Investigating the Effects of Exogenous and Endogenous 2-Arachidonoylglycerol on Retinal CB1 Cannabinoid Receptors and Reactive Microglia in Naive and Diseased Retina

Abstract

The endocannabinoid system (ECS) is a new target for the development of retinal disease therapeutics, whose pathophysiology involves neurodegeneration and neuroinflammation. The endocannabinoid 2-arachidonoylglycerol (2-AG) affects neurons and microglia by activating CB1/CB2 cannabinoid receptors (Rs). The aim of this study was to investigate the effects of 2-AG on the CB1R expression/downregulation and retinal neurons/reactive microglia, when administered repeatedly (4 d), in three different paradigms. These involved the 2-AG exogenous administration (a) intraperitoneally (i.p.) and (b) topically and (c) by enhancing the 2-AG endogenous levels via the inhibition (AM11920, i.p.) of its metabolic enzymes (MAGL/ABHD6). Sprague Dawley rats were treated as mentioned above in the presence or absence of CB1/CB2R antagonists and the excitatory amino acid, α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA). Immunohistochemistry, Western blot and a 2-AG level analyses were performed. The 2-AG repeated treatment (i.p.) induced the CB1R downregulation, abolishing its neuroprotective actions. However, 2-AG attenuated the AMPA-induced activation of microglia via the CB2R, as concurred by the AM630 antagonist effect. Topically administered 2-AG was efficacious as a neuroprotectant/antiapoptotic and anti-inflammatory agent. AM11920 increased the 2-AG levels providing neuroprotection against excitotoxicity and reduced microglial activation without affecting the CB1R expression. Our findings show that 2-AG, in the three paradigms studied, displays differential pharmacological profiles in terms of the downregulation of the CB1R and neuroprotection. All treatments, however, attenuated the activation of microglia via the CB2R activation, supporting the anti-inflammatory role of 2-AG in the retina.

Enantiomeric Agonists of the Type 2 Cannabinoid Receptor Reduce Retinal Damage during Proliferative Vitreoretinopathy and Inhibit Hyperactive Microglia In Vitro

“…28 Chronic administration of topical 2-AG attenuated α-amino-3hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA)-induced neurotoxicity in the retina, and the effect was dependent on microglial CB 2 receptors. 70 Furthermore, human retinal pigment epithelial cells treated with JWH-015 exhibited enhanced survival following hydrogen peroxide exposure in vitro. 71 Thus, stimulation of CB 2 can directly promote the survival of retinal pigment epithelial cells as well as retinal ganglion cells during neuroinflammation.…”

“…HU-308 treatment reduced the death of photoreceptor cells following light irradiation to mouse retinas . Chronic administration of topical 2-AG attenuated α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA)-induced neurotoxicity in the retina, and the effect was dependent on microglial CB 2 receptors . Furthermore, human retinal pigment epithelial cells treated with JWH-015 exhibited enhanced survival following hydrogen peroxide exposure in vitro .…”