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

Abstract

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.

Fig. 1.

Sample phrenic (Phr) and XII neurograms demonstrating the method for calculating the respiratory cycle. Inspiratory duration (T_I) was defined as the period between the onset of the phrenic inspiratory burst (vertical dashed line) and the time point when integrated phrenic activity amplitude reduced by 50% of the peak value (vertical solid line; see ref. 31). Onset of the XII burst was identified from the integrated XII trace (vertical dotted line) and then used to calculate the difference between phrenic and XII burst onset time. Amplitudes of the preinspiratory (pre-I) and inspiratory (I) XII bursts were measured as the peak height of the integrated XII signal during expiration and inspiration, respectively. Tonic XII bursting during the hypoxic challenge is indicated by the upward displacement of the integrated XII neurogram just before the pre-I burst onset. ∫ is moving time averaged or “integrated” neurogram.

Fig. 2.

Phrenic and XII neurogram activity during and following hypoxia. Peak height of the integrated (∫) I phrenic and XII burst and the pre-I XII burst are expressed relative to baseline activity (%baseline) in A. In B, the change (Δ) in amplitude is expressed relative to the maximum activity (Δ %max). Hypoxia induced more robust in increase in pre-I XII bursting compared with the I XII and phrenic responses. Following hypoxia, however, pre-I XII bursting rapidly returned to baseline values, whereas I XII and phrenic bursting remained elevated. Insets are provided for clarity and show an expanded view of the data recorded at 3-min posthypoxia. *P < 0.05 and **P < 0.01 vs. baseline. ##P < 0.01, difference between I phrenic and XII activity. ϕϕP < 0.01, difference between pre-I and I XII activity.

Fig. 3.

Distribution of XII motoneurons (MN) discharge onset relative to the respiratory cycle. XII MN discharge onset time was expressed as percentage of inspiratory (T_I) or expiratory (T_E) duration as measured from the phrenic neurogram (see Fig. 1). Onset time of expiratory-inspiratory (E-I, white) and inspiratory (I, gray) XII MNs was assessed during the baseline condition. Recruited, previously silent XII MNs (S, dark grey) were inactive during baseline and thus their onset time was assessed during the first 30 s of hypoxia.

Fig. 4.

Representative example of phrenic and XII neurograms (whole nerve recordings) and I XII MN bursting (single fiber recording) before, during, and after hypoxia. A: arterial blood pressure (BP) and both the raw and integrated phrenic and XII nerve signals. In addition, the discharge of a single I XII MN (XII MN) and its burst frequency (mean f, calculated in 100-ms bins) are shown in A, bottom. Hypoxia caused a progressive increase in phrenic and XII activity and a substantial increase in XII MN discharge frequency. Following hypoxia, phrenic and XII burst amplitude and I XII MN discharge frequency all remained elevated indicating posthypoxia short-term potentiation. B: expanded time scale traces showing a single neural breath from the areas marked in A, a–d. Both discharge frequency and spike numbers of I XII MNs were enhanced during and following hypoxia (B, b–d). Note that the I baseline pattern (Ba) transforms into an E-I pattern during hypoxia (Bc). To confirm that the recordings were from the same MN, the individual spikes from a–d are superimposed in Fig. 4B, bottom.

Fig. 5.

Representative example of phrenic and XII neurograms (whole nerve recordings) and E-I XII MN bursting (single fiber recording) before, during, and after hypoxia. Figure labels and orientation are the same as in Fig. 4. Note that the E-I MN begins bursting during the late E or pre-I period during baseline (Ba), and this pattern is maintained both during (Bb–c) and following hypoxia (Bd).

Fig. 6.

Influence of hypoxia on XII MN discharge frequency. Burst frequency was assessed during both the inspiratory (T_I; A) and expiratory periods (T_E; B) as well as the overall discharge frequency (reflecting both T_I and T_E; C). *P < 0.05 and **P < 0.01 vs. baseline. #P < 0.05 and ##P < 0.01, significant differences between E-I and I XII MN discharge frequency. ϕϕP < 0.01, difference between I and S XII MN discharge frequency.

Fig. 7.

Influence of hypoxia on the number of XII MN spikes per neural breath. Spike number was assessed during both the T_I (A) and T_E (B) as well as the total number of spikes per breath (T_I + T_E; C). **P < 0.01, data point is different that the baseline value. #P < 0.05 and ##P < 0.01, differences between E-I and I XII MNs. ϕϕP < 0.01, significant difference between I and S XII MNs.

Fig. 8.

The influence of hypoxia on XII MN discharge duration. The discharge duration (i.e., time between the first and last spike during each neural breath) was assessed during both the T_I (A) and T_E (B) as well as the total discharge duration (T_I + T_E; C). *P < 0.05 and **P < 0.01, differences compared with the baseline value. #P < 0.05 and ##P < 0.01, difference between E-I and I XII MNs. ϕP < 0.05 and ϕϕP < 0.01, significant difference between I and S XII MNs.

Fig. 9.

Representative example depicting recruitment of a previously silent XII MN during hypoxia, and the persistent bursting of this neuron following hypoxia. Labels and orientation are the same as in Figs. 4 and 5. Note that the MN (XII MN) initially begins to burst with an I pattern (Ba) but then adopts an E-I pattern (Bc). The previously silent neuron continues to burst upon return to normoxia (Bd).

Fig. 10.

Hypoxia-induced changes in MN burst frequency during the T_I and T_E phases. Hypoxia induced a progressive increase in both I and E discharge frequency in both E-I (A) and I (B) XII MNs. However, the relative enhancement of discharge frequency during hypoxia was substantially greater during the E phase. On the contrary, following hypoxia E bursting rapidly returns to baseline but I bursting remains enhanced. *P < 0.05 and **P < 0.01, differences from the baseline value. #P < 0.05 and ##P < 0.01, differences between I and E discharge frequency.