized Spontaneously Breathing Adult Male C57BL/6 Mice

Abstract

In chronic lung/airway disease, there is often a combination of systemic inflammation with an airway/lung‐mediated exacerbation. Under these conditions, patients may experience bouts of acute hypoxia; however, to date, there is limited information regarding the influence of this inflammation paradigm, including its accompanying neuroinflammation, on the acute hypoxic ventilatory response (HVR). To begin to address this issue, we used a ‘two‐hit’ lipopolysaccharide (LPS) administration protocol in which systemic LPS (ip, 3 mg/kg) was administered ~24 hr prior to an IT LPS (0.5 mg/kg) injection in spontaneously breathing adult male C57BL/6 mice, and at 2–6 hr post ‘two‐hit’ LPS administration, we recorded EMG activities from diaphragm (EMGDia) and genioglossus (EMGGG) muscles under baseline conditions (40% O2 in a balance of N2), during an acute bout of hypoxia (HVR, 10% O2 for 90 s), and during recovery from hypoxia. A similar recording protocol was used in control experiments in which mice received two‐hit ip‐IT saline injections. We found that ip‐IT LPS administration not only altered basal inspiratory motor output, but markedly altered the acute HVR. Specifically, ‘two‐hit’ LPS administration blunted the hypoxia‐induced amplitude response, such that amplitude increased by ≤10% (versus 30–50% increase in control mice), and produced an exaggerated frequency response, such that frequency increased by ≥30% (versus~15–20% increase in control mice). In addition, in LPS treated mice in which a dysrhythmic breathing pattern was observed, hypoxia either worsened the dysrhythmia or improved it to yield a more normal (but higher frequency) rhythm. Finally, ‘two‐hit’ LPS administration slightly altered the EMG burst pattern and attenuated the reduction in inspiratory burst complexity (as reflected by a decrease in the approximate entropy (ApEn) value) when compared to changes observed in control mice. During recovery from hypoxia, the inspiratory motor activity in both LPS treated and control mice returned toward baseline levels. These findings indicate that ‘two‐hit’ administration of LPS significantly alters the acute HVR, and further support the use of a ‘two‐hit’ administration approach for studying the role of lung/airway inflammation and its accompanying neuroinflammation in central respiratory control, including the chemical drive to breath.