Time for Zebrafish

it is now possible to effectively examine neural substrates underlying interval timing in zebrafish at single-cell resolution and in 3D. Sumbre et al. (2008) recently demonstrated that larval zebrafish (between 5 and 14 days post fertilization, or dpf) could follow temporal patterns of rhythmic stimuli both at the neuronal level in the optic tectum (a visual area), as well as at the behavioral level in its tail flips. The temporal rhythm was established by repeatedly presenting a 200 ms visual (light flash) conditioned stimulus (CS) at a fixed inter-stimulus interval (ISI) of 4, 6, or 10 s. It was found that the tectal neurons entrained to the CS, such that the calcium signals increased transiently and, more importantly, in synchrony with the ISI. Moreover, this synchronous neural activity pattern continued for several cycles even after the flashing CS terminated. The same study also revealed that the tectal neurons in a fish that is as young as 4 days (one out of seven tested fish), but not 3 days old, was able to keep repeating a temporal pattern established by the ISI. From a developmental perspective, it is intriguing that a 4–5 days old zebrafish brain is sophisticated enough to react to a visual stimulus and to follow its temporal patterns. In general, zebrafish embryos start hatching after 2–3 dpf (henceforth called larvae) and the larvae need to hunt for food after their yolk is depleted at 5 dpf. Therefore, it is reasonable to assume that the developing zebrafish brain is mature enough and ready to function after 5 dpf, with timing external stimuli a part of its abilities. During development, dopamine (DA) neurons in the zebrafish brain mature at different time points ranging from 1 dpf in the diencephalon to 3 dpf in the telencephalon (Mahler et al., 2010). The zebrafish telencephalon is composed of the pallium and the subpallium, which is teleost analog of cortico-basal-ganglia circuits in mammals (Rink and Wullimann, 2002). The DA neurotransmitter systems, especially the ones in cortico-striatal Time for zebrafish