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S. Yazulla, K. M. Studholme, S. Zimov; Cellular Redistribution of Endocannabinoid Degradative Enzymes, Monoglycerol Lipase and CycloOxygenase-2, in Retinas of Fatty Acid Amide Hydrolase Knockout Mice. Invest. Ophthalmol. Vis. Sci. 2009;50(13):1410.
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The activity of endocannabinoids (eCBs), anandamide (AEA) and 2-arachidonoly glycerol (2-AG) is affected by the distribution of inactivating enzymes. In vivo, AEA is hydrolyzed mainly by fatty acid amide hydrolase (FAAH), 2-AG mainly by monoglycerol lipase (MGL), while AEA and 2-AG are oxidized by cyclooxygenase 2 (COX-2). FAAH-immunoreactivity (IR) is present in all rod bipolar cells (RBC) and all ganglion cells. MGL-IR is present in all RBC and many cells in the ganglion cell layer (GCL). All displaced ganglion cells contain FAAH-IR and MGL-IR. In contrast, COX-2-IR is present in RBC and ON cone bipolar cells. The goal was to identify compensatory changes in the distribution of MGL-IR) and COX-2-IR in retinas of FAAH knockout mice.
Wild type (WT) C57BL/6J mice and FAAH knockout (KO) mice derived from the C57BL/6J strain were used. Standard single and double label immunofluorescence techniques were performed on cryostat sections obtained from isolated retinas fixed in 4% formaldehyde.
There was no difference in the frequency of RBCs, medium and large ganglion cells (identified by PKC-IR and MAP1-IR, respectively) in retinas of KO mice vs WT. In KO retinas there was an apparent increase in the frequency of MGL-IR in the GCL and the appearance of large COX-2-IR boutons in the distal IPL. Quantitation showed that there was a 50% increase in the frequency of cells in the GCL of KO retinas that were labeled by MGL-IR. The proportion of MGL-IR cells in the KO retina that contained MAP1-IR decreased to 69%from 84% in the WT, indicating that the increased frequency in MGL-IR was due to additional cells that were small ganglion cells and perhaps displaced amacrine cells. There was no change in MGL-IR in displaced ganglion cells. In WT mouse retina, two types of RBC label either for PKC only (32%) or double label for PKC and COX-2 (68%). In the KO, the density of COX-2 bipolar cells is reduced by 36%, but with a 66% reduction in double labeling of PKC with COX-2. An inference is that a populatoin of RBC in the WT lost COX-2 in KO mice. We conclude that there are three subtypes of RBC in mouse retina: PKC-IR only (32%), PKC/COX-2 stabile (22%) and PKC/COX-2 labile (44%) that are only unmasked in KO mice. Also, the appearance of COX-2 boutons in the distal IPL of KO mice suggests that a population of OFF cone bipolar cells express COX-2 in KO mice but not in WT.
The redistribution of MGL and COX-2 in retinas of FAAH KO mice compensates for the loss of FAAH and provides alternate pathways to terminate endocannabinoid activity. It is likely that both AEA and 2-AG serve as eCBs in the retina.
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