Kainic acid lesions of the superior olivary complex: a horseradish peroxidase study of surviving brain-stem projections.

Lesions of the superior olivary complex of the adult ferret were made by pressure injection of kainic acid (5 nM/microliters in Locke's solution) through a glass micropipette inserted into the lower brain-stem. Small injections of kainic acid (1.5 microliters) produced a localized loss of nerve cells in the superior olivary complex in the vicinity of the pipette tip without apparent damage to fibers of passage in the trapezoid body. The extent of neural damage was determined by the absence of cell bodies in Nissl-stained sections. Integrity of fibers in the trapezoid body and other decussating pathways of the auditory lower brain-stem was confirmed by retrograde transport of horseradish peroxidase (HRP) following large injections of the enzyme into the central nucleus of the inferior colliculus. Even animals with complete destruction of the superior olivary complex had a normal complement of HRP-labelled cells in all divisions of the cochlear nucleus (the dorsal, anteroventral and posteroventral nuclei) contralateral to the HRP injection. In cases with partial lesions, normal labelling was also seen in those parts of the superior olivary complex not directly destroyed by the kainic acid injection. There was no evidence of disruption of fibers of passage nor was there any indication of abnormal projections from the lower brain-stem to inferior colliculus even after prolonged survival times.