Plasticity in respiratory motor control - Selected contribution: Intermittent hypoxia induces phrenic long-term facilitation in carotid-denervated rats

Episodic hypoxia elicits a long-lasting augmentation of phrenic inspiratory activity known as long-term facilitation (LTF). We investigated the respective contributions of carotid chemoafferent neuron activation and hypoxia to the expression of LTF in urethane-anesthetized, vagotomized, paralyzed, and ventilated Sprague-Dawley rats. One hour after three 5-min isocapnic hypoxic episodes [arterial PO2 (Pa-O 2) = 40+/-5 Torr], integrated phrenic burst amplitude was greater than baseline in both carotid-denervated (n=8) and sham-operated (n=7) rats (P<0.05), indicating LTF. LTF was reduced in carotid-denervated rats relative to sham (P<0.05). In this and previous studies, rats were ventilated with hyperoxic gas mixtures (inspired oxygen fraction=0.5) under baseline conditions. To determine whether episodic hyperoxia induces LTF, phrenic activity was recorded under normoxic (Pa-O 2 = 90-100 Torr) conditions before and after three 5-min episodes of isocapnic hypoxia (Pa-O 2 = 40+/-5 Torr; n=6) or hyperoxia (Pa-O 2 > 470 Torr; n=6). Phrenic burst amplitude was greater than baseline 1 h after episodic hypoxia (P<0.05), but episodic hyperoxia had no detectable effect. These data suggest that hypoxia per se initiates LTF independently from carotid chemoafferent neuron activation, perhaps through direct central nervous system effects.