Identification of multiple groups of efferent vestibular neurons in the adult pigeon using horseradish peroxidase and DAPI.

This study was designed to investigate the efferent innervation of the pigeon labyrinth. Horseradish peroxidase (HRP) was injected and confined within the endolymphatic space of one labyrinth in 9 adult pigeons. The brain was perfused by transcardiac carotid catheterization and the HRP reacted by the tetramethylbenzidine (TMB) blue reaction process. Five different groups of HRP-labeled vestibular efferent neurons were identified. Three groups were located within the confines of the ipsilateral vestibular nuclear complex (in the lateral, tangential and descending nucleus) and two additional groups, each bilateral, were located in the reticular formation. In 9 additional pigeons, 4,6-diamidino-2-phenylindole (DAPI) was also injected and confined within the endolymphatic space of one labyrinth. DAPI-labeled cells were noted in 3 of the 5 locations (tangential nucleus, and both reticular groups) which in the other animals contained HRP-labeled cells. These findings raise the possibility of different physiological roles for the efferent vestibular groups in the ipsilateral vestibular nuclear complex and bilateral reticular formation.