Transneuronal labeling in hamster brainstem following lingual injections with herpes simplex virus-1

Brainstem projections to hypoglossal motoneurons innervating the intrinsic and extrinsic muscles of the tongue were determined using the transneuronal transfer of Herpes simplex virus-1. Injections of Herpes simplex virus-1 into the intrinsic muscles of the anterior tongue, the geniohyoid and styloglossus muscles each produced specific patterns of label within the hypoglossal nucleus that corresponded closely to the distributions of retrogradely labeled neurons produced by similar injections of horseradish peroxidase. With relatively short survival times, Herpes simplex virus-1 injections further labeled neurons in both the brainstem reticular formation lateral to the hypoglossal nucleus and in the nucleus of the solitary tract. Intrinsic lingual muscles injections of Herpes simplex virus-1 labeled reticular formation neurons distributed laterally along the entire anterior-posterior length of hypoglossal nucleus. In contrast, labeled reticular formation neurons in the immediate vicinity of the hypoglossal nucleus following extrinsic muscles injections, were located lateral to intermediate and anterior levels of hypoglossal nucleus. Thus, despite extensive areas of overlap, there was evidence for a differential distribution of pre-hypoglossal reticular formation neurons along the anterior-posterior axis associated with different lingual injections. Most of the overlap occurred anteriorly, at a level where the nucleus of the solitary tract abuts the fourth ventricle. The potential importance of this area is lingual integrative function was further suggested by camera lucida reconstructions that showed overlapping dendritic fields of labeled neurons in the reticular formation and nucleus of the solitary tract. The dendritic fields of other labeled neurons located more rostral and lateral in the reticular formation sometimes extended into the rostral (gustatory) nucleus of the solitary tract and spinal trigeminal nuclei, suggesting possible multisynaptic pathways through which tactile and gustatory information might influence hypoglossal nucleus. Not all injections of Herpes simplex virus-1 produced label in the hypoglossal nucleus. Some injections into the anterior tongue labeled neurons in the reticular formation near the exiting facial nerve, a region containing populations of preganglionic parasympathetic neurons. Other injections, particularly into the extrinsic lingual muscles, labeled brainstem neurons associated with the sympathetic nervous system, e.g. nuclei raphe magnus and pallidus, the rostral ventrolateral reticular formation, and neurons in the A5 region. These patterns of labeled neurons within the brainstem are suggestive of a differential autonomic innervation of the intrinsic and extrinsic muscles of the tongue.