Morphological changes in the neuronal substrate for the optokinetic reflex in albino ferrets

Albino mammals show very characteristic deficits in their optokinetic system, and albino ferrets are even optokinetically blind. To investigate the neuronal causes for this defect we compared the morphology of retinal slip cells in the pretectal nucleus of the optic tract and the dorsal terminal nucleus of the accessory optic system (NOT-DTN) in pigmented and albino ferrets (Mustela putorius furo) using retrograde tracing techniques. After tracer injections into the inferior olive, equal numbers of NOT-DTN neurons were retrogradely labelled in pigmented and albino animals. However, NOT-DTN cells in albino ferrets had fewer stem dendrites, and the cumulative dendritic length was reduced by 30% when compared with NOT-DTN neurons in pigmented animals. In addition, the prominent network formed by distal dendrites observed in the NOT-DTN of pigmented ferrets was largely diminished in albinos. Taken together with behavioural and physiological data, these findings indicate that the NOT-DTN as the main visuomotor interface in the optokinetic system is clearly defective in albino ferrets.