Biochemical interruption of membrane phospholipid renewal in retinal photoreceptor cells

Abstract

The rabbit retina's synthesis of new phosphatidylcholine from extracellular choline was interrupted by an intravitreal injection of the choline analogue hemicholinium-3. This disrupted the process by which new membrane is added to the rod photoreceptor outer segments and eventually caused outer segment degeneration. During the first 2 days after hemicholinium-3 was injected, rows of vesicles replaced the newly formed membrane discs at the outer segment's base. The region of vesicles then expanded, and the distal outer segment detached and was quickly phagocytosed by the pigment epithelium. Two weeks after hemicholinium-3 injection, all of the retina's outer segments had been lost and the inner segments were reduced in length. The threshold concentration of hemicholinium-3 was approximately 20 microM intraocularly. At this dose, the cell bodies, intracellular organelles, and synapses of the rod cells survived. A small group of amacrine cells, possibly those that synthesize acetylcholine, became pyknotic; but the other retinal neurons remained normal to both light and electron microscopy even upon exposure to intraocular concentrations as high as 1 mM. Biochemical experiments indicated that at 20 microM hemicholinium-3, the perturbation of choline metabolism is partial and transient. That it has major selective consequences for the outer segments probably reflects the large amount of new phospholipid required for renewal of their membranes. The selectivity of the lesion was also evidenced by electrophysiological activity recorded from hemicholinium-3-treated retinas. Hemicholinium-3 was injected in vivo, and at various times retinas were isolated and incubated in vitro. The normal components of the electroretinogram were observed, but its amplitude rapidly declined; 12 days after injection, no response to light could be detected. Spontaneous firing of single ganglion cells was observed at all times following hemicholinium-3 injection. As the outer segments degenerated and the threshold of the electroretinogram rose, the thresholds of the ganglion cells also rose, but normal ganglion cell receptive fields could sometimes be plotted. Fourteen days after injection, when all of the outer segments were gone, ganglion cell responses to bright light could still be recorded; their thresholds were about 3.5 log units above normal. This finding is consistent with a previous report of light-evoked responses in mice after outer segment degeneration.(ABSTRACT TRUNCATED AT 400 WORDS)