Feeding entrainment of the circadian clock in zebrafish and cavefish

Circadian clocks are normally synchronised (entrained) by periodic environmental cues (zeitgeber). Food availability for only a limited time each day is one of the stronger zeitgeber signal for circadian rhythms. This entrainment can be detected by the occurrence of the food-anticipatory activity (FAA), which consists in an increment on the locomotor activity few hours before mealtime. Recent investigations showed that food-entrainable oscillators are present in the circadian system of almost all vertebrates, including fish. We studied the effect of periodic food administration on the locomotor activity rhythms of 3 species of teleosts evolved in different environments: the zebrafish Danio rerio and the blind cavefish Astyanax mexicanus and Phreatichthys andruzzii. Adult fish were fed with different protocols (every 24, 36, 44, 48, 72 and 96 hours) for more than one month under constant darkness, and during this period, locomotor activity was measured. For all species we observed 1) the FAA, indicative of regulation by a food entrainable oscillator (FEO), 2) a strong entrainment of rhythmic locomotor activity, and 3) a freerunning activity under fasting. Taking together, our results point to that food is a stronger zeitgeber in cavefish than in zebrafish. To better understand the developed of feeding rhythms we established a protocol to study feeding rhythms in fish larvae using videotracking setup (Daniovision, Noldus). Preliminary results seem to indicate the onset of the feeding rhythms in young larvae. Several lines of evidence point to the existence of a FEO in vertebrates distinct from the light-entrainable oscillator (LEO). Fish could emerge as powerful models for the investigation of food entrainment. A comparative study involving zebrafish that possess both LEO and FEO and cavefish that has only the FEO could provide important insight into the basis of the evolution of the feeding entrainment mechanism in vertebrates.