The optokinetic nystagmus and ocular pigmentation of hypopigmented mouse mutants

Abstract

We tested the optokinetic nystagmus (OKN) reflex of various hypopigmented mutant mice and ultrastructurally examined the pigmentation of various ocular structures in these mutants. Using electron microscopy we examined the pigmentation of the choroid and retinal pigment epithelium (RPE) and measured the numerical density, volume density, and distribution of RPE melanosomes of mice with the following phenotypes: (1) wild type, (2) mutants that have abnormal or no OKN in response to horizontally moving, full‐field stimulation, and (3) other mutants that have normal OKN but reduced choroidal pigmentation. We also measured the OKN of all these mice in response to horizontally moving stimuli that were restricted to the nasal or to the temporal retina. We found that in the mutants with normal OKN the numerical density of melanosomes in the RPE was within the range found for wild type, while the numerical density was reduced for the mutants with abnormal OKN. For one mutant with normal RPE pigmentation and normal OKN, the choroidal pigmentation was nearly absent. For the genotypes with abnormal OKN the volume density of the RPE melanosomes and percent apical melanosomes were sometimes greater and sometimes less than normal. The OKN patterns of these mice fell into the following categories: (1) wild type; (2) field‐restriction dependent OKN with small following movements but no OKN in response to full‐field stimulation, normal OKN in response to stimulation of the nasal retina, and OKN of reversed direction in response to stimulation of the temporal retina; (3) oblique with slow oblique following movements and reduced numbers of OKNs with oblique quick phases in response to horizontally moving, full‐field stimulation, nearly normal OKN in response to stimulation of the nasal retina, and OKN of reversed direction in response to stimulation of the temporal retina. The horizontal component of the oblique response to full‐field stimulation was in the same direction for the two eyes, but the vertical component was in the opposite direction. (4) Slow, small amplitude, with no or very small following movements in response to full‐field stimulation, following movements in response to stimulation of the nasal retina and reversed “following” movements in response to stimulation of the temporal retina but few or no quick phases of the OKN for any stimulus condition. These results show that a variety of abnormalities of the OKN occur for hypopigmentation mutants of the mouse. Moreover, abnormality of the OKN is associated with a reduction of the numerical density of melanosomes in the RPE, but not with either a reduction of the volume of melanin in the RPE or hypopigmentation of the choroid.