Circadian clock protein Bmal1 regulates respiratory motor plasticity in male rats

Acute intermittent hypoxia (AIH) elicits respiratory motor plasticity in the phrenic, intercostal, and hypoglossal motor pools and has emerged as a promising therapeutic strategy to improve respiratory function in people with neuromuscular disorders that compromise breathing. Although we recently reported that time-of-day regulates moderate ([Formula: see text] ∼40-50 mmHg) AIH-induced respiratory motor plasticity, it is unknown whether diurnal effects on AIH-induced phrenic (pLTF) or ventilatory (vLTF) long-term facilitation are mediated via the endogenous circadian clock versus other factors. Since many biological rhythms are driven by the endogenous clock and clock genes (including the essential clock gene Bmal1) are rhythmically expressed in the phrenic motor system, we hypothesized that the molecular clock within respiratory motor neurons exerts time-of-day effects on pLTF and vLTF in Sprague Dawley rats (3-6 mo old males). Intrapleural injections of small-interfering RNAs (siRNAs) were used to selectively knock down Bmal1 within respiratory motor neurons by ∼30%. AIH consisting of 15, 1-min hypoxic episodes ([Formula: see text] = 0.09) was delivered in the midrest (i.e., light) or midactive (i.e., dark) phases, and pLTF (Δintegrated phrenic burst amplitude) and vLTF (ΔV̇e/V̇co₂) were assessed in rats given siRNAs targeting Bmal1 versus nontargeting controls. In midrest phase, pLTF was reduced, and vLTF was abolished in rats given siBmal1 versus nontargeting siRNA. However, siBmal1 had no significant effect on either pLTF or vLTF in the midactive phase. Thus, the phrenic motor neuron circadian clock regulates AIH-induced respiratory motor plasticity in a time-of-day-dependent manner. It is important to consider circadian biology in future studies of AIH-induced respiratory motor plasticity.NEW & NOTEWORTHY Although diurnal cycle influences respiratory motor plasticity elicited by acute intermittent hypoxia (AIH), it is unknown how endogenous circadian clock mechanisms contribute to time-of-day effects on plasticity. We report that knockdown of the circadian clock protein Bmal1 within respiratory motor neurons attenuates phrenic and ventilatory long-term facilitation in a manner dependent on diurnal phase. Thus, circadian biology is an important consideration for studies of respiratory motor plasticity.