In vitro physiological evidence of enhanced antioxidant and neuroprotective action of 2,3-dihydromelatonin, a melatonin analogue

As the capacity of the endogenous antioxidative system is limited, pharmacological treatment with antioxidants may help to protect neuronal tissue against increased amount of reactive oxygen species produced during oxidative stress. We attempted to improve resistance of rat hippocampal slices exposed to ischaemia in vitro (hypoxia (HYP) accompanied with decreased glucose concentration followed by reoxygenation (ROX)) and thus to diminish the impairment of synaptic transmission after HYP/ROX. We compared the protective features of the melatonin analogue 2,3-dihydromelatonin (2,3-DHM) with melatonin itself. In preliminary experiments, the compound 2,3-DHM compared to melatonin revealed enhanced antilipoperoxidation action in rat brain homogenates exposed to Fe/ascorbate system (−log IC50 = 4.76 ± 0.01 versus −log IC50 = 2.51 ± 0.02, respectively). In this study, 2,3-DHM (from 0.3 to 10 μmol l−1) applied at 30 min before the beginning of HYP and remaining all over the 6-min HYP as well as 20-min ROX, exerted a protective effect demonstrated by improvement of the population spike amplitude (PoS) recovery during ROX, with the maximum effect at 3 μmol l−1. In accordance with this, the ratio of irreversibly damaged slices after HYP/ROX was decreased in the groups treated with 2,3-DHM. Moreover, a significant delay of PoS decay during HYP (expressed as half time, t0.5) was revealed at 2,3-DHM concentration 1 and 3 μmol l−1). An equipotent effect of melatonin and 2,3-DHM was achieved by a 100-times lower concentration of the latter (0.3 and 1 μmol l−1) compared to that of melatonin (30 and 100 μmol l−1). Further, compared to the highest effect of 2,3-DHM in the concentration 3 μmol l−1 on the percentage of irreversibly damaged slices (only 20%), melatonin did not exert such pronounced effect, either in the concentration 30 or 100 μmol l−1 (67 and 50%, respectively). We conclude that hydrogenation of the melatonin molecule significantly improved its antihypoxic effect in our model of rat hippocampal slices exposed to ischaemic conditions in vitro, similarly as it enhanced the antilipoperoxidation action of 2,3-DHM in our previous studies. These findings suggest that 2,3-DHM deserves more attention concerning its neuroprotective effect in oxidative stress-associated tissue damage.