Endogenous and synthetic cannabinoids induce the downregulation of cannabinoid CB1 receptor in retina.

Endogenous and synthetic cannabinoids have been shown to provide neuroprotection to retinal neurons in acute animal models of retinopathy. Chronic exposure to cannabinoid receptor (CB1R) agonists has been reported to induce downregulation of the CB1R in brain and behavioral tolerance. The aim of this study was to investigate the effect of subchronic/chronic cannabinoid administration on CB1R downregulation in normal rat retina, its downstream prosurvival signaling and subsequent effect on retinal neuroprotection against AMPA excitotoxicity. Sprague-Dawley rats were administered intraperitoneally with vehicle (Control), the endogenous N-arachidonoyl ethanolamine (AEA), and the synthetic cannabinoids R-(+)-Methanandamide (MethAEA) and HU-210 daily (25, 50, 100 µg/kg) for four or fourteen days (4d/14d, subchronic/chronic administration, respectively). HU-210 was also administered acutely as follows, vehicle injection for 13 days and a single dose of HU-210 on the 14th day. Immunohistochemistry studies and Western blot analysis were employed to assess CB1R expression in control and AMPA treated retinas and cannabinoid induced changes in Akt and ERK1/2 phosphorylation (ph). Real time PCR was employed to examine the effect of MethAEA (50 mg/kg,4d) on CB1R mRNA expression. AEA, MethAEA and HU-210 attenuated CB1R expression in a dose-dependent manner (25, 50, 100 µg/kg), after subchronic and chronic administration. No effect was observed at the lower dose of 25 µg/kg. MethAEA (50 mg/kg,4d) attenuated CB1R mRNA expression. AM251 (CB1 antagonist/inverse agonist, 0.5 mg/kg,4d), administered prior to HU-210 (50 µg/kg,4d) inhibited CB1R downregulation. Chronic/subchronic treatments (50 µg/kg) of HU-210 and MethAEA reduced levels of ph-Akt and ph-Akt/ph-ERK1/2, respectively. AEA had no effect on ph-Akt nor ph-ERK1/2. All three cannabinoids (50 µg/kg,4d) failed to protect brain nitric oxide synthetase (bNOS) expressing amacrine cells against AMPA excitotoxicity, in agreement with the downregulation of CB1 receptor. At the lower doses of 12.5 and 25 µg/kg, HU-210 protected bNOS-expressing amacrine cells. This study provides novel information regarding agonist-induced CB1R downregulation in rat retina after subchronic/chronic cannabinoid treatment, and its effect on downstream prosurvival signaling and neuroprotection.

The Synthetic Microneurotrophin BNN27 Affects Retinal Function in Rats With Streptozotocin-Induced Diabetes.

BNN27, a C17-spiroepoxy derivative of DHEA, was shown to have antiapoptotic properties via mechanisms involving the nerve growth factor receptors (tropomyosin-related kinase A [TrkA]/neurotrophin receptor p75 [p75NTR]). In this study, we examined the effects of BNN27 on neural/glial cell function, apoptosis, and inflammation in the experimental rat streptozotocin (STZ) model of diabetic retinopathy (DR). The ability of BNN27 to activate the TrkA receptor and regulate p75NTR expression was investigated. BNN27 (2,10, and 50 mg/kg i.p. for 7 days) administration 4 weeks post-STZ injection (paradigm A) reversed the diabetes-induced glial activation and loss of function of amacrine cells (brain nitric oxide synthetase/tyrosine hydroxylase expression) and ganglion cell axons via a TrkA receptor (TrkAR)-dependent mechanism. BNN27 activated/phosphorylated the TrkAY490 residue in the absence but not the presence of TrkAR inhibitor and abolished the diabetes-induced increase in p75NTR expression. However, it had no effect on retinal cell death (TUNEL+ cells). A similar result was observed when BNN27 (10 mg/kg i.p.) was administered at the onset of diabetes, every other day for 4 weeks (paradigm B). However, BNN27 decreased the activation of caspase-3 in both paradigms. Finally, BNN27 reduced the proinflammatory (TNFα and IL-1ß) and increased the anti-inflammatory (IL-10 and IL-4) cytokine levels. These findings suggest that BNN27 has the pharmacological profile of a therapeutic for DR, since it targets both the neurodegenerative and inflammatory components of the disease.