RELATIONSHIP OF DESCENDING INFERIOR COLLICULUS PROJECTIONS TO OLIVOCOCHELAR NEURONS

With the objective of defining the relationship of descending inferior colliculus projections to the olivocochlear system in the guinea pig, inferior colliculus neurons were anterogradely labeled with Phaseolus vulgaris-leucoagglutinin and olivocochlear neurons were retrogradely labeled with horseradish peroxidase in the same brain sections. Inferi or colliculus neurons were found to project to many nuclei and regions of the hindbrain where olivocochlear neurons reside. The most substan tial of these descending projections was to the ipsilateral medioventr al periolivary region. Fewer descending projections terminated in the ipsilateral ventral nucleus of the lateral lemniscus, superior paraoli vary nucleus, and rostral periolivary region; and even fewer ipsilater al projections terminated in the area surrounding the lateral superior olive, caudal periolivary region, and the lateroventral periolivary r egion. Descending neurons of the inferior colliculus also project to t he contralateral hindbrain first via the lateral lemniscus and then th e trapezoid body, to terminate in the contralateral medioventral perio livary region, superior paraolivary nucleus, rostral periolivary regio n, and the ventral nucleus of the lateral lemniscus. In addition to th e projections into these regions that contain olivocochlear neurons, t here are varicosities of inferior colliculus neurons that appear to co ntact the olivocochlear neurons themselves, both ipsilaterally and con tralaterally, especially, but not only, in the ipsilateral medioventra l periolivary region. We therefore conclude that descending inferior c olliculus neurons do provide input to olivocochlear neurons and that t he input is not limited to olivocochlear neurons of the ipsilateral me dioventral periolivary region. However, given the robust nature of the projection to the ipsilateral medioventral periolivary region and the paucity of contacts observed in that region, we also conclude that th e olivocochlear neuron is not the major target of descending inferior colliculus projections. (C) 1993 Wiley-Liss, Inc.