Identification of vestibuloocular projection neurons in the developing chicken medial vestibular nucleus

Biocytin was injected into the oculomotor, trochlear, or abducens nucleus on one side using isolated chicken brainstem preparations or brain slices to identify the medial vestibular nucleus (MVN) neurons projecting to these targets. Oculomotor nucleus injections produced retrogradely labeled neurons in the contralateral ventrolateral MVN (MVN(VL)), with few labeled neurons in the ipsilateral MVN(VL) and rarely in the dorsomedial MVN on either side. Labeled MVN(VL) neurons were identified as stellate (95%) and elongate (5%) cells. Trochlear nucleus injections produced a similar pattern of MVN neuron labeling. Abducens nucleus injections resulted in retrogradely labeled stellate (87%) and elongate (13%) neurons in the MVN(VL), which had smaller cell bodies than those projecting to the oculomotor nucleus. Anteroposteriorly, labeled MVN(VL) neurons were coextensive with the tangential nucleus, with neurons projecting to the oculomotor nucleus distributed lateral to and intermixed with the more medially situated neurons projecting to the abducens nucleus. The fundamental pattern of vestibuloocular projecting neurons was similar at both embryonic ages studied, E16 and E13. In contrast to the case in mammals, where most vestibuloocular projection neurons reside within the MVN, most retrogradely labeled neurons in these chicken preparations were found within the ventrolateral vestibular, descending vestibular, and tangential nuclei. The morphological identification and mapping of vestibuloocular projection neurons in the chicken MVN described here represents the first step in a systematic evaluation of the relationship between avian vestibuloocular neuron structure and function.