Effects of Two‐Hit Lipopolysaccharide (LPS) Administration on Basal Inspiratory Motor Activity in Spontaneously Breathing Adult Male C57BL/6 Mice

Abstract

Ongoing studies designed to induce peripheral inflammation, and its accompanying neuroinflammation, have begun to examine various aspects of neural control of breathing. These studies have implemented either a systemic (ip) or intratracheal (IT) administration protocol using the bacterial endotoxin lipopolysaccharide (LPS) to produce models of sepsis and acute lung injury/acute respiratory distress syndrome (ALI/ARDS). In chronic lung disease, there is often a combination of systemic inflammation with an airway/lung‐mediated exacerbation; thus, a combined ip‐IT LPS administration protocol could better reflect chronic (lung/airway) disease states. Here, we implemented a ‘two‐hit’ 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, and examined the ventilatory behavior in both unanesthetized (recorded using whole body plethysmography) and urethane‐anesthetized (recorded as EMG activities from diaphragm (EMGDia) and genioglossus (EMGGG) muscles) spontaneously breathing adult male C57BL/6 mice at 2–6 hr post ‘two‐hit’ LPS administration. A two‐hit protocol using ip‐IT saline injections served as a control. We found that following ip‐IT LPS administration, (1) breathing frequency was initially increased (within ~2 hr post injections) after which it markedly declined by as much as ~50% of the pre‐LPS breathing rate, (2) inspiratory burst durations were minimally altered but expiratory durations were increased and highly variable, (3) the expiratory flow profile was altered to a pattern consistent with an obstructive disease phenotype, (4) a progressive alteration in breathing pattern that included periodic robust discharges in EMGGG as compared to EMGDia and periods of dysrhythmic breathing emerged, (5) the shape of the EMG bursts was altered with the peak in both EMGDia and EMGGG bursts being shifted to an earlier time point, and (6) the duration and location of EMGDia and EMGGG spectral activities were differentially affected, including an enhancement of MFO spectral power in a subset of mice. These findings indicate that ‘two‐hit’ administration of LPS not only markedly alters basal ventilatory behaviors, including inspiratory‐phase synchronization, but also may differentially affect the central neural mechanisms controlling diaphragm versus genioglossus muscle activities. Moreover, our data support using a ‘two‐hit’ administration approach as viable option for producing (chronic) lung disease and its accompanying neuroinflammation.