In vivo μελέτη της νευροπροστατευτικής δράσης της σωματοστατίνης στην αμφιβληστροειδική διεγερτοτοξικότητα

Abstract

An increasing number of clinical and laboratory studies have focused on the investigation of the pathophysiology and therapy of retinal diseases, such as diabetic retinopathy, glaucoma and age-related macular degeneration, that lead to visual impairment and blindness. Ischemia has been shown to be the underlying cause of these diseases. It leads to neovascularization and neuronal cell death. The cascade of events leading to the ischemia induced cell death involves increases in glutamate, the major excitatory neurotransmitter in retina, intracellular calcium ions and nitric oxide (NO) levels (Osborne et al., 2004). The neuropeptide somatostatin has been considered as a possible therapeutic target because of its well known inhibitory actions on Ca2+ channels and intracellular calcium levels (Reisine, 1990, Tallent et al., 1996). The aim of the present study was to investigate the neuroprotective effects of somatostatin in an in vivo model of retinal excitotoxicity, employing the excitatory amino acid AMPA. The intravitreal administration of AMPA (42nmol per rat eye) resulted in a loss of retinal neurons such as cholinergic amacrine, bNOS expressing amacrine and horizontal cells, 24 hours after its administration. An increase in apoptosis was also observed in the AMPA treated samples. Somatostatin levels and the expression of the sst2 and sst5 somatostatin receptor subtypes were not affected by the AMPA treatment. Intravitreal co-injection of AMPA with somatostatin or sst2 and sst5 selective analogs protected the retina from the AMPA induced toxicity in a dose dependent manner. The best protection was afforded by sst2 selective ligands since immunohistochemical studies showed that AMPA induced retinal cell loss was fully reversed. TUNEL studies confirmed the above results. The increase in apoptosis observed in the AMPA treated samples was drastically reduced in the presence of the somatostatinergic sst2 analogs. Intravitreal injection of the sst2 selective analogs after the administration of AMPA afforded partial protection. In order to study the mechanism involved in somatostatin’s neuroprotective actions against excitotoxicity, the involvement of the intracellular signaling pathway NO/cGMP was studied. The blockade of NO synthase and guanylate cyclase, the enzymes that catalyze the synthesis of nitric oxide and cGMP respectively, reversed the actions of the somatostatinergic analogs. These results offer new in vivo evidence supporting that sst2 and sst5 selective ligands, for the first time injected intravitreally, protect the retina from excitotoxic insults, and reinforce the theory of the involvement of NO/cGMP signaling in somatostatin’s protective effects. The present data encourage the therapeutic use of somatostatin either alone or in combination with other treatments presently applied (photocoagulation, anti-VEGFs) in the therapeutics of retinal diseases whose pathophysiology involves ischemia induced neurodegeneration or ischemia induced neovascularization. Further investigations are essential to study the pharmacokinetic properties of sst2 and sst5 selective analogs to assess the best conditions for optimum efficacy.