Preinspiratory and inspiratory hypoglossal motor output during hypoxia-induced plasticity in the rat.

Respiratory-related discharge in the hypoglossal (XII) nerve is composed of preinspiratory (pre-I) and inspiratory (I) activity. Our first purpose was to test the hypothesis that hypoxia-induced plasticity in XII motor output is differentially expressed in pre-I vs. I XII bursting. Short-term potentiation (STP) of XII motor output was induced in urethane-anesthetized, vagotomized, and ventilated rats by exposure to isocapnic hypoxia (PaO2 of approximately 35 Torr). Both pre-I and I XII discharge abruptly increased at beginning of hypoxia (i.e., acute hypoxic response), and the relative increase in amplitude was much greater for pre-I (507+/-46% baseline) vs. I bursting (257+/-16% baseline; P<0.01). In addition, STP was expressed in I but not pre-I bursting following hypoxia. Specifically, I activity remained elevated following termination of hypoxia but pre-I bursting abruptly returned to prehypoxia levels. Our second purpose was to test the hypothesis that STP of I XII activity results from recruitment of inactive or "silent" XII motoneurons (MNs) vs. rate coding of active MNs. Single fiber recordings were used to classify XII MNs as I, expiratory-inspiratory, or silent based on baseline discharge patterns. STP of I XII activity following hypoxia was associated with increased discharge frequency in active I and silent MNs but not expiratory-inspiratory MNs. We conclude that the expression of respiratory plasticity is differentially regulated between pre-I and I XII activity. In addition, both recruitment of silent MNs and rate coding of active I MNs contribute to increases in XII motor output following hypoxia.