Daily acute intermittent hypoxia upregulates mRNAs encoding proteins necessary for serotonin‐dependent respiratory motor plasticity and downregulates circadian clock gene Per1 mRNA

Abstract

The neural system controlling breathing has evolved mechanisms of neuroprotection to mitigate potential damage in response to decreased oxygen availability in addition to mechanisms of neuroplasticity that sustain life. Even brief episodes of low inspired O2– acute intermittent hypoxia (AIH) – elicit neuroplasticity in the form of phrenic long‐term facilitation (pLTF). Preconditioning with repeated AIH exposure elicits metaplasticity in spinal respiratory networks, expressed as an enhanced capacity for pLTF. Distinct cellular cascades underlying AIH‐induced pLTF have been elucidated, but important unanswered questions remain. For example, what is the cellular mechanism through which AIH preconditioning enhances pLTF? Two possible mechanisms include 1) transcriptional regulation of plasticity‐related molecules and 2) circadian clock regulation. We recently localized circadian clock genes in the phrenic motor circuit and these same genes are known to regulate multiple forms of neuroplasticity in other systems. Sustained oxygen deficiency dampens the amplitude of circadian clock gene expression and circadian behaviors. However, the influence of AIH on clock genes in the phrenic motor system or their impact on molecules necessary for AIH induced pLTF have not been assessed. In these preliminary experiments, we tested the hypotheses that 4 weeks of daily AIH: 1) increases endogenous expression mRNAs encoding molecules necessary for pLTF, including 5‐HT2a, 5‐HT2b, BDNF and TrkB; and 2) modulates the expression clock genes, including Bmal1, Per1, Per2. To determine the impact of daily AIH on plasticity and clock gene expression, ventral cervical (C3–C5) spinal cords (the major site of plasticity) were collected from adult male Sprague Dawley rats (n=12) exposed to normoxia [20.9%O2; n=6)] or daily acute intermittent hypoxia for 28 days (dAIH; 10, 5‐min episodes of 10.5% O2 per day, 5‐min normoxic intervals; n=6). Relevant transcripts were assessed via qRT‐PCR. dAIH increased mRNA for 5HT2a and BDNF (p<0.05), but there were no significant changes in mRNA for 5HT2b or TrkB. Per1, but not Bmal1 or Per2, was significantly decreased in dAIH treated rats versus controls (p<0.01). Thus, dAIH: 1) increases endogenous transcript levels for molecules involved in the seroton‐independent phrenic motor plasticity; and 2) modulates components the circadian clock in spinal regions associated with the phrenic motor nucleus. The functional significance of decreased Per1 is not yet known. However, we speculate that modulation of the circadian clock may regulate elements of phrenic motor plasticity. Greater understanding of relationships between the circadian clock and AIH‐induced respiratory motor plasticity may guide our efforts to harness the therapeutic potential of AIH as a means of restoring breathing in severe neuromuscular disorders that typically end life due to ventilatory failure.Support or Funding InformationNIH T32‐HL134621 and the UF McKnight Brain Institute.