DEVELOPMENT OF VENTRAL COCHLEAR NUCLEUS PROJECTIONS TO THE SUPERIOR OLIVARY COMPLEX IN GERBIL

The postnatal development of the projection from the ventral cochlear nucleus to the principal nuclei in the superior olivary complex in ger bil (Meriones unguiculatus) was studied in an age-graded series of pup s ranging from 0 to 18 days old. Small crystals of -dioctadecyl-3,3,3' ;3'-tetramethylindocarbocyanine perchlorate (DiI) were inserted into t he ventral cochlear nucleus of aldehyde-fixed brains, and the labeled projections were examined with epifluorescence microscopy, Selected se ctions were photooxidized in a solution of diaminobenzidine and subseq uently processed for electron microscopy to examine the development of labeled synapses in the target nuclei. Horseradish peroxidase was inj ected into the ventral cochlear nucleus of adult gerbils to assess the form and persistence of projections observed in the neonatal animals. In addition, electrophysiological responses to acoustic stimuli of si ngle units in the adult auditory brainstem were analyzed to confirm th e functionality of the novel projection from the ventral cochlear nucl eus to the contralateral lateral superior olive. By the day of birth ( PO), developing axons from the ventral cochlear nucleus have already e stablished highly ordered pathways to the three primary nuclei of the superior olivary complex: the ipsilateral lateral superior olive, the contralateral medial nucleus of the trapezoid body, and at the lateral and medial dendrites of the ipsilateral and contralateral medial supe rior olive, respectively. Developing axons from the ventral cochlear n ucleus that innervated the contralateral medial nucleus of the trapezo id body lacked the terminal morphology characteristic of the calyx of Held, but began to adopt a more characteristic form on P5. The mature calyx appeared around P14-16, Exuberant developmental projections to t opographically inappropriate areas of the superior olivary complex wer e not observed at the postnatal ages studied. In addition to the proje ctions of the ventral cochlear nucleus to the superior olivary complex described in other species, we observed the development and maintenan ce of a major direct projection from the ventral cochlear nucleus to t he contralateral lateral superior olive. On PO, ventral cochlear nucle us axons decussate;in the dorsal trapezoid body, form a plexus at the dorsal edge of the contralateral medial superior olive, and enter the ventrolateral limb of the contralateral lateral superior olive. Over t he next 2 weeks, fascicles of fibers form on the hilar and ventral asp ects of the ventrolateral limb. Fibers arising from these fascicles fo rm converging, but nonoverlapping, arborizations within the ventrolate ral limb at right angles-to the curvature of the nucleus. The medial r egion was devoid of labeled axons. The direct innervation of the contr alateral lateral superior olive was confirmed in the adult gerbil with anterograde horseradish peroxidase histochemistry and by the recordin g of excitatory responses in the innervated region to acoustic stimula tion of the contralateral ear. (C) 1995 Wiley-Liss, Inc.