Circadian Rhythms in Peripheral Serotonin in Mice

Most organisms possess biological circadian clocks which control and coordinate numerous physiological processes over each twenty-four hour day. Circadian oscillators play a role in generating biological rhythms and coordinating numerous processes with environmental stimuli (such as timing of a meal or exposure to light). In rodents it has been demonstrated that restricting the availability of food to a short window of time during the day can shift the phase of rhythmic oscillations in some peripheral organs (such as the gut and liver) while not affecting the phase of the light entrainable hypothalamic circadian pacemaker. We hypothesize that this paradigm of restricted feeding (RF) would result in a phase shift in the daily rhythm of the hormone serotonin within the serum and gut of mice compared with mice maintained on an ad libitum diet (AL). We also measured circulating levels of platelets since most of the serotonin in the blood is taken up and transported by platelets. Here we present data demonstrating the daily profile of serotonin and its major metabolite, 5HIAA, in serum and duodenum of AL or RF animals maintained under LD or free running conditions. As expected, motility rhythms entrained to the rhythm of food availability cues, whereas no statistically significant rhythm was detected in serum serotonin levels. In duodenum, serotonin and 5HIAA levels appeared to be rhythmic in AL mice only. Interestingly, a circadian rhythm in circulating platelets levels was detected only under RF conditions, but not in AL animals or in constant darkness. This suggests that food intake, but not light, may drive oscillations in platelet production or serve as a zeitgeber for entrainment. Further investigations of circadian control of serotonin biosynthesis and metabolism is ongoing.