Importance of the pineal gland, endogenous prostaglandins and sensory nerves in the gastroprotective actions of central and peripheral melatonin against stress-induced damage.

Melatonin attenuates acute gastric lesions induced by topical strong irritants because of scavenging of free radicals, but its role in the pathogenesis of stress-induced gastric lesions has been sparingly investigated. In this study we compared the effects of intragastric (i.g.) or intracerebroventricular (i.c.v.) administration of melatonin and its precursor, L-tryptophan, with or without concurrent treatment with luzindole, a selective antagonist of melatonin MT2 receptors, on gastric lesions induced by water immersion and restraint stress (WRS). The involvement of pineal gland, endogenous prostaglandins (PG) and sensory nerves in gastroprotective action of melatonin and L-tryptophan against WRS was studied in intact or pinealectomized rats or those treated with indomethacin or rofecoxib to suppress cyclooxygenase (COX)-1 and COX-2, respectively, and with capsaicin to induce functional ablation of the sensory nerves. In addition, the influence of i.c.v. and i.g. melatonin on gastric secretion was tested in a separate group of rats equipped with gastric fistulas. At 3.5 hr after the end of WRS, the number of gastric lesions was counted, the gastric blood flow (GBF) was determined by H2-gas clearance technique and plasma melatonin and gastrin levels were measured by specific radioimmunoassay (RIA). Biopsy mucosal samples were taken for determination of expression of mRNA for COX-1 and COX-2 by reverse transcriptase-polymerase chain reaction (RT-PCR) and of the mucosal generation of prostaglandin E2 (PGE2) by RIA. Melatonin applied i.g. (1.25-10 mg/kg) or i.c.v. (1.25-10 microg/kg) dose-dependently inhibited gastric acid secretion and significantly attenuated the WRS-induced gastric damage. This protective effect of melatonin was accompanied by a significant rise in the GBF and plasma melatonin and gastrin levels and in mucosal generation of PGE2. Pinealectomy, which suppressed plasma melatonin levels, aggravated the gastric lesions induced by WRS and these effects were counteracted by i.g. or i.c.v. application of melatonin. Luzindole abolished completely the gastroprotective effects of melatonin and L-tryptophan and attenuated significantly the rise in GBF evoked by the indoleamine and its precursor. Indomethacin and rofecoxib, which diminished PGE2 biosynthesis by c. 90 and 75% or capsaicin denervation, attenuated significantly melatonin- and L-tryptophan-induced protection and the rise in the GBF. Both the protection and the hyperemia were restored by addition of exogenous CGRP to capsaicin-denervated animals. COX-1 mRNA was detected by RT-PCR in the intact and melatonin-treated gastric mucosa, while COX-2 mRNA, which was undetectable in the intact gastric mucosa, appeared in WRS-exposed mucosa, especially in the melatonin-treated animals and this was accompanied by increased generation of PGE2 in gastric mucosa. Pinealectomy downregulated COX-2 mRNA and this effect was reversed by supplementation of pinealectomized animals with melatonin. We conclude that, (a) exogenous melatonin and its precursor, L-tryptophan, attenuates WRS-induced gastric lesions via interaction with MT2 receptors, (b) this protective action of melatonin is because of an enhancement of gastric microcirculation, probably mediated by PGE2 derived from COX-2 overexpression and activity, the activation of brain-gut axis involving CGRP released from sensory nerves, and the release of gastrin and (c) the pineal plays an important role in the limitation of WRS-induced gastric lesions via releasing melatonin, which exerts gastroprotective and hyperemic activities against stress ulcerogenesis.