Effect of chronic intermittent hypoxia on noradrenergic activation of hypoglossal motoneurons

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.