Effect of chronic intermittent hypoxia on noradrenergic activation of hypoglossal motoneurons

Abstract

In obstructive sleep apnea patients, elevated activity of the lingual muscles during wakefulness protects the upper airway against occlusions. A possibly related form of respiratory neuroplasticity is present in rats exposed to acute and chronic intermittent hypoxia (CIH). Since rats exposed to CIH have increased density of noradrenergic terminals and increased α(1)-adrenoceptor immunoreactivity in the hypoglossal (XII) nucleus, we investigated whether these anatomic indexes of increased noradrenergic innervation translate to increased sensitivity of XII motoneurons to noradrenergic activation. Adult male Sprague-Dawley rats were subjected to CIH for 35 days, with O(2) level varying between 24% and 7% with 180-s period for 10 h/day. They were then anesthetized, vagotomized, paralyzed, and artificially ventilated. The dorsal medulla was exposed, and phenylephrine (2 mM, 10 nl) and then the α(1)-adrenoceptor antagonist prazosin (0.2 mM, 3 × 40 nl) were microinjected into the XII nucleus while XII nerve activity (XIIa) was recorded. The area under integrated XIIa was measured before and at different times after microinjections. The excitatory effect of phenylephrine on XII motoneurons was similar in sham- and CIH-treated rats. In contrast, spontaneous XIIa was more profoundly reduced following prazosin injections in CIH- than sham-treated rats [to 21 ± 7% (SE) vs. 40 ± 8% of baseline, P < 0.05] without significant changes in central respiratory rate, arterial blood pressure, or heart rate. Thus, consistent with increased neuroanatomic measures of noradrenergic innervation of XII motoneurons following exposure to CIH, prazosin injections revealed a stronger endogenous noradrenergic excitatory drive to XII motoneurons in CIH- than sham-treated anesthetized rats.

Fig. 1.

Experimental protocol and sequence of phenylephrine (PE) and prazosin (PZ) microinjections into the hypoglossal (XII) nucleus and data collection. Gray bars represent 1-min intervals during which mean XII nerve activity (area under the curve), central respiratory rate, blood pressure, and heart rate were measured before and after microinjections. Arrows indicate times of PE and PZ injections into the XII nucleus during the course of the experimental protocol.

Fig. 2.

Effects of PE injections into the XII nucleus in rats exposed to chronic intermittent hypoxia (CIH) and sham-treated rats. A: integrated XII nerve activity (∫XIIa) prior to and following PE injection in a sham-treated rat. B: expanded segments of raw and integrated XIIa corresponding to the shaded areas in A. C: PE injection site for the experiment illustrated in A. Pontamine sky blue dye was injected with PE and is seen in the right XII nucleus (XIIn) in a neutral red-stained coronal section. Nucleus of the solitary tract (NTS) is shown as an orientation landmark. D and E: mean effects of PE injections into the XII nucleus on XIIa area under the curve (AUC) and central respiratory rate in CIH- and sham-treated rats. Values (means ± SE) for each measurement period are plotted at the time corresponding to the end of that period. Excitatory effect of PE on XIIa was of similar magnitude in both groups, and injections did not affect central respiratory rate. XIIa was significantly increased compared with the baseline level up to min 3 following PE injection when both treatment groups were combined (Friedman's ANOVA, Tukey-corrected multiple comparisons, P < 0.05).

Fig. 3.

Effects of PZ injections in CIH- and sham-treated rats. A: integrated XIIa prior to and following 3 successive injections of 40 nl of PZ into the XII nucleus at 3 rostrocaudal levels in a CIH-treated rat. Transient fluctuations of XIIa prior to the 2nd and 3rd PZ injections are caused by insertions of the microinjection pipette into the XII nucleus and the resulting mechanical stimulation of XII motoneurons. B: expanded segments of raw and integrated XIIa corresponding to the shaded areas in A. C and D: mean effects of PZ injections into the XII nucleus on XIIa AUC and central respiratory rate in CIH- and sham-treated rats. Values (means ± SE) for each measurement period are plotted at the time corresponding to the end of that period. *Significantly different from CIH (P < 0.05). PZ injections did not elicit changes in central respiratory rate.