Neuroprotection of photoreceptors by minocycline in light-induced retinal degeneration.
OBJECTIVE
Microglial cells have been found to play pivotal roles in various neuronal degenerative diseases such as Parkinson's and Alzheimer's diseases. Minocycline, a microglial inhibitor, has recently been shown to be neuroprotective in various models of cerebral ischemia and degenerative diseases of the brain. This study was conducted to evaluate the neuroprotective effect of minocycline and the role of microglia in light-induced retinal degeneration.


METHODS
BALB/cJ mice were exposed to intense green light for 3 hours and observed during 1, 3, or 7 days of dark recovery. The animals received intraperitoneal injections of minocycline or vehicle 1 day before exposure to light for 2, 4, or 8 days, depending on the periods of survival. Morphologic, morphometric, immunohistochemical, and electrophysiological studies were performed to evaluate the efficacy of minocycline in the amelioration of light-induced retinal degeneration and the possible involvement of microglial cells.


RESULTS
Minocycline treatment provided marked amelioration in the loss of photoreceptors in light-induced retinal degeneration, as evidenced by morphologic, morphometric, and electrophysiologic criteria. Morphologically, the minocycline-treated group showed markedly better preservation of the outer retina after exposure to light. Morphometrically, at 7 days after exposure to light, in the minocycline-treated animals, 89.1% of the normal-appearing photoreceptor nuclei remained, but in the retinas of the vehicle-control group only 38.0% of these nuclei remained. This difference was statistically significant (P < 0.001). At 7 days after exposure to light electroretinography (ERG) showed that minocycline significantly preserved the amplitudes of dark-adapted a- and b-wave and light-adapted b-wave, which were all significantly reduced after exposure to light. Concomitant with this protective effect, at 3 days after exposure to light, the CD11b(+) microglial cells in the outer nuclear layer (ONL) and subretinal space in the minocycline-treated group were significantly decreased (by 63.5%) when compared with those in the light-exposed, vehicle-treated control group (P < 0.01).


CONCLUSIONS
Minocycline is neuroprotective against light-induced loss of photoreceptors, possibly through the inhibition of retinal microglial activation.
Wilmer Eye Institute, Johns Hopkins University School of Medicine, Baltimore, Maryland 21287-9238, USA. czhang1@jhmi.edu
Cheng
C
Zhang
Bo
B
Lei
Tim T
TT
Lam
Fang
F
Yang
Debasish
D
Sinha
Mark O M
MO
Tso
D000818:Animals; D039481:CD11b Antigen / Q000378:metabolism; D003623:Dark Adaptation; D004596:Electroretinography; D007124:Immunoenzyme Techniques; D007274:Injections, Intraperitoneal; D008027:Light / Q000009:adverse effects; D051379:Mice; D008807:Mice, Inbred BALB C; D017628:Microglia / Q000187:drug effects / Q000378:metabolism / Q000473:pathology; D008911:Minocycline / Q000008:administration & dosage / Q000627:therapeutic use*; D018696:Neuroprotective Agents / Q000008:administration & dosage / Q000627:therapeutic use*; D020419:Photoreceptor Cells, Vertebrate / Q000187:drug effects* / Q000473:pathology / Q000528:radiation effects*; D011833:Radiation Injuries, Experimental / Q000188:drug therapy* / Q000209:etiology; D012162:Retinal Degeneration / Q000188:drug therapy* / Q000209:etiology / Q000473:pathology
D039481:CD11b Antigen; D018696:Neuroprotective Agents; D008911:Minocycline