REGULATION OF D-ASPARTATE RELEASE AND UPTAKE IN ADULT BRAIN-STEM AUDITORY NUCLEI AFTER UNILATERAL MIDDLE-EAR OSSICLE REMOVAL AND COCHLEAR ABLATION

In young adult guinea pigs, the effects of unilateral ossicle removal and cochlear ablation were determined on transmitter release from glut amatergic presynaptic endings and glutamate inactivation via uptake. ( i) D-[H-3]Aspartate release and uptake were measured in subdivisions o f the cochlear nucleus (CN) and in nuclei of the superior olive (SOC) and auditory midbrain (MB) up to 145 days after placing the lesions. A ctivities were compared to those from age-matched unlesioned controls. Fiber degeneration was visualized histologically, (ii) In the ipsilat eral CN, changes in release and uptake were governed by the type of le sion. Ossicle removal produced sparse pruning of fibers only after 112 days and decreased release and uptake at 145 days, consistent with re gulatory weakening of excitatory glutamatergic transmission. Cochlear ablation deafferented the CN, producing deficient release and uptake a t 2 days and abundant fiber degeneration at 7 days. Subsequently, the residual release and uptake increased in magnitude, consistent with st rengthening of excitatory glutamatergic transmission. (iii) In the con tralateral CN, after either lesion, changes in release and uptake usua lly matched those in the ipsilateral CN. Thus, the auditory pathway as sociated with the lesioned ear probably provided cues for the regulati on of synaptic strength in the contralateral CN, (iv) Both lesions inc reased release ill the SOC and MB, and uptake in the SOC, consistent w ith strengthening of excitatory glutamatergic transmission, Sparse fib er degeneration, suggesting axonal pruning, appeared in the SOC and MB after cochlear ablation. (v) The strengthening of excitatory glutamat ergic transmission may facilitate and maintain symptoms such as loudne ss recruitment and tinnitus which often accompany hearing loss. (C) 19 97 Academic Press.