Effect of melatonin on the retinal glutamate/glutamine cycle in the golden hamster retina.

Glutamate is the main excitatory neurotransmitter in the retina, but it is neurotoxic when present in excessive amounts. The metabolic dependence of glutamatergic neurons upon glia via the glutamate/glutamine cycle to provide the precursor for neurotransmitter glutamate is well established. 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. 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 alpha-(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. On the basis of 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.

Photic regulation of heme oxygenase activity in the golden hamster retina: involvement of dopamine.

The photic regulation of heme oxygenase (HO) activity was examined in the golden hamster retina. This enzymatic activity was significantly higher at midday than at midnight. When the hamsters were placed under constant darkness for 48 h and killed at subjective day or at subjective night, the differences in HO activity disappeared. Western blot analysis showed no differences in HO levels among these time points. Dopamine significantly increased this activity in retinas excised at noon or at midnight, with a higher sensitivity at night. The effect of dopamine was reversed by SCH 23390 but not by spiperone and clozapine and it was not reproduced by quinpirole. In vitro, the increase in HO activity found in retinas incubated under light for 1 h was significantly reduced by SCH 23390. Two cAMP analogs increased HO activity and their effect, as well as the effect of dopamine was blocked by H-89, a protein kinase A (PKA) inhibitor. Tin protoporphyrin IX, an HO inhibitor, significantly decreased cGMP accumulation with maximal effects during the day. Low concentrations of bilirubin decreased retinal thiobarbituric acid substances levels (an index of lipid peroxidation) in basal conditions and after exposing retinal cells to H2O2. These results suggest that hamster retinal HO activity is regulated by the photic stimulus, probably through a dopamine/cAMP/PKA dependent pathway.