Axonal projections and synapses from the supratrigeminal region to hypoglossal motoneurons in the rat

Neural circuits from the supratrigeminal region (Vsup) to the hypoglossal m otor nucleus were studied in rats using anterograde and retrograde neuroana tomical tracing methodologies. Iontophoretic injection of 10% biotinylated dextran amine (BDA) unilaterally into the Vsup anterogradely labeled axons and axon terminals bilaterally in the hypoglossal nucleus (XII) as well as other regions of the brainstem. In the ipsilateral XII, the highest density of BDA labeling was found in the dorsal compartment and the ventromedial s ubcompartment of the ventral compartment, where BDA labeling formed a dense , patchy distribution. Microinjection of 20% horseradish peroxidase (HRP) i psilaterally or bilaterally into the tongue resulted in retrograde labeling of XII motoneurons confined to the dorsal and ventral compartments of the hypoglossal motor nucleus. Under light microscopical examination, BDA-label ed terminals were observed closely apposing the somata and primary dendrite s of HRP-labeled hypoglossal motoneurons. Two hundred and sixty-five of the se BDA-labeled terminals were examined at the ultrastructural level. One hu ndred and twelve BDA-labeled axon terminals were observed synapsing with ei ther the somata (39%, 44/112) or the large or medium-size dendrites (61%, 6 8/112) of retrogradely labeled hypoglossal motoneurons. Axon terminals cont aining spherical vesicles (S-type) formed asymmetric synapses with HRP-labe led hypoglossal motoneuron dendrites. In contrast to this, F-F-type axon te rminals, containing flattened vesicles, formed symmetric synapses with bath the somata and dendrites of HRP-labeled hypoglossal motoneurons with a pre ponderance of the contacts on their somata. Axon terminals containing pleom orphic vesicles (F-p-type) were noted forming both symmetric and asymmetric synapses with HRP-labeled hypoglossal motoneuron somata and dendrites. The present study provides anatomical evidence of neuronal projections and syn aptic connections from the supratrigeminal region to hypoglossal motoneuron s. These data suggest that the supratrigeminal region, as one of the premot or neuronal pools of the hypoglossal nucleus, may coordinate and modulate t he activity of tongue muscles during oral motor behaviors. (C) 2001 Elsevie r Science B.V. All rights reserved.