Subcellular Localization of a 2-Arachidonoyl Glycerol Signaling Cassette in Retinal Ganglion Cell Axonal Growth In Vitro.

PURPOSE:

To investigate whether the subcellular distribution of endocannabinoid (eCB) system (ECS) components in growing RGC axons is consistent with the formation of eCB-enriched “hotspots” and the role of the ECS in RGC axonal growth.

METHODS:

We used immunocytochemistry and image analysis to quantify axonal expression of the ECS components diacylglycerol lipase alpha (DGLα), monoacylglycerol lipase (MGL), and cannabinoid receptor type 1 (CB1R) in a mouse retinal explant model. We tested whether pharmacologic antagonists of CB1R and inhibitors of eCB degradation modulate ECS component expression and axonal growth.

RESULTS:

DGLα expression was higher in the distal RGC axon than in the growth cone central domain (GCCD) (95% confidence interval [CI], 106.5%-122.4% at 15 μm proximal to the GCCD), whereas MGL expression in the same region was not significantly different (95% CI, 88.8%-102.1%). In more proximal axon segments, DGLα and MGL expression were both lower than in the GCCD, whereas CB1R expression was 2.5-fold higher in this region (95% CI, 220.3%-278.4% at 50 μm proximal to the GCCD). The presence of CB1R antagonist O-2050 disrupted profiles of ECS component expression and increased axonal growth (95% CI for the difference of median axon lengths 26.6-55.6 μM).

CONCLUSIONS:

Our results demonstrate an ECS topology in RGC axons that is consistent with formation of eCB-enriched hotspots and suggest that the ECS has a role in CB1R-dependent inhibition of RGC axonal growth in vitro.