Freeze-fracture studies of photoreceptor membranes: new observations bearing upon the distribution of cholesterol.

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Abstract
EM of replicas from freeze-fractured retinas [from Rana pipiens pipiens and Bufo maximus] exposed during or after fixation to the cholesterol-binding antibiotic filipin were performed. Characteristic filipin-induced perturbations throughout the disk and plasma membranes of retinal rod outer segments of various species were observed. A prolonged exposure to filipin in fixative enhances rather than reduces presumptive cholesterol detection in the vertebrate photoreceptor cell. In agreement with the pattern seen in a previous study (Andrews et Cohen, 1979), filipin-binding in membranes exhibiting particle-free patches seemed largely confined to these patches. Favorably fractured photoreceptors exhibited marked filipin-binding in apical inner segment plasma membrane topologically confluent with and proximate to the outer segment plasma membrane, which was comparatively free of filipin binding. A possible boundary between these differing membrane domains was suggested in a number of replicas exhibiting lower filipin binding to the apical plasma membrane of the inner segment in the area surrounding the cilium. This area contains a structure (Andrews, 1982) that resembles the active zones of the nerve terminals for the frog neuromuscular junction. These structures may function to direct vesicle fusion to occur near them, in a domain of membrane more closely resembling outer than inner segment plasma membrane. All disk membranes probably contain cholesterol, but the particle-free patches present in some disks trap cholesterol from contiguous particulate membrane regions. Contiguous inner and outer segment membranes may greatly differ in cholesterol content. The suggested higher cholesterol in the inner segment than in the outer segment plasma membrane may help direct newly inserted photopigment molecules to the outer segment.

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