Abnormal development of the locus coeruleus in Ear2(Nr2f6)-deficient mice impairs the functionality of the forebrain clock and affects nociception.

The orphan nuclear receptor Ear2 (Nr2f6) is transiently expressed in the rostral part of the rhombic lip in which the locus coeruleus (LC) arises. LC development, regulated by a signaling cascade (Mash1 --> Phox2b --> Phox2a), is disrupted in Ear2-/- embryos as revealed by an approximately threefold reduction in the number of Phox2a- and Phox2b-expressing LC progenitor cells. Mash1 expression in the rhombic lip, however, is unaffected, placing Ear2 in between Mash1 and Phox2a/b. Dopamine-beta-hydroxylase and tyrosine hydroxylase staining demonstrate that >70% of LC neurons are absent in the adult with agenesis affecting primarily the dorsal division of the LC. Normally, this division projects noradrenergic efferents to the cortex that appear to be diminished in Ear2-/- since the cortical concentration of noradrenaline is four times lower in these mice. The rostral region of the cortex is known to contain a circadian pacemaker regulating adaptability to light- and restricted food-driven entrainment. In situ hybridization establishes that the circadian expression pattern of the clock gene Period1 is abolished in the Ear2-/- forebrain. Behavioral experiments reveal that Ear2 mutants have a delayed entrainment to shifted light-dark cycles and adapt less efficiently to daytime feeding schedules. We propose that neurons in the dorsal division of LC contribute to the regulation of the forebrain clock, at least in part, through targeted release of noradrenaline into the cortical area.