GABA-IMMUNOREACTIVE AND GLYCINE-IMMUNOREACTIVE PROJECTIONS FROM THE SUPERIOR OLIVARY COMPLEX TO THE COCHLEAR NUCLEUS IN GUINEA-PIG

Retrograde transport of horseradish peroxidase was combined with immun ocytochemistry to identify the origins of potential gamma-aminobutyric acid (GABA) -ergic and glycinergic inputs to different subdivisions o f the cochlear nucleus. Projection neurons in the inferior colliculus, superior olivary complex, and contralateral cochlear nucleus were exa mined, but only those from the superior olivary complex contained sign ificant numbers of GABA- or glycine-immunoreactive neurons. The majori ty of these were in periolivary nuclei ipsilaterally, with a sizeable contribution from the contralateral ventral nucleus of the trapezoid b ody. Overall, 80% of olivary neurons projecting to the cochlear nucleu s were immunoreactive for GABA, glycine, or both. Most glycine-immunor eactive projection neurons were located ipsilaterally: in the lateral and ventral nuclei of the trapezoid body and the dorsal periolivary nu cleus. This suggests that glycine is the predominant neurotransmitter used by ipsilateral olivary projections. Most GABA-immunoreactive cell s were located bilaterally in the ventral nuclei of the trapezoid body . The contralateral olivary projection was primarily GABA-immunoreacti ve and provided almost half the GABA-immunoreactive projections to the cochlear nucleus. This suggests that GABA is the predominant neurotra nsmitter used by contralateral olivary projections. The present result s suggest that the superior olivary complex is the most important extr insic source of inhibitory inputs to the cochlear nucleus. Individual periolivary nuclei differ in the strength and the transmitter content of their projections to the cochlear nucleus and may perform different roles in acoustic processing in the cochlear nucleus. (C) 1997 Wiley- Liss, Inc.