Roles of nocturnal melatonin and the pineal gland in modulation of water-immersion restraint stress-induced gastric mucosal lesions in rats

The roles of melatonin and the pineal gland in the circadian variation of w ater-immersion restraint stress-induced gastric mucosal lesions in rats wer e investigated. Fasted rats were subjected to water-immersion restraint str ess during both the diurnal and nocturnal phases of a light:dark cycle. Pin ealectomized and sham-operated rats were also subjected to water-immersion restraint stress at night. The lesion area after 4 hr of stress during the dark phase was significantly lower than in light-phase controls. Pinealecto my increased the lesion area in the dark phase, compared to the sham operat ion, but this effect was counteracted by intracisternal melatonin preadmini stration at a dose of 100 ng/rat. Melatonin concentrations in control rats during the light phase were significantly increased 4 hr after water-immers ion restraint stress. In contrast, melatonin concentrations 4 hr after wate r-immersion restraint stress in the dark phase were significantly depressed compared with the control levels at the corresponding time. Melatonin leve ls after stress exposure were markedly decreased in pinealectomized rats as compared with sham-operated rats. These results suggest that circadian rhy thm has an important role in the formation of stress-induced gastric mucosa l lesions in rats and that melatonin responses to water-immersion restraint stress differ between day and night. The pineal gland modulates the stress response and melatonin contributes to gastric protection via a mechanism i nvolving the central nervous system.