Melatonin, a major
hormone of pineal gland, was recently shown to attenuate acute gastric lesions induced by strong irritants because of the scavenging of
free radicals but its role in
ulcer healing has been little investigated. In this study we compared the effects of intragastric (i.g.) administration of
melatonin and its precursor,
L-tryptophan, with or without concurrent treatment with
luzindole, a selective antagonist of
melatonin MT2 receptors, on healing of chronic
gastric ulcers induced by serosal application of
acetic acid (
ulcer area 28 mm2). The involvement of endogenous
prostaglandins (PG),
nitric oxide (NO) and sensory nerves in
ulcer healing action of
melatonin and
L-tryptophan was studied in rats treated with
indomethacin and
NG-nitro-L-arginine (L-NNA) to suppress, respectively, cyclo-
oxygenases (COX) and NO synthases or in those with functionally deactivated sensory nerves with
capsaicin. The influence of
melatonin on gastric secretion during
ulcer healing was tested in separate group of rats with
gastric ulcer equipped with gastric
fistulas (GF). At day 8 and 15 upon the
ulcer induction, the area of
gastric ulcers was measured by planimetry, the mucosal blood flow (GBF) was determined by H2-gas clearance technique and gastric
luminal NO2-/NO3- levels was assessed by Griess reaction. Plasma
melatonin and
gastrin levels were measured by specific radioimmunoassay (RIA). Biopsy mucosal samples were taken for expression of constitutive
NO-synthase (cNOS) and inducible NOS (iNOS) by
reverse transcriptase-polymerase chain reaction (RT-PCR).
Melatonin (2.5-20 mg/kg-d i.g.) and
L-tryptophan (25-100 mg/kg-d i.g.) dose-dependently accelerated
ulcer healing, the dose inhibiting by 50% (ED50) of
ulcer area being 10 and 115 mg/kg, respectively. This inhibitory effect of
melatonin (10 mg/kg-d i.g.) and
L-tryptophan (100 mg/kg-d i.g.) on
ulcer healing was accompanied by a significant rise in the GBF at
ulcer margin and an increase of plasma
melatonin.
luminal NO2-/NO3- and plasma
gastrin levels. Gastric acid and
pepsin outputs were significantly inhibited during the
ulcer healing in
melatonin-treated gastric mucosa as compared with those in vehicle-treated animals.
Luzindole abolished completely the healing effects of
melatonin and
L-tryptophan and attenuated significantly the rise in plasma
gastrin evoked by the
hormone and its precursor.
Indomethacin (5 mg/kg-d i.p). that blocked PG biosynthesis by 90% or
L-NAME (20 mg/kg i.v), inhibitor of NOS. that suppressed
luminal NO release, attenuated significantly
melatonin and
L-tryptophan-induced acceleration of
ulcer healing and accompanying rise in GBF at
ulcer margin and
luminal NO release. The
melatonin-induced acceleration of
ulcer healing,
hyperemia at
ulcer margin and increase in the release of NO were enhanced when
L-arginine but not D-
arginine was added to
L-NAME. The
ulcer healing and the GBF effects of
melatonin and
L-tryptophan were significantly impaired in rats with
capsaicin-induced
denervation of sensory nerves and both,
ulcer healing and the
hyperemia at
ulcer margin were restored in these rats by addition of exogenous CGRP to
melatonin and
L-tryptophan. Expression of cNOS
mRNA was detected by RT-PCR in the intact gastric mucosa as well as at the edge of
gastric ulcers treated with both, vehicle and
melatonin, while iNOS
mRNA that was undetectable in the intact gastric mucosa, appeared during
ulcer healing and especially this was strongly up-regulated in the
melatonin-treated gastric mucosa. We conclude that (1) exogenous
melatonin and that derived from its precursor,
L-tryptophan, accelerate
ulcer healing probably via interaction with MT2 receptors; (2) this
ulcer healing action is caused by an enhancement by
melatonin of the microcirculation at the
ulcer margin possibly mediated by COX-derived PG and NO because of overexpression of iNOS and (3)
gastrin, which exhibits trophic activity in the gastric mucosa and
calcitonin gene related peptide (CGRP), released from sensory nerves, may also contribute to the
ulcer healing action of
melatonin.