| Abstrakt: |
The effect of GABA on melatonin content in vitro was studied in the golden hamster retina. GABA significantly increased melatonin levels in a dose-dependent manner, its effect being reversed by a GABAA receptor antagonist, bicuculline, but not by saclofen, a GABAB antagonist. Moreover, an equimolar concentration of muscimol, a GABAA receptor agonist, significantly increased retinal melatonin content, whereas baclofen, a GABAB receptor agonist, was ineffective. The darkness-induced increase in melatonin content in vitro was inhibited by bicuculline, whereas saclofen was ineffective. Retinal GABA turnover rate was significantly higher at midnight than at midday. GABA significantly decreased cyclic AMP and increased cyclic GMP accumulation in the golden hamster retina. The effect of GABA on both nucleotide levels was reversed by bicuculline, but baclofen had no effect. Cyclic GMP analogues (i.e., 8-bromoguanosine 3′,5′-cyclic monophosphate and 2′-O-dibutyrylguanosine 3′,5′-cyclic monophosphate) significantly increased retinal melatonin content in vitro. Taken together, these results support the hypothesis that GABA may be important for the “dark message” in the hamster retina. [ABSTRACT FROM AUTHOR] |
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