| Abstract |
Somatostatin is a cyclic neuropeptide, which mediate diverse actions in the central and peripheral nervous systems. In the retina, somatostatin is localized in amacrine cells in the inner plexiform layer, in displaced amacrines cells and in sοme types of ganglion cells. Five somatostatin receptor subtypes have been recently cloned, namely SSTR1-5 and their localization in the retina studied (Thermos, 2003). Yet, the role of somatostatin in the retina remains to be ascertained. Immunohistochemical studies support the colocalization of the SSTR1 and SSTR2 receptors with the enzyme tyrosine hydroxylase, the marker for dopamine, in amacrine dopaminergic cells. Dopamine is one of the major neuromodulators in visual processes and in retinal circuitry. The aim of the present study was to determine the possible role of somatostatin as a regulator of dopamine release in rat retina. Retinas from female Sprague-Dawley rats (250-280 g) were employed. Retinas were detached and placed in a 24 well-plate which contained M-199 medium and incubated in a culture incubator at 37 oC (95% air and 5% CO2) under shaking conditions for one hour in order to stabilize the release of dopamine from the tissues. Subsequently, samples were collected (medium) from the culture every 20 min. The samples were treated with acidic activated alumina and the concentration of dopamine in each sample determined by high performance liquid chromatography (HPLC), in combination with an electrochemical detector. The three first samples (20-60 minutes) represent the basal release of dopamine. In the fourth incubation, pharmacological agents were added [somatostatin (10-4, 10-5, 10 -6 Μ), or specific analogs ΒΙΜ-23014 (SSTR2 agonist, 10-4, 10-5 Μ), CYN-154806 (SSTR2 antagonist 10-4 Μ with somatostatin 10-4 Μ), L-797.591 (SSTR1 agonist, 10-4 and 10-5 Μ), L-796.778 (SSTR3 agonist, 10-4 Μ)], and three to four more samples collected. Somatostatin increased dopamine levels in a concentration-dependent manner. The increase was statistical significant as compared to the basal release. The addition of the SSTR2 antagonist CYN-154806 blocked the somatostatin induced increase of dopamine, while the SSTR2 agonist ΒΙΜ-23014 mimicked the somatostatin effect. Also the activation of SSTR1 receptors evoked increase of dopamine release, while activation of SSTR3 receptors had no effect. These results support for the first time a regulatory role for somatostatin in the release of dopamine in rat retina.
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Αλληλεπιδράσεις Σωματοστατίνης - Ντοπαμίνης στον Αμφιβληστροειδή Αρουραίου
