George A. Bubenik

Gastrointestinal melatonin: localization, function, and clinical relevance.

The gastrointestinal tract of vertebrate species is a rich source of extrapineal melatonin. The concentration of melatonin in the gastrointestinal tissues surpasses blood levels by 10–100 times and there is at least 400× more melatonin in the gastrointestinal tract than in the pineal gland. The gastrointestinal tract contributes significantly to circulating concentrations of melatonin, especially during the daytime and melatonin may serve as an endocrine, paracrine, or autocrine hormone influencing the regeneration and function of epithelium, enhancing the immune system of the gut, and reducing the tone of gastrointestinal muscles. As binding sites for melatonin exhibit circadian variation in various species, it has been hypothesized that some melatonin found in the gastrointestinal tract might be of pineal origin. Unlike the photoperiodically regulated production of melatonin in the pineal, the release of gastrointestinal melatonin seems to be related to the periodicity of food intake. Phylogenetically, melatonin and its binding sites were detected in the gastrointestinal tract of lower vertebrates, birds, and mammals. Melatonin was found also in large quantities in the embryonic tissue of the mammalian and avian gastrointestinal tract. Food intake and, paradoxically, also long-term food deprivation resulted in an increase of tissue and plasma concentrations of melatonin. Melatonin release may have a direct effect on many gastrointestinal tissues but may also well influence the digestive tract indirectly, via the central nervous system and the sympathetic and parasympathetic nerves. Melatonin prevents ulcerations of gastrointestinal mucosa by an antioxidant action, reduction of secretion of hydrochloric acid, stimulation of the immune system, fostering epithelial regeneration, and increasing microcirculation. Because of its unique properties, melatonin could be considered for prevention or treatment of colorectal cancer, ulcerative colitis, gastric ulcers, irritable bowel syndrome, and childhood colic.

Melatonin in the retina and the Harderian gland. Ontogeny, diurnal variations and melatonin treatment.

Abstract Using highly specific antibodies, melatonin has been identified immunohistologically in the rat retina, and the Harderian gland. The first truly significant amount of retinal melatonin was already detected in the 2 day old pups. The amount of melatonin progressively increased with age reaching adult levels around the 20th day. Diurnal variations with higher night levels of melatonin have been found in the adult in both retina and Harderian gland. Intraperitoneal injection or subcutaneous implantation of melatonin in beeswax (150 μg/rat) resulted in a vast increase in melatonin content in the retina and the Harderian gland of the juvenile and adult rats. No sexual differences have been registered in any experimental group. The concept of melatonin synthesis at peripheral sites independent of pineal production, the involvement of light-dark rhythm in the regulation of pineal and extrapineal melatonin content and the possibility of uptake mechanisms or receptors for melatonin in the retina and Harderian gland are discussed.

Prospects of the clinical utilization of melatonin

This review summarizes the present knowledge on melatonin in several areas on physiology and discusses various prospects of its clinical utilization. Ever increasing evidence indicates that melatonin has an immuno-hematopoietic role. In animal studies, melatonin provided protection against gram-negative septic shock, prevented stress-induced immunodepression, and restored immune function after a hemorrhagic shock. In human studies, melatonin amplified the antitumoral activity of interleukin-2. Melatonin has been proven as a powerful cytostatic drug in vitro as well as in vivo. In the human clinical field, melatonin appears to be a promising agent either as a diagnostic or prognostic marker of neoplastic diseases or as a compound used either alone or in combination with the standard cancer treatment. Utilization of melatonin for treatment of rhythm disorders, such as those manifested in jet lag, shift work or blindness, is one of the oldest and the most successful clinical application of this chemical. Low doses of melatonin applied in controlled-release preparation were very effective in improving the sleep latency, increasing the sleep efficiency and rising sleep quality scores in elderly, melatonin-deficient insomniacs. In the cardiovascular system, melatonin seems to regulate the tone of cerebral arteries; melatonin receptors in vascular beds appear to participate in the regulation of body temperature. Heat loss may be the principal mechanism in the initiation of sleepiness caused by melatonin. The role of melatonin in the development of migraine headaches is at present uncertain but more research could result in new ways of treatment. Melatonin is the major messenger of light-dependent periodicity, implicated in the seasonal reproduction of animals and pubertal development in humans. Multiple receptor sites detected in brain and gonadal tissues of birds and mammals of both sexes indicate that melatonin exerts a direct effect on the vertebrate reproductive organs. In a clinical study, melatonin has been used successfully as an effective female contraceptive with little side effects. Melatonin is one of the most powerful scavengers of free radicals. Because it easily penetrates the blood-brain barrier, this antioxidant may, in the future, be used for the treatment of Alzheimer’s and Parkinson’s diseases, stroke, nitric oxide, neurotoxicity and hyperbaric oxygen exposure. In the digestive tract, melatonin reduced the incidence and severity of gastric ulcers and prevented severe symptoms of colitis, such as mucosal lesions and diarrhea.

Melatonin reduces the severity of dextran‐induced colitis in mice

Abstract: Melatonin administration reduces the severity of dextran sodium sulphate (DSS)‐induced colitis in mice. After 7 weeks of daily intraperitoneal melatonin administration (150 μg/kg), rectal bleeding and occult blood was eliminated in all mice in which colitis was induced by DSS. In addition the frequency and severity of focal lesions in the mucosa was dramatically reduced. Furthermore, weight loss and higher food consumption observed in DSS‐treated mice was reversed in DSS‐treated mice injected with melatonin. All treated groups exhibited significant alterations in goblet cell distribution as a result of DSS or melatonin administration. Surprisingly, serum melatonin levels were more than 10 times higher in mice that received DSS as compared to controls. The significant improvement in the conditions of melatonin‐treated mice might be due to its effect on the smooth muscles of the colon, the blood supply in the mucosa, its capability as an antioxidant and scavenger of free radicals, or its effect on the immune system of the gut. The higher plasma levels of melatonin in DSS‐treated mice might be due to a stress‐induced increase in the production of gastrointestinal (GIT) melatonin.

Localization, Physiological Significance and Possible Clinical Implication of Gastrointestinal Melatonin

The gastrointestinal tract (GIT) is a major source of extrapineal melatonin. In some animals, tissue concentrations of melatonin in the GIT surpass blood levels by 10–100 times and the digestive tract contributes significantly to melatonin concentrations in the peripheral blood, particularly during the day. Some melatonin found in the GIT may originate from the pineal gland, as the organs of the digestive system contain binding sites, which in some species exhibit circadian variation. Unlike the production of pineal melatonin, which is under the photoperiodic control, release of GI melatonin seems to be related to periodicity of food intake. Melatonin and melatonin binding sites were localized in all GI tissues of mammalian and avian embryos. Postnatally, melatonin was localized in the GIT of newborn mice and rats. Phylogenetically, melatonin and melatonin binding sites were detected in GIT of numerous mammals, birds and lower vertebrates. Melatonin is probably produced in the serotonin-rich enterochromaffin cells (EC) of the GI mucosa and can be released into the portal vein postprandially. In addition, melatonin can act as an autocrine or a paracrine hormone affecting the function of GI epithelium, lymphatic tissues of the immune system and the smooth muscles of the digestive tube. Finally, melatonin may act as a luminal hormone, synchronizing the sequential digestive processes. Higher peripheral and tissue levels of melatonin were observed not only after food intake but also after a long-term food deprivation. Such melatonin release may have a direct effect on the various GI tissues but may also act indirectly via the CNS; such action might be mediated by sympathetic or parasympathetic nerves. Melatonin can protect GI mucosa from ulceration by its antioxidant action, stimulation of the immune system and by fostering microcirculation and epithelial regeneration. Melatonin may reduce the secretion of pepsin and the hydrochloric acid and influence the activity of the myoelectric complexes of the gut via its action in the CNS. Tissue or blood levels of melatonin may serve as a marker of GI lesions or tumors. Clinically, melatonin has a potential for a prevention or treatment of colorectal cancer, ulcerative colitis, irritable bowel syndrome, children colic and diarrhea.

Melatonin concentrations in serum and tissues of porcine gastrointestinal tract and their relationship to the intake and passage of food

Abstract: Melatonin concentrations were determined in serum and 10 segments of the gastrointestinal tract (GIT) of 48 pigs (100 kg weight). The animals were fasted for 30 hr and then sacrificed 0, 1,2, 5, 10, and 20 hr after refeeding. Peak amount of gastric digesta (2,428 g) and ileum digesta (850 g) were observed 1 hr and 5 hr, after refeeding, respectively. Conversely, colon content reached a minimal weight (726 g) at 2 hr after refeeding. Serum levels of melatonin increased from 3.4 pg/ml to 15.5 pg/ ml (peak 5 hr after refeeding). Melatonin levels in GIT tissues before refeeding varied from 23.8 pg/g (stomach‐fundus) to 62.1 pg/g (rectum). Increasingly higher levels of melatonin were detected in the distal segments of the GIT. Higher melatonin levels after refeeding were observed in most GIT tissues except the rectum. In most tissues, peak melatonin values were detected 5 hr after refeeding. A significant change in weight of digesta across time (P<0.05) was detected in the stomach, ileum, and cecum. Similar changes in melatonin levels across time were found in most tissues except the esophagus, stomach (cardia and pylorus), and rectum. Adjacent GIT tissues exhibited similar (P<0.05) melatonin levels. The GIT melatonin levels correlated best with the variation of digesta weight in the ileum. In addition, the increase of serum melatonin levels correlated best with the increase of GIT melatonin levels in the distal part of the GIT. Our results suggest that melatonin produced in the ileum, cecum, and colon may contribute significantly to the short‐term increase of serum melatonin levels observed after refeeding.

Melatonin concentrations in the luminal fluid, mucosa, and muscularis of the bovine and porcine gastrointestinal tract

Abstract: Melatonin concentrations were measured in serum, luminal fluid, and tissues of the mucosa and muscularis of the entire bovine and porcine gastrointestinal tract (GIT). In both species, GIT levels profoundly exceeded serum levels. In pigs, melatonin was lowest in the luminal fluid and highest in the mucosa. No difference was found in various layers of bovine GIT. Compared to pigs, cows had higher melatonin levels in the stomach and ileum, but lower in the cecum and colon. There was no difference in melatonin levels between anterior and posterior segments of bovine GIT, whereas pigs exhibited several fold higher concentration of melatonin in the posterior segment (cecum and colon). Conversely, melatonin values in the anterior segment were significantly higher in cows, but in the posterior segments porcine values were higher. In cows, concentrations in the mucosa correlated with levels in the muscularis. Melatonin levels in the mucosa and muscularis were higher in the rumen and reticulum than in the omasum and abomasum. The species‐specific levels and a distinct distribution of melatonin in the layers of the digestive tube indicates that this indole may be involved in the modulation of gastrointestinal function of monogastric as well as polygastric ungulates, albeit in a different capacity.

The Effect of Serotonin, N‐Acetylserotonin, and Melatonin on Spontaneous Contractions of Isolated Rat Intestine

A dose‐dependent increase in tone and reduction in amplitude of contractions was observed after serotonin (5‐HT) was administered to isolated segments of rat ileum, incubated in Lockes solution at 38°C. Melatonin (M) reduced the tone but not the amplitude or frequency of contractions. Addition of M (administered in doses 20 to 100 × higher than 5‐HT) relieved the spasm induced by 5‐HT. Furthermore, pretreatment with M significantly reduced the 5‐HT effect. N‐acetylserotonin (NAS) exhibited delayed but similar effects to M. Neither M nor NAS could prevent or relieve acetylcholine‐induced contractions or influence relief of intestinal contractions by adrenaline. This indicates that 5‐HT and M act via a different mechanism than that of adrenaline and acetycholine system. Serotonin muscle receptor blocker methysergide reduced 5‐HT effect but was not able to abolish it completely. As methysergide could not reduce the muscle tone and did not relieve spasm caused by 5‐HT, it is speculated that M is not acting as antagonist of 5‐HT‐ stimulatory receptors but rather as agonist of 5‐HT‐inhibiting neuronal receptors.

Pinealectomy Reduces Melatonin Levels in the Serum but Not in the Gastrointestinal Tract of Rats

Melatonin levels were determined in serum and gastrointestinal tract (GIT) tissues of control (C), sham-pinelaectomized (SPx) and pinealectomized (Px) rats sacrificed in mid scotophase. Serum melatonin concentrations of Px rats exhibited the significantly lowest values (8.6 pg/ml), followed by SPx (20.1 pg/ml) and C (37.5 pg/ml) rats. In C, the ileum (542 pg/ml) and jejunum (531 pg/ml) exhibited the highest average GIT concentrations, followed by the colon (362 pg/ml), stomach (359 pg/ml) and cecum (164 pg/ml). However, only jejunum and ileum samples had significantly higher melatonin levels than cecum samples. There were no major differences between GIT melatonin levels in Px and C rats (range: 169-247 pg/ml). Statistically, pinealectomy did not influence melatonin levels in the GIT of rats. The findings support the hypothesis that melatonin concentrations in the tissues of the GIT are independent of pineal production.

The effect of food deprivation on brain and gastrointestinal tissue levels of tryptophan, serotonin, 5‐hydroxyindoleacetic acid, and melatonin

Abstract: In order to investigate the effect of food deprivation on the levels of indoles in the brain and the gastrointestinal tissues, we have determined tissue levels of tryptophan (TRP), serotonin (5‐HT), 5‐hydroxyindoleacetic acid (5‐H1AA), and melatonin in the brain and the gastrointestinal tract (GIT) of mice on ad libitum diet as well as in mice deprived of food for 24 and 48 hr. The reduction of food intake 1) had no effect on TRP levels in the brain, but increased TRP concentrations in the stomach and the gut, especially in the colon; 2) decreased 5‐HT levels in the brain, but increased values in the stomach and the intestines; 3) decreased 5‐HIAA levels in the brain, but increased them in the stomach and the intestines; 4) did not change 5‐HT conversion to 5‐HIAA in the brain, stomach, and the jejunum, but increased the conversion in the ileum and colon and; 5) increased melatonin levels in all tissues investigated, particularly in the stomach and the brain. The changes of indole levels induced by food deprivation were compared to their known function in the brain and the individual segments of the GIT. A possible serotonin‐melatonin antagonism in the brain and GIT function is considered.