Effect of Melatonin on the Glutamate/Glutamine Cycle Activity in the Golden Hamster Retina

Abstract

Abstract: : Purpose: Glutamate is the main excitatory neurotransmitter in the retina but it is neurotoxic when present in excessive amounts. Retinal glutamate synaptic concentrations are regulated by the activity of the glutamate/glutamine cycle. Since melatonin has been shown to be neuroprotective in several systems, in the present report, its effect on the glutamate/glutamine cycle activity was examined in the golden hamster retina Methods: Glutamine synthetase (GS) activity was assessed by a spectrophotometric assay, whereas glutamine uptake and release were assessed using [³H]–glutamine as a radioligand. In addition, glutaminase activity was measured through the conversion of [³H]–glutamine to [³H]–glutamate. Results: Melatonin (0.1 – 10 nM) significantly increased retinal glutamine synthetase activity but it did not affect L–glutamine release. A characterization of the hamster retinal L–glutamine uptake mechanism was performed. This mechanism was partly Na⁺–dependent, and it was significantly inhibited by 2–aminobicyclo (2,2,1) heptane 2–carboxylic acid (BCH, a selective antagonists for the L–type system) and by α–(methylamino)–isobutyric acid (MeAIB, substrate characteristic for the A –type transporter) suggesting the coexistence of these transport systems in the hamster retina. Melatonin (0.1 – 10 nM) significantly increased total glutamine uptake as well as the BCH and the MeAIB –insensitive transporters activity. On the other hand, melatonin significantly decreased retinal glutaminase activity. Conclusions: Based on these results, it might be presumed that hamster retinal glutamate /glutamine cycle activity is regulated by physiological concentrations of melatonin. Furthermore, these findings suggest that a treatment with melatonin could be considered as a new approach to handling glutamate–mediated neuronal degeneration.

Keywords: melatonin • signal transduction • neurotransmitters/neurotransmitter systems