Respiratory plasticity: differential actions of continuous and episodic hypoxia and hypercapnia

The objectives of this paper are: (1) to review advances in our understanding of the mechanisms of respiratory plasticity elicited by episodic versus continuous hypoxia in short to intermediate time domains (min to h); and (2) to present new data suggesting that different patterns of hypercapnia also elicit distinct forms of respiratory plasticity. Episodic, but not continuous hypoxia elicits long-term facilitation (LTF) of respiratory motor output. Phrenic LTF is a scrotonin-dependent central neural mechanism that requires; (a) activation of spinal serotonin receptors; and (b) spinal protein synthesis. Continuous and episodic hypercapnia also elicit different mechanisms of plasticity. Continuous, severe hypercapnia (25 min of similar to 10% inspired CO2) elicits long-term depression (LTD) of phrenic motor output (- 33 +/- 8% at 60 min post-hypercapnia) in anesthetized rats. In contrast, 3, 5 min hypercapnic episodes do not elicit LTD (9 +/- 17% at 60 min). We hypothesize that the response of respiratory motoneurons to serotonergic and noradrenergic modulation may contribute to pattern sensitivity to hypoxia and hypercapnia. (C) 2001 Elsevier Science B.V. All rights reserved.