Recurrent hypoxic exposure and reflex responses during development in the piglet

The effects of recurrent hypoxia on cardiorespiratory reflexes were charact erized in anesthetized piglets at 2-10 d (n = 15), 2-3 weeks (n = 11) and 8 -10 weeks (n = 8). Responses of phrenic and hypoglossal electroneurograms ( ENG(phr) and ENG(hyp)) to hypoxia (8% O-2, bal N-2, 5 min), hypercapnia (7% CO2 bal O-2, 5 min) and intravenous capsaicin were tested before and after recurrent exposure to 11 episodes of hypoxia (8% O-2 bal N-2, 5 min). In p iglets 2-10 d, ENG(phr) response to hypoxia declined in proportion to the n umber of hypoxic exposures; however, ENG(hyp) response to hypoxia was uncha nged. In piglets at 2-10 d, intracisternal injection of bicuculline (GABA, receptor antagonist) reversed effects of recurrent hypoxia on ENG(hpr) hypo xic response, eliminated apnea during hypoxia, as well as the delay in appe arance of ENG(phr) after hypoxia. The ENG(phr) response to 7% CO2 inhalatio n also decreased after recurrent hypoxia; however, the ENG(phr) response to C-fiber stimulation by capsaicin was unaltered. Piglets at 2-3 and 8-10 we eks were resistant to the depressive effects of recurrent hypoxia on respir atory reflex responses. We conclude that the response of the anesthetized n ewborn piglet to recurrent hypoxia is dominated by increasing inhibition of phrenic neuroelectrical output during successive hypoxic exposures. Centra l GABAergic inhibition may contribute significantly to the cumulative effec ts of repeated hypoxia in the newborn piglet experimental model. (C) 2000 E lsevier Science B.V. All rights reserved.