Activation of state-regulating neurochemical systems in newborn and embryonic chicks

•Newborn awake chicks show coordinated activation of arousal systems.•Arousal systems maturate and are able to support waking shortly after hatching.•One day before hatching, chick embryos show activation of sleep-regulatory systems.•These data support the presence of a sleep-like state in late-stage chick embryos.

Coordinated activity in different sets of widely-projecting neurochemical systems characterize waking (W) and sleep (S). How and when this coordination is achieved during development is not known. We used embryos and newborns of a precocial bird species (chickens) to assess developmental activation in different neurochemical systems using cFos expression, which has been extensively employed to examine cellular activation during S and W in adult mammals. Similarly to adult mammals, newborn awake chicks showed significantly higher cFos expression in W-active hypocretin/orexin (H/O), serotonergic Dorsal Raphe, noradrenergic Locus Coeruleus and cholinergic Laterodorsal and Pedunculopontine Tegmental (Ch-LDT/PT) neurons when compared to sleeping chicks. cFos expression was significantly correlated both between these systems, and with the amount of W. S-active melanin-concentrating hormone (MCH) neurons showed very low cFos expression with no difference between sleeping and awake chicks, possibly due to the very short duration of S episodes. In embryonic chicks, cFos expression was low or absent across all five systems at embryonic day (E) 12. Unexpectedly, a strong activation was seen at E16 in H/O neurons. The highest activation of Ch-LDT/PT (also S-active) and MCH neurons was seen at E20. These data suggest that maturation of arousal systems is achieved soon after hatching, while S-control networks are active in late chick embryos.