This review summarizes the numerous reports that have documented the neuroprotective actions of
melatonin in experimental models of
ischemia/reperfusion injury (
stroke). In these investigations, which have used three species (rat, gerbil, and cat),
melatonin was universally found to reduce brain damage that normally occurs as a consequence of the temporary interruption of blood flow followed by the reflow of oxygenated blood to the brain. The exogenous administration of
melatonin in these experimental
stroke models reduced
infarct volume, lowered the frequency of apoptosis, increased the number of surviving neurons, reduced reactive
gliosis, lowered the oxidation of neural
lipids and oxidatively damaged
DNA, induced bcl-2 gene expression (the activity of which improves cell survival), upregulated excision repair cross-complementing factor 6 (an essential gene for preferential
DNA excision repair), restrained
poly(ADP ribose)
synthetase (which depletes cellular
NAD resulting in the loss of
ATP) activity, and improved neurophysiologic outcomes. Under no circumstances did
melatonin exacerbate the damage associated with
ischemia/reperfusion injury. As well as the beneficial
pharmacologic actions of
melatonin, several studies show that a relative deficiency of endogenous
melatonin exaggerates neural damage due to
stroke; this suggests that even physiologic concentrations of
melatonin normally serve to protect the brain against damage. The primary action to explain
melatonin's protective effects may relate to its ubiquitous direct and indirect antioxidative actions, although other beneficial functions of
melatonin are not precluded.