Changes in Retinal N-Acylethanolamines and their Oxylipin Derivatives During the Development of Visual Impairment in a Mouse Model for Glaucoma

Abstract

Neurons are especially susceptible to oxidative damage, which is increasingly implicated in neurodegenerative disease. Certain N-acylethanolamines (NAEs) have been shown to protect neurons from oxidative stress. Since glaucoma may be considered a neurodegenerative disorder and the survival of retinal neurons could also be influenced by N-acylethanolamines, our goal was to quantify changes in certain N-acylethanolamine species and their oxylipin derivatives in the retina of a mouse model for glaucoma. We also sought to identify relationships between these and parameters of glaucoma disease development, specifically intraocular pressure, visual acuity, and contrast sensitivity. Five N-acylethanolamine species and three NAE oxylipin derivatives were quantified in retina from young and aged DBA/2Crl mice. N-Acylethanolamines and NAE-oxylipins in retinal extracts were quantified against deuterated standards by isotope dilution gas chromatography-mass spectrometry. Levels (nmol/g dry weight) of N-arachidonoylethanolamine (anandamide; NAE 20:4) were significantly (p = 0.008) decreased in aged (2.875 ± 0.6702) compared to young animals (5.175 ± 0.971). Conversely, the anandamide oxylipin, 15(S)-HETE ethanolamide (15(S)-HETE EA), was significantly (p = 0.042) increased in aged (0.063 ± 0.009) compared to young animals (0.039 ± 0.011). Enzymatic depletion of the anandamide pool by 15-lipoxygenase and consequent accumulation of 15(S)-HETE ethanolamine may contribute to decreased visual function in glaucomatous mice. Since N-acylethanolamines effectively attenuate glaucoma pathogenesis and associated visual impairment, our data provides additional rationale and novel targets for glaucoma therapies.

Keywords: Acylethanolamides; Anandamide; Contrast sensitivity; DBA/2; Intraocular pressure; Lipoxin; Lipoxygenase; N-Linoleoylethanolamine; N-Oleylethanolamine; Neuroprotection; Optokinetic reflex; Optomotor; Visual acuity.

Fig. 1

Intraocular pressure increases and visual function decreases in aged DBA/2Crl mice. a Aged (~9 m; n = 18 eyes) DBA/2Crl mice demonstrate significantly (P = 0.0022) elevated intraocular pressure (IOP) compared to young (7 w; n = 20 eyes) controls. b, c Aged mice have a significant (n = 20 eyes; p < 0.0001) deficit in visual acuity (VA) and contrast sensitivity (CS) compared to young controls. d IOP correlates moderately with visual function. e CS and VA correlate strongly with each other. Data are expressed as mean ± SD. IOP measurements are symbolized with triangles, VA with circles, and CS with squares. Open symbols represent young eyes and closed symbols aged eyes. Dashed lines represent the 95 % confidence interval

Fig. 2

Increased 16:0, 18:0, 18:2, and 20:4 N-acylethanolamine levels in the DBA/2Crl retina correlate with lower intraocular pressure and better visual function. a NAE 16:0 levels correlate moderately with visual acuity (VA) and intraocular pressure (IOP), but not significantly. There is a strong correlation that approaches statistical significance between NAE 16:0 and contrast sensitivity (CS). b NAE 18:0 levels correlate moderately with VA and CS, but not significantly. There is a strong, statistically significant correlation between NAE 18:0 and IOP. c NAE 18:1 levels do not correlate with VA, CS, or IOP. d NAE 18:2 levels correlate moderately with VA, but not significantly. There is a strong correlation that approaches statistical significance between NAE 18:2 and CS. A statistically significant, strong correlation is seen between NAE 18:2 and IOP. e NAE 20:4 levels correlate strongly and significantly with both VA and CS. There is a moderate correlation between NAE 20:4 and IOP that is not statistically significant. f Table of statistical values from correlations a–e. Data are expressed as mean ± SD. IOP measurements are symbolized with triangles, VA with circles, and CS with squares. Open symbols represent young eyes and closed symbols aged eyes. Dashed lines represent the 95 % confidence interval

Fig. 3

Increased levels of the N-acylethanolamine oxylipins 9NAE-HOD and 15(S)-HETE EA correlate with higher intraocular pressure and declining visual function. a 9NAE-HOD levels correlate moderately with intraocular pressure (IOP) and weakly with visual acuity (VA) and contrast sensitivity (CS), but not significantly. b 13NAE-HOD levels do not correlate with VA, CS, or IOP. c 15(S)-HETE EA levels correlate weakly with IOP, but not significantly. There is a strong correlation that approaches statistical significance between 15(S)-HETE EA and CS. A statistically significant, strong correlation is seen between 15(S)-HETE EA and VA. d Table of statistical values from correlations a–c. Data are expressed as mean ± SD. IOP measurements are symbolized with triangles, VA with circles, and CS with squares. Open symbols represent young eyes and closed symbols aged eyes. Dashed lines represent the 95 % confidence interval