Evidence That the Endocannabinoid System is Involved in Retinal Ganglion Cell Death Following High Intraocular Pressure-Induced Ischemia in Rat

Abstract

Purpose:: To evaluate whether the endocannabinoid system is involved in the mechanisms of retinal ganglion cell (RGC) death evoked by high intraocular pressure (IOP)-induced ischemia

Methods:: Anandamide (AEA) synthesis, transport, hydrolysis and AEA endogenous levels were assessed by means of high-performance liquid chromatography in the retina of rats undergoing 45 min. of ischemia followed by 12 h reperfusion. Under these experimental conditions, binding to cannabinoid (CB1R) and vanilloid (TRPV1) receptor was assessed using rapid filtration assays. To study the neuroprotective profile of drugs that interfere with the endocannabinoid system, RGC counting and real-time polymerase chain reactions for Thy-1 mRNA expression were used.

Results:: In rat retina, ischemia followed by reperfusion results in enhanced FAAH activity paralleled by a significant decrease of the endogenous AEA tone, whereas the AEA-membrane transporter or the AEA-synthase NAPE-PLD (N-acyl-phosphatidylethanolamines-hydrolyzing-phospholipase-D) were not affected. Retinal ischemia/reperfusion decreased the expression of cannabinoid (CB1) and vanilloid (TRPV1) receptors. Systemic administration of the FAAH inhibitor, URB597 (0.3 mg/kg), reduced enzyme activity and minimized RGC loss observed in ischemic/reperfused retinas. Similarly, intravitreal injection of the AEA stable analogue, R(+)-methanandamide (5 µl of a 1 mM sol.) 30 min prior to ischemia, reduced RGC loss and this was prevented by systemic administration of a CB1 or TRPV1 selective antagonists, e.g. SR141716 (3 mg/kg) and capsazepine (10 mg/kg) respectively.

Conclusions:: Our data indicate that retinal ischemia/reperfusion reduces endogenous AEA via enhanced FAAH activity. These events seem to be implicated in the RGC loss caused by high IOP-induced ischemia in rat.

Keywords: neuroprotection • retina: proximal (bipolar, amacrine, and ganglion cells) • ischemia