The recent literature contains reports of the action in dogs of serotonin which stimulates the mucin secretion of stomach and inhibits the acid secretion (White and Magee, 1958; Black et al.,1958; Whang, 1962).
In 1959 White et at. stated that the secretion of mucin was directly due to the action of serotonin and was not due to mechanical stimulation from peristalsis or the rubbing together of mucosal surfaces. The mucin has a protective effect on the
gastric mucosa, and also reduces arid secretion.
Paradoxically, however, others have reported the use of serotonin as an ulcerogenic agent (Nikodijevic and Vanov, 1960; Nikodijevic and Trajkov, 1963).
Brodie et al. in 1962 found a 70% incidence of gastric ulcers in rats which had been given 8 mg of serotonin per kg. Gastric acidity was not significantly changed.
Using 10 rats which had been fasted for 24 hours before the administration of serotonin we repeated Brodie's experiment, but were unable to duplicate his findings. The only change was a slight increase in the mucin content of the gastric juice.
The present experiment, using rats in which the pylorus had been closed by ligature, investigated the effect of serotonin in preventing, or protecting against, gastric ulceration.
A hundred and twenty albino rats, weighing 200 to 300 gm, of both sexes were used. The animals were starved for forty-eight to ninth-six hours depending upon the experimental schedule. During the starvation period, water was permitted ad libitum. Throughout the experimental period the animals were housed singly in cages with raised bottoms of wide wire mesh to avoid coprophagy. The pyloric ligature was placed by the procedure described by Shay et al. (1945).
Ninty-six rats were used in the first series of experiments. The first group, a control group of 16 rats was starved for 96 hours and sacrificed. This group did not have pyloric ligation. The second group of 14 rats after starving for 48 hours had ligation of the pylorus. The animals were sacrificed 5 hours after pyloric ligation. The third group of 20 rats was starved for 48 hours and then the pyloric ligature was placed. The animals were sacrificed 10 hours after pyloric ligation. The fourth group of 30 rats was starved and the pyloric ligature placed as in the second group, but the animals were sacrificed 20 hours after pyloric ligation. The fifth group of 16 rats were starved for 72 hours and the pyloric ligature placed as with the second group, and were sacrificed 20 hours after pyloric ligation.
Daily serotonin (8 mg/kg) was given subcutaneously to one-half of the animals in each group during starvation and after pyloric ligation, while the remaining one-half of the animals in each group served as the serotonin untreated control.
Twenty-four rats were used in the second series of experiments. The first group of six rats was the control, e.g., the rats were starved for 48 hours and then the pyloric ligation was performed. 20 hours after the pyloric ligation the rats were sacrificed. The second, third or fourth group of six rats each were starved and received the pyloric ligation as did the control group in this second experiment.
Following starvation and pyloric ligation the rats were given daily doses of atropine to the second group, morphine to the third group, and a bilateral vagotomy was done in the fourth group.
Under ether anesthesia, the rats were killed by exanguination from the femoral artery. The stomach was removed, trimmed free of adipose tissue, and then opened by cutting along the greater curvature. The gastric juice was collected. The specimens were then washed under tap water. The volumes of solid matter and juice were noted.
Free and total acid of the juice were titrated in the usual manner using Topfer's and phenolphthalein as the indicators and N/2O NaOH for titration. In some animals the gastric mucin was determined by the method of micro-Kjeldahl and pepsin activity by the method of Riggs and Stadie(1943).
Ulcerative gastric lesions in rats were produced by the procedure of Shay et al.(1945), which was a simple technique, namely pyloric ligation after starvation. The ulceration develops uniformly in the rumen, less of ten in the antrum, and least frequently in the body of the stomach. The lesions are most marked in the rumen. The administration of serotonin was effective in preventing the occurrence of gastric lesions Produced by pyloric ligation after starvation. When the rats were
starved for 48 hours and then the pyloric ligation left for 20 hours, the incidence of gastric ulceration was 92.9%. However, pretreatment of the animals with serotonin partially prevented the gastric lesions and markedly reduced the incidence to 45.5%. Though the incidence of gastric lesions depends upon the time elapse from either starvation or pyloric ligation to the time the animals were sacrificed, one can clearly see in every group in the thirst experiment that the administration of serotonin was effective in preventing the gastric lesions.
The acidity of gastric juice was considerably lower in the animals treated with serotonin than the nontreated control animals. It was true particularly in the group which had 48 hours of starvation and 5 or 10 hours of pyloric ligation. Here only a few ulcerative lesions were found. The determination of gastric contents was often impossible in the group having more than 48 hours of starvation and 5 hours of pyloric ligation . Mucin contents could not be determined in the other groups because of the extensive gastric ulceration.
In the second experiment the bilateral gastric vagotomy was completely effective in preventing the gastric lesions usually produced by 48 hours of starvation and 20 hours of pyloric ligation. Pretreatment using either atropine or morphine wad moderately effective in preventing the gastric lesions usually produced by the same procedure.
Microscopically, the majority of the rats showed one or more of ulcers in the rumen. However, the animals in the first group that were given serotonin did not show any evidence of ulcer. The severity and the numbers of ulcers differed from one group to another but, in general in each group the rats that were given serotonin showed less severe and less frequent ulcers than the control animals.
The first stage of the ulcer was characterized by an extreme thinning and later a breaking of the stratified squamous epithelium associated with the collection of acute inflammatory exudate. A moderate to marked degree of edema was present under the mucosa. The inflammatory exudative cells appeared to hug the epithelial portions of the mucosa. The ulcers of a moderate severity were characterized by thecomplete loss of the superficial epithelium with more wider areas of necrosis and
localized collections of polymorphonuclear leukocytes extending into the deeper portions of the gastric wall. Necrotic fibrinoid materials were frequently seenover the ulcer surface. Widely scattered inflammatory cells were noted along with the edema of the neighboring tissue. The most severe degree of ulcer was characterized by necrosis of the entire wall associated with an intense inflammatory reaction. The gastric wall at these points showed gangrene. The superficial layers often were covered by blood clot.
Although there was a considerable degree of individual variation, the studies,
after Periodic Acid Schiff reaction, revealed that mucus secretion was greater in
the animals that were given serotonin than in the control animals. Frequently,
patches of mucus were noted in the lumen over the gastric glands. Therefore, it is
concluded that serotonin is effective in preventing ulceration in the stomach by
its action of increasing mucin secretion and inhibiting gastric acid secretion.