Brainstem projections to the normal and noradrenergically hyperinnervated trigeminal motor nucleus

Abstract

The noradrenergic innervation of the trigeminal motor nucleus of the rat can be increased severalfold by neonatal treatment with the neurotoxin, 6-hydroxydopamine. The brainstem projections to the nucleus were studied by injecting HRP into the nucleus of normal and noradrenergically hyper-innervated rats. In order to identify the source of the noradrenergic inner-vation, the fluorescent dye, True Blue, was used as a retrograde tracer in combination with the glyoxylic acid histofluorescence method for catechol-amines. In both control and neonatally treated rats, the noradrenergic inner-vation of the motor nucleus' was shown to arise from an ipsilate group of cells located among the fibers of the lateral lemniscusjust rostral to the motor nucleus. Our results confirmed the high degree of specificity of nor-adrenergic innervation, which arises exclusively from this lateral tegmental noradrenergic cell group. During the process of sprouting, this specificity is maintained since only those noradrenergic cells normally innervating the nucleus were retrogradely labeled in neonatally treated animals. Other noradrenergic projections which are also increased in these animals, such as the nearby locus ceruleus innervation of the main sensory trigeminal nucleus, do not spread to the motor trigeminal nucleus. HRP-labeled nonadrenergic cells were concentrated dorsally, with scat-tered cells surrounding the nucleus. A similar distribution was observed contralateral to the injection site. The mesencephalic trigeminal nucleus was labeled only ipsilateral to the injection. The motor nucleus also receives an extensive bilateral input from the pontine and medullary reticular for-mation. The medial reticular formation nuclei, including nucleus pontis caudalis, nucleus gigantocellularis, and nucleus reticularis ventralis con-tained large labeled cells, which were especially numerous in the retrotri-geminal area. Smaller, lateral reticular formation neurons were concen-trated rostrally and ipsilaterally in the nucleus pontis lateralis. HRP retrograde labeling revealed no obvious change in the overall pattern of cells innervating the trigeminal motor nucleus following noradrenergic hy-perinncrvation.