Serotonin is synthesized by two distinct tryptophan hydroxylases, one in the brain and one in the periphery. The latter is known to be unable to cross the blood-brain barrier. These two serotonin systems have apparently independent functions, although the functions of peripheral serotonin have yet to be fully elucidated. In this study, we have investigated the physiological effect of peripheral serotonin on the concentrations of metabolites in the circulation and in the liver. After fasting, mice were ip injected with 1 mg serotonin. The plasma glucose concentration was significantly elevated between 60 and 270 min after the injection. In contrast, plasma triglyceride, cholesterol, and nonesterified fatty acid concentrations were decreased. The hepatic glycogen synthesis and concentrations were significantly higher at 240 min. At the same time, the hepatic triglyceride content was significantly lower than the basal levels noted before the serotonin injection, whereas the hepatic cholesterol content was significantly higher by 60 min after the injection. Furthermore, serotonin stimulated the contraction of the gallbladder and the excretion of bile. After the serotonin injection, there was a significant induction of apical sodium-dependent bile acid transporter expression, resulting in a decrease in the concentration of bile acids in the feces. Additionally, data are presented to show that the functions of serotonin are mediated through diverse serotonin receptor subtypes. These data indicate that peripheral serotonin accelerates the metabolism of lipid by increasing the concentration of bile acids in circulation.

Heat stress affects endocrine systems in cows. This study investigated changes in insulin and glucagon secretion between thermoneutral (TN; 18°C, relative humidity [RH] 60%) and hot (28°C, RH 60%) environments in lactating cows. Glucose, arginine, and butyrate were administered i.v. to four cows (mean, at 83 d postpartum) in each environment. Blood was collected via a jugular catheter at regular intervals. Heat exposure resulted in a marked increase in respiration rate and rectal temperature. A decrease in milk yield was also observed during heat exposure. Basal insulin concentrations were elevated, and basal glucose concentrations tended to be lower in the hot environment. Peak values of insulin and glucagon following the arginine injection were significantly higher in the hot than in the TN environment. The insulin peak value in response to the butyrate infusion was also higher during the heat exposure. However, insulin and glucagon responses to the glucose load were not affected by heat stress. The increase in plasma glucose concentration following arginine injection was inhibited by the heat exposure. In conclusion, heat stress resulted in a higher insulin secretion in lactating cows. Glucagon secretion in response to the arginine injection was enhanced, but the rise in plasma glucose was inhibited by heat exposure. These changes would be related to a reduction in milk yield during heat stress.

A better understanding of the behavior of individual grazing dairy cattle will assist in improving productivity and welfare. Global positioning systems (GPS) applied to cows could provide a means of monitoring grazing herds while overcoming the substantial efforts required for manual observation. Any model of behavioral prediction using GPS needs to be accurate and robust by accounting for inter-cow variation as well as atmospheric effects. We evaluated the performance using a series of machine learning algorithms on GPS data collected from 40 pasture-based dairy cows over 4 mo. A feature extraction step was performed on the collected raw GPS data, which resulted in 43 different attributes. The evaluated behaviors were grazing, resting, and walking. Classifier learners were built using 10 times 10-fold cross validation and tested on an independent test set. Results were evaluated using a variety of statistical significance tests across all parameters. We found that final model selection depended upon level of performance and model complexity. The classifier learner deemed most suitable for this particular problem was JRip, a rule-based learner (classification accuracy=0.85; false positive rate=0.10; F-measure=0.76; area under the receiver operating curve=0.87). This model will be used in further studies to assess the behavior and welfare of pasture-based dairy cows.

Two independent serotonin systems exist, one in the brain and the other in the periphery. Serotonin is a well known monoaminergic neurotransmitter in the central nervous system and it is known to regulate feeding behavior, meal size, and body weight. On the other hand, there is much less evidence for the role of serotonin as a gastrointestinal hormone, particularly with respect to its effects on glucose and lipid metabolism. This review summarizes our current understanding of the role of peripheral serotonin on glucose and lipid metabolism and the implications of this for further research. The enterochromaffin cells of the gastrointestinal tract produce peripheral serotonin postprandially. In mice, it induces a decrease in the concentration of circulating lipids as well as hyperglycemia and hyperinsulinemia through its action on several serotonin receptors. Further, serotonin metabolites act as endogenous agonists for peroxisome proliferator-activated receptor γ and serotonin accelerates adipocyte differentiation via serotonin receptor 2A and 2C. Studies of serotonin are likely to provide new insights into the field of lipid accumulation and metabolism. Recent studies show new physiological functions of peripheral serotonin, linked to glucose and lipid metabolism. Peripheral serotonin may serve as an attractive new therapeutic target for the treatment of metabolic disorders in the near future.