Inhibition of pontine noradrenergic A7 cells reduces hypoglossal nerve activity in rats

Noradrenergic (NE) excitatory drive maintains activity of hypoglossal (XII) motoneurons during wakefulness. In predisposed persons, sleep-related decrements of NE cell activity may contribute to hypotonia of upper airway muscles innervated by XII motoneurons. The goal of this study was to determine whether NE neurons of the pontine A7 group, an anatomically identified source of NE projections to the XII nucleus, provide significant, endogenous NE excitatory drive to XII motoneurons. In anesthetized rats, we microinjected clonidine (0.75 mM, 20-40 nl), an alpha(2)-adrenergic receptor agonist that inhibits pontine NE cells, aiming at the A7 region. Nine injections were placed within 0.4 mm from the A7 group identified using tyrosine hydroxylase immunohistochemistry: they reduced XII nerve activity by 31.3+/-2.8% (standard error) and decreased the central respiratory rate by 6%. Another 21 injections, including eight placed near NE cells of the sub-coeruleus region, were made at distances over 0.5 mm from the A7 group and they did not alter either XII nerve activity or respiratory rate. In control experiments, clonidine injections into the A7 group preceded by injections of an alpha(2)-receptor antagonist, RS-79948, did not change XII nerve activity. Four experiments with unilateral clonidine injections into the A7 region and with Fos immunohistochemistry used as a marker of cell activity revealed that the percentage of Fos-positive A7 cells was significantly reduced on the injected side. There was also a significant positive correlation between Fos expression in A7 cells and XII nerve activity. Thus, decrements of NE excitatory drive from the A7 group may significantly reduce XII nerve activity. In obstructive sleep apnea patients, in whom the muscles innervated by XII motoneurons act as upper airway dilators, this may contribute to sleep-related respiratory disorders.