Matthew M. LaVail

Retrograde Axonal Transport in the Central Nervous System

When horseradish peroxidase is injected into the optic tectum of a chick, axons of ganglion cells transport it centripetally to their cell bodies in the retina at a rate of about 72 millimeters per day. After intraocular injections in the young chick, the peroxidase is transported centripetally along efferent axons, and is concentrated in cell bodies within the isthmo-optic nucleus. This retrograde movement of protein from axon terminal to cell body suggests a possible mechanism by which neurons respond to their target areas.

Influence of eye pigmentation and light deprivation on inherited retinal dystrophy in the rat

The influence of eye pigmentation and light deprivation on inherited retinal dystrophy has been studied in Royal College of Surgeons (RCS) rats which are pink-eyed and in two congenic strains, RCS-p/+, which are black-eyed and RCS-c, which are albinos. The congenic animals are genetically similar to inbred RCS rats, differing only in pigmentation genes and other genes closely linked to the pigmentation loci. Progression of the disease has been analyzed in a series of animals cytologically with 1–2 μm plastic sections and biochemically by measurement of whole eye extractions of rhodopsin. When the rats are reared in cyclic light (12 hr light-12 hr dark; cage illumination less than 15 ft-c), the rate of photoreceptor degeneration in black-eyed rats is slowed from the rate in pink-eyed rats by about 10 days in the posterior retina. In the far peripheral retina, the disease is slowed by about 30–35 days in the superior half of the eye, along and above the horizontal meridian. No slowing occurs in the inferior half of the eye along the vertical meridian. When pink-eyed RCS and black-eyed RCS-p/+ rats are dark-reared, the pattern of degeneration in both is the same as in black-eyed rats reared in cyclic light. The rhodopsin content of eyes from black-eyed RCS-p/+ rats reared in cyclic light also is the same as that in pink-eyed rats reared in darkness. No difference was found between pink-eyed RCS and albino RCS-c retinas in the rate of the disease or in rhodopsin content. These findings indicate that (1) intrinsic differences exist in different regions of the retina in the rate of retinal dystrophy, (2) black eye pigment slows the progression of the disease as much as does dark-rearing in pink-eyed rats, (3) the very small amount of eye pigment in pink-eyed RCS rats is ineffectual in slowing the rate of the disease from that in albino RCS-c rats, and (4) dark-rearing does not slow the rate of the disease further in black-eyed rats. Additional features of retinal dystrophy in the RCS rat were observed. Some photoreceptor cells survive in clusters immediately adjacent to the optic nerve head and the ora serrata as late as day 96, long after most photoreceptors have disappeared. The rod outer segment debris (extra lamellar material), which is a characteristic of retinal dystrophy in the rat, shows a loss of basophilia and osmiophilia (“blanching”) beginning at days 32–35 in the apical region of the debris in the posterior retina. The debris becomes progressively more “blanched” until about day 96, when most of the debris has lost its basophilia in all regions of the eye; the “blanching” of the membranes correlates closely with the loss of rhodopsin from the eye. The issue of the source of the extra lamellar material is re-examined, and data are provided that indicate the material probably is formed entirely from the breakdown of rod outer segments.

Rod outer segment disc shedding in relation to cyclic lighting

Abstract Rod outer segment disc shedding and phagocytosis by the pigment epithelium has been studied by light microscopy in sections from plastic-embedded eyes taken from albino rats at different times of the day and night. Rats were reared in cyclic light (12 hr light-12 hr dark). Soon after the onset of light (0·5 hr) the number of large phagosomes in the pigment epithelium rose from a low level to a high level where it remained for about 2 hr. Thereafter, the number of large phagosomes dropped rapidly so that the number was low by 3–4 hr after the onset of light, and it remained low throughout the rest of the day and throughout the night. During the burst of disc shedding in the early morning, the number of large phagosomes was 2–5 times that found at other times of the day or night.

Rhodopsin content and rod outer segment length in albino rat eyes: Modification by dark adaptation

Abstract Whole eye rhodopsin content and rod outer segment length have been determined in albino rat eyes after different periods of dark adaptation, dark-rearing or cyclic light maintenance. The rhodopsin content in the eyes of dark-reared albino rats is approximately 50% higher than that in the eyes of littermates reared in cyclic light (in-cage illumination less than 15 ft-cd). The same increase in rhodopsin can be obtained in albino rats after only a 10-day dark adaptation period. Furthermore, the increased level of rhodopsin is the same as that in the eyes of pigmented rats reared in cyclic light. The increase in rhodopsin content in the albino rat eyes is due at least in part to an increase in rod outer segment length. The increase in length with dark adaptation and dark-rearing was somewhat variable, with an average increase of approximately 25%. No apparent increase was found in rod outer segment diameter, rod outer segment disc packing density or eye size. Rod outer segment lengths were consistently longer in the superior hemisphere of the eye than in the inferior hemisphere in both dark-adapted albino rats and pigmented rats maintained in cyclic light, but not in albino rats maintained in cyclic light.

Discrimination of light intensity by rats with inherited retinal degeneration: A behavioral and cytological study

Abstract RCS rats with inherited retinal degeneration suppressed lever-pressing behavior in response to onset of light at ages from about 6 months to more than 2 years. A threshold for light intensity was demonstrated. Control experiments established that the rats were responding to light rather than to some other stimulus, and that the response was mediated by the eyes. Electron microscopic examination of the retinas demonstrated numerous surviving photoreceptor cells which lacked outer segments but which did make synaptic contact with presumed bipolar and/or horizontal cell processes. Counts of conventional and ribbon synapses in the inner plexiform layer showed that the circuitry through the retinas was relatively well preserved. Pigment epithelial cells made tight and gap junctions with Muller cell processes, which in turn were apposed to the photoreceptor cells. Possible cellular mechanisms mediating visual behavior in RCS rats are discussed.

Role of the pigment epithelium in inherited retinal degeneration analyzed with experimental mouse chimeras

Abstract In the study of inherited photoreceptor cell degenerations, a difficult problem is to determine the site of the primary genetic defect. It could be either intrinsic to the photoreceptor cells, the pigment epithelial cells or both, or extrinsic to these cells or to the eye as a whole. This problem was studied in mice with inherited retinal degeneration ( rd rd ) by analyzing the interaction of mutant and normal pigment epithelium with the underlying normal or degenerated retina in the eyes of experimental chimeric mice. Chimeras were produced by fusing eight-cell embryos of albino SJL ( rd rd ) mice with those of pigmented C57BL 10 (+/+) mice. The eyes of the resulting chimeric mice at 1·5–26·5 months of age had patches of normal retina interspersed with patches lacking photoreceptor cells. Pigment epithelial cells of both normal and mutant strains, identified by the presence or absence of melanosomes, were found overlying areas of both normal and degenerated retina. The same findings were obtained using two other strain combinations, C3H (pigmented, rd rd ) fused with BALB c (albino, +/+) and SJL (albino, rd rd ) fused with C57BL 10 × CBA F 1 hybrids (pigmented, +/rd). The synthesis of rod outer segment dises proceeded normally in photoreceptor cells underlying mutant pigment epithelial cells, as determined by autoradiographic analysis, and phagosomes were found in both mutant and normal pigment epithelial cells. The findings indicate that the pigment epithelial cell is not the primary target of the mutant rd gene in the mouse and localize the site of mutant gene action to the neural retina, presumably but not necessarily, to the photoreceptor cells.