The auditory brainstem nuclei and some of their projections to the inferior colliculus in the North American opossum.

Afferent projections to the inferior colliculus in the North American opossum have been examined using the retrograde transport of horseradish peroxidase. Projections to primarily the contralateral inferior colliculus arise in the dorsal and ventral cochlear nuclei, the auditory nerve nucleus and the spinal trigeminal nucleus pars caudalis, while ipsilateral projections arise in the superior paraolivary nucleus, the ventral nucleus of the trapezoid body, the ventral nucleus of the lateral lemniscus, the paralemniscal nucleus, the deep layer of the superior colliculus and the parabrachial nucleus. Bilateral projections to the inferior colliculus originate within the dorsal column nuclei, the nucleus reticularis gigantocellularis pars ventralis, the lateral and medial superior olivary nuclei, the dorsal nucleus of the lateral lemniscus and the auditory cortex. Nissl, fiber and Golgi-stained preparations were used to study the neuronal organization of those auditory nuclei with projections to the inferior colliculus. Anterograde axonal degeneration and transport techniques revealed that the inferior colliculus is innervated differentially by the dorsal and ventral cochlear nucleus, the superior olivary complex and the auditory neocortex. Axons from the contralateral dorsal cochlear nucleus and the ipsilateral superior olivary complex innervate both the central nucleus and external cortex, whereas those from ventral cochlear nucleus and contralateral, superior olivary complex project to only the central nucleus. Projections from auditory cortex form the complement of those from the cochlear nuclei and superior olivary complex, that is, they terminate in a thin band overlying the dorsal cortex and the superficial layer of external cortex. Our results have been compared with those obtained from eutherian mammals and it is clear that there are striking similarities in neuronal organization and connectivity. Since the opossum is born 12 days after conception and has an extended development in an external pouch, it may be suited for developmental studies of the mammalian auditory connections and the behaviors dependent of them.