Age-related changes in serotonin in the hypoglossal nucleus of rat: implications for sleep-disordered breathing.

We are attempting to determine the neuronal factors that influence upper airway patency during sleep in the elderly. Serotonin has a facilitatory effect on hypoglossal motoneurons that innervate the tongue, and manipulations of the serotonergic system alter airway patency. We hypothesized that age-associated changes in serotonergic input to the hypoglossal nucleus might be a factor in the increased susceptibility to sleep-disordered breathing in the elderly. We used light microscopic immunocytochemistry to study the distribution of serotonin in the hypoglossal nucleus in young and old rats. Rats > 18 months had fewer serotonin immunoreactive axons and boutons in the hypoglossal nucleus than rats < 6 months. These data suggest that normal aging may result in a change in the availability of serotonin that results in decreased facilitation of hypoglossal motoneurons.