Amino acid uptake and release in the guinea pig cochlear nucleus after inferior colliculus ablation

This study attempts to determine if the neurons in the guinea pig inferior colliculus that project to the cochlear nucleus could use certain amino acid transmitters. The left inferior colliculus was ablated surgically to destroy projections descending to the cochlear nuclei. Nissl and silver stained sections of the brain stem indicated that this procedure destroyed most of the left inferior colliculus, but spared a small amount of collicular tissue ventrally and rostrally. Six to seven days after the ablation, degenerated fibers were present in the right inferior colliculus, in the left lateral lemniscus, and in the cochlear nucleus, bilaterally. Three to five days after the ablation, the uptake and electrically-evoked release of exogenous, radiolabeled D-aspartate, gamma-aminobutyrate, and glycine were measured in the three major subdivisions of the cochlear nucleus, the anteroventral, posteroventral, and dorsal divisions. These activities were compared to those in unlesioned controls. The ablation did not alter the uptake and release of the amino acids in the dorsal and posteroventral divisions of the cochlear nucleus. However, it lowered slightly (by 10-18%) the uptake and release of gamma-aminobutyrate and glycine in the anteroventral division, although the difference from the control group was not statistically significant. These findings suggest that most of the neurons in the inferior colliculus that project to the cochlear nucleus probably do not use glutamate, aspartate, gamma-aminobutyrate, or glycine as a transmitter. However, the possibility remains that a small proportion of the collicular projections to the anteroventral cochlear nucleus might use gamma-aminobutyrate or glycine as a transmitter.