Hypoglossal and reticular interneurons involved in oro-facial coordinationin the rat

Chewing, swallowing, breathing, and vocalization in mammals require precise coordination of tongue movements with concomitant activities of the mimeti c muscles. The neuroanatomic basis for this ore-facial coordination is not yet fully understood. After the stereotaxic microinjection of retrograde an d anterograde neuronal tracers (biotin-dextran, Fluoro-Ruby, Fluoro-Emerald , and Fluoro-Gold) into the facial and hypoglossal nuclei of the rat, we re port here a direct bilateral projection of hypoglossal internuclear interne urons onto facial motoneurons. We also confirm the existence of a small poo l of neurons in the dorsal part of the brainstem reticular formation that p roject ipsilaterally to both facial and hypoglossal nuclei. For precise tra cer injections, both motor nuclei were located and identified by the electr ical antidromic activation of their constituent motoneurons. Injections of retrograde tracers into the facial nucleus consistently labeled neurons in the hypoglossal nucleus. These neurons prevalently lay in the ipsilateral s ide, were small in size, and, like classic intrinsic hypoglossal local-circ uit interneurons, had several thin dendrites. Reverse experiments - injecti ons of anterograde tracers into the hypoglossal nucleus - labeled fine vari cose nerve fiber terminals in the facial nucleus. These fiber terminals wer e concentrated in the intermediate subdivision of the facial nucleus, with a strong ipsilateral prevalence. Double injections of different tracers int o the facial and the hypoglossal nuclei revealed a small, but constant, num ber of double-labeled neurons located predominantly ipsilateral in the caud al brainstem reticular formation. Hypoglossal internuclear interneurons pro jecting to the facial nucleus, as well as those neurons of the parvocellula r reticular formation that project to both facial and hypoglossal nuclei, c ould be involved in ore-facial coordination. (C) 2001 Wiley-Liss, Inc.