[STUDIES OF THE ANTIHYPOXIC AND ANTIAMNESTIC EFFECTS OF MELATONIN IN ANIMALS]

Experiments with mice showed that in a multitude of acute hypoxia models (normobaric hypoxic hypoxia with hypercapnia, hypobaric, hemic and histotaxic) the antihypoxic action of a single intra-abdominal dose of melatonin surpasses greatly amtisol, the standard antihypoxic agent. Single melatonin injection produced a strong antiamnestic action on various amnestic models (scopolamine-induced, acute normobaric hypoxia with hypercapnia, and a combination of extreme factors) which was much better than of pyracetame, a well-known nootropic (mind-stimulating) drug. Increase of the melatonin dose from 1 mg/kg to 20 mg/kg amplified both the antihypoxic and antiamnestic effects. Melatonin inhibited orthodromal population responses in surviving sections of rat's hippocampus (by 24 ± 3% at 2 mM; by 72 ± 6% at 5 mM). Besides, the inhibiting action of lusindol, a blocker of melatonin receptors MT1 and MT2--was virtually fully neutralized. Therefore, melatonin inhibits transmission within the Schaffer collateral--CA1 pyramidal neurons synapse by stimulation of melatonin receptors MT1 and MT2; higher melatonin concentrations (0.5 to 5 mM) enhance its effectiveness.