Interglial cell gap junctions increase in urethane-induced photoreceptor degeneration in rats.

Gap junctions are found between astrocytes in the inner retina of normal rats, but they are rare between MUHer cells or between astrocytes and Miiller cells in the inner retina. After photoreceptor degeneration induced by urethane treatment of newborn animals, morphologic alterations of glial cells occur in the inner retina. The Miiller cells withdraw from the inner limiting membrane, and the astrocytes hypertrophy and occupy the vitread surface of the inner limiting membrane. The frequency and size of the gap junctions between astrocytes increases with time in rats with urethane-induced photoreceptor degeneration, to a greater extent than expected from elaboration of additional astrocyte plasma membrane. The gap junction-profile length per glial cell membrane-contact length is 2.8 ±1.1 Mm/1000 ixm of membrane in 8-week-old normal animals; it increases to 18.9 ± 9.4 Mm/1000 fxm of membrane at 56 weeks of age in urethane-treated animals. The average size of the gap junction-profile length doubles during this same period. To the authors' knowledge this is the first study demonstrating pathologic changes in gap junctions in central nervous system tissue. The authors speculate that this up-regulation of gap junctions occurs in response to an altered extracellular ionic composition in an attempt to increase the lateral spatial buffering of K + by these cells. The relative location of glial cells in retina can determine, in part, the vulnerability of the retina to edema. Invest Ophthalmol Vis Sci 31:1690-1701,1990 The retinal vasculature has an intimate relationship with retinal neurons and glia in normal and pathologic responses. 12 Glial investment of the retinal vasculature is an important facet of its ultrastructure, 3 but the physiologic function 4 of the perivascular glia is not known. Three types of glial cells— Miiller cells, astrocytes, and microglia—have been identified in the retina, and each has a relationship with the retinal vasculature. 4 " 6 The radial Miiller fibers are considered to be analogous to glioependymal cells in the brain and are distributed throughout the retina, with the exception of the optic nerve head and fovea. 7 Fine processes of Miiller cells invest both large vessels and capillaries in the inner and outer retinal vessel beds. 3 Morphologic and functional polarity are exhibited by Miiller cells in that their "end-feet" processes (which line the vitreous) have the highest concentration of mitochondria and potassium channels, facilitating removal of K + from the extracellular space into the vitreous. 3 ' 89