INPUT FROM THE INFERIOR COLLICULUS TO MEDIAL OLIVOCOCHLEAR NEURONS INTHE RAT - A DOUBLE-LABEL STUDY WITH PHA-L AND CHOLERA-TOXIN

The inferior colliculus provides a strong descending influence capable of modulating the excitability levels of olivocochlear neurons (Rajan , 1990). In an attempt to anatomically demonstrate this pathway in rat s, an experimental paradigm was designed by which anterogradely transp orted Phaseolus vulgaris-leucoagglutinin (PHA-L), which delineates axo nal arbors, and retrogradely transported cholera toxin B subunit alone (CT-B) or conjugated to horseradish peroxidase (CT-HRP), which deline ate dendritic arbors, are visualized in the same brainstem sections. P HA-L was injected unilaterally into the central nucleus of the inferio r colliculus of adult rats 5-9 days prior to injection of CT-B or CT-H RP into either the contralateral or the ipsilateral cochlea. Descendin g collicular axons labeled with PHA-L densely innervate the Ventral nu cleus of the trapezoid body (VNTB), which contains neurons of the medi al olivocochlear system (MOCS), but do not enter the lateral superior olive, where the neurons of the lateral olivocochlear system (LOCS) ar e found. The collicular projection to VNTB is largely ipsilateral and supplies mostly the ventral half of the nucleus. Within VNTB, the coll icular fibers intermingle with dendrites and, to a lesser extent, cell bodies of MOCS. Collicular boutons, predominantly of the en passant t ype, are often observed in close apposition to dendrites and, less fre quently, cell bodies of both crossed and uncrossed MOCS. These light m icroscopic results suggest the existence of direct, synaptic contacts between descending collicular axons and ipsilateral crossed and uncros sed MOCS. Numerous collicular boutons were also seen at a distance fro m MOCS, suggesting that they establish synapses with other neuron type s of the VNTB that do not send their axons to the cochlea.