Studies on Endocannabinoid Levels in Normal and Glaucomatous Eyes

Abstract

Abstract: : Purpose: Synthetic cannabinoids and the endogenous ligands, endocannabinoids, have been reported to lower intraocular pressure. We investigated the levels of endocannabinoids 2–arachidonoylglycerol (2–AG), anandamide (AEA), and palmitoylethanolamide (PEA) as an "entourage compound" in different eye tissues from normal or glaucomatous donors and their distribution in mouse tissues. Methods: 2–AG, PEA, and AEA in tissues were extracted in chloroform/methanol. Deuterated standards d₅–2–AG, d₈–AEA, d₄–PEA were added as internal standards. The endocannabinoids were separated and subjected to liquid chromatography–mass spectrometry. Their amounts were quantified by isotope dilution and normalized per g wet tissue weight. Results:2–AG, PEA, and AEA were detected in all the human ocular tissues and mouse tissues examined. In human eyes, 2–AG levels were highest in the retina > ciliary body and choroid > iris and cornea. PEA and AEA levels in the iris were more than 3 times higher than their respective levels in the other ocular tissues. In human glaucomatous eyes compared to normal eyes, 2–AG levels were significantly decreased by 44% in the ciliary body and PEA levels by 40% in the ciliary body and by 54% in the choroid. The differences between 2–AG and AEA levels were less pronounced in the human eyes and mouse eyes compared to other mammalian tissues, where 2–AG levels typically exceed those of AEA by about 100–fold. In the present study, 2–AG was 73–fold more abundant than AEA in the mouse brain. In contrast to the other mouse tissues studied, PEA levels in the mouse eye were about 9–fold higher than 2–AG levels. Conclusions: The detection of 2–AG, PEA, and AEA in ocular tissues of normal or glaucomatous donors and in mouse eyes provided further support for the involvement of endocannabinoids in ocular physiology. In glaucomatous human eyes, the significantly reduced 2–AG and PEA levels in the ciliary body and PEA levels in the choroid suggest that pathological alterations in endocannabinoid levels may have a role in this disease state.

Keywords: lipids • pharmacology