EVIDENCE FOR GLUTAMATERGIC PROJECTIONS FROM THE COCHLEAR NUCLEUS TO THE SUPERIOR OLIVE AND THE VENTRAL NUCLEUS OF THE LATERAL LEMNISCUS

This study attempts to determine if projections ascending from the gui nea pig cochlear nucleus (CN) could be glutamatergic and/or aspartater gic. Multiple radio frequency lesions were made to ablate the right CN . The ablation was verified histologically. To identify the principal targets of CN efferents, silver impregnation methods were used to loca lize the preterminal degeneration of fibers in transverse sections of the brainstem 5 and 7 days after CN ablation. CN efferents projected h eavily to the lateral superior olive (LSO) ipsilaterally, the medial s uperior olive (MSG) bilaterally, and contralaterally to the medial (MN TB) and ventral (VNTB) nuclei of the trapezoid body, the ventral (VNLL ) and intermediate nuclei of the lateral lemniscus and the central nuc leus of the inferior colliculus (ICc). There were smaller projections to the lateral nucleus of the trapezoid body ipsilaterally, the dorsal and dorsomedial periolivary nuclei bilaterally, and the dorsal nucleu s of the lateral lemniscus contralaterally. There were sparse projecti ons to the VNLL and ICc ipsilaterally and the CN contralaterally, and a very sparse projection to the contralateral LSO. To determine if CN efferents were glutamatergic and/or aspartatergic, the fresh brainstem was sectioned transversely and samples of the LSO, MSG, MNTB, VNLL, a nd ICc were taken to measure the electrically evoked release and the u ptake of D-[H-3]Asp and [C-14] Gly Or [C-14]GABA 3-5 days after the CN ablation. The release studies suggest that only certain of the histol ogically identified projections ascending from the CN may be glutamate rgic and/or aspartatergic. CN ablation depressed D-[H-3]Asp release in the MSO bilaterally and in the contratateral MNTB and VNLL, suggestin g that the CN efferents to these nuclei may use glutamate or aspartate as a transmitter. It was unclear whether a marginal depression of D-[ H-3]Asp release in the ipsilateral LSO reflected the presence of gluta matergic CN projections to this nucleus. D-[H-3]Asp release in the ICc was unaffected, suggesting that CN efferents to this nucleus may not be glutamatergic. There were no deficits in D-[H-3]Asp uptake. [C-14]G ly release from the LSO and MSO was unchanged. [C-14]Gly uptake was un changed in the MSO and depressed only in the contralateral LSO, possib ly reflecting subnormal uptake activity in endings contributed by cont ralateral MNTB cells that had lost their CN efferents. [C-14]GABA upta ke in the MNTB, VNLL, and ICc was unchanged. [C-14]GABA release was un changed in the VNLL and ICc. [C-14]GABA release was depressed only in the contralateral MNTB, possibly reflecting the loss of a small comple ment of GABAergic CN efferents and the reaction of GABAergic projectio ns from the contralateral VNTB to their loss of CN efferents.