Auditory brainstem anomalies in albino cats: II. Neuronal atrophy in the superior olive.

In a previous paper (Brain Res., 260:1-9, 1983) we reported that albino cats show abnormal auditory brainstem evoked responses that appear to arise from structural defects in or near the superior olivary complex. In the present study, neuronal cross-sectional area in brainstem nuclei was compared in albino and normally pigmented adult cats. The albinos were true tyrosinase-negative (cc) and should not be confused with the deaf white cat (W); the albinos are not deaf. Neurons in the medial superior olivary nucleus (MSO) of albinos were, on average, 41% smaller than in pigmented animals; there was no overlap in the neuronal size distributions for the two groups of animals. Cell size in the lateral superior olive, medial nucleus of the trapezoid body, ventral nucleus of the lateral lemniscus, anteroventral cochlear nucleus, dorsal cochlear nucleus, and facial nucleus was also smaller (by 9-21%) in albinos than in pigmented animals but none of these differences was statistically reliable. In the abducens nucleus, neurons were 12% larger in albinos than in pigmented animals, demonstrating that neuronal size in the albinos is not uniformly smaller. Several lines of evidence suggest that the auditory system defects in albinos are related to abnormal pigmentation rather than to other gene effects. It is possible that a subtle pigment-related disruption of inner ear development in albinos results in a central cascade of atrophic changes along the auditory pathway.