| Abstract: | Immunohistochemical and retrograde tracing techniques were employed to demonstrate a changing pattern of adenosine deaminase (ADA) immunoreactivity in cranial motoneurons during their ontogenesis in the rat. Immunostaining for ADA was observed only in motoneurons of hypoglossal and facial motor nuclei and only at certain stages during development. Moreover, ADA immunoreactivity was restricted to subpopulations of motoneurons within each nucleus. In the hypoglossal nucleus ADA-immunostained neurons were seen only in the dorsal subnucleus, where they appeared at about 15 days of gestation, reached maximal staining intensity early after birth, and disappeared by the 25th postnatal day. In the facial motor nucleus, immunoreactive neurons were detected only in the intermediate subnucleus, where ADA immunostaining was first detected at 18 days of gestation and was maximal during the first few postnatal days, and in the lateral subnucleus, where immunostaining appeared perinatally. In both facial motor subnuclei, ADA immunoreactivity was no longer detectable by the 15th postnatal day. Retrograde tracing with WGA-HRP or fluorescent dye injected into various muscles of the face or tongue in young animals indicated that ADA-immunoreactive motoneurons in the hypoglossal and facial motor nuclei innervate retractor muscles of the tongue and perioral or nasal muscles, respectively. In view of the critical role of these muscles in suckling and sniffing behavior, it is suggested that metabolic pathways associated with ADA may be involved in the early maturation of the motoneurons projecting to these muscles. Alternatively, the transient presence of ADA in these neurons may reflect a developmental period during which purine nucleosides and/or nucleotides may serve as neuromodulators at their peripheral terminations. |
